第22型小腦萎縮症 (另一名為第19型小腦萎縮症) 主要為A型鉀離子通道 (KCND3 / Kv4.3) 的基因突變所造成的一種顯性退化性神經遺傳疾病。目前造成第22型小腦萎縮症致病機轉並不清楚，我的研究目的：(1) 建立第22型小腦萎縮症的果蠅模式；(2) 解析第22型小腦萎縮症的致病機轉。我們利用轉殖野生型KCND3wt, 突變KCND3G345V 及 KCND3ΔF277 蛋白建立第22型小腦萎縮症的果蠅模式，過量表現突變KCND3G345V 及 KCND3ΔF277 蛋白造成果蠅包括複眼退化、運動功能失調、壽命降低等性狀。由免疫螢光染色發現Kv4.3蛋白質位於內質網，野生型KCND3wt及突變KCND3G345V 及 KCND3ΔF227 核酸 (mRNA) 表現量相同，但突變Kv4.3較野生型Kv4.3蛋白表現量顯著大量降低，推測突變蛋白質因摺疊不正常而被水解，突變Kv4.3蛋白也較正常Kv4.3蛋白質引發更大的內質網壓力 (ER-stress) ，突變蛋白引發果蠅表現更多Xbp1s剪切核酸，及ER-stress感受標誌基因，突變蛋白引發持續性內質網壓力，也造成細胞死亡，表現p53H159N及抑制細胞凋亡蛋白DIAP I (Death-associated inhibitor of apoptosis 1) 則可抑制第22小腦萎縮症果蠅複眼感光細胞死亡，顯示突變Kv4.3蛋白造成的退化是細胞凋亡的結果，由於未折疊蛋白反應 (unfolded protein respose - upr) 是造成內質網壓力的主要原因之一，Xbp1s轉錄因子可促進內質網伴護蛋白表現以助蛋白質正摺疊，我們發現過量Xbp1s蛋白可改善疾病果蠅模式複眼退化，增加突變蛋白表現量，相反地，降低Xbp1s蛋白質表現量則顯著加劇突變Kv4.3蛋白質造成複眼退化，相同地，過量表現內質網伴護白Hsc70，則可顯著改善突變Kv4.3蛋白造成的危害，並增加突變蛋白表現量，顯示Xbp1s及Hsc70有助突變Kv4.3蛋白質折疊，而不受蛋白酶體水解。綜合上述結果，我們認為突變Kv4.3蛋白質引發的持續性內質網壓力為第22型小腦萎縮症的致病原因之一。

The spinocerebellar ataxia type 22 (SCA22), alternatively known as SCA19, is an autosomal dominant neurodegenerative disease caused by the mutations in the A-type potassium channel KCND3 / Kv4.3 gene. Currently, the disease mechanisms underlying the SCA22 is elusive. The objectives of the studies are: (1) Generating Drosophila models SCA22; (2) dissecting the pathomechanisms of the disease. We have generated Drosophila models for SCA19/22 by overexpressing mutant Kv4.3 proteins (KCND3G345V and KCND3ΔF227). The SCA19/22 fly models exhibit many age-dependent phenotypes, including degeneration, shorten lifespan, mobility defect, and neuronal apoptosis. Co-expression of a p53H159N or Drosophila inhibitor of apoptosis I (DIAPI) reduced rough eye phenotype in SCA22 fly models, indicating that mutant KCND3 induced apoptosis. Mutant KCND3G345V and KCND3ΔF277 induced retinal degenerative phenotypes can be rescued by the expression of the ER-stress sensor Xbp1s, suggesting that unfolded protein response (UPR) and Endoplasmic Reticulum stress (ER stress) might be involved in mutant KCND3 induced apoptosis. Overexpression of the ER-specific chaperone, Hsc70, improved the rough eye phenotype of SCA22 fly models. Compared to wild type KCND3wt, mutant KCND3G345V and KCND3ΔF277 proteins are less stable and are likely to be subjected for proteolytic cleavage because their protein levels are greatly reduced while the mRNA levels are compatible in flies.

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