The regulation of phosphorylation of τ in SY5Y neuroblastoma cells: the role of protein phosphatases

Abstract

In Alzheimer disease brain the microtubule associated protein (MAP) τ is abnormally hyperphosphorylated. The role of protein phosphatases (PP) in the regulation of phosphorylation of τ was studied in undifferentiated SY5Y cells. In cells treated with 10 nM okadaic acid (OA), a PP-2A/PP-1 inhibitor, the PP-1 and -2A activities decreased by 60% and 100% respectively and the activities of MAPKs, cdc2 kinase and cdk5, but not of GSK-3, increased. OA increased the phosphorylation of τ at Thr-231/Ser-235 and Ser-396/404, but not at Ser-262/356 or Ser-199/202. An increase in tyrosinated/detyrosinated tubulin ratio, a decrease in the microtubule binding activities of τ, MAP1b and MAP2, and cell death were observed. Treatment with 1 μm taxol partially inhibited the cell death. These data suggest (1) that OA induced hyperphosphorylation of τ is probably the result of activated MAPK and cdks in addition to decreased PP-2A and PP-1 activities and (2) that in SY5Y cells the OA induced cell death is associated with a decrease in stable microtubules.