Probes for MC4R

Abstract

Anorexigenic pro-opiomelanocortin (Pomc)/alpha-melanocyte stimulating hormone (αMSH) neurons of the hypothalamic melanocortin system function as key regulators of energy homeostasis, also controlling somatic growth across different species. However, the mechanisms of melanocortin-dependent growth control still remain ill-defined. Here, we reveal a thus-far-unrecognized structural and functional connection between Pomc neurons and the somatotropic hypothalamo-pituitary axis. Excessive feeding of larval zebrafish causes leptin resistance and reduced levels of the hypothalamic satiety mediator pomca. In turn, this leads to reduced activation of hypophysiotropic somatostatin (Sst)-neurons that express the melanocortin receptor Mc4r, elevated growth hormone (GH) expression in the pituitary, and enhanced somatic growth. Mc4r expression and αMSH responsiveness are conserved in Sst-expressing hypothalamic neurons of mice. Thus, acquired leptin resistance and attenuation of pomca transcription in response to excessive caloric intake may represent an ancient mechanism to promote somatic growth when food resources are plentiful.

Like most homeostatic systems, adiposity in mammals is defended between upper and lower boundary conditions. While leptin and melanocortin-4 receptor (MC4R) signaling are required for defending energy set point, mechanisms controlling upper and lower homeostatic boundaries are less well understood. In contrast to the MC4R, deletion of the MC3R does not produce measurable hyperphagia or hypometabolism under normal conditions. However, we demonstrate that MC3R is required bidirectionally for controlling responses to external homeostatic challenges, such as caloric restriction or calorie-rich diet. MC3R is also required for regulated excursion from set point, or rheostasis, during pregnancy. Further, we demonstrate a molecular mechanism: MC3R provides regulatory inputs to melanocortin signaling, acting presynaptically on agouti-related protein neurons to regulate γ-aminobutyric acid release onto anorexigenic MC4R neurons, exerting boundary control on the activity of MC4R neurons. Thus, the MC3R is a critical regulator of boundary controls on melanocortin signaling, providing rheostatic control on energy storage.

Hunger, driven by negative energy balance, elicits the search for and consumption of food. While this response is in part mediated by neurons in the hypothalamus, the role of specific cell types in other brain regions is less well defined. Here, we show that neurons in the dorsal raphe nucleus, expressing vesicular transporters for GABA or glutamate (hereafter, DRNVgat and DRNVGLUT3 neurons), are reciprocally activated by changes in energy balance and that modulating their activity has opposite effects on feeding-DRNVgat neurons increase, whereas DRNVGLUT3 neurons suppress, food intake. Furthermore, modulation of these neurons in obese (ob/ob) mice suppresses food intake and body weight and normalizes locomotor activity. Finally, using molecular profiling, we identify druggable targets in these neurons and show that local infusion of agonists for specific receptors on these neurons has potent effects on feeding. These data establish the DRN as an important node controlling energy balance. PAPERCLIP.

Understanding the neural framework behind appetite control is fundamental to developing effective therapies to combat the obesity epidemic. The paraventricular hypothalamus (PVH) is critical for appetite regulation, yet, the real-time, physiological response properties of PVH neurons to nutrients are unknown. Using a combination of fiber photometry, electrophysiology, immunohistochemistry, and neural manipulation strategies, we determined the population dynamics of four molecularly delineated PVH subsets implicated in feeding behavior: glucagon-like peptide 1 receptor (PVHGlp1r), melanocortin-4 receptor (PVHMc4r), oxytocin (PVHOxt), and corticotropin-releasing hormone (PVHCrh). We identified both calorie- and state-dependent sustained activity increases and decreases in PVHGlp1r and PVHCrh populations, respectively, while observing transient bulk changes of PVHMc4r, but no response in PVHOxt, neurons to food. Furthermore, we highlight the role of PVHGlp1r neurons in orchestrating acute feeding behavior, independent of the anti-obesity drug liraglutide, and demonstrate the indispensability of PVHGlp1r and PVHMc4r, but not PVHOxt or PVHCrh neurons, in body weight maintenance.

The central melanocortin system plays a crucial role in the control of energy balance. Although the decreased energy expenditure and increased adiposity of melanocortin-3 receptor (Mc3R)-null mice suggest the importance of Mc3R-regulated neurons in energy homeostasis, the roles for specific subsets of Mc3R neurons in energy balance have yet to be determined. Because the lateral hypothalamic area (LHA) contributes to the control of energy expenditure and feeding, we generated Mc3rcre mice to determine the roles of LHA Mc3R (Mc3RLHA) neurons in energy homeostasis. We found that Mc3RLHA neurons overlap extensively with LHA neuron markers that contribute to the control of energy balance (neurotensin, galanin, and leptin receptor) and project to brain areas involved in the control of feeding, locomotion, and energy expenditure, consistent with potential roles for Mc3RLHA neurons in these processes. Indeed, selective chemogenetic activation of Mc3RLHA neurons increased locomotor activity and augmented refeeding after a fast. Although the ablation of Mc3RLHA neurons did not alter food intake, mice lacking Mc3RLHA neurons displayed decreased energy expenditure and locomotor activity, along with increased body mass and adiposity. Thus, Mc3R neurons lie within LHA neurocircuitry that modulates locomotor activity and energy expenditure and contribute to energy balance control.