Differentiating Alzheimer’s Aβ Isoforms Coaggregated in Cerebrospinal Fluid via Single-Particle Imaging

Amyloid polymorphism can reflect Alzheimer's disease (AD) stages. This paper demonstrates that amyloid β (Aβ) peptides, primarily Aβ-40 and Aβ-42 (implicated in AD pathology), present in cerebrospinal fluid (CSF), can be differentiated, and their morphology studied in detail using fluorescence-based super-resolution and atomic force microscopy (AFM). An inhibitory effect of Aβ-40 on Aβ-42 protein aggregation, marked by Aβ-40 oligomers colocalizing along the Aβ-42 fibril backbone, was resolved at the single-particle level. Molecular dynamics simulations revealed that coaggregation is modulated by the ionic environment in CSF, where calcium ions form bridges between Glu residues of Aβ-40 and Aβ-42, known to stabilize the fibril structure. This ion-mediated tethering compacts Aβ-40 and kinetically traps the fibril-oligomer interface, thus reducing fibril elongation. The isoform-specific imaging method further allowed us to distinguish Aβ-40 and Aβ-42 aggregates from oligomers to mature fibrils in the CSF of AD patients, and the nanoscopic differences in aggregate sizes were quantified from the AFM topographs. Such a protein characterization approach, which is not limited by analyte size or shape and is capable of fingerprinting Aβ aggregates in CSF, could be used in clinical settings to monitor the progression of Alzheimer's disease and related pathologies.