GPCR, family 2, corticotropin releasing factor receptor, type 1 (GPCR_2_CRF1_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.

Corticotropin-releasing factor (CRF) is the principal neuroregulator of the hypothalamic-pituitary-adrenocortical axis, playing an important role in coordinating the endocrine, autonomic and behavioral responses to stress and immune challenge . The CRF receptor has been found in human cortex tissue, pituitary, brainstem and testis . The protein comprises 415 amino acid residues with the characteristic 7TM architecture of the secretin-like GPCR superfamily. Three isoforms (designated CRF-R1, CRF-R2 and CRF-R3) are produced as a result of alternative splicing of the same gene: CRF-R1 appears to be the predominant form; CRF-R3 does not bind to CRF with a high affinity . CRF and the related urocortin peptides (Ucn 1-3, also known as UCN, UCN2 and UCN3) mediate their actions through two CRF1 and CRF2 .

The sequence of the CRF-R is highly conserved from avian to mammalian species, the majority of the sequence divergence occuring in the putative signal peptide and extracellular N-terminal domain . Five additional amino acids are inserted in the N terminus of the avian receptor, and despite its overall similarity to the type 1 mammalian CRF-R, its ligand binding properties are similar to those of the type 2 receptor (i.e., has a higher affinity for urotensin I than for CRF) . This entry includes CRF1 receptor (CRF1R, also known as CRHR1), which is activated by CRF and Ucn1, is expressed in brain areas including the pituitary, hypothalamus, amygdala and cortex. It is an interesting target to develop drug treatments for stress-related conditions such as anxiety, depression and irritable bowel syndrome .