Significant Structural Differences between Transient Amyloid‐β Oligomers and Less‐Toxic Fibrils in Regions Known To Harbor Familial Alzheimer′s Mutations

Abstract

Small oligomers of the amyloid b (Ab) peptide, rather than the monomers or the fibrils, are suspected to initiate Alzheimer's disease (AD). However, their low concentration and transient nature under physiological conditions have made structural investigations difficult. A method for addressing such problems has been developed by combining rapid fluorescence techniques with slower two-dimensional solidstate NMR methods. The smallest Ab 40 oligomers that demonstrate a potential sign of toxicity, namely, an enhanced affinity for cell membranes, were thus probed. The two hydrophobic regions (residues 10-21 and 30-40) have already attained the conformation that is observed in the fibrils. However, the turn region (residues 22-29) and the N-terminal tail (residues 1-9) are strikingly different. Notably, ten of eleven known Ab mutants that are linked to familial AD map to these two regions. Our results provide potential structural cues for AD therapeutics and also suggest a general method for determining transient protein structures.

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

“…ssNMR studies were performed on aggregates of 7S ⌬G25 peptide containing isotopically enriched Val 18 , Phe 19 , Ala 21 , Gly 33 , and Leu 34 residues. These studies revealed that S ⌬G25 fibrils are also rich in ␤-sheet conformations, similar to the amyloid fibrils formed by S and full-length A␤ (20).…”

“…Interestingly, small oligomers still form, perhaps driven by the nonspecific intermolecular interactions of the hydrophobic residues. However, the oligomers have random coil architecture and cannot easily grow in size, whereas the oligomers of S and full-length A␤ have at least some ␤-sheet character, and they easily grow to form mature fibrils (7,11,18). The final fibrillar architecture of S ⌬G25 , obtained only at very high concentrations, shows that ␤-sheets do form, but the hairpin opens up, and the ␤-sheet is anti-parallel in nature.…”

“…The central part of this hairpin structure consists of a hydrophilic region flanked by hydrophobic ␤-sheet-forming segments at both ends (5)(6)(7)(8)(9)(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). Chemically constraining the side chains of Asp 23 and Lys 28 accelerates the kinetics of A␤ aggregation by stabilizing the hairpin structure (5).…”

“…In fact a contact between hydrophobic regions containing Phe 19 and Leu 34 is one of the earliest contacts formed during the aggregation of amyloid ␤ (18). In a generic hydrophobic aggregate, the burial of the hydrophobic surface can in principle proceed in an intermolecular fashion, which would appear to render the turn unnecessary.…”

Alzheimer’s Protective A2T Mutation Changes the Conformational Landscape of the Aβ1–42 Monomer Differently Than Does the A2V Mutation

“…A link between monomer misfolding and aggregation propensity and toxicity has been proposed based on experiments (13)(14)(15)(16) and simulations (17)(18)(19)(20)(21)(22)(23)(24)(25)(26) of the naturally occurring alloforms. Those alloforms contain substitutions that occur mainly within the turn region (residues [22][23][24][25][26][27][28][29][30]. Because of the disordered structure of the N-terminus in the fibril structure, the role of this region in aggregation, toxicity, and pathogenesis was considered for a long time to be negligible (27) and therefore remained largely unexplored.…”