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

ABSTRACT 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.