Antagonism Smoothened leads to Decrease, SMO relocation

While we know that entry to the cilia is tightly controlled, the exact mechanism of SMO ciliary trafficking is not fully understood. The PC is separated from the plasma membrane by the ciliary pockets and the transition zone which function together to regulate the movement of lipids and proteins in and out of the organelle (Goetz, Ocbina et al. 2009, Rohatgi and Snell 2010). The SHH receptor PTCH contains a ciliary localization sequence in its’ carboxy tail. Localization of PTCH to the PC is essential for inhibition of SMO as deletion of the CLS in PTCH prevents PTCH localization as well as inhibition of SMO (Kim, Hsia et al. 2015) (53). SMO also contains a CLS, but only accumulates in the PC upon ligand binding (Corbit, Aanstad et al. 2005). The entry of SMO into the PC is thought to occur either laterally through the ciliary pockets or internally via recycling endosomes (Milenkovic, Scott et al. 2009). Once inside the PC, SMO can diffuse freely, however it will usually accumulate in specific locations depending upon its’ activation state. Inactive SMO will accumulate more at the base of the PC while active SMO will accumulate in the tip of the PC (Milenkovic, Weiss et al. 2015).

An endogenous ligand for SMO has not been discovered although evidence for one exists and that PTCH controls SMO by controlling its’ availability or accessibility. To support this, it has been shown that PTCH and SMO do not physically interact (Chen and Struhl 1998). PTCH acts catalytically with SMO with one PTCH receptor capable of controlling many (~50) SMO receptors (Taipale, Cooper et al. 2002). Since PTCH includes a sterol sensing domain and shares characteristics of ancient bacterial transporters, a model of PTCH functioning by pumping a sterol-like MSO regulator has been proposed (Mukhopadhyay and Rohatgi 2014).  SMO is constitutively active in the absence of PTCH suggesting that the elusive molecule is an agonist (Rohatgi and Scott 2007). Conversely, the discovery that oxysterols bind to the CRD binding domain acting as positive modulators suggest that the molecule could be an agonist with PTCH functioning to sequester away or limit cellular concentration (Corcoran and Scott 2006, Nachtergaele, Mydock et al. 2012)

The activity of SMO is controlled by ligand binding (Kobilka 2007). Two separate binding pockets, one in the groove of the extracellular CRD and the other in the helices of the TMD have been identified (Nachtergaele, Mydock et al. 2012, Rana, Carroll et al. 2013, Wang, Wu et al. 2013, Byrne, Sircar et al. 2016, Huang, Zheng et al. 2018). These two binding pockets have been shown to interact in an allosteric manner (Nachtergaele, Mydock et al. 2012). The binding pocket in the helices of the TMD binds several SMO agonists including SAG as well as antagonists Vismodegib and Sonidegib. The CRD binding pocket binds cholesterol and its’ oxidized derivates (Byrne, Luchetti et al. 2018). The antagonist cyclopamine binds to the TMD binding pocket and inhibits SHH signal transduction. However, in mSMO carrying the mutations D477G/E552K that disable the TMD binding pocket, cyclopamine binds to the CRD pocket and activates the pathway (Huang, Nedelcu et al. 2016). To date several oxysterols including 20(S)-hydroxylcholesterol, 22(S)-hydroxylcholesterol, 7-keto-25-hydroxylcholesterol and 7-keto-27-hydroxylcholesterol have been identified as activators of SMO (Dwyer, Sever et al. 2007, Nachtergaele, Mydock et al. 2012, Myers, Sever et al. 2013). A binding site for 24(S),25-epoxycholesterol has been identified in the TMD pocket using cryo-EM of SMO in complex with 24(S),25-epoxycholesterol (Qi, Liu et al. 2019).      

While it is well understood that cyclopamine is an antagonist of SMO, contradictory in vivo data was found regarding whether cyclopamine blocks SMO relocation to the primary cilia. Rohatgi et al used NIH 3T3s cell and found that cyclopamine did not inhibit the accumulation of SMO in the cilia even when dosed at 5-10um (>10 fold above kd). All three antagonists inhibited SHH pathway transduction and target gene expression (Rohatgi, Milenkovic et al. 2009).  Corbit et al used a renal epithelial MDCK (Madin-Darby canine kidney) line was engineered to express Myc-tagged SMO. Following culture for 1hr in SHH conditioned media SMO presence in the primary cilium is upregulated while cells cultured in the presence of cyclopamine see a downregulation of SMO in the primary cilia (Corbit, Aanstad et al. 2005). Further work is required to determine if SMO antagonism via cyclopamine results in decrease in SMO relocation.

No studies identified

Relocation of SMO to the PC typically occurs within ~20 minutes of agonist stimulation (Arensdorf, Marada et al. 2016). No data was found on how fast antagonism of SMO will stop its’ relocation to the primary cilia.

None identified