Constitutive activity of the melanocortin-4 receptor is maintained by its N-terminal domain and plays a role in energy homeostasis in humans
Supriya Srinivasan, Cecile Lubrano-Berthelier, Cedric Govaerts, Franck Picard, Pamela Santiago, Bruce R. Conklin, Christian Vaisse
Supriya Srinivasan, Cecile Lubrano-Berthelier, Cedric Govaerts, Franck Picard, Pamela Santiago, Bruce R. Conklin, Christian Vaisse
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Article Metabolism

Constitutive activity of the melanocortin-4 receptor is maintained by its N-terminal domain and plays a role in energy homeostasis in humans

Abstract

The melanocortin-4 receptor (MC4R), a centrally expressed G protein–coupled receptor (GPCR), is essential for the maintenance of long-term energy balance in humans. Mutations in MC4R are the most common genetic cause of obesity. Since activation of this receptor leads to a decrease in food intake, MC4R is also a major therapeutic target for the treatment of obesity. Control of MC4R activity in vivo is modulated by the opposing effects of the anorexigenic agonist α–melanocyte-stimulating hormone (α-MSH) and the orexigenic antagonist agouti-related protein (AGRP). In addition, experiments in vitro have demonstrated that the human MC4R has an intrinsic constitutive activity on which AGRP also acts as an inverse agonist. The physiological role of this constitutive activity in the control of energy balance as well as the domain of the protein implicated in its maintenance are unknown. By systematically studying functional defects in naturally occurring MC4R mutations from obese patients, we defined a cluster of N-terminal mutations that selectively impair the constitutive activity of the receptor. Further characterization of this domain demonstrated that it functions as a tethered intramolecular ligand that maintains the constitutive activity of MC4R and may provide novel avenues for the design of drugs targeting this receptor. Our results also suggest that the tonic satiety signal provided by the constitutive activity of MC4R may be required for maintaining long-term energy homeostasis in humans.

Authors

Supriya Srinivasan, Cecile Lubrano-Berthelier, Cedric Govaerts, Franck Picard, Pamela Santiago, Bruce R. Conklin, Christian Vaisse

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N-terminal MC4R mutation does not impair ligand action. WT and mutant MC...
N-terminal MC4R mutation does not impair ligand action. WT and mutant MC4Rs were transiently transfected into HEK293 cells stably expressing luciferase under the control of a cAMP response element promoter. Cells were stimulated with increasing amounts of α-MSH (A and B) or with α-MSH 10–8 M and increasing concentrations of AGRP (C and D), and luci ferase activity was measured to generate dose response curves. Data were normalized to maximal α-MSH stimulation after subtraction of basal activity and were fitted by nonlinear regression. Each point represents the mean ± SEM of at least two independent experiments. The best-fit estimate of the EC50 (A and B) or the IC50 (C and D) and their 95% confidence intervals are indicated for each data set. There was no statistical difference between the WT EC50 and EC50 for each of the mutants (A and B) nor between the WT IC50 and IC50 for each of the mutants (C and D; for all in A–D, P > 0.05 in an F test under the null hypothesis “log EC50/IC50 same for WT and mutants”). RLU, relative luminescence units.