Supramolecular structural constraints on Alzheimer's β-amyloid fibrils from electron microscopy and solid-state nuclear magnetic resonance

We describe electron microscopy (EM), scanning transmission electron microscopy (STEM), and solid-state nuclear magnetic resonance (NMR) measurements on amyloid fibrils formed by the 42-residue beta-anlyloid peptide associated with Alzheimer's disease (Abeta(1-42)) and by residues 10-35 of the full-length peptide (Abeta(10-35)). These measurements place constraints on the supramolecular structure of the amyloid fibrils, especially the type of beta-sheets present in the characteristic amyloid cross-beta structural motif and the assembly of these beta-sheets into a fibril. EM images of negatively stained Abeta(10-35) fibrils and measurements of fibril mass per length (WPL) by STEM show a strong dependence of fibril morphology and MPL on pH. Abeta(10-35) fibrils formed at pH 3.7 are single "protofilaments" with MPL equal to twice the value expected for a single cross-beta layer. Abeta(10-35) fibrils formed at pH 7.4 are apparently pairs of protofilaments or higher order bundles. EM and STEM data for Abeta(1-42) fibrils indicate that protofilaments with MPL equal to twice the value expected for a single cross-beta layer are also formed by Abeta(1-42) and that these protofilaments exist singly and in pairs at pH 7.4. Solid-state NMR measurements of intermolecular distances in Abeta(10-35) fibrils, using multiple-quantum C-13 NMR, C-13-C-13 dipolar recoupling, and N-15-C-13 dipolar recoupling techniques, support the in-register parallel beta-sheet organization previously established by Lynn, Meredith, Botto, and co-wotkers [Benzinger et al. (1998) Proc. Nad. Acad. Sci. U.S.A. 95, 13407-13412; Benzingger et al. (2000) Biochemistry 39, 3491-3499] and show that this beta-sheet organization is present at pH 3.7 as well as pH 7.4 despite the differences in fibril morphology and MPL. Solid-state NMR measurements of intermolecular distances in Abeta(1-42) fibrils, which represent the first NMR data on Abeta(1-42) fibrils, also indicate an in-register parallel beta-sheet organization. These results, along with previously reported data on Abeta(1-40) fibrils, suggest that the supramolecular structures of Abeta(10-35), Abeta(1-40), and Abeta(1-42) fibrils are quite similar. A schematic structural model of these fibrils, consistent with known experimental EM, STEM, and solid-state, NMR data, is presented.