Uwe Beffert

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 Alzheimers 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.

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 Alzheimers 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 Alzheimers disease, from clinical studies to biochemical models, which may explain some of the complex neurobiology of this disorder.

Apolipoprotein E and β-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 Alzheimers 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.

β-Amyloid Peptides Increase the Binding and Internalization of Apolipoprotein E to Hippocampal Neurons

Abstract: The frequency of the ε4 allele of apolipoprotein E(apoE) is increased in late‐onset and sporadic forms of Alzheimers disease (AD). ApoE also binds to β‐amyloid (Aβ) and both proteins are found in AD plaques. To further investigate the potential interaction of apoE and Aβ in the pathogenesis of AD, we have determined the binding, internalization, and degradation of human apoE isoforms in the presence and absence of Aβ peptides to rat primary hippocampal neurons. We demonstrate that the lipophilic Aβ peptides, in particular Aβ1–42, Aβ1–40, and Aβ25–35, increase significantly apoE‐liposome binding to hippocampal neurons. For each Aβ peptide, the increase was significantly greater for the apoE4 isoform than for the apoE3 isoform. The most effective of the Aβ peptides to increase apoE binding, Aβ25–35, was further shown to increase significantly the internalization of both apoE3‐ and apoE4‐liposomes, without affecting apoE degradation. Conversely, Aβ1–40 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 Aβ peptides interact directly with apoE lipoproteins, which may then be transported together into neuronal cells through apoE receptors.

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

Both apolipoprotein E (apoE) and amyloid peptides are associated with Alzheimers 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 Alzheimers 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.

Putative Links Between Some of the Key Pathological Features of the Alzheimer's Brain

Alzheimers disease (AD) is characterized by the progressive impairment of memory and language abilities. Among the various neuropathological features of the AD brain possibly leading to these deficits, much research has focused on the role of senile plaques enriched in β-amyloid (Aβ) deposits and neurofibrillary tangles composed of phosphorylated tau proteins. Various groups have shown that very low concentrations of Aβ peptides can directly regulate cholinergic functions. It has been reported that rather high concentrations of Aβ decrease glucose uptake associated with neurotoxicity in hippocampal and cortical neurons. Aβ may limit the availability of acetyl CoA by reducing glucose uptake. As the cholinergic neuron is dependent, at least in part, on the same enzyme and upstream substrate (glucose) for neurotransmission and cellular metabolism, the impaired metabolism-glucose uptake reported in AD could render affected cholinergic neurons functionally compromised. It is suggested that Aβ peptides can rather specifically bind to neurons through the receptor for advanced glycosylation end products.

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 Alzheimers 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 Alzheimers 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.

Apolipoprotein E Uptake is Increased by Beta-Amyloid Peptides and Reduced by Blockade of the LDL Receptor

Apolipoprotein E (apoE) is a lipid-binding, 37 kDa, 299 amino acid glycoprotein involved in cholesterol and phospholipid transport and metabolism. ApoE mediates the uptake of lipid complexes through interaction with the apoB/ApoE (LDL) receptor and other receptors1. The LDL receptor pathway consists of cell surface binding of apoE-containing lipoproteins followed by internalization and degradation of the lipoprotein by a lysosomal pathway2. In the peripheral nervous system, apoE has been proposed to be involved in the transport of cholesterol in repair, growth and maintenance of membranes during development or following injury3,4. In the central nervous system, ApoE released from astrocytes in response to injury such as entorhinal cortex lesions in the rat plays a pivotal role in the redistribution of cholesterol and phospholipids during synaptic remodeling and compensatory synaptogenesis5,6.