The melanocortin-4 receptor (MC4R) is involved in energy homeostasis and is an important drug target for syndromic obesity. We report the structure of the antagonist SHU9119-bound human MC4R at 2.8-angstrom resolution. Ca2+ is identified as a cofactor that is complexed with residues from both the receptor and peptide ligand. Extracellular Ca2+ increases the affinity and potency of the endogenous agonist α-melanocyte–stimulating hormone at the MC4R by 37- and 600-fold, respectively. The ability of the MC4R crystallized construct to couple to ion channel Kir7.1, while lacking cyclic adenosine monophosphate stimulation, highlights a heterotrimeric GTP-binding protein (G protein)–independent mechanism for this signaling modality. MC4R is revealed as a structurally divergent G protein–coupled receptor (GPCR), with more similarity to lipidic GPCRs than to the homologous peptidic GPCRs.

Ablation of hypothalamic AgRP (Agouti-related protein) neurons is known to lead to fatal anorexia, whereas their activation stimulates voracious feeding and suppresses other motivational states including fear and anxiety. Despite the critical role of AgRP neurons in bidirectionally controlling feeding, there are currently no therapeutics available specifically targeting this circuitry. The melanocortin-3 receptor (MC3R) is expressed in multiple brain regions and exhibits sexual dimorphism of expression in some of those regions in both mice and humans. MC3R deletion produced multiple forms of sexually dimorphic anorexia that resembled aspects of human anorexia nervosa. However, there was no sexual dimorphism in the expression of MC3R in AgRP neurons, 97% of which expressed MC3R. Chemogenetic manipulation of arcuate MC3R neurons and pharmacologic manipulation of MC3R each exerted potent bidirectional regulation over feeding behavior in male and female mice, whereas global ablation of MC3R-expressing cells produced fatal anorexia. Pharmacological effects of MC3R compounds on feeding were dependent on intact AgRP circuitry in the mice. Thus, the dominant effect of MC3R appears to be the regulation of the AgRP circuitry in both male and female mice, with sexually dimorphic sites playing specialized and subordinate roles in feeding behavior. Therefore, MC3R is a potential therapeutic target for disorders characterized by anorexia, as well as a potential target for weight loss therapeutics.

Energy homeostasis is maintained by balancing energy intake and expenditure. Many signals regulating energy intake are conserved between the human and teleost. However, before this work, there was no sensitive highthroughput system to monitor energy expenditure in the teleost. We exploit the nonfluorescent and fluorescent properties of resazurin and its reduced form resorufin (alamarBlue Ò ) to monitor energy expenditure responses to drug application and genetic manipulation. We show that leptin, insulin, and alpha-melanocyte-stimulating hormone (a-MSH) increase energy expenditure dose dependently in the larval zebrafish. As previously established in the mouse, etomoxir, a carnitine palmitoyl transferase I inhibitor, blocks leptin-induced energy expenditure in the zebrafish. Metformin, the most commonly prescribed insulin sensitizer, increases the insulininduced metabolic rate. Using genetic knockdown, we observed that a-MSH treatment increases the metabolic rate, as does knockdown of the melanocortin antagonist, agouti-related protein. The agouti-related protein and multiple melanocortin receptors are shown to be involved in these effects. These studies confirm that aspects of hormonal regulation of energy expenditure are conserved in the teleost, and suggest that this assay may provide a unique tool to perform in vivo screens for drugs or genes that affect the metabolic rate, including insulin or leptin sensitizers.

The melanocortin MC4 receptor is a potential target for the development of drugs for both obesity and cachexia. Melanocortin MC4 receptor ligands known thus far are orthosteric agonists or antagonists, however the agonists, in particular, have generally exhibited unwanted side effects. For some receptors, allosteric modulators are expected to reduce side-effect profiles. To identify allosteric modulators of the melanocortin MC4 receptor, we created HEK293 cell lines coexpressing the human melanocortin MC4 receptor and a modified luciferase-based cAMP sensor. Monitoring luminescence as a readout of real-time intracellular cAMP concentration, we demonstrate this cell line is able to report melanocortin agonist responses, as well as inverse agonist response to the physiological AgRP peptide. Based on the MC4R-GLO cell line, we developed an assay that was shown to meet HTS standards (Z’=0.50). A pilot screen run on the Microsource Spectrum compound library (n= 2,000) successfully identified 62 positive modulators. This screen identified predicted families of compounds: β2AR agonists –the β2AR being endogenously expressed in HEK293 cells-, an adenylyl cyclase activator and finally a distribution of phosphodiesterase (PDE) inhibitors well characterized or recently identified. In this last category, we identified a structural family of coumarin-derived compounds (imperatorin, osthol and prenyletin), along with deracoxib, a drug in veterinary use for its COX2 inhibitory properties. This latter finding unveiled a new off-target mechanism of action for deracoxib as a PDE inhibitor. Overall, these data are the first report of an HTS for allosteric modulators for a Gs protein coupled receptor.

The melanocortin receptors (MC1R–MC5R) belong to class A G-protein-coupled receptors (GPCRs) and are known to have receptor-specific roles in normal and diseased states. Selectivity for MC4R is of particular interest due to its involvement in various metabolic disorders, including obesity, feeding regulation, and sexual dysfunctions. To further improve the potency and selectivity of MC4R (ant)­agonist peptide ligands, we designed and synthesized a series of cyclic peptides based on the recent crystal structure of MC4R in complex with the well-characterized antagonist SHU-9119 (Ac-Nle4-c­[Asp5-His6-DNal­(2′)7-Arg8-Trp9-Lys10]-NH2). These analogues were pharmacologically characterized in vitro, giving key insights into exploiting binding site subpockets to deliver more selective ligands. More specifically, the side chains of the Nle4, DNal(2′)7, and Trp9 residues in SHU-9119, as well as the amide linkage between the Asp5 and Lys10 side chains, were found to represent structural features engaging a hMC4R/hMC3R selectivity switch.