The Multifaceted Melanocortin Receptors

Abstract

The five known melanocortin receptors (MCs) have established physiological roles. With the exception of MC2, these receptors can behave unpredictably and since they are more widely expressed than their established roles would suggest, it is likely that they have other poorly characterized functions. The aim of this review is to discuss some of the less well-explored aspects of the four enigmatic members of this receptor family (MC1,3-5) and describe how these are multifaceted G-protein coupled receptors (GPCRs). These receptors appear to be promiscuous in that they bind several endogenous agonists (products of the proopiomelanocortin gene) and antagonists but with inconsistent relative affinities and effects. We propose that this is a result of post-translational modifications that determine receptor localization within nanodomains. Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins and other GPCRs,that can interact with the MCs to alter the availability of receptor at the cell surface as well as the intracellular signalling resulting from receptor activation. Different combinations of interacting proteins and MCs may therefore give rise to the complex and inconsistent functional profiles reported for the MCs. For further progress in understanding this family, improved characterization of tissue-specific functions is required. Current evidence for interactions of these receptors with a range of partners resulting in modulation of cell signalling suggests that each should be studied within the full context of their interacting partners. The role of physiological status in determining this context also remains to be characterized.

Proopiomelanocortin (POMC) and psychodermatology

“…13,14 Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins and other GPCRs, that can interact with the MCRs to alter the availability of receptor at the cell surface as well as the intracellular signalling resulting from receptor activation. 13 Different combinations of interacting proteins and MCRs may therefore give rise to the complex and inconsistent functional profiles reported for the MCRs. 13 There are a number of synthetic MCR agonists and antagonists, but these will not be discussed in much further detail.…”

“…13 Different combinations of interacting proteins and MCRs may therefore give rise to the complex and inconsistent functional profiles reported for the MCRs. 13 There are a number of synthetic MCR agonists and antagonists, but these will not be discussed in much further detail. 17,18…”

“…[65][66][67] The exact purpose of POMC-derived peptides in immunomodulation is still a large field requiring much further study. 1,2,4,6,7,10,11,[13][14][15][16][17][18]28,67,68 The HF has an unusual immune system. 67,68 The proximal epithelium of an anagen HF is an area of 'immune privilege', characterized primarily by a very low level of expression of major histocompatibility (MHC) class antigens.…”

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

“…The melanocortin system consists of a family of melanocortin peptides and a family of their receptors [ 43 ]. Melanocortins are a family of peptides derived from the 26 kDa proopiomelanocortin (POMC) precursor.…”

“…The specific effect exerted by MCR agonists depends on the subtype of the activated receptor and its tissue localization. MC1R is responsible for skin and hair pigmentation, MC2R is required for steroidogenesis in the adrenal cortex, MC3R and MC4R are involved in the control of food intake and behavior, and MC5R plays an important role in sebogenesis [ 43 ].…”