The phosphatase inhibitor okadaic acid induces a phosphorylated paired helical filament tau epitope in human LA-N-5 neuroblastoma cells.

Recently, a mitogen activated protein kinase has been implicated in the generation of a phosphorylated paired helical filament (PHF) epitope recognized by the monoclonal antibody AT8. This epitope consists of phosphorylated serines 199 and/or 202 of the human microtubule associated protein tau. Theoretically, aside from abnormal kinase activity, inhibition of phosphatase activity could also be involved in the abnormal phosphorylation status of the microtubule associated protein tau. To investigate this, we incubated LA-N-5 neuroblastoma cells with okadaic acid, a specific inhibitor of phosphatase 2A. We found that incubating neuroblastoma cells with okadaic acid induces the abnormally phosphorylated AT8 epitope. The effect of okadaic acid is time and dose dependent and is reversible. Our findings suggest that phosphatase activity is important in the regulation of the phosphorylation state of tau. Phosphatases may act directly on tau or may influence the activity of mitogen activated protein kinase. Incubation of LA-N-5 neuroblastoma cells with okadaic acid provides a cellular model in which the generation of a well-defined PHF-tau epitope can be investigated.