Anorexia and impaired glucose metabolism in mice with hypothalamic ablation of Glut4 neurons.

The central nervous system (CNS) uses glucose independent of insulin. Nonetheless, insulin receptors and insulin-responsive glucose transporters (Glut4) often colocalize in neurons (Glut4 neurons) in anatomically and functionally distinct areas of the CNS. The apparent heterogeneity of Glut4 neurons has thus far thwarted attempts to understand their function. To answer this question, we used Cre-dependent, diphtheria toxin-mediated cell ablation to selectively remove basal hypothalamic Glut4 neurons and investigate the resulting phenotypes. After Glut4 neuron ablation, mice demonstrate altered hormone and nutrient signaling in the CNS. Accordingly, they exhibit negative energy balance phenotype characterized by reduced food intake and increased energy expenditure, without locomotor deficits or gross neuronal abnormalities. Glut4 neuron ablation affects orexigenic melanin-concentrating hormone neurons but has limited effect on neuropeptide Y/agouti-related protein and proopiomelanocortin neurons. The food intake phenotype can be partially normalized by GABA administration, suggesting that it arises from defective GABAergic transmission. Glut4 neuron-ablated mice show peripheral metabolic defects, including fasting hyperglycemia and glucose intolerance, decreased insulin levels, and elevated hepatic gluconeogenic genes. We conclude that Glut4 neurons integrate hormonal and nutritional cues and mediate CNS actions of insulin on energy balance and peripheral metabolism.

Glut4 expression defines an insulin-sensitive hypothalamic neuronal population.

Insulin signaling in the CNS modulates satiety and glucose metabolism, but insulin target neurons are poorly defined. We have previously shown that ablation of insulin receptors (InsR) in Glut4-expressing tissues results in systemic abnormalities of insulin action. We propose that Glut4 neurons constitute an insulin-sensitive neuronal subset. We determined their gene expression profiles using flow-sorted hypothalamic Glut4 neurons. Gene ontology analyses demonstrated that Glut4 neurons are enriched in olfacto-sensory receptors, M2 acetylcholine receptors, and pathways required for the acquisition of insulin sensitivity. Following genetic ablation of InsR, transcriptome profiling of Glut4 neurons demonstrated impairment of the insulin, peptide hormone, and cAMP signaling pathways, with a striking upregulation of anion homeostasis pathway. Accordingly, hypothalamic InsR-deficient Glut4 neurons showed reduced firing activity. The molecular signature of Glut4 neurons is consistent with a role for this neural population in the integration of olfacto-sensory cues with hormone signaling to regulate peripheral metabolism.

Blunted refeeding response and increased locomotor activity in mice lacking FoxO1 in synapsin-Cre-expressing neurons.

Successful development of antiobesity agents requires detailed knowledge of neural pathways controlling body weight, eating behavior, and peripheral metabolism. Genetic ablation of FoxO1 in selected hypothalamic neurons decreases food intake, increases energy expenditure, and improves glucose homeostasis, highlighting the role of this gene in insulin and leptin signaling. However, little is known about potential effects of FoxO1 in other neurons. To address this question, we executed a broad-based neuronal ablation of FoxO1 using Synapsin promoter-driven Cre to delete floxed Foxo1 alleles. Lineage-tracing experiments showed that NPY/AgRP and POMC neurons were minimally affected by the knockout. Nonetheless, Syn-Cre-Foxo1 knockouts demonstrated a catabolic energy homeostatic phenotype with a blunted refeeding response, increased sensitivity to leptin and amino acid signaling, and increased locomotor activity, likely attributable to increased melanocortinergic tone. We confirmed these data in mice lacking the three Foxo genes. The effects on locomotor activity could be reversed by direct delivery of constitutively active FoxO1 to the mediobasal hypothalamus, but not to the suprachiasmatic nucleus. The data reveal that the integrative function of FoxO1 extends beyond the arcuate nucleus, suggesting that central nervous system inhibition of FoxO1 function can be leveraged to promote hormone sensitivity and prevent a positive energy balance.

Diabetes in mice with selective impairment of insulin action in Glut4-expressing tissues.

OBJECTIVE: Impaired insulin-dependent glucose disposal in muscle and fat is a harbinger of type 2 diabetes, but murine models of selective insulin resistance at these two sites are conspicuous by their failure to cause hyperglycemia. A defining feature of muscle and fat vis-à-vis insulin signaling is that they both express the insulin-sensitive glucose transporter Glut4. We hypothesized that diabetes is the result of impaired insulin signaling in all Glut4-expressing tissues. RESEARCH DESIGN AND METHODS: To test the hypothesis, we generated mice lacking insulin receptors at these sites ("GIRKO" mice), including muscle, fat, and a subset of Glut4-positive neurons scattered throughout the central nervous system. RESULTS: GIRKO mice develop diabetes with high frequency because of reduced glucose uptake in peripheral organs, excessive hepatic glucose production, and ß-cell failure. CONCLUSIONS: The conceptual advance of the present findings lies in the identification of a tissue constellation that melds cell-autonomous mechanisms of insulin resistance (in muscle/fat) with cell-nonautonomous mechanisms (in liver and ß-cell) to cause overt diabetes. The data are consistent with the identification of Glut4 neurons as a distinct neuroanatomic entity with a likely metabolic role.