小腦脊髓運動失調症第三型(Spinocerebellar ataxia type 3, SCA3)，又名為Machado-Joseph disease (MJD)，是種自體顯性的遺傳疾病，SCA3為小腦脊髓運動失調症眾多型中發生率最高的亞型。SCA3主要為位於染色體第14對長臂(14q24.3-q31)上的MJD1基因發生CAG三核苷酸重複序列的異常擴增突變所致，進而造成轉譯出含有異常麩醯胺酸重複(polyglutanine, polyQ)區域的ataxin-3蛋白(AT3)。AT3蛋白中的polyQ區域發生的擴增現象，同樣亦被發現於八種其他polyQ相關的神經退化性疾病中，然而對於polyQ蛋白產物之致病機轉則尚未十分明瞭。目前研究發現帶有異常擴增polyQ的AT3蛋白，可造成蛋白折疊的不穩定性增加，進而在神經細胞核與細胞質中產生蛋白堆積，形成細胞內含體(inclusion bodies)。為進一步研究AT3蛋白對於SCA3造成之神經致病機轉以及作為潛力藥物篩選平台，我們建立了AT3誘導表現的PC12細胞模式。從實驗結果發現，帶有異常擴增polyQ區域的AT3蛋白經由誘導表現後，於分化後PC12細胞的細胞核內與周圍，出現聚集性的內含體，我們並且觀察到ubiquitin亦共同存在於AT3內含體中的現象，顯示ubiquitin-proteasome system (UPS)的運作與SCA3神經退化性疾病有密切的關連性。我們也發現帶有致病範圍CTG擴增的AT3蛋白，對於分化的PC12細胞會造成神經毒性，導致細胞型態分化不健全，對於氧化壓力的抵禦敏感力亦降低，進而造成細胞存活率下降。在藥物投藥劑量試驗中，我們針對組蛋白去乙醯酶抑制劑(histone deacetylase inhibitor, HDACi)及抗氧化性藥物來評估其神經保護效果，結果顯示HDACi可有效維持細胞存活率，由此可推論基因表現調控參與SCA3病理機制的重要性。依據這些研究結果，希望可藉由此AT3誘導表現細胞模式，提供更多有關於SCA3的分子致病機制資訊，亦可幫助對於polyQ相關的神經退化性疾病之了解，並作為藥物篩選平台，發展出具潛力的治療性藥物與方法。

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is the most common autosomal dominant inherited ataxia. SCA3 is caused by a CAG trinucleotide expansion in the MJD1 gene, which is located on chromosome 14q24.3-q31, and translated into a polyglutamine (polyQ) stretch containing ataxin-3 (AT3) protein. Expansion of the glutamine domain in AT3, as well as in eight other members of polyQ neurodegenerative disease family, increases protein misfolding, results in aggregation and formation of nuclear and cytoplasmic inclusions. Inclusions of polyQ proteins are ubiquitylated and contain proteasomes, suggesting an attempt by the ubiquitin proteasome system (UPS) to degrade the misfolded protein. However, little is known about the correlation between aggregate formation and cell death. To further study the role of AT3 in SCA3 neuropathology, we have established AT3-inducible PC12 cells. This cell model should allow us to characterize the aggregation of full-length AT3 protein in living cells. Our results showed that AT3 mutant protein (75Q) expression in the PC12 cells intend to cause the formation of the nuclear or peri-nuclear aggregation. Futhermore, cell survival declined with the expression of extended polyQ AT3, especially under differentiated condition induced by NGF treatment. In addition, we found that neurotoxicity of expanded AT3 (Q75) could further cause unhealthy morphology and high sensitivity to oxidative stress. With this cell model, we have evaluated the neuron protective effect of several histone deacetylase (HDAC) inhibitors and antioxidant compounds. All of these HDAC inhibitors could sustain cell viability significantly when cells were treated with low dosage of these compounds. These results suggest that regulation of gene expression is involved in the neuropathology of SCA3. In conclusion, we have established an SCA3 inducible cell model which could be used as a platform for screening of potential therapeutic strategies for SCA3 and other polyQ-mediated neurodegenerative diseases.

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