C-terminal truncation of GSK-3β enhances its dephosphorylation by PP2A.

Glycogen synthase kinase-3β (GSK-3β) is the major tau kinase. Its phosphorylation at Ser9 suppresses the activity. In Alzheimer's disease (AD) brain, GSK-3β is truncated at the C terminus by overactivated calpain I, leading to an increase in its activity. However, the effect of truncation on its phosphorylation is unknown. We found here that in AD brain and in cultured cells, C-terminally truncated GSK-3β is less phosphorylated at Ser9 than the full-length enzyme. The truncation promotes GSK-3β nuclear translocation and enhances its interaction with protein phosphatase 2A (PP2A), leading to dephosphorylation. Thus, the truncation of GSK-3β may enhance its activity through Ser9 dephosphorylation by PP2A. Our findings shed new light on the role of calpain-GSK-3β-PP2A in tau pathogenesis of AD.