The replication of bluetongue virus

Sialic acid binding sites in VP2 of bluetongue virus and their use during virus entry

Bluetongue virus (BTV), a member of Orbivirus genus, is transmitted by biting midges (gnats, Culicoides sp) and is one of the most widespread animal pathogens, causing serious outbreaks in domestic animals, particularly in sheep, with high economic impact. The non-enveloped BTV particle is a double-capsid structure of seven proteins and a genome of ten double-stranded RNA segments. Although the outermost spike-like VP2 acts as the attachment protein during BTV entry, no specific host receptor has been identified for BTV. Recent high-resolution cryo-electron (cryoEM) structures and biological data have suggested that VP2 may interact with sialic acids (SAs). To confirm this, we have generated protein-based nanoparticles displaying multivalent VP2 and used them to probe glycan arrays. The data show that VP2 binds α2,3-linked SA with high affinity but also binds α2,6-linked SA. Further, Maackia Amurensis Lectin II (MAL II) and Sambucus Nigra Lectin (SNA), which specifically bind α2,3-linked and α2,6-linked SAs respectively, inhibited BTV infection and virus growth in susceptible sheep cells while SNA alone inhibited virus growth in Culicoides-derived cells. A combination of hydrogen deuterium exchange mass spectrometry and site-directed mutagenesis allowed the identification of the specific SA binding residues of VP2. This study provides direct evidence that sialic acids act as key receptor for BTV and that the outer capsid protein VP2 specifically binds SA during BTV entry in both mammalian and insect cells. Importance To date no receptor has been assigned for non-enveloped bluetongue virus. To determine if the outermost spike-like VP2 protein is responsible for host cell attachment via interaction with sialic acids, we first generated a protein-based VP2-nanoparticle, for the multivalent presentation of recombinant VP2 protein. Using nanoparticles-displaying VP2 to probe a glycan array, we identified that VP2 binds both α2,3-linked and α2,6-linked sialic acids. Lectin inhibitors targeting both linkages of sialic acids showed strong inhibition to BTV infection and progeny virus production in mammalian cells, however the inhibition was only seen with the lectin targeting α2,6-linked sialic acid in insect vector cells. In addition, we identified the VP2 sialic acid binding sites in the exposed tip domain. Our data provides direct evidence that sialic acids act as key receptors for BTV attachment and entry in to both mammalian and insect cells.

An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India

Bluetongue (BT) is an economically important, non-contagious viral disease of domestic and wild ruminants. BT is caused by BT virus (BTV) and it belongs to the genus Orbivirus and family Reoviridae. BTV is transmitted by Culicoides midges and causes clinical disease in sheep, white-tailed deer, pronghorn antelope, bighorn sheep, and subclinical manifestation in cattle, goats and camelids. BT is a World Organization for Animal Health (OIE) listed multispecies disease and causes great socio-economic losses. To date, 28 serotypes of BTV have been reported worldwide and 23 serotypes have been reported from India. Transplacental transmission (TPT) and fetal abnormalities in ruminants had been reported with cell culture adopted live-attenuated vaccine strains of BTV. However, emergence of BTV-8 in Europe during 2006, confirmed TPT of wild-type/field strains of BTV. Diagnosis of BT is more important for control of disease and to ensure BTV-free trade of animals and their products. Reverse transcription polymerase chain reaction, agar gel immunodiffusion assay and competitive enzyme-linked immunosorbent assay are found to be sensitive and OIE recommended tests for diagnosis of BTV for international trade. Control measures include mass vaccination (most effective method), serological and entomological surveillance, forming restriction zones and sentinel programs. Major hindrances with control of BT in India are the presence of multiple BTV serotypes, high density of ruminant and vector populations. A pentavalent inactivated, adjuvanted vaccine is administered currently in India to control BT. Recombinant vaccines with DIVA strategies are urgently needed to combat this disease. This review is the first to summarise the seroprevalence of BTV in India for 40 years, economic impact and pathobiology.

Pediatric Atypical Teratoid/Rhabdoid Tumors of the Brain: Identification of Metabolic Subgroups Using In Vivo 1H-MR Spectroscopy

BACKGROUND AND PURPOSE

Atypical teratoid/rhabdoid tumors are rare, aggressive central nervous system tumors that are predominantly encountered in very young children. Our aim was to determine whether in vivo metabolic profiles correlate with molecular features of central nervous system pediatric atypical teratoid/rhabdoid tumors.

MATERIALS AND METHODS

Twenty confirmed patients with atypical teratoid/rhabdoid tumors who underwent MR spectroscopy were included in this study. In vivo metabolite levels of atypical teratoid/rhabdoid tumors were compared with molecular subtypes assessed by achaete-scute homolog 1 expression. Additionally, brain-specific creatine kinase levels were determined in tissue samples.

RESULTS

In vivo creatine concentrations were higher in tumors that demonstrated achaete-scute homolog 1 expression compared with those without achaete-scute homolog 1 expression (3.42 ± 1.1 versus 1.8 ± 0.8 IU, P < .01). Additionally, levels of myo-inositol (mI) (9.0 ± 1.5 versus 4.7 ± 3.6 IU, P < .05) were significantly different, whereas lipids approached significance (44 ± 20 versus 80 ± 30 IU, P = .07) in these 2 cohorts. Higher brain-specific creatine kinase levels were observed in the cohort with achaete-scute homolog 1 expression (P < .05). Pearson correlation analysis showed a significant positive correlation of brain-specific creatine kinase with absolute creatine (P < .05) and myo-inositol (P < .05) concentrations.

CONCLUSIONS

In vivo MR spectroscopy may predict key molecular features of atypical teratoid/rhabdoid tumors at initial diagnosis, leading to timely patient risk stratification and accelerating the development of targeted therapies.

Function, Architecture, and Biogenesis of Reovirus Replication Neoorganelles

Most viruses that replicate in the cytoplasm of host cells form neoorganelles that serve as sites of viral genome replication and particle assembly. These highly specialized structures concentrate viral proteins and nucleic acids, prevent the activation of cell-intrinsic defenses, and coordinate the release of progeny particles. Reoviruses are common pathogens of mammals that have been linked to celiac disease and show promise for oncolytic applications. These viruses form nonenveloped, double-shelled virions that contain ten segments of double-stranded RNA. Replication organelles in reovirus-infected cells are nucleated by viral nonstructural proteins µNS and σNS. Both proteins partition the endoplasmic reticulum to form the matrix of these structures. The resultant membranous webs likely serve to anchor viral RNA⁻protein complexes for the replication of the reovirus genome and the assembly of progeny virions. Ongoing studies of reovirus replication organelles will advance our knowledge about the strategies used by viruses to commandeer host biosynthetic pathways and may expose new targets for therapeutic intervention against diverse families of pathogenic viruses.

Diagnosing bluetongue virus in domestic ruminants: current perspectives

This review provides an overview of current and potential new diagnostic techniques against bluetongue virus (BTV), an Orbivirus transmitted by arthropods that affects ruminants. Bluetongue is a disease currently notifiable to the World Organization for Animal Health (OIE), causing great economic losses due to decreased trade associated with bluetongue outbreaks and high mortality and morbidity. BTV cross-reacts with many antigenically related viruses including viruses that causes African Horse sickness and epizootic haemorrhagic disease of deer. Therefore, reliable diagnostic approaches to detect BTV among these other antigenically related viruses are used or being developed. The antigenic determinant for differentiation of virus species/serogroups among orbiviruses is the VP7 protein, meanwhile VP2 is serotype specific. Serologically, assays are established in many laboratories, based mainly on competitive ELISA or serum neutralization assay (virus neutralization assay [VNT]) although new techniques are being developed. Virus isolation from blood or semen is, additionally, another means of BTV diagnosis. Nevertheless, most of these techniques for viral isolation are time-consuming and expensive. Currently, reverse-transcription polymerase chain reaction (RT-PCR) panels or real-time RT-PCR are widely used methods although next-generation sequencing remains of interest for future virus diagnosis.

Entry of Bluetongue Virus Capsid Requires the Late Endosome-specific Lipid Lysobisphosphatidic Acid

The entry of viruses into host cells is one of the key processes of infection. The mechanisms of cellular entry for enveloped virus have been well studied. The fusion proteins as well as the facilitating cellular lipid factors involved in the viral fusion entry process have been well characterized. The process of non-enveloped virus cell entry, in comparison, remains poorly defined, particularly for large complex capsid viruses of the family Reoviridae, which comprises a range of mammalian pathogens. These viruses enter cells without the aid of a limiting membrane and thus cannot fuse with host cell membranes to enter cells. Instead, these viruses are believed to penetrate membranes of the host cell during endocytosis. However, the molecular mechanism of this process is largely undefined. Here we show, utilizing an in vitro liposome penetration assay and cell biology, that bluetongue virus (BTV), an archetypal member of the Reoviridae, utilizes the late endosome-specific lipid lysobisphosphatidic acid for productive membrane penetration and viral entry. Further, we provide preliminary evidence that lipid lysobisphosphatidic acid facilitates pore expansion during membrane penetration, suggesting a mechanism for lipid factor requirement of BTV. This finding indicates that despite the lack of a membrane envelope, the entry process of BTV is similar in specific lipid requirements to enveloped viruses that enter cells through the late endosome. These results are the first, to our knowledge, to demonstrate that a large non-enveloped virus of the Reoviridae has specific lipid requirements for membrane penetration and host cell entry.

Non-structural protein NS3/NS3a is required for propagation of bluetongue virus in Culicoides sonorensis

BACKGROUND

Bluetongue virus (BTV) causes non-contagious haemorrhagic disease in ruminants and is transmitted by Culicoides spp. biting midges. BTV encodes four non-structural proteins of which NS3/NS3a is functional in virus release. NS3/NS3a is not essential for in vitro virus replication. However, deletion of NS3/NS3a leads to delayed virus release from mammalian cells and largely reduces virus release from insect cells. NS3/NS3a knockout BTV in sheep causes no viremia, but induces sterile immunity and is therefore proposed to be a Disabled Infectious Single Animal (DISA) vaccine candidate. In the absence of viremia, uptake of this vaccine strain by blood-feeding midges would be highly unlikely. Nevertheless, unintended replication of vaccine strains within vectors, and subsequent recombination or re-assortment resulting in virulent phenotypes and transmission is a safety concern of modified-live vaccines.

METHODS

The role of NS3/NS3a in replication and dissemination of BTV1, expressing VP2 of serotype 2 within colonized Culicoides sonorensis midges was investigated. Virus strains were generated using reverse genetics and their growth was examined in vitro. A laboratory colony of C. sonorensis, a known competent BTV vector, was fed or injected with BTV with or without expressing NS3/NS3a and replication in the midge was examined using RT PCR. Crossing of the midgut infection barrier was examined by separate testing of midge heads and bodies.

RESULTS

Although the parental NS3/NS3a expressing strain was not able to replicate and disseminate within C. sonorensis after oral feeding, this virus was able to replicate efficiently when the midgut infection barrier was bypassed by intrathoracic injection, whereas the NS3/NS3a knockout mutant was unable to replicate. This demonstrates that NS3/NS3a is required for viral replication within Culicoides.

CONCLUSION

The lack of viremia and the inability to replicate within the vector, clearly demonstrate the inability of NS3/NS3a knockout DISA vaccine strains to be transmitted by midges.

Autophagy Activated by Bluetongue Virus Infection Plays a Positive Role in Its Replication

Bluetongue virus (BTV) is an important pathogen of wild and domestic ruminants. Despite extensive study in recent decades, the interplay between BTV and host cells is not clearly understood. Autophagy as a cellular adaptive response plays a part in many viral infections. In our study, we found that BTV1 infection triggers the complete autophagic process in host cells, as demonstrated by the appearance of obvious double-membrane autophagosome-like vesicles, GFP-LC3 dots accumulation, the conversion of LC3-I to LC3-II and increased levels of autophagic flux in BSR cells (baby hamster kidney cell clones) and primary lamb lingual epithelial cells upon BTV1 infection. Moreover, the results of a UV-inactivated BTV1 infection assay suggested that the induction of autophagy was dependent on BTV1 replication. Therefore, we investigated the role of autophagy in BTV1 replication. The inhibition of autophagy by pharmacological inhibitors (3-MA, CQ) and RNA interference (siBeclin1) significantly decreased viral protein synthesis and virus yields. In contrast, treating BSR cells with rapamycin, an inducer of autophagy, promoted viral protein expression and the production of infectious BTV1. These findings lead us to conclude that autophagy is activated by BTV1 and contributes to its replication, and provide novel insights into BTV-host interactions.

Trafficking of bluetongue virus visualized by recovery of tetracysteine-tagged virion particles

Bluetongue virus (BTV), a member of the Orbivirus genus in the Reoviridae family, is a double-capsid insect-borne virus enclosing a genome of 10 double-stranded RNA segments. Like those of other members of the family, BTV virions are nonenveloped particles containing two architecturally complex capsids. The two proteins of the outer capsid, VP2 and VP5, are involved in BTV entry and in the delivery of the transcriptionally active core to the cell cytoplasm. Although the importance of the endocytic pathway in BTV entry has been reported, detailed analyses of entry and the role of each protein in virus trafficking have not been possible due to the lack of availability of a tagged virus. Here, for the first time, we report on the successful manipulation of a segmented genome of a nonenveloped capsid virus by the introduction of tags that were subsequently fluorescently visualized in infected cells. The genetically engineered fluorescent BTV particles were observed to enter live cells immediately after virus adsorption. Further, we showed the separation of VP2 from VP5 during virus entry and confirmed that while VP2 is shed from virions in early endosomes, virus particles still consisting of VP5 were trafficked sequentially from early to late endosomes. Since BTV infects both mammalian and insect cells, the generation of tagged viruses will allow visualization of the trafficking of BTV farther downstream in different host cells. In addition, the tagging technology has potential for transferable application to other nonenveloped complex viruses.

IMPORTANCE

Live-virus trafficking in host cells has been highly informative on the interactions between virus and host cells. Although the insertion of fluorescent markers into viral genomes has made it possible to study the trafficking of enveloped viruses, the physical constraints of architecturally complex capsid viruses have imposed practical limitations. In this study, we have successfully genetically engineered the segmented RNA genome of bluetongue virus (BTV), a complex nonenveloped virus belonging to the Reoviridae family. The resulting fluorescent virus particles could be visualized in virus entry studies of both live and fixed cells. This is the first time a structurally complex capsid virus has been successfully genetically manipulated to generate virus particles that could be visualized in infected cells.

Structural constraints in the packaging of bluetongue virus genomic segments

The mechanism used by bluetongue virus (BTV) to ensure the sorting and packaging of its 10 genomic segments is still poorly understood. In this study, we investigated the packaging constraints for two BTV genomic segments from two different serotypes. Segment 4 (S4) of BTV serotype 9 was mutated sequentially and packaging of mutant ssRNAs was investigated by two newly developed RNA packaging assay systems, one in vivo and the other in vitro. Modelling of the mutated ssRNA followed by biochemical data analysis suggested that a conformational motif formed by interaction of the 5' and 3' ends of the molecule was necessary and sufficient for packaging. A similar structural signal was also identified in S8 of BTV serotype 1. Furthermore, the same conformational analysis of secondary structures for positive-sense ssRNAs was used to generate a chimeric segment that maintained the putative packaging motif but contained unrelated internal sequences. This chimeric segment was packaged successfully, confirming that the motif identified directs the correct packaging of the segment.

The molecular biology of Bluetongue virus replication

The members of Orbivirus genus within the Reoviridae family are arthropod-borne viruses which are responsible for high morbidity and mortality in ruminants. Bluetongue virus (BTV) which causes disease in livestock (sheep, goat, cattle) has been in the forefront of molecular studies for the last three decades and now represents the best understood orbivirus at a molecular and structural level. The complex nature of the virion structure has been well characterised at high resolution along with the definition of the virus encoded enzymes required for RNA replication; the ordered assembly of the capsid shell as well as the protein and genome sequestration required for it; and the role of host proteins in virus entry and virus release. More recent developments of Reverse Genetics and Cell-Free Assembly systems have allowed integration of the accumulated structural and molecular knowledge to be tested at meticulous level, yielding higher insight into basic molecular virology, from which the rational design of safe efficacious vaccines has been possible. This article is centred on the molecular dissection of BTV with a view to understanding the role of each protein in the virus replication cycle. These areas are important in themselves for BTV replication but they also indicate the pathways that related viruses, which includes viruses that are pathogenic to man and animals, might also use providing an informed starting point for intervention or prevention.

RNA interference targets arbovirus replication in Culicoides cells

Arboviruses are transmitted to vertebrate hosts by biting arthropod vectors such as mosquitoes, ticks, and midges. These viruses replicate in both arthropods and vertebrates and are thus exposed to different antiviral responses in these organisms. RNA interference (RNAi) is a sequence-specific RNA degradation mechanism that has been shown to play a major role in the antiviral response against arboviruses in mosquitoes. Culicoides midges are important vectors of arboviruses, known to transmit pathogens of humans and livestock such as bluetongue virus (BTV) (Reoviridae), Oropouche virus (Bunyaviridae), and likely the recently discovered Schmallenberg virus (Bunyaviridae). In this study, we investigated whether Culicoides cells possess an antiviral RNAi response and whether this is effective against arboviruses, including those with double-stranded RNA (dsRNA) genomes, such as BTV. Using reporter gene-based assays, we established the presence of a functional RNAi response in Culicoides sonorensis-derived KC cells which is effective in inhibiting BTV infection. Sequencing of small RNAs from KC and Aedes aegypti-derived Aag2 cells infected with BTV or the unrelated Schmallenberg virus resulted in the production of virus-derived small interfering RNAs (viRNAs) of 21 nucleotides, similar to the viRNAs produced during arbovirus infections of mosquitoes. In addition, viRNA profiles strongly suggest that the BTV dsRNA genome is accessible to a Dicer-type nuclease. Thus, we show for the first time that midge cells target arbovirus replication by mounting an antiviral RNAi response mainly resembling that of other insect vectors of arboviruses.

Bluetongue virus (BTV): propagation, quantification, and storage

As an obligate intracellular parasite, the genome of the Bluetongue virus (BTV) contains ten double-stranded RNA segments which are encapsidated by viral proteins, forming "transport vesicles" that can transmit the viral progeny cell-to-cell efficiently and that can also be transmitted animal-to-animal by a biting midge. BTV is a cytoplasmic virus, and its five major steps of viral infection: attachment, entry, uncoating, assembly, and release, occur only in the cytosol within the infected host cell. Viral replication, suppression of cellular processes, and subsequent pathological damage disrupt many cellular pathways, leading to cellular apoptosis. All of these steps are under very rapid, tight, and efficient control. BTV infects both domestic and wild ruminants, especially sheep, but not humans. BTV is also the prototype in the Orbivirus genus of the Reoviridae family, and has been studied very extensively for the last 25 years. The experimental protocols presented here describe most of the methods that have been used routinely and reproducibly in our lab for our studies of the BTV biosystems.

Oncolytic bluetongue viruses: promise, progress, and perspectives

Humans are sero-negative toward bluetongue viruses (BTVs) since BTVs do not infect normal human cells. Infection and selective degradation of several human cancer cell lines but not normal ones by five US BTV serotypes have been investigated. We determined the susceptibilities of many normal and human cancer cells to BTV infections and made comparative kinetic analyses of their cytopathic effects, survival rates, ultra-structural changes, cellular apoptosis and necrosis, cell cycle arrest, cytokine profiles, viral genome, mRNAs, and progeny titers. The wild-type US BTVs, without any genetic modifications, could preferentially infect and degrade several types of human cancer cells but not normal cells. Their selective and preferential BTV-degradation of human cancer cells is viral dose–dependent, leading to effective viral replication, and induced apoptosis. Xenograft tumors in mice were substantially reduced by a single intratumoral BTV injection in initial in vivo experiments. Thus, wild-type BTVs, without genetic modifications, have oncolytic potentials. They represent an attractive, next generation of oncolytic viral approach for potential human cancer therapy combined with current anti-cancer agents and irradiation.

A clathrin independent macropinocytosis-like entry mechanism used by bluetongue virus-1 during infection of BHK cells

Acid dependent infection of Hela and Vero cells by BTV-10 occurs from within early-endosomes following virus uptake by clathrin-mediated endocytosis (Forzan et al., 2007: J Virol 81: 4819–4827). Here we report that BTV-1 infection of BHK cells is also dependent on a low endosomal pH; however, virus entry and infection were not inhibited by dominant-negative mutants of Eps15, AP180 or the ‘aa’ splice variant of dynamin-2, which were shown to inhibit clathrin-mediated endocytosis. In addition, infection was not inhibited by depletion of cellular cholesterol, which suggests that virus entry is not mediated by a lipid-raft dependent process such as caveolae-mediated endocytosis. Although virus entry and infection were not inhibited by the dominant-negative dynamin-2 mutant, entry was inhibited by the general dynamin inhibitor, dynasore, indicating that virus entry is dynamin dependent. During entry, BTV-1 co-localised with LAMP-1 but not with transferrin, suggesting that virus is delivered to late-endosomal compartments without first passing through early-endosomes. BTV-1 entry and infection were inhibited by EIPA and cytochalasin-D, known macropinocytosis inhibitors, and during entry virus co-localised with dextran, a known marker for macropinocytosis/fluid-phase uptake. Our results extend earlier observations with BTV-10, and show that BTV-1 can infect BHK cells via an entry mechanism that is clathrin and cholesterol-independent, but requires dynamin, and shares certain characteristics in common with macropinocytosis.

Role of lipids on entry and exit of bluetongue virus, a complex non-enveloped virus

Non-enveloped viruses such as members of Picornaviridae and Reoviridae are assembled in the cytoplasm and are generally released by cell lysis. However, recent evidence suggests that some non-enveloped viruses exit from infected cells without lysis, indicating that these viruses may also utilize alternate means for egress. Moreover, it appears that complex, non-enveloped viruses such as bluetongue virus (BTV) and rotavirus interact with lipids during their entry process as well as with lipid rafts during the trafficking of newly synthesized progeny viruses. This review will discuss the role of lipids in the entry, maturation and release of non-enveloped viruses, focusing mainly on BTV.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Bluetongue virus: dissection of the polymerase complex

Bluetongue is a vector-borne viral disease of ruminants that is endemic in tropical and subtropical countries. Since 1998 the virus has also appeared in Europe. Partly due to the seriousness of the disease, bluetongue virus (BTV), a member of genus Orbivirus within the family Reoviridae, has been a subject of intense molecular study for the last three decades and is now one of the best understood viruses at the molecular and structural levels. BTV is a complex non-enveloped virus with seven structural proteins arranged in two capsids and a genome of ten double-stranded (ds) RNA segments. Shortly after cell entry, the outer capsid is lost to release an inner capsid (the core) which synthesizes capped mRNAs from each genomic segment, extruding them into the cytoplasm. This requires the efficient co-ordination of a number of enzymes, including helicase, polymerase and RNA capping activities. This review will focus on our current understanding of these catalytic proteins as derived from the use of recombinant proteins, combined with functional assays and the in vitro reconstitution of the transcription/replication complex. In some cases, 3D structures have complemented this analysis to reveal the fine structural detail of these proteins. The combined activities of the core enzymes produce infectious transcripts necessary and sufficient to initiate BTV infection. Such infectious transcripts can now be synthesized wholly in vitro and, when introduced into cells by transfection, lead to the recovery of infectious virus. Future studies thus hold the possibility of analysing the consequence of mutation in a replicating virus system.

Bluetongue virus entry into cells

Bluetongue virus (BTV) is a member of the Orbivirus genus within the Reoviridae family. Like those of other members of the family, BTV particles are nonenveloped and contain two distinct capsids, namely, an outer capsid and an inner capsid or core. The two outer capsid proteins, VP2 and VP5, are involved in BTV entry into cells and in the delivery of the transcriptionally active core to the target cell cytoplasm. However, very little is known about the precise mechanism of BTV entry. In this report, using RNA interference, we demonstrate that inhibition of the clathrin-dependent endocytic pathway correlates with reduced BTV internalization and subsequent replication. Furthermore, by using the ATPase inhibitor bafilomycin A1, we show that exposure of the virus to acidic pH is required for productive infection. Moreover, microscopic analysis of cells incubated with BTV indicated that the virus is internalized into early endosomes, where separation of the outer capsid and inner core occurs. Together, our data indicate that BTV undergoes low-pH-induced penetration in early endosomes following clathrin-mediated endocytosis from the plasma membrane, supporting a stepwise model for BTV entry and penetration.

Interaction between Bluetongue virus outer capsid protein VP2 and vimentin is necessary for virus egress

BACKGROUND

The VP2 outer capsid protein Bluetongue Virus (BTV) is responsible for receptor binding, haemagglutination and eliciting host-specific immunity. However, the assembly of this outer capsid protein on the transcriptionally active viral core would block transcription of the virus. Thus assembly of the outer capsid on the core particle must be a tightly controlled process during virus maturation. Earlier studies have detected mature virus particles associated with intermediate filaments in virus infected cells but the viral determinant for this association and the effect of disrupting intermediate filaments on virus assembly and release are unknown.

RESULTS

In this study it is demonstrated that BTV VP2 associates with vimentin in both virus infected cells and in the absence of other viral proteins. Further, the determinants of vimentin localisation are mapped to the N-terminus of the protein and deletions of amino acids between residues 65 and 114 are shown to disrupt VP2-vimentin association. Site directed mutation also reveals that amino acid residues Gly 70 and Val 72 are important in the VP2-vimentin association. Mutation of these amino acids resulted in a soluble VP2 capable of forming trimeric structures similar to unmodified protein that no longer associated with vimentin. Furthermore, pharmacological disruption of intermediate filaments, either directly or indirectly through the disruption of the microtubule network, inhibited virus release from BTV infected cells.

CONCLUSION

The principal findings of the research are that the association of mature BTV particles with intermediate filaments are driven by the interaction of VP2 with vimentin and that this interaction contributes to virus egress. Furthermore, i) the N-terminal 118 amino acids of VP2 are sufficient to confer vimentin interaction. ii) Deletion of amino acids 65-114 or mutation of amino acids 70-72 to DVD abrogates vimentin association. iii) Finally, disruption of vimentin structures results in an increase in cell associated BTV and a reduction in the amount of released virus from infected cells.

Pns4 of rice dwarf virus is a phosphoprotein, is localized around the viroplasm matrix, and forms minitubules

Rice dwarf virus (RDV), a member of the family Reoviridae, has a 12-segmented dsRNA genome. Seven segments, designated S1, S2, S3, S5, S7, S8, and S9, encode structural proteins, while the remainder encode nonstructural proteins. One of the nonstructural proteins, Pns4, which is encoded by S4, was characterized. Pns4 was a phosphorylatable substrate in a phosphorylation assay in vivo; it associated with large cytoplasmic fibrils and formed novel minitubules in infected cultured cells of its leafhopper insect vector, as revealed by immunofluorescence and immunoelectron microscopy. Early in infection, Pns4 was detected at the periphery of the viroplasm, and it was then observed on amorphous or fibrillar inclusions, which were identified as bundles of minitubules, at later stages of infection. Since viroplasms are believed to be the site of RDV replication, the intracellular location of Pns4 suggests that this protein might be involved in the process of assembly of the RDV virion.

Role of an arbovirus nonstructural protein in cellular pathogenesis and virus release

The insect-borne Bluetongue virus (BTV) is considered the prototypic Orbivirus, a member of the Reovirus family. One of the hallmarks of Orbivirus infection is the production of large numbers of intracellular tubular structures of unknown function. For BTV these structures are formed as the polymerization product of a single 64-kDa nonstructural protein, NS1, encoded by the viral double-stranded RNA genome segment 6. Although the NS1 protein is the most abundant viral protein synthesized in infected cells, its function has yet to be determined. One possibility is that NS1 tubules may be involved in the translocation of newly formed viral particles to the plasma membrane, and NS1-specific monoclonal antibodies have been shown to react with viral particles leaving infected cells. In the present study we generated a mammalian cell line that expresses a recombinant single-chain antibody fragment (scFv) derived from an NS1-specific monoclonal antibody (10B1) and analyzed the effect that this intracellular antibody has on BTV replication. Normally, BTV infection of mammalian cells in culture results in a severe cytopathic effect within 24 to 48 h postinfection manifested by cell rounding, apoptosis, and lytic release of virions into the culture medium. However, infection of scFv-expressing cells results in a marked reduction in the stability of NS1 and formation of NS1 tubules, a decrease in cytopathic effect, an increased release of infectious virus into the culture medium, and budding of virions from the plasma membrane. These results suggest that NS1 tubules play a direct role in the cellular pathogenesis and morphogenesis of BTV.

Bluetongue virus outer capsid proteins are sufficient to trigger apoptosis in mammalian cells

Bluetongue virus (BTV) is transmitted by Culicoides sp. biting midges to livestock, causing severe hemorrhagic disease in sheep, but is asymptomatic in the insect host. Similarly, BTV causes rapid cell death in infected mammalian cells in culture, whereas infections of insect cells are long-term and unapparent, despite productive virus replication. To assess whether apoptosis plays any role in these two distinct cell responses, we have investigated apoptosis in cultured insect and mammalian cells. Three different mammalian cell lines and three different insect cell lines including Culicoides variipennis (KC) cells were infected with BTV serotype 10, and the key apoptosis indicators of cell morphology, chromosomal DNA fragmentation, and caspase-3 activation were monitored. BTV infection induced apoptosis with the activation of the transcription factor nuclear factor kappaB (NF-kappaB) in all three mammalian cell lines. In contrast, no evidence for apoptosis was detected in any of the three insect cell lines in response to BTV infection. Using inhibitors of endosomal acidification and UV-inactivated virus, we established that virus uncoating, but not productive virus replication, is necessary for BTV to trigger apoptosis in mammalian cells. Intracellular expression of the viral outer capsid proteins VP2 and VP5 or the two major nonstructural proteins NS1 and NS2 was not sufficient to trigger an apoptotic response. However, extracellular treatment with a combination of purified recombinant VP2 and VP5, but not with each protein used separately, resulted in an apoptotic response. Virus- and VP2-VP5-stimulated apoptotic responses were both inhibited by inhibitors of endosomal acidification. Thus, for BTV the viral outer capsid proteins alone are sufficient to trigger apoptosis.

A capsid protein of nonenveloped Bluetongue virus exhibits membrane fusion activity

The outer capsid layer of Bluetongue virus, a member of the nonenveloped Reoviridae family, is composed of two proteins, a receptor-binding protein, VP2, and a second protein, VP5, which shares structural features with class I fusion proteins of enveloped viruses. In the replication cycle of Bluetongue virus VP5 acts as a membrane permeabilization protein that mediates release of viral particles from endosomal compartments into the cytoplasm. Here, we show that VP5 can also act as a fusion protein and induce syncytium formation when it is fused to a transmembrane anchor and expressed on the cell surface. Fusion activity is strictly pH-dependent and is triggered by short exposure to low pH. No cell-cell fusion is observed at neutral pH. Deletion of the first 40 amino acids, which can fold into two amphipathic helices, abolishes fusion activity. Syncytium formation by VP5 is inhibited in the presence of VP2 when it is expressed in a membrane-anchored form. The data indicate an interaction between the outer capsid protein VP2 and VP5 and show that VP5 undergoes pH-dependent conformational changes that render it capable of interacting with cellular membranes. More importantly, our data show that a membrane permeabilization protein of a nonenveloped virus can evolve into a fusion protein by the addition of an appropriate transmembrane anchor. The results strongly suggest that the mechanism of membrane permeabilization by VP5 and membrane fusion by viral fusion proteins require similar structural features and conformational changes.

Reovirus sigma NS and mu NS proteins form cytoplasmic inclusion structures in the absence of viral infection

Reovirus replication occurs in the cytoplasm of infected cells and culminates in the formation of crystalline arrays of progeny virions within viral inclusions. Two viral nonstructural proteins, sigma NS and micro NS, and structural protein sigma 3 form protein-RNA complexes early in reovirus infection. To better understand the minimal requirements of viral inclusion formation, we expressed sigma NS, mu NS, and sigma 3 alone and in combination in the absence of viral infection. In contrast to its concentration in inclusion structures during reovirus replication, sigma NS expressed in cells in the absence of infection is distributed diffusely throughout the cytoplasm and does not form structures that resemble viral inclusions. Expressed sigma NS is functional as it complements the defect in temperature-sensitive, sigma NS-mutant virus tsE320. In both transfected and infected cells, mu NS is found in punctate cytoplasmic structures and sigma 3 is distributed diffusely in the cytoplasm and the nucleus. The subcellular localization of mu NS and sigma 3 is not altered when the proteins are expressed together or with sigma NS. However, when expressed with micro NS, sigma NS colocalizes with mu NS to punctate structures similar in morphology to inclusion structures observed early in viral replication. During reovirus infection, both sigma NS and mu NS are detectable 4 h after adsorption and colocalize to punctate structures throughout the viral life cycle. In concordance with these results, sigma NS interacts with mu NS in a yeast two-hybrid assay and by coimmunoprecipitation analysis. These data suggest that sigma NS and mu NS are the minimal viral components required to form inclusions, which then recruit other reovirus proteins and RNA to initiate viral genome replication.

The core of bluetongue virus binds double-stranded RNA

Double-stranded RNA (dsRNA) viruses conceal their genome from the host to avoid triggering unfavorable cellular responses. The crystal structure of the core of one such virus, bluetongue virus, reveals an outer surface festooned with dsRNA. This may represent a deliberate strategy to sequester dsRNA released from damaged particles to prevent host cell shutoff.

Expression and functional characterization of bluetongue virus VP5 protein: role in cellular permeabilization

Segment 5 of bluetongue virus (BTV) serotype 10, which encodes the outer capsid protein VP5, was tagged with glutathione S-transferase and expressed by a recombinant baculovirus. The recombinant protein was subsequently purified to homogeneity, and its possible biological role in virus infection was investigated. Purified VP5 was able to bind mammalian cells but was not internalized, which indicates it is not involved in receptor-mediated endocytosis. The purified VP5 protein was shown to be able to permeabilize mammalian and Culicoides insect cells, inducing cytotoxicity. Sequence analysis revealed that VP5 possesses characteristic structural features (including two amino-terminal amphipathic helices) compatible with virus penetration activity. To assess the role of each feature in the observed cytotoxicity, a series of deleted VP5 molecules were generated, and their expression and biological activity was compared with the parental molecule. VP5 derivatives that included the two amphipathic helices exhibited cytotoxicity, while those that omitted these sequences did not. To confirm their role in membrane destabilization two synthetic peptides (amino acids [aa] 1 to 20 and aa 22 to 41) encompassing the two helices and an additional peptide representing the adjacent downstream sequences were also assessed for their effect on the cell membrane. Both helices, but not the downstream VP5 sequence, exhibited cytotoxicity with the most-amino-terminal helix (aa 1 to 20) showing a higher activity than the adjacent peptide (aa 22 to 41). Purified VP5 was shown to readily form trimers in solution, a feature of many proteins involved in membrane penetration. Taken together, these data support a role for VP5 in virus-cell penetration consistent with its revelation in the entry vesicle subsequent to cell binding and endocytosis.

Culicoides biting midges: their role as arbovirus vectors

Culicoides biting midges are among the most abundant of haematophagous insects, and occur throughout most of the inhabited world. Across this broad range they transmit a great number of assorted pathogens of human, and domestic and wild animals, but it is as vectors of arboviruses, and particularly arboviruses of domestic livestock, that they achieve their prime importance. To date, more than 50 such viruses have been isolated from Culicoides spp. and some of these cause diseases of such international significance that they have been allocated Office International des Epizooties (OIE) List A status. Culicoides are world players in the epidemiology of many important arboviral diseases. In this context this paper deals with those aspects of midge biology facilitating disease transmission, describes the factors controlling insect-virus interactions at the individual insect and population level, and illustrates the far-reaching effects that certain components of climate have upon the midges and, hence, transmission potential.

The loss of outer capsid protein P2 results in nontransmissibility by the insect vector of rice dwarf phytoreovirus

A transmission-defective (TD) isolate of rice dwarf phytoreovirus lacked the ability to infect cells when derived from the virus-free insect vector Nephotettix cincticeps. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified virus showed that among six structural proteins, the P2 outer capsid protein (encoded by genome segment S2) was absent from the TD isolate, whereas all six proteins were present in the transmission-competent (TC) isolate. P2 was not detected on immunoblots of rice plants infected with the TD isolate. Genome segment S2 and its transcript were detected in both TD and TC isolates. Sequence analysis of the S2 segment of the TD isolate revealed the presence of a termination codon due to a point mutation in the open reading frame, which might explain the absence of P2 in the TD isolate. These results demonstrate that the P2 protein is one of the factors essential for infection by the virus of vector cells and, thus, influences transmissibility by vector insects.

The pathogenesis of bluetongue virus infection of bovine blood cells in vitro: ultrastructural characterization

Cattle are proposed to be reservoir hosts of bluetongue virus (BTV) because infected animals typically have a prolonged cell-associated viremia. Enriched populations of bovine monocytes, erythrocytes and lymphocytes were inoculated with BTV serotype 10 (BTV 10) and the infected cells then were examined by transmission electron microscopy to characterize the interaction of BTV with bovine blood cells. Replication of BTV 10 in monocytes and stimulated (replicating) lymphocytes was morphologically similar to that which occurred in Vero cells, with formation of viral inclusion bodies and virus-specific tubules. In contrast, BTV 10 infection of unstimulated (non-replicating) lymphocytes and erythrocytes did not progress beyond adsorption, after which virus particles persisted in invaginations of the cell membrane. Studies with core particles and neutralizing monoclonal antibodies established that outer capsid protein VP2 is necessary for attachment of BTV 10 to erythrocytes. These in vitro virus-cell interactions provide a cogent explanation for the pathogenesis of BTV infection of cattle, especially the prolonged cell associated viremia that occurs in BTV-infected cattle.

Multiple glycoproteins synthesized by the smallest RNA segment (S10) of bluetongue virus

The genome of bluetongue virus, an orbivirus, consists of 10 double-stranded RNAs, each encoding at least one polypeptide. The smallest RNA segment (S10) encodes two minor nonstructural proteins, NS3 and NS3A, the structures and functions of which are not understood. We have expressed these two proteins in mammalian cells by using the T7 cytoplasmic transient expression system. Using a deletion mutant (lacking the first AUG initiation codon), we have demonstrated that the second initiation codon is used to initiate the synthesis of NS3A protein and that the two initiation codons are responsible for the synthesis not only of NS3 and NS3A but also of high-molecular-weight forms of both proteins. These higher-molecular-weight forms (GNS3 and GNS3A) are glycosylated. We have also demonstrated that the carbohydrate chains of GNS3 and GNS3A could be further modified by heterogeneous extension to polylactosaminoglycan forms. The glycosylated and nonglycosylated forms are found in similar intracellular locations in the Golgi complex. In the presence of cycloheximide, NS3 and NS3A immunofluorescence staining was pronounced in the Golgi complex, confirming that NS3 and NS3A are competent for transport to the Golgi apparatus after synthesis. We conclude that S10 gene products are integral membrane glycoproteins.

The complete sequence of genome segment 8 of bluetongue virus, serotype 1, which encodes the nonstructural protein, NS2

Bluetongue virus has a ten-segment double-stranded RNA genome, of which segment 8 encodes a nonstructural protein NS2. This protein is the only bluetongue viral protein to be phosphorylated and also has the ability to bind single-stranded RNA. At present, the function of NS2 is unknown and in order to analyse its characteristics in more detail, it was first necessary to obtain a full-length cDNA clone of the genome segment.