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Relationship: 2844

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Altered Bone Cell Homeostasis leads to Bone Remodeling

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Altered Bone Cell Homeostasis

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Bone Remodeling

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Deposition of energy leading to occurrence of bone loss	adjacent	Moderate	Low	Cataia Ives (send email)	Open for citation & comment

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
human	Homo sapiens	Low	NCBI
mouse	Mus musculus	High	NCBI
rat	Rattus norvegicus	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Male	High
Female	Moderate
Unspecific	Low

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
Adult	High
Juvenile	Moderate

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

The bone microenvironment is defined as a complex structural and biological system containing mesenchymal cells from different lineages; bone resident cells, such as osteoclasts, osteoblasts, and osteocytes; and the bone extracellular matrix. For bone structure to remain at a homeostatic level, osteoclasts and osteoblasts must act in unison so that bone resorption does not outpace bone formation, and vice versa. Osteoblasts differentiate from mesenchymal stem cells (MSCs) into pre-osteoblasts, then pre-osteoblasts migrate to the site of bone resorption where they become fully functioning osteoblasts capable of depositing new bone matrix (Donaubauer et al., 2020). Osteoclasts originate from hematopoietic stem cells (HSCs) in the bone marrow and their differentiation into pre-osteoclasts is stimulated by the release of cytokines by osteocytes, osteoblasts, and immune cells (Donauabauer et al., 2020). Imbalances in the regulation of osteoblast and osteoclast differentiation and proliferation results in altered bone cell homeostasis and consequent disruption to bone remodeling (Chatziravdeli et al., 2019; Donaubauer et al., 2020; Smith, 2020a; Smith, 2020b; Tian et al., 2017).

Altered bone cell homeostasis can be defined by an increase in osteoclast number and activity and a decrease in osteoblast number and activity, resulting in an imbalance in bone formation and resorption. Altered cell processes can increase osteoclast activity and decrease osteoblast activity and the production of the organic and inorganic components of the bone matrix. As a result of altered bone cell homeostasis, bone remodeling processes may be impacted. Each remodeling event, known as a basic multicellular unit (BMU), consists of osteoclasts, bone resorption cells, osteoblasts, and bone-forming cells (Raggatt & Partridge; Slyfield et al., 2012, Frost, 1966). The BMU activity can be assessed by examining parameters of dynamic bone histomorphometry. The structural model index (SMI) of bone tissue, which measures the proportion of rods and plates in trabecular bone, also serves as an important marker of bone structural changes (Shahnazari et al., 2012). A disruption in the activity of bone remodeling cells, such as bone MSCs, osteoblasts and osteoclasts, leads to dysfunction of bone cells and downstream altered bone remodeling (Wright et al., 2015; Zhang et al., 2018). The strict regulation of differentiation pathways that define osteoblast/osteoclastogenesis is essential for the maintenance of osteogenic balance and functioning of bone cells to bone remodeling.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

The strategy for collating the evidence on radiation stressors to support the relationship is described in Kozbenko et al 2022. Briefly, a scoping review methodology was used to prioritize studies based on a population, exposure, outcome, endpoint statement.

Evidence Map 2.0

ID	Experimental Design	Species	Upstream Observation	Downstream Observation	Citation (first author, year)	Notes

Evidence Map

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Title	First Author	Biological Plausibility	Dose Concordance	Temporal Concordance	Incidence Concordance

Biological Plausibility

Dose Concordance Evidence

Temporal Concordance Evidence

Incidence Concordance Evidence

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

None identified

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Modulating Factors	Details	Effects on the KER	References
Drug	Sclerostin (Wnt antagonist) suppression	Sclerostin, a Wnt antagonist, expression in adults is primarily restricted to osteocytes. The suppression of sclerostin was examined using Scl-Ab. Scl-Ab was found to completely reverse the effects of radiation on bone tissue. Scl-Ab injections not only blocked any structural deterioration, but also increased bone mass and improved bone quality in the radiated area to the same levels as in a non-radiated area with Scl-Ab treatment.	Chandra et al., 2017
Drug	Parathyroid hormone (PTH)1-34	Rats were given daily injections of human recombinant PTH (PTH1-34) to avoid the effects of ionizing radiation after being exposed to 16 Gy of X-rays. Compared to the irradiated group, rats treated with PTH1-34 had a 70.6% decrease in apoptotic osteoblasts (from 34 percent to 10 percent) and a 53% decrease in apoptosis in osteocytes.	Chandra et al., 2014
Age	Old age	Lower estrogen at old age is thought to contribute to higher osteoclast activity and increased bone resorption.	Pacheco and Stock, 2013

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

The following are a few examples of quantitative understanding of the relationship. All data that is represented is statistically significant unless otherwise indicated.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Dose/Incidence Concordance

Reference	Experiment Description	Result
Chandra et al., 2017	In vivo. An experiment was conducted on male C57BL/6 mice (8–10 weeks old) exposed to 8 Gy X-ray radiation at a rate of 1.65 Gy/min to analyze suppression of Sclerostin on irradiated bones. Osteoblast number over bone surface (Ob.N/BS), and structural model index (SMI) (bone remodeling markers) were measured.	The group without the sclerostin with a monoclonal antibody (Scl-Ab) injections experienced a 52% decrease in osteoblast number, and 26% increase in SMI.
Chandra et al., 2014	In vivo. 3-month-old female rats were irradiated with 16 Gy of X-rays, fractionated into two 8 Gy doses at a rate of 1.65 Gy/min. To analyze the effects of ionizing radiation-induced bone remodeling, histometric measurements of Ob.N/BS and osteoclast number over bone surface (Oc.N/BS) and BFR, MAR, and SMI (bone remodeling markers) were measured.	Ob.N/BS and Oc.N/BS was 75% and 50% lower in the irradiated group compared to the non-irradiated group, respectively. Ionizing radiation exposure also resulted in a ~100% decrease in both BFR and MAR, as well as a ~20% increase in SMI, at 28 days post-irradiation relative to non-irradiated controls.
Hui et al., 2014	In vivo. 20-week-old adult female mice were exposed to a single 16 Gy dose of X-rays. CTX (osteoclast marker), OCN (osteoblast marker) and MAR (bone remodeling marker) of the distal femurs of irradiated mice were measured.	Compared to the non-irradiated controls, CTX levels increased 38.2% by 3 days after radiation and OCN levels increased by 18.3% by 30 days after radiation. Mice experiencing a 16% decrease per day in MAR by 12-29 days post irradiation.
Wright et al., 2015	In vivo. The right hindlimbs of 20-week-old male C57BL/6 mice were irradiated with 2 Gy of X-rays at a rate of 1.6 Gy/min. Ob.N/BS and Oc.N/BS were measured to assess altered bone cell homeostasis and osteoid volume (OV/BV), osteoid surface (OS/BS), BFR, and MS/BS were measured to assess bone remodeling.	Compared to the control group, and contralateral group, bone marrow adiposity was increased in the irradiated group. Mineralized bone surface decreased in the irradiated group and unmineralized osteoid surface area was increased. Irradiation led to 46% increase in Oc.N/BS, a (n.s.) 15% increase in Ob.N/BS, a 33% decrease in BFR and a 20% decrease in MS/BS. In irradiated femurs OV/BV and OS/BS were increased compared to controls.
Yang et al., 2020	In vivo. Male 14-week-old transgenic mice were unloaded using tail suspension. The tibia of wildtype and transgenic mice were scanned at 28 days after un-loading. Bone cell markers including ALP activity, OCN, and TRAP-5b levels and bone remodeling markers such as MAR, BFR, and MS/BS were measured.	Analysis showed a 50% decrease in ALP activity, 47.5% decrease in OCN activity, and 4-fold increase in TRAP-5b by day 7. This was accompanied by a 23% decrease in MAR, a 33% decrease in BFR, and a 50% decrease in MS/BS under microgravity relative to control.
Lloyd et al., 2015	In vivo. 77-day-old female C57BL/6J mice were exposed to 12 days of microgravity conditions during spaceflight. Histological measurements were taken from the femur and proximal tibiae of the mice to study the effects of microgravity. These measurements consisted of indicators of bone cell function such as TRAP-5b and OCN and bone remodeling markers including MS/BS, MAR, BFR, and SMI	OCN was decreased by 40% in control groups and by nearly 50% in the spaceflight group. TRAP-5b levels were unchanged in the control group and were increased by 200% in the spaceflight group. There was a 33% decrease in periosteal BFR, a 32% decrease in periosteal MS/BS, and a 40% decrease in periosteal MAR. There was also a 40% decrease in endocortical BFR, a 29% decrease in endocortical MS/BS, and a 33% decrease in endocortical MAR. Lastly, there was a 50% decrease in trabecular BFR and a 6% increase in SMI.
Shahnazari et al., 2012	In vivo. 6-month-old adult male C57BL/6 and DBA/2 mice underwent hindlimb unloading for 1, 2, and 4 weeks to simulate the effects of microgravity. Measurements of calcified nodules and histological parameters were taken from cultured bone marrow cells and murine femurs, respectively. Levels of TRAP-positive cells (osteoclast marker) and BFR, MAR, MS/BS, and SMI (bone remodeling markers) were analyzed.	Compared to normally loaded controls, TRAP-positive osteoclasts increased by ~3.5-fold by week 1 of unloading and became non-significant after a week. By 1 week of unloading, there was a 70% and 60% decrease in calcified nodules in C57BL/6 and DBA/2 mice, respectively. While there was no significant change to BFR/BS in C57BL/6 mice, there was a ~33% decrease in DBA/2 mice at 2 weeks post-exposure. After 2 and 4 weeks, DBA/2 mice experienced significant decreases in MS/BS and MAR. SMI did not significantly change following unloading in either model.
Yotsumoto, Takeoka, and Yokoyama, 2010	In vivo. Eight-week-old male mice were tail-suspended. Deoxypyridinoline (DPD, osteoclast marker) and MAR, and BFR (bone remodeling markers) were measured to determine the effects of microgravity on bone remodeling.	Tail suspension resulted in a 50% decrease in OCN and 25% increase in DPD. This was accompanied by a 75% decrease in MAR and a 50% decrease in BFR under tail suspension.
Dehority et al., 1999	In vivo. Fifty-six 6-month-old virgin male Sprague-Dawley rats were unloaded using the hindlimb elevation model for 5 weeks. Osteoblast surface, BFR, and MAR (bone remodeling markers) levels were measured.	After 1 week of unloading, there was a 62.5% decrease in osteoblast surface, accompanied by an 80% decrease in BFR at the tibiofibular junction and a 33% decrease in MAR in the tibia after 2 weeks of unloading.
Matsumoto et al., 1998	In vivo. 6-week-old juvenile male rats underwent tail suspension for 14 days to simulate microgravity conditions. Histological measurements including osteoclast number, osteoblast surface and bone remodeling marker, MAR, of the femur and tibiae were measured.	Osteoclast number was 30% higher after tail suspension relative to controls at the same time point. Osteoblast surface was ~28% lower after tail suspension relative to controls. Tail suspension also resulted in a 48% decrease in periosteal MAR in the femur compared to baseline levels.
Wronski et al., 1987	In vivo. 84-day-old adult male, five large and six small, rats were exposed to microgravity conditions for 7 days during spaceflight. Osteoblast and osteoclast surface were measured along with BFR to assess altered bone cell homeostasis and bone remodeling, respectively.	Osteoclast surface increased 22% and osteoblast surface decreased 51% in large rats after spaceflight relative to controls. This was associated with a 34% decrease in BFR compared to the ground controls.
Bandstra et al., 2008	In vivo. 58-day-old, female, juvenile, C57BL/6J mice were exposed to whole-body irradiation with 0.5, 1, and 2 Gy of 250 MeV protons at a rate of 0.7 Gy/min. Histological measurements, including TRAP-5b, OCN (osteoclast markers) and periosteal BFR (Ps.BFR) and endosteal BFR (Ec.BFR) (bone remodeling marker) were measured.	All IR-induced changes to serum OCN and TRAP levels, along with BFR were non-significant compared to the control. TRAP-5b levels decreased in the 0.5 and 1 Gy group by 6% and 10%, respectively, and increased in the 2 Gy group by 2%. OCN levels were the same in the 1 Gy group and decreased in the 0.5 Gy and 2 Gy groups by 4%, and 18%, respectively. Ps.BFR increased by 5% and 14% after 0.5 and 1 Gy radiation, respectively; however, it remained unchanged post-2 Gy exposure. Ec.BFR decrease by 19%, 27%, and 21% after 0.5, 1, and 2 Gy, respectively.
Iwaniec et al., 2005	In vivo. 70-day-old female C56BL/6 F1 and DBA/2 mice underwent 1 week of hindlimb unloading to simulate microgravity conditions. Histological measurements were taken from the distal femur to study the effects of microgravity-induced bone remodeling. These measurements include BFR, an indicator of bone remodeling, and osteoblast and osteoclast surface, indicators of altered bone cell homeostasis.	Osteoclast surface was increased by 48% and osteoblast surface was decreased by 17% after hindlimb unloading. unloading. This was associated with a 43% decrease in BFR in wild type mice compared to control groups.

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Time Concordance

Reference	Experiment Description	Result
Shahnazari et al., 2012	In vivo. 6-month-old adult male C57BL/6 and DBA/2 mice underwent hindlimb unloading for 1, 2, and 4 weeks to simulate the effects of microgravity. Measurements of calcified nodules and histological parameters were taken from cultured bone marrow cells and murine femurs, respectively. Levels of TRAP-positive cells (osteoblast marker) and BFR, MAR, and MS (bone remodeling markers) were analyzed.	Compared to normally loaded controls, TRAP-positive osteoclasts increased by ~3.5-fold at week 1 of unloading but became non-significant after a week. Calcified nodule formation in both unloaded mouse models decreased significantly at all time points but progressively recovered from 1 to 4 weeks. C57BL/6 and DBA/2 mice saw maximum decreases of ~69% and ~61%, respectively, at 1 week of unloading. DBA/2 mice only experienced a significant decrease in BFR/BS at 2 weeks. BFR/BS in C57BL/6 mice did not change significantly at any time point. MS/BS and MAR both showed significant decreases in DBA/2 mice at 2 and 4 weeks.
Dehority et al., 1999	In vivo. Fifty-six 6-month-old male Sprague-Dawley rats were unloaded using the hindlimb elevation model for 5 weeks. Osteoblast surface (osteogenesis indicator), BFR, and MAR (bone remodeling markers) levels were measured.	Initial decrease in osteoblast surface at week 1 followed by a slight recovery at week 3 in unloaded rats; controls remained constant. At week 5 control rats showed a decrease in osteoblast surface and unloaded rats decreased to week 1 levels. BFR showed maximal decrease at week 2 of unloading and remained constant until week 4.
Hui et al., 2014	In vivo. 20-week-old adult female mice were exposed to a single 16 Gy dose of X-rays. CTX (osteoclast marker), OCN (osteoblast marker) and MAR (bone remodeling marker) of the distal femurs of irradiated mice were measured.	Compared to non-irradiated controls, CTX levels increased by 38.2% by 3 days after radiation. Irradiation resulted in the mice experiencing a 16% decrease per day in MAR by 12-29 days post irradiation.

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Not Identified

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Considerable evidence is available in mice and rats. The relationship has been demonstrated in vivo for both males and females, with more available evidence for males. In vivo evidence is derived from adolescents and adult models, with considerable evidence for adults.