Neurons of the human frontal cortex display apolipoprotein E immunoreactivity: implications for Alzheimer's disease

Relationships of brain cholesterol and cholesterol biosynthetic enzymes to Alzheimer's pathology and dementia in the CFAS population-derived neuropathology cohort

Altered cholesterol metabolism is implicated in brain ageing and Alzheimer's disease. We examined whether key genes regulating cholesterol metabolism and levels of brain cholesterol are altered in dementia and Alzheimer's disease neuropathological change (ADNC). Temporal cortex (n = 99) was obtained from the Cognitive Function and Ageing Study. Expression of the cholesterol biosynthesis rate-limiting enzyme HMG-CoA reductase (HMGCR) and its regulator, SREBP2, were detected using immunohistochemistry. Expression of HMGCR, SREBP2, CYP46A1 and ABCA1 were quantified by qPCR in samples enriched for astrocyte and neuronal RNA following laser-capture microdissection. Total cortical cholesterol was measured using the Amplex Red assay. HMGCR and SREBP2 proteins were predominantly expressed in pyramidal neurones, and in glia. Neuronal HMGCR did not vary with ADNC, oxidative stress, neuroinflammation or dementia status. Expression of HMGCR neuronal mRNA decreased with ADNC (p = 0.022) and increased with neuronal DNA damage (p = 0.049), whilst SREBP2 increased with ADNC (p = 0.005). High or moderate tertiles for cholesterol levels were associated with increased dementia risk (OR 1.44, 1.58). APOE ε4 allele was not associated with cortical cholesterol levels. ADNC is associated with gene expression changes that may impair cholesterol biosynthesis in neurones but not astrocytes, whilst levels of cortical cholesterol show a weak relationship to dementia status.

Apolipoprotein E in lipid metabolism and neurodegenerative disease

Dysregulation of lipid metabolism has emerged as a central component of many neurodegenerative diseases. Variants of the lipid transport protein, apolipoprotein E (APOE), modulate risk and resilience in several neurodegenerative diseases including late-onset Alzheimer's disease (LOAD). Allelic variants of the gene, APOE, alter the lipid metabolism of cells and tissues and have been broadly associated with several other cellular and systemic phenotypes. Targeting APOE-associated metabolic pathways may offer opportunities to alter disease-related phenotypes and consequently, attenuate disease risk and impart resilience to multiple neurodegenerative diseases. We review the molecular, cellular, and tissue-level alterations to lipid metabolism that arise from different APOE isoforms. These changes in lipid metabolism could help to elucidate disease mechanisms and tune neurodegenerative disease risk and resilience.

Differential and substrate-specific inhibition of γ-secretase by the C-terminal region of ApoE2, ApoE3, and ApoE4

Aberrant low γ-secretase activity is associated with most of the presenilin mutations that underlie familial Alzheimer's disease (fAD). However, the role of γ-secretase in the more prevalent sporadic AD (sAD) remains unaddressed. Here, we report that human apolipoprotein E (ApoE), the most important genetic risk factor of sAD, interacts with γ-secretase and inhibits it with substrate specificity in cell-autonomous manners through its conserved C-terminal region (CT). This ApoE CT-mediated inhibitory activity is differentially compromised in different ApoE isoforms, resulting in an ApoE2 > ApoE3 > ApoE4 potency rank order inversely correlating to their associated AD risk. Interestingly, in an AD mouse model, neuronal ApoE CT migrates to amyloid plaques in the subiculum from other regions and alleviates the plaque burden. Together, our data reveal a hidden role of ApoE as a γ-secretase inhibitor with substrate specificity and suggest that this precision γ-inhibition by ApoE may protect against the risk of sAD.

Expression and secretion of apoE isoforms in astrocytes and microglia during inflammation

Neuroinflammation is a common feature in neurodegenerative diseases, modulated by the Alzheimer's disease risk factor, apolipoprotein E (APOE). In the brain, apoE protein is synthesized by astrocytes and microglia. We examined primary cultures of astrocytes and microglia from human APOE (E2, E3, and E4) targeted-replacement mice. Astrocytes secreted two species of apoE, whereas cellular apoE consisted of only one. Both forms of secreted astrocytic apoE were bound during glycoprotein isolation, and enzymatic removal of glycans produced a convergence of the two forms of apoE to a single form; thus, the two species of astrocyte-secreted apoE are differentially glycosylated. Microglia released only a single species of apoE, while cellular apoE consisted of two forms; the secreted apoE and one of the two forms of cellular apoE were glycosylated. We treated the primary glia with either endogenous (TNFα) or exogenous (LPS) pro-inflammatory stimuli. While LPS had no effect on astrocytic apoE, APOE2, and APOE3 microglia increased release of apoE; APOE4 microglia showed no effect. APOE4 microglia showed higher baseline secretion of TNFα compared to APOE2 and APOE3 microglia. TNFα treatment reduced the secretion and cellular expression of apoE only in APOE4 astrocytes. The patterns of apoE species produced by astrocytes and microglia were not affected by inflammation. No changes in APOE mRNA were observed in astrocytes after both treatments. Together, our data demonstrate that astrocytes and microglia differentially express and secrete glycosylated forms of apoE and that APOE4 astrocytes and microglia are deficient in immunomodulation compared to APOE2 and APOE3.

ApoE Lipidation as a Therapeutic Target in Alzheimer's Disease

Apolipoprotein E (APOE) is the major cholesterol carrier in the brain, affecting various normal cellular processes including neuronal growth, repair and remodeling of membranes, synaptogenesis, clearance and degradation of amyloid β (Aβ) and neuroinflammation. In humans, the APOE gene has three common allelic variants, termed E2, E3, and E4. APOE4 is considered the strongest genetic risk factor for Alzheimer’s disease (AD), whereas APOE2 is neuroprotective. To perform its normal functions, apoE must be secreted and properly lipidated, a process influenced by the structural differences associated with apoE isoforms. Here we highlight the importance of lipidated apoE as well as the APOE-lipidation targeted therapeutic approaches that have the potential to correct or prevent neurodegeneration. Many of these approaches have been validated using diverse cellular and animal models. Overall, there is great potential to improve the lipidated state of apoE with the goal of ameliorating APOE-associated central nervous system impairments.

Common aspects between glaucoma and brain neurodegeneration

Neurodegeneration can be defined as progressive cell damage to nervous system cells, and more specifically to neurons, which involves morphologic alterations and progressive loss of function until cell death. Glaucoma exhibits many aspects of neurodegenerative disease. This review examines the pathogenesis of glaucoma, comparing it with that of Alzheimer's disease (AD) and Parkinson's disease (PD), highlighting their common features. Indeed, in all three diseases there are not only the same types of pathogenic events, but also similarities of temporal cadences that determine neuronal damage. All three age-related illnesses have oxidative damage and mitochondrial dysfunction as the first pathogenic steps. The consequence of these alterations is the death of visual neurons in glaucoma, cognitive neurons in AD and regulatory motor neurons (substantia nigra) in PD. The study of these common pathogenic events (oxidative stress, mitochondrial dysfunction, protein degradation, apoptosis and autophagy) leads us to consider common therapeutic strategies for the treatment and prevention of these diseases. Also, examination of the genetic aspects of the pathways involved in neurodegenerative processes plays a key role in shedding light on the details of pathogenesis and can suggest new treatments. This review discusses the common molecular aspects involved in these three oxidative-stress and age-related diseases.

Apo E4 Alleles and Impaired Olfaction as Predictors of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia that affects more than 5 million Americans. It is the only disease among the 10 causes of death that cannot be slowed or cured, thus raising the need for identification of early preclinical markers that could be the focus of preventative efforts. Although evidence is escalating that abnormalities in olfactory structure and function precede AD development and early cognitive impairments by one or more decades, the importance of olfaction is largely overlooked in AD, and such testing is not routinely performed in neurology clinics. Nevertheless, research using the olfactory model, has begun to advance our understanding of the preclinical pathophysiology of AD. Notably, an interesting series of studies is beginning to illuminate the relationship between Apolipoprotein E (ApoE) ε4 polymorphism and olfactory dysfunction and late-onset Alzheimer's disease. In this article, we reviewed present research on the significance of ApoE and olfaction to AD, summarized current studies on the associations and mechanisms of ApoE and olfactory dysfunction, and highlighted important gaps for future work to further advance the translational application of the olfactory paradigm to early, preclinical diagnosis and treatment of AD.

Biological Basis for Amyloidogenesis in Alzheimer's Disease

Certain cellular proteins normally soluble in the living organism under certain conditions form aggregates with a specific cross-β sheet structure called amyloid. These intra- or extracellular insoluble aggregates (fibers or plaques) are hallmarks of many neurodegenerative pathologies including Alzheimer's disease (AD), Huntington's disease, Parkinson's disease, prion disease, and other progressive neurological diseases that develop in the aging human central nervous system. Amyloid diseases (amyloidoses) are widespread in the elderly human population, a rapidly expanding demographic in many global populations. Increasing age is the most significant risk factor for neurodegenerative diseases associated with amyloid plaques. To date, nearly three dozen different misfolded proteins targeting brain and other organs have been identified in amyloid diseases and AD, the most prevalent neurodegenerative amyloid disease affecting over 15 million people worldwide. Here we (i) highlight the latest data on mechanisms of amyloid formation and further discuss a hypothesis on the amyloid cascade as a primary mechanism of AD pathogenesis and (ii) review the evolutionary aspects of amyloidosis, which allow new insight on human-specific mechanisms of dementia development.

Cerebrospinal Fluid Particles in Alzheimer Disease and Parkinson Disease

Human cerebrospinal fluid (CSF) contains diverse lipid particles, including lipoproteins that are distinct from their plasma counterparts and contain apolipoprotein (apo) E isoforms, apoJ, and apoAI, and extracellular vesicles, which can be detected by annexin V binding. The aim of this study was to develop a method to quantify CSF particles and evaluate their relationship to aging and neurodegenerative diseases. We used a flow cytometric assay to detect annexin V-, apoE-, apoAI-, apoJ-, and amyloid (A) β42-positive particles in CSF from 131 research volunteers who were neurologically normal or had mild cognitive impairment (MCI), Alzheimer disease (AD) dementia, or Parkinson disease. APOE ε4/ε4 participants had CSF apoE-positive particles that were more frequently larger but at an 88% lower level versus those in APOE ε3/ε3 or APOE ε3/ε4 patients; this finding was reproduced in conditioned medium from mouse primary glial cell cultures with targeted replacement of apoE. Cerebrospinal fluid apoE-positive and β-amyloid (Aβ42)-positive particle concentrations were persistently reduced one-third to one-half in middle and older age subjects; apoAI-positive particle concentration progressively increased approximately 2-fold with age. Both apoAI-positive and annexin V-positive CSF particle levels were reduced one-third to one-half in CSF of MCI and/or AD dementia patients versus age-matched controls. Our approach provides new methods to investigate CNS lipid biology in relation to neurodegeneration and perhaps develop new biomarkers for diagnosis or treatment monitoring.

Genetic predictors of outcome following traumatic brain injury

The nature of traumatic brain injury (TBI) has acute and chronic outcomes for those who survive. Over time, the chronic process of injury impacts multiple organ systems that may lead to disease. We discuss possible mechanisms and methodological issues in the context of candidate gene association studies using TBI patient populations. Because study population sizes have been generally limited, we discussed results on genes that have been the focus of independent studies. We also present a justification for testing more speculative candidate genes in recovery from TBI, such as those involved in circadian rhythm, to outline the importance of prioritizing functional variants in genes that may modulate recovery or provide neuroprotection from TBI. Finally, we provide a perspective on how future research will integrate population level genetic findings with the biological basis of disease in order to create a resource of predictive outcome measures for individual patients.

Stem cell derived basal forebrain cholinergic neurons from Alzheimer's disease patients are more susceptible to cell death

An early substantial loss of basal forebrain cholinergic neurons (BFCNs) is a constant feature of Alzheimer’s disease (AD) and is associated with deficits in spatial learning and memory. Induced pluripotent stem cells (iPSCs) derived from patients with AD as well as from normal controls could be efficiently differentiated into neurons with characteristics of BFCNs. We used BFCNs derived from iPSCs to model sporadic AD with a focus on patients with ApoE3/E4 genotypes (AD-E3/E4). BFCNs derived from AD-E3/E4 patients showed typical AD biochemical features evidenced by increased Aβ42/Aβ40 ratios. AD-E3/E4 neurons also exhibited altered responses to treatment with γ-secretase inhibitors compared to control BFCNs or neurons derived from patients with familial AD. BFCNs from patients with AD-E3/E4 also exhibited increased vulnerability to glutamate-mediated cell death which correlated with increased intracellular free calcium upon glutamate exposure. The ability to generate BFCNs with an AD phenotype is a significant step both for understanding disease mechanisms and for facilitating screening for agents that promote synaptic integrity and neuronal survival.

Roles of apolipoprotein E4 (ApoE4) in the pathogenesis of Alzheimer's disease: lessons from ApoE mouse models

ApoE4 (apolipoprotein E4) is the major known genetic risk factor for AD (Alzheimer's disease). In most clinical studies, apoE4 carriers account for 65–80% of all AD cases, highlighting the importance of apoE4 in AD pathogenesis. Emerging data suggest that apoE4, with its multiple cellular origins and multiple structural and biophysical properties, contributes to AD in multiple ways either independently or in combination with other factors, such as Aβ (amyloid β-peptide) and tau. Many apoE mouse models have been established to study the mechanisms underlying the pathogenic actions of apoE4. These include transgenic mice expressing different apoE isoforms in neurons or astrocytes, those expressing neurotoxic apoE4 fragments in neurons and human apoE isoform knock-in mice. Since apoE is expressed in different types of cells, including astrocytes and neurons, and in brains under diverse physiological and/or pathophysiological conditions, these apoE mouse models provide unique tools to study the cellular source-dependent roles of apoE isoforms in neurobiology and in the pathogenesis of AD. They also provide useful tools for discovery and development of drugs targeting apoE4's detrimental effects.

Function and comorbidities of apolipoprotein e in Alzheimer's disease

Alzheimer's disease (AD)—the most common type of dementia among the elderly—represents one of the most challenging and urgent medical mysteries affecting our aging population. Although dominant inherited mutation in genes involved in the amyloid metabolism can elicit familial AD, the overwhelming majority of AD cases, dubbed sporadic AD, do not display this Mendelian inheritance pattern. Apolipoprotein E (APOE), the main lipid carrier protein in the central nervous system, is the only gene that has been robustly and consistently associated with AD risk. The purpose of the current paper is thus to highlight the pleiotropic roles and the structure-function relationship of APOE to stimulate both the functional characterization and the identification of novel lipid homeostasis-related molecular targets involved in AD.

An apolipoprotein E4 fragment can promote intracellular accumulation of amyloid peptide beta 42

Apolipoprotein E (apoE) plays a crucial role in lipid transport in circulation and the brain. The apoE4 isoform is a major risk factor for Alzheimer's disease (AD). ApoE4 is more susceptible to proteolysis than other apoE isoforms and apoE4 fragments have been found in brains of AD patients. These apoE4 fragments have been hypothesized to be involved in the pathogenesis of AD, although the mechanism is not clear. In this study we examined the effect of lipid-free apoE4 on amyloid precursor protein processing and 40-amino-acid Aβ variant and 42-amino-acid Aβ variant levels in human neuroblastoma SK-N-SH cells. We discovered that a specific apoE4 fragment, apoE4[Δ(166-299)], can promote the cellular uptake of extracellular 40-amino-acid Aβ variant and 42-amino-acid Aβ variant either generated after amyloid precursor protein transfection or added exogenously. A longer length fragment, apoE4[Δ(186-299)], or full-length apoE4 failed to elicit this effect. ApoE4[Δ(166-299)] effected a 20% reduction of cellular sphingomyelin levels, as well as changes in cellular membrane micro-fluidity. Following uptake, approximately 50% of 42-amino-acid Aβ variant remained within the cell for at least 24 h, and led to increased formation of reactive oxygen species. Overall, our findings suggest a direct link between two early events in the pathogenesis of AD, apoE4 proteolysis and intraneuronal presence of amyloid beta peptide.

Mechanisms linking apolipoprotein E isoforms with cardiovascular and neurological diseases

PURPOSE OF REVIEW

The purpose of this review is to provide insights into recent advances in mechanisms linking apolipoprotein (apo) E isoforms to cardiovascular and neurological diseases.

RECENT FINDINGS

Human apoE has three common isoforms (apoE2, apoE3, and apoE4) with different structural and biophysical properties and different effects on lipid and neuronal homeostasis. ApoE is a protein constituent of different plasma lipoproteins and serves as a high-affinity ligand for several receptors. By interacting with its receptors, apoE mediates the clearance of different lipoproteins from the circulation. Absence or structural mutations of apoE cause significant disorders in lipid metabolism and cardiovascular disease. ApoE also has significant roles in neurobiology. ApoE4 is the major known genetic risk factor for Alzheimer's disease. It increases the occurrence and lowers the age of onset of Alzheimer's disease. ApoE4 carriers account for 65-80% of all Alzheimer's disease cases, highlighting the importance of apoE4 in Alzheimer's disease pathogenesis. ApoE4 has both amyloid beta-dependent and amyloid beta-independent roles in Alzheimer's disease pathogenesis.

SUMMARY

Emerging data suggest that apoE isoforms, with their multiple cellular origins and multiple structural and biophysical properties, contribute to cardiovascular and neurological diseases by interacting with different factors through various pathways.

Apolipoproteins in the brain: implications for neurological and psychiatric disorders

The brain is the most lipid-rich organ in the body and, owing to the impermeable nature of the blood-brain barrier, lipid and lipoprotein metabolism within this organ is distinct from the rest of the body. Apolipoproteins play a well-established role in the transport and metabolism of lipids within the CNS; however, evidence is emerging that they also fulfill a number of functions that extend beyond lipid transport and are critical for healthy brain function. The importance of apolipoproteins in brain physiology is highlighted by genetic studies, where apolipoprotein gene polymorphisms have been identified as risk factors for several neurological diseases. Furthermore, the expression of brain apolipoproteins is significantly altered in several brain disorders. The purpose of this article is to provide an up-to-date assessment of the major apolipoproteins found in the brain (ApoE, ApoJ, ApoD and ApoA-I), covering their proposed roles and the factors influencing their level of expression. Particular emphasis is placed on associations with neurological and psychiatric disorders.

Abeta-independent roles of apolipoprotein E4 in the pathogenesis of Alzheimer's disease

Human apolipoprotein (APO) E has three common isoforms that differentially affect lipid and neuronal homeostasis. APOE4, the major known genetic risk factor for Alzheimer's disease (AD), increases the occurrence and lowers the age of onset of AD. APOE4 carriers account for 65-80% of all AD cases, highlighting the importance of APOE4 in AD pathogenesis. Emerging data suggest that APOE4 contributes to AD through various pathways, some of which are dependent on amyloid-beta (Abeta). Although these Abeta-dependent roles of APOE4 have been widely studied, APOE4 has detrimental effects on neurons independent of Abeta: aberrant proteolysis of APOE4 generates neurotoxic fragments, stimulates Tau phosphorylation, which disrupts the cytoskeleton, and impairs mitochondrial function.

Apolipoprotein-E forms dimers in human frontal cortex and hippocampus

Background

Apolipoprotein-E (apoE) plays important roles in neurobiology and the apoE4 isoform increases risk for Alzheimer's disease (AD). ApoE3 and apoE2 are known to form disulphide-linked dimers in plasma and cerebrospinal fluid whereas apoE4 cannot form these dimers as it lacks a cysteine residue. Previous in vitro research indicates dimerisation of apoE3 has a significant impact on its functions related to cholesterol homeostasis and amyloid-beta peptide degradation. The possible occurrence of apoE dimers in cortical tissues has not been examined and was therefore assessed. Human frontal cortex and hippocampus from control and AD post-mortem samples were homogenised and analysed for apoE by western blotting under both reducing and non-reducing conditions.

Results

In apoE3 homozygous samples, ~12% of apoE was present as a homodimer and ~2% was detected as a 43 kDa heterodimer. The level of dimerisation was not significantly different when control and AD samples were compared. As expected, these dimerised forms of apoE were not detected in apoE4 homozygous samples but were detected in apoE3/4 heterozygotes at a level approximately 60% lower than seen in the apoE3 homozygous samples. Similar apoE3 dimers were also detected in lysates of SK-N-SH neuroblastoma cells and in freshly prepared rabbit brain homogenates. The addition of the thiol trapping agent, iodoacetamide, to block reactive thiols during both human and rabbit brain sample homogenisation and processing did not reduce the amount of apoE homodimer recovered. These data indicate that the apoE dimers we detected in the human brain are not likely to be post-mortem artefacts.

Conclusion

The identification of disulphide-linked apoE dimers in human cortical and hippocampal tissues represents a distinct structural difference between the apoE3 and apoE4 isoforms that may have functional consequences.

Cerebrospinal fluid apolipoprotein E concentration decreases after seizure

PURPOSE

In epilepsy, many studies were focus on apoE gene polymorphism and found APOE epsilon4 to be associated with earlier onset of temporal lobe epilepsy. There is not any study about apoE in cerebrospinal fluid (CSF) of epileptic patients, so we detect the CSF-apoE and determine whether it is changed after seizure.

METHODS

A total of 60 epileptic patients and 28 subjects with no evidence of any neurological diseases were studied. The concentrations of CSF-apoE were detected with enzyme-linked immunosorbent assay.

RESULTS

The CSF-apoE levels in epilepsy and control group were 5.78+/-2.15 mg/l and 13.60+/-12.11 mg/l, and there were statistical difference. In epilepsy group, the CSF-apoE concentration was 6.53+/-2.55 mg/l in male patients, and 4.98+/-1.21 mg/l in female. In secondary epilepsy group was 5.06+/-1.31 mg/l, and in idiopathic epilepsy was 6.04+/-2.34 mg/l. In different seizure types groups, including complex partial seizure (CPS), secondarily generalized tonic-clonic seizure (SGTC), generalized tonic-clonic seizure (GTCS), and absence seizure (AS), the mean concentrations of CSF-apoE were 6.62+/-3.13 mg/l, 5.21+/-1.22 mg/l, 5.00+/-1.09 mg/l and 7.25+/-1.88 mg/l, respectively.

CONCLUSIONS

CSF-apoE concentration decreases after seizures, correlated with the gender, etiological factor and seizure types.

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.

Isoform-specific proteolysis of apolipoprotein-E in the brain

Apolipoprotein-E (apoE) plays important roles in neurobiology and the apoE4 isoform increases risk for Alzheimer's disease (AD). ApoE peptides are biologically active and may be produced in the brain. It is unclear if apoE proteolysis is dependent on isoform or AD status and this was addressed here. Hippocampus, frontal cortex, occipital lobe and cerebellum samples were homogenized into fractions that were soluble in Tris-buffered saline (TBS), Triton X-100 or guanidine hydrochloride and analysed for apoE fragmentation by Western blotting. Approximately 20% of apoE3 was detected as fragments and this was predominantly as a 25 kDa peptide in TBS-soluble fractions. The concentration of TBS-soluble apoE fragments was two- to three-fold higher in apoE3 compared to apoE4 subjects. This difference was observed in all areas of the brain examined and was not related to AD status. Cathepsin-D treatment generated apoE fragments that were very similar to those detected in brain, however, no apoE isoform-specific differences in susceptibility to cathepsin-D proteolysis were detected. This indicates that proteolytic processing of apoE to form soluble fragments in the human brain is dependent on apoE isoform but not AD status.

Isoform-specific effects of ApoE on HSV immediate early gene expression and establishment of latency

Alzheimer's disease (AD) is a common and devastating neurodegenerative disease in which most cases are of unknown, sporadic origin. In addition to age, the most prevalent known risk factor for developing AD is carriage of the epsilon4 allele of Apolipoprotein E (ApoE). Carriage of the epsilon2 or epsilon3 allele of ApoE confers protection or no change in risk for AD, respectively. Latent herpes simplex virus type 1 (HSV-1) infection in the brain concurrent with ApoE4 carriage exacerbates risk for AD, suggesting that these two factors interact to promote neuronal dysfunction and degeneration in selective brain areas. Indeed, HSV-1 DNA has been found in regions primarily affected by AD, such as the temporal lobes, hippocampus, and neocortex. We hypothesize that HSV-1 infection in the background of ApoE4, but not ApoE2 or ApoE3, promotes an environment more conducive to neuronal degeneration. To investigate this idea, we have utilized transgenic mice that express human ApoE2, 3, or 4 alleles from astrocytes in a murine ApoE -/- background. We find that carriage of the different ApoE alleles dramatically affects HSV-1 immediate early gene expression as well as the establishment of latency. Both of these factors are poised to impact neuronal viability, inflammation, and viral spread. Our data support the concept that HSV-1 and ApoE4 interact to provide an environment conducive to the development and/or spread of AD.

Understanding the relationship between ApoE and HSV-1 and its possible significance in Alzheimer’s disease

Evaluation of: Miller RM, Federoff HJ: Isoform-specific effects of ApoE on HSV immediate early gene expression and establishment of latency. Neurobiol. Aging (2006) (Epub ahead of print). It is 100 years since Alzheimer's disease was first described and there is still no consensus of opinion regarding the etiology of its sporadic form. Herpes simplex virus type 1 (HSV-1), in conjunction with apolipoprotein E (ApoE), may well play a role in this form of the disease. In the evaluated article, Miller and Federoff try to determine whether the ApoE profile affects the expression of the HSV-1 immediate-early genes and the production of the virus’ latency-associated transcript. Their results show that overexpression of the intermediate-early genes and the establishment of latency are directly correlated with the ApoE genotype.

Apolipoprotein (apo) E4 and Alzheimer's disease: unique conformational and biophysical properties of apoE4 can modulate neuropathology

The unique structural and biophysical features of apolipoprotein (apo) E4 - domain interaction and molten globule formation - have been correlated with the detrimental effects of apoE4 in neuropathology. Two examples of how the structure of apoE4 determines the pathological outcome in neurons include apoE4 potentiation of amyloid beta-induced lysosomal leakage and apoptosis and the proteolytic cleavage of apoE synthesized by neurons. Thus, a new therapeutic target is to identify small molecules to modulate the inherent neuropathological structure of apoE4, i.e. to prevent domain interaction and to convert apoE4 to an apoE3-like molecule. A second therapeutic target is to inhibit the apoE-cleaving enzyme. This would prevent the generation of the reactive carboxyl-terminal fragments of apoE that enter the cytosol, disrupt the cytoskeleton, and cause neurodegeneration. ApoE4 is more susceptible than apoE3 to proteolytic cleavage and is thus more likely to cause detrimental effects in the central nervous system. It is predictable that apoE4 acts through various pathways to cause cognitive decline and neurodegeneration.

Profile and regulation of apolipoprotein E (ApoE) expression in the CNS in mice with targeting of green fluorescent protein gene to the ApoE locus

To study the profile and regulation of apolipoprotein E (apoE) expression in the CNS, we generated mice in which apoE expression can be detected in vivo with unprecedented sensitivity and resolution. cDNA encoding enhanced green fluorescent protein (EGFP) with a stop codon was inserted by gene targeting into the apoE gene locus (EGFPapoE) immediately after the translation initiation site. Insertion of EGFP into one apoE allele provides a real-time location marker of apoE expression in vivo; the remaining allele is sufficient to maintain normal cellular physiology. In heterozygous EGFPapoE mice, EGFP was highly expressed in hepatocytes and peritoneal macrophages. EGFP was also expressed in brain astrocytes; however some astrocytes (approximately 25%) expressed no EGFP, suggesting that a subset of these cells does not express apoE. EGFP was expressed in <10% of microglia after kainic acid treatment, suggesting that microglia are not a major source of brain apoE. Although hippocampal neurons did not express EGFP under normal conditions, kainic acid treatment induced intense expression of EGFP in injured neurons, demonstrating apoE expression in neurons in response to excitotoxic injury. The neuronal expression was confirmed by in situ hybridization of mouse apoE mRNA and by anti-apoE immunostaining. Smooth muscle cells of large blood vessels and cells surrounding small vessels in the CNS also strongly expressed EGFP, as did cells in the choroid plexus. EGFPapoE reporter mice will be useful for studying the regulation of apoE expression in the CNS and might provide insights into the diverse mechanisms of apoE4-related neurodegeneration.

Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer's disease

The premise of this review is that apolipoprotein (apo) E4 is much more than a contributing factor to neurodegeneration. ApoE has critical functions in redistributing lipids among CNS cells for normal lipid homeostasis, repairing injured neurons, maintaining synapto-dendritic connections, and scavenging toxins. In multiple pathways affecting neuropathology, including Alzheimer's disease, apoE acts directly or in concert with age, head injury, oxidative stress, ischemia, inflammation, and excess amyloid beta peptide production to cause neurological disorders, accelerating progression, altering prognosis, or lowering age of onset. We envision that unique structural features of apoE4 are responsible for apoE4-associated neuropathology. Although the structures of apoE2, apoE3, and apoE4 are in dynamic equilibrium, apoE4, which is detrimental in a variety of neurological disorders, is more likely to assume a pathological conformation. Importantly, apoE4 displays domain interaction (an interaction between the N- and C-terminal domains of the protein that results in a compact structure) and molten globule formation (the formation of stable, reactive intermediates with potentially pathological activities). In response to CNS stress or injury, neurons can synthesize apoE. ApoE4 uniquely undergoes neuron-specific proteolysis, resulting in bioactive toxic fragments that enter the cytosol, alter the cytoskeleton, disrupt mitochondrial energy balance, and cause cell death. Our findings suggest potential therapeutic strategies, including the use of "structure correctors" to convert apoE4 to an "apoE3-like" molecule, protease inhibitors to prevent the generation of toxic apoE4 fragments, and "mitochondrial protectors" to prevent cellular energy disruption.

Effect of domain interaction on apolipoprotein E levels in mouse brain

Apolipoprotein (apo) E4 is a risk factor for heart disease, Alzheimer's disease, and other forms of neurodegeneration, but the underlying mechanisms are unknown. Domain interaction, a structural property that distinguishes apoE4 from apoE2 and apoE3, results in more rapid turnover and lower plasma levels of apoE4. To determine whether domain interaction affects brain apoE levels, we analyzed brain homogenates from human apoE3 and apoE4 knock-in mice, wild-type mice, and Arg-61 apoE mice, in which domain interaction was introduced by gene targeting. As determined on Western blots, the hemibrain, cortex, hippocampus, and cerebellum of knock-in mice had 30-40% lower levels of apoE4 than apoE3, and Arg-61 mice had 25-50% lower apoE levels than wild-type mice. In the CSF, Arg-61 apoE level was 40% lower than the wild-type level. Arg-61 apoE mRNA levels were similar to or slightly higher than wild-type apoE mRNA levels. Thus, the lower Arg-61 apoE levels were not attributable to decreased mRNA levels. In culture medium from heterozygous Arg-61/wild-type and apoE4/apoE3 primary astrocytes, Arg-61 apoE and apoE4 levels were lower than wild-type apoE and apoE3, respectively, suggesting that primary astrocytes secrete lower amounts of Arg-61 apoE and apoE4. These results demonstrate that domain interaction is responsible for the lower levels of both human apoE4 and mouse Arg-61 apoE in mouse brain. Cells may recognize apoE4 and Arg-61 apoE as misfolded proteins and target them for degradation or accumulation. Thus, degradation/accumulation or lower levels of apoE4 may contribute to the association of apoE4 with Alzheimer's disease.

Lipid- and receptor-binding regions of apolipoprotein E4 fragments act in concert to cause mitochondrial dysfunction and neurotoxicity

Apolipoprotein (apo) E4, a 299-aa protein and a major risk factor for Alzheimer's disease, can be cleaved to generate C-terminal-truncated fragments that cause neurotoxicity in vitro and neurodegeneration and behavioral deficits in transgenic mice. To investigate this neurotoxicity, we expressed apoE4 with C- or N-terminal truncations or mutations in transfected Neuro-2a cells. ApoE4 (1-272) was neurotoxic, but full-length apoE4(1-299) and apoE4(1-240) were not, suggesting that the lipid-binding region (amino acids 241-272) mediates the neurotoxicity and that amino acids 273-299 are protective. A quadruple mutation in the lipid-binding region (I250A, F257A, W264R, and V269A) abolished the neurotoxicity of apoE4(1-272), and single mutations in the region of amino acids 273-299 (L279Q, K282A, or Q284A) made full-length apoE4 neurotoxic. Immunofluorescence staining showed that apoE4(1-272) formed filamentous inclusions containing phosphorylated tau in some cells and interacted with mitochondria in others, leading to mitochondrial dysfunction as determined by MitoTracker staining and flow cytometry. ApoE4(241-272) did not cause mitochondrial dysfunction or neurotoxicity, suggesting that the lipid-binding region alone is insufficient for neurotoxicity. Truncation of N-terminal sequences (amino acids 1-170) containing the receptor-binding region (amino acids 135-150) and triple mutations within that region (R142A, K146A, and R147A) abolished the mitochondrial interaction and neurotoxicity of apoE4(1-272). Further analysis showed that the receptor-binding region is required for escape from the secretory pathway and that the lipid-binding region mediates mitochondrial interaction. Thus, the lipid- and receptor-binding regions in apoE4 fragments act together to cause mitochondrial dysfunction and neurotoxicity, which may be important in Alzheimer's disease pathogenesis.

Sense and antisense transcripts of the apolipoprotein E gene in normal and ApoE knockout mice, their expression after spinal cord injury and corresponding human transcripts

The apolipoprotein E (ApoE) gene has been linked to maladies such as hypercholesterolemia, CNS injury and disease. In this study, we present evidence that, in addition to the known transcript (ApoE S1) that translates into ApoE, there are three additional transcripts in mice. Two of these transcripts, ApoE S2 and ApoE S3, which are predicted to be transmembrane proteins, are transcribed from the sense strand. ApoE AS1 is transcribed from the antisense strand and is complementary to exon 4 of ApoE S1. The open reading frame of ApoE AS1 is conserved between human and mouse. The antisense transcript falls within the region of the human epsilon 4 allele that has been linked to the familial onset form of Alzheimer's disease. We also demonstrate the expression of ApoE S3 and ApoE AS1 in ApoE knockout mice, and ApoE S1 and ApoE S2 do not get transcribed. We had previously identified ApoE S1 as being upregulated in mice after spinal cord injury. In this study, we show that in spinal cord-injured C57BL/6 mice, both ApoE S1 and ApoE S3 transcripts are 10-fold upregulated and the antisense ApoE AS1 is 100-fold upregulated compared with normal levels. Such data suggest that these alternate transcripts are involved in the molecular pathogenesis of CNS disease and perhaps in ApoE expression in general, as we show that ApoE S2 and AS1 are also transcribed in human.

Neuronal and astroglial alterations in the hippocampus of a mouse model for type 1 diabetes

The influence of diabetes mellitus on brain pathology is increasingly recognized. Previous contributions of our laboratory demonstrated in models of type 1 diabetes (nonobese diabetic and streptozotocin (STZ)-treated mice), a marked astrogliosis and neurogenesis deficit in hippocampus and increased expression of hypothalamic neuropeptides. In the present investigation, we further analyzed alterations of astroglia and neurons in the hippocampus of mice 1 month after STZ-induced diabetes. Results showed that these STZ-diabetic mice presented: (a) increased number of astrocytes positive for apolipoprotein-E (Apo-E), a marker of ongoing neuronal dysfunction; (b) abnormal expression of early gene products associated with neuronal activation, including a high number of Jun + neurons in CA1 and CA3 layers and dentate gyrus, and of Fos-expressing neurons in CA3 layer; (c) augmented activity of NADPH-diaphorase, linked to oxidative stress, in CA3 region. These data support the concept that uncontrolled diabetes leads to hippocampal pathology, which adjoin to changes in other brain structures such as hypothalamus and cerebral cortex.

Apolipoprotein E: diversity of cellular origins, structural and biophysical properties, and effects in Alzheimer's disease

Apolipoprotein E4 (apoE4) is a major risk factor for Alzheimer's disease (AD). Several hypotheses have been proposed to explain the association of the APOE epsilon4 allele with AD; however, the mechanisms underlying this association are largely unknown. Initially, apoE was thought to be synthesized primarily by astrocytes but not by neurons in the brain. However, subsequent studies have demonstrated that central nervous system neurons also express apoE under diverse physiological and pathological conditions. Detailed studies of the structure and biophysical properties of apoE isoforms have demonstrated unique properties distinguishing apoE4 from apoE3. Because the structural and biophysical properties of a protein determine how it functions under normal and abnormal conditions, apoE4, with its multiple cellular origins and multiple structural and biophysical properties, might contribute to the pathology of AD through several different mechanisms. Some of these mechanisms might be suitable targets for the development of new treatments for AD.

Marked regional differences of brain human apolipoprotein E expression in targeted replacement mice

We used three human apolipoprotein (apo) E targeted replacement mouse lines, each expressing one of the three common human apoE isoforms to study the pattern of apoE expression in the central nervous system (CNS). Immunocytochemistry on brain sections from all three lines of targeted replacement mice, wild type mice, African green monkeys, and humans show a predominantly glial pattern of apoE expression. The levels of human apoE protein in hippocampus and frontal cortex were similar between targeted replacement mice and non-demented human tissue. Within a given brain region, the levels of apoE were very similar amongst all three isoforms, which contrasts sharply with plasma, where apoE2 levels are 16-fold higher than apoE3 and E4 levels. Across brain regions, cerebellar apoE levels were significantly higher than cerebral apoE levels. In conclusion, we provide detailed analysis of a human apoE animal model system that recapitulates both the pattern and level of apoE expression in non-demented humans. The neurobiology of human apoE isoforms can now be studied in both the normal and post-injury state, since all apoE regulatory sequences are intact. Finally, the differences in apoE levels we observed may explain the regional vulnerability of neuronal degeneration in Alzheimer's disease.

Neuron-specific apolipoprotein e4 proteolysis is associated with increased tau phosphorylation in brains of transgenic mice

Apolipoprotein E (apoE) is found in amyloid plaques and neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) brains, but its role in their pathogenesis is unclear. Previously, we found C-terminal-truncated fragments of apoE in AD brains and showed that such fragments can cause neurodegeneration and can induce NFT-like inclusions in cultured neuronal cells and in transgenic mice. Here, we analyzed apoE fragmentation in brain tissue homogenates from transgenic mice expressing apoE3 or apoE4 in neurons [neuron-specific enolase (NSE)-apoE] or astrocytes [glial fibrillary acidic protein (GFAP)-apoE] by Western blotting. The C-terminal-truncated fragments of apoE accumulated, in an age-dependent manner, in the brains of NSE-apoE4 and, to a significantly lesser extent, NSE-apoE3 mice; however, no fragments were detected in GFAP-apoE3 or GFAP-apoE4 mice. In NSE-apoE mice, the pattern of apoE fragmentation resembled that seen in AD brains, and the fragmentation was specific for certain brain regions, occurring in the neocortex and hippocampus, which are vulnerable to AD-related neurodegeneration, but not in the less vulnerable cerebellum. Excitotoxic challenge with kainic acid significantly increased apoE fragmentation in NSE-apoE4 but not NSE-apoE3 mice. Phosphorylated tau (p-tau) also accumulated in an age-dependent manner in NSE-apoE4 mice and, to a much lesser extent, in NSE-apoE3 mice but not in GFAP-apoE3 or GFAP-apoE4 mice. Intraneuronal p-tau inclusions in the hippocampus were prominent in 21-month-old NSE-apoE4 mice but barely detectable in NSE-apoE3 mice. Thus, the accumulation of potentially pathogenic C-terminal-truncated fragments of apoE depends on both the isoform and the cellular source of apoE. Neuron-specific proteolytic cleavage of apoE4 is associated with increased phosphorylation of tau and may play a key role in the development of AD-related neuronal deficits.

Apolipoprotein E4 domain interaction occurs in living neuronal cells as determined by fluorescence resonance energy transfer

Apolipoprotein (apo) E4 is a major risk factor for Alzheimer disease. Although the mechanisms remain to be determined, the detrimental effects of apoE4 in neurobiology must be based on its unique structural and biophysical properties. One such property is domain interaction mediated by a salt bridge between Arg-61 in the N-terminal domain and Glu-255 in the C-terminal domain of apoE4. This interaction, which does not occur in apoE3 or apoE2, causes apoE4 to bind preferentially to certain lipoprotein particles in vitro and in vivo. Here we used fluorescence resonance energy transfer (FRET) to determine whether apoE4 domain interaction occurs in living neuronal cells. Neuro-2a cells were transfected with constructs encoding apoE3 or apoE4 in which yellow fluorescent protein (YFP) was fused to the N terminus, and cyan fluorescent protein (CFP) was fused to the C terminus. To generate a FRET signal that can be detected by spectrum confocal microscopy, the labeled N and C termini must be in close proximity (<100 A). FRET signals occurred in cells transfected with YFP-apoE4-CFP but not in those transfected with YFP-apoE3-CFP, suggesting that the N and C termini of apoE4 are in close proximity in living cells and that those of apoE3 are not. FRET signals did not occur in cells cotransfected with YFP-apoE4 and apoE4-CFP, suggesting that the FRET in YFP-apoE4-CFP-transfected cells was intramolecular. Mutation of Arg-61 to Thr or Glu-255 to Ala in apoE4, which disrupts domain interaction, abolished FRET in Neuro-2a cells, strongly suggesting that the FRET in YFP-apoE4-CFP cells was caused by domain interaction. ApoE4-producing cells secreted less phospholipid than apoE3-producing cells, but after disruption of domain interaction in apoE4, phospholipid secretion increased to the levels seen with apoE3, suggesting that domain interaction decreases the phospholipid-binding capacity of apoE4. Thus, apoE4 domain interaction occurs in living neuronal cells and may be a molecular basis for apoE4-related neurodegeneration.

Apolipoprotein E protects against oxidative stress in mixed neuronal-glial cell cultures by reducing glutamate toxicity

Apolipoprotein E (ApoE) deficiency has been shown to adversely affect outcome after transient cerebral ischemia and head trauma. Since oxidative stress contributes to these injuries, the ability of ApoE to reduce irreversible oxidative damage was studied in primary mixed neuronal-glial cell cultures. Cells (13-16 days in vitro) were exposed to 50 microM hydrogen peroxide (H2O2) for 30 min, and toxicity was determined by the release of lactate dehydrogenase (LDH) 24 h after exposure. The presence of recombinant human ApoE2 (100, 300, or 1000 nM) in the culture media partially protected against oxidative injury. This protection was not reversed by pre-treatment with receptor associated protein. The NMDA receptor antagonist, MK-801, also provided partial protection against H2O2 toxicity. The degree of protection was similar to that conferred by ApoE treatment. The protective effects of ApoE and MK-801 were not additive; no ApoE protection was observed in cultures treated with MK-801 prior to H2O2 exposure. ApoE treatment had no effect on H2O2 stimulated glutamate release, but did increase the rate of glutamate uptake via the high affinity glutamate transporter in H2O2 treated cultures. Pre-treatment with ApoE also conferred partial protection against glutamate-induced LDH release. Taken together, these findings suggest that ApoE protects mixed neuronal-glial cell cultures against irreversible oxidative injury from H2O2 by reducing secondary glutamate excitotoxicity.

Astroglial regulation of apolipoprotein E expression in neuronal cells. Implications for Alzheimer's disease

Although apolipoprotein (apo) E is synthesized in the brain primarily by astrocytes, neurons in the central nervous system express apoE, albeit at lower levels than astrocytes, in response to various physiological and pathological conditions, including excitotoxic stress. To investigate how apoE expression is regulated in neurons, we transfected Neuro-2a cells with a 17-kilobase human apoE genomic DNA construct encoding apoE3 or apoE4 along with upstream and downstream regulatory elements. The baseline expression of apoE was low. However, conditioned medium from an astrocytic cell line (C6) or from apoE-null mouse primary astrocytes increased the expression of both isoforms by 3-4-fold at the mRNA level and by 4-10-fold at the protein level. These findings suggest that astrocytes secrete a factor or factors that regulate apoE expression in neuronal cells. The increased expression of apoE was almost completely abolished by incubating neurons with U0126, an inhibitor of extracellular signal-regulated kinase (Erk), suggesting that the Erk pathway controls astroglial regulation of apoE expression in neuronal cells. Human neuronal precursor NT2/D1 cells expressed apoE constitutively; however, after treatment of these cells with retinoic acid to induce differentiation, apoE expression diminished. Cultured mouse primary cortical and hippocampal neurons also expressed low levels of apoE. Astrocyte-conditioned medium rapidly up-regulated apoE expression in fully differentiated NT2 neurons and in cultured mouse primary cortical and hippocampal neurons. Thus, neuronal expression of apoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the Erk kinase pathway in neurons.

Carboxyl-terminal-truncated apolipoprotein E4 causes Alzheimer's disease-like neurodegeneration and behavioral deficits in transgenic mice

Apolipoprotein (apo) E4 increases the risk and accelerates the onset of Alzheimer's disease (AD). However, the underlying mechanisms remain to be determined. We previously found that apoE undergoes proteolytic cleavage in AD brains and in cultured neuronal cells, resulting in the accumulation of carboxyl-terminal-truncated fragments of apoE that are neurotoxic. Here we show that this fragmentation is caused by proteolysis of apoE by a chymotrypsin-like serine protease that cleaves apoE4 more efficiently than apoE3. Transgenic mice expressing the carboxyl-terminal-cleaved product, apoE4(Delta272-299), at high levels in the brain died at 2-4 months of age. The cortex and hippocampus of these mice displayed AD-like neurodegenerative alterations, including abnormally phosphorylated tau (p-tau) and Gallyas silver-positive neurons that contained cytosolic straight filaments with diameters of 15-20 nm, resembling preneurofibrillary tangles. Transgenic mice expressing lower levels of the truncated apoE4 survived longer but showed impaired learning and memory at 6-7 months of age. Thus, carboxyl-terminal-truncated fragments of apoE4, which occur in AD brains, are sufficient to elicit AD-like neurodegeneration and behavioral deficits in vivo. Inhibiting their formation might inhibit apoE4-associated neuronal deficits.

Increased expression of neuronal apolipoprotein E in human brain with cerebral infarction

BACKGROUND AND PURPOSE

Cellular origin of apolipoprotein E (ApoE) in the human brain and its roles in physiological and pathological conditions remain to be clarified.

METHODS

Immunolocalization of ApoE was investigated in a series of autopsied human brains with or without infarction. ApoE expression was also estimated on immunoblot on protein extracts from autopsied brains and a cultured neuroblastoma cell line of human origin (GOTO) subjected to an oxidative stress induced by exposure to hydrogen peroxide (0.2 mmol/L).

RESULTS

In addition to astrocytes and microglia, neurons and degenerated axons in and around the ischemic foci contained ApoE-like immunoreactivity, which was more intense in recent ischemic foci. Immunoblot demonstrated an increase in expression of ApoE in brain extracts from ischemic lesion, and this increase was also pronounced in the cultured neuroblastoma cell line after the stress.

CONCLUSIONS

Accumulation of ApoE in neurons in and around ischemic foci of the human brain is related to an increase in ApoE synthesis in neurons, as seen in cultured neuronal cells after oxidative stress. Intrinsic regenerative activity of neuron in reaction to external insults may be related to this increase in ApoE of neuronal origin.

Apolipoprotein E expression after spinal cord injury in the mouse

Apolipoprotein E (apo-E), a protein involved in lipid metabolism and cholesterol transport, has been found to be up-regulated in CNS injury and is associated with Alzheimer's disease in humans. In this study, we show that apo-E is also up-regulated after complete spinal cord transection in the C57BL/6 mouse. In the uninjured cord, the cellular localization of apo-E protein is in astrocytes, in individual neurons throughout the laminae except for the dorsal horn, and in endothelial cells of capillaries in the immediate vicinity of those neurons. After injury, RNA levels are elevated as early as 4 days and reach a maximal level between 1 and 2 weeks. Protein levels follow closely but remain up-regulated beyond 3 weeks. Early on, the protein can be found in neutrophils and macrophages at the injury site and only at later times in astrocytes during the remodeling of white matter tracts, most prominently in degenerating parts of the fasciculus gracilis.

Genotype-related differences of hippocampal apolipoprotein E levels only in early stages of neuropathological changes in Alzheimer's disease

Inheritance of the epsilon4 allele of apolipoprotein E (APOE, gene; apoE, protein) represents the most common genetic risk factor for developing Alzheimer's disease (AD), but the role of apoE in AD pathogenesis is yet to be clarified. A number of studies investigating apoE expression and protein levels in AD brain in correlation to its genetic polymorphism has yielded controversial results. We designed our approach based on neuropathological characteristics of AD to investigate apoE levels in relation to the APOE genotype and AD-related neurofibrillary changes, and amyloid deposits. We determined hippocampal apoE levels by reducing sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting in 70 Braak-staged and APOE-genotyped autopsy brains. In our stage-, age- and gender-matched case sample, we found a significant increase of hippocampal apoE in the APOE epsilon3 homozygotes with beginning AD-related pathology (Braak stages I and II) compared with brain samples free of neurofibrillary changes and amyloid deposits. In the APOE epsilon4 allele carriers no such increase was found. In both genotype groups, severely affected brain samples with widespread neurofibrillary changes (Braak stages V and VI) and amyloid deposits (Braak stage C) showed low apoE levels comparable to those found in unaffected brain samples (Braak stage 0). Our data suggests that the isoform-specific impact of apoE on the development of AD may be of crucial importance only in the early stages of the disease. These stages are believed to represent phases of the disease in which the beginning neurodegeneration can be compensated by plastic reorganization.

Apolipoprotein E immunoreactivity in neurons and neurofibrillary degeneration of aged non-demented and Alzheimer's disease patients

Apolipoprotein E (ApoE) genotype is a risk factor for Alzheimer's disease (AD) but its relationship with neurofibrillary degeneration remains obscure. To further analyze this relationship, hippocampal, entorhinal, temporopolar, and insular cortices of 10 non-demented and 7 Alzheimer disease brains were studied with both light and electron microscopy. Focus was directed on pretangles and neurons starting to accumulate tangles. ApoE immunolabeling in neurons and tangles was independent of ApoE individual genotype. The majority of the neurons in all of the brains were ApoE-negative, but virtually every brain also contained groups of ApoE-immunoreactive neurons, with diffuse cytoplasmic labeling. Most of the ApoE-positive tangles were extracellular, but a few tangles were shown to be intraneuronal when studied ultrastructurally. No ApoE immunoreactivity was found in neuropil threads, as well as in neurites associated with senile plaques. Double protocols with both AT-8 and anti-ApoE antibodies, performed to determine whether ApoE-positive neurons were pretangle neurons, did not detect cytoplasmic AT-8 in ApoE-positive neurons. Though careful electron microscopy studies found ApoE reaction product in an occasional ApoE-positive pretangle-like neuron and a few intracellular tangles, these findings do not support that ApoE is necessary for the accumulation of hyperphosphorylated tau protein. The more consistent colocalization of anti-ApoE and AT-8 in extracellular tangles reveals that ApoE mainly binds to tangles once they are in the extracellular space, in a manner similar to that described for amyloid fibrils.

Synthesis and processing of apolipoprotein E in human brain cultures

Apolipoprotein E (apoE) plays a role in the distribution of lipid within many organs and cell types in the human body, including neurons and astrocytes of the central nervous system (CNS). The apoE4 isoform is also a genetic risk factor for late onset Alzheimer's disease (AD). However, the mechanism by which apoE is involved in AD is largely unknown. In order to understand how apoE is involved in the distribution of lipid in the CNS, we sought to investigate not only the origin of intraneuronal apoE, but the pathway by which it is processed once synthesized. We have established that human neurons can synthesize apoE in the presence of astrocytes, and that intracellular neuronal apoE is processed through the rough endoplasmic reticulum, golgi, and CD63-positive lysosomes where it may be stored before secretion. Our results also suggest that apoE synthesis is regulated by a feedback mechanism, controlled by the neuron itself. This regulatory mechanism may be essential to the maintenance of neuronal cholesterol concentrations and in turn membrane stability.

Susceptibility of transgenic mice expressing human apolipoprotein E to closed head injury: the allele E3 is neuroprotective whereas E4 increases fatalities

Apolipoprotein E, the major brain lipid-binding protein, is expressed in humans as three common isoforms (E2, E3 and E4). Previous studies revealed that the allele apolipoprotein E4 is a major genetic risk factor of Alzheimer's disease and that traumatic brain injury is associated with increased risk for developing this disease. Furthermore, it has been suggested that the effects of traumatic head injury and apolipoprotein E4 in Alzheimer's disease are synergistic. To test the hypothesis that the apolipoprotein E genotype affects susceptibility to brain injury, we subjected transgenic mice, expressing either human apolipoprotein E3 or human apolipoprotein E4 on a null mouse apolipoprotein E background and apolipoprotein E-deficient knockouts, to closed head injury and compared mortality, neurological recovery and the extent of brain damage of the survivors. More than 50% of the transgenic mice expressing human apolipoprotein E4 died following closed head injury, whereas only half as many of the transgenic mice expressing human apolipoprotein E3, and of the control and apolipoprotein E-deficient mice died during this period (P<0.02). A neurological severity score used for clinical assessment of the surviving mice up to 11 days after closed head injury revealed that the four mouse groups displayed similar severity of damage at 1h following injury. At three and 11 days post-injury, however, the neurological severity scores of the transgenic mice expressing human apolipoprotein E3 were significantly lower than those of the other three groups whose scores were similar, indicating better recovery of the transgenic mice expressing human apolipoprotein E3. Histopathological examination of the mice performed 11 days post-injury revealed, consistent with the above neurological results, that the size of the damaged brain area of the transgenic mice expressing human apolipoprotein E3 was smaller than that of the other head-injured groups. These findings show that transgenic mice expressing human apolipoprotein E4 are more susceptible than those expressing apolipoprotein E3 to closed head injury. We suggest that this effect is due to both a protective effect of apolipoprotein E3 and an apolipoprotein E4-related pathological function.

Synthesis and regulation of apolipoprotein E during the differentiation of human neuronal precursor NT2/D1 cells into postmitotic neurons

Recently, we showed expression of apolipoprotein E (apoE) in human neuronal-type cells such as neuroblastoma SK N SH-SY 5Y cells. In this model, a negative effect of neuronal differentiation on apoE synthesis was suspected. To check this hypothesis, we studied the regulation of apoE in human postmitotic neurons. The presence of apoE was investigated in undifferentiated human teratocarcinoma NT2/D1 (NT2) cells and during their differentiation into postmitotic hNT neurons induced by retinoic acid (RA) treatment. Before differentiation, apoE protein and mRNA were detected in NT2 cells by Western blotting and RT-PCR experiments. Immunofluorescence study showed that apoE was present in all cells. For longer times of RA treatment (3 weeks), the apoE labeling became heterogeneous: only some cells were immunopositive and among them were some differentiating cells in which apoE was located in both cellular body and neuritic process. Interestingly, terminally differentiated hNT cells no longer expressed apoE. These results demonstrate that neuronal precursor and differentiating cells were able to synthesize apoE while the fully neuronal differentiation exerted a negative effect on apoE neuronal expression. Our results are compatible with a weak expression of apoE in neurons of adult brains.

Prominent axonopathy and disruption of axonal transport in transgenic mice expressing human apolipoprotein E4 in neurons of brain and spinal cord

The epsilon 4 allele of the human apolipoprotein E gene (ApoE4) constitutes an important genetic risk factor for Alzheimer's disease. Recent experimental evidence suggests that human ApoE is expressed in neurons, in addition to being synthesized in glial cells. Moreover, brain regions in which neurons express ApoE seem to be most vulnerable to neurofibrillary pathology. The hypothesis that the expression pattern of human ApoE might be important for the pathogenesis of Alzheimer's disease was tested by generating transgenic mice that express human ApoE4 in neurons or in astrocytes of the central nervous system. Transgenic mice expressing human ApoE4 in neurons developed axonal degeneration and gliosis in brain and in spinal cord, resulting in reduced sensorimotor capacities. In these mice, axonal dilatations with accumulation of synaptophysin, neurofilaments, mitochondria, and vesicles were documented, suggesting impairment of axonal transport. In contrast, transgenic mice expressing human ApoE4 in astrocytes remained normal throughout life. These results suggest that expression of human ApoE in neurons of the central nervous system could contribute to impaired axonal transport and axonal degeneration. The possible contribution of hyperphosphorylation of protein Tau to the resulting phenotype is discussed.