BIPHASIC EFFECTS OF PHOSPHATASE INHIBITION ON ACCUMULATION OF TAU-PHOSPHO-ISOFORMS IN CULTURED CEREBELLAR NEURONS

Treatment of murine cerebellar cultures with the phosphatase inhibitor okadaic acid (OA) for 6-72hrs induced a time-and concentration-depend ent increase in phospho-isoforms of tau as ascertained by PHF-1 and AL Z-50 immunoreactivity and degeneration of cultures. The effects of OA were prevented, or attenuated at higher OA concentrations, by co-treat ment with the kinase inhibitor staurosporine (100nm). PHF-1 levels of cultures treated for 6hr with 50nm OA declined to those of controls wi thin 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 dephosph orylation of both tau and certain kinases that are themselves activate d by phosphorylation. These effects may respectively account for the b iphasic response observed in the present study: i.e., the immediate in crease 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 perpetuati on of (e.g.) MAP and/or CaM kinase activity. These findings underscore that disruption of the balance of kinase and phosphatase activities c an exert both chronic as well as acute effects on cellular constituent s including tau.