Characterization of ciliary targeting sequence of rat melanin-concentrating hormone receptor 1.

Melanin-concentrating hormone (MCH) is the natural peptide ligand for MCHR1 and MCHR2, which belong to the G protein-coupled receptor (GPCR) superfamily. The MCH-MCHR1 system is involved in the regulation of feeding, energy homeostasis and emotional processing in rodents. Recently, MCHR1 expression was discovered in neuronal immotile primary cilia of the central nervous system in mice. The cilium has an important chemosensory function in many types of cell and ciliary dysfunction is associated with cliopathies such as polycystic kidney disease, retinal dystrophy, and obesity. The targeting sequence of ciliary membrane proteins is thought to be unique. Although these sequences have been predicted in the cytoplasmic third loop and/or C-terminus of GPCRs, little is known about the characteristics of MCHR1. We thus explored the molecular mechanisms of MCHR1 targeting by transiently expressing a series of MCHR1 mutants into ciliated hRPE1 cells and evaluated the effects of these mutations on the ciliary localization of the heterologous receptor. This approach demonstrated that an Ala-to-Gly mutation (A242G) within the third intracellular loop induced a significant reduction in ciliary localization of the receptor without affecting the ciliogenesis. In contrast, no C-terminal truncation mutant had any effect on ciliary localization or cilia length. This study provides a potential molecular link between defective cilia and clinical manifestations such as obesity.