Effects of Insulin on synapse formation and function; A possible role for insulin resistance
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Insulin resistance is a major risk factor for metabolic syndrome, including Type II diabetes (T2D) and obesity. Most studies on insulin resistance focus on peripheral tissues, but little is known about insulin resistance in the brain. T2D is associated with a higher risk for cognitive decline. Here, we examined the effects of insulin on synapse formation and function using primary rat hippocampal neurons as a model system. Our aim was to characterize the effects of low or high levels of insulin on synapse maturation and synaptic vesicle release using live cell imaging. We also wanted to examine the factors responsible for causing insulin resistance in neurons and their role in synapse formation and function. We found impaired synaptic vesicle exocytosis in neurons grown under either low or high levels of insulin compared to control. A significant reduction in the levels of Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1?) were observed by Western blotting in neurons grown under conditions of insufficient insulin indicating decreased mitochondrial biogenesis. To investigate factors that may cause neuronal insulin resistance, we measured insulin responsiveness in neurons treated with leucine, high insulin or TNFalpha, all of which are known to contribute to insulin resistance in the periphery. Chronic treatment with leucine, high insulin or TNFalpha decreased insulin sensitivity and increased basal Akt phosphorylation in neurons deprived of insulin for 30 min. In addition, impaired vesicle exocytosis was observed in neurons treated with leucine but this defect was improved by 5-amino-1-?-D-ribofuranosyl-imidazole-4-carboxamide (AICAR), an AMP-dependent protein kinase (AMPK) activator. Overall, our results suggest that neurons require an optimum amount of insulin to undergo normal synaptic vesicle release. In addition, high levels of insulin can cause insulin resistance in neurons accompanied with impaired synaptic vesicle exocytosis. We also demonstrate that supplementation with leucine causes the neurons to become less insulin sensitive with defects in synaptic vesicle release. This provides an insight into the link between T2D and cognitive decline.