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

Abstract

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 radioimmunoassay and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the central nervous system. All agonists could bind and stimulate caMC3R to increase dose-dependently intracellular cAMP accumulation. Compared to hMC3R, culter MC3R showed higher constitutive activity, higher efficacies and Rmax to α-MSH, des-α-MSH, and ACTH. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax and Rmax 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 MRAP2a had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormone might play different roles in regulating culter development and growth.

Pharmacological Evaluation of Melanocortin 2 Receptor Accessory Protein 2 on Axolotl Neural Melanocortin Signaling

“…We found that axolotl MC3R exhibited a high constitutive activity, that was significantly decreased by MRAP2 (Figure 5E). MC3R was also reported to have high constitutive activity in other species, such as zebrafish (72), channel catfish (51), and topmouth culter (53). In contrast to our result, the constitutive activity of MC3R was reported to be enhanced by both MRAP and MRAP2 in the clawed frog (45).…”

“…Several studies have been conducted to evaluate the pharmacological profiles of MC3R/MC4R in the presence of MRAPs in various vertebrates, such as human ( 40 , 41 ), mouse ( 42 ), chicken ( 43 ), the clawed frog ( 44 – 46 ), zebrafish ( 47 , 48 ), snakehead ( 49 ), orange-spotted grouper ( 50 ), channel catfish ( 51 ), topmouth culter ( 52 , 53 ), and sea lamprey ( 54 ). For instance, human MRAP2 was found to promote the maximal activity of MC3R/MC4R stimulated by α-MSH, whereas the mutated MRAP2 variants failed to show the similar effect ( 40 ).…”

Functional Characterization of Melanocortin-3 Receptor in a Hibernating Cavefish Onychostoma macrolepis

“…MC3R plays an indispensable role in the regulation of energy homeostasis by controlling feeding efficiency and circadian rhythm [ 10 ]. To date, researches on MC3R have mainly focused on mammals; only six studies of Mc3r in fish have been reported, namely, blunt snout bream [ 18 ], zebrafish [ 16 , 19 ], spiny dogfish [ 3 ], stingray [ 20 ], channel catfish [ 12 ], and topmouth culter [ 21 ]. Given that Mc4r affects the weight loss and fat accumulation of Mexican cavefish in starvation [ 23 ], Mc3r, which functions similarly to Mc4r, is also speculated to play an important role in adapting fish to oligotrophic environments.…”

“…The lack of Mc3r in some teleosts, such as fugu [ 15 ], medaka [ 16 ], and stickleback [ 17 ], restricts the cloning and functional studies of this gene in fish. To date, only the MC3Rs from blunt snout bream [ 18 ], zebrafish [ 16 , 19 ], spiny dogfish [ 3 ], stingray [ 20 ], channel catfish [ 12 ], and topmouth culter [ 21 ] have been cloned and the latter five had their pharmacological functions demonstrated.…”

Regulation of Melanocortin-3 and -4 Receptors by Isoforms of Melanocortin-2 Receptor Accessory Protein 1 and 2

“…MRAP2 mutations are associated with severe obesity in humans [ 38 , 40 , 41 , 42 ]. MRAP2 regulates MC3R or MC4R signaling in mammals and other species [ 35 , 38 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ]. It has been reported that hMRAP2a either decreases [Nle4,D-Phe7]-α-MSH (NDP-MSH)-stimulated [ 35 ] or increases α-MSH-stimulated [ 50 ] cAMP generation of hMC3R and hMC4R.…”