Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Capillary cerebral amyloid angiopathy identifies a distinct APOE epsilon4-associated subtype of sporadic Alzheimer's disease

The deposition of amyloid beta-protein (Abeta) in the vessel wall, i.e., cerebral amyloid angiopathy (CAA), is associated with Alzheimer's disease (AD). Two types of CAA can be differentiated by the presence or absence of capillary Abeta-deposits. In addition, as in Alzheimer's disease, risk for capillary CAA is associated with the apolipoprotein E (APOE) epsilon4-allele. Because these morphological and genetic differences between the two types of AD-related CAA exist, the question arises as to whether there exist further differences between AD cases with and without capillary CAA and, if so, whether capillary CAA can be employed to distinguish and define specific subtypes of AD. To address this question, we studied AD and control cases both with and without capillary CAA to identify the following: (1) distinguishing neuropathological features; (2) alterations in perivascular protein expression; and (3) genotype-specific associations. More widespread Abeta-plaque pathology was observed in AD cases with capillary CAA than in those without. Expression of perivascular excitatory amino acid transporter 2 (EAAT-2/GLT-1) was reduced in cortical astrocytes of AD cases with capillary CAA in contrast to those lacking capillary Abeta-deposition and controls. Genetically, AD cases with capillary CAA were strongly associated with the APOE epsilon4 allele compared to those lacking capillary CAA and to controls. To further validate the existence of distinct types of AD we analyzed polymorphisms in additional apoE- and cholesterol-related candidate genes. Our results revealed an association between AD cases without capillary CAA (i.e., AD cases with CAA but lacking capillary CAA and AD cases without CAA) and the T-allele of the alpha(2)macroglobulin receptor/low-density lipoprotein receptor-related protein-1 (LRP-1) C766T polymorphism as opposed to AD cases with capillary CAA and non-AD controls. Taken together, these results indicate that AD cases with capillary CAA differ significantly from other AD cases both genetically and morphologically, thereby pointing to a specific capillary CAA-related and APOE epsilon4-associated subtype of AD.

Membrane rafts: a potential gateway for bacterial entry into host cells

Pathogenic bacteria have developed various mechanisms to evade host immune defense systems. Invasion of pathogenic bacteria requires interaction of the pathogen with host receptors, followed by activation of signal transduction pathways and rearrangement of the cytoskeleton to facilitate bacterial entry. Numerous bacteria exploit specialized plasma membrane microdomains, commonly called membrane rafts, which are rich in cholesterol, sphingolipids and a special set of signaling molecules which allow entry to host cells and establishment of a protected niche within the host. This review focuses on the current understanding of the raft hypothesis and the means by which pathogenic bacteria subvert membrane microdomains to promote infection.

The effects of cholesterol on learning and memory

Cholesterol is vital to normal brain function including learning and memory but that involvement is as complex as the synthesis, metabolism and excretion of cholesterol itself. Dietary cholesterol influences learning tasks from water maze to fear conditioning even though cholesterol does not cross the blood brain barrier. Excess cholesterol has many consequences including peripheral pathology that can signal brain via cholesterol metabolites, pro-inflammatory mediators and antioxidant processes. Manipulations of cholesterol within the central nervous system through genetic, pharmacological, or metabolic means circumvent the blood brain barrier and affect learning and memory but often in animals already otherwise compromised. The human literature is no less complex. Cholesterol reduction using statins improves memory in some cases but not others. There is also controversy over statin use to alleviate memory problems in Alzheimer's disease. Correlations of cholesterol and cognitive function are mixed and association studies find some genetic polymorphisms are related to cognitive function but others are not. In sum, the field is in flux with a number of seemingly contradictory results and many complexities. Nevertheless, understanding cholesterol effects on learning and memory is too important to ignore.

Oxidative stress up-regulates presenilin 1 in lipid rafts in neuronal cells

Oxidative stress is associated with beta-amyloid peptide (A beta) accumulation in the brains of Alzheimer's disease patients. A beta is generated upon the sequential proteolytic cleavage of transmembrane amyloid precursor protein (APP) by two membrane-bound proteases, beta-secretase (BACE1) and the gamma-secretase complex comprising presenilin 1 (PS1), nicastrin, APH-1 and PEN-2. Recent evidence suggests that significant amounts of BACE1 and gamma-secretase components localize in the cholesterol-rich region of membranes known as lipid rafts, where A beta production occurs preferentially. In this study, we investigated the effects of oxidative stress on the BACE1 and gamma-secretase components in lipid rafts using human neuroblastoma SH-SY5Y cells exposed to ethacrynic acid (EA), a compound that induces cellular glutathione depletion. Following exposure of cells to EA, heme oxygenase-1, a marker protein of oxidative stress, was strongly induced. Moreover, treatment with EA resulted in a significant increase in PS1 protein levels, but not those of nicastrin, APH-1, PEN-2 or BACE1, in both cell lysates and the lipid raft fraction. This increase in PS1 protein expression was prevented by co-treatment with an antioxidant, N-acetylcysteine (NAC). EA additionally induced a significant increase in PS1 mRNA expression, which was inhibited by NAC. Finally, EA treatment was found to promote A beta secretion from cells expressing Swedish mutant APP. It appears that in our cell culture model, oxidative stress enhances PS1 protein levels in lipid rafts via up-regulation of PS1 transcription, which may constitute the mechanism underlying the oxidative stress-associated promotion of A beta production.

A functional polymorphism in the HMGCR promoter affects transcriptional activity but not the risk for Alzheimer disease in Swedish populations

Variations in genes associated with cholesterol homeostasis have been reported to modify the risk of developing Alzheimer disease (AD). To date there have been few investigations into variations in genes directly involved in cholesterol biosynthesis and AD. We investigated the influence of the -911C>A polymorphism (rs3761740) in the hydroxy-methyl-glutaryl CoA reductase (HMGCR) gene promoter on basal and regulated transcription, plasma cholesterol levels and the association with AD. Under in vitro conditions the A allele was found to be significantly more responsive to SREBP-2 mediated regulation than the C allele. In an age and sex matched case-control study, the genotype distribution and allele frequency of this polymorphism were not associated with AD (OR=1.03; 95% CI=0.72-1.48). However, we did find evidence supporting an interaction between the HMGCR A allele, the APOE E4 allele and an altered risk of AD (OR=2.41; 95% CI=0.93-6.22).

Membrane biophysics and mechanics in Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disorder characterized by neuronal loss, cerebrovascular inflammation, and accumulation of senile plaques in the brain parenchyma and cerebral blood vessels. Amyloid-beta peptide (Abeta), a major component of senile plaques, has been shown to exert multiple toxic effects to neurons, astrocytes, glial cells, and brain endothelium. Oligomeric Abeta can disturb the structure and function of cell membranes and alter membrane mechanical properties, such as membrane fluidity and molecular order. Much of these effects are attributed to their capability to trigger oxidative stress and inflammation. In this review, we discuss the effects of Abeta on neuronal cells, astrocytes, and cerebral endothelial cells with special emphasis on cell membrane properties and cell functions.

Role of vascular risk factors and vascular dysfunction in Alzheimer's disease

Recent findings indicate that vascular risk factors and neurovascular dysfunction play integral roles in the pathogenesis of Alzheimer's disease. In addition to aging, the most common risk factors for Alzheimer's disease are apolipoprotein e4 allele, hypertension, hypotension, diabetes, and hypercholesterolemia. All of these can be characterized by vascular pathology attributed to conditions such as cerebral amyloid angiopathy and subsequent blood-brain barrier dysfunction. Many epidemiological, clinical, and pharmacotherapeutic studies have assessed the associations between such risk factors and Alzheimer's disease and have found positive associations between hypertension, hypotension, and diabetes mellitus. However, there are still many conflicting results from these population-based studies, and they should be interpreted carefully. Recognition of these factors and the mechanisms by which they contribute to Alzheimer's disease will be beneficial in the current treatment regimens for Alzheimer's disease and in the development of future therapies. Here we discuss vascular factors with respect to Alzheimer's disease and dementia and review the factors that give rise to vascular dysfunction and contribute to Alzheimer's disease.

Sterols and sphingolipids: dynamic duo or partners in crime?

One manner in which eukaryotic cells respond to their environments is by optimizing the composition and proportions of sterols and sphingolipids in membranes. The physical association of the planar ring of sterols with the acyl chains of phospholipids, particularly sphingolipids, produces membrane micro-heterogeneity that is exploited to coordinate several crucial pathways. We hypothesize that these lipid molecules play an integrated role in human disease; when one of the partners is mis-regulated, pathology frequently ensues. Sterols and sphingolipid levels are not coordinated by the action of a single master regulator, however the cross-talk between their metabolic pathways is considerable. We describe our perspectives on the key components of synthesis, catabolism and transport of these lipid partners with an emphasis on evolutionarily conserved reactions that produce disease states when defective.

Cardiovascular dementia - a different perspective

The number of dementia patients has been growing in recent years and dementia represents a significant threat to aging people all over the world. Recent research has shown that the number of people affected by Alzheimer's disease (AD) and dementia is growing at an epidemic pace. The rapidly increasing financial and personal costs will affect the world's economies, health care systems, and many families. Researchers are now exploring a possible connection among AD, vascular dementia (VD), diabetes mellitus (type 2, T2DM) and cardiovascular diseases (CD). This correlation may be due to a strong association of cardiovascular risk factors with AD and VD, suggesting that these diseases share some biologic pathways. Since heart failure is associated with an increased risk of AD and VD, keeping the heart healthy may prove to keep the brain healthy as well. The risk for dementia is especially high when diabetes mellitus is comorbid with severe systolic hypertension or heart disease. In addition, the degree of coronary artery disease (CAD) is independently associated with cardinal neuropathological lesions of AD. Thus, the contribution of T2DM and CD to AD and VD implies that cardiovascular therapies may prove useful in preventing AD and dementia.

Cholesterol reduces pardaxin's dynamics-a barrel-stave mechanism of membrane disruption investigated by solid-state NMR

While high-resolution 3D structures reveal the locations of all atoms in a molecule, it is the dynamics that correlates the structure with the function of a biological molecule. The complete characterization of dynamics of a membrane protein is in general complex. In this study, we report the influence of dynamics on the channel-forming function of pardaxin using chemical shifts and dipolar couplings measured from 2D broadband-PISEMA experiments on mechanically aligned phospholipids bilayers. Pardaxin is a 33-residue antimicrobial peptide originally isolated from the Red Sea Moses sole, Pardachirus marmoratus, which functions via either a carpet-type or barrel-stave mechanism depending on the membrane composition. Our results reveal that the presence of cholesterol significantly reduces the backbone motion and the tilt angle of the C-terminal amphipathic helix of pardaxin. In addition, a correlation between the dynamics-induced heterogeneity in the tilt of the C-terminal helix and the membrane disrupting activity of pardaxin by the barrel-stave mechanism is established. This correlation is in excellent agreement with the absence of hemolytic activity for the derivatives of pardaxin. These results explain the role of cholesterol in the selectivity of the broad-spectrum of antimicrobial activities of pardaxin.

Oxidative Stress Induced Mitochondrial Failure and Vascular Hypoperfusion as a Key Initiator for the Development of Alzheimer Disease

Mitochondrial dysfunction may be a principal underlying event in aging, including age-associated brain degeneration. Mitochondria provide energy for basic metabolic processes. Their decay with age impairs cellular metabolism and leads to a decline of cellular function. Alzheimer disease (AD) and cerebrovascular accidents (CVAs) are two leading causes of age-related dementia. Increasing evidence strongly supports the theory that oxidative stress, largely due to reactive oxygen species (ROS), induces mitochondrial damage, which arises from chronic hypoperfusion and is primarily responsible for the pathogenesis that underlies both disease processes. Mitochondrial membrane potential, respiratory control ratios and cellular oxygen consumption decline with age and correlate with increased oxidant production. The sustained hypoperfusion and oxidative stress in brain tissues can stimulate the expression of nitric oxide synthases (NOSs) and brain endothelium probably increase the accumulation of oxidative stress products, which therefore contributes to blood brain barrier (BBB) breakdown and brain parenchymal cell damage. Determining the mechanisms behind these imbalances may provide crucial information in the development of new, more effective therapies for stroke and AD patients in the near future.

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.

Transcritpional effects of S100B on neuroblastoma cells: perturbation of cholesterol homeostasis and interference on the cell cycle

S100B is a Ca2+ binding protein mainly secreted by astrocytes in the vertebrate brain that is considered a multifunctional cytokine and/or a damage-associated molecular pattern (DAMP) protein and a marker of brain injury and neurodegeneration when measured in different body fluids. It has been widely shown that this protein can exert diverse effects in neural cultures depending on its concentration, having detrimental effects at micromolar concentrations. The molecular mechanisms underlying this effect are still largely unknown. This study attempts to delineate the genome-wide gene expression analysis of the events associated with exposure to micromolar concentration of S100B in a human neuroblastoma cell line. In this experimental condition cells undergo a severe perturbation of lipid homeostasis along with cell cycle arrest. These mechanisms might reasonably mediate some aspects of the S100B-related detrimental effects of S100B, although obvious differences between mature neurons and neuroblastoma cells have to be considered.

Secretory phospholipase A2 type III enhances alpha-secretase-dependent amyloid precursor protein processing through alterations in membrane fluidity

In the non-amyloidogenic pathway, amyloid precursor protein (APP) is cleaved by alpha-secretases to produce alpha-secretase-cleaved soluble APP (sAPP(alpha)) with neuroprotective and neurotrophic properties; therefore, enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of Alzheimer's disease. Here, we demonstrate the effects of type III secretory phospholipase A(2) (sPLA(2)-III) on sAPP(alpha) secretion. Exposing differentiated neuronal cells (SH-SY5Y cells and primary rat neurons) to sPLA(2)-III for 24 h increased sAPP(alpha) secretion and decreased levels of Abeta(1-42) in SH-SY5Y cells, and these changes were accompanied by increased membrane fluidity. We further tested whether sPLA(2)-III-enhanced sAPP(alpha) release is due in part to the production of its hydrolyzed products, including arachidonic acid (AA), palmitic acid (PA), and lysophosphatidylcholine (LPC). Addition of AA but neither PA nor LPC mimicked sPLA(2)-III-induced increases in sAPP(alpha) secretion and membrane fluidity. Treatment with sPLA(2)-III and AA increased accumulation of APP at the cell surface but did not alter total expressions of APP, alpha-secretases, and beta-site APP cleaving enzyme. Taken together, these results support the hypothesis that sPLA(2)-III enhances sAPP(alpha) secretion through its action to increase membrane fluidity and recruitment of APP at the cell surface.

Clusterin: a forgotten player in Alzheimer's disease

Clusterin, also known as apolipoprotein J, is a versatile chaperone molecule which contains several amphipathic and coiled-coil alpha-helices, typical characteristics of small heat shock proteins. In addition, clusterin has three large intrinsic disordered regions, so-called molten globule domains, which can stabilize stressed protein structures. Twenty years ago, it was demonstrated that the expression of clusterin was clearly increased in Alzheimer's disease (AD). Later it was observed that clusterin can bind amyloid-beta peptides and prevent their fibrillization. Clusterin is also involved in the clearance of amyloid-beta peptides and fibrils by binding to megalin receptors and enhancing their endocytosis within glial cells. Clusterin is a complement inhibitor and can suppress complement activation observed in AD. Clusterin is also present in lipoprotein particles and regulates cholesterol and lipid metabolism of brain which is disturbed in AD. Clusterin is a stress-induced chaperone which is normally secreted but in conditions of cellular stress, it can be transported to cytoplasm where it can bind to Bax protein and inhibit neuronal apoptosis. Clusterin can also bind to Smad2/3 proteins and potentiate the neuroprotective TGFbeta signaling. An alternative splicing can produce a variant isoform of clusterin which can be translocated to nuclei where it induces apoptosis. The role of nuclear clusterin in AD needs to be elucidated. We will review here the extensive literature linking clusterin to AD and examine the recent progress in clusterin research with the respect to AD pathology. Though clusterin can be viewed as a multipotent guardian of brain, it is unable to prevent the progressive neuropathology in chronic AD.

Membrane topology of gp41 and amyloid precursor protein: interfering transmembrane interactions as potential targets for HIV and Alzheimer treatment

The amyloid precursor protein (APP), that plays a critical role in the development of senile plaques in Alzheimer disease (AD), and the gp41 envelope protein of the human immunodeficiency virus (HIV), the causative agent of the acquired immunodeficiency syndrome (AIDS), are single-spanning type-1 transmembrane (TM) glycoproteins with the ability to form homo-oligomers. In this review we describe similarities, both in structural terms and sequence determinants of their TM and juxtamembrane regions. The TM domains are essential not only for anchoring the proteins in membranes but also have functional roles. Both TM segments contain GxxxG motifs that drive TM associations within the lipid bilayer. They also each possess similar sequence motifs, positioned at the membrane interface preceding their TM domains. These domains are known as cholesterol recognition/interaction amino acid consensus (CRAC) motif in gp41 and CRAC-like motif in APP. Moreover, in the cytoplasmic domain of both proteins other α-helical membranotropic regions with functional implications have been identified. Recent drug developments targeting both diseases are reviewed and the potential use of TM interaction modulators as therapeutic targets is discussed.

[Should all old hypertensive subjects have Mini-Mental State Examination?]

Hypertension constitutes a recognized risk factor of vascular dementia but also of Alzheimer-type dementia. Various longitudinal studies showed that midlife blood pressure level is one of the factors conditioning the onset of dementia syndrome in late life. The high risk of dementia is linked to leukoaraiosis, vascular rigidity, microcirculation disorders, oxidative stress, blood pressure fluctuations including orthostatic hypotension and strokes, all of those being associated with hypertension. Numerous clinical trials showed the positive effect of effective treatment of hypertension on the prevention of cognitive disorders and dementias. Thus, screening and early management of dementia and cognitive decline, in particular in the hypertensive subject, are essential. The Mini-Mental State Examination (MMSE) is a major first-intention screening test because it allows a full assessment of cognitive aptitudes. If cognitive decline is suspected and the MMSE score is considered to be abnormal, the elderly subject must be sent to a specialist or a referent memory centre; the MMSE is only a first stage in the diagnostic reasoning. MMSE should be included in the follow-up of all hypertensive elderly subjects and should be performed once a year by the general practitioner.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

PPARγ2 polymorphism and human health

The nuclear hormone receptor peroxisome proliferator activated receptor gamma (PPARγ) is an important transcription factor regulating adipocyte differentiation, lipid and glucose homeostasis, and insulin sensitivity. Numerous genetic mutations of PPARγ have been identified and these mutations positively or negatively regulate insulin sensitivity. Among these, a relatively common polymorphism of PPARγ, Pro12Ala of PPARγ2, the isoform expressed only in adipose tissue has been shown to be associated with lower body mass index, enhanced insulin sensitivity, and resistance to the risk of type 2 diabetes in human subjects carrying this mutation. Subsequent studies in different ethnic populations, however, have revealed conflicting results, suggesting a complex interaction between the PPARγ2 Pro12Ala polymorphism and environmental factors such as the ratio of dietary unsaturated fatty acids to saturated fatty acids and/or between the PPARγ2 Pro12Ala polymorphism and genetic factors such as polymorphic mutations in other genes. In addition, this polymorphic mutation in PPARγ2 is associated with other aspects of human diseases, including cancers, polycystic ovary syndrome, Alzheimer disease and aging. This review will highlight findings from recent studies.

Time for a change in the research paradigm for Alzheimer's disease: the value of a chaotic matrix modeling approach

The amyloid cascade hypothesis, based on the genetic data from early onset, familial forms of the disease, has been the dominant model for many years and involves over production and deposition of the beta amyloid protein as causal in the disease process. However, it does not apply very well to the more common, later onset, sporadic form of the disease, where a wider range of factors appear to be involved in disease progression. Over recent years, data illustrating reciprocal interactions between the amyloid precursor protein (APP) and its various metabolites with many factors involved in normal synaptic plasticity have emerged. These feedback relationships have the potential to affect the complex kinase cascades involved in every aspect of neuronal function. Further, data regarding the multiple roles of the presenilins have the potential to allow the over expression and deposition of the amyloid beta protein to be both a cause and consequence of disease progression, with relevance in both sporadic and familial of Alzheimer's disease (AD). Disease progression might be better explained by a chaotic matrix of factors and raises the question again whether AD should be approached as a single entity or as a syndrome, with important consequences for disease identification and treatment.

Statins for the prevention of dementia

BACKGROUND

This is an update of a Cochrane review first published in 2001. At that stage there was insufficient evidence to recommend statins for the prevention of Alzheimer's disease (AD). The scope of this review has been expanded to include all forms of dementia.

OBJECTIVES

To assess the effects of statins in the prevention of dementia.

SEARCH STRATEGY

The Specialized Register of the Cochrane Dementia and Cognitive Improvement Group, The Cochrane Library, MEDLINE, EMBASE, PsycINFO, CINAHL and LILACS were searched on 10 October 2007 using the terms statin*, lovastatin*, pravastatin*, simvastatin*, fluvastatin*, atorvastatin* and rosuvastatin*. The CDCIG Register contains records from many healthcare databases, SIGLE, LILACS as well as many trials databases and is updated regularly.

SELECTION CRITERIA

Double-blind randomized placebo-controlled trials of statins in people at risk of AD and dementia.

DATA COLLECTION AND ANALYSIS

Two independent reviewers extracted and assessed data independently and agreement was reached after discussion. Adverse effects were noted.

MAIN RESULTS

Two trials were identified with 26,340 participants; HPS 2002 and PROSPER 2002. Age range was 40-82 years across the two studies, PROSPER 2002 included 5804 patients aged 70-82 years and HPS included 20,536 patients with 5806 at least 70 years old at study entry. Mean total cholesterol 5.9 mmol/l, LDL cholesterol 3.4 mmol/l at study entry with mean reduction in LDL cholesterol of 1.0 mmol/l in simvastatin treated patients compared to placebo in HPS 2002. Mean total cholesterol 5.7 mmol/l, LDL cholesterol 3.8 mmol/l at study entry with mean reduction in LDL cholesterol of 1.02 mmol/l in pravastatin treated patients compared to placebo in PROSPER 2002. Mean follow-up 3.2 years in PROSPER, 5 years in HPS 2002. Cognition was measured at different times and with different scales so could not be combined in a meta-analysis. There was no difference in incidence of dementia in HPS 2002 (31 cases in simvastatin group, 31 cases in placebo group) nor in performance on the modified Telephone Interview for Cognitive Status at final follow-up (23.7% simvastatin group cognitively impaired vs 24.2% in placebo group). There was no difference in cognition between groups either in relation to age at study entry or previous history of cerebrovascular disease. Cognitive function declined at the same rate in both treatment groups in PROSPER 2002, there was no significant difference between pravastatin treated and placebo groups in performance on letter digit codes, picture word learning test, Stroop and Mini Mental State Examination. There was no evidence that statins were detrimental to cognition.

AUTHORS' CONCLUSIONS

There is good evidence from RCTs that statins given in late life to individuals at risk of vascular disease have no effect in preventing AD or dementia. Biologically it seems feasible that statins could prevent dementia due to their role in cholesterol reduction and initial evidence from observational studies was very promising. Indication bias may have been a factor in these studies however and the evidence from subsequent RCTs has been negative.

Apolipoprotein E and its receptors in Alzheimer's disease: pathways, pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the epsilon4 allele of the apolipoprotein E (APOE) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-beta peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.

Therapeutic potential of statins in Alzheimer's disease

There is increasing evidence linking cholesterol metabolism with the neurofibrillary pathology of Alzheimer's disease (AD). Cholesterol and its transport have been shown to be involved in the regulation of amyloid production and tau hyperphosphorylation in the brain, while also contributing to intracranial vascular disease and cerebral ischemia. Statins inhibit HMG-CoA reductase, the enzyme that catalyzes the rate-limiting step in cholesterol biosynthesis thus inserting itself into the pathogenesis of AD. Numerous studies have examined the role of statins in the prevention of dementia and treatment of established AD. This paper describes the role of cholesterol in the pathogenesis of AD and explores how statins may influence this balance. A review of the epidemiological and clinical trials with statins in dementia and AD is also presented. While evidence from retrospective case control studies suggests a beneficial role of statins in the prevention of AD, a similar benefit has not been established in prospective cohort studies or clinical trials. The ability of statins in protecting against AD is not yet elucidated. In the near future recently completed but not yet reported randomized clinical trials will hopefully clarify the role of statins in the treatment of AD.

Oral pravastatin prolongs survival time of scrapie-infected mice

Statins are potent inhibitors of HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase in the cholesterol-biosynthesis pathway. They are either lipophilic (e.g. simvastatin) or hydrophilic [e.g. pravastatin (PRV)] compounds, considered mainly for long-term treatment of hypercholesterolaemic individuals. Beneficial effects of statins are not related exclusively to their lipid-lowering action; they also possess cholesterol-independent, pleiotropic effects (e.g. anti-inflammatory and antioxidant). Recent studies revealed that simvastatin treatment increased survival significantly in scrapie-infected mice. Although PRV treatment results in measurable drug levels in the mouse brain, the anti-prion effect of this compound has not been investigated. Therefore, we aimed to test the potential therapeutic action of PRV in a murine scrapie model. Our study showed that high-dose and long-term oral PRV treatment prolonged survival times of strain 139A scrapie-infected mice significantly (194 versus 177 days) in the absence of any obvious toxicity, suggesting that protective effects of statins may be independent of absolute solvent or water solubility of the drug.

Sterols in the central nervous system

PURPOSE OF REVIEW

Aberrations in cerebral cholesterol homeostasis can lead to severe neurological diseases and have been linked to Alzheimer's disease. Many proteins involved in peripheral cholesterol metabolism are also present in the brain. Yet, brain cholesterol metabolism is very different from that in the remainder of the body. This review reports on present insights into the regulation of cerebral cholesterol homeostasis, focusing on cholesterol trafficking between astrocytes and neurons.

RECENT FINDINGS

Astrocytes are a major site of cholesterol synthesis. They secrete cholesterol in the form of apolipoprotein E-containing HDL-like particles. After birth, neurons are thought to reduce their cholesterol synthesis and rely predominantly on astrocytes for their cholesterol supply. How exactly neurons regulate their cholesterol supply is largely unknown. A role for the brain-specific cholesterol metabolite, 24(S)-hydroxycholesterol, in this process was recently proposed. Recent findings strengthen the link between brain cholesterol metabolism and factors involved in synaptic plasticity, a process essential for learning and memory functions, as well as regeneration, which are affected in Alzheimer's disease.

SUMMARY

Insight into the regulation of cerebral cholesterol homeostasis will provide possibilities to modulate the key steps involved and may lead to the development of therapies for the prevention as well as treatment of neurodegenerative diseases such as Alzheimer's disease.

Serum lipids are related to Alzheimer's pathology in nursing home residents

BACKGROUND

Studies of associations between serum lipids and Alzheimer's disease (AD) or other dementias in the elderly show conflicting results, perhaps due to misclassification of the various dementias.

METHODS

For 358 nursing home residents, serum lipids were studied at admission and diagnoses established at autopsy. We used defined neuropathological criteria to distinguish the presence of AD and to avoid errors of clinical dementia assessment.

RESULTS

Residents with any AD pathology, as compared to those without AD pathology, had higher mean serum total cholesterol (TC; 200.4 vs. 185.9 mg/dl; p = 0.02) and higher mean low-density lipoprotein cholesterol (LDL; 124.5 vs. 111.5 mg/dl; p = 0.03). Further, mean TC, LDL and high-density lipoprotein cholesterol levels all increased progressively with increasing pathological certainty of AD (p for trend = 0.001, 0.02 and 0.02).

CONCLUSIONS

TC and LDL were significantly related to pathologically defined AD. If serum lipids have a role in the pathogenesis of AD, interventions may modify the course of disease.

Apolipoprotein E and Alzheimer's disease: molecular mechanisms and therapeutic opportunities

Multiple genetic and environmental factors are likely to contribute to the development of Alzheimer's disease (AD). The most important known risk factor for AD is presence of the E4 isoform of apolipoprotein E (apoE). Epidemiological studies demonstrated that apoE4 carriers have a higher risk and develop the disease and an early onset. Moreover, apoE4 is the only molecule that has been associated with all the biochemical disturbances characteristic of the disease: amyloid-beta (Abeta) deposition, tangle formation, oxidative stress, lipid homeostasis deregulation, synaptic plasticity loss and cholinergic dysfunction. This large body of evidence suggest that apoE is a key player in the pathogenesis of AD. This short review examines the current facts and hypotheses of the association between apoE4 and AD, as well as the therapeutic possibilities that apoE might offer for the treatment of this disease.

Substrate specificity of gamma-secretase and other intramembrane proteases

Gamma-Secretase is a promiscuous protease that cleaves bitopic membrane proteins within the lipid bilayer. Elucidating both the mechanistic basis of gamma-secretase proteolysis and the precise factors regulating substrate identification is important because modulation of this biochemical degradative process can have important consequences in a physiological and pathophysiological context. Here, we briefly review such information for all major classes of intramembranously cleaving proteases (I-CLiPs), with an emphasis on gamma-secretase, an I-CLiP closely linked to the etiology of Alzheimer's disease. A large body of emerging data allows us to survey the substrates of gamma-secretase to ascertain the conformational features that predispose a peptide to cleavage by this enigmatic protease. Because substrate specificity in vivo is closely linked to the relative subcellular compartmentalization of gamma-secretase and its substrates, we also survey the voluminous body of literature concerning the traffic of gamma-secretase and its most prominent substrate, the amyloid precursor protein.

Plasma membrane cholesterol content affects nitric oxide diffusion dynamics and signaling

Nitric oxide (NO) signaling is inextricably linked to both its physical and chemical properties. Due to its preferentially hydrophobic solubility, NO molecules tend to partition from the aqueous milieu into biological membranes. We hypothesized that plasma membrane ordering provided by cholesterol further couples the physics of NO diffusion with cellular signaling. Fluorescence lifetime quenching studies with pyrene liposome preparations showed that the presence of cholesterol decreased apparent diffusion coefficients of NO approximately 20-40%, depending on the phospholipid composition. Electrochemical measurements indicated that the diffusion rate of NO across artificial bilayer membranes were inversely related to cholesterol content. Sterol transport-defective Niemann-Pick type C1 (NPC1) fibroblasts exhibited increased plasma membrane cholesterol content but decreased activation of both intracellular soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein (VASP) phosphorylation at Ser(239) induced by exogenous NO exposure relative to their normal human fibroblast (NHF) counterparts. Augmentation of plasma membrane cholesterol in NHF diminished production of both cGMP and VASP phosphorylation elicited by NO to NPC1-comparable levels. Conversely, decreasing membrane cholesterol in NPC1 resulted in the augmentation in both cGMP and VASP phosphorylation to a level similar to those observed in NHF. Increasing plasma membrane cholesterol contents in NHF, platelets, erythrocytes and tumor cells also resulted in an increased level of extracellular diaminofluorescein nitrosation following NO exposure. These findings suggest that the impact of cholesterol on membrane fluidity and microdomain structure contributes to the spatial heterogeneity of NO diffusion and signaling.

Oxidative stress related changes in the brain of hypercholesterolemic rabbits

In recent years, there has been increasing evidence that cholesterol plays a role in the pathology of Alzheimer disease. Since hypercholesterolemia was reported to increase the levels of reactive oxygen species and Alzheimer disease has clearly involved an oxidative component, it is possible that hypercholesterolemia is via increased oxidant production facilitating the disease development of the neurodegenerative disease. Therefore, we tested in an established model of enhanced cholesterol feed in rabbits the effects of serum cholesterol increase on oxidative stress parameters as well in serum as in the brain. In addition to that we tested the effects of vitamin E on the cholesterol-induced oxidative stress. Since Alzheimer disease is largely connected with increased protein oxidation whereas cholesterol is rather connected with lipid peroxidation processes, we tested both protein carbonyl levels and the formation of malondialdehyde, a marker of lipid peroxidation. We could clearly demonstrate an increase in serum malondialdehyde due to high cholesterol feeding, which is accompanied by an increase in protein oxidation parameters in the brain, especially in the hippocampus. Therefore, we suggest that specific neuropathological changes occur during the feeding of hypercholesterolemic diet.