Inhibition of proteasome and Shaggy/Glycogen synthase kinase‐3β kinase prevents clearance of phosphorylated tau in Drosophila

Tauopathies, including Alzheimer's disease (AD), are a group of neurodegenerative disorders characterized by the presence of intraneuronal filamentous inclusions of abnormally phosphorylated tau protein. In AD brains, it has been shown that the level of abnormally phosphorylated tau is higher than in age‐matched control brains, suggesting that abnormally phosphorylated tau is resistant to degradation. By using a Drosophila model of tauopathy, we studied the relationship between tau phosphorylation and degradation. We showed that in vivo reduction of proteasome activity results in an accumulation of high‐molecular‐weight forms of hyperphosphorylated tau. We also found that glycogen synthase kinase (GSK)‐3β‐mediated hyperphosphorylated forms of tau are degradable by the proteasomal machinery. Unexpectedly, GSK‐3β inactivation resulted in a very large accumulation of high‐molecular‐weight species consisting of hyperphosphorylated tau, suggesting that, depending on the kinase(s) involved, tau phosphorylation state affects its degradation differently. We thus propose a model for tauopathies in which, depending on toxic challenges (e.g., oxidative stress, exposure to amyloid peptide, etc.), abnormal phosphorylation of tau by kinases distinct from GSK‐3β leads to progressive accumulation of hyperphosphorylated tau oligomers that are resistant to degradation. © 2006 Wiley‐Liss, Inc.