[Selective signaling pathway via feeding-related ciliary GPCR, melanin-concentrating hormone receptor 1]

G-protein-coupled receptors (GPCRs), which constitute a highly diverse family of seven transmembrane receptors, respond to external signals and regulate a variety of cellular and physiological processes. GPCRs are encoded by about 800 different genes in human and they represent the largest family of drug targets in clinical trials, which accounts for about 30% of approved drugs acting on 108 unique GPCRs. Signaling through GPCRs can be optimized by enriching receptors, selective binding partners, and downstream effectors in discrete cellular environment. The primary cilium is a ubiquitous organelle that functions as a sensory antenna for surrounding physical and chemical stimuli. Primary cilium's compartment is as little as 1/10,000th of the total cell volume. Therefore, the ciliary membrane is highly enriched for specific signaling molecules, allowing the primary cilium to organize signaling in a highly ordered microenvironment. Recently, a set of non-olfactory GPCRs such as somatostatin receptor 3 and melanin-concentrating hormone receptor 1 (MCHR1) have been found to be selectively targeted to cilia on several mammalian cell types including neuronal cells both in vitro and in vivo approaches. Moreover, investigations into the pathophysiology have implicated GPCR ciliary signaling in a number of developmental and cellular pathways. Thus, cilia are now considered as an increasingly important connection for GPCR signaling. This review summarizes our current understanding of the signaling pathways though ciliary GPCR, especially feeding- and mood-related GPCR MCHR1, along with specific biological phenomenon as cilia length shortening.