Lack of ABCA1 considerably decreases brain ApoE level and increases amyloid deposition in APP23 mice

Intracellular cholesterol homeostasis and amyloid precursor protein processing

Many preclinical and clinical studies have implied a role for cholesterol in the pathogenesis of Alzheimer's disease (AD). In this review we will discuss the movement of intracellular cholesterol and how normal distribution, transport, and export of cholesterol are vital for regulation of the AD related protein, Abeta. We focus on cholesterol distribution in the plasma membrane, transport through the endosomal/lysosomal system, control of cholesterol intracellular signaling at the endoplasmic reticulum and Golgi, the HMG-CoA reductase pathway and finally export of cholesterol from the cell.

Proton pump inhibitor lansoprazole is a nuclear liver X receptor agonist

The liver X receptors (LXRalpha and LXRbeta) are transcription factors that control the expression of genes primarily involved in cholesterol metabolism. In the brain, in addition to normal neuronal function, cholesterol metabolism is important for APP proteolytic cleavage, secretase activities, Abeta aggregation and clearance. Particularly significant in this respect is LXR mediated transcriptional control of APOE, which is the only proven risk factor for late onset Alzheimer's disease. Using a transactivation reporter assay for screening pharmacologically active compounds and off patent drugs we identified the proton pump inhibitor Lansoprazole as an LXR agonist. In secondary screens and counter-screening assays, it was confirmed that Lansoprazole directly activates LXR, increases the expression of LXR target genes in brain-derived human cell lines, and increases Abca1 and Apo-E protein levels in primary astrocytes derived from wild type but not LXRalpha/beta double knockout mice. Other PPIs activate LXR as well, but the efficiency of activation depends on their structural similarities to Lansoprazole. The identification of a widely used drug with LXR agonist-like activity opens the possibility for systematic preclinical testing in at least two diseases--Alzheimer's disease and atherosclerosis.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

Overexpression of low-density lipoprotein receptor in the brain markedly inhibits amyloid deposition and increases extracellular A beta clearance

Apolipoprotein E (APOE) is the strongest genetic risk factor for Alzheimer's disease (AD). Previous studies suggest that the effect of apoE on amyloid-beta (A beta) accumulation plays a major role in AD pathogenesis. Therefore, understanding proteins that control apoE metabolism may provide new targets for regulating A beta levels. LDLR, a member of the LDL receptor family, binds to apoE, yet its potential role in AD pathogenesis remains unclear. We hypothesized that LDLR overexpression in the brain would decrease apoE levels, enhance A beta clearance, and decrease A beta deposition. To test our hypothesis, we created several transgenic mice that overexpress LDLR in the brain and found that apoE levels in these mice decreased by 50%-90%. Furthermore, LDLR overexpression dramatically reduced A beta aggregation and enhanced A beta clearance from the brain extracellular space. Plaque-associated neuroinflammatory responses were attenuated in LDLR transgenic mice. These findings suggest that increasing LDLR levels may represent a novel AD treatment strategy.

Alzheimer's disease as homeostatic responses to age-related myelin breakdown

The amyloid hypothesis (AH) of Alzheimer's disease (AD) posits that the fundamental cause of AD is the accumulation of the peptide amyloid beta (Aβ) in the brain. This hypothesis has been supported by observations that genetic defects in amyloid precursor protein (APP) and presenilin increase Aβ production and cause familial AD (FAD). The AH is widely accepted but does not account for important phenomena including recent failures of clinical trials to impact dementia in humans even after successfully reducing Aβ deposits. Herein, the AH is viewed from the broader overarching perspective of the myelin model of the human brain that focuses on functioning brain circuits and encompasses white matter and myelin in addition to neurons and synapses. The model proposes that the recently evolved and extensive myelination of the human brain underlies both our unique abilities and susceptibility to highly prevalent age-related neuropsychiatric disorders such as late onset AD (LOAD). It regards oligodendrocytes and the myelin they produce as being both critical for circuit function and uniquely vulnerable to damage. This perspective reframes key observations such as axonal transport disruptions, formation of axonal swellings/sphenoids and neuritic plaques, and proteinaceous deposits such as Aβ and tau as by-products of homeostatic myelin repair processes. It delineates empirically testable mechanisms of action for genes underlying FAD and LOAD and provides "upstream" treatment targets. Such interventions could potentially treat multiple degenerative brain disorders by mitigating the effects of aging and associated changes in iron, cholesterol, and free radicals on oligodendrocytes and their myelin.

The role of apolipoprotein E in Alzheimer's disease

The epsilon4 allele of apolipoprotein E (APOE) is the major genetic risk factor for Alzheimer's disease (AD). Although there have been numerous studies attempting to elucidate the underlying mechanism for this increased risk, how apoE4 influences AD onset and progression has yet to be proven. However, prevailing evidence suggests that the differential effects of apoE isoforms on Abeta aggregation and clearance play the major role in AD pathogenesis. Other potential mechanisms, such as the differential modulation of neurotoxicity and tau phosphorylation by apoE isoforms as well as its role in synaptic plasticity and neuroinflammation, have not been ruled out. Inconsistent results among studies have made it difficult to define whether the APOE epsilon4 allele represents a gain of toxic function, a loss of neuroprotective function, or both. Therapeutic strategies based on apoE propose to reduce the toxic effects of apoE4 or to restore the physiological, protective functions of apoE.

Tenuifolin, an extract derived from tenuigenin, inhibits amyloid-beta secretion in vitro

AIM

Previous studies have shown that tenuigenin, a crude extract of Polygala tenuifolia Willd. that is commonly used in traditional Chinese herbal medicine for memory loss, can reduce the secretion of Abeta from cultured cells. However, the mechanism underlying this effect and the active compound derived from tenuigenin is unknown. In this study, a purified component of tenuigenin, tenuifolin, was examined and revealed to be an effective compound in vitro.

METHODS

Abeta secretion from three sets of COS-7 cells, each carrying a plasmid expressing a different form of APP was examined following the treatment with tenuifolin. Initially, tenuifolin was determined to have no inherent toxicity to either the transfected or wild type cells at the effective concentrations. Cells were then treated with 0.5-2.0 microg mL(-1) tenuifolin for 12 h and their media were examined via an ELISA for Abeta1-40 and Abeta-42.

RESULTS

We found that treatment with 2.0 microg mL(-1) tenuifolin significantly decreased Abeta secretion from COS-7 cells without altering the ratio of Abeta1-40 and Abeta-42. This effect is most probably due to inhibition of the beta-site APP cleaving enzyme as Abeta secretion was not inhibited from cells expressing the C99 fragment.

CONCLUSION

Tenuifolin is an effective compound from tenuigenin. We believe that this finding should lead the way for future experiments to determine the exact mechanism for tenuifolin's effect on Abeta secretion.

Greasing the wheels of Abeta clearance in Alzheimer's disease: the role of lipids and apolipoprotein E

Although apolipoprotein E (apoE) is the most common genetic risk factor for Alzheimer's Disease (AD), how apoE participates in AD pathogenesis remains incompletely understood. ApoE is also the major carrier of lipids in the brain. Here, we review studies showing that the lipidation status of apoE influences the metabolism of Abeta peptides, which accumulate as amyloid deposits in the neural parenchyma and cerebrovasculature. One effect of apoE is to inhibit the transport of Abeta across the blood-brain-barrier (BBB), particularly when apoE is lipidated. A second effect is to facilitate the proteolytic degradation of Abeta by neprilysin and insulin degrading enzyme (IDE), which is enhanced when apoE is lipidated. We also describe how apoE becomes lipidated and how this impacts Abeta metabolism. Specifically, genetic loss of the cholesterol transporter ABCA1 impairs apoE lipidation and promotes amyloid deposition in AD mouse models. ABCA1 catalyses the ATP-dependent transport of cholesterol and phospholipids from the plasma membrane to lipid-free apolipoproteins including apoE. Conversely, selective overexpression of ABCA1 increases apoE lipidation in the central nervous system (CNS) and eliminates the formation of amyloid plaques in vivo. Deficiency of Liver-X-Receptors (LXRs), transcription factors that stimulate ABCA1 and apoE expression, exacerbates AD pathogenesis in vivo, whereas treatment of AD mice with synthetic LXR agonists reduces amyloid load and improves cognitive performance. These studies provide new insights into the mechanisms by which apoE affects Abeta metabolism, and offer opportunities to develop novel therapeutic approaches to reduce the leading cause of dementia in the elderly.

Memory deficits in APP23/Abca1+/- mice correlate with the level of Aβ oligomers

ABCA1, a member of the ATP-binding cassette family of transporters, lipidates ApoE (apolipoprotein A) and is essential for the generation of HDL (high-density lipoprotein)-like particles in the CNS (central nervous system). Lack of Abca1 increases amyloid deposition in several AD (Alzheimer's disease) mouse models. We hypothesized that deletion of only one copy of Abca1 in APP23 (where APP is amyloid precursor protein) AD model mice will aggravate memory deficits in these mice. Using the Morris Water Maze, we demonstrate that 2-year-old Abca1 heterozygous APP23 mice (referred to as APP23/het) have impaired learning during acquisition, and impaired memory retention during the probe trial when compared with age-matched wild-type mice (referred to as APP23/wt). As in our previous studies, the levels of ApoE in APP23/het mice were decreased, but the differences in the levels of Aβ and thioflavin-S-positive plaques between both groups were insignificant. Importantly, dot blot analysis demonstrated that APP23/het mice have a significantly higher level of soluble A11-positive Aβ (amyloid β protein) oligomers compared with APP23/wt which correlated negatively with cognitive performance. To confirm this finding, we performed immunohistochemistry with the A11 antibody, which revealed a significant increase of A11-positive oligomer structures in the CA1 region of hippocampi of APP23/het. This characteristic region-specific pattern of A11 staining was age-dependent and was missing in younger APP23 mice lacking Abca1. In contrast, the levels of Aβ*56, as well as other low-molecular-mass Aβ oligomers, were unchanged among the groups. Overall, the results of the present study demonstrate that in aged APP23 mice memory deficits depend on Abca1 and are likely to be mediated by the amount of Aβ oligomers deposited in the hippocampus.

Potential mechanisms linking cholesterol to Alzheimer's disease-like pathology in rabbit brain, hippocampal organotypic slices, and skeletal muscle

Epidemiological, animal, and cellular studies suggest that abnormalities in cholesterol metabolism are important in the pathogenesis of Alzheimer's disease (AD), potentially by increasing amyloid-beta (Abeta) peptide levels. Accumulation of Abeta in the brain is suggested to play a key role in the neurodegenerative processes by triggering the hyperphosphorylation of tau and the neuronal death that develop in the course of AD. However, the mechanisms by which cholesterol increases Abeta levels are still ill-defined. Previous and ongoing work from our laboratory indicates that hypercholesterolemia leads to the increased neuronal content of cholesterol and increased levels and processing of the amyloid-beta protein precursor (AbetaPP). We also have found that the oxidized cholesterol metabolite, 27-hydroxycholesterol, increases Abeta levels in both organotypic hippocampal slices and in neuronal preparations cultured from adult rabbits. This cholesterol metabolite is predominantly formed in the circulation and, in contrast to cholesterol, has the ability to cross into the brain. These results may indicate that 27-hydroxycholesterol is the link between circulating cholesterol and AD-like pathology in the brain. We also have found pathological hallmarks in the skeletal muscle of cholesterol-fed rabbits that are suggestive of inclusion body myositis, a disease that shares some pathological similarities with AD.

Liver X Receptors as Therapeutic Targets for Managing Cholesterol: Implications for Atherosclerosis and Other Inflammatory Conditions

Atherosclerosis is a disease characterized by excess cholesterol and inflammation in the blood vessels. The liver X receptors (alpha and beta) are members of the nuclear hormone receptor family that are activated by endogenous cholesterol metabolites. These receptors are widely expressed with a tissue distribution that includes the liver, intestine and macrophage. Upon activation, these receptors have been shown to increase reverse cholesterol transport from the macrophage back to the liver to aid in the removal of excess cholesterol. More recently, they have also been shown to inhibit the inflammatory response in macrophages. These functions are accomplished through direct regulation of gene transcription. Herein, we will describe the key benefits and potential risks of targeting the LXRs for the treatment of atherosclerosis.

Differential effects of 24-hydroxycholesterol and 27-hydroxycholesterol on beta-amyloid precursor protein levels and processing in human neuroblastoma SH-SY5Y cells

Background

Activation of the liver × receptors (LXRs) by exogenous ligands stimulates the degradation of β-amyloid 1–42 (Aβ42), a peptide that plays a central role in the pathogenesis of Alzheimer's disease (AD). The oxidized cholesterol products (oxysterols), 24-hydroxycholesterol (24-OHC) and 27-hydroxycholesterol (27-OHC), are endogenous activators of LXRs. However, the mechanisms by which these oxysterols may modulate Aβ42 levels are not well known.

Results

We determined the effect of 24-OHC and/or 27-OHC on Aβ generation in SH-SY5Y cells. We found that while 27-OHC increases levels of Aβ42, 24-OHC did not affect levels of this peptide. Increased Aβ42 levels with 27-OHC are associated with increased levels of β-amyloid precursor protein (APP) as well as β-secretase (BACE1), the enzyme that cleaves APP to yield Aβ. Unchanged Aβ42 levels with 24-OHC are associated with increased levels of sAPPα, suggesting that 24-OHC favors the processing of APP to the non-amyloidogenic pathway. Interestingly, 24-OHC, but not 27-OHC, increases levels of the ATP-binding cassette transporters, ABCA1 and ABCG1, which regulate cholesterol transport within and between cells.

Conclusion

These results suggest that cholesterol metabolites are linked to Aβ42 production. 24-OHC may favor the non-amyloidogenic pathway and 27-OHC may enhance production of Aβ42 by upregulating APP and BACE1. Regulation of 24-OHC: 27-OHC ratio could be an important strategy in controlling Aβ42 levels in AD.

On enhancers and inhibitors of elastin-derived amyloidogenesis

Aims: The main aim of this study is to better understand the self-aggregation mechanism of amyloid-like elastin-derived fibers in order to design and produce new powerful drugs that will inhibit the onset of ‘amyloidosis’. Materials & methods: Atomic force microscopy (AFM), Congo Red birefringence assay and Thioflavin T fluorescence measurements were used to demonstrate the amyloid-like behavior of some fragments of elastin protein (exon 30 [EX30] and exon 28 [EX28]). Turbidimetry on apparent absorbance technique was used to investigate the effect either of enhancers or of inhibitors on the amyloidogenic elastin-like peptides. Circular-dichroism spectroscopy was used to study the secondary structures of the peptides. Results & discussion: We used Congo Red birefringence assay, Thioflavin T fluorescence measurements and AFM measurements that are used commonly to demonstrate the formation of amyloids. The elastin fibrillogenesis is amyloid-like. Then, the elastin amyloidogenesis is inhibited by particular pentapeptides. Conclusions: We have reported herein that the fibrillogenesis of elastin-derived EX28 and EX30 polypeptides is facilitated significantly by the effect of sodium taurocholate bile salt and is inhibited by a classical inhibitor of Aβ-amyloid peptide, such as KLVFF, as well as by novel inhibitors, designed by us on the basis of some elastin sequences.

Amyloid precursor protein mediates monocyte adhesion in AD tissue and apoE(-)/(-) mice

Amyloid precursor protein (APP) is a type 1 integral membrane protein, which is highly conserved and ubiquitously expressed. Numerous data suggest it functions in cellular adhesion. For example, APP binds components of the extracellular matrix to propagate intracellular signaling responses. In order to investigate adhesion-related changes in inflamed vasculature, brains from apolipoprotein E(-/-) (apoE(-/-)) mice were examined for changes related to APP then compared to human Alzheimer's disease (AD) brains. Cerebrovasculature from mouse apoE(-)/(-) and human AD brains revealed strong immunoreactivity for APP, APP phosphorylated at tyrosine residue 682 (pAPP) and Aβ. Further, Western blot analyses from mouse apoE(-/-) and AD brains showed statistically higher protein levels of APP, pAPP and increased APP association with the tyrosine kinase, Src. Lastly, utilizing a modified Stamper-Woodruff adhesion assay, we demonstrated that adhesion of monocytic cells to apoE(-/-) and AD brain endothelium is partially APP dependent. These data suggest that endothelial APP function coupled with increased Aβ production are involved in the vascular dysfunction associated with atherosclerosis and AD.

Transcriptional profiles of receptors and transporters involved in brain cholesterol homeostasis at the blood-brain barrier: use of an in vitro model

Brain is the most cholesterol rich organ of the whole body and recent studies suggest a role for the blood-brain barrier (BBB) in cerebral cholesterol homeostasis. Low density lipoprotein receptor (LDLR)-related receptors and ATP-binding Cassette (ABC) transporters play an important role in peripheral sterol homeostasis. The purpose of this study was to determine the mRNA expression profiles of ABC transporters (ABCA1, 2, 3, 7 and ABCG1) and low density lipoprotein receptor (LDLR)-related receptors (LDLR, vLDLR, LRP1, LRP2 and LRP8) in BBB endothelium using an in vitro co-culture model of bovine brain capillary endothelial cells (BCECs) and rat glial cells. All transcripts tested are expressed by BCECs and in capillary extract, except vLDLR. Glial cells influence ABCG1, A1, 2, 7 and LRP1 transcription, suggesting a role in cerebral lipid supply/elimination through the modulation of the expression of these transporters and receptors by these cells. Altogether, these results highlight the importance of glial input in the BBB transport phenotype for sterol homeostasis in the central nervous system, and confirm the importance of the BBB in this process.

Lipoprotein receptors and cholesterol in APP trafficking and proteolytic processing, implications for Alzheimer's disease

Amyloid-beta (Abeta) peptide accumulation in the brain is central to the pathogenesis of Alzheimer's disease (AD). Abeta is produced through proteolytic processing of a transmembrane protein, beta-amyloid precursor protein (APP), by beta- and gamma-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Abeta. Members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apoER2, interact with APP and regulate its endocytic trafficking. Additionally, APP trafficking and processing are greatly affected by cellular cholesterol content. In this review, we summarize the current understanding of the roles of lipoprotein receptors and cholesterol in APP trafficking and processing and their implication for AD pathogenesis and therapy.

Clinico-pathologic function of cerebral ABC transporters - implications for the pathogenesis of Alzheimer's disease

In recent years it has become evident that ABC transporters fulfill important barrier functions in normal organs and during disease processes. Most importantly, resistance to drugs in cancer cells led to intense oncological and pharmacological investigations in which researchers were able to highlight important pharmacological interactions of chemotherapeuticals with ABC transporter function. Recently, the development of neurodegenerative diseases and the maintenance of neuronal stem cells have been linked to the activity of ABC transporters. Here, we summarize findings from cell culture experiments, animal models and studies of patients with Alzheimer's disease. Furthermore, we discuss pharmacological interactions and computational methods for risk assessment.

Animal models of Alzheimer's disease and frontotemporal dementia

Insoluble protein aggregates have been linked to Alzheimer's disease (AD) and frontotemporal dementia (FTD). Recent work in transgenic mice has shed light on the role of these aggregates by identifying soluble oligomeric species that may interfere with essential cellular mechanisms at an early disease stage. This review summarizes what we have learned about the roles of these proteins from transgenic mice and invertebrate species such as flies and worms. Proteomic and transcriptomic analyses of tissue from these animal models have identified new molecules with crucial roles in disease. Moreover, transgenic animals have been instrumental in defining drug targets and designing novel therapeutic strategies. With advanced imaging techniques that can be used in both humans and mice an early, preclinical diagnosis of AD and FTD could be within reach.

ApoE promotes the proteolytic degradation of Abeta

Apolipoprotein E is associated with age-related risk for Alzheimer's disease and plays critical roles in Abeta homeostasis. We report that ApoE plays a role in facilitating the proteolytic clearance of soluble Abeta from the brain. The endolytic degradation of Abeta peptides within microglia by neprilysin and related enzymes is dramatically enhanced by ApoE. Similarly, Abeta degradation extracellularly by insulin-degrading enzyme is facilitated by ApoE. The capacity of ApoE to promote Abeta degradation is dependent upon the ApoE isoform and its lipidation status. The enhanced expression of lipidated ApoE, through the activation of liver X receptors, stimulates Abeta degradation. Indeed, aged Tg2576 mice treated with the LXR agonist GW3965 exhibited a dramatic reduction in brain Abeta load. GW3965 treatment also reversed contextual memory deficits. These data demonstrate a mechanism through which ApoE facilitates the clearance of Abeta from the brain and suggest that LXR agonists may represent a novel therapy for AD.

PPARgamma agonists as therapeutics for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid within the brain parenchyma and is accompanied by the impairment of neuronal metabolism and function, leading to extensive neuronal loss. The disease involves the perturbation of synaptic function, energy, and lipid metabolism. The development of amyloid plaques results in the induction of a microglial-mediated inflammatory response. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor whose biological actions are to regulate glucose and lipid metabolism and suppress inflammatory gene expression. Thus, agonists of this receptor represent an attractive therapeutic target for AD. There is now an extensive body of evidence that has demonstrated the efficacy of PPARgamma agonists in ameliorating disease-related pathology and improved learning and memory in animal models of AD. Recent clinical trials of the PPARgamma agonist rosiglitazone have shown significant improvement in memory and cognition in AD patients. Thus, PPARgamma represents an important new therapeutic target in treating AD.

ATP-binding cassette transporter A7 regulates processing of amyloid precursor protein in vitro

ATP-binding cassette transporter A7 (ABCA7) is expressed in the brain and, like its closest homolog ABCA1, belongs to the ABCA subfamily of full-length ABC transporters. ABCA1 promotes cellular cholesterol efflux to lipid-free apolipoprotein acceptors and also inhibits the production of neurotoxic beta-amyloid (Abeta) peptides in vitro. The potential functions of ABCA7 in the brain are unknown. This study investigated the ability of ABCA7 to regulate cholesterol efflux to extracellular apolipoprotein acceptors and to modulate Abeta production. The transient expression of ABCA7 in human embryonic kidney cells significantly stimulated cholesterol efflux (fourfold) to apolipoprotein E (apoE) discoidal lipid complexes but not to lipid-free apoE or apoA-I. ABCA7 also significantly inhibited Abeta secretion from Chinese hamster ovary cells stably expressing human amyloid precursor protein (APP) or APP containing the Swedish K670M671-->N670L671 mutations when compared with mock-transfected cells. Studies with fluorogenic substrates indicated that ABCA7 had no impact on alpha-, beta-, or gamma-secretase activities. Live cell imaging of Chinese hamster ovary cells expressing APP-GFP indicated an apparent retention of APP in a perinuclear location in ABCA7 co-transfected cells. These studies indicate that ABCA7 has the capacity to stimulate cellular cholesterol efflux to apoE discs and regulate APP processing resulting in an inhibition of Abeta production.

Overexpression of ABCA1 reduces amyloid deposition in the PDAPP mouse model of Alzheimer disease

APOE genotype is a major genetic risk factor for late-onset Alzheimer disease (AD). ABCA1, a member of the ATP-binding cassette family of active transporters, lipidates apoE in the CNS. Abca1(-/-) mice have decreased lipid associated with apoE and increased amyloid deposition in several AD mouse models. We hypothesized that mice overexpressing ABCA1 in the brain would have increased lipidation of apoE-containing lipoproteins and decreased amyloid deposition. To address these hypotheses, we created PrP-mAbca1 Tg mice that overexpress mouse Abca1 throughout the brain under the control of the mouse prion promoter. We bred the PrP-mAbca1 mice to the PDAPP AD mouse model, a transgenic line overexpressing a mutant human amyloid precursor protein. PDAPP/Abca1 Tg mice developed a phenotype remarkably similar to that seen in PDAPP/Apoe(-/-) mice: there was significantly less amyloid beta-peptide (Abeta) deposition, a redistribution of Abeta to the hilus of the dentate gyrus in the hippocampus, and an almost complete absence of thioflavine S-positive amyloid plaques. Analyses of CSF from PrP-mAbca1 Tg mice and media conditioned by PrP-mAbca1 Tg primary astrocytes demonstrated increased lipidation of apoE-containing particles. These data support the conclusions that increased ABCA1-mediated lipidation of apoE in the CNS can reduce amyloid burden and that increasing ABCA1 function may have a therapeutic effect on AD.

Fatty acids, lipid metabolism and Alzheimer pathology

Alzheimer's disease is the most common form of dementia in the elderly. The cause of Alzheimer's disease is still unknown and there is no cure for the disease yet despite 100 years of extensive research. Cardiovascular risk factors such as high serum cholesterol, presence of the Apolipoprotein epsilon4 (APOE epsilon4) allele and hypertension, play important roles in the development of Alzheimer's disease. We postulate that a combination of diet, lifestyle, vascular, genetic, and amyloid related factors, which enhance each other's contribution in the onset and course of Alzheimer's disease, will be more likely the cause of the disease instead of one sole mechanism. The possibility that the risk for Alzheimer's disease can be reduced by diet or lifestyle is of great importance and suggests a preventative treatment in Alzheimer's disease. Because of the great importance of lipid diets and metabolism in preventative treatment against both Alzheimer's disease and cardiovascular disease, long-chain polyunsaturated fatty acids from fish oil, ApoE genotype and cholesterol metabolism in correlation with Alzheimer's disease will be reviewed.

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. In vitro, conflicting reports exist as to whether ABCG1 promotes or impedes Abeta production. To clarify the in vivo roles of ABCG1 in Abeta metabolism and brain cholesterol homeostasis, we assessed neuropathological and cognitive outcome measures in PDAPP mice expressing excess transgenic ABCG1. A 6-fold increase in ABCG1 levels did not alter Abeta, amyloid, apolipoprotein E levels, cholesterol efflux, or cognitive performance in PDAPP mice. Furthermore, endogenous murine Abeta levels were unchanged in both ABCG1-overexpressing or ABCG1-deficient mice. These data argue against a direct role for ABCG1 in AD. However, excess ABCG1 is associated with decreased levels of sterol precursors and increased levels of SREBP-2 and HMG-CoA-reductase mRNA, whereas deficiency of ABCG1 leads to the opposite effects. Although functions for ABCG1 in cholesterol efflux and Abeta metabolism have been proposed based on results with cellular model systems, the in vivo role of this enigmatic transporter may be largely one of regulating the sterol biosynthetic pathway.

Pharmacogenomics in Alzheimer's disease

Pharmacological treatment in Alzheimer's disease (AD) accounts for 10-20% of direct costs, and fewer than 20% of AD patients are moderate responders to conventional drugs (donepezil, rivastigmine, galantamine, memantine), with doubtful cost-effectiveness. Both AD pathogenesis and drug metabolism are genetically regulated complex traits in which hundreds of genes cooperatively participate. Structural genomics studies demonstrated that more than 200 genes might be involved in AD pathogenesis regulating dysfunctional genetic networks leading to premature neuronal death. The AD population exhibits a higher genetic variation rate than the control population, with absolute and relative genetic variations of 40-60% and 0.85-1.89%, respectively. AD patients also differ in their genomic architecture from patients with other forms of dementia. Functional genomics studies in AD revealed that age of onset, brain atrophy, cerebrovascular hemodynamics, brain bioelectrical activity, cognitive decline, apoptosis, immune function, lipid metabolism dyshomeostasis, and amyloid deposition are associated with AD-related genes. Pioneering pharmacogenomics studies also demonstrated that the therapeutic response in AD is genotype-specific, with apolipoprotein E (APOE) 4/4 carriers the worst responders to conventional treatments. About 10-20% of Caucasians are carriers of defective cytochrome P450 (CYP) 2D6 polymorphic variants that alter the metabolism and effects of AD drugs and many psychotropic agents currently administered to patients with dementia. There is a moderate accumulation of AD-related genetic variants of risk in CYP2D6 poor metabolizers (PMs) and ultrarapid metabolizers (UMs), who are the worst responders to conventional drugs. The association of the APOE-4 allele with specific genetic variants of other genes (e.g., CYP2D6, angiotensin-converting enzyme [ACE]) negatively modulates the therapeutic response to multifactorial treatments affecting cognition, mood, and behavior. Pharmacogenetic and pharmacogenomic factors may account for 60-90% of drug variability in drug disposition and pharmacodynamics. The incorporation of pharmacogenetic/pharmacogenomic protocols to AD research and clinical practice can foster therapeutics optimization by helping to develop cost-effective pharmaceuticals and improving drug efficacy and safety.

Alzheimer disease models and human neuropathology: similarities and differences

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of Abeta peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of Abeta peptide, similar but not identical to the Abeta peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of Abeta, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in Abeta 42 levels, except for the Arctic mutation, which alters the Abeta sequence itself. Overexpressing wild-type APP alone (as in the murine models of human trisomy 21) causes no Abeta deposition in most mouse lines. Doubly (APP x mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of Abeta. The APP transgenic mice have raised new questions concerning the mechanisms of neuronal loss, the accumulation of Abeta in the cell body of the neurons, inflammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinetics of the changes. The connection between the symptoms, the lesions and the increase in Abeta oligomers has been found to be difficult to unravel. Neurofibrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau -/- background. A triply transgenic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of Abeta or of tau accumulation have been tested. A transgenic model may be analyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis.

ATP-binding cassette transporter A1 (ABCA1) deficiency does not attenuate the brain-to-blood efflux transport of human amyloid-beta peptide (1-40) at the blood-brain barrier

ATP-binding cassette transporter A1 (ABCA1) mediates apolipoprotein-dependent cholesterol release from cellular membranes. Recent studies using ABCA1 knockout mice have demonstrated that ABCA1 affects amyloid-beta peptide (A beta) levels in the brain and the production of senile plaque. Cerebral A beta(1-40) was eliminated from the brain to the circulating blood via the blood-brain barrier (BBB), which expresses ABCA1. Therefore, in the present study, we examined whether ABCA1 affects the brain-to-blood efflux transport of human A beta(1-40)(hA beta(1-40)) at the BBB. The apparent uptake of [125I]hA beta(1-40) into ABCA1-expressing HEK293 cells was not significantly different from that into parental HEK293 cells. In addition, the apparent uptake was not significantly affected even in the presence of apolipoprotein A-I as a cholesterol release acceptor. Moreover, [125I]hA beta(1-40) elimination from mouse brain across the BBB was not significantly different between ABCA1-deficient and wild-type mice 60 min after its administration into the cerebrum. These results suggest that ABCA1 does not directly transport hA beta(1-40) and a deficiency of ABCA1 does not attenuate the brain-to-blood efflux transport of hA beta(1-40) across the BBB.

A novel intronic polymorphism of ABCA1 gene reveals risk for sporadic Alzheimer's disease in Chinese

Recent genetic studies have shown that variants of the ATP-binding cassette transporter A1, ABCA1, may be implicated in the pathogenesis of Alzheimer's disease (AD). In this case-control study, a panel of 19 single nucleotide polymorphisms (SNP) (including three amino-acid-coding SNPs used for replication of previous work, and 16 newly selected intronic tag SNPs) was genotyped. Nominally significant single marker P-values were observed in four SNPs, with the highest score of 0.003 for rs2297404 (OR = 1.88, 95%CI 1.23-2.87). In addition, six distinct linkage disequilibrium (LD) blocks were detected. LD block1 harbored three nominally significant SNPs (rs2297404, rs2230808, and rs2020927), and showed a different haplotype structure in the affected and unaffected groups. Of the four haplotypes identified, haplotype2 (CAC) was more prevalent in the disease group (0.323 in AD vs. 0.202 in control); while haplotype1 (TGG) was over-represented in the healthy controls (0.595 in control vs. 0.493 in AD), indicating disease risk conferring possibility of haplotype2. After doubling the sample size, the three nominally significant SNPs were still significantly associated with AD. Although coding SNP (rs2230808) was confirmed to have a significant association with AD, prediction of the effects of an amino acid substitution SNP rs2230808 (R1587K) on the three-dimensional structure and function of the ABCA1 protein using PolyPhen program revealed that it is unlikely to be functionally significant. However, the adjacent rs2297404 in the same LD block is potentially functionally significant because of its position in the immediate vicinity of a splicing branch site. Further functional analysis of this polymorphism should be a high priority.

Cholesterol retention in Alzheimer's brain is responsible for high beta- and gamma-secretase activities and Abeta production

Alzheimer's disease (AD) is characterized by overproduction of A beta derived from APP cleavage via beta- and gamma-secretase pathway. Recent evidence has linked altered cholesterol metabolism to AD pathogenesis. In this study, we show that AD brain had significant cholesterol retention and high beta- and gamma-secretase activities as compared to age-matched non-demented controls (ND). Over one-half of AD patients had an apoE4 allele but none of the ND. beta- and gamma-secretase activities were significantly stimulated in vitro by 40 and 80 microM cholesterol in AD and ND brains, respectively. Both secretase activities in AD brain were more sensitive to cholesterol (40 microM) than those of ND (80 microM). Filipin-stained cholesterol overlapped with BACE and A beta in AD brain sections. Cholesterol (10-80 microM) added to N2a cultures significantly increased cellular cholesterol, beta- and gamma-secretase activities and A beta secretion. Similarly, addition of cholesterol (20-80 microM) to cell lysates stimulated both in vitro secretase activities. Ergosterol slightly decreased beta-secretase activity at 20-80 microM, but strongly inhibited gamma-secretase activity at 40 microM. Cholesterol depletion reduced cellular cholesterol, beta-secretase activity and A beta secretion. Transcription factor profiling shows that several key nuclear receptors involving cholesterol metabolism were significantly altered in AD brain, including decreased LXR-beta, PPAR and TR, and increased RXR. Treatment of N2a cells with LXR, RXR or PPAR agonists strongly stimulated cellular cholesterol efflux to HDL and reduced cellular cholesterol and beta-/gamma-secretase activities. This study provides direct evidence that cholesterol homeostasis is impaired in AD brain and suggests that altered levels or activities of nuclear receptors may contribute to cholesterol retention which likely enhances beta- and gamma-secretase activities and A beta production in human brain.

Expression profiling in APP23 mouse brain: inhibition of Abeta amyloidosis and inflammation in response to LXR agonist treatment

BACKGROUND

Recent studies demonstrate that in addition to its modulatory effect on APP processing, in vivo application of Liver X Receptor agonist T0901317 (T0) to APP transgenic and non-transgenic mice decreases the level of Abeta42. Moreover, in young Tg2576 mice T0 completely reversed contextual memory deficits. Compared to other tissues, the regulatory functions of LXRs in brain remain largely unexplored and our knowledge so far is limited to the cholesterol transporters and apoE. In this study we applied T0 to APP23 mice for various times and examined gene and protein expression. We also performed a series of experiments with primary brain cells derived from wild type and LXR knockout mice subjected to various LXR agonist treatments and inflammatory stimuli.

RESULTS

We demonstrate an upregulation of genes related to lipid metabolism/transport, metabolism of xenobiotics and detoxification. Downregulated genes are involved in immune response and inflammation, cell death and apoptosis. Additional treatment experiments demonstrated an increase of soluble apolipoproteins E and A-I and a decrease of insoluble Abeta. In primary LXRwt but not in LXRalpha-/-beta-/- microglia and astrocytes LXR agonists suppressed the inflammatory response induced by LPS or fibrillar Abeta.

CONCLUSION

The results show that LXR agonists could alleviate AD pathology by acting on amyloid deposition and brain inflammation. An increased understanding of the LXR controlled regulation of Abeta aggregation and clearance systems will lead to the development of more specific and powerful agonists targeting LXR for the treatment of AD.

Association of genetic variants of ABCA1 with Alzheimer's disease risk

ABCA1 plays key roles in cholesterol transport and apolipoprotein E (APOE) metabolism in the brain. To evaluate the relationship between ABCA1 genetic variants and Alzheimer's disease (AD), independently or in concert with the APOE epsilon4 allele, we examined three ABCA1 polymorphisms located in the coding region (R219K, I883M, and R1587K) and two ABCA1 polymorphisms in the promoter region (C-14T and C-477T) in a group of 372 Spanish AD patients and 440 controls. The ABCA1 219K, 883I, 1587R haplotype was significantly associated with AD, conferring a risk of 1.78 (P = 0.007). The ABCA1 C-14T polymorphism modified the risk of AD in an APOE epsilon4 allele-dependent fashion: in APOE epsilon4 carriers, homozygous for the ABCA1 -14T allele had 3.7 times higher risk of developing AD (OR = 13.99) than carriers of the ABCA1 -14CC and CT genotypes (OR = 3.79). These data suggest that the development of AD might be influenced by either a qualitative change of the ABCA1 protein caused by coding region variants (219K, 883I, and 1587R), or by a quantitative change in ABCA1 expression caused by promoter region variant (-14T) in concert with the APOE epsilon4 allele.

Serum amyloid P colocalizes with apolipoproteins in human atheroma: functional implications

Serum amyloid P (SAP) is a common component of human amyloid deposits and has been identified in atherosclerotic lesions. We investigated the extent of the colocalization of SAP with apolipoprotein A-I (apoA-I), apoB, apoC-II, and apoE in human coronary arteries and explored potential roles for SAP in these regions, specifically the effect of SAP on the rate of formation and macrophage recognition of amyloid fibrils composed of apoC-II. Analysis of 42 human arterial sections by immunohistochemistry and double label fluorescence microscopy demonstrated that SAP and apoA-I, apoB, apoC-II, and apoE were increased significantly in atherosclerotic lesions compared with nonatherosclerotic segments. SAP colocalized with all four apolipoproteins to a similar extent, whereas plaque macrophages were found to correlate most strongly with apoC-II and apoB. In vitro studies showed that SAP accelerated the formation of amyloid fibrils by purified apoC-II. Furthermore, SAP strongly inhibited the phagocytosis of apoC-II amyloid fibrils by primary macrophages and macrophage cell lines and blocked the resultant production of reactive oxygen species. The ability of SAP to accelerate apoC-II amyloid fibril formation and inhibit macrophage recognition of apoC-II fibrils suggests that SAP may modulate the inflammatory response to amyloid fibrils in atherosclerosis.

Elastic fibers and amyloid deposition in vascular tissue

Amyloid fibrils are associated with a large number of diseases, such as Alzheimer’s dementia and others. Evidence links Alzheimer’s dementia with vascular diseases and only few data connect amyloids and atherosclerosis and aging via deposits in the aortic intima. Recent results demonstrate that some elastin polypeptide sequences are also able to produce amyloid fibers. This finding could have useful implications in the study of amyloids in cardiovascular tissue whose main constituent is elastin. In this review, we have also outlined the main characterizing features regarding the structure of amyloid fibrils. Finally, we describe, as a future perspective, the design of proper inhibitors of amyloid deposition in vascular walls as potential therapeutic drugs.

Attenuation of neuroinflammation and Alzheimer's disease pathology by liver x receptors

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease that causes progressive cognitive impairment. The initiation and progression of AD has been linked to cholesterol metabolism and inflammation, processes that can be modulated by liver x receptors (LXRs). We show here that endogenous LXR signaling impacts the development of AD-related pathology. Genetic loss of either Lxralpha or Lxrbeta in APP/PS1 transgenic mice results in increased amyloid plaque load. LXRs regulate basal and inducible expression of key cholesterol homeostatic genes in the brain and act as potent inhibitors of inflammatory gene expression. Ligand activation of LXRs attenuates the inflammatory response of primary mixed glial cultures to fibrillar amyloid beta peptide (fAbeta) in a receptor-dependent manner. Furthermore, LXRs promote the capacity of microglia to maintain fAbeta-stimulated phagocytosis in the setting of inflammation. These results identify endogenous LXR signaling as an important determinant of AD pathogenesis in mice. We propose that LXRs may be tractable targets for the treatment of AD due to their ability to modulate both lipid metabolic and inflammatory gene expression in the brain.

Cholesterol metabolism, apolipoprotein E, adenosine triphosphate-binding cassette transporters, and Alzheimer's disease

PURPOSE OF REVIEW

Recent evidence suggests that cholesterol metabolism participates in the pathogenesis of Alzheimer's disease. Apolipoprotein E is the main lipid carrier in the brain and the best-established risk factor for late-onset Alzheimer's disease. Intracellular cholesterol levels influence the generation of amyloid-beta peptides, the toxic species thought to be a primary cause of Alzheimer's disease. Finally, compounds that modulate cholesterol metabolism affect amyloid-beta generation. This review summarizes data linking apolipoprotein E and adenosine triphosphate-binding cassette transporters to aspects of cholesterol metabolism and Alzheimer's disease pathogenesis.

RECENT FINDINGS

In vivo, the lipidation status of apolipoprotein E affects amyloid-beta burden in mice with Alzheimer's disease, which appears to caused by the modulation of amyloid-beta deposition or clearance rather than amyloid-beta production. State-of-the-art in-vivo assays reveal that amyloid-beta is cleared from the brain by multiple pathways. Members of the adenosine triphosphate-binding cassette superfamily of transporters regulate lipid homeostasis and apolipoprotein metabolism in the brain, and may affect Alzheimer's disease pathogenesis by modulating apolipoprotein E lipidation as well as intracellular sterol homeostasis.

SUMMARY

Proteins involved in brain cholesterol metabolism may affect the pathogenesis of Alzheimer's disease. Compounds that modulate the expression of these proteins may be of therapeutic benefit in Alzheimer's disease.

Pathway of biogenesis of apolipoprotein E-containing HDL in vivo with the participation of ABCA1 and LCAT

We have investigated the ability of apoE (apolipoprotein E) to participate in the biogenesis of HDL (high-density lipoprotein) particles in vivo using adenovirus-mediated gene transfer in apoA-I-/- (apolipoprotein A-I) or ABCA1-/- (ATP-binding cassette A1) mice. Infection of apoA-I-/- mice with 2x10(9) pfu (plaque-forming units) of an apoE4-expressing adenovirus increased both HDL and the triacylglycerol-rich VLDL (very-low-density lipoprotein)/IDL (intermediate-density lipoprotein)/LDL (low-density lipoprotein) fraction and generated discoidal HDL particles. ABCA1-/- mice treated similarly failed to form HDL particles, suggesting that ABCA1 is essential for the generation of apoE-containing HDL. Combined infection of apoA-I-/- mice with a mixture of adenoviruses expressing both apoE4 (2x10(9) pfu) and human LCAT (lecithin:cholesterol acyltransferase) (5x10(8) pfu) cleared the triacylglycerol-rich lipoproteins, increased HDL and converted the discoidal HDL into spherical HDL. Similarly, co-infection of apoE-/- mice with apoE4 and human LCAT corrected the hypercholesterolaemia and generated spherical particles, suggesting that LCAT is essential for the maturation of apoE-containing HDL. Overall, the findings indicate that apoE has a dual functionality. In addition to its documented functions in the clearance of triacylglycerol-rich lipoproteins, it participates in the biogenesis of HDL-sized apoE-containing particles. HDL particles generated by this pathway may account at least for some of the atheroprotective functions of apoE.

Apolipoprotein E levels in cerebrospinal fluid and the effects of ABCA1 polymorphisms

Background

Animal studies suggest that brain apolipoprotein E (apoE) levels influence amyloid-β (Aβ) deposition and thus risk for Alzheimer's disease (AD). We have previously demonstrated that deletion of the ATP-binding cassette A1 transporter (ABCA1) in mice causes dramatic reductions in brain and cerebrospinal fluid (CSF) apoE levels and lipidation. To examine whether polymorphisms in ABCA1 affect CSF apoE levels in humans, we measured apoE in CSF taken from 168 subjects who were 43 to 91 years old and were either cognitively normal or who had mild AD. We then genotyped the subjects for ten previously identified ABCA1 single nucleotide polymorphisms (SNPs).

Results

In all subjects, the mean CSF apoE level was 9.09 μg/ml with a standard deviation of 2.70 μg/ml. Levels of apoE in CSF samples taken from the same individual two weeks apart were strongly correlated (r² = 0.93, p < 0.01). In contrast, CSF apoE levels in different individuals varied widely (coefficient of variation = 46%). CSF apoE levels did not vary according to AD status, APOE genotype, gender or race. Average apoE levels increased with age by ~0.5 μg/ml per 10 years (r² = 0.05, p = 0.003). We found no significant associations between CSF apoE levels and the ten ABCA1 SNPs we genotyped. Moreover, in a separate sample of 1225 AD cases and 1431 controls, we found no association between the ABCA1 SNP rs2230806 and AD as has been previously reported.

Conclusion

We found that CSF apoE levels vary widely between individuals, but are stable within individuals over a two-week interval. AD status, APOE genotype, gender and race do not affect CSF apoE levels, but average CSF apoE levels increase with age. Given the lack of association between CSF apoE levels and genotypes for the ABCA1 SNPs we examined, either these SNPs do not affect ABCA1 function or if they do, they do not have strong effects in the CNS. Finally, we find no evidence for an association between the ABCA1 SNP rs2230806 and AD in a large sample set.

Follow-up mapping supports the evidence for linkage in the candidate region at 9q22 in the NIMH Alzheimer's disease Genetics Initiative cohort

Other than the APOE peak at 19q13, the 9q22 region was identified in our original genomic scan as the candidate region with the highest multipoint lod score (MLS) in the subset of late onset Alzheimer's Disease (AD) families (MLS = 2.9 at 101 cM) from the NIMH Genetics Initiative sample. We have now genotyped an additional 12 short tandem repeats (STR) in this region. Multipoint analysis shows the region remains significant with an increase in the peak MLS from 2.9 to 3.8 at 95 cM near marker D9S1815, and the 1 LOD interval narrows from 21.5 to 11 cM. HLOD scores also provide evidence for significant linkage (4.5 with an alpha = 31%) with a further narrowing of the region to 6.6 cM (92.2-98.8 cM). Single nucleotide polymorphisms (SNPs) in the Ubiquilin1 gene (UBQLN1), located at 83.3 cM, have been reported to be significantly associated to AD, accounting for a substantial portion of the original linkage signal [Bertram et al., 2005]. Our analyses of the higher resolution genotype data generated here provide further support for the existence of a least one additional locus on chromosome 9q22. In an effort to pinpoint this putative AD susceptibility gene, we have begun to analyze SNPs in other candidate genes in and around this narrowed region to test for additional associations to AD.

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 21q22.3, and individuals with Down syndrome (DS) typically manifest with Alzheimer's disease (AD) neuropathology in their 30s. Here, we demonstrate that expression of ABCG1 enhances amyloid-beta protein (Abeta) production in transfected HEK cells in a manner that requires functional cholesterol transporter activity. ABCG1-expressing cells also exhibit increased secreted APP (sAPP)alpha and sAPPbeta secretion and display increased cell surface-associated APP. These results suggest that ABCG1 increases the availability of APP as a secretase substrate for both the amyloidogenic and nonamyloidogenic pathways. In vivo, ABCG1 mRNA levels are 2-fold more abundant in DS brain compared with age- and sex-matched normal controls. Finally, both Abeta and sAPPalpha levels are increased in DS cortex relative to normal controls. These findings suggest that altered cholesterol metabolism and APP trafficking mediated by ABCG1 may contribute to the accelerated onset of AD neuropathology in DS.

ABCA1 polymorphisms and Alzheimer's disease

In our search for genetic factors related to the development of Alzheimer's disease, we have genotyped 332 pedigrees for three coding polymorphisms in the ABCA1 gene, two of which are known to alter plasma cholesterol levels, as well as a non-coding polymorphism within the promoter. We show an apparent weak association of rs2230806 (p-value=0.01) with the disease in a sibpair series of Alzheimer's disease that had shown previously evidence for linkage to the chromosome 9 locus where ABCA1 maps.

The LXR agonist TO901317 selectively lowers hippocampal Abeta42 and improves memory in the Tg2576 mouse model of Alzheimer's disease

Recent studies show that intracellular cholesterol levels can modulate the processing of amyloid precursor protein to Abeta peptide. Moreover, cholesterol-rich apoE-containing lipoproteins may also promote Abeta clearance. Agonists of the liver X receptor (LXR) transcriptionally induce genes involved in intracellular lipid efflux and transport, including apoE. Thus, LXR agonists have the potential to both inhibit APP processing and promote Abeta clearance. Here we show that LXR agonist, TO901317, increased hippocampal ABCA1 and apoE and decreased Abeta42 levels in APP transgenic mice. TO901317 had no significant effects on levels of Abeta40, full length APP, or the APP processing products. Next, we examined the effects of TO901317 in the contextual fear conditioning paradigm; TO901317 completely reversed the contextual memory deficit in these mice. These data demonstrate that LXR agonists do not directly inhibit APP processing but rather facilitate the clearance of Abeta42 and may represent a novel therapeutic approach to Alzheimer's disease.

Disease modifying therapy for AD?

Alzheimer's disease (AD) is the most common form of dementia in industrialized nations. If more effective therapies are not developed that either prevent AD or block progression of the disease in its very early stages, the economic and societal cost of caring for AD patients will be devastating. Only two types of drugs are currently approved for the treatment of AD: inhibitors of acetyl cholinesterase, which symptomatically enhance cognitive state to some degree but are not disease modifying; and the adamantane derivative, memantine. Memantine preferentially blocks excessive NMDA receptor activity without disrupting normal receptor activity and is thought to be a neuroprotective agent that blocks excitotoxicty. Memantine therefore may have a potentially disease modifying effect in multiple neurodegenerative conditions. An improved understanding of the pathogeneses of AD has now led to the identification of numerous therapeutic targets designed to alter amyloid beta protein (Abeta) or tau accumulation. Therapies that alter Abeta and tau through these various targets are likely to have significant disease modifying effects. Many of these targets have been validated in proof of concept studies in preclinical animal models, and some potentially disease modifying therapies targeting Abeta or tau are being tested in the clinic. This review will highlight both the promise of and the obstacles to developing such disease modifying AD therapies.

Role of ABCG1 and ABCA1 in regulation of neuronal cholesterol efflux to apolipoprotein E discs and suppression of amyloid-beta peptide generation

Maintenance of an adequate supply of cholesterol is important for neuronal function, whereas excess cholesterol promotes amyloid precursor protein (APP) cleavage generating toxic amyloid-beta (Abeta) peptides. To gain insights into the pathways that regulate neuronal cholesterol level, we investigated the potential for reconstituted apolipoprotein E (apoE) discs, resembling nascent lipoprotein complexes in the central nervous system, to stimulate neuronal [3H]cholesterol efflux. ApoE discs potently accelerated cholesterol efflux from primary human neurons and cell lines. The process was saturable (17.5 microg of apoE/ml) and was not influenced by APOE genotype. High performance liquid chromatography analysis of cholesterol and cholesterol metabolites effluxed from neurons indicated that <25% of the released cholesterol was modified to polar products (e.g. 24-hydroxycholesterol) that diffuse from neuronal membranes. Thus, most cholesterol (approximately 75%) appeared to be effluxed from neurons in a native state via a transporter pathway. ATP-binding cassette transporters ABCA1, ABCA2, and ABCG1 were detected in neurons and neuroblastoma cell lines and expression of these cDNAs revealed that ABCA1 and ABCG1 stimulated cholesterol efflux to apoE discs. In addition, ABCA1 and ABCG1 expression in Chinese hamster ovary cells that stably express human APP significantly reduced Abeta generation, whereas ABCA2 did not modulate either cholesterol efflux or Abeta generation. These data indicate that ABCA1 and ABCG1 play a significant role in the regulation of neuronal cholesterol efflux to apoE discs and in suppression of APP processing to generate Abeta peptides.

ABC transporters, neural stem cells and neurogenesis – a different perspective

Stem cells intrigue. They have the ability to divide exponentially, recreate the stem cell compartment, as well as create differentiated cells to generate tissues. Therefore, they should be natural candidates to provide a renewable source of cells for transplantation applied in regenerative medicine. Stem cells have the capacity to generate specific tissues or even whole organs like the blood, heart, or bones. A subgroup of stem cells, the neural stem cells (NSCs), is characterized as a self-renewing population that generates neurons and glia of the developing brain. They can be isolated, genetically manipulated and differentiated in vitro and reintroduced into a developing, adult or a pathologically altered central nervous system. NSCs have been considered for use in cell replacement therapies in various neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. Characterization of genes with tightly controlled expression patterns during differentiation represents an approach to understanding the regulation of stem cell commitment. The regulation of stem cell biology by the ATP-binding cassette (ABC) transporters has emerged as an important new field of investigation. As a major focus of stem cell research is in the manipulation of cells to enable differentiation into a targeted cell population; in this review, we discuss recent literatures on ABC transporters and stem cells, and propose an integrated view on the role of the ABC transporters, especially ABCA2, ABCA3, ABCB1 and ABCG2, in NSCs' proliferation, differentiation and regulation, along with comparisons to that in hematopoietic and other stem cells.

Role of ApoE in conformation-prone diseases and atherosclerosis

Three isoforms of human plasma apolipoprotein E (apoE) are ligands to lipoprotein receptors and influence in different manner the synthesis and catabolism of pro-atherogenic triglyceride-rich lipoproteins. Among three isoforms, the apoE4 isoform is associated with increased frequency of atherosclerosis and Alzheimer's disease (AD). The conformational transitions of beta-amyloid (Abeta) influenced by apoE and serum amyloid P (SAP) component are key events in AD development, the accumulation of intermediate diffusible and soluble oligomers of Abeta being of particular significance. SAP and apoE, in a different manner for the three isoforms, serve as "pathological" chaperones during the aggregation of Abeta considered as a conformation-prone process. In turn, apoE consisting of two domains self-associates in solution and intermediate structures differently populated for the three isoforms exist. The different structures of the three isoforms determine their different distribution among various plasma lipoproteins. The structural and metabolic consideration of the common apoE pathway(s) in two pathologies assumes four molecular targets for AD correction: (i) inhibition of the accumulation of diffusible soluble Abeta oligomers; (ii) inhibition of apoE synthesis and secretion by astrocytes, in particular, under lipid-lowering therapy; (iii) inhibition of the binding of apoE and/or SAP to Abeta; (iv) stimulation of the expression of cholesterol transporter ABCA1.