Real-Time Single Molecule Imaging Reveals Pathway-Selective Action of Aducanumab on Amyloid-β Aggregation

Aducanumab is an anti-amyloid-β (Aβ) antibody that binds aggregated Aβ species and has been reported to inhibit secondary nucleation on fibril surfaces. However, how antibody binding influences aggregate dynamics at the molecular level remains incompletely understood. Here, we used high-speed atomic force microscopy (HS-AFM) to directly visualize the interactions between aducanumab and Aβ42 aggregates in real time at single-molecule resolution. Aducanumab extensively decorated oligomers, protofibrils, and fibrils, with fibril binding depending on local surface structure. Consistent with its reported inhibition of secondary nucleation, bulk thioflavin T assays showed slowed aggregation. Nevertheless, in the continued presence of the antibody, protofibril formation proceeded─a pathway distinct from secondary nucleation. These observations reveal a pathway-selective mode of action: extensive surface binding and the suppression of secondary nucleation do not extend to protofibril formation, which proceeds in the continued presence of the antibody.