Membrane association of African horsesickness virus nonstructural protein NS3 determines its cytotoxicity

Evidence of Intragenic Recombination in African Horse Sickness Virus

Intragenic recombination has been described in various RNA viruses as a mechanism to increase genetic diversity, resulting in increased virulence, expanded host range, or adaptability to a changing environment. Orbiviruses are no exception to this, with intragenic recombination previously detected in the type species, bluetongue virus (BTV). African horse sickness virus (AHSV) is a double-stranded RNA virus belonging to the Oribivirus genus in the family Reoviridae. Genetic recombination through reassortment has been described in AHSV, but not through homologous intragenic recombination. The influence of the latter on the evolution of AHSV was investigated by analyzing the complete genomes of more than 100 viruses to identify evidence of recombination. Segment-1, segment-6, segment-7, and segment-10 showed evidence of intragenic recombination, yet only one (Segment-10) of these events was manifested in subsequent lineages. The other three hybrid segments were as a result of recombination between field isolates and the vaccine derived live attenuated viruses (ALVs).

Establishment of different plasmid only-based reverse genetics systems for the recovery of African horse sickness virus

In an effort to simplify and expand the utility of African horse sickness virus (AHSV) reverse genetics, different plasmid-based reverse genetics systems were developed. Plasmids containing cDNAs corresponding to each of the full-length double-stranded RNA genome segments of AHSV-4 under control of a T7 RNA polymerase promoter were co-transfected in cells expressing T7 RNA polymerase, and infectious AHSV-4 was recovered. This reverse genetics system was improved by reducing the required plasmids from 10 to five and resulted in enhanced virus recovery. Subsequently, a T7 RNA polymerase expression cassette was incorporated into one of the AHSV-4 rescue plasmids. This modified 5-plasmid set enabled virus recovery in BSR or L929 cells, thus offering the possibility to generate AHSV-4 in any cell line. Moreover, mutant and cross-serotype reassortant viruses were recovered. These plasmid DNA-based reverse genetics systems thus offer new possibilities for investigating AHSV biology and development of designer AHSV vaccine strains.

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An entirely plasmid-based reverse genetics system was developed for AHSV.

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Novel improvements were made that increases flexibility of AHSV plasmid-based reverse genetics.

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Virus recovery efficiency was increased by reducing plasmids required for rescue from 10 to 5.

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T7 RNA polymerase encoded by rescue plasmid backbone allows virus recovery in different cell lines.

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Recombinant wild-type AHSV, mutant and reassortant viruses were rescued from plasmid cDNA only.

Requirements and comparative analysis of reverse genetics for bluetongue virus (BTV) and African horse sickness virus (AHSV)

BACKGROUND

Bluetongue virus (BTV) and African horse sickness virus (AHSV) are distinct arthropod borne virus species in the genus Orbivirus (Reoviridae family), causing the notifiable diseases Bluetongue and African horse sickness of ruminants and equids, respectively. Reverse genetics systems for these orbiviruses with their ten-segmented genome of double stranded RNA have been developed. Initially, two subsequent transfections of in vitro synthesized capped run-off RNA transcripts resulted in the recovery of BTV. Reverse genetics has been improved by transfection of expression plasmids followed by transfection of ten RNA transcripts. Recovery of AHSV was further improved by use of expression plasmids containing optimized open reading frames.

RESULTS

Plasmids containing full length cDNA of the 10 genome segments for T7 promoter-driven production of full length run-off RNA transcripts and expression plasmids with optimized open reading frames (ORFs) were used. BTV and AHSV were rescued using reverse genetics. The requirement of each expression plasmid and capping of RNA transcripts for reverse genetics were studied and compared for BTV and AHSV. BTV was recovered by transfection of VP1 and NS2 expression plasmids followed by transfection of a set of ten capped RNAs. VP3 expression plasmid was also required if uncapped RNAs were transfected. Recovery of AHSV required transfection of VP1, VP3 and NS2 expression plasmids followed by transfection of capped RNA transcripts. Plasmid-driven expression of VP4, 6 and 7 was also needed when uncapped RNA transcripts were used. Irrespective of capping of RNA transcripts, NS1 expression plasmid was not needed for recovery, although NS1 protein is essential for virus propagation. Improvement of reverse genetics for AHSV was clearly demonstrated by rescue of several mutants and reassortants that were not rescued with previous methods.

CONCLUSIONS

A limited number of expression plasmids is required for rescue of BTV or AHSV using reverse genetics, making the system much more versatile and generally applicable. Optimization of reverse genetics enlarge the possibilities to rescue virus mutants and reassortants, and will greatly benefit the control of these important diseases of livestock and companion animals.

The effect of glycosylation on cytotoxicity of Ibaraki virus nonstructural protein NS3

The cytotoxicity of Ibaraki virus nonstructural protein NS3 was confirmed, and the contribution of glycosylation to this activity was examined by using glycosylation mutants of NS3 generated by site-directed mutagenesis. The expression of NS3 resulted in leakage of lactate dehydrogenase to the culture supernatant, suggesting the cytotoxicity of this protein. The lack of glycosylation impaired the transport of NS3 to the plasma membrane and resulted in reduced cytotoxicity. Combined with the previous observation that NS3 glycosylation was specifically observed in mammalian cells (Urata et al., Virus Research 2014), it was suggested that the alteration of NS3 cytotoxicity through modulating glycosylation is one of the strategies to achieve host specific pathogenisity of Ibaraki virus between mammals and vector arthropods.

VP2 Exchange and NS3/NS3a Deletion in African Horse Sickness Virus (AHSV) in Development of Disabled Infectious Single Animal Vaccine Candidates for AHSV

African horse sickness virus (AHSV) is a virus species in the genus Orbivirus of the family Reoviridae. There are nine serotypes of AHSV showing different levels of cross neutralization. AHSV is transmitted by species of Culicoides biting midges and causes African horse sickness (AHS) in equids, with a mortality rate of up to 95% in naive horses. AHS has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climates appear to be competent vectors for the related bluetongue virus (BTV). To control AHS, live-attenuated vaccines (LAVs) are used in Africa. We used reverse genetics to generate “synthetic” reassortants of AHSV for all nine serotypes by exchange of genome segment 2 (Seg-2). This segment encodes VP2, which is the serotype-determining protein and the dominant target for neutralizing antibodies. Single Seg-2 AHSV reassortants showed similar cytopathogenic effects in mammalian cells but displayed different growth kinetics. Reverse genetics for AHSV was also used to study Seg-10 expressing NS3/NS3a proteins. We demonstrated that NS3/NS3a proteins are not essential for AHSV replication in vitro. NS3/NS3a of AHSV is, however, involved in the cytopathogenic effect in mammalian cells and is very important for virus release from cultured insect cells in particular. Similar to the concept of the bluetongue disabled infectious single animal (BT DISA) vaccine platform, an AHS DISA vaccine platform lacking NS3/NS3a expression was developed. Using exchange of genome segment 2 encoding VP2 protein (Seg-2[VP2]), we will be able to develop AHS DISA vaccine candidates for all current AHSV serotypes.

IMPORTANCE African horse sickness virus is transmitted by species of Culicoides biting midges and causes African horse sickness in equids, with a mortality rate of up to 95% in naive horses. African horse sickness has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climates are supposed to be competent vectors. By using reverse genetics, viruses of all nine serotypes were constructed by the exchange of Seg-2 expressing the serotype-determining VP2 protein. Furthermore, we demonstrated that the nonstructural protein NS3/NS3a is not essential for virus replication in vitro. However, the potential spread of the virus by biting midges is supposed to be blocked, since the in vitro release of the virus was strongly reduced due to this deletion. VP2 exchange and NS3/NS3a deletion in African horse sickness virus were combined in the concept of a disabled infectious single animal vaccine for all nine serotypes.

Bluetongue virus without NS3/NS3a expression is not virulent and protects against virulent bluetongue virus challenge

Bluetongue is a disease in ruminants caused by the bluetongue virus (BTV), and is spread by Culicoides biting midges. Bluetongue outbreaks cause huge economic losses and death in sheep in several parts of the world. The most effective measure to control BTV is vaccination. However, both commercially available vaccines and recently developed vaccine candidates have several shortcomings. Therefore, we generated and tested next-generation vaccines for bluetongue based on the backbone of a laboratory-adapted strain of BTV-1, avirulent BTV-6 or virulent BTV-8. All vaccine candidates were serotyped with VP2 of BTV-8 and did not express NS3/NS3a non-structural proteins, due to induced deletions in the NS3/NS3a ORF. Sheep were vaccinated once with one of these vaccine candidates and were challenged with virulent BTV-8 3 weeks after vaccination. The NS3/NS3a knockout mutation caused complete avirulence for all three BTV backbones, including for virulent BTV-8, indicating that safety is associated with the NS3/NS3a knockout phenotype. Viraemia of vaccine virus was not detected using sensitive PCR diagnostics. Apparently, the vaccine viruses replicated only locally, which will minimize spread by the insect vector. In particular, the vaccine based on the BTV-6 backbone protected against disease and prevented viraemia of challenge virus, showing the efficacy of this vaccine candidate. The lack of NS3/NS3a expression potentially enables the differentiation of infected from vaccinated animals, which is important for monitoring virus spread in vaccinated livestock. The disabled infectious single-animal vaccine for bluetongue presented here is very promising and will be the subject of future studies.

Amino acid substitutions within the 2C coding sequence of Theiler's Murine Encephalomyelitis virus alter virus growth and affect protein distribution

Theiler's murine encephalomyelitis virus (TMEV) was used to investigate the distribution of P2 proteins in host cells and examine the effect of amino acid substitutions in conserved residues of the 2C protein on virus growth. The distribution of viral proteins 2B, 2C and 2BC with marker proteins of the endoplasmic reticulum (ER) and/or Golgi suggest an association with membranes of the secretory pathway. Similar results were obtained for truncated 2C and 2BC proteins with C-terminal deletions suggesting that the N-terminal region of the 2C protein is important in dictating distribution patterns. The significance of the high degree of conservation of this 2C region throughout the Picornaviridae was investigated by substituting conserved amino acid residues for alanine to create six mutant strains. Substitution mutations E(8)A, W(18)A and W(29)A abolished the ability of the virus to induce cytopathic effect (CPE) in BHK-21 cells. K(14)A, R(4)A and I(23)A delayed the onset and progression of CPE compared to the wild-type (WT) virus, and decreased virus yield. Immunofluorescence analysis of cells transiently expressing mutant 2C proteins revealed that the distribution of 2C was affected by substituting K(14), W(18) and I(23) for alanine indicating that specific conserved residues in 2C dictate protein distribution and virus growth.

Adaptive strategies of African horse sickness virus to facilitate vector transmission

African horse sickness virus (AHSV) is an orbivirus that is usually transmitted between its equid hosts by adult Culicoides midges. In this article, we review the ways in which AHSV may have adapted to this mode of transmission. The AHSV particle can be modified by the pH or proteolytic enzymes of its immediate environment, altering its ability to infect different cell types. The degree of pathogenesis in the host and vector may also represent adaptations maximising the likelihood of successful vectorial transmission. However, speculation upon several adaptations for vectorial transmission is based upon research on related viruses such as bluetongue virus (BTV), and further direct studies of AHSV are required in order to improve our understanding of this important virus.

Molecular epidemiology of the African horse sickness virus S10 gene

Between 2004 and 2006, 145 African horse sickness viruses (AHSV) were isolated from blood and organ samples submitted from South Africa to the Faculty of Veterinary Science, University of Pretoria. All nine serotypes were represented, with a range of 3–60 isolates per serotype. The RNA small segment 10 (S10) nucleotide sequences of these isolates were determined and the phylogeny investigated. AHSV, bluetongue virus (BTV) and equine encephalosis virus (EEV) all formed monophyletic groups and BTV was genetically closer to AHSV than EEV. This study confirmed the presence of three distinct S10 phylogenetic clades (α, β and γ). Some serotypes (6, 8 and 9 in α; 3 and 7 in β; 2 in γ) were restricted to a single clade, while other serotypes (1, 4 and 5) clustered into both the α and γ clades. Strong purifying selection was evident and a constant molecular clock was inappropriate. The S10 gene is the second most variable gene of the AHSV genome and the use of S10 in molecular epidemiology was illustrated by an AHS outbreak in the Western Cape in 2004. It was shown that two separate AHSV were circulating in the area, even though AHSV serotype 1 was the only isolate from the outbreak. The small size of the gene (755–764 bp) and conserved terminal regions facilitate easy and quick sequencing. The establishment of an S10 sequence database is important for characterizing outbreaks of AHS. It will be an essential resource for elucidating the epidemiology of AHS.

Identification of Transmembrane Domain of a Membrane Associated Protein NS5 of Dendrolimus punctatus Cytoplasmic Polyhedrosis Virus

We examined the intracellular localization of NS5 protein of Dendrolimus punctatus cytoplasmic polyhedrosis virus (DpCPV) by expressing NS5-GFP fusion protein and proteins from deletion mutants of NS5 in baculovirus recombinant infected insect Spodoptera frugiperda (Sf-9) cells. It was found that the NS5 protein was present at the plasma membrane of the cells, and that the N-terminal portion of the protein played a key role in the localization. A transmembrane region was identified to be present in the N-terminal portion of the protein, and the detailed transmembrane domain (SQIHMVWVKSGLVFF, 57-71aa) of N-terminal portion of NS5 was further determined, which was accorded with the predicted results, these findings suggested that NS5 might have an important function in viral life cycle.

Molecular analysis of the NS3/NS3A gene of Bluetongue virus isolates from the 1979 and 1998–2001 epizootics in Greece and their segregation into two distinct groups

The sequence of the genome segment 10 (Seg-10) encoding NS3/NS3A was determined for 19 field isolates of Bluetongue virus (BTV) of serotypes BTV-1, BTV-4, BTV-9 and BTV-16, derived from epizootics in Greece in the years 1979 and 1998-2001. The aim of the study was to define the molecular epidemiology of the virus in this part of the Mediterranean basin. On the basis of the Seg-10 sequences, the isolates grouped into two distinct phylogenetic clusters. These were Greek group I of solely serotype BTV-4 viruses, and Greek group II of serotypes BTV-1, BTV-9 and BTV-16 viruses. The isolates in Greek group I clustered with the Corsican and Tunisian BTV-2 serotypes and US group II strains of BTV-10 and BTV-13 serotypes, while those in Greek group II with Chinese, Indian and Australian viruses of different serotypes suggesting that viruses derived from two distinct ecosystems have caused BT incursions in Greece over the last 25 years. The NS3/NS3A sequences of most of the BTV-4 isolates were identical, irrespective of the year of isolation, geographical location and host species or tissue origin. Maximum of 15-16% nucleic acid sequence variation, but only 4% deduced amino acid substitution, were observed between groups I and II. Furthermore, the clustering of the NS3/NS3A sequences was independent of the viral serotype, indicating the occurrence of genome segment reassortment during the course of evolution of the viruses.

Extensive syncytium formation mediated by the reovirus FAST proteins triggers apoptosis-induced membrane instability

The fusion-associated small transmembrane (FAST) proteins of the fusogenic reoviruses are the only known examples of membrane fusion proteins encoded by non-enveloped viruses. While the involvement of the FAST proteins in mediating extensive syncytium formation in virus-infected and -transfected cells is well established, the nature of the fusion reaction and the role of cell-cell fusion in the virus replication cycle remain unclear. To address these issues, we analyzed the syncytial phenotype induced by four different FAST proteins: the avian and Nelson Bay reovirus p10, reptilian reovirus p14, and baboon reovirus p15 FAST proteins. Results indicate that FAST protein-mediated cell-cell fusion is a relatively non-leaky process, as demonstrated by the absence of significant [3H]uridine release from cells undergoing fusion and by the resistance of these cells to treatment with hygromycin B, a membrane-impermeable translation inhibitor. However, diminished membrane integrity occurred subsequent to extensive syncytium formation and was associated with DNA fragmentation and chromatin condensation, indicating that extensive cell-cell fusion activates apoptotic signaling cascades. Inhibiting effector caspase activation or ablating the extent of syncytium formation, either by partial deletion of the avian reovirus p10 ecto-domain or by antibody inhibition of p14-mediated cell-cell fusion, all resulted in reduced membrane permeability changes. These observations suggest that the FAST proteins do not possess intrinsic membrane-lytic activity. Rather, extensive FAST protein-induced syncytium formation triggers an apoptotic response that contributes to altered membrane integrity. We propose that the FAST proteins have evolved to serve a dual role in the replication cycle of these fusogenic non-enveloped viruses, with non-leaky cell-cell fusion initially promoting localized cell-cell transmission of the infection followed by enhanced progeny virus release from apoptotic syncytia and systemic dissemination of the infection.

The NS3 protein of bluetongue virus exhibits viroporin-like properties

Viroporins compose a group of small hydrophobic transmembrane proteins that can form hydrophilic pores through lipid bilayers. Viroporins have been implicated in promoting virus release from infected cells and in affecting cellular functions including protein trafficking and membrane permeability. Nonstructural protein 3 (NS3) of bluetongue virus has been shown previously to be important for efficient release of newly made virions from infected cells. In this report, we demonstrate that NS3 possesses properties commonly associated with viroporins. Our findings indicate that: (i) NS3 localizes to the Golgi apparatus and plasma membrane in transfected cells, (ii) NS3 can homo-oligomerize in transfected cells, (iii) targeting of NS3 to the Golgi apparatus and plasma membrane correlates with the enhanced permeability of cells to the translation inhibitor hygromycin B (hyg-B), (iv) amino acids 118-148 comprising transmembrane region 1 (TM1) of NS3 are critical for Golgi targeting and hyg-B permeability, and (v) deletion of amino acids 156-181 comprising transmembrane region 2 (TM2) of NS3 has little to no affect on Golgi targeting and hyg-B permeability. These viroporin-like properties may contribute to the role of NS3 in virus release and may have important implications for pathogenicity of bluetongue virus infections.

Modification of late membrane permeability in avian reovirus-infected cells: viroporin activity of the S1-encoded nonstructural p10 protein

Infection of chicken embryo fibroblasts by avian reovirus induces an increase in the permeability of the host plasma membrane at late, but not early, infection times. The absence of permeability changes at early infection times, as well as the dependence of late membrane modification on both viral protein synthesis and an active exocytic route, suggest that a virus-encoded membrane protein is required for avian reovirus to permeabilize cells. Further studies revealed that expression of nonstructural p10 protein in bacterial cells arrested cell growth and enhanced membrane permeability. Membrane leakiness was also observed following transient expression of p10 in BSC-40 monkey cells. Both its permeabilizing effect and the fact that p10 shares several structural and physical characteristics with other membrane-active viral proteins indicate that p10 is an avian reovirus viroporin. Furthermore, the fusogenic extracellular NH(2)-terminal domain of p10 appears to be dispensable for permeabilizing activity, because its deletion entirely abolished the fusogenic activity of p10, without affecting its ability to associate with cell membranes and to enhance membrane permeability. Similar properties have reported previously for immunodeficiency virus type I transmembrane glycoprotein gp41. Thus, like gp41, p10 appears to be a multifunctional protein that plays key roles in virus-host interaction.