Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant neurodegenerative disorder caused by polyglutamine-expanded ataxin-3. SCA3 neurodegeneration is found in the pontine nuclei and cerebellum. Polyglutamine-expanded ataxin-3-Q79 caused apoptotic death of cerebellar and pontine nuclei neurons by upregulating mRNA expression of pro-apoptotic Bax and activating mitochondria-mediated apoptotic cascade. Following various cellular stresses, transcription factor p53 promotes apoptotic neuronal death by enhancing the transcription of pro-apoptotic genes including Bax and PUMA. In the present study, cellular and animal models of SCA3 were used to test the hypothesis that mutant polyglutamine ataxin-3 upregulates Bax expression of cerebellar and pontine nuclei neurons by augmenting transcriptional activity of p53. Electrophoretic mobility shift assay (EMSA) indicated that p53 binding activity to Bax promoter sequence was significantly enhanced in cultured cerebellar neurons expressing mutant ataxin-3-Q79 and pontine nuclei and cerebellum of SCA3 transgenic mice expressing ataxin-3-Q79. The mRNA level of PUMA, a p53-inducible pro-apoptotic gene, was increased in the cerebellum and pontine nuclei of SCA3 transgenic mice and cultured cerebellar neurons expressing ataxin-3-Q79. Mutant polyglutamine ataxin-3 increased the protein level of active phospho-p53(Ser15) in cerebellar and pontine nuclei neurons without affecting mRNA or protein level of p53. Intraperitoneal administration of p53 inhibitor pifithrin-α significantly ameliorated neuronal death in the pontine nuclei of SCA3 transgenic mice. Our results suggest that polyglutamine-expanded ataxin-3 upregulates mRNA expression of Bax and PUMA and causes apoptotic death of affected neurons by enhancing phosphorylation and transcriptional activity of p53.
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