hMRAPa increases αMSH-induced hMC1R and hMC3R functional coupling and hMC4R constitutive activity

Abstract

Human melanocortin 2 receptor accessory protein (hMRAPa) is hypothesised to have functions beyond promoting human melanocortin 2 receptor (hMC2R) functional expression. To understand these potential functions, we exogenously co-expressed hMRAPa-FLAG with each of the five hMCR subtypes in HEK293 cells and assessed hMCR subtype coupling to adenylyl cyclase. We also co-expressed each HA-hMCR subtype with hMRAPa-FLAG to investigate their subcellular localisation. hMRAPa-FLAG enhanced a-melanocyte stimulating hormone (a-MSH)-stimulated hMC1R and hMC3R but reduced NDP-a-MSH-stimulated hMC5R, maximum coupling to adenylyl cyclase. hMRAPa-FLAG specifically increased hMC4R constitutive coupling to adenylyl cyclase despite not co-localising with the HA-hMC4R in the cell membrane. hMRAPa-FLAG co-localised with HA-hMC1R or HA-hMC3R in the perinuclear region, in cytoplasmic vesicles and at the plasma membrane, while it co-localised with HA-hMC2R, HA-hMC4R and HA-hMC5R predominantly in cytoplasmic vesicles. These diverse effects of hMRAPa indicate that hMRAPa could be an important modulator of the central and peripheral melanocortin systems if hMRAPa and any hMCR subtype co-express in the same cell. Key Words" GPCR accessory protein " constitutive activity " subcellular localisation " melanocortin 4 receptor

60 YEARS OF POMC: Regulation of feeding and energy homeostasis by α-MSH

“…In the zebrafish, neither mrap2a nor mrap2b alters MC3R signaling (Sebag et al 2013) suggesting that the role of MRAP2 in the regulation of feeding and body weight may be specific to its modulation of MC4R. However, other studies have shown that MRAP2 does alter mammalian MC3R signaling (Chan et al 2009, Kay et al 2013). The actual role of MRAP in the modulation of MC3R-regulated food intake and body weight, however, is still up for debate.…”

Adrenocorticotropic Hormone (ACTH) Responses Require Actions of the Melanocortin-2 Receptor Accessory Protein on the Extracellular Surface of the Plasma Membrane

“…MRAP and MC2 receptors appear to be at least partially colocalized by conventional fluorescence microscopy (2,3,5,(11)(12)(13). The MRAP dimer co-precipitates with MC2 receptors from whole cell lysates or from a pool of receptors isolated from the plasma membrane (4,7,10,14,15).…”

Pro‐Opiomelanocortin (POMC) Neurones, POMC‐Derived Peptides, Melanocortin Receptors and Obesity: How Understanding of this System has Changed Over the Last Decade

“…Shortly after this discovery, a paralogue of the MRAP gene was identified and these MRAPs became known as MRAP1 and MRAP2 genes. Although neither of these is required for functional expression of any of MC1R, MC3R, MC4R or MC5R subtypes, both MRAPs have been shown to interact with, and influence signalling through, each one of these melanocortin receptor subtypes overexpressed in heterologous cells in vitro . The physiological significance of these interactions was unclear until 2013 when two studies provided evidence that MRAP2 was a key player for body weight regulation .…”

“…Hence, it was concluded that Mrap2 is an important regulator of MC4R functional activity and that Mrap2 deletion in mice causes obesity partly as a result of reduced MC4R function . My research group found human MRAP2 to have no effect on human MC4R functional coupling to Gαs when these proteins are overexpressed in HEK293 or GT1‐7 cells (; R. Botha and K. G. Mountjoy, unpublished data). Whether MRAP2 is an important regulator of human body weight remains to be determined.…”