Rational drug design approaches targeting the brain melanocortin receptors

Obesity is a rapidly growing health problem in industrialized countries in the world and represents a major risk factor for type II diabetes, cardiovascular disease and stroke. The brain Melanocortin-3 and -4 receptors (MC3R and MC4R) have been implicated in the regulation of obesity and energy homeostasis. Extensive studies have been performed to understand the role of the MC4R in energy homeostasis, but little is known about the MC3R. One caveat is that compounds identified to date are not selective for the MC3R (i.e. > 500-fold). Ligands selectively targeting the MC3R can aid as in vivo tools to explore the physiological functions of this receptor which may result in the discovery of new treatment options for obesity. Rational drug design targeting G-Protein coupled receptors (GPCR) usually consists of an iterative process, combining ligand structure-activity relationship studies (SAR), site-directed receptor mutagenesis and GPCR homology modeling studies, which may ultimately result in the discovery of new selective ligands. Herein, traditional peptide SAR studies were undertaken that resulted in the identification of structural requirements underlying differential MC3R and MC4R receptor activation and inhibition mechanisms. Further, extensive site-directed mutagenesis studies of the MC3R led to the discovery of key receptor residues involved in ligand-receptor recognition and signaling processes, differentiation of agonist versus antagonist activity and MC3R/MC4R subtype specificity. In a dual pronged approach, tetrapeptide antagonists identified from above mentioned SAR studies were pharmacologically characterized at mutant MC3Rs identified from site-directed mutagenesis studies that switched antagonist into agonist function. These studies revealed important determinants suggesting distinct inhibition mechanisms of tetrapeptide antagonist ligands at the MC3R. Finally, the molecular mechanism was investigated by which a polymorphic human MC4R, discovered in an extremely obese patient, causes constitutive receptor activity. This study resulted in a better understanding of the activation mechanism of the MC4R, valuable for modeling of GPCRs in the active state and docking of agonist ligands. In summary, this research contributed significantly to the rational drug design of MC3R selective ligands potentially useful to unravel the myriad of physiological functions of this receptor subtype and to discover new agents for the treatment of obesity.