Acute rise in the concentration of free cytoplasmic calcium leads to dephosphorylation of the microtubule-associated protein tau

Abstract

The objective of this study was to asses the response of the microtubule-associated protein tau to acute rise in the concentration of free cytoplasmic calcium ([Ca 2+] i) in rat cortical neurons and mouse cerebellar granule cells in culture. One-hour exposure to glutamate (100 μM), N-methyl- d-aspartate (100 μM), KCl (50 mM), and ionomycin (5 μM) led to tau protein dephosphorylation as indicated by an appearance of additional faster moving bands on Western immunoblots with a phosphorylation-independent antibody and an increase in the tau-1 immunoreactivity associated with the appearance of an additional faster moving band. Lowering the extracellular concentration of Ca 2+ to less than 1 μM fully prevented the drug-induced tau protein dephosphorylation indicating a dependence on Ca 2+ influx from the extracellular environment. Administration of okadaic acid (inhibitor of phosphatase 1/2A) simultaneously with the above mentioned drugs decreased the drug-mediated dephosphorylation. Pre-incubation with okadaic acid fully prevented the dephosphorylation. Treatment with cypermethrin (inhibitor of phosphatase 2B) was without effect when administered either alone, simultaneously with the drugs, or pre-incubated. These findings indicate that, independently of the influx pathway, [Ca 2+] i elevation leads to dephosphorylation of the microtubule-associated protein tau and implicate phosphatase 1 and/or 2A in the process.