Localization of viral proteins in cells infected with bovine viral diarrhoea virus

Integrative Transcriptomics and Proteomics Analysis Provide a Deep Insight Into Bovine Viral Diarrhea Virus-Host Interactions During BVDV Infection

Bovine viral diarrhea virus (BVDV) is the causative agent of bovine viral diarrhea-mucosal disease (BVD-MD), an important viral disease in cattle that is responsible for extensive economic losses to the cattle industry worldwide. Currently, several underlying mechanisms involved in viral replication, pathogenesis, and evading host innate immunity of BVDV remain to be elucidated, particularly during the early stage of virus infection. To further explore the mechanisms of BVDV-host interactions, the transcriptomics and proteomics profiles of BVDV-infected MDBK cells were sequenced using RNA-seq and iTRAQ techniques, respectively, and followed by an integrative analysis. Compared with mock-infected MDBK cells, a total of 665 differentially expressed genes (DEGs) (391 down-regulated, 274 up-regulated) and 725 differentially expressed proteins (DEPs) (461 down-regulated, 264 up-regulated) were identified. Among these, several DEGs and DEPs were further verified using quantitative RT-PCR and western blot. Following gene ontology (GO) annotation and KEGG enrichment analysis, we determined that these DEGs and DEPs were significantly enriched in multiple important cellular signaling pathways including NOD-like receptor, Toll-like receptor, TNF, NF-κB, MAPK, cAMP, lysosome, protein processing in endoplasmic reticulum, lipid metabolism, and apoptosis signaling pathways. Significantly, the down-regulated DEGs and DEPs were predominantly associated with apoptosis-regulated elements, inflammatory factors, and antiviral elements that were involved in innate immunity, thus, indicating that BVDV could inhibit apoptosis and the expression of host antiviral genes to facilitate viral replication. Meanwhile, up-regulated DEGs and DEPs were primarily involved in metabolism and autophagy signaling pathways, indicating that BVDV could utilize the host metabolic resources and cell autophagy to promote replication. However, the potential mechanisms BVDV-host interactions required further experimental validation. Our data provide an overview of changes in transcriptomics and proteomics profiles of BVDV-infected MDBK cells, thus, providing an important basis for further exploring the mechanisms of BVDV-host interactions.

Characterization of Membrane Topology and Retention Signal of Pestiviral Glycoprotein E1

Pestiviruses are members of the family Flaviviridae, a group of enveloped viruses that bud at intracellular membranes. Pestivirus particles contain three glycosylated envelope proteins, E^rns, E1, and E2. Among them, E1 is the least characterized concerning both biochemical features and function. E1 from bovine viral diarrhea virus (BVDV) strain CP7 was analyzed with regard to its intracellular localization and membrane topology. Here, it is shown that even in the absence of other viral proteins, E1 is not secreted or expressed at the cell surface but localizes predominantly in the endoplasmic reticulum (ER). Using engineered chimeric transmembrane domains with sequences from E1 and vesicular stomatitis virus G protein, the E1 ER-retention signal could be narrowed down to six fully conserved polar residues in the middle part of the transmembrane domain of E1. Retention was observed even when several of these polar residues were exchanged for alanine. Mutations with a strong impact on E1 retention prevented recovery of infectious viruses when tested in the viral context. Analysis of the membrane topology of E1 before and after the signal peptide cleavage via a selective permeabilization and an in vivo labeling approach revealed that mature E1 is a typical type I transmembrane protein with a single span transmembrane anchor at its C terminus, whereas it adopts a hairpin-like structure with the C terminus located in the ER lumen when the precleavage situation is mimicked by blocking the cleavage site between E1 and E2.

IMPORTANCE The shortage of specific antibodies against E1, making detection and further analysis of E1 difficult, resulted in a lack of knowledge on E1 compared to E^rns and E2 with regard to biosynthesis, structure, and function. It is known that pestiviruses bud intracellularly. Here, we show that E1 contains its own ER retention signal: six fully conserved polar residues in the middle part of the transmembrane domain are shown to be the determinants for ER retention of E1. Moreover, those six polar residues could serve as a functional group that intensely affect the generation of infectious viral particles. In addition, the membrane topology of E1 has been determined. In this context, we also identified dynamic changes in membrane topology of E1 with the carboxy terminus located on the luminal side of the ER in the precleavage state and relocation of this sequence upon signal peptidase cleavage. Our work provides the first systematic analysis of the pestiviral E1 protein with regard to its biochemical and functional characteristics.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

Identification of protein inhibitor of activated STAT 4, a novel host interacting partner that involved in bovine viral diarrhea virus growth

Background

Bovine viral diarrhea virus (BVDV) belongs to the Flaviviridae family and the pestivius virus group. BVDV is responsible for significant economic loss in cattle industry worldwide because of reducing reproductive performance, increasing incidence of other diseases and mortality among young stock. The core (C) protein of the Flaviviridae family member is involved in host antiviral immune response through activation of related signaling pathways that affect the viral replication. However, the influence of C protein-interaction partners in BVDV infections is poorly defined.

Methods

To explore C-protein-interacting partners, yeast two-hybrid was used to screen the interaction protein of C protein using bovine peripheral blood mononuclear cell (PBMC) cDNA library. The co-immunoprecipitation and confocal assays were manipulated to determine the interaction between potential partners and C protein. Knockdown and overexpression of the partner were used to examine whether the C-protein-interacting partner plays a role in BVDV proliferation and virulence. Meanwhile, qRT-PCR and western blot assays were used to investigate the effect of C protein and C-protein-interacting partner on the immune response of host cells.

Results

We identified protein inhibitor of activated STAT 4 (PIAS4) as a novel interacting partner of the BVDV C protein. Co-immunoprecipitation and confocal assays demonstrated a strong interaction between C protein and PIAS4. Silencing of PIAS4 with small interfering RNA suppressed C protein expression and BVDV growth, while overexpression of PISA4 increased C protein expression and BVDV growth. The overexpression of PIAS4 increased the cell apoptosis. Meanwhile, the expressions of STAT4, SOCS3, IFITM, IFN-α were negatively regulated by the expression of PIAS4. The expression of C protein suppressed the antiviral proteins expression, and the inhibition effect was enhanced by interaction of PIAS4 and C protein. These results highlighted the beneficial properties of cellular PIAS4 for BVDV protein expression and growth.

Conclusions

This study provides reliable clues for understanding the roles of PIAS4 in the regulation of BVDV growth.

RNA-Seq based transcriptome analysis during bovine viral diarrhoea virus (BVDV) infection

Background

Bovine viral diarrhoea virus (BVDV) is the member of the genus Pestivirus within the Flaviviridae family and responsible for severe economic losses in the cattle industry. BVDV can employ ‘infect-and-persist’ strategy and ‘hit-and-run’ strategy to remain associated with hosts and thus contributes to BVDV circulation in cattle herds. BVDV have also evolved various strategies to evade the innate immunity of host. To further understand the mechanisms by which BVDV overcomes the host cell innate immune response and provide more clues for further understanding the BVDV-host interaction, in this descriptive study, we conducted a investigation of differentially expressed genes (DEGs) of the host during BVDV infection by RNA-Seq analysis.

Results

Our analysis identified 1297, 1732, 3072, and 1877 DEGs in the comparison groups mock vs. MDBK cells infected with BVDV post 2 h (MBV2h), mock vs. MBV6h, mock vs. MBV12h, and mock vs. MBV24h, respectively. The reproducibility and repeatability of the results were validated by RT-qPCR. Enrichment analyses of GO annotations and KEGG pathways revealed the host DEGs that are potentially induced by BVDV infection and may participate in BVDV-host interactions. Protein-protein interaction (PPI) network analyses identified the potential interactions among the DEGs. Our findings suggested that BVDV infection induced the upregulation of genes involved in lipid metabolism. The expression of genes that have antiviral roles, including ISG15, Mx1, OSA1Y, were found to be downregulated and are thus potentially associated with the inhibition of host innate immune system during BVDV infection. The expression levels of F3, C1R, KNG1, CLU, C3, FB, SERPINA5, SERPINE1, C1S, F2RL2, and C2, which belong to the complement and coagulation signalling cascades, were downregulated during BVDV infection, which suggested that the complement system might play a crucial role during BVDV infection.

Conclusion

In this descriptive study, our findings revealed the changes in the host transcriptome expression profile during BVDV infection and suggested that BVDV-infection induced altering the host’s metabolic network, the inhibition of the expression of antiviral proteins and genes within the complement system might be contributed to BVDV proliferation. The above findings provided unique insights for further studies on the mechanisms underlying BVDV-host interactions.

Fusogenicity of the Ghana Virus (Henipavirus: Ghanaian bat henipavirus) Fusion Protein is Controlled by the Cytoplasmic Domain of the Attachment Glycoprotein

The Ghana virus (GhV) is phylogenetically related to the zoonotic henipaviruses Nipah (NiV) and Hendra virus. Although GhV uses the highly conserved receptor ephrin-B2, the fusogenicity is restricted to cell lines of bat origin. Furthermore, the surface expression of the GhV attachment glycoprotein (G) is reduced compared to NiV and most of this protein is retained in the endoplasmic reticulum (ER). Here, we generated truncated as well as chimeric GhV G proteins and investigated the influence of the structural domains (cytoplasmic tail, transmembrane domain, ectodomain) of this protein on the intracellular transport and the fusogenicity following coexpression with the GhV fusion protein (F). We demonstrate that neither the cytoplasmic tail nor the transmembrane domain is responsible for the intracellular retention of GhV G. Furthermore, the cytoplasmic tail of GhV G modulates the fusogenicity of GhV F and therefore controls the species-restricted fusogenicity of the GhV surface glycoproteins.

Analyses of long non-coding RNAs and mRNA profiling through RNA sequencing of MDBK cells at different stages of bovine viral diarrhea virus infection

BACKGROUND

Bovine viral diarrhea virus (BVDV) infection is a dynamic and complex process that leads to significant economic losses in the dairy and cattle industries. However, our understanding of the protective and pathological mechanism underlying host infection is limited.

METHODS

To determine whether BVDV regulates specific activities of the host cell, the expression of long non-coding RNA (lncRNA) during BVDV NADL infection was studied by deep sequencing.

RESULTS

A total of 1236 lncRNA transcripts and 3261 mRNA transcripts were differentially regulated at 2h, 6h, and 18h post-infection. The lncRNAs shared same characteristics with other mammals in terms of exon length, number, expression level, and conservation. The Gene Ontology (GO) enrichment and KEGG pathway analyses showed that lncRNAs regulate immune reaction during BVDV infection. Thirteen differentially expressed genes in 18 hpi were selected and independently validated by reverse-transcription qPCR.

CONCLUSIONS

The present study is the first to provide insights into the biological connection of lncRNAs and BVDV, which can be further explored for the development of antiviral prevention strategies and in understanding persistent infection between viral and host components.

Retention and topology of the bovine viral diarrhea virus glycoprotein E2

Pestiviruses are enveloped viruses that bud intracellularly. They have three envelope glycoproteins, E^rns, E1, and E2. E2 is the receptor binding protein and the main target for neutralizing antibodies. Both E^rns and E2 are retained intracellularly. Here, E2 of the bovine viral diarrhea virus (BVDV) strain CP7 was used to study the membrane topology and intracellular localization of the protein. E2 is localized in the ER and there was no difference between E2 expressed alone or in the context of the viral polyprotein. The mature E2 protein was found to possess a single span transmembrane anchor. For the mapping of a retention signal CD72-E2 fusion proteins, as well as E2 alone were analysed. This confirmed the importance of the transmembrane domain and arginine 355 for intracellular retention, but also revealed a modulating effect on retention through the cytoplasmic tail of the E2 protein, especially through glutamine 370. Mutants with a strong impact on retention were tested in the viral context and we were able to rescue BVDV with certain mutations that in E2 alone impaired intracellular retention and lead to export of E2 to the cells surface.

Improved detection of bovine viral diarrhea virus in bovine lymphoid cell lines using PrimeFlow RNA assay

Bovine viral diarrhea virus (BVDV) infections, whether as acute, persistent or contributing to co-infections, result in significant losses for cattle producers. Although, BVDV can be identified readily by real-time PCR and ELISA, detection and quantification of viral infection at the single cell level is extremely difficult. Detection at the single lymphoid cell level is important due to the immunomodulation that accompanies BVDV infection. A novel PrimeFlow RNA assay using in-situ detection of BVDV was evaluated. The model used to develop this technique included three BL-3 cell lines with different infection statuses, one not infected with BVDV, one infected with BVDV and one dual infected with BVDV and bovine leukosis virus. Using RNA probes specific for the BVDV-2a N^pro-E^rns coding region, BVDV RNA was detected from both contaminated BL-3 cell lines by flow cytometry and fluorescent microscopy. This is the first report on in-situ detection of BVDV at the single-cell level.

Chimeric Pestivirus Experimental Vaccines

Chimeric pestiviruses have shown great potential as marker vaccine candidates against pestiviral infections. Exemplarily, we describe here the construction and testing of the most promising classical swine fever vaccine candidate "CP7_E2alf" in detail. The description is focused on classical cloning technologies in combination with reverse genetics.

Production of Monoclonal Antibody Against Recombinant Polypeptide From the Erns Coding Region of the Bovine Viral Diarrhea Virus

Background:

Bovine viral diarrhea (BVD) is an economically important cattle disease with a worldwide distribution. Detection and elimination of animals persistently infected (PI) with bovine viral diarrhea virus (BVDV) is essential for the control of BVD and eradication of BVDV. There are usually no pathognomonic clinical signs of BVDV infection. Diagnostic investigations therefore rely on laboratory-based detection of the virus, or virus-induced antigens or antibodies.

Objectives:

Erns as an immunogenic protein of BVDV, is genetically and antigenically conserved among different isolates and therefore, is a candidate antigen for development of the enzyme linked immunosorbent assay (ELISA) for serological studies or identification of PI animals. The aim of this study was to produce a monoclonal antibody (MAb) against recombinant Erns.

Materials and Methods:

For this purpose, recombinant maltose-binding protein (MBP)-Erns protein was expressed in Escherichia coli and purified using amylose resin chromatography column and used as an antigen in MAb production. Spleen cells of the immunized mice with the recombinant antigen were fused with SP2/0 myeloma cells. Next, culture supernatants of primary clones of fused cells were screened by indirect ELISA. After three rounds of cloning, the reactivity of the MAbs with recombinant and natural antigen was established by Western blotting.

Results:

Based on our results, MAb against recombinant Erns was produced and reacted successfully with recombinant and natural antigens.

Conclusions:

With regards to the role of Erns in the identification of PI animals, it appears that Erns recombinant antigen and the specific monoclonal antibodies produced against it may be suitable for developing BVDV laboratory diagnostic assays.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Morphogenesis of pestiviruses: new insights from ultrastructural studies of strain Giraffe-1

Knowledge on the morphogenesis of pestiviruses is limited due to low virus production in infected cells. In order to localize virion morphogenesis and replication sites of pestiviruses and to examine intracellular virion transport, a cell culture model was established to facilitate ultrastructural studies. Based on results of virus growth kinetic analysis and quantification of viral RNA, pestivirus strain Giraffe-1 turned out to be a suitable candidate for studies on virion generation and export from culture cells. Using conventional transmission electron microscopy and single-tilt electron tomography, we found virions located predominately in the lumen of the endoplasmic reticulum (ER) in infected cells and were able to depict the budding process of virions at ER membranes. Colocalization of the viral core protein and the envelope glycoprotein E2 with the ER marker protein disulfide isomerase (PDI) was demonstrated by immunogold labeling of cryosections. Moreover, pestivirions could be shown in transport vesicles and the Golgi complex and during exocytosis. Interestingly, viral capsid protein and double-stranded RNA (dsRNA) were detected in multivesicular bodies (MVBs), which implies that the endosomal compartment plays a role in pestiviral replication. Significant cellular membrane alterations such as those described for members of the Flavivirus and Hepacivirus genera were not found. Based on the gained morphological data, we present a consistent model of pestivirus morphogenesis.

A new type of intracellular retention signal identified in a pestivirus structural glycoprotein

Sorting of membrane proteins into intracellular organelles is crucial for cell function. Viruses exploit intracellular transport and retention systems to concentrate envelope proteins at the site of virus budding. In pestiviruses, a group of important pathogens of pigs and ruminants closely related to human hepatitis C virus, the E(rns) protein translated from the viral RNA is secreted from the infected cells and found in the serum of infected animals. Secretion of the protein is regarded as crucial for its function as a viral virulence factor associated with its RNase activity. However, ∼95% of the E(rns) molecules are retained within the infected cell. Fusion of different E(rns) fragments to the C terminus of CD72 allowed identification of a retention signal within the C-terminal 65 aa of the viral protein. This C-terminal sequence represents its membrane anchor and folds into an amphipathic helix binding in-plane to the membrane surface. Residues L183, I190, and L208 are important for intracellular location of E(rns). Presentation of the retention signal on the cytoplasmic instead of the luminal face of the ER membrane in CD8α fusion proteins still led to retention. Thus, E(rns) contains in its C-terminal amphipathic helix an intracellular retention signal that is active on both faces of the membrane.

Molecular characterization of RNA and protein synthesis during a one-step growth curve of bovine viral diarrhoea virus in ovine (SFT-R) cells

The aim of this study was to determine the kinetics of noncytopathic bovine viral diarrhoea virus (BVDV) multiplication and synthesis of BVDV specific RNA and proteins in ovine cells (SFT-R) during a one-step growth curve. The virus titre and RNA level were determined by focus-forming assay and real time RT-PCR. The RNA synthesis was detected by Northern blot while synthesis of E2 and NS3 proteins was assayed by immunohistochemistry and Western blot. The results showed that synthesis of viral RNA is initiated at 4h, NS3 and E2 proteins are detectable at 6-7h and the replication cycle is complete at 10-12h. Additionally, we provide evidence that NS2-3 protein was cleaved in ovine cells early during infection and in proliferated leukocytes of acutely infected sheep. This study showed that synthesis of BVDV RNA and proteins in ovine cells occurs at similar times as found in bovine cells.

Oxalate-induced activation of PKC-alpha and -delta regulates NADPH oxidase-mediated oxidative injury in renal tubular epithelial cells

Oxalate-induced oxidative stress contributes to cell injury and promotes renal deposition of calcium oxalate crystals. However, we do not know how oxalate stimulates reactive oxygen species (ROS) in renal tubular epithelial cells. We investigated the signaling mechanism of oxalate-induced ROS formation in these cells and found that oxalate significantly increased membrane-associated protein kinase C (PKC) activity while at the same time lowering cytosolic PKC activity. Oxalate markedly translocated PKC-alpha and -delta from the cytosol to the cell membrane. Pretreatment of LLC-PK1 cells with specific inhibitors of PKC-alpha or -delta significantly blocked oxalate-induced generation of superoxide and hydrogen peroxide along with NADPH oxidase activity, LDH release, lipid hydroperoxide formation, and apoptosis. The PKC activator PMA mimicked oxalate's effect on oxidative stress in LLC-PK1 cells as well as cytosol-to-membrane translocation of PKC-alpha and -delta. Silencing of PKC-alpha expression by PKC-alpha-specific small interfering RNA significantly attenuated oxalate-induced cell injury by decreasing hydrogen peroxide generation and LDH release. We believe this is the first demonstration that PKC-alpha- and -delta-dependent activation of NADPH oxidase is one of the mechanisms responsible for oxalate-induced oxidative injury in renal tubular epithelial cells. The study suggests that the therapeutic approach might be considered toward attenuating oxalate-induced PKC signaling-mediated oxidative injury in recurrent stone formers.

Formation of bovine viral diarrhea virus E1-E2 heterodimers is essential for virus entry and depends on charged residues in the transmembrane domains

The envelope of bovine viral diarrhea virus (BVDV) contains the glycoproteins Erns, E1 and E2. Complementation of a recombinant vesicular stomatitis virus (VSV) with BVDV glycoproteins resulted in infectious pseudotyped viruses. To elucidate the specific role of each of the single envelope glycoproteins during viral entry, pseudotypes were generated bearing the BVDV envelope proteins in different combinations. Pseudoviruses that contained E1 and E2 but not Erns were infectious, indicating that Erns is dispensable for virus entry. VSV/BVDV pseudotypes with chimeric proteins (the ectodomain of the BVDV glycoprotein and the transmembrane domain of the VSV-G protein) were not infectious. The fact that E1-E2 heterodimers were not detected if one of the proteins was chimeric indicated that the heterodimers are crucial for BVDV entry. It was shown by site-directed mutagenesis that the charged amino acids in the transmembrane domains of BVDV E1 (lysine and arginine) and the charged amino acid in the transmembrane domain of E2 (arginine) play a key role in heterodimer formation. Pseudoviruses bearing the mutation E2-R/A, where the charged amino acid was substituted by alanine, were not infectious, supporting the hypothesis that E1-E2 heterodimers are essential for BVDV entry.

Packaged replicons of bovine viral diarrhea virus are capable of inducing a protective immune response

Bovine viral diarrhea virus (BVDV) replicons with deletions within the capsid, E(RNS) or E1 encoding region were constructed and efficiently packaged with a helper cell line. High titres of packaged replicons were observed as early as 24 h after transfection, whereas no virus progeny could be detected after transfection of non-complementing cells. Infection of bovine cell cultures with rescued viruses resulted in one cycle of replication without release of infectious virus particles, and no genetic reversion of the generated viruses was detected. Packaged replicons with a deletion within the capsid-coding region were characterized in vivo in immunization and challenge trials. Following immunization of calves with the replication-deficient virus, neither virus shedding nor viremia was detected. After challenge infection with virulent BVDV, all vaccinates were completely protected from disease as measured by the absence of viremia and shedding of challenge virus, which indicated that a 'sterilizing immunity' could be induced with the generated replication-deficient packaged replicons.

CP7_E2alf: a safe and efficient marker vaccine strain for oral immunisation of wild boar against Classical swine fever virus (CSFV)

Wild boar are an important reservoir of Classical swine fever virus (CSFV) in several European countries, where most of the primary outbreaks in domestic pigs are directly related to the endemic disease situation in the wild boar population. Oral immunisation has been introduced as an additional control measure to accelerate CSF eradication in wild boar in Germany since 1993. Immunisation with an oral bait vaccine based on the conventionally attenuated live vaccine strain "C" proved to be safe and effective, but does not allow differentiation between infected and vaccinated animals. Therefore, we examined the vaccine efficacy of the recently constructed chimeric pestivirus CP7_E2alf, whose coding sequences for the major envelope protein E2 of BVDV strain CP7 are replaced by E2 of the CSFV strain Alfort187 [Reimann I, Depner K, Trapp S, Beer M. An avirulent chimeric pestivirus with altered cell tropism protects pigs against lethal infection with classical swine fever virus. Virology 2004;322(1):143-57]. Following oral immunisation of wild boar, CP7_E2alf proved to be completely avirulent. Furthermore, all vaccinees were fully protected from clinical disease after a highly virulent CSFV challenge infection. The immunised animals seroconverted within 3 weeks after vaccination for CSFV E2-specific and CSFV neutralising antibodies, whereas prior to challenge infection no antibodies against CSFV E(rns) were detected with an appropriate CSFV-specific marker ELISA test. Thus, the BVDV backbone of CP7_E2alf enables serological and genetic differentiation from wild type CSFV infection. In conclusion, CP7_E2alf represents the first efficient and safe marker vaccine candidate for oral immunisation of wild boar against CSFV.

Brefeldin A inhibits pestivirus release from infected cells, without affecting its assembly and infectivity

The enveloped bovine viral diarrhea virus (BVDV) is a member of the Pestivirus genus within the Flaviviridae family. While considerable information has been gathered on virus entry into the host cell, genome structure and protein function, little is known about pestivirus morphogenesis and release from cells. Here, we analyzed the intracellular localization, N-glycan processing and secretion of BVDV using brefeldin A (BFA), which blocks protein export from the endoplasmic reticulum (ER) and causes disruption of the Golgi complex with subsequent fusion of its cis and medial cisternae with the ER. BFA treatment of infected cells resulted in complete inhibition of BVDV secretion and increased co-localization of the envelope glycoproteins with the cis-Golgi marker GM 130. Processing of the N-linked glycans was affected by BFA, however, virus assembly was not perturbed and intracellular virions were fully infectious, suggesting that trafficking beyond the cis-Golgi is not a prerequisite for pestivirus infectivity.

The surface glycoprotein E2 of bovine viral diarrhoea virus contains an intracellular localization signal

The intracellular transport of the surface glycoprotein E2 of bovine viral diarrhoea virus was analysed by expressing the cloned gene in the absence of other viral proteins. Immunofluorescence analysis and surface biotinylation indicated that E2 is located in an early compartment of the secretory pathway and not transported to the cell surface. In agreement with this result, E2 was found to contain only high-mannose oligosaccharide side-chains but no N-glycans of the complex type. To define the intracellular localization signal of the E2 protein, chimeric proteins were generated. E2 chimeras containing the MT (membrane anchor plus carboxy-terminal domain) of the G protein of vesicular stomatitis virus (VSV) or of the F protein of bovine respiratory syncytial virus (BRSV) were transported to the cell surface. On the other hand, VSV G protein containing the MT domain of E2 was detected only in the ER, indicating that this domain contains an ER localization signal. A chimeric E2 protein, in which not the membrane anchor but only the carboxy-terminal end was replaced by the corresponding domain of the BRSV F protein, was also localized in the ER. Therefore, it was concluded that the membrane anchor contains the ER localization signal of E2. Interestingly, the ER export signal within the VSV G protein cytoplasmic tail was found to overrule the ER localization signal in the E2 protein membrane anchor.

An avirulent chimeric Pestivirus with altered cell tropism protects pigs against lethal infection with classical swine fever virus

A chimeric Pestivirus was constructed using an infectious cDNA clone of bovine viral diarrhea virus (BVDV) [J. Virol. 70 (1996) 8606]. After deletion of the envelope protein E2-encoding region, the respective sequence of classical swine fever virus (CSFV) strain Alfort 187 was inserted in-frame resulting in plasmid pA/CP7_E2alf. After transfection of in vitro-transcribed CP7_E2alf RNA, autonomous replication of chimeric RNA in bovine and porcine cell cultures was observed. Efficient growth of chimeric CP7_E2alf virus, however, could only be demonstrated on porcine cells, and in contrast to the parental BVDV strain CP7, CP7_E2alf only inefficiently infected and propagated in bovine cells. The virulence, immunogenicity, and "marker vaccine" properties of the generated chimeric CP7_E2alf virus were determined in an animal experiment using 27 pigs. After intramuscular inoculation of 1 x 10(7) TCID(50), CP7_E2alf proved to be completely avirulent, and neither viremia nor virus transmission to contact animals was observed; however, CSFV-specific neutralizing antibodies were detected from day 11 after inoculation. In addition, sera from all animals reacted positive in an E2-specific CSFV-antibody ELISA, but were negative for CSFV-E(RNS)-specific antibodies as determined with a CSFV marker ELISA. After challenge infection with highly virulent CSFV strain Eystrup, pigs immunized with CP7_E2alf were fully protected against clinical signs of CSFV infection, viremia, and shedding of challenge virus, and almost all animals scored positive in a CSFV marker ELISA. From our results, we conclude that chimeric CP7_E2alf may not only serve as a tool for a better understanding of Pestivirus attachment, entry, and assembly, but also represents an innocuous and efficacious modified live CSFV "marker vaccine".

Uncleaved NS2-3 is required for production of infectious bovine viral diarrhea virus

Despite increasing characterization of pestivirus-encoded proteins, functions for nonstructural (NS) proteins NS2, NS2-3, NS4B, and NS5A have not yet been reported. Here we investigated the function of bovine viral diarrhea virus (BVDV) uncleaved NS2-3. To test whether NS2-3 has a discrete function, the uncleaved protein was specifically abolished in two ways: first by inserting a ubiquitin monomer between NS2 and NS3, and second by placing an internal ribosome entry site between the two proteins (a bicistronic genome). In both cases, complete processing of NS2-3 prevented infectious virion formation without affecting RNA replication. We tested the hypothesis that uncleaved NS2-3 was involved in morphogenesis by creating a bicistronic genome in which NS2-3 was restored in the second cistron. With this genome, both uncleaved NS2-3 expression and particle production returned. We then investigated the minimal regions of the polyprotein that could rescue an NS2-3 defect by developing a trans-complementation assay. We determined that the expression of NS4A in cis with NS2-3 markedly increased its activity, while p7 could be supplied in trans. Based on these data, we propose a model for NS2-3 action in virion morphogenesis.

Differences in membrane association and sub-cellular distribution between NS2-3 and NS3 of bovine viral diarrhoea virus

The sub-cellular location and mechanism of membrane association of NS3 and NS2-3 polypeptides of bovine viral diarrhoea virus (BVDV) have been examined. Both NS3 and NS2-3 proteins were detected in post-nuclear membrane fractions but not in cytosolic fractions of BVDV infected cells; a proportion of NS3, but not NS2-3, could be dissociated from the membranes with 800 mM KCl or at pH 11. Following extraction with 1% Triton X-114, NS3 was predominantly present in the aqueous phase, but NS2-3 was only recovered in the detergent phase. Confocal microscopy showed that in BVDV infected cells, NS3 and/or NS2-3 co-localise with the endoplasmic reticulum (ER) protein, ERP60, but not Golgi or lysosomal proteins. Sub-cellular fractionation analysis demonstrated that NS2-3 was almost exclusively associated with the rough ER membrane but a significant proportion of NS3 was present in the smooth ER membrane fractions in addition to the rough ER membrane. These differences in the distribution of NS2-3 and NS3 on ER membranes in cells infected with cytopathogenic (CP) strains of BVDV were also observed using confocal microscopy and antibodies that are specific to either NS2 or NS3. This distinct distribution of NS3 and NS2-3 on the ER membrane has revealed a further difference between CP and non-cytopathogenic (NCP) strains of BVDV.

No caspase activation but overexpression of Bcl-2 in bovine cells infected with noncytopathic bovine virus diarrhoea virus

Cytopathic bovine viral diarrhoea viruses (cp BVDV) induce apoptosis in permissible cell cultures via the intrinsic pathway, which involves the mitochondria as key organelles. An important event is the irreversible opening of the permeability transition pore (PTP) and the breakdown of the transmembrane potential DeltaPsi(m). The resulting release of cytochrome C from the mitochondria serves as a trigger to form the apoptosome which then leads to caspase activation and cell death. In contrast, noncytopathic (ncp) BVDV do not seem to affect cells in vivo or in vitro, suggesting that they inhibit apoptosis. Interestingly, inhibition of caspases in cells infected with cp BVDV delayed the apoptotic cascade but did not prevent the cytopathic effect (CPE). This suggests that the induction of apoptosis and the processes finally leading to the CPE may proceed separately, implying that the inhibition of apoptosis by ncp BVDV has to start earlier in the cascade. In this study we show that in fact apoptosis inhibition in cells infected with ncp BVDV must occur at the mitochondrial level, before the activation of the caspase cascade occurs. To elucidate the role of mitochondria after infection of cells with ncp BVDV, expression of Bcl-2 and Bax were analysed. It was shown that while Bax expression was not affected, the anti-apoptotic Bcl-2 protein was upregulated, presumably suppressing initiation of cell death and enabling persistent infection in vitro.

Bovine viral diarrhea virus as a surrogate model of hepatitis C virus for the evaluation of antiviral agents

The identification and development of new antiviral agents that can be used to combat hepatitis C virus (HCV) infection has been complicated by both technical and logistic issues. There are few, if any, robust methods by which HCV virions can be grown in vitro. The development of HCV RNA replicons has been a great breakthrough that has allowed for the undertaking of significant screening efforts to identify inhibitors of HCV intracellular replication. However, since replicons do not undergo a complete replication cycle, drug screening programs and mechanism of action studies based solely on these assays will not identify compounds targeting either early (virion attachment, entry, uncoating) or late (virion assembly, egress) stages of the viral replication cycle. Drugs that negatively affect the infectivity of new virions will also not be identified using HCV RNA replicons. Bovine viral diarrhea virus (BVDV) shares a similar structural organization with HCV, and both viruses generally cause chronic long-term infections in their respective hosts. The BVDV surrogate model is attractive, since it is a virus-based system. It is easy to culture the virus in vitro, molecular clones are available for genetic studies, and the virus undergoes a complete replication cycle. Like HCV, BVDV utilizes the LDL receptor to enter cells, uses a functionally similar internal ribosome entry site (IRES) for translation, uses an NS4A cofactor with its homologous NS3 protease, has a similar NS3 helicase/NTPase, a mechanistically similar NS5B RNA-dependent RNA polymerase, and a seemingly equivalent mechanism of virion maturation, assembly and egress. While the concordance between drugs active in either BVDV or HCV is largely unknown at this time, BVDV remains a popular model system with which drugs can be evaluated for potential antiviral activity against HCV and in studies of drug mechanism of action.

The pestivirus Erns glycoprotein interacts with E2 in both infected cells and mature virions

E(rns) is a pestivirus envelope glycoprotein indispensable for virus attachment and infection of target cells. Unlike the other two envelope proteins E1 and E2, E(rns) lacks a transmembrane domain and a vast quantity is secreted into the medium of infected cells. The protein is also present in fractions of pure pestivirus virions, raising the important and intriguing question regarding the mechanism of its attachment to the pestivirus envelope. In this study a direct interaction between E(rns) and E2 glycoproteins was demonstrated in both pestivirus-infected cells and mature virions. By co- and sequential immunoprecipitation we showed that an E(rns)-E2 heterodimer is assembled very early after translation of the viral polyprotein and before its processing is completed. Our results suggest that E(rns) is attached to the pestivirus envelope via a direct interaction with E2 and explain the role of E(rns) in the initial virus-target cell interaction.

Trans-complementation of autonomously replicating Bovine viral diarrhea virus replicons with deletions in the E2 coding region

Autonomously replicating Bovine viral diarrhea virus (BVDV) genomes (replicons) were constructed from the full-length BVDV cDNA clone pA/BVDV/Ins- (G. Meyers et al., J. Virol. 70, 8606-8613, 1996). The sequences coding for envelope protein E2, for E2 without the C-terminal transmembrane region, or for E2 and nonstructural protein p7 were deleted, and the resulting mutants were tested for their ability to replicate after transfection. All deletion mutants were able to replicate and to express the inserted green fluorescent protein but did not produce infectious progeny virus in bovine kidney PT cells. The replicons were also tested for their ability to be trans-complemented in the bovine cell line PT_805, which constitutively expresses BVDV structural proteins. E2-negative BVDV mutants were complemented and >10(6) infectious units were obtained at 24 h after transfection. Complementing PT_805 cells could only inefficiently be infected using trans-complemented virions, however, and low levels of virus production were observed when complemented BVDV was passaged using PT_805 cells. Similarly, infection of PT_805 cells with BVDV was highly inefficient, but transfection of full-length BVDV NCP7 RNA into PT_805 resulted in 10,000-fold higher virus titers when compared to those obtained 24 h after transfection of parental PT cells. We concluded that self-replicating E2-deleted BVDV RNAs can be efficiently trans-complemented by constitutively expressed E2, and that expression of BVDV structural proteins markedly influences susceptibility of cells to BVDV infection as well as BVDV titers after transfection of full-length BVDV RNA.