orphan GPR126 and related proteins, group VIII adhesion GPCRs, member of the class B2 family of seven-transmembrane G protein-coupled receptors
Group VIII adhesion GPCRs include orphan GPCRs such as GPR56, GPR64, GPR97, GPR112, GPR114, and GPR126. GPR56 is involved in the regulation of oligodendrocyte development and myelination in the central nervous system via coupling to G(12/13) proteins, which leads to the activation of RhoA GTPase. GPR126, on the other hand, is required for Schwann cells, but not oligodendrocyte myelination in the peripheral nervous system. Gpr64 is mainly expressed in the epididymis of male reproductive tract, and targeted deletion of GPR64 causes sperm stasis and efferent duct blockage due to abnormal fluid reabsorption, resulting in male infertility. GPR64 is also overexpressed in Ewing's sarcoma (ES), as well as upregulated in other carcinomas from kidney, prostate or lung, and promotes invasiveness and metastasis in ES via the upregulation of placental growth factor (PGF) and matrix metalloproteinase (MMP) 1. GPR97 is identified as a lymphatic adhesion receptor that is specifically expressed in lymphatic endothelium, but not in blood vascular endothelium, and is shown to regulate migration of lymphatic endothelial cells via the small GTPases RhoA and cdc42. GPR112 is specifically expressed in normal enterochromatin cells and gastrointestinal neuroendocrine carcinoma cells, but its biological function is unknown. GPR114 is mainly found in granulocytes (polymorphonuclear leukocytes), and GPR114-transfected cells induced an increase in cAMP levels via coupling to G(s) protein. The adhesion receptors are characterized by the presence of large N-terminal extracellular domains containing multiple adhesion motifs, which play critical roles in cell-cell adhesion and cell-matrix interactions, that are coupled to a class B seven-transmembrane domain. Furthermore, almost all adhesion receptors, except GPR123, contain an evolutionarily conserved GPCR- autoproteolysis inducing (GAIN) domain that undergoes autoproteolytic processing at the GPCR proteolysis site (GPS) motif located immediately N-terminal to the first transmembrane region, to generate N- and C-terminal fragments (NTF and CTF), which may serve important biological functions.