Characterization of the heparin binding sites in human apolipoprotein E

ApoE3 R136S binds to Tau and blocks its propagation, suppressing neurodegeneration in mice with Alzheimer's disease

PSEN1 E280A carrier for the APOE3 Christchurch variant (R136S) is protected against Alzheimer's disease (AD) symptoms with a distinct anatomical pattern of Tau pathology. However, the molecular mechanism accounting for this protective effect remains incompletely understood. Here, we show that the ApoE3 R136S mutant strongly binds to Tau and reduces its uptake into neurons and microglia compared with ApoE3 wild type (WT), diminishing Tau fragmentation by asparagine endopeptidase (AEP), proinflammatory cytokines by Tau pre-formed fibrils (PFFs) or β-amyloid (Aβ), and neurotoxicity. Further, ApoE3 R136S demonstrates more robust effects in attenuating AEP activation and Tau PFF spreading in the brains of both 5xFAD and Tau P301S mice than in ApoE3 WT, leading to improved cognitive functions. Thus, our findings support the idea that ApoE3 R136S strongly binds Tau and decreases its cellular uptake, abrogating Tau pathology propagation in AD brains.

Recruitment of apolipoprotein E facilitates Herpes simplex virus 1 attachment and release

Human apolipoprotein E (ApoE) has been shown to play important roles during primary infection and pathogenesis of several viruses. Furthermore, epidemiological studies suggest that interactions between ApoE 4 and herpes simplex virus type-1 (HSV1) could associate with higher risk of Alzheimer's disease. Nevertheless, little is known about the ApoE-HSV1 interactions at molecular levels. Here, we investigate the effects of ApoE on HSV1 infection in vitro. Our results show that ApoE promotes HSV1 growth, which is attributed to the incorporation of ApoE into HSV1 particles. Using both biological and biophysical approaches, we conclude that ApoE-coated HSV1 demonstrates a more efficient attachment to and faster release from the cell surface. Mechanistic studies reveal that ApoE modifies HSV1 interactions with heparan sulfate, thereby modulating interactions between HSV1 and the cell surface. Overall, our results provide new insights into the roles of ApoE during HSV1 infections which may inspire future studies on Alzheimer's disease etiology.

APOE Christchurch enhances a disease-associated microglial response to plaque but suppresses response to tau pathology

BACKGROUND

Apolipoprotein E ε4 (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). A recent case report identified a rare variant in APOE, APOE3-R136S (Christchurch), proposed to confer resistance to autosomal dominant Alzheimer's Disease (AD). However, it remains unclear whether and how this variant exerts its protective effects.

METHODS

We introduced the R136S variant into mouse Apoe (ApoeCh) and investigated its effect on the development of AD-related pathology using the 5xFAD model of amyloidosis and the PS19 model of tauopathy. We used immunohistochemical and biochemical analysis along with single-cell spatial omics and bulk proteomics to explore the impact of the ApoeCh variant on AD pathological development and the brain's response to plaques and tau.

RESULTS

In 5xFAD mice, ApoeCh enhances a Disease-Associated Microglia (DAM) phenotype in microglia surrounding plaques, and reduces plaque load, dystrophic neurites, and plasma neurofilament light chain. By contrast, in PS19 mice, ApoeCh suppresses the microglial and astrocytic responses to tau-laden neurons and does not reduce tau accumulation or phosphorylation, but partially rescues tau-induced synaptic and myelin loss. We compared how microglia responses differ between the two mouse models to elucidate the distinct DAM signatures induced by ApoeCh. We identified upregulation of antigen presentation-related genes in the DAM response in a PS19 compared to a 5xFAD background, suggesting a differential response to amyloid versus tau pathology that is modulated by the presence of ApoeCh. Bulk proteomics show upregulated mitochondrial protein abundance with ApoeCh in 5xFAD mice, but reductions in mitochondrial and translation associated proteins in PS19 mice.

CONCLUSIONS

These findings highlight the ability of the ApoeCh variant to modulate microglial responses based on the type of pathology, enhancing DAM reactivity in amyloid models and dampening neuroinflammation to promote protection in tau models. This suggests that the Christchurch variant's protective effects likely involve multiple mechanisms, including changes in receptor binding and microglial programming.

Heparin-enriched plasma proteome is significantly altered in Alzheimer's disease

INTRODUCTION

Heparin binding proteins (HBPs) with roles in extracellular matrix assembly are strongly correlated to β-amyloid (Aβ) and tau pathology in Alzheimer's disease (AD) brain and cerebrospinal fluid (CSF). However, it remains challenging to detect these proteins in plasma using standard mass spectrometry-based proteomic approaches.

METHODS

We employed heparin-affinity chromatography, followed by off-line fractionation and tandem mass tag mass spectrometry (TMT-MS), to enrich HBPs from plasma obtained from AD (n = 62) and control (n = 47) samples. These profiles were then correlated to Aβ, tau and phosphorylated tau (pTau) CSF biomarkers and plasma pTau181 from the same individuals, as well as a consensus brain proteome network to assess the overlap with AD brain pathophysiology.

RESULTS

Heparin enrichment from plasma was highly reproducible, enriched well-known HBPs like APOE and thrombin, and depleted high-abundant proteins such as albumin. A total of 2865 proteins, spanning 10 orders of magnitude in abundance, were measured across 109 samples. Compared to the consensus AD brain protein co-expression network, we observed that specific plasma proteins exhibited consistent direction of change in both brain and plasma, whereas others displayed divergent changes, highlighting the complex interplay between the two compartments. Elevated proteins in AD plasma, when compared to controls, included members of the matrisome module in brain that accumulate with Aβ deposits, such as SMOC1, SMOC2, SPON1, MDK, OLFML3, FRZB, GPNMB, and the APOE4 proteoform. Additionally, heparin-enriched proteins in plasma demonstrated significant correlations with conventional AD CSF biomarkers, including Aβ, total tau, pTau, and plasma pTau181. A panel of five plasma proteins classified AD from control individuals with an area under the curve (AUC) of 0.85. When combined with plasma pTau181, the panel significantly improved the classification performance of pTau181 alone, increasing the AUC from 0.93 to 0.98. This suggests that the heparin-enriched plasma proteome captures additional variance in cognitive dementia beyond what is explained by pTau181.

CONCLUSION

These findings support the utility of a heparin-affinity approach coupled with TMT-MS for enriching amyloid-associated proteins, as well as a wide spectrum of plasma biomarkers that reflect pathological changes in the AD brain.

Human apolipoprotein E glycosylation and sialylation: from structure to function

Human apolipoprotein E (ApoE) was first identified as a polymorphic gene in the 1970s; however, the genetic association of ApoE genotypes with late-onset sporadic Alzheimer's disease (sAD) was only discovered 20 years later. Since then, intensive research has been undertaken to understand the molecular effects of ApoE in the development of sAD. Despite three decades' worth of effort and over 10,000 papers published, the greatest mystery in the ApoE field remains: human ApoE isoforms differ by only one or two amino acid residues; what is responsible for their significantly distinct roles in the etiology of sAD, with ApoE4 conferring the greatest genetic risk for sAD whereas ApoE2 providing exceptional neuroprotection against sAD. Emerging research starts to point to a novel and compelling hypothesis that the sialoglycans posttranslationally appended to human ApoE may serve as a critical structural modifier that alters the biology of ApoE, leading to the opposing impacts of ApoE isoforms on sAD and likely in the peripheral systems as well. ApoE has been shown to be posttranslationally glycosylated in a species-, tissue-, and cell-specific manner. Human ApoE, particularly in brain tissue and cerebrospinal fluid (CSF), is highly glycosylated, and the glycan chains are exclusively attached via an O-linkage to serine or threonine residues. Moreover, studies have indicated that human ApoE glycans undergo sialic acid modification or sialylation, a structural alteration found to be more prominent in ApoE derived from the brain and CSF than plasma. However, whether the sialylation modification of human ApoE has a biological role is largely unexplored. Our group recently first reported that the three major isoforms of human ApoE in the brain undergo varying degrees of sialylation, with ApoE2 exhibiting the most abundant sialic acid modification, whereas ApoE4 is the least sialylated. Our findings further indicate that the sialic acid moiety on human ApoE glycans may serve as a critical modulator of the interaction of ApoE with amyloid β (Aβ) and downstream Aβ pathogenesis, a prominent pathologic feature in AD. In this review, we seek to provide a comprehensive summary of this exciting and rapidly evolving area of ApoE research, including the current state of knowledge and opportunities for future exploration.

Heparin-enriched plasma proteome is significantly altered in Alzheimer's Disease

Introduction

Heparin binding proteins (HBPs) with roles in extracellular matrix assembly are strongly correlated to β-amyloid (Aβ) and tau pathology in Alzheimer's disease (AD) brain and cerebrospinal fluid (CSF). However, it remains challenging to detect these proteins in plasma using standard mass spectrometry-based proteomic approaches.

Methods

We employed heparin affinity chromatography, followed by off-line fractionation and tandem mass tag mass spectrometry (TMT-MS), to capture and enrich HBPs in plasma obtained from AD (n=62) and control (n=47) samples. These profiles were then correlated to a consensus AD brain proteome, as well as with Aβ, tau and phosphorylated tau (pTau) CSF biomarkers from the same individuals. We then leveraged published human postmortem brain proteome datasets to assess the overlap with the heparin-enriched plasma proteome.

Results

Heparin-enrichment from plasma was highly reproducible, enriched well-known HBPs like APOE and thrombin, and depleted high-abundance proteins such as albumin. A total of 2865 proteins, spanning 10 orders of magnitude were detectable. Utilizing a consensus AD brain protein co-expression network, we observed that specific plasma HBPs exhibited consistent direction of change in both brain and plasma, whereas others displayed divergent changes highlighting the complex interplay between the two compartments. Elevated HBPs in AD plasma, when compared to controls, included members of the matrisome module in brain that accumulate within Aβ deposits, such as SMOC1, SMOC2, SPON1, MDK, OLFML3, FRZB, GPNMB, and APOE. Additionally, heparin enriched plasma proteins demonstrated significant correlations with conventional AD CSF biomarkers, including Aβ, total tau, pTau, and plasma pTau from the same individuals.

Conclusion

These findings support the utility of a heparin-affinity approach for enriching amyloid-associated proteins, as well as a wide spectrum of plasma biomarkers that reflect pathological changes in the AD brain.

Peptidomic analysis of endogenous and bacterial protease activity in human plasma and wound fluids

Endogenous and bacterial proteases play important roles in wound healing and infection. Analysis of alterations in the low-molecular-weight peptidome by individual enzymes could therefore provide insight into proteolytic events occurring in wounds and may aid in the discovery of biomarkers. Using liquid chromatography with tandem mass spectrometry, we characterized the peptidome of plasma and acute wound fluids digested ex vivo with human (neutrophil elastase and cathepsin G) and bacterial proteases (Pseudomonas aeruginosa LasB and Staphyloccocus aureus V8). We identified over 100 protein targets for each enzyme and characterized enzyme specific peptides and cleavage patterns. Moreover, we found unique peptide regions in V8 digested samples that were also present in dressing extracts from S. aureus infected wounds. Finally, the work indicates that peptidomic analysis of qualitative differences of proteolytic activity of individual enzymes may aid in the discovery of potential diagnostic biomarkers for wound healing status.

Anti-inflammatory effect of ApoE23 on Salmonella typhimurium-induced sepsis in mice

Two independent experiments were performed with three groups each (sepsis control, sepsis, and sepsis with apoE23 treatment) to investigate the anti-inflammatory effect of apolipoprotein 23 (apoE23) in a mouse model of sepsis induced by S. typhimurium. Survival rates; plasma level variations in tumor necrosis factor (TNF)-α, interleukin (IL)-6, and lipopolysaccharide (LPS); S. typhimurium colony-forming units in the spleen tissue; and mRNA and protein expression levels of low-density lipoprotein receptor (LDLR), LDLR-related protein (LRP), syndecan-1, and scavenger receptor B1 were evaluated in the livers of mice from the three groups. Results found that the survival rate of septic mice treated with apoE23 was 100% within 48 h, while it was only 40% in septic mice without apoE23 treatment (P < 0.001). The plasma LPS, TNF-α, and IL-6 levels and the S. typhimurium load in mice in the apoE23-treated group were significantly lower than those in septic mice (P < 0.05). Moreover, apoE23 restored the downregulated expression of LDLR and LRP in the liver tissue of septic mice. So apoE23 exhibits an anti-inflammatory effect in the mouse model of S. typhimurium-induced sepsis. Further studies are required to understand the mechanisms underlying the anti-inflammatory effects of apoE23.

Apolipoprotein E Recognizes Alzheimer's Disease Associated 3-O Sulfation of Heparan Sulfate

Apolipoprotein E (ApoE)'s ϵ4 alle is the most important genetic risk factor for late onset Alzheimer's Disease (AD). Cell-surface heparan sulfate (HS) is a cofactor for ApoE/LRP1 interaction and the prion-like spread of tau pathology between cells. 3-O-sulfo (3-O-S) modification of HS has been linked to AD through its interaction with tau, and enhanced levels of 3-O-sulfated HS and 3-O-sulfotransferases in the AD brain. In this study, we characterized ApoE/HS interactions in wildtype ApoE3, AD-linked ApoE4, and AD-protective ApoE2 and ApoE3-Christchurch. Glycan microarray and SPR assays revealed that all ApoE isoforms recognized 3-O-S. NMR titration localized ApoE/3-O-S binding to the vicinity of the canonical HS binding motif. In cells, the knockout of HS3ST1-a major 3-O sulfotransferase-reduced cell surface binding and uptake of ApoE. 3-O-S is thus recognized by both tau and ApoE, suggesting that the interplay between 3-O-sulfated HS, tau and ApoE isoforms may modulate AD risk.

Heparan sulfate proteoglycan in Alzheimer's disease: aberrant expression and functions in molecular pathways related to amyloid-β metabolism

Alzheimer's disease (AD) is the most common form of dementia. Currently, there is no effective treatment for AD, as its etiology remains poorly understood. Mounting evidence suggests that the accumulation and aggregation of amyloid-β peptides (Aβ), which constitute amyloid plaques in the brain, is critical for initiating and accelerating AD pathogenesis. Considerable efforts have been dedicated to shedding light on the molecular basis and fundamental origins of the impaired Aβ metabolism in AD. Heparan sulfate (HS), a linear polysaccharide of the glycosaminoglycan family, co-deposits with Aβ in plaques in the AD brain, directly binds and accelerates Aβ aggregation, and mediates Aβ internalization and cytotoxicity. Mouse model studies demonstrate that HS regulates Aβ clearance and neuroinflammation in vivo. Previous reviews have extensively explored these discoveries. Here, this review focuses on the recent advancements in understanding abnormal HS expression in the AD brain, the structural aspects of HS-Aβ interaction, and the molecules involved in modulating Aβ metabolism through HS interaction. Furthermore, this review presents a perspective on the potential effects of abnormal HS expression on Aβ metabolism and AD pathogenesis. In addition, the review highlights the importance of conducting further research to differentiate the spatiotemporal components of HS structure and function in the brain and AD pathogenesis.

In Vitro Interaction of Melanoma-Derived Extracellular Vesicles with Collagen

Extracellular vesicles are now considered as active contributors to melanoma progression through their capacity to modify the tumor microenvironment and to favor the formation of a pre-metastatic niche. These prometastatic roles of tumor-derived EVs would pass through their interaction with the extracellular matrix (ECM) and its remodeling, in turn providing a substrate favoring persistent tumor cell migration. Nevertheless, the capacity of EVs to directly interact with ECM components is still questionable. In this study, we use electron microscopy and a pull-down assay to test the capacity of sEVs, derived from different melanoma cell lines, to physically interact with collagen I. We were able to generate collagen fibrils coated with sEVs and to show that melanoma cells release subpopulations of sEVs that can differentially interact with collagen.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer's disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell-cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Lipoproteins in the Central Nervous System: From Biology to Pathobiology

The brain, as one of the most lipid-rich organs, heavily relies on lipid transport and distribution to maintain homeostasis and neuronal function. Lipid transport mediated by lipoprotein particles, which are complex structures composed of apolipoproteins and lipids, has been thoroughly characterized in the periphery. Although lipoproteins in the central nervous system (CNS) were reported over half a century ago, the identification of APOE4 as the strongest genetic risk factor for Alzheimer's disease has accelerated investigation of the biology and pathobiology of lipoproteins in the CNS. This review provides an overview of the different components of lipoprotein particles, in particular apolipoproteins, and their involvements in both physiological functions and pathological mechanisms in the CNS.

Antibacterial and Anti-Inflammatory Effects of Apolipoprotein E

Apolipoprotein E (APOE) is a lipid-transport protein that functions as a key mediator of lipid transport and cholesterol metabolism. Recent studies have shown that peptides derived from human APOE display anti-inflammatory and antimicrobial effects. Here, we applied in vitro assays and fluorescent microscopy to investigate the anti-bacterial effects of full-length APOE. The interaction of APOE with endotoxins from Escherichia coli was explored using surface plasmon resonance, binding assays, transmission electron microscopy and all-atom molecular dynamics (MD) simulations. We also studied the immunomodulatory activity of APOE using in vitro cell assays and an in vivo mouse model in combination with advanced imaging techniques. We observed that APOE exhibits anti-bacterial activity against several Gram-negative bacterial strains of Pseudomonas aeruginosa and Escherichia coli. In addition, we showed that APOE exhibits a significant binding affinity for lipopolysaccharide (LPS) and lipid A as well as heparin. MD simulations identified the low-density lipoprotein receptor (LDLR) binding region in helix 4 of APOE as a primary binding site for these molecules via electrostatic interactions. Together, our data suggest that APOE may have an important role in controlling inflammation during Gram-negative bacterial infection.

In Silico Analysis of the Antagonist Effect of Enoxaparin on the ApoE4–Amyloid-Beta (Aβ) Complex at Different pH Conditions

Apolipoprotein E4 (ApoE4) is thought to increase the risk of developing Alzheimer’s disease. Several studies have shown that ApoE4-Amyloid β (Aβ) interactions can increment amyloid depositions in the brain and that this can be augmented at low pH values. On the other hand, experimental studies in transgenic mouse models have shown that treatment with enoxaparin significantly reduces cortical Aβ levels, as well as decreases the number of activated astrocytes around Aβ plaques. However, the interactions between enoxaparin and the ApoE4-Aβ proteins have been poorly explored. In this work, we combine molecular dynamics simulations, molecular docking, and binding free energy calculations to elucidate the molecular properties of the ApoE4-Aβ interactions and the competitive binding affinity of the enoxaparin on the ApoE4 binding sites. In addition, we investigated the effect of the environmental pH levels on those interactions. Our results showed that under different pH conditions, the closed form of the ApoE4 protein, in which the C-terminal domain folds into the protein, remains stabilized by a network of hydrogen bonds. This closed conformation allowed the generation of six different ApoE4-Aβ interaction sites, which were energetically favorable. Systems at pH5 and 6 showed the highest energetic affinity. The enoxaparin molecule was found to have a strong energetic affinity for ApoE4-interacting sites and thus can neutralize or disrupt ApoE4-Aβ complex formation.

The Unifying Hypothesis of Alzheimer's Disease: Heparan Sulfate Proteoglycans/Glycosaminoglycans Are Key as First Hypothesized Over 30 Years Ago

The updated "Unifying Hypothesis of Alzheimer's disease" (AD) is described that links all the observed neuropathology in AD brain (i.e., plaques, tangles, and cerebrovascular amyloid deposits), as well as inflammation, genetic factors (involving ApoE), "AD-in-a-Dish" studies, beta-amyloid protein (Aβ) as a microbial peptide; and theories that bacteria, gut microflora, gingivitis and viruses all play a role in the cause of AD. The common link is the early accumulation of heparan sulfate proteoglycans (HSPGs) and heparan sulfate glycosaminoglycans (GAGs). HS GAG accumulation and/or decreased HS GAG degradation is postulated to be the key initiating event. HS GAGs and highly sulfated macromolecules induce Aβ 1-40 (but not 1-42) to form spherical congophilic maltese-cross star-like amyloid core deposits identical to those in the AD brain. Heparin/HS also induces tau protein to form paired helical filaments (PHFs). Increased sulfation and/or decreased degradation of HSPGs and HS GAGs that occur due to brain aging leads to the formation of plaques and tangles in AD brain. Knockout of HS genes markedly reduce the accumulation of Aβ fibrils in the brain demonstrating that HS GAGs are key. Bacteria and viruses all use cell surface HS GAGs for entry into cells, including SARS-CoV-2. Bacteria and viruses cause HS GAGs to rapidly increase to cause near-immediate aggregation of Aβ fibrils. "AD-in-a-dish" studies use "Matrigel" as the underlying scaffold that spontaneously causes plaque, and then tangle formation in a dish. Matrigel mostly contains large amounts of perlecan, the same specific HSPG implicated in AD and amyloid disorders. Mucopolysaccharidoses caused by lack of specific HS GAG enzymes lead to massive accumulation of HS in lysosomal compartments in neurons and contribute to cognitive impairment in children. Neurons full of HS demonstrate marked accumulation and fibrillization of Aβ, tau, α-synuclein, and prion protein (PrP) in mucopolysaccharidosis animal models demonstrating that HS GAG accumulation is a precursor to Aβ accumulation in neurons. Brain aging leads to changes in HSPGs, including newly identified splice variants leading to increased HS GAG sulfation in the AD brain. All of these events lead to the new "Unifying Hypothesis of Alzheimer's disease" that further implicates HSPGs /HS GAGs as key (as first hypothesized by Snow and Wight in 1989).

APOE3-Jacksonville (V236E) variant reduces self-aggregation and risk of dementia

Apolipoprotein E (APOE) genetic variants have been shown to modify Alzheimer’s disease (AD) risk. We previously identified an APOE3 variant (APOE3-V236E), named APOE3-Jacksonville (APOE3-Jac), associated with healthy brain aging and reduced risk for AD and dementia with Lewy bodies (DLB). Herein, we resolved the functional mechanism by which APOE3-Jac reduces APOE aggregation and enhances its lipidation in human brains, as well as in cellular and biochemical assays. Compared to APOE3, expression of APOE3-Jac in astrocytes increases several classes of lipids in the brain including phosphatidylserine, phosphatidylethanolamine, phosphatidic acid, and sulfatide, critical for synaptic functions. Mice expressing APOE3-Jac have reduced amyloid pathology, plaque-associated immune responses, and neuritic dystrophy. The V236E substitution is also sufficient to reduce the aggregation of APOE4, whose gene allele is a major genetic risk factor for AD and DLB. These findings suggest that targeting APOE aggregation might be an effective strategy for treating a subgroup of individuals with AD and DLB.

The role of full-length apoE in clearance of Gram-negative bacteria and their endotoxins

ApoE is a well-known lipid-binding protein that plays a main role in the metabolism and transport of lipids. More recently, apoE-derived peptides have been shown to exert antimicrobial effects. Here, we investigated the antibacterial activity of apoE using in vitro assays, advanced imaging techniques, and in vivo mouse models. The formation of macromolecular complexes of apoE and endotoxins from Gram-negative bacteria was explored using gel shift assays, transmission electron microscopy, and CD spectroscopy followed by calculation of the α-helical content. The binding affinity of apoE to endotoxins was also confirmed by fluorescent spectroscopy detecting the quenching and shifting of tryptophan intrinsic fluorescence. We showed that apoE exhibits antibacterial activity particularly against Gram-negative bacteria such as Pseudomonas aeruginosa and Escherichia coli. ApoE protein folding was affected by binding of bacterial endotoxin components such as lipopolysaccharide (LPS) and lipid A, yielding similar increases in the apoE α-helical content. Moreover, high-molecular-weight complexes of apoE were formed in the presence of LPS, but not to the same extent as with lipid A. Together, our results demonstrate the ability of apoE to kill Gram-negative bacteria, interact with their endotoxins, which leads to the structural changes in apoE and the formation of aggregate-like complexes.

Coagulopathy-independent, bioinspired hemostatic materials: A full research story from preclinical models to a human clinical trial

Since the first report of underwater adhesive proteins of marine mussels in 1981, numerous studies have reported mussel-inspired synthetic adhesive polymers. However, none of them have developed up to human-level translational studies. Here, we report a sticky polysaccharide that effectively promotes hemostasis from animal bleeding models to first-in-human hepatectomy. We found that the hemostatic material instantly generates a barrier layer that seals hemorrhaging sites. The barrier is created within a few seconds by in situ interactions with abundant plasma proteins. Therefore, as long as patient blood contains proper levels of plasma proteins, hemostasis should always occur even in coagulopathic conditions. To date, insufficient tools have been developed to arrest coagulopathic bleedings originated from genetic disorders, chronic diseases, or surgical settings such as organ transplantations. Mussel-inspired adhesion chemistry described here provides a useful alternative to the use of fibrin glues up to a human-level biomedical application.

APOE2: protective mechanism and therapeutic implications for Alzheimer's disease

Investigations of apolipoprotein E (APOE) gene, the major genetic risk modifier for Alzheimer's disease (AD), have yielded significant insights into the pathogenic mechanism. Among the three common coding variants, APOE*ε4 increases, whereas APOE*ε2 decreases the risk of late-onset AD compared with APOE*ε3. Despite increased understanding of the detrimental effect of APOE*ε4, it remains unclear how APOE*ε2 confers protection against AD. Accumulating evidence suggests that APOE*ε2 protects against AD through both amyloid-β (Aβ)-dependent and independent mechanisms. In addition, APOE*ε2 has been identified as a longevity gene, suggesting a systemic effect of APOE*ε2 on the aging process. However, APOE*ε2 is not entirely benign; APOE*ε2 carriers exhibit increased risk of certain cerebrovascular diseases and neurological disorders. Here, we review evidence from both human and animal studies demonstrating the protective effect of APOE*ε2 against AD and propose a working model depicting potential underlying mechanisms. Finally, we discuss potential therapeutic strategies designed to leverage the protective effect of APOE2 to treat AD.

Apolipoprotein E-mediated regulation of selenoprotein P transportation via exosomes

Selenoprotein P (SELENOP), secreted from the liver, functions as a selenium (Se) supplier to other tissues. In the brain, Se homeostasis is critical for physiological function. Previous studies have reported that SELENOP co-localizes with the apolipoprotein E receptor 2 (ApoER2) along the blood-brain barrier (BBB). However, the mechanism underlying SELENOP transportation from hepatocytes to neuronal cells remains unclear. Here, we found that SELENOP was secreted from hepatocytes as an exosomal component protected from plasma kallikrein-mediated cleavage. SELENOP was interacted with apolipoprotein E (ApoE) through heparin-binding sites of SELENOP, and the interaction regulated the secretion of exosomal SELENOP. Using in vitro BBB model of transwell cell culture, exosomal SELENOP was found to supply Se to brain endothelial cells and neuronal cells, which synthesized selenoproteins by a process regulated by ApoE and ApoER2. The regulatory role of ApoE in SELENOP transport was also observed in vivo using ApoE-/- mice. Exosomal SELENOP transport protected neuronal cells from amyloid β (Aβ)-induced cell death. Taken together, our results suggest a new delivery mechanism for Se to neuronal cells by exosomal SELENOP.

Reconfiguring Nature's Cholesterol Accepting Lipoproteins as Nanoparticle Platforms for Transport and Delivery of Therapeutic and Imaging Agents

Apolipoproteins are critical structural and functional components of lipoproteins, which are large supramolecular assemblies composed predominantly of lipids and proteins, and other biomolecules such as nucleic acids. A signature feature of apolipoproteins is the preponderance of amphipathic α-helical motifs that dictate their ability to make extensive non-covalent inter- or intra-molecular helix-helix interactions in lipid-free states or helix-lipid interactions with hydrophobic biomolecules in lipid-associated states. This review focuses on the latter ability of apolipoproteins, which has been capitalized on to reconstitute synthetic nanoscale binary/ternary lipoprotein complexes composed of apolipoproteins/peptides and lipids that mimic native high-density lipoproteins (HDLs) with the goal to transport drugs. It traces the historical development of our understanding of these nanostructures and how the cholesterol accepting property of HDL has been reconfigured to develop them as drug-loading platforms. The review provides the structural perspective of these platforms with different types of apolipoproteins and an overview of their synthesis. It also examines the cargo that have been loaded into the core for therapeutic and imaging purposes. Finally, it lays out the merits and challenges associated with apolipoprotein-based nanostructures with a future perspective calling for a need to develop "zip-code"-based delivery for therapeutic and diagnostic applications.

Unbiased Identification of the Liposome Protein Corona using Photoaffinity-based Chemoproteomics

Protein adsorption to the surface of a nanoparticle can fundamentally alter the character, behavior, and fate of a nanoparticle in vivo. Current methods to capture the protein corona rely on physical separation techniques and are unable to resolve key, individual protein-nanoparticle interactions. As a result, the precise link between the "synthetic" and the "biological" identity of a nanoparticle remains unclear. Herein, we report an unbiased photoaffinity-based approach to capture, characterize, and quantify the protein corona of liposomes in their native state. Compared to conventional methods, our photoaffinity approach reveals markedly different interacting proteins as well as reduced total protein binding to liposome surfaces. Identified proteins do not follow protein abundancy patterns of human serum, as has been generally reported, but are instead dominated by soluble apolipoproteins-endogenous serum proteins that have evolved to recognize the lipidic surface of circulating lipoproteins. We believe our findings are the most accurate characterization of a liposome's biological identity but, more fundamentally, reveal liposome-protein binding is, in many cases, significantly less complex than previously thought.

Hepatitis C Virus Entry: An Intriguingly Complex and Highly Regulated Process

Hepatitis C virus (HCV) is a major cause of chronic hepatitis and liver disease worldwide. Its tissue and species tropism are largely defined by the viral entry process that is required for subsequent productive viral infection and establishment of chronic infection. This review provides an overview of the viral and host factors involved in HCV entry into hepatocytes, summarizes our understanding of the molecular mechanisms governing this process and highlights the therapeutic potential of host-targeting entry inhibitors.

Pro-metastatic functions of lipoproteins and extracellular vesicles in the acidic tumor microenvironment

Although the overall mortality in cancer is steadily decreasing, major groups of patients still respond poorly to available treatments. The key clinical challenge discussed here relates to the inherent capacity of cancer cells to metabolically adapt to hypoxic and acidic stress, resulting in treatment resistance and a pro-metastatic behavior. Hence, a detailed understanding of stress adaptive responses is critical for the design of more rational therapeutic strategies for cancer. We will focus on the emerging role of extracellular vesicles (EVs) and lipoprotein particles in cancer cell metabolic stress adaptation and how these pathways may constitute potential Achilles' heels of the cancer cell machinery and alternative treatment targets of metastasis. In this context, common extracellular lipid uptake mechanisms, involving specific cell-surface receptors and endocytic pathways, may operate during remodeling of acidic atherosclerotic plaques as well as the tumor microenvironment. The role of endocytosis in regulating the cellular response to hypoxic and acidic stress through spatial coordination of receptor proteins may be exploited for therapeutic purposes. As a consequence, molecular mechanisms of endocytosis have attracted increasing attention as potential targets for tumor specific delivery of therapeutic substances, such as antibody-drug conjugates. The identification of internalizing surface proteins specific to the acidic tumor niche remains an unmet need of high clinical relevance. Among the currently explored, acidosis-related, internalizing target proteins, we will focus on the cell-surface proteoglycan carbonic anhydrase 9.

The serine protease HtrA1 contributes to the formation of an extracellular 25-kDa apolipoprotein E fragment that stimulates neuritogenesis

Apolipoprotein-E (apoE) is a glycoprotein highly expressed in the brain, where it appears to play a role in lipid transport, β-amyloid clearance, and neuronal signaling. ApoE proteolytic fragments are also present in the brain, but the enzymes responsible for apoE fragmentation are unknown, and the biological activity of specific apoE fragments remains to be determined. Here we utilized SK-N-SH neuroblastoma cells differentiated into neurons with all-trans-retinoic acid (ATRA) to study extracellular apoE proteolysis. ApoE fragments were detectable in culture supernatants after 3 days, and their levels were increased for up to 9 days in the presence of ATRA. The concentration of apoE fragments was positively correlated with levels of the neuronal maturation markers (PSD95 and SMI32). The most abundant apoE fragments were 25- and 28-kDa N-terminal fragments that both contained sialylated glycosylation and bound to heparin. We detected apoE fragments only in the extracellular milieu and not in cell lysates, suggesting that an extracellular protease contributes to apoE fragmentation. Of note, siRNA-mediated knockdown of high-temperature requirement serine peptidase A1 (HtrA1) and a specific HtrA1 inhibitor reduced apoE 25-kDa fragment formation by 41 and 86%, respectively. Recombinant 25-kDa fragment apoE and full-length apoE both stimulated neuritogenesis in vitro, increasing neuroblastoma neurite growth by more than 2-fold over a 6-day period. This study provides a cellular model for assessing apoE proteolysis, indicates that HtrA1 regulates apoE 25-kDa fragment formation under physiological conditions, and reveals a new neurotrophic function for the apoE 25-kDa fragment.

Apolipoprotein E, Receptors, and Modulation of Alzheimer's Disease

Apolipoprotein E (apoE) is a lipid carrier in both the peripheral and the central nervous systems. Lipid-loaded apoE lipoprotein particles bind to several cell surface receptors to support membrane homeostasis and injury repair in the brain. Considering prevalence and relative risk magnitude, the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE4 contributes to AD pathogenesis by modulating multiple pathways, including but not limited to the metabolism, aggregation, and toxicity of amyloid-β peptide, tauopathy, synaptic plasticity, lipid transport, glucose metabolism, mitochondrial function, vascular integrity, and neuroinflammation. Emerging knowledge on apoE-related pathways in the pathophysiology of AD presents new opportunities for AD therapy. We describe the biochemical and biological features of apoE and apoE receptors in the central nervous system. We also discuss the evidence and mechanisms addressing differential effects of apoE isoforms and the role of apoE receptors in AD pathogenesis, with a particular emphasis on the clinical and preclinical studies related to amyloid-β pathology. Finally, we summarize the current strategies of AD therapy targeting apoE, and postulate that effective strategies require an apoE isoform-specific approach.

The Alzheimer's disease risk factors apolipoprotein E and TREM2 are linked in a receptor signaling pathway

Background

Triggering receptor expressed on myeloid cells 2 (TREM2) and apolipoprotein E (APOE) are genetically linked to Alzheimer’s disease. Here, we investigated whether human ApoE mediates signal transduction through human and murine TREM2 and sought to identify a TREM2-binding domain in human ApoE.

Methods

To investigate cell signaling through TREM2, a cell line was used which expressed an NFAT-inducible β-galactosidase reporter and human or murine TREM2, fused to CD8 transmembrane and CD3ζ intracellular signaling domains. ELISA-based binding assays were used to determine binding affinities of human ApoE isoforms to human TREM2 and to identify a TREM2-binding domain in ApoE.

Results

ApoE was found to be an agonist to human TREM2 with EC₅₀ in the low nM range, and to murine TREM2 with reduced potency. In the reporter cells, TREM2 expression was lower than in nontransgenic mouse brain. Human ApoE isoforms ε2, ε3, and ε4 bound to human TREM2 with K_d in the low nM range. The binding was displaced by an ApoE-mimetic peptide (amino acids 130–149).

Conclusions

An ApoE-mediated dose-dependent signal transduction through TREM2 in reporter cells was demonstrated, and a TREM2-binding region in ApoE was identified. The relevance of an ApoE-TREM2 receptor signaling pathway to Alzheimer’s disease is discussed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0835-4) contains supplementary material, which is available to authorized users.

Apolipoprotein E lipoprotein particles inhibit amyloid-β uptake through cell surface heparan sulphate proteoglycan

BACKGROUND

The accumulation, aggregation and deposition of amyloid-β (Aβ) peptides in the brain are central to the pathogenesis of Alzheimer's disease (AD). Alzheimer's disease risk increases significantly in individuals carrying one or two copies of APOE ε4 allele compared to individuals with an ε3/ε3 genotype. Growing evidence has demonstrated that apolipoprotein E (apoE) strongly influences AD pathogenesis by controlling Aβ aggregation and metabolism. Heparan sulphate proteoglycans (HSPGs) are abundant cell surface molecules that bind to both apoE and Aβ. HSPGs have been associated with Aβ aggregation and deposition. Although several lines of research have shown that apoE influences Aβ clearance in the brain, it is not clear how apoE influences HSPG-mediated cellular uptake of Aβ.

RESULTS

In this study, we show that apoE lipoprotein particles from conditioned media of immortalized astrocytes isolated from human APOE-targeted replacement (TR) mice significantly suppress cellular Aβ42 and Aβ40 uptake through cell surface HSPG. ApoE3 and apoE4 particles have similar binding affinity to heparin, while apoE4 particles are likely hypolipidated compared to apoE particles. We also found that the apoE particles antagonize Aβ binding to cell surface, and inhibited Aβ uptake in a concentration-dependent manner in Chinese hamster ovary (CHO) cells. While the effect was not apoE isoform-dependent, the suppressive effect of apoE particles on Aβ uptake was not observed in HSPG-deficient CHO cells. We further demonstrated that apoE particles reduced the internalization of Aβ in mouse primary neurons, an effect that is eliminated by the presence of heparin.

CONCLUSIONS

Taken together, our findings indicate that apoE particles irrespective of isoform inhibit HSPG-dependent cellular Aβ uptake. Modulating the ability of apoE particles to affect Aβ cellular uptake may hold promises for developing new strategies for AD therapy.

ApoE: In Vitro Studies of a Small Molecule Effector

Apolipoprotein E4 (apoE4), one of three isoforms of apoE, is the major risk factor for developing late onset Alzheimer's disease. The only differences among these isoforms (apoE2, apoE3, and apoE4) are single amino acid changes. Yet these proteins are functionally very different. One approach to ameliorating the effect of apoE4 with respect to Alzheimer's disease would be to find small molecular weight compounds that affect the behavior of apoE4. Few studies of this approach have been carried out in part because there was no complete structure of any full-length apoE isoform until 2011. Here, we focus on one small molecular weight compound, EZ-482, and explore the effects of its binding to apoE. Using hydrogen-deuterium exchange, we determined that EZ-482 binds to the C-terminal domains of both apoE3 and apoE4. The binding to apoE4, however, is accompanied by a unique N-terminal allosteric effect. Using fluorescence methods, we determined an apparent dissociation constant of approximately 8 μM. Although EZ-482 binds to the C-terminal domain, it blocks heparin binding to the N-terminal domain. The residues of apoE that bind heparin are the same as those involved in apoE binding to LDL and LRP-1 receptors. The methods and the data presented here may serve as a template for future studies using small molecular weight compounds to modulate the behavior of apoE.

Exploring the glycosaminoglycan-protein interaction network by glycan-mediated pull-down proteomics

Glycosaminoglycans (GAGs) are linear, highly sulfated polysaccharides expressed by almost all animal cells. They occur as soluble molecules, or form proteoglycans by being O-linked to different core proteins on the cell surface and in the extracellular matrix. Due to their ability to interact with diverse proteins and to modulate their biologic functions, GAGs are main drivers of mammalian biology. However, to the present day, the human GAG binding proteome has only been insufficiently explored. The aim of this study was therefore to investigate the human GAG binding proteome of different sources by using the major GAG classes as ligands, and to explore the GAG-binding selectivity of the human plasma proteome. For this purpose, proteins were pulled down from immobilized low molecular weight heparin, heparan sulfate, and dermatan sulfate under different conditions and were identified by nano-LC/MS². Four hundred and fifty eight human GAG binding proteins have been identified, whereas plasma proteins showed clear differences in their GAG-binding specificity/selectivity and affinity. We were able to differentiate between proteins that bound to all three glycan ligands and proteins that showed selective binding to one or two glycan ligands. Moreover, step-gradient salt elution revealed different binding affinities toward different GAG ligands. On top of proteins with well-known GAG-binding properties we have identified formerly unknown GAG binders. Functional annotation of the identified GAG-binding proteins showed clusters of proteins that are involved in a variety of biological processes. The method described here is well suited for identifying GAG-binding proteins and for comparing human subproteomes with respect to binding to different GAG classes.

Electronegative Low-density Lipoprotein Increases Coronary Artery Disease Risk in Uremia Patients on Maintenance Hemodialysis

Electronegative low-density lipoprotein (LDL) is a recognized factor in the pathogenesis of coronary artery disease (CAD) in the general population, but its role in the development of CAD in uremia patients is unknown. L5 is the most electronegative subfraction of LDL isolated from human plasma. In this study, we examined the distribution of L5 (L5%) and its association with CAD risk in uremia patients.The LDL of 39 uremia patients on maintenance hemodialysis and 21 healthy controls was separated into 5 subfractions, L1-L5, with increasing electronegativity. We compared the distribution and composition of plasma L5 between uremia patients and controls, examined the association between plasma L5% and CAD risk in uremia patients, and studied the effects of L5 from uremia patients on endothelial function.Compared to controls, uremia patients had significantly increased L5% (P < 0.001) and L5 that was rich in apolipoprotein C3 and triglycerides. L5% was significantly higher in uremia patients with CAD (n = 10) than in those without CAD (n = 29) (P < 0.05). Independent of other major CAD risk factors, the adjusted odds ratio for CAD was 1.88 per percent increase in plasma L5% (95% CI, 1.01-3.53), with a near-linear dose-response relationship. Compared with controls, uremia patients had decreased flow-mediated vascular dilatation. In ex vivo studies with preconstricted rat thoracic aortic rings, L5 from uremia patients inhibited acetylcholine-induced relaxation. In cultured human endothelial cells, L5 inhibited endothelial nitric oxide synthase activation and induced endothelial dysfunction.Our findings suggest that elevated plasma L5% may induce endothelial dysfunction and play an important role in the increased risk of CAD in uremia patients.

Several Human Liver Cell Expressed Apolipoproteins Complement HCV Virus Production with Varying Efficacy Conferring Differential Specific Infectivity to Released Viruses

Apolipoprotein E (ApoE), an exchangeable apolipoprotein, is necessary for production of infectious Hepatitis C virus (HCV) particles. However, ApoE is not the only liver-expressed apolipoprotein and the role of other apolipoproteins for production of infectious HCV progeny is incompletely defined. Therefore, we quantified mRNA expression of human apolipoproteins in primary human hepatocytes. Subsequently, cDNAs encoding apolipoproteins were expressed in 293T/miR-122 cells to explore if they complement HCV virus production in cells that are non-permissive due to limiting endogenous levels of human apolipoproteins. Primary human hepatocytes expressed high mRNA levels of ApoA1, A2, C1, C3, E, and H. ApoA4, A5, B, D, F, J, L1, L2, L3, L4, L6, M, and O were expressed at intermediate levels, and C2, C4, and L5 were not detected. All members of the ApoA and ApoC family of lipoproteins complemented HCV virus production in HCV transfected 293T/miR-122 cells, albeit with significantly lower efficacy compared with ApoE. In contrast, ApoD expression did not support production of infectious HCV. Specific infectivity of released particles complemented with ApoA family members was significantly lower compared with ApoE. Moreover, the ratio of extracellular to intracellular infectious virus was significantly higher for ApoE compared to ApoA2 and ApoC3. Since apolipoproteins complementing HCV virus production share amphipathic alpha helices as common structural features we altered the two alpha helices of ApoC1. Helix breaking mutations in both ApoC1 helices impaired virus assembly highlighting a critical role of alpha helices in apolipoproteins supporting HCV assembly. In summary, various liver expressed apolipoproteins with amphipathic alpha helices complement HCV virus production in human non liver cells. Differences in the efficiency of virus assembly, the specific infectivity of released particles, and the ratio between extracellular and intracellular infectivity point to distinct characteristics of these apolipoproteins that influence HCV assembly and cell entry. This will guide future research to precisely pinpoint how apolipoproteins function during virus assembly and cell entry.

ApoE: the role of conserved residues in defining function

The amino acid sequences of apolipoprotein E (apoE) from 63 different mammalian species have been downloaded from the protein database. The sequences were compared to human apoE4 to determine conserved and non-conserved sequences of amino acids. ApoE4 is the major risk factor for the development of late onset Alzheimer's disease while apoE3, which differs from apoE4 by a single amino acid change at position 112, poses little or no risk for the development of this disease. Thus, the two proteins appear to be structurally and functionally different. Seven highly conserved regions, representing approximately 47 amino acids (of 299) have been found. These regions are distributed throughout the protein and reflect ligand binding sites as well as regions proposed to be involved in the propagation of the cysteine-arginine change at position 112 to distant regions of the protein in the N- and C-terminal domains. Highly non-conserved regions are at the N- and C-terminal ends of the apoE protein.

Partial-filling affinity capillary electrophoresis and quartz crystal microbalance with adsorption energy distribution calculations in the study of biomolecular interactions with apolipoprotein E as interaction partner

Adsorption energy distribution (AED) calculations were successfully applied to partial-filling affinity capillary electrophoresis (PF-ACE) to facilitate more detailed studies of biomolecular interactions. PF-ACE with AED calculations was employed to study the interactions between two isoforms of apolipoprotein E (apoE) and dermatan sulfate (DS), and a quartz crystal microbalance (QCM) was used in combination with AED calculations to examine the interactions of the 15-amino-acid peptide fragment of apoE with DS. The heterogeneity of the interactions was elucidated. Microscale thermophoresis was used to validate the results. The interactions studied are of interest because, in vivo, apolipoprotein E localizes on DS-containing regions in the extracellular matrix of human vascular subendothelium. Two-site binding was demonstrated for the isoform apoE3 and DS, but only one-site binding for apoE2-DS. Comparable affinity constants were obtained for the apoE2-DS, apoE3-D3, and 15-amino-acid peptide of apoE-DS using the three techniques. The results show that combining AED calculations with modern biosensing techniques can open up another dimension in studies on the heterogeneity and affinity constants of biological molecules.

ApoE and Aβ in Alzheimer's disease: accidental encounters or partners?

Among the three human apolipoprotein E (apoE) isoforms, apoE4 increases the risk of Alzheimer's disease (AD). While transporting cholesterol is a primary function, apoE also regulates amyloid-β (Aβ) metabolism, aggregation, and deposition. Although earlier work suggests that different affinities of apoE isoforms to Aβ might account for their effects on Aβ clearance, recent studies indicate that apoE also competes with Aβ for cellular uptake through apoE receptors. Thus, several factors probably determine the variable effects apoE has on Aβ. In this Review, we examine biochemical, structural, and functional studies and propose testable models that address the complex mechanisms underlying apoE-Aβ interaction and how apoE4 may increase AD risk and also serve as a target pathway for therapy.

Hepatitis C virus entry

Hepatitis C virus (HCV) is a hepatotropic virus and a major cause of chronic hepatitis and liver disease worldwide. Initial interactions between HCV virions and hepatocytes are required for productive viral infection and initiation of the viral life cycle. Furthermore, HCV entry contributes to the tissue tropism and species specificity of this virus. The elucidation of these interactions is critical, not only to understand the pathogenesis of HCV infection, but also to design efficient antiviral strategies and vaccines. This review summarizes our current knowledge of the host factors required for the HCV-host interactions during HCV binding and entry, our understanding of the molecular mechanisms underlying HCV entry into target cells, and the relevance of HCV entry for the pathogenesis of liver disease, antiviral therapy, and vaccine development.

Apolipoproteins E and AV mediate lipoprotein clearance by hepatic proteoglycans

The heparan sulfate proteoglycan (HSPG) syndecan-1 (SDC1) acts as a major receptor for triglyceride-rich lipoprotein (TRL) clearance in the liver. We sought to identify the relevant apolipoproteins on TRLs that mediate binding to SDC1 and determine their clinical relevance. Evidence supporting ApoE as a major determinant arose from its enrichment in TRLs from mice defective in hepatic heparan sulfate (Ndst1f/fAlbCre⁺ mice), decreased binding of ApoE-deficient TRLs to HSPGs on human hepatoma cells, and decreased clearance of ApoE-deficient [³H]TRLs in vivo. Evidence for a second ligand was suggested by the faster clearance of ApoE-deficient TRLs after injection into WT Ndst1f/fAlbCre⁻ versus mutant Ndst1f/fAlbCre⁺ mice and elevated fasting and postprandial plasma triglycerides in compound Apoe⁻/⁻Ndst1f/fAlbCre⁺ mice compared with either single mutant. ApoAV emerged as a candidate based on 6-fold enrichment of ApoAV in TRLs accumulating in Ndst1f/fAlbCre⁺ mice, decreased binding of TRLs to proteoglycans after depletion of ApoAV or addition of anti-ApoAV mAb, and decreased heparan sulfate-dependent binding of ApoAV-deficient particles to hepatocytes. Importantly, disruption of hepatic heparan sulfate-mediated clearance increased atherosclerosis. We conclude that clearance of TRLs by hepatic HSPGs is atheroprotective and mediated by multivalent binding to ApoE and ApoAV.

Heparan sulfate dissociates serum amyloid A (SAA) from acute-phase high-density lipoprotein, promoting SAA aggregation

Inflammation-related (AA) amyloidosis is a severe clinical disorder characterized by the systemic deposition of the acute-phase reactant serum amyloid A (SAA). SAA is normally associated with the high-density lipoprotein (HDL) fraction in plasma, but under yet unclear circumstances, the apolipoprotein is converted into amyloid fibrils. AA amyloid and heparan sulfate (HS) display an intimate relationship in situ, suggesting a role for HS in the pathogenic process. This study reports that HS dissociates SAA from HDLs isolated from inflamed mouse plasma. Application of surface plasmon resonance spectroscopy and molecular modeling suggests that HS simultaneously binds to two apolipoproteins of HDL, SAA and ApoA-I, and thereby induce SAA dissociation. The activity requires a minimum chain length of 12-14 sugar units, proposing an explanation to previous findings that short HS fragments preclude AA amyloidosis. The results address the initial events in the pathogenesis of AA amyloidosis.

Biophysical analysis of apolipoprotein E3 variants linked with development of type III hyperlipoproteinemia

Background

Apolipoprotein E (apoE) is a major protein of the lipoprotein transport system that plays important roles in lipid homeostasis and protection from atherosclerosis. ApoE is characterized by structural plasticity and thermodynamic instability and can undergo significant structural rearrangements as part of its biological function. Mutations in the 136–150 region of the N-terminal domain of apoE, reduce its low density lipoprotein (LDL) receptor binding capacity and have been linked with lipoprotein disorders, such as type III hyperlipoproteinemia (HLP) in humans. However, the LDL-receptor binding defects for these apoE variants do not correlate well with the severity of dyslipidemia, indicating that these variants may carry additional properties that contribute to their pathogenic potential.

Methodology/Principal Findings

In this study we examined whether three type III HLP predisposing apoE3 variants, namely R136S, R145C and K146E affect the biophysical properties of the protein. Circular dichroism (CD) spectroscopy revealed that these mutations do not significantly alter the secondary structure of the protein. Thermal and chemical unfolding analysis revealed small thermodynamic alterations in each variant compared to wild-type apoE3, as well as effects in the reversibility of the unfolding transition. All variants were able to remodel multillamelar 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles, but R136S and R145C had reduced kinetics. Dynamic light scattering analysis indicated that the variant R136S exists in a higher-order oligomerization state in solution. Finally, 1-anilinonaphthalene-8-sulfonic acid (ANS) binding suggested that the variant R145C exposes a larger amount of hydrophobic surface to the solvent.

Conclusions/Significance

Overall, our findings suggest that single amino acid changes in the functionally important region 136–150 of apoE3 can affect the molecule's stability and conformation in solution and may underlie functional consequences. However, the magnitude and the non-concerted nature of these changes, make it unlikely that they constitute a distinct unifying mechanism leading to type III HLP pathogenesis.

Topology of human apolipoprotein E3 uniquely regulates its diverse biological functions

Human apolipoprotein E (apoE) is one of the major determinants in lipid transport, playing a critical role in atherosclerosis and other diseases. Binding to lipid and heparan sulfate proteoglycans (HSPG) induces apoE to adopt active conformations for binding to low-density lipoprotein receptor (LDLR) family. ApoE also interacts with beta amyloid peptide, manifests critical isoform-specific effects on Alzheimer's disease. Despite the importance of apoE in these major human diseases, the fundamental questions of how apoE adjusts its structure upon binding to regulate its diverse functions remain unsolved. We report the NMR structure of apoE3, displaying a unique topology of three structural domains. The C-terminal domain presents a large exposed hydrophobic surface that likely initiates interactions with lipids, HSPG, and beta amyloid peptides. The unique topology precisely regulates apoE tertiary structure to permit only one possible conformational adaptation upon binding and provides a double security in preventing lipid-free and partially-lipidated apoE from premature binding to apoE receptors during receptor biogenesis. This topology further ensures the optimal receptor-binding activity by the fully lipidated apoE during lipoprotein transport in circulation and in the brain. These findings provide a structural framework for understanding the structural basis of the diverse functions of this important protein in human diseases.

Human Plasma Very Low-Density Lipoproteins Are Stabilized by Electrostatic Interactions and Destabilized by Acidic pH

Very low-density lipoproteins (VLDL) are precursors of low-density lipoproteins (LDL, or “bad cholesterol”). Factors affecting structural integrity of VLDL are important for their metabolism. To assess the role of electrostatic interactions in VLDL stability, we determined how solvent ionic conditions affect the heat-induced VLDL remodeling. This remodeling involves VLDL fusion, rupture, and fission of apolipoprotein E-containing high-density lipoprotein-(HDL-) like particles similar to those formed during VLDL-to-LDL maturation. Circular dichroism and turbidity show that increasing sodium salt concentration in millimolar range reduces VLDL stability and its enthalpic component. Consequently, favorable electrostatic interactions stabilize VLDL. Reduction in pH from 7.4 to 6.0 reduces VLDL stability, with further destabilization detected at pH < 6, which probably results from titration of the N-terminal α-amino groups and free fatty acids. This destabilization is expected to facilitate endosomal degradation of VLDL, promote their coalescence into lipid droplets in atherosclerotic plaques, and affect their potential use as drug carriers.

A novel peptide derived from human apolipoprotein E is an inhibitor of tumor growth and ocular angiogenesis

Angiogenesis is a hallmark of tumor development and metastasis and now a validated target for cancer treatment. We previously reported that a novel dimer peptide (apoEdp) derived from the receptor binding region of human apolipoprotein E (apoE) inhibits virus-induced angiogenesis. However, its role in tumor anti-angiogenesis is unknown. This study demonstrates that apoEdp has anti-angiogenic property in vivo through reduction of tumor growth in a mouse model and ocular angiogenesis in a rabbit eye model. Our in vitro studies show that apoEdp inhibits human umbilical vein endothelial cell proliferation, migration, invasion and capillary tube formation. We document that apoEdp inhibits vascular endothelial growth factor-induced Flk-1 activation as well as downstream signaling pathways that involve c-Src, Akt, eNOS, FAK, and ERK1/2. These in vitro data suggest potential sites of the apoE dipeptide inhibition that could occur in vivo.

This is the first evidence that a synthetic dimer peptide mimicking human apoE has anti-angiogenesis functions and could be an anti-tumor drug candidate.

Molecular basis for the differences in lipid and lipoprotein binding properties of human apolipoproteins E3 and E4

Human apolipoprotein (apo) E4 binds preferentially to very low-density lipoproteins (VLDLs), whereas apoE3 binds preferentially to high-density lipoproteins (HDLs), resulting in different plasma cholesterol levels for the two isoforms. To understand the molecular basis for this effect, we engineered the isolated apoE N-terminal domain (residues 1-191) and C-terminal domain (residues 192-299) together with a series of variants containing deletions in the C-terminal domain and assessed their lipid and lipoprotein binding properties. Both isoforms can bind to a phospholipid (PL)-stabilized triolein emulsion, and residues 261-299 are primarily responsible for this activity. ApoE4 exhibits better lipid binding ability than apoE3 as a consequence of a rearrangement involving the segment spanning residues 261-272 in the C-terminal domain. The strong lipid binding ability of apoE4 coupled with the VLDL particle surface being ∼60% PL-covered is the basis for its preference for binding VLDL rather than HDL. ApoE4 binds much more strongly than apoE3 to VLDL but less strongly than apoE3 to HDL(3), consistent with apoE-lipid interactions being relatively unimportant for binding to HDL. The preference of apoE3 for binding to HDL(3) arises because binding is mediated primarily by interaction of the N-terminal helix bundle domain with the resident apolipoproteins that cover ∼80% of the HDL(3) particle surface. Thus, the selectivity in the binding of apoE3 and apoE4 to HDL(3) and VLDL is dependent upon two factors: (1) the stronger lipid binding ability of apoE4 relative to that of apoE3 and (2) the differences in the nature of the surfaces of VLDL and HDL(3) particles, with the former being largely covered with PL and the latter with protein.

Spectroscopic studies of GSK3{beta} phosphorylation of the neuronal tau protein and its interaction with the N-terminal domain of apolipoprotein E

Alzheimer disease neurons are characterized by extraneuronal plaques formed by aggregated amyloid-β peptide and by intraneuronal tangles composed of fibrillar aggregates of the microtubule-associated Tau protein. Tau is mostly found in a hyperphosphorylated form in these tangles. Glycogen synthase kinase 3β (GSK3β) is a proline-directed kinase generally considered as one of the major players that (hyper)phosphorylates Tau. The kinase phosphorylates mainly (Ser/Thr)-Pro motifs and is believed to require a priming activity by another kinase. Here, we use an in vitro phosphorylation assay and NMR spectroscopy to characterize in a qualitative and quantitative manner the phosphorylation of Tau by GSK3β. We find that three residues can be phosphorylated (Ser-396, Ser-400, and Ser-404) by GSK3β alone, without priming. Ser-404 is essential in this process, as its mutation to Ala prevents all activity of GSK3β. However, priming enhances the catalytic efficacy of the kinase, as initial phosphorylation of Ser-214 by the cAMP-dependent protein kinase (PKA) leads to the rapid modification by GSK3β of four regularly spaced additional sites. Because the regular incorporation of negative charges by GSK3β leads to a potential parallel between phospho-Tau and heparin, we investigated its interaction with the heparin/low density lipoprotein receptor binding domain of human apolipoprotein E. We indeed observed an interaction between the GSK3β-promoted regular phospho-pattern on Tau and the apolipoprotein E fragment but none in the absence of phosphorylation or the presence of an irregular phosphorylation pattern by the prolonged activity of PKA. Apolipoprotein E is therefore able to discriminate and interact with specific phosphorylation patterns of Tau.