Rationally designed antibodies as research tools to study the structure–toxicity relationship of amyloid-β oligomers

Alzheimer’s disease is associated with the aggregation of the amyloid-β peptide (Aβ), resulting in the deposition of amyloid plaques in brain tissue. Recent scrutiny of the mechanisms by which Aβ aggregates induce neuronal dysfunction has highlighted the importance of the Aβ oligomers of this protein fragment. Because of the transient and heterogeneous nature of these oligomers, however, it has been challenging to investigate the detailed mechanisms by which these species exert cytotoxicity. To address this problem, we demonstrate here the use of rationally designed single-domain antibodies (DesAbs) to characterize the structure–toxicity relationship of Aβ oligomers. For this purpose, we use Zn²⁺-stabilized oligomers of the 40-residue form of Aβ (Aβ₄₀) as models of brain Aβ oligomers and two single-domain antibodies (DesAb_18-24 and DesAb_34-40), designed to bind to epitopes at residues 18–24 and 34–40 of Aβ₄₀, respectively. We found that the DesAbs induce a change in structure of the Zn²⁺-stabilized Aβ₄₀ oligomers, generating a simultaneous increase in their size and solvent-exposed hydrophobicity. We then observed that these increments in both the size and hydrophobicity of the oligomers neutralize each other in terms of their effects on cytotoxicity, as predicted by a recently proposed general structure–toxicity relationship, and observed experimentally. These results illustrate the use of the DesAbs as research tools to investigate the biophysical and cytotoxicity properties of Aβ oligomers.