Tau Loss of Function, by Deletion or Aggregation, Contributes to Peripheral Insulin Resistance

Background: Several epidemiological data revealed an association between Alzheimer's disease (AD) and type 2 diabetes. Researchers concentrated on brain insulin resistance with little emphasis on the link between systemic insulin resistance and AD, despite the fact that the incidence of type 2 diabetes is higher in AD patients and that impairment in insulin signaling is a risk factor for AD. Objective: The goal of this study is to determine the role of systemic insulin resistance in the pathogenesis of Alzheimer's disease by evaluating the consequences of tau loss-of-function on peripheral insulin sensitivity. Methods: Primary hepatocytes isolated from transgenic mouse models (Tau KO, P301L) and wild type mice (C57BL/6) were evaluated for their insulin sensitivity using glucose uptake assays as well as biochemical analysis of insulin signaling markers. Results: Our data show that tau deletion or loss of function promotes peripheral insulin resistance as seen in primary hepatocytes isolated from Tau KO and P301L mice, respectively. Furthermore, exposure of wild-type primary hepatocytes to sub-toxic concentrations of tau oligomers results in a dose-dependent inhibition of glucose uptake, associated with downregulation of insulin signaling. Tau oligomers-induced inactivation of insulin signaling proteins was rescued by inhibition of p38 MAPK, suggesting the involvement of p38 MAPK. Conclusions: This is the first study testing tau role in peripheral insulin resistance at the cellular level using multiple transgenic mouse models. Moreover, this study suggests that tau should be functional for insulin sensitivity, therefore, any loss of function by deletion or aggregation would result in insulin resistance.