Structures of Aβ-Related Peptide-Monoclonal Antibody Complexes

Passive immunotherapy (PI) is being explored as a potential therapeutic against Alzheimer's disease. The most promising antibodies (Abs) used in PI target the EFRH motif of the A beta N-terminus. The monoclonal anti-A beta Ab PFA1 recognizes the EFRH epitope of A beta. PFA1 has a high affinity for A beta fibrils and protofibrils (0.1 nM), as well as good affinity for A beta monomers (20 nM). However, PFA1 binds the toxic N-terminally modified pyroglutamate peptide pyro-Glu3-A beta with a 77-fold loss in affinity compared to the WT A beta(1-8). Furthermore, our earlier work illustrated PFA1's potential for cross-reactivity. The receptor tyrosine kinase Ror2, which plays a role in skeletal and bone formation, possesses the EFRH sequence. PFA1 Fab binds the Ror2(518-525) peptide sequence REEFRHEA with a 3-fold enhancement over WT A beta(1-8). In this work, the crystal structures of the hybridoma-derived PFA1 Fab in complex with pyro-Glu3-A beta peptide and with a cross-reacting peptide from Ror2 have been determined at resolutions of 1.95 and 2.7 angstrom, respectively. As with wild-type A beta, these peptides bind to the Fab via a combination of charge- and shape-complementarity, hydrogen-bonding, and hydrophobic interactions. Comparison of the structures of the four peptides A beta(1-8), Gripl, pyro-Glu3-A beta(3-8), and Ror2 in complex with PFA1 shows that the greatest conformational flexibility occurs at residues 2 to 3 and 8 of the peptide. These structures provide a molecular basis of the specificity tolerance of PFA1 and its ability to recognize A beta N-terminal heterogeneity. The structures provide clues to improving mAb specificity and affinity for pyroglutamate A beta.