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

Amyloid beta peptide (A beta 42) 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 beta peptide (A beta 42) 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 (K (D) ) of 5.5 x 10(-6) M and a binding threshold of 6.25 x 10(-4) mu M for A beta 42 oligomers. The molecular docking simulations of A beta 42 to scFv AS revealed that scFv AS tends to approached A beta 42 oligomers and immature protofibrils mainly by their hydrophobic interaction and then drew A beta 42 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 A beta 42 and the hydrophobic interactions between scFv AS and the middle region (residues 20-33) of A beta 42. The combination of scFv AS with A beta 42 was realized likely through an induced-fit process.