Characterization of melanocortin receptors from stingray Dasyatis akajei, a cartilaginous fish.

Melanocortin (MC) systems are composed of MC peptides such as adrenocorticotropic hormone (ACTH), several molecular forms of melanocyte-stimulating hormones (MSHs) and MC receptors (MCRs). Here we demonstrated that the cartilaginous fish, Dasyatis akajei (stingray) expresses five subtypes of MCR genes-mc1r to mc5r-as in the case of teleost and tetrapod species. This is the first evidence showing the presence of the full repertoire of melanocortin receptors in a single of cartilaginous fish. Expression of respective stingray mcr cDNAs in Chinese hamster ovary cells revealed that Des-acetyl-α-MSH exhibited cAMP-producing activity indistinguishable to ACTH(1-24) on MC1R and MC2R, while the activity of Des-acetyl-α-MSH on MC3R, MC4R, and MC5R were similar to or slightly greater than that of ACTH(1-24). Notably, in contrast to the other vertebrates, MC2R did not require coexpression with a melanocortin receptor-2 accessory protein 1 (mrap1) cDNA for functional expression. One of the roles of MC system resides in regulation of the pituitary-interrenal (PI) axis-a homologue of tetrapod pituitary-adrenal axis. In stingray, interrenal tissues were shown to express mc2r and mc5r as major MCR genes. These results established the presence of functional PI axis in stingray at the level of receptor molecule. While MC2R participates in adrenal functions together with MRAP1 in tetrapod species, the fact that sensitivity of MC5R to Des-acetyl-α-MSH and ACTH(1-24) were two order of magnitude higher than MC2R without coexpression with MRAP1 suggested that MC5R could play a more important role than MC2R to transmit signals conveyed by ACTH and MSHs if MRAP1 is really absent in the stingray.