Assessing the Role of APOE in Glial Lipid Droplet Metabolism and Function

ABSTRACT Alzheimer’s disease (AD) is one of the predominant causes of disability and dependency among older people, and the third leading cause of death in the United States. Late-onset AD is the most common form, with more than 99% of AD cases occuring after age 65. The strongest genetic risk factor for developing late- onset AD is carrying the APOE4 allele. Apolipoprotein E (ApoE) is primarily expressed by glial cells in the brain and is the major protein component of lipoprotein particles secreted by astrocytes and microglia. Lipoprotein particles provide a bidirectional mechanism of lipid transport between glia and neurons. Many recent studies suggest that a disequilibrium in nervous system lipids is associated with increased risk of developing AD. However, the mechanisms by which ApoE4 affects cellular lipid homeostasis are incompletely understood. We recently discovered that in astrocytes, ApoE can traffic to cytoplasmic lipid droplets (LDs) rather than undergoing secretion on lipoprotein particles. We hypothesize that ApoE plays previously unrecognized roles in cellular lipid metabolism by acting directly on LDs in glia. Astrocytes play important roles in metabolizing peroxidated lipids, while microglia are the primary innate immune effector cells of the central nervous system. In Aim 1, we will test the effect of modulating ApoE in astrocytes on cellular lipid composition, metabolism, and peroxidation. In Aim 2, we will test the effect of modulating ApoE in microglia on cellular lipid composition, metabolism, and inflammation. In Aim 3, we will fill a key knowledge gap by developing a community resource visualizing and quantifying LDs in various brain regions in mice of different genotypes and ages. Together, these studies will lead to new insights about the cellular and molecular mechanism by which expression of ApoE4 leads to increased risk of late-onset AD. This work could also lead to the identification of novel drug targets for preventing and treating AD.