Monitoring oligomeric forms of amyloid beta in the early stages of aggregation using hybrid X-ray footprinting mass spectrometry with inline fluorescence

The aggregation-prone Amyloid Beta (Aβ) peptide has been the target of the majority of efforts to develop Alzheimer's Disease therapeutics, most of which have failed or have shown mixed results. It is no longer widely believed that buildup of amyloid fibrils is a causal factor in cognitive decline associated with the disease, and a growing body of evidence now suggests that low molecular weight oligomeric forms of Aβ may be the culprits in disease progression. However, while numerous Aβ fibril structures have been characterized, primarily by NMR and cryo-EM, obtaining structural information on oligomeric forms of Aβ that presumably precede and/or seed fibril formation has proved challenging. These transient forms are heterogeneous, and depend heavily on experimental conditions such as buffer, temperature, concentration, and degree of quiescence during measurement. Here, we present a new approach to delineating structural features of early-stage low molecular weight Aβ oligomers using a solvent accessibility assay in conjunction with inline fluorescence measurements. In this method, inline fluorescence is collected nearly simultaneously from samples during collection of X-ray footprinting mass spectrometry (XFMS) data. In the XFMS method, X-ray irradiation dissociates solvent water to produce hydroxyl radicals, which covalently modify solvent accessible side chains. Liquid chromatography-mass spectrometry is subsequently used to analyze the covalent modifications produced, and the data provide a “water map” at the single residue level, which is used to determine protein structure. This new hybrid approach circumvents issues in batch-to-batch variability of Aβ aggregation which has made this peptide so difficult to study, and provides a method to combine residue-specific structural information nearly simultaneously with global protein structure readouts from either intrinsic protein fluorescence or fluorophores such as Thioflavin T.