Single amino acid change in the receptor binding domain of mouse APOE results in altered protein dynamics and worsened AD‐related cognitive impairment

Background APOE genotype is the strongest genetic modifier of late‐onset Alzheimer’s disease (AD) in humans. In genetically diverse mice, natural variation in the Apoe locus predicts AD‐related cognitive decline, with C57BL/6J (B6) mice harboring a protective variant compared to DBA/2J (D2) (Neuner 2019, Neuron). The mechanisms by which protective and detrimental Apoe / APOE variants affect cognitive decline in AD are not fully understood yet, but likely could provide an opportunity for personalized treatment and prevention strategies. Method Here, we have used CRISPR/Cas9 gene editing to introduce the D2 Apoe variant into the B6 genome. We crossed these heterozygous APOE B6/E163D mice to B6 congenic heterozygous 5XFAD carriers in order to explore the effects of this single variant on AD‐related cognitive function, pathology, cell‐type specific gene dysregulation, and hippocampal and cortical network function. We also performed molecular dynamics modeling to ask how this variant affects APOE protein biophysics. Result 5XFAD‐APOE B6/E163D mice showed exacerbated cognitive dysfunction compared to 5XFAD‐APOE B6/B6 mice, indicating that the single D2 variant in Apoe significantly contributes to AD‐related cognitive decline. We did not observe differences in amyloid pathology or phosphorylated tau across Apoe genotypes, suggesting that the effect on cognitive function is not mediated by pathological protein aggregation. However, snRNAseq revealed that excitatory neuron gene expression was substantially dysregulated in 5XFAD‐APOE B6/E163D mice compared to other cell types; we then performed in vivo electrophysiology to characterize how this translates to circuit‐level neuronal dysfunction. Finally, computational modeling of protein structure and function showed that the APOE E163D variant results in protein states most closely resembling human APOE4, particularly in the presence of lipids, suggesting that APOE E163D may show impaired lipid binding and transport function. Conclusion C57BL/6J mice harbor a protective Apoe variant which confers resiliency to AD‐related cognitive dysfunction. This protection, and–by the same measure–the detrimental effects of the DBA/2J variant of Apoe (APOE E163D ), is likely ultimately mediated by altered neuronal gene expression and activity. We are currently exploring the intermediate mechanisms between APOE E163D and neuronal function; altered APOE protein states hint at impairments in lipid binding and transport, which may result in pathological lipid and protein accumulation affecting neuron function.