Many different peptides act as signaling molecules, including the proinflammatory peptide bradykinin, the protease enzyme thrombin, and the blood pressure regulating peptide angiotensin. While these three proteins are distinct in their sequence and physiology, and act through different cell surface receptors, they share in a common class of cell surface receptors called G-protein coupled receptors (GPCRs). Other polypeptide ligands of GPCRs include vasopressin, oxytocin, somatostatin, neuropeptide Y, GnRH, leutinizing hormone, follicle stimulating hormone, parathyroid hormone, orexins, urotensin II, endorphins, enkephalins, and many others. GPCRs are a broad and diverse gene family that respond not only to peptide ligands but also small molecule neurotransmitters (acetylcholine, dopamine, serotonin and adrenaline), light, odorants, taste, lipids, nucleotides, and ions. The main signaling mechanism used by GPCRs is to interact with G-protein GTPase proteins coupled to downstream second messenger systems including intracellular calcium release and cAMP production. The intracellular signaling systems used by peptide GPCRs are similar to those used by all GPCRs, and are typically classified according to the G-protein they interact with and the second messenger system that is activated. For Gs-coupled GPCRs, activation of the G-protein Gs by receptor stimulates the downstream activation of adenylate cyclase and the production of cyclic AMP, while Gi-coupled receptors inhibit cAMP production. One of the key results of cAMP production is activation of protein kinase A. Gq-coupled receptors stimulate phospholipase C, releasing IP3 and diacylglycerol. IP3 binds to a receptor in the ER to cause the release of intracellular calcium, and the subsequent activation of protein kinase C, calmodulin-dependent pathways. In addition to these second messenger signaling systems for GPCRs, GPCR pathways exhibit crosstalk with other signaling pathways including tyrosine kinase growth factor receptors and map kinase pathways. Transactivation of either receptor tyrosine kinases like the EGF receptor or focal adhesion complexes can stimulate ras activation through the adaptor proteins Shc, Grb2 and Sos, and downstream Map kinases activating Erk1 and Erk2. Src kinases may also play an essential intermediary role in the activation of ras and map kinase pathways by GPCRs.
