Ebola virus glycoprotein: proteolytic processing, acylation, cell tropism, and detection of neutralizing antibodies

Targeted retroviral vectors displaying a cleavage site-engineered hemagglutinin (HA) through HA-protease interactions

We report here a targeting method that exploits the expression pattern of cell surface proteases to induce gene delivery to specific tissues. We describe retroviral vectors harboring modified surface glycoproteins derived from an avian influenza virus hemagglutinin (HA) for which the cell entry properties, dependent on HA cleavage by producer cells, were conditionally blocked at a postbinding step by insertion of matrix metalloproteinase (MMP) substrates. We demonstrate that such vectors induce gene transfer, both in vitro and in mice harboring human tumor xenografts, only through contact with target cells expressing MMPs that cleave the substrate introduced into the recombinant HA. This selective gene transfer in MMP-rich cells was specifically inhibited by 1,10-phenanthroline, a broad-range MMP inhibitor. Importantly, such MMP-activatable vectors selectively transduced MMP-rich cells in heterogeneous populations containing MMP-rich and MMP-poor cells. These vectors will allow useful gene transfer applications into target cells exhibiting specific protease activities.

Ebola virus: unravelling pathogenesis to combat a deadly disease

Ebola virus (EBOV) causes severe haemorrhagic fever leading to up to 90% lethality. Increasingly frequent outbreaks and the placement of EBOV in the category A list of potential biothreat agents have boosted interest in this virus. Furthermore, development of new technologies (e.g. reverse genetics systems) and extensive studies on Ebola haemorrhagic fever (EHF) in animal models have substantially expanded the knowledge on the pathogenic mechanisms that underlie this disease. Two major factors in EBOV pathogenesis are the impairment of the immune response and vascular dysfunction. Here, we attempt to summarize the current knowledge on EBOV pathogenesis focusing on these two factors and on recent progress in the development of vaccines and potential therapeutics.

Heptad repeat 2 in herpes simplex virus 1 gH interacts with heptad repeat 1 and is critical for virus entry and fusion

Herpes simplex virus 1 (HSV-1) entry into cells and cell-cell fusion mediated by HSV-1 glycoproteins require four glycoproteins, gD, gB, gH, gL. Of these, gH is the only one that so far exhibits structural-functional features typical of viral fusion glycoproteins, i.e., a candidate fusion peptide and, downstream of it, a heptad repeat (HR) segment able to form a coiled coil, named HR-1. Here, we show that gH carries a functional HR-2 capable of physical interaction with HR-1. Specifically, mutational analysis of gH aimed at increasing or decreasing the ability of HR-2 to form a coiled coil resulted in an increase or decrease of fusion activity, respectively. HSV infection was modified accordingly. A mimetic peptide with the HR-2 sequence inhibited HSV-1 infection in a specific and dose-dependent manner. Circular dichroism spectroscopy showed that both HR-2 and HR-1 mimetic peptides adopt mainly random conformation in aqueous solution, while a decrease in peptide environmental polarity determines a conformational change, with a significant increase of the alpha-helical conformation content, in particular, for the HR-1 peptide. Furthermore, HR-1 and HR-2 mimetic peptides formed a stable complex, as revealed in nondenaturing electrophoresis and by circular dichroism. The mixture of HR-1 and HR-2 peptides reversed the inhibition of HSV infection exerted by the single peptides. Complex formation between HR-1 and HR-2 was independent of the presence of adjacent gH sequences and of additional glycoproteins involved in entry and fusion. Altogether, HR-2 adds to the features typical of class 1 fusion glycoproteins exhibited by HSV-1 gH.

Filovirus assembly and budding

Filoviruses belong to the order of negative-stranded non-segmented RNA viruses and are classified into two genera, Ebola and Marburg viruses. They have a characteristic filamentous shape, which is largely determined by the matrix protein VP40. Although VP40 is the main driving force for assembly and budding from the host cell, the production of infectious virus involves an intricate interplay between all viral structural proteins in addition to cellular factors, e.g., those that normally function in multi-vesicular body biogenesis. As a consequence, assembly and budding steps are defined to specific cellular compartments, and the recent progress in understanding how the different components are assembled into stable enveloped virus particles is reviewed.

Chimpanzee adenovirus vaccine protects against Zaire Ebola virus

This study evaluated the use of a chimpanzee-based adenovirus vaccine in mouse and Guinea pigs models of Zaire Ebola virus (ZEBOV) infection. Vaccine vector expressing the envelope glycoprotein of ZEBOV was created from the molecular clone of chimpanzee adenovirus pan7 (AdC7). AdC7 vaccine stimulated robust T and B cell responses to ZEBOV in naïve mice inducing complete protection to an otherwise lethal challenge of ZEBOV. Complete protection to Zaire Ebola virus was also observed in Guinea pigs vaccinated with a relatively low dose of AdC7 (5 x 10(9)/kg). Pre-existing immunity to AdHu5 was generated in mice following pre-exposure to AdHu5 or administration of pooled human immune globulin. Pre-existing immunity to human adenoviruses severely compromised the efficacy of the human AdHu5 vaccine but not the chimpanzee AdC7 vaccine. These results validate further development of Chimpanzee-based vaccine and highlight the impact of pre-existing immunity to the vaccine carrier.

Formation of vesicular stomatitis virus pseudotypes bearing surface proteins of hepatitis B virus

It has been difficult to propagate and titrate hepatitis B virus (HBV) in tissue culture. We examined whether vesicular stomatitis virus (VSV) pseudotypes bearing HBV surface (HBs) proteins infectious for human cell lines could be prepared. For this, expression plasmids for three surface proteins, L, M, and S, of HBV were made. 293T cells were then transfected with these plasmids either individually or in different combinations. 293T cells expressing HBs proteins were infected with VSVdeltaG*-G, a recombinant VSV expressing green fluorescent protein (GFP), to make VSV pseudotypes. Culture supernatants together with cells were harvested and sonicated for a short time. The infectivities of freshly harvested supernatants were determined by quantifying the number of cells expressing GFP after neutralization with anti-VSV serum and mouse monoclonal antibodies (MAbs) against HBs protein. Among 14 cell lines tested for susceptibility to HBV pseudotype samples, HepG2, JHH-7, and 293T cells were judged to be the most susceptible. Namely, the infectious units (IU) of the culture supernatant samples neutralized with anti-VSV in the absence and presence of anti-HBs S MAbs and titrated on HepG2 cells ranged from 1,000 to 4,000 IU/ml and 200 to 400 IU/ml, respectively, suggesting the presence of VSVdeltaG*(HBV) pseudotypes. This infectivity was inhibited by treatment with lactoferrin or dextran sulfate. Pretreatment of the cells with trypsin or tunicamycin inhibited plating of the pseudotype samples. The HBV pseudotypes can be used to analyze early steps of HBV infection, including the entry mechanism of HBV.

Use of influenza C virus glycoprotein HEF for generation of vesicular stomatitis virus pseudotypes

Influenza C virus contains two envelope glycoproteins: CM2, a putative ion channel protein; and HEF, a unique multifunctional protein that performs receptor-binding, receptor-destroying and fusion activities. Here, it is demonstrated that expression of HEF is sufficient to pseudotype replication-incompetent vesicular stomatitis virus (VSV) that lacks the VSV glycoprotein (G) gene. The pseudotyped virus showed characteristic features of influenza C virus with respect to proteolytic activation, receptor usage and cell tropism. Chimeric glycoproteins composed of HEF ectodomain and VSV-G C-terminal domains were efficiently incorporated into VSV particles and showed receptor-binding and receptor-destroying activities but, unlike authentic HEF, did not mediate efficient infection, probably because of impaired fusion activity. HEF-pseudotyped VSV efficiently infected polarized Madin-Darby canine kidney cells via the apical plasma membrane, whereas entry of VSV-G-complemented virus was restricted to the basolateral membrane. These findings suggest that pseudotyping of viral vectors with HEF might be useful for efficient apical gene transfer into polarized epithelial cells and for targeting cells that express 9-O-acetylated sialic acids.

Disulfide bond assignment of the Ebola virus secreted glycoprotein SGP

The non-structural glycoprotein (SGP) of Ebola virus (EboV) is secreted in large amounts from infected cells as a disulfide-linked homodimer. In this communication, highly purified SGP, derived from Vero E6 cultures infected with the Zaire species of EboV, was used to determine the correct localization of inter- and intrachain disulfide bonds. Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry analysis of proteolytic cleavage fragments indicates that all cysteines (six per monomeric unit) form unique disulfide bonds. Monomers of the SGP homodimer are joined in a parallel manner by two intersubunit disulfide bonds formed between paired N-terminal and C-terminal cysteines (C53-C53' and C306-C306'). The remaining cysteines are involved in intrachain disulfide bonding (paired as C108-C135 and C121-C147), which resembles the disulfide bond topology of fibronectin type II domains. The findings presented here provide the foundation for future studies aimed at defining the structural and functional properties of SGP.

Properties of replication-competent vesicular stomatitis virus vectors expressing glycoproteins of filoviruses and arenaviruses

Replication-competent recombinant vesicular stomatitis viruses (rVSVs) expressing the type I transmembrane glycoproteins and selected soluble glycoproteins of several viral hemorrhagic fever agents (Marburg virus, Ebola virus, and Lassa virus) were generated and characterized. All recombinant viruses exhibited rhabdovirus morphology and replicated cytolytically in tissue culture. Unlike the rVSVs with an additional transcription unit expressing the soluble glycoproteins, the viruses carrying the foreign transmembrane glycoproteins in replacement of the VSV glycoprotein were slightly attenuated in growth. Biosynthesis and processing of the foreign glycoproteins were authentic, and the cell tropism was defined by the transmembrane glycoprotein. None of the rVSVs displayed pathogenic potential in animals. The rVSV expressing the Zaire Ebola virus transmembrane glycoprotein mediated protection in mice against a lethal Zaire Ebola virus challenge. Our data suggest that the recombinant VSV can be used to study the role of the viral glycoproteins in virus replication, immune response, and pathogenesis.

Ectodomain shedding of the glycoprotein GP of Ebola virus

In this study, release of abundant amounts of the Ebola virus (EBOV) surface glycoprotein GP in a soluble form from virus-infected cells was investigated. We demonstrate that the mechanism responsible for the release of GP is ectodomain shedding mediated by cellular sheddases. Proteolytic cleavage taking place at amino-acid position D637 removes the transmembrane anchor and liberates complexes consisting of GP1 and truncated GP2 (GP(2delta)) subunits from the cell surface. We show that tumor necrosis factor alpha-converting enzyme (TACE), a member of the ADAM family of zinc-dependent metalloproteases, is involved in EBOV GP shedding. This finding shows for the first time that virus-encoded surface glycoproteins are substrates for ADAMs. Furthermore, we provide evidence that shed GP is present in significant amounts in the blood of virus-infected animals and that it may play an important role in the pathogenesis of infection by efficiently blocking the activity of virus-neutralizing antibodies.

Folate receptor alpha and caveolae are not required for Ebola virus glycoprotein-mediated viral infection

Folate receptor alpha (FRalpha) has been described as a factor involved in mediating Ebola virus entry into cells (6). Furthermore, it was suggested that interaction with FRalpha results in internalization and subsequent viral ingress into the cytoplasm via caveolae (9). Descriptions of cellular receptors for Ebola virus and its entry mechanisms are of fundamental importance, particularly with the advent of vectors bearing Ebola virus glycoprotein (GP) being utilized for gene transfer into cell types such as airway epithelial cells. Thus, the ability of FRalpha to mediate efficient entry of viral pseudotypes carrying GP was investigated. We identified cell lines and primary cell types such as macrophages that were readily infected by GP pseudotypes despite lacking detectable surface FRalpha, indicating that this receptor is not essential for Ebola virus infection. Furthermore, we find that T-cell lines stably expressing FRalpha are not infectible, suggesting that FRalpha is also not sufficient to mediate entry. T-cell lines lack caveolae, the predominant route of FRalpha-mediated folate metabolism. However, the coexpression of FRalpha with caveolin-1, the major structural protein of caveolae, was not able to rescue infectivity in a T-cell line. In addition, other cell types lacking caveolae are fully infectible by GP pseudotypes. Finally, a panel of ligands to and soluble analogues of FRalpha were unable to inhibit infection on a range of cell lines, questioning the role of FRalpha as an important factor for Ebola virus entry.

Generation of Marburg virus-like particles by co-expression of glycoprotein and matrix protein

Marburg virus (MARV), the causative agent of a severe hemorrhagic fever, has a characteristic filamentous morphology. Here we report that co-expression of MARV glycoprotein and matrix protein (VP40) in mammalian cells leads to spontaneous budding of filamentous particles strikingly similar to wild-type MARV. In addition, these particles elicit an immune response in BALB/c mice. The generation of non-replicating Marburg virus-like particles (VLPs) should significantly facilitate the research on molecular mechanisms of MARV assembly and release. Furthermore, VLPs may be an excellent vaccine candidate against Marburg infection.

Palmitoylation, membrane-proximal basic residues, and transmembrane glycine residues in the reovirus p10 protein are essential for syncytium formation

Avian reovirus and Nelson Bay reovirus are two unusual nonenveloped viruses that induce extensive cell-cell fusion via expression of a small nonstructural protein, termed p10. We investigated the importance of the transmembrane domain, a conserved membrane-proximal dicysteine motif, and an endodomain basic region in the membrane fusion activity of p10. We now show that the p10 dicysteine motif is palmitoylated and that loss of palmitoylation correlates with a loss of fusion activity. Mutational and functional analyses also revealed that a triglycine motif within the transmembrane domain and the membrane-proximal basic region were essential for p10-mediated membrane fusion. Mutations in any of these three motifs did not influence events upstream of syncytium formation, such as p10 membrane association, protein topology, or surface expression, suggesting that these motifs are more intimately associated with the membrane fusion reaction. These results suggest that the rudimentary p10 fusion protein has evolved a mechanism of inducing membrane merger that is highly dependent on the specific interaction of several different motifs with donor membranes. In addition, cross-linking, coimmunoprecipitation, and complementation assays provided no evidence for p10 homo- or heteromultimer formation, suggesting that p10 may be the first example of a membrane fusion protein that does not form stable, higher-order multimers.

Molecular mechanisms of filovirus cellular trafficking

The filoviruses, Ebola and Marburg, are two of the most pathogenic viruses, causing lethal hemorrhagic fever in humans. Recent discoveries suggest that filoviruses, along with other phylogenetically or functionally related viruses, utilize a complex mechanism of replication exploiting multiple cellular components including lipid rafts, endocytic compartments, and vacuolar protein sorting machinery. In this review, we summarize these recent findings and discuss the implications for vaccine and therapeutics development.

Vaccine research efforts for filoviruses

Ebola and Marburg viruses belong to the family Filoviridae, and cause acute, frequently fatal, haemorrhagic fever in humans and non-human primates. No vaccines are available for human use. This review describes the status of research efforts to develop vaccines for these viruses and to identify the immune mechanisms of protection. The vaccine approaches discussed include DNA-based vaccines, and subunit vaccines vectored by adenovirus, alphavirus replicons, and vaccinia virus.

Do lipid rafts mediate virus assembly and pseudotyping?

Co-infection of a host cell by two unrelated enveloped viruses can lead to the production of pseudotypes: virions containing the genome of one virus but the envelope proteins of both viruses. The selection of components during virus assembly must therefore be flexible enough to allow the incorporation of unrelated viral membrane proteins, yet specific enough to exclude the bulk of host proteins. This apparent contradiction has been termed the pseudotypic paradox. There is mounting evidence that lipid rafts play a role in the assembly pathway of non-icosahedral, enveloped viruses. Viral components are concentrated initially in localized regions of the plasma membrane via their interaction with lipid raft domains. Lateral interactions of viral structural proteins amplify the changes in local lipid composition which in turn enhance the concentration of viral proteins in the rafts. An affinity for lipid rafts may be the common feature of enveloped virus proteins that leads to the formation of pseudotypes.

Comparison of the protective efficacy of DNA and baculovirus-derived protein vaccines for EBOLA virus in guinea pigs

The filoviruses Ebola virus (EBOV) and Marburg virus (MARV) cause severe hemorrhagic fever in humans for which no vaccines are available. Previously, a priming dose of a DNA vaccine expressing the glycoprotein (GP) gene of MARV followed by boosting with recombinant baculovirus-derived GP protein was found to confer protective immunity to guinea pigs (Hevey et al., 2001. Vaccine 20, 568-593). To determine whether a similar prime-boost vaccine approach would be effective for EBOV, we generated and characterized recombinant baculoviruses expressing full-length EBOV GP (GP(1,2)) or a terminally-deleted GP (GPa-) and examined their immunogenicity in guinea pigs. As expected, cells infected with the GPa- recombinant secreted more GP(1) than those infected with the GP(1,2) recombinant. In lectin binding studies, the insect cell culture-derived GPs were found to differ from mammalian cell derived virion GP, in that they had no complex/hybrid N-linked glycans or glycans containing sialic acid. Despite these differences, the baculovirus-derived GPs were able to bind monoclonal antibodies to five distinct epitopes on EBOV GP, indicating that the antigenic structures of the proteins remain intact. As a measure of the ability of the baculovirus-derived proteins to elicit cell-mediated immune responses, we evaluated the T-cell stimulatory capacity of the GPa- protein in cultured human dendritic cells. Increases in cytotoxicity as compared to controls suggest that the baculovirus proteins have the capacity to evoke cell-mediated immune responses. Guinea pigs vaccinated with the baculovirus-derived GPs alone, or in a DNA prime-baculovirus protein boost regimen developed antibody responses as measured by ELISA and plaque reduction neutralization assays; however, incomplete protection was achieved when the proteins were given alone or in combination with DNA vaccines. These data indicate that a vaccine approach that was effective for MARV is not effective for EBOV in guinea pigs.

Biochemical and functional characterization of the Ebola virus VP24 protein: implications for a role in virus assembly and budding

The VP24 protein of Ebola virus is believed to be a secondary matrix protein and minor component of virions. In contrast, the VP40 protein of Ebola virus is the primary matrix protein and the most abundant virion component. The structure and function of VP40 have been well characterized; however, virtually nothing is known regarding the structure and function of VP24. Wild-type and mutant forms of VP24 were expressed in mammalian cells to gain a better understanding of the biochemical and functional nature of this viral protein. Results from these experiments demonstrated that (i) VP24 localizes to the plasma membrane and perinuclear region in both transfected and Ebola virus-infected cells, (ii) VP24 associates strongly with lipid membranes, (iii) VP24 does not contain N-linked sugars when expressed alone in mammalian cells, (iv) VP24 can oligomerize when expressed alone in mammalian cells, (v) progressive deletions at the N terminus of VP24 resulted in a decrease in oligomer formation and a concomitant increase in the formation of high-molecular-weight aggregates, and (vi) VP24 was present in trypsin-resistant virus like particles released into the media covering VP24-transfected cells. These data indicate that VP24 possesses structural features commonly associated with viral matrix proteins and that VP24 may have a role in virus assembly and budding.

Identification of protective epitopes on ebola virus glycoprotein at the single amino acid level by using recombinant vesicular stomatitis viruses

Ebola virus causes lethal hemorrhagic fever in humans, but currently there are no effective vaccines or antiviral compounds for this infectious disease. Passive transfer of monoclonal antibodies (MAbs) protects mice from lethal Ebola virus infection (J. A. Wilson, M. Hevey, R. Bakken, S. Guest, M. Bray, A. L. Schmaljohn, and M. K. Hart, Science 287:1664-1666, 2000). However, the epitopes responsible for neutralization have been only partially characterized because some of the MAbs do not recognize the short synthetic peptides used for epitope mapping. To identify the amino acids recognized by neutralizing and protective antibodies, we generated a recombinant vesicular stomatitis virus (VSV) containing the Ebola virus glycoprotein-encoding gene instead of the VSV G protein-encoding gene and used it to select escape variants by growing it in the presence of a MAb (133/3.16 or 226/8.1) that neutralizes the infectivity of the virus. All three variants selected by MAb 133/3.16 contained a single amino acid substitution at amino acid position 549 in the GP2 subunit. By contrast, MAb 226/8.1 selected three different variants containing substitutions at positions 134, 194, and 199 in the GP1 subunit, suggesting that this antibody recognized a conformational epitope. Passive transfer of each of these MAbs completely protected mice from a lethal Ebola virus infection. These data indicate that neutralizing antibody cocktails for passive prophylaxis and therapy of Ebola hemorrhagic fever can reduce the possibility of the emergence of antigenic variants in infected individuals.

Analysis of linear B-cell epitopes of the nucleoprotein of ebola virus that distinguish ebola virus subtypes

Ebola virus consists of four genetically distinguishable subtypes. We developed monoclonal antibodies (MAbs) to the nucleoprotein (NP) of Ebola virus Zaire subtype and analyzed their cross-reactivities to the Reston and Sudan subtypes. We further determined the epitopes recognized by these MAbs. Three MAbs reacted with the three major subtypes and recognized a fragment containing 110 amino acids (aa) at the C-terminal extremity. They did not show specific reactivities to any 10-aa short peptides in Pepscan analyses, suggesting that these MAbs recognize conformational epitope(s) located within this region. Six MAbs recognized a fragment corresponding to aa 361 to 461 of the NP. Five of these six MAbs showed specific reactivities in Pepscan analyses, and the epitopes were identified in two regions, aa 424 to 430 and aa 451 to 455. Three MAbs that recognized the former epitope region cross-reacted with all three subtypes, and one that recognized the same epitope region was Zaire specific. One MAb, which recognized the latter epitope region, was reactive with Zaire and Sudan subtypes but not with the Reston subtype. These results suggest that Ebola virus NP has at least two linear epitope regions and that the recognition of the epitope by MAbs can vary even within the same epitope region. These MAbs showing different subtype specificities might be useful reagents for developing an immunological system to identify Ebola virus subtypes.

Covalent modifications of the ebola virus glycoprotein

The role of covalent modifications of the Ebola virus glycoprotein (GP) and the significance of the sequence identity between filovirus and avian retrovirus GPs were investigated through biochemical and functional analyses of mutant GPs. The expression and processing of mutant GPs with altered N-linked glycosylation, substitutions for conserved cysteine residues, or a deletion in the region of O-linked glycosylation were analyzed, and virus entry capacities were assayed through the use of pseudotyped retroviruses. Cys-53 was the only GP(1) ( approximately 130 kDa) cysteine residue whose replacement resulted in the efficient secretion of GP(1), and it is therefore proposed that it participates in the formation of the only disulfide bond linking GP(1) to GP(2) ( approximately 24 kDa). We propose a complete cystine bridge map for the filovirus GPs based upon our analysis of mutant Ebola virus GPs. The effect of replacement of the conserved cysteines in the membrane-spanning region of GP(2) was found to depend on the nature of the substitution. Mutations in conserved N-linked glycosylation sites proved generally, with a few exceptions, innocuous. Deletion of the O-linked glycosylation region increased GP processing, incorporation into retrovirus particles, and viral transduction. Our data support a common evolutionary origin for the GPs of Ebola virus and avian retroviruses and have implications for gene transfer mediated by Ebola virus GP-pseudotyped retroviruses.

Evidence against Ebola virus sGP binding to human neutrophils by a specific receptor

The issue of whether Ebola secretory glycoprotein (sGP) binds to human neutrophils via the IgG Fc receptor IIIb (FcgammaRIIIb, CD16b) or other receptors has been controversial. To clarify this, FACS analysis, an sGP absorption assay, and direct binding of (125)I-sGP to neutrophils were performed. Results from FACS analysis demonstrated that limited washing conditions leads to the nonspecific formation of immune complexes on the neutrophil surface and this, but not a specific interaction between sGP and CD16b, is responsible for the previous observations. An sGP absorption assay also demonstrated that sGP is not specifically bound but is nonspecifically proteolysed by proteases released from neutrophils. Finally, there was no difference in (125)I-sGP binding to neutrophils compared to other control cell types. Taken together, these results demonstrate that neutrophils do not express a specific receptor for Ebola virus sGP. It is unlikely that sGP plays a role in the Ebola virus pathogenesis through interfering with the innate immunity by targeting neutrophils.

Association of the caveola vesicular system with cellular entry by filoviruses

The filoviruses Ebola Zaire virus and Marburg virus are believed to infect target cells through endocytic vesicles, but the details of this pathway are unknown. We used a pseudotyping strategy to investigate the cell biology of filovirus entry. We observed that specific inhibitors of the caveola system, including cholesterol-sequestering drugs and phorbol esters, inhibited the entry of filovirus pseudotypes into human cells. We also measured slower cell entry kinetics for both filovirus pseudotypes than for pseudotypes of vesicular stomatitis virus (VSV), which has been recognized to exploit the clathrin-mediated entry pathway. Finally, visualization by immunofluorescence and confocal microscopy revealed that the filovirus pseudotypes colocalized with the caveola protein marker caveolin-1 but that VSV pseudotypes did not. Collectively, these results provide evidence suggesting that filoviruses use caveolae to gain entry into cells.

Lipid raft microdomains: a gateway for compartmentalized trafficking of Ebola and Marburg viruses

Spatiotemporal aspects of filovirus entry and release are poorly understood. Lipid rafts act as functional platforms for multiple cellular signaling and trafficking processes. Here, we report the compartmentalization of Ebola and Marburg viral proteins within lipid rafts during viral assembly and budding. Filoviruses released from infected cells incorporated raft-associated molecules, suggesting that viral exit occurs at the rafts. Ectopic expression of Ebola matrix protein and glycoprotein supported raft-dependent release of filamentous, virus-like particles (VLPs), strikingly similar to live virus as revealed by electron microscopy. Our findings also revealed that the entry of filoviruses requires functional rafts, identifying rafts as the site of virus attack. The identification of rafts as the gateway for the entry and exit of filoviruses and raft-dependent generation of VLPs have important implications for development of therapeutics and vaccination strategies against infections with Ebola and Marburg viruses.

Ebola virus glycoproteins induce global surface protein down-modulation and loss of cell adherence

The Ebola virus envelope glycoprotein (GP) derived from the pathogenic Zaire subtype mediates cell rounding and detachment from the extracellular matrix in 293T cells. In this study we provide evidence that GPs from the other pathogenic subtypes, Sudan and Côte d'Ivoire, as well as from Reston, a strain thought to be nonpathogenic in humans, also induced cell rounding, albeit at lower levels than Zaire GP. Sequential removal of regions of potential O-linked glycosylation at the C terminus of GP1 led to a step-wise reduction in cell detachment without obviously affecting GP function, suggesting that such modifications are involved in inducing the detachment phenotype. While causing cell rounding and detachment in 293T cells, Ebola virus GP did not cause an increase in cell death. Indeed, following transient expression of GP, cells were able to readhere and continue to divide. Also, the rounding effect was not limited to 293T cells. Replication-deficient adenovirus vectors expressing Ebola virus GP induced the loss of cell adhesion in a range of cell lines and primary cell types, including those with proposed relevance to Ebola virus infection in vivo, such as endothelial cells and macrophages. In both transfected 293T and adenovirus-infected Vero cells, a reduction in cell surface expression of adhesion molecules such as integrin beta1 concurrent with the loss of cell adhesion was observed. A number of other cell surface molecules, however, including major histocompatibility complex class I and the epidermal growth factor receptor, were also down-modulated, suggesting a global mechanism for surface molecule down-regulation.

Reverse genetics demonstrates that proteolytic processing of the Ebola virus glycoprotein is not essential for replication in cell culture

Ebola virus, a prime example of an emerging pathogen, causes fatal hemorrhagic fever in humans and in nonhuman primates. Identification of major determinants of Ebola virus pathogenicity has been hampered by the lack of effective strategies for experimental mutagenesis. Here we exploit a reverse genetics system that allows the generation of Ebola virus from cloned cDNA to engineer a mutant Ebola virus with an altered furin recognition motif in the glycoprotein (GP). When expressed in cells, the GP of the wild type, but not of the mutant, virus was cleaved into GP1 and GP2. Although posttranslational furin-mediated cleavage of GP was thought to be an essential step in Ebola virus infection, generation of a viable mutant Ebola virus lacking a furin recognition motif in the GP cleavage site demonstrates that GP cleavage is not essential for replication of Ebola virus in cell culture.

The pathogenesis of Ebola hemorrhagic fever

Ebola virus causes lethal hemorrhagic disease in humans, yet there are still no satisfactory biological explanations to account for its extreme virulence. This review focuses on recent findings relevant to understanding the pathogenesis of Ebola virus infection and developing vaccines and effective therapy. The available data suggest that the envelope glycoprotein and the interaction of some viral proteins with the immune system are likely to play important roles in the extraordinary pathogenicity of this virus. There are also indications that genetically engineered vaccines, including plasmid DNA and viral vectors expressing Ebola virus proteins, and passive transfer of neutralizing antibodies could be feasible options for the control of Ebola virus-associated disease.

A system for functional analysis of Ebola virus glycoprotein

Ebola virus causes hemorrhagic fever in humans and nonhuman primates, resulting in mortality rates of up to 90%. Studies of this virus have been hampered by its extraordinary pathogenicity, which requires biosafety level 4 containment. To circumvent this problem, we developed a novel complementation system for functional analysis of Ebola virus glycoproteins. It relies on a recombinant vesicular stomatitis virus (VSV) that contains the green fluorescent protein gene instead of the receptor-binding G protein gene (VSVDeltaG*). Herein we show that Ebola Reston virus glycoprotein (ResGP) is efficiently incorporated into VSV particles. This recombinant VSV with integrated ResGP (VSVDeltaG*-ResGP) infected primate cells more efficiently than any of the other mammalian or avian cells examined, in a manner consistent with the host range tropism of Ebola virus, whereas VSVDeltaG* complemented with VSV G protein (VSVDeltaG*-G) efficiently infected the majority of the cells tested. We also tested the utility of this system for investigating the cellular receptors for Ebola virus. Chemical modification of cells to alter their surface proteins markedly reduced their susceptibility to VSVDeltaG*-ResGP but not to VSVDeltaG*-G. These findings suggest that cell surface glycoproteins with N-linked oligosaccharide chains contribute to the entry of Ebola viruses, presumably acting as a specific receptor and/or cofactor for virus entry. Thus, our VSV system should be useful for investigating the functions of glycoproteins from highly pathogenic viruses or those incapable of being cultured in vitro.