Requirements for cell rounding and surface protein down-regulation by Ebola virus glycoprotein

Identification and Potential Roles of Human MicroRNAs in Ebola Virus Infection and Disease Pathogenesis

Ebola virus (EBOV) is a highly pathogenic virus that causes a severe illness called Ebola virus disease (EVD). EVD has a high mortality rate and remains a significant threat to public health. Research on EVD pathogenesis has traditionally focused on host transcriptional responses. Limited recent studies, however, have revealed some information on the significance of cellular microRNAs (miRNAs) in EBOV infection and pathogenic mechanisms, but further studies are needed. Thus, this study aimed to identify and validate additional known and novel human miRNAs in EBOV-infected adult retinal pigment epithelial (ARPE) cells and predict their potential roles in EBOV infection and pathogenic mechanisms. We analyzed previously available small RNA-Seq data obtained from ARPE cells and identified 23 upregulated and seven downregulated miRNAs in the EBOV-infected cells; these included two novel miRNAs and 17 additional known miRNAs not previously identified in ARPE cells. In addition to pathways previously identified by others, these miRNAs are associated with pathways and biological processes that include WNT, FoxO, and phosphatidylinositol signaling; these pathways were not identified in the original study. This study thus confirms and expands on the previous study using the same datasets and demonstrates further the importance of human miRNAs in the host response and EVD pathogenesis during infection.

A rapid, high-throughput, viral infectivity assay using automated brightfield microscopy with machine learning

Infectivity assays are essential for the development of viral vaccines, antiviral therapies, and the manufacture of biologicals. Traditionally, these assays take 2-7 days and require several manual processing steps after infection. We describe an automated viral infectivity assay (AVIATM), using convolutional neural networks (CNNs) and high-throughput brightfield microscopy on 96-well plates that can quantify infection phenotypes within hours, before they are manually visible, and without sample preparation. CNN models were trained on HIV, influenza A virus, coronavirus 229E, vaccinia viruses, poliovirus, and adenoviruses, which together span the four major categories of virus (DNA, RNA, enveloped, and non-enveloped). A sigmoidal function, fit between virus dilution curves and CNN predictions, results in sensitivity ranges comparable to or better than conventional plaque or TCID50 assays, and a precision of ∼10%, which is considerably better than conventional infectivity assays. Because this technology is based on sensitizing CNNs to specific phenotypes of infection, it has potential as a rapid, broad-spectrum tool for virus characterization, and potentially identification.

Crimean-Congo haemorrhagic fever virus

Crimean-Congo haemorrhagic fever (CCHF) is a severe tick-borne illness with a wide geographical distribution and case fatality rates of 30% or higher. Caused by infection with the CCHF virus (CCHFV), cases are reported throughout Africa, the Middle East, Asia and southern and eastern Europe. The expanding range of the Hyalomma tick vector is placing new populations at risk for CCHF, and no licensed vaccines or specific antivirals exist to treat CCHF. Furthermore, despite cases of CCHF being reported annually, the host and viral determinants of CCHFV pathogenesis are poorly understood. CCHFV can productively infect a multitude of animal species, yet only humans develop a severe illness. Within human populations, subclinical infections are underappreciated and may represent a substantial proportion of clinical outcomes. Compared with other members of the Bunyavirales order, CCHFV has a more complex genomic organization, with many viral proteins having unclear functions in viral pathogenesis. In recent years, improved animal models have led to increased insights into CCHFV pathogenesis, and several antivirals and vaccines for CCHFV have shown robust efficacy in preclinical models. Translation of these insights and candidate therapeutics to the clinic will hopefully reduce the morbidity and mortality caused by CCHFV.

The Virus-Induced Cytopathic Effect

The cytopathic effect comprises the set of cellular alterations produced by a viral infection. It is of great relevance since it constitutes a direct marker of infection. Likewise, these alterations are often virus-specific which makes them a phenotypic marker for many viral species. All these characteristics have been used to complement the study of the dynamics of virus-cell interactions through the kinetic study of the progression of damage produced by the infection. Various approaches have been used to monitor the cytopathic effect, ranging from light microscopy, immunofluorescence assays, and direct labeling with fluorescent dyes, to plaque assay for the characterization of the infection over time. Here we address the relevance of the study of cytopathic effect and describe different experimental alternatives for its application.

RNF185 regulates proteostasis in Ebolavirus infection by crosstalk between the calnexin cycle, ERAD, and reticulophagy

Virus infection affects cellular proteostasis and provides an opportunity to study this cellular process under perturbation. The proteostasis network in the endoplasmic reticulum (ER) is composed of the calnexin cycle, and the two protein degradation pathways ER-associated protein degradation (ERAD) and ER-to-lysosome-associated degradation (ERLAD/ER-phagy/reticulophagy). Here we show that calnexin and calreticulin trigger Zaire Ebolavirus (EBOV) glycoprotein GP_1,2 misfolding. Misfolded EBOV-GP_1,2 is targeted by ERAD machinery, but this results in lysosomal instead of proteasomal degradation. Moreover, the ER Ub ligase RNF185, usually associated with ERAD, polyubiquitinates EBOV-GP_1,2 on lysine 673 via ubiquitin K27-linkage. Polyubiquinated GP_1,2 is subsequently recruited into autophagosomes by the soluble autophagy receptor sequestosome 1 (SQSTM1/p62), in an ATG3- and ATG5-dependent manner. We conclude that EBOV hijacks all three proteostasis mechanisms in the ER to downregulate GP_1,2 via polyubiquitination and show that this increases viral fitness. This study identifies linkages among proteostasis network components previously thought to function independently.

Little is known about how proteostasis is maintained during viral infection. Here, the authors identify unexpected crosstalk between the calnexin cycle, ERAD, and reticulophagy, resulting in suppression of ebolavirus glycoprotein expression.

Protein disulfide isomerases (PDIs) negatively regulate ebolavirus structural glycoprotein expression in the endoplasmic reticulum (ER) via the autophagy-lysosomal pathway

Zaire ebolavirus (EBOV) causes a severe hemorrhagic fever in humans and non-human primates with high morbidity and mortality. EBOV infection is dependent on its structural glycoprotein (GP), but high levels of GP expression also trigger cell rounding, detachment, and downregulation of many surface molecules that is thought to contribute to its high pathogenicity. Thus, EBOV has evolved an RNA editing mechanism to reduce its GP expression and increase its fitness. We now report that the GP expression is also suppressed at the protein level in cells by protein disulfide isomerases (PDIs). Although PDIs promote oxidative protein folding by catalyzing correct disulfide formation in the endoplasmic reticulum (ER), PDIA3/ERp57 adversely triggered the GP misfolding by targeting GP cysteine residues and activated the unfolded protein response (UPR). Abnormally folded GP was targeted by ER-associated protein degradation (ERAD) machinery and, unexpectedly, was degraded via the macroautophagy/autophagy-lysosomal pathway, but not the proteasomal pathway. PDIA3 also decreased the GP expression from other ebolavirus species but increased the GP expression from Marburg virus (MARV), which is consistent with the observation that MARV-GP does not cause cell rounding and detachment, and MARV does not regulate its GP expression via RNA editing during infection. Furthermore, five other PDIs also had a similar inhibitory activity to EBOV-GP. Thus, PDIs negatively regulate ebolavirus glycoprotein expression, which balances the viral life cycle by maximizing their infection but minimizing their cellular effect. We suggest that ebolaviruses hijack the host protein folding and ERAD machinery to increase their fitness via reticulophagy during infection.Abbreviations: 3-MA: 3-methyladenine; 4-PBA: 4-phenylbutyrate; ACTB: β-actin; ATF: activating transcription factor; ATG: autophagy-related; BafA1: bafilomycin A1; BDBV: Bundibugyo ebolavirus; CALR: calreticulin; CANX: calnexin; CHX: cycloheximide; CMA: chaperone-mediated autophagy; ConA: concanamycin A; CRISPR: clusters of regularly interspaced short palindromic repeats; Cas9: CRISPR-associated protein 9; dsRNA: double-stranded RNA; EBOV: Zaire ebolavirus; EDEM: ER degradation enhancing alpha-mannosidase like protein; EIF2AK3/PERK: eukaryotic translation initiation factor 2 alpha kinase 3; Env: envelope glycoprotein; ER: endoplasmic reticulum; ERAD: ER-associated protein degradation; ERN1/IRE1: endoplasmic reticulum to nucleus signaling 1; GP: glycoprotein; HA: hemagglutinin; HDAC6: histone deacetylase 6; HMM: high-molecular-mass; HIV-1: human immunodeficiency virus type 1; HSPA5/BiP: heat shock protein family A (Hsp70) member 5; IAV: influenza A virus; IP: immunoprecipitation; KIF: kifenesine; Lac: lactacystin; LAMP: lysosomal associated membrane protein; MAN1B1/ERManI: mannosidase alpha class 1B member 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MARV: Marburg virus; MLD: mucin-like domain; NHK/SERPINA1: alpha1-antitrypsin variant null (Hong Kong); NTZ: nitazoxanide; PDI: protein disulfide isomerase; RAVV: Ravn virus; RESTV: Reston ebolavirus; SARS-CoV: severe acute respiratory syndrome coronavirus; SBOV: Sudan ebolavirus; sGP: soluble GP; SQSTM1/p62: sequestosome 1; ssGP: small soluble GP; TAFV: Taï Forest ebolavirus; TIZ: tizoxanide; TGN: thapsigargin; TLD: TXN (thioredoxin)-like domain; Ub: ubiquitin; UPR: unfolded protein response; VLP: virus-like particle; VSV: vesicular stomatitis virus; WB: Western blotting; WT: wild-type; XBP1: X-box binding protein 1.

Structural and Functional Aspects of Ebola Virus Proteins

Ebola virus (EBOV), member of genus Ebolavirus, family Filoviridae, have a non-segmented, single-stranded RNA that contains seven genes: (a) nucleoprotein (NP), (b) viral protein 35 (VP35), (c) VP40, (d) glycoprotein (GP), (e) VP30, (f) VP24, and (g) RNA polymerase (L). All genes encode for one protein each except GP, producing three pre-proteins due to the transcriptional editing. These pre-proteins are translated into four products, namely: (a) soluble secreted glycoprotein (sGP), (b) Δ-peptide, (c) full-length transmembrane spike glycoprotein (GP), and (d) soluble small secreted glycoprotein (ssGP). Further, shed GP is released from infected cells due to cleavage of GP by tumor necrosis factor α-converting enzyme (TACE). This review presents a detailed discussion on various functional aspects of all EBOV proteins and their residues. An introduction to ebolaviruses and their life cycle is also provided for clarity of the available analysis. We believe that this review will help understand the roles played by different EBOV proteins in the pathogenesis of the disease. It will help in targeting significant protein residues for therapeutic and multi-protein/peptide vaccine development.

Bombyx mori nucleopolyhedrovirus F-like protein Bm14 is a factor for viral-induced cytopathic effects via regulating oxidative phosphorylation and cellular ROS levels

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly pathogenic to Bombyx mori, silkworm, which causes serious cytopathic effects (CPEs) during infection. However, the role of viral protein in the virus-induced CPEs remains unclear. Here, we discovered that BmNPV infection induced severe CPEs including titer-dependent cell floating and changes in cellular surface morphology. Further explorations revealed the involvement of F-like protein (Bm14), a viral envelope protein, in inducing cytotoxicity and detachment of adherent BmN cells, and its disruption significantly impaired the virus infection-mediated CPEs. Intriguingly, transcriptomic analysis identified the tight association of Bm14 deletion with the activation of cellular oxidative phosphorylation pathway, consistent with the elevated mitochondrial membrane potential (MMP) levels and ATP concentrations as well as reduced ROS levels. Collectively, our results characterized for the first time the novel role of Bm14 in accelerating viral-induced cytopathogenicity via suppressing the cellular oxidative phosphorylation levels and upregulating the ROS levels.

Efficient Expression and Processing of Ebola Virus Glycoprotein Induces Morphological Changes in BmN Cells but Cannot Rescue Deficiency of Bombyx Mori Nucleopolyhedrovirus GP64

Ebola virus (EBOV) disease outbreaks have resulted in many fatalities, yet no licensed vaccines are available to prevent infection. Recombinant glycoprotein (GP) production may contribute to finding a cure for Ebola virus disease, which is the key candidate protein for vaccine preparation. To explore GP_1,2 expression in BmN cells, EBOV-GP_1,2 with its native signal peptide or the GP64 signal peptide was cloned and transferred into a normal or gp64 null Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid via transposition. The infectivity of the recombinant bacmids was investigated after transfection, expression and localization of EBOV-GP were investigated, and cell morphological changes were analyzed by TEM. The GP64 signal peptide, but not the GP_1,2 native signal peptide, caused GP_1,2 localization to the cell membrane, and the differentially localized GP_1,2 proteins were cleaved into GP₁ and GP₂ fragments in BmN cells. GP_1,2 expression resulted in dramatic morphological changes in BmN cells in the early stage of infection. However, GP_1,2 expression did not rescue GP64 deficiency in BmNPV infection. This study provides a better understanding of GP expression and processing in BmN cells, which may lay a foundation for EBOV-GP expression using the BmNPV baculovirus expression system.

Filovirus Strategies to Escape Antiviral Responses

This chapter describes the various strategies filoviruses use to escape host immune responses with a focus on innate immune and cell death pathways. Since filovirus replication can be efficiently blocked by interferon (IFN), filoviruses have evolved mechanisms to counteract both type I IFN induction and IFN response signaling pathways. Intriguingly, marburg- and ebolaviruses use different strategies to inhibit IFN signaling. This chapter also summarizes what is known about the role of IFN-stimulated genes (ISGs) in filovirus infection. These fall into three categories: those that restrict filovirus replication, those whose activation is inhibited by filoviruses, and those that have no measurable effect on viral replication. In addition to innate immunity, mammalian cells have evolved strategies to counter viral infections, including the induction of cell death and stress response pathways, and we summarize our current knowledge of how filoviruses interact with these pathways. Finally, this chapter delves into the interaction of EBOV with myeloid dendritic cells and macrophages and the associated inflammatory response, which differs dramatically between these cell types when they are infected with EBOV. In summary, we highlight the multifaceted nature of the host-viral interactions during filoviral infections.

Site-specific glycosylation of Ebola virus glycoprotein by human polypeptide GalNAc-transferase 1 induces cell adhesion defects

The surface glycoprotein (GP) of Ebola virus causes many of the virus's pathogenic effects, including a dramatic loss of endothelial cell adhesion associated with widespread hemorrhaging during infection. Although the GP-mediated deadhesion depends on its extracellular mucin-like domain, it is unknown whether any, or all, of this domain's densely clustered O-glycosylation sites are required. It is also unknown whether any of the 20 distinct polypeptide GalNAc-transferases (ppGalNAc-Ts) that initiate mucin-type O-glycosylation in human cells are functionally required. Here, using HEK293 cell lines lacking specific glycosylation enzymes, we demonstrate that GP requires extended O-glycans to exert its deadhesion effect. We also identified ppGalNAc-T1 as largely required for the GP-mediated adhesion defects. Despite its profound effect on GP function, the absence of ppGalNAc-T1 only modestly reduced the O-glycan mass of GP, indicating that even small changes in the bulky glycodomain can cause loss of GP function. Indeed, protein-mapping studies identified a small segment of the mucin-like domain critical for function and revealed that mutation of five glycan acceptor sites within this segment are sufficient to abrogate GP function. Together, these results argue against a mechanism of Ebola GP-induced cell detachment that depends solely on ectodomain bulkiness and identify a single host-derived glycosylation enzyme, ppGalNAc-T1, as a potential target for therapeutic intervention against Ebola virus disease.

Characterization of Influenza Virus Pseudotyped with Ebolavirus Glycoprotein

We have produced a new Ebola virus pseudotype, E-S-FLU, that can be handled in biosafety level 1/2 containment for laboratory analysis. The E-S-FLU virus is a single-cycle influenza virus coated with Ebolavirus glycoprotein, and it encodes enhanced green fluorescence protein as a reporter that replaces the influenza virus hemagglutinin. MDCK-SIAT1 cells were transduced to express Ebolavirus glycoprotein as a stable transmembrane protein for E-S-FLU virus production. Infection of cells with the E-S-FLU virus was dependent on the Niemann-Pick C1 protein, which is the well-characterized receptor for Ebola virus entry at the late endosome/lysosome membrane. The E-S-FLU virus was neutralized specifically by an anti-Ebolavirus glycoprotein antibody and a variety of small drug molecules that are known to inhibit the entry of wild-type Ebola virus. To demonstrate the application of this new Ebola virus pseudotype, we show that a single laboratory batch was sufficient to screen a library (LOPAC¹²⁸⁰; Sigma) of 1,280 pharmacologically active compounds for inhibition of virus entry. A total of 215 compounds inhibited E-S-FLU virus infection, while only 22 inhibited the control H5-S-FLU virus coated in H5 hemagglutinin. These inhibitory compounds have very dispersed targets and mechanisms of action, e.g., calcium channel blockers, estrogen receptor antagonists, antihistamines, serotonin uptake inhibitors, etc., and this correlates with inhibitor screening results obtained with other pseudotypes or wild-type Ebola virus in the literature. The E-S-FLU virus is a new tool for Ebola virus cell entry studies and is easily applied to high-throughput screening assays for small-molecule inhibitors or antibodies.

IMPORTANCE Ebola virus is in the Filoviridae family and is a biosafety level 4 pathogen. There are no FDA-approved therapeutics for Ebola virus. These characteristics warrant the development of surrogates for Ebola virus that can be handled in more convenient laboratory containment to study the biology of the virus and screen for inhibitors. Here we characterized a new surrogate, named E-S-FLU virus, that is based on a disabled influenza virus core coated with the Ebola virus surface protein but does not contain any genetic information from the Ebola virus itself. We show that E-S-FLU virus uses the same cell entry pathway as wild-type Ebola virus. As an example of the ease of use of E-S-FLU virus in biosafety level 1/2 containment, we showed that a single production batch could provide enough surrogate virus to screen a standard small-molecule library of 1,280 candidates for inhibitors of viral entry.

A comparison of host gene expression signatures associated with infection in vitro by the Makona and Ecran (Mayinga) variants of Ebola virus

The Ebola virus (EBOV) variant Makona (which emerged in 2013) was the causative agent of the largest outbreak of Ebola Virus Disease recorded. Differences in virus-host interactions between viral variants have potential consequences for transmission, disease severity and mortality. A detailed profile of the cellular changes induced by the Makona variant compared with other Ebola virus variants was lacking. In this study, A549 cells, a human cell line with a robust innate response, were infected with the Makona variant or with the Ecran variant originating from the 1976 outbreak in Central Africa. The abundance of viral and cellular mRNA transcripts was profiled using RNASeq and differential gene expression analysis performed. Differences in effects of each virus on the expression of interferon-stimulated genes were also investigated in A549 NPro cells where the type 1 interferon response had been attenuated. Cellular transcriptomic changes were compared with those induced by human respiratory syncytial virus (HRSV), a virus with a similar genome organisation and replication strategy to EBOV. Pathway and gene ontology analysis revealed differential expression of functionally important genes; including genes involved in the inflammatory response, cell proliferation, leukocyte extravasation and cholesterol biosynthesis. Whilst there was overlap with HRSV, there was unique commonality to the EBOV variants.

Anti-Ebola therapies based on monoclonal antibodies: current state and challenges ahead

The 2014 Ebola outbreak, the largest recorded, took us largely unprepared, with no available vaccine or specific treatment. In this context, the World Health Organization declared that the humanitarian use of experimental therapies against Ebola Virus (EBOV) is ethical. In particular, an experimental treatment consisting of a cocktail of three monoclonal antibodies (mAbs) produced in tobacco plants and specifically directed to the EBOV glycoprotein (GP) was tested in humans, apparently with good results. Several mAbs with high affinity to the GP have been described. This review discusses our current knowledge on this topic. Particular emphasis is devoted to those mAbs that have been assayed in animal models or humans as possible therapies against Ebola. Engineering aspects and challenges for the production of anti-Ebola mAbs are also briefly discussed; current platforms for the design and production of full-length mAbs are cumbersome and costly.

The roles of ebolavirus glycoproteins in viral pathogenesis

Ebolaviruses are highly dangerous pathogens exhibiting extreme virulence in humans and nonhuman primates. The majority of ebolavirus species, most notably Zaire ebolavirus, can cause Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, in humans. EVD is associated with case-fatality rates as high as 90%, and there is currently no specific treatment or licensed vaccine available against EVD. Understanding the molecular biology and pathogenesis of ebolaviruses is important for the development of antiviral therapeutics. Ebolavirus encodes several forms of glycoproteins (GPs), which have some interesting characteristics, including the transcriptional editing coding strategy and extensive O-glycosylation modification, clustered in the mucin-like domain of GP1, full-length GP (GP_1,2), and shed GP. In addition to the canonical role of the spike protein, GP_1,2, in viral entry, ebolavirus GPs appear to have multiple additional functions, likely contributing to the complex pathogenesis of the virus. Here, we review the roles of ebolavirus GPs in viral pathogenesis.

Filovirus proteins for antiviral drug discovery: A structure/function analysis of surface glycoproteins and virus entry

This review focuses on the recent progress in our understanding of filovirus protein structure/function and its impact on antiviral research. Here we focus on the surface glycoprotein GP_1,2 and its different roles in filovirus entry. We first describe the latest advances on the characterization of GP gene-overlapping proteins sGP, ssGP and Δ-peptide. Then, we compare filovirus surface GP_1,2 proteins in terms of structure, synthesis and function. As they bear potential in drug-design, the discovery of small organic compounds inhibiting filovirus entry is a currently very active field. Although it is at an early stage, the development of antiviral drugs against Ebola and Marburg virus entry might prove essential to reduce outbreak-associated fatality rates through post-exposure treatment of both suspected and confirmed cases.

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The filovirus surface glycoprotein is the key player protein responsible for viral entry.

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Secreted forms of the glycoprotein have been suggested to participate to filovirus virus pathogenicity.

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Recent structural insights of the filovirus surface glycoprotein highlight new antiviral perspectives.

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Interesting compounds and innovative antiviral strategies emerge from research and development to inhibit filovirus entry.

Inhibition of Ebola virus glycoprotein-mediated cytotoxicity by targeting its transmembrane domain and cholesterol

The high pathogenicity of the Ebola virus reflects multiple concurrent processes on infection. Among other important determinants, Ebola fusogenic glycoprotein (GP) has been associated with the detachment of infected cells and eventually leads to vascular leakage and haemorrhagic fever. Here we report that the membrane-anchored GP is sufficient to induce the detachment of adherent cells. The results show that the detachment induced through either full-length GP1,2 or the subunit GP2 depends on cholesterol and the structure of the transmembrane domain. These data reveal a novel molecular mechanism in which GP regulates Ebola virus assembly and suggest that cholesterol-reducing agents could be useful as therapeutics to counteract GP-mediated cell detachment.

Shedding of Ebola Virus Surface Glycoprotein Is a Mechanism of Self-regulation of Cellular Cytotoxicity and Has a Direct Effect on Virus Infectivity

The surface glycoprotein (GP) is responsible for Ebola virus (EBOV) attachment and membrane fusion during virus entry. Surface expression of highly glycosylated GP causes marked cytotoxicity via masking of a wide range of cellular surface molecules, including integrins. Considerable amounts of surface GP are shed from virus-infected cells in a soluble truncated form by tumor necrosis factor α-converting enzyme. In this study, the role of GP shedding was investigated using a reverse genetics approach by comparing recombinant viruses possessing amino acid substitutions at the GP shedding site. Virus with an L635V substitution showed a substantial decrease in shedding, whereas a D637V substitution resulted in a striking increase in the release of shed GP. Variations in shedding efficacy correlated with observed differences in the amounts of shed GP in the medium, GP present in virus-infected cells, and GP present on virions. An increase in shedding appeared to be associated with a reduction in viral cytotoxicity, and, vice versa, the virus that shed less was more cytotoxic. An increase in shedding also resulted in a reduction in viral infectivity, whereas a decrease in shedding efficacy enhanced viral growth characteristics in vitro. Differences in shedding efficacy and, as a result, differences in the amount of mature GP available for incorporation into budding virions did not equate to differences in overall release of viral particles. Likewise, data suggest that the resulting differences in the amount of mature GP on the cell surface led to variations in the GP content of released particles and, as a consequence, in infectivity. In conclusion, fine-tuning of the levels of EBOV GP expressed at the surface of virus-infected cells via GP shedding plays an important role in EBOV replication by orchestrating the balance between optimal virion GP content and cytotoxicity caused by GP.

Human Ebola virus infection in West Africa: a review of available therapeutic agents that target different steps of the life cycle of Ebola virus

The recent outbreak of the human Zaire ebolavirus (EBOV) epidemic is spiraling out of control in West Africa. Human EBOV hemorrhagic fever has a case fatality rate of up to 90%. The EBOV is classified as a biosafety level 4 pathogen and is considered a category A agent of bioterrorism by Centers for Disease Control and Prevention, with no approved therapies and vaccines available for its treatment apart from supportive care. Although several promising therapeutic agents and vaccines against EBOV are undergoing the Phase I human trial, the current epidemic might be outpacing the speed at which drugs and vaccines can be produced. Like all viruses, the EBOV largely relies on host cell factors and physiological processes for its entry, replication, and egress. We have reviewed currently available therapeutic agents that have been shown to be effective in suppressing the proliferation of the EBOV in cell cultures or animal studies. Most of the therapeutic agents in this review are directed against non-mutable targets of the host, which is independent of viral mutation. These medications are approved by the Food and Drug Administration (FDA) for the treatment of other diseases. They are available and stockpileable for immediate use. They may also have a complementary role to those therapeutic agents under development that are directed against the mutable targets of the EBOV.

Less is more: Ebola virus surface glycoprotein expression levels regulate virus production and infectivity

The Ebola virus (EBOV) surface glycoprotein (GP1,2) mediates host cell attachment and fusion and is the primary target for host neutralizing antibodies. Expression of GP1,2 at high levels disrupts normal cell physiology, and EBOV uses an RNA-editing mechanism to regulate expression of the GP gene. In this study, we demonstrate that high levels of GP1,2 expression impair production and release of EBOV virus-like particles (VLPs) as well as infectivity of GP1,2-pseudotyped viruses. We further show that this effect is mediated through two mechanisms. First, high levels of GP1,2 expression reduce synthesis of other proteins needed for virus assembly. Second, viruses containing high levels of GP1,2 are intrinsically less infectious, possibly due to impaired receptor binding or endosomal processing. Importantly, proteolysis can rescue the infectivity of high-GP1,2-containing viruses. Taken together, our findings indicate that GP1,2 expression levels have a profound effect on factors that contribute to virus fitness and that RNA editing may be an important mechanism employed by EBOV to regulate GP1,2 expression in order to optimize virus production and infectivity.

IMPORTANCE

The Ebola virus (EBOV), as well as other members of the Filoviridae family, causes severe hemorrhagic fever that is highly lethal, with up to 90% mortality. The EBOV surface glycoprotein (GP1,2) plays important roles in virus infection and pathogenesis, and its expression is tightly regulated by an RNA-editing mechanism during virus replication. Our study demonstrates that the level of GP1,2 expression profoundly affects virus particle production and release and uncovers a new mechanism by which Ebola virus infectivity is regulated by the level of GP1,2 expression. These findings extend our understanding of EBOV infection and replication in adaptation of host environments, which will aid the development of countermeasures against EBOV infection.

A highly immunogenic fragment derived from Zaire Ebola virus glycoprotein elicits effective neutralizing antibody

In order to produce polyvalent vaccines based on single rVSV vector, we investigated the immunogenicity, antibody neutralizing activity, and antigenic determinant domain of Zaire Ebola's fragment MFL (aa 393-556) that contains furin site and internal fusion loop. Both the recombinant protein and the recombinant plasmid of fragment MFL elicited high levels of antibody, similar to those of Zaire Ebola GP (ZGP). The MFL fragment of ZGP also elicited high levels of neutralizing antibody and induced moderate cellular immune response in mice, as revealed by the proliferation and cytokine secretion of splenocytes. Through the analysis of the induction of neutralizing antibody by pVAX1-based recombinant plasmids that expressed truncated fragments of MFL, we found that the domain containing the internal fusion loop and the furin site was the major contributor of fragment MFL's immunogenicity. Furthermore, the rVSV-based bivalent vaccine expressing Sudan Ebola GP (SGP) and MFL fragment elicited efficient cross-immunity against ZGP and SGP with high levels of neutralizing antibody. Our results indicate that fragment MFL is an effective and novel antigen for the production of neutralizing antibody and polyvalent vaccines of Ebola virus.

Identification of continuous human B-cell epitopes in the VP35, VP40, nucleoprotein and glycoprotein of Ebola virus

Ebola virus (EBOV) is a highly virulent human pathogen. Recovery of infected patients is associated with efficient EBOV-specific immunoglobulin G (IgG) responses, whereas fatal outcome is associated with defective humoral immunity. As B-cell epitopes on EBOV are poorly defined, we sought to identify specific epitopes in four EBOV proteins (Glycoprotein (GP), Nucleoprotein (NP), and matrix Viral Protein (VP)40 and VP35). For the first time, we tested EBOV IgG+ sera from asymptomatic individuals and symptomatic Gabonese survivors, collected during the early humoral response (seven days after the end of symptoms) and the late memory phase (7–12 years post-infection). We also tested sera from EBOV-seropositive patients who had never had clinical signs of hemorrhagic fever or who lived in non-epidemic areas (asymptomatic subjects). We found that serum from asymptomatic individuals was more strongly reactive to VP40 peptides than to GP, NP or VP35. Interestingly, anti-EBOV IgG from asymptomatic patients targeted three immunodominant regions of VP40 reported to play a crucial role in virus assembly and budding. In contrast, serum from most survivors of the three outbreaks, collected a few days after the end of symptoms, reacted mainly with GP peptides. However, in asymptomatic subjects the longest immunodominant domains were identified in GP, and analysis of the GP crystal structure revealed that these domains covered a larger surface area of the chalice bowl formed by three GP₁ subunits. The B-cell epitopes we identified in the EBOV VP35, VP40, NP and GP proteins may represent important tools for understanding the humoral response to this virus and for developing new antibody-based therapeutics or detection methods.

An update on the use of antibodies against the filoviruses

Multiple recent, independent studies have confirmed that passively administered antibodies can provide effective postexposure therapy in nonhuman primates after exposure to an otherwise lethal dose of Ebola virus or Marburg virus. In this article, we review composition and performance of the antibody cocktails tested thus far, what is known about antibody epitopes on the viral glycoprotein target and ongoing research questions in further development of such cocktails for pre-exposure or emergency postexposure use.

Intracellular events and cell fate in filovirus infection

Marburg and Ebola viruses cause a severe hemorrhagic disease in humans with high fatality rates. Early target cells of filoviruses are monocytes, macrophages, and dendritic cells. The infection spreads to the liver, spleen and later other organs by blood and lymph flow. A hallmark of filovirus infection is the depletion of non-infected lymphocytes; however, the molecular mechanisms leading to the observed bystander lymphocyte apoptosis are poorly understood. Also, there is limited knowledge about the fate of infected cells in filovirus disease. In this review we will explore what is known about the intracellular events leading to virus amplification and cell damage in filovirus infection. Furthermore, we will discuss how cellular dysfunction and cell death may correlate with disease pathogenesis.

Anti-tetherin activities of HIV-1 Vpu and Ebola virus glycoprotein do not involve removal of tetherin from lipid rafts

BST-2/tetherin is an interferon-inducible host restriction factor that blocks the release of newly formed enveloped viruses. It is enriched in lipid raft membrane microdomains, which are also the sites of assembly of several enveloped viruses. Viral anti-tetherin factors, such as the HIV-1 Vpu protein, typically act by removing tetherin from the cell surface. In contrast, the Ebola virus glycoprotein (GP) is unusual in that it blocks tetherin restriction without apparently altering its cell surface localization. We explored the possibility that GP acts to exclude tetherin from the specific sites of virus assembly without overtly removing it from the cell surface and that lipid raft exclusion is the mechanism involved. However, we found that neither GP nor Vpu had any effect on tetherin's distribution within lipid raft domains. Furthermore, GP did not prevent the colocalization of tetherin and budding viral particles. Contrary to previous reports, we also found no evidence that GP is itself a raft protein. Together, our data indicate that the exclusion of tetherin from lipid rafts is not the mechanism used by either HIV-1 Vpu or Ebola virus GP to counteract tetherin restriction.

The role of antigen-presenting cells in filoviral hemorrhagic fever: gaps in current knowledge

The filoviruses, Ebola virus (EBOV) and Marburg virus (MARV), are highly lethal zoonotic agents of concern as emerging pathogens and potential bioweapons. Antigen-presenting cells (APCs), particularly macrophages and dendritic cells, are targets of filovirus infection in vivo. Infection of these cell types has been proposed to contribute to the inflammation, activation of coagulation cascades and ineffective immune responses characteristic of filovirus hemorrhagic fever. However, many aspects of filovirus–APC interactions remain to be clarified. Among the unanswered questions: What determines the ability of filoviruses to replicate in different APC subsets? What are the cellular signaling pathways that sense infection and lead to production of copious quantities of cytokines, chemokines and tissue factor? What are the mechanisms by which innate antiviral responses are disabled by these viruses, and how may these mechanisms contribute to inadequate adaptive immunity? A better understanding of these issues will clarify the pathogenesis of filoviral hemorrhagic fever and provide new avenues for development of therapeutics.

Genomic RNA editing and its impact on Ebola virus adaptation during serial passages in cell culture and infection of guinea pigs

Synthesis of the structural, surface glycoprotein (GP) of Ebola virus (EBOV) is dependent on transcriptional RNA editing phenomenon. Editing results in the insertion of an extra adenosine by viral polymerase at the editing site (7 consecutive template uridines) during transcription of GP gene of the wild-type virus (EBOV/7U). In this study, we demonstrate that passage of EBOV/7U in Vero E6 cells results in the appearance and rapid accumulation of a variant (EBOV/8U) containing an additional uridine at the editing site in the viral genome. EBOV/8U outgrows and eventually replaces the wild-type EBOV during 4-5 passages. On the contrary, infection of guinea pigs with EBOV/8U leads to the appearance and rapid predominance by EBOV/7U. These rapid conversions suggest that editing of the genomic RNA occurs at a higher frequency than previously thought. In addition, it indicates that the EBOV/7U phenotype has a selective advantage that is linked to controlled expression of GP and/or expression of secreted sGP, the primary gene product for wild-type EBOV. This study demonstrates the potential for insertion and deletion of uridines in the editing site of the EBOV genomic RNA, depending on environmental constraints.

Impact of Ebola mucin-like domain on antiglycoprotein antibody responses induced by Ebola virus-like particles

Ebola virus (EBOV) glycoprotein (GP), responsible for mediating host-cell attachment and membrane fusion, contains a heavily glycosylated mucin-like domain hypothesized to shield GP from neutralizing antibodies. To test whether the mucin-like domain inhibits the production and function of anti-GP antibodies, we vaccinated mice with Ebola virus-like particles (VLPs) that express vesicular stomatitis virus G, wild-type EBOV GP (EBGP), EBOV GP without its mucin-like domain (ΔMucGP), or EBOV GP with a Crimean-Congo hemorrhagic fever virus mucin-like domain substituted for the EBOV mucin-like domain (CMsubGP). EBGP-VLP immunized mice elicited significantly higher serum antibody titers toward EBGP or its mutants, as detected by western blot analysis, than did VLP-ΔMucGP. However, EBGP-, ΔMucGP- and CMsubGP-VLP immunized mouse sera contained antibodies that bound to cell surface-expressed GP at similar levels. Furthermore, low but similar neutralizing antibody titers, measured against a vesicular stomatitis virus (VSV) expressing EBGP or ΔMucGP, were present in EBGP, ΔMucGP, and CMsubGP sera, although a slightly higher neutralizing titer (2- to 2.5-fold) was detected in ΔMucGP sera. We conclude that the EBOV GP mucin-like domain can increase relative anti-GP titers, however these titers appear to be directed, at least partly, to denatured GP. Furthermore, removing the mucin-like domain from immunizing VLPs has modest impact on neutralizing antibody titers in serum.

The cytoplasmic domain of Marburg virus GP modulates early steps of viral infection

Marburg virus infection is mediated by the only viral surface protein, GP, a trimeric type I transmembrane protein. While its ectodomain mediates receptor binding and fusion of viral and cellular membranes and its transmembrane domain is essential for the recruitment of GP into budding particles by the matrix protein VP40, the role of the short cytoplasmic domain has remained enigmatic. Here we show that a missing cytoplasmic domain did not impair trimerization, intracellular transport, or incorporation of GP into infectious Marburg virus-like particles (iVLPs) but altered the glycosylation pattern as well as the recognition of GP by neutralizing antibodies. These results suggest that subtle conformational changes took place in the ectodomain. To investigate the function of the cytoplasmic domain during viral entry, a novel entry assay was established to monitor the uptake of filamentous VLPs by measuring the occurrence of luciferase-labeled viral nucleocapsids in the cytosol of target cells. This quantitative assay showed that the entry process of VLPs incorporating GP missing its cytoplasmic domain (GPΔCD) was impaired. Supporting these results, iVLPs incorporating a mutant GP missing its cytoplasmic domain were significantly less infectious than iVLPs containing wild-type GP. Taken together, the data indicate that the absence of the short cytoplasmic domain of Marburg virus GP may induce conformational changes in the ectodomain which impact the filoviral entry process.

Full-length Ebola glycoprotein accumulates in the endoplasmic reticulum

The Filoviridae family comprises of Ebola and Marburg viruses, which are known to cause lethal hemorrhagic fever. However, there is no effective anti-viral therapy or licensed vaccines currently available for these human pathogens. The envelope glycoprotein (GP) of Ebola virus, which mediates entry into target cells, is cytotoxic and this effect maps to a highly glycosylated mucin-like region in the surface subunit of GP (GP1). However, the mechanism underlying this cytotoxic property of GP is unknown. To gain insight into the basis of this GP-induced cytotoxicity, HEK293T cells were transiently transfected with full-length and mucin-deleted (Δmucin) Ebola GP plasmids and GP localization was examined relative to the nucleus, endoplasmic reticulum (ER), Golgi, early and late endosomes using deconvolution fluorescent microscopy. Full-length Ebola GP was observed to accumulate in the ER. In contrast, GPΔmucin was uniformly expressed throughout the cell and did not localize in the ER. The Ebola major matrix protein VP40 was also co-expressed with GP to investigate its influence on GP localization. GP and VP40 co-expression did not alter GP localization to the ER. Also, when VP40 was co-expressed with the nucleoprotein (NP), it localized to the plasma membrane while NP accumulated in distinct cytoplasmic structures lined with vimentin. These latter structures are consistent with aggresomes and may serve as assembly sites for filoviral nucleocapsids. Collectively, these data suggest that full-length GP, but not GPΔmucin, accumulates in the ER in close proximity to the nuclear membrane, which may underscore its cytotoxic property.

Steric shielding of surface epitopes and impaired immune recognition induced by the ebola virus glycoprotein

Many viruses alter expression of proteins on the surface of infected cells including molecules important for immune recognition, such as the major histocompatibility complex (MHC) class I and II molecules. Virus-induced downregulation of surface proteins has been observed to occur by a variety of mechanisms including impaired transcription, blocks to synthesis, and increased turnover. Viral infection or transient expression of the Ebola virus (EBOV) glycoprotein (GP) was previously shown to result in loss of staining of various host cell surface proteins including MHC1 and β1 integrin; however, the mechanism responsible for this effect has not been delineated. In the present study we demonstrate that EBOV GP does not decrease surface levels of β1 integrin or MHC1, but rather impedes recognition by steric occlusion of these proteins on the cell surface. Furthermore, steric occlusion also occurs for epitopes on the EBOV glycoprotein itself. The occluded epitopes in host proteins and EBOV GP can be revealed by removal of the surface subunit of GP or by removal of surface N- and O- linked glycans, resulting in increased surface staining by flow cytometry. Importantly, expression of EBOV GP impairs CD8 T-cell recognition of MHC1 on antigen presenting cells. Glycan-mediated steric shielding of host cell surface proteins by EBOV GP represents a novel mechanism for a virus to affect host cell function, thereby escaping immune detection.

The Ebola virus (EBOV) is a highly pathogenic virus that infects humans and non-human primates, causing severe disease or death in the majority of these cases. The interaction of this virus with its host on a cellular level is only just beginning to be understood. EBOV, like many viruses, affects the expression or function of several cell surface proteins, including adhesion factors and protein complexes responsible for allowing the immune system to recognize infected cells. Our group and others have previously shown that expression of the main viral glycoprotein of EBOV in cultured cells is sufficient to cause this disruption. Here we have identified the mechanism by which this disruption occurs. Heavily glycosylated domains of the EBOV glycoprotein form a steric shield over proteins at the cell surface. This steric interference blocks the detection of affected surface proteins using antibody reagents, but also has the functional effect of abrogating cell adhesion and preventing interactions with CD8 T cells. The results from this study highlight a novel mechanism for viral disruption of host cell surface protein functions and give insight to interactions between the Ebola virus and its host.

Antibody-mediated neutralization of Ebola virus can occur by two distinct mechanisms

Human Ebola virus causes severe hemorrhagic fever disease with high mortality and there is no vaccine or treatment. Antibodies in survivors occur early, are sustained, and can delay infection when transferred into nonhuman primates. Monoclonal antibodies (mAbs) from survivors exhibit potent neutralizing activity in vitro and are protective in rodents. To better understand targets and mechanisms of neutralization, we investigated a panel of mAbs shown previously to react with the envelope glycoprotein (GP). While one non-neutralizing mAb recognized a GP epitope in the nonessential mucin-like domain, the rest were specific for GP1, were neutralizing, and could be further distinguished by reactivity with secreted GP. We show that survivor antibodies, human KZ52 and monkey JP3K11, were specific for conformation-dependent epitopes comprising residues in GP1 and GP2 and that neutralization occurred by two distinct mechanisms; KZ52 inhibited cathepsin cleavage of GP whereas JP3K11 recognized the cleaved, fusion-active form of GP.

Ebolavirus glycoprotein structure and mechanism of entry

Ebolavirus (EBOV) is a highly virulent pathogen capable of causing a severe hemorrhagic fever with 50-90% lethality. The EBOV glycoprotein (GP) is the only virally expressed protein on the virion surface and is critical for attachment to host cells and catalysis of membrane fusion. Hence, the EBOV GP is a critical component of vaccines as well as a target of neutralizing antibodies and inhibitors of attachment and fusion. The crystal structure of the Zaire ebolavirus GP in its trimeric, prefusion conformation (3 GP(1) plus 3 GP(2)) in complex with a neutralizing antibody fragment, derived from a human survivor of the 1995 Kikwit outbreak, was recently determined. This is the first near-complete structure of any filovirus glycoprotein. The overall molecular architecture of the Zaire ebolavirus GP and its role in viral entry and membrane fusion are discussed in this article.

Ebolavirus glycoprotein GP masks both its own epitopes and the presence of cellular surface proteins

Ebolavirus (EBOV) is the etiological agent of a severe hemorrhagic fever with a high mortality rate. The spike glycoprotein (GP) is believed to be one of the major determinants of virus pathogenicity. In this study, we demonstrated the molecular mechanism responsible for the downregulation of surface markers caused by EBOV GP expression. We showed that expression of mature GP on the plasma membrane results in the masking of cellular surface proteins, including major histocompatibility complex class I. Overexpression of GP also results in the masking of certain antigenic epitopes on GP itself, causing an illusory effect of disappearance from the plasma membrane.

Tetherin-mediated restriction of filovirus budding is antagonized by the Ebola glycoprotein

Mammalian cells employ numerous innate cellular mechanisms to inhibit viral replication and spread. Tetherin, also known as Bst-2 or CD317, is a recently identified, IFN-induced, cellular response factor that blocks release of HIV-1 and other retroviruses from infected cells. The means by which tetherin retains retroviruses on the cell surface, as well as the mechanism used by the HIV-1 accessory protein Vpu to antagonize tetherin function and promote HIV-1 release, are unknown. Here, we document that tetherin functions as a broadly acting antiviral factor by demonstrating that both human and murine tetherin potently inhibit the release of the filovirus, Ebola, from the surface of cells. Expression of the Ebola glycoprotein (GP) antagonized the antiviral effect of human and murine tetherin and facilitated budding of Ebola particles, as did the HIV-1 Vpu protein. Conversely, Ebola GP could substitute for Vpu to promote HIV-1 virion release from tetherin-expressing cells, demonstrating a common cellular target for these divergent viral proteins. Ebola GP efficiently coimmunoprecipitated with tetherin, suggesting that the viral glycoprotein directly interferes with this host antiviral factor. These results demonstrate that tetherin is a cellular antiviral factor that restricts budding of structurally diverse enveloped viruses. Additionally, Ebola has evolved a highly effective strategy to combat this antiviral response elicited in the host during infection.