corticotropin-releasing factor receptor 1, member of the class B family of seven-transmembrane G protein-coupled receptors
The vertebrate corticotropin-releasing factor (CRF) receptors are predominantly expressed in central nervous system with high levels in cortex tissue, brain stem, and pituitary. They have two isoforms as a result of alternative splicing of the same receptor gene: CRF-R1 and CRF-R2, which differ in tissue distribution and ligand binding affinities. Recently, a third CRF receptor (CRF-R3) has been identified in catfish pituitary. The catfish CRF-R1 is highly homologous to CRF-R3. CRF is a 41-amino acid neuropeptide that plays a central role in coordinating neuroendocrine, behavioral, and autonomic responses to stress by acting as the primary neuroregulator of the hypothalamic-pituitary-adrenal axis, which controls the levels of cortisol and other stress related hormones. In addition, the CRF family of neuropeptides also includes structurally related peptides such as mammalian urocortin, fish urotensin I, and frog sauvagine. The actions of CRF and CRF-related peptides are mediated through specific binding to CRF-R1 and CRF-R2. CRF and urocortin 1 bind and activate mammalian CRF-R1 with similar high affinities. By contrast, urocortin 2 and urocortin 3 do not bind to CRF-R1 or stimulate CRF-R1-mediated cAMP formation. Urocortin 1 also shows high affinity for mammalian CRF-R2, whereas CRF has significantly lower affinity for this receptor. These evidence suggest that urocortin 1 is an endogenous ligand for CRF-R1 and CRF-R2. The CRF receptors are members of the B1 subfamily of class B GPCRs, also referred to as secretin-like receptor family, which includes receptors for polypeptide hormones of 27-141 amino-acid residues such as secretin, glucagon, glucagon-like peptide (GLP), calcitonin gene-related peptide, and parathyroid hormone (PTH). These receptors contain the large N-terminal extracellular domain (ECD), which plays a critical role in hormone recognition by binding to the C-terminal portion of the peptide. On the other hand, the N-terminal segment of the hormone induces receptor activation by interacting with the receptor transmembrane domains and connecting extracellular loops, triggering intracellular signaling pathways. All members of the B1 subfamily preferentially couple to G proteins of G(s) family, which positively stimulate adenylate cyclase, leading to increased intracellular cAMP formation and calcium influx. However, depending on its cellular location and function, CRF receptors can activate multiple G proteins, which can in turn stimulate different second messenger pathways.
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