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

Previous studies have shown that the activities of phosphoseryl/phosphothre onyl protein phosphatases (PP), especially PP-2A and PP-1 are compromised a nd 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, i n rats infused for one month with okadaic acid, a PP-2A/PP-1 inhibitor, hyp erphosphorylation of tau was detected at Ser 198/199/202 and Ser 396/404 wh ile inhibition of PP-2B by cyclosporin A resulted in an increase in tau pho sphorylation at Ser 262/356 and Ser 396/404. In both cases, abnormally phos phorylated tau was detected mainly in the mossy fibers and the pyramidal ce lls of the CA3 sector of the hippocampus by immunocytochemistry. In rats in jected with a single dose of okadaic acid into the left ventricle, PP-2A ac tivity was inhibited by about 50% whereas no significant decrease in PP-1 a ctivity was observed. The inhibition of PP-2A was observed 3 hr following t he injection and was maintained up to 24 hr, although slightly decreased. A n increase in tau phosphorylation was detected at Ser 262/356 and Ser 396/4 04 three hours after the injection, and was considerably decreased 24hr lat er. In metabolically active rat brain slices treated with fostriecin, a les s toxic PP-2A/PP-1 inhibitor, tau was found to be hyperphosphorylated at Se r 198/199/202 and Ser 262/356. These studies demonstrate (i) that the phosp horylation 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 metabolicall y active rat brain slices represent a new useful model, as an alternative t o live animals, to study the regulation of the phosphorylation of tau.