Sonic Hedgehog receptor Patched deficiency in astrocytes enhances glucose metabolism in mice.

ABSTRACT Objective Astrocytes are glial cells proposed as the main Sonic Hedgehog (Shh)-responsive cells in the adult brain. Their roles in mediating Shh functions are still poorly understood. In the hypothalamus, astrocytes support neuronal circuits implicated in the regulation of energy metabolism. Here, we investigated the impact of genetic activation of Shh signaling on hypothalamic astrocytes and characterized its effects on energy metabolism. Methods We analyzed the distribution of gene transcripts of the Shh pathway (Ptc, Gli1, Gli2, Gli3) in astrocytes using single molecule fluorescence in situ hybridization combined to immunohistofluorescence and of Shh peptides by Western blotting in the adult mouse hypothalamus. Based on the metabolic phenotype, we characterized Glast-CreERT2-YFP-Ptc-/- (YFP-Ptc-/-) mice and their controls over time and under high-fat-diet (HFD) to investigate the potential effects of conditional astrocytic deletion of the Shh receptor Patched (Ptc) on metabolic efficiency, insulin sensitivity and systemic glucose metabolism. Molecular and biochemical assays were used to analyze the alteration of key pathways modulating energy metabolism, insulin sensitivity, glucose uptake and inflammation. Primary astrocyte cultures were used to evaluate a potential role of Shh signaling in astrocytic glucose uptake. Results Shh peptides were the highest in the hypothalamic extracts of adult mice and a large population of hypothalamic astrocytes expressed Ptc and Gli1-3 mRNAs. Characterization of Shh signaling after conditional Ptc deletion in YFP-Ptc-/- mice revealed heterogeneity in hypothalamic astrocyte populations. Interestingly, the activation of Shh signaling in Glast+ astrocytes enhanced insulin responsiveness as evidenced by glucose and insulin tolerance tests. This effect was maintained over time and associated with lower blood insulin levels and was also observed under HFD. YFP-Ptc-/- mice exhibited a lean phenotype with the absence of body weight gain and a marked reduction of white and brown adipose tissues accompanied by increased whole body fatty acid oxidation. In contrast, food intake, locomotor activity and body temperature were not altered. At the cellular level, Ptc deletion did not affect glucose uptake in primary astrocyte cultures. In the hypothalamus, the activation of astrocytic Shh pathway was associated with the upregulation of transcripts coding for the insulin receptor and the Liver Kinase B1 (LKB1) after 4 weeks, and for the glucose transporter Glut-4 after 32 weeks. Conclusions Here, we define hypothalamic Shh action on astrocytes as a novel master regulator of energy metabolism. In the hypothalamus, astrocytic Shh signaling could be critically involved in preventing both aging- and obesity-related metabolic disorders.