1型糖尿病葡萄糖代谢与胰岛素信号传导途径异常导致大鼠海马Alzheimer样tau蛋白过度磷酸化修饰

Tau蛋白过度磷酸化是Alzheimer病(Alzheimer disease, AD)的一个重要病理特征.采用 I 型糖尿病大鼠模型,研究胰岛素信号传导途径及葡萄糖代谢失调对tau蛋白过度磷酸化的形成机制进行探讨.以同龄Wistar大鼠做对照（CTL），胰腺大部分切除造低胰岛素组（PX），STZ较大剂量一次性注射造1型糖尿病模型即低胰岛素高血糖组（T1DM）.葡萄糖氧化酶法检测血浆血糖，放免法检测血浆胰岛素，蛋白质印迹分析海马内总tau蛋白及tau蛋白上部分位点（Ser199、Thr212、Ser214、Ser396及Ser422）的磷酸化及神经细胞膜上葡萄糖转运子3（Glucose transport 3，GLUT3）水平.γ-32P-ATP和特异性底物肽检测海马内胰岛素信号传导系统中的关键酶糖原合成酶激酶-3β（Glycogen synthase kinase-3β, GSK-3β）活性.发现3组大鼠海马回总tau蛋白水平无显著差异，但以高血糖、低胰岛素血症为特征的T1DM组在tau蛋白Ser199、Thr212、Ser214、Ser396及Ser422位点上，呈现过度磷酸化状态，以低胰岛素血症为特征而血糖正常的PX组在位点Ser199、Thr212及Ser396上磷酸化程度比CTL组显著上升, 在位点Ser214及 Ser422上的磷酸化程度的改变不显著；T1DM及PX组大鼠海马 GSK-3β活性显著高于CTL组, 而GLUT3水平在T1DM和PX组均降低, 尤以T1DM组降低更显著.研究结果显示，胰岛素水平低下可能通过激活GSK-3β和下调细胞内葡萄糖代谢的双重作用引起脑内tau蛋白过度磷酸化.

Alzheimer-like Tau Phosphorylation Induced by Dysregulated Insulin Signal Transduction and Impaired Glucose Metabolism in Type 1 Diabetes Millitus

Abnormal hyperphosphorylation of tau plays a critical role in the pathogenesis of Alzheimer disease (AD) and decreased insulin and glucose are the causes of hyperphosphorylation of tau. The roles of dysregulation of insulin signaling and glucose metabolism in hyperphosphorylation of tau in type 1 diabetic rats were sdutied. Wistar rats were randomized into 3 groups, pancreatectomy (PX), type 1 diabetes mellitus (T1DM) and control (CTL ) group. The model of PX group characterized by low plasma insulin was made by partial (90%) pancreatectomy, and the model of T1DM group was made by single injection of STZ. The plasma insulin level was measured by RIA method, and the plasma glucose by glucose-oxidase method. Total tau level, the phosphorylation level of tau at individual phosphorylation sites (Ser199, Thr212, Ser214, Ser396 and Ser422) and glucose transport 3 (GLUT3) on neuronal membrane fractions were analyzed by Western blots. The activity of glycogen synthase kinase 3β (GSK-3 β), 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. It was found that neither pancreatectomy 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, Ser214, Ser396 and Ser422) in T1DM group. In PX group, Ser199,Thr212 and Ser396 of tau protein were hyperphosphorylated, but no significant changes at Ser214 and Ser422. The activity of GSK-3β was found to be increased dramatically in the hippocampi of rats in both PX and T1DM groups. The level of GLUT3 was also found to be decreased in both PX and T1DM groups, with a more dramatic decrease in T1DM group. These findings suggest that impaired insulin signaling could facilitate hyperphosphorylation of tau via both upregulation of GSK-3β and decreased intracellular glucose metabolism.