Role of the familial Dutch mutation E22Q in the folding and aggregation of the 15-28 fragment of the Alzheimer amyloid-beta protein

Abstract

Amyloid fibrils, large ordered aggregates of amyloid beta proteins (Abeta), are clinical hallmarks of Alzheimer's disease (AD). The aggregation properties of amyloid beta proteins can be strongly affected by single-point mutations at positions 22 and 23. The Dutch mutation involves a substitution at position 22 (E22Q) and leads to increased deposition rates of the AbetaE22Q peptide onto preseeded fibrils. We investigate the effect of the E22Q mutation on two key regions involved in the folding and aggregation of the Abeta peptide through replica exchange molecular dynamics simulations of the 15-28 fragment of the Abeta peptide. The Abeta15-28 peptide encompasses the 22-28 region that constitutes the most structured part of the Abeta peptide (the E22-K28 bend), as well as the central hydrophobic cluster (CHC) (segment 17-21), the primary docking site for Abeta monomers depositing onto fibrils. Our simulations show that the 22-28 bend is preserved in the Abeta(15-28) peptide and that the CHC, which is mostly unstructured, interacts with this bend region. The E22Q mutation does not affect the structure of the bend but weakens the interactions between the CHC and the bend. This leads to an increased population of beta-structure in the CHC. Our analysis of the fibril elongation reaction reveals that the CHC adopts a beta-strand conformation in the transition state ensemble, and that the E22Q mutation increases aggregation rates by lowering the barrier for Abeta monomer deposition onto a fibril. Thermodynamic signatures of this enhanced fibrillization process from our simulations are in good agreement with experimental observations.

Fig. 1.

Overlay of the central structures observed for Aβ21–30E22Q (transparent) and Aβ21–30WT. Residues D23–N27 are shown in stick representation; residues E22, Q22, and K28 are shown as lines. The backbone of the peptide is highlighted in gray. Hydrogen bonds are shown as dotted blue lines.

Fig. 2.

Contact maps for WT Aβ21–30 (bottom right) and the E22Q mutant (top left) obtained in the present simulations. The values correspond to the probability of having at least one pair of atoms in a given pair of side chains within 6 Å of one another.

Fig. 3.

All-atom representation of the most representative conformation of Aβ15–28 sampled in the present simulations. A large hydrophobic patch exposed to the solvent by residues of the central hydrophobic cluster is shown as surface.

Fig. 4.

Probability map of interresidue contacts computed in the present simulations for Aβ15–28WT (lower right triangle) and Aβ15–28E22Q (upper left triangle).

Fig. 5.

Transition state conformations of monomer deposition onto fibril identified in the present work. Two structural motifs, β-strands at positions H14–F20 and A30–V36, are shared by all observed conformations.

Fig. 6.

Cartoon illustrating how monomers of Aβ40 deposit onto amyloid fibrils. The time scale of this process is governed by the free energy difference between transition states and docked (monomeric) states, or the activation free energy ΔG^†. The activation free energy in Aβ40 with E22Q mutation is lowered compared with the WT sequence, ΔΔG^† > 0. The defining characteristics of transition state conformations are β-strands in segments H14–F20 and A30–V36. To reach these conformations, Aβ40 has to refold its central hydrophobic cluster segment L17–F20.

Fig. 7.

Deposition free energy. (a) Temperature dependence of the activation free energy for monomer deposition in Aβ15–28 and Aβ15–28E22Q as determined in the present simulations. (b) Free energy profile for Aβ15–28WT as a function of the RMS deviation from the transition state conformations over C^α atoms of L17–F20 segment.