iPSC Macrophages/microglia and genome-wide CRISPR/Cas9 screening to investigate lipid accumulation in the pathophysiology of Alzheimer's Disease

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BackgroundLipid accumulation in the cell cytoplasm of Alzheimer's brains was observed in the earliest descriptions of the disease. However, the vast majority of biomedical research in Alzheimer's disease (AD) has focused on the two gross histopathological targets of amyloid plaques and neurofibrillary tangles. The result of this research has been over 200 clinical trials focusing on these two targets that failed to generate an effective therapy. There are several reasons for this failure; among these are the poor understanding of the fundamental pathophysiology of Alzheimer's disease and the poor choice of disease models to more faithfully mimic the human cells most associated with AD pathology.Recently, GWAS studies have identified lipid metabolism as an important pathway in Alzheimer's disease (Jansen et al., 2019), with GWAS hits, such as Trem2 and ApoE, involved in lipid sensing and processing, in addition to regulation of inflammatory processes. A greater understanding of lipid regulation, and in particular of neuroinflammation, would be fundamental to understanding the pathophysiology of AD. Macrophages and microglia containing accumulations of lipids upon inflammation have been described (Nadjar, 2018). Lipid droplets, consisting of neutral lipids such as triacylglycerols and diacylglycerols, have been associated with inflammation and cytokine storage, but also with homeostatic functions, such as storage of fatty acids that could support phagocytosis and proper mitochondria functioning.All these studies strongly evidence that fatty acids can modulate microglial phagocytic activity. More studies are needed however to understand the mechanisms by which microglia interact with fatty acids and how they can shift microglia from one phenotype/function to another. Moreover, we still need to address the net outcome of these functional shifts for brain homeostasis, since depending on the context, increasing microglial phagocytosis might be beneficial (e.g. clearance of A plaques) or detrimental (e.g. phagocytosis of functional synapses).Research DesignThis project will investigate whether altering microglia lipid accumulation can improve microglial functions in human cell models. By modelling the disease effectively using patient-derived iPSC differentiated into microglia, targets affecting lipid accumulation will be identified through genome-wide CRISPR/Cas9 screening. Firstly, a robust assay for lipid droplet formation in human iPSC-derived microglia will be developed and comparisons between disease and isogenic lines explored. A genome-wide CRISPR/Cas9 screen will then be performed to identify important genes involved in lipid accumulation and potential targets for therapeutic validation. Further assessments will then be made of the consequences of lipid droplet formation and modulation by compounds on phagocytosis, ROS and cytokine production. Finally, the effect of lipid droplet modulation on complex co-culture models including neurons and astrocytes will be investigated.ImplicationUsing human iPSC-derived microglia will enable more faithful modelling of the cells associated with the disease, and by investigating this unexplored role of microglial lipid accumulation in the pathophysiology of AD, we aim to identify new targets that can be explored as potential new strategies for therapeutic intervention in Alzheimer's disease.ReferencesJansen, I. E. et al. 2019. Genome-wide meta-analysis identifies new loci and functional pathways influencing Alzheimer's disease risk. Nature Genetics 51(3), pp. 404-413. doi: 10.1038/s41588-018-0311-9Nadjar, A. 2018. Role of metabolic programming in the modulation of microglia phagocytosis by lipids. Prostaglandins Leukot Essent Fatty Acids 135, pp. 63-73. doi: 10.1016/j.plefa.2018.07.006