Effect of Peripheral Insulin Administration on Phosphorylation of Tau in the Brain.

Background: Abnormally hyperphosphorylated tau is the major protein of neurofibrillary tangles in Alzheimer's disease. Insulin activates PI3K-AKT signaling and regulates tau phosphorylation. Impaired brain insulin signaling is involved in Alzheimer's disease pathogenesis. However, the effect of peripheral insulin on tau phosphorylation is controversial.

Objective: In the present study, we determined the effect of peripheral insulin administration on tau phosphorylation in brain.

Methods: We intraperitoneally injected a super physiological dose of insulin to mice and analyzed PI3K-AKT signaling and tau phosphorylation in brains by western blots.

Results: We found that peripherally administered insulin activated the PI3K-AKT signaling pathway immediately in the liver, but not in the brain. Tau phosphorylation in the mouse brain was found to be first decreased (15 min) and then increased (30 min and 60 min) after peripheral insulin administration and these changes correlated inversely with body temperature and the level of brain protein O-GlcNAcylation. Maintaining body temperature of mice post peripheral insulin administration prevented the insulin/hypoglycemia-induced tau hyperphosphorylation after peripheral insulin administration.

Conclusion: These findings suggest that peripheral insulin can induce tau hyperphosphorylation through both hypothermia and downregulation of brain protein O-GlcNAcylation during hypoglycemia.