Effect of cholesterol reduction on receptor signaling in neurons

Diabetes mellitus is associated with a variety of complications, including alterations in the central nervous system (CNS). We have recently shown that diabetes results in a reduction of Cholesterol synthesis in the brain due to decreased Insulin stimulation of SREBP2-mediated Cholesterol synthesis in neuronal and glial cells. In the present study, we explored the effects of the decrease in Cholesterol on neuronal cell function using GT1-7 hypothalamic cells subjected to Cholesterol depletion in vitro using three independent methods: 1) exposure to methyl-β-cyclodextrin, 2) treatment with the HMG-CoA reductase inhibitor simvastatin, and 3) shRNA-mediated knockdown of SREBP2. All three methods produced 20-31% reductions in cellular Cholesterol content, similar to the decrease in Cholesterol synthesis observed in diabetes. All cholesterol-depleted neuron-derived cells, independent of the method of reduction, exhibited decreased phosphorylation/activation of IRS-1 and Akt following stimulation by Insulin, insulin-like growth factor-1, or the neurotrophins (NGF and BDNF). ERK phosphorylation/activation was also decreased after methyl-β-cyclodextrin and statin treatment but increased in cells following SREBP2 knockdown. In addition, Apoptosis in the presence of Amyloid-β was increased. Reduction in cellular Cholesterol also resulted in increased basal Autophagy and impairment of induction of Autophagy by glucose deprivation. Together, these data indicate that a reduction in neuron-derived Cholesterol content, similar to that observed in diabetic brain, creates a state of Insulin and growth factor resistance that could contribute to CNS-related complications of diabetes, including increased risk of neurodegenerative diseases, such as Alzheimer disease.