Molecular basis of ß-amyloid oligomer recognition with a conformational antibody fragment.

Oligomers are intermediates of the ß-amyloid (Aß) peptide fibrillogenic pathway and are putative pathogenic culprits in Alzheimer's disease (AD). Here we report the biotechnological generation and biochemical characterization of an oligomer-specific antibody fragment, KW1. KW1 not only discriminates between oligomers and other Aß conformations, such as fibrils or disaggregated peptide; it also differentiates between different types of Aß oligomers, such as those formed by Aß (1-40) and Aß (1-42) peptide. This high selectivity of binding contrasts sharply with many other conformational antibodies that interact with a large number of structurally analogous but sequentially different antigens. X-ray crystallography, NMR spectroscopy, and peptide array measurements imply that KW1 recognizes oligomers through a hydrophobic and significantly aromatic surface motif that includes Aß residues 18-20. KW1-positive oligomers occur in human AD brain samples and induce synaptic dysfunctions in living brain tissues. Bivalent KW1 potently neutralizes this effect and interferes with Aß assembly. By altering a specific step of the fibrillogenic cascade, it prevents the formation of mature Aß fibrils and induces the accumulation of nonfibrillar aggregates. Our data illuminate significant mechanistic differences in oligomeric and fibril recognition and suggest the considerable potential of KW1 in future studies to detect or inhibit specific types of Aß conformers.

Oligomer-targeting with a conformational antibody fragment promotes toxicity in Aß-expressing flies.

INTRODUCTION: The self-assembly of Aß peptides into a range of conformationally heterogeneous amyloid states represents a fundamental event in Alzheimer's disease. Within these structures oligomeric intermediates are considered to be particularly pathogenic. To test this hypothesis we have used a conformational targeting approach where particular conformational states, such as oligomers or fibrils, are recognized in vivo by state-specific antibody fragments. RESULTS: We show that oligomer targeting with the KW1 antibody fragment, but not fibril targeting with the B10 antibody fragment, affects toxicity in Aß-expressing Drosophila melanogaster. The effect of KW1 is observed to occur selectively with flies expressing Aß(1-40) and not with those expressing Aß(1-42) or the arctic variant of Aß(1-42) This finding is consistent with the binding preference of KW1 for Aß(1-40) oligomers that has been established in vitro. Strikingly, and in contrast to the previously demonstrated in vitro ability of this antibody fragment to block oligomeric toxicity in long-term potentiation measurements, KW1 promotes toxicity in the flies rather than preventing it. This result shows the crucial importance of the environment in determining the influence of antibody binding on the nature and consequences of the protein misfolding and aggregation. CONCLUSIONS: While our data support to the pathological relevance of oligomers, they highlight the issues to be addressed when developing inhibitory strategies that aim to neutralize these states by means of antagonistic binding agents.