Specificity of ABCA7-mediated lipid efflux and its effects on intracellular lipid metabolism in neural cells

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Project Summary/Abstract Alzheimer's disease (AD) dementia has emerged as a major burden on the healthcare system and a major barrier to human longevity, especially productive human longevity. Genetics of familial early-onset AD, pathological changes in the brain of sporadic late-onset AD patients and pathological changes in the brain of individuals with Down syndrome, who frequently have increased expression of amyloid precursor protein (APP) owing to partial trisomy of chromosome 21, all point to APP and its cleavage product amyloid β (Aβ) peptide as major causative factors in AD pathogenesis. However, other factors may also play a role. The genome-wide association studies (GWAS) of the architecture of common genetic predisposition to AD, genetic investigations of rare loss-function mutations and studies in mouse models of AD have implicated ATP-binding cassette transport subfamily A member 7 (ABCA7) in AD pathogenesis. A closely related protein, ATP-binding cassette transport subfamily A member 1 (ABCA1) has been implicated in AD in the GWAS and animal studies. In our preliminary data, we show that individuals with low ABCA7 protein levels have a greater risk for developing AD pathology. Both ABCA1 and ABCA7 mediate formation of high-density lipoprotein (HDL) particles from cell lipids. In preliminary data, we show that ABCA7- and ABCA1-HDL significantly differ in the lipid composition. Recently, we published the altered lipidostasis hypothesis that draws on the paradigms provided by ABCA1 and another ABC A transporter, ABCA4, and poses the existence of a neurodegenerative lipid that continuously arises during normal physiology and promotes AD pathogenesis when it is not eliminated from neural cells by ABCA7. ABCA1 and ABCA4 eliminate from specific cells deleterious lipids that these cells accumulate during normal function. The purpose of the present exploratory project is to conduct comparative ABCA7 vs ABCA1 studies in specificity of lipid efflux in neural cells and to test the prediction of the lipidostasis hypothesis of AD that in the neural tissue ABCA7 mediates efflux of a specific lipid species (i.e., the neurodegenerative lipid) that is not effluxed significantly by ABCA1. To test this prediction, we will use metabolic radioactive labeling of lipids/high performance thin layer chromatography (HPTLC) and multidimensional electrospray ionization `shotgun' mass spectrometry lipidomics to compare ABCA7- and ABCA1-mediated lipid efflux/HDL and the effects that this efflux/HDL formation has on the lipid composition of the plasma membrane and whole cell in microglia C20, astrocyte glioblastoma A172 and neuroblastoma SK-N- SH cells inducibly expressing ABCA7 or ABCA1.