Comparison of Alzheimer Aβ(1–40) and Aβ(1–42) amyloid fibrils reveals similar protofilament structures

Abstract

We performed mass-per-length (MPL) measurements and electron cryomicroscopy (cryo-EM) with 3D reconstruction on an A␤(1-42) amyloid fibril morphology formed under physiological pH conditions. The data show that the examined A␤(1-42) fibril morphology has only one protofilament, although two protofilaments were observed with a previously studied A␤(1-40) fibril. The latter fibril was resolved at 8 Å resolution showing pairs of ␤-sheets at the cores of the two protofilaments making up a fibril. Detailed comparison of the A␤(1-42) and A␤(1-40) fibril structures reveals that they share an axial twofold symmetry and a similar protofilament structure. Furthermore, the MPL data indicate that the protofilaments of the examined A␤(1-40) and A␤(1-42) fibrils have the same number of A␤ molecules per cross-␤ repeat. Based on this data and the previously studied A␤(1-40) fibril structure, we describe a model for the arrangement of peptides within the A␤(1-42) fibril.Alzheimer's disease ͉ electron microscopy ͉ prion ͉ protein folding A myloid fibrils are fibrillar polypeptide aggregates that consist of a cross-␤ structure (1, 2). They accumulate inside the human body in the course of aging and are associated with several debilitating conditions such as Alzheimer disease (AD) (1, 2). AD amyloid fibrils are formed from A␤ peptide, which occurs in isoforms of different length. The 40-residue peptide A␤(1-40) represents the most abundant A␤ isoform in the brain (3), while the 42-residue A␤(1-42) shows a significant increase with certain forms of AD (4). A␤ amyloid fibrils have been analyzed with various biophysical and biochemical techniques, such as solid-state NMR (NMR) spectroscopy, solution state NMR spectroscopy coupled with hydrogen/deuterium exchange, or mutagenesis (5-8). These analyses have provided a wealth of information about specific structural details of the peptide in the fibril, for example dihedral torsional angles, protection factors or distances between specific atoms. Atomic models for A␤ peptides and their assembly in different amyloid fibrils have been constructed based on such data (6, 8-10) but these models have not been confirmed by more direct 3D imaging methods.A hallmark of A␤ amyloid fibrils is their substantial polymorphism (11)(12)(13)(14). We have recently shown by transmission electron cryomicroscopy (cryo-EM) and 3D reconstruction that A␤(1-40) fibrils form a range of morphologies with almost continuously altering structural properties (13). Despite their different morphologies, the cross-sectional areas of the reconstructed fibrils were similar. The study suggested that the observed polymorphism may be the result of different packing of protofilaments that have the same basic structure. To obtain an image of a fibril at higher resolution, we established growth conditions that promote a specific A␤(1-40) fibril morphology (15) and selected fibrils with this morphology from electron micrographs for further processing. Using newly developed image processing tools (16), we obtained a 3D image at 8 Å...

Transformation of amyloid β(1–40) oligomers into fibrils is characterized by a major change in secondary structure

“…The initial and final states of Ab fibrillogenesis have been well established and characterized by biophysical methods [26,27,40,41], but conversely, the dynamic process leading to aggregation and to the formation of oligomeric intermediates is much less understood and still under investigation [24,42]. Tycko's group provided evidence for a parallel b-sheet organization in the fibrils using solid state NMR implicating residues 12-40 with a turn located around residues 21-30 [25,26,41].…”

“…This strand-turn-strand motif is packed by the strands side chains and the turn is called b-arc. Other structural organizations have been suggested for fibrils based on cryo electron microscopy images [27]. Nevertheless, in all these models, the b-sheets are described as parallel and therefore should give rise to similar infrared spectral features.…”

“…Biophysical studies describe fibrils as cross-b-structure filaments [25][26][27]. However, structural studies of oligomers and the conformational transitions taking place during the aggregation process are limited by the size range and transitory nature of these aggregates [28].…”

Hydrogen bonding involving side chain exchangeable groups stabilizes amyloid quarternary structure

“…These are based on MAS solidstate NMR experiments, [5][6][7][8][9][10][11] and cryo-electron microscopic image reconstructions. 12 Even though there is some controversy concerning the conformational space that Ab fibrils can adopt, all published models, including the 2-fold 7,10 and 3-fold 9 symmetric Ab 1-40 fibril structure suggested by Tycko, and Bertini and co-workers, agree on the basic building block which involves a b-sheet (b1, residues 12-24), a turn, and a second b-sheet (b2, residues [28][29][30][31][32][33][34][35][36][37][38][39][40]. Biophysical studies indicate a certain degree of conformational plasticity for the N-terminal b-sheet.…”

“…14 In all structural models, Ab peptides are arranged in a parallel fashion. An antiparallel arrangement is only observed for the Iowa mutant Ab-D23N, 15 and for truncated forms of the peptide such as Ab [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . 16 The fibril structure is stabilized in particular by hydrogen bonding interactions.…”