MC4R

MC4R is the hypothalamic class A GPCR that is the single most important genetic determinant of human body weight: heterozygous loss-of-function MC4R mutations account for up to 6% of severe early-onset obesity, making it the most common monogenic obesity gene identified. MC4R activation by α-MSH from POMC neurons suppresses appetite, increases energy expenditure, and modulates glucose homeostasis; AgRP from AgRP/NPY neurons is its endogenous inverse agonist. Setmelanotide (MC4R agonist) is FDA-approved for genetic obesity syndromes caused by upstream POMC pathway defects. MC4R is the primary target for anti-obesity peptide pharmacology and is expressed in the dorsal vagal complex, where it also modulates insulin secretion and gastrointestinal motility.

Receptor structure and signaling

MC4R (chromosome 18q22, 332 aa, intronless gene) is constitutively active to a significant degree - basal MC4R activity tonically suppresses food intake, and AgRP inverse agonism removes this brake rather than simply blocking agonist effects. The orthosteric binding pocket is formed by TM2 (Asp122), TM3 (Asp146), TM6 (Phe261), and ECL2, coordinating the HFRW core with Asp122 making the critical ionic contact with Arg of the pharmacophore. Primary signaling: Gs → adenylyl cyclase → cAMP → PKA → MAPK. Secondary: Gq/11 in some neuronal populations; β-arrestin-2 biased signaling → internalization. MC4R expressed in hypothalamic paraventricular nucleus (PVN) drives the anorexigenic response; in the spinal cord and autonomic ganglia, it modulates sympathetic tone and erectile function. MRAP2 (melanocortin receptor accessory protein 2) is an MC4R modulator - it enhances MC4R surface expression and potentiates agonist-induced cAMP responses. MC4R mutations >200 pathogenic variants described; most cluster in TM domains and cause receptor misfolding (pharmacoperone-rescuable) or impaired Gs coupling.

Approved drugs and research recipes

Setmelanotide (MC4R agonist peptide, cyclic: Ac-Arg-cyclo[Cys-Glu-His-D-Phe-Arg-Trp-Cys]-NH₂) is FDA-approved for obesity caused by POMC, PCSK1, or LEPR gene deficiency, achieving ~12–16% body weight loss in these genetic syndromes. It drives MC4R agonism downstream of the broken POMC/leptin processing machinery. Bremelanotide (PT-141, MC4R/MC3R agonist) is FDA-approved for female hypoactive sexual desire disorder via hypothalamic/spinal MC4R mechanisms. MTII (Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂) is the canonical research agonist. SHU9119 is the classic cyclic heptapeptide antagonist (also partial agonist at MC1R). For peptide research, the tractable recipes are: setmelanotide analogs with fatty acid conjugation for weekly SC dosing in broader obesity indications; MC4R-selective cyclic peptides exploiting the Asp146 contact vs. MC3R divergence; pharmacoperone peptides that rescue misfolded MC4R variants in early-onset obesity carriers; and biased MC4R agonists that maximize Gs/cAMP anorexigenic signaling over β-arrestin desensitization to improve sustained efficacy with minimal receptor downregulation.