Rapid, cell-free assay for membrane-active forms of amyloid-β.

Small oligomers of amyloid beta (Aβ) are suspected to be the key to Alzheimer's disease (AD). However, identifying these toxic species in the background of other similar but nontoxic Aβ aggregates has remained a challenge. Recent studies indicate that Aβ undergoes a global structural transition in an early step of aggregation. This transition is marked by a strong increase in its affinity for cell membranes, which suggests that the resultant oligomers could be the key to Aβ toxicity. Here we use this increased membrane affinity to develop a rapid, quantitative, cell-free assay for these bioactive oligomers. It uses fluorescence correlation spectroscopy of fluorescently labeled Aβ and requires only 30 s of measurement time. We also describe a simpler (though less rapid) assay based on the same principles, which uses a dialysis step followed by conventional fluorescence spectroscopy. Our results potentially provide a much-needed high-throughput assay for AD drug development.