Glia‐Selective Replacement of APOE4 with APOE2: Implications for AD‐Associated Pathologies

Abstract

AbstractBackgroundCompared 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. APOE’s strong risk profile and multitude of effects make it a promising therapeutic target. Here, we aimed to assess the therapeutic potential of allelic ‘switching’ by investigating the physiological changes associated with an inducible, in vivo APOE4 to APOE2 transition in a novel transgenic mouse model.MethodsThe APOE “switch mouse” (APOE4s2) uses the Cre‐loxP system to allow for inducible APOE allele switching from APOE4 to APOE2. These mice express a floxed human APOE4 coding region followed by the human APOE2 coding region. To study the effects of full body APOE switching, APOE4s2 mice were crossed to ubiquitous ROSA26‐CreERT1 mice. Gene expression and proteomic analyses were used to validate that the model worked as expected. Lipidomics and single‐cell RNAseq were used to measure physiological changes following the APOE4 to APOE2 allele switch.ResultmRNA and protein analyses confirm that APOE4s2 mice synthesize full‐length human APOE4/ApoE4 pre‐switch and that the inducible switch results in successful and efficient recombination and expression of human APOE2/ApoE2. Single‐cell RNAseq shows that global, genetic replacement of APOE4 with APOE2 results in distinct alterations to glial cell transcriptomes affecting pathways involved with lipid and mitochondrial metabolism, inflammation, blood‐brain barrier integrity, and amyloid beta. Excitingly, differentially expressed genes associated with an E4 to E2 ‘switch’ included several AD GWAS, and both aging‐ and AD‐associated pathways. Additionally, untargeted lipidomics reveals changes to several lipid classes in the brain, in particular glycerophospholipids.ConclusionTogether, these data suggest that a successful transition from APOE4 to APOE2 has broad impact on the cerebral transcriptome as well as peripheral and cerebral metabolism. Ongoing studies aim to determine whether cell‐specific replacement of APOE4 with APOE2 will rescue E4‐associated metabolic dysfunction, disease‐associated gene signatures, and AD pathology.