Synergistic Interactions between Alzheimer's Aβ40 and Aβ42 on the Surface of Primary Neurons Revealed by Single Molecule Microscopy

Two amyloid-beta peptides (A beta 40 and A beta 42) feature prominently in the extracellular brain deposits associated with Alzheimer's disease. While A beta 40 is the prevalent form in the cerebrospinal fluid, the fraction of A beta 42 increases in the amyloid deposits over the course of disease development. The low in vivo concentration (pM-nM) and metastable nature of A beta oligomers have made identification of their size, composition, cellular binding sites and mechanism of action challenging and elusive. Furthermore, recent studies have suggested that synergistic effects between A beta 40 and A beta 42 alter both the formation and stability of various peptide oligomers as well as their cytotoxicity. These studies often utilized A beta oligomers that were prepared in solution and at mu M peptide concentrations. The current work was performed using physiological A beta concentrations and single-molecule microscopy to follow peptide binding and association on primary cultured neurons. When the cells were exposed to a 1: 1 mixture of nM A beta 40:A beta 42, significantly larger membrane-bound oligomers developed compared to those formed from either peptide alone. Fluorescence resonance energy transfer experiments at the single molecule level reveal that these larger oligomers contained both A beta 40 and A beta 42, but that the growth of these oligomers was predominantly by addition of A beta 42. Both pure peptides form very few oligomers larger than dimers, but either membrane bound A beta 40/42 complex, or A beta 40, bind A beta 42 to form increasingly larger oligomers. These findings may explain how A beta 42-dominant oligomers, suspected of being more cytotoxic, develop on the neuronal membrane under physiological conditions.