| Abstract: | 第二型小腦脊髓運動失調症 (Spinocerebellar ataxia type2, SCA2) ，是體染色體顯性遺傳的罕見疾病，屬於漸進性神經退化性疾病。此遺傳疾病主要是ATXN-2 基因位於第12對染色體 (12q24.13的CAG三核苷酸序列異常擴增所引起，其轉譯出具有大量麩胺酸的ataxin-2蛋白堆積，導致神經細胞死亡。本文主要以分析正常人與SCA2病人之間細胞的差異，進一步了解SCA2的致病機轉。細胞因受到突變蛋白的刺激導致細胞凋亡(apoptosis)，Bcl2和Bax及熱休克蛋白是調控細胞凋亡的重要蛋白，透過西方墨點法實驗結果顯示SCA2中促進細胞凋亡的Bax蛋白表現量確實比LCL高，也發現SCA2細胞株中熱休克蛋白表現量比正常人的細胞株低。此外，RhoA/ ROCK訊息傳遞路徑中的RhoA與ROCK在SCA2細胞中表現量較高。進而推測促使細胞凋亡的訊息傳遞路徑是否經由RhoA與ROCK的活化刺激下游的mitogen-activated protein kinase (MAPK) 訊息路徑所調控。結果在SCA2病人的細胞株中其磷酸化的p38、JNK以及ERK1/2蛋白的表現量比正常人的細胞株高。此外，細胞自噬 (autophagy) 作用的標記蛋白–LC3、Atg5均在SCA2細胞中大量表現。依據以上實驗結果顯示病人的發病原因可能是細胞受到突變蛋白不正常堆積產生的壓力刺激，其中SCA2細胞熱休克蛋白表現量降低，將導致大量麩胺酸蛋白堆積，造成RhoA/ROCK訊息傳遞路徑活化、刺激下游MAPK磷酸化的p38、JNK以及ERK1/2的蛋白表現異常，Bax蛋白表現量上升，走向細胞凋亡，導致神經細胞退化。但是，SCA2細胞中LC3、Atg5大量表現，表示細胞亦產生自噬作用，啟動保護機制可能減緩神經細胞退化。Spinocerebellar ataxia type 2 (SCA2) , an autosomal dominant neurodegenerative disease, is caused by the expansion of a CAG triplet repeats located in the N-terminal coding region of the ATXN2 gene. Alleles of the ATXN2 gene that carry 13-31 CAG-trinucleotide repeats are present in normal individuals. Contrariwise, alleles with a CAG triplet repeat number of 31 and up to approximately 200 are present in patients with SCA2. Although the detail mechanism of pathogenesis is yet to be defined, neurotoxin, especially reactive oxygen species (ROS) released from aggregated mutant proteins, and formed stress granule (SG) may play a role in the pathogenic process. In this study, the lymphoblastoid cell lines (LCLs) isolated from SCA2 patients were utilized to compare with the wild-type lymphoblastoid cells. We investigated the crucial relationship between the expression of p-ERK1/2 and autophagy. Western blot results found the endogenous p-ERK1/2 and autophagy marker protein, Atg8 (LC3 class ΙΙ) were higher in SCA2 patients. Moreover, misfolding mutant ataxin aggregation causes spinocerebellar ataxia type 2 downregulate heat shock protein reduces neurotoxicity by promoting polyglutamine protein degradation. Heat-shock protein disfunction, like Hsp60, Hsp40, Hsp70 and Hsp27 consistently suppresses the formation of polyQ inclusion bodies and their toxicity. These results highlight the possibility that chaperones facilitate neuroprotection through several distinct mechanisms, but because small, diffusible, potentially toxic polyQ assemblies could not be evaluated, an essential role for the refolding activity of chaperones cannot be ruled out. The essential functions of Rho family GTPases in regulating neuronal growth cone formation, neurite outgrowth, and nervousmsystem development suggest that Rho GTPases have an important and conserved function in mediating neuronal survival and death. Furthermore, RhoA/ROCK signaling pathway marker protein, ROCK and RhoA were higher in SCA2 cells. Rho can provoke apoptosis via activation of either p38- or JNK-dependent signaling pathways which elicit apoptosis through activation of pro-apoptotic members of the Bcl-2 family of proteins lead to neurodegenerative. Otherwise, based on the above observations we hypothesized that the aggregated mutant Ataxin-2 protein may generate stress and form SG, which subsequently up-regulate Atg8 expression levels and ultimately lead to cell autophagy to slow down neurodegenerative. |
