Structural Correlates of Antibodies Associated with Acute Reversal of Amyloid β-related Behavioral Deficits in a Mouse Model of Alzheimer Disease

Abstract

Immunotherapy targeting of amyloid ␤ (A␤) peptide in transgenic mouse models of Alzheimer disease (AD) has been widely demonstrated to resolve amyloid deposition as well as associated neuronal, glial, and inflammatory pathologies. These successes have provided the basis for ongoing clinical trials of immunotherapy for treatment of AD in humans. Acute as well as chronic A␤-targeted immunotherapy has also been demonstrated to reverse A␤-related behavioral deficits assessing memory in AD transgenic mouse models. We observe that three antibodies targeting the same linear epitope of A␤, A␤ 3-7 , differ in their ability to reverse contextual fear deficits in Tg2576 mice in an acute testing paradigm. Reversal of contextual fear deficit by the antibodies does not correlate with in vitro recognition of A␤ in a consistent or correlative manner. To better define differences in antigen recognition at the atomic level, we determined crystal structures of Fab fragments in complex with A␤. The conformation of the A␤ peptide recognized by all three antibodies was highly related and is also remarkably similar to that observed in independently reported A␤:antibody crystal structures. Sequence and structural differences between the antibodies, particularly in CDR3 of the heavy chain variable region, are proposed to account for differing in vivo properties of the antibodies under study. These findings provide a structural basis for immunotherapeutic strategies targeting A␤ species postulated to underlie cognitive deficits in AD. Immunotherapy targeting the amyloid ␤ (A␤)2 peptide via either active (i.e. immunization with A␤ peptide, or fragments derived from it), or passive immunization (i.e. parenteral administration of anti-A␤ antibodies) has been widely demonstrated to be efficacious for modification of AD pathology (1, 2), as well as A␤-related behavioral deficits (3-5) in transgenic mouse models of Alzheimer disease (AD) (for reviews see Refs. 6, 7). These successes in pre-clinical studies have provided the basis for clinical trials of A␤ immunotherapy for treatment of AD in humans. Results from post-mortem histological evaluation of a limited sampling of patients from clinical trials of active immunotherapy with AN1792 provided initial corroborating evidence of pre-clinical findings with respect to reversal of plaque-associated AD pathology at autopsy in brains of treated patients (8 -13). Conclusive evidence for cognitive benefits stemming from reversal of pathology in AD patients undergoing anti-A␤ immunotherapy must await results from adequately powered Phase 3 clinical trial studies. Analysis of cognitive and functional outcomes in patients from Phase 1 and Phase 2 clinical trials provide evidence supporting improvement in some (13, 14), but not all (15) clinical measures of disease.A␤-associated behavioral deficits in transgenic mouse models of AD offer a potential surrogate of the cognitive and memory decline seen in AD patients (reviewed in Ref. 16). Arguments in support of this hypothesis stem from the fact that the b...

Heavy-Chain Complementarity-Determining Regions Determine Conformation Selectivity of Anti-Aβ Antibodies

“…The CDRs of the light chain, on the other hand, displayed a high degree of sequence similarity between all high-affinity antibodies, indicating that A ␤ affinity in general is associated with these regions. This idea is supported by the striking resemblance of the light-chain CDRs of our high-affinity antibodies and the antibodies 10D5 (IgG1), 12A11 (IgG1) and 12B4 (IgG2a), all binding to aa 3-7 of the A ␤ peptide [34,35] as well as the antibodies PFA1 (IgG2a) and PFA2 (IgG2a) [36] , where the CDR sequences differ only by a few amino acids.…”

“…This is further supported by the similarity between our pan-A ␤ antibody mAb1C3 and the antibody 12A11 (IgG1) [34] . These antibodies bind both monomeric and aggregated A ␤ and share 90% of their CDR sequences in the lightchain CDRs as well as in CDR1 and 2 of the heavy chain [35] . Furthermore, the extreme variability in sequence and length of the heavy-chain CDR3 in all antibodies suggests either that this region is not even in contact with the antigen [33] or that the slight variability in antigen recognition is mainly encoded in this CDR.…”

Amyloid Fibril Recognition with the Conformational B10 Antibody Fragment Depends on Electrostatic Interactions

“…1c) and engulf extended polypeptide chains within these cavities. 28,29 These antibodies were also shown to bind their antigens through a combination of positively and negatively charged CDR residues (Fig. 1d).…”

“…(c) Crystal structure of a fragment antigen binding (Fab) from an antibody that is sequence specific for the Aβ N-terminus (PDB code: 3IFN). 29 Aβ N-terminus is colored green; CDR residues are colored yellow (uncharged), blue (basic), and red (basic). (d) Surface representation of the same Fab fragment using the same color coding as in (c).…”

Synthesis, structure and properties of tris(1-ethyl-4-isopropyl-imidazolyl-κN)phosphine copper(II)

“…Please refer to Table 1 for additional crystal and refinement information. The structure was solved using SIR-97 [51] and refined using SHELXL-97 [52]. A direct-methods solution was calculated which provided most non-hydrogen atoms from the E-map.…”