Inhibition, Na+/I- symporter (NIS)

14797-73-0

VLTRZXGMWDSKGL-UHFFFAOYSA-M

Perchlorate

Perchlorate ion Perchlorate ion (ClO41-) Perchlorate ion(1-) Perchlorate(1-) Perchloric acid, ion(1-)

DTXSID6024252

14797-55-8

NHNBFGGVMKEFGY-UHFFFAOYSA-N

Nitrate

Nitrate (NO3-) Nitrate ion Nitrate ion (NO3-) Nitrate ion(1-) Nitrate(1-) Nitrates/nitrites Nitrato Nitric acid, ion(1-)

DTXSID5024217

302-04-5

ZMZDMBWJUHKJPS-UHFFFAOYSA-M

Thiocyanate

Thiocyanates Isothiocyanic acid, ion(1-) Rhodanide Thiocyanate (NCS1-) Thiocyanate anion Thiocyanate ion Thiocyanic acid, ion(1-) Thiocyanide

DTXSID8047763

51-52-5

KNAHARQHSZJURB-UHFFFAOYSA-N

6-Propyl-2-thiouracil

6-Propyl-2 thiouracil (PTU) 4(1H)-Pyrimidinone, 2,3-dihydro-6-propyl-2-thioxo- 2,3-Dihydro-6-propyl-2-thioxo-4(1H)-pyrimidinone 2-Mercapto-4-hydroxy-6-n-propylpyrimidine 2-Mercapto-4-hydroxy-6-propylpyrimidine 2-Mercapto-6-propylpyrimidin-4-ol 2-Thio-4-oxo-6-propyl-1,3-pyrimidine 2-Thio-6-propyl-1,3-pyrimidin-4-one 6-n-Propyl-2-thiouracil 6-n-Propylthiouracil 6-Propyl-2-thio-2,4(1H,3H)pyrimidinedione 6-Propylthiouracil NSC 6498 NSC 70461 Procasil Propacil propiltiouracilo Propycil Propyl-Thiorist Propylthiorit propylthiouracil Propylthiouracile Propyl-Thyracil Prothiucil Prothiurone Prothycil Prothyran Protiural Thiuragyl Thyreostat II URACIL, 4-PROPYL-2-THIO- Uracil, 6-propyl-2-thio-

DTXSID5021209

60-56-0

PMRYVIKBURPHAH-UHFFFAOYSA-N

Methimazole

2H-Imidazole-2-thione, 1,3-dihydro-1-methyl- 1,3-Dihydro-1-methyl-2H-imidazole-2-thione 1-Methyl-1,3-dihydroimidazole-2-thione 1-Methyl-1H-imidazole-2-thiol 1-Methyl-2-mercapto-1H-imidazole 1-Methyl-2-mercaptoimidazole 1-Methyl-4-imidazoline-2-thione 1-Methylimidazole-2(3H)-thione 1-Methylimidazole-2-thiol 1-Methylimidazole-2-thione 2-Mercapto-1-methyl-1H-imidazole 2-Mercapto-1-methylimidazole 2-Mercapto-N-methylimidazole 4-Imidazoline-2-thione, 1-methyl- Basolan Danantizol Favistan Frentirox Imidazole-2-thiol, 1-methyl- Mercaptazole Mercazole Mercazolyl Metazolo Methimazol Methylmercaptoimidazole Metothyrin Metothyrine Metotirin N-Methyl-2-mercaptoimidazole N-Methylimidazolethiol NSC 38608 Strumazol Tapazole Thacapzol Thiamazol thiamazole Thycapzol Thymidazol Thymidazole tiamazol

DTXSID4020820

PR:000015171

PR sodium/iodide cotransporter

CHEBI:60311

CHEBI thyroid hormone

CHEBI:30660

CHEBI thyroxine

UBERON:0001016

UBERON nervous system

GO:0008507

GO sodium:iodide symporter activity

GO:0006590

GO thyroid hormone generation

MP:0005475

MP abnormal circulating thyroxine level

GO:0010817

GO regulation of hormone levels

GO:0048856

GO anatomical structure development

MP:0003633

MP abnormal nervous system physiology

GO:0007611

GO learning or memory

GO:0050890

GO cognition

WIKI decreased

WIKI abnormal

Perchlorate

2016-11-29T18:42:26

Nitrate

2016-11-29T18:42:26

Thiocyanate

2016-11-29T18:42:26

Dysidenin

2016-11-29T18:42:26

Aryltrifluoroborates

2016-11-29T18:42:26

Econazole

2016-12-07T11:06:26

5-(N,N-hexamethylene) amiloride (HMA)

2016-12-07T11:06:59

Small molecules: ITB3, ITB4, ITB5, ITB9

2016-12-07T11:16:09

Propylthiouracil

2016-11-29T18:42:22

Methimazole

2016-11-29T18:42:19

Iodine deficiency

2017-03-26T11:37:44

WCS_9606

common toxicological species human

10116

NCBI rat

10090

NCBI mouse

WikiUser_24

Wikiuser:Migration Pig

7955

NCBI zebra fish

224340

NCBI Xenopus (Silurana) epitropicalis

WCS_8355

common ecological species African clawed frog

WCS_8355

common ecological species Xenopus laevis

WCS_7955

common ecological species zebrafish

WCS_90988

common ecological species fathead minnow

9823

NCBI Sus scrofa

WCS_9031

common ecological species chicken

10118

NCBI Rattus sp.

Inhibition, Na+/I- symporter (NIS)

Molecular

<h4>Evidence for Perturbation by Stressor</h4> <h5>Overview for Molecular Initiating Event</h5> Thyroid Disrupting Chemicals (TDCs) are defined as the xenobiotics that interfere with the thyroid axis with different outcomes for the organism. A very well-studied mechanism of action of the TDCs is the reduction of the circulating levels of THs by inhibiting hormone synthesis in the thyroid gland. For example, perchlorate is a very potent inhibitor of iodide uptake through the sodium/iodide symporter (Tonacchera et al., 2004). Perchlorate has been detected in human breast milk ranging from 1.4 to 92.2 mg μl–1 (10.5 μg l–1 mean) in 18 US states (Kirk et al. 2005), and 1.3 to 411 μg l–1 (9.1 μg l–1 median) in the Boston area, United States (Pearce et al. 2007). Perchlorate has also been detected in human colostrum of 46 women in the Boston area (from < 0.05 to 187.2 μmol l–1 (Leung et al. 2009)). The mechanism of perchlorate action is quite simple, as it is believed to be mediated only by the NIS inhibition (Dohan et al., 2007; Wolff, 1998). Additionally, thiocyanate and nitrate are two known inhibitors that have been found to reduce circulating TH levels (Blount et al., 2006; Steinhaus et al., 2007), but they are both less potent than perchlorate (Tonacchera et al., 2004). However, there are also contradictory results from other studies that showed no correlation between thyroid parameters and perchlorate levels in humans (Pearce et al., 2010; Amitai et al., 2007; Tellez et al., 2005). Co-occurrence of perchlorate, nitrate, and thiocyanate can alter thyroid function in pregnant women. Horton et al. (2015) have shown positive associations between the weighted sum of urinary concentrations of these three analytes and increased TSH, with perchlorate showing the largest weight in the index. Interestingly, De Groef et al. 2006 showed that nitrate and thiocyanate, acquired through drinking water or food, account for a much larger proportion of iodine uptake inhibition than perchlorate, suggesting that NIS inhibition and any potential downstream effect by perchlorate are highly dependent on the presence of other environmental NIS inhibitors and iodine intake itself (Leung et al., 2010). In particular, Tonacchera et al. (2004) showed that the relative potency of perchlorate to inhibit radioactive I− uptake by NIS is 15, 30 and 240 times that of thiocyanate, iodide, and nitrate respectively on a molar concentration basis. These data are in line with earlier studies in rats (Alexander and Wolff, 1996; Greer et al. 1966). Contradictory findings in these studies may therefore be a result of the confounding mixtures in the environment, masking the primary effect of perchlorate. Decreased iodine intake can decrease TH production, and therefore exposure to perchlorate might be particularly detrimental in iodine-deficient individuals (Leung et al. 2010). Moreover, biologically based dose-response modeling of the relationships among iodide status (e.g., dietary iodine levels), perchlorate dose, and TH production in pregnant women has shown that iodide intake has a profound effect on the likelihood that exposure to goitrogens will produce hypothyroxinemia (Lewandowski et al. 2015). During pregnancy TH requirements increase, particularly during the first trimester (Alexander et al. 2004; Leung et al. 2010), due to higher concentrations of thyroxine-binding globulin, placental T4 inner-ring deiodination leading to the inactive reverse T3 (rT3), and transfer of small amounts of T4 to the foetus (during the first trimester foetal thyroid function is absent). Moreover, glomerular filtration rate and clearance of proteins and other molecules are both increased during pregnancy, possibly causing increased renal iodide clearance and a decreased of circulating plasma iodine (Glinoer, 1997). Thus, even though the foetal thyroid can trap iodide by about 12 week of gestation (Fisher and Klein, 1981), high concentrations of maternal perchlorate may potentially decrease thyroidal iodine available to the foetus by inhibiting placental NIS (Leung et al. 2010). Consequences of TH deficiency depend on the developmental timing of the deficiency (Zoeller and Rovet, 2004). For instance, if the TH deficiency occurs during early pregnancy, offspring show visual attention, visual processing and gross motor skills deficits, while if it occurs later, offspring may show subnormal visual and visuospatial skills, along with slower response speeds and motor deficits. If TH insufficiency occurs after birth, language and memory skills are most predominantly affected (Zoeller and Rovet, 2004). Along this line, age and developmental stage are crucial in determining sensitivity to NIS inhibitors (e.g., perchlorate, thiocyanate, and nitrate). In this regard, McMullen et al. (2017) have shown that adolescent boys and girls, more than adults, represent vulnerable subpopulations to NIS symporter inhibitors. Altogether these studies indicate that age, gender, developmental stage, and dietary iodine levels can affect the impact of NIS inhibitors. Finally, ten more small simple-structured molecules were identified in a large screening study (Lecat-Guillet et al., 2008b) that could block iodide uptake by specifically disrupting NIS in a dose-dependent manner. These molecules were named Iodide Transport Blockers (ITBs). There are few organic molecules that lead to NIS inhibition but no direct interaction with NIS has been determined (Gerard et al., 1994; Kaminsky et al., 1991, Lindenthal et al., 2009). Up to date, only dysidenin, a toxin isolated from the marine sponge Dysidea herbacea, has been reported to specifically inhibit NIS (Van Sande et al., 2003). Finally, the aryltrifluoroborates were found to inhibit iodide uptake with an IC50 value of 0.4 μM on rat-derived thyroid cells (Lecat-Guillet et al., 2008a). The biological activity is rationalized by the presence of the BF3− ion as a minimal binding motif for substrate recognition at the iodide binding site. It has been also shown that many anions, such as ClO3-, SCN-, NO3-, ReO4-, TcO4- and in a lower extent Br- and BF4-, are also acting as NIS substrates and they enter the cell by the same transporter mechanism (Van Sande et al., 2003). It has been also shown that ClO4- is transferred by NIS with high affinity and is considered as one of its most potent inhibitors (Dohan et al., 2007). Most recently, the aryltrifluoroborates were also shown to inhibit NIS function (Lecat-Guillet et al., 2008a). A library of 17,020 compounds was tested by a radioactive screening method with high specificity using transfected mammalian cells (Lecat-Guillet et al., 2008b; 2007) for NIS inhibition evaluation. Further studies with the most powerful iodide transport blockers showed a high diversity in their structure and mode of action (Lindenthal et al., 2009). Apart from the human, functional NIS protein has been also identified in different species, including the rat (Dai et al., 1996), the mouse (Perron et al., 2001), the pig (Selmi-Ruby et al., 2003), zebrafish (Thienpont et al., 2011) and xenopus (amphibian) (Lindenthal et al., 2009). Mouse and rat NIS proteins contain 618 amino acid residues, while the human and pig variants contain 643. There are several NIS variants that produce three active proteins in the pig due to alternative splicing at mRNA sites that are not present on the other species (Selmi-Ruby et al., 2003). NIS orthologs are discussed in the review by Darrouzet's group (Darrouzet et al., 2014). Interestingly, functional differences have been identified between mouse or rat NIS (mNIS or rNIS, respectively) and human NIS (hNIS). The rat and themouse orthologs were shown to accumulate radioisotopes more efficiently than the human protein (Dayem et al., 2008; Heltemes et al., 2003). The molecular basis of these functional differences could be helpful for further characterization of NIS. Zhang and collaborators showed that rNIS is localized in a higher proportion at the plasma membrane than hNIS and the N-terminal region up to putative transmembrane helix TM7 appears to be involved in this difference (Zhang et al., 2005). These authors also reported differences in the kinetics of the Na+ binding, implicating the region spanning from TM4 to TM6 and Ser200 of hNIS. They, thus, proposed that this region could be involved in sodium binding (Zhang et al., 2005). In our laboratory, it was shown that the Vmax of the mouse protein is four times higher than the Vmax of the human protein when expressed in the same cell line (HEK-293) (Dayem et al., 2008; Darrouzet et al., 2014). The KmI value determined for hNIS (9.0 ± 0.8 μM) was significantly lower than the KmI for the mouse protein (26.4 ± 3.5 μM) whereas the KmNa values were not significantly different indicating that mNIS has a lower iodide affinity than hNIS. Similarly to the rat protein, mNIS is predominantly localized in the plasma membrane whereas the human ortholog is detected intracellularly in 40% of the cells in which it is expressed (Darrouzet et al., 2014). However, the difference in the Vmax values does not only seem to be related to the higher intracellular localization of hNIS. Using chimeric proteins between human and mouse NIS, we showed that the N-terminal region up to TM8 is most probably involved in iodide binding, and that the region from TM5 to the C terminus could play an important role in targeting the protein to the plasma membrane (Dayem et al., 2008). One of the long-term goals of these studies is the engineering of a chimeric NIS protein most suitable for gene therapy, i.e. preserving regions responsible for the high turnover rate and the efficient plasma membrane localization of the mouse proteinwhile replacing the immunogenic extracellular regions with those of the human ortholog. The porcine NIS gene gives rise to splice variants leading to three active NIS proteins with differences in their C-terminal extremities [4]. However, it is not known if these differences lead to distinct properties (Darrouzet et al., 2014). There is evidence that the MIE (NIS inhibition) is of relevance also for fish as an expression of the slc5a5 transcript (sodium/iodide co-transporter) has been described by various publications for the zebrafish embryo (see <a href="http://www.zfin.org/">www.zfin.org</a>). It has been demonstrated that NIS inhibitors in zebra fish lead also to a strong repression of thyroid hormone levels (Thienpont et al., 2011) and in xenopus (amphibian) to  inhibition of the iodide-induced current  (Lindenthal et al., 2009). Biological state: Sodium/Iodide symporter (NIS) is a key protein in the thyroid function and its role has been thoroughly investigated after the determination of its molecular identity a few decades ago (Dai et al., 1996). NIS is an intrinsic membrane glycoprotein and it belongs to the superfamily of sodium /solute symporters (SSS) and to the family of human transporters SLC5 (De La Vieja, 2000; Jung, 2002). Its molecular weight is 87 kDa and it contains 13 transmembrane domains that transport 2 sodium cations (Na+) for each iodide anion (I-) into the follicular thyroid cell (Dohan et al., 2003). The regulation of NIS protein function is usually cell- and tissue-specific (Hingorani et al., 2010) and it is done at the transcriptional and posttranslational levels, including epigenetic regulation (Darrouzet et al., 2014; Russo et al., 2011a). One of the major NIS regulators is the thyroid stimulating hormone (TSH), which has been shown to enhance NIS mRNA and protein expression, therefore it can contribute to restore and maintain iodide uptake activity (Saito et al., 1997; Kogai et al., 2000). At the posttranslational level TSH also contributes to NIS regulation but the specific mechanisms that underlie these effects are still under investigation (Riedel et al., 2001). Biological compartments: NIS protein is mainly found at the basolateral plasma membrane of the thyroid follicular cells (Dai et al., 1996), where it actively mediates the accumulation of iodide that is the main component of thyroid hormone synthesis and therefore is considered as a major regulator of thyroid homeostasis. NIS also mediates active I- transport in extrathyroidal tissues but it is commonly agreed that is regulated and processed differently in each tissue. Functional NIS protein has been found in salivary gland ductal cells (Jhiang et al., 1998; La Perle et al., 2013), in the mammary gland during lactation (Perron et al., 2001; Cho et al., 2000), lung epithelial cells (Fragoso et la., 2004), intestinal enterocytes (Nicola et al., 2009), stomach cells (Kotani et al., 1998), placenta (Bidart et al., 2000) and testicular cells (Russo et al. 2011b). Additionally, contradictory results have been obtained regarding the NIS expression in human kidney tissue (Lacroix et al., 2001; Spitzweg et al., 2001). In the case of the lactating breast, it is suggested that NIS serves the transfer of iodide in the cells and its subsequent accumulation in the milk, thereby supplying newborns with this component during this sensitive developmental period (Tazebay et al., 2000). Additionally, NIS mRNA has been detected in various other tissues, such as colon, ovaries, uterus, and spleen (Perron et al., 2001; Spitzweg et al., 1998; Vayre et al., 1999), but the functional NIS protein and the site of its localization has not been verified. General role in biology: The NIS is known in the field of thyroidology because of its ability to mediate the active transport of I- into the thyrocytes, which is the first and most crucial step for T3 and T4 biosynthesis (Dohan et al., 2000). NIS is located on the basolateral membrane of the thyrocytes and co-transports 2 sodium ions along with 1 iodide (2:1 stoichiometry). The electrochemical gradient of sodium serves as the driving force for iodide uptake and it is generated and maintained by the Na+/K+ ATPase pump, which is located in the same membrane of the thyrocytes. The iodide molecules, after their active transport in the cytoplasm, are passively translocated in the follicular lumen via the transporter protein pendrin and possibly other unknown efflux proteins that are located on the apical membrane (Bizhanova and Kopp, 2009). Subsequently, the thyroid hormones are synthesized in the follicular lumen by incorporating the accumulated iodide, a process which is significantly suppressed in case of NIS dysfunction or inhibition (reviewed in Spitzweg and Morris, 2010). NIS is the last thyroid-related component to be expressed during development at the 10th gestational week, which temporaly coincides with the onset of thyroid function and hormonogenesis (Szinnai et al., 2007). Albeit the localization of NIS is not fully completed at this stage, the iodide accumulation has already started. Mutations of NIS gene (SLCA5A) cause expression of non-functional NIS molecule leading to inability of the thyrocyte to accumulate iodide (Matsuda and Koshugi, 1997; Pohlenz et al., 1998), a condition called iodide transport defect (ITD). This is a rear autosomic recessive disease, which if not properly treated is clinically identified by congenital hypothyroidism, goiter, low I- uptake, low saliva/plasma I- ratio and mental impairment of varying degrees (Dohan et al., 2003). Up to date 13 mutations have been described in the NIS gene (Spitzweg and Morris, 2010) and each one of them produces mutants with different structure but in all cases non-functional. The extensive study after NIS molecular characterization and the numerous findings have convinced the scientists that is one of the most crucial components of the entire thyroid system. Additionally, after the realization that NIS could be also used as diagnostic and therapeutic tool for thyroid and non-thyroid cancers (Portulano et al., 2013) a new research activity concerning this specific mechanism has been initiated.

There are several methods that are used nowadays to detect the functionality of NIS but none of these methods is OECD validated (OECD Scoping document, 2017). The most well established methods are the following: 1. Measurement of radioiodide uptake (125I-) in NIS expressing cells. For this method the FRTL5 cell line is the most commonly used, as it endogenously express the NIS protein, but also NIS transfected cell lines have been successfully implemented in many cases (Lecat-Guillet et al., 2007; 2008b; Lindenthal et al., 2009). Once inhibitory activity is identified for a compound then further tests are performed in order to verify that the observed effect is specific due to NIS inhibition. This method has been also adapted in a high throughput format and has been already used for the screening of a chemical library of 17.020 compounds (Lecat-Guillet et al., 2008b). 2. More recently a non-radioactive method has been developed, which has been also adapted in a high throughput format (Waltz et al., 2010). It is a simple spectrophotometric assay for the determination of iodide uptake  using  rat thyroid-derived cells (FRTL5) based on the catalytic effect of iodide on the reduction of yellow cerium(IV) to colorless cerium(III) in the presence of arsenious acid (Sandell-Kolthoff reaction). The assay is fast, highly reproducible and equally sensitive with the radioiodine detection method. 3. A fluorescence-based method has been developed, which uses the variant  YFP-H148Q/I152L of the Yellow Fluorescent Protein (YFP) in order to detect the efflux of iodide into the rat FRTL5 cells. As a positive control perchlorate is used  as it is a well known  competitive inhibitor of iodide transport by NIS. Fluorescence of recombinant YFP-H148Q/I152L is  suppressed by perchlorate and iodide with similar affinities. Fluorescence changes in FRTL-5 cells are  Na+-dependent, consistent with the Na+-dependence of NIS activity.   It is supposed to be an innovative approach to detect the cellular uptake of perchlorate and characterize the kinetics of transport by NIS. This method needs further optimization, as YFP is not specific for iodide and thus binding of other ionic molecules could affect the results of the assay (Cianchetta et al., 2010; Rhoden et al., 2008; Di Bernarde et al., 2011). 4. In vivo 125I uptake assays is based on  immunofluorescence analyses of thyroid glands after the treatment of rat with excess I−,  injected with Ci Na125I  as previously described by Ferreira et al., 2005. Then the thyroid glands are  removed and weighed, and the amount of 125I in the thyroid gland is  measured in a γ-counter (PerkinElmer; model Wizard). The counts per minute in the thyroid gland are used to calculate the percentage of 125I in the thyroid gland, having in account that 100% corresponded to the counts per minute injected I− into the rat (Arriagada et al., 2015). 5. The U.S. EPA's Endocrine Disruptor Screening Program aims to use high-throughput assays and computational toxicology models to screen and prioritize chemicals that may disrupt the thyroid signaling pathway. Thyroid hormone biosynthesis requires active iodide uptake mediated by the sodium/iodide symporter (NIS). Monovalent anions, such as the environmental contaminant perchlorate, are competitive inhibitors of NIS, yet limited information exists for more structurally diverse chemicals. A novel cell line expressing human NIS, hNIS-HEK293TEPA, was used in a radioactive iodide uptake (RAIU) assay to identify inhibitors of NIS-mediated iodide uptake. The RAIU assay was optimized and performance evaluated with 12 reference chemicals comprising known NIS inhibitors and inactive compounds. An additional 39 chemicals including environmental contaminants were evaluated, with 28 inhibiting RAIU over 20% of that observed for solvent controls. Cell viability assays were performed to assess any confounding effects of cytotoxicity. RAIU and cytotoxic responses were used to calculate selectivity scores to group chemicals based on their potential to affect NIS. RAIU IC50 values were also determined for chemicals that displayed concentration-dependent inhibition of RAIU (≥50%) without cytotoxicity. Strong assay performance and highly reproducible results support the utilization of this approach to screen large chemical libraries for inhibitors of NIS-mediated iodide uptake (Hallinger et al., 2017). 6. This study (Wang et al., 2018) applied a previously validated high-throughput approach to screen for NIS inhibitors in the ToxCast phase I library, representing 293 important environmental chemicals. Here 310 blinded samples were screened in a tiered-approach using an initial single-concentration (100 μM) radioactive-iodide uptake (RAIU) assay, followed by 169 samples further evaluated in multi-concentration (0.001 μM−100 μM) testing in parallel RAIU and cell viability assays. A novel chemical ranking system that incorporates multi-concentration RAIU and cytotoxicity responses was also developed as a standardized method for chemical prioritization in current and future screenings. Representative chemical responses and thyroid effects of high-ranking chemicals are further discussed. This study significantly expands current knowledge of NIS inhibition potential in environmental chemicals and provides critical support to U.S. EPA’s Endocrine Disruptor Screening Program (EDSP) initiative to expand coverage of thyroid molecular targets, as well as the development of thyroid adverse outcome pathways (AOPs).

Apart from the human, functional NIS protein has been also identified in different species, including  the rat (Dai et al., 1996), the mouse (Perron et al., 2001), the pig (Selmi-Ruby et al., 2003), zebrafish (Thienpont et al., 2011) and in xenopus (amphibian)  (Lindenthal et al., 2009). Mouse and rat contain 618 amino acid residues, while the human and pig contain 643. There are several NIS variants that produce three active proteins in the pig due to alternative splicing at mRNA sites that are not present on the other species (Selmi-Ruby et al., 2003). NIS orthologs are discussed in the review by Darrouzet's group ( Darrouzet et al., 2014). Interestingly, functional differences have been identified between mouse or rat NIS (mNIS or rNIS, respectively) and human NIS (hNIS). The rat and themouse orthologs were shown to accumulate radioisotopes more efficiently than the human protein (Dayem et al., 2008; Heltemes et al., 2003). The molecular basis of these functional differences could be helpful for further characterization of NIS. Zhang and collaborators showed that rNIS is localized in a higher proportion at the plasma membrane than hNIS and the N-terminal region up to putative TM7 appears to be involved in this difference (Zhang et al., 2005). These authors also reported differences in the kinetics of the Na+ binding, implicating the region spanning from TM4 to TM6 and Ser200 of hNIS. They, thus, proposed that this region could be involved in sodium binding (Zhang et al., 2005). In our laboratory, it was shown that the Vmax of the mouse protein is four times higher than the Vmax of the human protein when expressed in the same cell line (HEK-293) (Dayem et al., 2008; Darrouzet et al., 2014). The KmI value determined for hNIS (9.0 ± 0.8 μM) was significantly lower than the KmI for the mouse protein (26.4 ± 3.5 μM) whereas the KmNa values were not significantly different. Similarly to the rat protein, mNIS is predominantly localized in the plasma membrane whereas the human ortholog is detected intracellularly in 40% of the cells in which it is expressed (Darrouzet et al., 2014). However, the difference in the Vmax values does not only seem to be related to the higher intracellular localization of hNIS. Using chimeric proteins between human and mouse NIS, we showed that the N-terminal region up to TM8 is most probably involved in iodide binding, and that the region from TM5 to the C terminus could play an important role in targeting the protein to the plasma membrane (Dayem et al., 2008). One of the long-term goals of these studies is the engineering of a chimeric NIS protein most suitable for gene therapy, i.e. preserving regions responsible for the high turnover rate and the efficient plasma membrane localization of the mouse proteinwhile replacing the immunogenic extracellular regions with those of the human ortholog. The porcine NIS gene gives rise to splice variants leading to three active NIS proteins with differences in their C-terminal extremities [4]. However, it is not known if these differences lead to distinct properties (Darrouzet et al., 2014). There is evidence that the MIE (NIS inhibition) is of relevance also for fish as an expression of the slc5a5 transcript (sodium/iodide co-transporter) has been described by various publications for the zebrafish embryo (see <a href="http://www.zfin.org">www.zfin.org</a>). It has been demonstrated that NIS inhibitors in zebra fish lead also to a strong repression of thyroid hormone levels (Thienpont et al., 2011) and in xenopus (amphibian) to  inhibition of the iodide-induced current  (Lindenthal et al., 2009).

CL:0002258

CL thyroid follicular cell

High Mixed

High

Pregnancy

High

Birth to < 1 month

High

During brain development

High High High High High Moderate Not Specified

Alexander EK, Marqusee E, Lawrence J, Jarolim P, Fischer GA, Larsen PR (2004). Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. N Engl J Med. Jul 15; 351(3):241-9. Alexander WD and Wolff J (1996). Thyroidal iodide transport VIII, Relation between transport goitrogenic and antigoitrogenic properties of certain anions. Endocrinology 78 581–590. Amitai Y, Winston G, Sack J, Wasser J, Lewis M, Blount BC, Valentin-Blasini L, Fisher N, Israeli A, Leventhal A. (2007). Gestational exposure to high perchlorate concentrations in drinking water and neonatal thyroxine levels. Thyroid. 17:843-850. Arriagada A.A, Eduardo Albornoz, Ma. Cecilia Opazo, Alvaro Becerra, Gonzalo Vidal, Carlos Fardella, Luis Michea, Nancy Carrasco, Felipe Simon, Alvaro A. Elorza, Susan M. Bueno, Alexis M. Kalergis, and Claudia A.  (2015).Excess Iodide Induces an Acute Inhibition of the Sodium/Iodide Symporter in Thyroid Male Rat Cells by Increasing Reactive Oxygen Species. Endocrinology. 2015 Apr; 156(4): 1540–1551. Bidart JM, Lacroix L, Evain-Brion D, Caillou B, Lazar V, Frydman R, Bellet D, Filetti S, Schlumberger M. (2000). Expression of Na+/I− symporter and Pendred syndrome genes in trophoblast cells. J Clin Endocrinol Metab 85:4367–4372. Bizhanova A, Kopp P. (2009). The sodium-iodide symporter NIS and pendrin in iodide homeostasis of the thyroid. Endocrinol 150:1084-1090. Blount BC, Pirkle JL, Osterloh JD, Valentin-Blasini L, Caldwell KL. (2006). Urinary perchlorate and thyroid hormone levels in adolescent and adult men and women living in the United States. Env Health Persp. 114:1865-1871. Cho JY, Leveille R, Kao R, Rousset B, Parlow AF, Burak WE Jr, Mazzaferri EL, Jhiang SM. (2000). Hormonal regulation of radioiodide uptake activity and Na+/I− symporter expression in mammary glands. J Clin Endocrinol Metab 85:2936–2943. Cianchetta S, di Bernardo J, Romeo G, Rhoden KJ. (2010). Perchlorate transport and ihnibition of the sodium iodide symporter measured with the yellow fluorescent protein variant YFP-H148Q/I152L. Toxicol App Pharmacol. 243:372-380. Dai G, Levy O, Carrasco N. (1996). Cloning and characterization of the thyroid iodide transporter. Nature 379:458–460. Darrouzet E, Lindenthal S, Marcellin D, Pellequer J, Pourcher T. (2014). The sodium/iodide symporter: state of the art of its molecular characterization. Biochim Biophys Acta 1838:244-253. Dayem M, Basquin C, Navarro V, Carrier P, Marsault R, Chang P, Huc S, Darrouzet E, Lindenthal S, Pourcher T. (2008). Comparison of expressed human and mouse sodium/iodide symporters reveals differences in transport properties and subcellular localization. J Endocrinol. 197:95–109. 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2019-04-04T08:52:00

Thyroid hormone synthesis, Decreased

TH synthesis, Decreased

Cellular

The thyroid hormones (TH), triiodothyronine (T3) and thyroxine (T4) are thyrosine-based hormones. Synthesis of THs is regulated by thyroid-stimulating hormone (TSH) binding to its receptor and thyroidal availability of iodine via the sodium iodide symporter (NIS). Other proteins contributing to TH production in the thyroid gland, including thyroperoxidase (TPO), dual oxidase enzymes (DUOX), and the transport protein pendrin are also necessary for iodothyronine production (Zoeller et al., 2007). The production of THs in the thyroid gland and resulting serum concentrations are controlled by a negatively regulated feedback mechanism. Decreased T4 and T3 serum concentrations activates the hypothalamus-pituitary-thyroid (HPT) axis which upregulates thyroid-stimulating hormone (TSH) that acts to increase production of additional THs (Zoeller and Tan, 2007). This regulatory system includes: 1) the hypothalamic secretion of the thyrotropin-releasing hormone (TRH); 2) the thyroid-stimulating hormone (TSH) secretion from the anterior pituitary; 3) hormonal transport by the plasma binding proteins; 4) cellular uptake mechanisms at the tissue level; 5) intracellular control of TH concentrations by deiodinating mechanisms; 6) transcriptional function of the nuclear TH receptor; and 7) in the fetus, the transplacental passage of T4 and T3 (Zoeller et al., 2007). TRH and the TSH primarily regulate the production of T4, often considered a “pro-hormone,” and to a lesser extent of T3, the transcriptionally active TH. Most of the hormone released from the thyroid gland into circulation is in the form of T4, while peripheral deiodination of T4 is responsible for the majority of circulating T3. Outer ring deiodination of T4 to T3 is catalyzed by the deiodinases 1 and 2 (DIO1 and DIO2), with DIO1 expressed mainly in liver and kidney, and DIO2 expressed in several tissues including the brain (Bianco et al., 2006). Conversion of T4 to T3 takes place mainly in the liver and kidney, but also in other target organs such as in the brain, the anterior pituitary, brown adipose tissue, thyroid and skeletal muscle (Gereben et al., 2008; Larsen, 2009).  In mammals, most evidence for the ontogeny of TH synthesis comes from measurements of serum hormone concentrations. And, importantly, the impact of xenobiotics on fetal hormones must include the influence of the maternal compartment since a majority of fetal THs are derived from maternal blood early in fetal life, with a transition during mid-late gestation to fetal production of THs that is still supplemented by maternal THs. In humans, THs can be found in the fetus as early as gestational weeks 10-12, and concentations rise continuously until birth. At term, fetal T4 is similar to maternal levels, but T3 remains 2-3 fold lower than maternal levels. In rats, THs can be detected in the fetus as early as the second gestational week, but fetal synthesis does not start until gestational day 17 with birth at gestational day 22-23. Maternal THs continue to supplement fetal production until parturition. (see Howdeshell, 2002; Santisteban and Bernal, 2005 for review). Due to the maternal factor, the life stage specific impact of TPO inhibition after exposure to environmental chemicals is complex (Ramhoj et al., 2022). Decreased TH synthesis in the thyroid gland may result from several possible molecular-initiating events (MIEs) including: 1) Disruption of key catalytic enzymes or cofactors needed for TH synthesis, including TPO, NIS, or dietary iodine insufficiency. Theoretically, decreased synthesis of Tg could also affect TH production (Kessler et al., 2008; Yi et al., 1997). Mutations in genes that encode requisite proteins in the thyroid may also lead to impaired TH synthesis, including mutations in pendrin associated with Pendred Syndrome (Dossena et al., 2011), mutations in TPO and Tg (Huang and Jap 2015), and mutations in NIS (Spitzweg and Morris, 2010). 2) Decreased TH synthesis in cases of clinical hypothyroidism may be due to Hashimoto's thyroiditis or other forms of thyroiditis, or physical destruction of the thyroid gland as in radioablation or surgical treatment of thyroid lymphoma. 3) It is possible that TH synthesis may also be reduced subsequent to disruption of the negative feedback mechanism governing TH homeostasis, e.g. pituitary gland dysfunction may result in a decreased TSH signal with concomitant T3 and T4 decreases. 4) More rarely, hypothalamic dysfunction can result in decreased TH synthesis.  Increased fetal TH levels are also possible. Maternal Graves disease, which results in fetal thyrotoxicosis (hyperthyroidism and increased serum T4 levels), has been successfully treated by maternal administration of TPO inhibitors (c.f., Sato et al., 2014).   It should be noted that different species and different life stages store different amounts of TH precursors and iodine within the thyroid gland. Thus, decreased TH synthesis via transient iodine insufficiency or inhibition of TPO may not affect TH release from the thyroid gland until depletion of stored iodinated Tg. Adult humans may store sufficient Tg-DIT residues to serve for several months to a year of TH demand (Greer et al., 2002; Zoeller, 2004). Neonates and infants have a much more limited supply of less than a week. While the TH system is highly conserved across vertebrates, there are some taxon-specific considerations. Zebrafish and fathead minnows are oviparous fish species in which maternal THs are transferred to the eggs and regulate early embryonic developmental processes during external (versus intra-uterine in mammals) development (Power et al., 2001; Campinho et al., 2014; Ruuskanen and Hsu, 2018) until embryonic TH synthesis is initiated. Maternal transfer of THs to the eggs has been demonstrated in zebrafish (Walpita et al., 2007; Chang et al., 2012) and fathead minnows (Crane et al., 2004; Nelson et al., 2016). Decreases in TH synthesis can only occur after initiation of embryonic TH synthesis. The components of the TH system responsible for TH synthesis are highly conserved across vertebrates and therefore interference with the same molecular targets compared to mammals can lead to decreased TH synthesis (TPO, NIS, etc.) in fish. Endogenous transcription profiles of thyroid-related genes in zebrafish and fathead minnow showed that mRNA coding for these genes is also maternally transferred and increasing expression of most transcripts during hatching and embryo-larval transition indicates a fully functional HPT axis in larvae (Vergauwen et al., 2018). Although the HPT axis is highly conserved, there are some differences between fish and mammals (Blanton and Specker, 2007; Deal and Volkoff, 2020). For example, in fish, corticotropin releasing hormone (CRH) often plays a more important role in regulating thyrotropin (TSH) secretion by the pituitary and thus TH synthesis compared to TSH-releasing hormone (TRH). Also, in most fish species thyroid follicles are more diffusely located in the pharyngeal region rather than encapsulated in a gland.

Decreased TH synthesis is often implied by measurement of TPO and NIS inhibition measured clinically and in laboratory models as these enzymes are essential for TH synthesis. Rarely is decreased TH synthesis measured directly, but rather the impact of chemicals on the quantity of T4 produced in the thyroid gland, or the amount of T4 present in serum is used as a marker of decreased T4 release from the thyroid gland (e.g., Romaldini et al., 1988). Methods used to assess TH synthesis include, incorporation of radiolabeled tracer compounds, radioimmunoassay, ELISA, and analytical detection.    Recently, amphibian thyroid explant cultures have been used to demonstrate direct effects of chemicals on TH synthesis, as this model contains all necessary synthesis enzymes including TPO and NIS (Hornung et al., 2010). For this work THs was measured by HPLC/ICP-mass spectometry. Decreased TH synthesis and release, using T4 release as the endpoint, has been shown for thiouracil antihyperthyroidism drugs including MMI, PTU, and the NIS inhibitor perchlorate (Hornung et al., 2010). Techniques for in vivo analysis of TH system disruption among other drug-related effects in fish were reviewed by Raldua and Piña (2014). TIQDT (Thyroxine-immunofluorescence quantitative disruption test) is a method that provides an immunofluorescent based estimate of thyroxine in the gland of zebrafish (Raldua and Babin, 2009; Thienpont et al., 2011; Jomaa et al., 2014; Rehberger et al., 2018).  Thienpont used this method with ~25 xenobiotics (e.g., amitrole, perchlorate, methimazole, PTU, DDT, PCBs). The method detected changes for all chemicals known to directly impact TH synthesis in the thyroid gland (e.g., NIS and TPO inhibitors), but not those that upregulate hepatic catabolism of T4. Rehberger et al. (2018) updated the method to enable simultaneous semi-quantitative visualization of intrafollicular T3 and T4 levels. Most often, whole body TH level measurements in fish early life stages are used as indirect evidence of decreased TH synthesis (Nelson et al., 2016; Stinckens et al., 2016; Stinckens et al., 2020). Analytical determination of TH levels by LC-MS is becoming increasingly available (Hornung et al., 2015). More recently, transgenic zebrafish with fluorescent thyroid follicles are being used to visualize the compensatory proliferation of the thyroid follicles following inhibition of TH synthesis among others (Opitz et al., 2012).

Taxonomic: This KE is plausibly applicable across vertebrates. Decreased TH synthesis resulting from TPO or NIS inhibition is conserved across vertebrate taxa, with in vivo evidence from humans, rats, amphibians, some fish species, and birds, and in vitro evidence from rat and porcine microsomes. Indeed, TPO and NIS mutations result in congenital hypothyroidism in humans (Bakker et al., 2000; Spitzweg and Morris, 2010), demonstrating the essentiality of TPO and NIS function toward maintaining euthyroid status. Though decreased serum T4 is used as a surrogate measure to indicate chemical-mediated decreases in TH synthesis, clinical and veterinary management of hyperthyroidism and Graves’ disease using propylthiouracil and methimazole, known to decrease TH synthesis, indicates strong evidence for chemical inhibition of TPO (Zoeller and Crofton, 2005). Life stage: Applicability to certain life stages may depend on the species and their dependence on maternally transferred THs during the earliest phases of development. The earliest life stages of teleost fish (e.g., fathead minnow, zebrafish) rely on maternally transferred THs to regulate certain developmental processes until embryonic TH synthesis is active (Power et al., 2001). In externally developing fish species, decreases in TH synthesis can only occur after initiation of embryonic TH synthesis. In zebrafish, Opitz et al. (2011) showed the formation of a first thyroid follicle at 55 hours post fertilization (hpf), Chang et al. (2012) showed a first significant TH increase at 120 hpf and Walter et al. (2019) showed clear TH production already at 72 hpf but did not analyse time points between 24 and 72 hpf. TPO inhibition in a homozygous knockout line abolished the T4 production in thyroid follicles of mutant zebrafish with phenotypic abnormalities occurring from 20 dpf onwards but not before 10 dpf (Fang et al., 2022). Therefore, it is still uncertain when exactly embryonic TH synthesis is activated and thus when exactly this process becomes sensitive to disruption. In fathead minnows, a significant increase of whole body TH levels was already observed between 1 and 2 dpf, which corresponds to the appearance of the thyroid anlage at 35 hpf prior to the first observation of thyroid follicles at 58 hpf (Wabuke-Bunoti and Firling, 1983). It currently remains unclear when exactly embryonic TH production is initiated in zebrafish. Sex: The KE is plausibly applicable to both sexes. THs are essential in both sexes and the components of the HPT-axis are identical in both sexes. There can however be sex-dependent differences in the sensitivity to the disruption of TH levels and the magnitude of the response. In humans, females appear more susceptible to hypothyroidism compared to males when exposed to certain halogenated chemicals (Hernandez‐Mariano et al., 2017; Webster et al., 2014). In adult zebrafish, Liu et al. (2019) showed sex-dependent changes in TH levels and mRNA expression of regulatory genes including corticotropin releasing hormone (crh), thyroid stimulating hormone (tsh) and deiodinase 2 after exposure to organophosphate flame retardants. The underlying mechanism of any sex-related differences remains unclear.

UBERON:0002046

UBERON thyroid gland

CL:0002258

CL thyroid follicular cell

High Male High Female

High

All life stages

High High Moderate High Moderate High

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General and Comparative Endocrinology, 323–324. https://doi.org/10.1016/j.ygcen.2022.114033 Gereben B, Zeöld A, Dentice M, Salvatore D, Bianco AC.  Activation and inactivation of thyroid hormone by deiodinases: local action with general consequences.  Cell Mol Life Sci. 2008 Feb;65(4):570-90 Greer MA, Goodman G, Pleus RC, Greer SE. Health effects assessment for environmental perchlorate contamination: the dose response for inhibition of thyroidal radioiodine uptake in humans. Environ Health Perspect. 2002. 110:927-937. Hernandez-Mariano JA, Torres-Sanchez L, Bassol-Mayagoitia S, Escamilla-Nunez M, Cebrian ME, Villeda-Gutierrez EA, Lopez-Rodriguez G, Felix-Arellano EE, Blanco-Munoz J. 2017. Effect of exposure to p,p '-dde during the first half of pregnancy in the maternal thyroid profile of female residents in a mexican floriculture area. Environmental Research. 156:597-604. Hornung MW, Degitz SJ, Korte LM, Olson JM, Kosian PA, Linnum AL, Tietge JE. 2010. Inhibition of thyroid hormone release from cultured amphibian thyroid glands by methimazole, 6-propylthiouracil, and perchlorate. Toxicol Sci 118:42-51. Hornung MW, Kosian PA, Haselman JT, Korte JJ, Challis K, Macherla C, Nevalainen E, Degitz SJ. 2015. In vitro, ex vivo, and in vivo determination of thyroid hormone modulating activity of benzothiazoles. Toxicological Sciences. 146(2):254-264. Howdeshell KL. 2002. A model of the development of the brain as a construct of the thyroid system. Environ Health Perspect. 110 Suppl 3:337-48. Huang CJ and Jap TS. 2015. A systematic review of genetic studies of thyroid disorders in Taiwan. J Chin Med Assoc. 78: 145-153. Jomaa B, Hermsen SAB, Kessels MY, van den Berg JHJ, Peijnenburg AACM, Aarts JMMJG, Piersma AH, Rietjens IMCM. 2014. Developmental toxicity of thyroid-active compounds in a zebrafish embryotoxicity test. Altex-Alternatives to Animal Experimentation. 31(3):303-317. Kessler J, Obinger C, Eales G. 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Ramhoj, L., Svingen, T., Fradrich, C., Rijntjes, E., Wirth, E.K., Pedersen, K., Kohrle, J., Axelstad, M., 2022. Perinatal exposure to the thyroperoxidase inhibitors methimazole and amitrole perturbs thyroid hormone system signaling and alters motor activity in rat offspring. Toxicology Letters 354, 44-55. Rehberger K, Baumann L, Hecker M, Braunbeck T. 2018. Intrafollicular thyroid hormone staining in whole-mount zebrafish (danio rerio) embryos for the detection of thyroid hormone synthesis disruption. Fish Physiology and Biochemistry. 44(3):997-1010. Romaldini JH, Farah CS, Werner RS, Dall'Antonia Júnior RP, Camargo RS. 1988.  "In vitro" study on release of cyclic AMP and thyroid hormone in autonomously functioning thyroid nodules.  Horm Metab Res.20:510-2. Ruuskanen S, Hsu BY. 2018. Maternal thyroid hormones: An unexplored mechanism underlying maternal effects in an ecological framework. Physiological and Biochemical Zoology. 91(3):904-916. Santisteban P, Bernal J. 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Thienpont B, Tingaud-Sequeira A, Prats E, Barata C, Babin PJ, Raldúa D.  Zebrafish eleutheroembryos provide a suitable vertebrate model for screening chemicals that impair thyroid hormone synthesis.  Environ Sci Technol. 2011. 45(17):7525-32. Vergauwen L, Cavallin JE, Ankley GT, Bars C, Gabriels IJ, Michiels EDG, Fitzpatrick KR, Periz-Stanacev J, Randolph EC, Robinson SL et al. 2018. Gene transcription ontogeny of hypothalamic-pituitary-thyroid axis development in early-life stage fathead minnow and zebrafish. General and Comparative Endocrinology. 266:87-100. Wabukebunoti MAN, Firling CE. 1983. The prehatching development of the thyroid-gland of the fathead minnow, pimephales-promelas (rafinesque). General and Comparative Endocrinology. 49(2):320-331. Walpita CN, Van der Geyten S, Rurangwa E, Darras VM. 2007. The effect of 3,5,3'-triiodothyronine supplementation on zebrafish (danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors. Gen Comp Endocrinol. 152(2-3):206-214. Walter KM, Miller GW, Chen XP, Yaghoobi B, Puschner B, Lein PJ. 2019. Effects of thyroid hormone disruption on the ontogenetic expression of thyroid hormone signaling genes in developing zebrafish (danio rerio). General and Comparative Endocrinology. 272:20-32. Webster GM, Venners SA, Mattman A, Martin JW. 2014. Associations between perfluoroalkyl acids (pfass) and maternal thyroid hormones in early pregnancy: A population-based cohort study. Environmental Research. 133:338-347. Yi X, Yamamoto K, Shu L, Katoh R, Kawaoi A. Effects of Propyithiouracil (PTU) Administration on the Synthesis and Secretion of Thyroglobulin in the Rat Thyroid Gland: A Quantitative Immuno-electron Microscopic Study Using Immunogold Technique. Endocr Pathol. 1997 Winter;8(4):315-325. Zoeller RT, Crofton KM. 2005.  Mode of action: developmental thyroid hormone insufficiency--neurological abnormalities resulting from exposure to propylthiouracil. Crit Rev Toxicol. 35:771-81 Zoeller RT, Tan SW, Tyl RW. 2007. General background on the hypothalamic-pituitary-thyroid (HPT) axis. Critical reviews in toxicology.  37:11-53. Zoeller RT.  Interspecies differences in susceptibility to perturbation of thyroid hormone homeostasis requires a definition of "sensitivity" that is informative for risk analysis. Regul Toxicol Pharmacol. 2004 Dec;40(3):380.   AOPWiki 2016-11-29T18:41:23

2022-11-04T09:25:39

Thyroxine (T4) in serum, Decreased

T4 in serum, Decreased

Tissue

All iodothyronines are derived from the modification of tyrosine molecules (Taurog, 2000). There are two biologically active thyroid hormones (THs) in serum, triiodothyronine (T3) and T4, and a few less active iodothyronines, reverse T3 (rT3),  and 3,3'-Diiodothyronine (3,5-T2). T4 is the predominant TH in circulation, comprising approximately 80% of the TH excreted from the thyroid gland in mammals and is the pool from which the majority of T3 in serum is generated (Zoeller et al., 2007). As such, serum T4 changes usually precede changes in other serum THs. Decreased thyroxine (T4) in serum results from one or more MIEs upstream and is considered a key biomarker of altered TH homeostasis (DeVito et al., 1999). Serum T4 is used as a biomarker of TH status because the circulatory system serves as the major transport and delivery system for TH delivery to tissues. The majority of THs in the blood are bound to transport proteins (Bartalena and Robbins, 1993). In serum, it is the unbound, or ‘free’ form of the hormone that is thought to be available for transport into tissues. Free hormones are approximately 0.03 and 0.3 percent for T4 and T3, respectively. There are major species differences in the predominant binding proteins and their affinities for THs (see below). However, there is broad agreement that changes in serum concentrations of THs is diagnostic of thyroid disease or chemical-induced disruption of thyroid homeostasis across vertebrates (DeVito et al., 1999; Miller et al., 2009; Zoeller et al., 2007; Carr and Patiño, 2011). Normal serum T4 reference ranges can be species and lifestage specific. In rodents, serum THs are low in the fetal circulation, increasing as the fetal thyroid gland becomes functional on gestational day 17, just a few days prior to birth. After birth serum hormones increase steadily, peaking at two weeks, and falling slightly to adult levels by postnatal day 21 (Walker et al., 1980; Harris et al., 1978; Goldey et al., 1995; Lau et al., 2003). Similarly, in humans, adult reference ranges for THs do not reflect the normal ranges for children at different developmental stages, with TH concentrations highest in infants, still increased in childhood, prior to a decline to adult levels coincident with pubertal development (Corcoran et al. 1977; Kapelari et al., 2008). In some frog species, there is an analogous peak in THs in tadpoles that starts around embryonic NF stage 56, peaks at stage 62 and the declines to lower levels by stage 56 (Sternberg et al., 2011; Leloup and Buscaglia, 1977).  Additionally, ample evidence is available from studies investigating responses to inhibitors of TH synthesis in fish. For example, Stinckens et al. (2020) showed reduced whole body T4 concentrations in zebrafish larvae exposed to 50 or 100 mg/L methimazole, a potent TPO inhibitor, from immediately after fertilization until 21 or 32 days of age. Exposure to 37 or 111 mg/L propylthiouracil also reduced T4 levels after exposure up to 14, 21 and 32 days in the same study. Walter et al. (2019) showed that propylthiouracil had no effect on T4 levels in 24h old zebrafish, but decreased T4 levels of 72h old zebrafish. This difference is probably due to the onset of embryonic TH production between the age of 24 and 72 hours (Opitz et al., 2011). Stinckens et al. (2016) showed that exposure to 2-mercaptobenzothiazole (MBT), an environmentally relevant TPO inhibitor, decreased whole body T4 levels in continuously exposed 5 and 32 day old zebrafish larvae. A high concentration of MBT also decreased whole body T4 levels in 6 day old fathead minnows, but recovery was observed at the age of 21 days although the fish were kept in the exposure medium (Nelson et al., 2016). Crane et al. (2006) showed decreased T4 levels in 28 day old fathead minnows continuously exposed to 32 or 100 µg/L methimazole.

Serum T3 and T4 can be measured as free (unbound) or total (bound + unbound). Free hormone concentrations are clinically considered more direct indicators of T4 and T3 activities in the body, but in animal studies, total T3 and T4 are typically measured. Historically, the most widely used method in toxicology is the radioimmunoassay (RIA). The method is routinely used in rodent endocrine and toxicity studies. The ELISA method is commonly used as a human clinical test method. Analytical determination of iodothyronines (T3, T4, rT3, T2) and their conjugates, through methods employing HPLC, liquid chromatography, immuno luminescence, and mass spectrometry are less common, but are becoming increasingly available (Hornung et al., 2015; DeVito et al., 1999; Baret and Fert, 1989; Spencer, 2013; Samanidou V.F et al., 2000; Rathmann D. et al., 2015 ). In fish early life stages most evidence for the ontogeny of thyroid hormone synthesis comes from measurements of whole body thyroid hormone levels using LC-MS techniques (Hornung et al., 2015) which are increasingly used to accurately quantify whole body thyroid hormone levels as a proxy for serum thyroid hormone levels (Nelson et al., 2016; Stinckens et al., 2016; Stinckens et al., 2020). It is important to note that thyroid hormones concentrations can be influenced by a number of intrinsic and extrinsic factors (e.g., circadian rhythms, stress, food intake, housing, noise) (see for example, Döhler et al., 1979). Any of these measurements should be evaluated for the relationship to the actual endpoint of interest, repeatability, reproducibility, and lower limits of quantification using a fit-for-purpose approach. This is of particular significance when assessing the very low levels of TH present in fetal serum. Detection limits of the assay must be compatible with the levels in the biological sample. All three of the methods summarized above would be fit-for-purpose, depending on the number of samples to be evaluated and the associated costs of each method. Both RIA and ELISA measure THs by an indirect methodology, whereas analytical determination is the most direct measurement available. All these methods, particularly RIA, are repeatable and reproducible.

Taxonomic: This KE is plausibly applicable across vertebrates and the overall evidence supporting taxonomic applicability is strong. THs are evolutionarily conserved molecules present in all vertebrate species (Hulbert, 2000; Yen, 2001). Moreover, their crucial role in zebrafish development, embryo-to-larval transition and larval-to-juvenile transition (Thienpont et al., 2011; Liu and Chan, 2002), and amphibian and lamprey metamorphoses is well established (Manzon and Youson, 1997; Yaoita and Brown, 1990; Furlow and Neff, 2006). Their role as environmental messenger via exogenous routes in echinoderms confirms the hypothesis that these molecules are widely distributed among the living organisms (Heyland and Hodin, 2004). However, the role of THs in the different species depends on the expression and function of specific proteins (e.g receptors or enzymes) under TH control and may vary across species and tissues. As such, extrapolation regarding TH action across species and developmental stages should be done with caution. With few exceptions, vertebrate species have circulating T4 (and T3) that are bound to transport proteins in blood. Clear species differences exist in serum transport proteins (Dohler et al., 1979; Yamauchi and Isihara, 2009). There are three major transport proteins in mammals; thyroid binding globulin (TBG), transthyretin (TTR), and albumin. In adult humans, the percent bound to these proteins is about 75, 15 and 10 percent, respectively (Schussler 2000).  In contrast, in adult rats the majority of THs are bound to TTR. Thyroid- binding proteins are developmentally regulated in rats. TBG is expressed in rats until approximately postnatal day (PND) 60, with peak expression occurring during weaning (Savu et al., 1989). However, low levels of TBG persist into adult ages in rats and can be experimentally induced by hypothyroidism, malnutrition, or caloric restriction (Rouaze-Romet et al., 1992). While these species differences impact TH half-life (Capen, 1997) and possibly regulatory feedback mechanisms, there is little information on quantitative dose-response relationships of binding proteins and serum hormones during development across different species. Serum THs are still regarded as the most robust measurable key event causally linked to downstream adverse outcomes. Life stage: The earliest life stages of teleost fish rely on maternally transferred THs to regulate certain developmental processes until embryonic TH synthesis is active (Power et al., 2001). As a result, T4 levels are not expected to decrease in response to exposure to inhibitors of TH synthesis during these earliest stages of development. In zebrafish, Opitz et al. (2011) showed the formation of a first thyroid follicle at 55 hours post fertilization (hpf), Chang et al. (2012) showed a first significant TH increase at 120 hpf and Walter et al. (2019) showed clear TH production already at 72 hpf but did not analyse time points between 24 and 72 hpf. In fathead minnows, a significant increase of whole body TH levels was already observed between 1 and 2 dpf, which corresponds to the appearance of the thyroid anlage at 35 hpf prior to the first observation of thyroid follicles at 58 hpf (Wabuke-Bunoti and Firling, 1983). It is still uncertain when exactly embryonic TH synthesis is activated and how this determines sensitivity to TH system disruptors. Sex: The KE is plausibly applicable to both sexes. THs are essential in both sexes and the components of the HPT-axis are identical in both sexes. There can however be sex-dependent differences in the sensitivity to the disruption of TH levels and the magnitude of the response. In humans, females appear more susceptible to hypothyroidism compared to males when exposed to certain halogenated chemicals (Hernandez‐Mariano et al., 2017; Webster et al., 2014). In adult zebrafish, Liu et al. (2019) showed sex-dependent changes in TH levels and mRNA expression of regulatory genes including corticotropin releasing hormone (crh), thyroid stimulating hormone (tsh) and deiodinase 2 after exposure to organophosphate flame retardants. The underlying mechanism of any sex-related differences remains unclear.

UBERON:0001977

UBERON serum

High Female High Male

High

All life stages

High High High Moderate Moderate High High High

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Tsh receptor function is required for normal thyroid differentiation in zebrafish. Molecular Endocrinology. 25(9):1579-1599. Power DM, Llewellyn L, Faustino M, Nowell MA, Bjornsson BT, Einarsdottir IE, Canario AV, Sweeney GE. 2001. Thyroid hormones in growth and development of fish. Comp Biochem Physiol C Toxicol Pharmacol. 130(4):447-459. Rathmann D, Rijntjes E, Lietzow J, Köhrle J. (2015) Quantitative Analysis of Thyroid Hormone Metabolites in Cell Culture Samples Using LC-MS/MS. Eur Thyroid J. Sep;4(Suppl 1):51-8. Rouaze-Romet M, Savu L, Vranckx R, Bleiberg-Daniel F, Le Moullac B, Gouache P, Nunez EA. 1992. Reexpression of thyroxine-binding globulin in postweaning rats during protein or energy malnutrition. Acta Endocrinol (Copenh).127:441-448. Samanidou VF, Kourti PV. (2009) Rapid HPLC method for the simultaneous monitoring of duloxetine, venlaflaxine, fluoxetine and paroxetine in biofluids. Bioanalysis. 2009 Aug;1(5):905-17. Savu L, Vranckx R, Maya M, Gripois D, Blouquit MF, Nunez EA. 1989. Thyroxine-binding globulin and thyroxinebinding prealbumin in hypothyroid and hyperthyroid developing rats. BiochimBiophys Acta. 992:379-384. Schneider S, Kaufmann W, Strauss V, van Ravenzwaay B.    Vinclozolin: a feasibility and sensitivity study of the ILSI-HESI F1-extended one-generation rat reproduction protocol. Regul Toxicol Pharmacol. 2011 Feb;59(1):91-100. Schussler, G.C. (2000). The thyroxine-binding proteins. Thyroid 10:141–149. Spencer, CA. (2013). Assay of thyroid hormone and related substances. In De Groot, LJ et al. (Eds). Endotext. South Dartmouth, MA Sternberg RM, Thoemke KR, Korte JJ, Moen SM, Olson JM, Korte L, Tietge JE, Degitz SJ Jr. Control of pituitary thyroid-stimulating hormone synthesis and secretion by thyroid hormones during Xenopus metamorphosis. Gen Comp Endocrinol. 2011. 173(3):428-37 Stinckens E, Vergauwen L, Blackwell BR, Anldey GT, Villeneuve DL, Knapen D. 2020. Effect of thyroperoxidase and deiodinase inhibition on anterior swim bladder inflation in the zebrafish. Environmental Science & Technology. 54(10):6213-6223. Stinckens E, Vergauwen L, Schroeder A, Maho W, Blackwell B, Witters H, Blust R, Ankley G, Covaci A, Villeneuve D et al. 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part ii: Zebrafish. Aquatic Toxicology. 173:204-217. Taurog A. 2005. Hormone synthesis. In: Werner and Ingbar’s The Thyroid: A Fundamental and Clinical Text (Braverman LE, Utiger RD, eds). Philadelphia:Lippincott, Williams and Wilkins, 47–81Walker P, Dubois JD, Dussault JH.  Free thyroid hormone concentrations during postnatal development in the rat.  Pediatr Res. 1980 Mar;14(3):247-9. Thienpont B, Tingaud-Sequeira A, Prats E, Barata C, Babin PJ, Raldúa D. Zebrafish eleutheroembryos provide a suitable vertebrate model for screening chemicals that impair thyroid hormone synthesis. Environ Sci Technol. 2011 Sep 1;45(17):7525-32. Wabukebunoti MAN, Firling CE. 1983. The prehatching development of the thyroid-gland of the fathead minnow, pimephales-promelas (rafinesque). General and Comparative Endocrinology. 49(2):320-331. Walter KM, Miller GW, Chen XP, Yaghoobi B, Puschner B, Lein PJ. 2019. Effects of thyroid hormone disruption on the ontogenetic expression of thyroid hormone signaling genes in developing zebrafish (danio rerio). General and Comparative Endocrinology. 272:20-32. Webster GM, Venners SA, Mattman A, Martin JW. 2014. Associations between perfluoroalkyl acids (pfass) and maternal thyroid hormones in early pregnancy: A population-based cohort study. Environmental Research. 133:338-347. Yamauchi K1, Ishihara A. Evolutionary changes to transthyretin: developmentally regulated and tissue-specific gene expression. FEBS J. 2009. 276(19):5357-66. Yaoita Y, Brown DD. (1990). A correlation of thyroid hormone receptor gene expression with amphibian metamorphosis. Genes Dev. 4:1917-1924. Yen PM. (2001). Physiological and molecular basis of thyroid hormone action. Physiol Rev. 81:1097-1142. Zoeller RT, Tan SW, Tyl RW. General background on the hypothalamic-pituitary-thyroid (HPT) axis. Crit Rev Toxicol. 2007 Jan-Feb;37(1-2):11-53 Zoeller, R. T., R. Bansal, et al. (2005). "Bisphenol-A, an environmental contaminant that acts as a thyroid hormone receptor antagonist in vitro, increases serum thyroxine, and alters RC3/neurogranin expression in the developing rat brain." Endocrinology 146(2): 607-612. AOPWiki 2016-11-29T18:41:23

2022-10-10T08:52:30

Thyroxine (T4) in neuronal tissue, Decreased

T4 in neuronal tissue, Decreased

Organ

Thyroid hormones (TH) are present in brain tissue of most vertebrate species, and thyroxine (T4) is converted to triiodothyronine (T3) locally in this tissue.  The amount of THs in brain is known to vary during development and to differ among brain regions (Calvo et al., 1990; Kester et al., 2004; Tu et al., 1999). In human cerebral cortex, T3 increases steadily from 13-weeks, reaching adult levels by 20 weeks post conception. This occurs despite very low and unchanging levels in fetal serum T3, when fetal serum T4 increases 3-fold over the same period. This indicates that T3 in fetal brain is locally generated from serum-derived T4 via the activity of deiodinases, primarily DIO2. DIO2 serves to convert T4 to T3. During this time in fetal development DIO3 activity, which converts T3 to the inactive reverse T3 (rT3), remains very low in cortex.  In contrast, in other brain regions including hippocampus and cerebellum, T3 remains low throughout early and mid-gestation and corresponds with high activity of DIO3 in these brain regions. In late gestation and after birth, DIO3 levels drop in hippocampus and cerebellum with a corresponding increase in T3 concentrations (Kester et al., 2004).  A similar spatial and temporal profile of deiodinase activity and corresponding brain hormone concentrations has been observed in rodent brain (Calvo et al., 1990; Tu et al., 1999). In the rat, either whole brain or cortex have been preferentially assessed due to the low levels of hormones present and the small tissue volumes make quantitification difficult. Brain T3 and T4 rise in parallel from gestational day 10 to gestational day 20 in rat. They are typically both quite low until gestational 17 with steep increases between GD18 and GD20 corresponding to the onset of fetal thyroid function (Calvo et al., 1990; Ruiz de Ono et al., 1988; Obergon et al., 1981). Just before birth, brain T3 and T4 concentrations are about one-third to one-half that of adult brain. Brain development in the early postnatal period in rat is roughly equivalent to the 3rd trimester in humans such that adult levels of T3 and T4 in brain are not reached in rodents until the 2nd-3rd postnatal week. For THs to gain access to brain tissue they need to cross the blood brain barrier (BBB) which regulates the active transport of TH into neurons. Many transporter proteins have been identified, and the monocarboxylate transporters (Mct8, Mct10) and anion-transporting polypeptide (OATP1c1) show the highest degree of affinity towards TH and are prevalent in brain (Jansen et al., 2007; Mayer et al., 2014).  Transporters express a distinct distribution pattern that varies by tissue and age (Friesema et al., 2005; Henneman et al., 2001; Visser et al., 2007; Heuer et al., 2005; Muller and Heuer, 2007). Although several transporters have been identified, current knowledge of cell specific profile of transporters is limited.  Most of the hormone transported across the blood brain barrier is in the form of T4, primarily through the cellular membrane transporters (e.g., OATP1c1 transporter) into the astrocyte (Visser and Visser, 2012; Sugiyama et al., 2003; Tohyama et al., 2004). Within the astrocyte, T4 is converted into T3 via the local activity of deiodinase 2 (DIO2) (Guadano-Ferraz et al., 1997).  A small amount of T3 may cross the blood brain barrier directly via the T3-specific transporter, MCT8 (Heuer et al., 2005). Although in mature brain T3 derives partially from the circulation and from the deiodination of T4, in the fetal brain T3 is exclusively a product of T4 deiodination (Calvo et al., 1990; Grijota-Martinez et al., 2011). In both cases, only the required amount of T3 is utilized in neurons and the excess is degraded by the neuron-specific deiodinase DIO3 (Tu et al., 1999; St. Germain et al., 2009; Hernandez et al., 2010). Both deiodinase and transporter expression in brain peak in different brain regions at different times in fetal and neonatal life (Kester et al., 2004; Bates et al., 1999; Muller and Heuer, 2014; Heuer, 2007). Collectively, these spatial and temporal patterns of transporter expression and deiodinase activity provide exquisite control of brain T3 available for nuclear receptor activation and regulated gene expression.

Radioimmunoassays (RIAs) are commonly used to detect TH in the brain (e.g., Obregon et al., 1982; Calvo et al., 1990; Morse et al., 1996; Bansal et al., 2005; Gilbert et al., 2013). The method (and minor variants) is well established in the published literature. However, it is not available in a simple 'kit' and requires technical knowledge of RIAs, thus has not been used in most routine toxicology studies. Evaluations in neuronal tissue are complicated by the difficulty of the fatty matrix, heterogeneity of regions within the brain, and low tissue concentrations and small tissue amounts especially in immature brain. Most often whole brain homogenates are assessed, obfuscating the known temporal and regional differences in brain hormone present. Two analytical techniques, LC- and HPLC-inductively coupled plasma–mass spectrometry have recently been used to measure brain concentrations of TH. These techniques have proven capable of measuring very low levels in whole-body homogenates of frog tadpoles at different developmental stages (e.g., Simon et al., 2002; Tietge et al., 2010). The assay detects I–, MIT, DIT, T4, T3, and rT3. More recently, Wang and Stapleton (2010) and Donzelli et al. (2016) used liquid chromatography-tandem mass spectrometry for the simultaneous analysis of five THs including thyroxine (T4), 3,3′,5-triidothyronine (T3), 3,3′,5′-triiodothyronine (rT3; reverse T3), 3,3′-diiodothyronine (3,3′-T2), and 3,5-diiodothyronine (3,5-T2) in serum and a variety of tissues including brain. These analytical methods require expensive equipment and technical expertise and as such are not routinely used.

THs are critical for normal brain development in most vertebrates, primarily documented empirically in mammalian species (Bernal, 2013).  However, there is compelling data that demonstrates the need for TH in brain development for many other taxa, including: birds, fish and frogs (Van Herck et al., 2013; Denver, 1998; Power et al., 2001). The most well known non-mammalian action of TH is to induce metamorphosis in amphibians and some fish species. However, there is a fundamental difference in the mechanisms by which T3 affects amphibian metamorphosis vs its role in mammalian brain development (Galton, 1983). In the rat, brain development proceeds, even if defective, despite the absence of TH. By contrast, TH administration to tadpoles induces early metamorphosis, whereas in its absence, tadpoles grow to extremely large size, but the metamorphosis program is never activated (Galton, 1983).

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Bansal R, You SH, Herzig CT, Zoeller RT (2005). Maternal thyroid hormone increases HES expression in the fetal rat brain: an effect mimicked by exposure to a mixture of polychlorinated biphenyls (PCBs). Brain Res Dev Brain Res 156:13-22. Bates JM, St Germain DL, Galton VA. Expression profiles of the three iodothyronine deiodinases, D1, D2, and D3, in the developing rat. Endocrinology. 1999 Feb;140(2):844-51. Bernal J. (2013). Thyroid Hormones in Brain Development and Function.  In: De Groot LJ, Chrousos G, Dungan K, Feingold KR, Grossman A, Hershman JM, Koch C, Korbonits M, McLachlan R, New M, Purnell J, Rebar R, Singer F, Vinik A, editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-2015. www.thyroidmanager.org Calvo R, Obregón MJ, Ruiz de Oña C, Escobar del Rey F, Morreale de Escobar G. (1990). Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3′-triiodothyronine in the protection of the fetal brain. J. Clin. Invest. 86:889-899. Chatonnet F., Picou F., Fauquier T., and Flamant F., (2011). Thyroid Hormone Action in Cerebellum and Cerebral Cortex Development, Journal of Thyroid Research, Volume 2011, Article ID 145762, 8 pages http://dx.doi.org/10.4061/2011/145762) Denver, RJ 1998 The molecular basis of thyroid hormone-dependent central nervous system remodeling during amphibian metamorphosis. Comparative Biochemistry and Physiology Part C: Pharmacology, Toxicology and Endocrinology, 119:219-228. Donzelli R, Colligiani D, Kusmic C, Sabatini M, Lorenzini L, Accorroni A, Nannipieri M, Saba A, Iervasi G, Zucchi R. Effect of Hypothyroidism and Hyperthyroidism on Tissue Thyroid Hormone Concentrations in Rat. Eur Thyroid J. 2016 Mar;5(1):27-34. Friesema EC, Jansen J, Milici C, Visser TJ (2005) Thyroid hormone transporters. Vitam Horm 70:137-167. Galton VH 1983 Thyroid hormone action in amphibian metamorphosis. In: Oppenheimer JH, Samuels HH (eds) Molecular Basis of Thyroid Hormone Action. Academic Press, New York, pp 445–483. Gilbert ME, Hedge JM, Valentin-Blasini L, Blount BC, Kannan K, Tietge J, Zoeller RT, Crofton KM, Jarrett JM, Fisher JW (2013) An animal model of marginal iodine deficiency during development: the thyroid axis and neurodevelopmental outcome. Toxicol Sci 132:177-195. Grijota-Martinez C, Diez D, Morreale de Escobar G, Bernal J, Morte B. (2011). Lack of action of exogenously administered T3 on the fetal rat brain despite expression of the monocarboxylate transporter 8. Endocrinology. 152:1713-1721. Guadano-Ferraz A, Obregon MJ, St Germain DL, Bernal J. (1997). The type 2 iodothyronine deiodinase is expressed primarily in glial cells in the neonatal rat brain. Proc Natl Acad Sci USA. 94: 10391–10396. Hennemann G, Docter R, Friesema EC, de Jong M, Krenning EP, Visser TJ. (2001). Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability. Endocr Rev. 22:451-476. Hernandez A, Quignodon L, Martinez ME, Flamant F, St Germain DL. Type 3 deiodinase deficiency causes spatial and temporal alterations in brain T3 signaling that are dissociated from serum thyroid hormone levels. Endocrinology. 2010 Nov;151(11):5550-8. Heuer H. (2007). The importance of thyroid hormone transporters for brain development and function. Best Pract Res Clin Endocrinol Metab. 21:265–276. Heuer H, Maier MK, Iden S, Mittag J, Friesema EC, Visser TJ, Bauer K. (2005). The monocarboxylate transporter 8 linked to human psychomotor retardation is highly expressed in thyroid hormone-sensitive neuron populations. Endocrinology 146:1701–1706. Jansen J, Friesema EC, Kester MH, Milici C, Reeser M, Gruters A, Barrett TG, Mancilla EE, Svensson J, Wemeau JL, Busi da Silva Canalli MH, Lundgren J, McEntagart ME, Hopper N, Arts WF, Visser TJ (2007) Functional analysis of monocarboxylate transporter 8 mutations identified in patients with X-linked psychomotor retardation and elevated serum triiodothyronine. J Clin Endocrinol Metab 92:2378-2381. Kester MH, Martinez de Mena R, Obregon MJ, Marinkovic D, Howatson A, Visser TJ, Hume R, Morreale de Escobar G. (2004). Iodothyronine levels in the human developing brain: major regulatory roles of iodothyronine deiodinases in different areas. J Clin Endocrinol Metab 89:3117–3128. Mayer S, Müller J, Bauer R, Richert S, Kassmann CM, Darras VM, Buder K, Boelen A, Visser TJ, Heuer H. Transporters MCT8 and OATP1C1 maintain murine brain thyroid hormone homeostasis. J Clin Invest. 2014 May 1;124(5):1987-99. Moog N.K., Entringer S., Heim Ch., Wadhwa PD., Kathmann N., Buss C. (2017). Influence of maternal thyroid hormones during gestation on fetal  brain development. Neuroscience, 2017, 342: 68–100. doi:10.1016/j.neuroscience.2015.09.0 Morse DC, Wehler EK, Wesseling W, Koeman JH, Brouwer A. Alterations in rat brain thyroid hormone status following pre- and postnatal exposure to polychlorinated biphenyls (Aroclor 1254). Toxicol Appl Pharmacol. 1996 Feb;136(2):269-79 Müller J, Heuer H. Expression pattern of thyroid hormone transporters in the postnatal mouse brain. Front Endocrinol (Lausanne). 2014 Jun 18:5:92. Obregon MJ, Mallol J, Escobar del Rey F, Morreale de Escobar G. (1981). Presence of l-thyroxine and 3,5,3-triiodo-l-thyronine in tissues from thyroidectomised rats. Endocrinology 109:908-913. Power DM, Llewellyn L, Faustino M, Nowell MA, Björnsson BT, Einarsdottir IE, Canario AV, Sweeney GE. Thyroid hormones in growth and development of fish. Comp Biochem Physiol C Toxicol Pharmacol. 2001 Dec;130(4):447-59. Ruiz de Oña C, Obregón MJ, Escobar del Rey F, Morreale de Escobar G. Developmental changes in rat brain 5'-deiodinase and thyroid hormones during the fetal period: the effects of fetal hypothyroidism and maternal thyroid hormones. Pediatr Res. 1988 Nov;24(5):588-94. Simon R, Tietge JE, Michalke B, Degitz S, Schramm KW. Iodine species and the endocrine system: thyroid hormone levels in adult Danio rerio and developing Xenopus laevis. Anal Bioanal Chem. 2002 Feb;372(3):481-5. St Germain DL, Galton VA, Hernandez A. (2009). Minireview: Defining the roles of the iodothyronine deiodinases: current concepts and challenges. Endocrinology. 150:1097-107. Sugiyama D, Kusuhara H, Taniguchi H, Ishikawa S, Nozaki Y, Aburatani H, Sugiyama Y. (2003). Functional characterization of rat brain-specific organic anion transporter (Oatp14) at the blood–brain barrier: high affinity transporter for thyroxine. J Biol Chem. 278:43489–43495. Tietge JE, Butterworth BC, Haselman JT, Holcombe GW, Hornung MW, Korte JJ, Kosian PA, Wolfe M, Degitz SJ. Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis. Aquat Toxicol. 2010 Jun 1;98(1):44-50. Tohyama K, Kusuhara H, Sugiyama Y. (2004). Involvement of multispecific organic anion transporter, Oatp14 (Slc21a14), in the transport of thyroxine across the blood-brain barrier. Endocrinology. 145: 4384–4391. Tu HM, Legradi G, Bartha T, Salvatore D, Lechan RM, Larsen PR. (1999). Regional expression of the type 3 iodothyronine deiodinase messenger ribonucleic acid in the rat central nervous system and its regulation by thyroid hormone. Endocrinology. 140: 784–790. Van Herck SL, Geysens S, Delbaere J, Darras VM. Regulators of thyroid hormone availability and action in embryonic chicken brain development. Gen Comp Endocrinol. 2013.190:96-104. Visser EW, Visser TJ. (2012). Finding the way into the brain without MCT8. J Clin Enodcrinol Metab. 97:4362-4365. Visser WE, Friesema EC, Jansen J, Visser TJ. (2007). Thyroid hormone transport by monocarboxylate transporters. Best Pract Res Clin Endocrinol Metab. 21:223–236. Wang, D. and Stapleton, HM. (2010) Analysis of thyroid hormones in serum by liquid chromatography -tandem mass spectrometry. Anal Bioanal Chem. 2010 Jul; 397(5): 1831–1839 AOPWiki 2016-11-29T18:41:23

2019-04-04T09:13:27

Altered, Neuroanatomy

Organ

TH signalling controls a number of major anatomical processes in development that if altered will lead to permanently abnormal brain morphology. These processes include differentiation of neurons and glia from progenitor cells, neuronal migration, and myelination of axonal processes. The evidence supporting a role of TH in these neurodevelopmental processes is strong. Evidence from human is primarily from studies of iodine-deficient children and children with congenital hypothyroidism (CH)(Zoeller and Rovet, 2004). Animal models using rats and mice, as well as in vitro studies, have provided ample evidence of TH control of these processes (Gilbert and Zoeller, 2010). Below are brief descriptions of the impact of TH insufficiency on two of these processes. Altered Neuronal Migration: Effects of TH insufficiency on specific developmental events are reflected in alterations in brain structure. Altered lamination and cellular morphology in cerebellum (Koibuchi and Chin, 2000; Morte et al., 2004; Farwell and Dubord-Tomasetti, 1999), hippocampus (Madeira et al., 1991), and the neocortex (Auso et al., 2003; Cuevas et al., 2005) have been documented. In addition presence of aberrantly placed neuronal cells in the corpus callosum have been described (Gilbert et al., 2014, Powell et al., 2012; Shibutani et al., 2009). Altered Axonal Myelination: Nerve conduction is accelerated by the insulation formed by oligodendrocytes of the myelin sheath that surround axons of many nerve fibers. Reduced size and altered composition of the white matter tracts throughout the brain, the most prominent of which is the corpus callosusm, are hallmarks of severe developmental hypothyroidism (Berbel et al., 1993, 1994; Ferreira et al., 2004; Gravel and Hawkes, 1990; Ibarrola and Rodriguez-Pena, 1997; Schnoover et al., 2005). In addition, more subtle abnormalities have been described in white matter tracks including corpus callosum and anterior commissure following more modest reductions in circulating levels of TH in the neonatal period (Sharlin et al., 2008).

Data in support of this key event have been collected using a wide variety of standard biochemical, histological and anatomical methods (eg., morphometrics, immunohistochemical staining, in situ hybridation) and imaging procedures. Many of methods applied to reveal anatomical abnormalities are routine neurohistopatholgical procedures similar to those recommended in EPA and OECD developmental neurotoxicity guidelines. Subtle changes in cytoarchitecure such as seen in the neocortex depend on more specialized birth dating procedures and staining techniques. Some alterations in brain structure are transient in nature and depend on appropriate timing for detection.

The majority of the evidence supporting this KE comes from rodent studies. However, amphibians display vast structural remodelling during metamorphosis that is TH-dependent and share common TH signaling pathways with rat brain development.

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Auso E, et al. A moderate and transient deficiency of maternal thyroid function at the beginning of fetal neocorticogenesis alters neuronal migration. Endocrinology 2004, 145(9), 4037-4047. Berbel, P., A. Guadano-Ferraz, et al. (1994). Role of thyroid hormones in the maturation of interhemispheric connections in rats. Behav Brain Res 64(1-2): 9-14. Berbel, P., A. Guadano-Ferraz, et al. (1993). Organization of auditory callosal connections in hypothyroid adult rats. Eur J Neurosci 5(11): 1465-78. Cuevas E, et al. Transient maternal hypothyroxinemia at onset of corticogenesis alters tangential migration of medial ganglionic eminence-derived neurons. Eur J Neurosci 2005, 22(3), 541-551. Farwell AP, Dubord-Tomasetti SA. Thyroid hormone regulates the extracellular organization of laminin on astrocytes. Endocrinology 1999, 140(11), 5014-5021. Ferreira, A. A., J. C. Nazario, et al. (2004). Effects of experimental hypothyroidism on myelin sheath structural organization. J Neurocytol 33(2): 225-31. Gilbert ME, Ramos RL, McCloskey DP, Goodman JH. Subcortical band heterotopia in rat offspring following maternal hypothyroxinaemia: structural and functional characteristics. J Neuroendocrinol. 2014 Aug;26(8):528-41. Gilbert M, Zoeller R. Thyroid hormone - impact on the developing brain: Possible mechanisms of neurotoxicity. In: Harry GJ T, HA ed. Neurotoxicology, 3rd edition Vol 3. New York: Informa Healthcare USA, Inc; 2010:79-111. Gravel C Hawkes R. Maturation of the corpus callosum of the rat: I. Influence of thyroid hormones on the topography of callosal projections. J Comp Neurol 1990, 291(1), 128-146. Ibarrola, N. and A. Rodriguez-Pena (1997). "Hypothyroidism coordinately and transiently affects myelin protein gene expression in most rat brain regions during postnatal development." Brain Res 752(1-2): 285-93. Koibuchi N, Chin WW. Thyroid hormone action and brain development. Trends Endocrinol Metab 2000, 11(4), 123-128. Madeira, MD, et al. Effects of hypothyroidism upon the granular layer of the dentate gyrus in male and female adult rats: a morphometric study. J Comp Neurol 1991, 314(1), 171-186. Morte B, et al. Aberrant maturation of astrocytes in thyroid hormone receptor alpha 1 knockout mice reveals an interplay between thyroid hormone receptor isoforms. Endocrinology 2004, 145(3), 1386-1391. Powell MH, Nguyen HV, Gilbert M, Parekh M, Colon-Perez LM, Mareci TH, Montie E. Magnetic resonance imaging and volumetric analysis: novel tools to study the effects of thyroid hormone disruption on white matter development. Neurotoxicology. 2012 Oct;33(5):1322-9. Schoonover, C. M., M. M. Seibel, et al. (2004). Thyroid hormone regulates oligodendrocyte accumulation in developing rat brain white matter tracts. Endocrinology 145(11): 5013-20. Sharlin DS, et al. The balance between oligodendrocyte and astrocyte production in major white matter tracts is linearly related to serum total thyroxine. Endocrinology 2008, 149(5), 2527-2536. Shibutani M, Woo GH, Fujimoto H, Saegusa Y, Takahashi M, Inoue K, Hirose M, Nishikawa A. Assessment of developmental effects of hypothyroidism in rats from in utero and lactation exposure to anti-thyroid agents. Reproductive toxicology (Elmsford, NY). 2009;28(3):297-307. Zoeller, R. T. and. Rovet, J. Timing of thyroid hormone action in the developing brain: clinical observations and experimental findings. J Neuroendocrinol, 2004; 16(10): 809-18. AOPWiki 2016-11-29T18:41:23

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Discrete parts of nerve cells, single neurons, simple circuits, and complex brain systems have been used to evaluate the impact of thyroid disruption on brain function. The nervous system functions as a highly integrated and organized communication and information processing network. Neurons transmit and receive information from sensory and motor organs, but the largest number of neuronal connections is with other neurons. This is largely accomplished by chemical transmission across the synaptic cleft, the space where the specialized ending of the presynaptic axon terminus of the transmitting neuron meets the specialized postsynaptic region of the neuron that is receiving that information. Activation of the presynaptic neuron to produce an action potential causes the release of neurotransmitter substances into the synaptic cleft. Most of the released neurotransmitters bind with molecules at receptors on the dendrites of the postsynaptic neuron. The chemical signal is then transduced back into an electrical impulse which travels in the receiving neuron and if of sufficient magnitude, fires the neuron, and the signal is transduced once again to a chemical signal in the next neuron along the pathway. The scale of neurophysiological techniques that can be used ranges along a broad continuum from the very small (e.g., individual ion channel fluxes) to the very large anatomical pathways (e.g., electroencephalograms) in both in vitro and in vivo preparations. A number of studies on TH and brain function have incorporated field potentials in the hippocampus, an area critical for certain forms of learning and memory, to probe functional integrity after TH insufficiency (e.g., Dong et al., 2005; Gilbert and Sui, 2006; Opazo et al., 2008; Vara et al., 2002). Field potentials have been recorded following thyroid disruption both in vivo and in vitro, at two discrete hippocampal synaptic regions, area CA1 and the dentate gyrus. Excitatory and inhibitory synaptic transmission are assessed by recording electrical field potentials (voltage changes across large populations of neurons) across a monosynaptic circuit (e.g. axons from cortical neurons that synapse on dentate gyrus granule cells) in response to electrical pulses delivered to the incoming pathway (Gilbert and Burdette, 1995). The synapses in this region of the brain are also intensely studied as they are imbued with a capacity for use-dependent plasticity, the best studied model being long-term potentiation (LTP). LTP is a model of information acquisition (learning) and storage (memory) at the synaptic level. It is induced by applying trains of stimulus pulses at high frequencies to the incoming pathway of the monosynaptic circuit and measuring the amplitude of the induced change in synaptic responsiveness that persists for hours, and in some instances days to weeks (Malenka and Nicoll, 1999; Martinez and Derrick, 1996; Gilbert and Burdette, 1995). The induction of LTP is believed to emulate, both at the synaptic and molecular level, the coincident firing of large numbers of neurons that are engaged during a learning event. The persistence of LTP emulates the duration of the memory of that learning event. Field potentials are recorded from slices of hippocampus taken from exposed animals, or from indwelling electrodes placed within the appropriate hippocampal field. One electrode is placed in the afferent fiber pathway (e.g., perforant path for dentate gyrus, Schaeffer collaterals for CA1 region) and a brief electrical pulse applied to stimulate these axons. A second electrode is placed in the synaptic or cell body region to record the activity evoked by the incoming pulse. Electrodes are placed visually according to established landmarks in in vitro preparations, and acccording to stereotaxic coordinates for in vivo preparations. Once accurately placed, a series of stimulus pulses at increasing stimulus current intensities are applied to the input pathway, and the response evoked in the receiving neuronal population is recorded. Excitatory Synaptic Transmission: Two measures, the excitatory postsynaptic potential (EPSP) and the population spike are derived from the compound field potential in response to a series of single pulse stimulations applied at increasing stimulus strengths. The function described by the relationship of current strength (input) and evoked response (output), the I-O curve is the measure of excitatory synaptic transmission. Inhibitory Synaptic Transmission: Pairs of stimulus pulses delivered in close temporal proximity is used to probe the integrity of inhibitory synaptic transmission. The response evoked by the second pulse of the pair at brief intervals (<30msec) arrives during the activation of feedback inhibitory loops in the hippocampus. An alteration in the degree of suppression to the 2nd pulse of the pair reflects altered inhibitory synaptic function. Long Term Potentiation (LTP): Synaptic plasticity in the form of LTP is assessed by delivering trains of high frequency stimulation to induce a prolonged augmentation of synaptic responsivity. Probe stimuli at mid-range stimulus strenghts are delivered before and after application of LTP-inducing trains. The degree of increase in EPSP and PS amplitude to the probe stimulus after train application, and the longevity of that induced enhancement are metrics of LTP. Additionally, contrasting I-O functions of excitatory synaptic transmission before and after (1-5 hours) LTP is induced is also a common measure of induced LTP. Assays of this type are fit for purpose, have been well accepted in the literature, and are reproducible across laboratories. The assay directly measures the key event of altered neurophysiological function.

The majority of evidence for this key event come from work in rodent species (i.e., rat, mouse). there is a moderate amount of evidence from other species.

UBERON:0000955

UBERON brain

High

Dong, J., H. Yin, et al. (2005). "Congenital iodine deficiency and hypothyroidism impair LTP and decrease C-fos and C-jun expression in rat hippocampus." Neurotoxicology 26(3): 417-26. Gilbert, M.E. and Burdette, L.J. (1995). Hippocampal Field Potentials: A Model System to Characterize Neurotoxicity. In Neurotoxicology: Approaches and Methods. L.W Chang and W. Slikker (Eds). Academic Press:New York, 183-204. Gilbert, M. E. and L. Sui (2006). "Dose-dependent reductions in spatial learning and synaptic function in the dentate gyrus of adult rats following developmental thyroid hormone insufficiency." Brain Res 1069(1): 10-22. Malenka RC, Nicoll RA (1999) Long-term potentiation--a decade of progress? Science, 285:1870-1874. Martinez, J.L. and Derrick, B.E. (1996). Long term potentiation and learning. Annual Review of Psychology, 47,173-203. Opazo MC, Gianini A, Pancetti F, Azkcona G, Alarcón L, Lizana R, Noches V, Gonzalez PA, Marassi MP, Mora S, Rosenthal D, Eugenin E, Naranjo D, Bueno SM, Kalergis AM, Riedel CA (2008), Maternal hypothyroxinemia impairs spatial learning and synaptic nature and function in the offspring. Endocrinology 149:5097-5106. Vara H, et al. Thyroid hormone regulates neurotransmitter release in neonatal rat hippocampus. Neuroscience 2002, 110(1), 19-28. AOPWiki 2016-11-29T18:41:23

2017-09-16T10:15:26

Cognitive Function, Decreased

Individual

Learning and memory depend upon the coordinated action of different brain regions and neurotransmitter systems constituting functionally integrated neural networks (D’Hooge and DeDeyn, 2001). Among the many brain areas engaged in the acquisition of, or retrieval of, a learned event, the hippocampal-based memory systems have received the most study. The main learning areas and pathways are similar in rodents and primates, including man (Eichenbaum, 2000; Stanton and Spear, 1990; Squire, 2004). In humans, the hippocampus is involved in recollection of an event’s rich spatial-temporal contexts and distinguished from simple semantic memory which is memory of a list of facts (Burgess et al., 2000). Hemispheric specialization has occurred in humans, with the left hippocampus specializing in verbal and narrative memories (i.e., context-dependent episodic or autobiographical memory) and the right hippocampus, more prominently engaged in visuo-spatial memory (i.e., memory for locations within an environment). The hippocampus is particularly critical for the formation of episodic memory, and autobiographical memory tasks have been developed to specifically probe these functions (Eichenbaun, 2000; Willoughby et al., 2014). In rodents, there is obviously no verbal component in hippocampal memory, but reliance on the hippocampus for spatial, temporal and contextual memory function has been well documented. Spatial memory deficits and fear-based context learning paradigms engage the hippocampus, amygdala, and prefrontal cortex (Eichenbaum, 2000; Shors et al., 2001; Samuels et al., 2011; Vorhees and Williams, 2014; D’Hooge and DeDeyn, 2001; Lynch, 2004; O’Keefe and Nadal, 1978). These tasks are impaired in animals with hippocampal dysfunction (O’Keefe and Nadal, 1978; Morris and Frey, 1987; Gilbert et al., 2016).

In rodents, a variety of tests of learning and memory have been used to probe the integrity of hippocampal function. These include tests of spatial learning like the radial arm maze (RAM), the Barnes maze, and most commonly, the Morris water maze (MWM). Test of novelty such as novel object recognition, and fear based context learning are also sensitive to hippocampal disruption. Finally, trace fear conditioning which incorporates a temporal component upon traditional amygdala-based fear learning engages the hippocampus. A brief description of these tasks follows. 1) RAM, Barnes, MWM are examples of spatial tasks in which animals are required to learn: the location of a food reward (RAM); an escape hole to enter a preferred dark tunnel from a brightly lit open field area (Barnes maze); or a hidden platform submerged below the surface of the water in a large tank of water (MWM) (Vorhees and Williams, 2014). 2) Novel Object recognition. This is a simpler task that can be used to probe recognition memory. Two objects are presented to animal in an open field on trial 1, and these are explored. On trial 2, one object is replaced with a novel object and time spent interacting with the novel object is taken evidence of memory retention (i.e., I have seen one of these objects before, but not this one. Cohen and Stackman, 2015). 3) Contextual Fear conditioning is a hippocampal based learning task in which animals are placed in a novel environment and allowed to explore for several minutes before delivery of an aversive stimulus, typically a mild foot shock. Upon reintroduction to this same environment in the future (typically 24-48 hours after original training), animals will limit their exploration, the context of this chamber being associated with an aversive event. The degree of suppression of activity after training is taken as evidence of retention, i.e., memory (Curzon et al., 2009). 4) Trace fear conditioning. Standard fear conditioning paradigms require animals to make an association between a neutral conditioning stimulus (CS, a light or a tone) and an aversive stimulus (US, a footshock). The unconditioned response (CR) that is elicited upon delivery of the footshock US is freezing behavior. With repetition of CS/US delivery, the previously neutral stimulus comes to elicit the freezing response. This type of learning is dependent on the amygdala, a brain region associated with, but distinct from the hippocampus. Introducing a brief delay between presentation of the neutral CS and the aversive US, a trace period, requires the engagement of the amygdala and the hippocampus (Shors et al., 2004). Most methods are well established in the published literature and many have been engaged to evaluate the effects of developmental thyroid disruption. The US EPA and OECD Developmental Neurotoxicity (DNT) Guidelines (OCSPP 870.6300 or OECD 426) both require testing of learning and memory (USEPA, 1998; OECD, 2007). These DNT Guidelines have been deemed valid to identify developmental neurotoxicity and adverse neurodevelopmental outcomes (Makris et al., 2009). A variety of standardized learning and memory tests have been developed for human neuropsychological testing. These include episodic autobiographical memory, word pair recognition memory; object location recognition memory. Some components of these tests have been incorporated in general tests of adult intelligence (IQ) such as the WAIS and the Wechsler. Modifications have been made and norms developed for incorporating of tests of learning and memory in children. Examples of some of these tests include: 1) Rey Osterieth Complex Figure (RCFT) which probes a variety of functions including as visuospatial abilities, memory, attention, planning, and working memory (Shin et al., 2006). 2) Children’s Auditory Verbal Learning Test (CAVLT) is a free recall of presented word lists that yields measures of Immediate Memory Span, Level of Learning, Immediate Recall, Delayed Recall, Recognition Accuracy, and Total Intrusions. (Lezak 1995; Talley, 1986).  3) Continuous Visual Memory Test (CVMT) measures visual learning and memory. It is a free recall of presented pictures/objects rather than words but that yields similar measures of Immediate Memory Span, Level of Learning, Immediate Recall, Delayed Recall, Recognition Accuracy, and Total Intrusions. (Lezak, 1984; 1994). 4) Story Recall from Wechsler Memory Scale (WMS) Logical Memory Test Battery, a standardized neurospychological test designed to measure memory functions (Lezak, 1994; Talley, 1986). 5) Autobiographical memory (AM) is the recollection of specific personal events in a multifaceted higher order cognitive process. It includes episodic memory- remembering of past events specific in time and place, in contrast to semantic autobiographical memory is the recollection of personal facts, traits, and general knowledge. Episodic AM is associated with greater activation of the hippocampus and a later and more gradual developmental trajectory. Absence of episodic memory in early life (infantile amnesia) is thought to reflect immature hippocampal function (Herold et al., 2015; Fivush, 2015). 6) Staged Autobiographical Memory Task. In this version of the AM test, children participate in a staged event involving a tour of the hospital, perform a series of tasks (counting footprints in the hall, identifying objects in wall display, buy lunch, watched a video). It is designed to contain unique event happenings, place, time, visual/sensory/perceptual details. Four to five months later, interviews are conducted using Children’s Autobiographical Interview and scored according to standardized scheme (Willoughby et al., 2014).

Basic forms of learning behavior such as habituation have been found in many taxa from worms to humans (Alexander, 1990). More complex cognitive processes such as executive function likely reside only in higher mammalian species such as non-human primates and humans.

High Male High Female

High

During brain development

High High High

Alexander RD (1990) Epigenetic rules and Darwinian algorithms: The adaptive study of learning and development. Ethology and Sociobiology 11:241-303. Bellinger DC (2012) A strategy for comparing the contributions of environmental chemicals and other risk factors to neurodevelopment of children. Environ Health Perspect 120:501-507. Burgess N (2002) The hippocampus, space, and viewpoints in episodic memory. Q J Exp Psychol A 55:1057-1080. Cohen, SJ and Stackman, RW. (2015). Assessing rodent hippocampal involvement in the novel object recognition task. A review. Behav. Brain Res. 285: 105-1176. Curzon P, Rustay NR, Browman KE. Cued and Contextual Fear Conditioning for Rodents. In: Buccafusco JJ, editor. Methods of Behavior Analysis in Neuroscience. 2nd edition. Boca Raton (FL): CRC Press/Taylor & Francis; 2009 D'Hooge R, De Deyn PP (2001) Applications of the Morris water maze in the study of learning and memory. Brain Res Brain Res Rev 36:60-90. Eichenbaum H (2000) A cortical-hippocampal system for declarative memory. Nat Rev Neurosci 1:41-50. Fivush R. The development of autobiographical memory. Annu Rev Psychol. 2011. 62:559-82. Gilbert ME, Sanchez-Huerta K, Wood C (2016) Mild Thyroid Hormone Insufficiency During Development Compromises Activity-Dependent Neuroplasticity in the Hippocampus of Adult Male Rats. Endocrinology 157:774-787. Gilbert ME, Sui L (2006) Dose-dependent reductions in spatial learning and synaptic function in the dentate gyrus of adult rats following developmental thyroid hormone insufficiency. Brain Res 1069:10-22. Herold, C, Lässer, MM, Schmid, LA, Seidl, U, Kong, L, Fellhauer, I, Thomann, PA, Essig, M and Schröder, J. (2015). Neuropsychology, Autobiographical Memory, and Hippocampal Volume in “Younger” and “Older” Patients with Chronic Schizophrenia. Front. Psychiatry, 6: 53. Lezak MD (1984) Neuropsychological assessment in behavioral toxicology--developing techniques and interpretative issues. Scand J Work Environ Health 10 Suppl 1:25-29. Lezak MD (1994) Domains of behavior from a neuropsychological perspective: the whole story. Nebr Symp Motiv 41:23-55. Lynch, M.A. (2004). Long-Term Potentiation and Memory. Physiological Reviews. 84:87-136. Makris SL, Raffaele K, Allen S, Bowers WJ, Hass U, Alleva E, Calamandrei G, Sheets L, Amcoff P, Delrue N, Crofton KM. A retrospective performance assessment of the developmental neurotoxicity study in support of OECD test guideline 426. Environ Health Perspect. 2009 Jan;117(1):17-25. Morris RG, Frey U. Hippocampal synaptic plasticity: role in spatial learning or the automaticrecording of attended experience? Philos Trans R Soc Lond B Biol Sci. 1997 Oct 29;352(1360):1489-503. Review O’Keefe, J. and Nadel, L. (1978). The Hippocampus as a Cognitive Map. Oxford: Oxford University Press. OECD. 2007. OECD guidelines for the testing of chemicals/ section 4: Health effects. Test no. 426: Developmental neurotoxicity study.  www.Oecd.Org/dataoecd/20/52/37622194.Pdf [accessed May 21, 2012]. Samuels BA, Hen R (2011) Neurogenesis and affective disorders. Eur J Neurosci 33:1152-1159. Shin, MS, Park, SY, Park, SR, Oeol, SH and Kwon, JS. (2006). Clinical and empirical appliations fo the Rey-Osterrieth complex figure test. Nature Protocols, 1: 892-899. Shors TJ, Miesegaes G, Beylin A, Zhao M, Rydel T, Gould E (2001) Neurogenesis in the adult is involved in the formation of trace memories. Nature 410:372-376.Squire LR (2004) Memory systems of the brain: a brief history and current perspective. Neurobiol Learn Mem 82:171-177. Stanton ME, Spear LP (1990) Workshop on the qualitative and quantitative comparability of human and animal developmental neurotoxicity, Work Group I report: comparability of measures of developmental neurotoxicity in humans and laboratory animals. Neurotoxicol Teratol 12:261-267. Talley, JL. (1986). Memory in learning disabled children: Digit span and eh Rey Auditory verbal learning test. Archives of Clinical Neuropsychology, Elseiver. U.S.EPA. 1998. Health effects guidelines OPPTS 870.6300 developmental neurotoxicity study. EPA Document 712-C-98-239.Office of Prevention Pesticides and Toxic Substances. Vorhees CV, Williams MT (2014) Assessing spatial learning and memory in rodents. ILAR J 55:310-332. Willoughby KA, McAndrews MP, Rovet JF. Accuracy of episodic autobiographical memory in children with early thyroid hormone deficiency using a staged event. Dev Cogn Neurosci. 2014. 9:1-11. AOPWiki 2016-11-29T18:41:24

2018-08-09T11:55:05

<upstream-id>60977fed-fb61-41b3-90af-942b2bfd73ba</upstream-id> <downstream-id>c51bccd4-a14e-4a0e-ac61-a844f50842a7</downstream-id>

Not Specified Not Specified

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b43008337b8> AOPWiki 2016-11-29T18:41:34

2016-12-03T16:37:58

<upstream-id>c51bccd4-a14e-4a0e-ac61-a844f50842a7</upstream-id> <downstream-id>79819d6f-e072-4fe9-9f1f-96d86a2d845c</downstream-id> Thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) are synthesized by NIS and TPO in the thyroid gland as iodinated thyroglobulin (Tg) and stored in the colloid of thyroid follicles across vertebrates. Secretion from the follicle into serum is a multi-step process. The first involves thyroid stimulating hormone (TSH) stimulation of the separation of the peptide linkage between Tg and TH. The next steps involve endocytosis of colloid, fusion of the endosome with the basolateral membrane of the thyrocyte, and finally release of TH into blood. More detailed descriptions of this process can be found in reviews by Braverman and Utiger (2012) and Zoeller et al. (2007).

The weight of evidence linking these two KEs of decreased TH synthesis and decreased T4 in serum is strong. It is commonly accepted dogma that decreased synthesis in the thyroid gland will result in decreased circulating TH (serum T4).

The biological relationship between two KEs in this KER is well understood and documented fact within the scientific community.

It is widely accepted that TPO inhibition leads to declines in serum T4 levels in adult mammals. This is due to the fact that the sole source for circulating T4 derives from hormone synthesis in the thyroid gland. Indeed, it has been known for decades that insufficient dietary iodine will lead to decreased serum TH concentrations due to inadequate synthesis. Strong qualitative and quantitative relationships exist between reduced TH synthesis and reduced serum T4 (Ekerot et al., 2013; Degon et al., 2008; Cooper et al., 1982; 1983; Leonard et al., 2016; Zoeller and Tan, 2007).  There is more limited evidence supporting the relationship between decreased TH synthesis and lowered circulating hormone levels during development.  Lu and Anderson (1994) followed the time course of TH synthesis, measured as thyroxine secretion rate, in non-treated pregnant rats and correlated it with serum T4 levels. Modeling of TH in the rat fetus demonstrates the quantitative relationship between TH synthesis and serum T4 concentrations (Hassan et al., 2017, 2020; Handa et al., 2021). Furthermore, a wide variety of drugs and chemicals that inhibit TPO are known to result in decreased release of TH from the thyroid gland, as well as decreased circulating TH concentrations. This is evidenced by a very large number of studies that employed a wide variety of techniques, including thyroid gland explant cultures, tracing organification of 131-I and in vivo treatment of a variety of animal species with known TPO inhibitors (King and May,1984; Atterwill et al., 1990; Brown et al., 1986; Brucker-Davis, 1998; Haselman et al., 2020; Hornung et al., 2010; Hurley et al., 1998; Kohrle, 2008; Tietge et al., 2010). Additionally, evidence is available from studies investigating responses to TPO inhibitors in fish. For example, Stinckens et al. (2020) showed reduced whole body T4 concentrations in zebrafish larvae exposed to 50 or 100 mg/L methimazole, a potent TPO inhibitor, from immediately after fertilization until 21 or 32 days of age. Exposure to 37 or 111 mg/L propylthiouracil also reduced T4 levels after exposure up to 14, 21 and 32 days in the same study. Walter et al. (2019) showed that propylthiouracil had no effect on T4 levels in 24h old zebrafish, but decreased T4 levels of 72h old zebrafish. This difference is probably due to the onset of embryonic TH production between the age of 24 and 72 hours (Opitz et al., 2011). Stinckens et al. (2016) showed that exposure to 2-mercaptobenzothiazole (MBT), an environmentally relevant TPO inhibitor, decreased whole body T4 levels in continuously exposed 5 and 32 day old zebrafish larvae. Several other studies have also shown that chemically induced Inhibition of TPO results in reduced TH synthesis in zebrafish (Van der Ven et al., 2006; Raldua and Babin, 2009; Liu et al., 2011; Thienpont et al., 2011; Rehberger et al., 2018). A high concentration of MBT also decreased whole body T4 levels in 6 day old fathead minnows, but recovery was observed at the age of 21 days although the fish were kept in the exposure medium (Nelson et al., 2016). Crane et al. (2006) showed decreased T4 levels in 28 day old fathead minnows continuously exposed to 32 or 100 µg/L methimazole. Temporal Evidence: In mammals, the temporal nature of this KER is applicable to all life stages, including development (Seed et al., 2005).  There are currently no studies that measured both TPO synthesis and TH production during development. However, the impact of decreased TH synthesis on serum hormones is similar across all ages in mammals. Good evidence for the temporal relationship comes from thyroid system modeling of the impacts of iodine deficiency and NIS inhibition (e.g., Degon et al., 2008; Fisher et al., 2013). In addition, recovery experiments have demonstrated that serum thyroid hormones recovered in athyroid mice following grafting of in-vitro derived follicles (Antonica et al., 2012). In Xenopus, it has been shown that depression of TH synthesis in the thyroid gland precedes depression of circulating TH within 7 days of exposure during pro-metamorphosis (Haselman et al., 2020).    In oviparous fish such as zebrafish and fathead minnow, the nature of this KER depends on the life stage since the earliest stages of embryonic development rely on maternal THs transferred to the eggs. Embryonic TH synthesis is activated later during embryo-larval development. (See Domain of applicability) Dose-response Evidence: Dose-response data is lacking from studies that include concurrent measures of both TH synthesis and serum TH concentrations. However, data is available demonstrating correlations between thyroidal TH and serum TH concentrations during gestation and lactation during development (Gilbert et al., 2013). This data was used to develop a rat quantitative biologically-based dose-response model for iodine deficiency (Fisher et al., 2013). In Xenopus, dose-responses were demonstrated in both thyroidal T4 and circulating T4 following exposure to three TPO inhibitors (Haselman et al., 2020).

There are no inconsistencies in this KER, but there are some uncertainties. The first uncertainty stems from the paucity of data for quantitative modeling of the relationship between the degree of synthesis decrease and resulting changes in circulating T4 concentrations. In addition, most of the data supporting this KER comes from inhibition of TPO, and there are a number of other processes (e.g., endocytosis, lysosomal fusion, basolateral fusion and release) that are not as well studied. For example, Kim et al. (2015) investigated the adverse effects of Triphenyl phosphate (TPP), a substance that disrupts the thyroid system. Therefore, Rat pituitary (GH3) and thyroid follicular cell lines (FRTL-5) were studied. In the GH3 cells, TPP led to an upregulation of the expression of important thyroid genes (tsh, tr alpha and tr beta) while T3, a positive control, downregulated the expression of these genes. In FRTL-5 cells, the expression of nis and tpo genes was significantly upregulated, suggesting that TPP stimulates TH synthesis in the thyroid gland. In zebrafish larvae at the age of 7 days post-fertilisation (dpf), TPP exposure resulted in a significant increase in T3 and T4 concentrations and the expression of genes involved in thyroid hormone synthesis. Exposure to TPP also significantly regulated the expression of genes involved in the metabolism (dio1), transport (ttr) and excretion (ugt1ab) of THs. The down-regulation of the crh and tsh genes in the zebrafish larvae suggests the activation of a central regulatory feedback mechanism that is triggered by the increased T3 levels in vivo. Taken together, these observations indicate that TPP increases TH concentrations in early life stages of zebrafish by disrupting central regulatory and hormone synthesis pathways.

During Xenopus metamorphosis, circulating T4 steadily increases to peak levels at metamorphic climax. Therefore, during Xenopus metamorphosis, this KER is operable at an increased rate as compared to a system that is maintaining steady circulating T4 levels through homeostatic control. In this case, developmental status is a modulating factor for the rates and trajectories of these KEs.

In rats, Hassan et al. (2020) demonstrated in vitro: ex vivo correlations of TPO inhibition using PTU and MMI and constructed a quantitative model relating level of TPO inhibition with changes in circulating T4 levels. They determined that 30% inhibition of TPO was sufficient to decrease circulating T4 levels by 20%. This is further supported by studies of Hassan et al. (2017) and Handa et al. (2021) In Xenopus, Haselman et al. (2020) collected temporal and dose-response data for both thyroidal and circulating T4 which showed strong qualitative concordance of the response-response relationship. A quantitative relationship exists there in, but is yet to be demonstrated mathematically in this species.

Fisher et al. (2013) published a quantitative biologically-based dose-response model for iodine deficiency in the rat. This model provides quantitative relationships for thyroidal T4 synthesis (iodine organification) and predictions of serum T4 concentrations in developing rats. There are other computational models that include thyroid hormone synthesis. Ekerot et al. (2012) modeled TPO, T3, T4 and TSH in dogs and humans based on exposure to myeloperoxidase inhibitors that also inhibit TPO.  This model was recently adapted for rats(Leonard et al., 2016) and Hassan et al (2017) have extended it to include the pregnant rat dam in response to TPO inhibition induced by PTU. While the original model predicted serum TH and TSH levels as a function of oral dose, it was not used to explicitly predict the relationship between serum hormones and TPO inhibition, or TH synthesis. Leonard et al. (2016) recently incorporated TPO inhibition into the model. Degon et al (2008) developed a human thyroid model that includes TPO, but does not make quantitative prediction of organification changes due to inhibition of the TPO enzyme. Further empirical support for the response-response relationship has been demonstrated in the amphibian model, Xenopus laevis, exposed to TPO inhibitors during pro-metamorphosis (Haselman et al., 2020) wherein temporal profiles were measured for both thyroidal and circulating T4.

Given that the thyroid gland contains follicular lumen space filled with stored thyroglobulin/T4, complete inhibition of thyroid hormone synthesis at a given point in time will not result in an instantaneous decrease in circulating T4. The system will be capable of maintaining sufficient circulating T4 levels until the gland stores are depleted. The time it takes to deplete stored hormone will greatly depend on species, developmental status and numerous other factors. In Xenopus, Haselman et al. (2020) demonstrated an approximately 5 day difference between a significant decrease in thyroidal T4 preceding a significant decrease in circulating T4 while exposed to a potent TPO inhibitor (MMI) continuously during pro-metamorphosis.

This KER is entirely influenced by the feedback loop between circulating T4 originating from the thyroid gland and circulating TSH originating from the pituitary. Intermediate biochemical processes exist within the hypothalamus to affirm feedback and coordinately release TSH from the pituitary. However, quantitative representations of these feedback processes are limited to models discussed previously. In Xenopus, circulating levels of T4 increase through pro-metamorphosis indicating a "release" of feedback to allow circulating levels of T4 to increase and drive metamorphic changes (Sternberg et al., 2011). This provides evidence that homeostatic control of feedback can be developmentally dependent, and likely species dependent.

High Male High Female

High

All life stages

High High High High Low Low

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b4308aa62b8> AOPWiki 2016-11-29T18:41:33

2022-10-10T08:56:38

<upstream-id>79819d6f-e072-4fe9-9f1f-96d86a2d845c</upstream-id> <downstream-id>64d85229-2572-4a1f-9ff8-e5d850a6ea57</downstream-id> In mammals, thyroxine (T4) in brain tissue is derived almost entirely from the circulating pool of T4 in blood. Transfer of free T4 (and to a lesser extent, T3) from serum binding proteins (thyroid binding globulin (TBG), transthyretin (TTR) and albumin; see McLean et al., 2017, for a recent review) into the brain requires transport across the blood brain barrier (BBB) and /or indirect transport from the cerebral spinal fluid (CSF) into the brain through the blood-CSF-barrier.  The blood vessels in rodents and humans expresses the main T4 transporter, MCT8, (Roberts et al. 2008), as does the choroid plexus which also expresses TTR and secretes the protein into the CSF (Alshehri et al. 2015). T4 entering the brain through the BBB is taken up into astrocytes via cell membrane iodothyronine transporters (e.g., organic anion-transporting polypeptides OATP), monocarboxylate transporter 8 (MCT8) (Visser et al., 2011).  In astrocytes, T4 is then deiodinated by Type II deiodinase to triiodothyronine (T3) (St Germain and Galton, 1997), which is then transported via other iodothyronine transporters (MCT8) into neurons (Visser et al., 2011). While some circulating T3 may be taken up into brain tissue directly from blood (Dratman et al., 1991), the majority of neuronal T3 comes from deiodination of T4 in astrocytes. Decreases in circulating T4 will eventually result in decreased brain T3 tissue concentrations. It is also known that Type II deiodinase can be up-regulated in response to decreased T4 concentrations to maintain tissue concentrations of T3 (Pedraza et al., 2007; Lavado-Autric et al., 2013; Morse et al., 1986), except in tanycytes of the paraventricular nucleus (Fekete and Lechan, 2014).

The weight of evidence linking reductions in circulating serum TH and reduced brain concentrations of TH is moderate. Many studies support this basic linkage. However, there are compensatory mechanisms (e.g., upregulation of deiodinases, transporters) that may alter the relationship between hormones in the periphery and hormone concentrations in the brain. There is limited information available on the quantitative relationship between circulating levels of TH, these compensatory processes, and neuronal T4 concentrations, especially during development. Furthermore, in certain conditions, such as iodine deficiency, the decreases in circulating hormone might have greater impacts on tissue levels of TH (see for instance, Escobar del Rey, et al., 1989).

The biological relationship between these two KEs is strong as it is well accepted dogma within the scientific community. There is no doubt that decreased circulating T4 leads to declines in tissue concentrations of T4 and T3 in a variety of tissues, including brain. However, compensatory mechanisms (e.g., increased expression of Type 2 deiodinase) may differ during different lifestages and across different tissues, especially in different brain regions.  Similarly, the degree to which serum TH must drop to overwhelm these compensatory responses has not been established.

Several studies have shown that tissue levels, including brain, of TH are proportional to serum hormone level (Oppenheimer, 1983; Morreale de Escobar et al., 1987; 1990; Calvo et al., 1992; Porterfield and Hendrich, 1992, 1993; Broedel et al., 2003). In thyroidectomized rats, brain concentrations of T4 were decreased and Type II deiodinase (DII) activity was increased. Both brain T3 and T4 as well as DII activity returned to normal following infusion of T4 (Escobar-Morreale et al., 1995; 1997). Animals treated with PTU, MMI, or iodine deficiency during development demonstrate both lower serum and lower brain TH concentrations (Escobar-Morreale et al 1995; 1997; Taylor et al., 2008; Bastian et al., 2012; 2014; Gilbert et al., 2013).  Compared to the wildtype, a mouse MCT8 knockout model has was shown to have decreased plasma T4, decreased uptake of T4 into the brain, and decreased brian T3 concentrations, as well as increased cortical diodinase Type 2 activity and increased plasma T3 concentrations (Mayerl et al., 2014; Barez-Lopez et al., 2016).  Temporal Evidence: The temporal relationship between serum T4 and T4 in growing neuronal tissue described in this KER is dependent on the developmental stage (Seed et al., 2005).  While all brain regions will be impacted by changes in serum hormones, brain concentrations will be a function of development stage and brain region. Data are available from thyroid hormone replacement studies that demonstrate recovery of fetal brain T3 and T4 levels (following low iodine diets or MMI exposure) to control levels after maternal thyroid hormone replacement or iodine supplementation (e.g., Calvo et al.,1990; Obregon et al., 1991). For example, Calvo et al. (1990) carried out a detailed study of the effects of TPO inhibition on serum and tissues levels of TH in gestating rats. Clear dose-dependent effects of T4 replacement, but not T3 replacement were seen in all maternal tissues. However, for fetal tissues, neither T4 nor T3, at any dose, could completely restore tissue TH levels to control levels. Dose-Response Evidence:  There is good evidence, albeit from a limited number of studies of the correlative relationship between circulating thyroid hormone concentrations and brain tissue concentrations during fetal and early postnatal development following maternal iodine deficient diets or chemical treatments that depress serum THs (c.f., Calvo et al., 1990; Obregon et al., 1991; Morse et al.,1996).

The fact that decreased serum TH results in lower brain TH concentrations is well accepted.  However, the ability of the developing brain to compensate for insuffiencies in serum TH has not been well studied.  Limited data is available that demonstrates that changes in local deiodination in the developing brain can compensate for chemical-induced alterations in TH concentrations (e.g., Calvo et al., 1990; Morse et al., 1996; Sharlin et al., 2010). And, there are likely different quantitative relationships between these two KEs depending on the compensatory ability based on both developmental stage and specific brain region (Sharlin et al., 2010). For these reasons, the empirical support for this linkage is rated as moderate The role of cellular transporters represents an additional uncertainly. In addition, future work on cellular transport mechanisms and deiodinase activity is likley to inform addition of new KEs and KERs between serum and brain T4.

While it is well established that decreased in serum TH levels result in decreased brain TH concentrations, particularly fetal brain concentrations, a major gap is the lack of empirical data that allow direct quantification of this relationship (Hassan et al., 2018). Recently, serum TH and brain TH were measured in fetal cortex and postnatal day 14 offspring following graded degrees of hypothyroidism induced by PTU (O’Shaughnessy et al., 2018). Results showed that brain levels TH levels at both ages were quantitatively related to serum T4 levels. Additional dose-response information is necessary to confirm these findings, and standardization of analysis for the measurements in these distinct matrices is crucial to allow comparisons to be made between independent experiments.

Moderate Male Moderate Female

High

During brain development

Moderate

All life stages

Moderate Moderate Moderate

The majority of the information on this KER comes from in vivo studies with rodents (mainly MCT8 knock-out mice and thyroidectomized rats) and histopathological analyses of human brain tissues derived from patients affected by AHDS (Allan-Herndon-Dudley syndrome). The evoluationary conservation of the transport of TH from circulation to the developing brain suggests, with some uncertainty, that this KER is also applicable to other mammalian species.

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b4308b910b0> AOPWiki 2016-11-29T18:41:33

2019-04-04T10:50:44

<upstream-id>79819d6f-e072-4fe9-9f1f-96d86a2d845c</upstream-id> <downstream-id>7afb76b3-8c88-4000-b0c7-e39d301c7a6f</downstream-id> Thyroid hormones (TH) are critical for normal development of the structure and function of the brain, including hippocampal development and cognitive function (Anderson et al., 2003; Bernal, 2007; Willoughby et al., 2014).   Brain concentrations of T4 are dependent on transfer of T4 from serum, through the vascular endothelia, into astrocytes.  In astrocytes, T4 is converted to T3 by deiodinase and subsequently transferred to neurons cellular membrane transporters. In the brain T3 controls transcription and translation of genes responsible for normal hippocampal structural and functional development. Clearly the brain circuitry controlling cognitive function is complex and is not solely accomplished by the functionality of the hippocampus. However, it is well documented that normal hippocampal structure and physiology are critical for the development of cognitive function. Thus, there is an indisputable indirect link between serum T4 and cognitive function.

The weight of evidence for this indirect relationship is strong. Alterations in serum TH concentrations are very well correlated with adverse impacts on cognitive behaviors such as learning and memory. This includes a large amount of literature, from more than four decades of research, that links hypothyroidism and/or hypothyroxinemia with alterations in spatial cognitive function, a hippocampal dependent behavior. A number of reviews are cited below that are primarily from humans and rodents, but this indirect relationship has also been shown for a number of other species. In humans, severe serum TH reductions that accompany congenital hypothyroidism dramatically impair brain function and lead to severe mental retardation. Lower global IQ scores, language delays and weak verbal skills, motor weakness, attentional deficits and learning impairments accompany low serum TH in children (Derksen-Lubsen and Verkerk 1996). Standard tests of IQ function in children born to mothers with even marginal hypothyoidism during pregnancy or in children with a defective thyroid gland who are then treated remain approximately 6 points below expected values. Selective deficits on visual spatial, motor, language, memory and attention tests are observed, the exact phenotype largely dependent on the developmental window over which the insufficiency occurred and the severity of the hormone deficit (Mirabella et al. 2000; Rovet 2002; Zoeller and Rovet 2004; Willoughby et al 2014). Indeed, this link is recognized as being so clinically important that T4 and TSH are monitored in all newborns in the US.    In rodent models, reductions in serum TH induced by TPO inhibitors such as MMI and PTU, when induced during development, lead to a variety of neurobehavioral impairments. These impairments can occur in the sensory, motor, and cognitive domains. The specific phenotype is dependent on both the window of exposure, the duration of exposure, and the severity of the hormone reduction (Zoeller and Rovet, 2004).  This includes more than four decades of work linking serum TH changes to alterations in hippocampal-dependent spatial behaviors (Akaike et al., 2004; Axelstand etal., 2008; Brosvic et al; Kawada et al, 1988; Friedhoff et al, 2000; Gilbert and Sui, 2006; Gilbert et al., 2016; Gilbert, 2011).

The biological plausibility of this KER is rated as strong. The relationship is consistent with the known biology of how the relationship between serum TH concentrations, brain TH concentrations, and TH control of brain development.

Empirical support for this KER is rated as strong. Empirical data from studies that measure serum TH concentrations and then assess alterations in cognitive function, including hippocampal dependent behaviors, is vast. The qualitative relationship between reduced serum hormone levels and adverse cognitive outcomes is well accepted in endocrinology, as well as developmental neuroendocrinology. Indeed, the relationship between serum T4 and T3 levels and adverse neurodevelopmental outcomes (e.g., IQ loss in children) is beyond reproach. Temporal Evidence: The temporal nature of this KER is developmental (Seed et al., 2005). It is a well-recognized fact that there are critical developmental windows for disruption of the serum THs that result in cognitive function.  In humans, hormone insufficiency that occurs in mid-pregnancy due to maternal drops in serum hormone, and that which occurs in late pregnancy due to disruptions in the fetal thyroid gland lead to different patterns of cognitive impairment (Zoeller and Rovet, 2004). In animal models, deficits in hippocampal-dependent cognitive tasks result from developmental, but not adult hormone deprivation (Gilbert and Sui, 2006; Gilbert et al., 2016; Axelstad et al, 2009; Gilbert, 2011; Opazo et al., 2008). Replacement studies have demonstrated that varying adverse neurobehavioral outcomes, including cognitive function, can be reduced or eliminated if T4 (and/or T3) treatment is given during the critical windows (e.g., Kawada et al., 1988; Goldey and Crofton, 1998; Reid et al., 2007). Dose-Response Evidence: An increasing amount of literature is now available that provides clear evidence of the ‘dose-response’ nature of this KER.  Most research over that last 40 years has employed high doses of chemicals, or chemicals plus thyroidectomies, that results in severe depletion of circulating thyroid hormones. More recently, researchers produced graded degrees of TH insufficiency in dams and pups by administering varying doses of chemicals and have correlated them to the dose-dependency of the observed effects.  This work has provided increased confidence in the relationship between serum TH decrements and a variety of neurodevelopmental impairments, and also to the specificity of the observed effects on brain development that is directly mediated by TH insufficiency (Goldey et al., 1995; Crofton, 2004; Gilbert and Sui, 2006; Gilbert, 2011; Bastian et al., 2014; Royland et al., 2008; Sharlin et al., 2008).

There are no inconsistencies in this KER, but there are some remaining uncertainties. It is widely accepted that changes in serum THs during development will result in alterations in behavior controlled by the hippocampus. This has been repeatedly demonstrated in animal models and in humans. A major uncertainty is the precise relationship between the degree, timing and duration of serum TH changes that leads to these behavioral deficits. Inconsistencies may also exist for chemicals other than classical TPO inhibitors that may reduce serum TH and induce impairments in cognitive function, but through action on other endocrine systems, or via direct action on the brain in the absence of an intervening endocrine action.

Except for a quantitative relationship between serum T4 and hearing loss in rodents (Crofton, 2004), there are no other reports of development of quantitative predictive models linking serum TH and adverse neurological outcomes. Insufficient data exist to develop a quantitative predictive model of adverse cognitive outcomes from serum TH concentrations. However, evidence from human studies suggests that decreases as low as 25% in serum T4 in pregnant women will yield small decrements in IQ in children (e.g., Haddow et al., 1995). Since publication of this seminal paper, several reports have appeared providing supportive if not direct confirmatory data on the association of reductions in maternal or early postnatal serum TH and adverse neurodevelopmental outcomes (e.g., Rovet and Willoughby, 2010, Wheeler et al., 2011, Willoughby et al., 2014, Wheeler et al., 2015; Pop et al., 1999, Pop et al., 2003, Kooistra et al., 2006, Henrichs et al., 2010, Korevaar et al., 2016). Based on these data, regulatory authorities have used 10 and/or 20% changes in serum T4 as a point of departure for hazard assessments in rodent studies (EPA, 2011).

High Male High Female

High

During brain development

High High High

There is a plethora of data supporting this KER in rats, mice, and humans.

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b4308d21e48> AOPWiki 2016-11-29T18:41:34

2018-08-11T19:44:38

<upstream-id>84c4a9c3-ad43-4062-becb-a9de6d80172a</upstream-id> <downstream-id>67eedfd5-3577-467d-b113-56296996fb44</downstream-id> Synaptic transmission and plasticity require the integrity of the anatomical substrate. The connectivity of axons emanating from one set of cells to synapse on the dendrties of the receiving cells must be intact for effective communication between neurons to be possible. Changes in the placement of cells within the network due to delays in neuronal migration, the absence of a full proliferation of dendritic arbors and spine upon which synaptic contacts are made, and the lagging of transmission of electrical impulses due to insufficient myelination will individually and cumulatively impair synaptic function. These anatomical alterations are among a host of many structural anomolies reported in various regions of the brain following severe developmental hypothyroidism. Although the primary evidence of synaptic transmission impairments in hypothyroid models have been limited to hippocampus, it is assumed that the role TH play in these processes is likely similar across different brain regions.

The weight of evidence supporting the relationship between structural abnormalities in brain induced by thyroid hormone insufficiency and altered synaptic function is moderate. Functional change as exemplified by alterations in synaptic transmission may be more easily detected that structural abnormalities. What is less than clear is the exact alignment between the neuroanatomical effects that have been identified with developmental hypothyroidism and the neurophysiological characteristics that have been described in hippocampus.

The biological plausibility of the known effects of TH insufficiency on brain structure having an impact on synaptic function and plasticity in brain is strong. Reductions in myelination of axons, cell number, dendritic arborization, synaptogenesis have been described in models of severe hormone deprivation. Because synaptic transmission relies on the integrity of contacts and the reilability of electical and chemical transmission between pre- and post-synaptic neurons, it is well accepted that interference on the anatomical levels will very much impact the functional output on the neurophysiological level.

Most of the information on developmental hypothyroidism and altered synaptic function has been provided by study of the hippocampus. Within the hippocampus, area CA1 has been investigated primarily with in vitro techniques, harvesting slices of hippocampus from exposed animals an measuring synaptic function across the Shaeffer Collateral-pyramidal cell synapse (Vara et al., Sui and Gilbert, 2003). Pyramidal neurons of hypothyroid animals have fewer synapses and an impoverished dendritic arbor (Rami et al., 1986; Madeira et al., 1992). The other major region in hippocampus investigated in hpothyroid models is the perforant path-dentate gyrus synapse (Gilbert, 2011). Granule cells are the principal cell type of the dentate gyrus and receive input from cortical neurons of the entorhinal cortex. TPO inhibitors like PTU and MMI decrease the volume of the granule cell layer, the density of cells within the layer, and estimates of total granule cell number (Madeira et al. 1991). Migration of granule cells from the proliferative zone to the granule cell layer is retarded by thyroid deficiency as is dendritic arborization and synaptogenesis assessed by immunohistochemistry for the synaptic protein, synaptophysin (Rami et al. 1986; Dong et al., 2013). In models of severe hormone insufficiency, synaptic transmission in both hippocampal subfields is dramatically compromised. Excitatory synaptic function is disrupted in area CA1 of slices taken from hypothyroid animals in the preweaning period (Vara et al., 2002; Sui and Gilbert 2003; Gilbert 2004; Dong et al., 2007). These changes in synaptic transmission are permanent as deficiencies are also evident in slices from adult animals exposed developmentally but tested following return to euthyroidism (Gilbert 2003; Sui et al. 2005). Reductions in the expression of synaptic proteins are associated with these deficites in synaptic transmission (Dong et al., 2013). Expression of the calcium-binding protein, parvalbumin, is reduced in the hippocampus and cortex of PTU-exposed rats and associated with reductions in paired pulse depression in hippocampus, a measure of synaptic inhibition (Gilbert et al., 2007; Berbel et al., 1998). Induction of hypothyroid state in the adult animals does not induce these changes in synaptic function (Gilbert et al., 2007; Sanchez-Heurta et al., 2014). Deficits in synaptic plasticity in area CA1 have also been reported following developmental TH insufficiency induced by TPO inhibitors by a number of laboratories (Opazo et al., 2008; Dong et al., 2005; Sui and Gilbert, 2003). In the dentate gyrus, most investigations have been performed in intact animals and as such examined primarily in adult offspring of hypothyroid dams. Suppression of excitatory and inhibitory synaptic function and deficits in synaptic plasticity have been demonstrated (Gilbert, 2003; 2011; Gilbert and Sui, 2006; Gilbert et al., 2007). Evidence of temporal concordance for antomical and neurophysiological deficits induced by TPO inhibition derives from reports of dose-dependent declines in synaptic function with moderate decreases in hormones during development (Gilbert, 2011) and the absence of similar deficiencies when hormone reductions occur in adulthood (Gilbert et al., 2007; Sanchez-Huerta et al., 2014).

The uncertainties that exist in the relationship between altered neuroanatomy and altered neurophysiology include the exact way in which a change in cell number, degree of myelination, reduced dendritric arborization and synaptogenesis may express itself neurophysiologically. Dose-dependent reductions in synaptic function in hippocampus have been demonstrated in models of moderate degrees of hormone reduction, but studies of the anatomical integrity of the specific cell populations examined electrophysiologically have largely been evaluated in models of severe hypothyroidism and often in brain regions distinct form the hippocampus.

There are insufficient data available to describe the quantitative linkages between altered neuroanatomy and deficits in synaptic transmission and plasticity.

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b42fc7cc738> AOPWiki 2016-11-29T18:41:33

2016-11-29T19:59:43

<upstream-id>67eedfd5-3577-467d-b113-56296996fb44</upstream-id> <downstream-id>7afb76b3-8c88-4000-b0c7-e39d301c7a6f</downstream-id> Cognitive function and impairments thereof are measured using behavioral techniques. It is well accepted that these alterations in behavior are the result of structural or functional changes in neurocircuitry. Functional impairments are often measured using field potentials of critical synaptic circuits in hippocampus and cortex. A number of studies have been performed in rodent models that reveal deficits in both excitatory and inhibitory synaptic transmission in the hippocampus as a result of developmental thyroid insufficiency. A well established model of memory at the synaptic levels is known as long-term potentiation (LTP). Deficiencies in LTP are generally regarded as potential substrates of learning and memory impairments.

A number of studies have consistently reported alterations in synaptic transmission resulting from developmental TH disruption and impairments in behavioral tasks assessing learning and memory. It is not clear if all behavioral impairments reported can be directly tied to synaptic dysfunction in the brain regions within which neurotransmission deficits have been recorded. It is not unreasonable to posit that the mechanisms of supporting synaptic transmission and synaptic strengthening are similar in different regions of the brain that support learning and memory and that demonstration at one site where it is most readily assessed implicates this mechanism may also be impaired at other sites as well.

It is well accepted that alterations in synaptic transmission and plasticity contribute to deficits in cognitive function. There are a few studies that have linked exposure to TPO inhibitors (e.g., PTU, MMI) and well as iodine deficient diets, to changes in serum TH levels and have measured both alterations in both synaptic function and cognitive behaviors (Opazo et al., 2009; Dong et al., 2005; Gilbert, 2011; Gilbert and Sui, 2006).

Developmental hypothyroidism reduces the functional integrity in brain regions critical for learning and memory. Neurophysiological indices of synaptic transmission of excitatory and inhibitory circuitry are impaired in hypothyroid animals. Two regions of the hippocampus, area CA1 and the dentate gyrus, have received the most study due to their ease of assess and the robustness of the recorded responses in ex vivo and in vivo preparations. Both regions exhibit alterations in excitatory and inhibitory synaptic transmission following reductions in serum TH in the pre and early postnatal period (Dong et al., 2005; Gilbert, 2011; Gilbert and Sui, 2006; Sui and Gilbert, 2003; Taylor et al., 2008; Vara et al, 2002). These deficits persist into adulthood long after recovery to euthyroid status (Gilbert and Sui; 2006; Gilbert, 2011). The latter observation indicates that these alterations represent permanent changes in brain function induced from transient hormones insufficiencies induced during critical window of development. Because the adult hippocampus is involved in learning and memory, it is a brain region of remarkable plasticity. Use-dependent synaptic plasticity is critical during brain development for synaptogenesis and fine tuning of synaptic connectivity. In the adult brain, similar plasticity mechanisms underlie use-dependency that underlies learning and memory as exhibited in long-term potentiation (LTP) model of synaptic memory. Hypothyroidism during development reduces the capacity for synaptic plasticity in juvenile and adult offspring (Taylor et al., 2008; Sui and Gilbert, 2003; Gilbert and Sui, 2006; Gilbert, 2011; Dong et al., 2005). In animal models of developmental hypothyroidism, deficits in passive avoidance learning, spatial learning, and operant conditioning (Davenport and Dorcey 1972; Schalock et al. 1979; Tamasy et al. 1986; Akaike et al. 1991; Brosvic et al, 2002), have been reported, but these early observations are often limited to animals suffering fairly severe hormonal deprivation. At these levels of hormone deprivation, clear evidence of synaptic dysfunction and impaired plasticity has been reported (Dong et al., 2005; Vara et al., 2002; Gilbert and Pazckowski, 2003). More recent data have demonstrated persistent impairments in a variety of learning tasks with more modest reductions in serum TH induced by TPO inhibitors (Axelstad et al., 2009; Darbra et al., 2003; 2004; Gilbert and Sui, 2006; Gilbert, 2011). Subtle impairments accompanying moderate levels of hormone insufficiency require more sensitive behavioral assessments and have been observed coincident with synaptic transmission and plasticity deficits (Gilbert, 2011). Temporal concordance of TH insufficiency and disrupted development, defined at many levels of biological organization. There are critical windows during development where permanent changes are affected. Hormone replacement studies have demonstrated that structural alterations in brain are reduced or eliminated if T4 (and/or T3) treatment is given during the critical windows (Goodman and Gilbert, 2008; Auso et al., 2004; Lavado-Autric et al., 2003; Berbel et al., 2010; Koibuchi and Chin, 2000). Induction of graded degrees of TH insufficiency in dams and pups by administering varying doses of TPO inhibitors have demonstrated, in the absence of overt signs of toxicity in dams or the pups, dose-dependent alterations in synaptic transmission, synaptic plasticity, and performance on a variety of learning and memory tasks (Gilbert and Sui, 2006; Gilbert, 2011; Dong et al., 2005; Gilbert et al., 2013; Sui and Gilbert, 2003; Taylor et al., 2008).

The direct relationship of alterations in synaptic function and specific cognitive deficits is difficult to ascertain given the many forms that learning and memory can take and the complexity of synaptic interactions in even the simplest brain circuit. Linking of neurophysiological assessments to learning and memory processes have, by necessity, been made across simple monosynaptic connections and largely focused on the hippocampus. Alterations in synaptic function, however, have been found in the absence of behavioral impairments (e.g., Gilbert et al., 2013; 2007). This may result from: 1) Measuring only one component in the complex brain circuitry that underlies 'cognition' 2) Behavioral tests typically used in large dose-response studies allow for processing of large numbers of animals and may not be sufficiently sensitive to detect subtle cognitive impairments 3) Behavioral tasks may be solved by a number of differnt strategies - animals develop alernative strategies as a consequence of developmental insult to compensate for impaired ability.

There is very limited information on the degree of change in synaptic activity required to alter cognitive behaviors. This is a result of the diversity of methods for measuring both physiology and cognitive function, that hamper cross-study analyses. This highlights the need to develop empirical data based models of this key relationship.

Synaptic transmission and plasticity are acheived via mechanisms common across taxonomies. LTP has been recorded in aplysia, lizards, turtles, birds, mice, guinea pigs, rabbits and rats.

#<Reference::ActiveRecord_Associations_CollectionProxy:0x00007b42fc8036c0> AOPWiki 2016-11-29T18:41:33

2016-11-29T20:09:24

XX Inhibition of Sodium Iodide Symporter and Subsequent Adverse Neurodevelopmental Outcomes in Mammals

NIS and Neurodevelopment

Evgeniia Kazymova

Mary Gilbert, National Health and Environmental Effects Research Laboratory, US EPA, RTP, NC USA <gilbert.mary@epa.gov> Anna Price, European Commission Joint Research Centre, Institute for Health and Consumer Protection, Ispra, Italy <anna.price@jrc.ec.europa.eu> Kevin Crofton, National Center for Computational Toxicology, US EPA, RTP, NC USA <crofton.kevin@epa.gov>

BY-SA

1.0

Thyroid Disrupting Chemicals (TDCs) are defined as the xenobiotics that interfere with the thyroid axis with different outcomes for the organism. A very well-studied mechanism of action of the TDCs is the reduction of the circulating levels of THs by inhibiting hormone synthesis in the thyroid gland. For example, perchlorate is a very potent inhibitor of iodide uptake through the sodium/iodide symporter (Tonacchera et al., 2004). Perchlorate has been detected in human breast milk ranging from 1.4 to 92.2 mg μl–1 (10.5 μg l–1 mean) in 18 US states (Kirk et al. 2005), and 1.3 to 411 μg l–1 (9.1 μg l–1 median) in the Boston area, United States (Pearce et al. 2007). Perchlorate has also been detected in human colostrum of 46 women in the Boston area (from < 0.05 to 187.2 μmol l–1 (Leung et al. 2009)). The mechanism of perchlorate action is quite simple, as it is believed to be mediated only by the NIS inhibition (Dohan et al., 2007; Wolff, 1998). Additionally, thiocyanate and nitrate are two known inhibitors that have been found to reduce circulating TH levels (Blount et al., 2006; Steinhaus et al., 2007), but they are both less potent than perchlorate (Tonacchera et al., 2004). However, there are also contradictory results from other studies that showed no correlation between thyroid parameters and perchlorate levels in humans (Pearce et al., 2010; Amitai et al., 2007; Tellez et al., 2005). Co-occurrence of perchlorate, nitrate, and thiocyanate can alter thyroid function in pregnant women. Horton et al. (2015) have shown positive associations between the weighted sum of urinary concentrations of these three analytes and increased TSH, with perchlorate showing the largest weight in the index. Interestingly, De Groef et al. 2006 showed that nitrate and thiocyanate, acquired through drinking water or food, account for a much larger proportion of iodine uptake inhibition than perchlorate, suggesting that NIS inhibition and any potential downstream effect by perchlorate are highly dependent on the presence of other environmental NIS inhibitors and iodine intake itself (Leung et al., 2010). In particular, Tonacchera et al. (2004) showed that the relative potency of perchlorate to inhibit radioactive I− uptake by NIS is 15, 30 and 240 times that of thiocyanate, iodide, and nitrate respectively on a molar concentration basis. These data are in line with earlier studies in rats (Alexander and Wolff, 1996; Greer et al. 1966). Contradictory findings in these studies may therefore be a result of the confounding mixtures in the environment, masking the primary effect of perchlorate. Decreased iodine intake can decrease TH production, and therefore exposure to perchlorate might be particularly detrimental in iodine-deficient individuals (Leung et al. 2010). Moreover, biologically based dose-response modeling of the relationships among iodide status (e.g., dietary iodine levels), perchlorate dose, and TH production in pregnant women has shown that iodide intake has a profound effect on the likelihood that exposure to goitrogens will produce hypothyroxinemia (Lewandowski et al. 2015). During pregnancy TH requirements increase, particularly during the first trimester (Alexander et al. 2004; Leung et al. 2010), due to higher concentrations of thyroxine-binding globulin, placental T4 inner-ring deiodination leading to the inactive reverse T3 (rT3), and transfer of small amounts of T4 to the foetus (during the first trimester foetal thyroid function is absent). Moreover, glomerular filtration rate and clearance of proteins and other molecules are both increased during pregnancy, possibly causing increased renal iodide clearance and a decreased of circulating plasma iodine (Glinoer, 1997). Thus, even though the foetal thyroid can trap iodide by about 12 week of gestation (Fisher and Klein, 1981), high concentrations of maternal perchlorate may potentially decrease thyroidal iodine available to the foetus by inhibiting placental NIS (Leung et al. 2010). Consequences of TH deficiency depend on the developmental timing of the deficiency (Zoeller and Rovet, 2004). For instance, if the TH deficiency occurs during early pregnancy, offspring show visual attention, visual processing and gross motor skills deficits, while if it occurs later, offspring may show subnormal visual and visuospatial skills, along with slower response speeds and motor deficits. If TH insufficiency occurs after birth, language and memory skills are most predominantly affected (Zoeller and Rovet, 2004). Along this line, age and developmental stage are crucial in determining sensitivity to NIS inhibitors (e.g., perchlorate, thiocyanate, and nitrate). In this regard, McMullen et al. (2017) have shown that adolescent boys and girls, more than adults, represent vulnerable subpopulations to NIS symporter inhibitors. Altogether these studies indicate that age, gender, developmental stage, and dietary iodine levels can affect the impact of NIS inhibitors. Finally, ten more small simple-structured molecules were identified in a large screening study (Lecat-Guillet et al., 2008b) that could block iodide uptake by specifically disrupting NIS in a dose-dependent manner. These molecules were named Iodide Transport Blockers (ITBs). There are few organic molecules that lead to NIS inhibition but no direct interaction with NIS has been determined (Gerard et al., 1994; Kaminsky et al., 1991). Up to date, only dysidenin, a toxin isolated from the marine sponge Dysidea herbacea, has been reported to specifically inhibit NIS (Van Sande et al., 2003). Finally, the aryltrifluoroborates were found to inhibit iodide uptake with an IC50 value of 0.4 μM on rat-derived thyroid cells (Lecat-Guillet et al., 2008a). The biological activity is rationalized by the presence of the BF3− ion as a minimal binding motif for substrate recognition at the iodide binding site. It has been also shown that many anions, such as ClO3-, SCN-, NO3-, ReO4-, TcO4- and in a lower extent Br- and BF4-, are also acting as NIS substrates and they enter the cell by the same transporter mechanism (Van Sande et al., 2003). It has been also shown that ClO4- is transferred by NIS with high affinity and is considered as one of its most potent inhibitors (Dohan et al., 2007). Most recently, the aryltrifluoroborates were also shown to inhibit NIS function (Lecat-Guillet et al., 2008a). A library of 17,020 compounds was tested by a radioactive screening method with high specificity using transfected mammalian cells (Lecat-Guillet et al., 2008b; 2007) for NIS inhibition evaluation. Further studies with the most powerful inhibitors showed a high diversity in their structure and mode of action (Lindenthal et al., 2009).

A prime example of impairments in cognitive function as the adverse outcome for regulatory action is developmental lead exposure and IQ function in children (Bellinger, 2012). In addition, testing for the impact of chemical expsoures on cognitive function, often including spatially-mediated behaviors, is an intergral part of both EPA and OECD developmental neurotoxicity guidelines (USEPA, 1998; OECD, 2007).

adjacent

Moderate

High adjacent

Moderate

High adjacent

Low

Moderate adjacent

Low

Moderate adjacent

Low

Moderate non-adjacent

Moderate

High

High Male High Female

High

Foetal

High

Perinatal

High High

AOPWiki 2016-11-29T18:41:16

2023-09-25T16:26:47