Regulation of Melanocortin-4 Receptor Pharmacology by Two Isoforms of Melanocortin Receptor Accessory Protein 2 in Topmouth Culter (Culter alburnus)

Melanocortin-4 receptor (MC4R) plays important roles in regulation of multiple physiological processes, and interaction of MC4R and melanocortin receptor accessory protein 2 (MRAP2) is suggested to play pivotal role in energy balance of vertebrates. Topmouth culter (Culter alburnus) is an economically important freshwater fish in China. Herein we cloned cultermc4r, mrap2a, andmrap2b. Cultermc4rconsisted of a 981 bp open reading frame encoding a protein of 326 amino acids. qRT-PCR revealed thatmc4r, mrap2a, andmrap2bwere primarily expressed in the central nervous system. In the periphery,mc4randmrap2bwere expressed more widely in the male, whilemrap2awas expressed more widely in the female. Culter MC4R could bind to four peptide agonists and increase intracellular cAMP production dose dependently. Culter MC4R was constitutively active in both cAMP and ERK1/2 pathways, which was differentially regulated by culter MRAP2a and MRAP2b. Culter MRAP2a significantly increased B(max)and decreased agonist-stimulated cAMP, while MRAP2b increased cell surface and total expression but did not affect B(max)and agonist-stimulated cAMP. These results will aid the investigation of the potential physiological processes that MC4R might be involved in topmouth culter.