Microglia contribute to polyG-dependent neurodegeneration in neuronal intranuclear inclusion disease

Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disease caused by GGC repeat expansion of NOTCH2NLC. Despite the identification of uN2CpolyG, a toxic polyglycine (polyG) protein, the pathogenic mechanisms of NIID remain unclear. Herein, we investigated the role of polyG by expressing wild-type NOTCH2NLC, expanded NOTCH2NLC with 100 GGC repeats (translating or not translating into uN2CpolyG), or mutated NOTCH2NLC that encodes a pure polyG without GGC-repeat RNA and C-terminal stretch (uN2CpolyG-dCT) in mice. Both uN2CpolyG and uN2CpolyG-dCT induced the formation of inclusions composed by filamentous materials and caused neurodegenerative phenotypes in mice, including impaired motor and cognitive performance, shortened lifespan, and pathological lesions such as white matter lesions, microgliosis, and astrogliosis. By contrast, the sole expression of GGC-repeat RNA was non-pathogenic. Combining with bulk and single-nuclei RNA-seq, we determined the common molecular signatures associated with expressing uN2CpolyG and uN2CpolyG-dCT in the brain, especially the up-regulation of inflammation and microglia markers whereas down-regulation of immediate early genes and splicing factors. Importantly, microglia-mediated inflammation was visualized in NIID patients by positron emission tomography, which correlated with white matter atrophy levels. Moreover, microglia ablation ameliorated neurodegeneration in uN2CpolyG-expressing mice but did not alter polyG inclusions. Together, these results demonstrate that polyG is critical for the pathogenesis of NIID, and microglia contribute to polyG-induced neurodegeneration.