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Understanding the molecular mechanisms behind the interactions of amyloid beta oligomers (AβO) with an AβO-specific antibody and the AβO-receptors Khorvash, Massih

Abstract

Alzheimer's disease (AD) is the most common cause of cognitive impairment. It is characterized by the presence of plaques in the brain consisting of protein fragments called amyloid beta (Aβ). Monomeric Aβ peptides can aggregate to form Aβ oligomers (AβOs), which assemble into Aβ fibrils that deposit as plaques. AβOs are the most neurotoxic of the three Aβ forms. AβO-specific antibodies have been developed to neutralize the toxicity of AβOs. We used computational methods to determine how an AβO-specific monoclonal antibody, 5E3 (m5E3), has a higher affinity for AβOs than Aβ monomers and Aβ fibrils. Our study provided proof of the effectiveness of AβO-specific antibodies as some of the AβOs start to disaggregate in the presence of 5E3. A single-chain antibody of m5E3 (ScFv5E3) was designed to conduct NMR-based structural analysis of how the ScFv5E3 binds to a peptide that mimics the Aβ epitope (mimotope). We verified the complementarity determining regions (CDR) of m5E3 by demonstrating the binding of ScFv5E3 to the mimotope using dot-blot and surface plasmon resonance techniques. We also developed a protocol that allowed us to produce a few hundred μg of ScFv5E3. However, this amount was insufficient to perform the NMR study. Nevertheless, the ScFv5E3 may prove effective as a therapeutic agent, as its small size may increase its ability to pass through the blood brain barrier and it is unlikely to cause vasogenic edema (which arises from the microglia-activating constant region of full-sized monoclonal antibodies (1)) The possible binding sites of AβOs on cell-surface receptors were determined based on sequence alignments of AβO-receptors with the CDRs of m5E3. Some of the predicted binding sites mapped to the regions of the receptors that have been suggested to be the binding sites of AβOs. This rational drug design approach may prove productive in discovering a therapeutic treatment for AD. The predicted binding sites may be used to design therapeutic inhibitor peptides and novel AβO-specific antibodies. The high sequence similarities among the CDRs of m5E3 and some of the AβO-receptors also suggest a mechanism for the neutralizing effect of m5E3 on the cytotoxicity of AβOs.

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