Topmouth culter melanocortin-3 receptor: regulation by two isoforms of melanocortin-2 receptor accessory protein 2

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by RIA, and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the CNS. All agonists could bind and stimulate caMC3R to increase dose dependently intracellular cAMP accumulation. Compared to human MC3R, culter MC3R showed higher constitutive activity, higher efficacies, and R-max to alpha-melanocyte-stimulating hormone (alpha-MSH), des-alpha-MSH, and adrenocorticotrophic hormone. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, B-max, and R-max of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2b had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormones might play different roles in regulating culter development and growth.

Automated summary

The study explored the expression and signaling of topmouth culter MC3R and its interaction with MRAP2a and MRAP2b. The results showed that culter MC3R exhibited higher expression in the CNS, and could bind and stimulate intracellular cAMP accumulation in a dose-dependent manner. MRAP2a and MRAP2b had different effects on basal cAMP production and cell surface expression of caMC3R, with MRAP2a significantly decreasing these parameters. Additionally, it was suggested that MRAP2a and MRAP2b might play a more important role in regulating MC3R/MC4R signaling during the larval period, while reduced expression of mc3r, mc4r, and pomc might be involved in modulation of MC3R/MC4R in adults.