Structural Polymorphism of polyG Inclusions Revealed by In Situ Cryo‐Electron Tomography

NIID (Neuronal intranuclear inclusion disease) is defined by ubiquitin- and p62-positive intranuclear inclusions, yet their native ultrastructure remains unclear. Using correlative cryo-electron tomography in primary cortical neurons and brain tissue from an NIID mouse model, we show that polyG inclusions are built from interconnected ribbon-like assemblies rather than canonical amyloid fibrils. PolyG populates multiple compartment-specific ribbon states, including a nuclear ribbon network enriched in 26S proteasomes and two cytoplasmic ribbon packing states with sharply different proteasome accessibility. In the cytoplasm, ribbon assemblies frequently contact endomembranes-particularly ER-like membranes-and these interactions coincide with membrane deformation, consistent with transcriptomic dysregulation of ER-stress responses-related genes. Together, these findings establish multiple ribbon states as a core feature of polyG aggregation and provide an in situ framework for linking NIID inclusion architecture to cellular interactions.