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Author ORCID Identifier

https://orcid.org/0009-0006-1628-2067

Date Available

5-31-2026

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Medicine

Department/School/Program

Physiology

Faculty

Lance Johnson

Faculty

Cata Velez-Ortega

Abstract

APOE exists as three common alleles (APOE2, APOE3, APOE4) that encode their corresponding protein isoforms (apoE2, apoE3, apoE4). Despite differing by only one or two single-amino acid substitutions, each APOE allele confers a distinct AD risk profile with APOE4 increasing risk by 28% and APOE2 decreasing risk by 14% compared to the most commonly expressed APOE3 allele. While traditionally considered “neutral,” APOE3 still accounts for ~36% of AD cases while APOE4 contributes ~57%, indicating that the majority of AD cases would not occur in the absence of the APOE3 and APOE4 alleles. Microglia, the resident phagocytic immune cells of the brain, are strongly implicated in AD, with several AD risk variants identified by genome-wide association studies (GWAS) linked to microglial function. Moreover, APOE genotype alters the inflammatory, metabolic, and phagocytic response of microglia in AD and AD-related models. Here, we sought to define the AD-relevant, context-specific responses of microglia expressing the different APOE alleles. First, we assessed the spatial transcriptomic microglial signatures of APOE3 and APOE4 mice crossed to the 5xFAD model of amyloidosis. We identified heterogeneous disease-associated microglial clusters that differentially associate with parenchymal versus vascular amyloid pathology, and demonstrate APOE-dependent differences, with APOE4 exhibiting more pronounced disease-associated microglial signatures. Second, we utilized an APOE “switch mouse” (APOE4s2), generated by our lab, to selectively transition microglia from APOE4 to APOE2 expression and observe the dynamic response of microglial APOE2 expression to de/remyelination. The APOE4s2 model uses the Cre-loxP system to allow for an inducible and conditional replacement of the APOE4 allele with the protective APOE2 allele in microglia specifically. Following allelic switching in microglia, mice were subjected to one of two de/remyelination paradigms, and the glial cell and lipid profiles were assessed. Following remyelination, microglia-specific APOE2 expression resulted in increased myelin, decreased microgliosis, and altered astrocytic lipid droplet load compared to mice expressing APOE4 in all cells. Finally, we examined how microglia-specific APOE2 allelic switching modulates the response to a Western Diet or inflammatory challenge in aged mice. We found that microglial APOE2 switching reduced phago-lysosomal microglia and modestly altered the lipid and metabolic profile following a Western Diet while enhancing expression of metabolic and immune-related pathways in response to inflammation. Together, these results suggest that microglial APOE expression is highly dynamic with both genotype and context regulating the immune, transcriptional, and metabolic profile of microglia and surrounding glial cells, thereby providing insight into how microglial APOE influences responses to AD-relevant challenges. We hope these data, along with the APOE4s2 model, will be a valuable resource for the AD/APOE research communities as therapeutic approaches targeting microglia and apoE continue to emerge.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.311

Archival?

Archival

Funding Information

This study was supported by the National Institutes of Health grants R01AG081421 (2023), R01AG080589 (2023), RF1NS118558 (2020), and T32 AG078110 (2022); a grant from the Central Nervous System Center for Biomedical Research Excellence P20 GM148326 (2023); and a grant from the Alzheimer’s Association ABA-22-972169 (2023).

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