Carbohydrate structure of Marburg virus glycoprotein

Glycan-Lectin Interactions in Cancer and Viral Infections and How to Disrupt Them

Glycan-lectin interactions play an essential role in different cellular processes. One of their main functions is involvement in the immune response to pathogens or inflammation. However, cancer cells and viruses have adapted to avail themselves of these interactions. By displaying specific glycosylation structures, they are able to bind to lectins, thus promoting pathogenesis. While glycan-lectin interactions promote tumor progression, metastasis, and/or chemoresistance in cancer, in viral infections they are important for viral entry, release, and/or immune escape. For several years now, a growing number of investigations have been devoted to clarifying the role of glycan-lectin interactions in cancer and viral infections. Various overviews have already summarized and highlighted their findings. In this review, we consider the interactions of the lectins MGL, DC-SIGN, selectins, and galectins in both cancer and viral infections together. A possible transfer of ways to target and disrupt them might lead to new therapeutic approaches in different pathological backgrounds.

Marburg virus regulates the IRE1/XBP1-dependent unfolded protein response to ensure efficient viral replication

Viruses regulate cellular signalling pathways to ensure optimal viral replication. During Marburg virus (MARV) infection, large quantities of the viral glycoprotein GP are produced in the ER; this may result in the activation of the unfolded protein response (UPR). The most conserved pathway to trigger UPR is initiated by IRE1. Activation of IRE1 results in auto-phosphorylation, splicing of the XBP1 mRNA and translation of the XBP1s protein. XBP1s binds cis-acting UPR elements (UPRE) which leads to the enhanced expression of genes which should restore ER homeostasis. XBP1u protein is translated, if IRE1 is not activated. Here we show that ectopic expression of MARV GP activated the IRE1-XBP1 axis of UPR as monitored by UPRE luciferase assays. However, while at 24 h of infection with MARV IRE1 was phosphorylated, expression of XBP1s was only slightly enhanced and UPRE activity was not detected. The IRE1-XBP1 axis was not active at 48 h p.i. Co-expression studies of MARV proteins demonstrated that the MARV protein VP30 suppressed UPRE activation. Co-immunoprecipitation analyses revealed an RNA-dependent interaction of VP30 with XBP1u. Knock-out of IRE1 supported MARV infection at late time points. Taken together, these results suggest that efficient MARV propagation requires specific regulation of IRE1 activity.

A Recombinant Rabies Virus Expressing the Marburg Virus Glycoprotein Is Dependent upon Antibody-Mediated Cellular Cytotoxicity for Protection against Marburg Virus Disease in a Murine Model

Marburg virus (MARV) is a filovirus related to Ebola virus (EBOV) associated with human hemorrhagic disease. Outbreaks are sporadic and severe, with a reported case mortality rate of upward of 88%. There is currently no antiviral or vaccine available. Given the sporadic nature of outbreaks, vaccines provide the best approach for long-term control of MARV in regions of endemicity. We have developed an inactivated rabies virus-vectored MARV vaccine (FILORAB3) to protect against Marburg virus disease. Immunogenicity studies in our labs have shown that a Th1-biased seroconversion to both rabies virus and MARV glycoproteins (GPs) is beneficial for protection in a preclinical murine model. As such, we adjuvanted FILORAB3 with glucopyranosyl lipid adjuvant (GLA), a Toll-like receptor 4 agonist, in a squalene-in-water emulsion. Across two different BALB/c mouse challenge models, we achieved 92% protection against murine-adapted Marburg virus (ma-MARV). Although our vaccine elicited strong MARV GP antibodies, it did not strongly induce neutralizing antibodies. Through both in vitro and in vivo approaches, we elucidated a critical role for NK cell-dependent antibody-mediated cellular cytotoxicity (ADCC) in vaccine-induced protection. Overall, these findings demonstrate that FILORAB3 is a promising vaccine candidate for Marburg virus disease.IMPORTANCE Marburg virus (MARV) is a virus similar to Ebola virus and also causes a hemorrhagic disease which is highly lethal. In contrast to EBOV, only a few vaccines have been developed against MARV, and researchers do not understand what kind of immune responses are required to protect from MARV. Here we show that antibodies directed against MARV after application of our vaccine protect in an animal system but fail to neutralize the virus in a widely used virus neutralization assay against MARV. This newly discovered activity needs to be considered more when analyzing MARV vaccines or infections.

Addicted to sugar: roles of glycans in the order Mononegavirales

Glycosylation is a biologically important protein modification process by which a carbohydrate chain is enzymatically added to a protein at a specific amino acid residue. This process plays roles in many cellular functions, including intracellular trafficking, cell-cell signaling, protein folding and receptor binding. While glycosylation is a common host cell process, it is utilized by many pathogens as well. Protein glycosylation is widely employed by viruses for both host invasion and evasion of host immune responses. Thus better understanding of viral glycosylation functions has potential applications for improved antiviral therapeutic and vaccine development. Here, we summarize our current knowledge on the broad biological functions of glycans for the Mononegavirales, an order of enveloped negative-sense single-stranded RNA viruses of high medical importance that includes Ebola, rabies, measles and Nipah viruses. We discuss glycobiological findings by genera in alphabetical order within each of eight Mononegavirales families, namely, the bornaviruses, filoviruses, mymonaviruses, nyamiviruses, paramyxoviruses, pneumoviruses, rhabdoviruses and sunviruses.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Comparison of N- and O-linked glycosylation patterns of ebolavirus glycoproteins

Ebolaviruses are emerging pathogens that cause severe and often fatal viral hemorrhagic fevers. Four distinct ebolaviruses are known to cause Ebola virus disease in humans. The ebolavirus envelope glycoprotein (GP_1,2) is heavily glycosylated, but the precise glycosylation patterns of ebolaviruses are largely unknown. Here we demonstrate that approximately 50 different N-glycan structures are present in GP_1,2 derived from the four pathogenic ebolaviruses, including high mannose, hybrid, and bi-, tri-, and tetra-antennary complex glycans with and without fucose and sialic acid. The overall N-glycan composition is similar between the different ebolavirus GP_1,2s. In contrast, the amount and type of O-glycan structures varies widely between ebolavirus GP_1,2s. Notably, this O-glycan dissimilarity is also present between two variants of Ebola virus, the original Yambuku variant and the Makona variant responsible for the most recent Western African epidemic. The data presented here should serve as the foundation for future ebolaviral entry and immunogenicity studies.

Current landscape of protein glycosylation analysis and recent progress toward a novel paradigm of glycoscience research

This review covers the basics and some applications of methodologies for the analysis of glycoprotein glycans. Analytical techniques used for glycoprotein glycans, including liquid chromatography (LC), capillary electrophoresis (CE), mass spectrometry (MS), and high-throughput analytical methods based on microfluidics, were described to supply the essentials about biopharmaceutical and biomarker glycoproteins. We will also describe the MS analysis of glycoproteins and glycopeptides as well as the chemical and enzymatic releasing methods of glycans from glycoproteins and the chemical reactions used for the derivatization of glycans. We hope the techniques have accommodated most of the requests from glycoproteomics researchers.

Ebola virus outbreak, updates on current therapeutic strategies

Filoviruses are enveloped negative‐sense single‐stranded RNA viruses, which include Ebola and Marburg viruses, known to cause hemorrhagic fever in humans with a case fatality of up to 90%. There have been several Ebola virus outbreaks since the first outbreak in the Democratic Republic of Congo in 1976 of which, the recent 2013–2015 epidemic in Guinea, Liberia, and Sierra Leone is the largest in recorded history. Within a few months of the start of the outbreak in December 2013, thousands of infected cases were reported with a significant number of deaths. As of March 2015, according to the Centers for Disease Control and Prevention, there have been nearly 25 000 suspected cases, with 15 000 confirmed by laboratory testing, and over 10 000 deaths. The large number of cases and the high mortality rate, combined with the lack of effective Food and Drug Administration‐approved treatments, necessitate the development of potent and safe therapeutic measures to combat the current and future outbreaks. Since the beginning of the outbreak, there have been considerable efforts to develop and characterize protective measures including vaccines and antiviral small molecules, and some have proven effective in vitro and in animal models. Most recently, a cocktail of monoclonal antibodies has been shown to be highly effective in protecting non‐human primates from Ebola virus infection. In this review, we will discuss what is known about the nature of the virus, phylogenetic classification, genomic organization and replication, disease transmission, and viral entry and highlight the current approaches and efforts, in the development of therapeutics, to control the outbreak. Copyright © 2015 John Wiley & Sons, Ltd.

A strategy for O-glycoproteomics of enveloped viruses--the O-glycoproteome of herpes simplex virus type 1

Glycosylation of viral envelope proteins is important for infectivity and interaction with host immunity, however, our current knowledge of the functions of glycosylation is largely limited to N-glycosylation because it is difficult to predict and identify site-specific O-glycosylation. Here, we present a novel proteome-wide discovery strategy for O-glycosylation sites on viral envelope proteins using herpes simplex virus type 1 (HSV-1) as a model. We identified 74 O-linked glycosylation sites on 8 out of the 12 HSV-1 envelope proteins. Two of the identified glycosites found in glycoprotein B were previously implicated in virus attachment to immune cells. We show that HSV-1 infection distorts the secretory pathway and that infected cells accumulate glycoproteins with truncated O-glycans, nonetheless retaining the ability to elongate most of the surface glycans. With the use of precise gene editing, we further demonstrate that elongated O-glycans are essential for HSV-1 in human HaCaT keratinocytes, where HSV-1 produced markedly lower viral titers in HaCaT with abrogated O-glycans compared to the isogenic counterpart with normal O-glycans. The roles of O-linked glycosylation for viral entry, formation, secretion, and immune recognition are poorly understood, and the O-glycoproteomics strategy presented here now opens for unbiased discovery on all enveloped viruses.

Information on site-specific O-glycosylation of viral envelope glycoproteins is generally very limited despite important functions. We present a powerful mass-spectrometry based strategy to globally identify O-glycosylation sites on viral envelope proteins of a given virus in the context of a productive infection. We successfully utilized the strategy to map O-linked glycosylation sites on the complex HSV-1 virus demonstrating that O-glycosylation is widely distributed on most envelope proteins. Moreover, we used genetically engineered keratinocytes lacking O-glycan elongation capacity to demonstrate that O-linked glycans are indeed important for HSV-1 biology as HSV-1 particles produced in these cells had significantly lower titers compared to wild-type keratinocytes. These tools enable wider discovery and detailed analysis of the role of site-specific O-glycosylation in virology.

Characterization of the envelope glycoprotein of a novel filovirus, lloviu virus

Lloviu virus (LLOV), a novel filovirus detected in bats, is phylogenetically distinct from viruses in the genera Ebolavirus and Marburgvirus in the family Filoviridae. While filoviruses are known to cause severe hemorrhagic fever in humans and/or nonhuman primates, LLOV is biologically uncharacterized, since infectious LLOV has never been isolated. To examine the properties of LLOV, we characterized its envelope glycoprotein (GP), which likely plays a key role in viral tropism and pathogenicity. We first found that LLOV GP principally has the same primary structure as the other filovirus GPs. Similar to the other filoviruses, virus-like particles (VLPs) produced by transient expression of LLOV GP, matrix protein, and nucleoprotein in 293T cells had densely arrayed GP spikes on a filamentous particle. Mouse antiserum to LLOV VLP was barely cross-reactive to viruses of the other genera, indicating that LLOV is serologically distinct from the other known filoviruses. For functional study of LLOV GP, we utilized a vesicular stomatitis virus (VSV) pseudotype system and found that LLOV GP requires low endosomal pH and cathepsin L, and that human C-type lectins act as attachment factors for LLOV entry into cells. Interestingly, LLOV GP-pseudotyped VSV infected particular bat cell lines more efficiently than viruses bearing other filovirus GPs. These results suggest that LLOV GP mediates cellular entry in a manner similar to that of the other filoviruses while showing preferential tropism for some bat cells.

Filovirus entry

A number of advances in recent years have significantly furthered our understanding of filovirus attachment and cellular tropism. For example, several cell-surface molecules have been identified as attachment factors with the potential to facilitate the in vivo targeting of particular cell types such as macrophages and hepatic cells. Furthermore, our knowledge of internalization and subsequent events during filovirus entry has also been widened, adding new variations to the paradigms for viral entry established for HIV and influenza. In particular, host cell factors such as endosomal proteases and the intracellular receptor Niemann-Pick C1 are now known to play a vital role in activating the membrane fusion potential of filovirus glycoproteins.

Assembly of the Marburg virus envelope

The key player to assemble the filamentous Marburg virus particles is the matrix protein VP40 which orchestrates recruitment of nucleocapsid complexes and the viral glycoprotein GP to the budding sites at the plasma membrane. Here, VP40 induces the formation of the viral particles, determines their morphology and excludes cellular proteins from the virions. Budding takes place at filopodia in non-polarized cells and at the basolateral cell pole in polarized epithelial cells. Molecular basis of how VP40 exerts its multifunctional role in these different processes is currently under investigation. Here we summarize recent data on structure-function relationships of VP40 and GP in connection with their function in assembly. Questions concerning the complex particle assembly, budding and release remaining enigmatic are addressed. Cytoplasmic domains of viral surface proteins often serve as a connection to the viral matrix protein or as binding sites for further viral or cellular proteins. A cooperation of MARV GP and VP40 building up the viral envelope can be proposed and is discussed in more detail in this review, as the cytoplasmic domain of GP represents an obvious interaction candidate because of its localization adjacent to the VP40 layer. Interestingly, truncation of the short cytoplasmic domain of GP neither inhibited interaction with VP40 nor incorporation of GP into progeny viral particles. Based on reverse genetics we generated recombinant virions expressing a GP mutant without the cytoplasmic tail. Investigations revealed attenuation in virus growth and an obvious defect in entry. Further investigations showed that the truncation of the cytoplasmic domain of GP impaired the structural integrity of the ectodomain, whichconsequently had impact on entry steps downstream of virus binding. Our data indicated that changes in the cytoplasmic domain are relayed over the lipid membrane to alter the function of the ectodomain.

Forty-five years of Marburg virus research

In 1967, the first reported filovirus hemorrhagic fever outbreak took place in Germany and the former Yugoslavia. The causative agent that was identified during this outbreak, Marburg virus, is one of the most deadly human pathogens. This article provides a comprehensive overview of our current knowledge about Marburg virus disease ranging from ecology to pathogenesis and molecular biology.

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.

C-type lectins do not act as functional receptors for filovirus entry into cells

Cellular C-type lectins have been reported to facilitate filovirus infection by binding to glycans on filovirus glycoprotein (GP). However, it is not clearly known whether interaction between C-type lectins and GP mediates all the steps of virus entry (i.e., attachment, internalization, and membrane fusion). In this study, we generated vesicular stomatitis viruses pseudotyped with mutant GPs that have impaired structures of the putative receptor binding regions and thus reduced ability to infect the monkey kidney cells that are routinely used for virus propagation. We found that infectivities of viruses with the mutant GPs dropped in C-type lectin-expressing cells, parallel with those in the monkey kidney cells, whereas binding activities of these GPs to the C-type lectins were not correlated with the reduced infectivities. These results suggest that C-type lectin-mediated entry of filoviruses requires other cellular molecule(s) that may be involved in virion internalization or membrane fusion.

Proposal for a revised taxonomy of the family Filoviridae: classification, names of taxa and viruses, and virus abbreviations

The taxonomy of the family Filoviridae (marburgviruses and ebolaviruses) has changed several times since the discovery of its members, resulting in a plethora of species and virus names and abbreviations. The current taxonomy has only been partially accepted by most laboratory virologists. Confusion likely arose for several reasons: species names that consist of several words or which (should) contain diacritical marks, the current orthographic identity of species and virus names, and the similar pronunciation of several virus abbreviations in the absence of guidance for the correct use of vernacular names. To rectify this problem, we suggest (1) to retain the current species names Reston ebolavirus, Sudan ebolavirus, and Zaire ebolavirus, but to replace the name Cote d'Ivoire ebolavirus [sic] with Taï Forest ebolavirus and Lake Victoria marburgvirus with Marburg marburgvirus; (2) to revert the virus names of the type marburgviruses and ebolaviruses to those used for decades in the field (Marburg virus instead of Lake Victoria marburgvirus and Ebola virus instead of Zaire ebolavirus); (3) to introduce names for the remaining viruses reminiscent of jargon used by laboratory virologists but nevertheless different from species names (Reston virus, Sudan virus, Taï Forest virus), and (4) to introduce distinct abbreviations for the individual viruses (RESTV for Reston virus, SUDV for Sudan virus, and TAFV for Taï Forest virus), while retaining that for Marburg virus (MARV) and reintroducing that used over decades for Ebola virus (EBOV). Paying tribute to developments in the field, we propose (a) to create a new ebolavirus species (Bundibugyo ebolavirus) for one member virus (Bundibugyo virus, BDBV); (b) to assign a second virus to the species Marburg marburgvirus (Ravn virus, RAVV) for better reflection of now available high-resolution phylogeny; and (c) to create a new tentative genus (Cuevavirus) with one tentative species (Lloviu cuevavirus) for the recently discovered Lloviu virus (LLOV). Furthermore, we explain the etymological derivation of individual names, their pronunciation, and their correct use, and we elaborate on demarcation criteria for each taxon and virus.

Identification of N-glycans from Ebola virus glycoproteins by matrix-assisted laser desorption/ionisation time-of-flight and negative ion electrospray tandem mass spectrometry

The larger fragment of the transmembrane glycoprotein (GP1) and the soluble glycoprotein (sGP) of Ebola virus were expressed in human embryonic kidney cells and the secreted products were purified from the supernatant for carbohydrate analysis. The N-glycans were released with PNGase F from within sodium dodecyl sulphate/polyacrylamide gel electrophoresis (SDS-PAGE) gels. Identification of the glycans was made with normal-phase high-performance liquid chromatography (HPLC), matrix-assisted laser desorption/ionisation mass spectrometry, negative ion electrospray ionisation fragmentation mass spectrometry and exoglycosidase digestion. Most glycans were complex bi-, tri- and tetra-antennary compounds with reduced amounts of galactose. No bisected compounds were detected. Triantennary glycans were branched on the 6-antenna; fucose was attached to the core GlcNAc residue. Sialylated glycans were present on sGP but were largely absent from GP1, the larger fragment of the transmembrane glycoprotein. Consistent with this was the generally higher level of processing of carbohydrates found on sGP as evidenced by a higher percentage of galactose and lower levels of high-mannose glycans than were found on GP1. These results confirm and expand previous findings on partial characterisation of the Ebola virus transmembrane glycoprotein. They represent the first detailed data on carbohydrate structures of the Ebola virus sGP.

Different potential of C-type lectin-mediated entry between Marburg virus strains

The glycoproteins (GPs) of filoviruses are responsible for virus entry into cells. It is known that GP interacts with cellular C-type lectins for virus attachment to cells. Since primary target cells of filoviruses express C-type lectins, C-type lectin-mediated entry is thought to be a possible determinant of virus tropism and pathogenesis. We compared the efficiency of C-type lectin-mediated entry between Marburg virus strains Angola and Musoke by using a vesicular stomatitis virus (VSV) pseudotype system. VSV pseudotyped with Angola GP (VSV-Angola) infected K562 cells expressing the C-type lectin, human macrophage galactose-type C-type lectin (hMGL), or dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) more efficiently than VSV pseudotyped with Musoke GP (VSV-Musoke). Unexpectedly, the binding affinity of the C-type lectins to the carbohydrates on GPs did not correlate with the different efficiency of C-type lectin-mediated entry. Site-directed mutagenesis identified the amino acid at position 547, which switched the efficiency of C-type lectin-mediated entry. In a three-dimensional model of GP, this amino acid was in close proximity to the putative site of cathepsin processing. Interestingly, the cathepsin inhibitors reduced the infectivity of VSV-Angola less efficiently than that of VSV-Musoke in C-type lectin-expressing K562 cells, whereas only a limited difference was found in control cells. The amino acid at position 547 was critical for the different effects of the inhibitors on the virus infectivities. These results suggest that the efficiency of C-type lectin-mediated entry of filoviruses is controlled not only by binding affinity between C-type lectins and GP but also by mechanisms underlying endosomal entry, such as proteolytic processing by the cathepsins.

Role of the transmembrane domain of marburg virus surface protein GP in assembly of the viral envelope

The major protein constituents of the filoviral envelope are the matrix protein VP40 and the surface transmembrane protein GP. While VP40 is recruited to the sites of budding via the late retrograde endosomal transport route, GP is suggested to be transported via the classical secretory pathway involving the endoplasmic reticulum, Golgi apparatus, and trans-Golgi network until it reaches the plasma membrane where most filoviral budding takes place. Since both transport routes target the plasma membrane, it was thought that GP and VP40 join there to form the viral envelope. However, it was recently shown that, upon coexpression of both proteins, GP is partially recruited into peripheral VP40-enriched multivesicular bodies, which contained markers of the late endosome. Accumulation of GP and VP40 in this compartment was presumed to play an important role in the formation of the filoviral envelope. Using a domain-swapping approach, we were able to show that the transmembrane domain of GP was essential and sufficient for (i) partial recruitment of chimeric glycoproteins into VP40-enriched multivesicular bodies and (ii) incorporation into virus-like particles (VLPs) that were released upon expression of VP40. Only those chimeric glycoproteins which were targeted to VP40-enriched endosomal multivesicular bodies were subsequently recruited into VLPs. These data show that the transmembrane domain of GP is critical for the mixing of VP40 and GP in multivesicular bodies and incorporation of GP into the viral envelope. Results further suggest that trapping of GP in the VP40-enriched late endosomal compartment is important for the formation of the viral envelope.

Mannose-binding lectin binds to Ebola and Marburg envelope glycoproteins, resulting in blocking of virus interaction with DC-SIGN and complement-mediated virus neutralization

Mannose-binding lectin (MBL), a serum lectin that mediates innate immune functions including activation of the lectin complement pathway, binds to carbohydrates expressed on some viral glycoproteins. In this study, the ability of MBL to bind to virus particles pseudotyped with Ebola and Marburg envelope glycoproteins was evaluated. Virus particles bearing either Ebola (Zaire strain) or Marburg (Musoke strain) envelope glycoproteins bound at significantly higher levels to immobilized MBL compared with virus particles pseudotyped with vesicular stomatitis virus glycoprotein or with no virus glycoprotein. As observed in previous studies, Ebola-pseudotyped virus bound to cells expressing the lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin). However, pre-incubation of virus with MBL blocked DC-SIGN-mediated binding to cells, suggesting that the two lectins bind at the same or overlapping sites on the Ebola glycoprotein. Neutralization experiments showed that virus pseudotyped with Ebola or Marburg (Musoke) glycoprotein was neutralized by complement, while the Marburg (Ravn strain) glycoprotein-pseudotyped virus was less sensitive to neutralization. Neutralization was partially mediated through the lectin complement pathway, since a complement source deficient in MBL was significantly less effective at neutralizing viruses pseudotyped with filovirus glycoproteins and addition of purified MBL to the MBL-deficient complement increased neutralization. These experiments demonstrated that MBL binds to filovirus envelope glycoproteins resulting in important biological effects and suggest that MBL can interact with filoviruses during infection in humans.

Multivesicular bodies as a platform for formation of the Marburg virus envelope

The Marburg virus (MARV) envelope consists of a lipid membrane and two major proteins, the matrix protein VP40 and the glycoprotein GP. Both proteins use different intracellular transport pathways: GP utilizes the exocytotic pathway, while VP40 is transported through the retrograde late endosomal pathway. It is currently unknown where the proteins combine to form the viral envelope. In the present study, we identified the intracellular site where the two major envelope proteins of MARV come together as peripheral multivesicular bodies (MVBs). Upon coexpression with VP40, GP is redistributed from the trans-Golgi network into the VP40-containing MVBs. Ultrastructural analysis of MVBs suggested that they provide the platform for the formation of membrane structures that bud as virus-like particles from the cell surface. The virus-like particles contain both VP40 and GP. Single expression of GP also resulted in the release of particles, which are round or pleomorphic. Single expression of VP40 led to the release of filamentous structures that closely resemble viral particles and contain traces of endosomal marker proteins. This finding indicated a central role of VP40 in the formation of the filamentous structure of MARV particles, which is similar to the role of the related Ebola virusVP40. In MARV-infected cells, VP40 and GP are colocalized in peripheral MVBs as well. Moreover, intracellular budding of progeny virions into MVBs was frequently detected. Taken together, these results demonstrate an intracellular intersection between GP and VP40 pathways and suggest a crucial role of the late endosomal compartment for the formation of the viral envelope.

Human macrophage C-type lectin specific for galactose and N-acetylgalactosamine promotes filovirus entry

Filoviruses cause lethal hemorrhagic disease in humans and nonhuman primates. An initial target of filovirus infection is the mononuclear phagocytic cell. Calcium-dependent (C-type) lectins such as dendritic cell- or liver/lymph node-specific ICAM-3 grabbing nonintegrin (DC-SIGN or L-SIGN, respectively), as well as the hepatic asialoglycoprotein receptor, bind to Ebola or Marburg virus glycoprotein (GP) and enhance the infectivity of these viruses in vitro. Here, we demonstrate that a recently identified human macrophage galactose- and N-acetylgalactosamine-specific C-type lectin (hMGL), whose ligand specificity differs from DC-SIGN and L-SIGN, also enhances the infectivity of filoviruses. This enhancement was substantially weaker for the Reston and Marburg viruses than for the highly pathogenic Zaire virus. We also show that the heavily glycosylated, mucin-like domain on the filovirus GP is required for efficient interaction with this lectin. Furthermore, hMGL, like DC-SIGN and L-SIGN, is present on cells known to be major targets of filoviruses (i.e., macrophages and dendritic cells), suggesting a role for these C-type lectins in viral replication in vivo. We propose that filoviruses use different C-type lectins to gain cellular entry, depending on the cell type, and promote efficient viral replication.

Identification and partial characterization of Taastrup virus: a newly identified member species of the Mononegavirales

We present a 8904-nt sequence of the central part of the RNA genome of a novel virus with a filovirus-like, nonidentical morphology named Taastrup virus (TV) detected in the leafhopper Psammotettix alienus. Sequence analysis identified five potential open reading frames (ORFs) and a complex pattern of homologies to various members of the Mononegavirales suggests a genome organization with the following order of genes: 3'-N-P-M-G-L-5'. Sequence analyses reveal an unusually large glycoprotein (G) containing both potential O-linked (14) and N-linked (9) glycosylation sites-a feature shared with the glycoproteins of Filoviridae and Pneumovirinae, and a nucleoprotein (N) with homology to the nucleoprotein of viral hemorrhagic septicemia virus (VHSV), a member of the Rhabdoviridae. Highly conserved domains were identified in the RNA-dependent RNA polymerase (L) between TV and other viruses within the order of Mononegavirales, and homology was found in particular with members of the Rhabdoviridae. The sequence similarities and the unique filovirus-like but nonidentical morphology unambiguously refer this newly identified virus to the order of Mononegavirales but to no family more than any, to other within the order.

DC-SIGN and DC-SIGNR bind ebola glycoproteins and enhance infection of macrophages and endothelial cells

Ebola virus exhibits a broad cellular tropism in vitro. In humans and animal models, virus is found in most tissues and organs during the latter stages of infection. In contrast, a more restricted cell and tissue tropism is exhibited early in infection where macrophages, liver, lymph node, and spleen are major initial targets. This indicates that cellular factors other than the broadly expressed virus receptor(s) modulate Ebola virus tropism. Here we demonstrate that the C-type lectins DC-SIGN and DC-SIGNR avidly bind Ebola glycoproteins and greatly enhance transduction of primary cells by Ebola virus pseudotypes and infection by replication-competent Ebola virus. DC-SIGN and DC-SIGNR are expressed in several early targets for Ebola virus infection, including dendritic cells, alveolar macrophages, and sinusoidal endothelial cells in the liver and lymph node. While DC-SIGN and DC-SIGNR do not directly mediate Ebola virus entry, their pattern of expression in vivo and their ability to efficiently capture virus and to enhance infection indicate that these attachment factors can play an important role in Ebola transmission, tissue tropism, and pathogenesis.

The role of DC-SIGN and DC-SIGNR in HIV and Ebola virus infection: can potential therapeutics block virus transmission and dissemination?

Sexual transmission of HIV requires that the virus crosses mucosal barriers and disseminates into lymphoid tissue, the major site of viral replication. To achieve this, HIV might engage DC-SIGN, a calcium dependent lectin that is expressed on mucosal dendritic cells (DCs), which binds avidly to HIV. DC-SIGN and other attachment factors are likely to account for the well-known ability of DCs to enhance infection of T cells by HIV. Attachment of HIV to DC-SIGN might thus enhance viral spread in mucosal tissues and, by taking advantage of the inherent capacity of DCs to migrate into lymphoid tissue, might promote viral dissemination within the host. DC-SIGN and a related molecule, termed DC-SIGNR, also enhance infection by Ebola virus. The expression of these lectins on early targets of Ebola virus infection, like liver endothelial cells and alveolar macrophages, suggests an important role for DC-SIGN and DC-SIGNR in the establishment of Ebola infection. This article reviews the interaction of DC-SIGN and DC-SIGNR with HIV and Ebola, discusses the mechanism of DC-SIGN-mediated viral transmission and examines how this process could be inhibited by potential therapeutics.

The Marburg virus surface protein GP is phosphorylated at its ectodomain

Marburg virus, a filovirus, contains only one transmembrane protein (GP) which is responsible for receptor recognition on target cells. GP, a type I membrane protein of approximately 220 kDa, is acylated and highly glycosylated carrying N- and O-linked sugar side chains. GP is transported through the exocytotic pathway toward the plasma membrane where budding of virions takes place. In the trans-Golgi network, GP is proteolytically activated by the prohormone convertase furin into two subunits GP(1) and GP(2). In the present paper, we provide evidence that GP undergoes an additional posttranslational modification; it is phosphorylated at its ectodomain. Phosphorylation takes place at serine residues between amino acid 260 and 273. The respective serines are located in conserved recognition sites for luminal protein kinases (protein kinase CK II and Golgi casein kinase). Consistent with this data, it was found that GP was phosphorylated in the Golgi apparatus of the expressing HeLa cells before cleavage of the molecule. GP is the first example of a viral glycoprotein with a phosphorylated ectodomain.

Characterization of N-glycans from mouse brain neural cell adhesion molecule

The N-glycosylation pattern of the neural cell adhesion molecule (NCAM), isolated from brains of newborn mice, has been analyzed. Following digestion with trypsin, generated glycopeptides were fractionated by serial immunoaffinity chromatography using immobilized monoclonal antibodies specifically recognizing polysialic acid (PSA) units or the HNK1-carbohydrate epitope. Subsequent analyses of the resulting (glyco)peptides by Edman degradation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed polysialylated glycans to be exclusively linked to glycosylation sites 5 (Asn(431)) and 6 (Asn(460)), whereas glycans carrying the HNK1-epitope could be assigned to sites 2 (Asn(297)), 5, 6, and, to a lesser extent, site 3 (Asn(329)). PSA-, HNK1-, and non-PSA/HNK1-glycan fractions were characterized by carbohydrate constituent and methylation analyses as well as MALDI-TOF-MS in conjunction with chromatographic fractionation techniques. The results revealed that the core structures of PSA-glycans represented predominantly fucosylated, partially sulfated 2,6-branched isomers of triantennary as well as tetraantennary complex-type glycans, whereas carbohydrate chains bearing the HNK1-epitope were dominated by diantennary species carrying in part bisecting GlcNAc residues. Non-PSA/HNK1-glycans exhibited a highly heterogeneous pattern of partially truncated, mostly diantennary structures being characterized by the presence of additional fucose, bisecting GlcNAc and/or sulfate residues. In conclusion, our results revealed that the glycosylation pattern of murine NCAM displays high structural and regional selectivity, which might play an important role in controlling the biological activities of this molecule.

Sorting of Marburg virus surface protein and virus release take place at opposite surfaces of infected polarized epithelial cells

Marburg virus, a filovirus, causes severe hemorrhagic fever with hitherto poorly understood molecular pathogenesis. We have investigated here the vectorial transport of the surface protein GP of Marburg virus in polarized epithelial cells. To this end, we established an MDCKII cell line that was able to express GP permanently (MDCK-GP). The functional integrity of GP expressed in these cells was analyzed using vesicular stomatitis virus pseudotypes. Further experiments revealed that GP is transported in MDCK-GP cells mainly to the apical membrane and is released exclusively into the culture medium facing the apical membrane. When MDCKII cells were infected with Marburg virus, the majority of GP was also transported to the apical membrane, suggesting that the protein contains an autonomous apical transport signal. Release of infectious progeny virions, however, took place exclusively at the basolateral membrane of the cells. Thus, vectorial budding of Marburg virus is presumably determined by factors other than the surface protein.

Adverse effects of MVA-T7 on the transport of Marburg virus glycoprotein

Expression of glycoproteins has been carried out successfully using recombinant vaccinia virus vectors. Especially attractive is the use of recombinant vaccinia viruses which express the DNA-dependent RNA polymerase of the phage T7 (T7-polymerase). The T7-polynerase drives the transcription of plasmid-based genes under the control of the T7 RNA polymerase promoter transfected into the infected cell. Comparison of two different recombinant vaccinia viruses, vTF7-3 and MVA-T7, revealed that post-translational processing of Marburg virus surface glycoprotein (GP) is impaired in the MVA-T7 but not in the vTF7-3 system. Influenza virus hemagglutinin, however, was transported and processed like the authentic protein in both systems. It is shown that transport of GP in the MVA-T7 system is not completely blocked, but the vast majority of molecules remained Endo H-sensitive. Only trace amounts evaded the endoplasmatic reticulum and reached the plasma membrane. Thus, the adverse effects of MVA-T7 on the processing of recombinant glycoproteins cannot be predicted, and correct processing has to be investigated for every expressed glycoprotein.

Proteolytic processing of Marburg virus glycoprotein

Processing of the transmembrane glycoprotein (GP) of Marburg virus involved the conversion of an endo H-sensitive, ER-specific form into an endo H-resistant, Golgi-specific precursor that was cleaved into GP(1) and GP(2). Cleavage was mediated by furin or another subtilisin-like endoprotease with similar substrate specificity as indicated by mutational analysis of the cleavage site and inhibition using peptidyl chloromethylketones. Mature GP consisted of disulfide-linked GP(1) and GP(2) subunits.

The glycoproteins of Marburg and Ebola virus and their potential roles in pathogenesis

Filoviruses cause systemic infections that can lead to severe hemorrhagic fever in human and non-human primates. The primary target of the virus appears to be the mononuclear phagocytic system. As the virus spreads through the organism, the spectrum of target cells increases to include endothelial cells, fibroblasts, hepatocytes, and many other cells. There is evidence that the filovirus glycoprotein plays an important role in cell tropism, spread of infection, and pathogenicity. Biosynthesis of the glycoprotein forming the spikes on the virion surface involves cleavage by the host cell protease furin into two disulfide linked subunits GP1 and GP2. GP1 is also shed in soluble form from infected cells. Different strains of Ebola virus show variations in the cleavability of the glycoprotein, that may account for differences in pathogenicity, as has been observed with influenza viruses and paramyxoviruses. Expression of the spike glycoprotein of Ebola virus, but not of Marburg virus, requires transcriptional editing. Unedited GP mRNA yields the nonstructural glycoprotein sGP, which is secreted extensively from infected cells. Whether the soluble glycoproteins GP1 and sGP interfere with the humoral immune response and other defense mechanisms remains to be determined.

Characterization of human vascular endothelial cadherin glycans

The glycosylation pattern of human vascular endothelial cadherin (VE-cadherin), purified from cultured human umbilical cord vein endothelial cells, was analyzed. VE-cadherin was metabolically radiolabeled with d-[6-(3)H]glucosamine, isolated by immunoprecipitation, purified by SDS-PAGE and in-gel digested with endoproteinase Asp N. Oligosaccharides were sequentially released from resulting glycopeptides and analyzed by chromatographic profiling. The results revealed that VE-cadherin carries predominantly sialylated diantennary and hybrid-type glycans in addition to some triantennary and high mannose-type species. Highly branched, tetraantennary oligosaccharides were found in trace amounts only. Immunohistochemical labeling of VE-cadherin and sialic acids displayed a codistribution along the intercellular junctions in endothelial cells of human umbilical arteries, veins, and cultured endothelial monolayers. Ca(2+)-depletion, performed on cultured endothelial cells, resulted in a reversible complete disappearance of VE-cadherin and of almost all sialic acid staining from the junctions. Sialidase treatment of whole cells caused a change of VE-cadherin immunofluorescence from a continuous and netlike superstructural organization to a scattered inconsistent one. Hence, cell surface sialic acids might play a role in VE-cadherin organization.

Analysis of the pre-S2 N- and O-linked glycans of the M surface protein from human hepatitis B virus

The surface antigen of hepatitis B virus comprises a nested set of small (S), middle (M), and large (L) proteins, all of which are partially glycosylated in their S domains. The pre-S2 domain, present only in M and L proteins, is further N-glycosylated at Asn-4 exclusively in the M protein. Since the pre-S2 N-glycan appears to play a crucial role in the secretion of viral particles, the M protein may be considered as a potential target for antiviral therapy. For characterization of the pre-S2 glycosylation, pre-S2 (glyco)peptides were released from native, patient-derived hepatitis B virus subviral particles by tryptic digestion, separated from remaining particles, purified by reversed-phase high performance liquid chromatography, and identified by amino acid and N-terminal sequence analysis as well as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Pre-S2 N-glycans were characterized by anion exchange chromatography, methylation analysis, and on target sequential exoglycosidase digestions in combination with MALDI-TOF-MS, demonstrating the presence of partially sialylated diantennary complex-type oligosaccharides. In addition, the pre-S2 domain of M protein, but not that of L protein, was found to be partially O-glycosylated by a Gal(beta1-3)GalNAcalpha-, Neu5Ac(alpha2-3)Gal(beta1-3)GalNAcalpha-, or GalNAcalpha-residue. The respective O-glycosylation site was assigned to Thr-37 by digestion with carboxypeptidases in combination with MALDI-TOF-MS and by quadrupole time-of-flight electrospray mass spectrometry. Analytical data further revealed that about 90% of M protein is N-terminally acetylated.

Host-cell-specific glycosylation of HIV-2 envelope glycoprotein

Neutral complex-type N-glycans of the envelope glycoprotein 120 of HIV-2, propagated in different host cells, display cell-type specific variations. In order to identify typical structural elements, glycans were analysed by gel filtration, by enzymic sequencing and, in part, by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The characteristic substituents of di- tri- and tetraantennary carbohydrate units thus observed include N-acetyllactosamine repeats, bisecting N-acetylglucosamine and fucose linked to the chitobiose core as well as to N-acetyllactosamine antennae. Each glycoprotein preparation displayed a characteristic set of glycoforms.

N-acetyl-beta-glucosaminidase accounts for differences in glycosylation of influenza virus hemagglutinin expressed in insect cells from a baculovirus vector

The hemagglutinin of fowl plague virus has been expressed in Spodoptera frugiperda (Sf9) cells and in Estigmene acrea cells by using a baculovirus vector. Structural analysis revealed that the endo-H-resistant N-glycans of HA from Sf9 cells were predominantly trimannosyl core oligosaccharides, whereas in E. acrea cells most of these cores were elongated by at least one terminal N-acetylglucosamine residue. To understand the difference in carbohydrate structures, enzymes involved in N-glycan processing have been analyzed. The results revealed that the different glycosylation patterns observed are due to an N-acetyl-beta-glucosaminidase activity that was found in Sf9 cells but not in E. acrea cells. This enzyme specifically used the GlcNAcMan(3)GlcNAc(2) oligosaccharide as a substrate. When N-acetyl-beta-glucosaminidase or alpha-mannosidase II was inhibited by specific inhibitors, the amount of terminal N-acetylglucosamine in hemagglutinin from Sf9 cells was significantly enhanced. These results demonstrate that N glycosylation in both cell lines follows the classical pathway up to the stage of GlcNAcMan(3)GlcNAc(2) oligosaccharide side chains. Whereas these structures are the end product in E. acrea cells, they are degraded in Sf9 cells to Man(3)GlcNAc(2) cores by N-acetyl-beta-glucosaminidase.

The virion glycoproteins of Ebola viruses are encoded in two reading frames and are expressed through transcriptional editing

In late 1994 and early 1995, Ebola (EBO) virus dramatically reemerged in Africa, causing human disease in the Ivory Coast and Zaire. Analysis of the entire glycoprotein genes of these viruses and those of other EBO virus subtypes has shown that the virion glycoprotein (130 kDa) is encoded in two reading frames, which are linked by transcriptional editing. This editing results in the addition of an extra nontemplated adenosine within a run of seven adenosines near the middle of the coding region. The primary gene product is a smaller (50-70 kDa), nonstructural, secreted glycoprotein, which is produced in large amounts and has an unknown function. Phylogenetic analysis indicates that EBO virus subtypes are genetically diverse and that the recent Ivory Coast isolate represents a new (fourth) subtype of EBO virus. In contrast, the EBO virus isolate from the 1995 outbreak in Kikwit, Zaire, is virtually identical to the virus that caused a similar epidemic in Yambuku, Zaire, almost 20 years earlier. This genetic stability may indicate that EBO viruses have coevolved with their natural reservoirs and do not change appreciably in the wild.

Filovirus-induced endothelial leakage triggered by infected monocytes/macrophages

The pathogenetic mechanisms underlying hemorrhagic fevers are not fully understood, but hemorrhage, activation of coagulation, and shock suggest vascular instability. Here, we demonstrate that Marburg virus (MBG), a filovirus causing a severe form of hemorrhagic fever in humans, replicates in human monocytes/macrophages, resulting in cytolytic infection and release of infectious virus particles. Replication also led to intracellular budding and accumulation of viral particles in vacuoles, thus providing a mechanism by which the virus may escape immune surveillance. Monocytes/macrophages were activated by MBG infection as indicated by tumor necrosis factor alpha (TNF-alpha) release. Supernatants of monocyte/macrophage cultures infected with MBG increased the permeability of cultured human endothelial cell monolayers. The increase in endothelial permeability correlated with the time course of TNF-alpha release and was inhibited by a TNF-alpha specific monoclonal antibody. Furthermore, recombinant TNF-alpha added at concentrations present in supernatants of virus-infected macrophage cultures increased endothelial permeability in the presence of 10 micron H2O2. These results indicate that TNF-alpha plays a critical role in mediating increased permeability, which was identified as a paraendothelial route shown by formation of interendothelial gaps. The combination of viral replication in endothelial cells (H.-J. Schnittler, F. Mahner, D. Drenckhahn, H.-D. Klenk, and H. Feldmann, J. Clin. Invest. 19:1301-1309, 1993) and monocytes/macrophages and the permeability-increasing effect of virus-induced cytokine release provide the first experimental data for a novel concept in the pathogenesis of viral hemorrhagic fever.

Emerging and reemerging of filoviruses

Filoviruses are causative agents of a hemorrhagic fever in man with mortalities ranging from 22 to 88%. They are enveloped, nonsegmented negative-stranded RNA viruses and are separated into two types, Marburg and Ebola, which can be serologically, biochemically and genetically distinguished. In general, there is little genetic variability among viruses belonging to the Marburg type. The Ebola type, however, is subdivided into at least three distinct subtypes. Marburg virus was first isolated during an outbreak in Europe in 1967. Ebola virus emerged in 1976 as the causative agent of two simultaneous outbreaks in southern Sudan and northern Zaire. The reemergence of Ebola, subtype Zaire, in Kikwit 1995 caused a worldwide sensation, since it struck after a sensibilization on the danger of Ebola virus disease. Person-to-person transmission by intimate contact is the main route of infection, but transmission by droplets and small aerosols among infected individuals is discussed. The natural reservoir for filoviruses remains a mystery. Filoviruses are prime examples for emerging pathogens. Factors that may be involved in emergence are international commerce and travel, limited experience in diagnosis and case management, import of nonhuman primates, and the potential of filoviruses for rapid evolution.

Differentiation of filoviruses by electron microscopy

Cultured monolayers of MA-104, Vero 76, SW-13, and DBS-FRhL-2 cells were infected with Marburg (MBG), Ebola-Sudan (EBO-S), Ebola-Zaire (EBO-Z), and Ebola-Reston (EBO-R) viruses (Filoviridae, Filovirus) and examined by electron microscopy to provide ultrastructural details of morphology and morphogenesis of these potential human pathogens. Replication of each filovirus was seen in all cell systems employed. Filoviral particles appeared to enter host cells by endocytosis. Filoviruses showed a similar progression of morphogenic events, from the appearance of nascent intracytoplasmic viral inclusions to formation of mature virions budded through plasma membranes, regardless of serotype or host cell. However, ultrastructural differences were demonstrated between MBG and other filoviruses. MBG virions recovered from culture fluids were uniformly shorter in mean unit length than EBO-S, EBO-Z, or EBO-R particles. Examination of filovirus-infected cells revealed that intermediate MBG inclusions were morphologically distinct from EBO-S, EBO-Z, and EBO-R inclusions. No structural difference of viral inclusion material was observed among EBO-S, EBO-Z, and EBO-R. Immunoelectron microscopy showed that the filoviral matrix protein (VP40) and nucleoprotein (NP) accumulated in EBO-Z inclusions, and were closely associated during viral morphogenesis. These details facilitate the efficient and definitive diagnosis of filoviral infections by electron microscopy.

The complete nucleotide sequence of the Popp (1967) strain of Marburg virus: a comparison with the Musoke (1980) strain

The nucleotide sequence of genomic RNA of Marburg virus strain Popp was determined. Strain Popp was isolated in 1967 during the first filoviral outbreak. The virus was purified from blood of infected guinea pigs in which it had been maintained. The length of the determined sequence was 19112 nucleotides. Amino acid sequences of seven known virion proteins were deduced. Nucleotide and amino acid sequences were compared with those of strain Musoke of Marburg virus isolated in 1980 in Kenya and purified from Vero cells. Homology between nucleotide sequences of two strains was 93.9%. Comparisons revealed conserved and variable regions of the nucleotide and amino acid sequences. The GP, the envelope protein of the virion, was found to be the most variable protein. The greatest differences in the protein were located in the supposedly external part of the molecule. Amino acid substitutions in the L protein, the main component of viral RNA-dependent RNA polymerase, were also distributed extremely non-randomly. It was shown that the non-coding regions of the genome were more variable than the coding ones; 37.6% of nucleotide differences corresponded to the former. 72.6% of nucleotide substitutions located in the coding regions were found to be at the third codon position.