Human neuroblastoma SH-SY5Y cells treated with okadaic acid express phosphorylated high molecular weight tau-immunoreactive protein species

Background: Early stages of Alzheimer's disease (AD) are characterized by high phosphorylation of microtubule-associated protein tau, which may result from the downregulation of protein phosphatases. New method In order to model phosphatase downregulation and analyze its effect on tau aggregation in vitro, we treated neuroblastoma SH-SY5Y cells with okadaic acid (OA), a protein phosphatase inhibitor, and examined high molecular weight phospho-tau species. Results and comparison with existing methods: OA treatment led to the appearance of heat-stable protein species with apparent molecular weight around 100 kDa, which were immunoreactive to anti-tau antibodies against phosphorylated Ser202 and Ser396. As these high molecular weight tau-immunoreactive proteins (HMW-TIPS) corresponded to the predicted size of two tau monomers, we considered the possibility that they represent phosphorylation-induced tau oligomers. We attempted to dissociate HMW-TIPs by urea and guanidine, as well as by alkaline phosphatase treatment, but HMW-TIPs were stable under all conditions tested. These characteristics resemble properties of certain sodium dodecyl sulfate (SDS)-resistant tau oligomers from AD brains. The absence of HMW-TIPs detection by anti-total tau antibodies Tau46, CP27 and Tau13 may be a consequence of epitope masking and protein truncation. Alternatively, HMW-TIPs may represent previously unreported phosphoproteins cross-reacting with tau. Conclusions: Taken together, our data provide a novel characterization of an OA-based cell culture model in which OA induces the appearance of HMW-TIPS. These findings have implications for further studies of tau under the conditions of protein phosphatase downregulation, aiming to explain mechanisms involved in early events leading to AD.