Pharmacological characterization and functional rescue of human melanocortin-4 receptor mutants

Abstract

The melanocortin-4 receptor (MC4R) plays an important role in regulating food intake and energy expenditure. Previous studies reported that the transmembrane domain 6 was important in activating several G protein-coupled receptors. To better understand the function of thirty-one residues in the transmembrane domain 6 of the MC4R, we performed alaninescanning mutagenesis. We identified residues that are important for cell surface expression, ligand binding, cAMP signaling, and residues for maintaining the wild type MC4R in inactive conformation. We also observed constitutive activation of the mitogen-activated protein kinase signaling and biased signaling of the MC4R. Up to now, approximately 170 mutations in the MC4R have been identified from obese patients. Functional studies of these mutants are important in understanding the role of the MC4R in causing obesity and in developing personalized medicine. We performed detailed functional studies on nine MC4R mutants (G55V, R165G, R165W, R165Q, C172R, F202L, M208V, I269N, and A303P) that were identified from Pima Indian heritage and Hispanic heritage. We showed that the majority of these mutants (six) (R165G, R165W, R165Q, C172R, I269N, and A303P) were completely or partially defective in cell surface expression. The intracellularly retained mutants may retain intrinsic function and become functional when coaxed to the cell surface. We therefore investigated whether small molecule ligands could act as pharmacological chaperones (pharmacoperones) promoting the proper folding of iii intracellularly retained MC4R mutants. Three MC4R ligands including two antagonists (ML00253764 and Ipsen 5i) and one agonist (THIQ) and eleven intracellularly retained MC4R mutants (S58C, N62S, I69R, P78L, C84R, G98R, Y157S, W174C, P260Q, F261S, and C271Y) were studied using different cell lines. We showed that the three small molecule ligands could act as pharmacoperones rescuing the cell surface expression and signaling of intracellularly retained MC4R mutants with various efficacies. In summary, we comprehensively studied the transmembrane domain 6 of the MC4R and nine MC4R mutants identified from obese patients. We also identified three pharmacoperones of the MC4R.