Functional Characteristics and Molecular Mechanism of a New scFv Antibody Against Aβ42 Oligomers and Immature Protofibrils.

Amyloid β peptide (Aβ42) is a major determinant of Alzheimer's disease (AD). In this study, we studied a novel single-chain variable fragment (scFv), AS, generated from an antibody library of AD patients, which recognized and bound specifically to medium-size amyloid β peptide (Aβ42) oligomers and immature protofibrils (25-55 kDa) and, more importantly, reduced their level by blocking their formation or inducing their disassembly. Consequently, scFv AS ameliorated or prevented their cytotoxicity and protected SH-SY5Y cells and primary cultured neurons in vitro from their damage in a concentration-dependent manner. Comparison of its cytotoxicity-inhibiting and cytotoxicity-neutralizing activities indicated that scFv AS displayed its protective effect on target cells mainly due to its cytotoxicity-inhibitory activity though it could also neutralize the cytotoxicity. We also found that scFv AS could efficiently cross the in vitro BBB model with a delivery efficiency of over 70% after a 60-min post-administration. The scFv AS was a monovalent antibody with an affinity constant (KD) of 5.5 × 10(-6) M and a binding threshold of 6.25 × 10(-4) μM for Aβ42 oligomers. The molecular docking simulations of Aβ42 to scFv AS revealed that scFv AS tends to approached Aβ42 oligomers and immature protofibrils mainly by their hydrophobic interaction and then drew Aβ42 molecule into the gap between VL and VH domains of scFv AS by hydrophilic interaction between scFv AS and the N-terminal region (residues 1-15) of Aβ42 and the hydrophobic interactions between scFv AS and the middle region (residues 20-33) of Aβ42. The combination of scFv AS with Aβ42 was realized likely through an induced-fit process.