The Mechanisms Involved In β-amyloid-induced Cell Apoptosis And The Antiapoptotic Effect Of Tau Proteins

Part 1 Tau protein antagonizes cell apoptosis through Akt pathwayTwo major neuropathological features of Alzheimer's disease (AD) are intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated microtubule-associated protein tau, and the extracellular senile plaques composed ofβ-amyloid (Aβ). Though exposing to a proapoptotic environment, most NFTs-bearing nourons degenerate chronically rather than go to apoptosis in AD patients and other taupathies, and the mechanisms are still elusive. Recently, we have found that tau hyperphosphorylation plays a role in anti-apoptosis of the cells. Here we investigated the underlying mechanisms involved in the protective effects of tau. We found that mouse Neuroblastoma 2a (N2a) cells overexpressing tau were more resistent to serum withdrawal-induced apoptosis than the wild type cells. The protective effect was positively correlated with the relatively increased phosphrylation level of tau. We also found that the Akt was activated in cells overexpressing tau. Simultaneously, the phosphorylation level of FOXO3 increased (representing decreased activity) and the expression of p53 and Bax decreased. The antiapoptotic effects of tau almost disappeared by inhibition of Akt. Upon serum starvation, the expression level of catalytic subunit of PI3K (p110) increased significantly in cells overexpressing tau compared with the control cells, whereas the relative level of the phosphorylated p85 was not changed. Additonally, we also detected tau proteins in the nuclear fraction of the cells, and starvation led to a significantly decreased level of nuclear tau proteins. Our findings suggest that tau leads the cells to escape apoptosis by activating PI3K/Akt pathway and the activation of PI3K involves the decreased level of nuclear tau proteins. Part 2β-?amyloid induces neuronal apoptosis with Akt/FOXO3/Bim-involved pathwayOne of the major neuropathological features of Alzheimer's disease (AD) is the formation of extracellular senile plaques, which are composed of beta-amyloid (Aβ). Aβi?s neurotoxic and it causes neuron apoptosis. Aβcan also decrease the activity of Akt, a well-known survival factor, in vivo and in vitro. FOXO is a down-stream substrate of Akt. Phosphorylation of FOXO at Thr32 by Akt restrains the transcription activity of FOXO and thus attenuates cell apoptosis through down-regulating the expression of proapoptotic factors. Here we investigated the involvement of Akt/FOXO in Aβ-induced apoptosis by using N2a/APPswe+PS1?9 cells, which overproduce Aβ. We found that the activity of Akt was reduced in N2a/APPswe+PS1?9 compared with the wild type control cells. Simultaneously, the FOXO3 was activated and the level of Bim, a proapoptotic transcript, was elevated. Moreover, the level of the apoptosis was increased in N2a/APPswe+PS1?9 cells. These data suggest that Aβmay induce neuron apoptosis through Akt/FOXO3/Bim-involved pathway. Part 3 Melatonin arrests peroxynitrite-induced tau hyperphosphorylationand the overactivation of protein kinases in rat brainAbstract: The purpose of this study was to examine the in vivo effect of melatonin (MEL) on peroxynitrite-induced tau hyperphosphorylation and the involvement of glycogen synthase kinase-3β(GSK-3β) and mitogenactivated protein kinase (MAPK) families. Melatonin was injected into the right cerebroventricle of the rats 1 hr before the bilateral hippocampal injection of 3-morpholino-sydnonimine chloride (SIN-1), the recognized donor of peroxynitrite. Thereafter, the phosphorylation level of tau and the activity of the kinases were analyzed. The injection of SIN-1 induced hyperphosphorylation of tau at pS396 epitope with a concomitant activation of GSK-3βand selective MAPK isoforms including p38α, p38β, and p38δbut not p38γ. The effect of peroxynitrite was confirmed using uric acid, a recognized scavenger of peroxynitrite. Preinjection of MEL significantly arrested the peroxynitrite-induced hyperphosphorylation of tau and the activation of GSK-3βand MAPKs. Melatonin also ameliorated peroxynitrite-induced oxidative stress. We conclude that MEL can efficiently arrest peroxynitrite-induced tau hyperphosphorylation, and the underlying mechanism may involve scavenging the reactive species and suppressing the activated GSK-3βand p38 MAPK family. Part 4 Effect of Intracerebroventricular Injection of LPS on tau Hyperhosphorylation in Rat HippocampusAbstract Objective: To investigate the effect of lipopolysaccharide (LPS) intracerebroventricular injection on hyperphosphorylation of tau. Methods: Western blot was used to analyze the hyperphosphorylation of tau from different rats killed at 2, 4 and 8 h respectively after LPS injection. Results: (1) Hyperphosphorylation of tau protein was seen after 2 h of LPS injection, then, the tau protein was dephosphorylated. (2) Intraperitoneal injection of LPS resulted in a significant increase in total level of tau. Conclusion: LPS affects tau expression and phosphorylation.