Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in neurons of the hypothalamus where it regulates feeding behavior. MC4R responds to an agonist, α-melanocyte-stimulating hormone (α-MSH) and to an antagonist/inverse agonist, agouti-related peptide (AgRP), which are released by upstream neurons. Binding to α-MSH leads to stimulation of receptor activity and suppression of food intake, whereas AgRP has opposite effects. In this respect, MC4R agonists are under investigation as therapy to treat or reverse obesity. However, in animal models of diet-induced obesity, treatment with most MC4R agonists leads to tachyphylaxis after a few days, which is thought to be a consequence of receptor desensitization or upregulation of MC4R antagonist AgRP. Here, we study the signaling, trafficking and desensitization of MC4R in response to different agonists in cultured neurons overexpressing the tagged receptor. We demonstrate that the constitutive, agonist independent endocytosis of MC4R is clathrin-dependent and cholesterol-dependent. We find that the α-MSH-dependent desensitization of MC4R occurs by a mechanism that includes attenuated post-endocytic recycling of the receptor, which is dependent on intact phosphorylation sites at the C-terminus. When we target α-MSH to the endoplasmic reticulum (ER) of neuroblastoma cells, we find that the ER-targeted α-MSH binds to MC4R at this location and stabilizes the receptor in a different active conformation than MC4R which initially binds to α-MSH at the plasma membrane. The active conformation that is stabilized by α-MSH binding to MC4R in the ER signals constantly and is resistant to desensitization and antagonism by AgRP. We find that, unlike α-MSH, the synthetic MC4R agonist melanotan-II (MT-II) does not induce decreased plasma membrane expression of MC4R after three hour exposure to agonist. Further, MC4R signaling induced by MT-II, unlike that induced by α-MSH, is not antagonized by AgRP, and signals constantly even after MT-II is removed from the media. In sum, we describe the signaling and desensitization properties of MC4R in response to different MC4R agonists. These different properties may underlie the different responses to MC4R agonists in vivo.
|Commitee:||Childs, Gwen V., Lupashin, Vladimir V., Tackett, Alan J.|
|School:||University of Arkansas for Medical Sciences|
|Department:||Interdisciplinary Biomedical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 76/01(E), Dissertation Abstracts International|
|Subjects:||Biology, Neurosciences, Cellular biology|
|Keywords:||Biased agonism, Desensitization, Gpcr, Mc4r, Obesity, Trafficking|
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