Insulin receptor signaling in the subfornical organ protects against the development of metabolic syndrome
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- Subfornical organ, insulin receptor, obesity, metabolic syndrome
Metabolic syndrome encompasses a combination of conditions including obesity, diabetes, and hypertension. Brain insulin resistance has emerged as a contributor to the development of metabolic syndrome, although the neural regions involved remain unclear. While most investigations have focused on insulin action in the hypothalamus, recent evidence suggests that the insulin receptor (IR) gene is also expressed in the subfornical organ (SFO); a circumventricular organ well known for cardiovascular/fluid regulation and recently recognized as a metabolic nucleus. We therefore hypothesized that IR signaling in the SFO is involved in metabolic regulation. We first examined protein levels of SFO IR in male C57Bl/6 mice (n=3) using immunohistochemistry, and observed that IR expressing cells are rich in the SFO. Co-immunohistochemistry further revealed heterogeneous cellular expression of the SFO IR, with 11.9 ± 2.2% of IR-ir detected on astrocytes (GFAP), 57.2 ± 2.6% on endothelial cells (TIE2), and 18.3 ± 0.8% on neurons (NeuN). Interestingly, neuronal expression of IR in the SFO was restricted to glutamatergic cells, but absent in GABAergic cells. To test the functional role of SFO IR, we next utilized mice harboring a conditional allele of the IR gene (IRfl/fl), and selectively knocked down the SFO IR via SFO-targeted delivery of an adeno-associated virus encoding Cre-recombinase (AAV-Cre-eGFP; n=4), or control vector (AAV-eGFP; n=3). Both groups remained on normal chow, and metabolic parameters were continuously monitored using indirect calorimetry for 12 weeks. Selective removal of SFO IR did not influence food and water intake, but resulted in a greater increase in body weight (e.g. 12 weeks: 27.9 ± 1.5 vs. 31.4 ± 1.4 g, AAV-eGFP vs. AAV-Cre-eGFP, ANOVA interaction p=0.0005). This was associated with a significantly lower energy expenditure (e.g. 12 week average: 12.5 ± 0.6 vs. 11.7 ± 0.2 kcal/hr/kg, AAV-eGFP vs. AAV-Cre-eGFP, ANOVA interaction p=0.013) and a slight reduction in ambulatory activity in AAV-Cre-eGFP mice relative to controls. Examination of regional adipose tissue also revealed a ~40% increase in overall adiposity following ablation of SFO IR (total adipose: 1.4 ± 0.4 vs. 2.2 ± 0.3 g, AAV-eGFP vs. AAV-Cre-eGFP, p=0.1). Whole body glucose clearance and insulin sensitivity were comparable between groups. These data demonstrate that ablation of SFO IRs under normal diet conditions results in a deleterious metabolic state. Moreover, these findings indicate a tonic metabolic regulatory role for SFO IR, and suggest that impairments in IR signaling in the SFO may contribute to a development of metabolic syndrome.