Rearranged genomic RNA segments offer a new approach to the reverse genetics of rotaviruses

A GFP-expressing minigenome of a chrysovirus replicating in fungi

The Fusarium graminearum virus China 9 (FgV-ch9) is a member of the genus Betachrysovirus in the Chrysoviridae family and causes hypovirulence in its host, Fusarium graminearum, the causal agent of Fusarium head blight. Although insights into viral biology of FgV-ch9 have expanded in recent years, questions regarding the function of virus-encoded proteins, cis-acting elements, and virus transmission are yet to be answered. Therefore, we developed a tool for the establishment of an artificial 6th segment of FgV-ch9, which encodes a GFP gene flanked by the non-translated regions of FgV-ch9 segment 1. Subsequently, we have proved successful encapsidation of this artificial segment into virus particles as well as its horizontal transmission. Expression of GFP was further verified via immunoassay and life cell imaging. Thus far, we were able to establish for the first time a mini-replicon system for segmented dsRNA viruses replicating in fungi.

Recent advances in Reovirales viruses reverse genetics research

Reovirales are segmented double-strand RNA viruses with a broad host range that pose a serious threat to human and animal health. However, there are numerous viral species within the Reovirales, some of which have lagged behind other RNA viruses in the study of their biology due to the lack of an effective reverse genetics (RG) system. The RG systems are the most powerful tools for studying viral protein function, viral gene expression regulation, viral pathogenesis, and the generation of engineered vaccines. Recently, several entirely plasmid-based RG systems have been developed for several members of the Reovirales. This review outlines the development and future direction of the RG system for the best studied Reovirales viruses.

Using Species a Rotavirus Reverse Genetics to Engineer Chimeric Viruses Expressing SARS-CoV-2 Spike Epitopes

Species A rotavirus (RVA) vaccines based on live attenuated viruses are used worldwide in humans. The recent establishment of a reverse genetics system for rotoviruses (RVs) has opened the possibility of engineering chimeric viruses expressing heterologous peptides from other viral or microbial species in order to develop polyvalent vaccines. We tested the feasibility of this concept by two approaches. First, we inserted short SARS-CoV-2 spike peptides into the hypervariable region of the simian RV SA11 strain viral protein (VP) 4. Second, we fused the receptor binding domain (RBD) of the SARS-CoV-2 spike protein, or the shorter receptor binding motif (RBM) nested within the RBD, to the C terminus of nonstructural protein (NSP) 3 of the bovine RV RF strain, with or without an intervening Thosea asigna virus 2A (T2A) peptide. Mutating the hypervariable region of SA11 VP4 impeded viral replication, and for these mutants, no cross-reactivity with spike antibodies was detected. To rescue NSP3 mutants, we established a plasmid-based reverse genetics system for the bovine RV RF strain. Except for the RBD mutant that demonstrated a rescue defect, all NSP3 mutants delivered endpoint infectivity titers and exhibited replication kinetics comparable to that of the wild-type virus. In ELISAs, cell lysates of an NSP3 mutant expressing the RBD peptide showed cross-reactivity with a SARS-CoV-2 RBD antibody. 3D bovine gut enteroids were susceptible to infection by all NSP3 mutants, but cross-reactivity with SARS-CoV-2 RBD antibody was only detected for the RBM mutant. The tolerance of large SARS-CoV-2 peptide insertions at the C terminus of NSP3 in the presence of T2A element highlights the potential of this approach for the development of vaccine vectors targeting multiple enteric pathogens simultaneously. IMPORTANCE We explored the use of rotaviruses (RVs) to express heterologous peptides, using SARS-CoV-2 as an example. Small SARS-CoV-2 peptide insertions (<34 amino acids) into the hypervariable region of the viral protein 4 (VP4) of RV SA11 strain resulted in reduced viral titer and replication, demonstrating a limited tolerance for peptide insertions at this site. To test the RV RF strain for its tolerance for peptide insertions, we constructed a reverse genetics system. NSP3 was C-terminally tagged with SARS-CoV-2 spike peptides of up to 193 amino acids in length. With a T2A-separated 193 amino acid tag on NSP3, there was no significant effect on the viral rescue efficiency, endpoint titer, and replication kinetics. Tagged NSP3 elicited cross-reactivity with SARS-CoV-2 spike antibodies in ELISA. We highlight the potential for development of RV vaccine vectors targeting multiple enteric pathogens simultaneously.

Re-Examining Rotavirus Innate Immune Evasion: Potential Applications of the Reverse Genetics System

Rotaviruses represent one of the most successful pathogens in the world, with high infectivity and efficient transmission between the young of many animal species, including humans. To overcome host defenses, rotaviruses have evolved a plethora of strategies to effectively evade the innate immune response, establish initial infection in the small intestine, produce progeny, and shed into the environment. Previously, studying the roles and relative contributions of specific rotaviral factors in innate immune evasion had been challenging without a plasmid-only reverse genetics system. Although still in its infancy, current reverse genetics technology will help address important research questions regarding rotavirus innate immune evasion, host range restriction, and viral pathogenesis. In this review, we summarize the current knowledge about the antiviral host innate immune defense mechanisms, countermeasures of rotavirus-encoded factors, and strategies to better understand these interactions using the rotavirus reverse genetics system.

Rotavirus research: 2014-2020

Rotaviruses are major causes of acute gastroenteritis in infants and young children worldwide and also cause disease in the young of many other mammalian and of avian species. During the recent 5-6 years rotavirus research has benefitted in a major way from the establishment of plasmid only-based reverse genetics systems, the creation of human and other mammalian intestinal enteroids, and from the wide application of structural biology (cryo-electron microscopy, cryo-EM tomography) and complementary biophysical approaches. All of these have permitted to gain new insights into structure-function relationships of rotaviruses and their interactions with the host. This review follows different stages of the viral replication cycle and summarizes highlights of structure-function studies of rotavirus-encoded proteins (both structural and non-structural), molecular mechanisms of viral replication including involvement of cellular proteins and lipids, the spectrum of viral genomic and antigenic diversity, progress in understanding of innate and acquired immune responses, and further developments of prevention of rotavirus-associated disease.

FAST Proteins: Development and Use of Reverse Genetics Systems for Reoviridae Viruses

Reverse genetics systems for viruses, the technology used to generate gene-engineered recombinant viruses from artificial genes, enable the study of the roles of the individual nucleotides and amino acids of viral genes and proteins in infectivity, replication, and pathogenicity. The successful development of a reverse genetics system for poliovirus in 1981 accelerated the establishment of protocols for other RNA viruses important for human health. Despite multiple efforts, rotavirus (RV), which causes severe gastroenteritis in infants, was refractory to reverse genetics analysis, and the first complete reverse genetics system for RV was established in 2017. This novel technique involves use of the fusogenic protein FAST (fusion-associated small transmembrane) derived from the bat-borne Nelson Bay orthoreovirus, which induces massive syncytium formation. Co-transfection of a FAST-expressing plasmid with complementary DNAs encoding RV genes enables rescue of recombinant RV. This review focuses on methodological insights into the reverse genetics system for RV and discusses applications and potential improvements to this system.

Rotavirus reverse genetics: A tool for understanding virus biology

Rotaviruses (RVs) are considered to be one of the most common causes of viral gastroenteritis in young children and infants worldwide. Before recent developments, studies on rotavirus biology have suffered from the lack of an effective reverse genetics (RG) system to generate recombinant rotaviruses and study the precise roles of the viral proteins in the context of RV infection. Lately a fully-tractable plasmid-only based RG system for rescuing recombinant rotaviruses has been developed leading to a breakthrough in the RV field. Since then, the reproducibility and improvements of this technology have led to the generation of several recombinant rotaviruses with modifications on different gene segments, which has allowed the manipulation of viral genes to characterise the precise roles of viral proteins during RV replication cycle or to encode exogenous proteins for different purposes. This review will recapitulate the different RG approaches developed so far, highlighting any similarities, differences and limitations of the systems as well as the gene segments involved. The review will further summarise the latest recombinant rotaviruses generated using the plasmid-only based RG system showing the enormous potentials of this technique to shed light on the still unanswered questions in rotavirus biology.

Relationships between RNA topology and nucleocapsid structure in a model icosahedral virus

The process of genome packaging in most of viruses is poorly understood, notably the role of the genome itself in the nucleocapsid structure. For simple icosahedral single-stranded RNA viruses, the branched topology due to the RNA secondary structure is thought to lower the free energy required to complete a virion. We investigate the structure of nucleocapsids packaging RNA segments with various degrees of compactness by small-angle x-ray scattering and cryotransmission electron microscopy. The structural differences are mild even though compact RNA segments lead on average to better-ordered and more uniform particles across the sample. Numerical calculations confirm that the free energy is lowered for the RNA segments displaying the larger number of branch points. The effect is, however, opposite with synthetic polyelectrolytes, in which a star topology gives rise to more disorder in the capsids than a linear topology. If RNA compactness and size account in part for the proper assembly of the nucleocapsid and the genome selectivity, other factors most likely related to the host cell environment during viral assembly must come into play as well.

Human Rotavirus Reverse Genetics Systems to Study Viral Replication and Pathogenesis

Human rotaviruses (HuRVAs) are highly important causes of acute gastroenteritis in infants and young children worldwide. A lack of reliable and reproducible reverse genetics systems for HuRVAs has limited a proper understanding of HuRVA biology and also the rational design of live-attenuated vaccines. Since the development of the first reverse genetics system for RVAs (partially plasmid-based reverse genetics system) in 2006, there have been many efforts with the goal of generating infectious recombinant HuRVAs entirely from cloned cDNAs. However, the establishment of a HuRVA reverse genetics system was very challenging until 2019. This review article provides an overview of the historical background of the recent development of long-awaited HuRVA reverse genetics systems, beginning with the generation of recombinant human-simian reassortant RVAs with the aid of a helper virus in 2006 and the generation of recombinant animal (simian) RVAs in a helper virus-free manner in 2017, and culminating in the generation of recombinant HuRVAs entirely from plasmid cDNAs in 2019. Notably, the original HuRVA reverse genetics system has already been optimized to increase the efficiency of virus generation. Although the application of HuRVA reverse genetics systems has only just been initiated, these technologies will help to answer HuRVA research questions regarding viral replication and pathogenicity that could not be addressed before, and to develop next-generation vaccines and intestine-specific rotaviral vectors.

Recent advances in rotavirus reverse genetics and its utilization in basic research and vaccine development

Rotaviruses are segmented double-stranded RNA viruses with a high frequency of gene reassortment, and they are a leading cause of global diarrheal deaths in children less than 5 years old. Two-thirds of rotavirus-associated deaths occur in low-income countries. Currently, the available vaccines in developing countries have lower efficacy in children than those in developed countries. Due to added safety concerns and the high cost of current vaccines, there is a need to develop cost-effective next-generation vaccines with improved safety and efficacy. The reverse genetics system (RGS) is a powerful tool for investigating viral protein functions and developing novel vaccines. Recently, an entirely plasmid-based RGS has been developed for several rotaviruses, and this technological advancement has significantly facilitated novel rotavirus research. Here, we review the recently developed RGS platform and discuss its application in studying infection biology, gene reassortment, and development of vaccines against rotavirus disease.

Rotavirus reverse genetics systems: Development and application

Rotaviruses (RVs) cause acute gastroenteritis in infants and young children. Since 2006, live-attenuated vaccines have reduced the number of RV-associated deaths; however, RV is still responsible for an estimated 228,047 annual deaths worldwide. RV, a member of the family Reoviridae, has an 11-segmented double-stranded RNA genome contained within a non-enveloped, triple layered virus particle. In 2017, a long-awaited helper virus-free reverse genetics system for RV was established. Since then, numerous studies have reported the generation of recombinant RVs; these studies verify the robustness of reverse genetics systems. This review provides technical insight into current reverse genetics systems for RVs, as well as discussing basic and applied studies that have used these systems.

Rapid generation of rotavirus single-gene reassortants by means of eleven plasmid-only based reverse genetics

Reassortment is an important mechanism in the evolution of group A rotaviruses (RVAs), yielding viruses with novel genetic and phenotypic traits. The classical methods for generating RVA reassortants with the desired genetic combinations are laborious and time-consuming because of the screening and selection processes required to isolate a desired reassortant. Taking advantage of a recently developed RVA reverse genetics system based on just 11 cloned cDNAs encoding the RVA genome (11 plasmid-only system), we prepared a panel of simian SA11-L2 virus-based single-gene reassortants, each containing 1 segment derived from human KU virus of the G1P[8] genotype. It was shown that there was no gene-specific restriction of the reassortment potential. In addition to these 11 single-gene reassortants, a triple-gene reassortant with KU-derived core-encoding VP1–3 gene segments with the SA11-L2 genetic background, which make up a virion composed of the KU-based core, and SA11-L2-based intermediate and outer layers, could also be prepared with the 11 plasmid-only system. Finally, for possible clinical application of this system, we generated a series of VP7 reassortants representing all the major human RVA G genotypes (G1–4, G9 and G12) efficiently. The preparation of each of these single-gene reassortants was achieved within just 2 weeks. Our results demonstrate that the 11 plasmid-only system allows the rapid and reliable generation of RVA single-gene reassortants, which will be useful for basic research and clinical applications.

Generation of Infectious Recombinant Human Rotaviruses from Just 11 Cloned cDNAs Encoding the Rotavirus Genome

The generation of recombinant group A rotaviruses (RVAs) entirely from cloned cDNAs has been described only for a single animal RVA strain, simian SA11-L2. We recently developed an optimized RVA reverse genetics system based on only RVA cDNAs (11-plasmid system), in which the concentration of cDNA plasmids containing the NSP2 and NSP5 genes is 3- or 5-fold increased in relation to that of the other plasmids. Based on this approach, we generated a recombinant human RVA (HuRVA)-based monoreassortant virus containing the VP4 gene of the simian SA11-L2 virus using the 11-plasmid system. In addition to this monoreassortant virus, authentic HuRVA (strain KU) was also generated with the 11-plasmid system with some modifications. Our results demonstrate that the 11-plasmid system involving just RVA cDNAs can be used for the generation of recombinant HuRVA and recombinant HuRVA-based reassortant viruses.IMPORTANCE Human group A rotavirus (HuRVA) is a leading pathogen causing severe diarrhea in young children worldwide. In this paper, we describe the generation of recombinant HuRVA (strain KU) from only 11 cloned cDNAs encoding the HuRVA genome by reverse genetics. The growth properties of the recombinant HuRVA were similar to those of the parental RVA, providing a powerful tool for better understanding of HuRVA replication and pathogenesis. Furthermore, the ability to manipulate the genome of HuRVAs "to order" will be useful for next-generation vaccine production for this medically important virus and for the engineering of clinical vectors expressing any foreign genes.

Rotavirus Infection Alters Splicing of the Stress-Related Transcription Factor XBP1

XBP1 is a stress-regulated transcription factor also involved in mammalian host defenses and innate immune response. Our investigation of XBP1 RNA splicing during rotavirus infection revealed that an additional XBP1 RNA (XBP1es) that corresponded to exon skipping in the XBP1 pre-RNA is induced depending on the rotavirus strain used. We show that the translation product of XBP1es (XBP1es) has trans-activation properties similar to those of XBP1 on ER stress response element (ERSE) containing promoters. Using monoreassortant between ES⁺ ("skipping") and ES^- ("nonskipping") strains of rotavirus, we show that gene 7 encoding the viral translation enhancer NSP3 is involved in this phenomenon and that exon skipping parallels the nuclear relocalization of cytoplasmic PABP. We further show, using recombinant rotaviruses carrying chimeric gene 7, that the ES⁺ phenotype is linked to the eIF4G-binding domain of NSP3. Because the XBP1 transcription factor is involved in stress and immunological responses, our results suggest an alternative way to activate XBP1 upon viral infection or nuclear localization of PABP.IMPORTANCE Rotavirus is one of the most important pathogens causing severe gastroenteritis in young children worldwide. Here we show that infection with several rotavirus strains induces an alternative splicing of the RNA encoding the stressed-induced transcription factor XBP1. The genetic determinant of XBP1 splicing is the viral RNA translation enhancer NSP3. Since XBP1 is involved in cellular stress and immune responses and since the XBP1 protein made from the alternatively spliced RNA is an active transcription factor, our observations raise the question of whether alternative splicing is a cellular response to rotavirus infection.

[Reverse genetics of rotaviruses: Generation of recombinant human rotaviruses from just 11 cDNAs encoding the rotavirus genome]

An entirely plasmid-based reverse genetics system for animal rotavirus was established very recently. We improved the reverse genetics system to generate recombinant rotavirus by transfecting only 11 T7 plasmids for its 11 genes under the condition of increasing the ratio (3- or 5-fold) of the cDNA plasmids for NSP2 and NSP5 genes (11-plasmid system). Utilizing this highly efficient system, we engineered the first infectious recombinant rotaviruses harboring fluorescent (EGFP and mCherry) protein genes. In addition to these recombinant animal viruses, the first infectious recombinant human rotavirus (strain KU (G1P[8])) was also generated with the 11-plasmid system with some modifications. The availability of recombinant human rotaviruses will provide a genetic platform for a better understanding of the replication, pathogenicity, and other biological characteristics of this medically important virus and enable the rational development of next-generation human rotavirus vaccines.

Generation of Recombinant Rotaviruses Expressing Fluorescent Proteins by Using an Optimized Reverse Genetics System

An entirely plasmid-based reverse genetics system for rotaviruses was established very recently. We improved the reverse genetics system to generate recombinant rotavirus by transfecting only 11 cDNA plasmids for its 11 gene segments under the condition of increasing the ratio of the cDNA plasmids for NSP2 and NSP5 genes. Utilizing this highly efficient system, we then engineered infectious recombinant rotaviruses expressing bioluminescent (NanoLuc luciferase) and fluorescent (enhanced green fluorescent protein [EGFP] and mCherry) reporters. These recombinant rotaviruses expressing reporters remained genetically stable during serial passages. Our reverse genetics approach and recombinant rotaviruses carrying reporter genes will be great additions to the tool kit for studying the molecular virology of rotavirus and for developing future next-generation vaccines and expression vectors.IMPORTANCE Rotavirus is one of the most important pathogens causing severe gastroenteritis in young children worldwide. In this paper, we describe a robust and simple reverse genetics system based on only rotavirus cDNAs and its application for engineering infectious recombinant rotaviruses harboring bioluminescent (NanoLuc) and fluorescent (EGFP and mCherry) protein genes. This highly efficient reverse genetics system and recombinant group A rotaviruses expressing reporters could be powerful tools for the study of different aspects of rotavirus replication. Furthermore, they may be useful for next-generation vaccine production for this medically important virus.

Molecular and biological characterization of the human-bovine rotavirus-based reassortant rotavirus

Rotaviruses (RV) are the leading cause of acute infantile gastroenteritis, associated with elevated mortality in low-income countries. Morbidity and mortality, length and rates of hospitalization due to RV gastroenteritis are dropping. Improving the quality of newborns life is an ongoing challenge for health-care providers. In this study, homemade reassortant human-bovine rotavirus was developed and biological activity and molecular characterization of candidate vaccine were evaluated for the vaccine stability. Virus titration and purification of reassortant rotavirus strains were evaluated by plaque assays, electropherotyping. The genetic stability after first, third and sixth passage was by sequencing. Due to WHO recommendation, developingment of national capacity for vaccine production in appropriate quantities and at affordable prices is the cornerstone of developing global vaccination policies. Such studies are critical to producing national vaccines and modeling herd protection.

Reverse Genetics System Demonstrates that Rotavirus Nonstructural Protein NSP6 Is Not Essential for Viral Replication in Cell Culture

The use of overlapping open reading frames (ORFs) to synthesize more than one unique protein from a single mRNA has been described for several viruses. Segment 11 of the rotavirus genome encodes two nonstructural proteins, NSP5 and NSP6. The NSP6 ORF is present in the vast majority of rotavirus strains, and therefore the NSP6 protein would be expected to have a function in viral replication. However, there is no direct evidence of its function or requirement in the viral replication cycle yet. Here, taking advantage of a recently established plasmid-only-based reverse genetics system that allows rescue of recombinant rotaviruses entirely from cloned cDNAs, we generated NSP6-deficient viruses to directly address its significance in the viral replication cycle. Viable recombinant NSP6-deficient viruses could be engineered. Single-step growth curves and plaque formation of the NSP6-deficient viruses confirmed that NSP6 expression is of limited significance for RVA replication in cell culture, although the NSP6 protein seemed to promote efficient virus growth.IMPORTANCE Rotavirus is one of the most important pathogens of severe diarrhea in young children worldwide. The rotavirus genome, consisting of 11 segments of double-stranded RNA, encodes six structural proteins (VP1 to VP4, VP6, and VP7) and six nonstructural proteins (NSP1 to NSP6). Although specific functions have been ascribed to each of the 12 viral proteins, the role of NSP6 in the viral replication cycle remains unknown. In this study, we demonstrated that the NSP6 protein is not essential for viral replication in cell culture by using a recently developed plasmid-only-based reverse genetics system. This reverse genetics approach will be successfully applied to answer questions of great interest regarding the roles of rotaviral proteins in replication and pathogenicity, which can hardly be addressed by conventional approaches.

A Point Mutation in the Rhesus Rotavirus VP4 Protein Generated through a Rotavirus Reverse Genetics System Attenuates Biliary Atresia in the Murine Model

Rotavirus infection is one of the most common causes of diarrheal illness in humans. In neonatal mice, rhesus rotavirus (RRV) can induce biliary atresia (BA), a disease resulting in inflammatory obstruction of the extrahepatic biliary tract and intrahepatic bile ducts. We previously showed that the amino acid arginine (R) within the sequence SRL (amino acids 445 to 447) in the RRV VP4 protein is required for viral binding and entry into biliary epithelial cells. To determine if this single amino acid (R) influences the pathogenicity of the virus, we generated a recombinant virus with a single amino acid mutation at this site through a reverse genetics system. We demonstrated that the RRV mutant (RRV^VP4-R446G) produced less symptomatology and replicated to lower titers both in vivo and in vitro than those seen with wild-type RRV, with reduced binding in cholangiocytes. Our results demonstrate that a single amino acid change in the RRV VP4 gene influences cholangiocyte tropism and reduces pathogenicity in mice.IMPORTANCE Rotavirus is the leading cause of diarrhea in humans. Rhesus rotavirus (RRV) can also lead to biliary atresia (a neonatal human disease) in mice. We developed a reverse genetics system to create a mutant of RRV (RRV^VP4-R446G) with a single amino acid change in the VP4 protein compared to that of wild-type RRV. In vitro, the mutant virus had reduced binding and infectivity in cholangiocytes. In vivo, it produced fewer symptoms and lower mortality in neonatal mice, resulting in an attenuated form of biliary atresia.

Entirely plasmid-based reverse genetics system for rotaviruses

Rotaviruses (RVs) are highly important pathogens that cause severe diarrhea among infants and young children worldwide. The understanding of the molecular mechanisms underlying RV replication and pathogenesis has been hampered by the lack of an entirely plasmid-based reverse genetics system. In this study, we describe the recovery of recombinant RVs entirely from cloned cDNAs. The strategy requires coexpression of a small transmembrane protein that accelerates cell-to-cell fusion and vaccinia virus capping enzyme. We used this system to obtain insights into the process by which RV nonstructural protein NSP1 subverts host innate immune responses. By insertion into the NSP1 gene segment, we recovered recombinant viruses that encode split-green fluorescent protein-tagged NSP1 and NanoLuc luciferase. This technology will provide opportunities for studying RV biology and foster development of RV vaccines and therapeutics.

[A plasmid-based reverse genetics system for rotaviruses]

Rotavirus (RV), a non-enveloped icosahedral virus containing eleven gene segments of double-stranded RNA, is the leading cause of severe, acute diarrhea among infants and young children worldwide. Safe and effective rotavirus vaccines have been available since 2006, and have markedly reduced the number of deaths by severe gastroenteritis. However, rotaviruses are still responsible for approximately 200,000 deaths annually worldwide. Reverse genetics systems for the manipulation of viral genomes are a powerful approach for studying viral replication and pathogenesis, and for developing vaccines and viral vectors. The understanding of the molecular mechanisms underlying RV pathogenesis, or development of next generation vaccines, has been hampered by the lack of a complete reverse genetics system. Recently, we developed a novel reverse genetics system which enabled recovery of recombinant RVs entirely from cloned cDNAs. This new strategy requires co-expression of a small transmembrane protein that accelerates cell-to-cell fusion and vaccinia virus capping enzyme. In this review, the strategies and history of the development of reverse genetics systems for the family Reoviridae are described.

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

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

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

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

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

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

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

Genomic changes detected after serial passages in cell culture of virulent human G1P[8] rotaviruses

Serial passages of a virulent mouse rotavirus in cell cultures caused a loss of virulence in mice. To gain insight into the genomic mutations in human rotavirus during cell culture and its attenuation in humans, we serially passaged three wild type human G1P[8] rotavirus strains (Wa, DC3695, DC5685) derived from diarrheal stool samples up to 60 times in two different cell cultures (human colon adenocarcinoma cell line: HT29, and primary African green monkey kidney cells: primary AGMK). We sequenced the whole genomes of 60 times-passaged strains and compared them with those of the original viruses. Most substitutions were detected in VP4, followed by substitutions in VP7 and NSP4 genes. Substitution at amino acid 385 in the putative VP4 fusion domain and substitution T45M in NSP4 genes were detected in all AGMK-passaged strains, respectively. These genomic changes are likely to correlate with a loss of rotavirus virulence in humans.

[Rotaviruses]

Rotavirus, a member of the family Reoviridae, was identified as the leading etiological agent of severe gastroenteritis in infants and young children in 1973. The rotavirus genome is composed of 11 gene segments of double-stranded (ds)RNA. During the last 40 years, a large amount of basic research on rotavirus structure, genome, antigen, replication, pathogenesis, epidemiology, immune responses, and evolution has been accumulated. This article reviews the fundamental aspects of rotavirology including recent important achievements in research.

Reassortment in segmented RNA viruses: mechanisms and outcomes

Segmented RNA viruses are widespread in nature and include important human, animal and plant pathogens, such as influenza viruses and rotaviruses. Although the origin of RNA virus genome segmentation remains elusive, a major consequence of this genome structure is the capacity for reassortment to occur during co-infection, whereby segments are exchanged among different viral strains. Therefore, reassortment can create viral progeny that contain genes that are derived from more than one parent, potentially conferring important fitness advantages or disadvantages to the progeny virus. However, for segmented RNA viruses that package their multiple genome segments into a single virion particle, reassortment also requires genetic compatibility between parental strains, which occurs in the form of conserved packaging signals, and the maintenance of RNA and protein interactions. In this Review, we discuss recent studies that examined the mechanisms and outcomes of reassortment for three well-studied viral families - Cystoviridae, Orthomyxoviridae and Reoviridae - and discuss how these findings provide new perspectives on the replication and evolution of segmented RNA viruses.

Challenging the Roles of NSP3 and Untranslated Regions in Rotavirus mRNA Translation

Rotavirus NSP3 is a translational surrogate of the PABP-poly(A) complex for rotavirus mRNAs. To further explore the effects of NSP3 and untranslated regions (UTRs) on rotavirus mRNAs translation, we used a quantitative in vivo assay with simultaneous cytoplasmic NSP3 expression (wild-type or deletion mutant) and electroporated rotavirus-like and standard synthetic mRNAs. This assay shows that the last four GACC nucleotides of viral mRNA are essential for efficient translation and that both the NSP3 eIF4G- and RNA-binding domains are required. We also show efficient translation of rotavirus-like mRNAs even with a 5’UTR as short as 5 nucleotides, while more than eleven nucleotides are required for the 3’UTR. Despite the weak requirement for a long 5’UTR, a good AUG environment remains a requirement for rotavirus mRNAs translation.

An Inhibitory Motif on the 5'UTR of Several Rotavirus Genome Segments Affects Protein Expression and Reverse Genetics Strategies

Rotavirus genome consists of eleven segments of dsRNA, each encoding one single protein. Viral mRNAs contain an open reading frame (ORF) flanked by relatively short untranslated regions (UTRs), whose role in the viral cycle remains elusive. Here we investigated the role of 5'UTRs in T7 polymerase-driven cDNAs expression in uninfected cells. The 5'UTRs of eight genome segments (gs3, gs5-6, gs7-11) of the simian SA11 strain showed a strong inhibitory effect on the expression of viral proteins. Decreased protein expression was due to both compromised transcription and translation and was independent of the ORF and the 3'UTR sequences. Analysis of several mutants of the 21-nucleotide long 5'UTR of gs 11 defined an inhibitory motif (IM) represented by its primary sequence rather than its secondary structure. IM was mapped to the 5' terminal 6-nucleotide long pyrimidine-rich tract 5'-GGY(U/A)UY-3'. The 5' terminal position within the mRNA was shown to be essentially required, as inhibitory activity was lost when IM was moved to an internal position. We identified two mutations (insertion of a G upstream the 5'UTR and the U to A mutation of the fifth nucleotide of IM) that render IM non-functional and increase the transcription and translation rate to levels that could considerably improve the efficiency of virus helper-free reverse genetics strategies.

Nonstructural protein Pns4 of rice dwarf virus is essential for viral infection in its insect vector

Background

Rice dwarf virus (RDV), a plant reovirus, is mainly transmitted by the green rice leafhopper, Nephotettix cincticeps, in a persistent-propagative manner. Plant reoviruses are thought to replicate and assemble within cytoplasmic structures called viroplasms. Nonstructural protein Pns4 of RDV, a phosphoprotein, is localized around the viroplasm matrix and forms minitubules in insect vector cells. However, the functional role of Pns4 minitubules during viral infection in insect vector is still unknown yet.

Methods

RNA interference (RNAi) system targeting Pns4 gene of RDV was conducted. Double-stranded RNA (dsRNA) specific for Pns4 gene was synthesized in vitro, and introduced into cultured leafhopper cells by transfection or into insect body by microinjection. The effects of the knockdown of Pns4 expression due to RNAi induced by synthesized dsRNA from Pns4 gene on viral replication and spread in cultured cells and insect vector were analyzed using immunofluorescence, western blotting or RT-PCR assays.

Results

In cultured leafhopper cells, the knockdown of Pns4 expression due to RNAi induced by synthesized dsRNA from Pns4 gene strongly inhibited the formation of minitubules, preventing the accumulation of viroplasms and efficient viral infection in insect vector cells. RNAi induced by microinjection of dsRNA from Pns4 gene significantly reduced the viruliferous rate of N. cincticeps. Furthermore, it also strongly inhibited the formation of minitubules and viroplasms, preventing efficient viral spread from the initially infected site in the filter chamber of intact insect vector.

Conclusions

Pns4 of RDV is essential for viral infection and replication in insect vector. It may directly participate in the functional role of viroplasm for viral replication and assembly of progeny virions during viral infection in leafhopper vector.

Mycoreovirus genome rearrangements associated with RNA silencing deficiency

Mycoreovirus 1 (MyRV1) has 11 double-stranded RNA genome segments (S1 to S11) and confers hypovirulence to the chestnut blight fungus, Cryphonectria parasitica. MyRV1 genome rearrangements are frequently generated by a multifunctional protein, p29, encoded by a positive-strand RNA virus, Cryphonectria hypovirus 1. One of its functional roles is RNA silencing suppression. Here, we explored a possible link between MyRV1 genome rearrangements and the host RNA silencing pathway using wild-type (WT) and mutant strains of both MyRV1 and the host fungus. Host strains included deletion mutants of RNA silencing components such as dicer-like (dcl) and argonaute-like (agl) genes, while virus strains included an S4 internal deletion mutant MyRV1/S4ss. Consequently, intragenic rearrangements with nearly complete duplication of the three largest segments, i.e. S1, S2 and S3, were observed even more frequently in the RNA silencing-deficient strains Δdcl2 and Δagl2 infected with MyRV1/S4ss, but not with any other viral/host strain combinations. An interesting difference was noted between genome rearrangement events in the two host strains, i.e. generation of the rearrangement required prolonged culture for Δagl2 in comparison with Δdcl2. These results suggest a role for RNA silencing that suppresses genome rearrangements of a dsRNA virus.

Complete genome characterization of recent and ancient Belgian pig group A rotaviruses and assessment of their evolutionary relationship with human rotaviruses

Group A rotaviruses (RVAs) are an important cause of diarrhea in young pigs and children. An evolutionary relationship has been suggested to exist between pig and human RVAs. This hypothesis was further investigated by phylogenetic analysis of the complete genomes of six recent (G2P[27], G3P[6], G4P[7], G5P[7], G9P[13], and G9P[23]) and one historic (G1P[7]) Belgian pig RVA strains and of all completely characterized pig RVAs from around the globe. In contrast to the large diversity of genotypes found for the outer capsid proteins VP4 and VP7, a relatively conserved genotype constellation (I5-R1-C1-M1-A8-N1-T7-E1-H1) was found for the other 9 genes in most pig RVA strains. VP1, VP2, VP3, NSP2, NSP4, and NSP5 genes of porcine RVAs belonged to genotype 1, which is shared with human Wa-like RVAs. However, for most of these gene segments, pig strains clustered distantly from human Wa-like RVAs, indicating that viruses from both species have entered different evolutionary paths. However, VP1, VP2, and NSP3 genes of some archival human strains were moderately related to pig strains. Phylogenetic analysis of the VP6, NSP1, and NSP3 genes, as well as amino acid analysis of the antigenic regions of VP7, further confirmed this evolutionary segregation. The present results also indicate that the species barrier is less strict for pig P[6] strains but that chances for successful spread of these strains in the human population are hampered by the better adaptation of pig RVAs to pig enterocytes. However, future surveillance of pig and human RVA strains is warranted.

IMPORTANCE

Rotaviruses are an important cause of diarrhea in many species, including pigs and humans. Our understanding of the evolutionary relationship between rotaviruses from both species is limited by the lack of genomic data on pig strains. In this study, recent and ancient Belgian pig rotavirus isolates were sequenced, and their evolutionary relationship with human Wa-like strains was investigated. Our data show that Wa-like human and pig strains have entered different evolutionary paths. Our data indicate that pig P[6] strains form the most considerable risk for interspecies transmission to humans. However, efficient spread of pig strains in the human population is most likely hampered by the adaptation of some crucial viral proteins to the cellular machinery of pig enterocytes. These data allow a better understanding of the risk for direct interspecies transmission events and the emergence of pig rotaviruses or pig-human reassortants in the human population.

Identification of luteolin as enterovirus 71 and coxsackievirus A16 inhibitors through reporter viruses and cell viability-based screening

Hand, foot and mouth disease (HFMD) is a common pediatric illness mainly caused by infection with enterovirus 71 (EV71) and coxsackievirus A16 (CA16). The frequent HFMD outbreaks have become a serious public health problem. Currently, no vaccine or antiviral drug for EV71/CA16 infections has been approved. In this study, a two-step screening platform consisting of reporter virus-based assays and cell viability‑based assays was developed to identify potential inhibitors of EV71/CA16 infection. Two types of reporter viruses, a pseudovirus containing luciferase-encoding RNA replicons encapsidated by viral capsid proteins and a full-length reporter virus containing enhanced green fluorescent protein, were used for primary screening of 400 highly purified natural compounds. Thereafter, a cell viability-based secondary screen was performed for the identified hits to confirm their antiviral activities. Three compounds (luteolin, galangin, and quercetin) were identified, among which luteolin exhibited the most potent inhibition of viral infection. In the cell viability assay and plaque reduction assay, luteolin showed similar 50% effective concentration (EC₅₀) values of about 10 μM. Luteolin targeted the post-attachment stage of EV71 and CA16 infection by inhibiting viral RNA replication. This study suggests that luteolin may serve as a lead compound to develop potent anti-EV71 and CA16 drugs.

Transfection of exogenous rotavirus rearranged RNA segments in cells infected with a WT rotavirus results in subsequent gene rearrangements

Group A rotaviruses, members of the family Reoviridae, are a major cause of infantile acute gastroenteritis. The rotavirus genome consists of 11 dsRNA segments. In some cases, an RNA segment is replaced by a rearranged RNA segment, which is derived from its standard counterpart by partial sequence duplication. It has been shown that some rearranged segments are preferentially encapsidated into viral progenies after serial passages in cell culture. Based on this characteristic, a reverse genetics system was used previously to introduce exogenous segment 7 rearrangements into an infectious rotavirus. This study extends this reverse genetics system to RNA segments 5 and 11. Transfection of exogenous rotavirus rearranged RNA segment 5 or 11 into cells infected with a WT helper rotavirus (bovine strain RF) resulted in subsequent gene rearrangements in the viral progeny. Whilst recombinant viruses were rescued with an exogenous rearranged segment 11, the exogenous segment was modified by a secondary rearrangement. The occurrence of spontaneous rearrangements of WT or exogenous segments is a major hindrance to the use of this reverse genetics approach.

Virulence-associated genome mutations of murine rotavirus identified by alternating serial passages in mice and cell cultures

Although significant clinical efficacy and safety of rotavirus vaccines were recently revealed in many countries, the mechanism of their attenuation is not well understood. We passaged serially a cell culture-adapted murine rotavirus EB strain in mouse pups or in cell cultures alternately and repeatedly and fully sequenced all 11 genes of 21 virus samples passaged in mice or in cell cultures. Sequence analysis revealed that mouse-passaged viruses that regained virulence almost consistently acquired four kinds of amino acid (aa) substitutions in VP4 and substitution in aa 37 (Val to Ala) in NSP4. In addition, they gained and invariably conserved the 3' consensus sequence in NSP1. The molecular changes occurred along with the acquisition of virulence during passages in mice and then disappeared following passages in cell cultures. Intraperitoneal injection of recombinant NSP4 proteins confirmed the aa 37 site as important for its diarrheagenic activity in mice. These genome changes are likely to be correlated with rotavirus virulence.

IMPORTANCE

Serial passage of a virulent wild-type virus in vitro often results in loss of virulence of the virus in an original animal host, while serial passage of a cell culture-adapted avirulent virus in vivo often gains virulence in an animal host. Actually, live attenuated virus vaccines were originally produced by serial passage in cell cultures. Although clinical efficacy and safety of rotavirus vaccines were recently revealed, the mechanism of their attenuation is not well understood. We passaged serially a murine rotavirus by alternating switch of host (mice or cell cultures) repeatedly and sequenced the eleven genes of the passaged viruses to identify mutations associated with the emergence or disappearance of virulence. Sequence analysis revealed that changes in three genes (VP4, NSP1, and NSP4) were associated with virulence in mice. Intraperitoneal injection of recombinant NSP4 proteins confirmed its diarrheagenic activity in mice. These genome changes are likely to be correlated with rotavirus virulence.

Further characterisation of rotavirus cores: Ss(+)RNAs can be packaged in vitro but packaging lacks sequence specificity

Rotavirus (RV) cores were released from double-layered particles (DLPs) by high concentrations of CaCl2, purified and 'opened' by treatment with EDTA or EGTA. Under appropriate in vitro conditions DLPs have been shown to have transcriptase and 'open cores' replicase activity. Furthermore, it has been demonstrated that transcriptase activity and infectivity of native cores can be restored by transcapsidation with VP6, VP7 and VP4. The missing link for particle reconstitution in vitro has been the manipulation of 'open cores' to become functionally active cores again. The experiments described here were undertaken with the aim of exploring packaging of RV RNAs into opened cores in vitro. Rotavirus cores were opened by approximately 200μM EGTA, leading to the release of genomic dsRNA. Conversely, RV cores were found to be stable in the presence of minimum concentrations of Ca(2+), Mg(2+), spermidine(3+) and cobalthexamine(3+) of between 40 and 300 μM. Aggregates of purified cores were resolved in the presence of 0.3mM deoxycholate (minimum concentration). Core shells opened with EGTA were reconstituted by the addition of di- or trivalent cations within 2 min of the opening procedure. Addition of purified, baculovirus recombinant-expressed VP6 to native and reconstituted cores led to the formation of DLPs or DLP-like particles, which upon transfection into MA104 cells were infectious. The rescued infectivity likely originated in part from unopened and in part from reconstituted cores. Radiolabelled RV (+) ssRNAs could be packaged into reconstituted cores and DLPs, as indicated by resistance to RNase I digestion. The packaging reaction was, however, not RV RNA sequence-specific, since unrelated ssRNAs, such as those transcribed from HIV-2 cDNAs, were also packaged. The kinetics of packaging of homologous and heterologous RNAs were similar, as evidenced by competitive packaging assays. None of the packaged in vitro engineered RNA segments has so far been rescued into infectious virus.

Experimental pathways towards developing a rotavirus reverse genetics system: synthetic full length rotavirus ssRNAs are neither infectious nor translated in permissive cells

At present the ability to create rationally engineered mutant rotaviruses is limited because of the lack of a tractable helper virus-free reverse genetics system. Using the cell culture adapted bovine RV RF strain (G6P6 [1]), we have attempted to recover infectious RV by co-transfecting in vitro transcribed ssRNAs which are identical in sequence to the positive sense strand of each of the 11 dsRNA genomic segments of the RF strain. The RNAs were produced either from cDNAs cloned by a target sequence-independent procedure, or from purified double layered RV particles (DLPs). We have validated their translational function by in vitro synthesis of (35)S-labelled proteins in rabbit reticulocyte lysates; all 11 proteins encoded by the RV genome were expressed. Transfection experiments with DLP- or cDNA-derived ssRNAs suggested that the RNAs do not act independently as mRNAs for protein synthesis, once delivered into various mammalian cell lines, and exhibit cytotoxicity. Transfected RNAs were not infectious since a viral cytopathic effect was not observed after infection of MA104 cells with lysates from transfected cells. By contrast, an engineered mRNA encoding eGFP was expressed when transfected under identical conditions into the same cell lines. Co-expression of plasmids encoding NSP2 and NSP5 using a fowlpox T7 polymerase recombinant virus revealed viroplasm-like structure formation, but this did not enable the translation of transfected RV ssRNAs. Attempts to recover RV from ssRNAs transcribed intracellularly from transfected cDNAs were also unsuccessful and suggested that these RNAs were also not translated, in contrast to successful translation from a transfected cDNA encoding an eGFP mRNA.

Engineering recombinant reoviruses with tandem repeats and a tetravirus 2A-like element for exogenous polypeptide expression

We tested a strategy for engineering recombinant mammalian reoviruses (rMRVs) to express exogenous polypeptides. One important feature is that these rMRVs are designed to propagate autonomously and can therefore be tested in animals as potential vaccine vectors. The strategy has been applied so far to three of the 10 MRV genome segments: S3, M1, and L1. To engineer the modified segments, a 5' or 3' region of the essential, long ORF in each was duplicated, and then exogenous sequences were inserted between the repeats. The inner repeat and exogenous insert were positioned in frame with the native protein-encoding sequences but were separated from them by an in-frame "2A-like" sequence element that specifies a cotranslational "stop/continue" event releasing the exogenous polypeptide from the essential MRV protein. This design preserves a terminal region of the MRV genome segment with essential activities in RNA packaging, assortment, replication, transcription, and/or translation and alters the encoded MRV protein to a limited degree. Recovery of rMRVs with longer inserts was made more efficient by wobble-mutagenizing both the inner repeat and the exogenous insert, which possibly helped via respective reductions in homologous recombination and RNA structure. Immunogenicity of a 300-aa portion of the simian immunodeficiency virus Gag protein expressed in mice by an L1-modified rMRV was confirmed by detection of Gag-specific T-cell responses. The engineering strategy was further used for mapping the minimal 5'-terminal region essential to MRV genome segment S3.

Generation of genetically stable recombinant rotaviruses containing novel genome rearrangements and heterologous sequences by reverse genetics

The rotavirus (RV) genome consists of 11 segments of double-stranded RNA (dsRNA). Typically, each segment contains 5' and 3' untranslated regions (UTRs) that flank an open reading frame (ORF) encoding a single protein. RV variants with segments of atypical size owing to sequence rearrangements have been described. In many cases, the rearrangement originates from a partial head-to-tail sequence duplication that initiates after the stop codon of the ORF, leaving the protein product of the segment unaffected. To probe the limits of the RV genome to accommodate additional genetic sequence, we used reverse genetics to insert duplications (analogous to synthetic rearrangements) and heterologous sequences into the 3' UTR of the segment encoding NSP2 (gene 8). The approach allowed the recovery of recombinant RVs that contained sequence duplications (up to 200 bp) and heterologous sequences, including those for FLAG, the hepatitis C virus E2 epitope, and the internal ribosome entry site of cricket paralysis virus. The recombinant RVs grew to high titer (>10(7) PFU/ml) and remained genetically stable during serial passage. Despite their longer 3' UTRs, rearranged RNAs of recombinant RVs expressed wild-type levels of protein in vivo. Competitive growth experiments indicated that, unlike RV segments with naturally occurring sequence duplications, genetically engineered segments were less efficiently packaged into progeny viruses. Thus, features of naturally occurring rearranged segments, other than their increased length, contribute to their enhanced packaging phenotype. Our results define strategies for developing recombinant RVs as expression vectors, potentially leading to next-generation RV vaccines that induce protection against other infectious agents.

[Reverse genetics system of rotaviruses: development and application for analysis of VP4 spike protein]

The rotavirus genome is composed of 11 gene segments of double-stranded (ds)RNA. Reverse genetics is the powerful and ideal methodology for the molecular analysis of virus biology, which enables the virus genome to be artificially manipulated. Although reverse genetics systems exist for nearly all major groups of RNA viruses, development of such a system for rotaviruses is more challenging owing in part to the technical complexity of manipulation of their multi-segmented genome. A breakthrough in the field of rotavirus reverse genetics came in 2006, when we established the first reverse genetics system for rotaviruses, which is a partially plasmid-based system that permits replacement of a viral gene segment with the aid of a helper virus. Although this helper virus-driven system is technically limited and gives low levels of recombinant viruses, it allows alteration of the rotavirus genome, thus contributing to our understanding of these medically important viruses. In this review, I describe the development and application of our rotavirus reverse genetics system, and its future perspectives.

Development of a Coxsackievirus A16 neutralization assay based on pseudoviruses for measurement of neutralizing antibody titer in human serum

Serum neutralizing antibody titers are indicative of protective immunity against Coxsackievirus A16 (CV-A16) and Enterovirus 71 (EV71), the two main etiological agents of hand, foot and mouth disease (HFMD), and provide the basis for evaluating vaccine efficacy. The current CV-A16 neutralization assay based on inhibition of cytopathic effects requires manual microscopic examination, which is time-consuming and labor-intensive. In this study, a high-throughput neutralization assay was developed by employing CV-A16 pseudoviruses expressing luciferase for detecting infectivity in rhabdomyosarcoma (RD) cells and measuring serum viral neutralizing antibodies. Without the need to use infectious CV-A16 strains, the neutralizing antibody titer against CV-A16 could be determined within 15 h by measuring luciferase signals by this assay. The pseudovirus CV-A16 neutralization assay (pCNA) was validated by comparison with a conventional CV-A16 neutralization assay (cCNA) in testing 174 human serum samples collected from children (age <5 years). The neutralizing antibody titers determined by these two assays were well correlated (R² = 0.7689). These results suggest that the pCNA can serve as a rapid and objective procedure for the measurement of neutralizing antibodies against CV-A16.

Rotavirus viroplasm fusion and perinuclear localization are dynamic processes requiring stabilized microtubules

Rotavirus viroplasms are cytosolic, electron-dense inclusions corresponding to the viral machinery of replication responsible for viral template transcription, dsRNA genome segments replication and assembly of new viral cores. We have previously observed that, over time, those viroplasms increase in size and decrease in number. Therefore, we hypothesized that this process was dependent on the cellular microtubular network and its associated dynamic components. Here, we present evidence demonstrating that viroplasms are dynamic structures, which, in the course of an ongoing infection, move towards the perinuclear region of the cell, where they fuse among each other, thereby gaining considerably in size and, simultaneouly, explaining the decrease in numbers. On the viral side, this process seems to depend on VP2 for movement and on NSP2 for fusion. On the cellular side, both the temporal transition and the maintenance of the viroplasms are dependent on the microtubular network, its stabilization by acetylation, and, surprisingly, on a kinesin motor of the kinesin-5 family, Eg5. Thus, we provide for the first time deeper insights into the dynamics of rotavirus replication, which can explain the behavior of viroplasms in the infected cell.

Genetics and reverse genetics of rotavirus

Rotavirus is a member of the family Reoviridae, which have genomes consisting of 10-12 double-stranded RNA segments. The functions of proteins encoded by each segment of the rotavirus genome have been studied extensively by several methods including reassortants, temperature-sensitive mutants, isolates with rearranged RNA segments, RNAi analysis, and other procedures. However, as found for most RNA viruses, the technique of reverse genetics is required for precise genotype/phenotype correlation, for the analysis of the role of specific mutation in replication process and pathogenesis, and for the development of vectors and vaccines. In 2006, we presented the first description of a reverse genetics system for rotavirus, although a helper virus and a selection system are required. Since then, two other approaches have been reported for rotavirus reverse genetics, both requiring the presence of a helper virus. A tractable, helper virus-free reverse genetics system for rotavirus has not been developed so far, in contrast to the recent developments of plasmid only-based reverse genetics systems for other members of the Reoviridae.

Comparative analysis of Reoviridae reverse genetics methods

Effective methods to engineer the segmented, double-stranded RNA genomes of Reoviridae viruses have only recently been developed. Mammalian orthoreoviruses (MRV) and bluetongue virus (BTV) can be recovered from entirely recombinant reagents, significantly improving the capacity to study the replication, pathogenesis, and transmission of these viruses. Conversely, rotaviruses (RVs), which are the major etiological agent of severe gastroenteritis in infants and children, have thus far only been modified using single-segment replacement methods. Reoviridae reverse genetics techniques universally rely on site-specific initiation of transcription by T7 RNA polymerase to generate the authentic 5' end of recombinant RNA segments, but they vary in how the RNAs are introduced into cells: recombinant BTV is recovered by transfection of in vitro transcribed RNAs, whereas recombinant MRV and RV RNAs are transcribed intracellularly from transfected plasmid cDNAs. Additionally, several parameters have been identified in each system that are essential for recombinant virus recovery. Generating recombinant BTV requires the use of 5' capped RNAs and is enhanced by multiple rounds of RNA transfection, suggesting that translation of viral proteins is likely the rate-limiting step. For RV, the efficiency of recovery is almost entirely dependent on the strength of the selection mechanism used to isolate the single-segment recombinant RV from the unmodified helper virus. The reverse genetics methods for BTV and RV are presented and compared to the previously described MRV methods. Analysis and comparison of each method suggest several key lines of research that might lead to a reverse genetics system for RV, analogous to those used for MRV and BTV.

Rotavirus variant replicates efficiently although encoding an aberrant NSP3 that fails to induce nuclear localization of poly(A)-binding protein

The rotavirus (RV) non-structural protein NSP3 forms a dimer that has binding domains for the translation initiation factor eIF4G and for a conserved 3'-terminal sequence of viral mRNAs. Through these activities, NSP3 has been proposed to promote viral mRNA translation by directing circularization of viral polysomes. In addition, by disrupting interactions between eIF4G and the poly(A)-binding protein (PABP), NSP3 has been suggested to inhibit translation of host polyadenylated mRNAs and to stimulate relocalization of PABP from the cytoplasm to the nucleus. Herein, we report the isolation and characterization of SA11-4Fg7re, an SA11-4F RV derivative that contains a large sequence duplication initiating within the genome segment (gene 7) encoding NSP3. Our analysis showed that mutant NSP3 (NSP3m) encoded by SA11-4Fg7re is almost twice the size of the wild-type protein and retains the capacity to dimerize. However, in comparison to wild-type NSP3, NSP3m has a decreased capacity to interact with eIF4G and to suppress the translation of polyadenylated mRNAs. In addition, NSP3m fails to induce the nuclear accumulation of PABP in infected cells. Despite the defective activities of NSP3m, the levels of viral protein and progeny virus produced in SA11-4Fg7re- and SA11-4F-infected cells were indistinguishable. Collectively, these data are consistent with a role for NSP3 in suppressing host protein synthesis through antagonism of PABP activity, but also suggest that NSP3 functions may have little or no impact on the efficiency of virus replication in widely used RV-permissive cell lines.