Apolipoprotein C-I expression in the brain in Alzheimer's disease.

The H2 allele of apolipoprotein (apo) C-I is associated with Alzheimer's disease (AD). However, this association is potentially confounded by the linkage disequilibrium of H2 with the epsilon2 and epsilon4 alleles of apoE and of H1 with the epsilon3 allele. To establish plausibility for a direct role for apoC-I in AD, we compared apoC-I and apoE protein and mRNA levels in postmortem specimens of frontal cortex and hippocampus from AD patients with levels in nondemented controls. In H2-allelic individuals (usually also epsilon4 carriers), apoC-I mRNA levels were strikingly lower with AD (by 65%, P < 0.05), but apoC-I protein levels in AD were significantly higher (by 34%, P < 0.05). The opposite direction of the apoC-I mRNA and apoC-I protein level changes in AD in the epsilon4/H2 genotype may reflect decreased clearance of CNS lipoproteins associated with apoE4. In H1/H1 (usually epsilon3/epsilon3) individuals, both apoC-I protein and mRNA were lower in AD. ApoC-I protein levels in hippocampus were nearly twice those in frontal cortex. Immunohistochemistry of hippocampus revealed colocalization of apoC-I protein with the astrocytic marker GFAP. In addition, cultured human astrocytes expressed the mRNA for apoC-I. This study confirms apoC-I expression in the CNS and identifies astrocytes as the source of apoC-I. In addition, it has revealed differences in apoC-I expression based on site, genotype, and disease status that may reflect a role for apoC-I in the pathogenesis of AD.

Oxidative insults are associated with apolipoprotein E genotype in Alzheimer's disease brain.

The epsilon4 allele of the apolipoprotein E gene (APOE) is associated with sporadic and familial late-onset Alzheimer's disease (AD). Oxidative stress is believed to play an important role in neuronal dysfunction and cell death in AD. We now provide evidence that in the hippocampus of AD, the level of thiobarbituric acid-reactive substances (TBARS) and the APOE genotype are linked. Within AD cases, the levels of TBARS were found to be higher among epsilon4 carriers while the apoE protein concentrations were lower. The relationship between the levels of TBARS and apoE proteins was corroborated by the results from the APOE-deficient mice, in which the levels of TBARS were higher than those in wild-type mice. Among AD cases, tissues from patients with the epsilon4 allele of APOE displayed lower activities of catalase and glutathione peroxidase and lower concentration of glutathione than tissues from patients homozygous for the epsilon3 allele of APOE. Together these data demonstrate that, in AD, the epsilon4 allele of APOE is associated with higher oxidative insults.

Apolipoprotein E receptors: linking brain development and Alzheimer's disease.

Alzheimer's disease is a debilitating neurodegenerative disorder that afflicts an increasing part of our ageing population. An isoform of apolipoprotein E, a protein that mediates the transport of lipids and cholesterol in the circulatory system, predisposes carriers of this allele to the common late-onset form of the disease. How this protein is related to a neurodegenerative disorder is an enigma. Mounting evidence indicates that apolipoprotein E receptors, which are abundantly expressed in most neurons in the central nervous system, also fulfill critical functions during brain development and may profoundly influence the pathogenesis of Alzheimer's disease.

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.

Oxidative damage and protection by antioxidants in the frontal cortex of Alzheimer's disease is related to the apolipoprotein E genotype.

A great number of epidemiological studies have demonstrated that the frequency of the epsilon4 allele of the apolipoprotein E gene (APOE) is markedly higher in sporadic and in familial late onset Alzheimer disease (AD). In the frontal cortex of AD patients, oxidative damage is elevated. We address the hypothesis that the APOE genotype and reactive oxygen-mediated damage are linked in the frontal cortex of AD patients. We have related the APOE genotype to the levels of lipid oxidation (LPO) and to the antioxidant status, in frontal cortex tissues from age-matched control and AD cases with different APOE genotypes. LPO levels were significantly elevated in tissues from Alzheimer's cases which are homozygous for the epsilon4 allele of APOE, compared to AD epsilon3/epsilon3 cases and controls. Activities of enzymatic antioxidants, such as catalase and glutathione peroxidase (GSH-PX), were also higher in AD cases with at least one epsilon4 allele of APOE, while superoxide dismutase (SOD) activity was unchanged. In the frontal cortex, the concentration of apoE protein was not different between controls and AD cases, and was genotype independent. The Ginkgo biloba extract (EGb 761), the neurosteroid dehydroepiandrosterone (DHEA) and human recombinant apoE3 (hapoE3rec) were able to protect control, AD epsilon3/epsilon3 and epsilon3/epsilon4 cases against hydrogen peroxide/iron-induced LPO, while hapoE4rec was completely ineffective. Moreover, EGb 761 and DHEA had no effect in homozygous epsilon4 cases. These results demonstrate that oxidative stress-induced injury and protection by antioxidants in the frontal cortex of AD cases are related to the APOE genotype.

Apolipoprotein E isoform-specific reduction of extracellular amyloid in neuronal cultures.

Both apolipoprotein E (apoE) and amyloid peptides are associated with Alzheimer's disease (AD). Using primary hippocampal neurons, we demonstrate that apoE is capable of reducing potentially toxic extracellular amyloid peptides, likely through a receptor mediated mechanism. We hypothesize that isoform-specific differences in apoE-mediated amyloid clearance and intracellular accumulation may be responsible, at least in part, for the increased number of amyloid plaques observed in apoE epsilon4 allele AD individuals.

The polymorphism in exon 3 of the low density lipoprotein receptor-related protein gene is weakly associated with Alzheimer's disease.

The low density lipoprotein receptor-related protein (LRP) gene is a candidate gene for Alzheimer's disease (AD) due to its role as a receptor for apolipoprotein E (apoE), a major genetic risk factor for late-onset familial and sporadic AD. Recently, several studies have reported a correlation between a polymorphism (C766T) in exon 3 of LRP and AD. We examined this polymorphism in a Caucasian population of 225 neuropathologically confirmed cases with AD and 187 elderly cases without any AD neuropathological changes. We found that the exon 3 LRP C/C genotype was slightly but not significantly higher in the AD group when compared to the control group. A meta-analysis of previous studies revealed only a weak correlation of this polymorphism with AD (odds ratio 1.34, [95% CI 1.16-1.54], P < 0.0001). These data indicate that the polymorphism in exon 3 of LRP is only a minor risk factor for AD and that another locus on chromosome 12 is likely responsible for the associations observed in other studies.

Brain lipoprotein metabolism and its relation to neurodegenerative disease.

Lipoproteins are macromolecular complexes composed of lipids and proteins. The role of these complexes is to provide cells of the organism with lipids to be used as a source of energy, building blocks for biomembrane synthesis, and lipophilic molecules (e.g., steroid hormones and vitamin E) for other physiological purposes, such as cell signaling and antioxidative mechanisms. Lipoproteins also promote the cellular efflux of cholesterol for its disposal into bile. Thus, lipoproteins play an important role in the maintenance of lipid homeostasis throughout the organism. Accordingly, lipoprotein particles have been found circulating in blood, lymph, and interstitial fluid. Despite the existence of the blood-brain barrier, lipoprotein particles have been shown to be also present in the cerebrospinal fluid (CSF). Although a portion of their protein components may filter through the barrier from the vascular compartment, experimental evidence indicates that these particles originate from the nervous tissue. The other protein components include apolipoproteins E, J, and D, and these have been shown to be synthesized by cells within the central nervous system (CNS). Furthermore, it was shown that lipoprotein particles can be isolated from the conditioned medium of astrocytic cultures. The differences in size, structure, and composition of in vitro assembled particles compared with those isolated from the CSF suggest that the particles are modified following their secretion in vivo. This is supported by observations that lipoprotein-modifying enzymes and transfer proteins are also present within CNS tissue and CSF. The fate of CSF lipoproteins is unclear but is probably related to the turnover and clearance of lipids from the CNS or, alternatively, the particles may be recaptured and recycled back into the CNS tissue. The presence of several cell surface receptors for apoE-containing lipoproteins on ependymal cells, as well as on neurons and glial cells, supports this notion and suggests that the isolated brain possesses its own system to maintain local lipid homeostasis. This is further exemplified by the salvage and recycling of lipids shown to occur following a lesion in order to allow surviving neurons to sprout and reestablish lost synapses. Not much is currently known about lipoprotein metabolism in neurodegenerative diseases, but lipid alterations have been repeatedly reported in Alzheimer brains in which neuronal loss and deafferentation are major features. Although the mechanism underlying the link between the epsilon4 allele of the apolipoprotein E gene and Alzheimer's disease is presently unclear, it may well be postulated that it is related to disturbances in brain lipoprotein metabolism.

ApoE associated with lipid has a reduced capacity to inhibit beta-amyloid fibril formation.

Apolipoprotein E (apoE) and the beta-amyloid peptide are both found in the senile plaques associated with Alzheimer's disease. Several studies have recently determined that apoE can prevent beta-amyloid fibril (Abetaf) formation in vitro. In vivo, apoE is normally associated with a lipid source. We show that both recombinant and purified plasma apoE inhibit Abetaf formation in a dose-dependent manner. The three major apoE isoforms were equipotent at inhibiting Abetaf formation. ApoE associated with either cholesterol-containing liposomes or very low density lipoproteins displayed less potent inhibition of Abetaf formation than their unlipidated counterparts. These results indicate that the presence of associated lipids reduce the ability of apoE to inhibit the formation of Abetaf in vitro.

The neurobiology of apolipoproteins and their receptors in the CNS and Alzheimer's disease.

The importance of apolipoproteins in the central nervous system became increasingly clear with the association in 1993 of the epsilon4 allele of apolipoprotein E with familial and sporadic late-onset Alzheimer's disease. Apolipoprotein E is a ligand for several receptors, most of which are found to some extent in the brain. This review summarizes the various apolipoproteins and lipoprotein receptors found in the brain. A growing body of evidence now implicates irregular lipoprotein metabolism in several neurodegenerative disorders. We then focus on research linking apolipoprotein E and Alzheimer's disease, from clinical studies to biochemical models, which may explain some of the complex neurobiology of this disorder.

Beta-amyloid peptides increase the binding and internalization of apolipoprotein E to hippocampal neurons.

The frequency of the epsilon4 allele of apolipoprotein E (apoE) is increased in late-onset and sporadic forms of Alzheimer's disease (AD). ApoE also binds to beta-amyloid (A beta) and both proteins are found in AD plaques. To further investigate the potential interaction of apoE and A beta in the pathogenesis of AD, we have determined the binding, internalization, and degradation of human apoE isoforms in the presence and absence of A beta peptides to rat primary hippocampal neurons. We demonstrate that the lipophilic A beta peptides, in particular A beta(1-42), A beta(1-40), and A beta(25-35), increase significantly apoE-liposome binding to hippocampal neurons. For each A beta peptide, the increase was significantly greater for the apoE4 isoform than for the apoE3 isoform. The most effective of the A beta peptides to increase apoE binding, A beta(25-35), was further shown to increase significantly the internalization of both apoE3- and apoE4-liposomes, without affecting apoE degradation. Conversely, A beta(1-40) uptake by hippocampal neurons was shown to be increased in the presence of apoE-liposomes, more so in the presence of the apoE4 than the apoE3 isoform. These results provide evidence that A beta peptides interact directly with apoE lipoproteins, which may then be transported together into neuronal cells through apoE receptors.

Apolipoprotein E, plaques, tangles and cholinergic dysfunction in Alzheimer's disease.

Apolipoprotein E is a plasma cholesterol and phospholipid transporter which plays a central role in lipoprotein metabolism in the brain. Apolipoprotein E is a polymorphic protein with three common alleles in the general population, designated epsilon 2, epsilon 3 and epsilon 4 coding for proteins ApoE2, ApoE3 and ApoE4, respectively. Recent findings have demonstrated a significant relationship between the epsilon 4 allele and late onset familial and sporadic Alzheimer's disease. We examined several classical neuropathological hallmarks of Alzheimer's disease to determine whether they might be related to apolipoprotein E genotype: the presence of intracellular neurofibrillary tangles, extracellular senile plaques, and the attenuation of choline acetyltransferase activity. Significant correlations were found between epsilon 4 allele copy number and senile plaque density in the frontal, parietal and fusiform cortical areas. Similarly, significant correlations were also found with increased neurofibrillary tangle number in the frontal and fusiform cortex. Interestingly, there was an inverse correlation between the epsilon 4 allele with temporal cortical choline acetyltransferase activity. To further define the specific function of ApoE4, cultured rat hippocampal neurons were used to investigate interactions involving beta-amyloid protein. In this model, ApoE4 (but not ApoE2) was able to reverse the neuroprotective effects of beta-amyloid. ApoE3 was demonstrated to increase the internalization of beta-amyloid peptide into these neurons. Taken together, these results support the involvement of ApoE4 in the pathogenesis of Alzheimer's disease and also provide some explanations for the possible function of this protein.

Increased levels of statin, a marker of cell cycle arrest, in response to hippocampal neuronal injury.

Injured neurons in the CNS are known to synthesize high levels of proliferation related oncogene products and heat shock proteins without dividing. Statin is a cell cycle regulated nuclear phosphoprotein, selectively associated with the non-proliferative state in a wide variety of cell types. In the present study, neuronal statin was examined following lethal or sublethal neuronal injuries in the hippocampus of Alzheimer's disease patients, in rats receiving kainate lesions to the dorsal hippocampus and in entorhinal cortex lesioned rats. Immunolabelling of nuclear statin showed that statin immunoreactivity increased preferentially in CA1 pyramidal neurons of the hippocampus in Alzheimer's disease. In kainate lesioned rats, statin immunoreactivity was markedly induced in the CA3 hippocampal region in association with neuronal loss. Entorhinal cortex lesioned rats showed a transient induction of statin between 2 and 6 days post lesion in CA1 neurons. However, cell counts in entorhinal cortex lesioned rats remained unaltered in the CA1 and granule cell layers during the entire 30 day time course, indicating that increased statin levels are not secondary to neuronal degeneration and are not necessarily accompanied by irreversible neuronal death. It is concluded that, in addition to proliferation related gene products, neuronal injury induces an increase in levels of statin, a nuclear marker of cell cycle arrest. Furthermore, statin may be a potentially useful marker of injurious neuronal stress, even under conditions that do not necessarily lead to irreversible cell death.