NMR-based site-resolved profiling of ?-amyloid misfolding reveals structural transitions from pathologically relevant spherical oligomer to fibril

Increasing evidence highlights the central role of neurotoxic oligomers of the 42-residue-long ?-amyloid (A?42) in Alzheimer's disease (AD). However, very limited information is available on the structural transition from oligomer to fibril, particularly for pathologically relevant amyloids. To the best of our knowledge, we present here the first site-specific structural characterization of A?42 misfolding, from toxic oligomeric assembly yielding a similar conformation to an AD-associated A?42 oligomer, into a fibril. Transmission EM (TEM) analysis revealed that a spherical amyloid assembly (SPA) of A?42 with a 15.6 ? 2.1-nm diameter forms in a ?30-?m A?42 solution after a ?10-h incubation at 4 ?C, followed by a slow conversion into fibril at ?180 h. Immunological analysis suggested that the SPA has a surface structure similar to that of amylospheroid (ASPD), a patient-derived toxic A? oligomer, which had a diameter of 10?15 nm in negative-stain TEM. Solid-state NMR analyses indicated that the SPA structure involves a ?-loop-? motif, which significantly differed from the triple-? motif observed for the A?42 fibril. The comparison of the C-13 chemical shifts of SPA with those of the fibril prepared in the above conditions and interstrand distance measurements suggested a large conformational change involving rearrangements of intermolecular ?-sheet into in-register parallel ?-sheet during the misfolding. A comparison of the SPA and ASPD C-13 chemical shifts indicated that SPA is structurally similar to the ASPD relevant to AD. These observations provide insights into the architecture and key structural transitions of amyloid oligomers relevant for AD pathology.