Statin effects on cholesterol micro-domains in brain plasma membranes

Simvastatin enhances learning and memory independent of amyloid load in mice

OBJECTIVE

Normal aging is often associated with a decline in learning and memory functions. This decline is manifested to a much greater extent in Alzheimer's disease. Recent studies have indicated statins, a class of cholesterol-lowering drugs, as a potential therapy for Alzheimer's disease. Our objective was to determine whether administering a statin drug (simvastatin) would protect against the development of behavioral deficits in an established mouse model of Alzheimer's disease.

METHODS

Tg2576 mice and their nontransgenic littermates were treated with simvastatin and assessed by behavioral tests and biochemical analyses.

RESULTS

Simvastatin treatment not only reversed learning and memory deficits in the Tg2576 mice, but also enhanced learning and memory in the nontransgenic mice. Moreover, levels of amyloid beta protein in the brains of treated mice did not differ from those of untreated mice. Simvastatin treatment was associated with increased expression levels of protein kinase B (Akt) and endothelial nitric oxide synthase in the mouse brain.

INTERPRETATION

Our findings demonstrate that the effects of simvastatin on learning and memory are independent of amyloid beta protein levels. The mechanisms by which simvastatin exerts its beneficial effects may be related to modulation of signaling pathways in memory formation.

Alzheimer disease--no target for statin treatment. A mini review

Nosologically, Alzheimer disease (AD) is not a single disorder. A minority of around 400 families worldwide can be grouped as hereditary in origin, whereas the majority of all Alzheimer cases (approx. 25 million worldwide) are sporadic in origin. In the pathophysiology of the latter type, a number of susceptibility genes contribute to the disease among which are allelic abnormalities of the apolipoprotein E4 gene pointing to a link between disturbed cholesterol metabolism and sporadic AD. Cholesterol is a main component of membrane composition enriched in microdomains and is functionally linked to the proteolytic processing of amyloid precursor protein (APP). In sporadic AD, a marked diminution of both membrane phospholipids and cholesterol has been found. Evidence has been provided that high plasma cholesterol may protect from AD. In contrast to these well documented abnormalities observed in AD patients, it was assumed that an elevated cholesterol concentration might favour the generation of beta-amyloid and, thus, AD. However, a series of in vitro-and in vivo-studies did not provide evidence for the assumption that an enhanced cholesterol concentration increased betaA4-production. A harsh reduction of membrane cholesterol only caused a "beneficial" effect of APP metabolism. However, this experimentally induced condition may not be compatible to sporadic AD. The application of statins in sporadic AD did not yield results to assume that this therapeutic strategy may prevent or treat successfully sporadic AD.

Statins inhibit the dimerization of beta-secretase via both isoprenoid- and cholesterol-mediated mechanisms

We have previously reported that protein lipidation in the form of palmitoylation and farnesylation is critical for the production of Abeta (amyloid beta-peptide), the dimerization of beta-secretase and its trafficking into cholesterol-rich microdomains. As statins influence these lipid modifications in addition to their effects on cholesterol biosynthesis, we have investigated the effects of lovastatin and SIMVA (simvastatin) at a range of concentrations chosen to distinguish different cellular effects on Abeta production and beta-secretase structure and its localization in bHEK cells [HEK-293 cells (human embryonic kidney cells) transfected with the Asp-2 gene plus a polyhistidine coding tag] cells. We have compared the changes brought about by statins with those brought about by the palmitoylation inhibitor cerulenin and the farnesyltransferase inhibitor CVFM (Cys-Val-Phe-Met). The statin-mediated reduction in Abeta production correlated with an inhibition of beta-secretase dimerization into its more active form at all concentrations of statin investigated. These effects were reversed by the administration of mevalonate, showing that these effects were mediated via 3-hydroxy-3-methylglutaryl-CoA-dependent pathways. At low (1 microM) statin concentrations, reduction in Abeta production and inhibition of beta-secretase dimerization were mediated by inhibition of isoprenoid synthesis. At high (>10 microM) concentrations of statins, inhibition of beta-secretase palmitoylation occurred, which we demonstrated to be regulated by intracellular cholesterol levels. There was also a concomitant concentration-dependent change in beta-secretase subcellular trafficking. Significantly, Abeta release from cells was markedly higher at 50 microM SIMVA than at 1 microM, whereas these concentrations resulted in similar reductions in total Abeta production, suggesting that low-dose statins may be more beneficial than high doses for the therapeutic treatment of Alzheimer's disease.

Proteomic identification of lower apolipoprotein A-I in Alzheimer's disease

Many researches have been trying to find the potential biomarkers for Alzheimer's disease (AD). We hereby used the proteomics method to search for protein expression differences in the serum between AD patients and controls. We enrolled 59 AD patients and 74 age- and sex-matched controls in this study. Ten AD patients and 10 controls were selected for proteomic analysis. Apolipoprotein A-I (ApoA-I) was found to have a lower expression in the AD group by a proteomics two-dimensional gel electrophoresis study. We further measured the serum ApoA-I level which was significantly lower in the AD patients (112.29 +/- 21.33 mg/dl) in comparison to the controls (144.53 +/- 19.91 mg/dl; p < 0.0002). Lower serum ApoA-I levels might be a potential biomarker for AD.

Effect of pravastatin, simvastatin and atorvastatin on the phagocytic activity of mouse peritoneal macrophages

Since cholesterol and lipid content may affect cell membrane fluidity, we assumed that treatment of mice with lipid lowering statins would enhance the engulfing capacity of their macrophages. Four groups of animals were examined. Group I-treated with pravastatin, group II--with simvastatin--both drugs in a dosage of 40 mg/kg daily, 5 days/week for a total of 3 weeks. Mice in group III received atorvastatin 5 mg/kg for the same time period. Group IV--untreated animals serving as controls. The phagocytic capacity of the peritoneal macrophages was evaluated by their ability to engulf latex particles. In addition, the mitogen response of the peripheral blood mononuclear cells (PBMC) and splenocytes to Con A and PHA was examined. Compared to the controls, the percentage of phagocyting cells in pravastatin-treated mice was enhanced by 18%, with simvastatin--by 24% and in atorvastatin-treated animals by 8%. The three statins increased the phagocytic index by 79.5%, 88.8% and 62%, respectively. The mitogen response of splenocytes from mice treated with the three statins to Con A increased by 68%, 48% and by 40%, respectively. Compared with the controls the response to PHA was higher in animals treated with pravastatin (84%), simvastatin (73%) and atorvastatin (57%). The response of PBMC from statin-treated animals to both mitogens did not differ from that of the controls. The results suggest that statins, at least those hereby investigated, may exert a beneficial effect on the immune function of the macrophages.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

Cholesterol in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia and affects up to 15 million people worldwide. Although no single cause of AD has been identified, recent research has suggested that several pathogenetic factors influence risk and expression. A growing amount of evidence underscores a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques. Excess brain cholesterol has been associated with increased formation and deposition of amyloid-beta peptide from amyloid precursor protein. Cholesterol-lowering statins have become a focus of research in AD. Genetic polymorphisms associated with pivotal points in cholesterol metabolism in brain tissues may contribute to the risk and pathogenesis of AD. In this review, we summarise current knowledge of the role of cholesterol metabolism in the pathogenesis of AD and examine the potential of statins in the prevention and treatment of AD.

The role of seladin-1/DHCR24 in cholesterol biosynthesis, APP processing and Abeta generation in vivo

The cholesterol-synthesizing enzyme seladin-1, encoded by the Dhcr24 gene, is a flavin adenine dinucleotide-dependent oxidoreductase and regulates responses to oncogenic and oxidative stimuli. It has a role in neuroprotection and is downregulated in affected neurons in Alzheimer's disease (AD). Here we show that seladin-1-deficient mouse brains had reduced levels of cholesterol and disorganized cholesterol-rich detergent-resistant membrane domains (DRMs). This was associated with inefficient plasminogen binding and plasmin activation, the displacement of beta-secretase (BACE) from DRMs to APP-containing membrane fractions, increased beta-cleavage of APP and high levels of Abeta peptides. In contrast, overexpression of seladin-1 increased both cholesterol and the recruitment of DRM components into DRM fractions, induced plasmin activation and reduced both BACE processing of APP and Abeta formation. These results establish a role of seladin-1 in the formation of DRMs and suggest that seladin-1-dependent cholesterol synthesis is involved in lowering Abeta levels. Pharmacological enhancement of seladin-1 activity may be a novel Abeta-lowering approach for the treatment of AD.

Fluvastatin Alters Psychomotor Performance and Daily Activity but not the Spatial Memory in Rats

Statins, inhibitors of cholesterol synthesis for treating dyslipidemia and preventing cardiovascular complications, have been shown to alter central nervous system functions. Our aim was to investigate the effects of the fluvastatin, a member of statin family, on psychomotor performance, daily activity and spatial memory. Sprague-Dawley rats were treated with fluvastatin (n = 8) or placebo as a control (n = 11) regardless of sex. Fluvastatin (7.5 mg/kg) was administered orally once a day for four weeks, while the control group was administered only placebo. Psychomotor performance was measured by rotarod tests. No significant difference was observed in the fluvastatin group over the course of weeks, but the control group preferred to stay on the device shorter times (p < 0.05). For the first three weeks of the drug administration there was a statistical difference between the groups, however no difference was found after the 4th week. There was no difference in the Barnes maze spatial memory test between the groups and also within the groups over the course of time. Daily activity tests revealed that stereotypical and vertical movements of the fluvastatin group were significantly less than the control group in all four weeks. Ambulatory movements and the distances taken by the fluvastatin group were decreased significantly over the course of time (p < 0.005 and p < 0.001, respectively), but the control group did not reveal any significant change. Our results suggest that fluvastatin altered psychomotor performance and daily activity in rats, but it did not affect the spatial memory. These behavioral changes might be associated with alterations in the composition of the brain lipids caused by fluvastatin.

Simvastatin regulates oligodendroglial process dynamics and survival

Simvastatin, a lipophilic statin that crosses the blood-brain barrier, is being evaluated as a potential therapy for multiple sclerosis (MS) due to its anti-inflammatory properties. We assessed the effects of simvastatin on cultures of rat newborn and human fetal oligodendrocyte progenitor cells (OPCs) and human adult mature oligodendrocytes (OLGs) with respect to cellular events pertaining to myelin maintenance and repair. Short-term simvastatin treatment of OPCs (1 day) induced robust process extension, enhanced differentiation to a mature phenotype, and decreased spontaneous migration. These effects were reversed by isoprenoid products and mimicked with an inhibitor of Rho kinase (ROCK), the downstream effector of the isoprenylated protein RhoA GTPase. Prolonged treatment (2 days) caused process retraction that was rescued by cholesterol, and increased cell death (4 days) partially rescued by either cholesterol or isoprenoid co-treatment. In comparison, simvastatin treatment of human mature OLGs required a longer initial time course (2 days) to induce significant process outgrowth, mimicked by inhibiting ROCK. Prolonged treatment of mature OLGs was associated with process retraction (6 days) and increased cell death (8 days). Human-derived OPCs and mature OLGs demonstrated an increased sensitivity to simvastatin relative to the rodent cells, responding to nanomolar versus micromolar concentrations. Our findings indicate the importance of considering the short- and long-term effects of systemic immunomodulatory therapies on neural cells affected by the MS disease process. (c) 2006 Wiley-Liss, Inc.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Post-translational processing of beta-secretase in Alzheimer's disease

Beta-amyloid is released into the brains of Alzheimer's patients, where it aggregates and causes damage to neurons. It is cleaved proteolytically from a large transmembrane glycoprotein amyloid precursor protein by a membrane-bound protease, known as beta-secretase identified previously as the acid protease, Asp-2. We have shown previously that beta-secretase is up-regulated by increased intracellular cholesterol, and down-regulated by cholesterol biosynthesis inhibition. Here we show using mass spectrometry that discrete changes in the glycosylation and palmitoylation of beta-secretase occur when cells expressing it are treated with statins.

[Direct neuronal effects of statins]

Statins, i.e. HMG-CoA reductase inhibitors, reduce the risk of stroke and may have therapeutic potential for other neurologic diseases, including multiple sclerosis and Alzheimer's disease. In addition to lowering cholesterol levels, statins exert a number of cholesterol-independent (pleiotropic) effects. While endothelial, anti-thrombotic, anti-inflammatory, and immunomodulatory, i.e. peripheral, effects of statins are well known, little is known about the direct effects on neurons. This may be of clinical relevance because some statins are able to cross the blood-brain barrier. Recent experimental studies demonstrate that statins reduce the activity of neuronal glutamate receptors and protect neurons from excitotoxic insults. At higher doses, however, statins may also inhibit neurite sprouting and even induce neuronal apoptosis.

Statin treatment and a disease-specific pattern of β-amyloid peptides in Alzheimer’s disease

According to the amyloid cascade hypothesis, sporadic Alzheimer's disease (AD) is caused by the production and aggregation of beta-amyloid (Abeta), and the production of Abeta has recently been linked to the metabolism of cholesterol. We have previously published clinical studies where the effect of statin treatment on Abeta production has been investigated. No effect on Abeta was found, which is in disagreement with cell and animal studies. In the present study we investigated the effect of statin treatment on a disease-specific pattern consisting of a C-terminally-truncated quintet of Abeta peptides. Nineteen patients with AD were treated with simvastatin for 12 months and the quintet of Abeta peptides were analysed in cerebrospinal fluid before and after treatment. Also included was a group of 15 untreated patients with AD. We found that the Abeta peptide pattern at baseline was in agreement with earlier findings; however, we did not find any change in the Abeta peptide pattern after statin treatment. We suggest that clinical studies with extended treatment periods are performed where higher dosages of statins are used. We also believe that the pleiotropic effects of statins should be investigated further in order to elucidate the connection between Alzheimer's disease and statin treatment.

Trafficking of normal stem cells and metastasis of cancer stem cells involve similar mechanisms: pivotal role of the SDF-1-CXCR4 axis

The alpha-chemokine stromal-derived factor (SDF)-1 and the G-protein-coupled seven-span transmembrane receptor CXCR4 axis regulates the trafficking of various cell types. In this review, we present the concept that the SDF-1-CXCR4 axis is a master regulator of trafficking of both normal and cancer stem cells. Supporting this is growing evidence that SDF-1 plays a pivotal role in the regulation of trafficking of normal hematopoietic stem cells (HSCs) and their homing/retention in bone marrow. Moreover, functional CXCR4 is also expressed on nonhematopoietic tissue-committed stem/progenitor cells (TCSCs); hence, the SDF-1-CXCR4 axis emerges as a pivotal regulator of trafficking of various types of stem cells in the body. Furthermore, because most if not all malignancies originate in the stem/progenitor cell compartment, cancer stem cells also express CXCR4 on their surface and, as a result, the SDF-1-CXCR4 axis is also involved in directing their trafficking/metastasis to organs that highly express SDF-1 (e.g., lymph nodes, lungs, liver, and bones). Hence, we postulate that the metastasis of cancer stem cells and trafficking of normal stem cells involve similar mechanisms, and we discuss here the common molecular mechanisms involved in these processes. Finally, the responsiveness of CXCR4+ normal and malignant stem cells to an SDF-1 gradient may be regulated positively/primed by several small molecules related to inflammation which enhance incorporation of CXCR4 into membrane lipid rafts, or may be inhibited/blocked by small CXCR4 antagonist peptides. Consequently, strategies aimed at modulating the SDF-1-CXCR4 axis could have important clinical applications both in regenerative medicine to deliver normal stem cells to the tissues/organs and in clinical hematology/oncology to inhibit metastasis of cancer stem cells.

Mechanistic and epidemiologic considerations in the evaluation of adverse birth outcomes following gestational exposure to statins

The cholesterol-lowering "statin" drugs are contraindicated in pregnancy, but few data exist on their safety in human gestation. We reviewed case reports for patterns suggesting drug-related effects on prenatal development and considered a variety of mechanisms by which such effects, if confirmed, might occur. This uncontrolled case series included all FDA reports of statin exposures during gestation, as well as others from the literature and from manufacturers. Exposures and outcomes were reviewed and were tabulated by individual drug. Age-specific rates of exposure to each drug among women of child-bearing age were estimated. Of 214 ascertained pregnancy exposures, 70 evaluable reports remained after excluding uninformative cases. Among 31 adverse outcomes were 22 cases with structural defects, 4 cases of intrauterine growth restriction, and 5 cases of fetal demise. There were two principal categories of recurrent structural defects: cerivastatin and lovastatin were associated with four reports of severe midline CNS defects; simvastatin, lovastatin, and atorvastatin were all associated with reports of limb deficiencies, including two similar complex lower limb defects reported following simvastatin exposure. There were also two cases of VACTERL association among the limb deficiency cases. All adverse outcomes were reported following exposure to cerivastatin, simvastatin, lovastatin, or atorvastatin, which are lipophilic and equilibrate between maternal and embryonic compartments. None were reported following exposure to pravastatin, which is minimally present in the embryo. Statins reaching the embryo may down-regulate biosynthesis of cholesterol as well as many important metabolic intermediates, and may have secondary effects on sterol-dependent morphogens such as Sonic Hedgehog. The reported cases display patterns consistent with dysfunction of cholesterol biosynthesis and Sonic Hedgehog activity. Controlled studies are needed to investigate the teratogenicity of individual drugs in this class.

Cholesterol-lowering therapy evokes time-limited changes in serotonergic transmission

A number of studies have reported an increased risk for violent deaths and depression in subjects with reduced serum cholesterol concentrations. Links with hypothesized impairment of serotonin neurotransmission have not been satisfactorily tested. In this investigation, the serum and membrane cholesterol, microviscosity of erythrocyte membranes, platelet serotonin uptake, and clinical parameters were determined during pharmacotherapy of 17 hypercholesterolemic patients. A significant decrease in serum cholesterol and a nonsignificant decrease in membrane cholesterol concentration were found after 2 months of simvastatin therapy. Serotonin transporter (SERT) activity was significantly increased following 1 month of simvastatin; the tendency to decrease the initial increase in SERT activity was evident following 2 months of therapy. Both membrane cholesterol and SERT activity returned to pre-treatment levels after more than 1 year of therapy. Microviscosity of plasma membranes, impulsivity, empathy, adventure, sensation seeking, and depressed mood were not markedly changed. These data indicate that long-term therapy has different effects on serotonin transmission from short-term (1- to 2-month) therapy. A significant increase in SERT activity was detected only during the first month of simvastatin therapy. This finding suggests that within this period some patients could be vulnerable to depression, violence, or suicide.

Protection by cholesterol-extracting cyclodextrins: a role for N-methyl-d-aspartate receptor redistribution

Cyclodextrins (CDs) are cyclic oligosaccharides composed of a lipophilic central cavity and a hydrophilic outer surface. Some CDs are capable of extracting cholesterol from cell membranes and can affect function of receptors and proteins localized in cholesterol-rich membrane domains. In this report, we demonstrate the neuroprotective activity of some CD derivatives against oxygen-glucose deprivation (OGD), N-methyl-D-aspartic acid (NMDA) and glutamate in cortical neuronal cultures. Although all CDs complexed with NMDA or glutamate, only beta-, methylated beta- and sulfated beta-CDs displayed neuroprotective activity and lowered cellular cholesterol. Only CDs that lowered cholesterol levels redistributed the NMDA receptor NR2B subunit, PSD-95 (postsynaptic density protein 95 kDa) and neuronal nitric oxide synthase (nNOS) from Triton X-100 insoluble membrane domains to soluble fractions. Cholesterol repletion counteracted the ability of methylated beta-CD to protect against NMDA toxicity, and reversed NR2B, PSD-95 and nNOS localization to Triton X-100 insoluble membrane fraction. Surprisingly, neuroprotective CDs had minimal effect on NMDA receptor-mediated increases in intracellular Ca(2+) concentration ([Ca(2+)](i)), but did suppress OGD-induced increases in [Ca(2+)](i). beta-CD, but not Mbeta-CD, also caused a slight block of NMDA-induced currents, suggesting a minor contribution to neuroprotection by direct action on NMDA receptors. Taken together, data suggest that cholesterol extraction from detergent-resistant microdomains affects NMDA receptor subunit distribution and signal propagation, resulting in neuroprotection of cortical neuronal cultures against ischemic and excitotoxic insults. Since cholesterol-rich membrane domains exist in neuronal postsynaptic densities, these results imply that synaptic NMDA receptor subpopulations underlie excitotoxicity, which can be targeted by CDs without affecting overall neuronal Ca(2+) levels.

Simvastatin inhibits T‐cell activation by selectively impairing the function of Ras superfamily GTPases

Statins are widely used hypocholesterolemic drugs that inhibit 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme of the mevalonate pathway whose biosynthetic end product is cholesterol. In addition to lowering circulating cholesterol, statins perturb the composition of cell membranes, resulting in disruption of lipid rafts, which function as signaling platforms in immunoreceptor signaling. Furthermore, by inhibiting protein prenylation, a process also dependent on mevalonate, statins block membrane targeting and hence activity of small GTPases, which control multiple pathways triggered by these receptors. T-cell activation is crucially dependent on Ras, Rho and Rab GTPases. Furthermore TCR signaling is orchestrated at lipid rafts, identifying T-cells as potential cellular targets of statins. Here we report that simvastatin suppresses T-cell activation and proliferation as the result of its capacity to inhibit HMG-CoA reductase. T-cell treatment with simvastatin does not affect intracellular cholesterol levels or raft integrity nor, accordingly, the initial tyrosine phosphorylation-dependent cascade. Conversely, inhibition of protein prenylation by simvastatin results in a dramatic impairment in the pathways regulated by small GTPases, including the Ras/MAP kinase pathway, the Rac/stress kinase pathway, and the Rab-dependent pathway of receptor endocytosis. The results identify Ras superfamily GTPases as strategic molecular targets in T-cell immunosuppression by statins.

Cholesterol and Alzheimer's disease: statins, cholesterol depletion in APP processing and Abeta generation

Molecular and more specifically subcellular analyses of the neurodegenerative mechanisms involved in Alzheimer's disease (AD) had been considered most of the time an interplay of proteins and genes. However, some of the observations could not be explained this way. Recently, a number of research groups found the missing link ... lipids! Among the variety of lipids that had been investigated, most investigations had been focused on cholesterol and some derivatives. A recent statistic found that for every primary research article on AD and cholesterol/statins, approximately two reviews were published. This clearly reflects as much the interest in this topic, as it gives evidence that this field is still in its juvenile phase and most aspects have yet to be covered or clarified. To date there is evidently no final answer to whether this approach will eventually provide a therapeutic solution to treat or prevent AD. At the end of the day such answers can only be obtained from clinical studies and to date only two studies with a suitable design have published their results, one of them with preliminary results only. This review focuses on what we know about the cellular mechanisms involved in the AD-lipid connection and what kinds of problematic issues; theoretical and practical, are at hand.

Inhibition of toll-like receptor and cytokine signaling--a unifying theme in ischemic tolerance

Cerebral ischemia triggers acute inflammation, which exacerbates primary brain damage. Activation of the innate immune system is an important component of this inflammatory response. Inflammation occurs through the action of proinflammatory cytokines, such as TNF, IL-1 beta and IL-6, that alter blood flow and increase vascular permeability, thus leading to secondary ischemia and accumulation of immune cells in the brain. Production of these cytokines is initiated by signaling through Toll-like receptors (TLRs) that recognize host-derived molecules released from injured tissues and cells. Recently, great strides have been made in understanding the regulation of the innate immune system, particularly the signaling mechanisms of TLRs. Negative feedback inhibitors of TLRs and inflammatory cytokines have now been identified and characterized. It is also evident that lipid rafts exist in membranes and play a role in receptor-mediated inflammatory signaling events. In the present review, using this newly available large body of knowledge, we take a fresh look at studies of ischemic tolerance. Based on this analysis, we recognize a striking similarity between ischemic tolerance and endotoxin tolerance, an immune suppressive state characterized by hyporesponsiveness to lipopolysaccharide (LPS). In view of this analogy, and considering recent discoveries related to molecular mechanisms of endotoxin tolerance, we postulate that inhibition of TLR and proinflammatory cytokine signaling contributes critically to ischemic tolerance in the brain and other organs. Ischemic tolerance is a protective mechanism induced by a variety of preconditioning stimuli. Tolerance can be established with two temporal profiles: (i) a rapid form in which the trigger induces tolerance to ischemia within minutes and (ii) a delayed form in which development of protection takes several hours or days and requires de-novo protein synthesis. The rapid form of tolerance is achieved by direct interference with membrane fluidity, causing disruption of lipid rafts leading to inhibition of TLR/cytokine signaling pathways. In the delayed form of tolerance, the preconditioning stimulus first triggers the TLR/cytokine inflammatory pathways, leading not only to inflammation but also to simultaneous upregulation of feedback inhibitors of inflammation. These inhibitors, which include signaling inhibitors, decoy receptors, and anti-inflammatory cytokines, reduce the inflammatory response to a subsequent episode of ischemia. This novel interpretation of the molecular mechanism of ischemic tolerance highlights new avenues for future investigation into the prevention and treatment of stroke and related diseases.

Chronic administration of statins alters multiple gene expression patterns in mouse cerebral cortex

Statins have been reported to lower the risk of developing Alzheimer's disease; however, the mechanism of this potentially important neuroprotective action is not understood. Lowering cholesterol levels does not appear to be the primary mechanism. Statins have pleiotropic effects in addition to lowering cholesterol, and statins may act on several different pathways involving distinct gene expression patterns that would be difficult to determine by focusing on a few genes or their products in a single study. In addition, gene expression patterns may be specific to a particular statin. To understand the molecular targets of statins in brain, DNA microarrays were used to identify gene expression patterns in the cerebral cortex of mice chronically treated with lovastatin, pravastatin, and simvastatin. Furthermore, brain statin levels were determined using liquid chromatography/tandem mass spectrometry. These studies revealed 15 genes involved in cell growth and signaling and trafficking that were similarly changed by all three statins. Overall, simvastatin had the greatest influence on expression as demonstrated by its ability to modify the expression of 23 genes in addition to those changed by all three drugs. Of particular interest was the expression of genes associated with apoptotic pathways that were altered by simvastatin. Reverse transcription-polymerase chain reaction experiments confirmed the microarray findings. All three drugs were detected in the cerebral cortex, and acute experiments revealed that statins are relatively rapidly removed from the brain. These results provide new insight into possible mechanisms for the potential efficacy of statins in reducing the risk of Alzheimer's disease and lay the foundation for future studies.

Cholesterol modulates ligand binding and G-protein coupling to serotonin(1A) receptors from bovine hippocampus

The serotonin(1A) (5-HT(1A)) receptor is an important member of the superfamily of seven-transmembrane domain G-protein-coupled receptors. We have examined the modulatory role of cholesterol on the ligand binding activity and G-protein coupling of the bovine hippocampal 5-HT(1A) receptor by depleting cholesterol from native membranes using methyl-beta-cyclodextrin (MbetaCD). Removal of cholesterol from bovine hippocampal membranes using varying concentrations of MbetaCD results in a concentration-dependent reduction in specific binding of the agonist 8-OH-DPAT to 5-HT(1A) receptors. This is accompanied by alterations in binding affinity and sites obtained from analysis of binding data. Importantly, cholesterol depletion affected G-protein-coupling of the receptor as monitored by the GTP-gamma-S assay. The concomitant changes in membrane order were reported by changes in fluorescence polarization of membrane probes such as DPH and TMA-DPH, which are incorporated at different locations (depths) in the membrane. Replenishment of membranes with cholesterol led to recovery of ligand binding activity as well as membrane order to a considerable extent. Our results provide evidence, for the first time, that cholesterol is necessary for ligand binding and G-protein coupling of this important neurotransmitter receptor. These results could have significant implications in understanding the influence of the membrane lipid environment on the activity and signal transduction of other G-protein-coupled transmembrane receptors.

Can dementia be prevented? Brain aging in a population-based context

As a consequence of global aging of the human population, the occurrence of cognitive impairment and dementia is rapidly becoming a significant burden for medical care and public health systems. By the year 2020, the WHO predicts there will be nearly 29 million demented people in both developed and developing countries. Primary and secondary prevention of dementia through individual and population-level interventions could reduce this imminent risk. Vascular risk factors such as type 2 diabetes, hypertension, dietary fat intake, high cholesterol, and obesity have emerged as important influences on the risk of both vascular and Alzheimer's dementia. Understanding the reasons for differences between populations in genetic vulnerability and environmental exposures may help to identify modifiable risk factors that may lead to effective prevention of vascular and Alzheimer's dementia.

Cell-based screen of HMG-CoA reductase inhibitors and expression regulators using LC–MS

We present an integrated consolidation of previously reported methods for screening hydroxymethylglutaryl-coenzyme A reductase (HMGR) inhibitors in 96-well microtiter plates with rapid workup using established mammalian cell lines and liquid chromatography-mass spectrometry analysis. Inhibitors as well as expression regulators of HMGR (inducers or repressors) can be screened. To validate the method, three competitive inhibitors of HMGR (lovastatin, simvastatin, and atorvastatin), as well as a potent sterol repressor of HMGR synthesis (25-hydroxycholesterol), were assayed on two cell lines: HepG2, a human hepatic derived cell line, and L cells, a subline of NCTC clone 929 mouse fibroblasts. The direct inhibition of HMGR by statins, induction of HMGR synthesis by the same statins following incubation with the cells, and repression of HMGR synthesis by 25-hydroxycholesterol were confirmed.

Gene delivery by dendrimers operates via a cholesterol dependent pathway

Understanding the cellular uptake and intracellular trafficking of dendrimer-DNA complexes is an important prerequisite for improving the transfection efficiency of non-viral vector-mediated gene delivery. Dendrimers are synthetic polymers used for gene transfer. Although these cationic molecules show promise as versatile DNA carriers, very little is known about the mechanism of gene delivery. This paper investigates how the uptake occurs, using an endothelial cell line as model, and evaluates whether the internalization of dendriplexes takes place randomly on the cell surface or at preferential sites such as membrane rafts. Following extraction of plasma membrane cholesterol, the transfection efficiency of the gene delivered by dendrimers was drastically decreased. Replenishment of membrane cholesterol restored the gene expression. The binding and especially internalization of dendriplexes was strongly reduced by cholesterol depletion before transfection. However, cholesterol removal after transfection did not inhibit expression of the delivered gene. Fluorescent dendriplexes co-localize with the ganglioside GM1 present into membrane rafts in both an immunoprecipitation assay and confocal microscopy studies. These data strongly suggest that membrane cholesterol and raft integrity are physiologically relevant for the cellular uptake of dendrimer-DNA complexes. Hence these findings provide evidence that membrane rafts are important for the internalization of non-viral vectors in gene therapy.

Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer's disease

A hypothetical model of Alzheimer's disease (AD) as a uniquely human brain disorder rooted in its exceptional process of myelination is presented. Cortical regions with the most protracted development are most vulnerable to AD pathology, and this protracted development is driven by oligodendrocytes, which continue to differentiate into myelin producing cells late into the fifth decade of life. The unique metabolic demands of producing and maintaining their vast myelin sheaths and synthesizing the brain's cholesterol supply make oligodendrocytes especially susceptible to a variety of insults. Their vulnerability increases with increasing age at differentiation as later-differentiating cells myelinate increasing numbers of axonal segments. These vulnerable late-differentiating cells drive the protracted process of intracortical myelination and by increasing local cholesterol and iron levels, progressively increase the toxicity of the intracortical environment forming the basis for the age risk factor for AD. At older ages, the roughly bilaterally symmetrical continuum of oligodendrocyte vulnerability manifests as a progressive pattern of myelin breakdown that recapitulates the developmental process of myelination in reverse. The ensuing homeostatic responses to myelin breakdown further increase intracortical toxicity and results in the relentless progression and non-random anatomical distribution of AD lesions that eventually cause neuronal dysfunction and degeneration. This process causes a slowly progressive disruption of neural impulse transmission that degrades the temporal synchrony of widely distributed neural networks underlying normal brain function. The resulting network "disconnections" first impact functions that are most dependent on large-scale synchronization including higher cognitive functions and formation of new memories. Multiple genetic and environmental risk factors (e.g. amyloid beta-peptide and free radical toxicity, head trauma, anoxia, cholesterol levels, etc.) can contribute to the cognitive deficits observed in aging and AD through their impact on the life-long trajectory of myelin development and breakdown. This development-to-degeneration model is testable through imaging and post mortem methods and highlights the vital role of myelin in impulse transmission and synchronous brain function. The model offers a framework that explains the anatomical distribution and progressive course of AD pathology, some of the failures of promising therapeutic interventions, and suggests further testable hypotheses as well as novel approaches for intervention efforts.

Membrane cholesterol modulates dihydropyridine receptor function in mice fetal skeletal muscle cells

Caveolae and transverse (T-) tubules are membrane structures enriched in cholesterol and glycosphingolipids. They play an important role in receptor signalling and myogenesis. The T-system is also highly enriched in dihydropyridine receptors (DHPRs), which control excitation-contraction (E-C) coupling. Recent results have shown that a depletion of membrane cholesterol alters caveolae and T-tubules, yet detailed functional studies of DHPR expression are lacking. Here we studied electrophysiological and morphological effects of methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering drug, on freshly isolated fetal skeletal muscle cells. Exposure of fetal myofibres to 1-3 mM MbetaCD for 1 h at 37 degrees C led to a significant reduction in caveolae and T-tubule areas and to a decrease in cell membrane electrical capacitance. In whole-cell voltage-clamp experiments, the L-type Ca(2+) current amplitude was significantly reduced, and its voltage dependence was shifted approximately 15 mV towards more positive potentials. Activation and inactivation kinetics were slower in treated cells than in control cells and stimulation by a saturating concentration of Bay K 8644 was enhanced. In addition, intramembrane charge movement and Ca(2+) transients evoked by a depolarization were reduced without a shift of the midpoint, indicating a weakening of E-C coupling. In contrast, T-type Ca(2+) current was not affected by MbetaCD treatment. Most of the L-type Ca(2+) conductance reduction and E-C coupling weakening could be explained by a decrease of the number of DHPRs due to the disruption of caveolae and T-tubules. However, the effects on L-type channel gating kinetics suggest that membrane cholesterol content modulates DHPR function. Moreover, the significant shift of the voltage dependence of L-type current without any change in the voltage dependence of charge movement and Ca(2+) transients suggests that cholesterol differentially regulates the two functions of the DHPR.

Hypolipidemic Drugs Can Change the Composition of Rat Brain Lipids

Hypolipidemic drugs are potent serum cholesterol lowering agents used for prevention of coronary heart disease. In addition to their cholesterol lowering effect, these drugs exhibit both pleiotropic beneficial and various neurological side effects. Therefore, we analysed effect of the hypolipidemic drugs, fenofibrate and statins, on membrane lipid composition in the rat brain tissue. Male Wistar rats were given 0.1 mg of fenofibrate, lovastatin, pravastatin, fluvastatin or placebo (control) once daily for six weeks. In rats treated with lovastatin or pravastatin, decreased cholesterol and increased ceramide monohexoside contents in the brain tissue were observed in comparison with control. Treatment with fluvastatin or lovastatin resulted in increased sphingomyelin and decreased diphosphatidylglycerol contents. The most important changes in the fatty acid profile were observed in ceramide monohexosides; treatment with fluvastatin decreased the content of saturated and increased the content of polyunsaturated fatty acids. Fenofibrate treatment led to decreased content of saturated fatty acids in phosphatidylethanolamines. In conclusion, statin treatment resulted in the decreased content of cholesterol and diphosphatidylglycerol associated with the increased content of sphingolipids in the rat brain tissue. As cholesterol and sphingolipids are important components of brain membranes, the observed alterations in the composition brain lipids might be involved in genesis of neurological and mental symptoms following statin therapy.

Membrane Lipid Homeostasis

The lipid matrix of biological membranes is composed of a complex mixture of polar lipids. It has been estimated that more than 600 distinct molecular species of lipid are constituents of biological membranes. This rather remarkable feature raises the questions of why such complexity is required when barrier properties and many protein functions can be reconstituted with relatively simple lipid systems. Secondly, the molecular species composition of morphologically distinct membranes appears to be preserved within fairly narrow limits. The biochemical mechanism(s) responsible for this homeostasis are not fully understood. This review examines the origin of membrane lipid complexity, the methods that are currently employed to measure and detect lipid molecular species and the biochemical reactions associated with the turnover of membrane lipids in resting and stimulated cells.