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, … , Bruce R. Conklin, Christian Vaisse
Supriya Srinivasan, … , Bruce R. Conklin, Christian Vaisse
Published October 15, 2004
Citation Information: J Clin Invest. 2004;114(8):1158-1164. https://doi.org/10.1172/JCI21927.
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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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The N-terminal domain of MC4R acts as a tethered ligand to maintain cons...
The N-terminal domain of MC4R acts as a tethered ligand to maintain constitutive activity of the receptor. (A) Cell-surface expression of WT MC4R and the ATG24 receptor. Empty vector (pcDNA 3.1) was transfected as a control to detect background. (B) The ratio of activity (as measured by cAMP accumulation) to cell-surface expression (as measured by ELISA) was measured in the WT and the ATG24-MC4R. (C) After stimulation with 10 μM α-MSH, cAMP accumulation was measured in cells transfected with WT MC4R, ATG24-MC4R, or empty vector (pcDNA 3.1). (D) HEK293 cells were cotransfected with the WT receptor or the ATG24 receptor and either the WT N-terminus–CD8 fusion protein or the R18C N-terminus–CD8-fusion protein. The ratio of receptor to fusion protein was maintained at 1:10. To measure constitutive activity of the WT and ATG24 receptors alone, pcDNA 3.1 was transfected in place of the fusion proteins to keep the amount of total transfected DNA the same. Inset: Cell-surface expression of WT MC4R and the ATG24 receptor. Empty vector (pcDNA 3.1) was transfected as a control to detect background.