Basic Science and Pathogenesis

Background: Compared to the 'neutral' E3, the E4 allele of Apolipoprotein E (APOE) confers up to a 15-fold increase in Alzheimer's Disease (AD) risk. Conversely, the neuroprotective E2 allele decreases AD risk by a similar degree. Here, we aimed to assess the therapeutic potential of cell-type specific allelic 'switching' by investigating the physiological and neuropathological changes associated with an inducible, in vivo APOE4 to APOE2 transition in astrocytes using a novel transgenic mouse model METHOD: The APOE "switch mouse" (APOE4s2) uses the Cre-loxP system to allow for inducible APOE allele switching from E4 to E2. These mice express a floxed human APOE4 coding region followed by the human APOE2 coding region. Allelic discrimination (RT-PCR) and mass spec-based proteomic analyses were employed to validate the E4 to E2 transition. Single-cell RNAseq and Xenium In Situ were used to measure transcriptomic changes following the astrocytic E4 to E2 allele switch. Behavioral measures and neuropathological analyses were applied to assess the effects of an early-life vs. mid-life allelic switch on AD pathology.

Result: mRNA and protein analyses confirm that APOE4s2 mice synthesize full-length human APOE4 pre-switch, and that tamoxifen induces an efficient recombination and expression of human APOE2 in target tissues. Single-cell RNAseq and Xenium reveal that, genetic replacement of astrocytic APOE4 with APOE2 results in distinct alterations to glial cell transcriptomes affecting pathways involved with metabolism, inflammation, and amyloid beta. Neuropathological analyses show that an astrocyte-specific E4 to E2 'switch' significantly decreases total amyloid burden, even once severe amyloidosis has already begun (mid-life 'switch'), and that the astrocytic E4 to E2 transition improves cognition. Additionally, amyloid-associated astro- and micro-gliosis are decreased, and mice with E2-expressing astrocytes had less MHCII expressing microglia and a decrease in plaque-associated ApoE.

Conclusion: Together, these data suggest that a successful transition from E4 to E2 in astrocytes has broad impact on the cerebral transcriptome and that an astrocyte-specific E4 to E2 'switch' improves multiple AD-associated pathologies at different points across the AD pathophysiological timeline.