Melanocortin-4 receptor in swamp eel (Monopterus albus): Cloning, tissue distribution, and pharmacology

Melanocortin-4 receptor (MC4R) plays critical roles in the regulation of various physiological processes, such as energy homeostasis, reproduction and sexual function, cardiovascular function, and other functions in mammals. Although the functions of the MC4R in fish have not been extensively studied, the importance of MC4R in regulation of piscine energy expenditure and sexual functions is emerging. Swamp eel (Monopterus albus) is an economically and evolutionarily important fish widely distributed in tropics and subtropics. We cloned swamp eel mc4r (mamc4r), consisting of a 981 bp open reading frame encoding a protein of 326 amino acids. The sequence of maMC4R was homologous to those of several teleost MC4Rs. Phylogenetic and chromosomal synteny analyses showed that maMC4R was closely related to piscine MC4Rs. qRT-PCR revealed that mc4r transcripts were highly expressed in brain and gonads of swamp eel. The maMC4R was further demonstrated to be a functional receptor by pharmacological studies. Four agonists, α-melanocyte stimulating hormone (α-MSH), β-MSH, [Nle⁴, D-Phe⁷]-α-MSH (NDP-MSH), and adrenocorticotropin, could bind to maMC4R and induce intracellular cAMP production dose-dependently. Small molecule agonist THIQ allosterically bound to maMC4R and exerted its effect. Similar to other fish MC4Rs, maMC4R also exhibited significantly increased basal activity compared with that of human MC4R. The high basal activity of maMC4R could be decreased by inverse agonist ML00253764, suggesting that maMC4R was indeed constitutively active. The availability of maMC4R and its pharmacological characteristics will facilitate the investigation of its function in regulating diverse physiological processes in swamp eel.