Assessment of structural diversity of ß-amyloid with genetically engineered antibody fragments
Several antibodies targeting β-amyloid (Aβ) conformers have been studied so far, but the mechanism of their action is still poorly understood. I described several VHH antibody domains, selected with the use of a camelid antibody library. Using various biophysical (e.g., electron microscopy, surface plasmon resonance) and biochemical methods (e.g., enzyme-linked immunosorbent assay, immunoblot), I assessed the molecular mechanism of the binding, and the conformational specificity of particular VHH antibodies to their respective targets. The recognition of Aβ40 by B10, KW2, and KW3 proceeds in a conformation-specific manner; these antibodies bind fibrils, but they interact neither with Aβ oligomers, nor with the disaggregated Aβ peptide. Analysis of the B10 recognition mechanism of Aβ fibrils showed that it involves electrostatic interactions, as well as complementarity of flat surfaces on the antibody and the fibril. Hence, B10 binds to fibrils possessing the following properties: a regular cross-β sheet structure, and a regular distribution of anionic surface charges. KW1 interacts only with a certain type of Aβ40 oligomers. In contrast, oligomers derived either from Aβ42, or from non-Aβ peptides are not recognised. None of the analysed features of oligomers (i.e., polypeptide origin, morphological appearance, presence of parallel or anti-parallel β-sheet structure) predisposes oligomers to act as a KW1 ligand. The recognition of Aβ40 oligomers might be based on hydrophobic interactions, as suggested by the analysis of the crystal structure of KW1. The mechanism of VHH antibody recognition proposed here is based on ionic (B10-fibrils) and hydrophobic (KW1-oligomers) interactions. This provides information about the surface structure of particular Aβ aggregates. Furthermore, the VHH binders, capable of distinguishing between different conformational states of Aβ, and may prove useful in an assessment of the pathological relevance of Aβ.