Expression of Ebolavirus glycoprotein on the target cells enhances viral entry

Minor electrostatic changes robustly increase VP40 membrane binding, assembly, and budding of Ebola virus matrix protein derived virus-like particles

Ebola virus (EBOV) is a filamentous negative-sense RNA virus, which causes severe hemorrhagic fever. There are limited vaccines or therapeutics for prevention and treatment of EBOV, so it is important to get a detailed understanding of the virus lifecycle to illuminate new drug targets. EBOV encodes for the matrix protein, VP40, which regulates assembly and budding of new virions from the inner leaflet of the host cell plasma membrane (PM). In this work, we determine the effects of VP40 mutations altering electrostatics on PM interactions and subsequent budding. VP40 mutations that modify surface electrostatics affect viral assembly and budding by altering VP40 membrane-binding capabilities. Mutations that increase VP40 net positive charge by one (e.g., Gly to Arg or Asp to Ala) increase VP40 affinity for phosphatidylserine and phosphatidylinositol 4,5-bisphosphate in the host cell PM. This increased affinity enhances PM association and budding efficiency leading to more effective formation of virus-like particles. In contrast, mutations that decrease net positive charge by one (e.g., Gly to Asp) lead to a decrease in assembly and budding because of decreased interactions with the anionic PM. Taken together, our results highlight the sensitivity of slight electrostatic changes on the VP40 surface for assembly and budding. Understanding the effects of single amino acid substitutions on viral budding and assembly will be useful for explaining changes in the infectivity and virulence of different EBOV strains, VP40 variants that occur in nature, and for long-term drug discovery endeavors aimed at EBOV assembly and budding.

Minor changes in electrostatics robustly increase VP40 membrane binding, assembly, and budding of Ebola virus matrix protein derived virus-like particles

Ebola virus (EBOV) is a filamentous negative-sense RNA virus which causes severe hemorrhagic fever. There are limited vaccines or therapeutics for prevention and treatment of EBOV, so it is important to get a detailed understanding of the virus lifecycle to illuminate new drug targets. EBOV encodes for the matrix protein, VP40, which regulates assembly and budding of new virions from the inner leaflet of the host cell plasma membrane (PM). In this work we determine the effects of VP40 mutations altering electrostatics on PM interactions and subsequent budding. VP40 mutations that modify surface electrostatics affect viral assembly and budding by altering VP40 membrane binding capabilities. Mutations that increase VP40 net positive charge by one (e.g., Gly to Arg or Asp to Ala) increase VP40 affinity for phosphatidylserine (PS) and PI(4,5)P2 in the host cell PM. This increased affinity enhances PM association and budding efficiency leading to more effective formation of virus-like particles (VLPs). In contrast, mutations that decrease net positive charge by one (e.g., Gly to Asp) lead to a decrease in assembly and budding because of decreased interactions with the anionic PM. Taken together our results highlight the sensitivity of slight electrostatic changes on the VP40 surface for assembly and budding. Understanding the effects of single amino acid substitutions on viral budding and assembly will be useful for explaining changes in the infectivity and virulence of different EBOV strains, VP40 variants that occur in nature, and for long-term drug discovery endeavors aimed at EBOV assembly and budding.

Pseudotyped Viruses for Marburgvirus and Ebolavirus

Marburg virus (MARV) and Ebola virus (EBOV) of the Filoviridae family are the most lethal viruses in terms of mortality rate. However, the development of antiviral treatment is hampered by the requirement for biosafety level-4 (BSL-4) containment. The establishment of BSL-2 pseudotyped viruses can provide important tools for the study of filoviruses. This chapter summarizes general information on the filoviruses and then focuses on the construction of replication-deficient pseudotyped MARV and EBOV (e.g., lentivirus system and vesicular stomatitis virus system). It also details the potential applications of the pseudotyped viruses, including neutralization antibody detection, the study of infection mechanisms, the evaluation of antibody-dependent enhancement, virus entry inhibitor screening, and glycoprotein mutation analysis.

Identification of filovirus entry inhibitors targeting the endosomal receptor NPC1 binding site

Filoviruses, mainly consisting of Ebola viruses (EBOV) and Marburg viruses (MARV), are enveloped negative-strand RNA viruses which can infect humans to cause severe hemorrhagic fevers and outbreaks with high mortality rates. The filovirus infection is mediated by the interaction of viral envelope glycoprotein (GP) and the human endosomal receptor Niemann-Pick C1 (NPC1). Blocking this interaction will prevent the infection. Therefore, we utilized an In silico screening approach to conduct virtual compound screening against the NPC1 receptor-binding site (RBS). Twenty-six top-hit compounds were purchased and evaluated by in vitro cell based inhibition assays against pseudotyped or replication-competent filoviruses. Two classes (A and U) of compounds were identified to have potent inhibitory activity against both Ebola and Marburg viruses. The IC₅₀ values are in the lower level of micromolar concentrations. One compound (compd-A) was found to have a sub-micromolar IC₅₀ value (0.86 μM) against pseudotyped Marburg virus. The cytotoxicity assay (MTT) indicates that compd-A has a moderate cytotoxicity level but the compd-U has much less toxicity and the CC₅₀ value was about 100 μM. Structure-activity relationship (SAR) study has found some analogs of compd-A and -U have reduced the toxicity and enhanced the inhibitory activity. In conclusion, this work has identified several qualified lead-compounds for further drug development against filovirus infection.

MHC class II transactivator CIITA induces cell resistance to Ebola virus and SARS-like coronaviruses

Recent outbreaks of Ebola virus (EBOV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have exposed our limited therapeutic options for such diseases and our poor understanding of the cellular mechanisms that block viral infections. Using a transposon-mediated gene-activation screen in human cells, we identify that the major histocompatibility complex (MHC) class II transactivator (CIITA) has antiviral activity against EBOV. CIITA induces resistance by activating expression of the p41 isoform of invariant chain CD74, which inhibits viral entry by blocking cathepsin-mediated processing of the Ebola glycoprotein. We further show that CD74 p41 can block the endosomal entry pathway of coronaviruses, including SARS-CoV-2. These data therefore implicate CIITA and CD74 in host defense against a range of viruses, and they identify an additional function of these proteins beyond their canonical roles in antigen presentation.

Discovery and Structural Optimization of 4-(Aminomethyl)benzamides as Potent Entry Inhibitors of Ebola and Marburg Virus Infections

The recent Ebola epidemics in West Africa underscore the great need for effective and practical therapies for future Ebola virus outbreaks. We have discovered a new series of remarkably potent small molecule inhibitors of Ebola virus entry. These 4-(aminomethyl)benzamide-based inhibitors are also effective against Marburg virus. Synthetic routes to these compounds allowed for the preparation of a wide variety of structures, including a conformationally restrained subset of indolines (compounds 41-50). Compounds 20, 23, 32, 33, and 35 are superior inhibitors of Ebola (Mayinga) and Marburg (Angola) infectious viruses. Representative compounds (20, 32, and 35) have shown good metabolic stability in plasma and liver microsomes (rat and human), and 32 did not inhibit CYP3A4 nor CYP2C9. These 4-(aminomethyl)benzamides are suitable for further optimization as inhibitors of filovirus entry, with the potential to be developed as therapeutic agents for the treatment and control of Ebola virus infections.

Optimization of 4-Aminopiperidines as Inhibitors of Influenza A Viral Entry That Are Synergistic with Oseltamivir

Vaccination is the most prevalent prophylactic means for controlling seasonal influenza infections. However, an effective vaccine usually takes at least 6 months to develop for the circulating strains. Therefore, new therapeutic options are needed for the acute treatment of influenza infections to control this virus and prevent epidemics/pandemics from developing. We have discovered fast-acting, orally bioavailable acylated 4-aminopiperidines with an effective mechanism of action targeting viral hemagglutinin (HA). Our data show that these compounds are potent entry inhibitors of influenza A viruses. We present docking studies that suggest an HA binding site for these inhibitors on H5N1. Compound 16 displayed a significant decrease of viral titer when evaluated in the infectious assays with influenza virus H1N1 (A/Puerto Rico/8/1934) or H5N1 (A/Vietnam/1203/2004) strains and the oseltamivir-resistant strain with the most common H274Y mutation. In addition, compound 16 showed significant synergistic activity with oseltamivir in vitro.

Reporter Assays for Ebola Virus Nucleoprotein Oligomerization, Virion-Like Particle Budding, and Minigenome Activity Reveal the Importance of Nucleoprotein Amino Acid Position 111

For highly pathogenic viruses, reporter assays that can be rapidly performed are critically needed to identify potentially functional mutations for further study under maximal containment (e.g., biosafety level 4 [BSL-4]). The Ebola virus nucleoprotein (NP) plays multiple essential roles during the viral life cycle, yet few tools exist to study the protein under BSL-2 or equivalent containment. Therefore, we adapted reporter assays to measure NP oligomerization and virion-like particle (VLP) production in live cells and further measured transcription and replication using established minigenome assays. As a proof-of-concept, we examined the NP-R111C substitution, which emerged during the 2013‒2016 Western African Ebola virus disease epidemic and rose to high frequency. NP-R111C slightly increased NP oligomerization and VLP budding but slightly decreased transcription and replication. By contrast, a synthetic charge-reversal mutant, NP-R111E, greatly increased oligomerization but abrogated transcription and replication. These results are intriguing in light of recent structures of NP oligomers, which reveal that the neighboring residue, K110, forms a salt bridge with E349 on adjacent NP molecules. By developing and utilizing multiple reporter assays, we find that the NP-111 position mediates a complex interplay between NP's roles in protein structure, virion budding, and transcription and replication.

Mining of Ebola virus entry inhibitors identifies approved drugs as two-pore channel pore blockers

Two-pore channels (TPCs) are Ca2+-permeable ion channels localised to the endo-lysosomal system where they regulate trafficking of various cargoes including viruses. As a result, TPCs are emerging as important drug targets. However, their pharmacology is ill-defined. There are no approved drugs to target them. And their mechanism of ligand activation is largely unknown. Here, we identify a number of FDA-approved drugs as TPC pore blockers. Using a model of the pore of human TPC2 based on recent structures of mammalian TPCs, we virtually screened a database of ~1500 approved drugs. Because TPCs have recently emerged as novel host factors for Ebola virus entry, we reasoned that Ebola virus entry inhibitors may exert their effects through inhibition of TPCs. Cross-referencing hits from the TPC virtual screen with two recent high throughput anti-Ebola screens yielded approved drugs targeting dopamine and estrogen receptors as common hits. These compounds inhibited endogenous NAADP-evoked Ca2+ release from sea urchin egg homogenates, NAADP-mediated channel activity of TPC2 re-routed to the plasma membrane, and PI(3,5)P2-mediated channel activity of TPC2 expressed in enlarged lysosomes. Mechanistically, single channel analyses showed that the drugs reduced mean open time consistent with a direct action on the pore. Functionally, drug potency in blocking TPC2 activity correlated with inhibition of Ebola virus-like particle entry. Our results expand TPC pharmacology through the identification of approved drugs as novel blockers, support a role for TPCs in Ebola virus entry, and provide insight into the mechanisms underlying channel regulation. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Immunogenicity of a Candidate Ebola Hemorrhagic Fever Vaccine in Mice Based on Controlled In Vitro Expression of Ebolavirus Glycoprotein

Ebolavirus (EBOV) is the etiology of Ebola hemorrhagic fever (EHF). A major EHF outbreak in 2014-2015 in West Africa claimed >11,000 lives. A licensed vaccine is not available for EHF, although several vaccines have undergone clinical trials. We developed a human adenovirus (Ad) serotype 5-based candidate EHF vaccine based on controlled expression of the EBOV (Makona strain) glycoprotein (GP) as the immunogen. Two clones, AdGP72 and AdGP75, and a control Ad515 vector, were generated and tested for protein expression in vitro and immunogenicity in mice. Eight groups of mice were immunized with three doses of buffer, Ad515, AdGP72, and AdGP75, by two different dose regimens. Three different antigens (AdGP75-infected Vero E6 cell extract and two baculovirus expressed EBOV GP antigens, namely, GP alone or GP with EBOV VP40) were used to evaluate the immune response. Expression studies indicated that full-length GP was cleaved into its component subunits when expressed in mammalian cells through the Ad vectors. Moreover, in coimmunoprecipitation studies, EBOV GP was found to be associated with VP40 when expressed in baculoviruses. The candidate vaccines were immunogenic in mice, as evaluated by enzyme-linked immunosorbent assay using mammalian- or baculovirus-derived antigens. Further characterization and development of the candidate vaccines are warranted.

Electron-Beam-Lithographed Nanostructures as Reference Materials for Label-Free Scattered-Light Biosensing of Single Filoviruses

Optical biosensors based on scattered-light measurements are being developed for rapid and label-free detection of single virions captured from body fluids. Highly controlled, stable, and non-biohazardous reference materials producing virus-like signals are valuable tools to calibrate, evaluate, and refine the performance of these new optical biosensing methods. To date, spherical polymer nanoparticles have been the only non-biological reference materials employed with scattered-light biosensing techniques. However, pathogens like filoviruses, including the Ebola virus, are far from spherical and their shape strongly affects scattered-light signals. Using electron beam lithography, we fabricated nanostructures resembling individual filamentous virions attached to a biosensing substrate (silicon wafer overlaid with silicon oxide film) and characterized their dimensions with scanning electron and atomic force microscopes. To assess the relevance of these nanostructures, we compared their signals across the visible spectrum to signals recorded from Ebola virus-like particles which exhibit characteristic filamentous morphology. We demonstrate the highly stable nature of our nanostructures and use them to obtain new insights into the relationship between virion dimensions and scattered-light signal.

Ebola virus requires phosphatidylinositol (3,5) bisphosphate production for efficient viral entry

For entry, Ebola virus (EBOV) requires the interaction of its viral glycoprotein with the cellular protein Niemann-Pick C1 (NPC1) which resides in late endosomes and lysosomes. How EBOV is trafficked and delivered to NPC1 and whether this is positively regulated during entry remain unclear. Here, we show that the PIKfyve-ArPIKfyve-Sac3 cellular complex, which is involved in the metabolism of phosphatidylinositol (3,5) bisphosphate (PtdIns(3,5)P₂), is critical for EBOV infection. Although the expression of all subunits of the complex was required for efficient entry, PIKfyve kinase activity was specifically critical for entry by all pathogenic filoviruses. Inhibition of PIKfyve prevented colocalization of EBOV with NPC1 and led to virus accumulation in intracellular vesicles with characteristics of early endosomes. Importantly, genetically-encoded phosphoinositide probes revealed an increase in PtdIns(3,5)P₂-positive vesicles in cells during EBOV entry. Taken together, our studies suggest that EBOV requires PtdIns(3,5)P₂ production in cells to promote efficient delivery to NPC1.

Chemically Modified Human Serum Albumin Potently Blocks Entry of Ebola Pseudoviruses and Viruslike Particles

Ebola virus (EBOV), the causative pathogen of the deadly Ebola virus disease (EVD), can be transmitted via contact with EVD patients, including sexual contact with EVD survivors. At present, no licensed vaccine or therapeutic is available. In this study, we compared eight anhydride-modified proteins for their entry-inhibitory activity against the pseudovirus (PsV) carrying the envelope glycoprotein (GP) of the EBOV Zaire or Sudan species (Zaire PsV and Sudan PsV, respectively). We found that 3-hydroxyphthalic anhydride-modified human serum albumin (HP-HSA) was the most effective in inhibiting the entry of both Zaire PsV and Sudan PsV, with the 50% effective concentration being at the nanomolar level and with HP-HSA being more potent than EBOV-neutralizing antibody MIL77-2 (4G7, a component antibody of the ZMapp drug cocktail). The combination of HP-HSA and MIL77-2 exhibited a synergistic effect. HP-HSA had no obvious in vitro or in vivo toxicity. The EBOV PsV entry-inhibitory activity of HP-HSA remained intact after storage at 45°C for 8 weeks, suggesting that HP-HSA has the potential for worldwide use, including tropical regions in African countries, as either a therapeutic to treat EBOV infection or a prophylactic microbicide to prevent the sexual transmission of EBOV.

An enzymatic assay based on luciferase Ebola virus-like particles for evaluation of virolytic activity of antimicrobial peptides

Antimicrobial peptides are currently considered as promising antiviral compounds. Current assays to evaluate the effectivity of peptides against enveloped viruses based on liposomes or hemolysis are encumbered by the artificial nature of liposomes or distinctive membrane composition of used erythrocytes. We propose a novel assay system based on enzymatic Ebola virus-like particles containing sensitive luciferase reporter. The assay was validated with several cationic and anionic peptides and compared with lentivirus inactivation and hemolytic assays. The assay is sensitive and easy to perform in standard biosafety level laboratory with potential for high-throughput screens. The use of virus-like particles in the assay provides a system as closely related to the native viruses as possible eliminating some issues associated with other more artificial set ups. We have identified CAM-W (KWKLWKKIEKWGQGIGAVLKWLTTWL) as a peptide with the greatest antiviral activity against infectious lentiviral vectors and filoviral virus-like particles.

Ebola Virus Glycoprotein with Increased Infectivity Dominated the 2013-2016 Epidemic

The magnitude of the 2013-2016 Ebola virus disease (EVD) epidemic enabled an unprecedented number of viral mutations to occur over successive human-to-human transmission events, increasing the probability that adaptation to the human host occurred during the outbreak. We investigated one nonsynonymous mutation, Ebola virus (EBOV) glycoprotein (GP) mutant A82V, for its effect on viral infectivity. This mutation, located at the NPC1-binding site on EBOV GP, occurred early in the 2013-2016 outbreak and rose to high frequency. We found that GP-A82V had heightened ability to infect primate cells, including human dendritic cells. The increased infectivity was restricted to cells that have primate-specific NPC1 sequences at the EBOV interface, suggesting that this mutation was indeed an adaptation to the human host. GP-A82V was associated with increased mortality, consistent with the hypothesis that the heightened intrinsic infectivity of GP-A82V contributed to disease severity during the EVD epidemic.

A cell-based high-throughput protocol to screen entry inhibitors of highly pathogenic viruses with Traditional Chinese Medicines

Emerging viruses such as Ebola virus (EBOV), Lassa virus (LASV), and avian influenza virus H5N1 (AIV) are global health concerns. Since there is very limited options (either vaccine or specific therapy) approved for humans against these viruses, there is an urgent need to develop prophylactic and therapeutic treatments. Previously we reported a high‐throughput screening (HTS) protocol to identify entry inhibitors for three highly pathogenic viruses (EBOV, LASV, and AIV) using a human immunodeficiency virus–based pseudotyping platform which allows us to perform the screening in a BSL‐2 facility. In this report, we have adopted this screening protocol to evaluate traditional Chinese Medicines (TCMs) in an effort to discover entry inhibitors against these viruses. Here we show that extracts of the following Chinese medicinal herbs exhibit potent anti‐Ebola viral activities: Gardenia jasminoides Ellis, Citrus aurantium L., Viola yedoensis Makino, Prunella vulgaris L., Coix lacryma‐jobi L. var. mayuen (Roman.) Stapf, Pinellia ternata (Thunb.) Breit., and Morus alba L. This study represents a proof‐of‐principle investigation supporting the suitability of this assay for rapid screening TCMs and identifying putative entry inhibitors for these viruses. J. Med. Virol. 89:908–916, 2017. © 2016 Wiley Periodicals, Inc.

An Ebola Virus-Like Particle-Based Reporter System Enables Evaluation of Antiviral Drugs In Vivo under Non-Biosafety Level 4 Conditions

Ebola virus (EBOV) is a highly contagious lethal pathogen. As a biosafety level 4 (BSL-4) agent, however, EBOV is restricted to costly BSL-4 laboratories for experimentation, thus significantly impeding the evaluation of EBOV vaccines and drugs. Here, we report an EBOV-like particle (EBOVLP)-based luciferase reporter system that enables the evaluation of anti-EBOV agents in vitro and in vivo outside BSL-4 facilities. Cotransfection of HEK293T cells with four plasmids encoding the proteins VP40, NP, and GP of EBOV and firefly luciferase (Fluc) resulted in the production of Fluc-containing filamentous particles that morphologically resemble authentic EBOV. The reporter EBOVLP was capable of delivering Fluc into various cultured cells in a GP-dependent manner and was recognized by a conformation-dependent anti-EBOV monoclonal antibody (MAb). Significantly, inoculation of mice with the reporter EBOVLP led to the delivery of Fluc protein into target cells and rapid generation of intense bioluminescence signals that could be blocked by the administration of EBOV neutralizing MAbs. This BSL-4-free reporter system should facilitate high-throughput screening for anti-EBOV drugs targeting viral entry and efficacy testing of candidate vaccines.

IMPORTANCE

Ebola virus (EBOV) researches have been limited to costly biosafety level 4 (BSL-4) facilities due to the lack of animal models independent of BSL-4 laboratories. In this study, we reveal that a firefly luciferase-bearing EBOV-like particle (EBOVLP) with typical filamentous EBOV morphology is capable of delivering the reporter protein into murine target cells both in vitro and in vivo Moreover, we demonstrate that the reporter delivery can be inhibited both in vitro and in vivo by a known anti-EBOV protective monoclonal antibody, 13C6. Our work provides a BSL-4-free system that can facilitate the in vivo evaluation of anti-EBOV antibodies, drugs, and vaccines. The system may also be useful for mechanistic study of the viral entry process.

A parallel genome-wide RNAi screening strategy to identify host proteins important for entry of Marburg virus and H5N1 influenza virus

BACKGROUND

Genome-wide RNAi screening has been widely used to identify host proteins involved in replication and infection of different viruses, and numerous host factors are implicated in the replication cycles of these viruses, demonstrating the power of this approach. However, discrepancies on target identification of the same viruses by different groups suggest that high throughput RNAi screening strategies need to be carefully designed, developed and optimized prior to the large scale screening.

METHODS

Two genome-wide RNAi screens were performed in parallel against the entry of pseudotyped Marburg viruses and avian influenza virus H5N1 utilizing an HIV-1 based surrogate system, to identify host factors which are important for virus entry. A comparative analysis approach was employed in data analysis, which alleviated systematic positional effects and reduced the false positive number of virus-specific hits.

RESULTS

The parallel nature of the strategy allows us to easily identify the host factors for a specific virus with a greatly reduced number of false positives in the initial screen, which is one of the major problems with high throughput screening. The power of this strategy is illustrated by a genome-wide RNAi screen for identifying the host factors important for Marburg virus and/or avian influenza virus H5N1 as described in this study.

CONCLUSIONS

This strategy is particularly useful for highly pathogenic viruses since pseudotyping allows us to perform high throughput screens in the biosafety level 2 (BSL-2) containment instead of the BSL-3 or BSL-4 for the infectious viruses, with alleviated safety concerns. The screening strategy together with the unique comparative analysis approach makes the data more suitable for hit selection and enables us to identify virus-specific hits with a much lower false positive rate.

Role of EXT1 and Glycosaminoglycans in the Early Stage of Filovirus Entry

Filoviruses, including both Ebola virus (EBOV) and Marburg virus (MARV), can infect humans and other animals, causing hemorrhagic fever with a high mortality rate. Entry of these viruses into the host is mediated by a single filoviral glycoprotein (GP). GP is composed of two subunits: GP1, which is responsible for attachment and binding to receptor(s) on susceptible cells, and GP2, which mediates viral and cell membrane fusion. Although numerous host factors have been implicated in the entry process, the initial attachment receptor(s) has not been well defined. In this report, we demonstrate that exostosin 1 (EXT1), which is involved in biosynthesis of heparan sulfate (HS), plays a role in filovirus entry. Expression knockdown of EXT1 by small interfering RNAs (siRNAs) impairs GP-mediated pseudoviral entry and that of infectious EBOV and MARV in tissue cultured cells. Furthermore, HS, heparin, and other related glycosaminoglycans (GAGs), to different extents, can bind to and block GP-mediated viral entry and that of infectious filoviruses. These results strongly suggest that HS and other related GAGs are attachment receptors that are utilized by filoviruses for entry and infection. These GAGs may have therapeutic potential in treating EBOV- and MARV-infected patients.

IMPORTANCE

Infection by Ebola virus and Marburg virus can cause severe illness in humans, with a high mortality rate, and currently there is no FDA-approved vaccine or therapeutic treatment available. The ongoing 2014 outbreak in West Africa underscores a lack of our understanding in the infection and pathogenesis of these viruses and the urgency of drug discovery and development. In this study, we provide several pieces of evidence that demonstrate that heparan sulfate and other closely related glycosaminoglycans are the molecules that are used by filoviruses for initial attachment. Furthermore, we demonstrate that these glycosaminoglycans can block entry of and infection by filoviruses. Thus, this work provides mechanistic insights on the early step of filoviral infection and suggests a possible therapeutic option for diseases caused by filovirus infection.

A comparative high-throughput screening protocol to identify entry inhibitors of enveloped viruses

Emerging and reemerging human viral pathogens pose great public health concerns since therapeutics against these viruses are limited. Thus, there is an urgent need to develop novel drugs that can block infection of either a specific virus or a number of viruses. Viral entry is thought to be an ideal target for potential therapeutic prevention. One of the challenges of developing antivirals is that most of these viruses are highly pathogenic and therefore require high biosafety-level containment. In this study, we have adopted a comparative high-throughput screening protocol to identify entry inhibitors for three enveloped viruses (Marburg virus, influenza virus H5N1, and Lassa virus) using a human immunodeficiency virus-based pseudotyping platform. We demonstrate the utility of this approach by screening a small compound library and identifying putative entry inhibitors for these viruses. One major advantage of this protocol is to reduce the number of false positives in hit selection, and we believe that the protocol is useful for inhibitor screening for many enveloped viruses.

Multiple cationic amphiphiles induce a Niemann-Pick C phenotype and inhibit Ebola virus entry and infection

Ebola virus (EBOV) is an enveloped RNA virus that causes hemorrhagic fever in humans and non-human primates. Infection requires internalization from the cell surface and trafficking to a late endocytic compartment, where viral fusion occurs, providing a conduit for the viral genome to enter the cytoplasm and initiate replication. In a concurrent study, we identified clomiphene as a potent inhibitor of EBOV entry. Here, we screened eleven inhibitors that target the same biosynthetic pathway as clomiphene. From this screen we identified six compounds, including U18666A, that block EBOV infection (IC₅₀ 1.6 to 8.0 µM) at a late stage of entry. Intriguingly, all six are cationic amphiphiles that share additional chemical features. U18666A induces phenotypes, including cholesterol accumulation in endosomes, associated with defects in Niemann–Pick C1 protein (NPC1), a late endosomal and lysosomal protein required for EBOV entry. We tested and found that all six EBOV entry inhibitors from our screen induced cholesterol accumulation. We further showed that higher concentrations of cationic amphiphiles are required to inhibit EBOV entry into cells that overexpress NPC1 than parental cells, supporting the contention that they inhibit EBOV entry in an NPC1-dependent manner. A previously reported inhibitor, compound 3.47, inhibits EBOV entry by blocking binding of the EBOV glycoprotein to NPC1. None of the cationic amphiphiles tested had this effect. Hence, multiple cationic amphiphiles (including several FDA approved agents) inhibit EBOV entry in an NPC1-dependent fashion, but by a mechanism distinct from that of compound 3.47. Our findings suggest that there are minimally two ways of perturbing NPC1-dependent pathways that can block EBOV entry, increasing the attractiveness of NPC1 as an anti-filoviral therapeutic target.

Ebola virus exploits a monocyte differentiation program to promote its entry

Antigen-presenting cells (APCs) are critical targets of Ebola virus (EBOV) infection in vivo. However, the susceptibility of monocytes to infection is controversial. Studies indicate productive monocyte infection, and yet monocytes are also reported to be resistant to EBOV GP-mediated entry. In contrast, monocyte-derived macrophages and dendritic cells are permissive for both EBOV entry and replication. Here, freshly isolated monocytes are demonstrated to indeed be refractory to EBOV entry. However, EBOV binds monocytes, and delayed entry occurs during monocyte differentiation. Cultured monocytes spontaneously downregulate the expression of viral entry restriction factors such as interferon-inducible transmembrane proteins, while upregulating the expression of critical EBOV entry factors cathepsin B and NPC1. Moreover, these processes are accelerated by EBOV infection. Finally, ectopic expression of NPC1 is sufficient to rescue entry into an undifferentiated, normally nonpermissive monocytic cell line. These results define the molecular basis for infection of APCs and suggest means to limit APC infection.

Filoviruses utilize glycosaminoglycans for their attachment to target cells

Filoviruses are the cause of severe hemorrhagic fever in human and nonhuman primates. The envelope glycoprotein (GP), responsible for both receptor binding and fusion of the virus envelope with the host cell membrane, has been demonstrated to interact with multiple molecules in order to enhance entry into host cells. Here we have demonstrated that filoviruses utilize glycosaminoglycans, and more specifically heparan sulfate proteoglycans, for their attachment to host cells. This interaction is mediated by GP and does not require the presence of the mucin domain. Both the degree of sulfation and the structure of the carbohydrate backbone play a role in the interaction with filovirus GPs. This new step of filovirus interaction with host cells can potentially be a new target for antiviral strategies. As such, we were able to inhibit filovirus GP-mediated infection using carrageenan, a broad-spectrum microbicide that mimics heparin, and also using the antiviral dendrimeric peptide SB105-A10, which interacts with heparan sulfate, antagonizing the binding of the virus to cells.

Exposure to ozone modulates human airway protease/antiprotease balance contributing to increased influenza A infection

Exposure to oxidant air pollution is associated with increased respiratory morbidities and susceptibility to infections. Ozone is a commonly encountered oxidant air pollutant, yet its effects on influenza infections in humans are not known. The greater Mexico City area was the primary site for the spring 2009 influenza A H1N1 pandemic, which also coincided with high levels of environmental ozone. Proteolytic cleavage of the viral membrane protein hemagglutinin (HA) is essential for influenza virus infectivity. Recent studies suggest that HA cleavage might be cell-associated and facilitated by the type II transmembrane serine proteases (TTSPs) human airway trypsin-like protease (HAT) and transmembrane protease, serine 2 (TMPRSS2), whose activities are regulated by antiproteases, such as secretory leukocyte protease inhibitor (SLPI). Based on these observations, we sought to determine how acute exposure to ozone may modulate cellular protease/antiprotease expression and function, and to define their roles in a viral infection. We utilized our in vitro model of differentiated human nasal epithelial cells (NECs) to determine the effects of ozone on influenza cleavage, entry, and replication. We show that ozone exposure disrupts the protease/antiprotease balance within the airway liquid. We also determined that functional forms of HAT, TMPRSS2, and SLPI are secreted from human airway epithelium, and acute exposure to ozone inversely alters their expression levels. We also show that addition of antioxidants significantly reduces virus replication through the induction of SLPI. In addition, we determined that ozone-induced cleavage of the viral HA protein is not cell-associated and that secreted endogenous proteases are sufficient to activate HA leading to a significant increase in viral replication. Our data indicate that pre-exposure to ozone disrupts the protease/antiprotease balance found in the human airway, leading to increased influenza susceptibility.

An enzymatic assay for detection of viral entry

This unit describes a viral entry assay where a beta-lactamase reporter protein fused to the matrix protein of either influenza (BlaM1) or ebola virus (BlaVP40) is packaged as a structural component into virus-like particles (VLPs). The Bla reporter is released upon fusion with target cells and can be detected in live cells by flow cytometry, microscopy, or a fluorometric plate reader for utility in high-throughput screening approaches. The transfer of Bla to a target cell by BlaM1 or BlaVP40 VLPs requires the presence of influenza hemagglutinin (HA) and neuraminidase (NA) or EboV glycoprotein (GP), respectively. This straightforward assay has broad application for studying the entry steps of enveloped viruses, especially those that require high levels of biosafety containment.

Expression of an immunogenic Ebola immune complex in Nicotiana benthamiana

Filoviruses (Ebola and Marburg viruses) cause severe and often fatal haemorrhagic fever in humans and non-human primates. The US Centers for Disease Control identifies Ebola and Marburg viruses as 'category A' pathogens (defined as posing a risk to national security as bioterrorism agents), which has lead to a search for vaccines that could prevent the disease. Because the use of such vaccines would be in the service of public health, the cost of production is an important component of their development. The use of plant biotechnology is one possible way to cost-effectively produce subunit vaccines. In this work, a geminiviral replicon system was used to produce an Ebola immune complex (EIC) in Nicotiana benthamiana. Ebola glycoprotein (GP1) was fused at the C-terminus of the heavy chain of humanized 6D8 IgG monoclonal antibody, which specifically binds to a linear epitope on GP1. Co-expression of the GP1-heavy chain fusion and the 6D8 light chain using a geminiviral vector in leaves of N. benthamiana produced assembled immunoglobulin, which was purified by ammonium sulphate precipitation and protein G affinity chromatography. Immune complex formation was confirmed by assays to show that the recombinant protein bound the complement factor C1q. Size measurements of purified recombinant protein by dynamic light scattering and size-exclusion chromatography also indicated complex formation. Subcutaneous immunization of BALB/C mice with purified EIC resulted in anti-Ebola virus antibody production at levels comparable to those obtained with a GP1 virus-like particle. These results show excellent potential for a plant-expressed EIC as a human vaccine.

Nrf2 expression modifies influenza A entry and replication in nasal epithelial cells

Influenza infection is a major cause of morbidity and mortality worldwide, especially during pandemics outbreaks. Emerging data indicate that phase II antioxidant enzyme pathways could play a role in virus-associated inflammation and immune clearance. While Nrf2-dependent gene expression is known to modify inflammation, a mechanistic role in viral susceptibility and clearance has yet to be elucidated. Therefore, we utilized differentiated human nasal epithelial cells (NEC) and an enzymatic virus-like particle entry assay, to examine the role Nrf2-dependent gene expression has on viral entry and replication. Herein, lentiviral vectors that express Nrf2-specific short hairpin (sh)-RNA effectively decreased both Nrf2 mRNA and Nrf2 protein expression in transduced human NEC from healthy volunteers. Nrf2 knockdown correlated with a significant increase in influenza virus entry and replication. Conversely, supplementation with the potent Nrf2 activators sulforaphane (SFN) and epigallocatechin gallate (EGCG) significantly decreased viral entry and replication. The suppressive effects of EGCG on viral replication were abolished in cells with knocked-down Nrf2 expression, suggesting a causal relationship between the EGCG-induced activation of Nrf2 and the ability to protect against viral infection. Interestingly, the induction of Nrf2 via nutritional supplements SFN and EGCG increased antiviral mediators/responses: RIG-I, IFN-β, and MxA at baseline in the absence of infection. Our data indicate that there is an inverse relationship between the levels of Nrf2 expression and the viral entry/replication. We also demonstrate that supplementation with Nrf2-activating antioxidants inhibits viral replication in human NEC, which may prove to be an attractive therapeutic intervention. Taken together, these data indicate potential mechanisms by which Nrf2-dependent gene expression regulates susceptibility to influenza in human epithelial cells.

Ebolavirus delta-peptide immunoadhesins inhibit marburgvirus and ebolavirus cell entry

With the exception of Reston and Lloviu viruses, filoviruses (marburgviruses, ebolaviruses, and "cuevaviruses") cause severe viral hemorrhagic fevers in humans. Filoviruses use a class I fusion protein, GP(1,2), to bind to an unknown, but shared, cell surface receptor to initiate virus-cell fusion. In addition to GP(1,2), ebolaviruses and cuevaviruses, but not marburgviruses, express two secreted glycoproteins, soluble GP (sGP) and small soluble GP (ssGP). All three glycoproteins have identical N termini that include the receptor-binding region (RBR) but differ in their C termini. We evaluated the effect of the secreted ebolavirus glycoproteins on marburgvirus and ebolavirus cell entry, using Fc-tagged recombinant proteins. Neither sGP-Fc nor ssGP-Fc bound to filovirus-permissive cells or inhibited GP(1,2)-mediated cell entry of pseudotyped retroviruses. Surprisingly, several Fc-tagged Δ-peptides, which are small C-terminal cleavage products of sGP secreted by ebolavirus-infected cells, inhibited entry of retroviruses pseudotyped with Marburg virus GP(1,2), as well as Marburg virus and Ebola virus infection in a dose-dependent manner and at low molarity despite absence of sequence similarity to filovirus RBRs. Fc-tagged Δ-peptides from three ebolaviruses (Ebola virus, Sudan virus, and Taï Forest virus) inhibited GP(1,2)-mediated entry and infection of viruses comparably to or better than the Fc-tagged RBRs, whereas the Δ-peptide-Fc of an ebolavirus nonpathogenic for humans (Reston virus) and that of an ebolavirus with lower lethality for humans (Bundibugyo virus) had little effect. These data indicate that Δ-peptides are functional components of ebolavirus proteomes. They join cathepsins and integrins as novel modulators of filovirus cell entry, might play important roles in pathogenesis, and could be exploited for the synthesis of powerful new antivirals.

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.

Characterization of the receptor-binding domain of Ebola glycoprotein in viral entry

Ebola virus infection causes severe hemorrhagic fever in human and non-human primates with high mortality. Viral entry/infection is initiated by binding of glycoprotein GP protein on Ebola virion to host cells, followed by fusion of virus-cell membrane also mediated by GP. Using an human immunodeficiency virus (HIV)-based pseudotyping system, the roles of 41 Ebola GP1 residues in the receptor-binding domain in viral entry were studied by alanine scanning substitutions. We identified that four residues appear to be involved in protein folding/structure and four residues are important for viral entry. An improved entry interference assay was developed and used to study the role of these residues that are important for viral entry. It was found that R64 and K95 are involved in receptor binding. In contrast, some residues such as I170 are important for viral entry, but do not play a major role in receptor binding as indicated by entry interference assay and/or protein binding data, suggesting that these residues are involved in post-binding steps of viral entry. Furthermore, our results also suggested that Ebola and Marburg viruses share a common cellular molecule for entry.

Cell adhesion-dependent membrane trafficking of a binding partner for the ebolavirus glycoprotein is a determinant of viral entry

Ebolavirus is a hemorrhagic fever virus associated with high mortality. Although much has been learned about the viral lifecycle and pathogenesis, many questions remain about virus entry. We recently showed that binding of the receptor binding region (RBR) of the ebolavirus glycoprotein (GP) and infection by GP pseudovirions increase on cell adhesion independently of mRNA or protein synthesis. One model to explain these observations is that, on cell adhesion, an RBR binding partner translocates from an intracellular vesicle to the cell surface. Here, we provide evidence for this model by showing that suspension 293F cells contain an RBR binding site within a membrane-bound compartment associated with the trans-Golgi network and microtubule-organizing center. Consistently, trafficking of the RBR binding partner to the cell surface depends on microtubules, and the RBR binding partner is internalized when adherent cells are placed in suspension. Based on these observations, we reexamined the claim that lymphocytes, which are critical for ebolavirus pathogenesis, are refractory to infection because they lack an RBR binding partner. We found that both cultured and primary human lymphocytes (in suspension) contain an intracellular pool of an RBR binding partner. Moreover, we identified two adherent primate lymphocytic cell lines that bind RBR at their surface and strikingly, support GP-mediated entry and infection. In summary, our results reveal a mode of determining viral entry by a membrane-trafficking event that translocates an RBR binding partner to the cell surface, and they suggest that this process may be operative in cells important for ebolavirus pathogenesis (e.g., lymphocytes and macrophages).

An enzymatic virus-like particle assay for sensitive detection of virus entry

A viral entry assay where a beta-lactamase reporter protein fused to the influenza matrix protein-1 (BlaM1) is packaged as a structural component into influenza virus-like particles (VLPs) is described. The Bla reporter is released upon fusion with target cells and can be detected in live cells by flow cytometry, microscopy, or fluorometric plate reader for utility in high-throughput screening approaches. The production of BlaM1 VLPs and subsequent transfer of Bla activity to target cells requires the presence of influenza hemagglutinin (HA) and neuraminidase (NA). In addition, transfer of Bla by the VLPs can be blocked by an influenza neutralizing antibody, is impeded by a chemical inhibitor of influenza virus entry, and requires HA that is cleaved by a protease specific for its cleavage site. An analogous VLP system also was developed for Ebola (EBOV) and Marburg (MARV) viruses, demonstrating that this straightforward assay has broad application for studying the entry steps of enveloped viruses, especially those that require high levels of biosafety containment.