Pal et al. describe how a G protein–coupled receptor (GPCR) that inhibits sonic hedgehog (Shh) signaling is removed from the primary cilium in response to activation of the pathway.
Shh signaling is associated with trafficking of proteins into and out of the primary cilium. For example, upon activation of the pathway, the signal transducer Smoothened accumulates in the cilium, while the constitutively active orphan GPCR Gpr161, which suppresses Shh signaling, is quickly expelled. How Gpr161 is removed from the cilium is unknown; GPCRs are generally removed from the plasma membrane by clathrin-mediated endocytosis, but clathrin only localizes to the base of the cilium—at the ciliary pocket—rather than in the cilium itself.
Pal et al. found that Gpr161’s removal from the cilium is triggered by the accumulation of active Smoothened and is dependent on the kinase Grk2 and the β-arrestin family of proteins that link GPCRs to the clathrin machinery. Active Smoothened promoted Gpr161’s interaction with β-arrestins inside the cilium. Deleting β-arrestin1/2, or mutating the β-arrestin binding site in Gpr161, prevented the receptor’s disappearance from cilia and suppressed Shh signaling. Knocking down clathrin also inhibited Gpr161’s removal from the cilium, suggesting that, after associating with β-arrestins inside the cilium, Gpr161 is linked to the endocytic machinery at the ciliary pocket.
Senior author Saikat Mukhopadhyay now wants to investigate the physiological consequences of blocking Gpr161’s removal from the cilium, particularly in the hippocampus and cerebellum where Shh signaling is crucial for neurodevelopment.