Low efficiency of the 5' nontranslated region of hepatitis A virus RNA in directing cap-independent translation in permissive monkey kidney cells

Enhanced replication of a hepatitis A virus vaccine strain via adaptation in Vero cells

Purpose

Hepatitis A virus (HAV) production has been limited by its slow replication rate and reliance on diploid cell lines like MRC-5, which present challenges in scalability, passage limitations, and serum-free culture conditions. This study aimed to develop an HAV vaccine strain with enhanced replication capacity.

Materials and Methods

We generated a reverse genetically modified HAV vaccine strain (RG-HAV) and adapted it to Vero cells through sequential culturing. Replication rates of RG-HAV and a commercially used strain, HM-175, were compared in Vero and MRC-5 cells. Nucleotide sequences, including coding and non-coding regions like the internal ribosomal entry site (IRES), were analyzed. Structural assessments included 3-dimensional modeling of IRES and relative codon deoptimization analysis of the capsid. Immunogenicity was evaluated by measuring HAV-specific antibody responses in mice.

Results

Vero-adapted RG-HAV achieved a 30-fold increase in production yield compared to initial transfection. In Vero cells, RG-HAV peaked at 15 days post-infection, compared to 20 days for HM-175. In MRC-5 cells, RG-HAV and HM-175 reached peak production at 10 and 15 days, respectively. RG-HAV produced over 5-fold more HAV in Vero cells and 8-fold more in MRC-5 cells than HM-175. Sequence analysis revealed nine amino acid differences in RG-HAV structural proteins and five nucleotide changes in the type III IRES region, potentially enhancing IRES functionality. Immunization with inactivated RG-HAV with alum hydroxide induced HAV-specific antibody responses in mice.

Conclusion

RG-HAV offers enhanced replication and production yields, supporting its potential in advancing HAV vaccine development.

Hepatovirus translation requires PDGFA-associated protein 1, an eIF4E-binding protein regulating endoplasmic reticulum stress responses

The overexpression and misfolding of viral proteins in the endoplasmic reticulum (ER) may cause cellular stress, thereby inducing a cytoprotective, proteostatic host response involving phosphorylation of eukaryotic translation initiation factor 2 subunit alpha (eIF2α). Here, we show that hepatitis A virus, a positive-strand RNA virus responsible for infectious hepatitis, adopts a stress-resistant, eIF2α-independent mechanism of translation to ensure the synthesis of viral proteins within the infected liver. Cap-independent translation directed by the hepatovirus internal ribosome entry site and productive hepatovirus infection of mice both require platelet-derived growth factor subunit A (PDGFA)-associated protein 1 (PDAP1), a small phosphoprotein of unknown function with eIF4E-binding activity. PDAP1 also interacts with eIF1A and is essential for translating stress-resistant host messenger RNAs that evade the proteostatic response to ER stress and that encode proteins promoting the survival of stressed cells.

Hepatitis A virus infection

Hepatitis A is a vaccine-preventable infection caused by the hepatitis A virus (HAV). Over 150 million new infections of hepatitis A occur annually. HAV causes an acute inflammatory reaction in the liver that usually resolves spontaneously without chronic sequelae. However, up to 20% of patients experience a prolonged or relapsed course and <1% experience acute liver failure. Host factors, such as immunological status, age, pregnancy and underlying hepatic diseases, can affect the severity of disease. Anti-HAV IgG antibodies produced in response to HAV infection persist for life and protect against re-infection; vaccine-induced antibodies against hepatitis A confer long-term protection. The WHO recommends vaccination for individuals at higher risk of infection and/or severe disease in countries with very low and low hepatitis A virus endemicity, and universal childhood vaccination in intermediate endemicity countries. To date, >25 countries worldwide have implemented such programmes, resulting in a reduction in the incidence of HAV infection. Improving hygiene and sanitation, rapid identification of outbreaks and fast and accurate intervention in outbreak control are essential to reducing HAV transmission.

Cap-Independent Circular mRNA Translation Efficiency

Recently, the mRNA platform has become the method of choice in vaccine development to find new ways to fight infectious diseases. However, this approach has shortcomings, namely that mRNA vaccines require special storage conditions, which makes them less accessible. This instability is due to the fact that the five-prime and three-prime ends of the mRNA are a substrate for the ubiquitous exoribonucleases. To address the problem, circular mRNAs have been proposed for transgene delivery as they lack these ends. Notably, circular RNAs do not have a capped five-prime end, which makes it impossible to initiate translation canonically. In this review, we summarize the current knowledge on cap-independent translation initiation methods and discuss which approaches might be most effective in developing vaccines and other biotechnological products based on circular mRNAs.

Nonlytic Quasi-Enveloped Hepatovirus Release Is Facilitated by pX Protein Interaction with the E3 Ubiquitin Ligase ITCH

Hepatoviruses are atypical hepatotropic picornaviruses that are released from infected cells without lysis in small membranous vesicles. These exosome-like, quasi-enveloped virions (eHAV) are infectious and the only form of hepatitis A virus (HAV) found circulating in blood during acute infection. eHAV is released through multivesicular endosomes in a process dependent on endosomal sorting complexes required for transport (ESCRT). Capsid protein interactions with the ESCRT-associated Bro1 domain proteins, ALG-2-interacting protein X (ALIX) and His domain-containing protein tyrosine phosphatase (HD-PTP), which are both recruited to the pX domain of 1D (VP1pX), are critical for this process. Previous proteomics studies suggest pX also binds the HECT domain, NEDD4 family E3 ubiquitin ligase, ITCH. Here, we confirm this interaction and show ITCH binds directly to the carboxy-terminal half of pX from both human and bat hepatoviruses independently of ALIX. A small chemical compound (compound 5) designed to disrupt interactions between WW domains of NEDD4 ligases and substrate molecules blocked ITCH binding to pX and demonstrated substantial antiviral activity against HAV. CRISPR deletion or small interfering RNA (siRNA) knockdown of ITCH expression inhibited the release of a self-assembling nanocage protein fused to pX and also impaired the release of eHAV from infected cells. The release could be rescued by overexpression of wild-type ITCH, but not a catalytically inactive ITCH mutant. Despite this, we found no evidence that ITCH ubiquitylates pX or that eHAV release is strongly dependent upon Lys residues in pX. These data indicate ITCH plays an important role in the ESCRT-dependent release of quasi-enveloped hepatovirus, although the substrate molecule targeted for ubiquitylation remains to be determined. IMPORTANCE Mechanisms underlying the cellular release of quasi-enveloped hepatoviruses are only partially understood, yet play a crucial role in the pathogenesis of this common agent of viral hepatitis. Multiple NEDD4 family E3 ubiquitin ligases, including ITCH, have been reported to promote the budding of conventional enveloped viruses but are not known to function in the release of HAV or other picornaviruses from infected cells. Here, we show that the unique C-terminal pX extension of the VP1 capsid protein of HAV interacts directly with ITCH and that ITCH promotes eHAV release in a manner analogous to its role in budding of some conventional enveloped viruses. The catalytic activity of ITCH is required for efficient eHAV release and may potentially function to ubiquitylate the viral capsid or activate ESCRT components.

Picornavirus translation strategies

The genome of viruses classified as picornaviruses consists of a single monocistronic positive strand RNA. The coding capacity of these RNA viruses is rather limited, and thus, they rely on the cellular machinery for their viral replication cycle. Upon the entry of the virus into susceptible cells, the viral RNA initially competes with cellular mRNAs for access to the protein synthesis machinery. Not surprisingly, picornaviruses have evolved specialized strategies that successfully allow the expression of viral gene products, which we outline in this review. The main feature of all picornavirus genomes is the presence of a heavily structured RNA element on the 5´UTR, referred to as an internal ribosome entry site (IRES) element, which directs viral protein synthesis as well and, consequently, triggers the subsequent steps required for viral replication. Here, we will summarize recent studies showing that picornavirus IRES elements consist of a modular structure, providing sites of interaction for ribosome subunits, eIFs, and a selective group of RNA‐binding proteins.

RNA-Binding Proteins as Regulators of Internal Initiation of Viral mRNA Translation

Viruses are obligate intracellular parasites that depend on the host’s protein synthesis machinery for translating their mRNAs. The viral mRNA (vRNA) competes with the host mRNA to recruit the translational machinery, including ribosomes, tRNAs, and the limited eukaryotic translation initiation factor (eIFs) pool. Many viruses utilize non-canonical strategies such as targeting host eIFs and RNA elements known as internal ribosome entry sites (IRESs) to reprogram cellular gene expression, ensuring preferential translation of vRNAs. In this review, we discuss vRNA IRES-mediated translation initiation, highlighting the role of RNA-binding proteins (RBPs), other than the canonical translation initiation factors, in regulating their activity.

Current Practice in Bicistronic IRES Reporter Use: A Systematic Review

Bicistronic reporter assays have been instrumental for transgene expression, understanding of internal ribosomal entry site (IRES) translation, and identification of novel cap-independent translational elements (CITE). We observed a large methodological variability in the use of bicistronic reporter assays and data presentation or normalization procedures. Therefore, we systematically searched the literature for bicistronic IRES reporter studies and analyzed methodological details, data visualization, and normalization procedures. Two hundred fifty-seven publications were identified using our search strategy (published 1994-2020). Experimental studies on eukaryotic adherent cell systems and the cell-free translation assay were included for further analysis. We evaluated the following methodological details for 176 full text articles: the bicistronic reporter design, the cell line or type, transfection methods, and time point of analyses post-transfection. For the cell-free translation assay, we focused on methods of in vitro transcription, type of translation lysate, and incubation times and assay temperature. Data can be presented in multiple ways: raw data from individual cistrons, a ratio of the two, or fold changes thereof. In addition, many different control experiments have been suggested when studying IRES-mediated translation. In addition, many different normalization and control experiments have been suggested when studying IRES-mediated translation. Therefore, we also categorized and summarized their use. Our unbiased analyses provide a representative overview of bicistronic IRES reporter use. We identified parameters that were reported inconsistently or incompletely, which could hamper data reproduction and interpretation. On the basis of our analyses, we encourage adhering to a number of practices that should improve transparency of bicistronic reporter data presentation and improve methodological descriptions to facilitate data replication.

Pathogenicity and virulence of hepatitis A virus

Hepatitis A is an acute infection of the liver, which is mostly asymptomatic in children and increases the severity with age. Although in most patients the infection resolves completely, in a few of them it may follow a prolonged or relapsed course or even a fulminant form. The reason for these different outcomes is unknown, but it is generally accepted that host factors such as the immunological status, age and the occurrence of underlaying hepatic diseases are the main determinants of the severity. However, it cannot be ruled out that some virus traits may also contribute to the severe clinical outcomes. In this review, we will analyze which genetic determinants of the virus may determine virulence, in the context of a paradigmatic virus in terms of its genomic, molecular, replicative, and evolutionary features.

Advances for the Hepatitis A Virus Antigen Production Using a Virus Strain With Codon Frequency Optimization Adjustments in Specific Locations

The available cell-adapted hepatitis A virus (HAV) strains show a very slow replication phenotype hampering the affordable production of antigen. A fast-growing strain characterized by the occurrence of mutations in the internal ribosome entry site (IRES), combined with changes in the codon composition has been selected in our laboratory. A characterization of the IRES activity of this fast-growing strain (HM175-HP; HP) vs. its parental strain (HM175; L0) was assessed in two cell substrates used in vaccine production (MRC-5 and Vero cells) compared with the FRhK-4 cell line in which its selection was performed. The HP-derived IRES was significantly more active than the L0-derived IRES in all cells tested and both IRES were more active in the FRhK-4 cells. The translation efficiency of the HP-derived IRES was also much higher than the L0-derived IRES, particularly, in genes with a HP codon usage background. These results correlated with a higher virus production in a shorter time for the HP strain compared to the L0 strain in any of the three cell lines tested, and of both strains in the FRhK-4 cells compared to Vero and MRC-5 cells. The addition of wortmannin resulted in the increase of infectious viruses and antigen in the supernatant of FRhK-4 infected cells, independently of the strain. Finally, the replication of both strains in a clone of FRhK-4 cells adapted to grow with synthetic sera was optimal and again the HP strain showed higher yields.

Cell Culture Systems and Drug Targets for Hepatitis A Virus Infection

Hepatitis A virus (HAV) infection is one of the major causes of acute hepatitis, and this infection occasionally causes acute liver failure. HAV infection is associated with HAV-contaminated food and water as well as sexual transmission among men who have sex with men. Although an HAV vaccine has been developed, outbreaks of hepatitis A and life-threatening severe HAV infections are still observed worldwide. Therefore, an improved HAV vaccine and anti-HAV drugs for severe hepatitis A should be developed. Here, we reviewed cell culture systems for HAV infection, and other issues. This review may help with improving the HAV vaccine and developing anti-HAV drugs.

The Critical Role of Codon Composition on the Translation Efficiency Robustness of the Hepatitis A Virus Capsid

Hepatoviruses show an intriguing deviated codon usage, suggesting an evolutionary signature. Abundant and rare codons in the cellular genome are scarce in the human hepatitis A virus (HAV) genome, while intermediately abundant host codons are abundant in the virus. Genotype–phenotype maps, or fitness landscapes, are a means of representing a genotype position in sequence space and uncovering how genotype relates to phenotype and fitness. Using genotype–phenotype maps of the translation efficiency, we have shown the critical role of the HAV capsid codon composition in regulating translation and determining its robustness. Adaptation to an environmental perturbation such as the artificial induction of cellular shutoff—not naturally occurring in HAV infection—involved movements in the sequence space and dramatic changes of the translation efficiency. Capsid rare codons, including abundant and rare codons of the cellular genome, slowed down the translation efficiency in conditions of no cellular shutoff. In contrast, rare capsid codons that are abundant in the cellular genome were efficiently translated in conditions of shutoff. Capsid regions very rich in slowly translated codons adapt to shutoff through sequence space movements from positions with highly robust translation to others with diminished translation robustness. These movements paralleled decreases of the capsid physical and biological robustness, and resulted in the diversification of capsid phenotypes. The deviated codon usage of extant hepatoviruses compared with that of their hosts may suggest the occurrence of a virus ancestor with an optimized codon usage with respect to an unknown ancient host.

A conserved RNA structural motif for organizing topology within picornaviral internal ribosome entry sites

Picornaviral IRES elements are essential for initiating the cap-independent viral translation. However, three-dimensional structures of these elements remain elusive. Here, we report a 2.84-Å resolution crystal structure of hepatitis A virus IRES domain V (dV) in complex with a synthetic antibody fragment-a crystallization chaperone. The RNA adopts a three-way junction structure, topologically organized by an adenine-rich stem-loop motif. Despite no obvious sequence homology, the dV architecture shows a striking similarity to a circularly permuted form of encephalomyocarditis virus J-K domain, suggesting a conserved strategy for organizing the domain architecture. Recurrence of the motif led us to use homology modeling tools to compute a 3-dimensional structure of the corresponding domain of foot-and-mouth disease virus, revealing an analogous domain organizing motif. The topological conservation observed among these IRESs and other viral domains implicates a structured three-way junction as an architectural scaffold to pre-organize helical domains for recruiting the translation initiation machinery.

Cellular entry and uncoating of naked and quasi-enveloped human hepatoviruses

Many ‘non-enveloped’ viruses, including hepatitis A virus (HAV), are released non-lytically from infected cells as infectious, quasi-enveloped virions cloaked in host membranes. Quasi-enveloped HAV (eHAV) mediates stealthy cell-to-cell spread within the liver, whereas stable naked virions shed in feces are optimized for environmental transmission. eHAV lacks virus-encoded surface proteins, and how it enters cells is unknown. We show both virion types enter by clathrin- and dynamin-dependent endocytosis, facilitated by integrin β₁, and traffic through early and late endosomes. Uncoating of naked virions occurs in late endosomes, whereas eHAV undergoes ALIX-dependent trafficking to lysosomes where the quasi-envelope is enzymatically degraded and uncoating ensues coincident with breaching of endolysosomal membranes. Neither virion requires PLA2G16, a phospholipase essential for entry of other picornaviruses. Thus naked and quasi-enveloped virions enter via similar endocytic pathways, but uncoat in different compartments and release their genomes to the cytosol in a manner mechanistically distinct from other Picornaviridae.

The Hepatitis A virus is a common cause of liver disease in humans. It is unable to multiply on its own so it needs to enter the cells of its host and hijack them to make new virus particles.

Infected human cells produce two different types of Hepatitis A particles. The first, known as ‘naked’ virus particles, consist of molecules of ribonucleic acid (or RNA for short) that are surrounded by a protein shell. Naked virus particles are shed in the feces of infected individuals and are very stable, allowing the virus to spread in the environment to find new hosts.

At the same time, a second type of particle, known as the ‘quasi-enveloped’ virus, circulates in the blood of the infected individual. In a quasi-enveloped particle, the RNA and protein shell are completely enclosed within a membrane that is released from the host cell. This membrane protects the protein shell from human immune responses, enabling quasi-enveloped virus particles to spread in a stealthy fashion within the liver.

It was not clear how these two different types of virus particle are both able to enter cells despite their surface being so different. To address this question, Rivera-Serrano et al. used a microscopy approach to observe Hepatitis A particles infecting human liver cells.

The experiments showed that both types of virus particle actually use similar routes. First, the external membrane of the cell folded around the particles, creating a vesicle that trapped the viruses and brought them within the cell. Inside these vesicles, the naked virus particles soon fell apart, and their RNA was released directly into the interior of the cell.

However, the vesicles that carried quasi-enveloped virus travelled further into the cell and eventually delivered their contents to a specialized compartment, the lysosome, where the virus membrane was degraded. This caused the quasi-enveloped viruses to fall apart and release their RNA into the cell more slowly than the naked particles.

Several viruses, such as the one that causes polio, also have quasi-enveloped forms. Studying how these particles are able to infect human cells while hiding behind membranes borrowed from the host may help us target these viruses better.

Hepatitis A Virus Genome Organization and Replication Strategy

Hepatitis A virus (HAV) is a positive-strand RNA virus classified in the genus Hepatovirus of the family Picornaviridae It is an ancient virus with a long evolutionary history and multiple features of its capsid structure, genome organization, and replication cycle that distinguish it from other mammalian picornaviruses. HAV proteins are produced by cap-independent translation of a single, long open reading frame under direction of an inefficient, upstream internal ribosome entry site (IRES). Genome replication occurs slowly and is noncytopathic, with transcription likely primed by a uridylated protein primer as in other picornaviruses. Newly produced quasi-enveloped virions (eHAV) are released from cells in a nonlytic fashion in a unique process mediated by interactions of capsid proteins with components of the host cell endosomal sorting complexes required for transport (ESCRT) system.

Hepatitis A Virus Codon Usage: Implications for Translation Kinetics and Capsid Folding

Codon usage bias is universal to all genomes. Hepatitis A virus (HAV) codon usage is highly biased and deoptimized with respect to its host. Accordingly, HAV is unable to induce cellular translational shutoff and its internal ribosome entry site (IRES) is inefficient. Codon usage deoptimization may be seen as a hawk (host cell) versus dove (HAV) game strategy for accessing transfer RNA (tRNA). HAV avoids use of abundant host cell codons and thereby eludes competition for the corresponding tRNAs. Instead, codons that are abundant or rare in cellular messenger RNAs (mRNAs) are used relatively rarely in its genome, although intermediately abundant host cell codons are abundant in the viral genome. Rare codons in the capsid coding region slow down the translation elongation rate, and in doing so intrinsically modulate capsid folding, which is critical to the stability of a virus transmitted through the fecal-oral route. HAV is a paradigmatic example of what has been proposed as a codon usage "code" for protein structure.

Viral internal ribosomal entry sites: four classes for one goal

To ensure efficient propagation, viruses need to rapidly produce viral proteins after cell entrance. Since viral genomes do not encode any components of the protein biosynthesis machinery, viral proteins must be produced by the host cell. To hi-jack the host cellular translation, viruses use a great variety of distinct strategies. Many single-stranded positive-sensed RNA viruses contain so-called internal ribosome entry sites (IRESs). IRESs are structural RNA motifs that have evolved to specific folds that recruit the host ribosomes on the viral coding sequences in order to synthesize viral proteins. In host canonical translation, recruitment of the translation machinery components is essentially guided by the 5' cap (m⁷ G) of mRNA. In contrast, IRESs are able to promote efficient ribosome assembly internally and in cap-independent manner. IRESs have been categorized into four classes, based on their length, nucleotide sequence, secondary and tertiary structures, as well as their mode of action. Classes I and II require the assistance of cellular auxiliary factors, the eukaryotic intiation factors (eIF), for efficient ribosome assembly. Class III IRESs require only a subset of eIFs whereas Class IV, which are the more compact, can promote translation without any eIFs. Extensive functional and structural investigations of IRESs over the past decades have allowed a better understanding of their mode of action for viral translation. Because viral translation has a pivotal role in the infectious program, IRESs are therefore attractive targets for therapeutic purposes. WIREs RNA 2018, 9:e1458. doi: 10.1002/wrna.1458 This article is categorized under: Translation > Ribosome Structure/Function Translation > Translation Mechanisms RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Improving virus production through quasispecies genomic selection and molecular breeding

Virus production still is a challenging issue in antigen manufacture, particularly with slow-growing viruses. Deep-sequencing of genomic regions indicative of efficient replication may be used to identify high-fitness minority individuals suppressed by the ensemble of mutants in a virus quasispecies. Molecular breeding of quasispecies containing colonizer individuals, under regimes allowing more than one replicative cycle, is a strategy to select the fittest competitors among the colonizers. A slow-growing cell culture-adapted hepatitis A virus strain was employed as a model for this strategy. Using genomic selection in two regions predictive of efficient translation, the internal ribosome entry site and the VP1-coding region, high-fitness minority colonizer individuals were identified in a population adapted to conditions of artificially-induced cellular transcription shut-off. Molecular breeding of this population with a second one, also adapted to transcription shut-off and showing an overall colonizer phenotype, allowed the selection of a fast-growing population of great biotechnological potential.

L protease from foot and mouth disease virus confers eIF2-independent translation for mRNAs bearing picornavirus IRES

The leader protease (L(pro)) from foot-and-mouth disease virus (FMDV) has the ability to cleave eIF4G, leading to a blockade of cellular protein synthesis. In contrast to previous reports, our present findings demonstrate that FMDV L(pro) is able to increase translation driven by FMDV IRES. Additionally, inactivation of eIF2 subsequent to phosphorylation induced by arsenite or thapsigargin in BHK cells blocks protein synthesis directed by FMDV IRES, whereas in the presence of L(pro), significant translation is found under these conditions. This phenomenon was also observed in cell-free systems after induction of eIF2 phosphorylation by addition of poly(I:C).

Hepatitis A virus adaptation to cellular shutoff is driven by dynamic adjustments of codon usage and results in the selection of populations with altered capsids

Hepatitis A virus (HAV) has a highly biased and deoptimized codon usage compared to the host cell and fails to inhibit host protein synthesis. It has been proposed that an optimal combination of abundant and rare codons controls the translation speed required for the correct capsid folding. The artificial shutoff host protein synthesis results in the selection of variants containing mutations in the HAV capsid coding region critical for folding, stability, and function. Here, we show that these capsid mutations resulted in changes in their antigenicity; in a reduced stability to high temperature, low pH, and biliary salts; and in an increased efficacy of cell entry. In conclusion, the adaptation to cellular shutoff resulted in the selection of large-plaque-producing virus populations.

IMPORTANCE

HAV has a naturally deoptimized codon usage with respect to that of its cell host and is unable to shut down the cellular translation. This fact contributes to the low replication rate of the virus, in addition to other factors such as the highly inefficient internal ribosome entry site (IRES), and explains the outstanding physical stability of this pathogen in the environment mediated by a folding-dependent highly cohesive capsid. Adaptation to artificially induced cellular transcription shutoff resulted in a redeoptimization of its capsid codon usage, instead of an optimization. These genomic changes are related to an overall change of capsid folding, which in turn induces changes in the cell entry process. Remarkably, the adaptation to cellular shutoff allowed the virus to significantly increase its RNA uncoating efficiency, resulting in the selection of large-plaque-producing populations. However, these populations produced much-debilitated virions.

Hepatitis A virus: host interactions, molecular epidemiology and evolution

Infection with hepatitis A virus (HAV) is the commonest viral cause of liver disease and presents an important public health problem worldwide. Several unique HAV properties and molecular mechanisms of its interaction with host were recently discovered and should aid in clarifying the pathogenesis of hepatitis A. Genetic characterization of HAV strains have resulted in the identification of different genotypes and subtypes, which exhibit a characteristic worldwide distribution. Shifts in HAV endemicity occurring in different parts of the world, introduction of genetically diverse strains from geographically distant regions, genotype displacement observed in some countries and population expansion detected in the last decades of the 20th century using phylogenetic analysis are important factors contributing to the complex dynamics of HAV infections worldwide. Strong selection pressures, some of which, like usage of deoptimized codons, are unique to HAV, limit genetic variability of the virus. Analysis of subgenomic regions has been proven useful for outbreak investigations. However, sharing short sequences among epidemiologically unrelated strains indicates that specific identification of HAV strains for molecular surveillance can be achieved only using whole-genome sequences. Here, we present up-to-date information on the HAV molecular epidemiology and evolution, and highlight the most relevant features of the HAV-host interactions.

Translation directed by hepatitis A virus IRES in the absence of active eIF4F complex and eIF2

Translation directed by several picornavirus IRES elements can usually take place after cleavage of eIF4G by picornavirus proteases 2A^pro or L^pro. The hepatitis A virus (HAV) IRES is thought to be an exception to this rule because it requires intact eIF4F complex for translation. In line with previous results we report that poliovirus (PV) 2A^pro strongly blocks protein synthesis directed by HAV IRES. However, in contrast to previous findings we now demonstrate that eIF4G cleavage by foot-and-mouth disease virus (FMDV) L^pro strongly stimulates HAV IRES-driven translation. Thus, this is the first observation that 2A^pro and L^pro exhibit opposite effects to what was previously thought to be the case in HAV IRES. This effect has been observed both in hamster BHK and human hepatoma Huh7 cells. In addition, this stimulation of translation is also observed in cell free systems after addition of purified L^pro. Notably, in presence of this FMDV protease, translation directed by HAV IRES takes place when eIF2α has been inactivated by phosphorylation. Our present findings clearly demonstrate that protein synthesis directed by HAV IRES can occur when eIF4G has been cleaved and after inactivation of eIF2. Therefore, translation directed by HAV IRES without intact eIF4G and active eIF2 is similar to that observed with other picornavirus IRESs.

Hepatitis A virus evolution and the potential emergence of new variants escaping the presently available vaccines

Hepatitis A is the most common infection of the liver worldwide and is fecal-orally transmitted. Its incidence tends to decrease with improvements in hygiene conditions but at the same time its severity increases. Hepatitis A virus is the causative agent of acute hepatitis in humans and belongs to the Hepatovirus genus in the Picornaviridae family, and it has very unique characteristics. This article reviews some molecular and biological properties that allow the virus to live in a very quiescent way and to build an extremely stable capsid that is able to persist in and out of the body. Additionally, the relationship between the genomic composition and the structural and antigenic properties of the capsid is discussed, and the potential emergence of antigenic variants is evaluated from an evolutionary perspective.

Translation without eIF2 promoted by poliovirus 2A protease

Poliovirus RNA utilizes eIF2 for the initiation of translation in cell free systems. Remarkably, we now describe that poliovirus translation takes place at late times of infection when eIF2 is inactivated by phosphorylation. By contrast, translation directed by poliovirus RNA is blocked when eIF2 is inactivated at earlier times. Thus, poliovirus RNA translation exhibits a dual mechanism for the initiation of protein synthesis as regards to the requirement for eIF2. Analysis of individual poliovirus non-structural proteins indicates that the presence of 2A^pro alone is sufficient to provide eIF2 independence for IRES-driven translation. This effect is not observed with a 2A^pro variant unable to cleave eIF4G. The level of 2A^pro synthesized in culture cells is crucial for obtaining eIF2 independence. Expression of the N-or C-terminus fragments of eIF4G did not stimulate IRES-driven translation, nor provide eIF2 independence, consistent with the idea that the presence of 2A^pro at high concentrations is necessary. The finding that 2A^pro provides eIF2-independent translation opens a new and unsuspected area of research in the field of picornavirus protein synthesis.

A detailed comparative analysis on the overall codon usage patterns in hepatitis A virus

Hepatitis A virus (HAV) is a hepatotropic member of the family Picornaviridae. HAV has several unique biological characteristics that distinguish it from other members of this family. Recent and previous studies revealed that codon usage plays a key role in HAV replication and evolution. In this study, the patterns of synonymous codon usage in HAV have been studied through multivariate statistical methods on 30 complete open reading frames (ORFs) from the available 30 full-length HAV sequences. Effective number of codons (ENC) indicates that the overall extent of codon usage bias in HAV genomes is significant. The relative dinucleotide abundances suggest that codon usage in HAV can also be strongly influenced by underlying biases in dinucleotide frequencies. These factors strongly correlated with the first major axis of correspondence analysis (COA) on relative synonymous codon usage (RSCU). The distribution of the HAV ORFs along the plane defined by the first two major axes in COA showed that different genotypes are located at different places in the plane, suggesting that HAV codon usage is also reflecting an evolutionary process. It has been very recently described that fine-tuning translation kinetics selection also contributes to codon usage bias of HAV. The results of these studies suggest that HAV genomic biases are the result of the co-evolution of genome composition, controlled translation kinetics and probably the ability to escape the antiviral cell responses.

Fine-tuning translation kinetics selection as the driving force of codon usage bias in the hepatitis A virus capsid

Hepatitis A virus (HAV), the prototype of genus Hepatovirus, has several unique biological characteristics that distinguish it from other members of the Picornaviridae family. Among these, the need for an intact eIF4G factor for the initiation of translation results in an inability to shut down host protein synthesis by a mechanism similar to that of other picornaviruses. Consequently, HAV must inefficiently compete for the cellular translational machinery and this may explain its poor growth in cell culture. In this context of virus/cell competition, HAV has strategically adopted a naturally highly deoptimized codon usage with respect to that of its cellular host. With the aim to optimize its codon usage the virus was adapted to propagate in cells with impaired protein synthesis, in order to make tRNA pools more available for the virus. A significant loss of fitness was the immediate response to the adaptation process that was, however, later on recovered and more associated to a re-deoptimization rather than to an optimization of the codon usage specifically in the capsid coding region. These results exclude translation selection and instead suggest fine-tuning translation kinetics selection as the underlying mechanism of the codon usage bias in this specific genome region. Additionally, the results provide clear evidence of the Red Queen dynamics of evolution since the virus has very much evolved to re-adapt its codon usage to the environmental cellular changing conditions in order to recover the original fitness.

Each organism has a specific codon usage signature. Translational selection i.e., selection for the codon adaptation to the tRNA pools, is one of the driving forces of codon bias. In the virus world, this implies an adjustment of the virus codon usage to that of the host cell. Hepatitis A virus appears as an exception to the rule, with a highly deoptimized codon usage, suggesting that translational selection is not the underlying mechanism of its codon bias. However, since the virus lacks a mechanism of cellular protein synthesis inhibition, the deoptimized codon usage may be envisaged as a hawk (cell) and dove (hepatitis A virus) competition strategy for tRNAs and translational selection as well. To confirm this possibility, we artificially induced cell protein synthesis shut-off, thus increasing the tRNA pool availability for the virus, and we took advantage of the quasispecies dynamics to elucidate changes in its codon usage. Virus adaptation to the drug results in a re-deoptimization of codon usage in the capsid region, suggesting a requirement of a slow translation rate, i.e., a translation kinetic selection, instead of a translational selection associated with an optimization of the codon usage. Translation kinetics control is based on the right combination of codons (common and rare) that allows a regulated ribosome traffic rate ensuring the proper protein folding. Capsid folding is critical for a virus transmitted through the fecal-oral route with long extracorporeal periods.

Hepatitis A virus (HAV) packaging size limit

Background

Hepatitis A virus (HAV), an atypical Picornaviridae that causes acute hepatitis in humans, grows poorly in cell culture and in general does not cause cytopathic effect. Foreign sequences have been inserted into different parts of the HAV genome. However, the packaging size limit of HAV has not been determined. The purpose of the present study is to investigate the maximum size of additional sequences that the HAV genome can tolerate without loosing infectivity.

Results

In vitro T7 polymerase transcripts of HAV constructs containing a 456-nt fragment coding for a blasticidin (Bsd) resistance gene, a 1,098-nt fragment coding for the same gene fused to GFP (GFP-Bsd), or a 1,032-nt fragment containing a hygromycin (Hyg) resistance gene cloned into the 2A-2B junction of the HAV genome were transfected into fetal Rhesus monkey kidney (FRhK4) cells. After antibiotic selection, cells transfected with the HAV construct containing the resistance gene for Bsd but not the GFP-Bsd or Hyg survived and formed colonies. To determine whether this size limitation was due to the position of the insertion, a 606 bp fragment coding for the Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) sequence was cloned into the 5' nontranslated (NTR) region of HAV. The resulting HAV-IRES retained the EMCV IRES insertion for 1-2 passages. HAV constructs containing both the EMCV IRES at the 5' NTR and the Bsd-resistance gene at the 2A-2B junction could not be rescued in the presence of Bsd but, in the absence of antibiotic, the rescued viruses contained deletions in both inserted sequences.

Conclusion

HAV constructs containing insertions of approximately 500-600 nt but not 1,000 nt produced viable viruses, which indicated that the HAV particles can successfully package approximately 600 nt of additional sequences and maintain infectivity.

Bridging IRES elements in mRNAs to the eukaryotic translation apparatus

IRES elements are highly structured RNA sequences that function to recruit ribosomes for the initiation of translation. In contrast to the canonical cap-binding, ribosome-scanning model, the mechanism of IRES-mediated translation initiation is not well understood. IRES elements, first discovered in viral RNA genomes, were subsequently found in a subset of cellular RNAs as well. Interestingly, these cellular IRES-containing mRNAs appear to play important roles during conditions of cellular stress, development, and disease (e.g., cancer). It has been shown for viral IRESes that some require specific IRES trans-acting factors (ITAFs), while others require few if any additional proteins and can bind ribosomes directly. Current studies are aimed at elucidating the mechanism of IRES-mediated translation initiation and features that may be common or differ greatly among cellular and viral IRESes. This review will explore IRES elements as important RNA structures that function in both cellular and viral RNA translation and the significance of these structures in providing an alternative mechanism of eukaryotic translation initiation.

Divergent picornavirus IRES elements

Internal ribosome entry site (IRES) elements were first identified about 20 years ago within the 5' untranslated region of picornavirus RNAs. They direct a cap-independent mechanism of translation initiation on the viral RNA. Within the picornavirus family it is now known that there are four classes of IRES element which vary in size (450-270 nt), they also have different, complex, secondary structures and distinct requirements for cellular proteins to allow them to function. This review describes the features of each class of picornavirus IRES element but focuses on the characteristics of the most recently described group, initially identified within the porcine teschovirus-1 RNA, which has strong similarities to the IRES elements from within the genomes of hepatitis C virus and the pestiviruses which are members of the flavivirus family. The selection of the initiation codon by these distinct IRES elements is also discussed.

Poly(A) binding protein, C-terminally truncated by the hepatitis A virus proteinase 3C, inhibits viral translation

Proteolytic cleavage of translation initiation factors is a means to interfere with mRNA circularization and to induce translation arrest during picornaviral replication or apoptosis. It was shown that the regulated cleavages of eukaryotic initiation factor (eIF) 4G and poly(A)-binding protein (PABP) by viral proteinases correlated with early and late arrest of host cap-dependent and viral internal ribosome entry site (IRES)-dependent translation, respectively. Here we show that in contrast to coxsackievirus, eIF4G is not a substrate of proteinase 3C of hepatitis A virus (HAV 3C(pro)). However, PABP is cleaved by HAV 3C(pro) in vitro and in vivo, separating the N-terminal RNA-binding domain (NTD) of PABP from the C-terminal protein-interaction domain. In vitro, NTD has a dominant negative effect on HAV IRES-dependent translation and an enhanced binding affinity to the RNA structural element pY1 in the 5' nontranslated region of the HAV RNA that is essential for viral genome replication. The results point to a regulatory role of PABP cleavage in RNA template switching of viral translation to RNA synthesis.

Hepatitis C virus expression and interferon antiviral action is dependent on PKR expression

Interferon (IFN)-inducible double-stranded RNA-activated protein kinase (PKR) is thought to play a key antiviral role against hepatitis C virus (HCV). However, demonstrating the importance of PKR expression on HCV protein synthesis in the presence or absence of IFN has proven difficult in vivo. In the present experiment, full-length HCV constructs were transiently transfected into two cell lines stably expressing T7 RNA polymerase. HCV expression was monitored under conditions of upregulated or downregulated PKR expression. In addition, IFN was monitored during downregulation of PKR. HCV expression effectively increased PKR expression, as well as that of its regulated proteins. PKR was obviously knocked down by PKR-specific siRNA, which resulted in significantly increased HCV core protein levels. Conversely, over-expression of PKR significantly suppressed HCV core levels in both cell lines. Furthermore, IFN induced high levels of PKR, whereas downregulation of PKR reversed IFN's antiviral effects and increased HCV core levels. Based on these results, it appears that HCV protein expression is directly dependent on PKR expression. PKR is antiviral toward HCV and responsible for IFN's effect against HCV.

Codon usage and replicative strategies of hepatitis A virus

Hepatitis A virus (HAV), the prototype of genus Hepatovirus, has many biological characteristics that distinguish it from other members of the Picornaviridae family. Among these it is worth of note the need for an intact eIF4G factor for the initiation of translation and thus the inability to shut down host protein synthesis by a similar mechanism as in other picornaviruses. Consequently, HAV must inefficiently compete for the cellular translational machinery and this may explain its poor growth in cell culture. In this context of virus/cell competition HAV has strategically adopted a naturally highly deoptimized codon usage. Accordingly, a low protein synthesis may be expected with those proteins involved in RNA replication existing at limiting concentrations. Thus, a very low translation rate and a very low RNA replication rate may play a role in escaping to host cell defenses, allowing the virus to grow in a quiescent way. This could explain the high specific infectivity of HAV in spite of its naturally deoptimized codon usage, which would indicate non-abortive infections due to the antiviral cell response. Additionally, the deoptimized codon usage conveys in the use of abundant and rare codons. Many clusters of such rare codons are present in the capsid surface playing a seminal role in the highly cohesive stability of the HAV virion. Thus, the slow translation rate, resulting from the accumulation of rare codons, is likely to contribute to the highly stable viral capsid necessary for a prolonged survival outside the host body.

Apoptosis induced by a cytopathic hepatitis A virus is dependent on caspase activation following ribosomal RNA degradation but occurs in the absence of 2'-5' oligoadenylate synthetase

We have presented previously evidence that the cytopathogenic 18f strain of hepatitis A virus (HAV) induced degradation of ribosomal RNA (rRNA) in infected cells [Arch. Virol. 148 (2003) 1275–1300]. In contrast, the non-cytopathogenic parent virus HM175 clone 1 had no effect on rRNA integrity. We present here data showing that rRNA degradation is followed by apoptosis accompanied by characteristic DNA laddering in the cytoplasm of 18f infected cells. The DNA laddering coincided with the detection of caspase 3 and PARP-1 cleavage and was dependent upon activation of the caspase pathway, since treatment with Z-VAD-FMK, a pan-caspase inhibitor, inhibited both events. RNase L mRNA was present in both virus-infected and uninfected cells. Messenger RNA for the interferon inducible enzyme 2′–5′ oligoadenylate synthetase (2′–5′ OAS), which polymerizes ATP into 2′–5′ oligo adenylate (2–5A, the activator of RNase L) in the presence of double-stranded RNA, was not detected following virus infection. 2′–5′ OAS mRNA was induced by treatment of the cells with interferon-β (IFN-β). IFN-β mRNA was marginally induced following infection. However, phosphorylated STAT 1, a key regulator of interferon-stimulated gene transcription was not detected in virus infected cells. STAT 1 phosphorylation in response to IFN treatment was lower in virus-infected cells, compared to uninfected cells treated with interferon, suggesting that 18f virus infection interferes with interferon signaling. The results suggest that 18f infection causes the induction of a 2–5A independent RNase L like activity.

Signals in hepatitis A virus P3 region proteins recognized by the ubiquitin-mediated proteolytic system

The hepatitis A virus 3C protease and 3D RNA polymerase are present in low concentrations in infected cells. The 3C protease was previously shown to be rapidly degraded by the ubiquitin/26S proteasome system and we present evidence here that the 3D polymerase is also subject to ubiquitination-mediated proteolysis. Our results show that the sequence (32)LGVKDDWLLV(41) in the 3C protease serves as a protein destruction signal recognized by the ubiquitin-protein ligase E3alpha and that the destruction signal for the RNA polymerase does not require the carboxyl-terminal 137 amino acids. Both the viral 3ABCD polyprotein and the 3CD diprotein were also found to be substrates for ubiquitin-mediated proteolysis. Attempts to determine if the 3C protease or the 3D polymerase destruction signals trigger the ubiquitination and degradation of these precursors yielded evidence suggesting, but not unequivocally proving, that the recognition of the 3D polymerase by the ubiquitin system is responsible.

Subgenomic hepatitis C virus replicons inducing expression of a secreted enzymatic reporter protein

We constructed dicistronic, subgenomic hepatitis C virus (HCV) replicons in which the sequence encoding the human immunodeficiency virus (HIV) tat protein was placed in the upstream cistron, between the HCV 5'NTR and a picornaviral 2A proteinase sequence fused to the selectable marker Neo. Stably transformed Huh7 cells expressing secreted alkaline phosphatase (SEAP) under transcriptional control of the HIV LTR promoter actively secreted SEAP following transfection with these replicon RNAs. Extracellular SEAP activity correlated closely with intracellular HCV RNA levels, as determined by Northern blotting and real-time RT-PCR analysis. These RNAs replicated efficiently despite the absence of core-protein-coding sequence downstream of the HCV IRES. The replication efficiency of replicons derived from the HCV-N strain of HCV was significantly greater than those derived from Con1 in transiently transfected cells. Using this reporter system, we have demonstrated significant differences in the response to interferon alpha-2b in cell lines containing replicons derived from these two strains of HCV.

Unique characteristics of a picornavirus internal ribosome entry site from the porcine teschovirus-1 talfan

The teschoviruses constitute a recently defined picornavirus genus. Most of the genome sequence of the porcine teschovirus-1 (PTV) Talfan and several other strains is known. We now demonstrate that initiation of protein synthesis occurs at nucleotide (nt) 412 on the PTV Talfan RNA and that nt 1 to 405 contains an internal ribosome entry site (IRES) that functions efficiently in vitro and within mammalian cells. In comparison with other picornavirus IRES elements, the PTV IRES is relatively short and lacks a significant polypyrimidine tract near the 3' end. Expression of an enterovirus 2A protease, which induces cleavage of eIF4G within the translation initiation complex eIF4F, has little effect on the PTV IRES activity within BHK cells. The PTV IRES has a unique set of properties and represents a new class of picornavirus IRES element.

Replication of a hepatitis A virus replicon detected by genetic recombination in vivo

Unlike other picornaviruses, hepatitis A virus (HAV) replicates so inefficiently in cell culture that the study of its RNA biosynthesis presents a major experimental challenge. To assess viral RNA replication independent of particle formation, a subgenomic replicon representing a self-replicating RNA was constructed by replacing the P1 domain encoding the capsid proteins with the firefly luciferase sequence. Although translation of the HAV replicon was as efficient as a similar poliovirus replicon, the luciferase activity derived from replication of the HAV construct was more than 100-fold lower than that of poliovirus. The replication capacity of the HAV replicon was clearly demonstrated by its ability to recombine genetically with a non-viable, full-length HAV genome that served as capsid donor and thus to rescue a fully infectious virus. In contrast to a replication-deficient replicon, co-expression of the genetically marked and replication-competent HAV replicon with several lethally mutated HAV genomes resulted in the successful rescue of infectious HAV with a unique genetic marker. Our data suggest: (i) that autonomous HAV RNA replication does not require sequences for the HAV structural proteins; and (ii) that low-level genome replication can unequivocally be demonstrated by the rescue of infectious virus after co-expression with non-viable genomes.

Association between severity of type A hepatitis and nucleotide variations in the 5' non-translated region of hepatitis A virus RNA: strains from fulminant hepatitis have fewer nucleotide substitutions

BACKGROUND

Type A hepatitis is still a considerable problem in both underdeveloped and developed countries. Why some patients progress to fulminant type A hepatitis and others do not is unclear.

AIMS

To determine if nucleotide differences in the genome of hepatitis A virus (HAV) are responsible for the range of clinical severities, we analysed the 5' non-translated region (5'NTR) of the HAV genome, which has an internal ribosomal entry site and is important for cap independent translation of the viral message.

METHODS

Serum samples from 84 Japanese patients with sporadic type A hepatitis from five distant regions of Japan, comprising 12 patients with fulminant hepatitis (FH), 13 with severe acute hepatitis (AHs), and 59 with acute hepatitis (AH), were examined for HAV RNA. The fragment between nucleotides 75 and 638 of the 5'NTR was amplified by reverse transcription-polymerase chain reaction, and the nucleotide sequence was determined by direct sequencing.

RESULTS

Comparison of sequences of the 5'NTR revealed relatively fewer nucleotide substitutions in FH and AHs patients compared with the considerable sequence variations found in strains of AH. This tendency was most prominent between nucleotides 200 and 500. Strains from FH and AHs cases had fewer nucleotide substitutions (p<0.001) in this region.

CONCLUSIONS

Nucleotide variations in the central portion of the 5'NTR of HAV may influence the severity of type A hepatitis.

Replication of subgenomic hepatitis A virus RNAs expressing firefly luciferase is enhanced by mutations associated with adaptation of virus to growth in cultured cells

Replication of hepatitis A virus (HAV) in cultured cells is inefficient and difficult to study due to its protracted and generally noncytopathic cycle. To gain a better understanding of the mechanisms involved, we constructed a subgenomic HAV replicon by replacing most of the P1 capsid-coding sequence from an infectious cDNA copy of the cell culture-adapted HM175/18f virus genome with sequence encoding firefly luciferase. Replication of this RNA in transfected Huh-7 cells (derived from a human hepatocellular carcinoma) led to increased expression of luciferase relative to that in cells transfected with similar RNA transcripts containing a lethal premature termination mutation in 3D(pol) (RNA polymerase). However, replication could not be confirmed in either FrhK4 cells or BSC-1 cells, cells that are typically used for propagation of HAV. Replication was substantially slower than that observed with replicons derived from other picornaviruses, as the basal luciferase activity produced by translation of input RNA did not begin to increase until 24 to 48 h after transfection. Replication of the RNA was reversibly inhibited by guanidine. The inclusion of VP4 sequence downstream of the viral internal ribosomal entry site had no effect on the basal level of luciferase or subsequent increases in luciferase related to its amplification. Thus, in this system this sequence does not contribute to viral translation or replication, as suggested previously. Amplification of the replicon RNA was profoundly enhanced by the inclusion of P2 (but not 5' noncoding sequence or P3) segment mutations associated with adaptation of wild-type virus to growth in cell culture. These results provide a simple reporter system for monitoring the translation and replication of HAV RNA and show that critical mutations that enhance the growth of virus in cultured cells do so by promoting replication of viral RNA in the absence of encapsidation, packaging, and cellular export of the viral genome.

Efficient translation of mRNAs in influenza A virus-infected cells is independent of the viral 5' untranslated region

We test the hypothesis that the translation machinery in cells infected by influenza A virus efficiently translates only mRNAs that possess the influenza viral 5' untranslated region (5'-UTR) by introducing mRNAs directly into the cytoplasm of infected cells. This strategy avoids effects due to the inhibition of the nuclear export of cellular mRNAs mediated by the viral NS1 protein. In one approach, we transfect in vitro synthesized mRNAs into infected cells and demonstrate that these mRNAs are efficiently translated whether or not they possess the influenza viral 5'-UTR. In the second approach, an mRNA is synthesized endogenously in the cytoplasm of influenza A virus infected cells by a constitutively expressed T7 RNA polymerase. Although this mRNA is uncapped and lacks the influenza viral 5'-UTR sequence, it is efficiently translated in infected cells via an internal ribosome entry site. We conclude that the translation machinery in influenza A virus infected cells is capable of efficiently translating all mRNAs and that the switch from cellular to virus-specific protein synthesis that occurs during infection results from other processes.

Cellular COPII proteins are involved in production of the vesicles that form the poliovirus replication complex

Poliovirus (PV) replicates its genome in association with membranous vesicles in the cytoplasm of infected cells. To elucidate the origin and mode of formation of PV vesicles, immunofluorescence labeling with antibodies against the viral vesicle marker proteins 2B and 2BC, as well as cellular markers of the endoplasmic reticulum (ER), anterograde transport vesicles, and the Golgi complex, was performed in BT7-H cells. Optical sections obtained by confocal laser scanning microscopy were subjected to a deconvolution process to enhance resolution and signal-to-noise ratio and to allow for a three-dimensional representation of labeled membrane structures. The mode of formation of the PV vesicles was, on morphological grounds, similar to the formation of anterograde membrane traffic vesicles in uninfected cells. ER-resident membrane markers were excluded from both types of vesicles, and the COPII components Sec13 and Sec31 were both found to be colocalized on the vesicular surface, indicating the presence of a functional COPII coat. PV vesicle formation during early time points of infection did not involve the Golgi complex. The expression of PV protein 2BC or the entire P2 and P3 genomic region led to the production of vesicles carrying a COPII coat and showing the same mode of formation as vesicles produced after PV infection. These results indicate that PV vesicles are formed at the ER by the cellular COPII budding mechanism and thus are homologous to the vesicles of the anterograde membrane transport pathway.

Detailed analysis of the requirements of hepatitis A virus internal ribosome entry segment for the eukaryotic initiation factor complex eIF4F

The hepatitis A virus (HAV) internal ribosome entry segment (IRES) is unique among the picornavirus IRESs in that it is inactive in the presence of either the entero- and rhinovirus 2A or aphthovirus Lb proteinases. Since these proteinases both cleave eukaryotic initiation factor 4G (eIF4G) and HAV IRES activity could be rescued in vitro by addition of eIF4F to proteinase-treated extracts, it was concluded that the HAV IRES requires eIF4F containing intact eIF4G. Here, we show that the inability of the HAV IRES to function with cleaved eIF4G cannot be attributed to inefficient binding of the cleaved form of eIF4G by the HAV IRES. Indeed, the binding of both intact eIF4F and the C-terminal cleavage product of eIF4G to the HAV IRES was virtually indistinguishable from their binding to the encephalomyocarditis virus IRES, as assessed by UV cross-linking and filter retention assays. Rather, we show that HAV IRES activity requires, either directly or indirectly, components of the eIF4F complex which interact with the N-terminal fragment of eIF4G. Effectively, HAV IRES activity, but not that of the human rhinovirus IRES, was sensitive to the rotavirus nonstructural protein NSP3 [which displaces poly(A)-binding protein from the eIF4F complex], to recombinant eIF4E-binding protein (which prevents the association of the cap binding protein eIF4E with eIF4G), and to cap analogue.

Activity of the hepatitis A virus IRES requires association between the cap-binding translation initiation factor (eIF4E) and eIF4G

The question of whether translation initiation factor eIF4E and the complete eIF4G polypeptide are required for initiation dependent on the IRES (internal ribosome entry site) of hepatitis A virus (HAV) has been examined using in vitro translation in standard and eIF4G-depleted rabbit reticulocyte lysates. In agreement with previous publications, the HAV IRES is unique among all picornavirus IRESs in that it was inhibited if translation initiation factor eIF4G was cleaved by foot-and-mouth disease L-proteases. In addition, the HAV IRES was inhibited by addition of eIF4E-binding protein 1, which binds tightly to eIF4E and sequesters it, thus preventing its association with eIF4G. The HAV IRES was also inhibited by addition of m(7)GpppG cap analogue, irrespective of whether the RNA tested was capped or not. Thus, initiation on the HAV IRES requires that eIF4E be associated with eIF4G and that the cap-binding pocket of eIF4E be empty and unoccupied. This suggests two alternative models: (i) initiation requires a direct interaction between an internal site in the IRES and eIF4E/4G, an interaction which involves the cap-binding pocket of eIF4E in addition to any direct eIF4G-RNA interactions; or (ii) it requires eIF4G in a particular conformation which can be attained only if eIF4E is bound to it, with the cap-binding pocket of the eIF4E unoccupied.

Analysis of full-length hepatitis A virus genome in sera from patients with fulminant and self-limited acute type A hepatitis

BACKGROUND/AIMS

Type A hepatitis still poses a considerable problem worldwide. Why some patients progress to fulminant type A hepatitis and others do not is still unknown. To examine whether genomic differences of hepatitis A virus (HAV) are responsible for the severity of the disease, we analyzed the whole HAV genomes from patients with fulminant and self-limited acute type A hepatitis.

METHODS

Sera from three patients with sporadic type A fulminant hepatitis (FH) and three patients with acute hepatitis (AH) were examined for HAV RNA. Full-length nucleotide sequences were determined using long reverse transcription polymerase chain reaction, 5' and 3' rapid amplification of cDNA ends methods, and direct sequencing. The amino acid sequences were deduced from the nucleotide sequences.

RESULTS

HAV RNA was detected in all six patients examined. From the sequence of viral protein 1/2A, all cases were revealed to be genotype IA. By comparing with genotype IA, wild-type HAV strain GBM, the analysis of whole genomes from the six cases showed no specific substitutions between FH and AH. Completely identical nucleotide sequences were observed at 3' non-translated region (NTR) in all six cases. In 5'NTR, less nucleotide substitutions were found in FH than in AH, and in the non-structural protein 2B region, a little more amino acid substitutions seemed to be found in FH than in AH.

CONCLUSIONS

This study showed that full-length HAV could be analyzed from serum samples. Although there were no unique nucleotide or amino acid substitutions, possible associations were suggested between the severity of type A hepatitis and the nucleotide substitutions in 5'NTR and the amino acid substitutions in 2B.

The influence of downstream protein-coding sequence on internal ribosome entry on hepatitis C virus and other flavivirus RNAs

Some studies suggest that the hepatitis C virus (HCV) internal ribosome entry site (IRES) requires downstream 5' viral polyprotein-coding sequence for efficient initiation of translation, but the role of this RNA sequence in internal ribosome entry remains unresolved. We confirmed that the inclusion of viral sequence downstream of the AUG initiator codon increased IRES-dependent translation of a reporter RNA encoding secretory alkaline phosphatase, but found that efficient translation of chloramphenicol acetyl transferase (CAT) required no viral sequence downstream of the initiator codon. However, deletion of an adenosine-rich domain near the 5' end of the CAT sequence, or the insertion of a small stable hairpin structure (deltaG = -18 kcal/mol) between the HCV IRES and CAT sequences (hpCAT) substantially reduced IRES-mediated translation. Although translation could be restored to both mutants by the inclusion of 14 nt of the polyprotein-coding sequence downstream of the AUG codon, a mutational analysis of the inserted protein-coding sequence demonstrated no requirement for either a specific nucleotide or amino acid-coding sequence to restore efficient IRES-mediated translation to hpCAT. Similar results were obtained with the structurally and phylogenetically related IRES elements of classical swine fever virus and GB virus B. We conclude that there is no absolute requirement for viral protein-coding sequence with this class of IRES elements, but that there is a requirement for an absence of stable RNA structure immediately downstream of the AUG initiator codon. Stable RNA structure immediately downstream of the initiator codon inhibits internal initiation of translation but, in the case of hpCAT, did not reduce the capacity of the RNA to bind to purified 40S ribosome subunits. Thus, stable RNA structure within the 5' proximal protein-coding sequence does not alter the capacity of the IRES to form initial contacts with the 40S subunit, but appears instead to prevent the formation of subsequent interactions between the 40S subunit and viral RNA in the vicinity of the initiator codon that are essential for efficient internal ribosome entry.

Characterization of recombinant hepatitis A virus genomes containing exogenous sequences at the 2A/2B junction

Hepatitis A virus (HAV) differs from other members of the family Picornaviridae in that the cleavage of the polyprotein at the 2A/2B junction, commonly considered to be the primary polyprotein cleavage by analogy with other picornaviruses, is mediated by 3C(pro), the only proteinase encoded by the virus. However, it has never been formally demonstrated that the 2A/2B junction is the site of primary cleavage, and the actual function of the 2A sequence, which lacks homology with sequence of other picornaviruses, remains unknown. To determine whether 2A functions in cis as a precursor with the nonstructural proteins, we constructed dicistronic HAV genomes in which a heterologous picornaviral internal ribosome entry site was inserted at the 2A/2B junction. Transfection of permissive FRhK-4 cells with these dicistronic RNAs failed to result in the rescue of infectious virus, indicating a possible cis replication function spanning the 2A/2B junction. However, infectious virus was recovered from recombinant HAV genomes containing exogenous protein-coding sequences inserted in-frame at the 2A/2B junction and flanked by consensus 3C(pro) cleavage sites. The replication of these recombinants was less efficient than that of the parent virus but was variable and not dependent upon the length of the inserted sequence. An HAV recombinant containing a 420-nt insertion encoding the bleomycin resistance protein Zeo was stable for up to five passages in cell culture. Inserted sequences were deleted from replicating viruses, but this did not result from homologous recombination at the flanking 3C(pro) cleavage sites, since the 5' and 3' segments of the inserted sequence were retained in the deletion mutants. These results indicate that the HAV polyprotein can tolerate an insertion at the 2A/2B junction and that the 2A polypeptide does not function in cis as a 2AB precursor. Recombinant HAV genomes containing foreign protein-coding sequences inserted at the 2A/2B junction are novel and potentially useful protein expression vectors.

PCR-SSCP analysis of 5'-nontranslated region of hepatitis A viral RNA: comparison with clinicopathological features of hepatitis A

The recent development of the sensitive reverse transcription-polymerase chain reaction (RT-PCR) method allowed us to detect the presence of hepatitis A virus (HAV) RNA in sera from hepatitis A patients. To determine whether differences in HAV are related to the wide range of clinical severity, we used PCR-single strand conformation polymorphism (SSCP) to analyze the amplified product of the 5'-nontranslated region (5'NTR), where relatively homologous sequences are reported and the internal ribosomal entry site is considered to exist, from these various levels of hepatitis A. Twenty-seven patients admitted to Chiba University Hospital (between 1988 and 1997) were examined for HAV RNA in their sera by RT-PCR with primers located at 5'NTR of HAV RNA. The nucleotide (nt) sequence of a central part (nt 277-551) of 5'NTR was amplified and the product was examined by PCR-SSCP. HAV RNA was detected in all 27 hepatitis A patients examined: in 3 with fulminant hepatitis, 2 with severe acute hepatitis, and 22 with self-limited acute hepatitits. The amplified sequence revealed by PCR-SSCP showed that samples from fulminant and severe acute hepatitis patients had similar mobility in the gel, whereas that from acute hepatitis patients demonstrated a considerable variety in mobility patterns. From the analysis of the amplified product of 5'NTR of HAV, the bands are likely to be similar in the more severe forms of hepatitis A compared to those from uncomplicated self-limited acute hepatitis.