Mechanisms of Apolipoprotein E Isoform Conferred Susceptibility and Resistance to Alzheimer's Disease
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Susceptibility to sporadic Alzheimer's disease (AD) is foremost modulated by APOE genotype. A single copy of the APOE ε4 allele endows a ~3 fold increase in AD risk, and 2 ε4 copies effect a ~15 fold increase, while an ε2 allele halves AD risk compared to ε3 homozygotes. Neither etiology of ε4 deleterious effect nor ε2 conferred protection is fully explained. Past work primarily focused on the role of apoE in Aβ deposition showed thatapoE in isoform-dependent fashion binds Aβ peptides, facilitates assembly of Aβ into amyloid fibrils and promotes formation of parenchymal plaques and vascular deposits in the rank order of E4>>E3>E2, while the Apoe gene knockout in APP transgenic mice precludes formation of fibrillar Aβ deposits. In addition to the well-studied catalytic effect on fibrillization and deposition of Aβ, there is evidence of apoE isoform-specific effect on the clearance of Aβ from the brain extracellular (or interstitial) space, modulation of microglia inflammatory response, and regulation of synaptic plasticity and neuronal network function, which all may contribute to the differential effect of APOE genotype on AD susceptibility. Though relationship between APOE genotype and variable rate of Aβ clearance from the brain interstitial space is well recognized, how apoE isoforms differentially engage this process and whether it depends on direct apoE/Aβ binding remains disputed. Our preliminary microdialysis experiments indicate substantial degree of binding between apoE and Aβ in the brain interstitial fluid (ISF) of Tg2576 and PDAPP mice while application of specific apoE/Aβ antagonist, dramatically increases unbound Aβ level. Based on these data we hypothesize that apoE isoforms differentially bind Aβ in the ISF and that pharmacological targeting of the apoE/Aβ interaction may enhance Aβ clearance and prevent Aβ oligomerization. This hypothesis will be explored in Specific Aim I using APPSW/PS1dE9/apoE-TR mice (APP/E-TR) with targeted replacement (TR) of the mouse Apoe gene for various human APOE alleles and APP/E-/- mice subjected to various in vivo microdialysis experiments. Specific Aim II will investigate how APOE genotype influences inflammatory microglia response. Our preliminary studies show greater microglia activation in APP/E4 mice in response to Aβ deposition and anti-Aβ passive immunization than in APP/E2 and APP/E3 mice. We thus hypothesize that ineffective Aβ phagocytosis and deleterious microglia activation can be an independent mechanism of ε4 allele conferred susceptibility to AD. This hypothesis will be explored by functional phagocytosis and transcriptomics studies of primary CNS microglia isolated from apoE-TR and apoE-/- mice of various ages and from APP/E-TR and APP/E-/- mice. Transcriptome assessment will include RT-qPCR of pro- and anti-inflammatory cytokines and unbiased RNA-Seq to identify signaling pathways differentially activated by apoE isoforms in microglia. Specific Aim III, using aged apoE-TR mice and APP/E-TR mice, will investigate how apoE isoforms in Aβ-independent and Aβ-dependent way modulate Reelin-Apoer2/Vldlr signaling, which regulates synaptic plasticity and neuronal network integrity.
