The cardiovirulent phenotype of coxsackievirus B3 is determined at a single site in the genomic 5' nontranslated region

Baicalein suppresses Coxsackievirus B3 replication by inhibiting caspase-1 and viral protease 2A

Myocarditis is an inflammatory disease of the cardiac muscle and one of the primary causes of dilated cardiomyopathy. Group B coxsackievirus (CVB) is one of the leading causative pathogens of viral myocarditis, which primarily affects children and young adults. Due to the lack of vaccines, the development of antiviral medicines is crucial to controlling CVB infection and the progression of myocarditis. In this study, we investigated the antiviral effect of baicalein, a flavonoid extracted from Scutellaria baicaleinsis. Our results demonstrated that baicalein treatment significantly reduced cytopathic effect and increased cell viability in CVB3-infected cells. In addition, significant reductions in viral protein 3D, viral RNA, and viral particles were observed in CVB3-infected cells treated with baicalein. We found that baicalein exerted its inhibitory effect in the early stages of CVB3 infection. Baicalein also suppressed viral replication in the myocardium and effectively alleviated myocarditis induced by CVB3 infection. Our study revealed that baicalein exerts its antiviral effect by inhibiting the activity of caspase-1 and viral protease 2A. Taken together, our findings demonstrate that baicalein has antiviral activity against CVB3 infection and may serve as a potential therapeutic option for the myocarditis caused by enterovirus infection.

Major Group-B Enterovirus populations deleted in the noncoding 5' region of genomic RNA modulate activation of the type I interferon pathway in cardiomyocytes and induce myocarditis

Major 5'-terminally deleted (5'TD) RNA forms of group-B coxsackievirus (CVB-5'TD) has been associated with myocarditis in both mice and humans. Although it is known that interferon-β (IFN-β) signaling is critical for an efficient innate immune response against CVB-induced myocarditis, the link between CVB-5'TD RNA forms and type I IFN signaling in cardiomyocytes remains to be explored. In a mouse model of CVB3/28-induced myocarditis, major early-emerging forms of CVB-5'TD RNA have been characterized as replicative viral populations that impair IFN-β production in the heart. Synthetic CVB3/28 RNA forms mimicking each of these major 5'TD virus populations were transfected in mice and have been shown to modulate innate immune responses in the heart and to induce myocarditis in mice. Remarkably, transfection of synthetic viral RNA with deletions in the secondary structures of the 5'-terminal CVB3 RNA domain I, modifying stem-loops "b", "c" or "d", were found to impair IFN-β production in human cardiomyocytes. In addition, the activation of innate immune response by Poly(I:C), was found to restore IFN-β production and to reduce the burden of CVB-5'TD RNA-forms in cardiac tissues, thereby reducing the mortality rate of infected mice. Overall, our results indicate that major early-emerging CVB3 populations deleted in the domain I of genomic RNA, in the 5' noncoding region, modulate the activation of the type I IFN pathway in cardiomyocytes and induce myocarditis in mice. These findings shed new light on the role of replicative CVB-5'TD RNA forms as key pathophysiological factors in CVB-induced human myocarditis.

Enteroviral 2C protein is an RNA-stimulated ATPase and uses a two-step mechanism for binding to RNA and ATP

The enteroviral 2C protein is a therapeutic target, but the absence of a mechanistic framework for this enzyme limits our understanding of inhibitor mechanisms. Here, we use poliovirus 2C and a derivative thereof to elucidate the first biochemical mechanism for this enzyme and confirm the applicability of this mechanism to other members of the enterovirus genus. Our biochemical data are consistent with a dimer forming in solution, binding to RNA, which stimulates ATPase activity by increasing the rate of hydrolysis without impacting affinity for ATP substantially. Both RNA and DNA bind to the same or overlapping site on 2C, driven by the phosphodiester backbone, but only RNA stimulates ATP hydrolysis. We propose that RNA binds to 2C driven by the backbone, with reorientation of the ribose hydroxyls occurring in a second step to form the catalytically competent state. 2C also uses a two-step mechanism for binding to ATP. Initial binding is driven by the α and β phosphates of ATP. In the second step, the adenine base and other substituents of ATP are used to organize the active site for catalysis. These studies provide the first biochemical description of determinants driving specificity and catalytic efficiency of a picornaviral 2C ATPase.

A Single Mutation in the Cryptic AUG (cAUG) Affects In Vitro Translation and Replication Efficiencies and In Vivo Virulence of Coxsackievirus B3 (CVB3)

The 5'UTR of the genomic RNA of CVB3, unusually long and rich on highly structured secondary structure, contains a conserved cis acting RNA element named the cryptic AUG (cAUG), where the cellular 48S complex is formed. In this study, we investigate the role of this cAUG in CVB3 translation, replication, and virulence. Mutant viral sub-genomic replicon RNA was constructed by site-directed mutagenesis. We characterize in vitro translation and replication efficiencies and in vivo virulence of a cAUG mutant in comparison with wild-type strain. UV-cross-linking assay and Real-Time PCR were used, respectively, to detect binding host proteins and to quantify viral production. Secondary structures of domain containing the cAUG site were studied and compared. The results suggest that introduced mutation in the CVB3 5'UTR affects in vitro and ex vivo viral translation which cannot be rescued by compensatory mutations. A reduced interaction of the La and PCBP2 translation initiation factors with cAUG residue of mutant was revealed. Decreasing production of viral mutant RNA was also demonstrated. Furthermore, secondary structure prediction reveals changes in the ribosome binding sites of the cAUG moiety of mutant sense strand RNA and no alterations in the structure of wild type, suggesting that cAUG mutation specifically affects the secondary structure of the sense RNA strand. Taken together, AUG integrity influences the efficiency of ribosome recruitment through IRES element and the capacity of replication.

Early Emergence of 5' Terminally Deleted Coxsackievirus-B3 RNA Forms Is Associated with Acute and Persistent Infections in Mouse Target Tissues

Major EV-B populations characterized by 5′ terminal deletions (5′TD) have been shown to be associated with the development of myocarditis and type 1 diabetes in mice or humans. To date, the dynamics of EV-B 5′TD-RNA forms’ emergence during the course of infection and their impact on cellular functions remain unclear. Using a RACE-PCR approach in CVB3/28-infected mouse organs, we showed an early (3 days post infection, DPI) emergence of major 5′TD populations associated with minor full-length RNA forms. Viral replication activities with infectious particle production were associated with heart, liver, and pancreas acute inflammatory lesions, whereas clearance of viral RNA without organ lesions was observed in the brain, lung, intestines, and muscles from 3 to 7 DPI. At 28 DPI, low viral RNA levels, +/-RNA ratios < 5 associated with viral protein 1 expression revealed a persistent infection in the heart and pancreas. This persistent infection was characterized by molecular detection of only 5′TD RNA forms that were associated with dystrophin cleavage in the heart and insulin production impairment in beta-pancreatic cells. These results demonstrated that major EV-B 5′TD RNA forms can be early selected during systemic infection and that their maintenance may drive EV-induced acute and persistent infections with target cell dysfunctions.

Persistent Enterovirus Infection: Little Deletions, Long Infections

Enteroviruses have now been shown to persist in cell cultures and in vivo by a novel mechanism involving the deletion of varying amounts of the 5′ terminal genomic region termed domain I (also known as the cloverleaf). Molecular clones of coxsackievirus B3 (CVB3) genomes with 5′ terminal deletions (TD) of varying length allow the study of these mutant populations, which are able to replicate in the complete absence of wildtype virus genomes. The study of TD enteroviruses has revealed numerous significant differences from canonical enteroviral biology. The deletions appear and become the dominant population when an enterovirus replicates in quiescent cell populations, but can also occur if one of the cis-acting replication elements of the genome (CRE-2C) is artificially mutated in the element’s stem and loop structures. This review discusses how the TD genomes arise, how they interact with the host, and their effects on host biology.

Development of Group B Coxsackievirus as an Oncolytic Virus: Opportunities and Challenges

Oncolytic viruses have emerged as a promising strategy for cancer therapy due to their dual ability to selectively infect and lyse tumor cells and to induce systemic anti-tumor immunity. Among various candidate viruses, coxsackievirus group B (CVBs) have attracted increasing attention in recent years. CVBs are a group of small, non-enveloped, single-stranded, positive-sense RNA viruses, belonging to species human Enterovirus B in the genus Enterovirus of the family Picornaviridae. Preclinical studies have demonstrated potent anti-tumor activities for CVBs, particularly type 3, against multiple cancer types, including lung, breast, and colorectal cancer. Various approaches have been proposed or applied to enhance the safety and specificity of CVBs towards tumor cells and to further increase their anti-tumor efficacy. This review summarizes current knowledge and strategies for developing CVBs as oncolytic viruses for cancer virotherapy. The challenges arising from these studies and future prospects are also discussed in this review.

Enterovirus Persistence in Cardiac Cells of Patients With Idiopathic Dilated Cardiomyopathy Is Linked to 5' Terminal Genomic RNA-Deleted Viral Populations With Viral-Encoded Proteinase Activities

BACKGROUND

Group B enteroviruses are common causes of acute myocarditis, which can be a precursor of chronic myocarditis and dilated cardiomyopathy, leading causes of heart transplantation. To date, the specific viral functions involved in the development of dilated cardiomyopathy remain unclear.

METHODS

Total RNA from cardiac tissue of patients with dilated cardiomyopathy was extracted, and sequences corresponding to the 5' termini of enterovirus RNAs were identified. After next-generation RNA sequencing, viral cDNA clones mimicking the enterovirus RNA sequences found in patient tissues were generated in vitro, and their replication and impact on host cell functions were assessed on primary human cardiac cells in culture.

RESULTS

Major enterovirus B populations characterized by 5' terminal genomic RNA deletions ranging from 17 to 50 nucleotides were identified either alone or associated with low proportions of intact 5' genomic termini. In situ hybridization and immunohistological assays detected these persistent genomes in clusters of cardiomyocytes. Transfection of viral RNA into primary human cardiomyocytes demonstrated that deleted forms of genomic RNAs displayed early replication activities in the absence of detectable viral plaque formation, whereas mixed deleted and complete forms generated particles capable of inducing cytopathic effects at levels distinct from those observed with full-length forms alone. Moreover, deleted or full-length and mixed forms of viral RNA were capable of directing translation and production of proteolytically active viral proteinase 2A in human cardiomyocytes.

CONCLUSIONS

We demonstrate that persistent viral forms are composed of B-type enteroviruses harboring a 5' terminal deletion in their genomic RNAs and that these viruses alone or associated with full-length populations of helper RNAs could impair cardiomyocyte functions by the proteolytic activity of viral proteinase 2A in cases of unexplained dilated cardiomyopathy. These results provide a better understanding of the molecular mechanisms that underlie the persistence of EV forms in human cardiac tissues and should stimulate the development of new therapeutic strategies based on specific inhibitors of the coxsackievirus B proteinase 2A activity for acute and chronic cardiac infections.

Albumin Enhances the Rate at Which Coxsackievirus B3 Strain 28 Converts to A-Particles

Three strains of coxsackievirus B3 (CVB3) differ by single mutations in capsid protein VP1 or VP3 and also differ in stability at 37°C in tissue culture medium. Among these strains, the CVB3/28 parent strain has been found to be uniquely sensitive to a component in fetal bovine serum (FBS) identified as serum albumin. In cell culture medium, serum increased the rate of CVB3/28 conversion to noninfectious particles at least 2-fold. The effect showed a saturable dose response. Rates of conversion to noninfectious virus with high concentrations of soluble coxsackievirus and adenovirus receptor (sCAR) were similar with and without FBS, but FBS amplified the catalytic effect of 100 nM sCAR nearly 3-fold. Such effects in other systems are due to nonessential activating cofactors.IMPORTANCE A factor other than the virus receptor expressed by target cells has been found to accelerate the loss of an enterovirus (CVB3/28) infectious titer, with little effect on nearly identical mutant strains. The destabilizing factor in fetal bovine serum, identified as albumin, does not interfere with the catalytic activity of soluble receptor at saturating receptor concentrations and amplifies the catalytic activity of the soluble receptor at a concentration that otherwise produces about one-third the saturated receptor-catalyzed rate of virus decay. This finding evidences the possibility that other virus-"priming" ligands may also be nonessential activating cofactors that serve to accelerate receptor-catalyzed viral eclipse.

A second open reading frame in human enterovirus determines viral replication in intestinal epithelial cells

Human enteroviruses (HEVs) of the family Picornaviridae, which comprises non-enveloped RNA viruses, are ubiquitous worldwide. The majority of EV proteins are derived from viral polyproteins encoded by a single open reading frame (ORF). Here, we characterize a second ORF in HEVs that is crucial for viral intestinal infection. Disruption of ORF2p expression decreases the replication capacity of EV-A71 in human intestinal epithelial cells (IECs). Ectopic expression of ORF2p proteins derived from diverse enteric enteroviruses sensitizes intestinal cells to the replication of ORF2p-defective EV-A71 and respiratory enterovirus EV-D68. We show that the highly conserved WIGHPV domain of ORF2p is important for ORF2p-dependent viral intestinal infection. ORF2p expression is required for EV-A71 particle release from IECs and can support productive EV-D68 infection in IECs by facilitating virus release. Our results indicate that ORF2p is a determining factor for enteric enterovirus replication in IECs.

A reverse genetics system for enterovirus D68 using human RNA polymerase I

Human enterovirus D68 (EV-D68) is a highly contagious virus, which causes respiratory tract infections. However, no effective vaccines are currently available for controlling EV-D68 infection. Here, we developed a reverse genetics system to recover EV-D68 minireplicons and infectious EV-D68 from transfected plasmids using the RNA polymerase I (Pol I) promoter. The EV-D68 minireplicons contained the luciferase reporter gene, which flanked by the non-coding regions of the EV-D68 RNA. The luciferase signals could be detected in cells after transfection and Pol I promoter-mediated luciferase signal was significantly stronger than that mediated by the T7 promoter. Furthermore, recombinant viruses were generated by transfecting plasmids that contained the genomic RNA segments of EV-D68, under the control of Pol I promoter into 293T cells or RD cells. On plaque morphology and growth kinetics, the rescued virus and parental virus were indistinguishable. In addition, we showed that the G394C mutation disrupts the viral 5'-UTR structure and suppresses the viral cap-independent translation. This reverse genetics system for EV-D68 recovery can greatly facilitate research into EV-D68 biology. Moreover, this system could accelerate the development of EV-D68 vaccines and anti-EV-D68 drugs.

Functional Consequences of RNA 5'-Terminal Deletions on Coxsackievirus B3 RNA Replication and Ribonucleoprotein Complex Formation

Group B coxsackieviruses are responsible for chronic cardiac infections. However, the molecular mechanisms by which the virus can persist in the human heart long after the signs of acute myocarditis have abated are still not completely understood. Recently, coxsackievirus B3 strains with 5'-terminal deletions in genomic RNAs were isolated from a patient suffering from idiopathic dilated cardiomyopathy, suggesting that such mutant viruses may be the forms responsible for persistent infection. These deletions lacked portions of 5' stem-loop I, which is an RNA secondary structure required for viral RNA replication. In this study, we assessed the consequences of the genomic deletions observed in vivo for coxsackievirus B3 biology. Using cell extracts from HeLa cells, as well as transfection of luciferase replicons in two types of cardiomyocytes, we demonstrated that coxsackievirus RNAs harboring 5' deletions ranging from 7 to 49 nucleotides in length can be translated nearly as efficiently as those of wild-type virus. However, these 5' deletions greatly reduced the synthesis of viral RNA in vitro, which was detected only for the 7- and 21-nucleotide deletions. Since 5' stem-loop I RNA forms a ribonucleoprotein complex with cellular and viral proteins involved in viral RNA replication, we investigated the binding of the host cell protein PCBP2, as well as viral protein 3CD^pro, to deleted positive-strand RNAs corresponding to the 5' end. We found that binding of these proteins was conserved but that ribonucleoprotein complex formation required higher PCBP2 and 3CD^pro concentrations, depending on the size of the deletion. Overall, this study confirmed the characteristics of persistent CVB3 infection observed in heart tissues and provided a possible explanation for the low level of RNA replication observed for the 5'-deleted viral genomes-a less stable ribonucleoprotein complex formed with proteins involved in viral RNA replication.IMPORTANCE Dilated cardiomyopathy is the most common indication for heart transplantation worldwide, and coxsackie B viruses are detected in about one-third of idiopathic dilated cardiomyopathies. Terminal deletions at the 5' end of the viral genome involving an RNA secondary structure required for RNA replication have been recently reported as a possible mechanism of virus persistence in the human heart. These mutations are likely to disrupt the correct folding of an RNA secondary structure required for viral RNA replication. In this report, we demonstrate that transfected RNAs harboring 5'-terminal sequence deletions are able to direct the synthesis of viral proteins, but not genomic RNAs, in human and murine cardiomyocytes. Moreover, we show that the binding of cellular and viral replication factors to viral RNA is conserved despite genomic deletions but that the impaired RNA synthesis associated with terminally deleted viruses could be due to destabilization of the ribonucleoprotein complexes formed.

MOPS and coxsackievirus B3 stability

Study of coxsackievirus B3 strain 28 (CVB3/28) stability using MOPS to improve buffering in the experimental medium revealed that MOPS (3-morpholinopropane-1-sulfonic acid) increased CVB3 stability and the effect was concentration dependent. Over the pH range 7.0-7.5, virus stability was affected by both pH and MOPS concentration. Computer-simulated molecular docking showed that MOPS can occupy the hydrophobic pocket in capsid protein VP1 where the sulfonic acid head group can form ionic and hydrogen bonds with Arg95 and Asn211 near the pocket opening. The effects of MOPS and hydrogen ion concentrations on the rate of virus decay were modeled by including corresponding parameters in a recent kinetic model. These results indicate that MOPS can directly associate with CVB3 and stabilize the virus, possibly by altering capsid conformational dynamics.

Complete genome sequence of a coxsackievirus B3 recombinant isolated from an aseptic meningitis outbreak in eastern China

Coxsackievirus B3 (CV-B3) has frequently been associated with aseptic meningitis outbreaks in China. To identify sequence motifs related to aseptic meningitis and to construct an infectious clone, the genome sequence of 08TC170, a representative strain isolated from cerebrospinal fluid (CSF) samples from an outbreak in Shandong in 2008, was determined, and the coding regions for P1-P3 and VP1 were aligned. The first 21 and last 20 residues were "TTAAAACAGCCTGTGGGTTGT" and "ATTCTCCGCATTCGGTGCGG", respectively. The whole genome consisted of 7401 nucleotides, sharing 80.8 % identity with the prototype strain Nancy and low sequence similarity with members of clusters A-C. In contrast, 08TC170 showed high sequence similarity to members of cluster D. An especially high level of sequence identity (≥97.7 %) was found within a branch constituted by 08TC170 and four Chinese strains that clustered together in all of the P1-P3 phylogenic trees. In addition, 08TC170 also possessed a close relationship to the Hong Kong strain 26362/08 in VP1. Similarity plot analysis showed that 08TC170 was most similar to the Chinese CV-B3 strain SSM in P1 and the partial P2 coding region but to the CV-B5 or E-6 strain in 2C and following regions. A T277A mutation was found in 08TC170 and other strains isolated in 2008-2010, but not in strains isolated before 2008, which had high sequence similarity and formed the cluster A277. The results suggested that 08TC170 was the product of both intertypic recombination and point mutation, whose effects on viral neurovirulence will be investigated in a further study. The high homology between 08TC170 and other strains revealed their co-circulation in mainland China and Hong Kong and indicates that further surveillance is needed.

Three capsid amino acids notably influence coxsackie B3 virus stability

Coxsackievirus B3 strain 28 (CVB3/28) is less stable at 37 °C than eight other CVB3 strains with which it has been compared, including four in this study. In a variant CVB3/28 population selected for increased stability at 37 °C, the capsid proteins of the stable variant differed from the parental CVB3/28 by two mutations in Vp1 and one mutation in Vp3, each of which resulted in altered protein sequences. Each of the amino acid changes was individually associated with a more stable virus. Competition between CVB3/28 and a more stable derivative of the strain showed that propagation of the less stable virus was favoured in receptor-rich HeLa cells.

Sequence specificity for uridylylation of the viral peptide linked to the genome (VPg) of enteroviruses

Enteroviruses (EV) uridylylate a peptide, VPg, as the first step in their replication. VPgpUpU, found free in infected cells, serves as the primer for RNA elongation. The abilities of four polymerases (3D(pol)), from EV-species A-C, to uridylylate VPgs that varied by up to 60% of their residues were compared. Each 3D(pol) was able to uridylylate all five VPgs using polyA RNA as template, while showing specificity for its own genome encoded peptide. All 3D(pol) uridylylated a consensus VPg representing the physical chemical properties of 31 different VPgs. Thus the residues required for uridylylation and the enzymatic mechanism must be similar in diverse EV. As VPg-binding sites differ in co-crystal structures, the reaction is probably done by a second 3D(pol) molecule. The conservation of polymerase residues whose mutation reduces uridylylation but not RNA elongation is compared.

Isolation and genetic characterization of enterovirus in patients with febrile rash illness

Measles and rubella virus are usually considered as the causative agents in patients with febrile rash illness (FRI). However, investigators have identified that enteroviruses are also associated with FRI, and the present study was undertaken to investigate this association. In the study, 20 throat swab samples were collected from patients suffering from rash and fever between April and July in 2013. The 20 samples tested negative for measles virus, but all were positive when reverse transcription-polymerase chain reaction was performed using enterovirus universal primers. Specifically, five tested positive for Coxsackie B3 virus (CVB3). The titers of the five CVB3 isolates were 10^6.5, 10^6.4, 10^7.5, 10^6.5 and 10^6.5 tissue culture infectious dose₅₀/100 µl respectively. The partial VP1 sequences of the five CVB3 isolates were identical to each other and were closely associated with the CVB3/MKP and CVB3/Macocy strains, which are known to induce myocarditis and neonatal diseases of the central nervous system. In conclusion, CVB3 may cause symptoms of fever and rash and should be differentiated from measles, rubella and other infectious pathogens. The five CVB3 isolates described in the study were genetically similar to each other and to other local CVB3 strains. The results provide further data on the viral pathogen spectrum associated with FRI.

Major alteration in coxsackievirus B3 genomic RNA structure distinguishes a virulent strain from an avirulent strain

Coxsackievirus B3 (CV-B3) is a cardiovirulent enterovirus that utilizes a 5′ untranslated region (5′UTR) to complete critical viral processes. Here, we directly compared the structure of a 5′UTR from a virulent strain with that of a naturally occurring avirulent strain. Using chemical probing analysis, we identified a structural difference between the two 5′UTRs in the highly substituted stem-loop II region (SLII). For the remainder of the 5′UTR, we observed conserved structure. Comparative sequence analysis of 170 closely related enteroviruses revealed that the SLII region lacks conservation. To investigate independent folding and function, two chimeric CV-B3 strains were created by exchanging nucleotides 104–184 and repeating the 5′UTR structural analysis. Neither the parent SLII nor the remaining domains of the background 5′UTR were structurally altered by the exchange, supporting an independent mechanism of folding and function. We show that the attenuated 5′UTR lacks structure in the SLII cardiovirulence determinant.

Use of the 5' untranslated region and VP1 region to examine the molecular diversity in enterovirus B species

Human enteroviruses evolve quickly. The 5' untranslated region (UTR) is fundamentally important for efficient viral replication and for virulence; the VP1 region correlates well with antigenic typing by neutralization, and can be used for virus identification and evolutionary studies. In order to investigate the molecular diversity in EV-B species, the 5' UTR and VP1 regions were analysed for 208 clinical isolates from a single public-health laboratory (serving New South Wales, Australia), representing 28 EV-B types. Sequences were compared with the 5' UTR and VP1 regions of 98 strains available in GenBank, representing the same 28 types. The genetic relationships were analysed using two types of software (mega and BioNumerics). The sequence analyses of the 5' UTR and VP1 regions of 306 EV-B strains demonstrated that: (i) comparing the two regions gives strong evidence of epidemiological linkage of strains in some serotypes; (ii) the intraserotypic genetic variation within each gene reveals that they evolve distinctly largely due to their different functions; and (iii) mutation and possible recombination in the two regions play significant roles in the molecular diversity of EV-B. Understanding the tempo and pattern of molecular diversity and evolution is of great importance in the pathogenesis of EV-B enteroviruses, information which will assist in disease prevention and control.

Role of RNA structure motifs in IRES-dependent translation initiation of the coxsackievirus B3: new insights for developing live-attenuated strains for vaccines and gene therapy

Internal ribosome entry site (IRES) elements are highly structured RNA sequences that function to recruit ribosomes for the initiation of translation. In contrast to the canonical cap-binding, the mechanism of IRES-mediated translation initiation is still poorly understood. Translation initiation of the coxsackievirus B3 (CVB3), a causative agent of viral myocarditis, has been shown to be mediated by a highly ordered structure of the 5' untranslated region (5'UTR), which harbors an IRES. Taking into account that efficient initiation of mRNA translation depends on temporally and spatially orchestrated sequence of RNA-protein and RNA-RNA interactions, and that, at present, little is known about these interactions, we aimed to describe recent advances in our understanding of molecular structures and biochemical functions of the translation initiation process. Thus, this review will explore the IRES elements as important RNA structures and the significance of these structures in providing an alternative mechanism of translation initiation of the CVB3 RNA. Since translation initiation is the first intracellular step during the CVB3 infection cycle, the IRES region provides an ideal target for antiviral therapies. Interestingly, the 5' and 3'UTRs represent promising candidates for the study of CVB3 cardiovirulence and provide new insights for developing live-attenuated vaccines.

Kinetic and structural analysis of coxsackievirus B3 receptor interactions and formation of the A-particle

The coxsackievirus and adenovirus receptor (CAR) has been identified as the cellular receptor for group B coxsackieviruses, including serotype 3 (CVB3). CAR mediates infection by binding to CVB3 and catalyzing conformational changes in the virus that result in formation of the altered, noninfectious A-particle. Kinetic analyses show that the apparent first-order rate constant for the inactivation of CVB3 by soluble CAR (sCAR) at physiological temperatures varies nonlinearly with sCAR concentration. Cryo-electron microscopy (cryo-EM) reconstruction of the CVB3-CAR complex resulted in a 9.0-Å resolution map that was interpreted with the four available crystal structures of CAR, providing a consensus footprint for the receptor binding site. The analysis of the cryo-EM structure identifies important virus-receptor interactions that are conserved across picornavirus species. These conserved interactions map to variable antigenic sites or structurally conserved regions, suggesting a combination of evolutionary mechanisms for receptor site preservation. The CAR-catalyzed A-particle structure was solved to a 6.6-Å resolution and shows significant rearrangement of internal features and symmetric interactions with the RNA genome.

IMPORTANCE

This report presents new information about receptor use by picornaviruses and highlights the importance of attaining at least an ∼9-Å resolution for the interpretation of cryo-EM complex maps. The analysis of receptor binding elucidates two complementary mechanisms for preservation of the low-affinity (initial) interaction of the receptor and defines the kinetics of receptor-catalyzed conformational change to the A-particle.

Visceral pathology of acute systemic injury in newborn mice on the onset of Coxsackie virus infection

Coxsackievirus B (CVB) is a significant pathogen of neonatal diseases with severe systemic involvement and high mortality. Hence, it is essential to develop a CVB-induced acute systemic disease model on newborn mouse and study the injury at the onset phase. In this work, a clinical strain of CVB3, Nancy, and its variant strain, Macocy, were adopted in 24 hour old neonates by oral infection. The pathological changes in the heart, liver and lung tissues were analyzed by pathology assays. In situ end labeling assay for programmed cell death was carried out for liver tissues. The data on fatality and infection rates and pathology scores were analyzed statistically. The genomic sequences of the two strains were aligned. The model represented the manifest clinical syndromes of hepatitis, pneumonia and myocardial injury at the onset phase, in which massive numbers of hepatocytes had undergone programmed cell death. Statistical and pathological analysis indicated that the myocardial injury was mild, whereas the liver and lung were more severe. The fatality rate, infection and pathology of the two CVB strains were the same. Therefore, two nucleotide mutations in the 5' UTR and four amino acid mutations in polyprotein, which did not alter virulence, were shown. By peroral CVB infection of neonatal mice, we developed an acute systemic disease model for studying visceral pathology and systemic disease. At the onset of acute neonatal systemic disease, the hepatitis and pneumonia may be the dominant reason of death, as the injury of liver and lung is more severe than that of heart.

Nucleotide differences of coxsackievirus B3 and chronic myocarditis

The in vivo mechanisms in chronic myocarditis remain unclear. The aim of the current study was to clarify the genomic difference of amyocarditic (CB3O) and myocarditic (CB3M) coxsackievirus B3 (CB3) and the pathogenesis of in vivo mechanisms in chronic myocarditis. We examined the histopathology of CB3-inoculated wild-type (WT) and severe combined immunodeficient (SCID) mice with and without adoptive transfer of lymphocytes. There were no differences in viral growth between CB3O and CB3M. There were four to six nucleotide differences in the sequence of CB3O in comparison with the known CB3M. The difference in virus sequence between CB3O and CB3M was very minimal. The changes were located in 1A, 1C, and 1D regions, which encode the structural capsid proteins. Definite myocarditis developed in WT C3H (H-2(k)) inoculated with CB3M. On the contrary, trivial or mild myocarditis occurred in WT C3H mice inoculated with CB3O. In SCID C3H and SCID C57BL/6 (H-2(b)) mice, definite myocarditis developed by inoculation with both CB3O and CB3M. Myocardial lesion was less severe in the mice infected with CB3O than in those with CB3M. After anti-CD8 antibody treatment, myocarditis was easily induced in mice originally showing resistance to infection. In addition, chronic myocarditis developed in CB3O-infected SCID C3H mice reconstituted with CB3M-sensitized splenocytes of WT C3H mice. The development of chronic myocarditis primarily depends on the presence or absence of the virus genome, and secondarily on the complex interaction between virus virulence and immunological background of the host. CB3 infection may cause chronic myocarditis with ongoing inflammation with or without viral persistence.

In vitro molecular characterization of RNA-proteins interactions during initiation of translation of a wild-type and a mutant Coxsackievirus B3 RNAs

Translation initiation of Coxsackievirus B3 (CVB3) RNA is directed by an internal ribosome entry site (IRES) within the 5' untranslated region. Host cell factors involved in this process include some canonical translation factors and additional RNA-binding proteins. We have, previously, described that the Sabin3-like mutation (U475 → C) introduced in CVB3 genome led to a defective mutant with a serious reduction in translation efficiency. With the aim to identify proteins interacting with CVB3 wild-type and Sabin3-like IRESes and to study interactions between HeLa cell or BHK-21 protein extracts and CVB3 RNAs, UV-cross-linking assays were performed. We have observed a number of proteins that specifically interact with both RNAs. In particular, molecular weights of five of these proteins resemble to those of the eukaryotic translation initiation factors 4G, 3b, 4B, and PTB. According to cross-linking patterns obtained, we have demonstrated a better affinity of CVB3 RNA binding to BHK-21 proteins and a reduced interaction of the mutant RNA with almost cellular polypeptides compared to the wild-type IRES. On the basis of phylogeny of some initiation factors and on the knowledge of the initiation of translation process, we focused on the interaction of both IRESes with eIF3, p100 (eIF4G), and 40S ribosomal subunit by filter-binding assays. We have demonstrated a better affinity of binding to the wild-type CVB3 IRES. Thus, the reduction efficiency of the mutant RNA to bind to cellular proteins involved in the translation initiation could be the reason behind inefficient IRES function.

Neutralizing activity induced by the attenuated coxsackievirus B3 Sabin3-like strain against CVB3 infection

Coxsackievirus B3 (CVB3) causes viral myocarditis, and can ultimately result in dilated cardiomyopathy. There is no vaccine available for clinical use. In the present work, we assessed whether the Sabin3-like mutant of CVB3 could induce a protective immunity against virulent CVB3 Nancy and CVB4 E2 strains in mice by both oral and intraperitoneal (IP) routes. Serum samples, taken from mice inoculated with Sabin3-like, were assayed in vitro for their anti-CVB3 neutralizing activity. CVB3 Sabin3-like was highly attenuated in vivo and was able to induce an anti-CVB3 activity of the serum. However, at 4 days post-CVB3 challenge, significant increased titers of CVB3 neutralizing antibodies were detectable in the sera of immunized mice over the next 6 days. Non-immunized mice challenged with CVB3 Nancy had no anti-CVB3 activity in their sera until 10 days post-infection. CVB3 Nancy induced higher viral titers than did the mutant strain. There was no variation of the neutralizing activity of serum taken from mice immunized with CVB3 Sabin3-like and challenged with CVB4 E2, compared to non-immunized mice. Despite the fact that CVB3 and CVB4 are closely related viruses, virus-neutralizing activity clearly distinguish between these viruses. A variable and limited amount of pancreatic inflammation was seen in some mice 10 days after Sabin3-like inoculation by IP route, whereas there was no evidence of pancreatic damage in mice inoculated by oral route. All immunized mice were protected from myocarditis and pancreatitis at 8 days post-challenge with CVB3 or CVB4 E2. These findings strongly suggest that the mutant strain could be considered a candidate for an attenuated CVB3 vaccine.

Ribosomal Initiation Complex Assembly within the Wild-Strain of Coxsackievirus B3 and Live-Attenuated Sabin3-like IRESes during the Initiation of Translation

Coxsackievirus B3 (CVB3) is an enterovirus of the family of Picornaviridae. The Group B coxsackieviruses include six serotypes (B1 to B6) that cause a variety of human diseases, including myocarditis, meningitis, and diabetes. Among the group B, the B3 strain is mostly studied for its cardiovirulence and its ability to cause acute and persistent infections. Translation initiation of CVB3 RNA has been shown to be mediated by a highly ordered structure of the 5'-untranslated region (5'UTR), which harbors an internal ribosome entry site (IRES). Translation initiation is a complex process in which initiator tRNA, 40S and 60S ribosomal subunits are assembled by eukaryotic initiation factors (eIFs) into an 80S ribosome at the initiation codon of the mRNA. We have previously addressed the question of whether the attenuating mutations of domain V of the poliovirus IRES were specific for a given genomic context or whether they could be transposed and extrapolated to a genomic related virus, i.e., CVB3 wild-type strain. In this context, we have described that Sabin3-like mutation (U473→C) introduced in CVB3 genome led to a defective mutant with a serious reduction in translation efficiency. In this study, we analyzed the efficiency of formation of ribosomal initiation complexes 48S and 80S through 10%-30% and 10%-50% sucrose gradients using rabbit reticulocyte lysates (RRLs) and stage-specific translation inhibitors: 5'-Guanylyl-imidodiphosphate (GMP-PNP) and Cycloheximide (CHX), respectively. We demonstrated that the interaction of 48S and 80S ribosomal complexes within the mutant CVB3 RNA was abolished compared with the wild-type RNA by ribosome assembly analysis. Taken together, it is possible that the mutant RNA was unable to interact with some trans-acting factors critical for enhanced IRES function.

HeLa cell heterogeneity and coxsackievirus B3 cytopathic effect: implications for inter-laboratory reproducibility of results

Concerns over cell line identities and contamination have led investigators to acquire fresh stocks of HeLa CCL-2 cells, but results with the HeLa CCL-2 cells do not always reproduce results with HeLa cells that have long history in the laboratory. When used for TCID(50) assays of Coxsackievirus B3/28 (CVB3/28), HeLa CCL-2 cells returned titers for CVB3/28 that were more than ten-fold lower than titers obtained using laboratory HeLa cells. The viral cytopathic effect was less distinct in the HeLa CCL-2 cultures, suggestive of a mixed population of cells with varied susceptibility to viral cytopathic effect. Analysis of short tandem repeat markers confirmed the identities of the cell lines as HeLa. Subpopulations in the HeLa CCL-2 culture, separated easily based on the speed with which they were released by trypsin-EDTA, differed in their susceptibilities to CVB3/28 cytopathic effect, and in their expression of the Coxsackievirus and adenovirus receptor (CAR). The distinctions between Lab HeLa and HeLa CCL-2 cells were less obvious when infected with CVB3/RD, a strain selected for growth in RD cells. Results that differ among laboratories may be due to the use of HeLa cell strains with different histories, and experiments using HeLa CCL-2 available from the American Type Culture Collection are probably incapable of reproducing many of the published studies of Coxsackievirus that have used HeLa cells with laboratory-dependent histories.

An attenuated coxsackievirus b3 vector: a potential tool for viral tracking study and gene delivery

Cardiomyocytes are quite resistant to gene transfer using standard techniques. We developed an expression vector carrying an attenuated but infectious and replicative coxsackievirus B3 (CVB3) genome, and unique ClaI-StuI cloning sites for an exogenous gene, whose product can be released from the nascent viral polyprotein by 2A(pro) cleavage. This vector was tested as an expression vehicle for green fluorescent protein (GFP). The vector transiently expressed GFP in cell cultures for at least ten passages and delivered functional GFP to the infected cardiomyocytes for at least 6 days. Moreover, the recombinant viruses showed virulence attenuation in vitro and in vivo. The findings suggest that the recombinant CVB3 vector could be a useful tool for viral tracking study and delivering exogenous proteins to cardiomyocytes.

Caspase-8 and FLIP regulate RIG-I/MDA5-induced innate immune host responses to picornaviruses

Picornaviruses are small, nonenveloped, positive-stranded RNA viruses, which cause a wide range of animal and human diseases, based on their distinct tissue and cell type tropisms. Myocarditis, poliomyelitis, hepatitis and the common cold are the most significant human illnesses caused by picornaviruses. The host response to picornaviruses is complex, and the damage to tissues occurs not only from direct viral replication within infected cells. Picornaviruses exhibit an exceptional ability to evade the early innate immune response, resulting in chronic infection and autoimmunity. This review discusses the detailed aspects of the early innate host response to picornaviruses infection mediated by RIG-I-like helicases, their adaptor, mitochondrial ant iviral signaling protein, innate immune-induced apoptosis, and the role of caspase-8 and its regulatory paralog, FLIP, in these processes.

The crystal structure of a coxsackievirus B3-RD variant and a refined 9-angstrom cryo-electron microscopy reconstruction of the virus complexed with decay-accelerating factor (DAF) provide a new footprint of DAF on the virus surface

The coxsackievirus-adenovirus receptor (CAR) and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). The first described DAF-binding isolate was obtained during passage of the prototype strain, Nancy, on rhabdomyosarcoma (RD) cells, which express DAF but very little CAR. Here, the structure of the resulting variant, CVB3-RD, has been solved by X-ray crystallography to 2.74 Å, and a cryo-electron microscopy reconstruction of CVB3-RD complexed with DAF has been refined to 9.0 Å. This new high-resolution structure permits us to correct an error in our previous view of DAF-virus interactions, providing a new footprint of DAF that bridges two adjacent protomers. The contact sites between the virus and DAF clearly encompass CVB3-RD residues recently shown to be required for binding to DAF; these residues interact with DAF short consensus repeat 2 (SCR2), which is known to be essential for virus binding. Based on the new structure, the mode of the DAF interaction with CVB3 differs significantly from the mode reported previously for DAF binding to echoviruses.

Viral quasispecies evolution

Evolution of RNA viruses occurs through disequilibria of collections of closely related mutant spectra or mutant clouds termed viral quasispecies. Here we review the origin of the quasispecies concept and some biological implications of quasispecies dynamics. Two main aspects are addressed: (i) mutant clouds as reservoirs of phenotypic variants for virus adaptability and (ii) the internal interactions that are established within mutant spectra that render a virus ensemble the unit of selection. The understanding of viruses as quasispecies has led to new antiviral designs, such as lethal mutagenesis, whose aim is to drive viruses toward low fitness values with limited chances of fitness recovery. The impact of quasispecies for three salient human pathogens, human immunodeficiency virus and the hepatitis B and C viruses, is reviewed, with emphasis on antiviral treatment strategies. Finally, extensions of quasispecies to nonviral systems are briefly mentioned to emphasize the broad applicability of quasispecies theory.

Coxsackievirus B3 used as a gene therapy vector to express functional FGF2

Current gene therapies are predominantly based on a handful of viral vectors. The limited choice of delivery vectors has been one of the stumbling blocks to the advancement of gene therapy. Therefore, the development of novel recombinant vectors should facilitate the application of gene therapies. In this study, we examined coxsackievirus B3 (CVB3) as a novel recombinant vector for the delivery and expression of a foreign gene in vitro and in vivo. A recombinant CVB3 complementary DNA was constructed by inserting a gene encoding human fibroblast growth factor 2 (FGF2). The recombinant virus (CVB3-FGF2) efficiently expressed FGF2 in HeLa cells and human cardiomyocytes in vitro and in mouse hindlimbs in vivo. The injection of the recombinant virus into mice with ischemic hindlimbs protected the hindlimbs from ischemic necrosis. CVB3-FGF2 injection significantly improved the blood flow in the ischemic limbs for over 3 weeks compared with that in the phosphate-buffered saline- or CVB3-injected controls, suggesting that FGF2 expressed from CVB3-FGF2 is functional and therapeutically effective. The virulence of CVB3 was also drastically attenuated in the recombinant virus. Thus, CVB3 can be modified to express a functional foreign protein, supporting its use as a novel viral vector for gene therapy.

A single nucleotide in stem loop II of 5'-untranslated region contributes to virulence of enterovirus 71 in mice

Background

Enterovirus 71 (EV71) has emerged as a neuroinvasive virus responsible for several large outbreaks in the Asia-Pacific region while virulence determinant remains unexplored.

Principal Findings

In this report, we investigated increased virulence of unadapted EV71 clinical isolate 237 as compared with isolate 4643 in mice. A fragment 12 nucleotides in length in stem loop (SL) II of 237 5′-untranslated region (UTR) visibly reduced survival time and rate in mice was identified by constructing a series of infectious clones harboring chimeric 5′-UTR. In cells transfected with bicistronic plasmids, and replicon RNAs, the 12-nt fragment of isolate 237 enhanced translational activities and accelerated replication of subgenomic EV71. Finally, single nucleotide change from cytosine to uridine at base 158 in this short fragment of 5′-UTR was proven to reduce viral translation and EV71 virulence in mice. Results collectively indicated a pivotal role of novel virulence determinant C158 on virus translation in vitro and EV71 virulence in vivo.

Conclusions

These results presented the first reported virulence determinant in EV71 5′-UTR and first position discovered from unadapted isolates.

Molecular characterization of enterovirus 71 and coxsackievirus A16 using the 5′ untranslated region and VP1 region

Enterovirus 71 (EV71) and coxsackievirus A16 (CVA16) are members of the species Human enterovirus A, and are both major and independent aetiological agents of hand-foot-and-mouth disease. The human enterovirus (HEV) 5′ untranslated region (UTR) is fundamentally important for efficient virus replication and for virulence, whilst the VP1 region correlates well with antigenic typing by neutralization, and can be used for virus identification and evolutionary studies. A comparison was undertaken of the 5′UTR and VP1 nucleotide sequences of five EV71 clinical isolates and 10 CVA16 clinical isolates from one laboratory with the 5′UTR and VP1 sequences of 104 EV71 strains and 45 CVA16 strains available in GenBank. The genetic relationships were analysed using standard phylogenetic methods. The EV71 phylogenetic analysis showed that the VP1 sequences were clustered into three genogroups, A, B and C, with genogroups B and C further divided into five subgenogroups, B1–B5 and C1–C5, respectively. All EV71 strains were clustered similarly in the 5′UTR and VP1 trees, except for one Taiwanese strain, which demonstrated different clustering in the two trees, suggesting a recombination event in the phylogeny. The CVA16 phylogenetic analysis showed that the VP1 sequences were clustered into two genogroups, A and B, with genogroup B further divided into B1 (B1a and B1b), B2 and a possible B3; and that a similar pattern and grouping of all strains were displayed in the 5′UTR tree. This study demonstrated that comparing the two regions provides evidence of epidemiological linkage of HEV-A strains, and that mutation in the two regions plays a vital role in the evolution of these viruses. The combination of molecular typing and phylogenetic sequence analysis will be beneficial in both individual patient diagnosis and public health measures.

Variations of coxsackievirus B3 capsid primary structure, ligands, and stability are selected for in a coxsackievirus and adenovirus receptor-limited environment

While group B coxsackieviruses (CVB) use the coxsackievirus and adenovirus receptor (CAR) as the receptor through which they infect susceptible cells, some CVB strains are known for their acquired capacity to bind other molecules. The CVB3/RD strain that emerged from a CVB3/Nancy population sequentially passaged in the CAR-poor RD cell line binds decay-accelerating factor (DAF) (CD55) and CAR. A new strain, CVB3/RDVa, has been isolated from RD cells chronically infected with CVB3/RD and binds multiple molecules in addition to DAF and CAR. The capsid proteins of CVB3/RD differ from those of CVB3/28, a cloned strain that binds only CAR, by only four amino acids, including a glutamate/glutamine dimorphism in the DAF-binding region of the capsid. The capsid proteins of CVB3/RD and CVB3/RDVa differ by seven amino acids. The ability of CVB3/RDVa to bind ligands in addition to CAR and DAF may be attributed to lysine residues near the icosahedral 5-fold axes of symmetry. Considered with differences in the stability of the CVB3 strains, these traits suggest that in vitro selection in a CAR-limited environment selects for virus populations that can associate with molecules on the cell surface and survive until CAR becomes available to support infection.

Altered interactions between stem-loop IV within the 5' noncoding region of coxsackievirus RNA and poly(rC) binding protein 2: effects on IRES-mediated translation and viral infectivity

Coxsackievirus B3 (CVB3) is a causative agent of viral myocarditis, meningitis, pancreatitis, and encephalitis. Much of what is known about the coxsackievirus intracellular replication cycle is based on the information already known from a well-studied and closely related virus, poliovirus. Like that of poliovirus, the 5' noncoding region (5' NCR) of CVB3 genomic RNA contains secondary structures that function in both viral RNA replication and cap-independent translation initiation. For poliovirus IRES-mediated translation, the interaction of the cellular protein PCBP2 with a major secondary structure element (stem-loop IV) is required for gene expression. Previously, the complete secondary structure of the coxsackievirus 5' NCR was determined by chemical structure probing and overall, many of the RNA secondary structures bear significant similarity to those of poliovirus; however, the functions of the coxsackievirus IRES stem-loop structures have not been determined. Here we report that a CVB3 RNA secondary structure, stem-loop IV, folds similarly to poliovirus stem-loop IV and like its enterovirus counterpart, coxsackievirus stem-loop IV interacts with PCBP2. We used RNase foot-printing to identify RNA sequences protected following PCBP2 binding to coxsackievirus stem-loop IV. When nucleotide substitutions were separately engineered at two sites in coxsackievirus stem-loop IV to reduce PCBP2 binding, inhibition of IRES-mediated translation was observed. Both of these nucleotide substitutions were engineered into full-length CVB3 RNA and upon transfection into HeLa cells, the specific infectivities of both constructs were reduced and the recovered viruses displayed small-plaque phenotypes and slower growth kinetics compared to wild type virus.

1A and 3D gene sequences of coxsackievirus B3 strain CC: variation and phylogenetic analysis

Coxsackievirus B3 (CVB3) was thought to be the most common causative agent of life-threatening viral myocarditis. Coxsackievirus B3 strain CC (CVB3-CC) was isolated in China; however, no sequence data are available. The 1A and 3D regions of CVB3-CC were sequenced and phylogenetic analysis was done with reference to ten other CVB3 strains and all 36 prototype strains of human enterovirus B (HEV-B). Sequence analysis showed that the 1A gene region of CVB3-CC consisted of 207 nucleotides, encoding 69 amino acids; and the 3D gene region was comprised of 1386 nucleotides, encoding 462 amino acids. Variation analysis showed that the 3D gene of CVB3 strain CC varied the least among the two regions. Phylogenetic tree analysis of the 1A and 3D regions indicated that CVB3-CC clustered together with CVB3 Nancy strain suggesting that there may be a close evolutionary relationship between the two strains. Incongruity was observed between the non-structural protein gene and the structural protein gene trees, according to the topological structure, indicating that recombination was occurred among these strains.

Interactions between multiple genetic determinants in the 5' UTR and VP1 capsid control pathogenesis of chronic post-viral myopathy caused by coxsackievirus B1

Mice infected with coxsackievirus B1 Tucson (CVB1(T)) develop chronic, post-viral myopathy (PVM) with clinical manifestations of hind limb muscle weakness and myositis. The objective of the current study was to establish the genetic basis of myopathogenicity in CVB1(T). Using a reverse genetics approach, full attenuation of PVM could only be achieved by simultaneously mutating four sites located at C706U in the 5' untranslated region (5' UTR) and at Y87F, V136A, and T276A in the VP1 capsid. Engineering these four myopathic determinants into an amyopathic CVB1(T) variant restored the ability to cause PVM. Moreover, these same four determinants controlled PVM expression in a second strain of mice, indicating that the underlying mechanism is operational in mice of different genetic backgrounds. Modeling studies predict that C706U alters both local and long range pairing in the 5' UTR, and that VP1 determinants are located on the capsid surface. However, these differences did not affect viral titers, temperature stability, pH stability, or the antibody response to virus. These studies demonstrate that PVM develops from a complex interplay between viral determinants in the 5' UTR and VP1 capsid and have uncovered intriguing similarities between genetic determinants that cause PVM and those involved in pathogenesis of other enteroviruses.

Mutation variants generated from nonvirulent coxsackievirus B3 acquire virulence phenotypes by active virus replication

OBJECTIVE

To understand coxsackievirus B3 (CVB3) virulence at the molecular level.

METHOD

A mutation library was generated from noncardiovirulent CVB3/0. Highly virulent mutation variants were recovered and characterized both phenotypically and genotypically.

RESULTS

The variants consistently caused destruction of multiple tissues together with active virus production and induced severe mortality in vivo. The extent of infectious virus generation was directly correlated with that of histopathological aberration. Genotypic analysis of the entire genome indicated that the virulent viruses encode nucleotide substitutions in the 5'-nontranslated region, which have previously been identified in other virulent CVB3s.

CONCLUSION

The present study provides evidence that particular nucleotide substitutions in the 5'-nontranslated region of nonvirulent CVB3 can lead to active virus replication, which is sufficient to induce virulence.

Role of GNRA Motif Mutations within Stem-Loop V of Internal Ribosome Entry Segment in Coxsackievirus B3 Molecular Attenuation

The lengthy 5' nontranslated region of coxsackievirus B3 (CVB3) forms a highly ordered secondary structure containing an internal ribosome entry segment (IRES), which plays an important role in controlling viral translation and pathogenesis. The stem-loop V (SL-V) of this IRES contains a large lateral bulge loop which encompasses two conserved GNRA motifs. In this study, we analyzed the effects of point mutations within the GNRA motifs of the CVB3 IRES. We characterized in vitro virus production and translation efficiency and we tested in vivo virulence of two CVB3 mutants produced by site-directed mutagenesis. The GNAA1 and GNAA2 RNAs displayed decreased translation initiation efficiency when translated in rabbit reticulocyte lysates. This translation defect was correlated with reduced yields of infectious virus particles in HeLa cells in comparison with the wild type. When inoculated orally into Swiss mice, both mutant viruses were avirulent and caused neither inflammation nor necrosis in hearts. These results highlight the important role of the GNRA motifs within the SL-V of the IRES of CVB3, in directing translation initiation.

Replication of coxsackievirus B3 in primary cell cultures generates novel viral genome deletions

Coxsackievirus B3 (CVB3) generates 5'-terminally deleted genomes (TDs) during replication in murine hearts. We show here that CVB3 populations with TDs can also be generated within two to three passages of CVB3 in primary, but not immortalized, cell cultures. Deletions of less than 49 nucleotides increase in size during passage, while 5' TDs of 49 nucleotides appear to be the maximum deletion size. The cellular environment of contact-inhibited primary cell cultures or the myocardium in vivo is sufficient for the selection of 5' TDs over undeleted genomes.

Molecular analysis of the role of IRES stem-loop V in replicative capacities and translation efficiencies of Coxsackievirus B3 mutants

Coxsackievirus B3 (CVB3) is a principal viral cause of acute myocarditis in humans and has been implicated in the pathogenesis of dilated cardiomyopathy. The natural genetic determinants of cardiovirulence for CVB3 have not been identified, although using strains engineered in the laboratory, it has been demonstrated elsewhere that, for several wild-type CB3 strains, the primary molecular determinant of cardiovirulence phenotype localizes to the 5' nontranslated region (5'NTR) and capsid. Stable RNA tetraloop motifs are found frequently in biologically active RNAs. These motifs carry out a wide variety of functions in RNA folding, in RNA-RNA and RNA-protein interactions. A great deal of knowledge about the structures and functions of tetraloop motifs has accumulated largely due to intensive theoretical, biochemical, and biophysical studies on one most frequently occurring family of tetraloop sequences, namely, the GNRA sequence, especially the GNAA sequence conserved in all enteroviruses. Here in this study, through construction of CVB3 chimeric mutants, the predicted stem loop (SL) V within the 5'NTR has been identified as important in determining viral cardiovirulence. Replication assays in HeLa cell monolayers revealed that wild-type CVB3 virus and two of the six mutants constructed here grow efficiently, whereas other mutant viruses replicate poorly. Furthermore, the in vitro translation products from these mutants and wild-type CVB3, demonstrated that the two mutants who replicate efficiently, translated at relatively equivalent amount than the wild-type. However, other mutants demonstrated a low efficiency in their production of protein when translated in a Rabbit Reticulocytes Lysats.

Endogenous low-level expression of the coxsackievirus and adenovirus receptor enables coxsackievirus B3 infection of RD cells

Cells in which the appropriate viral receptor cannot be detected may paradoxically act as a host to the virus. For example, RD cells are often considered to be non-permissive for infection with coxsackievirus and adenovirus receptor (CAR)-dependent group B coxsackieviruses (CVB), insofar as inoculated cell monolayers show little or no cytopathic effect (CPE) and immunohistological assays for CAR have been consistently negative. Supernatants recovered from RD cells exposed to CVB, however, contained more virus than was added in the initial inoculum, indicating that productive virus replication occurred in the monolayer. When infected with a recombinant CVB type 3 (CVB3) chimeric strain expressing S-Tag within the viral polyprotein, 4–11 % of RD cells expressed S-Tag over 48 h. CAR mRNA was detected in RD cells by RT-PCR, and CAR protein was detected on Western blots of RD lysates; both were detected at much lower levels than in HeLa cells. Receptor blockade by an anti-CAR antibody confirmed that CVB3 infection of RD cells was mediated by CAR. These results show that some RD cells in the culture population express CAR and can thereby be infected by CVB, which explains the replication of CAR-dependent CVB in cell types that show little or no CPE and in which CAR has not previously been detected. Cells within cultures of cell types that have been considered non-permissive may express receptor transiently, leading to persistent replication of virus within the cultured population.

The substitution U475 --> C with Sabin3-like mutation within the IRES attenuate Coxsackievirus B3 cardiovirulence

The Sabin3 mutation in the viral RNA plays an important role in directing attenuation phenotype of Sabin vaccine strain of poliovirus type 1 (PV1). We previously described that Sabin3-like mutation introduced in Coxsackievirus B3 (CVB3) genome led to a defective mutant. However, this mutation do not led to destruction of secondary structure motif C within the stem-loop V of CVB3 RNA because of the presence of one nucleotide difference (C --> U) in the region encompassing the Sabin3 mutation at nucleotides 471 of PV1 and 475 of CVB3 RNA. In order to reproduce the same sequence of PV1 sabin3 vaccine strain, we introduce in this study an additional mutation (U475 --> C) to CVB3 Sabin3-like mutant. Our results demonstrated that Sabin3-like+C mutant displayed a decreased translation initiation defects when translated in cell-free system. This translation initiation defect was correlated with reduced yields of infectious virus particles in HeLa cells in comparison with Sabin3-like mutant and wild-type CVB3 viruses. Inoculation of Swiss mice with mutant viruses resulted in no inflammatory heart disease when compared to heart of mice infected with wild-type. Theses findings indicate that the double mutant could be exploited for the development of a live attenuated vaccine against CVB3.