Measurement of the attachment and assembly of small amyloid-ß oligomers on live cell membranes at physiological concentrations using single-molecule tools.

ABSTRACT

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