PolyG Fibrils Coalesce Into Nuclear Ribbons That Engage Proteostasis Machinery in Neuronal Intranuclear Inclusion Disease.

Neuronal intranuclear inclusion disease (NIID) arises from GGC repeat expansions in NOTCH2NLC. These expanded repeats produce polyglycine (polyG) proteins, and the accumulation of these polyG proteins in neuronal nuclei serves as the characteristic pathological hallmark of NIID. However, the native cellular ultrastructure of polyG and its contribution to pathology remain poorly understood. Here, using a transgenic NIID mouse model, we extract polyG assemblies from diseased brain and characterize their architecture by cryo-electron tomography (cryo-ET). We further examine their native organization by tracer-guided in situ cryo-ET in vitrified mouse brain. We find that polyG forms highly branched ∼5 nm fibrils that laterally coalesce into densely packed ribbons, which represent the predominant polyG state within neuronal nuclei in situ. In parallel, proximity-dependent labeling coupled to mass spectrometry reveals selective enrichment of proteostasis factors-including proteasome subunits and molecular chaperones-at polyG assemblies in mouse brain. Consistent with this, cryo-ET visualizes proteasome-like particles decorating ribbon-shaped surfaces and edges in cells. Together, these findings uncover an unexpected ribbon-shaped supramolecular architecture for a low-complexity disease protein and suggest that nuclear polyG ribbons act as scaffolds that engage proteostasis machinery, providing mechanistic insight into NIID.