GPCR, family 2, glucagon-like peptide-1/glucagon receptor (GPCR_2_GLP1/glucagon_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.

The glucagon receptor (GR) plays a central role in regulating the level of blood glucose by controlling the rate of hepatic glucose production and insulin secretion . GR is expressed predominantly in liver, kidney, adrenal, lung and stomach, with lower levels of expression detected in brown and white adipose tissue, cerebellum, duodenum and heart . Their role in the control of blood glucose concentrations makes glucagon and GR especially important to studies of diabetes, in which the loss of control over blood glucose concentrations clinically defines the disease . GR is similar to the secretin-like receptor superfamily. It can transduce signals leading to the accumulation of two different second messengers - i.e., both cAMP and calcium .

Glucagon-like peptide-1 (GLP-1), which is encoded by the glucagon gene and released from the gut in response to nutrients, is a potent stimulator of glucose-induced insulin secretion and proinsulin gene expression of pancreatic beta-cells . In humans, GLP-I exerts its physiological effect as an incretin. Patients with insulinoma tumors show uncontrolled insulin hypersecretion . The GLP-I receptor binds GLP-1 with high affinity and couples to activation of adenylate cyclase . The receptor specifically binds GLP-1 and not peptides of related structure and function, such as glucagon, gastric inhibitory peptide, VIP or secretin . It is thought that GLP-I might have effects beyond the pancreas, including the cardiovascular and central nervous systems, where a receptor with the same ligand-binding specificity is found .