A structural model for Alzheimer's beta -amyloid fibrils based on experimental constraints from solid state NMR

Abstract

We present a structural model for amyloid fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta(1-40)), based on a set of experimental constraints from solid state NMR spectroscopy. The model additionally incorporates the cross-beta structural motif established by x-ray fiber diffraction and satisfies constraints on Abeta(1-40) fibril dimensions and mass-per-length determined from electron microscopy. Approximately the first 10 residues of Abeta(1-40) are structurally disordered in the fibrils. Residues 12-24 and 30-40 adopt beta-strand conformations and form parallel beta-sheets through intermolecular hydrogen bonding. Residues 25-29 contain a bend of the peptide backbone that brings the two beta-sheets in contact through sidechain-sidechain interactions. A single cross-beta unit is then a double-layered beta-sheet structure with a hydrophobic core and one hydrophobic face. The only charged sidechains in the core are those of D23 and K28, which form salt bridges. Fibrils with minimum mass-per-length and diameter consist of two cross-beta units with their hydrophobic faces juxtaposed.

Fig 1.

(a) Transmission electron microscope images of negatively stained amyloid fibrils after 14-day incubation of a 0.5 mM Aβ_1–40 solution. A 3× expansion (Inset) shows fibrils with the smallest diameters observed. (b) 2D ¹³C-¹³C chemical shift correlation spectrum of Aβ_1–40 fibril sample SU7, showing resonance assignment paths for the seven uniformly ¹⁵N- and ¹³C-labeled residues in this sample. (c) Expansion of the aliphatic region of the 2D spectrum of SU7. (d) Aliphatic region of the 2D ¹³C-¹³C chemical shift correlation spectrum of Aβ_1–40 fibril sample SU6.

Fig 2.

¹³C NMR linewidths for CO, Cα, and Cβ sites in Aβ_1–40 fibrils, determined from 2D solid state NMR spectra as in Fig. 1. Linewidths of 2.5 ppm or less indicate well-ordered conformations. Larger linewidths in the N-terminal segment indicate structural disorder.

Fig 3.

Solid state NMR data on DLn Aβ_1–40 fibril samples with ¹³C labels at the indicated backbone carbonyl sites. These data constrain the φ and ψ angles of the second labeled residue. (a) fpRFDR-CT data and simulations for φ = 40° (solid line), 80° (dashed line), 120° (dot-dashed line), and 160° (dotted line). Simulations are scaled and baseline-corrected to match the first and last experimental data points. (b) DQCSA data and simulations for φ, ψ = −70°, −40° (green); 70°, −65° (red); and −165°, 135° (black).

Fig 4.

Structural model for Aβ_1–40 fibrils, consistent with solid state NMR constraints on the molecular conformation and intermolecular distances and incorporating the cross-β motif common to all amyloid fibrils. Residues 1–8 are considered fully disordered and are omitted. (a) Schematic representation of a single molecular layer, or cross-β unit. The yellow arrow indicates the direction of the long axis of the fibril, which coincides with the direction of intermolecular backbone hydrogen bonds. The cross-β unit is a double-layered structure, with in-register parallel β-sheets formed by residues 12–24 (orange ribbons) and 30–40 (blue ribbons). (b) Central Aβ_1–40 molecule from the energy-minimized, five-chain system, viewed down the long axis of the fibril. Residues are color-coded according to their sidechains as hydrophobic (green), polar (magenta), positive (blue), or negative (red).

Fig 5.

(a) Cross section of an Aβ_1–40 fibril with the minimal MPL indicated by scanning transmission electron microscopy (13, 29), formed by juxtaposing the hydrophobic faces of two cross-β units from Fig. 4. Residues 1–8 are included with randomly assigned conformations. (b) Possible mode of lateral association to generate fibrils with greater MPL and greater cross-sectional dimensions.