Elsevier

Neurobiology of Disease

Volume 14, Issue 3, December 2003, Pages 557-566
Neurobiology of Disease

Regular article
Atypical role of proximal caspase-8 in truncated Tau-induced neurite regression and neuronal cell death

https://doi.org/10.1016/j.nbd.2003.08.017Get rights and content

Abstract

Abnormal Tau protein is known to be closely associated with several neurodegenerative diseases. Previously, we showed that Tau was cleaved by caspase-3 to generate the cleavage product lacking the C-terminus (ΔTau-1) during neuronal cell death. Here we characterized caspase-8-dependent neurotoxicity of the truncated Tau. Introduction of ΔTau-1 into primary hippocampal neurons induced loss of neurites in a caspase-dependent manner. Caspase-8 and -6 were proteolytically activated during ΔTau-1-triggered neuronal cell death, which was suppressed by IETD-fmk, caspase-8 inhibitor. Direct targeting of caspase-8 and its associated FADD with antisense approaches and transient expression of their dominant-negative mutants reduced ΔTau-1-induced apopotosis. Cells deficient in caspase-8, but not caspase-3, became sensitized to ΔTau-1-mediated toxicity upon reconstitution with caspase-8. In addition, ectopic expression of mitochondrial antiapoptotic Bcl-2, Bcl-XL, or inactive caspase-9 short form suppressed ΔTau-1 toxicity. These results suggest that the truncated Tau protein activates proximal caspase-8 through FADD as a necessary step leading to neuronal cell death and neurite regression, contributing to the progression of abnormal Tau-associated neurodegeneracy.

Introduction

Abnormal regulation of apoptosis has been observed in a number of clinical conditions including autoimmunity, cancer, and neurodegenerative diseases such as Alzheimer's disease (AD) and Huntington's disease Portera-Cailliau et al., 1995, Ugolini et al., 1997, O'Reilly and Strasser, 1999, Tatton and Olanow, 1999. Such observation of apoptosis leads to a hypothesis that programmed cell death may play a critical role during AD pathogenesis (Kitamura et al., 1998). While there has been no clear evidence of apoptosis or activation of caspase in animal models of AD (Vila and Przedborski, 2003), evidence for the involvement of apoptosis in the human AD pathogenesis, such as the fragmentation of nuclear DNA, activation of caspase-3, -8, and -9, and cleavage of APP by caspase-3, has been accumulated Su et al., 1994a, Su et al., 1994b; Gervais et al., 1999, Selznick et al., 1999, Rohn et al., 2001b, Rohn et al., 2002.

Tau, a microtubule-binding protein, plays a key role in microtubule stabilization and axon growth. Abnormally modified Tau protein is known as a pathogen in several neuronal degenerative diseases including AD Goedert, 1993, Novak et al., 1993, Reynolds et al., 1997, Canu et al., 1998, Wang et al., 1998, Garcia and Cleveland, 2001. Development of neurofibrillary tangles closely correlates with the pathology of AD, while it is not clear whether the tangles are the initial lesions or by-products of nerve cell injury (Goedert, 1993). As a major component of the paired helical filaments (PHF) in the neurofibrillary tangles, the C-terminus of Tau appears to contribute to microtubule organization and PHF core formation in vitro Kondo et al., 1988, Lee et al., 1989, Brandt and Lee, 1993, Yanagawa et al., 1998. Recently, it was reported that Tau was cleaved by caspase-3 and calpain during neuronal apoptosis in vitro Canu et al., 1998, Fasulo et al., 2000, Chung et al., 2001. A truncated form of Tau was also found in the brains of Alzheimer's patients (Novak, 1994). It is thus conceivable that the truncated or abnormal Tau protein may modulate cell death which is associated with dementia such as AD and non-AD neurofibrillary diseases (Garcia and Cleveland, 2001). However, a death-signaling pathway triggered by such abnormal Tau is unknown.

In this study, we found that the truncated Tau-induced cell death employed caspase-8 as a major player. In vitro functional characterization analysis showed that forced expression of the truncated Tau protein caused caspase-dependent neurite regression of the primary cultured neurons. Unexpectedly, proximal caspase-8 and its adaptor FADD were required in the ΔTau-1-triggered cell death and the signaling of the Tau toxicity was transduced to caspase-9 by a mitochondrial event-dependent process. These results imply an atypical role of caspase-8 in abnormal Tau-associated neuronal degeneracy.

Section snippets

Materials

Caspase inhibitors, Ile-Glu-Thr-Asp-fluoromethylketone (IETD-fmk), Val-Glu-Ile-Asp-fluoromethylketone (VEID-fmk), Tyr-Val-Ala-Asp-fluoromethylketone (YVAD-fmk), acetyl-Asp-Glu-Val-Asp-chloromethylketone (DEVD-cmk), Leu-Glu-His-Asp-fluoromethylketone (LEHD-fmk), and acetylz-Val-Ala-Asp-floromethylketone (z-VAD-fmk) were purchased from Bachem (Torrance, CA). Okadaic acid and calpeptin were from Sigma (St. Louis, MO).

Antibodies

Anti-caspase-2, caspase-3, caspase-6, caspase-9, and caspase-10 antibodies were

Truncated tau protein induces caspase-dependent neurite loss of hippocampal neurons

We have previously shown that Tau was cleaved at residue 421 by caspase-3 and the 50-kDa cleavage product was proapoptotic (Chung et al., 2001). To characterize the toxic effects of the truncated Tau, rat hippocampal neuron cultures were prepared from gestation stage E17, maintained in vitro for 6 days, and then transiently transfected with GFP, Tau-GFP, or ΔTau-1-GFP expression plasmid (Fig. 1A). Typical morphology of the hippocampal neurons expressing ΔTau-1-GFP appeared shrunk as compared

Acknowledgements

This work was supported by a grant (HMP-01-PJ8-PG6-01NE01-0003) of the 01 Good Health R&D Program, Korean Ministry of Health and Welfare.

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