Cell cycle arrest enhances the in vitro cellular toxicity of the truncated Machado–Joseph disease gene product with an expanded polyglutamine stretch

Abstract

Machado–Joseph disease (MJD) is an inherited neurodegenerative disorder caused by the expansion of the polyglutamine stretch in the MJD gene-encoded protein, ataxin-3. Using a series of deletion constructs expressing ataxin-3 fragments with expanded polyglutamine stretches, we observed aggregate formation and cell death in cultured BHK-21 cells. The cytotoxic effect of N-terminal-truncated ataxin-3 with the expanded polyglutamine tract was enhanced under serum starvation culture, in which cells were arrested in the G₀/G₁ phase. Coexpression of p21^{waf1/cip1/sdi1}, a cyclin–Cdk inhibitor that induced cell cycle arrest in the G₁ phase, also increased the cell death susceptibility produced by the mutant ataxin-3 fragment in BHK-21 cells. The elevated susceptibility to cell death in the G₀/G₁ phase was confirmed in nerve growth factor-treated, postmitotic neuronal PC12 cells compared with undifferentiated proliferating PC12 cells. These results strongly suggest that the cellular toxicity of truncated ataxin-3 with an expanded polyglutamine stretch is enhanced by cell cycle arrest in the G₀/G₁ phase. Mutant ataxin-3 may confer a higher susceptibility to cell death on cells in the G₀/G₁ phase.