Tau phosphorylation and neuronal apoptosis induced by the blockade of PP2A preferentially involve GSK3β

Overactivation of GSK3 beta (glycogen synthase kinase-3 beta) and downregulation of PP2A (protein phosphatase-2A) have been proposed to be involved in the abnormal tau phosphorylation and aggregation in Alzheimer's disease (AD). GSK3 beta and PP2A signaling pathways were reported to be interconnected. Targeting tau kinases was suggested to represent a therapeutic strategy for AD. Here, tau phosphorylation and neuronal apoptosis were induced in cortical cultured neurons by the inhibition of PP2A by okadaic acid (OKA). In this in vitro model of 'tau pathology' and neurodegeneration, we tested whether GSK3 beta and other tau kinases including DYRK1A and CDK5 were implicated. Our results show that the inhibitors of GSK3 beta, lithium and 6-BIO (6-bromoindirubin-3'-oxime), prevented OKA-induced tau phosphorylation and neuronal apoptosis. The implication of GSK3 beta in these OKA-induced effects was confirmed by its silencing by hairpin siRNA. By contrast, inhibition of DYRK1A (dual-specificity tyrosine-phosphorylation regulated kinase-1A) and CDK5 (cyclin-dependent kinase-5) reversed OKA-induced tau phosphorylation at certain sites but failed to prevent neuronal apoptosis. These results indicate that OKA-induced effects, especially neuronal apoptosis, are preferentially mediated by GSK3 beta. Furthermore, since chronic exposure to lithium and 6-BIO might be deleterious for neurons, we tested the effect of a new 6-BIO derivative, 6-BIBEO (6-bromoindirubin-3'(2-bromoethyl)-oxime), which is much less cytotoxic and more selectively inhibits GSK3 beta compared to lithium and 6-BIO. We show that 6-BIBEO efficiently reversed OKA-induced tau phosphorylation and neuronal apoptosis. It will be interesting to test neuroprotection by 6-BIBEO in an in vivo model of tau pathology and neurodegeneration. (C) 2011 Elsevier B.V. All rights reserved.