Fellowship for Farmer: Apolipoprotein E4 Alters Cerebral Lipid Metabolism

From the initial report of Alzheimer’s Disease (AD) to recent transgenic AD mouse models, a phenomenon of increased lipid droplet (LD) formation in glia of diseased brain has been described yet poorly studied. Once thought to be simple depots of fat storage, LDs have now been implicated in diseases of abberant metabolism. As the search for disease modifying therapies for AD continues, a deeper understanding of the metabolic derangements in AD individuals is critical. The apolipoprotein E gene is the strongest genetic predictor of the development of late onset AD (LOAD). In humans, there are three major isoforms of apolipoprotein E (apoE): E2, E3, and E4. E4 confers between a 2 (heterozygous) to 15 fold (homozygous) increase in risk of LOAD compared to E3. ApoE is primarily secreted by astrocytes and serves as the main lipid carrier in the brain. Lipid uptake and metabolism in astrocytes therefore relies on apoE. In this study we characterize an increase in LD formation in astrocytes expressing human E4. Compared to E3 expressing astrocytes, E4 astrocytes exhibit significantly greater numbers of LDs. Additionally, E4 astrocytes exhibited more LDs than E3 astrocytes at varying concentrations of oleic acid supplementation. A preliminary analysis of mouse brains sectioned and stained for LDs showed that mice expressing human E4 have substantially more LDs than E3 along the lateral ventricles, as well as in cortical regions. Finally, metabolic studies were conducted to measure uptake and oxidation of fatty acids (14C palmitate) in astrocytes. It was found that E4 astrocytes take up palmitate at a higher rate than E3 astrocytes, and furthermore oxidize less palmitate compared to E3. This data suggests a mismanagement of lipid homeostasis in the E4 brain and provides the justification for the current proposal. In order to dissect the apoE contributions to lipid uptake in astrocytes, we will use pharmacologic and genetic inhibitors of LDLR and fatty acid transporters and measure intracellular lipid storage. Secondly, we will study how apoE modulates lipid oxidation and lipid droplet activation by tracing labeled palmitate to its labeled by products. Thirdly, we will translate our findings to human tissue by correlating AD pathology and APOE genotype with LD presence in astrocytes and vascular endothelium. Completion of this proposal will be a breakthrough in our understanding of apoE’s role in cerebral lipid metabolism. Determining the mechanisms contributing to LD formation in E4 glia could offer insights into potential therapies to mitigate disease risk.