Coupling of Mammalian Target of Rapamycin with Phosphoinositide 3-Kinase Signaling Pathway Regulates Protein Phosphatase 2A- and Glycogen Synthase Kinase-3β-dependent Phosphorylation of Tau

Thomas Georg Ohm,Frank Albert,Volker Meske

doi:10.1074/jbc.m704292200

Thomas Georg Ohm, Frank Albert + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m704292200

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Abstract

Tau is an important microtubule-stabilizing protein in neurons. In its hyperphosphorylated form, Tau protein loses its ability to bind to microtubules and then accumulates and is part of pathological lesions characterizing tauopathies, e.g. Alzheimer disease. Glycogen synthase kinase-3beta (GSK-3beta), antagonized by protein phosphatase 2A (PP2A), regulates Tau phosphorylation at many sites. Diabetes mellitus is linked to an increased risk of developing Alzheimer disease. This could be partially caused by dysregulated GSK-3beta. In a long term experiment (-16 h) using primary murine neuron cultures, we interfered in the insulin/phosphoinositide 3-kinase (PI3K) (LY294002 treatment and insulin boost) and mammalian target of rapamycin (mTor) (AICAR and rapamycin treatment) signaling pathways and examined consequent changes in the activities of PP2A, GSK-3beta, and Tau phosphorylation. We found that the coupling of PI3K with mTor signaling, in conjunction with a regulatory interaction between PP2A and GSK-3beta, changed activities of both enzymes always in the same direction. These balanced responses seem to ensure the steady Tau phosphorylation at GSK/PP2A-dependent sites observed over a long period of time (>/=6 h). This may help in preventing severe changes in Tau phosphorylation under conditions when neurons undergo transient fluctuations either in insulin or nutrient supply. On the other hand, the investigation of Tau protein at Ser-262 showed that interference in the insulin/PI3K and mTor signaling potentially influenced the Tau phosphorylation status at sites where only one of two enzymes (in this case PP2A) is involved in the regulation.

Highlights

Among the various Tau kinases glycogen synthase kinase-3␤ (GSK-3␤) is of particular interest
Phosphorylation at the epitopes recognized by the antibodies AT8, PHF1, and Tau-1 showed strong dependence on both the phosphatase 2A (PP2A) and the GSK-3␤ activity (Fig. 1)
Inhibition of the GSK-3␤ with lithium chloride led to a massive decline of phosphorylated Tau at AT8 and PHF1 epitope accompanied by the increase of Tau protein positive for Tau-1 antibody (Fig. 1)

Summary

Introduction

Among the various Tau kinases glycogen synthase kinase-3␤ (GSK-3␤) is of particular interest. We detected sustained changes (up to 16 h) in both the specific activity of PP2A and in the phosphorylation of activity-regulating sites of Akt (Ser-473), GSK-3␤ (Ser-9), and mTor (Ser-2448). The investigation of Tau at Ser-262 showed that interferences in the insulin/PI3K and mTor signaling potentially affects phosphorylation when only one of the enzymes, in this case the PP2A, is involved in the regulation.

Results

Conclusion