ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Abstract

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Keywords: ACTHR; MC2R; mutagenesis; mutation; site directed; specificity.

Figure 1

Schematic representation of the proopiomelanocortin precursor, the products of its proteolysis, and known posttranslational modifications. Melanocortins are colored green, signal peptide is colored red, and other peptides are colored blue. Posttranslational modification sites are marked with vertical black lines.

Figure 2

Sequence alignment of melanocortin (MC) peptides. All MCs share the conserved –M–X–H–F–R–W– motif, which serves as the pharmacophore for receptor binding (8, 10). The –K–K–R–R–P– motif within adrenocorticotropic hormone (ACTH) is considered the second pharmacophore and is required for successful activation of adrenocorticotropic hormone receptor (26).

Figure 3

Formation of functionally active adrenocorticotropic hormone receptor (ACTHR). The functional form of ACTHR is a hetero-hexameric structure formed from two molecules of ACTHR and four molecules of MRAP1. The antiparallel dimer of MRAP1 is formed within the endoplasmic reticulum, where it couples to one molecule of ACTHR. Afterward, this complex is transported to the Golgi where it “dimerizes,” thus forming the hetero-hexameric functional ACTHR (128). Symbols representing ACTHR and MRAP1 were selected to highlight the protein secondary structure. The depicted intermolecular interactions are therefore general and do not reflect the actual situation in detail.

Figure 4

Amino acid sequence alignment of human melanocortin receptors (MCRs). +--+ and vertical dashes mark transmembrane domain boundaries, acquired from GPCRdb (http://gpcrdb.org/). *Indicates residues that form the “backbone” of the MCR-binding pocket. ^{^}Indicates residues that are unique to adrenocorticotropic hormone receptor (ACTHR). Red boxes indicate amino acid residues that, according to the model created by Pogozheva et al. (148) and Chai et al. (149), form the ligand-binding cavity of MC4R.

Figure 5

Cartoon representation of the backbone of the MC4R model created by Pogozheva et al. (148) and Chai et al. (149). (A) View from the side and top on the surface of the ligand-binding cavity that contains several acidic (red) and multiple aromatic residues (blue) located in groups at opposite sides of the cavity and forming acidic- and aromatic-binding pockets, respectively. (B) Close-up view of the binding cavity without the surface layer.

Figure 6

Snake-like plot of adrenocorticotropic hormone receptor (ACTHR). Forked structures represent glycosylation. –S–S– represents disulfide bonds. Circles with a red outline represent naturally occurring missense mutations (labels enclose information on the resulting residue replacement). Red labels indicate residues located on the extracellular part of the receptor. Green labels indicate residues located within the central part of transmembrane domains. Blue labels indicate residues located within the intracellular part of receptor. Dark green circles represent residues that form the backbone of the melanocortin receptor (MCR)-binding pocket. Orange circles represent residues equivalent to those forming the ligand-binding cavity of MC4R based on the model created by Pogozheva et al. (148) and Chai et al. (149). Light green circles represent residues that Yang et al. (146) identified as important for ligand recognition. Circles shaded in red, pink, and light purple represent residues equivalent to those that, according to the ADRB2 model (163), directly interact with Gα, are involved in Gα coupling-related intramolecular interactions, and that form the surface of the Gα-binding cavity, respectively. Circles shaded in black represent S/T residues that are crucial for ACTHR expression and functional regulation. Circles with angled cross-hatching represent residues that are unique to ACTHR (yet conserved in other MCRs).

Figure 7

Cartoon representation of the crystal structure of bovine rhodopsin (150). EL2 (magenta) forms a lid-like structure over the ligand-binding pocket that prevents any external molecular interference with the covalently bound ligand.