OKADAIC ACID INDUCES HYPERPHOSPHORYLATION OF Tau PROTEIN IN THE HIPPOCAMPAL NEURONS OF THE RAT

Objective To explore the effect of Okadaic acid(OA),a protein phosphatase inhibitor,on Tau hyperphosphorylation and neuronal microtubule system in the rat hippocampus. Methods OA in a 1.5?μl volume was injected into unilateral dorsal part of rat hippocampal CA1 region in OA Model groups at 12h,24h,48h and 2 weeks before sample collection,respectively.DMSO treatment alone was defined as vehicle control group.The time-course-depend pathological changes and Tau protein hyperphosphorylation were observed by Bielschowsky staining,immunocytochemistry(AT8,Tau-pThr 231)and Western-blot with Tau antibodies(Tau-5,AT8,Tau-pThr 231 and Tau-pSer 396).The activity of PP2A was also assayed. Results 1.After injection with OA,the results of Bielschowsky staining showed that the axons of hippocampal neurons became anachromasis,and the thickened and other pathological changes of neurofibrils in hippocampus were observed,however,the neurofibrils were arranged in an order in the normal and vehicle groups.2.The immunochemistry staining of brain slice showed that the numbers of Tau-AT8 and Tau-pThr 231 immuno-positive stained cells in OA groups were markedly increased(P0.05)comparing to those in normal and vehicle control groups.For Western-blot detect,hyperphosphorylation of Tau at the sites of Ser 199/202,Thr 231 and Ser 396 was increased comparing to vehicle control groups.Phosphorylation of Ser 199/202(AT8)reached peak at 12h after injection,then declined at 24h,48h and 2 weeks later.Phosphorylation of Thr 231 and Ser 396 increased gradually,reached peak at 48h,and became weak at 2 weeks.The total Tau(Tau5)increased when Tau was much more phosphorylated at the sites of Thr 231 and Ser 396.3. Activity of PP2A decreased significantly after OA injection(P0.01),it came to the bottom at 48h,and recovered at 2 weeks later.Conclusion OA could induce Tau hyperphosphorylation in the hippocampal neurons of the rat brain.