b-amyloid peptide accumulation, tau hyperphosphorylation, and synapse loss are characteristic neuropathological symptoms of Alzheimer's disease (AD). Tau hyperphosphorylation is suggested to inhibit the association of tau with microtubules, making microtubules unstable and causing neurodegeneration. The mechanism of tau phosphorylation in AD brain, therefore, is of considerable significance. Although PHF-tau is phosphorylated at over 40 Ser/Thr sites, Ser(262) phosphorylation was shown to mediate b-amyloid neurotoxicity and formation of toxic tau lesions in the brain. In vitro, PKA is one of the kinases that phosphorylates tau at Ser(262), but the mechanism by which it phosphorylates tau in AD brain is not very clear. 14-3-3z is associated with neurofibrillary tangles and is upregulated in AD brain. In this study, we show that 14-3-3z promotes tau phosphorylation at Ser(262) by PKA in differentiating neurons. When overexpressed in rat hippocampal primary neurons, 14-3-3z causes an increase in Ser(262) phosphorylation, a decrease in the amount of microtubule-bound tau, a reduction in the amount of polymerized microtubules, as well as microtubule instability. More importantly, the level of pre-synaptic protein synaptophysin was significantly reduced. Downregulation of synaptophysin in 14-3-3z overexpressing neurons was mitigated by inhibiting the proteosome, indicating that 14-3-3z promotes proteosomal degradation of synaptophysin. When 14-3-3z overexpressing neurons were treated with the microtubule stabilizing drug taxol, tau Ser(262) phosphorylation decreased and synaptophysin level was restored. Our data demonstrate that overexpression of 14-3-3z accelerates proteosomal turnover of synaptophysin by promoting the destabilization of microtubules. Synaptophysin is involved in synapse formation and neurotransmitter release. Our results suggest that 14-3-3z may cause synaptic pathology by reducing synaptophysin levels in the brains of patients suffering from AD.
