Basic Science and Pathogenesis

The rare APOE3-Christchurch (APOE3Ch) variant is associated with remarkable protective effects, delaying the age at onset of PSEN1-E280A driven autosomal dominant Alzheimer's disease (AD). While much of the research on APOE3Ch has focused on its role in AD, little is known about how this variant impacts APOE biology under non-disease conditions. Using cortical organoids as a model system, we aim to investigate the fundamental mechanisms by which APOE3Ch alters APOE biology, including its effects on metabolism and cellular interactions, to better understand its unique properties and potential implications for broader applications.

Cortical organoids were derived from the isogenic neurodegeneration iPSC line series, with either the APOE3 or APOE3Ch genotype. Single-cell RNA sequencing (scRNA-seq) was performed on three independent batches of 6-month-old cerebral organoids to identify pathways differentially regulated by APOE3Ch. To further investigate APOE lipidation and its effects on lipid metabolism, lipidomics analysis was conducted on 8-month-old cerebral organoids, offering additional insights into APOE3Ch-specific biological mechanisms. Validation was also carried out using iPSC-derived astrocytes and neurons to confirm key findings.

ScRNA-seq revealed differences in cell populations and astrocyte subtypes between APOE3 and APOE3Ch organoids, with mitochondrial-related pathways differentially regulated in APOE3Ch cerebral organoids. These findings were validated in iPSC-derived astrocytes. APOE3Ch iPSC-derived astrocytes exhibited increased APOE secretion compared to APOE3. They also demonstrated enhanced mitochondrial function, as evidenced by increased spare respiratory capacity measured by Seahorse stress assays. Furthermore, APOE3Ch astrocytes showed reduced lipid droplet formation following oleic acid treatment. Lipidomics analysis of APOE3Ch cerebral organoids revealed decreased cholesterol ester levels, suggesting alterations in lipid metabolism.

Our findings provide novel insights into how the APOE3Ch variant alters APOE biology, revealing differences in mitochondrial function and lipid metabolism. The differential regulation of mitochondrial pathways in APOE3Ch cerebral organoids, along with the enhanced mitochondrial capacity and altered lipid metabolism observed in APOE3Ch astrocytes, highlights the unique properties of this variant. These results offer valuable mechanistic understanding that may inform future therapeutic strategies targeting APOE biology, particularly in the context of AD.