[P3–182]: PROPAGATIVE TOXIC Aβ OLIGOMER SUBCLASS IDENTIFIED AND DEFINED BY SOLVENT ORIENTATION OF THE LYSINE 28 SIDE CHAIN

Oligomers of Amyloid-β (AβO) are deemed key in synaptotoxicity and amyloid seeding of Alzheimer's disease (AD). However, the heterogeneous and dynamic nature of AβO, and inadequate markers for AβO subtypes, have stymied effective AβO identification and therapeutic targeting in vivo. We identified an AβO-subclass epitope defined by constrained sidechain orientation of serine-asparagine-lysine (cSNK), not displayed by Aβ monomers and fibrils. Molecular dynamics simulations confirmed the epitope restriction of cSNK to AβOs. Specificity of a mouse monoclonal antibody raised against the AβO cSNK epitope was evaluated by surface plasmon resonance (SPR), dot blotting/immunoblotting, immunoprecipitation, immunohistochemistry and immunoelectron microscopy. The percent of AβO falling into the cSNK subclass was defined by acute peripheral infusion of anti-AβO in aged animals of two AD mouse models (Tg2576 and APP/PS1) followed by soluble Aβ aggregate quantification. Behavioural toxicity of AβO was evaluated in vivo by: 1) AβO intraventricularly co-injected with the anti-AbO antibody into C57BL/6 wild-type mice followed by Novel Object Recognition testing, and 2) Chronic cSNK peptide immunization of APP/PS1 mice starting at 3 months of age and continuing monthly until sacrifice at 12 months, with Cued and Contextual Fear Conditioning testing at 11 months. Immunobloting and immunohistochemistry identifed AβO sensitive cells and brain regions. A mouse monoclonal antibody targeting AβO recognizes ∼50–60 kDa SDS-resistant Aβ soluble assemblages in AD brain. Acute peripheral infusion of anti-AβO in AD mouse models reduced soluble brain Aβ aggregates by 20–30%. Chronic cSNK peptide immunization of APP/PS1 mice generated an anti-AβO IgG1 response without epitope spreading to Aβ monomers or fibrils, and was accompanied by preservation of global PSD95 expression and improved cued fear memory. The anti-AβO IgG1 reduced amygdalar plaque burdens in female mice, with no effect on hippocampal plaque. A parsimonious interpretation of the results is that antibody-accessible brain AβO, a minority of total AβO, participate in AD synaptic architectural disruption, with specific toxicity for post-synaptic dendrites, and seeding of Aβ fibrillogensis via the hippocampal-amygdala connectome. The IgG1 response indicates local neutralization/clearance, without the participation of Fc receptor-bearing cells. We demonstrate that pathogenic activities of AβO subclasses can be defined in vivo by immunological subtraction.

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