Role Of GSK-3 In The Pathogenesis Of Alzheimer's Disease And Effective Prevention

Alzheimer's disease (AD) is the most popular dementia in aged people. The main pathological hallmarks in AD are the formation of intracellular neurofibrillary tangles consisted of hyperphosphorylated tau, extracellular senile plaques consisted ofβ-amyloid (Aβ) peptide, neuronal loss caused by dystrophic neuritis. The main character of AD in clinical is progressive memory loss. Previously studies had revealed that the dysfunction of synaptic plasticity is the primary reason for learning/memory deficits in AD.Glycogen synthase kinase-3 (GSK-3) is a crucial protein kinase which is necessary for the development and maturity of neuron. It is also known that GSK-3 plays an important role in the pathogenesis of AD, such as induces tau hyperphosphorylation, increases the production of Aβand leads to AD-like learning/memory deficits. However, the mechanisms of GSK-3 induce AD-like learning/memory deficits is still unknown.In the present study, we aimed to investigate the role of GSK-3 in synaptic plasticity, especially on the effect of GSK-3 activation to LTP in our previously reported animal model with AD-like learning/memory deficits. We also explore the underlying mechanisms through the changing of presynaptic neurotransmitter releasing, axonal transport, morphological changes in post-synpase and protein synthesis. The main results are as following:Part I Activation of glycogen synthase kinase-3 inhibits long term potentiation with synapse-associated impairmentsActivation of glycogen synthase kinase-3 (GSK-3) can cause memory deficits as seen in Alzheimer's disease (AD), the most common age-associated dementia, but the mechanism is not understood. Here, we found that activation of GSK-3 by wortmannin or transient overexpression of wild type GSK-3β(wt-GSK-3β) could suppress the induction of long term potentiation (LTP) in rat hippocampus, while simultaneous inhibition of GSK-3 by lithium or SB216763 or transient expression of a dominant negative GSK-3βmutant (dn-GSK-3β) preserved the LTP. With activation of GSK-3, prominent LTP-associated synapse impairments including less presynaptic active zone, thinner postsynaptic density (PSD) and broader synaptic cleft were observed in the hippocampal slices after high frequency stimulation (HFS). In presynaptic level, the release of glutamate and the expression/clustering of synapsin I, a synaptic vesicle protein playing an important role in neurotransmitter release, decreased markedly upon upregulation of GSK-3. In vitro studies further demonstrated that GSK-3 inhibited the expression of SynI independent of HFS. In postsynaptic level, the expression of PSD93 and NR2A/B proteins decreased significantly when GSK-3 was activated. These synaptic impairments were attenuated when GSK-3 was simultaneously inhibited by LiCl or SB216763 or transient expression of dnGSK-3. We conclude that upregulation of GSK-3 impairs the synaptic plasticity both functionally and structurally, which may underlie the GSK-3-involved memory deficits.Part II Activation GSK-3 retards calcium dependent exocytosis and the dissociation of synaptophysin I/VAMP2 complexGSK-3 plays an important role in the development and maturation of neurons, but it is still not clear about its role in neuronal physiological functions. We found in this study that overexpressing wild type GSK-3β(wt-GSK-3β) inhibits the exocytosis in hippocampus neurons, and this inhibition mainly on the full-fusion style of vesicles. We also found that upregulation of GSK-3 phosphorylated the synprint site of P/Q calcium channel and reduced the calcium influx to trigger exocytosis. Moreover, the reduction in calcium concentration in presynapse retarded the dissociation of synaptophysin I/VAMP2 complex, which might be the molecular mechanism of exocytosis inhibition by GSK-3 activation. We conclude that upregulation GSK-3 can inhibit exocytosis through phosphorylating calcium channel in presynapse and decreasing calcium influx, indicating dysfunction of vesicle associated proteins. Part III Activation GSK-3 inhibits anterograde axonal transport of VAMP2 mediated by kinesin light chain and its underlying mechanismsGSK-3 is one of important kinase which is crucial in the development and mature in the neurons. Previously study had revealed that activated GSK-3 negative regulation axonal transport through phosphorylation kinesin light chain. We found in this study that overexpressing GSK-3 retards the axonal transport of VAMP2 and decreases the level of VAMP2 in distal axon. We also found that the inhibition of GSK-3 to axonal transport mainly on the inhibition to expression of KLC. We conclude that GSK-3 retards anterograde axonal transport via decreasing the expression of KLCPart IV 17β-estradiol attenuates tau hyperphosphorylation through glycogen synthase kinase-3βinhibition independent of protein kinase BDecline of estrogen is associated with a higher incidence of Alzheimer's disease (AD), the most common age-associated dementia characterized pathologically with formation of numerous neurofibrillary tangles and senile plaques. The major components in the tangles and plaques are respectively the hyperphosphorylated microtubule-associated protein tau andβ-amyloid (Aβ). Previous studies have demonstrated that estrogen can efficiently attenuate Aβ-induced toxicities. However, the effect of estrogen on tau phosphorylation and the underlying mechanisms are elusive. Here, we treated the neuro2A (N2a) cells with wortmannin (Wort) and GF-109203X (GFX) to activate glycogen synthase kinase-3 (GSK-3) and thus to induce tau hyperphosphorylation. We found that 17β-estradiol (βE2) could attenuate Wort/GFX-induced tau hyperphosphorylation at multiple AD-related sites, including Ser396/404, Thr231, Thr205, Ser199/202. Simultaneously, it increased the level of the Ser9-phosphorylated (inactive) GSK-3β. The effect ofβE2 on GSK-3βwas confirmed by transient overexpression of GSK-3β. To study whether the protective effect ofβE2 on GSK-3β and tau phosphorylation involves protein kinase B (Akt), an upstream effector of GSK-3, we transiently expressed the dominant negative Akt (dnAkt) in the cells. The results showed thatβE2 could attenuate Wort/GFX-induced GSK-3βactivation and tau hyperphosphorylation with Akt-independent manner. It is suggested thatβE2 may arrest AD-like tau hyperphosphorylation through directly targeting GSK-3β.