Solid-state NMR Reveals a Close Structural Relationship between Amyloid-β Protofibrils and Oligomers

Abstract

Background: Little tertiary structure information is available for the toxic intermediates in the amyloid-␤ (A␤) fibrillation process. Results: A␤ protofibrils show tertiary contacts between Glu-22 and Ile-31, which are not present in mature fibrils. Conclusion: A␤ protofibrils share tertiary structure features with oligomers but not with mature fibrils. Significance: A␤ protofibrils must undergo a major structural reorientation in the development of mature A␤ fibrils.We have studied tertiary contacts in protofibrils and mature fibrils of amyloid-␤ (A␤) peptides using solid-state NMR spectroscopy. Although intraresidue contacts between Glu-22 and Ile-31 were found in A␤ protofibrils, these contacts were completely absent in mature A␤ fibrils. This is consistent with the current models of mature A␤ fibrils. As these intramolecular contacts have also been reported in A␤ oligomers, our measurements suggest that A␤ protofibrils are structurally more closely related to oligomers than to mature fibrils. This suggests that some structural alterations have to take place on the pathway from A␤ oligomers/protofibrils to mature fibrils, in agreement with a model that suggests a conversion of intramolecular hydrogen-bonded structures of A␤ oligomers to the intermolecular stabilized mature fibrils (Hoyer, W., Grönwall, C., Jonsson, A., Ståhl, S., and Härd, T. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 5099 -5104).Alzheimer disease is characterized by extracellular deposition of plaques of amyloid-␤ (A␤) 2 peptides in the brain (1). These protein aggregates are composed of mature A␤ fibrils, which represent the end product of a long, complex, and not well understood fibrillation process (2, 3). The fibrillation pathway initiates with soluble unstructured monomeric A␤ peptides, which are converted into oligomers, protofibrils, and finally into mature fibrils (4 -6). Recently, interest in the transient A␤ intermediate structures has been growing rapidly because these species are considered to represent the cytotoxic intermediates in Alzheimer disease (7). In addition to the well studied structure (8 -12) and dynamics (13) of mature A␤ fibrils, a growing amount of data for oligomers (14 -19) and protofibrils (20, 21) has become available. With regard to the secondary structure elements, these studies revealed that oligomers and protofibrils already exhibit the characteristic two ␤-strand sections connected by a short loop also present in mature A␤ fibrils. However, there are several significant differences between theses species. For instance, the first ␤-strand of A␤ oligomers and protofibrils is significantly shorter than that in mature A␤ fibrils and has to elongate during the conversion from protofibrils to mature fibrils (20). In addition, many questions about the tertiary structure, the fibrillation process, and the conversion from one intermediate to another are still unanswered.Härd and co-workers have resolved the structure of A␤(1-40) oligomers stabilized by an Affibody and also proposed a model for the arrangement of t...

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

“…proposed a β-hairpin structure for Aβ(1–40) protofibrils stabilized by an antibody-derived fusion protein. 35 The structure was reported to include a close proximity of E22 and I31 that is inconsistent with Aβ(1–40) fibril models but consistent with a β-hairpin. 35 Another solid state NMR study by Doi et.…”

“…It is often thought that fibrils are the eventual stable products of Aβ aggregation and that oligomers and protofibrils exist transiently. 32 Consequently, different non-fibrillar structures have been argued to be either “on-pathway” 33–35 or “off-pathway” 28 to fibril formation. Molecules within off-pathway aggregates must dissociate into monomers before eventually being incorporated into fibrils, whereas on-pathway aggregates can convert to fibril structures without dissociation.…”

“…have shown solid state NMR evidence that protofibrils could be composed of molecules that experience intra-molecular hydrogen bonding between distinct β-strands and form antiparallel β-sheets. 35, 36 While Scheidt et. al .…”

Antiparallel Triple-strand Architecture for Prefibrillar Aβ42 Oligomers

“…Ahmed et al (16) interpreted the high frequency FTIR peak at 1675-1695 cm Ϫ1 as side chain vibrations of arginine, asparagine, and glutamine in their A␤42 oligomers. Detailed structural information such as inter-residue distances is very limited (16,58). Currently there are no structural models of oligomers based on extensive experimental restraints.…”

Steric Crowding of the Turn Region Alters the Tertiary Fold of Amyloid-β18–35 and Makes It Soluble

“…The central part of this hairpin structure consists of a hydrophilic region flanked by hydrophobic ␤-sheet-forming segments at both ends (5-10). This particular shape is believed to be dictated by the specific pattern of hydrophobic and charged residues in the A␤ sequence and has been identified even in soluble A␤ aggregates (7,(11)(12)(13)(14)(15)(16)(17), including the very early stage oligomers of A␤ (18 , and stabilizes the soluble monomeric and oligomeric assemblies of A␤ (19). This hairpin turn thus seems to be a critical factor dictating the self-assembly of A␤ under physiological conditions.…”