Hypothalamic melanocortin-4 receptors on astrocytes mediate inflammation and body weight homeostasis

Abstract

World-wide, nearly 40% of the adult population is classified as clinically obese. Central inflammation is highly correlated with obesity and increases morbidity and deterioration of health. The hypothalamus is a brain region that governs many facets of energy homeostasis, and melanocortins in the hypothalamus both decrease feeding and increase metabolism via melanocortin-4 receptors (MC4Rs). Although MC4Rs are present on neurons and astrocytes (aMC4R) previous work has focused almost exclusively on the neuronal population with the contribution of aMC4R on these processes largely unknown. Our objective was to determine the effects of hypothalamic aMC4R deletion on central and peripheral inflammation, as well as feeding and body weight homeostasis. Adult MC4R fl/fl mice were microinjected with an astrocyte-specific promoter driving Cre-expression (AAV-GFAP-GFP-Cre) or AAV-control (AAV-GFAP-GFP; n=4-7/group/sex) to produce a hypothalamic knock-down of aMC4R (KD). Body weight and composition were monitored throughout the study, and indirect calorimetry was conducted at 1 and 4 weeks after AAV injection. Acquisition of operant self-administration of palatable food was also examined. Mice were euthanized 7-8 weeks post AAV injection and brain and tissue samples were collected. We observed a significant increase in body weight, feeding, and energy balance in the KD group compared to control group. Inflammation was significantly increased centrally in KD mice within the hypothalamus, but not peripherally within serum. Additionally, aMC4R KD mice trended towards an increased reward learning for palatable food. This is the first demonstration that hypothalamic aMC4R, independent of neuronal MC4R, is important in modulating inflammation as well as contributing to energy balance. These results provide an integral understanding of the aMC4R system that will provide the foundation for future studies investigating the role of aMC4R in various disease states.