| dc.description.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. | en_US |
