Site specific NMR characterization of abeta-40 oligomers cross seeded by abeta-42 oligomers.

Extracellular accumulation of ß amyloid peptides of 40 (Aß40) and 42 residues (Aß42) has been considered as one of the hallmarks in the pathology of Alzheimer's disease. In this work, we are able to prepare oligomeric aggregates of Aß with uniform size and monomorphic structure. Our experimental design is to incubate Aß peptides in reverse micelles (RMs) so that the peptides could aggregate only through a single nucleation process and the size of the oligomers is confined by the physical dimension of the reverse micelles. The hence obtained Aß oligomers (AßOs) are 23 nm in diameter and they belong to the category of high molecular-weight (MW) oligomers. The solid-state NMR data revealed that Aß40Os adopt the structural motif of ß-loop-ß but the chemical shifts manifested that they may be structurally different from low-MW AßOs and mature fibrils. From the thioflavin-T results, we found that high-MW Aß42Os can accelerate the fibrillization of Aß40 monomers. Our protocol allows performing cross-seeding experiments among oligomeric species. By comparing the chemical shifts of Aß40Os cross seeded by Aß42Os and those of Aß40Os prepared in the absence of Aß42Os, we observed that the chemical states of E11, K16, and E22 were altered, whereas the backbone conformation of the ß-sheet region near the C-terminus was structurally invariant. The use of reverse micelles allows hitherto the most detailed characterization of the structural variability of Aß40Os.