Melanocortin 4 Receptor Becomes an ACTH Receptor by Coexpression of Melanocortin Receptor Accessory Protein 2

Abstract

Melanocortin 2 receptor (MC2R) is the only canonical ACTH receptor. Its functional expression requires the presence of an accessory protein, known as melanocortin receptor 2 accessory protein 1 (MRAP1). The vertebrate genome exhibits a paralogue gene called MRAP2, which is duplicated in zebrafish (MRAP2a and MRAP2b), although its function remains unknown. In this paper, we demonstrate that MRAP2a enables MC4R, a canonical MSH receptor, to be activated by ACTH with a similar sensitivity to that exhibited by MC2R. Both proteins physically interact and are coexpressed in the neurons of the preoptic area, a key region in the control of the energy balance and hypophyseal secretion in fish. ACTH injections inhibit food intake in wild-type zebrafish but not in fish lacking functional MC4R. Both MRAP1 and MRAP2a are hormonally regulated, suggesting that these proteins are substrates for feed-back regulatory pathways of melanocortin signaling. Fasting has no effect on the central expression of MRAP2a but stimulates MRAP2b expression. This protein interacts and is colocalized with MC4R in the tuberal hypothalamic neurons but has no effect on the pharmacologic profile of MC4R. However, MRPA2b is able to decrease basal reporter activity in cell lines expressing MC4R. It is plausible that MRAP2b decreases the constitutive activity of the MC4R during fasting periods, driving the animal toward a positive energy balance. Our data indicate that MRAP2s control the activity of MC4R, opening up new pathways for the regulation of melanocortin signaling and, by extension, for the regulation of the energy balance and obesity.

Replacement of short segments within transmembrane domains of MC2R disrupts retention signal

“…It has been observed that the EC 50 value of the cAMP response after MC4R stimulation with a-MSH was 16.6-fold higher in the presence of MRAPb than in its absence, which corresponds to the observations of Chan et al (2009). We also observed 3.4-fold higher cAMP response sensitivity when MC4R was coexpressed with MRAPb and stimulated with ACTH than when MC4R was expressed alone (Table 1), which has been previously reported by Josep Agulleiro et al (2013) to be also the case for the zebrafish MC4R. Taken together, these findings indicate that MRAPb not only selectively reduces the amount of MC4R in the cell membrane, but also negatively affects the receptor's ability to respond to stimulation with a-MSH, while leaving intact or even increasing its ability to respond to stimulation with ACTH.…”

MOLECULAR EVOLUTION OF GPCRS: Melanocortin/melanocortin receptors

“…However, in this instance, the interaction increases the zebrafish Mc4r sensitivity for activation by a-MSH in a manner nearly identical to the interaction between mouse MRAP2 and mouse MC4R (Asai et al 2013). In addition, co-expression of zebrafish mc4r with zebrafish mrap2a in HEK-293 cells results in an increase in the sensitivity of zebrafish MC4R for ACTH(1-24) (Agulleiro et al 2013). Thus, zebrafish Mrap2b provides another mechanism for regulating the ligand selectivity of zebrafish Mc4r, and as a result modulating the feeding behavior and growth of older zebrafish.…”

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

“…As for POMC, its mRNA abundance increased post-prandially in rainbow trout (Gong and Björnsson, 2014), medaka (Chisada et al, 2014), and Atlantic halibut (Gomes et al, 2015). The mRNA levels of AgRP increased with food deprivation in hypothalamus of goldfish (Cerdá-Reverter and Peter, 2003), zebrafish (Song et al, 2003), carp (Zhong et al, 2013), and sea bass (Agulleiro et al, 2013), though not in Atlantic salmon (Murashita et al, 2009) whereas no post-feeding changes occurred in medaka (Chisada et al, 2014). AgRP mRNA levels also increased in hypothalamus of GH-transgenic carp that also displayed increased food intake (Zhong et al, 2013).…”