GPCR, family 2, vasoactive intestinal peptide receptor (GPCR_2_VIP_rcpt)
Type: Family

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 .

The secretin-like GPCRs include secretin , calcitonin , parathyroid hormone/parathyroid hormone-related peptides and vasoactive intestinal peptide , all of which activate adenylyl cyclase and the phosphatidyl-inositol-calcium pathway. These receptors contain seven transmembrane regions, in a manner reminiscent of the rhodopsins and other receptors believed to interact with G-proteins (however there is no significant sequence identity between these families, the secretin-like receptors thus bear their own unique '7TM' signature). Their N-terminal is probably located on the extracellular side of the membrane and potentially glycosylated. This N-terminal region contains a long conserved region which allows the binding of large peptidic ligand such as glucagon, secretin, VIP and PACAP; this region contains five conserved cysteines residues which could be involved in disulphide bond. The C-terminal region of these receptor is probably cytoplasmic. Every receptor gene in this family is encoded on multiple exons, and several of these genes are alternatively spliced to yield functionally distinct products.

Vasoactive intestinal polypeptide (VIP) has a wide physiological profile. In the periphery, it induces relaxation in smooth muscle; inhibits secretion in certain tissues, but stimulates secretion in others; and modulates activity of cells in the immune system. In the CNS, it has a range of both excitatory and inhibitory actions. VIP receptors are distributed widely in the periphery, and occur throughout the gastrointestinal tract and genitourinary system, other smooth muscles and secretory glands. In the CNS, they are found abundantly in, e.g. the cortex, hippocampus and thalamus. All VIP receptors activate adenylyl cyclase.

There are two structurally distinct receptors that recognise VIP peptides and pituitary adenylate cyclase activating polypeptide (PACAP) with similar affinities (PACAP/VIPR-1, PACAP/VIPR-2), as well as a specific receptor for the PACAP peptide (PACAP-1). RNA transcripts for all three receptor types are found in human heart, brain and adipose tissue . VIPR-1 is constitutively expressed, while the expression of VIPR-2 is induced only following stimulation through the TCR-associated CD3 complex . VIP induces the expression of the VIPR-2 gene in the absence of additional stimuli. Differential expression and regulation of the two VIP receptors in T lymphocytes suggests different physiological roles in mediating the immunomodulatory activities of VIP and related neuropeptides . PACAP type I receptors are present in the hypothalamus and anterior pituitary, where they regulate the release of adrenocorticotropin, luteinising hormone, growth hormone and prolactin, and in the adrenal medulla, where they regulate the release of epinephrine . The receptors are also found in high concentrations in testicular germ cells, where they may regulate spermatogenesis, and in some transformed cell lines, such as the rat pancreatic acinar carcinoma cell AR4-2J .