Alzheimer-like Hyperphosphorylation Of Tau In Hippocampus Of Rats With Type 2 Diabetes

Objective: Abnormally hyperphosphorylation of tau plays a critical role in the pathogenesis of Alzheimer disease (AD) and type 2 diabetes is a known risk factor of AD. Mechanisms that control this relationship remain unknown. In this present study, firstly, we studied the phosphorylation of tau in insulin resistant and type 2 diabetic rats.Materials and Methods: The models of insulin resistance (IR) were feed with high glucose, high fat and high protein for 8 weeks, and the models of T2DM group were randomly taken from IR rats, and then injected STZ. All models were controlled by the rats with the same age and normal foods feed(CTL). The plasma insulin was examined by RIA method, and the plasma glucose by glucose-oxidase method. The indexes of insulin resistance were calculated by HOMA-IR. Total tau, some phosphorylation sites of tau (Ser199, Thr212, Ser214, Ser396 and Ser422) were analyzed by Western blots.The expression of tau protein [Ser396/404] was observed by immunohistochemical assay.Results: We found that the plasma insulin level was remarkably increased in IR and T2DM group as compared to CTL group, and plasma glucose level in T2DM was increased to ~3 times of the CTL, but that in IR group was only marginally increased. Insulin resistance, which calculated by HOMA-IR was significantly increased in IR and T2DM group as compared to CTL group. Neither high fat, high glucose and high protein diet nor STZ injection changed the total level of tau protein in the hippocampus of rats. However, tau was found to be hyperphosphorylated at several AD-related phosphorylation sites (Ser199,Thr212 and Ser396) in IR and T2DM group. In T2DM group, Ser214 and Ser422 of tau protein were hyperphosphorylated, but no significant changes in IR group. Data of immunohistochemistry showed that the expression of phosphorylated tau modestly increased in CA3 sector of hippocampus in IR and T2DM group compared with CTL group.Conclusions: These findings suggest (1) that tau protein in hippocampus was hyperphosphrylated in IR and T2DM rats, and (2) insulin resistance and high plasma glucose in type 2 diabetes may be the causes of the hyperphosphorylation of tau protein. Objective: tau protein was Alzheimer-like hyperphosphorylation in hippocampus of rats with insulin resistance or type 2 diabetes. We therefore studied the ways which lead to hyperphosphorylation of tau by insulin signal transduction and glucose metabolism.Materials and Methods: The models of insulin resistance (IR) were feed with high glucose, high fat and high protein for 8 weeks, and the models of T2DM group were randomly taken from IR rats, and then injected STZ. All models were controlled by the rats with the same age and normal foods feed(CTL). The binding activities between insulin and insulin receptor were determined by SPR. LiCl, which blocked activation of GSK-3β, were Intrahippocampal injection in rats randomly taken from IR and T2DM group. The plasma insulin was examined by RIA method, and the plasma glucose by glucose-oxidase method. The indexes of insulin resistance were calculated by HOMA-IR. The activity of glycogen synthase kinase 3β(GSK-3β), a key component of insulin signal transduction pathway and a known tau kinase in the hippocampus of rats was determined by usingγ-32P-ATP and the specific peptide substrate. Total tau, some phosphorylation sites of tau (Ser199, Thr212 and Ser214), insulin receptors and glucose transport 3 (GLUT3) on neuronal membrane fractions were analyzed by Western blots.Results: We found that LiCl Intrahippocampal injection did not change plasma glucose, plasma insulin or insulin resistance in IR or T2DM group. Theβ-subunit of the membrane insulin receptor was found to be reduced in the brains of insulin resistant and type 2 diabetic rats, and the binding activities between insulin and insulin receptors on the neuronal membrane in type 2 diabetic rats were dramatically decreased. The activity of GSK-3βwas found to be increased in the brains of both insulin resistant and diabetic rats. Intrahippocampal injection of LiCl blocked activation of GSK-3βin both groups, but only blocked hyperphosphorylation of tau in the insulin resistant rats. In additional, the glucose transport 3(GLUT3) on the membrane was found to be reduced only in the brains of type 2 diabetic rats.Conclusions: These findings suggest that (1) type 2 diabetes increased the probability of AD by increased insulin resistance and consequent upregulation of GSK-3β, which leads to hyperphosphorylation of tau, and (2) impaired glucose metabolism may also contribute to tau hyperphosphorylation in type 2 diabetes. Objective: We studied the phosphorylation of tau protein in hippocampus of obese rats, and changes of phosphorylation after effect by rosiglitazone. Materials and methods: Wistar rats were randomized into 3 groups. A control group (CTL) was fed with normal food, as well as high sugar, high fat and high protein diet was given to an insulin resistant group (IR) and a rosiglitazone group (TZD) for 8 weeks. Rosiglitazone was then administered by garage 3mg·kg-1 daily for 4 weeks to TZD group. The plasma insulin was examined by RIA method, and the plasma glucose by glucose-oxidase method. The indexes of insulin resistance were calculated by HOMA-IR. Total tau, some phosphorylation sites of tau (Ser199, Thr212, Ser214, Ser396 and Ser422) were analyzed by Western blots. The activity of glycogen synthase kinase 3β, a key component of insulin signal transudation pathway and a known tau kinase, in hippocampus of rats was examined byγ-32P-ATP and special substrate of it.Results: There are no differences among IR, TZD and CTL group in plasma glucose. Plasma insulin was significantly higher in IR group than in CTL group, but no difference between TZD and CTL group. Insulin resistance, which was calculated by HOMA-IR, was significantly higher in IR than in TZD and CTL group. Tau protein is hyperphosphorylated at several AD-related phosphorylation sites (Ser199, Thr212, Ser214, Ser396 and Ser422) in IR group as well as Ser199, Thr212, Ser396 and Ser422 in TZD group, but no difference between TZD and CTL group in Thr214. The activity of GSK-3βwas found to be increased in the brains of rats in both IR and TZD group. Conclusions: These findings suggest: (1) insulin resistance increased the probality of AD by downregulation of insulin signal transduction and the consequent upregulation of GSK-3β, which leads to hyperphosphorylation od tau protein in insulin resistant rats. (2) The hyperphosphorylation of tau could be lower by Rosiglitazone, but do not through the pathway of lower the activities of GSK-3β.