Inhibition of protein phosphatases and regulation of tau phosphorylation in rat brain

Previous studies have shown that the activities of phosphoseryl/phosphothreonyl protein phosphatases (PP), especially PP-2A and PP-1 are compromised and the microtubule associated protein tau is abnormally hyperphosphorylated in Alzheimer's disease (AD) brain. In this study we investigated the role of PP-2A and PP-2B in the regulation of tau phosphorylation in rat brain, in rats infused for one month with okadaic acid, a PP-2A/PP-1 inhibitor, hyperphosphorylation of tau was detected at Ser 198/199/202 and Ser 396/404 while inhibition of PP-2B by cyclosporin A resulted in an increase in tau phosphorylation at Ser 262/356 and Ser 396/404. In both cases, abnormally phosphorylated tau was detected mainly in the mossy fibers and the pyramidal cells of the CA3 sector of the hippocampus by immunocytochemistry. In rats injected with a single dose of okadaic acid into the left ventricle, PP-2A activity was inhibited by about 50% whereas no significant decrease in PP-1 activity was observed. The inhibition of PP-2A was observed 3 hr following the injection and was maintained up to 24 hr, although slightly decreased. An increase in tau phosphorylation was detected at Ser 262/356 and Ser 396/404 three hours after the injection, and was considerably decreased 24hr later. In metabolically active rat brain slices treated with fostriecin, a less toxic PP-2A/PP-1 inhibitor, tau was found to be hyperphosphorylated at Ser 198/199/202 and Ser 262/356. These studies demonstrate (i) that the phosphorylation of tau at Ser 198/199/202 is regulated by PP-2A, and at Ser 262/356 and Ser 396/404 by both PP-2A and PP-2B; and (ii) that the metabolically active rat brain slices represent a new useful model, as an alternative to live animals, to study the regulation of the phosphorylation of tau.