Glycogen synthase kinase-3 beta (GSK3 beta) is recognized as one of major kinases to phosphorylate tau in Alzheimer's disease (AD), thus lots of AD drug discoveries target GSK3 beta. However, the inactive form of GSK3 beta which is phosphorylated at serine-9 is increased in AD brains. This is also inconsistent with phosphorylation status of other GSK3 beta substrates, such as beta-catenin and collapsin response mediator protein-2 (CRMP2) since their phosphorylation is all increased in AD brains. Thus, we addressed this paradoxical condition of AD in rat neurons treated with okadaic acid (OA) which inhibits protein phosphatase-2A (PP2A) and induces tau hyperphosphorylation and cell death. Interestingly, OA also induces phosphorylation of GSK3 beta at serine-9 and other substrates including tau, beta-catenin and CRMP2 like in AD brains. In this context, we observed that GSK3 beta inhibitors such as lithium chloride and 6-bromoindirubin-3'-monoxime (6-BIO) reversed those phosphorylation events and protected neurons. These data suggest that GSK3 beta may still have its kinase activity despite increase of its phosphorylation at serine-9 in AD brains at least in PP2A-compromised conditions and that GSK3 beta inhibitors could be a valuable drug candidate in AD. (C) 2010 Elsevier Inc. All rights reserved.