Highly Sensitive FRET-FCS Detects Amyloid β-Peptide Oligomers in Solution at Physiological Concentrations

Oligomers formed by the amyloid beta-peptide (A beta) are pathogens in Alzheimers disease. Increased knowledge on the oligomerization process is crucial for understanding the disease and for finding treatments. Ideally, A beta oligomerization should be studied in solution and at physiologically relevant concentrations, but most popular techniques of today are not capable of such analyses. We demonstrate here that the combination of FOrster Resonance Energy Transfer and Fluorescence Correlation Spectroscopy (FRET-FCS) has a unique ability to detect small subpopulations of FRET-active molecules and oligomers. FRET-FCS could readily detect a FRET-active oligonucleotide present at levels as low as 0.5% compared to FRET-inactive dye molecules. In contrast, three established fluorescence fluctuation techniques (FCS, FCCS, and PCH) required fractions between 7 and 11%. When applied to the analysis of A beta, FRET-FCS detected oligomers consisting of less than 10 A beta molecules, which coexisted with the monomers at fractions as low as 2 +/- 2%. Thus, we demonstrate for the first time direct detection of small fractions of A beta oligomers in solution at physiological concentrations. This ability of FRET-FCS could be an indispensable tool for studying biological oligomerization processes, in general, and for finding therapeutically useful oligomerization inhibitors.