Human APOE variants in Alzheimer’s Disease and type III hyperlipoproteinemia: insights from Long-Evans knock-in rat models with humanized App and APOE

Abstract

APOE is a major genetic factor in late-onset Alzheimer's disease (LOAD), with APOE4 significantly increasing risk, APOE3 acting as a neutral isoform, and APOE2 offering protective effects. The primary hypothesis links APOE isoforms to LOAD through their impact on Aβ production and deposition, which is thought to be related to their effects on lipid metabolism. Specifically, APOE4 enhances Aβ production and accumulation in amyloid plaques more than APOE3. In contrast, APOE3-Aβ complexes, which promote Aβ clearance and reduce Aβ aggregation, are approximately 20 times more prevalent than APOE4-Aβ complexes, highlighting differences in their functional interactions. APOE is also important in lipid metabolism, affecting both peripheral and central systems. It is involved in the metabolism of lipoproteins and plays a key role in triglyceride and cholesterol regulation. APOE2 is notably associated with Hyperlipoproteinemia type III (HLP), which is characterized by mixed hypercholesterolemia and hypertriglyceridemia due to impaired binding to Low Density Lipoproteins receptors. To explore the impact of human APOE isoforms on lipid metabolism and LOAD, we developed Long-Evans rats with the rat Apoe gene replaced by human APOE2, APOE3, or APOE4. These rats were crossed with those carrying a humanized App allele, enabling the expression of human Aβ, which is more aggregation-prone than rodent Aβ. This model offers a significant advancement for studying APOE-Aβ interactions. We found that APOE2^hAβ rats had the highest levels of APOE in serum and brain, with no significant transcriptional differences among isoforms, suggesting variations in protein translation or stability. Elevated Aβ43 levels in male APOE4^hAβ rats compared to APOE2^hAβ rats highlight the model’s utility for amyloid pathology studies. Additionally, a lipidomic analysis of 222 lipid molecular species in serum samples showed that APOE2^hAβ rats displayed elevated triglycerides and cholesterol, making them a valuable model for studying HLP. These rats also exhibited elevated levels of phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine, sphingomyelin, and lysophosphatidylcholine. Minimal differences in lipid profiles between APOE3^hAβ and APOE4^hAβ rats reflect findings from mouse models. Future studies will include comprehensive lipidomic analyses in various CNS regions to further validate these models and explore the effects of APOE isoforms on lipid metabolism in relation to AD pathology.