An integrated strategy to correlate aggregation state, structure and toxicity of Aβ 1-42 oligomers

Despite great efforts, it is not known which oligomeric population of amyloid beta (A beta) peptides is the main neurotoxic mediator in Alzheimer's disease. In vitro and in vivo experiments are challenging, mainly because of the high aggregation tendency of A beta (in particular of A beta 1-42 peptide), as well as because of the dynamic and non covalent nature of the prefibrillar aggregates. As a step forward in these studies, an analytical platform is here proposed for the identification and characterization of A beta 1-42 oligomeric populations resulting from three different sample preparation protocols. To preserve the transient nature of aggregates, capillary electrophoresis is employed for monitoring the oligomerization process in solution until fibril precipitation, which is probed by transmission electron microscopy. Based on characterization studies by ultrafiltration and SDS-PAGE/Western Blot, we find that low molecular weight oligomers build up over time and form bigger aggregates ( > dodecamers) and that the kinetics strongly depends on sample preparations. The use of phosphate buffer results to be more aggregating, since trimers are the smallest species found in solution, whereas monomers and dimers are obtained by solubilizing A beta 1-42 in a basic mixture. For the first time, attenuated total reflection-Fourier transform infrared spectroscopy is used to assign secondary structure to the separated oligomers. Random coil and/or a-helix are most abundant in smaller species, whereas beta-sheet is the predominant conformation in bigger aggregates, which in turn are demonstrated to be responsible for A beta 1-42 toxicity.