Secondary structure determination of the conserved 98-base sequence at the 3' terminus of hepatitis C virus genome RNA

A diagnostic target in the 3' untranslated region of hepatitis C virus genome

INTRODUCTION

Hepatitis C virus (HCV) causes hepatitis worldwide. Detection of HCV infection is important for diagnosis and treatment. The most frequently used method for diagnosis is the detection of the 5' untranslated (5' UTR) conserved target using polymerase chain reaction (PCR). Unfortunately, the detection of the 5'UTR is not completely reliable. The 5'UTR may not be the best target for nucleic acid detection because of its complex secondary structures and the variations of different HCV strains. An alternative target is the 3'UTR. The purpose is to evaluate the diagnostic potential of the 3'UTR of HCV.

METHODS AND RESULTS

RNA was extracted from sixty-three serum samples. The 3'UTR and 5'UTR were amplified. Within the fifty-nine anti-HCV-positive samples, targeting the HCV 3'UTR and 5'UTR resulted in amplification in 40 (67.8%) and 54 (91.5%) of the samples, respectively. Five samples that were not amplified by 5'UTR detection were found positive by 3'UTR detection.

CONCLUSION

These results indicate that PCR detection of the HCV 3'UTR appears less effective than detection of the 5'UTR. Nevertheless, the detection of several regions of the HCV genome by multiplex PCR may improve the accuracy of HCV detection.

Conformational dynamics of the hepatitis C virus 3'X RNA

The 3' end of the hepatitis C virus genome is terminated by a highly conserved, 98 nt sequence called 3'X. This untranslated structural element is thought to regulate several essential RNA-dependent processes associated with infection. 3'X has two proposed conformations comprised of either three or two stem-loop structures that result from the different base-pairing interactions within the first 55 nt. Here, we used single-molecule Förster resonance energy transfer spectroscopy to monitor the conformational status of fluorescently labeled constructs that isolate this region of the RNA (3'X55). We observed that 3'X55 can adopt both proposed conformations and the relative abundance of them can be modulated by either solution conditions or nucleotide deletions. Furthermore, interconversion between the two conformations takes place over the course of several hours. The simultaneous existence of two slowly interconverting conformations may help prime individual copies of the viral genome for either viral protein or RNA synthesis, thereby minimizing conflicts between these two competing processes.

Comprehensive survey of conserved RNA secondary structures in full-genome alignment of Hepatitis C virus

Hepatitis C virus (HCV) is a plus-stranded RNA virus that often chronically infects liver hepatocytes and causes liver cirrhosis and cancer. These viruses replicate their genomes employing error-prone replicases. Thereby, they routinely generate a large 'cloud' of RNA genomes (quasispecies) which-by trial and error-comprehensively explore the sequence space available for functional RNA genomes that maintain the ability for efficient replication and immune escape. In this context, it is important to identify which RNA secondary structures in the sequence space of the HCV genome are conserved, likely due to functional requirements. Here, we provide the first genome-wide multiple sequence alignment (MSA) with the prediction of RNA secondary structures throughout all representative full-length HCV genomes. We selected 57 representative genomes by clustering all complete HCV genomes from the BV-BRC database based on k-mer distributions and dimension reduction and adding RefSeq sequences. We include annotations of previously recognized features for easy comparison to other studies. Our results indicate that mainly the core coding region, the C-terminal NS5A region, and the NS5B region contain secondary structure elements that are conserved beyond coding sequence requirements, indicating functionality on the RNA level. In contrast, the genome regions in between contain less highly conserved structures. The results provide a complete description of all conserved RNA secondary structures and make clear that functionally important RNA secondary structures are present in certain HCV genome regions but are largely absent from other regions. Full-genome alignments of all branches of Hepacivirus C are provided in the supplement.

OUP accepted manuscript

Subdomain 5BSL3.2 of hepatitis C virus RNA lies at the core of a network of distal RNA–RNA contacts that connect the 5′ and 3′ regions of the viral genome and regulate the translation and replication stages of the viral cycle. Using small-angle X-ray scattering and NMR spectroscopy experiments, we have determined at low resolution the structural models of this subdomain and its distal complex with domain 3′X, located at the 3′-terminus of the viral RNA chain. 5BSL3.2 adopts a characteristic ‘L’ shape in solution, whereas the 5BSL3.2–3′X distal complex forms a highly unusual ‘Y’-shaped kissing junction that blocks the dimer linkage sequence of domain 3′X and promotes translation. The structure of this complex may impede an effective association of the viral polymerase with 5BSL3.2 and 3′X to start negative-strand RNA synthesis, contributing to explain the likely mechanism used by these sequences to regulate viral replication and translation. In addition, sequence and shape features of 5BSL3.2 are present in functional RNA motifs of flaviviruses, suggesting conserved regulatory processes within the Flaviviridae family.

Hepatitis C Viral Replication Complex

The life cycle of the hepatitis C virus (HCV) can be divided into several stages, including viral entry, protein translation, RNA replication, viral assembly, and release. HCV genomic RNA replication occurs in the replication organelles (RO) and is tightly linked to ER membrane alterations containing replication complexes (proteins NS3 to NS5B). The amplification of HCV genomic RNA could be regulated by the RO biogenesis, the viral RNA structure (i.e., cis-acting replication elements), and both viral and cellular proteins. Studies on HCV replication have led to the development of direct-acting antivirals (DAAs) targeting the replication complex. This review article summarizes the viral and cellular factors involved in regulating HCV genomic RNA replication and the DAAs that inhibit HCV replication.

Hepatitis C Virus Replication

Replication and amplification of the viral genome is a key process for all viruses. For hepatitis C virus (HCV), a positive-strand RNA virus, amplification of the viral genome requires the synthesis of a negative-sense RNA template, which is in turn used for the production of new genomic RNA. This process is governed by numerous proteins, both host and viral, as well as distinct lipids and specific RNA elements within the positive- and negative-strand RNAs. Moreover, this process requires specific changes to host cell ultrastructure to create microenvironments conducive to viral replication. This review will focus on describing the processes and factors involved in facilitating or regulating HCV genome replication.

The Role of the RNA-RNA Interactome in the Hepatitis C Virus Life Cycle

RNA virus genomes are multifunctional entities endowed with conserved structural elements that control translation, replication and encapsidation, among other processes. The preservation of these structural RNA elements constraints the genomic sequence variability. The hepatitis C virus (HCV) genome is a positive, single-stranded RNA molecule with numerous conserved structural elements that manage different steps during the infection cycle. Their function is ensured by the association of protein factors, but also by the establishment of complex, active, long-range RNA-RNA interaction networks-the so-called HCV RNA interactome. This review describes the RNA genome functions mediated via RNA-RNA contacts, and revisits some canonical ideas regarding the role of functional high-order structures during the HCV infective cycle. By outlining the roles of long-range RNA-RNA interactions from translation to virion budding, and the functional domains involved, this work provides an overview of the HCV genome as a dynamic device that manages the course of viral infection.

Structure and function analysis of the essential 3'X domain of hepatitis C virus

The 3'X domain of hepatitis C virus has been reported to control viral replication and translation by modulating the exposure of a nucleotide segment involved in a distal base-pairing interaction with an upstream 5BSL3.2 domain. To study the mechanism of this molecular switch, we have analyzed the structure of 3'X mutants that favor one of the two previously proposed conformations comprising either two or three stem-loops. Only the two-stem conformation was found to be stable and to allow the establishment of the distal contact with 5BSL3.2, and also the formation of 3'X domain homodimers by means of a universally conserved palindromic sequence. Nucleotide changes disturbing the two-stem conformation resulted in poorer replication and translation levels, explaining the high degree of conservation detected for this sequence. The switch function attributed to the 3'X domain does not occur as a result of a transition between two- and three-stem conformations, but likely through the sequestration of the 5BSL3.2-binding sequence by formation of 3'X homodimers.

Syntheses and Binding Testing of N1-Alkylamino-Substituted 2-Aminobenzimidazole Analogues Targeting the Hepatitis C Virus Internal Ribosome Entry Site

A series of 2-aminobenzimidazole analogues have been synthesised and tested for binding to a previously established RNA target for viral translation inhibitors in the internal ribosome entry site (IRES) of the hepatitis C virus (HCV). Synthesis of new inhibitor compounds followed a highly convergent strategy which allowed for incorporation of diverse tertiary amino substituents in high overall yields (eight-steps, 4–22%). Structure–activity relationship (SAR) studies focussed on the tertiary amine substituent involved in hydrogen bonding with the RNA backbone at the inhibitor binding site. The SAR study was further correlated with in silico docking experiments. Analogous compounds showed promising activities (half maximal effective concentration, EC50: 21–89µM). Structures of the synthesised analogues and a correlation to their mode of binding, provided the opportunity to explore parameters required for selective targeting of the HCV IRES at the subdomain IIa which acts as an RNA conformational switch in HCV translation.

An unexpected RNA distal interaction mode found in an essential region of the hepatitis C virus genome

The 3’X tail is a functionally essential 98-nt sequence located at the 3′-end of the hepatitis C virus (HCV) RNA genome. The domain contains two absolutely conserved dimer linkage sequence (DLS) and k nucleotide segments involved in viral RNA dimerization and in a distal base-pairing interaction with stem-loop 5BSL3.2, respectively. We have previously shown that domain 3’X forms an elongated structure comprising two coaxially stacked SL1’ and SL2’ stem-loops. This conformation favors RNA dimerization by exposing a palindromic DLS segment in an apical loop, but buries in the upper stem of hairpin SL2’ the k nucleotides involved in the distal contact with 5BSL3.2. Using nuclear magnetic resonance spectroscopy and gel electrophoresis experiments, here we show that the establishment of the complex between domain 3’X and stem-loop 5BSL3.2 only requires a rearrangement of the nucleotides forming the upper region of subdomain SL2’. The results indicate that the interaction does not occur through a canonical kissing loop mechanism involving the unpaired nucleotides of two terminal loops, but rather involves a base-paired stem and an apical loop and may result in a kissing three-way junction. On the basis of this information we suggest how the 3’X tail switches between monomer, homodimer and heterodimer states to regulate the HCV viral cycle.

Potential of the Other Genetic Information Coded by the Viral RNA Genomes as Antiviral Target

In addition to the protein coding information, viral RNA genomes code functional information in structurally conserved units termed functional RNA domains. These RNA domains play essential roles in the viral cycle (e.g., replication and translation). Understanding the molecular mechanisms behind their function is essential to understanding the viral infective cycle. Further, interfering with the function of the genomic RNA domains offers a potential means of developing antiviral strategies. Aptamers are good candidates for targeting structural RNA domains. Besides its potential as therapeutics, aptamers also provide an excellent tool for investigating the functionality of RNA domains in viral genomes. This review briefly summarizes the work carried out in our laboratory aimed at the structural and functional characterization of the hepatitis C virus (HCV) genomic RNA domains. It also describes the efforts we carried out for the development of antiviral aptamers targeting specific genomic domains of the HCV and the human immunodeficiency virus type-1 (HIV-1).

SilentMutations (SIM): A tool for analyzing long-range RNA-RNA interactions in viral genomes and structured RNAs

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We developed a tool to analyze the effect of multiple point mutations on the secondary structures of two interacting viral RNAs.

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Our tool simulates destructive and compensatory mutants of two key regions from a single-stranded RNA.

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The simulated mutants can be utilized for the combinatorial in vitro analysis of RNA–RNA interactions.

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We predicted potential mutants for in vitro validation experiments of influenza A virus and hepatitis C virus interactions.

A single nucleotide change in the coding region can alter the amino acid sequence of a protein. In consequence, natural or artificial sequence changes in viral RNAs may have various effects not only on protein stability, function and structure but also on viral replication.

In recent decades, several tools have been developed to predict the effect of mutations in structured RNAs such as viral genomes or non-coding RNAs. Some tools use multiple point mutations and also take coding regions into account. However, none of these tools was designed to specifically simulate the effect of mutations on viral long-range interactions.

Here, we developed SilentMutations (SIM), an easy-to-use tool to analyze the effect of multiple point mutations on the secondary structures of two interacting viral RNAs. The tool can simulate disruptive and compensatory mutants of two interacting single-stranded RNAs. This allows a fast and accurate assessment of key regions potentially involved in functional long-range RNA–RNA interactions and will eventually help virologists and RNA-experts to design appropriate experiments.

SIM only requires two interacting single-stranded RNA regions as input. The output is a plain text file containing the most promising mutants and a graphical representation of all interactions.

We applied our tool on two experimentally validated influenza A virus and hepatitis C virus interactions and we were able to predict potential double mutants for in vitro validation experiments.

The source code and documentation of SIM are freely available at github.com/desiro/silentMutations.

Signals Involved in Regulation of Hepatitis C Virus RNA Genome Translation and Replication

Hepatitis C virus (HCV) preferentially replicates in the human liver and frequently causes chronic infection, often leading to cirrhosis and liver cancer. HCV is an enveloped virus classified in the genus Hepacivirus in the family Flaviviridae and has a single-stranded RNA genome of positive orientation. The HCV RNA genome is translated and replicated in the cytoplasm. Translation is controlled by the Internal Ribosome Entry Site (IRES) in the 5' untranslated region (5' UTR), while also downstream elements like the cis-replication element (CRE) in the coding region and the 3' UTR are involved in translation regulation. The cis-elements controlling replication of the viral RNA genome are located mainly in the 5'- and 3'-UTRs at the genome ends but also in the protein coding region, and in part these signals overlap with the signals controlling RNA translation. Many long-range RNA-RNA interactions (LRIs) are predicted between different regions of the HCV RNA genome, and several such LRIs are actually involved in HCV translation and replication regulation. A number of RNA cis-elements recruit cellular RNA-binding proteins that are involved in the regulation of HCV translation and replication. In addition, the liver-specific microRNA-122 (miR-122) binds to two target sites at the 5' end of the viral RNA genome as well as to at least three additional target sites in the coding region and the 3' UTR. It is involved in the regulation of HCV RNA stability, translation and replication, thereby largely contributing to the hepatotropism of HCV. However, we are still far from completely understanding all interactions that regulate HCV RNA genome translation, stability, replication and encapsidation. In particular, many conclusions on the function of cis-elements in HCV replication have been obtained using full-length HCV genomes or near-full-length replicon systems. These include both genome ends, making it difficult to decide if a cis-element in question acts on HCV replication when physically present in the plus strand genome or in the minus strand antigenome. Therefore, it may be required to use reduced systems that selectively focus on the analysis of HCV minus strand initiation and/or plus strand initiation.

NF90-NF45 is a selective RNA chaperone that rearranges viral and cellular riboswitches: biochemical analysis of a virus host factor activity

The heterodimer NF90-NF45 is an RNA-binding protein complex that modulates the expression of various cellular mRNAs on the post-transcriptional level. Furthermore, it acts as a host factor that supports the replication of several RNA viruses. The molecular mechanisms underlying these activities have yet to be elucidated. Recently, we showed that the RNA-binding capabilities and binding specificity of NF90 considerably improves when it forms a complex with NF45. Here, we demonstrate that NF90 has a substrate-selective RNA chaperone activity (RCA) involving RNA annealing and strand displacement activities. The mechanism of the NF90-catalyzed RNA annealing was elucidated to comprise a combination of 'matchmaking' and compensation of repulsive charges, which finally results in the population of dsRNA products. Heterodimer formation with NF45 enhances 'matchmaking' of complementary ssRNAs and substantially increases the efficiency of NF90's RCA. During investigations of the relevance of the NF90-NF45 RCA, the complex was shown to stimulate the first step in the RNA replication process of hepatitis C virus (HCV) in vitro and to stabilize a regulatory element within the mRNA of vascular endothelial growth factor (VEGF) by protein-guided changes of the RNAs' structures. Thus, our study reveals how the intrinsic properties of an RNA-binding protein determine its biological activities.

Theory Meets Experiment: Metal Ion Effects in HCV Genomic RNA Kissing Complex Formation

The long-range base pairing between the 5BSL3. 2 and 3′X domains in hepatitis C virus (HCV) genomic RNA is essential for viral replication. Experimental evidence points to the critical role of metal ions, especially Mg²⁺ ions, in the formation of the 5BSL3.2:3′X kissing complex. Furthermore, NMR studies suggested an important ion-dependent conformational switch in the kissing process. However, for a long time, mechanistic understanding of the ion effects for the process has been unclear. Recently, computational modeling based on the Vfold RNA folding model and the partial charge-based tightly bound ion (PCTBI) model, in combination with the NMR data, revealed novel physical insights into the role of metal ions in the 5BSL3.2-3′X system. The use of the PCTBI model, which accounts for the ion correlation and fluctuation, gives reliable predictions for the ion-dependent electrostatic free energy landscape and ion-induced population shift of the 5BSL3.2:3′X kissing complex. Furthermore, the predicted ion binding sites offer insights about how ion-RNA interactions shift the conformational equilibrium. The integrated theory-experiment study shows that Mg²⁺ ions may be essential for HCV viral replication. Moreover, the observed Mg²⁺-dependent conformational equilibrium may be an adaptive property of the HCV genomic RNA such that the equilibrium is optimized to the intracellular Mg²⁺ concentration in liver cells for efficient viral replication.

The Chimpanzee Model of Viral Hepatitis: Advances in Understanding the Immune Response and Treatment of Viral Hepatitis

Chimpanzees (Pan troglodytes) have contributed to diverse fields of biomedical research due to their close genetic relationship to humans and in many instances due to the lack of any other animal model. This review focuses on the contributions of the chimpanzee model to research on hepatitis viruses where chimpanzees represented the only animal model (hepatitis B and C) or the most appropriate animal model (hepatitis A). Research with chimpanzees led to the development of vaccines for HAV and HBV that are used worldwide to protect hundreds of millions from these diseases and, where fully implemented, have provided immunity for entire generations. More recently, chimpanzee research was instrumental in the development of curative therapies for hepatitis C virus infections. Over a span of 40 years, this research would identify the causative agent of NonA,NonB hepatitis, validate the molecular tools for drug discovery, and provide safety and efficacy data on the therapies that now provide a rapid and complete cure of HCV chronic infections. Several cocktails of antivirals are FDA approved that eliminate the virus following 12 weeks of once-per-day oral therapy. This represents the first cure of a chronic viral disease and, once broadly implemented, will dramatically reduce the occurrence of cirrhosis and liver cancer. The recent contributions of chimpanzees to our current understanding of T cell immunity for HCV, development of novel therapeutics for HBV, and the biology of HAV are reviewed. Finally, a perspective is provided on the events leading to the cessation of the use of chimpanzees in research and the future of the chimpanzees previously used to bring about these amazing breakthroughs in human healthcare.

The 5BSL3.2 Functional RNA Domain Connects Distant Regions in the Hepatitis C Virus Genome

Viral genomes are complexly folded entities that carry all the information required for the infective cycle. The nucleotide sequence of the RNA virus genome encodes proteins and functional information contained in discrete, highly conserved structural units. These so-called functional RNA domains play essential roles in the progression of infection, which requires their preservation from one generation to the next. Numerous functional RNA domains exist in the genome of the hepatitis C virus (HCV). Among them, the 5BSL3.2 domain in the cis-acting replication element (CRE) at the 3' end of the viral open reading frame has become of particular interest given its role in HCV RNA replication and as a regulator of viral protein synthesis. These functionalities are achieved via the establishment of a complex network of long-distance RNA-RNA contacts involving (at least as known to date) the highly conserved 3'X tail, the apical loop of domain IIId in the internal ribosome entry site, and/or the so-called Alt region upstream of the CRE. Changing contacts promotes the execution of different stages of the viral cycle. The 5BSL3.2 domain thus operates at the core of a system that governs the progression of HCV infection. This review summarizes our knowledge of the long-range RNA-RNA interaction network in the HCV genome, with special attention paid to the structural and functional consequences derived from the establishment of different contacts. The potential implications of such interactions in switching between the different stages of the viral cycle are discussed.

Nuclear Magnetic Resonance Study of RNA Structures at the 3'-End of the Hepatitis C Virus Genome

The 3'-end of the genomic RNA of the hepatitis C virus (HCV) embeds conserved elements that regulate viral RNA synthesis and protein translation by mechanisms that have yet to be elucidated. Previous studies with oligo-RNA fragments have led to multiple, mutually exclusive secondary structure predictions, indicating that HCV RNA structure may be context-dependent. Here we employed a nuclear magnetic resonance (NMR) approach that involves long-range adenosine interaction detection, coupled with site-specific ²H labeling, to probe the structure of the intact 3'-end of the HCV genome (385 nucleotides). Our data reveal that the 3'-end exists as an equilibrium mixture of two conformations: an open conformation in which the 98 nucleotides of the 3'-tail (3'X) form a two-stem-loop structure with the kissing-loop residues sequestered and a closed conformation in which the 3'X rearranges its structure and forms a long-range kissing-loop interaction with an upstream cis-acting element 5BSL3.2. The long-range kissing species is favored under high-Mg²⁺ conditions, and the intervening sequences do not affect the equilibrium as their secondary structures remain unchanged. The open and closed conformations are consistent with the reported function regulation of viral RNA synthesis and protein translation, respectively. Our NMR detection of these RNA conformations and the structural equilibrium in the 3'-end of the HCV genome support its roles in coordinating various steps of HCV replication.

Three-dimensional structure of the 3'X-tail of hepatitis C virus RNA in monomeric and dimeric states

The 3'X domain is a 98-nt region located at the 3' end of hepatitis C virus genomic RNA that plays essential functions in the viral life cycle. It contains an absolutely conserved, 16-base palindromic sequence that promotes viral RNA dimerization, overlapped with a 7-nt tract implicated in a distal contact with a nearby functional sequence. Using small angle X-ray scattering measurements combined with model building guided by NMR spectroscopy, we have studied the stoichiometry, structure, and flexibility of domain 3'X and two smaller subdomain sequences as a function of ionic strength, and obtained a three-dimensional view of the full-length domain in its monomeric and dimeric states. In the monomeric form, the 3'X domain adopted an elongated conformation containing two SL1' and SL2' double-helical stems stabilized by coaxial stacking. This structure was significantly less flexible than that of isolated subdomain SL2' monomers. At higher ionic strength, the 3'X scattering envelope nearly doubled its size, reflecting the formation of extended homodimers containing an antiparallel SL2' duplex flanked by coaxially stacked SL1' helices. Formation of these dimers could initialize and/or regulate the packaging of viral RNA genomes into virions.

The chaperone-like activity of the hepatitis C virus IRES and CRE elements regulates genome dimerization

The RNA genome of the hepatitis C virus (HCV) establishes a network of long-distance RNA-RNA interactions that direct the progression of the infective cycle. This work shows that the dimerization of the viral genome, which is initiated at the dimer linkage sequence (DLS) within the 3'UTR, is promoted by the CRE region, while the IRES is a negative regulatory partner. Using differential 2'-acylation probing (SHAPE-dif) and molecular interference (HMX) technologies, the CRE activity was found to mainly lie in the critical 5BSL3.2 domain, while the IRES-mediated effect is dependent upon conserved residues within the essential structural elements JIIIabc, JIIIef and PK2. These findings support the idea that, along with the DLS motif, the IRES and CRE are needed to control HCV genome dimerization. They also provide evidences of a novel function for these elements as chaperone-like partners that fine-tune the architecture of distant RNA domains within the HCV genome.

The HCV Replicase Complex and Viral RNA Synthesis

Replication of hepatitis C virus (HCV) is tightly linked to membrane alterations designated the membranous web, harboring the viral replicase complex. In this chapter we describe the morphology and 3D architecture of the HCV-induced replication organelles, mainly consisting of double membrane vesicles, which are generated by a concerted action of the nonstructural proteins NS3 to NS5B. Recent studies have furthermore identified a number of host cell proteins and lipids contributing to the biogenesis of the membranous web, which are discussed in this chapter. Viral RNA synthesis is tightly associated with these membrane alterations and mainly driven by the viral RNA dependent RNA polymerase NS5B. We summarize our current knowledge of the structure and function of NS5B, the role of cis-acting replication elements at the termini of the genome in regulating RNA synthesis and the contribution of additional viral and host factors to viral RNA synthesis, which is still ill defined.

The cis-acting replication element of the Hepatitis C virus genome recruits host factors that influence viral replication and translation

The cis-acting replication element (CRE) of the hepatitis C virus (HCV) RNA genome is a region of conserved sequence and structure at the 3' end of the open reading frame. It participates in a complex and dynamic RNA-RNA interaction network involving, among others, essential functional domains of the 3' untranslated region and the internal ribosome entry site located at the 5' terminus of the viral genome. A proper balance between all these contacts is critical for the control of viral replication and translation, and is likely dependent on host factors. Proteomic analyses identified a collection of proteins from a hepatoma cell line as CRE-interacting candidates. A large fraction of these were RNA-binding proteins sharing highly conserved RNA recognition motifs. The vast majority of these proteins were validated by bioinformatics tools that consider RNA-protein secondary structure. Further characterization of representative proteins indicated that hnRNPA1 and HMGB1 exerted negative effects on viral replication in a subgenomic HCV replication system. Furthermore DDX5 and PARP1 knockdown reduced the HCV IRES activity, suggesting an involvement of these proteins in HCV translation. The identification of all these host factors provides new clues regarding the function of the CRE during viral cycle progression.

HCV RNA traffic and association with NS5A in living cells

The spatiotemporal dynamics of Hepatitis C Virus (HCV) RNA localisation are poorly understood. To address this we engineered HCV genomes harbouring MS2 bacteriophage RNA stem-loops within the 3'-untranslated region to allow tracking of HCV RNA via specific interaction with a MS2-Coat-mCherry fusion protein. Despite the impact of these insertions on viral fitness, live imaging revealed that replication of tagged-HCV genomes induced specific redistribution of the mCherry-tagged-MS2-Coat protein to motile and static foci. Further analysis showed that HCV RNA was associated with NS5A in both static and motile structures while a subset of motile NS5A structures was devoid of HCV RNA. Further investigation of viral RNA traffic with respect to lipid droplets (LDs) revealed HCV RNA-positive structures in close association with LDs. These studies provide new insights into the dynamics of HCV RNA traffic with NS5A and LDs and provide a platform for future investigations of HCV replication and assembly.

RNA chaperones encoded by RNA viruses

RNAs are functionally diverse macromolecules whose proper functions rely strictly upon their correct tertiary structures. However, because of their high structural flexibility, correct folding of RNAs is challenging and slow. Therefore, cells and viruses encode a variety of RNA remodeling proteins, including helicases and RNA chaperones. In RNA viruses, these proteins are believed to play pivotal roles in all the processes involving viral RNAs during the life cycle. RNA helicases have been studied extensively for decades, whereas RNA chaperones, particularly virus-encoded RNA chaperones, are often overlooked. This review describes the activities of RNA chaperones encoded by RNA viruses, particularly the ones identified and characterized in recent years, and the functions of these proteins in different steps of viral life cycles, and presents an overview of this unique group of proteins.

The conserved 3'X terminal domain of hepatitis C virus genomic RNA forms a two-stem structure that promotes viral RNA dimerization

The 3′X domain of hepatitis C virus is a strongly conserved structure located at the 3′ terminus of the viral genomic RNA. This domain modulates the replication and translation processes of the virus in conjunction with an upstream 5BSL3.2 stem–loop, and contains a palindromic sequence that facilitates RNA dimerization. Based on nuclear magnetic resonance spectroscopy and gel electrophoresis, we report here that domain 3′X adopts a structure composed of two stem–loops, and not three hairpins or a mixture of folds, as previously proposed. This structure exposes unpaired terminal nucleotides after a double-helical stem and palindromic bases in an apical loop, favoring genomic RNA replication and self-association. At higher ionic strength the domain forms homodimers comprising an intermolecular duplex of 110 nucleotides. The 3′X sequences can alternatively form heterodimers with 5BSL3.2. This contact, reported to favor translation, likely involves local melting of one of the 3′X stem–loops.

Effects of short RNA structural analogues against hepatitis C virus genotypes 2, 3 and 4 in replicon cells

OBJECTIVE

To determine whether computer-predicted short RNA structural analogues could inhibit hepatitis C virus (HCV) genotype 2a, 3a and 4a replication in cultured cells.

METHODS

Short RNA sequences, X12, X12a and X12b, designed to be identical in secondary structure to the X region in the 3'-untranslated region (3'-UTR) of the HCV 1b genome, as well as shorter stem-loop components of X region, were inserted into a plasmid and transfected into separate Huh7.5 human hepatoma cells stably transfected with subgenomic replicons for genotypes 2a, 3a and 4a. All replicons included a firefly luciferase reporter gene. After 48 h of plasmid transfection, the inhibition of HCV replication was determined by HCV RNA isolation and quantification by real-time polymerase chain reaction and luciferase assays.

RESULTS

All the secondary structural analogues to genotype 1b X region cross-inhibited genotype 2a, 3a and 4a replicons. The maximum inhibition by genotype 1b X region structural analogues was obtained against genotype 2a cells in which X12, X12a and X12b inhibited replication by 30%, 63% and 72%, respectively (P < 0.05 for all), compared to an unrelated hepatitis B viral analogue.

CONCLUSIONS

Despite substantial sequence dissimilarity, HCV RNA genotype 1b X region analogues cross-inhibited the replication of HCV genotypes 2a, 3a and 4a. Particular conformations and not the sequence of the stem-loops of the X region are involved in HCV replication.

Conserved RNA secondary structures and long-range interactions in hepatitis C viruses

Hepatitis C virus (HCV) is a hepatotropic virus with a plus-strand RNA genome of ∼9.600 nt. Due to error-prone replication by its RNA-dependent RNA polymerase (RdRp) residing in nonstructural protein 5B (NS5B), HCV isolates are grouped into seven genotypes with several subtypes. By using whole-genome sequences of 106 HCV isolates and secondary structure alignments of the plus-strand genome and its minus-strand replication intermediate, we established refined secondary structures of the 5' untranslated region (UTR), the cis-acting replication element (CRE) in NS5B, and the 3' UTR. We propose an alternative structure in the 5' UTR, conserved secondary structures of 5B stem-loop (SL)1 and 5BSL2, and four possible structures of the X-tail at the very 3' end of the HCV genome. We predict several previously unknown long-range interactions, most importantly a possible circularization interaction between distinct elements in the 5' and 3' UTR, reminiscent of the cyclization elements of the related flaviviruses. Based on analogy to these viruses, we propose that the 5'-3' UTR base-pairing in the HCV genome might play an important role in viral RNA replication. These results may have important implications for our understanding of the nature of the cis-acting RNA elements in the HCV genome and their possible role in regulating the mutually exclusive processes of viral RNA translation and replication.

Secondary Structural Elements of the HCV X-region Involved in Viral Replication

BACKGROUND AND AIMS

The noncoding regions in the 3'-untranslated region (UTR) of the hepatitis C virus (HCV) genome contain secondary structures that are important for replication. The aim of this study was to identify detailed conformational elements of the X-region involved in HCV replication.

METHODS

Ribonucleic acid (RNA) structural analogs X94, X12, and X12c were constructed to have identical conformation but 94%, 12%, and 0% sequence identity, respectively, to the X region of HCV genotype 2a. Effects of structural analogs on replication of HCV genotypes 1b and 2a HCV RNA were studied by quantitative reverse transcriptase polymerase chain reaction.

RESULTS

In replicon BB7 cells, a constitutive replication model, HCV RNA levels decreased to 55%, 52%, 53%, and 54% after transfection with expression plasmids generating RNA structural analogs 5B-46, X-94, X-12, and X-12c, respectively (p<0.001 for all). In an HCV genotype 2a infection model, RNA analogs 5B-46, X-94, and X-12 in hepatic cells inhibited replication to 11%, 9%, and 12%, respectively. Because the X-12 analog was only 12% identical to the corresponding sequence of HCV genotype 2a, the sequence per se, or antisense effects were unlikely to be involved.

CONCLUSIONS

The data suggest that conformation of secondary structures in 3'-UTR of HCV RNA genome is required for HCV replication. Stable expression of RNA analogs predicted to have identical stem-loop structures might inhibit HCV infection of hepatocytes in liver and may represent a novel approach to design anti-HCV agents.

Inhibition of HCV translation by disrupting the structure and interactions of the viral CRE and 3' X-tail

A phylogenetically conserved RNA structure within the NS5B coding region of hepatitis C virus functions as a cis-replicating element (CRE). Integrity of this CRE, designated SL9266 (alternatively 5BSL3.2), is critical for genome replication. SL9266 forms the core of an extended pseudoknot, designated SL9266/PK, involving long distance RNA–RNA interactions between unpaired loops of SL9266 and distal regions of the genome. Previous studies demonstrated that SL9266/PK is dynamic, with ‘open’ and ‘closed’ conformations predicted to have distinct functions during virus replication. Using a combination of site-directed mutagenesis and locked nucleic acids (LNA) complementary to defined domains of SL9266 and its interacting regions, we have explored the influence of this structure on genome translation and replication. We demonstrate that LNAs which block formation of the closed conformation inhibit genome translation. Inhibition was at least partly independent of the initiation mechanism, whether driven by homologous or heterologous internal ribosome entry sites or from a capped message. Provision of SL9266/PK in trans relieved translational inhibition, and mutational analysis implied a mechanism in which the closed conformation recruits a cellular factor that would otherwise suppresses translation. We propose that SL9266/PK functions as a temporal switch, modulating the mutually incompatible processes of translation and replication.

Hallmarks of hepatitis C virus in equine hepacivirus

Equine hepacivirus (EHcV) has been identified as a closely related homologue of hepatitis C virus (HCV) in the United States, the United Kingdom, and Germany, but not in Asian countries. In this study, we genetically and serologically screened 31 serum samples obtained from Japanese-born domestic horses for EHcV infection and subsequently identified 11 PCR-positive and 7 seropositive serum samples. We determined the full sequence of the EHcV genome, including the 3' untranslated region (UTR), which had previously not been completely revealed. The polyprotein of a Japanese EHcV strain showed approximately 95% homology to those of the reported strains. HCV-like cis-acting RNA elements, including the stem-loop structures of the 3' UTR and kissing-loop interaction were deduced from regions around both UTRs of the EHcV genome. A comparison of the EHcV and HCV core proteins revealed that Ile(190) and Phe(191) of the EHcV core protein could be important for cleavage of the core protein by signal peptide peptidase (SPP) and were replaced with Ala and Leu, respectively, which inhibited intramembrane cleavage of the EHcV core protein. The loss-of-function mutant of SPP abrogated intramembrane cleavage of the EHcV core protein and bound EHcV core protein, suggesting that the EHcV core protein may be cleaved by SPP to become a mature form. The wild-type EHcV core protein, but not the SPP-resistant mutant, was localized on lipid droplets and partially on the lipid raft-like membrane in a manner similar to that of the HCV core protein. These results suggest that EHcV may conserve the genetic and biological properties of HCV.

IMPORTANCE

EHcV, which shows the highest amino acid or nucleotide homology to HCV among hepaciviruses, was previously reported to infect horses from Western, but not Asian, countries. We herein report EHcV infection in Japanese-born horses. In this study, HCV-like RNA secondary structures around both UTRs were predicted by determining the whole-genome sequence of EHcV. Our results also suggest that the EHcV core protein is cleaved by SPP to become a mature form and then is localized on lipid droplets and partially on lipid raft-like membranes in a manner similar to that of the HCV core protein. Hence, EHcV was identified as a closely related homologue of HCV based on its genetic structure as well as its biological properties. A clearer understanding of the epidemiology, genetic structure, and infection mechanism of EHcV will assist in elucidating the evolution of hepaciviruses as well as the development of surrogate models for the study of HCV.

Role of N-linked glycans in the interactions of recombinant HCV envelope glycoproteins with cellular receptors

Hepatitis C virus (HCV) infection is a major cause of chronic hepatitis and hepatocellular carcinoma. It infects human liver cells through several cellular protein receptors including CD81, SR-BI, claudin-1, and occludin. Previous reports also show that lectin receptors can mediate HCV recognition and entry. The envelope proteins of HCV (E1 and E2) are heavily glycosylated, further indicating the possible roles of lectin receptor-virus interaction in HCV infection. However, there is limited study investigating the relationship of HCV envelope glycoproteins and lectin as well as non-lectin receptors. Here we used surface plasmon resonance to examine the binding affinity of different glycoforms of recombinant HCV envelope protein to receptors and inspected the infectivity and assembly of HCV pseudoparticles composed of different glycoforms of envelope proteins. Our results indicated that DC-SIGN, L-SIGN, and Langerin had higher affinity to recombinant HCV envelope proteins in the presence of calcium ions than non-lectin receptors, and envelope proteins with Man8/9 N-glycans showed approximate 10-fold better binding to lectin receptors than envelope proteins with Man5 and complex type N-glycans. Interestingly, comparing among glycoforms, recombinant envelope proteins with Man5 N-glycans showed the highest binding affinity when interacting with non-lectin receptors. In summary, the glycans on HCV envelope protein play a modulatory role in HCV assembly and infection and direct HCV-receptor interaction, which mediates viral entry in different cells. Receptors with high affinity to HCV envelope proteins may be considered as targets for development of a therapeutic strategy against HCV.

The role of microRNAs in hepatitis C virus replication and related liver diseases

Hepatitis C virus (HCV) infection is a worldwide health problem and is one of the main causes of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma (HCC). However, only limited therapeutic options and no vaccines are currently available against HCV infection. Recent studies of microRNAs (miRNAs), which are able to regulate HCV replication and its related liver diseases by directly interacting with the HCV genome or indirectly controlling virus-associated host pathways, have broadened our understanding of the HCV life cycle. HCV utilizes host cellular miRNAs and modulates expression of miRNAs in infected hepatocytes for its infection and propagation. Moreover, such miRNAs directly or indirectly alter HCV replication efficiency and induce liver diseases including liver fibrosis, cirrhosis, or HCC. Representatively, miR-122 directly modulates the HCV life cycle by increasing HCV translation and genomic RNA stability. Recently, a phase IIa clinical trial with miravirsen, an LNA form of antimiR-122 oligonucleotides, showed significant reduction in serum HCV levels in patients chronically infected with HCV with no detectible evidence of resistance. In addition to miR-122, other miRNAs involved in the regulation of HCV propagation could be targeted in strategies to modulate HCV replication and pathogenesis. In this review, we summarize the features of miRNAs critical for HCV replication and HCV-mediated liver abnormalities and briefly discuss their potential application as therapeutic reagents for the treatment of HCV infection and its related diseases.

Origin of hepatitis C virus genotype 3 in Africa as estimated through an evolutionary analysis of the full-length genomes of nine subtypes, including the newly sequenced 3d and 3e

We characterized the full-length genomes of nine hepatitis C virus genotype 3 (HCV-3) isolates: QC7, QC8, QC9, QC10, QC34, QC88, NE145, NE274 and 811. To the best of our knowledge, NE274 and NE145 were the first full-length genomes for confirming the provisionally assigned subtypes 3d and 3e, respectively, whereas 811 represented the first HCV-3 isolate that had its extreme 3' UTR terminus sequenced. Based on these full-length genomes, together with 42 references representing eight assigned subtypes and an unclassified variant of HCV-3, and 10 sequences of six other genotypes, a timescaled phylogenetic tree was reconstructed after an evolutionary analysis using a coalescent Bayesian procedure. The results indicated that subtypes 3a, 3d and 3e formed a subset with a common ancestor dated to ~202.89 [95% highest posterior density (HPD): 160.11, 264.6] years ago. The analysis of all of the HCV-3 sequences as a single lineage resulted in the dating of the divergence time to ~457.81 (95% HPD: 350.62, 587.53) years ago, whereas the common ancestor of all of the seven HCV genotypes dated to ~780.86 (95% HPD: 592.15, 1021.34) years ago. As subtype 3h and the unclassified variant were relatives, and represented the oldest HCV-3 lineages with origins in Africa and the Middle East, these findings may indicate the ancestral origin of HCV-3 in Africa. We speculate that the ancestral HCV-3 strains may have been brought to South Asia from Africa by land and/or across the sea to result in its indigenous circulation in that region. The spread was estimated to have occurred in the era after Vasco da Gama had completed his expeditions by sailing along the eastern coast of Africa to India. However, before this era, Arabians had practised slave trading from Africa to the Middle East and South Asia for centuries, which may have mediated the earliest spread of HCV-3.

End-to-end crosstalk within the hepatitis C virus genome mediates the conformational switch of the 3'X-tail region

The hepatitis C virus (HCV) RNA genome contains multiple structurally conserved domains that make long-distance RNA-RNA contacts important in the establishment of viral infection. Microarray antisense oligonucleotide assays, improved dimethyl sulfate probing methods and 2' acylation chemistry (selective 2'-hydroxyl acylation and primer extension, SHAPE) showed the folding of the genomic RNA 3' end to be regulated by the internal ribosome entry site (IRES) element via direct RNA-RNA interactions. The essential cis-acting replicating element (CRE) and the 3'X-tail region adopted different 3D conformations in the presence and absence of the genomic RNA 5' terminus. Further, the structural transition in the 3'X-tail from the replication-competent conformer (consisting of three stem-loops) to the dimerizable form (with two stem-loops), was found to depend on the presence of both the IRES and the CRE elements. Complex interplay between the IRES, the CRE and the 3'X-tail region would therefore appear to occur. The preservation of this RNA-RNA interacting network, and the maintenance of the proper balance between different contacts, may play a crucial role in the switch between different steps of the HCV cycle.

The Role of MicroRNA in Hepatitis C Virus Replication

Hepatitis C virus (HCV) infection is a major global health problem. There is no effective vaccine and the current treatment regimen with pegylated interferon α and ribavirin is associated with significant adverse events. Therefore, there is an urgent need to identify new antiviral targets for HCV therapy. In recent years, a growing number of microRNAs (miRNAs) have been reported to be able to regulate HCV replication and infection by interacting with the HCV genome directly or by regulating host innate immunity to build a nonspecific antiviral state within cells. In this review, we discuss HCV virology and standard of care followed by miRNA in general, and then give a brief overview of miRNAs involved in HCV infection and discuss their potential application as a therapeutic option for the treatment of HCV infection.

Unmasking the information encoded as structural motifs of viral RNA genomes: a potential antiviral target

RNA viruses show enormous capacity to evolve and adapt to new cellular and molecular contexts, a consequence of mutations arising from errors made by viral RNA-dependent RNA polymerase during replication. Sequence variation must occur, however, without compromising functions essential for the completion of the viral cycle. RNA viruses are safeguarded in this respect by their genome carrying conserved information that does not code only for proteins but also for the formation of structurally conserved RNA domains that directly perform these critical functions. Functional RNA domains can interact with other regions of the viral genome and/or proteins to direct viral translation, replication and encapsidation. They are therefore potential targets for novel therapeutic strategies. This review summarises our knowledge of the functional RNA domains of human RNA viruses and examines the achievements made in the design of antiviral compounds that interfere with their folding and therefore their function.

Hepatitis C virus RNA replication

Genome replication is a crucial step in the life cycle of any virus. HCV is a positive strand RNA virus and requires a set of nonstructural proteins (NS3, 4A, 4B, 5A, and 5B) as well as cis-acting replication elements at the genome termini for amplification of the viral RNA. All nonstructural proteins are tightly associated with membranes derived from the endoplasmic reticulum and induce vesicular membrane alterations designated the membranous web, harboring the viral replication sites. The viral RNA-dependent RNA polymerase NS5B is the key enzyme of RNA synthesis. Structural, biochemical, and reverse genetic studies have revealed important insights into the mode of action of NS5B and the mechanism governing RNA replication. Although a comprehensive understanding of the regulation of RNA synthesis is still missing, a number of important viral and host determinants have been defined. This chapter summarizes our current knowledge on the role of viral and host cell proteins as well as cis-acting replication elements involved in the biogenesis of the membranous web and in viral RNA synthesis.

Sequence and structural analysis of 3' untranslated region of hepatitis C virus, genotype 3a, from pakistani isolates

BACKGROUND

Hepatitis C virus (HCV) is the cause of high morbidity and mortality worldwide, inflicting around one million people in Pakistan alone. The HCV genomic RNA harbors conserved structural elements that are indispensable for its replication. The 3' untranslated region (UTR) contains several of these elements essentially involved in regulating the major steps of the viral life cycle.

OBJECTIVES

Differences in regulatory elements of HCV may contribute towards differential infectivity of local isolates. The present study explicates sequence analysis and secondary structure prediction of HCV 3'UTR region of subtype 3a from Pakistan to characterize this particular region.

PATIENTS AND METHODS

HCV 3'UTR region was amplified, cloned and sequenced from five different patients. Sequence and structural analysis was performed and phylogenetic analysis was carried out using the 3'UTR sequence reported in NCBI nucleotide data base (http://www.ncbi.nlm.nih.gov/nuccore) by other studies.

RESULTS

Sequence analysis of the amplified fragment from five patients indicated that the 3'UTR is composed of 214-235 nts. Its sequence contains a type-specific variable region followed by a poly U/UC region and a highly conserved X-tail of 98 nts. The variable region reported here has 26 nts and one stem loop at the secondary structure that differentiate it from HCV genotype 1a ( GT1a) 3'UTR which contains additional 14 nts and two stem loops. The poly U/UC region varied in length (100-79 nts) and nucleotide sequence within the Pakistani isolates, and among different genotypes. Some substitutions found in the X-tail do not affect secondary structure of this element suggesting that this region might play an important role in replication, stabilization and packaging of HCV genome. Additionally, U residues are not present at the end of the X-tail in Pakistani 3a isolates as otherwise reported for the variants of genotype 1b.

CONCLUSIONS

Sequence and structural diversity of the 3'UTR variable region and Poly U/UC region found in the local isolates indicate specificity in the regulating elements of 3'UTR that might be associated with differential replication efficacy of the HCV Pakistani isolates. The study necessitates functional characterization of these regulating elements to elucidate variable viral efficiency and pathogenicity associated with inter-geographical isolates.

Systematic analysis of enhancer and critical cis-acting RNA elements in the protein-encoding region of the hepatitis C virus genome

Hepatitis C virus (HCV) causes chronic hepatitis, cirrhosis, and liver cancer. cis-acting RNA elements of the HCV genome are critical for translation initiation and replication of the viral genome. We hypothesized that the coding regions of nonstructural proteins harbor enhancer and essential cis-acting replication elements (CRE). In order to experimentally identify new cis RNA elements, we utilized an unbiased approach to introduce synonymous substitutions. The HCV genome coding for nonstructural proteins (nucleotide positions 3872 to 9097) was divided into 17 contiguous segments. The wobble nucleotide positions of each codon were replaced, resulting in 33% to 41% nucleotide changes. The HCV genome containing one of each of 17 mutant segments (S1 to S17) was tested for genome replication and infectivity. We observed that silent mutations in segment 13 (S13) (nucleotides [nt] 7457 to 7786), S14 (nt 7787 to 8113), S15 (nt 8114 to 8440), S16 (nt 8441 to 8767), and S17 (nt 8768 to 9097) resulted in impaired genome replication, suggesting CRE structures are enriched in the NS5B region. Subsequent high-resolution mutational analysis of NS5B (nt 7787 to 9289) using approximately 51-nucleotide contiguous subsegment mutant viruses having synonymous mutations revealed that subsegments SS8195-8245, SS8654-8704, and SS9011-9061 were required for efficient viral growth, suggesting that these regions act as enhancer elements. Covariant nucleotide substitution analysis of a stem-loop, JFH-SL9098, revealed the formation of an extended stem structure, which we designated JFH-SL9074. We have identified new enhancer RNA elements and an extended stem-loop in the NS5B coding region. Genetic modification of enhancer RNA elements can be utilized for designing attenuated HCV vaccine candidates.

Hepatitis C virus RNA: molecular switches mediated by long-range RNA-RNA interactions?

Multiple conserved structural cis-acting regulatory elements have been recognized both in the coding and untranslated regions (UTRs) of the hepatitis C virus (HCV) genome. For example, the cis-element 5BSL3.2 in the HCV-coding region has been predicted to use both its apical and internal loops to interact with the X RNA in the 3'-UTR, with the IIId domain in the 5'-UTR and with the Alt sequence in the coding region. Additionally, the X RNA region uses a palindromic sequence that overlaps the sequence required for the interaction with 5BSL3.2, to dimerize with another HCV genome. The ability of the 5BSL3.2 and X RNA regions to engage in multi-interactions suggests the existence of one or more molecular RNA switches which may regulate different steps of the HCV life cycle. In this study, we used biophysical methods to characterize the essential interactions of these HCV cis-elements at the molecular level. Our results indicate that X RNA interacts with 5BSL3.2 and another X RNA molecule by adopting two different conformations and that 5BSL3.2 engages simultaneously in kissing interactions using its apical and internal loops. Based on these results, we propose a mode of action for possible molecular switches involving the HCV RNA.

Uridine composition of the poly-U/UC tract of HCV RNA defines non-self recognition by RIG-I

Viral infection of mammalian cells triggers the innate immune response through non-self recognition of pathogen associated molecular patterns (PAMPs) in viral nucleic acid. Accurate PAMP discrimination is essential to avoid self recognition that can generate autoimmunity, and therefore should be facilitated by the presence of multiple motifs in a PAMP that mark it as non-self. Hepatitis C virus (HCV) RNA is recognized as non-self by RIG-I through the presence of a 5'-triphosphate (5'-ppp) on the viral RNA in association with a 3' poly-U/UC tract. Here we define the HCV PAMP and the criteria for RIG-I non-self discrimination of HCV by examining the RNA structure-function attributes that impart PAMP function to the poly-U/UC tract. We found that the 34 nucleotide poly-uridine "core" of this sequence tract was essential for RIG-I activation, and that interspersed ribocytosine nucleotides between poly-U sequences in the RNA were required to achieve optimal RIG-I signal induction. 5'-ppp poly-U/UC RNA variants that stimulated strong RIG-I activation efficiently bound purified RIG-I protein in vitro, and RNA interaction with both the repressor domain and helicase domain of RIG-I was required to activate signaling. When appended to 5'-ppp RNA that lacks PAMP activity, the poly-U/UC U-core sequence conferred non-self recognition of the RNA and innate immune signaling by RIG-I. Importantly, HCV poly-U/UC RNA variants that strongly activated RIG-I signaling triggered potent anti-HCV responses in vitro and hepatic innate immune responses in vivo using a mouse model of PAMP signaling. These studies define a multi-motif PAMP signature of non-self recognition by RIG-I that incorporates a 5'-ppp with poly-uridine sequence composition and length. This HCV PAMP motif drives potent RIG-I signaling to induce the innate immune response to infection. Our studies define a basis of non-self discrimination by RIG-I and offer insights into the antiviral therapeutic potential of targeted RIG-I signaling activation.

A twist in the tail: SHAPE mapping of long-range interactions and structural rearrangements of RNA elements involved in HCV replication

The RNA structure and long-range interactions of the SL9266 cis-acting replication element located within the NS5B coding region of hepatitis C virus (HCV) were determined using selective 2′-hydroxyl acylation analysed by primer extension. Marked differences were found in the long-range interactions of SL9266 when the two widely used genotype 2a JFH-1 (HCVcc) and genotype 1b Con1b sub-genomic replicon systems were compared. In both genomes, there was evidence for interaction of the sub-terminal bulge loop of SL9266 and sequences around nucleotide 9110, though the replication phenotype of genomes bearing mutations that disrupted this interaction was fundamentally different. In contrast, a ‘kissing loop’ interaction between the terminal loop of SL9266 and sequences in the 3′-untranslated X-tail was only detectable in JFH-1-based genomes. In the latter, where both long-range interactions are present, they were independent, implying that SL9266 forms the core of an extended pseudoknot. The presence of the ‘kissing loop’ interaction inhibited the formation of SL9571 in the 3′-X-tail, an RNA structure implicated in genome replication. We propose that, SL9266 may contribute a switch function that modulates the mutually incompatible translation and replication events that must occur for replication of the positive-strand RNA genome of HCV.

Turning hepatitis C into a real virus

The reality of hepatitis C is inescapable for the estimated 130 million people worldwide chronically infected with the virus. Yet this pathogen has been notoriously difficult to move from the infected individual into experimental systems, and each advance--from the identification of the infectious agent to its culture and study--has been a significant challenge. As a result of unrelenting technical hurdles, preventative and therapeutic options have been slow to reach hepatitis C patients. More than 35 years since the recognition of the disease, there is no vaccine available, and the only approved treatment, a combination of pegylated interferon-alpha (IFN-α) and ribavirin, is frequently ineffective. Decades of research, however, have resulted in systematic progress and much is now known about this once elusive pathogen. Most importantly, key breakthroughs have stimulated drug discovery, and the first generation of specifically targeted antiviral inhibitors is poised to enter the market. This review provides a look back at progress in developing tractable model systems for this important agent of chronic hepatitis.

Analysis of the RNA chaperoning activity of the hepatitis C virus core protein on the conserved 3'X region of the viral genome

The core protein of hepatitis c virus (HCV) is a structural protein with potent RNA chaperoning activities mediated by its hydrophilic N-terminal domain D1, which is thought to play a key role in HCV replication. To further characterize the core chaperoning properties, we studied the interactions between core D1 and the conserved HCV 3'X genomic region required for genome replication. To this end, we monitored the real-time annealing kinetics of native and mutated fluorescently labelled 16-nt palindromic sequence (DLS) and 27-nt Stem Loop II (SL2) from X with their respective complementary sequences. Core D1 and peptides consisting of the core basic domains were found to promote both annealing reactions and partly switch the loop-loop interaction pathway, which predominates in the absence of peptide, towards a pathway involving the stem termini. The chaperone properties of the core D1 peptides were found to be mediated through interaction of their basic clusters with the oligonucleotide phosphate groups, in line with the absence of high affinity site for core on HCV genomic RNA. The core ability to facilitate the interconversion between different RNA structures may explain how this protein regulates RNA structural transitions during HCV replication.

Functional RNA elements in the dengue virus genome

Dengue virus (DENV) genome amplification is a process that involves the viral RNA, cellular and viral proteins, and a complex architecture of cellular membranes. The viral RNA is not a passive template during this process; it plays an active role providing RNA signals that act as promoters, enhancers and/or silencers of the replication process. RNA elements that modulate RNA replication were found at the 5′ and 3′ UTRs and within the viral coding sequence. The promoter for DENV RNA synthesis is a large stem loop structure located at the 5′ end of the genome. This structure specifically interacts with the viral polymerase NS5 and promotes RNA synthesis at the 3′ end of a circularized genome. The circular conformation of the viral genome is mediated by long range RNA-RNA interactions that span thousands of nucleotides. Recent studies have provided new information about the requirement of alternative, mutually exclusive, structures in the viral RNA, highlighting the idea that the viral genome is flexible and exists in different conformations. In this article, we describe elements in the promoter SLA and other RNA signals involved in NS5 polymerase binding and activity, and provide new ideas of how dynamic secondary and tertiary structures of the viral RNA participate in the viral life cycle.

Interference of hepatitis C virus replication in cell culture by antisense peptide nucleic acids targeting the X-RNA

The RNA-dependent RNA polymerase (RdRp) of hepatitis C virus (HCV) is the essential catalytic enzyme for viral genome replication. It initiates minus-strand RNA synthesis from a highly conserved 98-nt sequence, called the X-RNA, at the 3'-end of the plus-strand viral genome. In this study, we evaluated the antiviral effects of peptide nucleic acids (PNAs) targeting the X-RNA. Our in vitro RdRp assay results showed that PNAs targeting the three major stem-loop (SL) domains of X-RNA can inhibit RNA synthesis initiation. Delivery of X-RNA-targeted PNAs by fusing the PNAs to cell-penetrating peptides (CPPs) into HCV-replicating cells effectively suppressed HCV replication. Electrophoretic mobility shift assays revealed that the PNA targeting the SL3 region at the 5'-end of X-RNA dissociated the viral RdRp from the X-RNA. Furthermore, delivery of the SL3-targeted PNA into HCV-infected cells resulted in the suppression of HCV RNA replication without activation of interferon β expression. Collectively, our results indicate that the HCV X-RNA can be effectively targeted by CPP-fused PNAs to block RNA-protein and/or RNA-RNA interactions essential for viral RNA replication and identify X-RNA SL3 as an RdRp binding site crucial for HCV replication. In addition, the ability to inhibit RNA synthesis initiation by targeting HCV X-RNA using antisense PNAs suggests their promising therapeutic potential against HCV infection.

A comprehensive structure-function comparison of hepatitis C virus strain JFH1 and J6 polymerases reveals a key residue stimulating replication in cell culture across genotypes

The hepatitis C virus (HCV) genotype 2a isolate JFH1 represents the only cloned HCV wild-type sequence capable of efficient replication in cell culture as well as in vivo. Previous reports have pointed to NS5B, the viral RNA-dependent RNA polymerase (RdRp), as a major determinant for efficient replication of this isolate. To understand the contribution of the JFH1 NS5B gene at the molecular level, we aimed at conferring JFH1 properties to NS5B from the closely related J6 isolate. We created intragenotypic chimeras in the NS5B regions of JFH1 and J6 and compared replication efficiency in cell culture and RdRp activity of the purified proteins in vitro, revealing more than three independent mechanisms conferring the role of JFH1 NS5B in efficient RNA replication. Most critical was residue I405 in the thumb domain of the polymerase, which strongly stimulated replication in cell culture by enhancing overall de novo RNA synthesis. A structural comparison of JFH1 and J6 at high resolution indicated a clear correlation of a closed-thumb conformation of the RdRp and the efficiency of the enzyme at de novo RNA synthesis, in accordance with the proposal that I405 enhances de novo initiation. In addition, we identified several residues enhancing replication independent of RdRp activity in vitro. The functional properties of JFH1 NS5B could be restored by a few single-nucleotide substitutions to the J6 isolate. Finally, we were able to enhance the replication efficiency of a genotype 1b isolate with the I405 mutation, indicating that this mechanism of action is conserved across genotypes.

Hepatitis C virus genomic RNA dimerization is mediated via a kissing complex intermediate

With over 200 million people infected with hepatitis C virus (HCV) worldwide, there is a need for more effective and better-tolerated therapeutic strategies. The HCV genome is a positive-sense; single-stranded RNA encoding a large polyprotein cleaved at multiple sites to produce at least ten proteins, among them an error-prone RNA polymerase that confers a high mutation rate. Despite considerable overall sequence diversity, in the 3'-untranslated region of the HCV genomic RNA there is a 98-nucleotide (nt) sequence named X RNA, the first 55 nt of which (X55 RNA) are 100% conserved among all HCV strains. The X55 region has been suggested to be responsible for in vitro dimerization of the genomic RNA in the presence of the viral core protein, although the mechanism by which this occurs is unknown. In this study, we analyzed the X55 region and characterized the mechanism by which it mediates HCV genomic RNA dimerization. Similar to a mechanism proposed previously for the human immunodeficiency 1 virus (HIV-1) genome, we show that dimerization of the HCV genome involves formation of a kissing complex intermediate, which is converted to a more stable extended duplex conformation in the presence of the core protein. Mutations in the dimer linkage sequence loop sequence that prevent RNA dimerization in vitro significantly reduced but did not completely ablate the ability of HCV RNA to replicate or produce infectious virus in transfected cells.