ACUTE RISE IN THE CONCENTRATION OF FREE CYTOPLASMIC CALCIUM LEADS TO DEPHOSPHORYLATION OF THE MICROTUBULE-ASSOCIATED PROTEIN-TAU

The objective of this study was to asses the response of the microtubu le-associated protein tau to acute rise in the concentration of free c ytoplasmic calcium ([Ca2+](i)) in rat cortical neurons and mouse cereb ellar granule cells in culture. One-hour exposure to glutamate (100 mu M), N-methyl-D-aspartate (100 mu M), KCl (50 mM), and ionomycin (5 mu M) led to tau protein dephosphorylation as indicated by an appearance of additional faster moving bands on Western immunoblots with a phosp horylation-independent antibody and an increase in the tau-1 immunorea ctivity associated with the appearance of an additional faster moving band. Lowering the extracellular concentration of Ca2+ to less than 1 mu M fully prevented the drug-induced tau protein dephosphorylation in dicating a dependence on Ca2+ influx from the extracellular environmen t. Administration of okadaic acid (inhibitor of phosphatase 1/2A) simu ltaneously with the above mentioned drugs decreased the drug-mediated dephosphorylation. Pre-incubation with okadaic acid fully prevented th e dephosphorylation. Treatment with cypermethrin (inhibitor of phospha tase 2B) was without effect when administered either alone, simultaneo usly with the drugs, or pre-incubated. These findings indicate that, i ndependently of the influx pathway, [Ca2+](i) elevation leads to depho sphorylation of the microtubule-associated protein tau and implicate p hosphatase 1 and/or 2A in the process.