Mitotic-like tau phosphorylation by p25-Cdk5 kinase complex

Among tau phosphorylation sites, some phosphoepitopes referred to as abnormal ones are exclusively found on tau aggregated into filaments in Alzheimer's disease. Recent data suggested that molecular mechanisms similar to those encountered during mitosis may play a role in abnormal tau phosphorylation. In particular, TG-3 phosphoepitope is associated with early stages of neurofibrillary tangles (NFTs). In this study, we reported a suitable cell model consisting of SH-SY5Y cells stably transfected with an inducible p25 expression vector. It allows investigation of tau phosphorylation by p25-Cdk5 kinase complex in a neuronal context and avoiding p25-induced cytotoxicity. Immunoblotting analyses showed that p25-Cdk5 strongly phosphorylates tau protein not only at the AT8 epitope but also at the AT180 epitope and at the Alzheimer's mitotic epitope TG-3. Further biochemical analyses showed that abnormal phosphorylated tau accumulated in cytosol as a microtubule-free form, suggesting its impact on tau biological activity. Since tau abnormal phosphorylation occurred in dividing cells, TG-3 immunoreactivity was also investigated in differentiated neuronal ones, and both TG-3-immunoreactive tau and nucleolin, another early marker for NFT, were also generated. These data suggest that p25-Cdk5 is responsible for the mitotic-like phosphoepitopes present in NFT and argue for a critical role of Cdk5 in neurodegenerative mechanisms.