The in-tissue molecular architecture of β-amyloid pathology in the mammalian brain.

Amyloid plaques composed of extracellular focal deposition of Aβ fibrils are a hallmark of Alzheimer's disease (AD). Cryo-EM structures of Aβ fibrils purified from human AD brain tissue post mortem have recently been determined. However, the molecular architecture of amyloid plaques in the context of fresh, unfixed mammalian brain tissue is unknown. Here, using cryogenic correlated light and electron tomography we report the native, in situ molecular architecture of Aβ fibrils in the brain of a mouse model containing the Arctic familial AD mutation (AppNL-G-F) and an atomic model of Arctic Aβ fibril purified from the brains of these animals. We show that in-tissue Aβ fibrils are arranged in a lattice or in parallel bundles within a plaque, and are interdigitated by subcellular compartments, exosomes, extracellular droplets and extracellular multilamellar bodies. At the atomic level, the Arctic Aβ fibril differs significantly from earlier structures of Aβ amyloid extracted from AppNL-F mice models and human AD brain tissue, showing a striking effect of the Arctic mutation (E22G) on fibril structure. Cryo-electron tomography of ex vivo purified and in-tissue amyloid revealed an ensemble of additional fibrillar species, including thin protofilament-like rods and branched fibrils. Together, these results provide a structural model for the dense network architecture that characterises β-amyloid plaque pathology.