Huntington’s disease (HD) is a progressive incurable neurodegenerative disorder characterized by motor and neuropsychiatric symptoms. It is caused by expansion of a cytosine–adenine–guanine triplet in the N-terminal domain of exon 1 in the huntingtin (HTT) gene that codes for an expanded polyglutamine stretch in the protein product which becomes aggregation prone. The mutant Htt (mHtt) aggregates are associated with components of the ubiquitin–proteasome system, suggesting that mHtt is marked for proteasomal degradation and that, for reasons still debated, are not properly degraded. We used a novel HD rat model, proteomic analysis, and long-term live neuronal imaging to characterize the effects of ubiquitination on aggregation of mHtt and subsequent cellular responses. We identified two lysine residues, 6 and 9, in the first exon of mHtt that are specifically ubiquitinated in striatal and cortical brain tissues of mHtt-transgenic animals. Expression of mHtt exon 1 lacking these ubiquitination sites in cortical neurons and cultured cells was found to slow aggregate appearance rates and reduce their size but at the same time increase the number of much smaller and less visible ones. Importantly, expression of this form of mHtt was associated with elevated death rates. Proteomic analysis indicated that cellular reactions to mHtt expression were weaker in cells expressing the lysineless protein, possibly implying a reduced capacity to cope with the proteotoxic stress. Taken together, the findings suggest a novel role for ubiquitination—attenuation of the pathogenic effect of mHtt.

亨廷顿舞蹈病（HD）是一种以运动和神经精神症状为特征的进行性无法治愈的神经退行性疾病。这是由于亨廷顿（HTT）基因外显子1的N端结构域中的胞嘧啶-腺嘌呤-鸟嘌呤三联体的扩张引起的，该三联体编码蛋白质产物中的聚谷氨酰胺伸展，从而易于聚集。突变的Htt（mHtt）聚集体与泛素-蛋白酶体系统的组分相关，这表明mHtt被标记为蛋白酶体降解，并且出于尚有争议的原因，并未适当降解。我们使用新型的高清大鼠模型，蛋白质组学分析和长期的实时神经元成像来表征泛素化对mHtt聚集和后续细胞反应的影响。我们确定了两个赖氨酸残基6和9 在mHtt的第一个外显子中特异存在于mHtt转基因动物的纹状体和皮质脑组织中。发现在皮质神经元和培养细胞中缺少这些泛素化位点的mHtt外显子1的表达减慢了聚集出现率并减小了其大小，但同时却增加了更小，更不可见的数量。重要的是，这种形式的mHtt的表达与死亡率升高相关。蛋白质组学分析表明，在表达无赖氨酸蛋白的细胞中，细胞对mHtt表达的反应较弱，这可能意味着其应对蛋白毒性应激的能力降低。综上所述，这些发现提示了泛素化的新作用-减轻mHtt的致病作用。