Cholesterol is increased in the exofacial leaflet of synaptic plasma membranes of human apolipoprotein E4 knock-in mice

Inheritance of the apolipoprotein (apoE) epsilon4 allele is a risk factor for developing Alzheimer's disease (AD). The purpose of the present study was to determine effects of apoE-isoforms on the transbilayer distribution of cholesterol in synaptic plasma membranes (SPM) using mice expressing human apoE3 and apoE4. Total SPM cholesterol levels did not differ among the wild-type and apoE3 and apoE4 knock-in mice. However, a striking difference was observed in the transbilayer distribution of SPM cholesterol. ApoE4 knock-in mice showed an approximately 2-fold increase in exofacial leaflet cholesterol compared with apoE3 knock-in mice and wild-type mice. The results of this study suggest that pathogenic effects of apoE4 on AD development could be closely linked to alteration of cholesterol distribution in SPM.     

Role of cholesterol in amyloid cascade: cholesterol-dependent modulation of tau phosphorylation and mitochondrial function

Apolipoprotein E (apoE) alleles are important genetic risk factors for Alzheimer's disease (AD), with the ɛ 4 allele increasing and the ɛ 2 allele decreasing the risk of developing AD. ApoE is the major apolipoprotein that modulates cholesterol transport in the central nervous system, cholesterol being an essential component of membranes for maintaining their structure and functions. Epidemiological studies have suggested a link between serum cholesterol levels and AD development and the potential therapeutic effectiveness of statins for AD; and furthermore, biological studies have shown that amyloid β -protein (A β ) secretion is modulated by cellular cholesterol level. However, other lines of evidence show controversial results. In addition to the role of cholesterol in A β generation, different interactions of cholesterol with A β and its role in AD pathogenesis have been shown, i.e. A β affects cholesterol dynamics in neurons, and altered cholesterol metabolism in turn leads to neurodegeneration with abnormally phosphorylated tau (tauopathy). In this review, the reciprocal interactions between cholesterol and A β , and the role of cholesterol in tauopathy are discussed. The isoform-specific involvement of apoE in this cascade, in which high-density lipoprotein-like particles are generated and supplied to neurons to maintain cholesterol homeostasis, is also discussed.     

Effects of the ApoE epsilon4 allele on olfactory function in Down syndrome

The present study investigated the effects of the apolipoprotein E (ApoE) epsilon4 allele, a risk factor for Alzheimer's disease (AD), on olfactory function in Down syndrome (DS). Brain areas critical to olfactory processing, particularly the entorhinal cortex, show the earliest neuropathological changes in AD. Functionally, odor identification has been shown to be impaired in AD and in persons with the epsilon4 allele. DS is also a risk factor for AD. Thus, we hypothesized greater impairment in epsilon4 positive DS participants. Olfactory function was assessed with the San Diego Odor Identification Test in 34 participants with DS and 34 normal controls. Genomic DNA was prepared from blood samples to obtain ApoE status for the DS participants. Results indicate (1) that participants with DS had significant deficits in olfactory functioning; and (2) that among DS participants, those with an epsilon4 allele had poorer odor identification than those without an epsilon4 allele. The results support the hypothesis that individuals with DS who have an additional genetic risk factor for AD, the ApoE epsilon4 allele, exhibit greater deficits in odor identification. Areas of the brain involved in odor identification may be particularly affected in individuals with DS who carry the epsilon4 allele.     

The isoform-specific pathological effects of apoE4 in vivo are prevented by a fish oil (DHA) diet and are modified by cholesterol

Apolipoprotein E4 (apoE4) is the most prevalent genetic risk factor for Alzheimer's disease (AD). Epidemiological studies revealed that consumption of docosahexaenoic acid (DHA: 22 : 6 (ω3)), a major brain polyunsaturated fatty acid, is protective for AD and that elevated cholesterol levels are an AD risk factor. We presently investigated the extent to which the pathological effects of apoE4 in vivo can be prevented by consuming fish oil (DHA) or can be modified by cholesterol. Accordingly, apoE3- and apoE4-targeted replacement mice were subjected, following weaning, to a fish oil diet enriched in DHA and to a cholesterol-containing diet under regular and enriched environments. Cholesterol metabolism in the hippocampus and the corresponding phospholipid and fatty acid levels were affected by fish oil (DHA) and cholesterol diets and by environmental stimulation. Importantly, cholesterol metabolism and the fatty acid levels were not affected by apoE4. The phospholipid levels were, however, affected by apoE4. This effect was most pronounced in the cholesterol-fed mice and was abolished by the fish oil (DHA) diet. ApoE4 elevated hippocampal intraneuronal amyloid-β levels under regular conditions and lowered them following environmental stimulation, relative to those of the apoE3 mice. ApoE4 also elevated the levels of the presynaptic transporters Vglut and Vgat, and decreased behavioral performance in an object recognition test. Importantly, all of these apoE4 phenotypes were abolished by the fish oil (DHA) diet, whereas the cholesterol diet modified them. These findings suggest that a fish oil (DHA) diet could be used to attenuate the effects of apoE4 in AD.     

APOE epsilon 3/ epsilon 4 heterozygotes have an elevated proportion of apolipoprotein E4 in cerebrospinal fluid relative to plasma,independent of Alzheimer's disease diagnosis

Inheritance of the apolipoprotein E (APOE) epsilon 4 allele is associated with an increased risk of Alzheimer's disease (AD). However, the risk of AD in APOE epsilon 3/ epsilon 4 heterozygotes is variable. We tested the hypothesis that the risk of AD in APOE epsilon 3/ epsilon 4 heterozygotes was linked to the relative levels of expression of apoE4 versus apoE3 protein. We measured the apoE4 isoform and total apoE using two specific enzyme-linked immunosorbent assay (ELISA) kits in three cohorts of plasma samples and two cohorts of cerebrospinal fluid samples from AD, mild cognitive impairment, and control subjects. The apoE4 ELISAs were specific as they did not detect apoE in APOE epsilon 3/epsilon 3 homozygotes and were comparable to the total apoE ELISAs in APOE epsilon 4/ epsilon 4 homozygotes. In APOE epsilon 3/ epsilon 4 individuals, the ratio of apoE4 to total apoE levels was 30-40% in plasma, suggesting a decreased production or an increased metabolism of apoE4 compared to apoE3. Surprisingly, the ratio in the CSF was reversed, with apoE4 accounting for 60-70% of the total apoE. The proportion of apoE4 in these cases did not vary by diagnosis, age of onset, or duration of AD. We conclude that the proportion of apoE4 in plasma is not predictive of AD risk in APOE epsilon 3/epsilon 4 individuals. However, the greater proportion of apoE4 in the cerebrospinal fluid suggests differential production or metabolism of the protein in the central nervous system (CNS), with the apoE4 isoform dominating.     

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.     

Interaction of the H63D mutation in the hemochromatosis gene with the apolipoprotein E epsilon 4 allele modulates age at onset of Alzheimer's disease

The H63D mutation in the hemochromatosis gene (HFE) has recently been considered as a risk factor in Alzheimer's disease (AD) with advancing age at onset of the disease, independently of the apolipoprotein E (ApoE) epsilon4 allele effect. We examined the distribution of the H63D mutation and ApoE genotypes as a function of age at AD onset in 328 patients with sporadic AD. Our data show that the mutant H63D allele potentially interacts with the ApoE epsilon4 allele to significantly reduce age at onset of AD compared to ApoE epsilon4 carriers alone, but has no effect on age at onset in ApoE epsilon4 non-carriers.     

Apolipoprotein E and beta-amyloid levels in the hippocampus and frontal cortex of Alzheimer's disease subjects are disease-related and apolipoprotein E genotype dependent.

The epsilon4 allele of apolipoprotein E (apoE) is associated with increased risk for the development of Alzheimer's disease (AD), possibly due to interactions with the beta-amyloid (Abeta) protein. The mechanism by which these two proteins are linked to AD is still unclear. To further assess their potential relationship with the disease, we have determined levels of apoE and Abeta isoforms from three brain regions of neuropathologically confirmed AD and non-AD tissue. In two brain regions affected by AD neuropathology, the hippocampus and frontal cortex, apoE levels were found to be decreased while Abeta(1-40) levels were increased. Levels of apoE were unchanged in AD cerebellum. Furthermore, levels of apoE and Abeta(1-40) were found to be apoE genotype dependent, with lowest levels of apoE and highest levels of Abeta(1-40) occurring in epsilon4 allele carriers. These results suggest that reduction in apoE levels may give rise to increased deposition of amyloid peptides in AD brain.     

Apolipoprotein E in Taiwan Chinese patients with dementia

To clarify whether Alzheimer's disease (AD) and vascular dementia (VaD) share the same risk factors in Taiwan Chinese patients. Using the criteria of the NINCDS- ADRDA and NINDS-AIREN, 154 AD patients, 30 VaD patients, and 112 controls were enrolled. Their apolipoprotein E (ApoE) genes, extracted from peripheral blood leukocytes, were analyzed. The epsilon4 allele frequency was significantly higher in AD patients than in the control group. The odds ratio of carrying at least one copy of the epsilon4 allele in AD patients is 2.7 compared with control subjects. There was no significant difference between the VaD patients and the control subjects in their ApoE epsilon4 or epsilon2 allele frequency. The present study demonstrates a strong association between the ApoE epsilon4 allele and AD, but not between the ApoE epsilon4 allele and VaD. This suggests that AD and VaD do not share the same pathogenesis and deserve further investigation.     

Dietary omega 3 polyunsaturated fatty acids and Alzheimer's disease: interaction with apolipoprotein E genotype

Epidemiological studies suggest a protective role of omega-3 poly-unsaturated fatty acids (n-3 PUFA) against Alzheimer's disease (AD). However, most intervention studies of supplementation with n-3 PUFA have yielded disappointing results. One reason for such discordant results may result from inadequate targeting of individuals who might benefit from the supplementation, in particular because of their genetic susceptibility to AD. The ε4 allele of the apolipoprotein E gene (ApoE) is a genetic risk factor for late-onset AD. ApoE plays a key role in the transport of cholesterol and other lipids involved in brain composition and functioning. The action of n-3 PUFA on the aging brain might therefore differ according to ApoE polymorphism. The aim of this review is to examine the interaction between dietary fatty acids and ApoE genotype on the risk for AD. Carriers of the ε4 allele tend to be the most responsive to changes in dietary fat and cholesterol. Conversely, several epidemiological studies suggest a protective effect of long-chain n-3 PUFA on cognitive decline only in those who do not carry ε4 but with inconsistent results. An intervention study showed that only non-carriers had increased concentrations of long-chain n-3 PUFA in response to supplementation. The mechanisms underlying this gene-by-diet interaction on AD risk may involve impaired fatty acids and cholesterol transport, altered metabolism of n-3 PUFA, glucose or ketones, or modification of other risk factors of AD in ε4 carriers. Further research is needed to explain the differential effect of n-3 PUFA on AD according to ApoE genotype.     

Herpes simplex encephalitis: involvement of apolipoprotein E genotype

It was previously found that herpes simplex type 1 virus (HSV1) when present in the brain, is a risk factor for Alzheimer's disease in carriers of the type 4 allele of the gene for apolipoprotein E (apoE epsilon4), and apoE epsilon4 is a risk factor for herpes labialis. Whether a specific allele of the gene is involved in susceptibility to another disorder caused by HSV1-herpes simplex encephalitis (HSE)-has now been investigated. DNA was prepared from formalin-fixed, paraffin-embedded blocks of specimens from the brain or spleen of 14 United Kingdom patients with HSE, confirmed by necropsy, and from the CSF of seven United Kingdom clinical patients with HSV1 in their CSF detected by polymerase chain reaction (PCR). ApoE genotype of the DNA from blocks was determined by seminested PCR, and of the DNA from CSF by one step PCR, followed by restriction endonuclease digestion. The apoE allele frequencies were compared with values previously obtained for 238 normal people from the United Kingdom. The apoE epsilon2 allele frequency of the patients with HSE was 26%, significantly higher than the value of 7% for the normal subjects (OR=4.6, 95% confidence interval (95% CI) 2. 0-10.8). The apoE epsilon3 and epsilon4 allele frequencies did not differ significantly between the two groups. Thus, it seems that apoE epsilon2 is a risk factor for HSE.     

Apolipoprotein E epsilon4 allele frequency in elderly depressed patients with and without cerebrovascular disease

Late-onset depression (LOD) could be a very early manifestation of Alzheimer's disease (AD), although contradictory results have been reported. Cerebrovascular disease (CVD) may favor the development of LOD, and that the particular forms of vascular depression should be individualized. The Apolipoprotein E (ApoE) epsilon4 allele was shown to be a risk factor for AD. Its role in LOD is controversial, while it is still unknown in vascular depression. Our objective was to clarify the relationship between ApoE epsilon4 allele and LOD in patients with and without CVD. We examined the ApoE phenotypes in a sample of 311 subjects: 50 with vascular LOD, 24 with LOD without CVD, 115 with AD and 122 normal controls (NC). The study of the ApoE epsilon4 allele frequency showed significant differences between: AD group and the vascular LOD and NC groups; LOD group without CVD compared with NC group (p<0.05 to 0.001). The frequency of the epsilon4 allele in the LOD group without CVD did not differ significantly from the AD group, similarly the frequency of the epsilon4 allele in the vascular LOD group was not different from that in NC. The study suggests an association between the ApoE epsilon4 allele and the LOD without CVD. These patients could be at risk of developing AD by an epsilon4-dependent pathway. In contrast, the results show no association between the presence of ApoE epsilon4 allele and vascular depression and provide further evidence in support of the concept that ApoE epsilon4 allele is not associated with clinical CVD.     

Inheritance of the ApoE epsilon4 allele increases the rate of brain atrophy in dementia patients.

We investigated the influence of the apolipoprotein (ApoE) epsilon4 allele on the rate of brain atrophy in patients with clinical dementia and in subjects at risk for dementia. Eighty-one subjects, consecutively referred to a memory clinic due to symptoms of dementia, went through a comprehensive examination, including cerebral magnetic resonance imaging. After an initial investigation these subjects were divided into one of six diagnostic groups; Alzheimer's disease (AD, n = 23), objective cognitive impairment (OCI, n = 27), subjective cognitive impairment (SCI, n = 17), vascular dementia (VaD), frontotemporal dementia (FTD) and unspecified dementia (USD). The last three groups were joined into one diagnostic group designated 'other dementia' (OD, altogether n = 14). In order to study the progression of cognitive impairment as well as the rate of atrophy in different brain regions all subjects were reinvestigated after an average period of 16 months. Interest was focused on investigating if those subjects with one or two epsilon4 alleles differed in either dementia progression or rate of brain atrophy compared to those without the epsilon4 allele. We found that the ApoE epsilon4 carriers had a statistically significantly larger increase in ventricular volume as compared with the ApoE epsilon4 noncarriers. In all diagnostic groups the ApoE epsilon4 carriers showed a greater rate of ventricular volume increase, as compared to the noncarriers. However, this difference was statistically significant only for the OD subjects. No statistical significant changes over time were seen for whole brain volume or volume of the temporal lobes and the medial temporal lobes. The diagnostic groups differed in dementia progression with the AD subjects having the most pronounced reduction in MMSE scores as compared to subjects at risk for AD (OCI and SCI subjects). The presence of ApoE epsilon4 allele did not influence the change in MMSE in any of the diagnostic groups.     

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.     

Apolipoprotein E and Alzheimer disease: an update on genetic and functional analyses

Exceptional advances have been made in understanding the genetics of how common polymorphisms of the apolipoprotein E gene influence the risk and age of onset of Alzheimer disease (AD). The major genetic susceptibility locus for the common forms of AD, there are 3 common alleles, designated epsilon2, epsilon3, and epsilon4. The inheritance of each dose of APOE4 increases the risk of disease and decreases the age of onset; conversely, the APOE2 allele appears to be protective, by lowering the risk of disease and increasing the age of onset. Testing for the APOE4 allele can be a clinically useful tool in the early diagnosis of cognitively impaired patients suspected of having AD. The APOE4 allele also negatively influences functional recovery following a variety of brain insults. What remains in the study of apolipoprotein E is an explanation of how minor changes in a protein can produce such striking differences in risk and age of onset. In vitro and animal model studies strongly suggest that brain apolipoprotein E is a multifunctional molecule, with potential roles in amyloid deposition and clearance, microtubule stability, intracellular signaling, immune modulation, glucose metabolism, oxidative stress, and other cellular processes. While the relevance of these proposed functions to the etiology of AD remains a mystery, these and other hypotheses will be tested as the field of apoE neurobiology grows, adding relevant new data to the functions of apoE in health and in the pathogenic mechanisms leading to AD.     

Apolipoprotein E4, cholinergic integrity and the pharmacogenetics of Alzheimer's disease

Recent evidence indicates that apolipoprotein E (apoE) plays a central role in the brain's response to injury. The coordinated expression of apoE and its receptors (the so-called LDL [low density lipoprotein] receptor family) appears to regulate the transport and internalization of cholesterol and phospholipids during the early phase of the re-innervation process in the adult brain. During dendritic remodelling and synaptogenesis, neurons progressively repress the synthesis of cholesterol in favour of cholesterol internalization through the apoE/LDL receptor pathway. The discovery a few years ago, that the apolipoprotein epsilon 4 allele found in 15% of the normal population is strongly linked to both sporadic and familial late-onset Alzheimer's disease (AD), raises the possibility that a dysfunction of the lipid transport system associated with compensatory sprouting and synaptic remodelling could be central to the AD process. The role of apoE in the central nervous system is particularly important in relation to the cholinergic system, which relies to a certain extent on the integrity of phospholipid homeostasis in neurons. Recent evidence obtained by 4 independent research teams indicates that apo epsilon 4 allele directly affects cholinergic activity in the brain of AD subjects. It was also shown to modulate the drug efficacy profile of several cholinomimetic and noncholinomimetic drugs used for the treatment of AD patients.