Comparing the structure and stability of diverse amyloid beta oligomers using fluorescence correlation spectroscopy

Soluble oligomers of amyloid beta (Aβ) play a central role in Alzheimer's Disease pathology, however the detailed mechanisms of Aβ oligomer toxicity remain under investigation. One challenge in measuring oligomer size or cytotoxicity is the variability between published protocols that describe distinct oligomers, and the likely scenario that there are multiple pathological oligomers rather than a single disease-causing complex. Given the heavy use of dilution and incubation in these protocols, a systematic review covering how different oligomeric concentrations and preparation protocols affect oligomeric size and aggregation propensity is essential for standardizing assays of amyloid toxicity. To begin addressing this gap, we have systematically characterized and compared two distinct amyloid beta oligomers derived from either a homogeneous detergent or from cell media using fluorescence correlation spectroscopy (FCS). We observe temporal changes in size and stability of these oligomers over a wide range of concentrations and attempt to codify their similarities and differences. Significant changes between oligomer populations are observed over hours, with more diluted complexes falling apart. These observations also demonstrate that protein oligomer behavior is highly specific to their environments, which must be accounted for in biological assays, especially in the presence of cell media. Accounting for such discrepancies will also enable high-throughput screening of compounds that inhibit Aβ oligomer cytotoxicity with high reproducibility, which is a crucial component of drug discovery in the context of neurodegeneration and aging.