Ciliary signaling in development and disease
Basal suppression of Shh signaling by Gpr161
The primary cilium is critical for Shh signaling in dorso-ventral neural tube patterning (Goetz and Anderson, 2010). Shh signaling culminates in processing of the bifunctional Gli transcription factors to generate Gli activators or repressors in a cilium-dependent manner. Addition of Shh results in removal of Patched (Ptch1), the Shh receptor from cilia, and accumulation of Smoothened (Smo), a frizzled family GPCR in the compartment.Smo determines activation of the pathway by promoting generation of Gli2 activator.
In addition to Gli activator, Gli repressors (mainly Gli3R) are generated in a cilium-, protein kinase A- (PKA), and suppressor of fused (Sufu)-dependent manner upon proteolytic processing (Mukhopadhyay and Rohatgi, 2014), referred to as the basal repression machinery of Shh signaling. However, the pathways determining cAMP-PKA signaling in the cillium are not well understood and are critical toward addressing the role of negative regulation of the pathway.
Limb and skeletal development
The role of basal suppression of Shh pathway and its interaction with Indian hedgehog (Ihh) signaling during limb/skeletal morphogenesis is not well understood. We discovered that forelimb buds are not formed in Gpr161 knockout mice embryos despite establishment of prospective limb fields. Limb-specific deletion of Gpr161 resulted in prematurely expanded Shh signaling and ectopic Shh-dependent patterning defects to cause polysyndactyly. In addition, endochondral bone formation in forearms, including formation of both trabecular bone and bone collar was prevented.
Endochondral bone formation defects resulted from accumulation of proliferating round/periarticular-like chondrocytes, lack of differentiation into columnar chondrocytes, and corresponding absence of Ihh signaling. Gpr161 deficiency in craniofacial mesenchyme also prevented intramembranous bone formation in calvarium. Defects in limb patterning, endochondral and intramembranous skeletal morphogenesis were suppressed in the absence of cilia. Overall, Gpr161 promotes forelimb formation, regulates limb patterning, prevents periarticular chondrocyte proliferation, and drives osteoblastogenesis in intramembranous bones in a cilium-dependent manner (Hwang et al., 2018).
Cerebellum development and medulloblastoma pathogenesis.
Shh determines cerebellar granule cell (GC) progenitor proliferation and medulloblastoma pathogenesis. However, pathways regulating GC progenitors during embryogenesis before Shh production by Purkinje neurons and their roles in tumorigenesis remain unclear. By deleting Gpr161 in mouse neural stem cells or GC progenitors, we establish Gpr161 as a tumor suppressor in Shh-subtype medulloblastoma. Irrespective of Shh production in cerebellum, Gpr161 deletion increased downstream activity of the Shh pathway by restricting Gli3-mediated repression, causing more extensive generation and proliferation of GC progenitors.
Moreover, earlier deletion of Gpr161 during embryogenesis increased tumor incidence and severity. GC overproduction during embryogenesis from Gpr161 deletion was cilia-dependent, unlike normal development. Low GPR161 expression correlated with poor survival of SHH-subtype medulloblastoma patients. Together, Gpr161 restricts GC production by preventing premature and Shh-dependent pathway activity, highlighting importance of basal pathway suppression in tumorigenesis (Shimada et al., 2018).
Goetz, S.C., and Anderson, K.V. (2010). The primary cilium: a signalling centre during vertebrate development. Nat Rev Genet 11, 331-344.
Hwang, S.H., White, K.A., Somatilaka, B.N., Shelton, J.M., Richardson, J.A., and Mukhopadhyay, S. (2018). The G protein-coupled receptor Gpr161 regulates forelimb formation, limb patterning and skeletal morphogenesis in a primary cilium-dependent manner. Development 145.
Mukhopadhyay, S., and Rohatgi, R. (2014). G-protein-coupled receptors, Hedgehog signaling and primary cilia. Semin Cell Dev Biol 33, 63-72.
Mukhopadhyay, S., Wen, X., Ratti, N., Loktev, A., Rangell, L., Scales, S.J., and Jackson, P.K. (2013). The ciliary G-protein-coupled receptor Gpr161 negatively regulates the Sonic hedgehog pathway via cAMP signaling. Cell 152, 210-223.
Shimada, I.S., Hwang, S.H., Somatilaka, B.N., Wang, X., Skowron, P., Kim, J., Kim, M., Shelton, J.M., Rajaram, V., Xuan, Z., et al. (2018). Basal Suppression of the Sonic Hedgehog Pathway by the G-Protein-Coupled Receptor Gpr161 Restricts Medulloblastoma Pathogenesis. Cell Rep 22, 1169-1184.