Involvement Of PI3K/Akt Signal Pathway In Tau Protein’s Anti-apoptosis

One of the typical pathological features of Alzheimer’s disease(AD) is abnormal phosphorylated tau protein to form neurofibrillary tangles(NFTs). Studies have shown that NFTs-bearing neurons can escape rapid cell apoptosis caused by a variety of noxious stimulation, and degenerate chronically. PI3K/Akt signaling pathway is closely related to cell survival and metabolism, and participates in the onset of inflammation, tumor and degenerative disease process, but the role of PI3K/Akt signaling pathway in the mechanisms of tau protein with anti-apoptosis is not clear. Here we investigated the subcellular localization of tau protein in the rat cortical neuron and the mouse Neuroblastoma 2a(N2a) cell transfected with the full- length human tau441 isoform plasmid by immunoelectron microscopic colloidal gold technique; we checked the distribution and the expression level of several important molecules of PI3K/Akt signal pathway in total protein, cytoplasmic fraction and nuclear fraction in the N2 a cell with serum-withdrawal-starvation(simulating nociceptive stimulus) by western blotting, to explore the role and underlying mechanisms of PI3K/Akt signal in the process of the anti-apoptotic effects of tau protein. The results were as follows:(1) immunoelectron microscope colloidal gold technique showed that tau protein distributed in cytoplasm, nuclear and nuclear membrane of rat neurons and N2a(N2a/tau441) cells transiently transfected tau441. Western blot technique indicated that there was tau protein in cytoplasm, nuclear and membrane of N2a(N2a/tau441) cells.(2) the activity of Akt in the total proteins in serum-deprived wild type cells(N2a/wt), N2a/vector and N2a/tau441 cells declined compared with the control groups which were cultured with serum, and the activity of Akt in serum-deprived group which transfected tau441 declined more slightly compared with the other two groups, which suggested that when cells were stimulated by apoptosis overexpression of tau can inhibit the decline of the activity of Akt; the levels of total Akt in cytoplasm protein had no significant changes between vector group and tau441 group following serum withdrawal, but the level of p-Akt ser473 in tau441 group were significantly reduced; the levels of total Akt of tau441 group in nuclear protein slightly elevated, without statistical significance compared with the vector group, while the levels of p-Akt ser473 increased significantly, with statistical significance. this stated that when cell endured nociceptive stimulus, that overexpression of tau restrained the decline of Akt activity in total protein was mainly through increasing the nuclear Akt activity;(3) with serum starvation treatment, the level of GSK-3β(glycogen synthase kinase 3 beta, important downstream molecules of PI3K/Akt, also being one of the important kinase of tau phosphorylation in AD) of total protein is no significant difference between vector and tau441 group, but the level of p-GSK-3β ser9(inactivated GSK-3β) of tau441 group declined obviously, indicating that starvation treatment caused the GSK-3β activities of tau441 group in total protein was increased; the level of GSK-3β of cytoplasm protein in tau441 group was slightly higher, but without statistical significance, and the level of p-GSK-3β ser9 was significantly lower than that of vector group. The level of total GSK-3β of the nuclear protein in two groups was no obvious difference, and the level of p-GSK-3β ser9 of the nuclear protein in tau441 groups increased significantly. This suggested with apoptosis induction, the activities of GSK-3β in cell overexpressing tau was mainly due to that the activities of GSK-3β was increasing extremely significantly in its cytoplasm protein, and its activity in the nuclear was falling;(4) we transfected different fragments of tau plasmid-tau441, tau(1-220), tau(221-441) into N2 a cells, and the total content of Akt of three groups after serum-withdrawal-treatment has no difference,but the content of p-Akt ser473 of the tau441 or tau(1-220) plasmid group was significantly higher than that of the tau(221-441) group, while there was no obvious difference between the tau(1-220) and the tau441 groups. The total Akt had no statistical significance of difference in the cytoplasm and nuclear of the cells which transfected tau(1-220) or tau(221-441), but in the cytoplasm and nuclear protein, p-Akt ser473 level of tau(1-220) group was significantly higher than that of the tau(221-441) group, which suggested that the first half of tau–tau(1-220) containing suspected sequence which can take tau protein into the nucleus played an important role in influencing Akt activity. The above results suggested that when cells were induced by apoptosis, the overexpression of tau through inhibiting the decreasing of PI3K/Akt signaling pathway activity prevent cells from apoptosis rapidly and its mechanism was that as though harmful stimulation caused the PI3K/Akt signaling pathways in cells was suppressed, but the activity of the pathway significantly increased in the nuclear of the cell overexpressing tau. It needs further research to study how tau causes the pathway activation in the nuclear. These results provide a new research data for the pathogenesis of AD.