Time course of virus-specific macromolecular synthesis during rubella virus infection in Vero cells

Dissecting Rubella Placental Infection in an In Vitro Trophoblast Model

Vertical transmission of rubella virus (RuV) occurs at a high rate during the first trimester of pregnancy. The modes of vertical transmission including the response of trophoblasts to RuV are not well understood. Here, RuV-trophoblast interaction was studied in the BeWo trophoblast cell line. Analysis included early and late time-point kinetics of virus infection rate and the antiviral innate immune response at mRNA and protein level. BeWo characteristics were addressed through metabolic activity by extracellular flux analysis and syncytiotrophoblast formation through incubation with forskolin. We found that RuV infection of BeWo led to profuse type III interferon (IFN) production. Transfecting trophoblast cells with dsRNA analog induced an increase in the production of type I IFN-β and type III IFNs; however, this did not occur in RuV-infected BeWo trophoblasts. IFN-β and to a lesser extent type III IFN-λ1 were inhibitory to RuV. While no significant metabolic alteration was detected, RuV infection reduced the cell number in the monolayer culture in comparison to the mock control and resulted in detached and floating cells. Syncytia formation restricted RuV infection. The use of BeWo as a relevant cell culture model for infection of trophoblasts highlights cytopathogenicity in the absence of a type I IFN response as a pathogenic alteration by RuV.

Exogenous Rubella Virus Capsid Proteins Enhance Virus Genome Replication

Enhanced replication of rubella virus (RuV) and replicons by de novo synthesized viral structural proteins has been previously described. Such enhancement can occur by viral capsid proteins (CP) alone in trans. It is not clear whether the CP in the virus particles, i.e., the exogenous CP, modulate viral genome replication. In this study, we found that exogenous RuV CP also enhanced viral genome replication, either when used to package replicons or when mixed with RNA during transfection. We demonstrated that CP does not affect the translation efficiency from genomic (gRNA) or subgenomic RNA (sgRNA), the intracellular distribution of the non-structural proteins (NSP), or sgRNA synthesis. Significantly active RNA replication was observed in transfections supplemented with recombinant CP (rCP), which was supported by accumulated genomic negative-strand RNA. rCP was found to restore replication of a few mutants in NSP but failed to fully restore replicons known to have defects in the positive-strand RNA synthesis. By monitoring the amount of RuV RNA following transfection, we found that all RuV replicon RNAs were well-retained in the presence of rCP within 24 h of post-transfection, compared to non-RuV RNA. These results suggest that the exogenous RuV CP increases efficiency of early viral genome replication by modulating the stage(s) prior to and/or at the initiation of negative-strand RNA synthesis, possibly through a general mechanism such as protecting viral RNA.

Improved diagnostic and multiplex RT-qPCR for detecting rubella viral RNA

An examination of the nucleic acid sequence alignment of 48 full-length rubella virus genomes revealed that the 5' terminus of the genome is more conserved than the commonly used detection windows for rubella virus RNA located in the E1 protein coding region, suggesting that the 5' terminus could be a target for improving detection of all rubella virus genotypes. Two candidate primer sets were tested and the window between nucleotides (nts) 98 and 251 was found to have the greatest analytical sensitivity for detection of different genotypes. The new method had a limit of detection of four copies of rubella RNA per reaction with high specificity. The average coefficient variation of Ct was 2.2%. Concordance between the new method and currently used method, based on testing 251 clinical specimens collected from a rubella outbreak, was 99.4%. The assay was further improved upon by the incorporation of detection of both rubella virus RNA and mRNA from a cellular reference gene in a multiplex format. The multiplex format did not reduce the sensitivity or the reproducibility of rubella RNA detection and, of 60 specimens tested, the concordance between the single target and multiplex assays was 85.0%. To assess the utility of the multiplex assay for molecular surveillance, 62 rubella IgM positive serum samples from a rubella outbreak were tested, and eleven tested positive using the multiplex method while none were positive using the method targeting E1. These results show that the assay based on the new detection window near the 5' terminus of the genome can improve the detection of rubella virus for the purpose of molecular surveillance and case confirmation, with the added benefit of improved efficiency due to multiplexing.

Low Susceptibility of Rubella Virus in First-Trimester Trophoblast Cell Lines

We recently published an article about myelin oligodendrocyte glycoprotein-independent rubella infection of keratinocytes in vitro, in which first-trimester trophoblast cells were shown as rubella virus (RuV)-resistant. Given an incident rate as high as 90% of congenital rubella syndrome in the first eight weeks of pregnancy, the RuV infection of first-trimester trophoblasts is considered key to opening the gate to transplacental transmission mechanisms. Therefore, with this study, we aimed to verify the susceptibility/resistance of first-trimester trophoblast cell lines, HTR-8/SVneo and Swan.71, against RuV. Cells cultured on multi-well plates were challenged with a RuV clinical strain at a multiplicity of infection from 5 to 10 for 3 h. The infectivity was investigated by immunofluorescence (IF) assay and flow cytometry (FCM) analysis. Supernatants collected during the post-infection period were used to determine virus-progeny production. The scattered signaling of RuV infection of these cells was noted by IF assay, and the FCM analysis showed an average of 4–5% of gated cells infected with RuV. In addition, a small but significant production of virus progeny was also observed. In conclusion, by employing appropriate approaches, we determined the low infectivity of RuV in first-trimester trophoblast cell lines but not resistance as in our previous report.

Role of Inflammation in Virus Pathogenesis during Pregnancy

Viral infections during pregnancy lead to a spectrum of maternal and fetal outcomes, ranging from asymptomatic disease to more critical conditions presenting with severe maternal morbidity, stillbirth, preterm birth, intrauterine growth restriction, and fetal congenital anomalies, either apparent at birth or later in life. In this article, we review the pathogenesis of several viral infections that are particularly relevant in the context of pregnancy and intrauterine inflammation. Understanding the diverse mechanisms employed by viral pathogens as well as the repertoire of immune responses induced in the mother may help to establish novel therapeutic options to attenuate changes in the maternal-fetal interface and prevent adverse pregnancy outcomes.

Molecular aspects of the teratogenesis of rubella virus

Rubella or German measles is an infection caused by rubella virus (RV). Infection of children and adults is usually characterized by a mild exanthematous febrile illness. However, RV is a major cause of birth defects and fetal death following infection in pregnant women. RV is a teratogen and is a major cause of public health concern as there are more than 100,000 cases of congenital rubella syndrome (CRS) estimated to occur every year. Several lines of evidence in the field of molecular biology of RV have provided deeper insights into the teratogenesis process. The damage to the growing fetus in infected mothers is multifactorial, arising from a combination of cellular damage, as well as its effect on the dividing cells. This review focuses on the findings in the molecular biology of RV, with special emphasis on the mitochondrial, cytoskeleton and the gene expression changes. Further, the review addresses in detail, the role of apoptosis in the teratogenesis process.

Rubella Viruses Shift Cellular Bioenergetics to a More Oxidative and Glycolytic Phenotype with a Strain-Specific Requirement for Glutamine

The flexible regulation of cellular metabolic pathways enables cellular adaptation to changes in energy demand under conditions of stress such as posed by a virus infection. To analyze such an impact on cellular metabolism, rubella virus (RV) was used in this study. RV replication under selected substrate supplementation with glucose, pyruvate, and glutamine as essential nutrients for mammalian cells revealed its requirement for glutamine. The assessment of the mitochondrial respiratory (based on the oxygen consumption rate) and glycolytic (based on the extracellular acidification rate) rate and capacity by respective stress tests through Seahorse technology enabled determination of the bioenergetic phenotype of RV-infected cells. Irrespective of the cellular metabolic background, RV infection induced a shift of the bioenergetic state of epithelial cells (Vero and A549) and human umbilical vein endothelial cells to a higher oxidative and glycolytic level. Interestingly there was a RV strain-specific, but genotype-independent demand for glutamine to induce a significant increase in metabolic activity. While glutaminolysis appeared to be rather negligible for RV replication, glutamine could serve as donor of its amide nitrogen in biosynthesis pathways for important metabolites. This study suggests that the capacity of RVs to induce metabolic alterations could evolve differently during natural infection. Thus, changes in cellular bioenergetics represent an important component of virus-host interactions and could complement our understanding of the viral preference for a distinct host cell population.IMPORTANCE RV pathologies, especially during embryonal development, could be connected with its impact on mitochondrial metabolism. With bioenergetic phenotyping we pursued a rather novel approach in virology. For the first time it was shown that a virus infection could shift the bioenergetics of its infected host cell to a higher energetic state. Notably, the capacity to induce such alterations varied among different RV isolates. Thus, our data add viral adaptation of cellular metabolic activity to its specific needs as a novel aspect to virus-host evolution. In addition, this study emphasizes the implementation of different viral strains in the study of virus-host interactions and the use of bioenergetic phenotyping of infected cells as a biomarker for virus-induced pathological alterations.

Gene expression profiling of rubella virus infected primary endothelial cells of fetal and adult origin

BACKGROUND

Rubella virus (RV) infection is usually a mild illness in children and adults. However, maternal infection during the first trimester of pregnancy can lead to congenital rubella syndrome (CRS) in the infant. Fetuses with CRS show damage to the endothelium of the heart and blood vessels; thus, it has been speculated that the clinical manifestations associated with CRS may be a result of endothelial cells persistently infected with RV. Here, we compared the effects of RV infection on gene expression in primary endothelial cells of fetal (HUVEC) and of adult (HSaVEC) origin by transcriptional profiling.

RESULTS

More than 75 % of the genes differentially regulated following RV infection were identical in both cell types. Gene Ontology (GO) analysis of these commonly regulated genes showed an enrichment of terms involved in cytokine production and cytokine regulation. Increased accumulation of inflammatory cytokines following RV infection was verified by protein microarray. Interestingly, the chemokine CCL14, which is implicated in supporting embryo implantation at the fetal-maternal interface, was down-regulated following RV infection only in HUVEC. Most noticeably, when analyzing the uniquely regulated transcripts for each cell type, GO term-based cluster analysis of the down-regulated genes of HUVEC revealed an enrichment of the GO terms "sensory organ development", "ear development" and "eye development".

CONCLUSION

Since impairment in vision and hearing are the most prominent clinical manifestations observed in CRS patients, the here detected down-regulated genes involved in the development of sensory organs sheds light on the molecular mechanisms that may contribute to the teratogenic effect of RV.

[The life cycle of Rubella Virus]

Rubella virus (RV), an infectious agent of rubella, is the sole member of the genus Rubivirus in the family of Togaviridae. RV has a positive-stranded sense RNA as a genome. A natural host of RV is limited to human, and rubella is considered to be a childhood disease in general. When woman is infected with RV during early pregnancy, her fetus may develop severe birth defects known as congenital rubella syndrome. In this review, the RV life cycle from the virus entry to budding is illustrated in comparison with those of member viruses of the genus alphavirus in the same family. The multiple functions of the RV capsid protein are also introduced.

Persistent infection of human fetal endothelial cells with rubella virus

Cardiovascular abnormalities are the leading cause of neonatal death among patients with congenital rubella syndrome (CRS). Although persistence of rubella virus (RV) in fetal endothelium has been repeatedly suggested as a possible cause of cardiovascular birth defects, evidence of the permissiveness of fetal endothelial cells to RV is lacking. In this study we evaluated the ability of RV to infect and persist in primary fetal endothelial cells derived from human umbilical vein (HUVEC). We found that wild type (wt) low passage clinical RV productively infected HUVEC cultures without producing cytopathology or ultrastructural changes. RV did not inhibit host cell protein synthesis, cell proliferation, or interfere with the cell cycle. Persistently infected cultures were easily established at low and high multiplicities of infection (MOI) with both laboratory and wt clinical RV strains. However, synchronous infections of entire HUVEC monolayers were only observed with clinical RV strains. The release of infectious virions into media remained at consistently high levels for several subcultures of infected HUVEC. The results indicate that macrovascular fetal endothelial cells are highly permissive to RV and allow slow persistent RV replication. The findings provide more evidence for the suggestion that vascular pathologies in CRS are triggered by persistent rubella virus infection of the endothelium.

Binding of cellular p32 protein to the rubella virus P150 replicase protein via PxxPxR motifs

A proline-rich region (PRR) within the rubella virus (RUBV) P150 replicase protein that contains three SH3 domain-binding motifs (PxxPxR) was investigated for its ability to bind cell proteins. Pull-down experiments using a glutathione S-transferase-PRR fusion revealed PxxPxR motif-specific binding with human p32 protein (gC1qR), which could be mediated by either of the first two motifs. This finding was of interest because p32 protein also binds to the RUBV capsid protein. Binding of p32 to P150 was confirmed and was abolished by mutation of the first two motifs. When mutations in the first two motifs were introduced into a RUBV cDNA infectious clone, virus replication was significantly impaired. However, virus RNA synthesis was found to be unaffected, and subsequent immunofluorescence analysis of RUBV-infected cells revealed co-localization of p32 and P150 but little overlap of p32 with RNA replication complexes, indicating that p32 does not participate directly in virus RNA synthesis. Thus, the role of p32 in RUBV replication remains unresolved.

Analysis of subcellular G3BP redistribution during rubella virus infection

Rubella virus (RUBV) replicates slowly and to low titre in vertebrate cultured cells, with minimal cytopathology. To determine whether a cellular stress response is induced during such an infection, the formation of Ras-GAP-SH3 domain-binding protein (G3BP)-containing stress granules (SGs) in RUBV-infected cells was examined. Late in infection, accumulation of G3BP granules was detected, albeit in fewer than half of infected cells. Active virus RNA replication was required for induction of these granules, but they were found to differ from SGs induced by arsenite treatment both in composition (they did not uniformly contain other SG proteins, such as PABP and TIA-1) and in resistance to cycloheximide treatment. Thus, bona fide SGs do not appear to be induced during RUBV infection. The distribution of G3BP, either on its own or in granules, did not overlap with that of dsRNA-containing replication complexes, indicating that it played no role in virus RNA synthesis. However, G3BP did co-localize with viral ssRNAs in perinuclear clusters, suggesting an interaction that could possibly be important in a post-replicative role in virus replication, such as encapsidation.

Identification of the myelin oligodendrocyte glycoprotein as a cellular receptor for rubella virus

Rubella virus (RV) is a highly transmissible pathogenic agent that causes the disease rubella. Maternal RV infection during early pregnancy causes the death of the fetus or congenital rubella syndrome in infants. However, the cellular receptor for RV has not yet been identified. In this study, we found that the myelin oligodendrocyte glycoprotein (MOG) specifically bound to the E1 envelope glycoprotein of RV, and an antibody against MOG could block RV infection. Most importantly, we also showed that ectopic expression of MOG on the cell surface of 293T cells rendered this nonpermissive cell line permissive for RV entry and replication. Thus, this study has identified a cellular receptor for RV and suggests that blocking the MOG attachment site of RV may be a strategy for molecular intervention of RV infection.

The Rubella virus capsid is an anti-apoptotic protein that attenuates the pore-forming ability of Bax

Apoptosis is an important mechanism by which virus-infected cells are eliminated from the host. Accordingly, many viruses have evolved strategies to prevent or delay apoptosis in order to provide a window of opportunity in which virus replication, assembly and egress can take place. Interfering with apoptosis may also be important for establishment and/or maintenance of persistent infections. Whereas large DNA viruses have the luxury of encoding accessory proteins whose primary function is to undermine programmed cell death pathways, it is generally thought that most RNA viruses do not encode these types of proteins. Here we report that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis. Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur. Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation. Importantly, data from reverse genetic studies are consistent with a scenario in which the anti-apoptotic activity of capsid protein is important for virus replication. If so, this would be among the first demonstrations showing that blocking apoptosis is important for replication of an RNA virus. Finally, it is tempting to speculate that other slowly replicating RNA viruses employ similar mechanisms to avoid killing infected cells.

Among the variety of defense systems employed by mammalian cells to combat virus infection, apoptosis or programmed cell death is the most drastic response. Some large DNA viruses encode proteins whose sole function is to block apoptosis. Conversely, very little is known about whether RNA viruses encode analogous proteins. In many cases, RNA viruses are able to replicate before cell death occurs, which may be one reason why so little thought has been given to this topic. However, a number of RNA viruses, some of which are important human pathogens, have slow replication cycles and it stands to reason that they must block apoptosis during this time period. Here we show that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. Data from reverse genetic experiments suggest that the anti-apoptotic function of a virus-encoded protein is important for replication of an RNA virus. We anticipate that other slowly replicating RNA viruses may employ similar mechanisms and, as such, these studies have implications for development of novel anti-virals and vaccines.

Rubella virus capsid protein: a small protein with big functions

Virus replication occurs in the midst of a life or death struggle between the virus and the infected host cell. To limit virus replication, host cells can activate a number of antiviral pathways, the most drastic of which is programmed cell death. Whereas large DNA viruses have the luxury of encoding accessory proteins whose main function is to interfere with host cell defences, the genomes of RNA viruses are not large enough to encode proteins of this type. Recent studies have revealed that proteins encoded by RNA viruses often play multiple roles in the battles between viruses and host cells. In this article, we discuss the many functions of the rubella virus capsid protein. This protein has well-defined roles in virus assembly, but recent research suggests that it also functions to modulate virus replication and block host cell defences.

Predominant inflammatory cytokine secretion pattern in response to two doses of live rubella vaccine in healthy vaccinees

We conducted a population-based study on 738 schoolchildren who received two doses of rubella vaccine in order to determine cytokine secretion patterns and their associations with demographic and clinical variables. The results showed a robust rubella-specific inflammatory cytokine response characterized by high median [inter-quartile range (IQR)] secretion levels (in pg/mL) of IL-6 [3681.0 (3160.0, 4052.0)], GM-CSF [28.0 (23.6, 32.6)], and TNF-alpha [29.7 (-7.0, 89.2)]. We also detected modest levels of rubella-specific secretion of Th1 cytokines IL-2 and IFN-gamma, while IL-12p40 was undetectable. In contrast, rubella-specific Th2 responses were hardly detectable. Age at vaccination, enrollment, and time elapsed between last vaccination and enrollment was significantly associated with the outcome of IL-2, IL-6, and IFN-gamma secretion. These results suggest an immune-deviation or "skewing" from Th1/Th2 cytokine patterns towards a predominant inflammatory response upon in vitro rubella virus stimulation.

Rubella virus capsid protein interacts with poly(a)-binding protein and inhibits translation

During virus assembly, the capsid proteins of RNA viruses bind to genomic RNA to form nucleocapsids. However, it is now evident that capsid proteins have additional functions that are unrelated to nucleocapsid formation. Specifically, their interactions with cellular proteins may influence signaling pathways or other events that affect virus replication. Here we report that the rubella virus (RV) capsid protein binds to poly(A)-binding protein (PABP), a host cell protein that enhances translational efficiency by circularizing mRNAs. Infection of cells with RV resulted in marked increases in the levels of PABP, much of which colocalized with capsid in the cytoplasm. Mapping studies revealed that capsid binds to the C-terminal half of PABP, which interestingly is the region that interacts with other translation regulators, including PABP-interacting protein 1 (Paip1) and Paip2. The addition of capsid to in vitro translation reaction mixtures inhibited protein synthesis in a dose-dependent manner; however, the capsid block was alleviated by excess PABP, indicating that inhibition of translation occurs through a stoichiometric mechanism. To our knowledge, this is the first report of a viral protein that inhibits protein translation by sequestration of PABP. We hypothesize that capsid-dependent inhibition of translation may facilitate the switch from viral translation to packaging RNA into nucleocapsids.

Analysis of gene expression in fetal and adult cells infected with rubella virus

Congenital infection with rubella virus (RUB) leads to persistent infection and congenital defects and we showed previously that primary human fetal fibroblasts did not undergo apoptosis when infected with RUB, which could promote fetal virus persistence [Adamo, P., Asís, L., Silveyra, P., Cuffini, C., Pedranti, M., Zapata, M., 2004. Rubella virus does not induce apoptosis in primary human embryo fibroblasts cultures: a possible way of viral persistence in congenital infection. Viral Immunol. 17, 87-100]. To extend this observation, gene chip analysis was performed on a line of primary human fetal fibroblasts (10 weeks gestation) and a line of human adult lung fibroblasts (which underwent apoptosis in response to RUB infection) to compare gene expression in infected and uninfected cells. A total of 632 and 516 genes were upregulated or downregulated in the infected fetal and adult cells respectively in comparison to uninfected cells, however only 52 genes were regulated in both cell types. Although the regulated genes were different, across functional gene categories the patterns of gene regulation were similar. In general, regulation of pro- and anti-apoptotic genes following infection appeared to favor apoptosis in the adult cells and lack of apoptosis in the fetal cells, however there was a greater relative expression of anti-apoptotic genes and reduced expression of pro-apoptotic genes in uninfected fetal cells versus uninfected adult cells and thus the lack of apoptosis in fetal cells following RUB infection was also due to the prevailing background of gene expression that is antagonistic to apoptosis. In support of this hypothesis, it was found that of a battery of five chemicals known to induce apoptosis, two induced apoptosis in the adult cells, but not in fetal cells, and two induced apoptosis more rapidly in the adult cells than in fetal cells (the fifth did not induce apoptosis in either). A robust interferon-stimulated gene response was induced following infection of both fetal and adult cells and many of the genes upregulated in both cell types were those involved in establishment of an antiviral state; this is the first demonstration of an interferon response at this early stage of human embryonic development. In both fetal and adult cells, interferon controlled but did not eliminate virus spread and apoptosis was not induced in infected fetal cells in the absence of interferon. In addition to the interferon response, chemokines were induced in both infected fetal and adult cells. Thus, it is possible that fetal damage following congenital RUB infection, which involves cell proliferation and differentiation, could be due to induction of the innate immune response as well as frank virus infection.

Rubella virus pseudotypes and a cell-cell fusion assay as tools for functional analysis of the rubella virus E2 and E1 envelope glycoproteins

The rubivirus Rubella virus contains the two envelope glycoproteins E2 and E1 as a heterodimeric spike complex embedded in its lipid envelope. The functions of both proteins, especially of E2, in the process of viral entry are still not entirely understood. In order to dissect E2 and E1 entry functions from post-entry steps, pseudotypes of lentiviral vectors based on Simian immunodeficiency virus were used. C-terminally modified E2 and E1 variants successfully pseudotyped lentiviral vector particles. This is the first report to show that not only E1, but also E2, is able to mediate infectious viral entry. Furthermore, a cell-cell fusion assay was used to further clarify membrane-fusion activities of E2 and E1 as one of the early steps of infection. It was demonstrated that the capsid protein, when coexpressed in cis, enhances the degree of E2- and E1-mediated cell-cell fusion.

Analyses of phosphorylation events in the rubella virus capsid protein: role in early replication events

The Rubella virus capsid protein is phosphorylated prior to virus assembly. Our previous data are consistent with a model in which dynamic phosphorylation of the capsid regulates its RNA binding activity and, in turn, nucleocapsid assembly. In the present study, the process of capsid phosphorylation was examined in further detail. We show that phosphorylation of serine 46 in the RNA binding region of the capsid is required to trigger phosphorylation of additional amino acid residues that include threonine 47. This residue likely plays a direct role in regulating the binding of genomic RNA to the capsid. We also provide evidence which suggests that the capsid is dephosphorylated prior to or during virus budding. Finally, whereas the phosphorylation state of the capsid does not directly influence the rate of synthesis of viral RNA and proteins or the assembly and secretion of virions, the presence of phosphate on the capsid is critical for early events in virus replication, most likely the uncoating of virions and/or disassembly of nucleocapsids.

Evaluation of cis-acting elements in the rubella virus subgenomic RNA that play a role in its translation

The subgenomic (SG) mRNA of rubella virus (RUB) contains the structural protein open reading frame (SP-ORF) that is translated to produce the three virion structural proteins: capsid (C) and glycoproteins E2 and E1. RUB expression vectors have been developed that express heterologous genes from the SG RNA, including replicons which replace the SP-ORF with a heterologous gene, and these expression vectors are candidate vaccine vectors. In the related alphaviruses, translational enhancing elements have been identified in both the 5' untranslated region (UTR) of the SG RNA and the N-terminal region of the C gene. To optimize expression from RUB vectors, both the 5'UTR of the SG RNA and the C gene were surveyed for translational enhancing elements using both plasmids and replicons expressing reporter genes from the SG RNA. In replicons, the entire 5'UTR was necessary for translation; interestingly, when plasmids were used the 5'UTR was dispensable for optimal translation. The RUB C gene contains a predicted long stem-loop starting 62 nts downstream from the initiation codon (SLL) that has a structure and stability similar to SL's found in the C genes of two alphaviruses, Sindbis virus (SIN) and Semliki Forest virus, that have been shown to enhance translation of the SG RNA in infected cells. However, a series of fusions of various lengths of the N-terminus of the RUB C protein with reporter genes showed that the SLL had an attenuating effect on translation that was overcome by mutagenesis that destabilized the SLL or by adding downstream sequences of the C gene to the fusion. Thus, for optimal expression efficiency from RUB expression vectors, only the 5'UTR of the SG RNA is required. Further investigation of the differing effects of the SLL on RUB and alphavirus SG RNA translation revealed that the SIN and RUB SLLs could enhance translation when expressed from a SIN cytopathic replicon, but not when expressed from a plasmid, a RUB replicon, or a SIN noncytopathic replicon. Thus, the SLL only functions in a "cytopathic environment" in which cell translation has been altered.

Interactions between rubella virus capsid and host protein p32 are important for virus replication

The distribution and morphology of mitochondria are dramatically affected during infection with rubella virus (RV). Expression of the capsid, in the absence of other viral proteins, was found to induce both perinuclear clustering of mitochondria and the formation of electron-dense intermitochondrial plaques, both hallmarks of RV-infected cells. We previously identified p32, a host cell mitochondrial matrix protein, as a capsid-binding protein. Here, we show that two clusters of arginine residues within capsid are required for stable binding to p32. Mutagenic ablation of the p32-binding site in capsid resulted in decreased mitochondrial clustering, indicating that interactions with this cellular protein are required for capsid-dependent reorganization of mitochondria. Recombinant viruses encoding arginine-to-alanine mutations in the p32-binding region of capsid exhibited altered plaque morphology and replicated to lower titers. Further analysis indicated that disruption of stable interactions between capsid and p32 was associated with decreased production of subgenomic RNA and, consequently, infected cells produced significantly lower amounts of viral structural proteins under these conditions. Together, these results suggest that capsid-p32 interactions are important for nonstructural functions of capsid that include regulation of virus RNA replication and reorganization of mitochondria during infection.

Rubella virus capsid protein modulation of viral genomic and subgenomic RNA synthesis

The ratio of the subgenomic (SG) to genome RNA synthesized by rubella virus (RUB) replicons expressing the green fluorescent protein reporter gene (RUBrep/GFP) is substantially higher than the ratio of these species synthesized by RUB (4.3 for RUBrep/GFP vs. 1.3-1.4 for RUB). It was hypothesized that this modulation of the viral RNA synthesis was by one of the virus structural protein genes and it was found that introduction of the capsid (C) protein gene into the replicons as an in-frame fusion with GFP resulted in an increase of genomic RNA production (reducing the SG/genome RNA ratio), confirming the hypothesis and showing that the C gene was the moiety responsible for the modulation effect. The N-terminal one-third of the C gene was required for the effect of be exhibited. A similar phenomenon was not observed with the replicons of Sindbis virus, a related Alphavirus. Interestingly, modulation was not observed when RUBrep/GFP was co-transfected with either other RUBrep or plasmid constructs expressing the C gene, demonstrating that modulation could occur only when the C gene was provided in cis. Mutations that prevented translation of the C protein failed to modulate RNA synthesis, indicating that the C protein was the moiety responsible for modulation; consistent with this conclusion, modulation of RNA synthesis was maintained when synonymous codon mutations were introduced at the 5' end of the C gene that changed the C gene sequence without altering the amino acid sequence of the C protein. These results indicate that C protein translated in proximity of viral replication complexes, possibly from newly synthesized SG RNA, participate in regulating the replication of viral RNA.

Rubella virus capsid protein modulates viral genome replication and virus infectivity

The structural proteins (SP) of the Togaviridae can be deleted in defective interfering RNAs. The dispensability of viral SP has allowed construction of noninfectious viral expression vectors and replicons from viruses of the Alphavirus and Rubivirus genera. Nevertheless, in this study, we found that the SP of rubella virus (RUB) could enhance expression of reporter genes from RUB replicons in trans. SP enhancement required capsid protein (CP) expression and was not due to RNA-RNA recombination. Accumulation of minus- and plus-strand RNAs from replicons was observed in the presence of SP, suggesting that SP specifically affects RNA synthesis. By using replicons containing an antibiotic resistance gene, we found 2- to 50-fold increases in the number of cells surviving selection in the presence of SP. The increases depended significantly on the amount of transfected RNA. Small amounts of RNA or templates that replicated inefficiently showed more enhancement. The infectivity of infectious RNA was increased by at least 10-fold in cells expressing CP. Moreover, virus infectivity was greatly enhanced in such cells. In other cells that expressed higher levels of CP, RNA replication of replicons was inhibited. Thus, depending on conditions, CP can markedly enhance or inhibit RUB RNA replication.

Structural maturation of rubella virus in the Golgi complex

Rubella virus (RUB) assembles its replication complexes (RCs) in modified organelles of endo‐lysosomal origin, known as cytopathic vacuoles (CPVs). These peculiar structures are key elements of RUB factories, where rough endoplasmic reticulum, mitochondria, and Golgi are recruited. Bicistronic RUB replicons expressing an antibiotic resistance gene either in the presence or the absence of the RUB capsid (C) gene were used to study the structure of RCs in transfected cells. Confocal microscopy showed that the RUB replicase components P90 and P150 localized to CPVs, as did double‐stranded RNA (dsRNA), a marker for RNA synthesis. Electron microscopy (EM) showed that replicons generated CPVs containing small vesicles and large vacuoles, similar to CPVs from RUB‐infected cells and that the replicase proteins were sufficient for organelle recruitment. Some of these CPVs contained straight membranes. When cross‐sectioned, these rigid membranes appeared to be sheets of closely packed proteins. Immuno‐EM revealed that these sheets, apparently in contact with the cytosol, contained both P150 and P90, as well as dsRNA, and thus could be two‐dimensional arrays of functional viral replicases. Labelling of dsRNA after streptolysin‐O permeabilization showed that replication of viral genome takes place on the cytoplasmic side of CPVs. When present, C accumulated around CPVs. Mitochondrial protein P32 was detected within modified CPVs, the first demonstration of involvement of this protein, which interacts with C, with RCs.

Structural maturation of rubella virus in the Golgi complex

Rubella virus is a small enveloped virus that assembles in association with Golgi membranes. Freeze-substitution electron microscopy of rubella virus-infected cells revealed a previously unrecognized virion polymorphism inside the Golgi stacks: homogeneously dense particles without a defined core coexisting with less dense, mature virions that contained assembled cores. The homogeneous particles appear to be a precursor form during the virion morphogenesis process as the forms with mature morphology were the only ones detected inside secretory vesicles and on the exterior of cells. In mature virions potential remnants of C protein membrane insertion were visualized as dense strips connecting the envelope with the internal core. In infected cells Golgi stacks were frequently seen close to cytopathic vacuoles, structures identified as the sites for viral RNA replication, along with the rough endoplasmic reticulum and mitochondria. These associations could facilitate the transfer of viral genomes from the cytopathic vacuoles to the areas of rubella assembly in Golgi membranes.

Rescue of rubella virus replication-defective mutants using vaccinia virus recombinant expressing rubella virus nonstructural proteins

The genome of rubella virus (RV) is translated into a polyprotein precusor, p200, of the nonstructural proteins (NSPs). This is proteolytically processed by a viral-encoded protease into two mature products, p150 and p90. p150 contains sequence corresponding to the predicted methyltransferase and protease activities, while p90 has sequence for the proposed helicase and RNA-dependent RNA polymerase activities. Processing of p200 is essential for RV viral replication. RV NSPs are responsible for viral RNA replication, in which a full-length negative-strand RNA serves as the intermediate for the replication of positive-strand genomic RNA and the transcription of subgenomic RNA. Previously we demonstrated that p200 synthesizes negative- but not positive-strand RNA, and that cleavage products p150/p90 are required for efficient production of positive-strand RNA. To determine whether p150 or p90 alone or together is involved in positive-strand RNA synthesis, vaccinia virus recombinants expressing individual NSPs were constructed and characterized. These were used in in vivo rescue experiments to complement replication-defective mutants in virus replication. A protease-inactive mutant was rescued by p200 or p150 provided in trans by using vaccinia virus recombinants. Thus this protease can function in trans. Rescue of cleavage-defective mutant by either p200 alone, or p150 plus p90 but not by p150 or p90 alone suggests that p150 and p90 function together as a replication complex in positive-strand RNA synthesis.

Mutations in the GDD motif of rubella virus putative RNA-dependent RNA polymerase affect virus replication

Rubella virus (RV) nonstructural proteins are translated as a p200 polyprotein that undergoes proteolytic cleavage into p150 and p90. From conserved amino acid sequence motifs in polypeptides, p90 has been proposed to be the RV RNA-dependent RNA polymerase (RdRp). To test whether the conserved GDD motif is involved in RdRp catalytic activity, three different alanine substitutions were introduced into it. Substitution of glycine by alanine (G1966A) resulted in impaired virus infectivity. Alteration of either aspartate residue completely abolished virus replication. A fully infectious variant was isolated from the G1966A mutant. Sequencing analysis showed that the alanine residue substituted in G1966A mutant had reverted to glycine in this variant. Complementation experiments were carried out to rescue the replication-defective RNA carrying G1966A, D1967A, or D1968A mutations. The defective RNA with G1966A mutation in p90 replicated efficiently when the helper genome that supplied a wild-type p90 was provided in trans. However, the replication-defective RNA with D1967A or D1968A was not rescued by supplementation of p90 in trans. Our studies support the idea that the GDD motif is critical for RV replication and p90 function as RV RdRp.

Rubella virus RNA replication is cis-preferential and synthesis of negative- and positive-strand RNAs is regulated by the processing of nonstructural protein

Rubella virus (RV) genome encodes nonstructural protein (NSP) in a large open reading frame at its 5' end. It is translated into p200 and further processed into p150 and p90. The NSPs are responsible for viral RNA replication, during which a full-length negative-strand RNA serves as the intermediate for the replication of positive-strand genomic RNA and the transcription of subgenomic RNA. Using complementation experiments, we demonstrated that RV negative-strand RNA is synthesized preferentially in cis while positive-strand RNAs can be synthesized both in cis and in trans but with higher efficiency in cis. During virus infection, negative-strand RNA accumulates until 10 hours postinfection (hpi) and remains nearly constant thereafter. In contrast, positive-strand RNAs (both genomic and subgenomic RNA) do not increase much before 10 hpi and accumulate rapidly thereafter. Previously we demonstrated that p200 synthesizes negative- but not positive-strand RNA, whereas cleavage products p150/p90 are required for efficient production of positive-strand RNAs. In this study, we present evidence demonstrating that a higher concentration of p150/p90 is associated with lower production of negative-strand RNA. Our data support the hypothesis that p200 is the principal replicase for negative-strand RNA, as is p150/p90 for positive-strand RNA. The switch from the synthesis of negative- to positive-strand RNA is thus regulated by NSP processing, which not only activates the efficient production of positive-strand RNA, but also disables negative-strand RNA synthesis. A mechanism for NSP translation, processing, and regulation of RV RNA synthesis is proposed.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Infectious cDNA clone of the RA27/3 vaccine strain of Rubella virus

Rubella virus (RUB), a small plus-strand RNA virus, is a significant human pathogen. The RA27/3 vaccine strain of RUB is one of the most successful live attenuated vaccines developed. In this article, we report the construction of an RA27/3 infectious clone, a complete cDNA copy of the RA27/3 genome that can be transcribed in vitro to generate infectious RNA molecules. Virus generated from such in vitro transcripts was phenotypically similar to RA27/3 virus. To investigate the attenuation of the RA27/3 strain, a series of chimeras was made by the insertion of different fragments of the RA27/3 genome into an infectious clone based on the Therien wild-type strain of RUB. Analysis of the resulting chimeric viruses revealed that the pattern of RA27/3 attenuation in cell culture is complex: attenuating elements in the RA27/3 genome were found in the 5' untranslated region (UTR), a region of the nonstructural proteins containing the protease motif and the capsid gene. Within the 5' UTR, the attenuation determinant was mapped to nt 7. Surprisingly, these analyses also revealed a potentiating mutation at nt 164 of the RA27/3 genome. Although this determinant was within the coding sequences of the nonstructural proteins, the encoded amino acid had no effect on cell culture phenotype and thus the determinant may operate at the level of RNA structure. In addition to investigation of the mechanisms of RA27/3 attenuation, the availability of the RA27/3 infectious clone offers the opportunity for strict genetic control over RUB vaccine manufacturing, for development of novel DNA-based vaccines against RUB, and for development of recombinant RUB vaccines that also target other diseases.

Mutational analysis of the rubella virus nonstructural polyprotein and its cleavage products in virus replication and RNA synthesis

Rubella virus nonstructural proteins, translated from input genomic RNA as a p200 polyprotein and subsequently processed into p150 and p90 by an intrinsic papain-like thiol protease, are responsible for virus replication. To examine the effect of p200 processing on virus replication and to study the roles of nonstructural proteins in viral RNA synthesis, we introduced into a rubella virus infectious cDNA clone a panel of mutations that had variable defective effects on p200 processing. The virus yield and viral RNA synthesis of these mutants were examined. Mutations that completely abolished (C1152S and G1301S) or largely abolished (G1301A) cleavage of p200 resulted in noninfectious virus. Mutations that partially impaired cleavage of p200 (R1299A and G1300A) decreased virus replication. An RNase protection assay revealed that all of the mutants synthesized negative-strand RNA as efficiently as the wild type does but produced lower levels of positive-strand RNA. Our results demonstrated that processing of rubella virus nonstructural protein is crucial for virus replication and that uncleaved p200 could function in negative-strand RNA synthesis, whereas the cleavage products p150 and p90 are required for efficient positive-strand RNA synthesis.

Intracellular distribution of rubella virus nonstructural protein P150

Antiserum prepared against an amino-terminal fragment of rubella virus (RUB) nonstructural polyprotein was used to study RUB-infected Vero cells. Replicase protein P150 was associated with vesicles and vacuoles of endolysosomal origin and later with large, convoluted, tubular membrane structures. Newly incorporated bromouridine was associated with the same structures and specifically with small membrane invaginations, spherules, indicating that these structures may be the sites of viral RNA synthesis.

Involvement of a p53-dependent pathway in rubella virus-induced apoptosis

In light of the important role of apoptotic cell death in the pathogenesis of several viral infections, we asked whether the cytopathogenicity evoked by rubella virus (RV) might also involve apoptotic mechanisms. The To-336 strain of RV induced apoptosis in Vero and RK-13 cells, but not in fibroblast cell lines. UV-inactivated RV virions did not elicit the apoptotic response, indicating that productive infection is required for the induction of cell death. Both p53 and p21 protein levels were highly elevated in RV-infected Vero cells. The level of p21 mRNA was increased, while expression of the p53 gene was unaffected by RV infection. A dominant-negative p53 mutant (p53(W248)) conferred partial protection from RV-induced apoptosis. These data implicate a p53-dependent apoptotic pathway in the cytopathogenicity of RV, thereby suggesting a mechanism by which RV exerts its teratogenic effects.

Effects of mutations in the rubella virus E1 glycoprotein on E1-E2 interaction and membrane fusion activity

Rubella virus (RV) virions contain two glycosylated membrane proteins, E1 and E2, that exist as a heterodimer and form the viral spike complexes on the virion surface. Formation of an E1-E2 heterodimer is required for transport of E1 out of the endoplasmic reticulum lumen to the Golgi apparatus and plasma membrane. To investigate the nature of the E1-E2 interaction, we have introduced mutations in the internal hydrophobic region (residues 81 to 109) of E1. Substitution of serine at Cys82 (mutant C82S) or deletion of this hydrophobic domain (mutant dt) of E1 resulted in a disruption of the E1 conformation that ultimately affected E1-E2 heterodimer formation and cell surface expression of both E1 and E2. Substitution of either aspartic acid at Gly93 (G93D) or glycine at Pro104 (P104G) was found to impair neither E1-E2 heterodimer formation nor the transport of E1 and E2 to the cell surface. Fusion of RV-infected cells is induced by a brief treatment at a pH below 6. 0. To test whether this internal hydrophobic domain is involved in the membrane fusion activity of RV, transformed BHK cell lines expressing either wild-type or mutant spike proteins were exposed to an acidic pH and polykaryon formation was measured. No fusion activity was observed in the C82S, dt, and G93D mutants; however, the wild type and the P104G mutant exhibited fusogenic activities, with greater than 60% and 20 to 40% of the cells being fused, respectively, at pH 4.8. These results suggest that it is likely that the region of E1 between amino acids 81 and 109 is involved in the membrane fusion activity of RV and that it may be important for the interaction of that protein with E2 to form the E1-E2 heterodimer.

Rubella virus induces apoptosis in culture cells

The replication of rubella virus (RUB) in Vero cells, an adherent cell line, results in apoptotic death of infected cells as detected by chromatin fragmentation assays. In infected cultures, virtually all of the cells that had become detached (a hallmark feature of RUB-induced cytopathology) were apoptotic; they were predominantly dead as shown by propidium iodide and trypan blue exclusion tests. In contrast, the majority of the cells in the infected monolayers that remained adherent were alive and contained intact chromatin. Thus simple counting of detached cells in the medium is a convenient way of measuring the extent of RUB-induced apoptosis. RUB-induced cytopathology was inhibited by z-VAD-fmk, an inhibitor of caspases that are involved in the execution stages of apoptosis, confirming the induction of apoptosis by RUB. The lack of apoptotic adherent cells (maximally 1% at any time point through 6 days postinfection) indicates that the induction of apoptosis is asynchronous since cells become uniformly virus antigen-positive by day 2 postinfection. To elucidate whether this asynchronicity and the ability of RUB to persistently infect Vero cells were due to a suppression of apoptosis, we examined whether RUB can suppress chemically induced apoptosis. Staurosporine (ST) was found to be an efficient inducer of apoptosis in Vero cells. ST treatment of RUB-infected and RUB persistently infected cells resulted in a much higher proportion of detached cells, higher even than in Vero cells treated with ST alone. This indicates that RUB does not suppress ST-induced apoptosis and, rather, that ST and RUB acted cumulatively in inducing apoptosis, possibly indicating that they use different induction pathways.

Proteolytic processing of rubella virus nonstructural proteins

The genomic RNA of rubella virus contains two long open reading frames (ORF), a 5'-proximal ORF that codes for the nonstructural proteins and a 3'-proximal ORF that encodes the structural proteins. The cDNA encoding the nonstructural protein ORF of the wild-type M33 strain of rubella virus has been obtained and sequenced. Comparison between the nonstructural proteins of the M33 and Therien strains of rubella virus revealed a 98% homology in nucleotide sequence and 98.1% in deduced amino acid sequence. To examine the processing of rubella virus nonstructural protein, the complete nonstructural protein ORF was expressed in BHK cells using a pSFV expression vector. Three nonstructural protein products (p200, p150, and p90) with molecular weights of 200, 150, and 90 kDa were identified using antisera raised against synthetic peptides corresponding to regions of the nonstructural proteins. p200 is the polyprotein precursor, while p150 and p90 are the cleavage products. Site-directed mutagenesis of the Cys-1151 residue (one of the catalytic dyad residues of the viral protease) and of the Gly-1300 residue (the viral protease cleavage site) abrogated protease activity and p200 precursor cleavage, respectively. Coexpression of mutant constructs in BHK cells indicated that rubella virus protease can function both in cis and in trans.

Rubella virus replication complexes are virus-modified lysosomes

Replication complexes are membrane-bound cytoplasmic vacuoles involved in rubella virus (RV) replication. These structures can be identified by their characteristic morphology at the electron microscopy (EM) level and by their association with double-stranded (ds) RNA in immunogold labeling EM studies. Although these virus-induced structures bear some resemblance to lysosomes, their exact nature and origin are unknown. In this study, the localization of two lysosomal markers, lysosomal-associated membrane protein (Lamp-1) and acid phosphatase, relative to the replication complexes was examined by light and electron microscopy. Confocal microscopy using antibodies to dsRNA and Lamp-1 showed colocalization of these two markers in the cytoplasm of RV-infected cells. Immunogold labeling EM studies using antibodies to Lamp-1 confirmed that Lamp-1 was associated with RV replication complexes. EM histochemical studies demonstrated the presence of acid phosphatase in the vacuoles of RV replication complexes. Taken together, these studies show that RV replication complexes are virus-modified lysosomes.

Effects of defined mutations in the 5' nontranslated region of rubella virus genomic RNA on virus viability and macromolecule synthesis

The 5' end of the genomic RNA of rubella virus (RUB) contains a 14-nucleotide (nt) single-stranded leader (ss-leader) followed by a stem-and-loop structure [5'(+)SL] (nt 15 to 65), the complement of which at the 3' end of the minus-strand RNA [3'(-)SL] has been proposed to function as a promoter for synthesis of genomic plus strands. A second intriguing feature of the 5' end of the RUB genomic RNA is the presence of a short (17 codons) open reading frame (ORF) located between nt 3 and 54; the ORF encoding the viral nonstructural proteins (NSPs) initiates at nt 41 in an alternate translational frame. To address the functional significance of these features, we compared the 5'-terminal sequences of six different strains of RUB, with the result that the short ORF is preserved (although the coding sequence is not conserved) as is the stem part of both the 5'(+)SL and 3'(-)SL, while the upper loop part of both structures varies. Next, using Robo302, an infectious cDNA clone of RUB, we introduced 31 different mutations into the 5'-terminal noncoding region, and their effects on virus replication and macromolecular synthesis were examined. This mutagenesis revealed that the short ORF is not essential for virus replication. The AA dinucleotide at nt 2 and 3 is of critical importance since point mutations and deletions that altered or removed both of these nucleotides were lethal. None of the other mutations within either the ss-leader or the 5'(+)SL [and accordingly within the 3'(-)SL], including deletions of up to 15 nt from the 5'(+)SL and three different multiple-point mutations that lead to destabilization of the 5'(+)SL, were lethal. Some of the mutations within both ss-leader and the 5'(+)SL resulted in viruses that grew to lower titers than the wild-type virus and formed opaque and/or small plaques; in general mutations within the stem had a more profound effect on viral phenotype than did mutations in either the ss-leader or upper loop. Mutations in the 5'(+)SL, but not in the ss-leader, resulted in a significant reduction in NSP synthesis, indicating that this structure is important for efficient translation of the NSP ORF. In contrast, viral plus-strand RNA synthesis was unaffected by the 5'(+)SL mutations as well as the ss-leader mutations, which argues against the proposed function of the 3'(-)SL as a promoter for initiation of the genomic plus-strand RNA.

Improvement of the specific infectivity of the rubella virus (RUB) infectious clone: determinants of cytopathogenicity induced by RUB map to the nonstructural proteins

A plasmid, Robo102, which contains a cDNA copy of the rubella virus (RUB) genomic RNA from which infectious transcripts can be synthesized in vitro, was recently developed (C. Y. Wang, G. Dominguez, and T. K. Frey, J. Virol. 68:3550-3557, 1994). To increase the specific infectivity of Robo102 transcripts (approximately 5 plaques/10 microg of transcripts), a modified reverse transcription-PCR method was used to amplify nearly 90% of the RUB genome in three fragments, which were then used to replace the corresponding fragments in Robo102. Replacement of a fragment covering nucleotides (nt) 5352 to 9759 of the RUB genome yielded a construct, Robo202, which produced highly infectious transcripts (10(4) plaques/microg), indicating the presence of an unrecognized deleterious mutation (or mutations) in this region of the Robo102 cDNA. Robo102 was based on the w-Therien strain of RUB, which forms opaque plaques in Vero cells, while the PCR replacement fragments were generated from a variant, f-Therien, which produces clear plaques in Vero cells. Although Robo202 contains over 4,000 nt from f-Therien, Robo202 virus produces opaque plaques. However, when the other two PCR fragments amplified from f-Therien (nt 1 to 1723 and nt 2800 to 5352) were introduced into Robo202, the resulting construct, Robo302, yielded transcripts that produced a virus that formed clear plaques. This indicates that the determinants of plaque morphology map to the regions of the genome covered by these two fragments, both of which are in the nonstructural open reading frame. Generation of Robo202/302 chimeras indicated that the most 5' terminal fragment (nt. 1 to 1723) had the greatest effect on plaque morphology. The plaque morphology was correlated with the ability of the viruses to kill infected cells. The only difference at the molecular level detected among the viruses was that the more cytopathic viruses produced more nonstructural proteins than did the less cytopathic viruses. This finding, as well as the mapping of the genetic determinants to the region of the genome encoding these proteins, indicates that the nonstructural proteins can mediate cell killing.

Characterization of defective-interfering RNAs of rubella virus generated during serial undiluted passage

During serial undiluted passage of rubella virus (RUB) in Vero cells, two species of defective-interfering (DI) RNAs of approximately 7000 and 800 nucleotides (nts) in length were generated (Frey, T. K., and Hemphill, M. L., Virology 164, 22-29, 1988). In this study, these DI RNAs were characterized by molecular cloning, hybridization with probes of defined sequence, and primer extension. The 7000-nt DI RNA species were found to be authentic DI RNAs which contain a single 2500- to 2700-nt deletion in the structural protein open reading frame (ORF) region of the genome. The 800-nt RNAs were found to be subgenomic DI RNAs synthesized from the large DI RNA templates. Analysis of the extent of the deletions using a reverse-transcription-PCR protocol revealed that the 3' end of the deletions did not extend beyond the 3' terminal 244 nts of the genome. The 5' end of the deletions did not extend into the nonstructural protein ORF; however, DI RNAs in which the subgenomic start site was deleted were present. Following serial undiluted passage of seven independent stocks of RUB, this was the only pattern of DI RNAs generated. DI RNAs of 2000 to 3000 nt in length were the majority DI RNA species in a persistently infected line of Vero cells, showing that other types of RUB DI RNAs can be generated and selected. However, when supernatant from the persistently infected cells was passaged, the only DI RNAs present after two passages were 7000 nts in length, indicating that this species has a selective advantage over other types of DI RNAs during serial passage.

Construction of rubella virus genome-length cDNA clones and synthesis of infectious RNA transcripts

Plasmids containing a complete cDNA copy of the rubella virus (RUB) genomic RNA were constructed. Transfection into cell culture of genome-length RNA transcribed in vitro from one of these cDNA clones, Robo102, resulted in the production of virus which preserved the genetic and phenotypic characteristics of the parental virus from which the cDNA clone was derived. Prior to construction of the RUB genome-length cDNA clones, the 5'-terminal sequence of the RUB genomic RNA was determined to be 5'CAAUGG...3' following the cap structure. Analysis of the specific infectivity of RUB genomic RNA isolated from virions revealed that in Vero cells, the specific infectivity of RUB genomic RNA is roughly equivalent to that of Sindbis virus genomic RNA. In RUB virion RNA preparations, the subgenomic RNA was detected. It was demonstrated that subgenomic RNA was packaged into RUB virions; however, the presence of the subgenomic RNA was not essential for infectivity of the genomic RNA.

Molecular biology of rubella virus

This chapter summarizes the present medical significance of rubella virus. Rubella virus infection is systemic in nature and the accompanying symptoms are generally benign, the most pronounced being a mild rash of short duration. The most common complication of rubella virus infection is transient joint involvement such as polyarthralgia and arthritis. The primary health impact of rubella virus is that it is a teratogenic agent. The vaccination strategy is aimed at elimination of rubella and includes both universal vaccination of infants at 15 months of age with the trivalent measles, mumps, rubella (MMR) vaccine and specific targeting with the rubella vaccine of seronegative women planning pregnancy and seronegative adults who could come in contact with women of childbearing age, although it is recommended that any individual over the age of 12 months without evidence of natural infection or vaccination be vaccinated. Medically, the current challenge posed by rubella virus is to achieve complete vaccination coverage to prevent resurgences.

Replication complexes associated with the morphogenesis of rubella virus

Thin section electron microscopy was used to investigate cellular changes associated with the replication of rubella virus (RV) in Vero cells and to compare these changes to those of the related alphavirus, Semliki Forest virus (SFV). Conspicuous membrane-bound cytoplasmic vacuoles analogous to the alphavirus replication complexes were observed in RV infected cells but not in mock infected cells. The vacuoles were characterised by membrane-bound vesicles measuring about 60 nm which often displayed an irregular dense core and/or a network of fibres. These vesicles were morphologically distinct from RV particles and were generally located at regular intervals on the inner side of the surrounding membrane of the RV replication complex. Degenerating cellular material was often found in the membrane-bound vacuole of a replication complex. The replication complexes were intimately associated with the rough endoplasmic reticulum (RER), which was localised 45-75 nm from the surrounding membrane of the replication complex. Parallel studies of replication complexes in SFV infected cells did not reveal such an intimate association with the RER. RV replication complexes appeared as early as 8 h post infection (p.i.), before detection of RV particles by electron microscopy, and their peak production at 24 h p.i. coincided with the time of maximum virus titre.

Oligomerization of the structural proteins of rubella virus

Rubella virus contains, in addition to its RNA genome, a nucleocapsid protein (C) and two membrane proteins (E2 and E1). We have studied the association of these proteins during viral assembly and when expressed from cDNA constructs. The C protein was found to dimerize very shortly after synthesis; this dimer became disulfide-linked in the virion. Formation of the dimer was independent of the presence of other RV proteins. The membrane glycoproteins formed an E2E1 heterodimer, a minor fraction of which was also found to be disulfide-linked in the virion. This heterodimer also formed when the two proteins were coexpressed from cloned cDNA. Formation of the heterodimer preceded the transport of E2 to the Golgi, as judged by modification of the protein by Golgi-located enzymes. In the absence of E2, the E1 protein was slowly converted to high molecular weight aggregates.

Detection of rubella virus gene sequences by enzymatic amplification and direct sequencing of amplified DNA

We developed a rapid and sensitive polymerase chain reaction (PCR) assay for detecting and identifying rubella virus (RV). A segment of the RV gene which encodes the E1 membrane glycoprotein of RV was selected as a target for PCR amplification. Single-stranded viral RNA, extracted from infected cells or released from virions, was used as a template for reverse transcription followed by PCR amplification with two different sets of primer pairs, one nested within the other. The amplified E1 gene sequences were detected in ethidium bromide-stained agarose minigels, and their identities were verified by restriction enzyme digestion and hybridization to a probe directed at a site within the PCR target. Single-stranded DNA generated by asymmetric amplification of the target was directly sequenced by using fluorescent dideoxy-terminators and an automated procedure in order to confirm the target sequence. This PCR assay provides a rapid confirmatory test for the detection of RV by cell culture and appears to have considerable potential for the direct detection of RV in clinical specimens. The strategy used in the development of this PCR assay should be useful for developing other diagnostic PCR assays for viruses.

pH-dependent solubility shift of rubella virus capsid protein

The mechanism of capsid uncoating in rubella virus and other togaviridae is not well understood. This study presents data which suggest that rubella virus capsid undergoes a structural change from having hydrophilic to hydrophobic properties, between pH 5 and 5.5. Such a conformational change would allow capsid uncoating to occur within the lysosome, allowing RNA penetration to occur upon fusion of the viral envelope with the limiting membrane of the lysosome.

Effect of antiviral antibody on maintenance of long-term rubella virus persistent infection in Vero cells

A Vero cell line with a long-term rubella virus persistent infection was maintained for 45 weeks in the presence of anti-rubella virus antibody of sufficient titer to completely neutralize the virus in the culture fluid to determine the effect of the presence of antibody on the maintenance of the persistent infection. Prior to antibody treatment, virus was continuously detected as plaque-forming units in the persistently infected culture fluid. Virus clones that were plaque purified from the persistently infected culture fluid were temperature sensitive and exhibited a reduced efficiency of replication and ability to induce cytopathic effects in Vero cells at the persistently infected culture temperature compared with the standard virus used to initiate the persistently infected culture. Defective interfering RNAs were the major intracellular virus-specific RNA species present in the persistently infected cells. Treatment with antibody failed to cure the persistently infected culture of virus, and the cells retained the ability to release virus after antibody treatment was discontinued. Interestingly, the presence of antibody led to the selection of a population of virus which was markedly less cytopathic for Vero cells than the virus population which was selected during persistent infection in the absence of antibody.

Sequence of the genome RNA of rubella virus: evidence for genetic rearrangement during togavirus evolution

The nucleotide sequence of the rubella virus (RUB) genomic RNA was determined. The RUB genomic RNA is 9757 nucleotides in length [excluding the poly(A) tail] and has a G/C content of 69.5%, the highest of any RNA virus sequenced to data. The RUB genomic RNA contains two long open reading frames (ORFs), a 5'-proximal ORF of 6656 nucleotides and a 3'-proximal ORF of 3189 nucleotides which encodes the structural proteins. Thus, the genomic organization of RUB is similar to that of alphaviruses, the other genus of the Togavirus family, and the 5'-proximal ORF of RUB therefore putatively codes for the nonstructural proteins. Sequences homologous to three regions of nucleotide sequence highly conserved among alphaviruses (a stem-and-loop structure at the 5' end of the genome, a 51-nucleotide conserved sequence near the 5' end of the genome, and a 20-nucleotide conserved sequence at the subgenomic RNA start site) were found in the RUB genomic RNA. Amino acid sequence comparisons between the nonstructural ORF of RUB and alphaviruses revealed only one short (122 amino acids) region of significant homology, indicating that these viruses are only distantly related. This region of homology is located at the NH2 terminus of nsP3 in the alphavirus genome. The RUB nonstructural protein ORF contains two global amino acid motifs conserved in a large number of positive-polarity RNA viruses, a motif indicative of helicase activity and a motif indicative of replicase activity. The order of the helicase motif and the nsP3 homology region in the RUB genome is reversed with respect to the alphavirus genome indicating that a genetic rearrangement has occurred during the evolution of these viruses.