The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

Abstract

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Figure 1

Sequence alignment of bovine rhodopsin (RHO) with human MC1R, MC3R, and MC4R. The ELs and intracellular loops are boxed (note that the C terminus of rhodopsin is not complete). The divergent N termini are not shown here. The most conserved residues in each TM (50 residue) are in red and are boxed individually. The highly conserved Cys residues at the top of TM3 and EL2 in rhodopsin that form a disulfide bridge are highlighted in gold. The potential palmitoylation sites at the C termini are shaded in light yellow. In MCRs, TM5 is two residues shorter than rhodopsin; otherwise, EL2 would consist of only two residues, which is not likely. Note the following unique characteristics of the MCRs, including the absence of the highly conserved disulfide bridge between the EL2 and the top of TM3 (shaded in gold for rhodopsin). The intracellular loops and ELs of the MCRs are short, especially EL2, making MCRs some of the shortest members in the GPCR superfamily. Highly conserved Pro in TM5 and Asn in TM7 (in the NPxxY motif) in family A GPCRs are substituted by a Met and an Asp, respectively, in all MCRs (shaded in rose). In other family A GPCRs, this Asn is proposed to interact with the highly conserved Asp in TM2. In MCRs, Asp is present at both loci, unlikely to interact with each other. GenBank accession numbers for the genes are: bovine rhodopsin, NM_001014890; human MC1R, NM_002386; human MC3R, NM_019888; and human MC4R, NM_005912. IL, Intracellular loop.

Figure 2

Constitutive activation of two naturally occurring MC4R mutations, H76R and D146N. The basal activities of H76R and D146N are 10- to 15-fold higher than the basal activity of WT MC4R. The small molecule inverse agonist ML00253764 partially decreases the basal activities of the two mutants and the WT MC4R. cAMP levels were measured with RIA (496).

Figure 3

Naturally occurring missense, nonsense, and inframe deletion mutations of the MC4R identified from various patient cohorts. The two polymorphisms that confer protection from obesity are indicated with red filling and letters. The mutations that cause constitutive activation are indicated with blue filling and letters. References describing these mutations are listed in the text. [Reprinted, with minor modification, with permission from Y. X. Tao: Prog Mol Biol Transl Sci 88:173–204, 2009 (435). ©Elsevier 2009.]

Figure 4

Molecular classification of naturally occurring MC4R mutations. See the text for detailed description. [Reprinted with permission from Y. X. Tao: Mol Cell Endocrinol 239:1–14, 2005 (339). ©Elsevier 2005.]

Figure 5

Pharmacological rescue of six intracellularly retained MC4R mutants by ML00253764. Human embryonic kidney 293 cells stably transfected with WT or mutant MC4Rs or empty vector pcDNA3.1 (as a negative control) were incubated with 10 μm ML00253764 for 24 h at 37 C. Cells were then stained with anti-myc monoclonal antibody to visualize the c-myc epitope at the N terminus of the MC4R by confocal microscopy. Detailed procedures are described in Ref. .