Differential Signaling Profiles of MC4R Mutations with Three Different Ligands

The melanocortin 4 receptor (MC4R) is a key player in hypothalamic weight regulation and energy expenditure as part of the leptin–melanocortin pathway. Mutations in this G protein coupled receptor (GPCR) are the most common cause for monogenetic obesity, which appears to be mediated by changes in the anorectic action of MC4R via G_S-dependent cyclic adenosine-monophosphate (cAMP) signaling as well as other signaling pathways. To study potential bias in the effects of MC4R mutations between the different signaling pathways, we investigated three major MC4R mutations: a G_S loss-of-function (S127L) and a G_S gain-of-function mutant (H158R), as well as the most common European single nucleotide polymorphism (V103I). We tested signaling of all four major G protein families plus extracellular regulated kinase (ERK) phosphorylation and β-arrestin2 recruitment, using the two endogenous agonists, α- and β-melanocyte stimulating hormone (MSH), along with a synthetic peptide agonist (NDP-α-MSH). The S127L mutation led to a full loss-of-function in all investigated pathways, whereas V103I and H158R were clearly biased towards the G_q/11 pathway when challenged with the endogenous ligands. These results show that MC4R mutations can cause vastly different changes in the various MC4R signaling pathways and highlight the importance of a comprehensive characterization of receptor mutations.

Hypothalamic POMC or MC4R deficiency impairs counterregulatory responses to hypoglycemia in mice

Objective

Life-threatening hypoglycemia is a major limiting factor in the management of diabetes. While it is known that counterregulatory responses to hypoglycemia are impaired in diabetes, molecular mechanisms underlying the reduced responses remain unclear. Given the established roles of the hypothalamic proopiomelanocortin (POMC)/melanocortin 4 receptor (MC4R) circuit in regulating sympathetic nervous system (SNS) activity and the SNS in stimulating counterregulatory responses to hypoglycemia, we hypothesized that hypothalamic POMC as well as MC4R, a receptor for POMC derived melanocyte stimulating hormones, is required for normal hypoglycemia counterregulation.

Methods

To test the hypothesis, we induced hypoglycemia or glucopenia in separate cohorts of mice deficient in either POMC or MC4R in the arcuate nucleus (ARC) or the paraventricular nucleus of the hypothalamus (PVH), respectively, and measured their circulating counterregulatory hormones. In addition, we performed a hyperinsulinemic-hypoglycemic clamp study to further validate the function of MC4R in hypoglycemia counterregulation. We also measured Pomc and Mc4r mRNA levels in the ARC and PVH, respectively, in the streptozotocin-induced type 1 diabetes mouse model and non-obese diabetic (NOD) mice to delineate molecular mechanisms by which diabetes deteriorates the defense systems against hypoglycemia. Finally, we treated diabetic mice with the MC4R agonist MTII, administered stereotaxically into the PVH, to determine its potential for restoring the counterregulatory response to hypoglycemia in diabetes.

Results

Stimulation of epinephrine and glucagon release in response to hypoglycemia or glucopenia was diminished in both POMC- and MC4R-deficient mice, relative to their littermate controls. Similarly, the counterregulatory response was impaired in association with decreased hypothalamic Pomc and Mc4r expression in the diabetic mice, a phenotype that was not reversed by insulin treatment which normalized glycemia. In contrast, infusion of an MC4R agonist in the PVH restored the counterregulatory response in diabetic mice.

Conclusion

In conclusion, hypothalamic Pomc as well as Mc4r, both of which are reduced in type 1 diabetic mice, are required for normal counterregulatory responses to hypoglycemia. Therefore, enhancing MC4R function may improve hypoglycemia counterregulation in diabetes.

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Hypothalamic POMC as well as MC4R is necessary to counteract hypoglycemia.

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Type 1 diabetic mice exhibit a reduced Pomc and Mc4r expression in the hypothalamus.

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Insulin treatment does not restore Pomc and Mc4r expression in diabetic mice.

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MC4R agonist improves hypoglycemia counterregulation in diabetic mice.

Characterization of MC4R Regulation of the Kir7.1 Channel Using the Tl+ Flux Assay

The family of inward rectifying potassium channels (Kir channels) plays crucial roles in the regulation of heart rhythms, renal excretion, insulin release, and neuronal activity. Their dysfunction has been attributed to numerous diseases such as cardiac arrhythmia, kidney failure and electrolyte imbalance, diabetes mellitus, epilepsy, retinal degeneration, and other neuronal disorders. We have recently demonstrated that the melanocortin-4 receptor (MC4R), a Gα_s-coupled GPCR, regulates Kir7.1 activity through a mechanism independent of Gα_s and cAMP. In contrast to the many other members of the Kir channel family, less is known about the biophysical properties, regulation, and physiological functions of Kir7.1. In addition to using conventional patch clamp techniques, we have employed a high-throughput Tl⁺ flux assay to further investigate the kinetics of MC4R-Kir7.1 signaling in vitro. Here, we discuss the employment of the Tl⁺ flux assay to study MC4R -mediated regulation of Kir7.1 activity and to screen compounds for drug discovery.