| Description: |
G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups []. The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence []. The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) [, , , , ]. GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice []. Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs [].GPCR family 3 receptors (also known as family C) are structurally similar to other GPCRs, but do not show any significant sequence similarity and thus represent a distinct group. Structurally they are composed of four elements; an N-terminal signal sequence; a large hydrophilic extracellular agonist-binding region containing several conserved cysteine residues which could be involved in disulphide bonds; a shorter region containing seven transmembrane domains; and a C-terminal cytoplasmic domain of variable length []. Family 3 members include the metabotropic glutamate receptors, the extracellular calcium-sensing receptors, the gamma-amino-butyric acid (GABA) type B receptors, and the vomeronasal type-2 receptors [, , , ]. As these receptors regulate many important physiological processes they are potentially promising targets for drug development.The metabotropic glutamate receptors are functionally and pharmacologically distinct from the ionotropic glutamate receptors. They are coupled to G-proteins and stimulate the inositol phosphate/Ca2+intracellular signalling pathway [, , , ]. At least eight sub-types of metabotropic receptor (GRM1-8) have been identified in cloning studies. The sub-types differ in their agonist pharmacology and signal transduction pathways.GRM7 mRNA has a widespread distribution in many neuronal cells of the central nervoussystem, with highest levels in the hippocampal formation, cerebral cortexand cerebellum [, ]; it is thus different from the more limitedly expressedGRM4 or GRM6 mRNA []. GRM7 and GRM4 are believed to correspond to theputative L-2-amino-4-phosphonobutyrate receptor, which plays an importantrole in modulation of glutamate transmission in the CNS []. |
