Treatment of murine cerebellar cultures with the phosphatase inhibitor okadaic acid (OA) for 6-72hrs induced a time-and concentration-dependent increase in phospho-isoforms of tau as ascertained by PHF-1 and ALZ-50 immunoreactivity and degeneration of cultures. The effects of OA were prevented, or attenuated at higher OA concentrations, by co-treatment with the kinase inhibitor staurosporine (1OOnm). PHF-1 levels of cultures treated for 6hr with 50nm OA declined to those of controls within 6hr after OA removal of OA. However, by 24hr after OA removal, a resurgence in increased PHF-1 immunoreactivity was observed. Previous studies indicate that OA treatment can be expected to prevent dephosphorylation of both tau and certain kinases that are themselves activated by phosphorylation.
These effects may respectively account for the biphasic response observed in the present study: i.e., the immediate increase in PHF-1 immunoreactivity may be derived from inhibition of tau dephosphorylation, while the delayed accumulation of PHF-1 may result from heightened tau phosphorylation fostered by OA-induced perpetuation of (e.g.) MAP and/or CaM kinase activity. These findings underscore that disruption of the balance of kinase and phosphatase activities can exert both chronic as well as acute effects on cellular constituents including tau.