ApoE expression across the CNS: Who, What, Where, When, and How (much)?

Apolipoprotein E (APOE) is the strongest genetic risk factor for late-onset Alzheimer’s disease (AD) and encodes a major lipid transporter in the central nervous system (CNS). Although apoE has been studied extensively, fundamental questions remain regarding its expression across the CNS. For example, which CNS cell types express APOE? Where is APOE expression enriched across brain regions? When is APOE dynamically regulated – across development, aging, injury, and neurodegeneration? How much apoE is produced at the mRNA and protein level – particularly across the common E2, E3, and E4 isoforms? Here, we synthesize transcriptomic, spatial, biochemical, and genetic evidence to provide an organized framework for the “who, what, where, when, and how (much)” of CNS APOE/apoE. By assessing large-scale single-cell datasets along with in vivo and cell-type–specific genetic studies, we highlight that astrocytes are the predominant source of CNS apoE at baseline, while microglia, oligodendrocyte-lineage cells, vascular-associated populations, and border-interface macrophages exhibit context-dependent induction that is strongly influenced by aging, injury, and AD-related pathology. We also evaluate unresolved discrepancies, such as the extent of neuronal APOE expression and its contribution to total brain apoE, and describe how – across modalities – apoE abundance often follows an isoform-dependent hierarchy (typically E2 > E3 > E4 at the protein level) while transcript–protein discordance and model-specific effects underscore substantial post-transcriptional and contextual regulation. Finally, we discuss how clarifying spatiotemporal, cell-specific, and isoform-dependent apoE expression will enable better informed and more precise apoE-targeted therapeutic strategies.