Mutations in the hypothalamic expressed MC4R gene are the most frequent cause of monogenetic obesity. This Gαs-Protein coupled receptor (GPCR) is activated by endogenous ligands α- and β-MSH and is inhibited by the only known endogenous inverse agonist and antagonist Agouti related peptide (AgRP). Naturally occurring mutations help to understand activation mechanisms of the human MC4R.
We previously described a constitutively activating MC4R mutation (H158R) found heterozygously in a normal weight proband. This mutation is located in the second intracellular loop and causes a six times higher basal activity, a higher maximal stimulation and a slightly decreased EC50 after stimulation with MSH-ligands when compared to the WT-MC4R. The increased basal activity can be dose dependently reduced by stimulation with the inverse agonist AgRP. When the WT-MC4R is stimulated with α-MSH in presence of increasing AgRP concentrations, an increase of EC50 is observed, which can not be seen for the H158R mutant, explaining the normal phenotype of the mutation carrier. The histidine at position 158 is conserved in the MC4Rs of 70 species and also in all 5 human melanocortin receptors but not generally in GPCRs. Exchanging the histidine to further amino acids by site directed mutagenesis decipher the crucial role of histidine 158 for receptor silencing. Additionally, in contrast to activating mutation H158R we describe three inactivating mutations at serine residues located in the third transmembrane domain, which were found in obese patients. S136F causes a loss of function although properly expressed on the cell surface and shows a dominant negative effect when co-transfected transiently with WT-MC4R. S127L is a partial loss of function mutation with normal cell surface expression pattern. Molecular details of modifications on the MC4R structure and signaling properties caused by these mutants are evaluated using molecular modelling driven side chain substitutions. Furthermore, S139R resulted in a complete loss of function and impaired membrane trafficking. Exchanging S139 to isoleucine partially rescued these effects. In a computer-generated receptor model, substitution of S139 with the bulky and charged arginine disrupts the conserved interaction of serine 85 in TM2 with tryptophan 174 in TM4 and forms a new constraining interaction to TM4 thus leading to inadequate folding of the receptor. These new findings reveal deeper insight into the intramolecular mechanism of activity regulation at the human MC4R.