Glycogen Synthase Kinase 3β Induces Caspase-cleaved Tau Aggregation in Situ

Jae-Hyeon Cho,Gail V.W Johnson

doi:10.1074/jbc.m403364200

Jae-Hyeon Cho, Gail V.W Johnson

Open Access

https://doi.org/10.1074/jbc.m403364200

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Abstract

Tau is a substrate of caspases, and caspase-cleaved tau has been detected in Alzheimer's disease brain but not in control brain. Furthermore, in vitro studies have revealed that caspase-cleaved tau is more fibrillogenic than full-length tau. Considering these previous findings, the purpose of this study was to determine how the caspase cleavage of tau affected tau function and aggregation in a cell model system. The effects of glycogen synthase kinase 3 beta (GSK3 beta), a well established tau kinase, on these processes also were examined. Tau or tau that had been truncated at Asp-421 to mimic caspase cleavage (Tau-D421) was transfected into cells with or without GSK3 beta, and phosphorylation, microtubule binding, and tau aggregation were examined. Tau-D421 was not as efficiently phosphorylated by GSK3 beta as full-length tau. Tau-D421 efficiently bound microtubules, and in contrast to the full-length tau, co-expression with GSK3 beta did not result in a reduction in the ability of Tau-D421 to bind microtubules. In the absence of GSK3 beta, neither Tau-D421 nor full-length tau formed Sarkosyl-insoluble inclusions. However, in the presence of GSK3 beta, Tau-D421, but not full-length tau, was present in the Sarkosyl-insoluble fraction and formed thioflavin-S-positive inclusions in the cell. Nonetheless, co-expression of GSK3 beta and Tau-D421 did not result in an enhancement of cell death. These data suggest that a combination of phosphorylation events and caspase activation contribute to the tau oligomerization process in Alzheimer's disease, with GSK3 beta-mediated tau phosphorylation preceding caspase cleavage.

Highlights

In addition to A␤ deposition, the accumulation of polymeric filaments of tau as intracellular neurofibrillary tangles is an essential feature of Alzheimer’s disease brain
Tau or tau that had been truncated at Asp-421 to mimic caspase cleavage (Tau-D421) was transfected into cells with or without GSK3␤, and phosphorylation, microtubule binding, and tau aggregation were examined
Truncation of Tau at Asp-421 Tau Attenuates GSK3␤-mediated Phosphorylation—To mimic caspase cleavage, tau constructs truncated at Asp-421 were made from T4L and T4 [11] and are referred to as T4L-D421 and T4-D421, respectively

Summary

Introduction

In addition to A␤ deposition, the accumulation of polymeric filaments of tau as intracellular neurofibrillary tangles is an essential feature of Alzheimer’s disease brain. In these studies tau with exons 2, 3, and 10 (T4L) and tau without exons 2 and 3 (T4) were truncated at Asp-421 to mimic caspase cleavage (T4L-D421 and T4-D421, respectively) Both T4L-D421 and T4-D421 were not phosphorylated as efficiently by GSK3␤ as the full-length tau constructs both in situ and in vitro. Even in the presence of GSK3␤, full-length tau was not detected in the Sarkosyl-insoluble fraction In this model system expression of T4L-D421 or T4-D421 tau did not result in an increase in cell death. These data demonstrate for the first time that a combination of phosphorylation events and caspase cleavage results in formation of Sarkosyl-insoluble tau aggregates in an in situ model system

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