Abundant Aβ fibrils in ultracentrifugal supernatants of aqueous extracts from Alzheimer’s disease brains

Soluble aggregates of amyloid-β (Aβ), often called oligomers, are believed to be principal drivers of neurotoxicity, spreading of pathology, and symptoms in Alzheimer’s disease (AD), but little is known about their structures in human brain. Aβ oligomers have been defined as aggregates found in supernatants following ultracentrifugation of aqueous extracts. We now report the unexpected presence of abundant Aβ fibrils in high-speed supernatants from AD brains that were extracted by soaking in aqueous buffer. The fibrils did not appear to form during extract preparation, and their numbers by EM correlated with ELISA quantification of aggregated Aβ42. Cryo-EM structures of Aβ fibrils from aqueous extracts were identical to those from sarkosyl-insoluble AD brain homogenates. The fibrils in aqueous extracts were immunolabeled by lecanemab, an Aβ aggregate-directed antibody reported to improve cognitive outcomes in AD. We conclude that Aβ fibrils are abundant in aqueous extracts from AD brains and have the same structures as those from amyloid plaques. These findings have implications for understanding the nature of Aβ oligomers and for designing oligomer-preferring therapeutic antibodies. ### Competing Interest Statement DJS is a director of Prothena Biosciences; the other authors declare no competing interests.