Hamano Tadanori, Gendron Tania F, Ko Li-Wen, Yen Shu-Hui
Department of Neuroscience, Mayo Clinic College of Medicine Jacksonville, FL, USA.
Int J Clin Exp Pathol. 2009 Jun 15;2(6):561-73.
Tauopathies are characterized by accumulation of filamentous tau aggregates. These aggregates can be recapitulated in transfectant M1C overproducing wild-type human brain tau 4R0N via the tetracycline off (TetOff) inducible expression mechanism. To determine the contribution of proteasomes to tau degradation and aggregation, we exposed M1C cells to epoxomicin (Epx; 2-50 nM) or MG132 (0.5 microM) on the 3(rd) or 4(th) day of a 5-day TetOff induction and demonstrated a reduction of proteasomal activity. Cultures treated with 2 nM Exp showed accumulation of full-length tau without affecting ubiquitin and beta-catenin immunoblotting profiles. In contrast, cells treated with 10, 50 nM Epx or MG132 displayed changes in ubiquitin or beta-catenin immunoblotting profiles and extensive tau degradation/truncation. The increase of tau degradation/truncation was accompanied with accumulation of oligomers and sarkosyl-insoluble aggregates of tau, augmented thioflavin-binding and activation of caspases and calpains. Truncated, oligomeric and sarkosyl-insoluble tau derivatives appeared with caspase-specific cleavage and their production was diminished when pretreated with a pan-caspase inhibitor. The results demonstrate (i) a dose-dependent, opposite effect of proteasome inhibition on tau processing, (ii) the participation of proteasome-dependent, ubiquitination-independent mechanisms in tau degradation and aggregation, and (iii) the promotion of tau aggregation by caspase-mediated tau degradation/truncation.
tau蛋白病的特征是丝状tau聚集体的积累。通过四环素关闭（TetOff）诱导表达机制，在过量产生野生型人脑中tau 4R0N的转染子M1C中可以重现这些聚集体。为了确定蛋白酶体对tau降解和聚集的作用，我们在5天TetOff诱导的第3天或第4天将M1C细胞暴露于环氧霉素（Epx；2 - 50 nM）或MG132（0.5 microM），并证明蛋白酶体活性降低。用2 nM Epx处理的培养物显示全长tau积累，而不影响泛素和β-连环蛋白免疫印迹图谱。相比之下，用10、50 nM Epx或MG132处理的细胞显示泛素或β-连环蛋白免疫印迹图谱发生变化，并且tau广泛降解/截断。tau降解/截断的增加伴随着tau寡聚体和 Sarkosyl不溶性聚集体的积累、硫黄素结合增加以及半胱天冬酶和钙蛋白酶的激活。截断的、寡聚的和Sarkosyl不溶性tau衍生物出现半胱天冬酶特异性切割，并且在用泛半胱天冬酶抑制剂预处理时其产生减少。结果表明：（i）蛋白酶体抑制对tau加工具有剂量依赖性的相反作用；（ii）蛋白酶体依赖性、非泛素化依赖性机制参与tau降解和聚集；（iii）半胱天冬酶介导的tau降解/截断促进tau聚集。
