TBP功能喪失為聚麩醯胺神經退化性疾病之共同致病因素：與氧化壓力之關聯

Abstract

在許多由聚麩醯胺擴增造成之神經退化性疾病(polyQ diseases) 之研究中指出TATA box binding protein (TBP) 被蛋白質包涵體捕捉而失去作用。不僅如此，其中一種疾病：第十七型脊髓小腦共濟失調症(SCA17) 正是由TBP本身的聚麩醯胺擴增所引起，進而形成具有神經毒性的蛋白質包涵體並影響下游基因。然而擴增的聚麩醯胺片段如何影響TBP功能，以及功能受到影響的TBP與SCA17的致病機制之間的關係依然不明。為深入探討此致病機制，在本研究中，我們建立新的SCA17果蠅模式，其確實展現出蛋白質聚集、運動能力降低以及壽命減短等退化症狀。此外，聚麩醯胺擴增之TBP不只會形成蛋白質聚集且其本身之DNA結合力以及轉錄活性也較低，聚麩醯胺擴增之TBP更進一步會干擾正常TBP的功能，據此推測TBP功能缺失為第十七型小腦萎縮症之病因之一。TBP突變果蠅顯現類似SCA17模式果蠅之神經退化症狀，且第十七型小腦萎縮症果蠅模式複眼的退化性狀在TBP功能缺失突變背景下更為突出，証實TBP功能之降低為SCA17致病機制的一環。另TBP表現降低更加劇第三型脊髓小腦萎縮症以及亨丁頓氏舞蹈症複眼感光細胞之退化。顯見TBP功能喪失可能為聚麩醯胺擴增所造成之神經退化的共同因子。TBP功能喪失在小鼠胚胎造成細胞凋亡，但原因不明。本研究發現果蠅TBP缺失不只造成神經退化亦能觀察到細胞凋亡，故以果蠅來探討TBP在神經退化與細胞凋亡之關聯。藉由微陣列基因分析篩選出能分解過氧化氫的prx2540-2基因在TBP突變的果蠅頭部中表現顯著減少。在果蠅中降低prx2540-2之表現亦產生類似TBP突變果蠅之神經退化症狀。此外，聚麩醯胺擴增所引起的退化性神經疾病果蠅模式與TBP突變之果蠅其頭部的過氧化氫濃度亦較高。故由TBP功能喪失引起prx2540-2表現下降進而造成的氧化壓力增加可能為聚麩醯胺擴增所造成之神經退化的原因之一。

TATA box binding protein (TBP) has been implicated in many polygluatmine (polyQ) induced neurodegenerations as it is sequestered and inactivated in polyQ proteins containing inclusions. Unlike most polyQ mediated neuropathies, spinocerebellar ataxia 17 (SCA17) is resulted from the abnormally expanded polyQ tract of TBP itself. Previous studies have shown that polyQ expanded TBP forms neurotoxic inclusions and affects downstream genes. However, how expanded polyQ tracts affect the function of TBP and the link between dysfunction of TBP and SCA17 are not clear. In this study, we generate novel Drosophila models of SCA17 that recapitulate pathological features, including aggregate formation, mobility defects and premature death. In addition to forming neurotoxic aggregates, we showed that polyQ-expanded TBP loses its intrinsic DNA-binding and transcription abilities. Dysfunctional TBP also disrupts the function of normal TBP. Moreover, flies expressing polyQ-expanded TBP exhibited enhanced retinal degeneration and heterozygous dTbp amorph mutant flies exhibited SCA17-like phenotypes, suggesting that loss of TBP function may contribute to SCA17 pathogenesis. We also determined that the downregulation of TBP activity enhances retinal degeneration in the fly models of SCA3 and Huntington’s disease, indicating that the deactivation of TBP is likely to play a common role in polyQ-induced neurodegeneration. Moreover, inactivation of TBP has been reported to cause apoptosis in mice. Nevertheless, the mechanism by which deactivated TBP leads to apoptosis is illusive. We found that Drosophila TBP mutants exhibit both neurodegeneration but also apoptosis phenotypes, indicating that Drosophila is a suitable model for unraveling the links between neurodegeneration and apoptosis. Through gene profiling experiments we have identified that prx2540-2, a peroxiredoxin (Prx) encoded gene whose gene product catalyzes the reduction of hydrogen peroxide (H2O2), is reduced greatly in the heads of dTbp mutants. Down-regulation of prx2540-2 generates similar neuropathies as seen in dTbp mutant flies. In contrast, expression of prx2540-2 reverses the above phenotypes in dTbp mutants, demonstrating that prx2540-2 acts downstream of dTbp. Additionally, the concentration of H2O2 is higher in the brains of dTbp mutants and polyQ disease models. Therefore, downregulation of prx2540-2 may mediate the neurodegeneration in polyQ disorders through increasing the oxidative stress in neuronal tissues.