Determination of the Interaction and Pharmacological Modulation of MCHR1 Signaling by the C-Terminus of MRAP2 Protein

The MRAP2 accessory protein directly interacts with melanocortin-3 receptor to enhance signaling

The central melanocortin system links nutrition to energy expenditure, with melanocortin-4 receptor (MC4R) controlling appetite and food intake, and MC3R regulating timing of sexual maturation, rate of linear growth and lean mass accumulation. Melanocortin-2 receptor accessory protein-2 (MRAP2) is a single transmembrane protein that interacts with MC4R to potentiate it's signalling, and human mutations in MRAP2 cause obesity. Previous studies have been unable to consistently show whether MRAP2 affects MC3R activity. Here we used single-molecule pull-down (SiMPull) to confirm that MC3R and MRAP2 interact in HEK293 cells. Analysis of fluorescent photobleaching steps showed that MC3R and MRAP2 readily form heterodimers most commonly with a 1:1 stoichiometry. Human single-nucleus and spatial transcriptomics show MRAP2 is co-expressed with MC3R in hypothalamic neurons with important roles in energy homeostasis and appetite control. Functional analyses showed MRAP2 enhances MC3R cAMP signalling, impairs β-arrestin recruitment, and reduces internalization in HEK293 cells. Structural homology models revealed putative interactions between the two proteins and alanine mutagenesis of five MRAP2 and three MC3R transmembrane residues significantly reduced MRAP2 effects on MC3R signalling. Finally, we showed genetic variants in MRAP2 that have been identified in individuals that are overweight or obese prevent MRAP2's enhancement of MC3R-driven signalling. Thus, these studies reveal MRAP2 as an important regulator of MC3R function and provide further evidence for the crucial role of MRAP2 in energy homeostasis.

MRAP2 Inhibits β-Arrestin-2 Recruitment to the Prokineticin Receptor 2

Melanocortin receptor accessory protein 2 (MRAP2) is a membrane protein that binds multiple G protein-coupled receptors (GPCRs) involved in the control of energy homeostasis, including prokineticin receptors. These GPCRs are expressed both centrally and peripherally, and their endogenous ligands are prokineticin 1 (PK1) and prokineticin 2 (PK2). PKRs couple all G-protein subtypes, such as Gαq/11, Gαs, and Gαi, and recruit β-arrestins upon PK2 stimulation, although the interaction between PKR2 and β-arrestins does not trigger receptor internalisation. MRAP2 inhibits the anorexigenic effect of PK2 by binding PKR1 and PKR2. The aim of this work was to elucidate the role of MRAP2 in modulating PKR2-induced β-arrestin-2 recruitment and β-arrestin-mediated signalling. This study could allow the identification of new specific targets for potential new drugs useful for the treatment of the various pathologies correlated with prokineticin, in particular, obesity.

MC4R in Central and Peripheral Systems

Obesity has emerged as a critical and urgent health burden during the current global pandemic. Among multiple genetic causes, melanocortin receptor-4 (MC4R), involved in food intake and energy metabolism regulation through various signaling pathways, has been reported to be the lead genetic factor in severe and early onset obesity and hyperphagia disorders. Most previous studies have illustrated the roles of MC4R signaling in energy intake versus expenditure in the central system, while some evidence indicates that MC4R is also expressed in peripheral systems, such as the gut and endocrine organs. However, its physiopathological function remains poorly defined. This review aims to depict the central and peripheral roles of MC4R in energy metabolism and endocrine hormone homeostasis, the diversity of phenotypes, biased downstream signaling caused by distinct MC4R mutations, and current drug development targeting the receptor.