Functional analysis of RNA structures present at the 3' extremity of the murine norovirus genome: the variable polypyrimidine tract plays a role in viral virulence

Highs and Lows in Calicivirus Reverse Genetics

In virology, the term reverse genetics refers to a set of methodologies in which changes are introduced into the viral genome and their effects on the generation of infectious viral progeny and their phenotypic features are assessed. Reverse genetics emerged thanks to advances in recombinant DNA technology, which made the isolation, cloning, and modification of genes through mutagenesis possible. Most virus reverse genetics studies depend on our capacity to rescue an infectious wild-type virus progeny from cell cultures transfected with an "infectious clone". This infectious clone generally consists of a circular DNA plasmid containing a functional copy of the full-length viral genome, under the control of an appropriate polymerase promoter. For most DNA viruses, reverse genetics systems are very straightforward since DNA virus genomes are relatively easy to handle and modify and are also (with few notable exceptions) infectious per se. This is not true for RNA viruses, whose genomes need to be reverse-transcribed into cDNA before any modification can be performed. Establishing reverse genetics systems for members of the Caliciviridae has proven exceptionally challenging due to the low number of members of this family that propagate in cell culture. Despite the early successful rescue of calicivirus from a genome-length cDNA more than two decades ago, reverse genetics methods are not routine procedures that can be easily extrapolated to other members of the family. Reports of calicivirus reverse genetics systems have been few and far between. In this review, we discuss the main pitfalls, failures, and delays behind the generation of several successful calicivirus infectious clones.

Production of infectious reporter murine norovirus by VP2 trans-complementation

Human norovirus (HuNoV) causes gastroenteritis, a disease with no effective therapy or vaccine, and does not grow well in culture. Murine norovirus (MNV) easily replicates in cell cultures and small animals and has often been used as a model to elucidate the structural and functional characteristics of HuNoV. An MNV plasmid-based reverse genetics system was developed to produce the modified recombinant virus. In this study, we attempted to construct the recombinant virus by integrating a foreign gene into MNV ORF3, which encodes the minor structural protein VP2. Deletion of VP2 expression abolished infectious particles from MNV cDNA clones, and supplying exogenous VP2 to the cells rescued the infectivity of cDNA clones without VP2 expression. In addition, the coding sequence of C-terminal ORF3 was essential for cDNA clones compensated with VP2 to produce infectious particles. Furthermore, the recombinant virus with exogenous reporter genes in place of the dispensable region of ORF3 was propagated when VP2 was constitutively supplied. Our findings indicate that foreign genes can be transduced into the norovirus ORF3 region when VP2 is supplied and that successive propagation of modified recombinant norovirus could lead to the development of norovirus-based vaccines or therapeutics.IMPORTANCEIn this study, we revealed that some of the coding regions of ORF3 could be replaced by a foreign gene and infectious virus could be produced when VP2 was supplied. Propagation of this virus depended on VP2 being supplied in trans, indicating that this virus could infect only once. Our findings help to elucidate the functions of VP2 in the virus lifecycle and to develop other caliciviral vectors for recombinant attenuated live enteric virus vaccines or therapeutics tools.

Recent insights into reverse genetics of norovirus

Norovirus is the leading cause of viral gastroenteritis globally, and poses substantial threats to public health. Despite substantial progress made in preventing norovirus diseases, the lack of a robust virus culture system has hampered biological research and effective strategies to combat this pathogen. Reverse genetic system is the technique to generate infectious viruses from cloned genetic constructs, which is a powerful tool for the investigation of viral pathogenesis and for the development of novel drugs and vaccines. The strategies of reverse genetics include bacterial artificial chromosomes, vaccinia virus vectors, and entirely plasmid-based systems. Since each strategy has its pros and cons, choosing appropriate approaches will greatly improve the efficiency of virus rescue. Reverse genetic systems that have been employed for norovirus greatly extend its life cycle and facilitate the development of medical countermeasures. In this review, we summarize the current knowledge on the structure, transmission, genetic evolution and clinical manifestations of norovirus, and describe recent advances in the studies of norovirus reverse genetics as well as its future prospects for therapeutics and vaccine development.

Protein Nucleotidylylation in +ssRNA Viruses

Nucleotidylylation is a post-transcriptional modification important for replication in the picornavirus supergroup of RNA viruses, including members of the Caliciviridae, Coronaviridae, Picornaviridae and Potyviridae virus families. This modification occurs when the RNA-dependent RNA polymerase (RdRp) attaches one or more nucleotides to a target protein through a nucleotidyl-transferase reaction. The most characterized nucleotidylylation target is VPg (viral protein genome-linked), a protein linked to the 5' end of the genome in Caliciviridae, Picornaviridae and Potyviridae. The nucleotidylylation of VPg by RdRp is a critical step for the VPg protein to act as a primer for genome replication and, in Caliciviridae and Potyviridae, for the initiation of translation. In contrast, Coronaviridae do not express a VPg protein, but the nucleotidylylation of proteins involved in replication initiation is critical for genome replication. Furthermore, the RdRp proteins of the viruses that perform nucleotidylylation are themselves nucleotidylylated, and in the case of coronavirus, this has been shown to be essential for viral replication. This review focuses on nucleotidylylation within the picornavirus supergroup of viruses, including the proteins that are modified, what is known about the nucleotidylylation process and the roles that these modifications have in the viral life cycle.

Norovirus VPg Binds RNA through a Conserved N-Terminal K/R Basic Patch

The viral protein genome-linked (VPg) of noroviruses is a multi-functional protein that participates in essential roles during the viral replication cycle. Predictive analyses indicate that murine norovirus (MNV) VPg contains a disordered N-terminal region with RNA binding potential. VPg proteins were expressed with an N-terminal spidroin fusion protein in insect cells and the interaction with RNA investigated by electrophoretic mobility shift assays (EMSA) against a series of RNA probes (pentaprobes) representing all possible five nucleotide combinations. MNV VPg and human norovirus (HuNV) VPg proteins were directly bound to RNA in a non-specific manner. To identify amino acids involved in binding to RNA, all basic (K/R) residues in the first 12 amino acids of MNV VPg were mutated to alanine. Removal of the K/R amino acids eliminated RNA binding and is consistent with a K/R basic patch RNA binding motif within the disordered N-terminal region of norovirus VPgs. Finally, we show that mutation of the K/R basic patch required for RNA binding eliminates the ability of MNV VPg to induce a G0/G1 cell cycle arrest.

Structures and Functions of the 3' Untranslated Regions of Positive-Sense Single-Stranded RNA Viruses Infecting Humans and Animals

The 3′ untranslated region (3′ UTR) of positive-sense single-stranded RNA [ssRNA(+)] viruses is highly structured. Multiple elements in the region interact with other nucleotides and proteins of viral and cellular origin to regulate various aspects of the virus life cycle such as replication, translation, and the host-cell response. This review attempts to summarize the primary and higher order structures identified in the 3′UTR of ssRNA(+) viruses and their functional roles.

Viral replicons as valuable tools for drug discovery

RNA viruses can cause severe diseases such as dengue, Lassa, chikungunya and Ebola. Many of these viruses can only be propagated under high containment levels, necessitating the development of low containment surrogate systems such as subgenomic replicons and minigenome systems. Replicons are self-amplifying recombinant RNA molecules expressing proteins sufficient for their own replication but which do not produce infectious virions. Replicons can persist in cells and are passed on during cell division, enabling quick, efficient and high-throughput testing of drug candidates that act on viral transcription, translation and replication. This review will explore the history and potential for drug discovery of hepatitis C virus, dengue virus, respiratory syncytial virus, Ebola virus and norovirus replicon and minigenome systems.

Replicative fitness recuperation of a recombinant murine norovirus - in vitro reciprocity of genetic shift and drift

Noroviruses are recognized as the major cause of non-bacterial gastroenteritis in humans. Molecular mechanisms driving norovirus evolution are the accumulation of point mutations and recombination. Recombination can create considerable changes in a viral genome, potentially eliciting a fitness cost, which must be compensated via the adaptive capacity of a recombinant virus. We previously described replicative fitness reduction of the first in vitro generated WU20-CW1 recombinant murine norovirus, RecMNV. In this follow-up study, RecMNV's capability of replicative fitness recuperation and genetic characteristics of RecMNV progenies at early and late stages of an adaptation experiment were evaluated. Replicative fitness regain of the recombinant was demonstrated via growth kinetics and plaque size differences between viral progenies prior to and post serial in vitro passaging. Point mutations at consensus and sub-consensus population levels of early and late viral progenies were characterized via next-generation sequencing and putatively associated to fitness changes. To investigate the effect of genomic changes separately and in combination in the context of a lab-generated inter-MNV infectious virus, mutations were introduced into a recombinant WU20-CW1 cDNA for subsequent DNA-based reverse genetics recovery. We thus associated fitness loss of RecMNV to a C7245T mutation and functional VP2 (ORF3) truncation and demonstrated individual and cumulative compensatory effects of one synonymous OFR2 and two non-synonymous ORF1 consensus-level mutations acquired during successive rounds of in vitro replication. Our data provide evidence of viral adaptation in a controlled environment via genetic drift after genetic shift induced a fitness cost of an infectious recombinant norovirus.

[Review Norovirus]

Noroviruses commonly cause infectious gastroenteritis and massive food poisoning. There is an urgent need to elucidate the infection mechanism of noroviruses and to develop vaccines and therapeutic drugs. In addition to human disease, noroviruses have been implicated in animal disease. Noroviruses that cause murine diseases can be propagated in strained cultured cells, and for many years, murine norovirus has been used as a model for human noroviruses that could not be propagated in cultured cells. That model and advances in technology have been instrumental in basic studies of noroviruses. From structural biology, noroviruses undergo dynamic shape changes to improve their infectivity when they infect cells. New culture techniques have made human intestinal organoids available for studying the mechanisms of pathogenic expression of human noroviruses in the intestinal tract, mechanisms of infection growth, and the search for receptor molecules. Vaccines and antivirals using human intestinal organoids are under active development, and some are already in clinical trials. In this paper, I review the latest research results, vaccine development, and other advances from the history of norovirus discovery.

Noroviruses subvert the core stress granule component G3BP1 to promote viral VPg-dependent translation

Knowledge of the host factors required for norovirus replication has been hindered by the challenges associated with culturing human noroviruses. We have combined proteomic analysis of the viral translation and replication complexes with a CRISPR screen, to identify host factors required for norovirus infection. The core stress granule component G3BP1 was identified as a host factor essential for efficient human and murine norovirus infection, demonstrating a conserved function across the Norovirus genus. Furthermore, we show that G3BP1 functions in the novel paradigm of viral VPg-dependent translation initiation, contributing to the assembly of translation complexes on the VPg-linked viral positive sense RNA genome by facilitating ribosome recruitment. Our data uncovers a novel function for G3BP1 in the life cycle of positive sense RNA viruses and identifies the first host factor with pan-norovirus pro-viral activity.

Parallel molecular evolution and adaptation in viruses

Parallel molecular evolution is the independent evolution of the same genotype or phenotype from distinct ancestors. The simple genomes and rapid evolution of many viruses mean they are useful model systems for studying parallel evolution by natural selection. Parallel adaptation occurs in the context of several viral behaviours, including cross-species transmission, drug resistance, and host immune escape, and its existence suggests that at least some aspects of virus evolution and emergence are repeatable and predictable. We introduce examples of virus parallel evolution and summarise key concepts. We outline the difficulties in detecting parallel adaptation using virus genomes, with a particular focus on phylogenetic and structural approaches, and we discuss future approaches that may improve our understanding of the phenomenon.

Human astroviruses: in silico analysis of the untranslated region and putative binding sites of cellular proteins

Human astrovirus (HAstV) constitutes a major cause of acute gastroenteritis in children. The viral 5' and 3' untranslated regions (UTR) have been involved in the regulation of several molecular mechanisms. However, in astrovirues have been less characterized. Here, we analyzed the secondary structures of the 5' and 3' UTR of HAstV, as well as their putative target sites that might be recognized by cellular factors. To our knowledge, this is the first bioinformatic analysis that predicts the HAstV 5' UTR secondary structure. The analysis showed that both the UTR sequence and secondary structure are highly conserved in all HAstVs analyzed, suggesting their regulatory role of viral activities. Notably, the UTRs of HAstVs contain putative binding sites for the serine/arginine-rich factors SRSF2, SRSF5, SRSF6, SRSF3, and the multifunctional hnRNPE2 protein. More importantly, putative binding sites for PTB were localized in single-stranded RNA sequences, while hnRNPE2 sites were localized in double-stranded sequence of the HAstV 5' and 3' UTR structures. These analyses suggest that the combination of SRSF proteins, hnRNPE2 and PTB described here could be involved in the maintenance of the secondary structure of the HAstVs, possibly allowing the recruitment of the replication complex that selects and recruits viral RNA replication templates.

Targeting macrophage- and intestinal epithelial cell-specific microRNAs against norovirus restricts replication in vivo

Until recently, our understanding of the cellular tropism of human norovirus (HuNoV), a major cause of viral gastroenteritis, has been limited. Immune cells and intestinal epithelial cells (IECs) have been proposed as targets of HuNoV replication in vivo, although the contribution of each to pathogenesis and transmission is unknown. Murine norovirus (MNV) is widely used as a surrogate model for HuNoV, as it replicates in cultured immune cells. The importance of the complete MNV immune cell tropism in vivo has not been determined. Recent work has linked replication in IECs to viral persistence in vivo. MNV provides a model to assess the relative contribution of each cell tropism to viral replication in immunocompetent native hosts. Here we exploited cell-specific microRNAs to control MNV replication, through insertion of microRNA target sequences into the MNV genome. We demonstrated the utility of this approach for MNV in vitro by selectively reducing replication in microglial cells, using microglial-specific miR-467c. We then showed that inserting a target sequence for the haematopoietic-specific miR-142-3p abrogated replication in a macrophage cell line. The presence of a target sequence for either miR-142-3p or IEC miR-215 significantly reduced viral secretion during the early stages of a persistent infection in immunocompetent mice, confirming that both cell types support viral replication in vivo. This study provides additional evidence that MNV shares the IEC tropism of HuNoVs in vivo, and now provides a model to dissect the contribution of replication in each cell type to viral pathogenesis and transmission in a native host.

Molecular characterization of a novel bat-associated circovirus with a poly-T tract in the 3' intergenic region

The family Circoviridae comprises a large group of small circular single-stranded DNA viruses with several members causing severe pig and poultry diseases. In recent years the number of new viruses within the family has had an explosive increase showing a high level of genetic diversity and a broad host range. In this report we describe two more circoviruses identified from bats in Yunnan and Heilongjiang provinces in China. Full genome sequencing has revealed that these bat associated circoviruses (bat ACV) should be classified as new species within the genus Circovirus based on the demarcation criteria of the International Committee on the Taxonomy of Viruses (ICTV). The most striking result is the novel finding of a 21-28 nt polythymidine (poly-T) tract in the 3' terminal intergenic region of bat ACV isolates from Heilongjiang province. To understand its role in viral replication, a wild type bat ACV and a mutated version with the entire poly-T deleted were rescued through construction of infectious clones. Replication comparison in vitro showed that the poly-T is not essential for viral replication. Identification of additional bat ACV isolates and study of their biological characteristics will be the main task in future to understand the potential roles of bats in transmission of circoviruses to terrestrial mammals and humans.

Structure and Function of Caliciviral RNA Polymerases

Caliciviruses are a leading agent of human and animal gastroenteritis and respiratory tract infections, which are growing concerns in immunocompromised individuals. However, no vaccines or therapeutics are yet available. Since the rapid rate of genetic evolution of caliciviruses is mainly due to the error-prone nature of RNA-dependent RNA polymerase (RdRp), this article focuses on recent studies of the structures and functions of RdRp from caliciviruses. It also provides recent advances in the interactions of RdRp with virion protein genome-linked (VPg) and RNA and the structural and functional features of its precursor.

Norovirus Infection: Replication, Manipulation of Host, and Interaction with the Host Immune Response

Noroviruses (NoVs) belong to the Caliciviridae family of viruses and are responsible for causing the majority of gastroenteritis outbreaks worldwide. In the past decade, research on NoV biology has intensified because of the discovery of murine NoV and subsequently the first cell culture system and small animal model for NoV replication and pathogenesis. In this review, we discuss the current literature on NoV biology, focusing particularly on NoV replication and the interaction between NoV and the host immune response. Understanding the NoV replication cycle and its interaction with cellular processes and innate immune immunity will help develop molecular targets to control human NoV infection and prevent outbreaks. In addition to the innate immune response, we have documented the current efforts to develop NoV vaccines to control outbreaks.

Molecular chaperone Hsp90 is a therapeutic target for noroviruses

Human noroviruses (HuNoV) are a significant cause of acute gastroenteritis in the developed world, and yet our understanding of the molecular pathways involved in norovirus replication and pathogenesis has been limited by the inability to efficiently culture these viruses in the laboratory. Using the murine norovirus (MNV) model, we have recently identified a network of host factors that interact with the 5' and 3' extremities of the norovirus RNA genome. In addition to a number of well-known cellular RNA binding proteins, the molecular chaperone Hsp90 was identified as a component of the ribonucleoprotein complex. Here, we show that the inhibition of Hsp90 activity negatively impacts norovirus replication in cell culture. Small-molecule-mediated inhibition of Hsp90 activity using 17-DMAG (17-dimethylaminoethylamino-17-demethoxygeldanamycin) revealed that Hsp90 plays a pleiotropic role in the norovirus life cycle but that the stability of the viral capsid protein is integrally linked to Hsp90 activity. Furthermore, we demonstrate that both the MNV-1 and the HuNoV capsid proteins require Hsp90 activity for their stability and that targeting Hsp90 in vivo can significantly reduce virus replication. In summary, we demonstrate that targeting cellular proteostasis can inhibit norovirus replication, identifying a potential novel therapeutic target for the treatment of norovirus infections.

IMPORTANCE

HuNoV are a major cause of acute gastroenteritis around the world. RNA viruses, including noroviruses, rely heavily on host cell proteins and pathways for all aspects of their life cycle. Here, we identify one such protein, the molecular chaperone Hsp90, as an important factor required during the norovirus life cycle. We demonstrate that both murine and human noroviruses require the activity of Hsp90 for the stability of their capsid proteins. Furthermore, we demonstrate that targeting Hsp90 activity in vivo using small molecule inhibitors also reduces infectious virus production. Given the considerable interest in the development of Hsp90 inhibitors for use in cancer therapeutics, we identify here a new target that could be explored for the development of antiviral strategies to control norovirus outbreaks and treat chronic norovirus infection in immunosuppressed patients.

Nuclear proteins hijacked by mammalian cytoplasmic plus strand RNA viruses

Plus strand RNA viruses that replicate in the cytoplasm face challenges in supporting the numerous biosynthetic functions required for replication and propagation. Most of these viruses are genetically simple and rely heavily on co-opting cellular proteins, particularly cellular RNA-binding proteins, into new roles for support of virus infection at the level of virus-specific translation, and building RNA replication complexes. In the course of infectious cycles many nuclear-cytoplasmic shuttling proteins of mostly nuclear distribution are detained in the cytoplasm by viruses and re-purposed for their own gain. Many mammalian viruses hijack a common group of the same factors. This review summarizes recent gains in our knowledge of how cytoplasmic RNA viruses use these co-opted host nuclear factors in new functional roles supporting virus translation and virus RNA replication and common themes employed between different virus groups.

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Nuclear shuttling host proteins are commonly hijacked by RNA viruses to support replication.

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A limited group of ubiquitous RNA binding proteins are commonly hijacked by a broad range of viruses.

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Key virus proteins alter roles of RNA binding proteins in different stages of virus replication.

Functions of the 5' and 3' ends of calicivirus genomes

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Noroviruses are now recognized as the most common cause of viral gastroenteritis.

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The 5′ and 3′ ends of caliciviruses genome fold into characteristic structures conserved within the family.

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The tirmini of calicivirus genome is involved in recruiting host factors to the replication complex.

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The 5′ and 3′ ends of the MNV genome have been shown to interact with host proteins and further stabilize this interaction.

The Caliciviridae family of small positive sense RNA viruses contains a diverse range of pathogens of both man and animals. The molecular mechanisms of calicivirus genome replication and translation have not been as widely studied as many other RNA viruses. With the relatively recent development of robust cell culture and reverse genetics systems for several members of the Caliciviridae family, a more in-depth analysis of the finer detail of the viral life cycle has now been obtained. As a result, the identification and characterization of the role of RNA structures in the calicivirus life cycle has also been possible. This review aims to summarize the current state of knowledge with respect to the role of RNA structures at the termini of calicivirus genomes.

Genetic characteristics of the complete feline kobuvirus genome

We sequenced the complete genome of a feline kobuvirus and determined relationships with other kobuviruses. This kobuvirus has an 8,269-nucleotide-long RNA genome, excluding the poly(A) tail. The genome contains a 7,311-bp open reading frame (ORF) encoding a putative polyprotein precursor of 2,437 amino acids, a 717-bp 5'-untranslated region (UTR), and a 241-bp 3'-UTR. The L protein sequence was found to be the most variable region in the feline kobuvirus genome. Interestingly, the 5'-UTR B and C stem-loops were conserved as observed with other kobuviruses; however, a secondary structure corresponding to stem-loop A was not found in the full length 5'-UTR sequence. Phylogenetic tree analysis showed that kobuviruses can be divided into 3 main groups. The feline kobuvirus belongs to the Aichivirus A species containing Aichivirus, mouse kobuvirus, and canine kobuvirus.

Mapping overlapping functional elements embedded within the protein-coding regions of RNA viruses

Identification of the full complement of genes and other functional elements in any virus is crucial to fully understand its molecular biology and guide the development of effective control strategies. RNA viruses have compact multifunctional genomes that frequently contain overlapping genes and non-coding functional elements embedded within protein-coding sequences. Overlapping features often escape detection because it can be difficult to disentangle the multiple roles of the constituent nucleotides via mutational analyses, while high-throughput experimental techniques are often unable to distinguish functional elements from incidental features. However, RNA viruses evolve very rapidly so that, even within a single species, substitutions rapidly accumulate at neutral or near-neutral sites providing great potential for comparative genomics to distinguish the signature of purifying selection. Computationally identified features can then be efficiently targeted for experimental analysis. Here we analyze alignments of protein-coding virus sequences to identify regions where there is a statistically significant reduction in the degree of variability at synonymous sites, a characteristic signature of overlapping functional elements. Having previously tested this technique by experimental verification of discoveries in selected viruses, we now analyze sequence alignments for ∼700 RNA virus species to identify hundreds of such regions, many of which have not been previously described.

Advances in norovirus biology

Human noroviruses are a major cause of epidemic and sporadic gastroenteritis worldwide and can chronically infect immunocompromised patients. Efforts to develop effective vaccines and antivirals have been hindered by the uncultivable nature and extreme genetic diversity of human noroviruses. Although they remain a particularly challenging pathogen to study, recent advances in norovirus animal models and in vitro cultivation systems have led to an increased understanding of norovirus molecular biology and replication, pathogenesis, cell tropism, and innate and adaptive immunity. Furthermore, clinical trials of vaccines consisting of nonreplicating virus-like particles have shown promise. In this review, we summarize these recent advances and discuss controversies in the field, which is rapidly progressing toward generation of antiviral agents and increasingly effective vaccines.

Host-cell factors involved in the calicivirus replicative cycle

ABSTRACT: 

The Caliciviridae includes small positive-sense, ssRNA viruses, which infect both animals and humans and cause a wide range of diseases. Human caliciviruses are considered the leading cause of outbreaks and sporadic cases of viral gastroenteritis worldwide. Caliciviruses are nonenveloped with a positive-sense, ssRNA genome. As with other positive-sense, ssRNA viruses, they require interactions between viral components and host-cellular factors at different steps along the viral life cycle. Although knowledge about the role of host-cell proteins in the Caliciviridae life cycle remains modest, evidence on this topic is rapidly emerging. This article compiles and discusses the information regarding the involvement of host-cellular factors in the various stages of the calicivirus replication process, emphasizing factors that might be involved in viral translation and/or RNA replication.

New insights into functional roles of the polypyrimidine tract-binding protein

Polypyrimidine Tract Binding Protein (PTB) is an intensely studied RNA binding protein involved in several post-transcriptional regulatory events of gene expression. Initially described as a pre-mRNA splicing regulator, PTB is now widely accepted as a multifunctional protein shuttling between nucleus and cytoplasm. Accordingly, PTB can interact with selected RNA targets, structural elements and proteins. There is increasing evidence that PTB and its paralog PTBP2 play a major role as repressors of alternatively spliced exons, whose transcription is tissue-regulated. In addition to alternative splicing, PTB is involved in almost all steps of mRNA metabolism, including polyadenylation, mRNA stability and initiation of protein translation. Furthermore, it is well established that PTB recruitment in internal ribosome entry site (IRES) activates the translation of picornaviral and cellular proteins. Detailed studies of the structural properties of PTB have contributed to our understanding of the mechanism of RNA binding by RNA Recognition Motif (RRM) domains. In the present review, we will describe the structural properties of PTB, its paralogs and co-factors, the role in post-transcriptional regulation and actions in cell differentiation and pathogenesis. Defining the multifunctional roles of PTB will contribute to the understanding of key regulatory events in gene expression.

Norovirus gene expression and replication

Noroviruses are small, positive-sense RNA viruses within the family Caliciviridae, and are now accepted widely as a major cause of acute gastroenteritis in both developed and developing countries. Despite their impact, our understanding of the life cycle of noroviruses has lagged behind that of other RNA viruses due to the inability to culture human noroviruses (HuNVs). Our knowledge of norovirus biology has improved significantly over the past decade as a result of numerous technological advances. The use of a HuNV replicon, improved biochemical and cell-based assays, combined with the discovery of a murine norovirus capable of replication in cell culture, has improved greatly our understanding of the molecular mechanisms of norovirus genome translation and replication, as well as the interaction with host cell processes. In this review, the current state of knowledge of the intracellular life of noroviruses is discussed with particular emphasis on the mechanisms of viral gene expression and viral genome replication.

Progress towards the prevention and treatment of norovirus infections

Noroviruses are now recognized as the major cause of acute gastroenteritis in the developed world, yet our ability to prevent and control infection is limited. Recent work has highlighted that, while typically an acute infection in the population, immunocompromised patients often experience long-term infections that may last many years. This cohort of patients and those regularly exposed to infectious material, for example, care workers and others, would benefit greatly from the development of a vaccine or antiviral therapy. While a licensed vaccine or antiviral has yet to be developed, work over the past 10 years in this area has intensified and trials with a vaccine candidate have proven promising. Numerous antiviral targets and small molecule inhibitors that have efficacy in cell culture have now been identified; however, further studies in this area are required in order to make these suitable for clinical use.

Norovirus genome circularization and efficient replication are facilitated by binding of PCBP2 and hnRNP A1

Sequences and structures within the terminal genomic regions of plus-strand RNA viruses are targets for the binding of host proteins that modulate functions such as translation, RNA replication, and encapsidation. Using murine norovirus 1 (MNV-1), we describe the presence of long-range RNA-RNA interactions that were stabilized by cellular proteins. The proteins potentially responsible for the stabilization were selected based on their ability to bind the MNV-1 genome and/or having been reported to be involved in the stabilization of RNA-RNA interactions. Cell extracts were preincubated with antibodies against the selected proteins and used for coprecipitation reactions. Extracts treated with antibodies to poly(C) binding protein 2 (PCBP2) and heterogeneous nuclear ribonucleoprotein (hnRNP) A1 significantly reduced the 5'-3' interaction. Both PCBP2 and hnRNP A1 recombinant proteins stabilized the 5'-3' interactions and formed ribonucleoprotein complexes with the 5' and 3' ends of the MNV-1 genomic RNA. Mutations within the 3' complementary sequences (CS) that disrupt the 5'-3'-end interactions resulted in a significant reduction of the viral titer, suggesting that the integrity of the 3'-end sequence and/or the lack of complementarity with the 5' end is important for efficient virus replication. Small interfering RNA-mediated knockdown of PCBP2 or hnRNP A1 resulted in a reduction in virus yield, confirming a role for the observed interactions in efficient viral replication. PCBP2 and hnRNP A1 induced the circularization of MNV-1 RNA, as revealed by electron microscopy. This study provides evidence that PCBP2 and hnRNP A1 bind to the 5' and 3' ends of the MNV-1 viral RNA and contribute to RNA circularization, playing a role in the virus life cycle.

Influence of genome-scale RNA structure disruption on the replication of murine norovirus--similar replication kinetics in cell culture but attenuation of viral fitness in vivo

Mechanisms by which certain RNA viruses, such as hepatitis C virus, establish persistent infections and cause chronic disease are of fundamental importance in viral pathogenesis. Mammalian positive-stranded RNA viruses establishing persistence typically possess genome-scale ordered RNA secondary structure (GORS) in their genomes. Murine norovirus (MNV) persists in immunocompetent mice and provides an experimental model to functionally characterize GORS. Substitution mutants were constructed with coding sequences in NS3/4- and NS6/7-coding regions replaced with sequences with identical coding and (di-)nucleotide composition but disrupted RNA secondary structure (F1, F2, F1/F2 mutants). Mutants replicated with similar kinetics to wild-type (WT) MNV3 in RAW264.7 cells and primary macrophages, exhibited similar (highly restricted) induction and susceptibility to interferon-coupled cellular responses and equal replication fitness by serial passaging of co-cultures. In vivo, both WT and F1/F2 mutant viruses persistently infected mice, although F1, F2 and F1/F2 mutant viruses were rapidly eliminated 1-7 days post-inoculation in competition experiments with WT. F1/F2 mutants recovered from tissues at 9 months showed higher synonymous substitution rates than WT and nucleotide substitutions that potentially restored of RNA secondary structure. GORS plays no role in basic replication of MNV but potentially contributes to viral fitness and persistence in vivo.

Genetic characterization of a reptilian calicivirus (Cro1)

Background

Vesiviruses in the family Caliciviridae infect a broad range of animal hosts including mammals, birds, fish, amphibians and reptiles. The vesivirus Cro1 strains were isolated from diseased snakes in the San Diego zoo in 1978 and reported as the first caliciviruses found in reptiles. The goal of this study was to characterize the Cro1 strain 780032I that was isolated in cell culture from a rock rattlesnake (Crotalus lepidus) in the original outbreak.

Results

We re-amplified the original virus stock in Vero cells, and determined its full-length genome sequence. The Cro1 genome is 8296 nucleotides (nt) in length and has a typical vesivirus organization, with three open reading frames (ORF), ORF1 (5643 nt), ORF2 (2121 nt), and ORF3 (348 nt) encoding a nonstructural polyprotein, the major capsid protein precursor, and a minor structural protein, respectively. Phylogenetic analysis of the full-length genome sequence revealed that the Cro1 virus clustered most closely with the VESV species of the genus Vesivirus, but was genetically distinct (82-83% identities with closest strains).

Conclusions

This is the first description of a full-length genome sequence from a reptile calicivirus (Cro1). The availability of the Cro1 genome sequence should facilitate investigation of the molecular mechanisms involved in Cro1 virus evolution and host range.

Identification of RNA-protein interaction networks involved in the norovirus life cycle

Human noroviruses are one of the major causes of acute gastroenteritis in the developed world, yet our understanding of their molecular mechanisms of genome translation and replication lags behind that for many RNA viruses. Due to the nonculturable nature of human noroviruses, many related members of the Caliciviridae family of small RNA viruses are often used as model systems to dissect the finer details of the norovirus life cycle. Murine norovirus (MNV) has provided one such system with which to study the basic mechanisms of norovirus translation and replication in cell culture. In this report we describe the use of riboproteomics to identify host factors that interact with the extremities of the MNV genome. This network of RNA-protein interactions contains many well-characterized host factors, including PTB, La, and DDX3, which have been shown to play a role in the life cycle of other RNA viruses. By using RNA coimmunoprecipitation, we confirmed that a number of the factors identified using riboproteomics are associated with the viral RNA during virus replication in cell culture. We further demonstrated that RNA inhibition-mediated knockdown of the intracellular levels of a number of these factors inhibits or slows norovirus replication in cell culture, allowing identification of new intracellular targets for this important group of pathogens.

High-resolution functional profiling of the norovirus genome

Human noroviruses (HuNoV) are a major cause of nonbacterial gastroenteritis worldwide, yet details of the life cycle and replication of HuNoV are relatively unknown due to the lack of an efficient cell culture system. Studies with murine norovirus (MNV), which can be propagated in permissive cells, have begun to probe different aspects of the norovirus life cycle; however, our understanding of the specific functions of the viral proteins lags far behind that of other RNA viruses. Genome-wide functional profiling by insertional mutagenesis can reveal protein domains essential for replication and can lead to generation of tagged viruses, which has not yet been achieved for noroviruses. Here, transposon-mediated insertional mutagenesis was used to create 5 libraries of mutagenized MNV infectious clones, each containing a 15-nucleotide sequence randomly inserted within a defined region of the genome. Infectious virus was recovered from each library and was subsequently passaged in cell culture to determine the effect of each insertion by insertion-specific fluorescent PCR profiling. Genome-wide profiling of over 2,000 insertions revealed essential protein domains and confirmed known functional motifs. As validation, several insertion sites were introduced into a wild-type clone, successfully allowing the recovery of infectious virus. Screening of a number of reporter proteins and epitope tags led to the generation of the first infectious epitope-tagged noroviruses carrying the FLAG epitope tag in either NS4 or VP2. Subsequent work confirmed that epitope-tagged fully infectious noroviruses may be of use in the dissection of the molecular interactions that occur within the viral replication complex.

Identification and complete genome characterization of a novel picornavirus in turkey (Meleagris gallopavo)

Members of the family Picornaviridae are important pathogens of humans and animals, although compared with the thousands of known bird species (>10 000), only a few (n = 11) picornaviruses have been identified from avian sources. This study reports the metagenomic detection and complete genome characterization of a novel turkey picornavirus from faecal samples collected from eight turkey farms in Hungary. Using RT-PCR, both healthy (two of three) and affected (seven of eight) commercial turkeys with enteric and/or stunting syndrome were shown to be shedding viruses in seven (88 %) of the eight farms. The viral genome sequence (turkey/M176/2011/HUN; GenBank accession no. JQ691613) shows a high degree of amino acid sequence identity (96 %) to the partial P3 genome region of a picornavirus reported recently in turkey and chickens from the USA and probably belongs to the same species. In the P1 and P2 regions, turkey/M176/2011/HUN is related most closely to, but distinct from, the kobuviruses and turdivirus 1. Complete genome analysis revealed the presence of characteristic picornaviral amino acid motifs, a potential type II-like 5' UTR internal ribosome entry site (first identified among avian-origin picornaviruses) and a conserved, 48 nt long 'barbell-like' structure found at the 3' UTR of turkey/M176/2011/HUN and members of the picornavirus genera Avihepatovirus and Kobuvirus. The general presence of turkey picornavirus - a novel picornavirus species - in faecal samples from healthy and affected turkeys in Hungary and in the USA suggests the worldwide occurrence and endemic circulation of this virus in turkey farms. Further studies are needed to investigate the aetiological role and pathogenic potential of this picornavirus in food animals.

Reverse genetics mediated recovery of infectious murine norovirus

Human noroviruses are responsible for most cases of human gastroenteritis (GE) worldwide and are recurrent problem in environments where close person-to-person contact cannot be avoided ^{1, 2}. During the last few years an increase in the incidence of outbreaks in hospitals has been reported, causing significant disruptions to their operational capacity as well as large economic losses. The identification of new antiviral approaches has been limited due to the inability of human noroviruses to complete a productive infection in cell culture ³. The recent isolation of a murine norovirus (MNV), closely related to human norovirus ⁴ but which can be propagated in cells ⁵ has opened new avenues for the investigation of these pathogens ^{6, 7}.

MNV replication results in the synthesis of new positive sense genomic and subgenomic RNA molecules, the latter of which corresponds to the last third of the viral genome (Figure 1). MNV contains four different open reading frames (ORFs), of which ORF1 occupies most of the genome and encodes seven non-structural proteins (NS1-7) released from a polyprotein precursor. ORF2 and ORF3 are contained within the subgenomic RNA region and encode the capsid proteins (VP1 and VP2, respectively) (Figure 1). Recently, we have identified that additional ORF4 overlapping ORF2 but in a different reading frame is functional and encodes for a mitochondrial localised virulence factor (VF1) ⁸.

Replication for positive sense RNA viruses, including noroviruses, takes place in the cytoplasm resulting in the synthesis of new uncapped RNA genomes. To promote viral translation, viruses exploit different strategies aimed at recruiting the cellular protein synthesis machinery ^9-11. Interestingly, norovirus translation is driven by the multifunctional viral protein-primer VPg covalently linked to the 5' end of both genomic and subgenomic RNAs ^12-14. This sophisticated mechanism of translation is likely to be a major factor in the limited efficiency of viral recovery by conventional reverse genetics approaches.

Here we report two different strategies based on the generation of murine norovirus-1 (referred to as MNV herewith) transcripts capped at the 5' end. One of the methods involves both in vitro synthesis and capping of viral RNA, whereas the second approach entails the transcription of MNV cDNA in cells expressing T7 RNA polymerase. The availability of these reverse genetics systems for the study of MNV and a small animal model has provided an unprecedented ability to dissect the role of viral sequences in replication and pathogenesis ^15-17.

Involvement of heterogeneous nuclear ribonucleoproteins in viral multiplication

The study of virus–host interactions is a major goal in molecular virology and provides new effective targets for antiviral therapies. Heterogeneous nuclear ribonucleoproteins (hnRNPs) constitute a group of cellular RNA-binding proteins localized predominantly within the nucleus, which participate in gene transcription and subsequent RNA post-transcriptional modifications. The interaction between hnRNPs and viral components was extensively demonstrated, as well as the ability of virus infections to alter the intracellular localization or the level of expression of different hnRNPs. The involvement of these proteins in the replication of numerous viruses including members from the Retroviridae, Flaviviridae, Coronaviridae, Arenaviridae, Rhabdoviridae, Papillomaviridae, Orthomyxoviridae, Picornaviridae, Togaviridae and Herpesviridae families, has been reported. In order to gain an increased understanding of the interactions between virus and cell that result in the productive infection of the latter, in this review we discuss the main findings about the role of hnRNPs in different steps of viral replication, such as RNA synthesis, translation, RNA processing and egress of newly assembled progeny virus.

Development of a strand specific real-time RT-qPCR assay for the detection and quantitation of murine norovirus RNA

Murine norovirus (MNV), currently the only norovirus that efficiently replicates in cell culture, is often used as a model system to understand the molecular mechanisms of norovirus replication. MNV is a single stranded positive sense RNA virus of the Caliciviridae family. Replication of MNV involves the synthesis of both full length genomic and sub-genomic RNAs. The replication of these RNAs involves the synthesis of negative strand intermediates. To understand the molecular mechanism of RNA replication and the role of viral and host factors in virus replication, it is necessary to quantify accurately both positive and negative sense RNA molecules of the viral RNA during replication. Increasingly, strand specific reverse transcription-quantitative PCR (RT-qPCR) is becoming the method of choice for this kind of quantitation. Many strategies have been developed to avoid the false priming property of reverse transcriptase and to amplify specifically one strand in the presence of excess opposite strand. In this report, a SYBR based, real time RT-qPCR assay was developed to detect and quantify specifically the negative and the positive sense RNAs of MNV genomic RNA. This assay is based on using a tagged RT primer containing a non-viral sequence at the 5' end of the viral strand specific sequence. This non-viral sequence is then used to amplify selectively the strand specific cDNA at the PCR stage. This assay can be used for a range of MNV strains including MNV-1 and 3, as these are now widely accepted for use in molecular studies. The specificity of this assay was determined by its ability to quantify one strand in the presence of up to 10(6) copies of competitor opposite sense RNA. Using this assay, the production of both strands of MNV-1 RNA was monitored during viral single step growth curve.

Development of a reverse-genetics system for murine norovirus 3: long-term persistence occurs in the caecum and colon

Human noroviruses (HuNoV) are a major cause of viral gastroenteritis worldwide, yet, due to the inability to propagate HuNoV in cell culture, murine norovirus (MNV) is typically used as a surrogate to study norovirus biology. MNV-3 represents an attractive strain to study norovirus infections in vivo because it establishes persistence in wild-type mice, yet causes symptoms resembling gastroenteritis in immune-compromised STAT1(-/-) mice. The lack of reverse-genetics approaches to recover genetically defined MNV-3 has limited further studies on the identification of viral sequences that contribute to persistence. Here we report the establishment of a combined DNA-based reverse-genetics and mouse-model system to study persistent MNV-3 infections in wild-type (C57BL/6) mice. Viral RNA and infectious virus were detected in faeces for at least 56 days after inoculation. Strikingly, the highest concentrations of viral RNA during persistence were detected in the caecum and colon, suggesting that viral persistence is maintained in these tissues. Possible adaptive changes arising during persistence in vivo appeared to accumulate in the minor capsid protein (VP2) and the viral polymerase (NS7), in contrast with adaptive mutations selected during cell-culture passages in RAW264.7 cells that appeared in the major capsid protein (VP1) and non-structural protein NS4. This system provides an attractive model that can be readily used to identify viral sequences that contribute to persistence in an immunocompetent host and to more acute infection in an immunocompromised host, providing new insights into the biology of norovirus infections.

Protruding domain of capsid protein is necessary and sufficient to determine murine norovirus replication and pathogenesis in vivo

Human noroviruses (HuNoVs) are the major cause of epidemic, nonbacterial gastroenteritis worldwide. Due to the lack of a tractable model system and the inability to grow HuNoVs in cell culture, factors required for the norovirus (NoV) life cycle and pathogenesis in the host remain largely unknown. The discovery of murine norovirus (MNV) and the development of reverse-genetics systems for this virus provide an opportunity to study these aspects of NoV infection in vitro and in vivo. Previous studies identified a single amino acid at residue 296 in the protruding (P) domain of the capsid protein that is responsible for determining the virulence of the MNV clone MNV1.CW1 in 12956/SvEv background STAT1-deficient (STAT1(-/-)) mice. In this report, we identified and characterized another determinant of lethality in the P domain that is necessary and sufficient to determine the replication and pathogenesis of the MNV clones MNV1.CW3 and CR6.STL1 in C57BL/6 background STAT1(-/-) mice. Furthermore, we describe how the role of residue 296 in MNV virulence differs between STAT1(-/-) mouse strains. We also describe potential interactions between subdomains of the P domain, as well as between other virus elements, which facilitate recovery of MNV using a reverse-genetics system.

Norovirus regulation of the innate immune response and apoptosis occurs via the product of the alternative open reading frame 4

Small RNA viruses have evolved many mechanisms to increase the capacity of their short genomes. Here we describe the identification and characterization of a novel open reading frame (ORF4) encoded by the murine norovirus (MNV) subgenomic RNA, in an alternative reading frame overlapping the VP1 coding region. ORF4 is translated during virus infection and the resultant protein localizes predominantly to the mitochondria. Using reverse genetics we demonstrated that expression of ORF4 is not required for virus replication in tissue culture but its loss results in a fitness cost since viruses lacking the ability to express ORF4 restore expression upon repeated passage in tissue culture. Functional analysis indicated that the protein produced from ORF4 antagonizes the innate immune response to infection by delaying the upregulation of a number of cellular genes activated by the innate pathway, including IFN-Beta. Apoptosis in the RAW264.7 macrophage cell line was also increased during virus infection in the absence of ORF4 expression. In vivo analysis of the WT and mutant virus lacking the ability to express ORF4 demonstrated an important role for ORF4 expression in infection and virulence. STAT1-/- mice infected with a virus lacking the ability to express ORF4 showed a delay in the onset of clinical signs when compared to mice infected with WT virus. Quantitative PCR and histopathological analysis of samples from these infected mice demonstrated that infection with a virus not expressing ORF4 results in a delayed infection in this system. In light of these findings we propose the name virulence factor 1, VF1 for this protein. The identification of VF1 represents the first characterization of an alternative open reading frame protein for the calicivirus family. The immune regulatory function of the MNV VF1 protein provide important perspectives for future research into norovirus biology and pathogenesis.

This report describes the identification and characterization of a novel protein of unknown function encoded by a mouse virus genetically similar to human noroviruses. This gene is unique to the mouse virus and occupies the same part of the genome that codes for the major capsid protein. The protein that we have described as virulence factor 1 (VF1) is found in all murine norovirus isolates, absent in all human strains but is indeed expressed during infection. Its expression enables MNV-1 to establish efficient infection of its natural host through interference with interferon-mediated response pathways and apoptosis. Our data would indicate that the VF1 protein is multi-functional with an ability to modulate the host's response to infection. Murine noroviruses are frequently used firstly as a model to study human norovirus replication and pathogenesis, studies hampered by their inability to replicate in cell culture. Secondly, persistent infection of laboratory animals with murine norovirus may affect other models of disease using experimental mice. The role of VF1 in infection and pathology in the differential outcome of infection is the source of continued research in our laboratory.

Nucleolin interacts with the feline calicivirus 3' untranslated region and the protease-polymerase NS6 and NS7 proteins, playing a role in virus replication

Cellular proteins play many important roles during the life cycle of all viruses. Specifically, host cell nucleic acid-binding proteins interact with viral components of positive-stranded RNA viruses and regulate viral translation, as well as RNA replication. Here, we report that nucleolin, a ubiquitous multifunctional nucleolar shuttling phosphoprotein, interacts with the Norwalk virus and feline calicivirus (FCV) genomic 3' untranslated regions (UTRs). Nucleolin can also form a complex in vitro with recombinant Norwalk virus NS6 and -7 (NS6/7) and can be copurified with the analogous protein from feline calicivirus (p76 or NS6/7) from infected feline kidney cells. Nucleolin RNA levels or protein were not modified during FCV infection; however, as a consequence of the infection, nucleolin was seen to relocalize from the nucleoli to the nucleoplasm, as well as to the perinuclear area where it colocalizes with the feline calicivirus NS6/7 protein. In addition, antibodies to nucleolin were able to precipitate viral RNA from feline calicivirus-infected cells, indicating a direct or indirect association of nucleolin with the viral RNA during virus replication. Small interfering RNA (siRNA)-mediated knockdown of nucleolin resulted in a reduction of the cytopathic effect and virus yield in CrFK cells. Taken together, these results demonstrate that nucleolin is a nucleolar component that interacts with viral RNA and NS6/7 and is required for feline calicivirus replication.

[Survival strategies of human norovirus]

Human norovirus is a mutatable non-enveloped RNA virus capable of causing acute gastroenteritis in humans. Thus far, no experimental systems can propagate this virus in large amounts. Recent progresses in viral genomics and bioinformatics have led to a better understanding of molecular evolution of this virus in human populations. In addition, progresses in studies of the related noroviruses, those are replicable in laboratory systems, have led to a rapid accumulation of information on structural biology of norovirus. Furthermore, progresses in public health and water environment researches have led to a better understanding of viral ecology. In this review, I will first summarize fundamental characteristics of norovirus and its molecules. Then, I will summarize structure and molecular evolution of norovirus GII/4 subtype, which is now responsible for majorities of norovirus outbreaks in the world. Finally I will discuss survival strategies of human norovirus in nature by integrating the information.

Development of an optimized RNA-based murine norovirus reverse genetics system

Murine norovirus (MNV), identified in 2003, is the only norovirus which replicates efficiently in tissue culture and as a result has been used extensively as a model for human noroviruses, a major cause of acute gastroenteritis. The current report describes the generation of a new approach to reverse genetics recovery of genetically defined MNV that relies on the transfection of in vitro transcribed capped RNA directly into cells. The use of the recently developed ScriptCap post-transcriptional enzymatic capping system, followed by optimized Neon mediated electroporation of the highly permissive RAW 264.7 cells, resulted in the rapid and robust recovery of infectious MNV. Transfection of cells capable of supporting virus replication but not permissive to virus infection, namely human or hamster kidney cells, also resulted in robust recovery of infectious virus without subsequent amplification by multiple rounds of re-infection. This latter system may provide a reproducible method to measure the specific infectivity of mutant norovirus RNA allowing the accurate quantitation of the effect of mutations on norovirus replication.

Antiviral strategies to control calicivirus infections

Caliciviridae are human or non-human pathogenic viruses with a high diversity. Some members of the Caliciviridae, i.e. human pathogenic norovirus or rabbit hemorrhagic disease virus (RHDV), are worldwide emerging pathogens. The norovirus is the major cause of viral gastroenteritis worldwide, accounting for about 85% of the outbreaks in Europe between 1995 and 2000. In the United States, 25 million cases of infection are reported each year. Since its emergence in 1984 as an agent of fatal hemorrhagic diseases in rabbits, RHDV has killed millions of rabbits and has been dispersed to all of the inhabitable continents.

In view of their successful and apparently increasing emergence, the development of antiviral strategies to control infections due to these viral pathogens has now become an important issue in medicine and veterinary medicine. Antiviral strategies have to be based on an understanding of the epidemiology, transmission, clinical symptoms, viral replication and immunity to infection resulting from infection by these viruses. Here, we provide an overview of the mechanisms underlying calicivirus infection, focusing on the molecular aspects of replication in the host cell. Recent experimental data generated through an international collaboration on structural biology, virology and drug design within the European consortium VIZIER is also presented. Based on this analysis, we propose antiviral strategies that may significantly impact on the epidemiological characteristics of these highly successful viral pathogens.