P3‐214: Impaired insulin signal transduction in neurons may be involved in β‐amyloid‐induced hyperphosphorylation of tau

Background: Alzheimer’s disease (AD) is pathologically characterized by the presence of senile plaque and neurofibrillary tangle, which consist of ß-amyloid (Aß) and hyperphosphorylated tau, respectively. The mechanism(s) by which Aß deposition leads to hyperphosphorylation of tau remain unknown. Recent epidemiological results have suggested a close rink between AD and type 2 diabetes mellitus (T2DM). T2DM is characterized by insulin resistance and hyperinsulinemia associated with insulin signal dysfunction. Insulin signal transduction is mediated by glycogen synthase kinase 3ß (GSK3ß), which is known as a tau phospharylation kinase. With this background, we hypothesized that insulin signaling dysfunction may be involved in the pathophisiology of Aß-induced hyperphosphorylation of tau. Methods:We established a co-culture system in which Neuro2a stably expressing human tau (4R1N)(N2a MAPT) or rat primary cultured neurons (recipient cells) are co-incubated with HEK293 cells (host cells) expressing the Swedish mutant of APP. By this assay, we examined the effect of Aß overproduction on tau phosphorylation in the recipient cells. We next examined the effects of Aß42 overproduction and Aß oligomer on tau phosphorylation by expressing the familial AD-linked PSEN1 mutants (deltaT440 and L166P) and APP deltaE693 mutant. To assess the role of insulin signal transduction in the Aß-induced tau phosphorylation, we investigated the activation of downstream molecules of insulin signaling including phosphorylated Akt and GSK3ß upon insulin stimulation. Results: When N2a MAPT and rat primary cultured cortical neurons were co-incubated with the host cells producing a large amount of Aß, the levels of phosphorylated tau determined by immunoblot with the AT8 antibody were substantially increased. Similar enhancement of tau phosphorylation occurs in the recipient cells when co-incubated with the host cells expressing the PSEN1 mutants and APP deltaE693. We next examined insulin signal transduction upon insulin stimulation in the recipient N2a cells and primary culture neurons. The levels of phospho-Akt and phosho-GSK3ß were not efficiently activated upon insulin stimulation when co-incubated with the host cells producing a large amount of Aß. Conclusions: These results suggest that Aß accumulation might cause abnormal tau phosphorylation through impaired insulin signal transduction and that improvement of insulin signal transduction could be a new therapeutic target of AD treatment.