APOE effects on glial lipid metabolism and 25-hydroxycholesterol: Effects on aging and AD-related pathology
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PROJECT SUMMARY (APOE U19: OVERALL) The overarching goal of this U19 project is to comprehensively understand the biology and pathobiology of apolipoprotein E (apoE) in aging and Alzheimer’s disease (AD) to inform therapeutic strategies. The ε4 allele of the APOE gene (APOE4) is the strongest genetic risk factor for AD impacting 50-70% of all AD patients, while the ε2 allele is protective compared to the common ε3 allele. APOE4 is also a strong risk factor for age-related cognitive decline and vascular cognitive impairment. To integrate existing knowledge and address critical gaps, we propose a unified ApoE Cascade Hypothesis that the structural differences and related biochemical properties among the three apoE isoforms initiate their differential effects on a cascade of events at the cellular and systems levels ultimately impacting aging-related pathogenic conditions including AD. Towards this, we have assembled a multi-disciplinary team to synergize expertise and resources across multiple institutions. By integrating five interactive Projects and seven robust Cores, we will create a nexus for apoE-related aging research, sharing the knowledge, expertise and resources with the broader scientific community. Project 1 will work closely with Core B to address the structural and biochemical properties of the three apoE isoforms to generate insights for functional outcomes. Projects 2, 3 and 4 will interactively study how apoE isoforms expressed in astrocytes, microglia, or vascular mural cells impact lipid metabolism, glial and vascular functions, AD-related pathologies, and cellular and molecular pathways using conditional mouse models and systems- based approaches. These studies will generate cell type-specific apoE/lipoprotein particles that will be collected through in vivo microdialysis for structural and biochemical studies. Project 5 will carry out genomic and genetic analyses to identify modifiers of APOE-related age at onset of AD. Studies in Projects 2-5 will be interactively supplemented by neuropathological studies using postmortem brains from healthy aging studies or with AD pathologies (Core C), biomarker studies using both human and mouse biospecimens (Core D), and functional studies using human iPSC-derived cellular and organoid models (Core E). This U19 proposal is supported by a comprehensive Multi-Omics Core (Core F) for centralized proteomics, lipidomics, and metabolomics studies on various animal and iPSC models, as well as human postmortem brains and fluid biospecimens. The Bioinformatics, Biostatistics, and Data Management Core (Core G) will provide critical supports for analyzing large datasets including those from single-cell RNA-seq and biostatistics supports to ensure scientific rigor. Core G will also work closely with the Administrative Core (Core A) to maintain an ApoE Web Portal designated as EPAAD where knowledge, resources, and data will be shared with the scientific community. Core A will also organize annual ApoE Symposium to promote collaboration and engage the ApoE Community. As such, this U19 will drive a team-based effort to generate essential knowledge to guide disease- modifying therapies for AD and other aging-related conditions.
PROJECT SUMMARY (APOE U19: PROJECT 2) Project 2 seeks to improve understanding of how apolipoprotein E (apoE) isoforms influence glial lipid metabolism and inflammatory responses underlying immune-mediated damage in the brain in Alzheimer’s disease (AD) and aging-related pathologies. Evidence indicates that the apoE protein not only affects Aβ deposition and structure, it strongly influences microglial-activation, Aβ-induced neuritic dystrophy, and tau- mediated neurodegeneration. ApoE produced by both astrocytes and microglia plays an important role in these effects, and microglia are critical effectors of neurodegeneration. In alignment with the ApoE Cascade Hypothesis of this U19, we anticipate that structural differences and related biochemical properties among the apoE isoforms are likely to mediate the differential effects of apoE on the brain’s innate immune response. Factors such as lipid metabolism and sterol processing, can influence microglial reactivity in ways that are relevant to AD pathogenesis. Recent evidence indicates that apoE and aging critically influence the process of cholesterol and lipid clearance in the brain, specifically in microglia. Accumulation of lipid debris in microglia under different conditions is linked with microglial activation and inflammasome-mediated damage. There is also literature and we have preliminary data that 25-hydroxycholesterol (25HC), a known oxysterol immunomodulator, is produced in the brain by microglia and can modulate cholesterol metabolism as well as the microglial cytokine response in an apoE isoform-dependent manner in vitro. These data lead us to hypothesize that apoE-dependent regulation and clearance of cholesterol esters and other lipids specifically in microglia regulates the microglial inflammatory response in aging and AD. We further hypothesize that this microglial response including 25HC production impacts Aβ-induced local damage, Aβ- induced tau spreading, and tau-mediated neurodegeneration in an apoE, age, and sex-dependent fashion. We propose these aims: Aim 1: Determine the effects of apoE and apoE isoforms on cholesterol, cholesterol esters, and other lipids in astrocytes and microglia during aging, in the setting of Aβ and tau pathology, and with LXR agonist stimulation. Aim 2: Assess the effects of the immunomodulatory oxysterol, 25- hydroxycholesterol (25HC) on Aβ-mediated tau-spreading, tau-mediated neurodegeneration, and age-related brain phenotypes and how these are modified by apoE. Aim 3: Assess the effect of apoE isoform on the lipidomic profile of astrocytes, microglia, and secreted apoE-containing lipoproteins as well as the mechanistic relationship between lipid droplet formation, apoE signaling, and inflammation in vitro. We will also assess the mechanism of the effects of ch25h on apoE-mediated neurodegeneration in vitro. Impact/Integration: Project 2 will work closely with Projects 1, 3, 5, Core B, Core D, Core E, Core F, and Core G on production, purification, and characterization of apoE particles and lipids from mouse glia in vitro and in vivo and iPSC derived glia in vitro. Histological analysis in Project 2 will be done with the Neuropathology Core (Core C).
