Differential effects of lovastatin treatment on brain cholesterol levels in normal and apoE-deficient mice

Statins Use in Alzheimer Disease: Bane or Boon from Frantic Search and Narrative Review

Alzheimer’s disease (AD) was used to describe pre-senile dementia to differentiate it from senile dementia, which develops in the adult age group of more than 65 years. AD is characterized by the deposition of amyloid beta (Aβ) plaque and tau-neurofibrillary tangles (TNTs) in the brain. The neuropathological changes in AD are related to the deposition of amyloid plaques, neurofibrillary tangles, and progression of neuroinflammation, neuronal mitochondrial dysfunction, autophagy dysfunction, and cholinergic synaptic dysfunction. Statins are one of the main cornerstone drugs for the management of cardiovascular disorders regardless of dyslipidemia status. Increasing the use of statins, mainly in the elderly groups for primary and secondary prevention of cardiovascular diseases, may affect their cognitive functions. Extensive and prolonged use of statins may affect cognitive functions in healthy subjects and dementia patients. Statins-induced cognitive impairments in both patients and health providers had been reported according to the post-marketing survey. This survey depends mainly on sporadic cases, and no cognitive measures were used. Evidence from prospective and observational studies gives no robust conclusion regarding the beneficial or detrimental effects of statins on cognitive functions in AD patients. Therefore, this study is a narrative review aimed with evidences to the beneficial, detrimental, and neutral effects of statins on AD.

Cholesterol in myasthenia gravis

The cholinergic neuromuscular junction is the paradigm peripheral synapse between a motor neuron nerve ending and a skeletal muscle fiber. In vertebrates, acetylcholine is released from the presynaptic site and binds to the nicotinic acetylcholine receptor at the postsynaptic membrane. A variety of pathologies among which myasthenia gravis stands out can impact on this rapid and efficient signaling mechanism, including autoimmune diseases affecting the nicotinic receptor or other synaptic proteins. Cholesterol is an essential component of biomembranes and is particularly rich at the postsynaptic membrane, where it interacts with and modulates many properties of the nicotinic receptor. The profound changes inflicted by myasthenia gravis on the postsynaptic membrane necessarily involve cholesterol. This review analyzes some aspects of myasthenia gravis pathophysiology and associated postsynaptic membrane dysfunction, including dysregulation of cholesterol metabolism in the myocyte brought about by antibody-receptor interactions. In addition, given the extensive therapeutic use of statins as the typical cholesterol-lowering drugs, we discuss their effects on skeletal muscle and the possible implications for MG patients under chronic treatment with this type of compound.

Lovastatin Differentially Regulates α7 and α4 Neuronal Nicotinic Acetylcholine Receptor Levels in Rat Hippocampal Neurons

Neuronal α7 and α4β2 are the predominant nicotinic acetylcholine receptor (nAChR) subtypes found in the brain, particularly in the hippocampus. The effects of lovastatin, an inhibitor of cholesterol biosynthesis, on these two nAChRs endogenously expressed in rat hippocampal neuronal cells were evaluated in the 0.01-1 µM range. Chronic (14 days) lovastatin treatment augmented cell-surface levels of α7 and α4 nAChRs, as measured by fluorescence microscopy and radioactive ligand binding assays. This was accompanied in both cases by an increase in total protein receptor levels as determined by Western blots. At low lovastatin concentrations (10-100 nM), the increase in α4 nAChR in neurites was higher than in neuronal cell somata; the opposite occurred at higher (0.5-1 µM) lovastatin concentrations. In contrast, neurite α7 nAChRs raised more than somatic α7 nAChRs at all lovastatin concentrations tested. These results indicate that cholesterol levels homeostatically regulate α7 and α4 nAChR levels in a differential manner through mechanisms that depend on statin concentration and receptor localization. The neuroprotective pleomorphic effects of statins may act by reestablishing the homeostatic equilibrium.

Pleiotropic effects of statins on brain cells

Starting with cholesterol homeostasis, the first part of the review addresses various aspects of cholesterol metabolism in neuronal and glial cells and the mutual crosstalk between the two cell types, particularly the transport of cholesterol from its site of synthesis to its target loci in neuronal cells, discussing the multiple mechanistic aspects and transporter systems involved. Statins are next analyzed from the point of view of their chemical structure and its impingement on their pharmacological properties and permeability through cell membranes and the blood-brain barrier in particular. The following section then discusses the transcriptional effects of statins and the changes they induce in brain cell genes associated with a variety of processes, including cell growth, signaling and trafficking, uptake and synthesis of cholesterol. We review the effects of statins at the cellular level, analyzing their impact on the cholesterol composition of the nerve and glial cell plasmalemma, neurotransmitter receptor mobilization, myelination, dendritic arborization of neurons, synaptic vesicle release, and cell viability. Finally, the role of statins in disease is exemplified by Alzheimer and Parkinson diseases and some forms of epilepsy, both in animal models and in the human form of these pathologies.

Evidence for benefit of statins to modify cognitive decline and risk in Alzheimer's disease

Background

Despite substantial research and development investment in Alzheimer’s disease (AD), effective therapeutics remain elusive. Significant emerging evidence has linked cholesterol, β-amyloid and AD, and several studies have shown a reduced risk for AD and dementia in populations treated with statins. However, while some clinical trials evaluating statins in general AD populations have been conducted, these resulted in no significant therapeutic benefit. By focusing on subgroups of the AD population, it may be possible to detect endotypes responsive to statin therapy.

Methods

Here we investigate the possible protective and therapeutic effect of statins in AD through the analysis of datasets of integrated clinical trials, and prospective observational studies.

Results

Re-analysis of AD patient-level data from failed clinical trials suggested by trend that use of simvastatin may slow the progression of cognitive decline, and to a greater extent in ApoE4 homozygotes. Evaluation of continual long-term use of various statins, in participants from multiple studies at baseline, revealed better cognitive performance in statin users. These findings were supported in an additional, observational cohort where the incidence of AD was significantly lower in statin users, and ApoE4/ApoE4-genotyped AD patients treated with statins showed better cognitive function over the course of 10-year follow-up.

Conclusions

These results indicate that the use of statins may benefit all AD patients with potentially greater therapeutic efficacy in those homozygous for ApoE4.

Tau pathology induces intraneuronal cholesterol accumulation

Epidemiologic and experimental data suggest the involvement of cholesterol metabolism in the development and progression of Alzheimer disease and Niemann-Pick type C disease, but not of frontotemporal dementias. In these 3 neurodegenerative diseases, however, protein tau hyperphosphorylation and aggregation into neurofibrillary tangles are observed. To elucidate the relationship between cholesterol and tau, we compared sterol levels of neurons burdened with neurofibrillary tangles with those of their unaffected neighbors using semiquantitative filipin fluorescence microscopy in mice expressing P301L mutant human tau (a well-described model of FTDP-17) and in P301L transgenic mice lacking apolipoprotein E (the major cholesterol transporter in the brain). Cellular unesterified cholesterol was higher in neurons affected by tau pathology irrespective of apolipoprotein E deficiency. This argues for an impact of tau pathology on cellular cholesterol homeostasis. We suggest that there is a bidirectional mode of action: Disturbances in cellular cholesterol metabolism may promote tau pathology, but tau pathology may also alter neuronal cholesterol homeostasis; once it is established, a vicious cycle may promote neurofibrillary tangle formation.

Role of cholesterol in APP metabolism and its significance in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disorder believed to be initiated by accumulation of amyloid β (Aβ)-related peptides derived from proteolytic processing of amyloid precursor protein (APP). Research over the past two decades provided a mechanistic link between cholesterol and AD pathogenesis. Genetic polymorphisms in genes regulating the pivotal points in cholesterol metabolism have been suggested to enhance the risk of developing AD. Altered neuronal membrane cholesterol level and/or subcellular distribution have been implicated in aberrant formation, aggregation, toxicity, and degradation of Aβ-related peptides. However, the results are somewhat contradictory and we still do not have a complete understanding on how cholesterol can influence AD pathogenesis. In this review, we summarize our current understanding on the role of cholesterol in regulating the production/function of Aβ-related peptides and also examine the therapeutic potential of regulating cholesterol homeostasis in the treatment of AD pathology.

Cholesterol level influences opioid signaling in cell models and analgesia in mice and humans

Cholesterol regulates the signaling of µ-opioid receptor in cell models, but it has not been demonstrated in mice or humans. Whether cholesterol regulates the signaling by mechanisms other than supporting the entirety of lipid raft microdomains is still unknown. By modulating cholesterol-enriched lipid raft microdomains and/or total cellular cholesterol contents in human embryonic kidney cells stably expressing µ-opioid receptor, we concluded that cholesterol stabilized opioid signaling both by supporting the lipid raft's entirety and by facilitating G protein coupling. Similar phenomena were observed in the primary rat hippocampal neurons. In addition, reducing the brain cholesterol level with simvastatin impaired the analgesic effect of opioids in mice, whereas the opioid analgesic effect was enhanced in mice fed a high-cholesterol diet. Furthermore, when the records of patients were analyzed, an inverse correlation between cholesterol levels and fentanyl doses used for anesthesia was identified, which suggested the mechanisms above could also be applicable to humans. Our results identified the interaction between opioids and cholesterol, which should be considered in clinics as a probable route for drug-drug interaction. Our studies also suggested that a low cholesterol level could lead to clinical issues, such as the observed impairment in opioid functions.

Prescription of lipophilic statins to Alzheimer's disease patients: some controversies to consider

Alzheimer's disease (AD) is the most common disorder causing cognitive decline in old age. It is a progressive and irreversible neuropathology with a diagnosis often missed or delayed. Cholesterol represents an important determinant of the physical state of biological membranes and in AD brains, specific changes in its membrane-ordering and Raft-organizing effects take place. A recent publication shows downregulation of Seladin-1 (selective Alzheimer's disease indicator, also called DHCR24), which catalyzes the last step of cholesterol biosynthesis in affected neurons in AD. Postmortem analysis of AD brains revealed a loss in membrane cholesterol content and this finding makes the therapeutical use of statins (especially the lipophilic ones) quite a lot controversial. Some clinical studies suggest that risk of Alzheimer's disease is substantially reduced in users of statins; however, because these studies are not randomized trials, they provide insufficient evidence to recommend statin family therapy.

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.

Cholesterol regulates volume-sensitive osmolyte efflux from human SH-SY5Y neuroblastoma cells following receptor activation

The ability of cholesterol to modulate receptor-mediated increases in the volume-dependent release of the organic osmolyte, taurine, has been examined. Depletion of cholesterol from SH-SY5Y neuroblastoma by preincubation of the cells with 5 mM methyl-beta-cyclodextrin (CD) for 10 min resulted in a 40 to 50% reduction in cholesterol and an enhancement of the ability of proteinase-activated receptor (PAR) 1, muscarinic cholinergic receptor (mAChR), and sphingosine 1-phosphate receptor to stimulate taurine efflux, when monitored under hypoosmotic conditions. Basal (swelling-induced) release of taurine was also enhanced by cholesterol depletion, but less markedly. Both basal- and receptor-mediated increases in taurine efflux were mediated via a volume-sensitive organic osmolyte and anion channel in control and cholesterol-depleted cells. Studies with the PAR-1 and mAChR receptor subtypes indicated that the stimulatory effect of CD pretreatment could be reversed by incubation of the cells with either CD/cholesterol or CD/5-cholesten-3alpha-ol donor complexes and that cholesterol depletion increased agonist efficacy, but not potency. The ability of cholesterol depletion to promote the PAR-1 receptor-mediated stimulation of osmolyte release was most pronounced under conditions of isotonicity or mild hypotonicity. In contrast to CD pretreatment, preincubation of the cells with cholesterol oxidase, a condition under which lipid microdomains are also disrupted, had no effect on either basal- or receptor-stimulated taurine efflux. Taken together, the results suggest that cholesterol regulates receptor-mediated osmolyte release via its effects on the biophysical properties of the plasma membrane, rather than its presence in lipid microdomains.

Regulation of central nervous system cholesterol homeostasis by the liver X receptor agonist TO-901317

The nuclear oxysterol receptors known as liver X receptors (LXR) are responsible for regulating genes involved in cholesterol homeostasis. Two subtypes exist, LXR-alpha and LXR-beta, both of which are expressed in the central nervous system (CNS). Activation of LXR causes an increase in mechanisms involved in cholesterol efflux, including ATP-binding cassette transporters (ABC)-A1 and ABC-G1. Although LXR agonists have been found to induce gene expression for ABC-A1 and ABC-G1 in the CNS, no functional response has been recorded. In this study, we show that an LXR agonist (TO-901317) increases protein levels of the cholesterol transporters ABC-A1 and ABC-G1, as well as the cholesterol chaperone protein apolipoprotein E (apoE). These changes are associated with a decrease in cholesterol levels from isolated cortical synaptosomal plasma membranes. LXR activation occurs in response to increased cholesterol levels in cells. However, while data exists on how LXR alter cholesterol efflux, there is no data on whether these receptors alter production of cholesterol. We found that TO-901317 increases HMG-CoA reductase activity, the rate-limiting step of cholesterol production in vivo. This finding was duplicated in an in vitro system. Although high concentrations (>1 microM) of TO-901317 were required to induce HMG-CoA reductase activity, these changes only occurred in the presence of apolipoprotein AI, suggesting that cholesterol efflux is required for this effect to occur. These data show that the LXR agonist TO-901317 is capable of reducing cholesterol in neurons of the CNS.

Impact of HMG-CoA reductase inhibition on brain pathology

Over the past two decades, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (HMGCRIs), originally designed to lower cholesterol blood levels, have been found to affect GTPase signaling during normal intracellular tasks. This finding has prompted use of these drugs in pathological situations, where such signaling processes need to be manipulated. Here, we review recent progress on the outcome of modulating GTPase signaling after inhibition of protein prenylation by HMGCRIs. We also discuss current controversies over the direct implications of these cholesterol-lowering agents on cholesterol-rich membrane lipid rafts and associated signaling. By reviewing these two different cellular events and the evidence from clinical studies, an overall assessment can be made of the concept of interfering with the HMG-CoA reductase pathway in different brain pathologies. We thereby provide a rational link between the benefit of applying HMGCRIs in brain pathologies, such as multiple sclerosis, Alzheimer's disease and stroke, and the impact on signaling in specific cell types crucial to disease pathogenesis.

Bcl-2 upregulation and neuroprotection in guinea pig brain following chronic simvastatin treatment

The present study determined if chronic simvastatin administration in vivo would provide neuroprotection in brain cells isolated from guinea pigs after challenge with the Bcl-2 inhibitor HA 14-1 or the NO donor sodium nitroprusside (SNP). Bcl-2 levels were significantly increased in brains of simvastatin-treated guinea pigs while levels of the pro-apoptotic protein Bax were significantly reduced. The ratio of Bax/Bcl-2, being a critical factor of the apoptotic state of cells, was significantly reduced in simvastatin-treated animals. Cholesterol levels in the brain remained unchanged in the simvastatin group. Brain cells isolated from simvastatin-treated guinea pigs were significantly less vulnerable to mitochondrial dysfunction and caspase-activation. These results provide new insight into potential mechanisms for the protective actions of statins within the CNS where programmed cell death has been implicated.

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.

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.

Tissue-specific effects of statins on the expression of heme oxygenase-1 in vivo

Heme oxygenase-1 (HO-1) plays a central role in antioxidant and anti-inflammatory actions, which may be mediated through its formation of biliverdin/bilirubin and carbon monoxide. HMG-CoA reductase inhibitors (statins) induce in vitro HO-1 expression and are reported to have pleiotropic benefits that reduce oxidative stress in the vasculature. We characterized the effects of statins on in vivo HO-1 expression in various extravascular tissues: liver, lung, brain, and heart. Adult mice were orally administered simvastatin, lovastatin, atorvastatin, or rosuvastatin. HO activity significantly increased in a statin- and tissue-specific manner, with all statins increasing heart and lung activity within 24 h. Significant elevations of HO-1 protein and mRNA were also observed in heart and lung after atorvastatin treatment. We conclude that in vivo HO-1 induction is statin- and tissue-specific. Through this pathway, statins may confer antioxidant and anti-inflammatory actions in the vasculature and extravascular systems.

Cholesterol and Alzheimer's disease—is there a relation?

The predominating theory on the pathophysiology of Alzheimer's disease (AD) concerns the mis-metabolism of amyloid precursor protein (APP). As a result of this mis-metabolism, there is an increased production of the 42 amino acid form of beta-amyloid (Abeta42) that rapidly will form oligomers that initiates a cascade of events leading to the accumulation of amyloid plaques. Commonly recognised as vascular factors, hypertension, hypercholesterolemia and diabetes and the inheritance of the epsilon4 allele of the APOE gene, are also risk factors for AD. These risks have been found to promote the production of Abeta42. An association between cholesterol and the development of AD was suggested in the early 1990s and ever since, an increasing amount of research has confirmed that there is a link between cholesterol and the development of AD. A high cholesterol levels in mid-life is a risk for AD and statins, i.e., cholesterol-lowering drugs, reduce this risk. Statins may not only inhibit enzymes involved in the endogenous synthesis of cholesterol but also affect enzymes involved in Abeta metabolism, i.e., alpha-secretase and beta-secretase. This normalises the breakdown of APP thereby promoting the non-amyloidogenic pathway. In this review, investigations focusing on cholesterol and Alzheimer's disease are presented.

Isoprenoids and Alzheimer's disease: a complex relationship

Cholesterol metabolism has been linked to Alzheimer's disease (AD) neuropathology, which is characterized by amyloid plaques, neurofibrillary tangles and neuroinflammation. Indeed, the use of statins, which inhibit cholesterol and isoprenoid biosynthesis, as potential AD therapeutics is under investigation. Whether statins offer benefit for AD will be determined by the outcome of large, placebo-controlled, randomized clinical trials. However, their use as pharmacological tools has delineated novel roles for isoprenoids in AD. Protein isoprenylation regulates multiple cellular and molecular events and here we review the complex roles of isoprenoids in AD-relevant processes and carefully evaluate isoprenoid pathways as potential AD therapeutic targets.

The link between cholesterol and Alzheimer's disease

A leading hypothesis on the pathophysiology of Alzheimer's disease (AD) is the mis-metabolism of amyloid precursor protein. This mis-metabolism causes the 42-amino acid form of A beta(Abeta42) to form oligomers that in turn start a chain of events leading to the accumulation of amyloid plaques. Vascular factors such as hypertension, hypercholesterolemia and diabetes as well as the inheritance of the epsilon4 allele of the ApoE gene are risk factors for AD. These risks are thought to promote the production of beta-amyloid (Abeta). An association between cholesterol and the development of AD was suggested in 1994 and since then, research has confirmed a link between cholesterol and the development of AD. A high cholesterol level in mid-life is a risk for AD and statins i.e. cholesterol-lowering drugs, reduce this risk. Statins inhibit enzymes involved in the endogenous synthesis of cholesterol and evidence is mounting that they also affect enzymes in Abeta metabolism i.e. beta-secretase. This normalises the breakdown of the precursor of Abeta, amyloid precursor protein, thereby promoting the nonamyloidogenic pathway. This review focusses on the link between cholesterol and Alzheimer's disease.

X-linked adrenoleukodystrophy mice demonstrate abnormalities in cholesterol metabolism

The neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD) is caused by ABCD1 mutations and characterized by very long-chain fatty acid (VLCFA) accumulation. Cholesterol-lowering normalized VLCFA in fibroblasts and plasma of X-ALD patients. We show that in cultured cells, cholesterol-loading induces ABCD1. In X-ALD mice, plasma cholesterol is elevated and not further increasable by cholesterol-feeding, whereas hepatic HMG-CoA reductase and Abcd2 are downregulated. Upon cholesterol modulation, brain VLCFA increased in X-ALD mice, but decreased in controls. In murine X-ALD fibroblasts, cholesterol-lowering did not normalize VLCFA. Thus, ALDP-deficiency and VLCFA are linked to cholesterol but species differences complicate evaluating cholesterol-lowering drugs in X-ALD mice.

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.

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.

ApoAI deficiency results in marked reductions in plasma cholesterol but no alterations in amyloid-beta pathology in a mouse model of Alzheimer's disease-like cerebral amyloidosis

Epidemiological studies suggest links between cholesterol metabolism and Alzheimer's disease (AD), with hypercholesterolemia associated with increased AD risk, and use of cholesterol-lowering drugs associated with decreased risk. Animal models using cholesterol-modifying dietary or pharmacological interventions demonstrate similar findings. Proposed mechanisms include effects of cholesterol on the metabolism of amyloid-beta (Abeta), the protein that deposits in AD brain. To investigate the effect of genetic alterations in plasma cholesterol on Abeta pathology, we crossed the PDAPP transgenic mouse model of AD-like cerebral amyloidosis to apolipoprotein AI-null mice that have markedly reduced plasma cholesterol levels due to a virtual absence of high density lipoproteins, the primary lipoprotein in mice. Interestingly and in contrast to models using non-physiological high fat diets or cholesterol-lowering drugs to modify plasma cholesterol, we observed no differences in Abeta pathology in PDAPP mice of the various apoAI genotypes despite robust differences in plasma cholesterol levels between the groups. Absence of apoAI also resulted in reductions in brain but not cerebrospinal fluid cholesterol, but had no effect on brain apolipoprotein E levels. These and other data suggest that it is perhaps the level of brain apolipoprotein E, not cholesterol per se, that plays a primary role in brain Abeta metabolism.

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 and the Biology of Alzheimer's Disease

Recent results implicating cholesterol metabolism in the pathophysiology of Alzheimer's disease (AD) bring cholesterol to the forefront of AD research. Research from genetics, epidemiology, and cell biology all converge, suggesting that cholesterol plays a central role in the biology of amyloid precursor protein and the toxic peptide generated by its cleavage, beta-amyloid (Abeta). The ability of cholesterol to modulate Abeta production suggests opportunities for therapeutic intervention, although the functional significance underlying the connection between cholesterol and Abeta remains to be investigated.

The cholesterol-lowering drug probucol increases apolipoprotein E production in the hippocampus of aged rats: implications for Alzheimer's disease

Several recent epidemiological studies have proposed that cholesterol-lowering drug Statin may provide protection against Alzheimer's disease (AD). Probucol is a non-Statin cholesterol-lowering drug and a potent inducer of apolipoprotein E (apoE) production in peripheral circulation. A recent clinical study using Probucol in elderly AD subjects revealed a concomitant stabilisation of cognitive symptoms and significant increases in apoE levels in the cerebral spinal fluid in these patients. To gain insight into the mechanisms underlying these effects, we treated a cohort of aged male rats (26-month-old) with oral dose of Probucol for 30 days. Specifically, we examined the effects of Probucol on apoE production and its receptors (low density lipoprotein receptor [LDLr] and low density lipoprotein receptor-related protein [LRP]), astroglial marker of cell damage (glial fibrillary acidic protein [GFAP]), markers of neuronal synaptic plasticity and integrity (synaptosomal associated protein of 25 kDa [SNAP-25] and synaptophysin) as well as cholesterol biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase [HMGCoAr]) in the hippocampus. We report that Probucol induces the production of apoE and one of its main receptors, LRP, increases HMGCoAr (rate-limiting enzyme in cholesterol synthesis), substantially attenuates age-related increases in glial activation, and induces production of synaptic marker SNAP-25, a molecule commonly associated with synaptogenesis and dendritic remodeling. These findings suggest that Probucol could promote neural and synaptic plasticity to counteract the synaptic deterioration associated with brain aging through an apoE/LRP-mediated system. Consistent with the beneficial effects of other cholesterol-lowering drugs such as the Statin, Probucol could also offers additional benefits based on apoE neurobiology.

Amyloid beta-protein interactions with membranes and cholesterol: causes or casualties of Alzheimer's disease

Amyloid beta-protein (Abeta) is thought to be one of the primary factors causing neurodegeneration in Alzheimer's disease (AD). This protein is an amphipathic molecule that perturbs membranes, binds lipids and alters cell function. Several studies have reported that Abeta alters membrane fluidity but the direction of this effect has not been consistently observed and explanations for this lack of consistency are proposed. Cholesterol is a key component of membranes and cholesterol interacts with Abeta in a reciprocal manner. Abeta impacts on cholesterol homeostasis and modification of cholesterol levels alters Abeta expression. In addition, certain cholesterol lowering drugs (statins) appear to reduce the risk of AD in human subjects. However, the role of changes in the total amount of brain cholesterol in AD and the mechanisms of action of statins in lowering the risk of AD are unclear. Here we discuss data on membranes, cholesterol, Abeta and AD, and propose that modification of the transbilayer distribution of cholesterol in contrast to a change in the total amount of cholesterol provides a cooperative environment for Abeta synthesis and accumulation in membranes leading to cell dysfunction including disruption in cholesterol homeostasis.

Statin effects on cholesterol micro-domains in brain plasma membranes

Recent epidemiological studies revealed inhibitors of the hydroxymethylglutaryl-coenzyme A reductase, so-called statins, to be effective in lowering the prevalence of Alzheimer's disease (AD). In vitro, statins strongly reduced the cellular amyloid beta-protein load by modulating the processing of the amyloid beta precursor protein. Both observations are probably linked to cellular cholesterol homeostasis in brain. So far, little is known about brain effects of statins. Recently, we could demonstrate that treatment of mice with the lipophilic compound lovastatin resulted in a discrete reduction of brain membrane cholesterol levels. To follow up these findings, we subsequently carried out a further in vivo study including lovastatin and simvastatin as lipophilic agents, as well as pravastatin as a hydrophilic compound, focussing on their efficiency to affect subcellular membrane cholesterol pools in synaptosomal plasma membranes of mice. In contrast to the hydrophilic pravastatin, the lipophilic lovastatin and simvastatin strongly reduced the levels of free cholesterol in SPM. Interestingly, lovastatin and pravastatin but not simvastatin significantly reduced cholesterol levels in the exofacial membrane leaflet. These changes were accompanied by modified membrane bulk fluidity. All three statins reduced the expression of the raft marker protein flotillin. Alterations in transbilayer cholesterol distribution have been suggested as the underlying mechanism that forces amyloidogenic processing of APP in AD. Thus, our data give some first insight in the mode of action of statins to reduce the prevalence of AD in clinical trials.

The role of cholesterol in pathogenesis of Alzheimer's disease: dual metabolic interaction between amyloid beta-protein and cholesterol

The implication that cholesterol plays an essential role in the pathogenesis of Alzheimer's disease (AD) is based on the 1993 finding that the presence of apolipoprotein E (apoE) allele epsilon;4 is a strong risk factor for developing AD. Since apoE is a regulator of lipid metabolism, it is reasonable to assume that lipids such as cholesterol are involved in the pathogenesis of AD. Recent epidemiological and biochemical studies have strengthened this assumption by demonstrating the association between cholesterol and AD, and by proving that the cellular cholesterol level regulates synthesis of amyloid beta-protein (Abeta). Yet several studies have demonstrated that oligomeric Abeta affects the cellular cholesterol level, which in turn has a variety of effects on AD related pathologies, including modulation of tau phosphorylation, synapse formation and maintenance of its function, and the neurodegenerative process. All these findings suggest that the involvement of cholesterol in the pathogenesis of AD is dualistic-it is involved in Abeta generation and in the amyloid cascade, leading to disruption of synaptic plasticity, promotion of tau phosphorylation, and eventual neurodegeneration. This review article describes recent findings that may lead to the development of a strategy for AD prevention by decreasing the cellular cholesterol level, and also focuses on the impact of Abeta on cholesterol metabolism in AD and mild cognitive impairment (MCI), which may result in promotion of the amyloid cascade at later stages of the AD process.

Plasma membrane cholesterol controls the cytotoxicity of Alzheimer's disease AbetaP (1-40) and (1-42) peptides

Cell degeneration in Alzheimer's disease is mediated by a toxic mechanism that involves interaction of the AbetaP peptide with the plasma membrane of the target cell. We report here that PC12 cells become resistant to the cytotoxic action of AbetaP when incubated in a medium that enriches cholesterol levels of the surface membrane. On the other hand, making cholesterol-deficient membranes by either cholesterol extraction with cyclodextrin or by inhibiting de novo synthesis of cholesterol makes PC12 cells more vulnerable to the action of AbetaP. Increasing cholesterol content of PS liposomes also suppresses AbetaP-dependent liposome aggregation. We suggest that by modifying the fluidity of the neuronal membranes, cholesterol modulates the incorporation and pore formation of AbetaP into cell membranes. This idea is supported by our finding that the enhanced cytotoxicity generated by lowering the membrane cholesterol content can be reversed by AbetaP calcium channel blockers Zn2+ and tromethamine.

Cholesterol attenuates the membrane perturbing properties of beta-amyloid peptides

Growing evidence indicates a significant linkage between Abeta and cholesterol metabolism, although the exact role of cholesterol in brain aging and in the pathogenesis of AD is still unknown. Recently, in vitro and in vivo modification of cell cholesterol and its effect on Abeta-generation became a straight focus in the research of AD. In the present study, we discretely modulated the cholesterol contents of neuronal membranes from mice of different ages in vivo and in vitro using lovastatin and methyl-beta-cyclodextrin, respectively. The aim of the study was to investigate whether this modulation results in altered physico-chemical membrane properties. Therefore, we performed membrane fluidity measurements using three fluorescent dyes labeling different membrane regions. Furthermore, we evaluated the effects of cholesterol modulation on the membrane disturbing properties of Abeta. Modulation of membrane cholesterol content in vivo and in vitro was linked to changes in membrane properties. Very interestingly, cholesterol content of in vitro modulated neuronal membranes was negatively correlated with the membrane perturbing effects of Abeta.

Profile of cholesterol-related sterols in aged amyloid precursor protein transgenic mouse brain

Cholesterol is implicated to play a role in Alzheimer disease pathology. Therefore, the concentrations of cholesterol, its precursors, and its degradation products in brain homogenates of aging wild-type and beta-amyloid precursor protein transgenic mice carrying the Swedish mutation (APP23) were analyzed. Among the sterols measured, lanosterol is the first common intermediate of two different pathways, which use either desmosterol or lathosterol as the predominant precursors for de novo synthesis of brain cholesterol. In young mice, cholesterol is mainly synthesized via the desmosterol pathway, while in aged mice, lathosterol is the major precursor. 24S-hydroxycholesterol (cerebrosterol), which plays a key role in the removal of cholesterol from the brain, modestly increased during aging. No differences in the levels of cholesterol, its precursors, or its metabolites were found between wild-type and APP23 transgenic mice. Moreover, the levels of the exogenous plant sterols campesterol and sitosterol were significantly elevated in the brains of APP23 animals at age 12 and 18 months. This time point coincides with abundant plaque formation.

Cholesterol, A beta and Alzheimer's disease

Statins have been used for many years for the treatment of hypercholesterolemia. They lower low-density lipoprotein (LDL) cholesterol, increase high-density lipoprotein (HDL) levels and are considered to be very safe. Recently, a set of potential new applications was identified for statins. In the future, these drugs could be used to treat Alzheimer's disease (AD). Past studies have suggested a link between AD and lipids and a series of reports has recently been published that significantly tightens this link and also provides some explanations at the cellular level. This review focuses on these recent developments and perspectives that appear to link cholesterol, beta-amyloid and AD.

3-hydroxy-3-methylglutaryl-coenzyme A reductase mRNA in Alzheimer and control brain

Statins are widely used pharmaceutical agents which lower plasma cholesterol by inhibiting the rate controlling enzyme 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase. One epidemiological study suggests that statin therapy may provide protection against Alzheimer disease (AD). The aim of the present study was to determine the relative expression of HMG-CoA reductase mRNAs in various areas of brain as well as in peripheral organs and to compare values in AD and control cases. High levels of the mRNA were found in all areas of brain but no obvious differences were found between AD and controls. We conclude that brain has a robust capacity to synthesize cholesterol which appears to be unaffected by AD pathology.