Effect Of Excitatory Neurotransmission On Tau Expression And Phosphorylation, And Its Role In Alzheimer's Disease

Excitatory neurotransmission is the basic function of neurons; N-Methyl-D-aspartate receptor (NMDAR) is one of the predominant excitatory ionotropic receptors in human brain, which plays important roles in learning and memory, synaptic plasticity, and neuronal development. However, excessive activation of NMDA receptors-induced glutamate accumulation in synaptic cleft also causes neuronal toxicity, which is now realized to be an upstream factor of many neurodegenerative diseases. Alzheimer's disease is the most common disease causing adult dementia; abnormal tau phosphorylation and accumulation is the characteristic pathological changes in AD brain. Till now the relationship between excitatory neurotransmission and tauopathy has not yet been elucidated. In the present study, we explored the effect of excitatory neurotransmission on tau expression and phosphorylation, and its role in AD-like neurodegeneration.In part I, we explored the changes of tau phosphorylation in neurons with short term activation of synaptic transmission. Nearly half of the glutamatergic neurons are "gluzinergic" neurons, which release glutamate and zinc simultaneously upon excitation. Our results suggest that synaptically released zinc promotes tau hyperphosphorylation through protein phosphatase2A (PP2A) inhibition. This finding explains the possible underlying mechamism for that location of zinc containing glutamatergic neurons is largely overlapped with neurofibrillary tangles (NFTs, which is formed by hyperphosphorylated tau) in AD susceptible brain regions such as hippocampus, amygdala, entorhinal and temporal cortex.In part II, we explored the effect of chronic activation of NMD A receptors, especially extrasynaptic NMDA receptors (E-NMDAR) on tau protein expression. We reported for the first time that prolonged activation of E-NMDAR dramatically increased total, phosphorylated and dephosphorylated tau levels, with simultaneous increase of tau mRNA level, while chronic activation of synaptic NMDAR showed no effect on tau protein expression. Activation of E-NMDA receptors in tau knockout mice neurons resulted in less neuronal death, increased level of survival signaling molecules such as active extracellular signal-regulated kinase (ERK) and phosphorylated CREB. These results indicate that tau overexpression mediates the neuronal toxicity of E-NMDAR activation. E-NMDAR may be a promising therapeutic target for AD and related tauopathy.In a summary, the present study focused on the effects of short or long term activation of glutamatergic synaptic transmission on tau expression and phosphorylation, the results disclosed new clues for the role of excessive activation of NMDA receptors in AD pathogenesis.Part â… Synaptic released zinc promotes tau hyperphosphorylation by inhibition of PP2AHyperphosphorylated tau is the major component of neurofibrillary tangles in Alzheimer's disease (AD), and the tangle distribution is largely overlapped with zinc containing glutamatergic neurons, suggesting that zinc released in synaptic terminals may play a role in tau phosphorylation. To explore this possibility, we treated the cultured hippocampal slices or primary neurons with glutamate or Bic/4-AP to increase the synaptic activity with or without pre-treatment of zinc chelators, and then detected the phosphorylation levels of tau. We found that glutamate or Bic/4-AP treatment caused tau hyperphosphorylation at multiple AD-related sites, including Ser396, Ser404, Thr231, and Thr205, while application of intracellular or extracellular zinc chelators, or blockade of zinc release by extracellular calcium omission almost abolished the synaptic activity-associated tau hyperphosphorylation. The zinc release and translocation of excitatory synapses in the hippocampus was detected, and zinc-induced tau hyperphosphorylation was also observed in cultured brain slices incubated with exogenously supplemented zinc. Tau hyperphosphorylation induced by synaptic activity was strongly associated with inactivation of protein phosphatase2A (PP2A), and this inactivation can be reversed by pre-treatment of zinc chelator. Together, these results suggest that synaptically released zinc promotes tau hyperphosphorylation through PP2A inhibition. Part â…¡Effect of extrasynaptic NMDA receptors on tau protein expression and its role in neurodegenerationSynaptic and extrasynaptic NMDA receptors (E-NMDAR) play distinct roles in neuronal plasticity, gene regulation, and neuronal survival. Activation of E-NMDA receptors may induce neuronal degeneration and neuronal death, however, the underlying mechanisms remain elusive.In the present study,, we reported for the first time that in cultured primary cortical neurons, activation of E-NMDA receptors dramatically increased the protein levels of total, phosphorylated and dephosphorylated tau, as well as the tau mRNA levels, which was accompanied with axonal degeneration and decreased neuronal survival. Memantine, a selective antagonist of E-NMDA receptors, reversed E-NMDAR-induced tau expression. Activation of E-NMDA receptors in tau knockout mice neurons resulted in less neuronal death, increased survival signaling molecules such as active extracellular signal-regulated kinases (ERk) and phosphorylated CREB.These results suggested that tau overexpression may mediate the toxicity of E-NMDA receptor activation; selective targeting E-NMDA receptor may be a promising therapeutic strategy for tauopathy like Alzheimer's Disease (AD), and tau may also be a target for the development of therapy for neuronal excitoxicity.