Amyloid-β oligomers, curvilinear and annular assemblies, imaged by cryo-ET, cryo-EM, and AFM.

Prefibrillar structures of the amyloid-β (Aβ) peptide are central to cytotoxicity in Alzheimer's disease. Time-resolved imaging of oligomers has enabled quantification of their extension. A snapshot of these prefibrillar assemblies has been characterized using a combination of cryo-electron tomography (cryo-ET), cryo-electron microscopy (cryo-EM) single-particle analysis, and atomic force microscopy (AFM). A highly consistent diameter for all curvilinear protofibrils and oligomers of 2.8 nanometers suggests that these assemblies are structural extensions from the smaller oligomers. In situ AFM confirms that spherical oligomers template and extend over time into curvilinear protofibrils. Furthermore, their basic cross section suggests that amyloid fibrils might be initiated by the lateral binding of two curvilinear protofibrils. Cryo-ET/EM single particles also reveal ring-shaped annular assemblies. These have a central internal channel, ~1.4 nanometers in diameter, which is capable of traversing lipid membranes. Large conductance recorded using patch-clamp electrophysiology matches the internal diameter of the Aβ annular architecture.