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 β-amyloid peptide associated with Alzheimer's disease (Aβ1 - 42) and by residues 10−35 of the full-length peptide (Aβ10 - 35). These measurements place constraints on the supramolecular structure of the amyloid fibrils, especially the type of β-sheets present in the characteristic amyloid cross-β structural motif and the assembly of these β-sheets into a fibril. EM images of negatively stained Aβ10 - 35 fibrils and measurements of fibril mass per length (MPL) by STEM show a strong dependence of fibril morphology and MPL on pH. Aβ10 - 35 fibrils formed at pH 3.7 are single “protofilaments” with MPL equal to twice the value expected for a single cross-β layer. Aβ10 - 35 fibrils formed at pH 7.4 are apparently pairs of protofilaments or higher order bundles. EM and STEM data for Aβ1 - 42 fibrils indicate that protofilaments with MPL equal to twice the value expected for a single cross-β layer are also formed by Aβ1 - 42 and that these protofilaments exist singly and in pairs at pH 7.4. Solid-state NMR measurements of intermolecular distances in Aβ10 - 35 fibrils, using multiple-quantum 13C NMR, 13C−13C dipolar recoupling, and 15N−13C dipolar recoupling techniques, support the in-register parallel β-sheet organization previously established by Lynn, Meredith, Botto, and co-workers [Benzinger et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 13407−13412; Benzinger et al. (2000) Biochemistry 39, 3491−3499] and show that this β-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 Aβ1 - 42 fibrils, which represent the first NMR data on Aβ1 - 42 fibrils, also indicate an in-register parallel β-sheet organization. These results, along with previously reported data on Aβ1 - 40 fibrils, suggest that the supramolecular structures of Aβ10 - 35, Aβ1 - 40, and Aβ1 - 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.