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

Vaccines against Group B Coxsackieviruses and Their Importance

The group B coxsackieviruses (CVBs) exist in six serotypes (CVB1 to CVB6). Disease associations have been reported for most serotypes, and multiple serotypes can cause similar diseases. For example, CVB1, CVB3, and CVB5 are generally implicated in the causation of myocarditis, whereas CVB1 and CVB4 could accelerate the development of type 1 diabetes (T1D). Yet, no vaccines against these viruses are currently available. In this review, we have analyzed the attributes of experimentally tested vaccines and discussed their merits and demerits or limitations, as well as their impact in preventing infections, most importantly myocarditis and T1D.

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.

Attenuated strain of CVB3 with a mutation in the CAR-interacting region protects against both myocarditis and pancreatitis

Coxsackievirus B3 (CVB3), is commonly implicated in myocarditis, which can lead to dilated cardiomyopathy, in addition to causing acute pancreatitis and meningitis. Yet, no vaccines are currently available to prevent this infection. Here, we describe the derivation of a live attenuated vaccine virus, termed mutant (Mt) 10, encoding a single amino acid substitution H790A within the viral protein 1, that prevents CVB3 infection in mice and protects from both myocarditis and pancreatitis in challenge studies. We noted that animals vaccinated with Mt 10 developed virus-neutralizing antibodies, predominantly containing IgG2a and IgG2b, and to a lesser extent IgG3 and IgG1. Furthermore, by using major histocompatibility complex class II dextramers and tetramers, we demonstrated that Mt 10 induces antigen-specific T cell responses that preferentially produce interferon-γ. Finally, neither vaccine recipients nor those challenged with the wild-type virus revealed evidence of autoimmunity or cardiac injury as determined by T cell response to cardiac myosin and measurement of circulating cardiac troponin I levels, respectively. Together, our data suggest that Mt 10 is a vaccine candidate that prevents CVB3 infection through the induction of neutralizing antibodies and antigen-specific T cell responses, the two critical components needed for complete protection against virus infections in vaccine studies.

The Antiviral and Antitumor Effects of Defective Interfering Particles/Genomes and Their Mechanisms

Defective interfering particles (DIPs), derived naturally from viral particles, are not able to replicate on their own. Several studies indicate that DIPs exert antiviral effects via multiple mechanisms. DIPs are able to activate immune responses and suppress virus replication cycles, such as competing for viral replication products, impeding the packaging, release and invasion of viruses. Other studies show that DIPs can be used as a vaccine against viral infection. Moreover, DIPs/DI genomes display antitumor effects by inducing tumor cell apoptosis and promoting dendritic cell maturation. With genetic modified techniques, it is possible to improve its safety against both viruses and tumors. In this review, a comprehensive discussion on the effects exerted by DIPs is provided. We further highlight the clinical significance of DIPs and propose that DIPs can open up a new platform for antiviral and antitumor therapies.

Construction and characterization of a full-length cDNA infectious clone of emerging porcine Senecavirus A

A full-length cDNA infectious clone, pKS15-01-Clone, was constructed from an emerging Senecavirus A (SVA; strain KS15-01). To explore the potential use as a viral backbone for expressing marker genes, the enhanced green fluorescent protein (EGFP)-tagged reporter virus (vKS15-01-EGFP) was generated using reverse genetics. Compared to the parental virus, the pKS15-01-Clone derived virus (vKS15-01-Clone) replicated efficiently in vitro and in vivo, and induced similar levels of neutralizing antibody and cytokine responses in infected animals. In contrast, the vKS15-01-EGFP virus showed impaired growth ability and induced lower level of immune response in infected animals. Lesions on the dorsal snout and coronary bands were observed in all pigs infected by parental virus KS15-01, but not in pigs infected with vKS15-01-Clone or vKS15-01-EGFP viruses. These results demonstrated that the infectious clone and EGFP reporter virus could be used as important tools in further elucidating the SVA pathogenesis and development of control measures.

In vitro-reduced translation efficiency of coxsackievirus B3 Sabin3-like strain is correlated to impaired binding of cellular initiation factors to viral IRES RNA

Coxsackievirus B3 (CVB3) causes viral myocarditis and can ultimately result in dilated cardiomyopathy. There is no vaccine available for clinical use. Translation initiation of CVB3 RNA is directed by an internal ribosome entry site within the 5'-untranslated region. We have previously described that Sabin3-like mutation (U(473) to C) introduced in CVB3 genome led to a defective mutant with a serious reduction in translation efficiency. In the present study, we analyzed, in vitro, the effect of the Sabin3-like mutation on the binding affinity of RNA domain V to some standard translation initiation factors: eIF4G, eIF3b, and eIF4B by filter-binding assays and UV-crosslink assays. We have demonstrated that this single-nucleotide exchange impairs the binding affinity of these cellular factors within the mutant RNA. These data indicate how this decisive Sabin3-like mutation mediates viral translation attenuation. Taken together, these findings strongly suggest that the mutant strain could be considered a candidate for an attenuated CVB3 vaccine.

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.

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.

Molecular Analysis of RNA-RNA Interactions between 5' and 3' Untranslated Regions during the Initiation of Translation of a Cardiovirulent and a Live-Attenuated Coxsackievirus B3 Strains

Coxsackievirus B3 (CVB3) is a causative agent of viral myocarditis, meningitis and pancreatitis. CVB3 overcome their host cells by usurping the translation machinery to benefit viral gene expression. This is accomplished through alternative translation initiation in a cap independent manner at the viral internal ribosomal entry site. The 5′ untranslated region (5′UTR) of CVB3 genomic RNA is highly structured. It is the site of multiple RNA-protein and RNA-RNA interactions and it plays a critical role during translation initiation. Similar to the 5′UTR, CVB3 3′ untranslated region (3′UTR) also contains secondary structural elements consisting of three stem-loops followed by a poly (A) tail sequence. Long-range RNA-RNA interactions between 5′ and 3′ ends of some viral genomes have been observed. Because of their dual role in translation and replication, the 5′ and 3′UTRs represent promising candidates for the study of CVB3 cardiovirulence. Taking into account that efficient initiation of mRNA translation depends on a temporally and spatially orchestrated sequence of protein-protein, protein-RNA and RNA-RNA interactions, and that, at present, little is known about RNA-RNA interactions between CVB3 5′ and 3′UTRs, we aimed in the present study, to assess a possible RNA-RNA interaction between 5′ and 3′UTRs during the initiation of translation of a wild-type and a previously characterized mutant (Sabin3-like) CVB3 strains and to investigate the effect of the Sabin3-like mutation on these potential interactions. For this purpose, “Electrophoretic Mobility Shift” assays were carried out. Data obtained did not show any RNA-RNA direct interactions between the 5′- and 3′- ends. Therefore, we can suggest that the possible mechanism by which 3′UTR enhances CVB3 IRES activity may be by bridging the 5′ to the 3′ end through RNA-protein interaction and not through RNA-RNA direct contact. However, these findings need to be confirmed by carrying out further experiments.

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.

Cryptic AUG is important for 48S ribosomal assembly during internal initiation of translation of coxsackievirus B3 RNA

We have investigated the possible role of a conserved cis-acting element, the cryptic AUG, present in the 5' UTR of coxsackievirus B3 (CVB3 ) RNA. CVB3 5' UTR contains multiple AUG codons upstream of the initiator AUG, which are not used for the initiation of translation. The 48S ribosomal assembly takes place upstream of the cryptic AUG. We show here that mutation in the cryptic AUG results in reduced efficiency of translation mediated by the CVB3 IRES; mutation also reduces the interaction of mutant IRES with a well characterized IRES trans-acting factor, the human La protein. Furthermore, partial silencing of the La gene showed a decrease in IRES activity in the case of both the wild-type and mutant. We have demonstrated here that the interaction of the 48S ribosomal complex with mutant RNA was weaker compared with wild-type RNA by ribosome assembly analysis. We have also investigated by chemical and enzymic modifications the possible alteration in secondary structure in the mutant RNA. Results suggest that the secondary structure of mutant RNA was only marginally altered. Additionally, we have demonstrated by generating compensatory and non-specific mutations the specific function of the cryptic AUG in internal initiation. Results suggest that the effect of the cryptic AUG is specific and translation could not be rescued. However, a possibility of tertiary interaction of the cryptic AUG with other cis-acting elements cannot be ruled out. Taken together, it appears that the integrity of the cryptic AUG is important for efficient translation initiation by the CVB3 IRES RNA.

Polypyrimidine tract-binding protein interacts with coxsackievirus B3 RNA and influences its translation

We have investigated the possible role of trans-acting factors interacting with the untranslated regions (UTRs) of coxsackievirus B3 (CVB3) RNA. We show here that polypyrimidine tract-binding protein (PTB) binds specifically to both 5' and 3' UTRs, but with different affinity. We have demonstrated that PTB is a bona fide internal ribosome entry site (IRES) trans-acting factor (ITAF) for CVB3 RNA by characterizing the effect of partial silencing of PTB ex vivo in HeLa cells. Furthermore, IRES activity in BSC-1 cells, which are reported to have a very low level of endogenous PTB, was found to be significantly lower than that in HeLa cells. Additionally, we have mapped the putative contact points of PTB on the 5' and 3' UTRs by an RNA toe-printing assay. We have shown that the 3' UTR is able to stimulate CVB3 IRES-mediated translation. Interestingly, a deletion of 15 nt at the 5' end or 14 nt at the 3' end of the CVB3 3' UTR reduced the 3' UTR-mediated enhancement of IRES activity ex vivo significantly, and a reduced interaction was shown with PTB. It appears that the PTB protein might help in circularization of the CVB3 RNA by bridging the ends necessary for efficient translation of the viral RNA.

Mutations at nucleotides 573 and 579 within 5'-untranslated region augment the virulence of coxsackievirus B1

Two mutants of coxsackievirus B1 (CVB1), CVB1c and CVB1e, with mutations in the stem-loop H of 5'-UTR were generated by site-directed mutagenesis. The A at nt579 of CVB1c was substituted by G. The U at nt573 and A at nt579 of CVB1e were substituted by A and G. The virulences of these mutants had been assessed by means of cytopathic effect (CPE), plaque formation, one-step growth curve, and 50% lethal dose (LD50) assays. The pathogenesis of these mutants was evaluated by attacking suckling Balb/c mice. Plaque assay and one-step growth curve showed that the replication of CVB1c and CVB1e on HeLa cells was significantly faster than that of their prototype CVB1n. Data of CPE assay, LD50, and pathological examination showed that CVB1c and CVB1e were more virulent than CVB1n. These data showed that mutation at nt579 (A-->G) alone and mutations at nt579 (A-->G) and nt573 (U-->A) together within 5'-UTR caused significant augment of the virulence and pathogenesis of coxsackievirus B1, and suggested that nt573 and nt579 might be molecular determinants for the virulence of coxsackievirus B1.