Intravenously injected human apolipoprotein A-I rapidly enters the central nervous system via the choroid plexus.

Background: Brain lipoprotein metabolism is dependent on lipoprotein particles that resemble plasma high-density lipoproteins but that contain apolipoprotein (apo) E rather than apoA-I as their primary protein component. Astrocytes and microglia secrete apoE but not apoA-I; however, apoA-I is detectable in both cerebrospinal fluid and brain tissue lysates. The route by which plasma apoA-I enters the central nervous system is unknown.

LCAT deficiency does not impair amyloid metabolism in APP/PS1 mice.

A key step in plasma HDL maturation from discoidal to spherical particles is the esterification of cholesterol to cholesteryl ester, which is catalyzed by LCAT. HDL-like lipoproteins in cerebrospinal fluid (CSF) are also spherical, whereas nascent lipoprotein particles secreted from astrocytes are discoidal, suggesting that LCAT may play a similar role in the CNS. In plasma, apoA-I is the main LCAT activator, while in the CNS, it is believed to be apoE.

Hormonal modulators of glial ABCA1 and apoE levels.

Apolipoprotein E (apoE) is the major lipid carrier in the central nervous system. As apoE plays a major role in the pathogenesis of Alzheimer disease (AD) and also mediates repair pathways after several forms of acute brain injury, modulating the expression, secretion, or function of apoE may provide potential therapeutic approaches for several neurological disorders. Here we show that progesterone and a synthetic progestin, lynestrenol, significantly induce apoE secretion from human CCF-STTG1 astrocytoma cells, whereas estrogens and the progesterone metabolite allopregnanolone have negligible effects.

The LXR agonist GW3965 increases apoA-I protein levels in the central nervous system independent of ABCA1.

Lipoprotein metabolism in the central nervous system (CNS) is based on high-density lipoprotein-like particles that use apoE as their predominant apolipoprotein rather than apoA-I. Although apoA-I is not expressed in astrocytes and microglia, which produce CNS apoE, apoA-I is reported to be expressed in porcine brain capillary endothelial cells and also crosses the blood-brain barrier (BBB). These mechanisms allow apoA-I to reach concentrations in cerebrospinal fluid (CSF) that are approximately 0.

ATP-binding cassette transporter A1 mediates the beneficial effects of the liver X receptor agonist GW3965 on object recognition memory and amyloid burden in amyloid precursor protein/presenilin 1 mice.

The cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) moves lipids onto apolipoproteins including apolipoprotein E (apoE), which is the major cholesterol carrier in the brain and an established genetic risk factor for late-onset Alzheimer disease (AD). In amyloid mouse models of AD, ABCA1 deficiency exacerbates amyloidogenesis, whereas ABCA1 overexpression ameliorates amyloid load, suggesting a role for ABCA1 in Aβ metabolism. Agonists of liver X receptors (LXR), including GW3965, induce transcription of several genes including ABCA1 and apoE, and reduce Aβ levels and improve cognition in AD mice.

Overexpression of human ABCG1 does not affect atherosclerosis in fat-fed ApoE-deficient mice.

Objective: The purpose of this study was to evaluate the effects of whole body overexpression of human ABCG1 on atherosclerosis in apoE(-/-) mice.

Methods And Results: We generated BAC transgenic mice in which human ABCG1 is expressed from endogenous regulatory signals, leading to a 3- to 7-fold increase in ABCG1 protein across various tissues. Although the ABCG1 BAC transgene rescued lung lipid accumulation in ABCG1(-/-) mice, it did not affect plasma lipid levels, macrophage cholesterol efflux to HDL, atherosclerotic lesion area in apoE(-/-) mice, or levels of tissue cholesterol, cholesterol ester, phospholipids, or triglycerides.

ABCG1 influences the brain cholesterol biosynthetic pathway but does not affect amyloid precursor protein or apolipoprotein E metabolism in vivo.

Cholesterol homeostasis is of emerging therapeutic importance for Alzheimer's disease (AD). Agonists of liver-X-receptors (LXRs) stimulate several genes that regulate cholesterol homeostasis, and synthetic LXR agonists decrease neuropathological and cognitive phenotypes in AD mouse models. The cholesterol transporter ABCG1 is LXR-responsive and highly expressed in brain.

The cholesterol transporter ABCG1 modulates the subcellular distribution and proteolytic processing of beta-amyloid precursor protein.

Although intracellular cholesterol levels are known to influence the proteolysis of beta-amyloid precursor protein (APP), the effect of specific genes that regulate cholesterol metabolism on APP processing remains poorly understood. The cholesterol transporter ABCG1 facilitates cholesterol efflux to HDL and is expressed in brain. Notably, the human ABCG1 gene maps to chromosome 21q22.

Elevated plasma triglyceride levels precede amyloid deposition in Alzheimer's disease mouse models with abundant A beta in plasma.

Dietary or pharmacological manipulation of plasma lipids markedly influences amyloid deposition in animal models of Alzheimer's Disease (AD). However, it is not known whether baseline plasma lipids in AD models differ from wild-type littermates throughout the natural history of disease. To address this question, we measured plasma total cholesterol and triglyceride levels over time in three transgenic AD mouse models in the absence of dietary or pharmacological treatments.

Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain.

ABCA1 is a cholesterol transporter that is widely expressed throughout the body. Outside the central nervous system (CNS), ABCA1 functions in the biogenesis of high-density lipoprotein (HDL), where it mediates the efflux of cholesterol and phospholipids to apolipoprotein (apo) A-I. Deficiency of ABCA1 results in lack of circulating HDL and greatly reduced levels of apoA-I.

The ATP-binding cassette transporter 1 mediates lipid efflux from Sertoli cells and influences male fertility.

The liver X receptor/retinoid X receptor (LXR/RXR)-regulated gene ABCA1 effluxes cellular cholesterol and phospholipid to apolipoprotein A1 (apoA1), which is the rate-limiting step in high-density lipoprotein synthesis. The RXR pathway plays a critical role in testicular lipid trafficking, and RXRbeta-deficient male mice are sterile and accumulate lipids in Sertoli cells. Here, we demonstrate that ABCA1 mRNA and protein are abundant in Sertoli cells, whereas germ cells express little ABCA1.