PHOSPHATASE INHIBITION IN HUMAN NEUROBLASTOMA-CELLS ALTERS TAU ANTIGENICITY AND RENDERS IT INCOMPETENT TO ASSOCIATE WITH EXOGENOUS MICROTUBULES

The abnormal cytoskeletal organization observed in Alzheimer's disease has been suggested to arise from hyperphosphorylation of tau and the resultant elimination of its ability to associate with microtubules, T his possibility has been supported by a number of studies under cell-f ree conditions utilizing various kinases, phosphatases and their corre sponding inhibitors each, and by treatment of intact cells with kinase and phosphatase activators and inhibitors, However, in studies utiliz ing intact cells, it remained difficult to attribute microtubule compr omise specifically to tau hyperphosphorylation due to potential influe nce of inhibitors on tubulin and/or other microtubule-associated prote ins, which themselves possess assembly-regulatory phosphorylation site s, To address this difficulty, we subjected SH-SY-5Y human neuroblasto ma cells to treatment with the phosphatase inhibitor okadaic acid (OA) , which has been previously demonstrated to depolymerize microtubules in these cells, OA induced an increase in tau hyperphosphorylation as evidenced by an increase in Alz-50 immunoreactivity and a correspondin g decrease in Tau-l immunoreactivity, When tau-enriched fractions from OA-treated cells were incubated under microtubule assembly-promoting conditions with twice-cycled, tau-free preparations of bovine brain tu bulin not exposed to OA, Alz-50-immunoreactive tau isoforms displayed a marked (49%) reduction in ability to co-assemble with bovine microtu bules as compared with Tau-land 5E2-immunoreactive isoforms, These dat a indicate that hyperphosphorylated tau has a reduced capacity to asso ciate with microtubules, and support the hypothesis that tau hyperphos phorylation may underlie microtubule breakdown in Alzheimer's disease.