Apolipoprotein E structure: insights into function

Lipid-Binding Proteins in Brain Health and Disease

A proper lipid management is paramount for a healthy brain. Lipid homeostasis alterations are known to be causative or risk factors for many neurodegenerative diseases, or key elements in the recovery from nervous system injuries of different etiology. In addition to lipid biogenesis and catabolism, non-enzymatic lipid-binding proteins play an important role in brain function and maintenance through aging. Among these types of lipoproteins, apolipoprotein E has received much attention due to the relationship of particular alleles of its gene with the risk and progression of Alzheimer's disease. However, other lipid-binding proteins whose role in lipid homeostasis and control are less known need to be brought to the attention of both researchers and clinicians. The aim of this review is to cover the knowledge of lipid-managing proteins in the brain, with particular attention to new candidates to be relevant for brain function and health.

Plasma ApoE elevations are associated with NAFLD: The PREVEND Study

Non-alcoholic fatty liver disease (NAFLD) is featured by increased plasma very low density lipoproteins (VLDL). The extent to which plasma apolipoprotein E (ApoE) levels are elevated in NAFLD is unclear. We determined whether plasma ApoE is elevated in subjects with suspected NAFLD. Plasma ApoE and genotypes were determined in 6,762 participants of the Prevention of Renal and Vascular End-Stage Disease (PREVEND) cohort. A Fatty Liver Index (FLI) ≥ 60 was used as a proxy of NAFLD. A total of 1,834 participants had a FLI ≥ 60, which coincided with increased triglycerides, non-HDL cholesterol, ApoB and ApoE (all P<0.001). In multivariable linear regression analysis, plasma ApoE levels were positively associated with an elevated FLI when taking account of ApoE genotypes and other clinical and laboratory covariates (fully adjusted model: β = 0.201, P<0.001). Stratified analysis for ApoE genotypes (ApoE ε3ε3 homozygotes, ApoE ε2 carriers, and ApoE ε3ε4 and ε4ε4 carriers combined), also showed positive associations of plasma ApoE levels with an elevated FLI in each group (all P<0.001). In conclusion, it is suggested that NAFLD is characterized by increased plasma ApoE levels, even when taking account of the various ApoE genotypes. Increased plasma ApoE may contribute to altered VLDL metabolism and to increased atherosclerosis susceptibility in NAFLD.

Human apolipoprotein E promotes hepatitis B virus infection and production

Hepatitis B virus (HBV) is a common cause of liver diseases, including chronic hepatitis, steatosis, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). HBV chronically infects about 240 million people worldwide, posing a major global health problem. The current standard antiviral therapy effectively inhibits HBV replication but does not eliminate the virus unlike direct-acting antivirals (DAA) for curing hepatitis C. Our previous studies have demonstrated that human apolipoprotein E (apoE) plays important roles in hepatitis C virus infection and morphogenesis. In the present study, we have found that apoE is also associated with HBV and is required for efficient HBV infection. An apoE-specific monoclonal antibody was able to capture HBV similar to anti-HBs. More importantly, apoE monoclonal antibody could effectively block HBV infection, resulting in a greater than 90% reduction of HBV infectivity. Likewise, silencing of apoE expression or knockout of apoE gene by CRISPR/Cas9 resulted in a greater than 90% reduction of HBV infection and more than 80% decrease of HBV production, which could be fully restored by ectopic apoE expression. However, apoE silencing or knockout did not significantly affect HBV DNA replication or the production of nonenveloped (naked) nucleocapsids. These findings demonstrate that human apoE promotes HBV infection and production. We speculate that apoE may also play a role in persistent HBV infection by evading host immune response similar to its role in the HCV life cycle and pathogenesis. Inhibitors interfering with apoE biogenesis, secretion, and/or binding to receptors may serve as antivirals for elimination of chronic HBV infection.

Apolipoprotein E and Alzheimer disease: pathobiology and targeting strategies

Polymorphism in the apolipoprotein E (APOE) gene is a major genetic risk determinant of late-onset Alzheimer disease (AD), with the APOE*ε4 allele conferring an increased risk and the APOE*ε2 allele conferring a decreased risk relative to the common APOE*ε3 allele. Strong evidence from clinical and basic research suggests that a major pathway by which APOE4 increases the risk of AD is by driving earlier and more abundant amyloid pathology in the brains of APOE*ε4 carriers. The number of amyloid-β (Aβ)-dependent and Aβ-independent pathways that are known to be differentially modulated by APOE isoforms is increasing. For example, evidence is accumulating that APOE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. In addition, APOE4 is either pathogenic or shows reduced efficiency in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. Here, we review the recent progress in clinical and basic research into the role of APOE in AD pathogenesis. We also discuss how APOE can be targeted for AD therapy using a precision medicine approach.

Apolipoprotein E4, inhibitory network dysfunction, and Alzheimer's disease

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD), increasing risk and decreasing age of disease onset. Many studies have demonstrated the detrimental effects of apoE4 in varying cellular contexts. However, the underlying mechanisms explaining how apoE4 leads to cognitive decline are not fully understood. Recently, the combination of human induced pluripotent stem cell (hiPSC) modeling of neurological diseases in vitro and electrophysiological studies in vivo have begun to unravel the intersection between apoE4, neuronal subtype dysfunction or loss, subsequent network deficits, and eventual cognitive decline. In this review, we provide an overview of the literature describing apoE4's detrimental effects in the central nervous system (CNS), specifically focusing on its contribution to neuronal subtype dysfunction or loss. We focus on γ-aminobutyric acid (GABA)-expressing interneurons in the hippocampus, which are selectively vulnerable to apoE4-mediated neurotoxicity. Additionally, we discuss the importance of the GABAergic inhibitory network to proper cognitive function and how dysfunction of this network manifests in AD. Finally, we examine how apoE4-mediated GABAergic interneuron loss can lead to inhibitory network deficits and how this deficit results in cognitive decline. We propose the following working model: Aging and/or stress induces neuronal expression of apoE. GABAergic interneurons are selectively vulnerable to intracellularly produced apoE4, through a tau dependent mechanism, which leads to their dysfunction and eventual death. In turn, GABAergic interneuron loss causes hyperexcitability and dysregulation of neural networks in the hippocampus and cortex. This dysfunction results in learning, memory, and other cognitive deficits that are the central features of AD.

Apolipoprotein E binds to and reduces serum levels of DNA-mimicking, pyrrolated proteins

Lysine N-pyrrolation, converting lysine residues to N ^ϵ-pyrrole-l-lysine, is a recently discovered post-translational modification. This naturally occurring reaction confers electrochemical properties onto proteins that potentially produce an electrical mimic to DNA and result in specificity toward DNA-binding molecules such as anti-DNA autoantibodies. The discovery of this unique covalent protein modification provides a rationale for establishing the molecular mechanism and broad functional significance of the formation and regulation of N ^ϵ-pyrrole-l-lysine-containing proteins. In this study, we used microbeads coupled to pyrrolated or nonpyrrolated protein to screen for binding activities of human serum-resident nonimmunoglobin proteins to the pyrrolated proteins. This screen identified apolipoprotein E (apoE) as a protein that innately binds the DNA-mimicking proteins in serum. Using an array of biochemical assays, we observed that the pyrrolated proteins bind to the N-terminal domain of apoE and that oligomeric apoE binds these proteins better than does monomeric apoE. Employing surface plasmon resonance and confocal microscopy, we further observed that apoE deficiency leads to significant accumulation of pyrrolated serum albumin and is associated with an enhanced immune response. These results, along with the observation that apoE facilitates the binding of pyrrolated proteins to cells, suggest that apoE may contribute to the clearance of pyrrolated serum proteins. Our findings uncover apoE as a binding target of pyrrolated proteins, providing a key link connecting covalent protein modification, lipoprotein metabolism, and innate immunity.

Apolipoprotein E associated with reconstituted high-density lipoprotein-like particles is protected from aggregation

Apolipoprotein E (APOE) genotype determines Alzheimer's disease (AD) susceptibility, with the APOE ε4 allele being an established risk factor for late‐onset AD. The ApoE lipidation status has been reported to impact amyloid‐beta (Aβ) peptide metabolism. The details of how lipidation affects ApoE behavior remain to be elucidated. In this study, we prepared lipid‐free and lipid‐bound ApoE particles, mimicking the high‐density lipoprotein particles found in vivo, for all three isoforms (ApoE2, ApoE3, and ApoE4) and biophysically characterized them. We find that lipid‐free ApoE in solution has the tendency to aggregate in vitro in an isoform‐dependent manner under near‐physiological conditions and that aggregation is impeded by lipidation of ApoE.

REP 2139: Antiviral Mechanisms and Applications in Achieving Functional Control of HBV and HDV Infection

Nucleic acid polymers (NAPs) are broad spectrum antiviral agents whose antiviral activity in hepatitis B virus (HBV) infection is derived from their ability to block the release of the hepatitis B virus surface antigen (HBsAg). This pharmacological activity blocks replenishment of HBsAg in the circulation, allowing host mediated clearance. This effect has important clinical significance as the clearance of circulating HBsAg dramatically potentiates the ability of immunotherapies to restore functional control of HBV infection which persists after antiviral therapy is removed. These effects are reproducible in preclinical evaluations and in several clinical trials that have evaluated the activity of the lead NAP, REP 2139, in monotherapy and in combination with immunotherapy in hepatitis B e antigen (HBeAg) negative and HBeAg positive HBV infection and also in HBeAg negative HBV/hepatitis D virus (HDV) coinfection. These antiviral effects of REP 2139 are achieved in the absence of any direct immunostimulatory effect in the liver and also without any discernible direct interaction with viral components. The search for the host protein interaction with NAPs that drives their antiviral effects is ongoing, and the interaction targeted by REP 2139 within infected cells has not yet been elucidated. This article provides an updated review of available data on the effects of REP 2139 in HBV and HDV infection and the ability of REP 2139-based combination therapy to achieve functional control of HBV and HDV infection in patients.

Hidden Aggregation Hot-Spots on Human Apolipoprotein E: A Structural Study

Human apolipoprotein E (apoE) is a major component of lipoprotein particles, and under physiological conditions, is involved in plasma cholesterol transport. Human apolipoprotein E found in three isoforms (E2; E3; E4) is a member of a family of apolipoproteins that under pathological conditions are detected in extracellular amyloid depositions in several amyloidoses. Interestingly, the lipid-free apoE form has been shown to be co-localized with the amyloidogenic Aβ peptide in amyloid plaques in Alzheimer’s disease, whereas in particular, the apoE4 isoform is a crucial risk factor for late-onset Alzheimer’s disease. Evidence at the experimental level proves that apoE self-assembles into amyloid fibrilsin vitro, although the misfolding mechanism has not been clarified yet. Here, we explored the mechanistic insights of apoE misfolding by testing short apoE stretches predicted as amyloidogenic determinants by AMYLPRED, and we computationally investigated the dynamics of apoE and an apoE–Αβ complex. Our in vitro biophysical results prove that apoE peptide–analogues may act as the driving force needed to trigger apoE aggregation and are supported by the computational apoE outcome. Additional computational work concerning the apoE–Αβ complex also designates apoE amyloidogenic regions as important binding sites for oligomeric Αβ; taking an important step forward in the field of Alzheimer’s anti-aggregation drug development.

Native Mass Spectrometry, Ion Mobility, Electron-Capture Dissociation, and Modeling Provide Structural Information for Gas-Phase Apolipoprotein E Oligomers

Apolipoprotein E (apoE) is an essential protein in lipid and cholesterol metabolism. Although the three common isoforms in humans differ only at two sites, their consequences in Alzheimer's disease (AD) are dramatically different: only the ε4 allele is a major genetic risk factor for late-onset Alzheimer's disease. The isoforms exist as a mixture of oligomers, primarily tetramer, at low μM concentrations in a lipid-free environment. This self-association is involved in equilibrium with the lipid-free state, and the oligomerization interface overlaps with the lipid-binding region. Elucidation of apoE wild-type (WT) structures at an oligomeric state, however, has not yet been achieved. To address this need, we used native electrospray ionization and mass spectrometry (native MS) coupled with ion mobility (IM) to examine the monomer and tetramer of the three WT isoforms. Although collision-induced unfolding (CIU) cannot distinguish the WT isoforms, the monomeric mutant (MM) of apoE3 shows higher stability when submitted to CIU than the WT monomer. From ion-mobility measurements, we obtained the collision cross section and built a coarse-grained model for the tetramer. Application of electron-capture dissociation (ECD) to the tetramer causes unfolding starting from the C-terminal domain, in good agreement with solution denaturation data, and provides additional support for the C4 symmetry structure of the tetramer.

The Role of ApoE in HCV Infection and Comorbidity

Hepatitis C virus (HCV) is an RNA virus that can efficiently establish chronic infection in humans. The overlap between the HCV replication cycle and lipid metabolism is considered to be one of the primary means by which HCV efficiently develops chronic infections. In the blood, HCV is complex with lipoproteins to form heterogeneous lipo-viro-particles (LVPs). Furthermore, apolipoprotein E (ApoE), which binds to receptors during lipoprotein transport and regulates lipid metabolism, is localized on the surface of LVPs. ApoE not only participate in the attachment and entry of HCV on the cell surface but also the assembly and release of HCV viral particles from cells. Moreover, in the blood, ApoE can also alter the infectivity of HCV and be used by HCV to escape recognition by the host immune system. In addition, because ApoE can also affect the antioxidant and immunomodulatory/anti-inflammatory properties of the host organism, the long-term binding and utilization of host ApoE during chronic HCV infection not only leads to liver lipid metabolic disorders but may also lead to increased morbidity and mortality associated with systemic comorbidities.

HCV Pit Stop at the Lipid Droplet: Refuel Lipids and Put on a Lipoprotein Coat before Exit

The replication cycle of the liver-tropic hepatitis C virus (HCV) is tightly connected to the host lipid metabolism, during the virus entry, replication, assembly and egress stages, but also while the virus circulates in the bloodstream. This interplay coins viral particle properties, governs viral cell tropism, and facilitates immune evasion. This review summarizes our knowledge of these interactions focusing on the late steps of the virus replication cycle. It builds on our understanding of the cell biology of lipid droplets and the biosynthesis of liver lipoproteins and attempts to explain how HCV hijacks these organelles and pathways to assemble its lipo-viro-particles. In particular, this review describes (i) the mechanisms of viral protein translocation to and from the lipid droplet surface and the orchestration of an interface between replication and assembly complexes, (ii) the importance of the triglyceride mobilization from the lipid droplets for HCV assembly, (iii) the interplay between HCV and the lipoprotein synthesis pathway including the role played by apolipoproteins in virion assembly, and finally (iv) the consequences of these complex virus–host interactions on the virion composition and its biophysical properties. The wealth of data accumulated in the past years on the role of the lipid metabolism in HCV assembly and its imprint on the virion properties will guide vaccine design efforts and reinforce our understanding of the hepatic lipid metabolism in health and disease.

The Role of APOE4 in Disrupting the Homeostatic Functions of Astrocytes and Microglia in Aging and Alzheimer's Disease

APOE4 is the greatest genetic risk factor for late-onset Alzheimer’s disease (AD), increasing the risk of developing the disease by 3-fold in the 14% of the population that are carriers. Despite 25 years of research, the exact mechanisms underlying how APOE4 contributes to AD pathogenesis remain incompletely defined. APOE in the brain is primarily expressed by astrocytes and microglia, cell types that are now widely appreciated to play key roles in the pathogenesis of AD; thus, a picture is emerging wherein APOE4 disrupts normal glial cell biology, intersecting with changes that occur during normal aging to ultimately cause neurodegeneration and cognitive dysfunction. This review article will summarize how APOE4 alters specific pathways in astrocytes and microglia in the context of AD and the aging brain. APOE itself, as a secreted lipoprotein without enzymatic activity, may prove challenging to directly target therapeutically in the classical sense. Therefore, a deeper understanding of the underlying pathways responsible for APOE4 toxicity is needed so that more tractable pathways and drug targets can be identified to reduce APOE4-mediated disease risk.

Peach Kernel Oil Downregulates Expression of Tissue Factor and Reduces Atherosclerosis in ApoE knockout Mice

Atherosclerosis is the pathological process in arteries due to the plaque formation that is responsible for several diseases like heart disease, stroke and peripheral arterial disease. In this study, we performed in vitro and in vivo assays to evaluate the potential anti-atherosclerosis activity of peach kernel oil. For the in vitro assay, we incubated human umbilical vein endothelial cells (HUVEC) with tumor necrosis factor-α (TNF-α) to induce tissue factors (TF, an essential mediator of hemostasis and trigger of thrombosis) elevation. We found that TNF-α-induced TF elevation was suppressed by peach kernel oil in a dose-dependent manner at both mRNA and protein levels. Peach kernel oil can significantly improve HUVEC viability, protect the endothelial cells, which achieved the goal of prevention of thrombotic diseases. For the in vivo assay, we investigated the effect and mechanism of peach kernel oil on preventing atherosclerotic lesion formation in ApoE knockout mice. Results show that peach kernel oil could reduce total cholesterol, triglyceride, low-density lipoprotein cholesterol levels, elevate the high-density lipoprotein cholesterol level in serum, and reduce the area of the aortic atherosclerotic lesions in high-fat diet fed ApoE knockout mice. Moreover, peach kernel oil treatment can significantly down regulate the expression of TF protein to inhibit the formation of atherosclerotic plaque. In conclusion, peach kernel oil may be a potential health food to prevent atherosclerosis in cardiovascular diseases.

An Integrative Overview of Non-Amyloid and Non-Tau Pathologies in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease that devastates the lives of its victims, and challenges the family members and health care infrastructures that care for them. Clinically, attempts to understand AD have focused on trying to predict the presence of, and more recently demonstrate the presence of, its characteristic amyloid plaque and neurofibrillary tangle pathologies. Fundamental research has also traditionally focused on understanding the generation, content, and pathogenicity of plaques and tangles, but in addition to this there is now an emerging independent interest in other molecular phenomena including apolipoprotein E, lipid metabolism, neuroinflammation, and mitochondrial function. While studies emphasizing the role of these phenomena have provided valuable AD insights, it is interesting that at the molecular level these entities extensively intertwine and interact. In this review, we provide a brief overview of why apolipoprotein E, lipid metabolism, neuroinflammation, and mitochondrial research have become increasingly ascendant in the AD research field, and present the case for studying these phenomena from an integrated perspective.

Comprehensive review of mechanisms of pathogenesis involved in Alzheimer's disease and potential therapeutic strategies

AD is a progressive neurodegenerative disorder and a leading cause of dementia in an aging population worldwide. The enormous challenge which AD possesses to global healthcare makes it as urgent as ever for the researchers to develop innovative treatment strategies to fight this disease. An in-depth analysis of the extensive available data associated with the AD is needed for a more comprehensive understanding of underlying molecular mechanisms and pathophysiological pathways associated with the onset and progression of the AD. The currently understood pathological and biochemical manifestations include cholinergic, Aβ, tau, excitotoxicity, oxidative stress, ApoE, CREB signaling pathways, insulin resistance, etc. However, these hypotheses have been criticized with several conflicting reports for their involvement in the disease progression. Several issues need to be addressed such as benefits to cost ratio with cholinesterase therapy, the dilemma of AChE selectivity over BChE, BBB permeability of peptidic BACE-1 inhibitors, hurdles related to the implementation of vaccination and immunization therapy, and clinical failure of candidates related to newly available targets. The present review provides an insight to the different molecular mechanisms involved in the development and progression of the AD and potential therapeutic strategies, enlightening perceptions into structural information of conventional and novel targets along with the successful applications of computational approaches for the design of target-specific inhibitors.

Thermodynamic destabilization and aggregation propensity as the mechanism behind the association of apoE3 mutants and lipoprotein glomerulopathy

Lipoprotein glomerulopathy (LPG) is a rare renal disease, characterized by lipoprotein thrombi in glomerular capillaries. A series of apoE mutations have been associated with LPG development. We previously showed that three mutants based on apoE3 sequence, in which an arginine was substituted by proline, are thermodynamically destabilized and aggregation-prone. To examine whether other LPG-associated apoE3 mutations induce similar effects, we characterized three nonproline LPG-associated apoE3 mutations, namely, R25C (apoEKyoto), R114C (apoETsukuba), and A152D (apoELasVegas). All three apoE3 variants are found to have significantly reduced helical content and to be thermodynamically destabilized, both in lipid-free and lipoprotein-associated form, and to expose a larger portion of hydrophobic surface to the solvent compared with WT apoE3. Furthermore, all three apoE3 variants are aggregation-prone, as shown by dynamic light-scattering measurements and by their enhanced capacity to bind the amyloid probe thioflavin T. Overall, our data suggest that the LPG-associated apoE3 mutations R25C, R114C, and A152D induce protein misfolding, which may contribute to protein aggregation in glomerular capillaries. The similar effects of both LPG-associated proline and nonproline mutations on apoE3 structure suggest that the thermodynamic destabilization and enhanced aggregation of apoE3 may constitute a common underlying mechanism behind the pathogenesis of LPG.

Amyloid-peptide β 42 Enhances the Oligomerization and Neurotoxicity of apoE4: The C-terminal Residues Leu279, Lys282 and Gln284 Modulate the Structural and Functional Properties of apoE4

Apolipoprotein E4 (apoE4), one of the three apoE isoforms, is the strongest factor for raising the risk for late-onset Alzheimer's disease (AD) and has been proposed to play a major role in AD pathogenesis. Amyloid-peptide β 42 (Aβ42) has also been proposed to affect neuronal degeneration and AD pathogenesis, possibly by interacting with apoE. Previous studies have shown that the functions of apoE forms can be dictated by their structural and biophysical properties. Here we show that apoE4 can form SDS-stable oligomers, possibly reflecting aggregated forms, which increase following incubation of apoE4 with Aβ42. In addition, extracellular apoE4 is cytotoxic for human neuroblastoma SK-N-SH cells, while Aβ42 enhances the cytotoxicity of apoE4. Carboxyl-terminal point mutations L279Q, K282A or Q284A reduced the capacity of apoE4 to form SDS-stable oligomers, as well as its cytotoxicity, both in the absence and presence of Aβ42. Structural and thermodynamic analyses showed that all three apoE4 mutants have significantly increased α-helical and decreased β-sheet content, have reduced portion of hydrophobic surfaces exposed to the solvent and have a reduced conformational stability during chemical denaturation. Overall, our data highlight a pathogenic role of apoE4 that could be linked to the capacity of the protein to form oligomeric species especially in the presence of Aβ42 and to induce cytotoxicity. Carboxyl-terminal residues L279, K282 or Q284 appear to be involved in the conformation of apoE4 that may underlie the protein's functional properties related to neurotoxicity.

Soluble LR11 competes with amyloid β in binding to cerebrospinal fluid-high-density lipoprotein

BACKGROUND

LR11 is a member of the low-density lipoprotein (LDL) receptor family with high expression in neurons. Some cell surface LR11 is cleaved and secreted into the cerebrospinal fluid (CSF) as soluble LR11 (sLR11). Patients with Alzheimer's disease (AD), particularly apolipoprotein E4 carriers, have high CSF-sLR11 and low CSF-amyloid β (Aβ) concentrations. Therefore, we assessed whether sLR11 is bound to CSF-high-density lipoprotein (HDL) and whether sLR11 competes with Aβ in binding to apoE in CSF-HDL.

METHODS

We measured CSF-sLR11 concentrations (50 controls and 16 patients with AD) using enzyme immunoassay. sLR11 and apoE distribution in the CSF was evaluated using non-denaturing two-dimensional gel electrophoresis (N-2DGE). ApoE bound to sLR11 or Aβ was identified using co-immunoprecipitation assay.

RESULTS

CSF-sLR11 concentrations were higher in patients with AD than controls (adjusted for sLR11 using phospholipid). N-2DGE analysis showed that sLR11 and Aβ comigrated with a large apoE-containing CSF-HDL. Moreover, fewer apoE was bound to Aβ when a higher amount of apoE was bound to sLR11 in patients with AD who presented with ε4/4.

CONCLUSION

sLR11 binds to CSF-HDL and competes with Aβ in binding to apoE in CSF-HDL, indicating that sLR11 affects Aβ clearance via CSF-HDL.

The APOE ε4 allele is associated with a reduction in FEV1/FVC in women: A cross-sectional analysis of the Long Life Family Study

INTRODUCTION

Murine studies have shown that apolipoprotein E modulates pulmonary function during development, aging, and allergen-induced airway disease. It is not known whether the polymorphic human APOE gene influences pulmonary function.

OBJECTIVES

We assessed whether an association exists between the polymorphic human APOE ε2, ε3, and ε4 alleles and pulmonary function among participants in the Long Life Family Study.

METHODS

Data from 4,468 Caucasian subjects who had genotyping performed for the APOE ε2, ε3, and ε4 alleles were analyzed, with and without stratification by sex. Statistical models were fitted considering the effects of the ε2 allele, defined as ε2/2 or ε2/3 genotypes, and the ε4 allele, defined as ε3/4 or ε4/4 genotypes, which were compared to the ε3/3 genotype.

RESULTS

The mean FEV1/FVC ratio (the forced expiratory volume in one second divided by the forced vital capacity) was lower among women with the ε4 allele as compared to women with the ε3/3 genotype or the ε2 allele. Carriage of the APOE ε4 allele was associated with FEV1/FVC, which implied lower values. Further analysis showed that the association primarily reflected women without lung disease who were older than 70 years. The association was not mediated by lipid levels, smoking status, body mass index, or cardiovascular disease.

CONCLUSIONS

This study for the first time identifies that the APOE gene is associated with modified lung physiology in women. This suggests that a link may exist between the APOE ε4 allele, female sex, and a reduction in the FEV1/FVC ratio in older individuals.

The effects of human leukocyte antigen DRB1*13 and apolipoprotein E on age-related variability of synchronous neural interactions in healthy women

BACKGROUND

Age-related brain changes are well-documented and influenced by genetics. Extensive research links apolipoprotein E (apoE) to brain function, with the E4 allele serving as a risk factor for brain disease, including Alzheimer's disease, and the E2 allele conferring protection. Recent evidence also supports protective effects of another gene, human leukocyte antigen (HLA) DRB1*13, on brain disease and age-related brain atrophy in cognitively healthy adults. Here we investigated the effects of apoE and HLA DRB1*13 on brain function by examining changes in neural network properties with age in healthy adults.

METHODS

One hundred seventy-eight cognitively healthy women (28-99 y old) underwent a magnetoencephalography scan and provided a blood sample for genetic analysis. Age-related changes in neural network variability in genetic subgroups of DRB1*13 × apoE genotype combinations were assessed using linear regression of network variability against age.

FINDINGS

For individuals lacking a DRB1*13 allele and/or carrying an apoE4 allele, network variability increased significantly with age. In contrast, no such increase was observed in the presence of DRB1*13 and/or apoE2.

INTERPRETATION

These findings extend previous research documenting the protective effect of DRB1*13 on brain structure to include protection against age-related changes in brain function, and demonstrate similar protective effects on neural network variability for either DRB1*13 or apoE2. These protective effects could be due to reduction or elimination of factors known to disrupt brain function, including neuroinflammation and amyloid beta protein.

FUNDING

U.S. Department of Veterans Affairs, and University of Minnesota.

Regulation of Apolipoprotein E Trafficking by Hepatitis C Virus-Induced Autophagy

Apolipoprotein E (ApoE) plays an important role in the maturation and infectivity of hepatitis C virus (HCV). By analyzing the subcellular localization of ApoE in Huh7 hepatoma cells that harbored an HCV subgenomic RNA replicon, we found that ApoE colocalized with autophagosomes. This colocalization was marginally detected in HCV-infected cells, apparently due to the depletion of ApoE by HCV, as treatment with bafilomycin A1 (BafA1), a vacuolar ATPase inhibitor that inhibits autophagic protein degradation, partially restored the ApoE level and enhanced its colocalization with autophagosomes in HCV-infected cells. The role of HCV-induced autophagy in the degradation of ApoE was further supported by the observations that nutrient starvation, which induces autophagic protein degradation, led to the loss of ApoE in HCV subgenomic RNA replicon cells and that the knockdown of ATG7, a protein essential for the formation of autophagic vacuoles, increased the ApoE level in cells with productive HCV replication. Interestingly, the inhibition of autophagy by ATG7 knockdown reduced the colocalization of ApoE with the HCV E2 envelope protein and the HCV titers released from cells. In contrast, the treatment of cells with BafA1 enhanced the colocalization of ApoE and HCV E2 and increased both intracellular and extracellular HCV titers. These results indicated that autophagy played an important role in the trafficking of ApoE in HCV-infected cells. While it led to autophagic degradation of ApoE, it also promoted the interaction between ApoE and HCV E2 to enhance the production of infectious progeny viral particles.IMPORTANCE Hepatitis C virus (HCV) is one of the most important human pathogens. Its virion is associated with apolipoprotein E (ApoE), which enhances its infectivity. HCV induces autophagy to enhance its replication. In this report, we demonstrate that autophagy plays an important role in the trafficking of ApoE in HCV-infected cells. This leads to the degradation of ApoE by autophagy. However, if the autophagic protein degradation is inhibited, ApoE is stabilized and colocalized with autophagosomes. This leads to its enhanced colocalization with the HCV E2 envelope protein and increased production of infectious progeny viral particles. If autophagy is inhibited by suppressing the expression of ATG7, a gene essential for the formation of autophagosomes, the colocalization of ApoE with E2 is reduced, resulting in the reduction of progeny viral titers. These results indicate an important role of autophagy in the transport of ApoE to promote the production of infectious HCV particles.

Remnant-Like Particle Cholesterol, Low-Density Lipoprotein Triglycerides, and Incident Cardiovascular Disease

BACKGROUND

Hypertriglyceridemia is associated with increased remnant-like particle cholesterol (RLP-C) and triglycerides in low-density lipoprotein (LDL-TG). Recent studies have focused on atherogenicity of RLP-C, with few data on LDL-TG.

OBJECTIVES

The aim of this study was to examine associations of RLP-C and LDL-TG with incident cardiovascular disease (CVD) events and genetic variants in the ARIC (Atherosclerosis Risk In Communities) study.

METHODS

Fasting plasma RLP-C and LDL-TG levels were measured in 9,334 men and women without prevalent CVD. Participants were followed for incident CVD events (coronary heart disease and ischemic stroke) for up to 16 years. Associations between LDL-TG and RLP-C levels and genetic variants were assessed by whole-exome sequencing using single-variant analysis for common variants and gene-based burden tests for rare variants; both an unbiased and a candidate gene approach were explored.

RESULTS

RLP-C and LDL-TG levels were correlated with triglyceride levels (r = 0.85 and r = 0.64, p < 0.0001). In minimally adjusted analyses, RLP-C and LDL-TG were associated with CVD risk, but in models adjusted for traditional risk factors including lipids, only LDL-TG was associated with incident CHD (hazard ratio: 1.28; 95% confidence interval: 1.10 to 1.50) and stroke (hazard ratio: 1.47; 95% confidence interval: 1.13 to 1.92). A common APOE variant, rs7412, had the strongest association with LDL-TG and RLP-C (p < 5 × 10-8).

CONCLUSIONS

RLP-C and LDL-TG levels were predictive of CVD and associated with APOE variants. LDL-TG may represent a marker of dysfunctional remnant lipoprotein metabolism associated with increased CVD risk. Further research is needed to determine whether LDL-TG plays a causal role in CVD and may be a target for therapy.

Aging and Apolipoprotein E in HIV Infection

With the implementation of increasingly effective antiretroviral therapy (ART) over the past three decades, individuals infected with HIV live a much longer life. HIV infection is no longer a terminal but rather a chronic disease. However, the lifespan of infected individuals remains shorter than that of their uninfected peers. Even with ART, HIV infection may potentiate “premature” aging. Organ-associated disease and systemic syndromes that occur in treated HIV-infection are like that of older, uninfected individuals. Brain aging may manifest as structural changes or neurocognitive impairment that are beyond the chronological age. The spectrum of neurological, cognitive, and motor deficiencies, currently described as HIV-associated neurocognitive disorders (HAND), may reflect earlier onset of mechanisms common to HIV infection and aging (accelerated aging). HAND could also reflect the neurological impact of HIV infection superimposed on comorbidities linked to age and chronic inflammation, leading to a higher prevalence of neurocognitive impairment across the age span (accentuated aging). In addition, apolipoprotein E (ApoE), one of the most influential host risk factors for developing Alzheimer’s disease, has been implicated in the development of HAND. But studies differ as to whether ApoE is relevant, and whether age and ApoE interact to impair brain function in the HIV-infected patient. What is clear is that HIV-infected individuals are living longer with HIV, and therefore factors related to aging and health need to be examined in the context of current, effective ART. This review addresses the recent evidence for the influence of aging and ApoE on HIV-associated neurocognitive impairment.

A flow cytometry-based in vitro assay reveals that formation of apolipoprotein E (ApoE)-amyloid beta complexes depends on ApoE isoform and cell type

Apolipoprotein E (ApoE) is a secreted apolipoprotein with three isoforms, E2, E3, and E4, that binds to lipids and facilitates their transport in the extracellular environment of the brain and the periphery. The E4 allele is a major genetic risk factor for the sporadic form of Alzheimer's disease (AD), and studies of human brain and mouse models have revealed that E4 significantly exacerbates the deposition of amyloid beta (Aβ). It has been suggested that this deposition could be attributed to the formation of soluble ApoE isoform-specific ApoE-Aβ complexes. However, previous studies have reported conflicting results regarding the directionality and strength of those interactions. In this study, using a series of flow cytometry assays that maintain the physiological integrity of ApoE-Aβ complexes, we systematically assessed the association of Aβ with ApoE2, E3, or E4. We used ApoE secreted from HEK cells or astrocytes overexpressing ApoE fused with a GFP tag. As a source of soluble Aβ peptide, we used synthetic Aβ40 or Aβ42 or physiological Aβ secreted from CHO cell lines overexpressing WT or V717F variant amyloid precursor protein (APP). We observed significant interactions between the different ApoE isoforms and Aβ, with E4 interacting with Aβ more strongly than the E2 and E3 isoforms. We also found subtle differences depending on the Aβ type and the ApoE-producing cell type. In conclusion, these results indicate that the strength of the ApoE-Aβ association depends on the source of Aβ or ApoE.

Phenotype and genotype predictors of BMI variability among European adults

Background/Objective

Obesity is a complex and multifactorial disease resulting from the interactions among genetics, metabolic, behavioral, sociocultural and environmental factors. In this sense, the aim of the present study was to identify phenotype and genotype variables that could be relevant determinants of body mass index (BMI) variability.

Subjects/Methods

In the present study, a total of 1050 subjects (798 females; 76%) were included. Least angle regression (LARS) analysis was used as regression model selection technique, where the dependent variable was BMI and the independent variables were age, sex, energy intake, physical activity level, and 16 polymorphisms previously related to obesity and lipid metabolism.

Results

The LARS analysis obtained the following formula for BMI explanation: (64.7 + 0.10 × age [years] + 0.42 × gender [0, men; 1, women] + −40.6 × physical activity [physical activity level] + 0.004 × energy intake [kcal] + 0.74 × rs9939609 [0 or 1–2 risk alleles] + −0.72 × rs1800206 [0 or 1–2 risk alleles] + −0.86 × rs1801282 [0 or 1–2 risk alleles] + 0.87 × rs429358 [0 or 1–2 risk alleles]. The multivariable regression model accounted for 21% of the phenotypic variance in BMI. The regression model was internally validated by the bootstrap method (r² original data set = 0.208, mean r² bootstrap data sets = 0.210).

Conclusion

In conclusion, age, physical activity, energy intake and polymorphisms in FTO, APOE, PPARG and PPARA genes are significant predictors of the BMI trait.

Brain APOE expression quantitative trait loci-based association study identified one susceptibility locus for Alzheimer's disease by interacting with APOE ε4

Some studies have demonstrated interactions of AD-risk single nucleotide polymorphisms (SNPs) in non-APOE regions with APOE genotype. Nevertheless, no study reported interactions of expression quantitative trait locus (eQTL) for APOE with APOE genotype. In present study, we included 9286 unrelated AD patients and 8479 normal controls from 12 cohorts of NIA Genetics of Alzheimer’s Disease Data Storage Site (NIAGADS) and Alzheimer’s Disease Neuroimaging Initiative (ADNI). 34 unrelated brain eQTLs for APOE were compiled from BRAINEAC and GTEx. We used multi-covariate logistic regression analysis to identify eQTLs interacted with APOE ε4. Adjusted for age and gender, substantia nigra eQTL rs438811 for APOE showed significantly strong interaction with APOE ε4 status (OR, 1.448; CI, 1.124–1.430; P-value = 7.94 × 10⁻⁶). APOE ε4-based sub-group analyses revealed that carrying one minor allele T of rs438811 can increase the opportunity of developing to AD by 26.75% in APOE ε4 carriers but not in non-carriers. We revealed substantia nigra eQTL rs438811 for APOE can interact with APOE ε4 and confers risk in APOE ε4 carriers only.

Charged Residues in the C-Terminal Domain of Apolipoprotein A-I Modulate Oligomerization

Charged residues of the C-terminal domain of human apolipoprotein A-I (apoA-I) were targeted by site-directed mutagenesis. A series of mutant proteins was engineered in which lysine residues (Lys 195, 206, 208, 226, 238, and 239) or glutamate residues (Glu 234 and 235) were replaced by glutamine. The amino acid substitutions did not result in changes in secondary structure content or protein stability. Cross-linking and size-exclusion chromatography showed that the mutations resulted in reduced self-association, generating a predominantly monomeric apoA-I when five or six lysine residues were substituted. The rate of phosphatidylcholine vesicle solubilization was enhanced for all variants, with approximately a threefold rate enhancement for apoA-I lacking Lys 206, 208, 238, and 239, or Glu 234 and 235. Single or double mutations did not change the ability to protect lipolyzed low density lipoprotein from aggregation, but variants lacking >4 lysine residues were less effective in preventing lipoprotein aggregation. ApoA-I mediated cellular lipid efflux from wild-type mice macrophage foam cells was decreased for the variant with five lysine mutations. However, this protein was more effective in releasing cellular phosphatidylcholine and sphingomyelin from Abca1-null mice macrophage foam cells. This suggests that the mutations caused changes in the interaction with ABCA1 transporters and that membrane microsolubilization was primarily responsible for lipid efflux in cells lacking ABCA1. Taken together, this study indicates that ionic interactions in the C-terminal domain of apoA-I favor self-association and that monomeric apoA-I is more active in solubilizing phospholipid bilayers.

Class I HDAC inhibition is a novel pathway for regulating astrocytic apoE secretion

Despite the important role of apolipoprotein E (apoE) secretion from astrocytes in brain lipid metabolism and the strong association of apoE4, one of the human apoE isoforms, with sporadic and late onset forms of Alzheimer's disease (AD) little is known about the regulation of astrocytic apoE. Utilizing annotated chemical libraries and a phenotypic screening strategy that measured apoE secretion from a human astrocytoma cell line, inhibition of pan class I histone deacetylases (HDACs) was identified as a mechanism to increase apoE secretion. Knocking down select HDAC family members alone or in combination revealed that inhibition of the class I HDAC family was responsible for enhancing apoE secretion. Knocking down LXRα and LXRβ genes revealed that the increase in astrocytic apoE in response to HDAC inhibition occurred via an LXR-independent pathway. Collectively, these data suggest that pan class I HDAC inhibition is a novel pathway for regulating astrocytic apoE secretion.

Apolipoprotein E isoform dependently affects Tat-mediated HIV-1 LTR transactivation

Background

Apolipoprotein E (ApoE) is the major carrier protein that mediates the transport and delivery of cholesterol and other lipids in the brain. Three isoforms of ApoE (ApoE2, ApoE3, ApoE4) exist in humans, and their relative expression levels impact HIV-1 infection, HIV-1/AIDS disease progression, and cognitive decline associated with HIV-1-associated neurocognitive disorder. Because HIV-1 Tat, a viral protein essential for HIV-1 replication, can bind to low-density lipoprotein receptor-related protein 1 (LRP1) that controls ApoE uptake in the brain, we determined the extent to which different isoforms of ApoE affected Tat-mediated HIV-1 LTR transactivation.

Methods

Using U87MG glioblastoma cells expressing LTR-driven luciferase, we determined the extent to which LRP1 as well as ApoE2, ApoE3, and ApoE4 affected Tat-mediated HIV-1 LTR transactivation.

Results

A specific LRP1 antagonist and siRNA knockdown of LRP1 both restricted significantly Tat-mediated LTR transactivation. Of the three ApoEs, ApoE4 was the least potent and effective at preventing HIV-1 Tat internalization and at decreasing Tat-mediated HIV-1 LTR transactivation. Further, Tat-mediated LTR transactivation was attenuated by an ApoE mimetic peptide, and ApoE4-induced restriction of Tat-mediated LTR transactivation was potentiated by an ApoE4 structure modulator that changes ApoE4 into an ApoE3-like phenotype.

Conclusions

These findings help explain observed differential effects of ApoEs on HIV-1 infectivity and the prevalence of HAND in people living with HIV-1 infection and suggest that ApoE mimetic peptides and ApoE4 structure modulator might be used as a therapeutic strategy against HIV-1 infection and associated neurocognitive disorders.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1129-1) contains supplementary material, which is available to authorized users.

Effect of human very low-density lipoproteins on cardiotrophin-like cytokine factor 1 (CLCF1) activity

The cytokines CLCF1 and CNTF are ligands for the CNTF receptor and the apolipoprotein E (ApoE) receptor sortilin. Both share structural similarities with the N-terminal domain of ApoE, known to bind CNTF. We therefore evaluated whether ApoE or ApoE-containing lipoproteins interact with CLCF1 and regulate its activity. We observed that CLCF1 forms complexes with the three major isoforms of ApoE in co-immunoprecipitation and proximity assays. FPLC analysis of mouse and human sera mixed with CLCF1 revealed that CLCF1 co-purifies with plasma lipoproteins. Studies with sera from ApoE^-/- mice indicate that ApoE is not required for CLCF1-lipoprotein interactions. VLDL- and LDL-CLCF1 binding was confirmed using proximity and ligand blots assays. CLCF1-induced STAT3 phosphorylation was significantly reduced when the cytokine was complexed with VLDL. Physiological relevance of our findings was asserted in a mouse model of oxygen-induced retinopathy, where the beneficial anti-angiogenic properties of CLCF1 were abrogated when co-administrated with VLDL, indicating, that CLCF1 binds purified lipoproteins or lipoproteins in physiological fluids such as serum and behave as a "lipocytokine". Albeit it is clear that lipoproteins modulate CLCF1 activity, it remains to be determined whether lipoprotein binding directly contributes to its neurotrophic function and its roles in metabolic regulation.

APOE gene and neuropsychiatric disorders and endophenotypes: A comprehensive review

The Apolipoprotein E (APOE) gene is one of the main candidates in neuropsychiatric genetics, with hundreds of studies carried out in order to explore the possible role of polymorphisms in the APOE gene in a large number of neurological diseases, psychiatric disorders, and related endophenotypes. In the current article, we provide a comprehensive review of the structural and functional aspects of the APOE gene and its relationship with brain disorders. Evidence from genome-wide association studies and meta-analyses shows that the APOE gene has been significantly associated with several neurodegenerative disorders. Cellular and animal models show growing evidence of the key role of APOE in mechanisms of brain plasticity and behavior. Future analyses of the APOE gene might find a possible role in other neurological diseases and psychiatric disorders and related endophenotypes. © 2016 Wiley Periodicals, Inc.

Atomistic Insights into Structural Differences between E3 and E4 Isoforms of Apolipoprotein E

Among various isoforms of Apolipoprotein E (ApoE), the E4 isoform (ApoE4) is considered to be the strongest risk factor for Alzheimer's disease, whereas the E3 isoform (ApoE3) is neutral to the disease. Interestingly, the sequence of ApoE4 differs from its wild-type ApoE3 by a single amino acid C112R in the 299-amino-acid-long sequence. Hence, the puzzle remains: how a single-amino-acid difference between the ApoE3 and ApoE4 sequences can give rise to structural dissimilarities between the two isoforms, which can potentially lead to functional differences with significant pathological consequences. The major obstacle in addressing this question has been the lack of a 3D atomistic structure of ApoE4 to date. In this work, we resolve the issue by computationally modeling a plausible atomistic 3D structure of ApoE4. Our microsecond-long atomistic simulations elucidate key structural differences between monomeric ApoE3 and ApoE4, which renders ApoE4 thermodynamically less stable, less structured, and topologically less rigid compared to ApoE3. Consistent with an experimental report of the molten globule state of ApoE4, simulations identify multiple partially folded intermediates for ApoE4, which are implicated in the stronger aggregation propensity of ApoE4.

Perspectives of ruthenium(ii) polyazaaromatic photo-oxidizing complexes photoreactive towards tryptophan-containing peptides and derivatives

This review focuses on recent advances in the search for Ru^II polyazaaromatic complexes as molecular photoreagents for tryptophan-containing peptides and proteins, in view of future biomedical applications.

Apolipoprotein E4 exacerbates ethanol-induced neurotoxicity through augmentation of oxidative stress and apoptosis in N2a-APP cells

Neuronal loss is a prominent phenomenon in Alzheimer's disease (AD) and alcohol-induced brain damage. Alcohol abuse is associated with an increased risk of AD, regardless of the type of alcoholic beverage. Furthermore, the detrimental effect of excessive alcohol consumption on the risk of AD is exacerbated among people carrying apolipoprotein E (APOE) ε4 allele, the major genetic risk factor for AD. However, how APOE ε4 and alcohol abuse synergistically enhance the possibility of AD is unclear. Here we show that in N2a cells stably expressing human APP695 (N2a-APP), high-concentration ethanol-induced neurotoxicity was significantly augmented in the presence of apoE4 protein, compared with apoE3 protein. Early and late apoptotic cells were apparently more in cells treated with the combination of apoE4 and ethanol, compared with that of apoE3 and ethanol. Inhibition of apoptosis using a pan-caspase inhibitor z-vad resulted in abolishment of the apoE isoform-specific effect on high-concentration ethanol-induced neurotoxicity. Moreover, compared with apoE3, apoE4 augmented ethanol-induced cellular oxidative stress, and pre-incubation with a reactive oxygen species (ROS) scavenger NAC abrogated the specific effect of apoE4 on ethanol-induced neurotoxicity. Taken together, our results for the first time demonstrate that apoE4 and high-concentration ethanol synergistically enhance neurotoxicity through elevating cellular oxidative stress and increasing neuronal apoptosis, and support the notion that avoiding excessive alcohol consumption can help to prevent AD especially in APOE ε4 carriers.

Peptide-Level Interactions between Proteins and Small-Molecule Drug Candidates by Two Hydrogen-Deuterium Exchange MS-Based Methods: The Example of Apolipoprotein E3

We describe a platform utilizing two methods based on hydrogen-deuterium exchange (HDX) coupled with mass spectrometry (MS) to characterize interactions between a protein and a small-molecule ligand. The model system is apolipoprotein E3 (apoE3) and a small-molecule drug candidate. We extended PLIMSTEX (protein-ligand interactions by mass spectrometry, titration, and H/D exchange) to the regional level by incorporating enzymatic digestion to acquire binding information for peptides. In a single experiment, we not only identified putative binding sites, but also obtained affinities of 6.0, 6.8, and 10.6 μM for the three different regions, giving an overall binding affinity of 7.4 μM. These values agree well with literature values determined by accepted methods. Unlike those methods, PLIMSTEX provides site-specific binding information. The second approach, modified SUPREX (stability of unpurified proteins from rates of H/D exchange) coupled with electrospray ionization (ESI), allowed us to obtain detailed understanding about apoE unfolding and its changes upon ligand binding. Three binding regions, along with an additional site, which may be important for lipid binding, show increased stability (less unfolding) upon ligand binding. By employing a single parameter, ΔC_1/2%, we compared relative changes of denaturation between peptides. This integrated platform provides information orthogonal to commonly used HDX kinetics experiments, providing a general and novel approach for studying protein-ligand interactions.

Hepatitis C Virus Strain-Dependent Usage of Apolipoprotein E Modulates Assembly Efficiency and Specific Infectivity of Secreted Virions

Hepatitis C virus (HCV) is extraordinarily diverse and uses entry factors in a strain-specific manner. Virus particles associate with lipoproteins, and apolipoprotein E (ApoE) is critical for HCV assembly and infectivity. However, whether ApoE dependency is common to all HCV genotypes remains unknown. Therefore, we compared the roles of ApoE utilizing 10 virus strains from genotypes 1 through 7. ApoA and ApoC also support HCV assembly, so they may contribute to virus production in a strain-dependent fashion. Transcriptome sequencing (RNA-seq) revealed abundant coexpression of ApoE, ApoB, ApoA1, ApoA2, ApoC1, ApoC2, and ApoC3 in primary hepatocytes and in Huh-7.5 cells. Virus production was examined in Huh-7.5 cells with and without ApoE expression and in 293T cells where individual apolipoproteins (ApoE1, -E2, -E3, -A1, -A2, -C1, and -C3) were provided in trans All strains were strictly ApoE dependent. However, ApoE involvement in virus production was strain and cell type specific, because some HCV strains poorly produced infectious virus in ApoE-expressing 293T cells and because ApoE knockout differentially affected virus production of HCV strains in Huh-7.5 cells. ApoE allelic isoforms (ApoE2, -E3, and -E4) complemented virus production of HCV strains to comparable degrees. All tested strains assembled infectious progeny with ApoE in preference to other exchangeable apolipoproteins (ApoA1, -A2, -C1, and -C3). The specific infectivity of HCV particles was similar for 293T- and Huh-7.5-derived particles for most strains; however, it differed by more than 100-fold in some viruses. Collectively, this study reveals strain-dependent and host cell-dependent use of ApoE during HCV assembly. These differences relate to the efficacy of virus production and also to the properties of released virus particles and therefore govern viral fitness at the level of assembly and cell entry.IMPORTANCE Chronic HCV infections are a major cause of liver disease. HCV is highly variable, and strain-specific determinants modulate the response to antiviral therapy, the natural course of infection, and cell entry factor usage. Here we explored whether host factor dependency of HCV in particle assembly is modulated by strain-dependent viral properties. We showed that all examined HCV strains, which represent all seven known genotypes, rely on ApoE expression for assembly of infectious progeny. However, the degree of ApoE dependence is modulated in a strain-specific and cell type-dependent manner. This indicates that HCV strains differ in their assembly properties and host factor usage during assembly of infectious progeny. Importantly, these differences relate not only to the efficiency of virus production and release but also to the infectiousness of virus particles. Thus, strain-dependent features of HCV modulate ApoE usage, with implications for virus fitness at the level of assembly and cell entry.

Evolution of human apolipoprotein E (APOE) isoforms: Gene structure, protein function and interaction with dietary factors

Apolipoprotein E (APOE) is a member of the vertebrate protein family of exchangeable apolipoproteins that is characterized by amphipathic α-helices encoded by multiple nucleotide tandem repeats. Its equivalent in flying insects - apolipophorin-III - shares structural and functional commonalities with APOE, suggesting the possibility of an evolutionary relationship between the proteins. In contrast to all other known species, human APOE is functionally polymorphic and possesses three major allelic variants (ε4, ε3 and ε2). The present review examines the current knowledge on APOE gene structure, phylogeny and APOE protein topology as well as its human isoforms. The ε4 allele is associated with an increased age-related disease risk but is also the ancestral form. Despite increased mortality in the elderly, ε4 has not become extinct and is the second-most common allele worldwide after ε3. APOE ε4, moreover, shows a non-random geographical distribution, and similarly, the ε2 allele is not homogenously distributed among ethnic populations. This likely suggests the existence of selective forces that are driving the evolution of human APOE isoforms, which may include differential interactions with dietary factors. To that effect, micronutrients such as vitamin D and carotenoids or dietary macronutrient composition are elucidated with respect to APOE evolution.

Isoform- and cell type-specific structure of apolipoprotein E lipoparticles as revealed by a novel Forster resonance energy transfer assay

Apolipoprotein E (apoE) has an important role in the pathogenesis of Alzheimer's disease with its three isoforms having distinct effects on disease risk. Here, we assessed the conformational differences between those isoforms using a novel flow cytometry-Forster resonance energy transfer (FRET) assay. We showed that the conformation of intracellular apoE within HEK cells and astrocytes adopts a directional pattern; in other words, E4 adopts the most closed conformation, E2 adopts the most open conformation, and E3 adopts an intermediate conformation. However, this pattern was not maintained upon secretion of apoE from astrocytes. Intermolecular interactions between apoE molecules were isoform-specific, indicating a great diversity in the structure of apoE lipoparticles. Finally, we showed that secreted E4 is the most lipidated isoform in astrocytes, suggesting that increased lipidation acts as a folding chaperone enabling E4 to adopt a closed conformation. In conclusion, this study gives insights into apoE biology and establishes a robust screening system to monitor apoE conformation.

Association between polymorphisms in the promoter region of the apolipoprotein E (APOE) gene and Alzheimer's disease: A meta-analysis

Several studies have evaluated the role of polymorphisms in the promoter region of APOE gene that encodes apolipoprotein E (APOE) and the susceptibility to Alzheimer's disease (AD). The aim of this literature review and meta-analysis was to investigate the relationship between the APOE promoter region single nucleotide polymorphisms (SNPs) (rs449647, -491A/T; rs769446, -427T/C and rs405509 -219T/G) and the risk of developing AD. Eligible controlled studies published up to November 2016 were retrieved from main online scientific and medical databases. Odds ratio (OR) and 95 % confidence interval (CI) were used to calculate the strength of the relationship. A total of 23 publications (19 for rs449647, ten for rs769446 and ten for rs405509) were retrieved that included 5,703 patients with AD and 5,692 controls. The C allele of the rs769446 variant of the promoter region of APOE gene was significantly associated with an increase of risk of AD (OR = 1.271, 95 % CI = 1.114-1.449, P < 0.0001), while other genetic models of this variant were not related with susceptibility to AD. Rs449647 and rs405509 polymorphisms of APOE gene were not associated with an increase of risk of AD. The findings of this literature review and meta-analysis have shown that rs769446 polymorphism in the promoter region of APOE gene could be a risk factor for AD. Future large-scale studies on the role of polymorphisms in the promoter region of APOE gene in AD are still awaited.

A mechanism for lipid binding to apoE and the role of intrinsically disordered regions coupled to domain-domain interactions

Relative to the apolipoprotein E (apoE) E3 allele of the APOE gene, apoE4 strongly increases the risk for the development of late-onset Alzheimer's disease. However, apoE4 differs from apoE3 by only a single amino acid at position 112, which is arginine in apoE4 and cysteine in apoE3. It remains unclear why apoE3 and apoE4 are functionally different. Described here is a proposal for understanding the functional differences between these two isoforms with respect to lipid binding. A mechanism is proposed that is based on the full-length monomeric structure of the protein, on hydrogen-deuterium exchange mass spectrometry data, and on the role of intrinsically disordered regions to control protein motions. It is proposed that lipid binds between the N-terminal and C-terminal domains and that separation of the two domains, along with the presence of intrinsically disordered regions, controls this process. The mechanism explains why apoE3 differs from apoE4 with respect to different lipid-binding specificities, why lipid increases the binding of apoE to its receptor, and why specific residues are conserved.