Uncovering APOE4 Matrisome endophenotypes using human iPSC-based models

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PROJECT SUMMARY Alzheimer disease (AD), the most prevalent age-related dementia, is characterized by widespread β-amyloid accumulation, neurofibrillary tangles, neuroinflammation, and frank neuronal and synaptic loss. Recent AD GWAS studies have uncovered several risk single nucleotide polymorphisms (SNPs) within genes predominantly expressed in glia. Notably, the Apolipoprotein E (APOE) ε4 allele is the strongest genetic risk factor for AD; APOE ɛ4/ɛ4 (APOE 44) increases AD risk by 15-fold compared to APOE ɛ3/ɛ3 (APOE 33) and is predominantly expressed in astrocytes, but also upregulated in the recently identified disease-associated microglia phenotype. Although the relationship between APOE4 and AD risk is well-established, the mechanisms underlying this effect in particular human brain cell types are not entirely clear. To comprehensively assess the effect of human APOE4 on human brain cell types, we characterized the APOE4 genotype-phenotype relationship in four brain cell types: microglia, astrocytes, brain microvascular endothelial cells and mixed cortical cultures derived from human induced pluripotent stem cells (iPSCs). Global transcriptome analyses reveal that APOE4 drives enhanced Matrisome gene sets associated with chemotaxis, glial activation and lipid metabolism in astrocytes from iPSCs- derived mixed cortical cultures, and this signature is confirmed in AD brain cell-type deconvoluted transcriptome data–a phenotype from astrocytes when co-cultured with neurons. We hypothesize that matrisome signals derived from APOE4 astrocytes induce defects in neuronal activity and microglia function. We also hypothesize that interactions of Siglec receptor-sialic acid containing glycosaminoglycan ligands by CD33 risk genotype and APOE4 glia promote plaque formation and neuroinflammation. To test these hypotheses, human “brain-in-a- dish” model systems, composed of neurons, astrocytes and microglia derived from isogenic iPSCs will be utilized. In aim 1, we will perform single cell transcriptomic analysis to confirm matrisome signals in 2D and 3D models and determine how altered matrisome signaling by APOE4 impacts neuronal activity and microglia phenotype at baseline and in the context of AD neuropathological features. In aim 2, we will determine the molecular mechanism of synergistic effects of APOE4 and Siglec receptor CD33 risk in a single cell transcriptomic level, and its effects on plaque formation and neuroinflammation. In aim 3, we will identify compounds to reverse altered astrocyte matrisome signaling phenotypes using drug screening platforms. The goal of this project is to assess functional consequences of APOE4 driven matrisome endophenotypes and understand the mechanism of glycoprotein receptor risk crosstalk with APOE4 matrisome signals. Further this proposed research project sets out to uncover potential therapeutic drugs to tackle APOE4 driven matrisome endophenotypes found in APOE4 carrier AD patients.