Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disorder caused by the expansion of GGC trinucleotide repeats in NOTCH2NLC gene. Despite identifying uN2CpolyG, a toxic polyglycine (polyG) protein translated by expanded GGC repeats, the exact pathogenic mechanisms of NIID remain unclear. In this study, we investigated the role of polyG by expressing various forms of NOTCH2NLC in mice: the wild-type, the expanded form with 100 GGC repeats (either translating or not translating into uN2CpolyG), and the mutated form that encodes a pure polyG without GGC-repeat RNA and the C-terminal stretch (uN2CpolyG-dCT). Both uN2CpolyG and uN2CpolyG-dCT induced the formation of inclusions composed by filamentous materials and resulted in neurodegenerative phenotypes in mice, including impaired motor and cognitive performance, shortened lifespan, and pathologic lesions such as white-matter lesions, microgliosis, and astrogliosis. In contrast, expressing GGC-repeat RNA alone was non-pathogenic. Through bulk and single-nuclei RNA sequencing, we identified common molecular signatures linked to the expression of uN2CpolyG and uN2CpolyG-dCT, particularly the upregulation of inflammation and microglia markers, and the downregulation of immediate early genes and splicing factors. Importantly, microglia-mediated inflammation was visualized in NIID patients using positron emission tomography, correlating with levels of white-matter atrophy. Furthermore, microglia ablation ameliorated neurodegenerative phenotypes and transcriptional alterations in uN2CpolyG-expressing mice but did not affect polyG inclusions. Together, these results demonstrate that polyG is crucial for the pathogenesis of NIID and highlight the significant role of microglia in polyG-induced neurodegeneration.

神经元核内包涵体病 （NIID） 是一种由 NOTCH2NLC 基因中 GGC 三核苷酸重复序列扩增引起的神经退行性疾病。尽管鉴定出 uN2CpolyG，一种由扩增的 GGC 重复序列翻译的有毒聚甘氨酸 （polyG） 蛋白，但 NIID 的确切致病机制仍不清楚。在这项研究中，我们通过在小鼠中表达各种形式的 NOTCH2NLC 来研究 polyG 的作用：野生型、具有 100 个 GGC 重复序列的扩增形式（翻译或不翻译成 uN2CpolyG），以及编码没有 GGC 重复序列 RNA 的纯 polyG 和 C 末端延伸的突变形式 （uN2CpolyG-dCT）。uN2CpolyG 和 uN2CpolyG-dCT 均诱导由丝状物质组成的包涵体的形成，并导致小鼠神经退行性表型，包括运动和认知能力受损、寿命缩短以及白质病变、小胶质细胞增生和星形胶质细胞增生等病理病变。相比之下，单独表达 GGC 重复 RNA 是非致病性的。通过大体和单核 RNA 测序，我们确定了与 uN2CpolyG 和 uN2CpolyG-dCT 表达相关的常见分子特征，特别是炎症和小胶质细胞标志物的上调，以及即刻早期基因和剪接因子的下调。重要的是，使用正电子发射断层扫描在 NIID 患者中可视化了小胶质细胞介导的炎症，与白质萎缩水平相关。此外，小胶质细胞消融改善了表达 uN2CpolyG 的小鼠的神经退行性表型和转录改变，但不影响 polyG 包涵体。 总之，这些结果表明 polyG 对 NIID 的发病机制至关重要，并强调了小胶质细胞在 polyG 诱导的神经退行性变中的重要作用。