Insulin is a principal metabolic hormone. It regulates a plethora of metabolic pathways in sensitive peripheral tissues. Recent studies show insulin signaling plays fundamental roles in the brain. Loss of insulin action in different neuronal and glial cells contributes to a variety of metabolic and behavioral abnormalities in mice. In particular, loss of astrocytic insulin signaling leads to impaired brain glucose sensing and increased depressive-like behavior in mice. How insulin signaling modulates astrocytic cellular functions and neural circuits is not completely known. Using next-generation RNA sequencing, we show that insulin triggers a robust transcriptional regulation on multiple metabolic pathways in primary astrocytes. As expected, insulin increased expression of genes involved in cholesterol biosynthesis, important for supporting synaptic integrity and remodeling. Further, autophagy is a quintessential pathway subjected to insulin regulation, indicating the role of insulin action on proteostasis in astrocytes. Thus, insulin suppresses the expression of most critical genes involved in autophagy, including p62, Ulk1/2, several Atg genes. The inhibitory effect is potent, as 1 nM is sufficient to suppress gene expression. To determine whether insulin receptors (IR) are required for such regulation, we examined the insulin response in astrocytes without endogenous IR. IR loss only partially reverses insulin-dependent suppression of autophagic genes, indicating that insulin may act through closely related IGF-1 receptors in astrocytes.
In summary, insulin potently suppresses autophagy in astrocytes through transcriptional regulation, indicating a potential important role of astrocytic insulin signaling on regulating proteostasis in accordance with systemic nutritional status. Impairment of this regulation in insulin resistance and diabetes may affect the modulation of neural circuits by astrocytes and contribute to neurological complications linked to diabetes.
S.Geffken: None. C.Smith: None. H.Lee: None. W.Cai: None.