Smoothened

Function

Cellular localization plays an essential role in the function of SMO, which anchors to the cell membrane as a 7-pass transmembrane protein. Stimulation of the patched 12-pass transmembrane receptor by the sonic hedgehog ligand leads to translocation of SMO to the primary cilium in vertebrates in a process that involves the exit of patched from the primary cilium, where it normally localizes in its unstimulated state. Vertebrate SMO that is mutated in the domain required for ciliary localisation often cannot contribute to hedgehog pathway activation. Conversely, SMO can become constitutively localized to the primary cilium and potentially activate pathway signaling constitutively as a result of a tryptophan to leucine mutation in the aforementioned domain. SMO has been shown to move during patched stimulation from the plasma membrane near the primary cilium to the ciliary membrane itself via a lateral transport pathway along the membrane, as opposed to via directed transport by vesicles. The cAMP-PKA pathway is known to promote the lateral movement of SMO and hedgehog signal transduction in general. In invertebrates like Drosophila, SMO does not organize at cilia and instead is generally translocated to the plasma membrane following hedgehog binding to patched.

After cellular localization, SMO must additionally be activated by a distinct mechanism in order to stimulate hedgehog signal transduction, but that mechanism is unknown. There is evidence for the existence of an unidentified endogenous ligand that binds SMO and activates it. It is believed that mutations in SMO can mimic the ligand-induced conformation of SMO and activate constitutive signal transduction.

SMO plays a key role in transcriptional repression and activation by the zinc-finger transcription factor Cubitus interruptus (Ci; known as Gli in vertebrates). When the hedgehog pathway is inactive, a complex of Fused (Fu), Suppressor of Fused (Sufu), and the kinesin motor protein Costal-2 (Cos2) tether Ci to microtubules. In this complex, Cos2 promotes proteolytic cleavage of Ci by activating hyperphosphorylation of Ci and subsequent recruitment of ubiquitin ligase; the cleaved Ci goes on to act as a repressor of hedgehog-activated transcription. However, when hedgehog signaling is active, Ci remains intact and acts as a transcriptional activator of the same genes that its cleaved form suppresses. SMO has been shown to bind Costal-2 and play a role in the localization of the Ci complex and prevention of Ci cleavage. Additionally, it is known that vertebrate SMO contributes to the activation of Gli as a transcription factor via association with ciliary structures such as Evc2, but these mechanisms are not fully understood.

Endogenous activation

A leading hypothesis in the field is that Patched regulates Smoothened by regulating access to cholesterol or a related substance. It has been proposed that cholesterol activates Smo, and subsequently Hh signaling, by entering the active site through a hydrophobic "oxysterol tunnel," which can adopt open or closed conformations to allow for activation or inactivation of Smo, respectively, due to allowed sterol binding. Shh would work by inhibiting Ptc, which would increase accessible cholesterol concentrations and allow for the activation of Smo and transmission of the Hh signal. A recent crystal structure has identified two sterol binding sites in Smo, but which site is endogenously regulated by Ptc remains to be determined. The potential sites of regulation include the extracellular cysteine-rich domain (CRD) of Smo, as well as a site deep within the transmembrane domain (TMD).

Due to the abundance of cholesterol in the plasma membrane (up to 50 mole %), it has also been proposed that Ptc regulates the activity of Smo by controlling cholesterol accessibility specifically within the membrane of the primary cilia, which contains a less abundant, and therefore more readily regulated pool of accessible cholesterol.

Typically, upon activation and release of inhibition by Ptc, Smo will relocate to the primary cilia and Ptc will diffuse out of the ciliary membrane. Upon inactivation, Smo no longer becomes concentrated in the ciliary membrane, This hypothesis is supported by methods which can increase or deplete the accessible cholesterol pool, with a subsequent increase or decrease in Hh signaling. This accessible cholesterol pool has been shown to be distinct from the general plasma membrane cholesterol pool in being available for protein interaction and cell uptake. The ciliary membrane has also been shown to contain lower levels of accessible cholesterol due to sequestering of cholesterol by sphingomyelin. In addition to cholesterol's role as a Hh pathway agonist, it has been shown that cholesterol levels within the ciliary membrane rapidly increase upon treatment with Shh only in the presence of Ptc, further suggesting Ptc regulation of accessible cholesterol as the mechanism behind Smo activation/inhibition. Additionally, Molecular Dynamics simulations suggest that vismodegib inhibits Smo through a conformational change that prevents cholesterol from binding. This suggests the hypothesis that Ptc functions by preventing Smo access to cholesterol, and upon Ptc inhibition by Shh, Smo gains access to cholesterol and is subsequently activated, transmitting the Hh signal.

Role in disease

SMO can function as an oncogene. Activating SMO mutations can lead to unregulated activation of the hedgehog pathway and serve as driving mutations for cancers such as medulloblastoma, basal-cell carcinoma, pancreatic cancer, and prostate cancer. As such, SMO is an attractive cancer drug target, along with the many hedgehog pathway agonists and antagonists that are known to directly target SMO.

Cholesterol is known to be crucial in regulating the overall hedgehog pathway, and congenital mutations in cholesterol synthesis pathways can inactivate SMO specifically, leading to developmental disorders. For example, oxysterol 20(S)-OHC is known to activate vertebrate SMO by binding the cysteine rich domain near its extracellular amino-terminal region. In the context of cancer, 20(S)-OHC is the target of a proposed anti-cancer oxysterol binding inhibitor.

Agonists

• Smoothened agonist (SAG)
• Purmorphamine
• Oxysterols including 20(S)-OHC and 20(S)-yne

Antagonists

• Cyclopamine
• Itraconazole
• Vismodegib (Erivedge), a smoothened receptor inhibitor for the treatment of basal-cell carcinoma, being investigated for the treatment of other types of cancer
• SANT-1
• Sonidegib
• Patidegib
• Glasdegib
• Jervine

References

References

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