On Wednesday, December 11, 2024 Lesley Golden successfully defended her dissertation and earned her doctoral degree in physiology. Congratulations, Dr. Golden!
Cell-Type Specific APOE4 to APOE2 ‘Switching’ in Astrocytes and Microglia Alters Alzheimer’s Disease Neuropathology
Apolipoprotein E (APOE) is the strongest genetic risk factor for late-onset Alzheimer’s disease. APOE exists in three common protein isoforms throughout the population: ApoE2 (E2), ApoE3 (E3), and ApoE4 (E4). When compared to the ‘neutral’ and most common E3 allele, the E4 allele confers up to a 33-fold increase in Alzheimer’s Disease (AD) risk. Conversely, the neuroprotective E2 allele decreases AD risk by up to 67%. Here, we aimed to determine the therapeutic potential of cell-type specific APOE allele ‘switching’ and explore potential mechanisms therein. To do so, we assessed the physiological and neuropathological changes associated with an in vivo APOE4 to APOE2 transition selectively in astrocytes or microglia using a novel transgenic mouse model.
The APOE “switch mouse” (APOE4s2) generated by our lab uses the Cre-loxP system to allow for an inducible replacement of the E4 allele with the neuroprotective E2 allele. These mice express a floxed human APOE4 coding region followed by the human APOE2 coding region. Preliminary data confirms that the APOE4s2 mice synthesize a full-length human ApoE4 that is identical to the ApoE protein synthesized by commonly used transgenic ‘targeted-replacement’ (TR) ApoE4 mice pre-switch, and that the tamoxifen induced ‘switch’ results in ApoE2 expression. Physiological phenotyping, gene expression measures, immunohistochemistry, and proteomic analyses further show that tamoxifen administration successfully results in efficient recombination and expression of human APOE2/ApoE2 in various cell types of interest (dependent on the Cre promoter): i) all ApoE producing cells throughout the body, ii) astrocytes, and iii) microglia. Single-cell RNAseq was used to measure gene expression changes following the full-body or astrocyte-specific E4 to E2 allele switch. Both a global and astrocyte-specific transition to ApoE2 expression resulted in alterations to several CNS cell-type transcriptomes, namely glial cells. Many of the genes impacted by global and astrocyte-specific APOE allele replacement were involved with AD-related pathways. Additionally, we found that astrocyte-specific replacement of APOE4 with APOE2 resulted in distinct cell-autonomous and non-autonomous alterations to the transcriptome that overlapped with what was observed following a full-body APOE ‘switch’.
To further characterize how cell-type specific APOE4 allele replacement impacted AD pathology, APOE4s2 mice with astrocyte or microglia specific Cre-recombinase were crossed with the 5xFAD model of amyloidosis. Tamoxifen was administered after the onset of pathology, to induce astrocyte or microglia exclusive expression of APOE2. Behavioral measures and neuropathological analyses were applied to assess the effects of a cell-specific late-stage allelic switch on AD pathology. Neuropathological analyses show while both an astrocyte- and microglia-specific E4 to E2 ‘switch’ significantly decreases total amyloid burden, even once severe amyloidosis has already begun, there are unique cell-type driven responses in gliosis and cognitive outcomes. Together, these data suggest that a successful transition from E4 to E2 has broad impact on the cerebral transcriptome and that glia-specific E4 to E2 ‘switching’ improves multiple AD-associated pathologies. Overall, this first-of-its kind, proof of concept data provides insight that will be valuable for guiding development of future APOE directed therapies, and we hope that this model will be a valuable resource for the AD/ApoE research communities.