Interactions of apolipoprotein E genotype, total cholesterol level, age, and sex in prediction of Alzheimer's disease: a case-control study

Dyslipidemia and the risk of Alzheimer's disease

Whether cholesterol is implicated in the pathogenesis of Alzheimer's disease (AD) is still controversial. Several studies that explored the association between lipids and/or lipid-lowering treatment and AD indicate a harmful effect of dyslipidemia on AD risk. The findings are supported by genetic linkage and association studies that have clearly identified several genes involved in cholesterol metabolism or transport as AD susceptibility genes, including apolipoprotein E (APOE), apolipoprotein J (APOJ, CLU), ATP-binding cassette subfamily A member 7(ABCA7), and sortilin-related receptor (SORL1). Functional cell biology studies further support a critical involvement of lipid raft cholesterol in the modulation of Aβ precursor protein processing by β-secretase and γ-secretase resulting in altered Aβ production. However, conflicting evidence comes from epidemiological studies showing no or controversial association between dyslipidemia and AD risk, randomized clinical trials observing no beneficial effect of statin therapy, and cell biology studies suggesting that there is little exchange between circulating and brain cholesterol, that increased membrane cholesterol level is protective by inhibiting loss of membrane integrity through amyloid cytotoxicity, and that cellular cholesterol inhibits colocalization of β-secretase 1 and Aβ precursor protein in nonraft membrane domains, thereby increasing generation of plasmin, an Aβ-degrading enzyme. The aim of this article is to provide a comprehensive review of the findings of epidemiological, genetic, and cell biology studies aiming to elucidate the role of cholesterol in the pathogenesis of AD.

Links between copper and cholesterol in Alzheimer's disease

Altered copper homeostasis and hypercholesterolemia have been identified independently as risk factors for Alzheimer's disease (AD). Abnormal copper and cholesterol metabolism are implicated in the genesis of amyloid plaques and neurofibrillary tangles (NFT), which are two key pathological signatures of AD. Amyloidogenic processing of a sub-population of amyloid precursor protein (APP) that produces Aβ occurs in cholesterol-rich lipid rafts in copper deficient AD brains. Co-localization of Aβ and a paradoxical high concentration of copper in lipid rafts fosters the formation of neurotoxic Aβ:copper complexes. These complexes can catalytically oxidize cholesterol to generate H₂O₂, oxysterols and other lipid peroxidation products that accumulate in brains of AD cases and transgenic mouse models. Tau, the core protein component of NFTs, is sensitive to interactions with copper and cholesterol, which trigger a cascade of hyperphosphorylation and aggregation preceding the generation of NFTs. Here we present an overview of copper and cholesterol metabolism in the brain, and how their integrated failure contributes to development of AD.

Cochrane review on 'Statins for the treatment of dementia'

OBJECTIVES

This review aimed to assess the clinical efficacy and tolerability of statins in the treatment of dementia.

METHODS

We searched the Specialized Register of the Cochrane Dementia and Cognitive Improvement Group, The Cochrane Library, MEDLINE, EMBASE, PsycINFO, CINAHL and LILACS, as well as many trials registries and grey literature sources (27 October 2008). Double-blind, randomized controlled trials of statins given for at least 6 months in people with a diagnosis of dementia were included. Two independent authors extracted and assessed data independently against the inclusion criteria. Data were pooled where appropriate and entered into a meta-analysis.

RESULTS

Three studies were identified (748 participants, age range 50-90 years). All patients had a diagnosis of probable or possible Alzheimer's disease according to standard criteria, and most patients were established on a cholinesterase inhibitor. Change in Alzheimer's Disease Assessment Scale cognitive subscale from baseline was a primary outcome in three studies; when data were pooled, statins did not provide any beneficial effect in this cognitive measure (mean difference -1.12; 95% confidence interval -3.99, 1.75; p = 0.44). All studies provided a change in Mini-Mental State Examination from baseline; there was no significant benefit from statins in this cognitive measure when the data were pooled (mean difference -1.53; 95% confidence interval -3.28; 0.21, p = 0.08). There were no studies identified assessing the role of statins in treatment of vascular dementia. There was no evidence that statins were detrimental to cognition.

CONCLUSIONS

There is insufficient evidence to recommend statins for the treatment of dementia.

Alzheimer’s disease, cerebrovascular disease, and the β-amyloid cascade

Alzheimer’s disease (AD), considered the commonest neurodegenerative cause of dementia, is associated with hallmark pathologies including extracellular amyloid-β protein (Aβ) deposition in extracellular senile plaques and vessels, and intraneuronal tau deposition as neurofibrillary tangles. Although AD is usually categorized as neurodegeneration distinct from cerebrovascular disease (CVD), studies have shown strong links between AD and CVD. There is evidence that vascular risk factors and CVD may accelerate Aβ 40-42 production/ aggregation/deposition and contribute to the pathology and symptomatology of AD. Aβ deposited along vessels also causes cerebral amyloid angiopathy. Amyloid imaging allows in vivo detection of AD pathology, opening the way for prevention and early treatment, if disease-modifying therapies in the pipeline show safety and efficacy. In this review, we review the role of vascular factors and Aβ, underlining that vascular risk factor management may be important for AD prevention and treatment.

KIF6 719Arg allele is associated with statin effects on cholesterol levels in amnestic mild cognitive impairment and Alzheimer's disease patients

KIF6 719Arg allele carriers are thought to have a greater lipid lowering response from statin therapy than non-carriers. Given the continued interest in the relationship between cholesterol, statin use, amnestic mild cognitive impairment (aMCI), and Alzheimer's disease (AD), investigating the role of KIF6 719Arg carrier status in these relationships may be of importance. Data from 86 patients (36 aMCI, 50 AD) with an average age of 76.87 ± 8.22 years were used for this study. Total cholesterol (TC), low density lipoprotein (LDL), high density lipoprotein (HDL), and triglycerides were the outcome variables. 719Arg carriers taking statins had significantly lower TC (p < 0.001) and LDL (p < 0.001) levels than 719Arg carriers not taking statins. In addition, 719Arg carriers not taking statins had significantly higher TC (p = 0.004) and LDL (p < 0.001) than 719Arg non-carriers taking statins. Additional analyses indicated that ApoE ε4 carrier status and statin use interaction is also associated with lower TC (p = 0.04), but not LDL (p = 0.06). The interaction between 719Arg and ApoE ε4 carrier status on TC and LDL was not significant. This study is the first to demonstrate an association between lower cholesterol levels and statin use among KIF6 719Arg allele carriers with aMCI and AD. Accounting for 719Arg carrier status may be important in future studies investigating the link between cholesterol and AD and also for AD and aMCI clinical trials using statins as a treatment.

Dyslipidemia and dementia: current epidemiology, genetic evidence, and mechanisms behind the associations

The role of cholesterol in the etiology of Alzheimer's disease (AD) is still controversial. Some studies exploring the association between lipids and/or lipid lowering treatment and AD indicate a harmful effect of dyslipidemia and a beneficial effect of statin therapy on AD risk. The findings are supported by genetic linkage and association studies that have clearly identified several genes involved in cholesterol metabolism or transport as AD susceptibility genes, including apolipoprotein E, apolipoprotein J, and the sortilin-related receptor. Functional cell biology studies support a critical involvement of lipid raft cholesterol in the modulation of amyloid-β protein precursor (AβPP) processing by β- and γ-secretase resulting in altered amyloid-β production. Contradictory evidence comes from epidemiological studies showing no or controversial association between dyslipidemia and AD risk. Additionally, cell biology studies suggest that there is little exchange between circulating and brain cholesterol, that increased membrane cholesterol is protective by inhibiting loss of membrane integrity through amyloid cytotoxicity, and that cellular cholesterol inhibits co-localization of BACE1 and AβPP in non-raft membrane domains, thereby increasing generation of plasmin, an amyloid-β-degrading enzyme. The aim of this review is to summarize the findings of epidemiological and cell biological studies to elucidate the role of cholesterol in AD etiology.

Improving Alzheimer's disease phase II clinical trials

Over the past 30 years, many drugs have been studied as possible treatments for Alzheimer's disease, but only four have demonstrated sufficient efficacy to be approved as treatments, of which three are in the same class. This lack of success has raised questions both in the pharmaceutical industry and academia about the future of Alzheimer's disease therapy. The high cost and low success rate of drug development across many disease areas can be attributed, in large part, to late-stage clinical failures (Schachter and Ramoni, Nat Rev Drug Discov 2007;6:107-8). Thus, identifying in phase II, or preferably phase I, drugs that are likely to fail would have a dramatic impact on the costs associated with developing new drugs. With this in mind, the Alzheimer's Association convened a Research Roundtable on June 23 and 24, 2011, in Washington, DC, bringing together scientists from academia, industry, and government regulatory agencies to discuss strategies for improving the probability of phase II trial results predicting success when considering the go/no-go decision-making process leading to the initiation of phase III.

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.

Upstream transcription factor 1 (USF1) polymorphisms associate with Alzheimer's disease-related neuropathological lesions: Tampere Autopsy Study

The apolipoprotein E (APOE) gene associates with Alzheimer's disease (AD) and cholesterol levels. Upstream transcription factor 1 (USF1) regulates lipid metabolism genes, including APOE, and the AD Aβ-precursor protein. We investigated associations between 6 haplotype-tagging USF1 single-nucleotide polymorphisms (and haplotypes) and AD-related neuropathological lesions [senile plaques (SP), neurofibrillary tangles (NFT) ] in an autopsy series comprising 603 cases (ages 0-97, mean 62 years, 215 women) that died out-of-hospital. In age- and APOE-adjusted analyses, the minor G-allele of rs2774276, previously linked to elevated cholesterol, associated with late-stage burnt out SP among women and early non-neuritic SP among men. The G-allele of the previously unreported rs10908821 showed significant risk of having SP, especially neuritic and burnt out SP, among women but not men. USF1 haplotype GCGCAC carriers (risk alleles of rs2774276 and rs10908821) associated with SP risk, especially neuritic and late-stage burnt out SP, among women but not men. Younger CCGCAC carriers (risk allele of rs2774276 and protective of rs10908821) were more likely to have non-neuritic and diffuse SP. Conversely, USF1 CCGCAC haplotype carriers had lower NFT prevalence among 65+ year-olds. These results suggest USF1 has an independent but gender- and age-associated effect on AD-related brain lesion development.

Isoprenoids and related pharmacological interventions: potential application in Alzheimer's disease

Two major isoprenoids, farnesyl pyrophosphate and geranylgeranyl pyrophosphate, serve as lipid donors for the posttranslational modification (known as prenylation) of proteins that possess a characteristic C-terminal motif. The prenylation reaction is catalyzed by prenyltransferases. The lipid prenyl group facilitates to anchor the proteins in cell membranes and mediates protein-protein interactions. A variety of important intracellular proteins undergo prenylation, including almost all members of small GTPase superfamilies as well as heterotrimeric G protein subunits and nuclear lamins. These prenylated proteins are involved in regulating a wide range of cellular processes and functions, such as cell growth, differentiation, cytoskeletal organization, and vesicle trafficking. Prenylated proteins are also implicated in the pathogenesis of different types of diseases. Consequently, isoprenoids and/or prenyltransferases have emerged as attractive therapeutic targets for combating various disorders. This review attempts to summarize the pharmacological agents currently available or under development that control isoprenoid availability and/or the process of prenylation, mainly focusing on statins, bisphosphonates, and prenyltransferase inhibitors. Whereas statins and bisphosphonates deplete the production of isoprenoids by inhibiting the activity of upstream enzymes, prenyltransferase inhibitors directly block the prenylation of proteins. As the importance of isoprenoids and prenylated proteins in health and disease continues to emerge, the therapeutic potential of these pharmacological agents has expanded across multiple disciplines. This review mainly discusses their potential application in Alzheimer's disease.

Genome scan of age-at-onset in the NIMH Alzheimer disease sample uncovers multiple loci, along with evidence of both genetic and sample heterogeneity

Alzheimer's disease (AD) is a common neurodegenerative disorder of late life with a complex genetic basis. Although several genes are known to play a role in rare early onset AD, only the APOE gene is known to have a high contribution to risk of the common late-onset form of the disease (LOAD, onset >60 years). APOE genotypes vary in their AD risk as well as age-at-onset distributions, and it is likely that other loci will similarly affect AD age-at-onset. Here we present the first analysis of age-at-onset in the NIMH LOAD sample that allows for both a multilocus trait model and genetic heterogeneity among the contributing sites, while at the same time accommodating age censoring, effects of known genetic covariates, and full pedigree and marker information. The results provide evidence for genomic regions not previously implicated in this data set, including regions on chromosomes 7q, 15, and 19p. They also affirm evidence for loci on chromosomes 1q, 6p, 9q, 11, and, of course, the APOE locus on 19q, all of which have been reported previously in the same sample. The analyses failed to find evidence for linkage to chromosome 10 with inclusion of unaffected subjects and extended pedigrees. Several regions implicated in these analyses in the NIMH sample have been previously reported in genome scans of other AD samples. These results, therefore, provide independent confirmation of AD loci in family-based samples on chromosomes 1q, 7q, 19p, and suggest that further efforts towards identifying the underlying causal loci are warranted.

Activation of α-secretase cleavage

Alpha-secretase-mediated cleavage of the amyloid precursor protein (APP) releases the neuroprotective APP fragment sαAPP and prevents amyloid β peptide (Aβ) generation. Moreover, α-secretase-like cleavage of the Aβ transporter 'receptor for advanced glycation end products' counteracts the import of blood Aβ into the brain. Assuming that Aβ is responsible for the development of Alzheimer's disease (AD), activation of α-secretase should be preventive. α-Secretase-mediated APP cleavage can be activated via several G protein-coupled receptors and receptor tyrosine kinases. Protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, cAMP and calcium are activators of receptor-induced α-secretase cleavage. Selective targeting of receptor subtypes expressed in brain regions affected by AD appears reasonable. Therefore, the PACAP receptor PAC1 and possibly the serotonin 5-HT(6) receptor subtype are promising targets. Activation of APP α-secretase cleavage also occurs upon blockade of cholesterol synthesis by statins or zaragozic acid A. Under physiological statin concentrations, the brain cholesterol content is not influenced. Statins likely inhibit Aβ production in the blood by α-secretase activation which is possibly sufficient to inhibit AD development. A disintegrin and metalloproteinase 10 (ADAM10) acts as α-secretase on APP. By targeting the nuclear retinoic acid receptor β, the expression of ADAM10 and non-amyloidogenic APP processing can be enhanced. Excessive activation of ADAM10 should be avoided because ADAM10 and also ADAM17 are not APP-specific. Both ADAM proteins cleave various substrates, and therefore have been associated with tumorigenesis and tumor progression.

Cholesterol level and statin use in Alzheimer disease: I. Review of epidemiological and preclinical studies

During the last 2 decades, evidence has accumulated that a high cholesterol level may increase the risk of developing Alzheimer disease (AD). With the global use of statins to treat hypercholesterolemia, this finding has led to the anticipation that statins could prove useful in treating or preventing AD. However, the results of work on this topic are inconsistent: some studies find beneficial effects, but other studies do not. In this first segment of a 2-part review, we examine the complex preclinical and clinical literature on cholesterol level and AD. First, we review epidemiological research on cholesterol level and the risk of AD and discuss the relevance of discrepancies among studies with regard to participants' age and clinical status. Second, we assess studies correlating cholesterol level with neuropathological AD type. The potential molecular mechanisms for the apparent adverse effects of cholesterol on the development of AD are then discussed. Third, we review preclinical studies of statin use and AD. Therefore, this first part of our review provides the background and rationale for investigating statins as potential therapeutic agents in patients with AD, the subject of the second part.

Modeling cholesterol metabolism by gene expression profiling in the hippocampus

An important part of the challenge of building models of biochemical reactions is determining reaction rate constants that transform substrates into products. We present a method to derive enzymatic kinetic values from mRNA expression levels for modeling biological networks without requiring further tuning. The core metabolic reactions of cholesterol in the brain, particularly in the hippocampus, were simulated. To build the model the baseline mRNA expression levels of genes involved in cholesterol metabolism were obtained from the Allen Mouse Brain Atlas. The model is capable of replicating the trends of relative cholesterol levels in Alzheimer's and Huntington's diseases; and reliably simulated SLOS, desmosterolosis, and Dhcr14/Lbr knockout studies. A sensitivity analysis correctly uncovers the Hmgcr, Idi2 and Fdft1 sites that regulate cholesterol homeostasis. Overall, our model and methodology can be used to pinpoint key reactions, which, upon manipulation, may predict altered cholesterol levels and reveal insights into potential drug therapy targets under diseased conditions.

Interactive effects of physical activity and APOE-ε4 on BOLD semantic memory activation in healthy elders

Evidence suggests that physical activity (PA) is associated with the maintenance of cognitive function across the lifespan. In contrast, the apolipoproteinE-ε4 (APOE-ε4) allele, a genetic risk factor for Alzheimer's disease (AD), is associated with impaired cognitive function. The objective of this study was to examine the interactive effects of PA and APOE-ε4 on brain activation during memory processing in older (ages 65-85) cognitively intact adults. A cross-sectional design was used with four groups (n=17 each): (1) Low Risk/Low PA; (2) Low Risk/High PA; (3) High Risk/Low PA; and (4) High Risk/High PA. PA level was based on self-reported frequency and intensity. AD risk was based on presence or absence of an APOE-ε4 allele. Brain activation was measured using event-related functional magnetic resonance imaging (fMRI) while participants performed a famous name discrimination task. Brain activation subserving semantic memory processing occurred in 15 functional regions of interest. High PA and High Risk were associated with significantly greater semantic memory activation (famous>unfamiliar) in 6 and 3 of the 15 regions, respectively. Significant interactions of PA and Risk were evident in 9 of 15 brain regions, with the High PA/High Risk group demonstrating greater semantic memory activation than the remaining three groups. These findings suggest that PA selectively increases memory-related brain activation in cognitively intact but genetically at-risk elders. Longitudinal studies are required to determine whether increased semantic memory processing in physically active at-risk individuals is protective against future cognitive decline.

Gene-environment interaction research and transgenic mouse models of Alzheimer's disease

The etiology of the sporadic form of Alzheimer's disease (AD) remains largely unknown. Recent evidence has suggested that gene-environment interactions (GxE) may play a crucial role in its development and progression. Whereas various susceptibility loci have been identified, like the apolipoprotein E4 allele, these cannot fully explain the increasing prevalence of AD observed with aging. In addition to such genetic risk factors, various environmental factors have been proposed to alter the risk of developing AD as well as to affect the rate of cognitive decline in AD patients. Nevertheless, aside from the independent effects of genetic and environmental risk factors, their synergistic participation in increasing the risk of developing AD has been sparsely investigated, even though evidence points towards such a direction. Advances in the genetic manipulation of mice, modeling various aspects of the AD pathology, have provided an excellent tool to dissect the effects of genes, environment, and their interactions. In this paper we present several environmental factors implicated in the etiology of AD that have been tested in transgenic animal models of the disease. The focus lies on the concept of GxE and its importance in a multifactorial disease like AD. Additionally, possible mediating mechanisms and future challenges are discussed.

Type 2 diabetes mellitus, dyslipidemia, and Alzheimer's disease

The prevalence of Alzheimer's disease (AD) is increasing rapidly, heightening the importance of finding effective preventive therapies for this devastating disease. Midlife vascular risk factors, including type 2 diabetes mellitus (T2DM), have been associated with increased risk of AD decades later and may serve as targets for AD prevention. Studies to date suggest that T2DM and hyperinsulinemia increase risk for AD, possibly through their effects on amyloid-beta metabolism and cerebrovascular dysfunction - two early findings in preclinical AD pathology. This paper reviews the evidence supporting a relationship between T2DM, hyperinsulinemia, and diabetic dyslipidemia on the development of AD, discusses DM treatment trials and their preliminary results on cognitive function, and proposes some strategies for optimizing future AD prevention trial design.

Statins for the treatment of dementia

BACKGROUND

The use of statin therapy in established Alzheimer's disease (AD) or vascular dementia (VaD) is a relatively unexplored area. In AD ss-amyloid protein (Ass) is deposited in the form of extracellular plaques and previous studies have determined Ass generation is cholesterol dependent. Hypercholesterolaemia has also been implicated in the pathogenesis of VaD. Due to the role of statins in cholesterol reduction it is biologically plausible they may be efficacious in the treatment of AD and dementia.

OBJECTIVES

To assess the clinical efficacy and tolerability of statins in the treatment of dementia.

SEARCH STRATEGY

We searched the Specialized Register of the Cochrane Dementia and Cognitive Improvement Group, The Cochrane Library, MEDLINE, EMBASE, PsycINFO, CINAHL and LILACS, as well as many trials registries and grey literature sources (27 October 2008).

SELECTION CRITERIA

Double-blind, randomized controlled trials of statins given for at least six months in people with a diagnosis of dementia.

DATA COLLECTION AND ANALYSIS

Two independent authors extracted and assessed data independently against the inclusion criteria. Data were pooled where appropriate and entered into a meta-analysis.

MAIN RESULTS

Three studies were identified (748 participants, age range 50-90 years). All patients had a diagnosis of probable or possible AD according to standard criteria and most patients were established on a cholinesterase inhibitor. Treatment in ADCLT 2005 consisted of 80mg atorvastatin compared to placebo for 52 weeks, serum low density lipoprotein (LDL) cholesterol was reduced by 54% in the atorvastatin group. Treatment in Simons 2002 consisted of 40mg simvastatin compared to placebo for 26 weeks, serum LDL cholesterol was reduced by 52% in the simvastatin group. Treatment in LEADe 2010 consisted of 80mg atorvastatin compared to placebo for 72 weeks, LDL cholesterol was reduced by 50.2% by month 3 and remained constant through month 18. Change in Alzheimer's Disease Assessment Scale- cognitive subscale (ADAS-Cog) from baseline was a primary outcome in 3 studies; when data were pooled there was considerable heterogeneity so the random effects model was used, statins did not provide any beneficial effect in this cognitive measure [mean difference -1.12, 95% CI -3.99, 1.75, p = 0.44]. All studies provided change in Mini Mental State Examination (MMSE) from baseline; again random effects model was used due to considerable heterogeneity: there was no significant benefit from statins in this cognitive measure when the data were pooled [mean difference -1.53, 95% CI -3.28, 0.21, p = 0.08]. There was some evidence that patients on statins in ADCLT 2005 maintained better cognitive function if serum cholesterol was high at baseline, MMSE was higher at baseline or if they had an apolipoprotein E4 allele present. This would need to be confirmed in larger studies however. Treatment related adverse effects were available from two studies, LEADe 2010 and Simons 2002; when data were pooled there was no significant difference between statins and placebo [odds ratio 2.45, 95% CI 0.69, 8.62, p = 0.16]. There was no significant difference in global function, behaviour or activities of daily living in the statin and placebo groups. One large randomised controlled trial (RCT) ( CLASP 2008) has not yet published its results. There were no studies identified assessing role of statins in treatment of VaD. There was no evidence that statins were detrimental to cognition.

AUTHORS' CONCLUSIONS

There is insufficient evidence to recommend statins for the treatment of dementia. Analysis from the studies available, including one large RCT, indicate statins have no benefit on the outcome measures ADAS-Cog or MMSE. We need to await full results from CLASP 2008 before we can be certain. This Cochrane review will be updated as these results become available.

Neurovascular changes measured by time-of-flight MR angiography in cholesterol-fed rabbits with cortical amyloid beta-peptide accumulation

PURPOSE

To test the hypothesis that narrowing of cranial blood vessels in cholesterol-fed rabbits is a function of the duration of the high cholesterol diet. Such neurovascular changes, caused by elevated serum cholesterol, are linked to stroke and Alzheimer's disease risk.

MATERIALS AND METHODS

Four groups of New Zealand White rabbits were studied. Six were fed a normal diet, 19 were fed a 2% cholesterol diet with 0.12 ppm copper in the drinking water for 8 weeks, 10 weeks, or 12 weeks. Time-of-flight (TOF) MR angiography (MRA) at 3 Tesla was used to measure arterial diameters in 11 vessels. Previously published data for amyloid beta-peptide (Abeta) accumulation in the brains measured postmortem were correlated to vessel diameters. Ventricular volumes of rabbits were measured on group-averaged data.

RESULTS

Several vessel diameters decreased with cholesterol diet duration. The posterior communicating arteries showed the largest significant effect. Abeta accumulation was inversely correlated with arterial diameter. Ventricular volumes between the normal diet and 12 weeks cholesterol-fed groups were not significantly different.

CONCLUSION

Reduction in vessel diameter of medium-sized vessels but not large vessels was measured in these hypercholesterolemic rabbits. The vessel diameter narrowing and cortical Abeta deposition occurred before measurable ventricular enlargement.

ACAT inhibition and amyloid beta reduction

Alzheimer's disease (AD) is a devastating neurodegenerative disorder. Accumulation and deposition of the beta-amyloid (Abeta) peptide generated from its larger amyloid precursor protein (APP) is one of the pathophysiological hallmarks of AD. Intracellular cholesterol was shown to regulate Abeta production. Recent genetic and biochemical studies indicate that not only the amount, but also the distribution of intracellular cholesterol is critical to regulate Abeta generation. Acyl-coenzyme A: cholesterol acyl-transferase (ACAT) is a family of enzymes that regulates the cellular distribution of cholesterol by converting membrane cholesterol into hydrophobic cholesteryl esters for cholesterol storage and transport. Using pharmacological inhibitors and transgenic animal models, we and others have identified ACAT1 as a potential therapeutic target to lower Abeta generation and accumulation. Here we discuss data focusing on ACAT inhibition as an effective strategy for the prevention and treatment of AD.

The effects of cholesterol on learning and memory

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

Roles of cholesterol and lipids in the etiopathogenesis of Alzheimer's disease

Alzheimer's disease is the principal cause of dementia throughout the world and the fourth cause of death in developed economies.This brain disorder is characterized by the formation of brain protein aggregates, namely, the paired helical filaments and senile plaques. Oxidative stress during life, neuroinflamamtion, and alterations in neuron-glia interaction patterns have been also involved in the etiopathogenesis of this disease. In recent years, cumulative evidence has been gained on the involvement of alteration in neuronal lipoproteins activity, as well as on the role of cholesterol and other lipids in the pathogenesis of this neurodegenerative disorder. In this review, we analyze the links between changes in cholesterol homeostasis, and the changes of lipids of major importance for neuronal activity and Alheimer's disease. The investigation on the fine molecular mechanisms underlying the lipids influence in the etiopathogenesis of Alzheimer's disease may shed light into its treatment and medical management.

Is there a relationship between high C-reactive protein (CRP) levels and dementia?

Inflammation is believed to play a pivotal role in dementia, but its role is still unclear. The aim of our study was to analyze the interplay among markers of inflammation, such as fibrinogen and high CRP levels, and dementia. First, we performed a cross-sectional study comparing markers of inflammation between 99 patients affected by dementia (mean age: 83.0+/-0.6 years) and 99 controls (mean age: 83.9+/-0.7 years). Then, we analyzed the relationship between inflammation and dementia in the same population composed by 34 Alzheimer's disease (AD) patients (mean age: 83.4+/-0.8 years), 64 vascular dementia (VaD) patients (mean age: 82.7+0.8 years) and 99 controls. Patients affected by dementia had higher CRP levels than controls (2.6+/-+/-0.2 vs. 0.7 + 0.1 p < 0.001, respectively). AD patients had higher CRP levels than VaD patients (4.2 + 0.6 vs. 1.7+/-0.2, p < 0.001, respectively). Stepwise multiple logistic regression analysis showed that dementia (odds ratio=OR=4.965, 95% confidence interval=Cl=1.402-13.23, p=0.004), fibrinogen (OR=1.011, Cl=1.007-1.015, p<0.001), and age (OR=1.158, Cl=1.063-1.261, p<0.001) are independently correlated with high levels of CRP. The study suggests that inflammation may have a pathogenetic role in AD.

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

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

Neuronutrition and Alzheimer's disease

Alzheimer's disease (AD) is a complex neurological disorder resulting from both genetic and environmental factors with the latter being particularly important for the sporadic form of the disease. As such, diets rich in saturated fatty acids and alcohol, and deficient in antioxidants and vitamins appear to promote the onset of the disease, while diets rich in unsaturated fatty acids, vitamins, antioxidants, and wine likely suppress its onset. In addition, evidence suggests that diets rich in polyphenols and some spices suppress the onset of AD by scavenging free radicals and preventing oxidative damage. Metal ions are known to catalyze the production of free radicals and induce mental retardation or dementia, and several studies have also identified metals such as Pb, Fe, Al, Cu, and Zn in AD pathogenesis. While specific metal chelators have been tested for therapy, they have not been very successful, probably due to their late administration, i.e., after brain damage has been triggered. Since several dietary polyphenols are known to chelate metals, their routine use may also be protective against the onset of AD. In this review, we summarize beneficial dietary techniques in the fight against AD.

Statin use and the risk of Alzheimer's disease: the MIRAGE study

BACKGROUND

The aim of this study was to examine the association between statin use before the onset of Alzheimer's disease (AD) symptoms and risk of having AD, and to explore the potential impact of APOE genotype and race on this association.

METHODS

Data were collected through standardized, validated questionnaires from 895 subjects with probable or definite AD by research criteria, and 1,483 of their nondemented relatives in this family-based, case-control study of AD patients and their relatives enrolled at 15 research centers from 1996 through 2002. To minimize temporal and prescription biases, exposure to statin use within each family was ignored in the one year before the first appearance of AD symptoms in that family's affected member. Associations were estimated using generalized estimating equations for a logistic model, adjusting for age, sex, race, education, history of heart disease, stroke, diabetes, smoking and APOE genotype.

RESULTS

Statin use was associated with lowered odds of having AD (adjusted odds ratio [OR], 0.61; 95% confidence interval [CI], 0.38 to 0.98). Nonstatin cholesterol-lowering medications were not associated significantly with lowered odds of having had AD (adjusted OR, 1.7; 95% CI, 0.61 to 5.0).

CONCLUSIONS

Statin medications were associated with lowered risk of AD in this population. Neither African-American race, nor the presence of the APOE 4 allele modified the statin-AD association.

Will testing for apolipoprotein E assist in tailoring dementia risk reduction? A review

This paper aims to systematically review the influence of apolipoprotein E (ApoE) on the effects of potentially modifiable mid and late life risk factors for dementia. Scopus, Medline, PubMed, PsycINFO, and HuGE databases were searched up to November 2008. Two independent reviewers selected 94 articles from 13,122 results. Results suggest the deleterious effect of current smoking is limited only to persons without ApoE epsilon 4 (4 out of 4 studies), ApoE epsilon 4 increases the risk of dementia associated with greater fat consumption, particularly saturated fats (3 out of 4 studies), and increases the protective effect against dementia associated with HRT use (3 out of 5 with one of the non-significant studies suggesting a trend). There was evidence that ApoE does not modify the risk of dementia associated with measures of, and treatments for CVD, other dietary factors, and estradiol levels. There was inconsistent or contradictory evidence for other environmental factors reviewed. There is insufficient evidence for the recommendation of ApoE testing to assist with tailoring risk reduction recommendations for dementia.

ReviewGenetics, lifestyle and the roles of amyloid β and oxidative stress in Alzheimer’s disease

This paper reviews a wide range of recent studies that have linked AD-associated biochemical and physiological changes with oxidative stress and damage. Some of these changes include disruptions in metal ion homeostasis, mitochondrial damage, reduced glucose metabolism, decreased intracellular pH and inflammation. Although the changes mentioned above are associated with oxidative stress, in most cases, a cause and effect relationship is not clearcut, as many changes are interlinked. Increases in the levels of Abeta peptides, the main protein components of the cerebral amyloid deposits of AD, have been demonstrated to occur in inherited early-onset forms of AD, and as a result of certain environmental and genetic risk factors. Abeta peptides have been shown to exhibit superoxide dismutase activity, producing hydrogen peroxide which may be responsible for the neurotoxicity exhibited by this peptide in vitro. This review also discusses the biochemical aspects of oxidative stress, antioxidant defence mechanisms, and possible antioxidant therapeutic measures which may be effective in counteracting increased levels of oxidative stress. In conclusion, this review provides support for the theory that damage caused by free radicals and oxidative stress is a primary cause of the neurodegeneration seen in AD with Abeta postulated as an initiator of this process.

Statins for the prevention of dementia

BACKGROUND

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

OBJECTIVES

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

SEARCH STRATEGY

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

SELECTION CRITERIA

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

DATA COLLECTION AND ANALYSIS

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

MAIN RESULTS

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

AUTHORS' CONCLUSIONS

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

Age-related influence of the HDL receptor SR-BI on synaptic plasticity and cognition

Dysregulated cholesterol metabolism is a major risk factor for atherosclerosis and other late-onset disorders, such as Alzheimer's disease. The scavenger receptor, class B, type I (SR-BI) is critical in maintaining the homeostasis of cholesterol and alpha-tocopherol. SR-BI binds high-density lipoproteins (HDL) and mediates the selective transfer of cholesteryl esters and alpha-tocopherol from circulating HDL to cells. SR-BI is also involved in reverse cholesterol transport from peripheral tissues into the liver. Previous studies using SR-BI genetic knockout mice indicated that the deletion of SR-BI resulted in an accelerated onset of atherosclerosis. We hypothesized that SR-BI-dependent lipid dysregulation might disrupt brain function leading to cognitive impairment. Here, we report that very old SR-BI knockout mice show deficient synaptic plasticity (long-term potentiation) in the CA1 region of the hippocampus. Very old SR-BI KO mice also display selective impairments in recognition memory and spatial memory. Thus, SR-BI influences neural and cognitive processes, a finding that highlights the contribution of cholesterol and alpha-tocopherol homeostasis in proper cognitive function.

Novel N-terminal cleavage of APP precludes Abeta generation in ACAT-defective AC29 cells

A common pathogenic event that occurs in all forms of Alzheimer's disease is the progressive accumulation of amyloid beta-peptide (Abeta) in brain regions responsible for higher cognitive functions. Inhibition of acyl-coenzyme A: cholesterol acyltransferase (ACAT), which generates intracellular cholesteryl esters from free cholesterol and fatty acids, reduces the biogenesis of the Abeta from the amyloid precursor protein (APP). Here we have used AC29 cells, defective in ACAT activity, to show that ACAT activity steers APP either toward or away from a novel proteolytic pathway that replaces both alpha and the amyloidogenic beta cleavages of APP. This alternative pathway involves a novel cleavage of APP holoprotein at Glu281, which correlates with reduced ACAT activity and Abeta generation in AC29 cells. This sterol-dependent cleavage of APP occurs in the endosomal compartment after internalization of cell surface APP. The resulting novel C-terminal fragment APP-C470 is destined to proteasomal degradation limiting the availability of APP for the Abeta generating system. The proportion of APP molecules that are directed to the novel cleavage pathway is regulated by the ratio of free cholesterol and cholesteryl esters in cells. These results suggest that subcellular cholesterol distribution may be an important regulator of the cellular fate of APP holoprotein and that there may exist several competing proteolytic systems responsible for APP processing within the endosomal compartment.

Total serum cholesterol levels and Alzheimer's dementia in patients with Down syndrome

BACKGROUND

The risk for dementia in Alzheimer's disease (DAD) in adults with Down syndrome (DS) is higher than in the general adult population. Hypercholesterolaemia has been reported as a risk factor for DAD in the general population. This study investigated the role of serum cholesterol levels in the onset of DAD in the DS population.

METHODS

This study investigated total serum cholesterol levels in 179 DS persons (with and without DAD). The possible association between Apolipoprotein E and amyloid beta1-40 and beta1-42 levels was also investigated.

RESULTS

No statistically significant association was found between total serum cholesterol levels and dementia in AD or with amyloid beta levels. However for DS adults with an apoE epsilon4 allele significantly higher serum cholesterol levels were found.

CONCLUSION

Hypercholesterolaemia is not a risk factor for DAD in persons with DS. However, DS persons with an apoE epsilon4 allele are susceptible to high serum cholesterol. Such individuals should be screened on a regular basis.

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

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

Direct and potent regulation of gamma-secretase by its lipid microenvironment

gamma-Secretase is an unusual and ubiquitous aspartyl protease with an intramembrane catalytic site that cleaves many type-I integral membrane proteins, most notably APP and Notch. Several reports suggest that cleavage of APP to produce the Abeta peptide is regulated in part by lipids. As gamma-secretase is a multipass protein complex with 19 transmembrane domains, it is likely that the local lipid composition of the membrane can regulate gamma-activity. To determine the direct contribution of the lipid microenvironment to gamma-secretase activity, we purified the human protease from overexpressing mammalian cells, reconstituted it in vesicles of varying lipid composition, and examined the effects of individual phospholipids, sphingolipids, cholesterol, and complex lipid mixtures on substrate cleavage. A conventional gamma-activity assay was modified to include a detergent-removal step to facilitate proteoliposome formation, and this increased baseline activity over 2-fold. Proteoliposomes containing sphingolipids significantly increased gamma-secretase activity over a phosphatidylcholine-only baseline, whereas the addition of phosphatidylinositol significantly decreased activity. Addition of soluble cholesterol in the presence of phospholipids and sphingolipids robustly increased the cleavage of APP- and Notch-like substrates in a dose-dependent manner. Reconstitution of gamma-secretase in complex lipid mixtures revealed that a lipid raft-like composition supported the highest level of activity compared with other membrane compositions. Taken together, these results demonstrate that membrane lipid composition is a direct and potent modulator of gamma-secretase and that cholesterol, in particular, plays a major regulatory role.

Statins reduce neuronal alpha-synuclein aggregation in in vitro models of Parkinson's disease

Aggregation of alpha-synuclein (alpha-syn) is believed to play a critical role in the pathogenesis of disorders such as dementia with Lewy bodies and Parkinson's disease. The function of alpha-syn remains unclear, although several lines of evidence suggest that alpha-syn is involved in synaptic vesicle trafficking probably via lipid binding. Moreover, interactions with cholesterol and lipids have been shown to be involved in alpha-syn aggregation. In this context, the main objective of this study was to determine if statins--cholesterol synthesis inhibitors--might interfere with alpha-syn accumulation in cellular models. For this purpose, we studied the effects of lovastatin, simvastatin, and pravastatin on the accumulation of alpha-syn in a stably transfected neuronal cell line and in primary human neurons. Statins reduced the levels of alpha-syn accumulation in the detergent insoluble fraction of the transfected cells. This was accompanied by a redistribution of alpha-syn in caveolar fractions, a reduction in oxidized alpha-syn, and enhanced neurite outgrowth. In contrast, supplementation of the media with cholesterol increased alpha-syn aggregation in detergent insoluble fractions of transfected cells and was accompanied by reduced neurite outgrowth. Taken together, these results suggest that regulation of cholesterol levels with cholesterol inhibitors might be a novel approach for the treatment of Parkinson's disease.

Activity-regulated cytoskeleton-associated protein in rodent brain is down-regulated by high fat diet in vivo and by 27-hydroxycholesterol in vitro

Growing evidence strongly suggests that high fat diet (HFD) has an important role in some neurodegenerative disorders, including Alzheimer's disease (AD). To identify new cellular pathways linking hypercholesterolemia and neurodegeneration, we analyzed the effects of HFD on gene expression in mouse brain. Using cDNA microarrays and real time RT-PCR, we found that HFD has a mild, but significant effect on the expression of several genes. The altered genes include molecules linked to AD pathology and others of potential interest for neurodegeneration. We further investigated the effect of HFD on the activity-regulated cytoskeleton-associated protein (Arc). Expression of Arc was decreased in cerebral cortex and hippocampus of HFD-fed animals. From the known regulatory mechanisms of Arc expression, HFD reduced N-methyl-D-aspartate receptor (NMDAR) activity, as seen by decreases in tyrosine phosphorylation of NMDAR2A and levels of NMDAR1. Additionally, we demonstrated that 27-hydroxycholesterol, a cholesterol metabolite that enters the brain from the blood, decreases Arc levels as well as NMDAR and Src kinase activities in rat primary hippocampal neurons. Finally, we showed that Arc levels are decreased in the cortex of AD brains. We propose that one of the mechanisms, by which hypercholesterolemia contributes to neurodegenerative diseases, could be through Arc down-regulation caused by 27-hydroxycholesterol.

Implications of age and diet on canine cerebral cortex transcription

Mechanisms contributing to age-related cognitive decline are poorly defined. Thus, we used canine microarrays to compare gene expression profiles of brain tissue from geriatric and young adult dogs. Cerebral cortex samples were collected from six geriatric (12-year old) and six young adult (1-year old) female beagles after being fed one of two diets (animal protein-based versus plant-protein based) for 12 months. RNA samples were hybridized to Affymetrix GeneChip Canine Genome Arrays. Statistical analyses indicated that the age had the greatest impact on gene expression, with 963 transcripts differentially expressed in geriatric dogs. Although not as robust as age, diet affected mRNA abundance of 140 transcripts. As demonstrated in aged rodents and humans, geriatric dogs had increased expression of genes associated with inflammation, stress response, and calcium homeostasis and decreased expression of genes associated with neuropeptide signaling and synaptic transmission. In addition to its existing strengths, availability of gene sequence information and commercial microarrays make the canine a powerful model for studying the effects of aging on cognitive function.

Lifestyle-related factors in predementia and dementia syndromes

Cognitive decline and dementia have a deep impact on the health and quality of life of older subjects and their caregivers. Since the therapeutic options currently available have demonstrated limited efficacy, the search for preventive strategies for cognitive decline and dementia are mandatory. A possible role of lifestyle-related factors was recently proposed for age-related changes of cognitive function, predementia syndromes and the cognitive decline of degenerative (Alzheimer's disease [AD]) or vascular origin. At present, cumulative evidence suggests that vascular risk factors may be important in the development of mild cognitive impairment (MCI), dementia and AD. Moderate alcohol drinking has been proposed as a protective factor against MCI and dementia in several longitudinal studies, but contrasting findings also exist. The Mediterranean diet could therefore be an interesting model with which to further study the association between dietary patterns and cognitive functioning, given the suggested role of many components of this diet (monounsaturated fatty acids, polyunsaturated fatty acids, cereals and red wine) in contrasting cognitive impairment and dementia. The association between low education and predementia and dementia syndromes is supported by the majority of studies, but very few studies have investigated whether this association may be attributed with lifestyle factors that covary with education. Studies in the literature seem to identify in physical exercise one promising strategy in decreasing cognitive decline, but some of the limitations of these studies do not allow us to draw definite conclusions. At present, in older subjects, healthy diets, antioxidant supplements, the prevention of nutritional deficiencies, and moderate physical activity could be considered the first line of defense against the development and progression of predementia and dementia syndromes. However, in most cases, these were only observational studies, and results are awaited from large multicenter randomized clinical trials in older persons that may clarify the possible synergy, for example, between moderate exercise, physical activity and healthy Mediterranean diet on cognition in the elderly.

Geranylgeranyl pyrophosphate stimulates gamma-secretase to increase the generation of Abeta and APP-CTFgamma

Cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases results in generation of the amyloid-beta protein (Abeta), which is characteristically deposited in the brain of Alzheimer's disease patients. Inhibitors of 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase (the statins) reduce levels of cholesterol and isoprenoids such as geranylgeranyl pyrophosphate (GGPP). Previous studies have demonstrated that cholesterol increases and statins reduce Abeta levels mostly by regulating beta-secretase activity. In this study, we focused on the role of geranylgeranyl isoprenoids GGPP and geranylgeraniol (GGOH) in regulating Abeta production. Our data show that the inhibition of GGPP synthesis by statins plays an important role in statin-mediated reduction of Abeta secretion. Consistent with this finding, the geranylgeranyl isoprenoids preferentially increase the yield of Abeta of 42 residues (Abeta42) in a dose-dependent manner. Our studies further demonstrated that geranylgeranyl isoprenoids increase the yield of APP-CTFgamma (a.k.a. AICD) as well as Abeta by stimulating gamma-secretase-mediated cleavage of APP-CTFalpha and APP-CTFbeta in vitro. Furthermore, GGOH increases the levels of the active gamma-secretase complex in the detergent-insoluble membrane fraction along with its substrates, APP-CTFalpha and APP-CTFbeta. Our results indicate that geranylgeranyl isoprenoids may be an important physiological facilitator of gamma-secretase activity that can foster production of the pathologically important Abeta42.

Decreased nicotinic receptors and cognitive deficit in rats intracerebroventricularly injected with beta-amyloid peptide(1-42) and fed a high-cholesterol diet

To investigate whether the changes in nicotinic receptors (nAChRs) and in learning and memory associated with Alzheimer's disease (AD) are influenced by both beta-amyloid peptide (Abeta) and cholesterol in vivo, we examined the effects of intracerebroventricular injection of Abeta(1-42) and/or a high-cholesterol diet on brain levels of nAChRs and learning and memory in rats. The levels of nAChR subunit proteins and the corresponding mRNA were measured by Western blotting and RT-PCR, respectively; and learning and memory were evaluated with the Morris Water Maze examination. Injection of Abeta(1-42) resulted in deposition of this peptide, activation of astrocytes, decreased levels of the alpha7 and alpha4 protein subunits of the nAChR, and elevated expression of alpha7 mRNA, as well as impaired learning and spatial memory. A high-cholesterol diet activated astrocytes and, more importantly, potentiated the toxic effects of Abeta on nAChR subunit levels and on learning and memory. These findings may be highly relevant to the mechanisms underlying the cognitive deficits associated with AD.