The Alzheimer's Amyloid-β(1–42) Peptide Forms Off-Pathway Oligomers and Fibrils That Are Distinguished Structurally by Intermolecular Organization

Abstract

Increasing evidence suggests that soluble aggregates of amyloid-β (Aβ) initiate the neurotoxicity that eventually leads to dementia in Alzheimer's disease. Knowledge on soluble aggregate structures will enhance our understanding of the relationship between structures and toxicities. Our group has reported a stable and homogeneous preparation of Aβ(1-42) oligomers that has been characterized by various biophysical techniques. Here, we have further analyzed this species by solid state nuclear magnetic resonance (NMR) spectroscopy and compared NMR results to similar observations on amyloid fibrils. NMR experiments on Aβ(1-42) oligomers reveal chemical shifts of labeled residues that are indicative of β-strand secondary structure. Results from two-dimensional dipolar-assisted rotational resonance experiments indicate proximities between I31 aliphatic and F19 aromatic carbons. An isotope dilution experiment further indicates that these contacts between F19 and I31 are intermolecular, contrary to models of Aβ oligomers proposed previously by others. For Aβ(1-42) fibrils, we observed similar NMR lineshapes and inter-side-chain contacts, indicating similar secondary and quaternary structures. The most prominent structural differences between Aβ(1-42) oligomers and fibrils were observed through measurements of intermolecular (13)C-(13)C dipolar couplings observed in PITHIRDS-CT experiments. PITHIRDS-CT data indicate that, unlike fibrils, oligomers are not characterized by in-register parallel β-sheets. Structural similarities and differences between Aβ(1-42) oligomers and fibrils suggest that folded β-strand peptide conformations form early in the course of self-assembly and that oligomers and fibrils differ primarily in schemes of intermolecular organization. Distinct intermolecular arrangements between Aβ(1-42) oligomers and fibrils may explain why this oligomeric state appears off-pathway for monomer self-assembly to fibrils.

Structural, morphological, and functional diversity of amyloid oligomers

“…Subsequent NMR analysis determined that in this oligomer population, C-terminal b-sheet (residues 22-39) was included into the secondary structure formation, whereas the N-terminal half of the peptide was unstructured due to the EGCG binding [32]. In Ab 42 oligomers formed in the presence of SDS, residues 25-40 were found to form b-strands that were arranged in the antiparallel fashion, whereas the structure of the N-terminal half of the peptide was unclear [14,18]. An EPR investigation of oligomers prepared under the similar conditions indicated gradual increase in the structural order towards the C-terminus of the peptide and antiparallel arrangement of b-strands [18].…”

“…In Ab 42 oligomers formed in the presence of SDS, residues 25-40 were found to form b-strands that were arranged in the antiparallel fashion, whereas the structure of the N-terminal half of the peptide was unclear [14,18]. An EPR investigation of oligomers prepared under the similar conditions indicated gradual increase in the structural order towards the C-terminus of the peptide and antiparallel arrangement of b-strands [18]. NMR analysis of yet another population of Ab 42 oligomers ('globulomers') also formed in the presence of SDS suggested the presence of a dimeric structural unit consisting of a pair of in-register parallel b-strands formed by residues 34-40 extending from a b-hairpin formed by antiparallel arrangement of two additional b-strand regions (18-23 and 28-33) [33] ( Fig.…”

Structural differences between amyloid beta oligomers

“…However, several structural models of amyloid oligomers have been proposed, some of them based on high-resolution x-ray crystallography or NMR data [15,33,34]. We have previously proposed a classification of Aβ and other amyloid oligomers into structural classes based on their reactivity with conformation dependent antibodies [12].…”

Antiparallel β-Sheet Structure within the C-Terminal Region of 42-Residue Alzheimer's Amyloid-β Peptides When They Form 150-kDa Oligomers

“…Here we report further on a 150 kDa Aβ(1–42) oligomer (~30–35 Aβ(1–42) molecules) that is isolated as a stable and homogenous preparation. 23 The isolation protocol involves aggregation in dilute SDS and is similar to that used to prepare distinct Aβ oligomers called globulomers. 27 Globulomers do not appear to be intermediates leading to fibril formation because globulomers do not readily convert to fibrils and do not accelerate fibril formation when added as potential seeds to initially monomeric Aβ(1–42) solutions.…”