Measurement of the Attachment and Assembly of Small Amyloid-β Oligomers on Live Cell Membranes at Physiological Concentrations Using Single-Molecule Tools

It is thought that the pathological cascade in Alzheimer's disease is initiated by the formation of amyloid-beta (A beta) peptide complexes on cell membranes. However, there is considerable debate about the nature of these complexes and the type of solution-phase A beta aggregates that may contribute to their formation. Also, it is yet to be shown that A beta attaches strongly to living cell membranes, and that this can happen at low, physiologically relevant A beta concentrations. Here, we simultaneously measure the aggregate size and fluorescence lifetime of fluorescently labeled A beta(1-40) on and above the membrane of cultured PC12 cells at near-physiological concentrations. We find that at 350 nM A beta concentration, large ( 10 nm average hydrodynamic radius) assemblies of codiffusing, membrane-attached A beta molecules appear on the cell membrane together with a near-monomeric species. When the extracellular concentration is 150 nM, the membrane contains only the smaller species, but with a similar degree of attachment. At both concentrations, the extracellular solution contains only small (similar to 2.3 nm average hydrodynamic radius) A beta oligomers or monomers. We conclude that at near-physiological concentrations only the small oligomeric species are relevant, they are capable of attaching to the cell membrane, and they assemble in situ to form much larger complexes.