Alzheimer's Disease (AD) is the most common cause of dementia, affecting nearly 6 million Americans and continuing to increase in prevalence with the aging population. AD causes progressive memory loss and cognitive impairment that can eventually lead to the total inability to perform daily functions. The brain is the most cholesterol-rich and lipid-diverse organ in the body and depends on tight regulation of lipid metabolism and transport to maintain proper neural signaling transduction and cognitive function. Several large GWAS have associated the ABCA7 gene locus with AD, with variants that are predicted to have reduced function associated with increased AD risk. ABCA7 is a member of the ATP-binding cassette transporter subfamily A (ABCA), with well described functions as membrane phospholipid (PL) translocases and mediators of cholesterol and lipid efflux from cells. The most characterized ABCA family member is ABCA1, which has been extensively characterized regarding its role in cellular phosphatidylcholine (PC) and cholesterol efflux to extracellular acceptors apolipoproteins apoA1 and apoE. ABCA7 is highly homologous to ABCA1, suggesting that ABCA? mediates transport of certain lipids from inside to outside the cell in response to acceptors like apoA1 and apoE. However, the specific lipids transported by ABCA7 and the mechanisms by which it modulates AD risk have not been established. GWAS of plasma metabolites identified a significant association of the ABCA7 gene locus with certain ceramide species. Lipidomic profiling of Abca7-/- mouse brain revealed dysregulation in several lipid classes, including ceramides, which coincided with impaired cognitive functions, suggesting a functional role of these lipids in memory and cognition. While ABCA1 is also expressed in the brain, the cellular expression of ABCA1 and ABCA 7 is different, and genetic variation at ABCA1 does not carry the same risk of AD at the population level. Therefore, it is hypothesized in this proposal, that ABCA7, in contrast to ABCA1, translocates and promotes cellular efflux of specific lipid species in specific brain cell types in an AD-protective manner, and that reduced ABCA? activity leads to cellular accumulation of toxic lipid species, contributing to neural cell dysfunction and AD. To test this hypothesis, the following research aims are proposed: 1) Determine the specific lipid species affected by deletion of ABCA7 (compared with ABCA1) in three different iPSC-derived brain cells (neurons, microglia, and astrocytes), and establish the effect of ABCA 7 deletion on AD-relevant functions of iPSC derived brain cells; 2) Identify ABCA7 protein domains that differentiate ABCA7 from ABCA1 with regard to specificity of lipid translocase activities; 3) Characterize the functional effects of naturallyoccurring ABCA7 coding variants that are significantly associated with AD.