I1PP2A affects Tau phosphorylation via association with the catalytic subunit of protein phosphatase 2A

Abstract

In Alzheimer disease (AD) brain, the level of I1PP2A, a 249-amino acid long endogenous inhibitor of protein phosphatase 2A (PP2A), is increased, the activity of the phosphatase is decreased, and the microtubule-associated protein Tau is abnormally hyperphosphorylated. However, little is known about the detailed regulatory mechanism by which PP2A activity is inhibited by I1PP2A and the consequent events in mammalian cells. In this study, we found that both I1PP2A and its N-terminal half I1PP2A(1-120), but neither I 1PP2A(1-163) nor I1PP2A(164-249), inhibited PP2A activity in vitro, suggesting an autoinhibition by amino acid residues 121-163 and its neutralization by the C-terminal region. Furthermore, transfection of NIH3T3 cells produced a dose-dependent inhibition of PP2A activity by I1PP2A. I1PP2A and PP2A were found to colocalize in PC12 cells. I1PP2A could only interact with the catalytic subunit of PP2A (PP2Ac) and had no interaction with the regulatory subunits of PP2A (PP2A-A or PP2A-B) using a glutathione S-transferase-pulldown assay. The interaction was further confirmed by coimmunoprecipitation of I1PP2A and PP2Ac from lysates of transiently transfected NIH3T3 cells. The N-terminal isotype specific region of I1PP2A was required for its association with PP2Ac as well as PP2A inhibition. In addition, the phosphorylation of Tau was significantly increased in PC12/Tau441 cells transiently transfected with full-length I 1PP2A and with PP2Ac-interacting I1PP2A deletion mutant 1-120 (I1PP2AΔC2). Double immunofluorescence staining showed that I1PP2A and I1PP2AΔC2 increased Tau phosphorylation and impaired the microtubule network and neurite outgrowth in PC12 cells treated with nerve growth factor. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.