A retention signal necessary and sufficient for endoplasmic reticulum localization maps to the transmembrane domain of hepatitis C virus glycoprotein E2

The red alga Porphyridium as a host for molecular farming: Efficient production of immunologically active hepatitis C virus glycoprotein

Microalgae are promising production platforms for the cost-effective production of recombinant proteins. We have recently established that the red alga Porphyridium purpureum provides superior transgene expression properties, due to the episomal maintenance of transformation vectors as multicopy plasmids in the nucleus. Here, we have explored the potential of Porphyridium to synthesize complex pharmaceutical proteins to high levels. Testing expression constructs for a candidate subunit vaccine against the hepatitis C virus (HCV), we show that the soluble HCV E2 glycoprotein can be produced in transgenic algal cultures to high levels. The antigen undergoes faithful posttranslational modification by N-glycosylation and is recognized by conformationally selective antibodies, suggesting that it adopts a proper antigenic conformation in the endoplasmic reticulum of red algal cells. We also report the experimental determination of the structure of the N-glycan moiety that is attached to glycosylated proteins in Porphyridium. Finally, we demonstrate the immunogenicity of the HCV antigen produced in red algae when administered by injection as pure protein or by feeding of algal biomass.

Landscape of protein-protein interactions during hepatitis C virus assembly and release

Assembly of infectious hepatitis C virus (HCV) particles requires multiple cellular proteins including for instance apolipoprotein E (ApoE). To describe these protein-protein interactions, we performed an affinity purification mass spectrometry screen of HCV-infected cells. We used functional viral constructs with epitope-tagged envelope protein 2 (E2), protein (p) 7, or nonstructural protein 4B (NS4B) as well as cells expressing a tagged variant of ApoE. We also evaluated assembly stage-dependent remodeling of protein complexes by using viral mutants carrying point mutations abrogating particle production at distinct steps of the HCV particle production cascade. Five ApoE binding proteins, 12 p7 binders, 7 primary E2 interactors, and 24 proteins interacting with NS4B were detected. Cell-derived PREB, STT3B, and SPCS2 as well as viral NS2 interacted with both p7 and E2. Only GTF3C3 interacted with E2 and NS4B, highlighting that HCV assembly and replication complexes exhibit largely distinct interactomes. An HCV core protein mutation, preventing core protein decoration of lipid droplets, profoundly altered the E2 interactome. In cells replicating this mutant, E2 interactions with HSPA5, STT3A/B, RAD23A/B, and ZNF860 were significantly enhanced, suggesting that E2 protein interactions partly depend on core protein functions. Bioinformatic and functional studies including STRING network analyses, RNA interference, and ectopic expression support a role of Rad23A and Rad23B in facilitating HCV infectious virus production. Both Rad23A and Rad23B are involved in the endoplasmic reticulum (ER)-associated protein degradation (ERAD). Collectively, our results provide a map of host proteins interacting with HCV assembly proteins, and they give evidence for the involvement of ER protein folding machineries and the ERAD pathway in the late stages of the HCV replication cycle.IMPORTANCEHepatitis C virus (HCV) establishes chronic infections in the majority of exposed individuals. This capacity likely depends on viral immune evasion strategies. One feature likely contributing to persistence is the formation of so-called lipo-viro particles. These peculiar virions consist of viral structural proteins and cellular lipids and lipoproteins, the latter of which aid in viral attachment and cell entry and likely antibody escape. To learn about how lipo-viro particles are coined, here, we provide a comprehensive overview of protein-protein interactions in virus-producing cells. We identify numerous novel and specific HCV E2, p7, and cellular apolipoprotein E-interacting proteins. Pathway analyses of these interactors show that proteins participating in processes such as endoplasmic reticulum (ER) protein folding, ER-associated protein degradation, and glycosylation are heavily engaged in virus production. Moreover, we find that the proteome of HCV replication sites is distinct from the assembly proteome, suggesting that transport process likely shuttles viral RNA to assembly sites.

Establishment and methodological evaluation of a chemiluminescence assay for detection of anti-envelope protein (E1, E2) antibodies in the serum of hepatitis C virus-infected patients

BACKGROUND

To establish a chemiluminescence method for detecting anti-E1 and anti-E2 antibodies in the serum of patients with hepatitis C virus (HCV) infection.

METHODS

The microplate was coated with recombinant envelope proteins E1 and E2 by indirect method, respectively, and the kits for detecting anti-E1 and anti-E2 antibodies were prepared. The methodological indexes were evaluated.

RESULTS

The methodological indexes of the kits were as follows: precision test (the variation coefficient of anti-E1 antibody 6.71%-8.95% for within run and 9.91%-12.16% for between run, the variation coefficient of anti-E2 antibody 6.06%-8.44% for within run and 10.77%-13.98% for between run, respectively). The blank limit and detection limit were 1.18 RLIR and 3.16 RLIR for the anti-E1 antibody, and 1.26 RLIR and 3.32 RLIR for the anti-E2 antibody, respectively. The correlation coefficients (r) of anti-E1 and anti-E2 were 0.9963 and 0.9828, the analysis and measurement ranges (AMR) were 1.66-41.28 RLIR and 1.55-19.46 RLIR, and the average recovery was 96.4% and 93.7%, respectively. The rheumatoid factor and other positive serum samples had no interference or cross-reaction to the test, and the kits were stable within 15 months. The positive rates of anti-E1 and anti-E2 antibodies in 45 patients with HCV infection were 35.6% (16/45) and 44.4% (20/45), respectively.

CONCLUSIONS

The kits for detecting anti-E1 and anti-E2 meet the requirements of methodology, and can be used in screening diagnosis, disease monitoring, prognosis evaluation, disease mechanism, and epidemiological studies of HCV infection. The HCV envelope proteins E1 and E2 have an immune response in HCV-infected patients.

Influence of glycosylation on the immunogenicity and antigenicity of viral immunogens

A key aspect of successful viral vaccine design is the elicitation of neutralizing antibodies targeting viral attachment and fusion glycoproteins that embellish viral particles. This observation has catalyzed the development of numerous viral glycoprotein mimetics as vaccines. Glycans can dominate the surface of viral glycoproteins and as such, the viral glycome can influence the antigenicity and immunogenicity of a candidate vaccine. In one extreme, glycans can form an integral part of epitopes targeted by neutralizing antibodies and are therefore considered to be an important feature of key immunogens within an immunization regimen. In the other extreme, the existence of peptide and bacterially expressed protein vaccines shows that viral glycosylation can be dispensable in some cases. However, native-like glycosylation can indicate native-like protein folding and the presence of conformational epitopes. Furthermore, going beyond native glycan mimicry, in either occupancy of glycosylation sites or the glycan processing state, may offer opportunities for enhancing the immunogenicity and associated protection elicited by an immunogen. Here, we review key determinants of viral glycosylation and how recombinant immunogens can recapitulate these signatures across a range of enveloped viruses, including HIV-1, Ebola virus, SARS-CoV-2, Influenza and Lassa virus. The emerging understanding of immunogen glycosylation and its control will help guide the development of future vaccines in both recombinant protein- and nucleic acid-based vaccine technologies.

Production and immunogenicity of different prophylactic vaccines for hepatitis C virus (Review)

Hepatitis C virus (HCV) infection is a global health challenge, and prophylactic vaccines are the most effective way to eliminate the infection. To date, numerous forms of preventive vaccines have entered the clinical trial stage, including the virus-like particle (VLP) vaccine, recombinant subunit vaccine, peptide vaccine and nucleic acid vaccine. The rational design makes it easier to obtain specific vaccine structures with a broad spectrum and strong immunogenicity. Different vaccine antigens can evoke different immune responses, including humoral and T-cell immune responses, and can be produced using different expression systems, such as bacteria, yeast, mammals, plants, insects or parasites. Intracellular and insoluble production and a narrow immune spectrum are two difficulties that limit the application of vaccines. The present study summarizes the immunogenicity of different preventive vaccines, evaluates the characteristics of different expression systems used for vaccine production, and analyzes the strategies to enhance the secretion and immune spectrum of vaccine proteins.

Hepatitis C virus E2 envelope glycoprotein produced in Nicotiana benthamiana triggers humoral response with virus-neutralizing activity in vaccinated mice

Chronic infection with hepatitis C virus (HCV) remains a leading cause of liver-related pathologies and a global health problem, currently affecting more than 71 million people worldwide. The development of a prophylactic vaccine is much needed to complement the effective antiviral treatment available and achieve HCV eradication. Current strategies focus on increasing the immunogenicity of the HCV envelope glycoprotein E2, the major target of virus-neutralizing antibodies, by testing various expression systems or manipulating the protein conformation and the N-glycosylation pattern. Here we report the first evidence of successful production of the full-length HCV E2 glycoprotein in Nicotiana benthamiana, by using the Agrobacterium-mediated transient expression technology. Molecular and functional analysis showed that the viral protein was correctly processed in plant cells and achieved the native folding required for binding to CD81, one of the HCV receptors. N-glycan analysis of HCV-E2 produced in N. benthamiana and mammalian cells indicated host-specific trimming of mannose residues and possibly, protein trafficking. Notably, the plant-derived viral antigen triggered a significant immune response in vaccinated mice, characterized by the presence of antibodies with HCV-neutralizing activity. Together, our study demonstrates that N. benthamiana is a viable alternative to costly mammalian cell cultures for the expression of complex viral antigens and supports the use of plants as cost-effective production platforms for the development of HCV vaccines.

Characterization of Membrane Topology and Retention Signal of Pestiviral Glycoprotein E1

Pestiviruses are members of the family Flaviviridae, a group of enveloped viruses that bud at intracellular membranes. Pestivirus particles contain three glycosylated envelope proteins, E^rns, E1, and E2. Among them, E1 is the least characterized concerning both biochemical features and function. E1 from bovine viral diarrhea virus (BVDV) strain CP7 was analyzed with regard to its intracellular localization and membrane topology. Here, it is shown that even in the absence of other viral proteins, E1 is not secreted or expressed at the cell surface but localizes predominantly in the endoplasmic reticulum (ER). Using engineered chimeric transmembrane domains with sequences from E1 and vesicular stomatitis virus G protein, the E1 ER-retention signal could be narrowed down to six fully conserved polar residues in the middle part of the transmembrane domain of E1. Retention was observed even when several of these polar residues were exchanged for alanine. Mutations with a strong impact on E1 retention prevented recovery of infectious viruses when tested in the viral context. Analysis of the membrane topology of E1 before and after the signal peptide cleavage via a selective permeabilization and an in vivo labeling approach revealed that mature E1 is a typical type I transmembrane protein with a single span transmembrane anchor at its C terminus, whereas it adopts a hairpin-like structure with the C terminus located in the ER lumen when the precleavage situation is mimicked by blocking the cleavage site between E1 and E2.

IMPORTANCE The shortage of specific antibodies against E1, making detection and further analysis of E1 difficult, resulted in a lack of knowledge on E1 compared to E^rns and E2 with regard to biosynthesis, structure, and function. It is known that pestiviruses bud intracellularly. Here, we show that E1 contains its own ER retention signal: six fully conserved polar residues in the middle part of the transmembrane domain are shown to be the determinants for ER retention of E1. Moreover, those six polar residues could serve as a functional group that intensely affect the generation of infectious viral particles. In addition, the membrane topology of E1 has been determined. In this context, we also identified dynamic changes in membrane topology of E1 with the carboxy terminus located on the luminal side of the ER in the precleavage state and relocation of this sequence upon signal peptidase cleavage. Our work provides the first systematic analysis of the pestiviral E1 protein with regard to its biochemical and functional characteristics.

Interaction of Pestiviral E1 and E2 Sequences in Dimer Formation and Intracellular Retention

Pestiviruses contain three envelope proteins: E^rns, E1, and E2. Expression of HA-tagged E1 or mutants thereof showed that E1 forms homodimers and -trimers. C123 and, to a lesser extent, C171, affected the oligomerization of E1 with a double mutant C123S/C171S preventing oligomerization completely. E1 also establishes disulfide linked heterodimers with E2, which are crucial for the recovery of infectious viruses. Co-expression analyses with the HA-tagged E1 wt/E1 mutants and E2 wt/E2 mutants demonstrated that C123 in E1 and C295 in E2 are the critical sites for E1/E2 heterodimer formation. Introduction of mutations preventing E1/E2 heterodimer formation into the full-length infectious clone of BVDV CP7 prevented the recovery of infectious viruses, proving that C123 in E1 and C295 in E2 play an essential role in the BVDV life cycle, and further support the conclusion that heterodimer formation is the crucial step. Interestingly, we found that the retention signal of E1 is mandatory for intracellular localization of the heterodimer, so that absence of the E1 retention signal directs the heterodimer to the cell surface even though the E2 retention signal is still present. The covalent linkage between E1 and E2 plays an essential role for this process.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

Spatiotemporal Coupling of the Hepatitis C Virus Replication Cycle by Creating a Lipid Droplet- Proximal Membranous Replication Compartment

The hepatitis C virus (HCV) is a major cause of chronic liver disease, affecting around 71 million people worldwide. Viral RNA replication occurs in a membranous compartment composed of double-membrane vesicles (DMVs), whereas virus particles are thought to form by budding into the endoplasmic reticulum (ER). It is unknown how these steps are orchestrated in space and time. Here, we established an imaging system to visualize HCV structural and replicase proteins in live cells and with high resolution. We determined the conditions for the recruitment of viral proteins to putative assembly sites and studied the dynamics of this event and the underlying ultrastructure. Most notable was the selective recruitment of ER membranes around lipid droplets where structural proteins and the viral replicase colocalize. Moreover, ER membranes wrapping lipid droplets were decorated with double membrane vesicles, providing a topological map of how HCV might coordinate the steps of viral replication and virion assembly.

Role of Hepatitis C Virus Envelope Glycoprotein E1 in Virus Entry and Assembly

Hepatitis C virus (HCV) glycoproteins E1 and E2 form a heterodimer to constitute viral envelope proteins, which play an essential role in virus entry. E1 does not directly interact with host receptors, and its functions in viral entry are exerted mostly through its interaction with E2 that directly binds the receptors. HCV enters the host cell via receptor-mediated endocytosis during which the fusion of viral and host endosomal membranes occurs to release viral genome to cytoplasm. A putative fusion peptide in E1 has been proposed to participate in membrane fusion, but its exact role and underlying molecular mechanisms remain to be deciphered. Recently solved crystal structures of the E2 ectodomains and N-terminal of E1 fail to reveal a classical fusion-like structure in HCV envelope glycoproteins. In addition, accumulating evidence suggests that E1 also plays an important role in virus assembly. In this mini-review, we summarize current knowledge on HCV E1 including its structure and biological functions in virus entry, fusion, and assembly, which may provide clues for developing HCV vaccines and more effective antivirals.

Development of Preventive Vaccines for Hepatitis C Virus E1/E2 Protein

Hepatitis C virus (HCV) is responsible for a vast majority of liver failure cases. HCV is a kind of blood disease estimated to chronically infect 3% of the worlds population, causing significant morbidity and mortality. Therefore, a complete knowledge of humoral responses against HCV, resulting antibodies, and virus-receptor and virus-antibody interactions, are essential to design a vaccine. HCV epitopes or full sequence of HCV proteins can induce HCV specific immune responses. In fact, structural proteins are usually the main target of humoral responses and non-structural proteins are usually the main target of cellular responses. Hence, various vaccines based on distinct antigenic combinations are developed to prevent HCV infection and the current study tried to summarize them.

Multiplex flow cytometry-based assay to study the breadth of antibody responses against E1E2 glycoproteins of hepatitis C virus

Hepatitis C virus (HCV) infection is a major global public health problem. Early induction of cross-reactive neutralizing antibodies during acute infection correlates with the spontaneous clearance of HCV. Understanding the antibody response in multiple subjects in large-scale studies would greatly benefit vaccine development. To determine the breadth of a polyclonal-serum antibody response, and or, the monoclonal antibodies against the different HCV E1E2 genotypes, we developed a quick and high throughput flow cytometry assay using fluorescent cell barcoding to distinguish cells transfected with different E1E2 sequences in a single measurement. HCV-specific antibodies recognizing conformational epitopes were tested for binding to cells transfected with E1E2 from six genotypes. In this assay, 1500 samples can be analyzed for specific binding to 6 different HCV E1E2 sequences within 8h. Plasma of HCV infected subjects were tested in our assay allowing us to determine the breadth of their antibody response. In summary, we developed a quick and high throughput assay to study the specificity of an antibody response against multiple HCV E1E2 sequences simultaneously. This assay can also be used to facilitate the discovery of novel antibodies, and because other flavi- and picornaviruses have similar intracellular assembly mechanisms, this approach can be used to study the antibody response against such viruses.

An Optimized Hepatitis C Virus E2 Glycoprotein Core Adopts a Functional Homodimer That Efficiently Blocks Virus Entry

The hepatitis C virus (HCV) envelope glycoprotein E2 is the major target of broadly neutralizing antibodies in vivo and is the focus of efforts in the rational design of a universal B cell vaccine against HCV. The E2 glycoprotein exhibits a high degree of amino acid variability which localizes to three discrete regions: hypervariable region 1 (HVR1), hypervariable region 2 (HVR2), and the intergenotypic variable region (igVR). All three variable regions contribute to immune evasion and/or isolate-specific structural variations, both important considerations for vaccine design. A high-resolution structural definition of the intact HCV envelope glycoprotein complex containing E1 and E2 remains to be elucidated, while crystallographic structures of a recombinant E2 ectodomain failed to resolve HVR1, HVR2, and a major neutralization determinant adjacent to HVR1. To obtain further information on E2, we characterized the role of all three variable regions in E2 ectodomain folding and function in the context of a recombinant ectodomain fragment (rE2). We report that removal of the variable regions accelerates binding to the major host cell receptor CD81 and that simultaneous deletion of HVR2 and the igVR is required to maintain wild-type CD81-binding characteristics. The removal of the variable regions also rescued the ability of rE2 to form a functional homodimer. We propose that the rE2 core provides novel insights into the role of the variable motifs in the higher-order assembly of the E2 ectodomain and may have implications for E1E2 structure on the virion surface. IMPORTANCE Hepatitis C virus (HCV) infection affects ∼2% of the population globally, and no vaccine is available. HCV is a highly variable virus, and understanding the presentation of key antigenic sites at the virion surface is important for the design of a universal vaccine. This study investigates the role of three surface-exposed variable regions in E2 glycoprotein folding and function in the context of a recombinant soluble ectodomain. Our data demonstrate the variable motifs modulate binding of the E2 ectodomain to the major host cell receptor CD81 and have an impact on the formation of an E2 homodimer with high-affinity binding to CD81.

Hepatitis C Virus Is Released via a Noncanonical Secretory Route

We analyzed hepatitis C virus (HCV) morphogenesis using viral genomes encoding a mCherry-tagged E1 glycoprotein. HCV-E1-mCherry polyprotein expression, intracellular localization, and replication kinetics were comparable to those of untagged HCV, and E1-mCherry-tagged viral particles were assembled and released into cell culture supernatants. Expression and localization of structural E1 and nonstructural NS5A followed a temporospatial pattern with a succinct decrease in the number of replication complexes and the appearance of E1-mCherry punctae. Interaction of the structural proteins E1, Core, and E2 increased at E1-mCherry punctae in a time-dependent manner, indicating that E1-mCherry punctae represent assembled or assembling virions. E1-mCherry did not colocalize with Golgi markers. Furthermore, the bulk of viral glycoproteins within released particles revealed an EndoH-sensitive glycosylation pattern, indicating an absence of viral glycoprotein processing by the Golgi apparatus. In contrast, HCV-E1-mCherry trafficked with Rab9-positive compartments and inhibition of endosomes specifically suppressed HCV release. Our data suggest that assembled HCV particles are released via a noncanonical secretory route involving the endosomal compartment.

IMPORTANCE

The goal of this study was to shed light on the poorly understood trafficking and release routes of hepatitis C virus (HCV). For this, we generated novel HCV genomes which resulted in the production of fluorescently labeled viral particles. We used live-cell microscopy and other imaging techniques to follow up on the temporal dynamics of virus particle formation and trafficking in HCV-expressing liver cells. While viral particles and viral structural protein were found in endosomal compartments, no overlap of Golgi structures could be observed. Furthermore, biochemical and inhibitor-based experiments support a HCV release route which is distinguishable from canonical Golgi-mediated secretion. Since viruses hijack cellular pathways to generate viral progeny, our results point toward the possible existence of a not-yet-described cellular secretion route.

Hepatitis C virus Broadly Neutralizing Monoclonal Antibodies Isolated 25 Years after Spontaneous Clearance

Hepatitis C virus (HCV) is world-wide a major cause of liver related morbidity and mortality. No vaccine is available to prevent HCV infection. To design an effective vaccine, understanding immunity against HCV is necessary. The memory B cell repertoire was characterized from an intravenous drug user who spontaneously cleared HCV infection 25 years ago. CD27+IgG+ memory B cells were immortalized using BCL6 and Bcl-xL. These immortalized B cells were used to study antibody-mediated immunity against the HCV E1E2 glycoproteins. Five E1E2 broadly reactive antibodies were isolated: 3 antibodies showed potent neutralization of genotype 1 to 4 using HCV pseudotyped particles, whereas the other 2 antibodies neutralized genotype 1, 2 and 3 or 1 and 2 only. All antibodies recognized non-linear epitopes on E2. Finally, except for antibody AT12-011, which recognized an epitope consisting of antigenic domain C /AR2 and AR5, all other four antibodies recognized epitope II and domain B. These data show that a subject, who spontaneously cleared HCV infection 25 years ago, still has circulating memory B cells that are able to secrete broadly neutralizing antibodies. Presence of such memory B cells strengthens the argument for undertaking the development of an HCV vaccine.

HCV induces transforming growth factor β1 through activation of endoplasmic reticulum stress and the unfolded protein response

HCV replication disrupts normal endoplasmic reticulum (ER) function and activates a signaling network called the unfolded protein response (UPR). UPR is directed by three ER transmembrane proteins including ATF6, IRE1, and PERK. HCV increases TGF-β1 and oxidative stress, which play important roles in liver fibrogenesis. HCV has been shown to induce TGF-β1 through the generation of reactive oxygen species (ROS) and p38 MAPK, JNK, ERK1/2, and NFκB-dependent pathways. However, the relationship between HCV-induced ER stress and UPR activation with TGF-β1 production has not been fully characterized. In this study, we found that ROS and JNK inhibitors block HCV up-regulation of ER stress and UPR activation. ROS, JNK and IRE1 inhibitors blocked HCV-activated NFκB and TGF-β1 expression. ROS, ER stress, NFκB, and TGF-β1 signaling were blocked by JNK specific siRNA. Knockdown IRE1 inhibited JFH1-activated NFκB and TGF-β1 activity. Knockdown of JNK and IRE1 blunted JFH1 HCV up-regulation of NFκB and TGF-β1 activation. We conclude that HCV activates NFκB and TGF-β1 through ROS production and induction of JNK and the IRE1 pathway. HCV infection induces ER stress and the UPR in a JNK-dependent manner. ER stress and UPR activation partially contribute to HCV-induced NF-κB activation and enhancement of TGF-β1.

Comparative Proteomics Reveals Important Viral-Host Interactions in HCV-Infected Human Liver Cells

Hepatitis C virus (HCV) poses a global threat to public health. HCV envelop protein E2 is the major component on the virus envelope, which plays an important role in virus entry and morphogenesis. Here, for the first time, we affinity purified E2 complex formed in HCV-infected human hepatoma cells and conducted comparative mass spectrometric analyses. 85 cellular proteins and three viral proteins were successfully identified in three independent trials, among which alphafetoprotein (AFP), UDP-glucose: glycoprotein glucosyltransferase 1 (UGT1) and HCV NS4B were further validated as novel E2 binding partners. Subsequent functional characterization demonstrated that gene silencing of UGT1 in human hepatoma cell line Huh7.5.1 markedly decreased the production of infectious HCV, indicating a regulatory role of UGT1 in viral lifecycle. Domain mapping experiments showed that HCV E2-NS4B interaction requires the transmembrane domains of the two proteins. Altogether, our proteomics study has uncovered key viral and cellular factors that interact with E2 and provided new insights into our understanding of HCV infection.

Claudin-6 and Occludin Natural Variants Found in a Patient Highly Exposed but Not Infected with Hepatitis C Virus (HCV) Do Not Confer HCV Resistance In Vitro

The clinical course of Hepatitis C Virus (HCV) infection is highly variable between infected individual hosts: up to 80% of acutely HCV infected patients develop a chronic infection while 20% clear infection spontaneously. Spontaneous clearance of HCV infection can be predicted by several factors, including symptomatic acute infection, favorable IFNL3 polymorphisms and gender. In our study, we explored the possibility that variants in HCV cell entry factors might be involved in resistance to HCV infection. In a same case patient highly exposed but not infected by HCV, we previously identified one mutation in claudin-6 (CLDN6) and a rare variant in occludin (OCLN), two tight junction proteins involved in HCV entry into hepatocytes. Here, we conducted an extensive functional study to characterize the ability of these two natural variants to prevent HCV entry. We used lentiviral vectors to express Wildtype or mutated CLDN6 and OCLN in different cell lines and primary human hepatocytes. HCV infection was then investigated using cell culture produced HCV particles (HCVcc) as well as HCV pseudoparticles (HCVpp) expressing envelope proteins from different genotypes. Our results show that variants of CLDN6 and OCLN expressed separately or in combination did not affect HCV infection nor cell-to-cell transmission. Hence, our study highlights the complexity of HCV resistance mechanisms supporting the fact that this process probably not primarily involves HCV entry factors and that other unknown host factors may be implicated.

In Vitro Coinfection and Replication of Classical Swine Fever Virus and Porcine Circovirus Type 2 in PK15 Cells

Increasing clinical lines of evidence have shown the coinfection/superinfection of porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV). Here, we investigated whether PCV2 and CSFV could infect the same cell productively by constructing an in vitro coinfection model. Our results indicated that PCV2-free PK15 cells but not ST cells were more sensitive to PCV2, and the PK15 cell line could stably harbor replicating CSFV (PK15-CSFV cells) with a high infection rate. Confocal and super-resolution microscopic analysis showed that PCV2 and CSFV colocalized in the same PK15-CSFV cell, and the CSFV E2 protein translocated from the cytoplasm to the nucleus in PK15-CSFV cells infected with PCV2. Moreover, PCV2-CSFV dual-positive cells increased gradually in PK15-CSFV cells in a PCV2 dose-dependent manner. In PK15-CSFV cells, PCV2 replicated well, and the production of PCV2 progeny was not influenced by CSFV infection. However, CSFV reproduction decreased in a PCV2 dose-dependent manner. In addition, cellular apoptosis was not strengthened in PK15-CSFV cells infected with PCV2 in comparison with PCV2-infected PK15 cells. Moreover, using this coinfection model we further demonstrated PCV2-induced apoptosis might contribute to the impairment of CSFV HCLV strain replication in coinfected cells. Taken together, our results demonstrate for the first time the coinfection/superinfection of PCV2 and CSFV within the same cell, providing an in vitro model to facilitate further investigation of the underlying mechanism of CSFV and PCV2 coinfection.

The Replacement of 10 Non-Conserved Residues in the Core Protein of JFH-1 Hepatitis C Virus Improves Its Assembly and Secretion

Hepatitis C virus (HCV) assembly is still poorly understood. It is thought that trafficking of the HCV core protein to the lipid droplet (LD) surface is essential for its multimerization and association with newly synthesized HCV RNA to form the viral nucleocapsid. We carried out a mapping analysis of several complete HCV genomes of all genotypes, and found that the genotype 2 JFH-1 core protein contained 10 residues different from those of other genotypes. The replacement of these 10 residues of the JFH-1 strain sequence with the most conserved residues deduced from sequence alignments greatly increased virus production. Confocal microscopy of the modified JFH-1 strain in cell culture showed that the mutated JFH-1 core protein, C10M, was present mostly at the endoplasmic reticulum (ER) membrane, but not at the surface of the LDs, even though its trafficking to these organelles was possible. The non-structural 5A protein of HCV was also redirected to ER membranes and colocalized with the C10M core protein. Using a Semliki forest virus vector to overproduce core protein, we demonstrated that the C10M core protein was able to form HCV-like particles, unlike the native JFH-1 core protein. Thus, the substitution of a few selected residues in the JFH-1 core protein modified the subcellular distribution and assembly properties of the protein. These findings suggest that the early steps of HCV assembly occur at the ER membrane rather than at the LD surface. The C10M-JFH-1 strain will be a valuable tool for further studies of HCV morphogenesis.

Genetic Diversity Underlying the Envelope Glycoproteins of Hepatitis C Virus: Structural and Functional Consequences and the Implications for Vaccine Design

In the 26 years since the discovery of Hepatitis C virus (HCV) a major global research effort has illuminated many aspects of the viral life cycle, facilitating the development of targeted antivirals. Recently, effective direct-acting antiviral (DAA) regimens with >90% cure rates have become available for treatment of chronic HCV infection in developed nations, representing a significant advance towards global eradication. However, the high cost of these treatments results in highly restricted access in developing nations, where the disease burden is greatest. Additionally, the largely asymptomatic nature of infection facilitates continued transmission in at risk groups and resource constrained settings due to limited surveillance. Consequently a prophylactic vaccine is much needed. The HCV envelope glycoproteins E1 and E2 are located on the surface of viral lipid envelope, facilitate viral entry and are the targets for host immunity, in addition to other functions. Unfortunately, the extreme global genetic and antigenic diversity exhibited by the HCV glycoproteins represents a significant obstacle to vaccine development. Here we review current knowledge of HCV envelope protein structure, integrating knowledge of genetic, antigenic and functional diversity to inform rational immunogen design.

New Insights into the Understanding of Hepatitis C Virus Entry and Cell-to-Cell Transmission by Using the Ionophore Monensin A

In our study, we characterized the effect of monensin, an ionophore that is known to raise the intracellular pH, on the hepatitis C virus (HCV) life cycle. We showed that monensin inhibits HCV entry in a pangenotypic and dose-dependent manner. Monensin induces an alkalization of intracellular organelles, leading to an inhibition of the fusion step between viral and cellular membranes. Interestingly, we demonstrated that HCV cell-to-cell transmission is dependent on the vesicular pH. Using the selective pressure of monensin, we selected a monensin-resistant virus which has evolved to use a new entry route that is partially pH and clathrin independent. Characterization of this mutant led to the identification of two mutations in envelope proteins, the Y297H mutation in E1 and the I399T mutation in hypervariable region 1 (HVR1) of E2, which confer resistance to monensin and thus allow HCV to use a pH-independent entry route. Interestingly, the I399T mutation introduces an N-glycosylation site within HVR1 and increases the density of virions and their sensitivity to neutralization with anti-apolipoprotein E (anti-ApoE) antibodies, suggesting that this mutation likely induces conformational changes in HVR1 that in turn modulate the association with ApoE. Strikingly, the I399T mutation dramatically reduces HCV cell-to-cell spread. In summary, we identified a mutation in HVR1 that overcomes the vesicular pH dependence, modifies the biophysical properties of particles, and drastically reduces cell-to-cell transmission, indicating that the regulation by HVR1 of particle association with ApoE might control the pH dependence of cell-free and cell-to-cell transmission. Thus, HVR1 and ApoE are critical regulators of HCV propagation.

IMPORTANCE

Although several cell surface proteins have been identified as entry factors for hepatitis C virus (HCV), the precise mechanisms regulating its transmission to hepatic cells are still unclear. In our study, we used monensin A, an ionophore that is known to raise the intracellular pH, and demonstrated that cell-free and cell-to-cell transmission pathways are both pH-dependent processes. We generated monensin-resistant viruses that displayed different entry routes and biophysical properties. Thanks to these mutants, we highlighted the importance of hypervariable region 1 (HVR1) of the E2 envelope protein for the association of particles with apolipoprotein E, which in turn might control the pH dependency of cell-free and cell-to-cell transmission.

Development of an infectious surrogate hepatitis C virus based on a recombinant vesicular stomatitis virus expressing hepatitis C virus envelope glycoproteins and green fluorescent protein

To develop surrogate viruses for hepatitis C virus (HCV), we previously produced recombinant vesicular stomatitis viruses (rVSVs) lacking glycoprotein G but instead expressing chimeric HCV E1/E2 fused to G. These rVSVs were not infectious in HCV-susceptible hepatoma cells. In this study, to develop an infectious surrogate HCV based on an rVSV (vesicular stomatitis virus [VSV]/HCV), we generated a novel rVSV encoding the native E1/E2 (H77 strain) and green fluorescent protein (GFP) instead of G. Here, we showed that this VSV/HCV efficiently infected human hepatoma cells, including Huh7 human hepatoma cells, expressed GFP in these cells, and propagated, but did not do so in nonsusceptible BHK-21 cells. The infectivity of VSV/HCV, measured as the number of foci of GFP-positive cells, was specifically reduced by the addition of chimpanzee anti-HCV serum, anti-E2 antibody, or anti-CD81 antibody to the cultures. When sera obtained from HCV-infected or uninfected patients were added, infection was selectively inhibited only by the sera of HCV-infected patients. These data together suggest that this infectious GFP-expressing VSV/HCV could be a useful tool for studying the mechanisms of HCV entry into cells and for assessing potential inhibitors of viral entry, including neutralizing antibodies.

Identification of a novel drug lead that inhibits HCV infection and cell-to-cell transmission by targeting the HCV E2 glycoprotein

Hepatitis C Virus (HCV) infects 200 million individuals worldwide. Although several FDA approved drugs targeting the HCV serine protease and polymerase have shown promising results, there is a need for better drugs that are effective in treating a broader range of HCV genotypes and subtypes without being used in combination with interferon and/or ribavirin. Recently, two crystal structures of the core of the HCV E2 protein (E2c) have been determined, providing structural information that can now be used to target the E2 protein and develop drugs that disrupt the early stages of HCV infection by blocking E2’s interaction with different host factors. Using the E2c structure as a template, we have created a structural model of the E2 protein core (residues 421–645) that contains the three amino acid segments that are not present in either structure. Computational docking of a diverse library of 1,715 small molecules to this model led to the identification of a set of 34 ligands predicted to bind near conserved amino acid residues involved in the HCV E2: CD81 interaction. Surface plasmon resonance detection was used to screen the ligand set for binding to recombinant E2 protein, and the best binders were subsequently tested to identify compounds that inhibit the infection of Huh-7 cells by HCV. One compound, 281816, blocked E2 binding to CD81 and inhibited HCV infection in a genotype-independent manner with IC50’s ranging from 2.2 µM to 4.6 µM. 281816 blocked the early and late steps of cell-free HCV entry and also abrogated the cell-to-cell transmission of HCV. Collectively the results obtained with this new structural model of E2c suggest the development of small molecule inhibitors such as 281816 that target E2 and disrupt its interaction with CD81 may provide a new paradigm for HCV treatment.

The association of hepatitis C virus glycoproteins with apolipoproteins E and B early in assembly is conserved in lipoviral particles

In patients chronically infected with hepatitis C virus and in the HCV cell culture system (HCVcc), it is known that highly infectious virus particles have low to very low buoyant densities. These low densities have been attributed to the association of HCV with lipoprotein components, which occur during the viral morphogenesis. The resulting hybrid particles are known as lipoviral particles (LVP); however, very little is known about how these particles are created. In our study, we used Huh7.5 cells to investigate the intracellular association between envelope proteins and apolipoproteins B and E (ApoB and ApoE, respectively). In particular, we were interested in the role of this association in initiating LVP morphogenesis. Co-immunoprecipitation assays revealed that ApoB, ApoE, and HCV glycoproteins formed a protein complex early in the HCV lifecycle. Confocal analyses of naïve, E1E2-transduced and HCVcc-infected cells showed that HCV glycoproteins, ApoB and ApoE were found strongly colocalized only in the endoplasmic reticulum. We also found that HCV glycoproteins, ApoB and ApoE were already associated with intracellular infectious viral particles and, furthermore, that the protein complex was conserved in the infectious viral particles present in the supernatant of infected Huh7.5 cells. The association of HCV glycoproteins with ApoE was also evidenced in the HCVpp system, using the non-hepatic HEK293T cell line. We suggest that the complex formed by HCV E1E2, ApoB, and ApoE may initiate lipoviral particle morphogenesis.

Interaction of hepatitis C virus core protein with janus kinase is required for efficient production of infectious viruses

Chronic hepatitis C virus (HCV) infection is responsible for the development of liver cirrhosis and hepatocellular carcinoma. HCV core protein plays not only a structural role in the virion morphogenesis by encapsidating a virus RNA genome but also a non-structural role in HCV-induced pathogenesis by blocking innate immunity. Especially, it has been shown to regulate JAK-STAT signaling pathway through its direct interaction with Janus kinase (JAK) via its proline-rich JAK-binding motif (⁷⁹PGYPWP⁸⁴). However, little is known about the physiological significance of this HCV core-JAK association in the context of the virus life cycle. In order to gain an insight, a mutant HCV genome (J6/JFH1-79A82A) was constructed to express the mutant core with a defective JAK-binding motif (⁷⁹AGYAWP⁸⁴) using an HCV genotype 2a infectious clone (J6/JFH1). When this mutant HCV genome was introduced into hepatocarcinoma cells, it was found to be severely impaired in its ability to produce infectious viruses in spite of its robust RNA genome replication. Taken together, all these results suggest an essential requirement of HCV core-JAK protein interaction for efficient production of infectious viruses and the potential of using core-JAK blockers as a new anti-HCV therapy.

Incorporation of hepatitis C virus E1 and E2 glycoproteins: the keystones on a peculiar virion

Hepatitis C virus (HCV) encodes two envelope glycoproteins, E1 and E2. Their structure and mode of fusion remain unknown, and so does the virion architecture. The organization of the HCV envelope shell in particular is subject to discussion as it incorporates or associates with host-derived lipoproteins, to an extent that the biophysical properties of the virion resemble more very-low-density lipoproteins than of any virus known so far. The recent development of novel cell culture systems for HCV has provided new insights on the assembly of this atypical viral particle. Hence, the extensive E1E2 characterization accomplished for the last two decades in heterologous expression systems can now be brought into the context of a productive HCV infection. This review describes the biogenesis and maturation of HCV envelope glycoproteins, as well as the interplay between viral and host factors required for their incorporation in the viral envelope, in a way that allows efficient entry into target cells and evasion of the host immune response.

Generation of infectious HCV pseudo typed particles and its utilization for studying the role of CD81 & SRBI receptors in HCV infection

Hepatitis C virus (HCV) entry into isolated primary liver cells and cell lines requires interaction with the cell surface receptors. The study of HCV attachment with host cell surface receptors has been hindered by the unavailability of competent cell culture based system for HCV propagation. This problem has been overcome by the development of genetically tagged infectious HCV pseudo particles (HCVpp) harboring unmodified E1 and E2 glycoproteins. Studies using cell binding assays together with infection assays using HCVpp have shown that CD81 and scavenger receptor (SRBI) are actively involved in binding with envelope proteins facilitating the viral entrance process. This paper aimed to develop HCVpp of local HCV 3a Pakistani isolate and to study the viral tropism role of CD81 and SRBI receptors in HCV infectivity. HCV E1 and E2 genes were amplified and cloned in mammalian expression vector pcDNA 3.1/myc. The expressing plasmid of HCV E1-E2 glycoprotein in native form was co-transfected into 293FT cells with lentiviral packaging plasmid encoding the MLV Gag-Pol core proteins, and a packaging competent MLV-derived genome (pMLVYCMV-Luc) encoding the luciferase marker protein to produce infectious HCVpp. Anti-CD81 antibody (CBL579), anti-SRBI type II antibody (sc-20441) HCV anti-E2 mouse IgG1 (sc-65457) and HCV anti-E1 antibody mouse IgG1 (sc-65459) were used in this setup. We showed that primary site of viral replication is liver which involve CD81 and SRBI receptors for HCV gp-dependent infection with HCVpp. This is the preliminary reported cell cultured based mechanism from Pakistan which facilitated functional studies of different antiviral agents. Understanding of this technique will help in development of new antiviral therapeutics focusing on earlier steps of HCV life cycle. We have developed infectious pseudo particles of local 3a-isolate and concluded that a number of liver-specific surface proteins function along with CD81 and SRBI receptor regarding HCV infectivity. To endeavors and to identify this liver specific co-receptor molecule(s) will provide insights into the role of these molecules in the initial steps of HCV life cycle.

Recent advances in HCV entry

ABSTRACT: 

The elucidation of the mechanisms by which HCV infects hepatocytes and replicates has been paramount for identifying therapeutic targets and developing the highly efficacious antiviral drugs from which we benefit today. The earliest stage of HCV infection is viral entry, a process in which a complex interplay is thought to occur between host molecules (including glycosaminoglycans, low-density lipoprotein receptor, CD81, SR-B1, CLDN1, OCLN, EGF receptor, ephrin type A receptor 2 and transferrin receptor 1) and envelope viral glycoproteins E1 and E2. The wealth of experimental data produced in the field of HCV entry is summarized in a proposed mechanism, updated to include the most recently published data on the topic. Compounds with putative entry-blocking and/or entry-inhibiting activity in vitro and in vivo are also briefly reviewed.

Characterization of hepatitis C virus intra- and intergenotypic chimeras reveals a role of the glycoproteins in virus envelopment

Hepatitis C virus (HCV) is highly variable and associated with chronic liver disease. Viral isolates are grouped into seven genotypes (GTs). Accumulating evidence indicates that viral determinants in the core to NS2 proteins modulate the efficiency of virus production. However, the role of the glycoproteins E1 and E2 in this process is currently poorly defined. Therefore, we constructed chimeric viral genomes to explore the role of E1 and E2 in HCV assembly. Comparison of the kinetics and efficiency of particle production by intragenotypic chimeras highlighted core and p7 as crucial determinants for efficient virion release. Glycoprotein sequences, however, had only a minimal impact on this process. In contrast, in the context of intergenotypic HCV chimeras, HCV assembly was profoundly influenced by glycoprotein genes. On the one hand, insertion of GT1a-derived (H77) E1-E2 sequences into a chimeric GT2a virus (Jc1) strongly suppressed virus production. On the other hand, replacement of H77 glycoproteins within the GT1a-GT2a chimeric genome H77/C3 by GT2a-derived (Jc1) E1-E2 increased infectious particle production. Thus, within intergenotypic chimeras, glycoprotein features strongly modulate virus production. Replacement of Jc1 glycoprotein genes by H77-derived E1-E2 did not grossly affect subcellular localization of core, E2, and NS2. However, it caused an accumulation of nonenveloped core protein and increased abundance of nonenveloped core protein structures with slow sedimentation. These findings reveal an important role for the HCV glycoproteins E1 and E2 in membrane envelopment, which likely depends on a genotype-specific interplay with additional viral factors.

Virion assembly and release

Hepatitis C Virus (HCV) particles exhibit several unusual properties that are not found in other enveloped RNA viruses, most notably their low buoyant density and interaction with serum lipoproteins. With the advent of systems to grow HCV in cell culture, the molecular basis of HCV particle assembly and release can now be addressed. The process of virus assembly involves protein-protein interactions between viral structural and nonstructural proteins and the coordinated action of host factors. This chapter reviews our current understanding of these interactions and factors.

The signal sequence of type II porcine reproductive and respiratory syndrome virus glycoprotein 3 is sufficient for endoplasmic reticulum retention

The glycoprotein 3 (GP3) of type II porcine reproductive and respiratory syndrome virus has the characteristic domains of a membrane protein. However, this protein has been reported to be retained in the endoplasmic reticulum (ER) rather than transported to the plasma membrane of the cell. In this study, we performed confocal laser scanning microscopy analysis of variants of GP3 and foundthat the signal sequence of the GP3 led to confinement of GP3 in the ER, while the functional ortransmembrane domain did not affect its localization. Based on these results, we concludedthat the signal sequence of GP3 contains the ER retention signal, which might play an important role in assembly of viral proteins.

Hepatitis C virus: virology and life cycle

Hepatitis C virus (HCV) is a positive sense, single-stranded RNA virus in the Flaviviridae family. It causes acute hepatitis with a high propensity for chronic infection. Chronic HCV infection can progress to severe liver disease including cirrhosis and hepatocellular carcinoma. In the last decade, our basic understanding of HCV virology and life cycle has advanced greatly with the development of HCV cell culture and replication systems. Our ability to treat HCV infection has also been improved with the combined use of interferon, ribavirin and small molecule inhibitors of the virally encoded NS3/4A protease, although better therapeutic options are needed with greater antiviral efficacy and less toxicity. In this article, we review various aspects of HCV life cycle including viral attachment, entry, fusion, viral RNA translation, posttranslational processing, HCV replication, viral assembly and release. Each of these steps provides potential targets for novel antiviral therapeutics to cure HCV infection and prevent the adverse consequences of progressive liver disease.

Broadly neutralizing immune responses against hepatitis C virus induced by vectored measles viruses and a recombinant envelope protein booster

Hepatitis C virus (HCV) infection remains a serious public health problem worldwide. Treatments are limited, and no preventive vaccine is available. Toward developing an HCV vaccine, we engineered two recombinant measles viruses (MVs) expressing structural proteins from the prototypic HCV subtype 1a strain H77. One virus directs the synthesis of the HCV capsid (C) protein and envelope glycoproteins (E1 and E2), which fold properly and form a heterodimer. The other virus expresses the E1 and E2 glycoproteins separately, with each one fused to the cytoplasmic tail of the MV fusion protein. Although these hybrid glycoproteins were transported to the plasma membrane, they were not incorporated into MV particles. Immunization of MV-susceptible, genetically modified mice with either vector induced neutralizing antibodies to MV and HCV. A boost with soluble E2 protein enhanced titers of neutralizing antibody against the homologous HCV envelope. In animals primed with MV expressing properly folded HCV C-E1-E2, boosting also induced cross-neutralizating antibodies against two heterologous HCV strains. These results show that recombinant MVs retain the ability to induce MV-specific humoral immunity while also eliciting HCV neutralizing antibodies, and that anti-HCV immunity can be boosted with a single dose of purified E2 protein. The use of MV vectors could have advantages for pediatric HCV vaccination.

Identification and targeting of an interaction between a tyrosine motif within hepatitis C virus core protein and AP2M1 essential for viral assembly

Novel therapies are urgently needed against hepatitis C virus infection (HCV), a major global health problem. The current model of infectious virus production suggests that HCV virions are assembled on or near the surface of lipid droplets, acquire their envelope at the ER, and egress through the secretory pathway. The mechanisms of HCV assembly and particularly the role of viral-host protein-protein interactions in mediating this process are, however, poorly understood. We identified a conserved heretofore unrecognized YXXΦ motif (Φ is a bulky hydrophobic residue) within the core protein. This motif is homologous to sorting signals within host cargo proteins known to mediate binding of AP2M1, the μ subunit of clathrin adaptor protein complex 2 (AP-2), and intracellular trafficking. Using microfluidics affinity analysis, protein-fragment complementation assays, and co-immunoprecipitations in infected cells, we show that this motif mediates core binding to AP2M1. YXXΦ mutations, silencing AP2M1 expression or overexpressing a dominant negative AP2M1 mutant had no effect on HCV RNA replication, however, they dramatically inhibited intra- and extracellular infectivity, consistent with a defect in viral assembly. Quantitative confocal immunofluorescence analysis revealed that core's YXXΦ motif mediates recruitment of AP2M1 to lipid droplets and that the observed defect in HCV assembly following disruption of core-AP2M1 binding correlates with accumulation of core on lipid droplets, reduced core colocalization with E2 and reduced core localization to trans-Golgi network (TGN), the presumed site of viral particles maturation. Furthermore, AAK1 and GAK, serine/threonine kinases known to stimulate binding of AP2M1 to host cargo proteins, regulate core-AP2M1 binding and are essential for HCV assembly. Last, approved anti-cancer drugs that inhibit AAK1 or GAK not only disrupt core-AP2M1 binding, but also significantly inhibit HCV assembly and infectious virus production. These results validate viral-host interactions essential for HCV assembly and yield compounds for pharmaceutical development.

Novel antiviral strategies are needed to combat the HCV pandemic. Understanding viral-host determinants involved in mediating assembly is critical for the development of drugs targeting this stage of the HCV life cycle. We identified a conserved heretofore unrecognized tyrosine motif within core, which is homologous to sorting signals within host cargo proteins that are recognized by AP2M1, a membrane trafficking protein. By combining novel proteomic techniques with molecular virology, RNAi, dominant-interfering, and pharmacological approaches, we show that this motif mediates core binding to AP2M1 and HCV assembly and that AP2M1 is essential for HCV assembly. Core's tryosine motif recruits AP2M1 to lipid droplets, the site of viral assembly, and disruption of core-AP2M1 binding alters core's sub-cellular localization and colocalization with the envelope protein E2. AAK1 and GAK, host kinases that stimulate AP2M1's interactions with cargo proteins, regulate core-AP2M1 binding and are essential for HCV assembly. Last, we discovered kinase inhibitors, which bind AAK1 or GAK and disrupt core-AP2M1 binding and HCV assembly. Our study provides insight into mechanisms of HCV assembly and yields candidate targets and compounds for pharmaceutical development. Since some of the discovered compounds are approved anti-cancer drugs, there may be an opportunity for repurposing them as antivirals.

A physical interaction between viral replicase and capsid protein is required for genome-packaging specificity in an RNA virus

Genome packaging is functionally coupled to replication in RNA viruses pathogenic to humans (Poliovirus), insects (Flock house virus [FHV]), and plants (Brome mosaic virus [BMV]). However, the underlying mechanism is not fully understood. We have observed previously that in FHV and BMV, unlike ectopically expressed capsid protein (CP), packaging specificity results from RNA encapsidation by CP that has been translated from mRNA produced from replicating genomic RNA. Consequently, we hypothesize that a physical interaction with replicase increases the CP specificity for packaging viral RNAs. We tested this hypothesis by evaluating the molecular interaction between replicase protein and CP using a FHV-Nicotiana benthamiana system. Bimolecular fluorescence complementation in conjunction with fluorescent cellular protein markers and coimmunoprecipitation assays demonstrated that FHV replicase (protein A) and CP physically interact at the mitochondrial site of replication and that this interaction requires the N-proximal region from either amino acids 1 to 31 or amino acids 32 to 50 of the CP. In contrast to the mitochondrial localization of CP derived from FHV replication, ectopic expression displayed a characteristic punctate pattern on the endoplasmic reticulum (ER). This pattern was altered to relocalize the CP throughout the cytoplasm when the C-proximal hydrophobic domain was deleted. Analysis of the packaging phenotypes of the CP mutants defective either in protein A-CP interactions or ER localization suggested that synchronization between protein A-CP interaction and its subcellular localization is imperative to confer packaging specificity.

A new type of intracellular retention signal identified in a pestivirus structural glycoprotein

Sorting of membrane proteins into intracellular organelles is crucial for cell function. Viruses exploit intracellular transport and retention systems to concentrate envelope proteins at the site of virus budding. In pestiviruses, a group of important pathogens of pigs and ruminants closely related to human hepatitis C virus, the E(rns) protein translated from the viral RNA is secreted from the infected cells and found in the serum of infected animals. Secretion of the protein is regarded as crucial for its function as a viral virulence factor associated with its RNase activity. However, ∼95% of the E(rns) molecules are retained within the infected cell. Fusion of different E(rns) fragments to the C terminus of CD72 allowed identification of a retention signal within the C-terminal 65 aa of the viral protein. This C-terminal sequence represents its membrane anchor and folds into an amphipathic helix binding in-plane to the membrane surface. Residues L183, I190, and L208 are important for intracellular location of E(rns). Presentation of the retention signal on the cytoplasmic instead of the luminal face of the ER membrane in CD8α fusion proteins still led to retention. Thus, E(rns) contains in its C-terminal amphipathic helix an intracellular retention signal that is active on both faces of the membrane.

Hepatitis C virus host cell entry

The hepatitis C virus (HCV) is a major medical problem with at least 130 million infected individuals worldwide. Over the last decade multiple host factors required for HCV cell entry have been identified, but a detailed understanding of their mechanistic interplay remains elusive. Nonetheless, recent advances in defining species-specific barriers of HCV transmission have allowed the identification of a minimal set of entry factors that are required for HCV infection of rodent cells and has culminated in an animal model that recapitulates HCV entry in vivo. A detailed understanding of the viral uptake pathway is imperative to define new drug targets allowing for more effective intervention against this devastating disease.

The role of neutralizing antibodies in hepatitis C virus infection

Hepatitis C virus (HCV) is a blood-borne virus estimated to infect around 170 million people worldwide and is, therefore, a major disease burden. In some individuals the virus is spontaneously cleared during the acute phase of infection, whilst in others a persistent infection ensues. Of those persistently infected, severe liver diseases such as cirrhosis and primary liver cancer may develop, although many individuals remain asymptomatic. A range of factors shape the course of HCV infection, not least host genetic polymorphisms and host immunity. A number of studies have shown that neutralizing antibodies (nAb) arise during HCV infection, but that these antibodies differ in their breadth and mechanism of neutralization. Recent studies, using both mAbs and polyclonal sera, have provided an insight into neutralizing determinants and the likely protective role of antibodies during infection. This understanding has helped to shape our knowledge of the overall structure of the HCV envelope glycoproteins--the natural target for nAb. Most nAb identified to date target receptor-binding sites within the envelope glycoprotein E2. However, there is some evidence that other viral epitopes may be targets for antibody neutralization, suggesting the need to broaden the search for neutralization epitopes beyond E2. This review provides a comprehensive overview of our current understanding of the role played by nAb in HCV infection and disease outcome and explores the limitations in the study systems currently used. In addition, we briefly discuss the potential therapeutic benefits of nAb and efforts to develop nAb-based therapies.

Hepatitis C virus assembly imaging

Hepatitis C Virus (HCV) assembly process is the least understood step in the virus life cycle. The functional data revealed by forward and reverse genetics indicated that both structural and non-structural proteins are involved in the assembly process. Using confocal and electron microscopy different groups determined the subcellular localization of different viral proteins and they identified the lipid droplets (LDs) as the potential viral assembly site. Here, we aim to review the mechanisms that govern the viral proteins recruitment to LDs and discuss the current model of HCV assembly process. Based on previous examples, this review will also discuss advanced imaging techniques as potential means to extend our present knowledge of HCV assembly process.

Trafficking of hepatitis C virus core protein during virus particle assembly

Hepatitis C virus (HCV) core protein is directed to the surface of lipid droplets (LD), a step that is essential for infectious virus production. However, the process by which core is recruited from LD into nascent virus particles is not well understood. To investigate the kinetics of core trafficking, we developed methods to image functional core protein in live, virus-producing cells. During the peak of virus assembly, core formed polarized caps on large, immotile LDs, adjacent to putative sites of assembly. In addition, LD-independent, motile puncta of core were found to traffic along microtubules. Importantly, core was recruited from LDs into these puncta, and interaction between the viral NS2 and NS3-4A proteins was essential for this recruitment process. These data reveal new aspects of core trafficking and identify a novel role for viral nonstructural proteins in virus particle assembly.

Hepatitis C virus (HCV) infects almost 200 million people worldwide, causing both acute and chronic liver disease. Although some antiviral treatments exist, they are not fully effective against all HCV genotypes and have serious side effects. In order to develop more effective treatment strategies, a better understanding of how HCV replicates in infected cells is required. In our study, we developed methods to visualize early steps in HCV particle assembly by fluorescently labeling core protein, a structural component of the virus. Soon after protein translation, core trafficked to the surface of large, immobile lipid droplets that were adjacent to sites of virus assembly. Core was also observed in highly motile puncta that traveled along microtubules. By using inhibitors of virus assembly and assembly-deficient viral mutants, we showed that core is recruited from lipid droplets into these puncta, and that this process was mediated by the interaction of HCV nonstructural proteins. Our work describes new methods to study the trafficking of core protein in infected cells, allowing us to better define aspects of infectious HCV particle assembly.

The hepatitis C virus E1 glycoprotein undergoes productive folding but accelerated degradation when expressed as an individual subunit in CHO cells

Hepatitis C Virus E1E2 heterodimers are components of the viral spike. Although there is a general agreement on the necessity of the co-expression of both E1 and E2 on a single coding unit for their productive folding and assembly, in a previous study using an in vitro system we obtained strong indications that E1 can achieve folding in absence of E2. Here, we have studied the folding pathway of unescorted E1 from stably expressing CHO cells, compared to the folding observed in presence of the E2 protein. A DTT-resistant conformation is achieved by E1 in both situations, consistent with the presence of an E2-independent oxidative pathway. However, while the E1E2 heterodimer is stable inside cells, E1 expressed alone is degraded within a few hours. On the other hand, the oxidation and stability of individually expressed E2 subunits is dependent on E1 co-expression. These data are consistent with E1 and E2 assisting each other for correct folding via different mechanisms: E2 assists E1 by stabilizing a semi-native conformation meanwhile E1 drives E2 towards a productive folding pathway.

Impact of the autophagy machinery on hepatitis C virus infection

Autophagy is a cellular process that catabolizes cytoplasmic components and maintains energy homeostasis. As a stress response, the autophagy machinery interconnects a wide range of cellular pathways, enhancing the spread of certain pathogens while limiting others, and has become a highly active research area over the past several years. Independent laboratories have recently reported that autophagy vesicles accumulate in hepatitis C virus (HCV) infected cells and that autophagy proteins can function as proviral factors required for HCV replication. In this review, we summarize what is currently known about the interplay between autophagy and HCV and the possible mechanisms whereby autophagy proteins might favor HCV propagation.

Comprehensive linker-scanning mutagenesis of the hepatitis C virus E1 and E2 envelope glycoproteins reveals new structure-function relationships

Despite extensive research, many details about the structure and functions of hepatitis C virus (HCV) glycoproteins E1 and E2 are not fully understood, and their crystal structure remains to be determined. We applied linker-scanning mutagenesis to generate a panel of 34 mutants, each containing an insertion of 5 aa at a random position within the E1E2 sequence. The mutated glycoproteins were analysed by using a range of assays to identify regions critical for maintaining protein conformation, E1E2 complex assembly, CD81 receptor binding, membrane fusion and infectivity. The results, while supporting previously published data, provide several interesting new findings. Firstly, insertion at amino acid 587 or 596 reduced E1E2 heterodimerization without affecting reactivity with some conformation-sensitive mAbs or with CD81, thus implicating these residues in glycoprotein assembly. Secondly, insertions within a conserved region of E2, between amino acid residues 611 and 631, severely disrupted protein conformation and abrogated binding of all conformation-sensitive antibodies, suggesting that the structural integrity of this region is critical for the correct folding of E2. Thirdly, an insertion at Leu-682 specifically affected membrane fusion, providing direct evidence that the membrane-proximal ‘stem’ of E2 is involved in the fusion mechanism. Overall, our results show that the HCV glycoproteins generally do not tolerate insertions and that there are a very limited number of sites that can be changed without dramatic loss of function. Nevertheless, we identified two E2 insertion mutants, at amino acid residues 408 and 577, that were infectious in the murine leukemia virus-based HCV pseudoparticle system.

Scavenger receptor class B type I and the hypervariable region-1 of hepatitis C virus in cell entry and neutralisation

Hepatitis C virus (HCV) infection is a leading cause of chronic liver disease worldwide and represents a major public health problem. Viral attachment and entry - the first encounter of the virus with the host cell - are major targets of neutralising immune responses. Thus, a detailed understanding of the HCV entry process offers interesting opportunities for the development of novel therapeutic strategies. Different cellular or soluble host factors mediate HCV entry, and considerable progress has been made in recent years to decipher how they induce HCV attachment, internalisation and membrane fusion. Among these factors, the scavenger receptor class B type I (SR-BI/SCARB1) is essential for HCV replication in vitro, through its interaction with the HCV E1E2 surface glycoproteins and, more particularly, the HVR1 segment located in the E2 protein. SR-BI is an interesting receptor because HCV, whose replication cycle intersects with lipoprotein metabolism, seems to exploit some aspects of its physiological functions, such as cholesterol transfer from high-density lipoprotein (HDL), during cell entry. SR-BI is also involved in neutralisation attenuation and therefore could be an important target for therapeutic intervention. Recent results suggest that it should be possible to identify inhibitors of the interaction of HCV with SR-BI that do not impair its important physiological properties, as discussed in this review.

The transmembrane domain of the molecular chaperone Cosmc directs its localization to the endoplasmic reticulum

The molecular basis for retention of integral membrane proteins in the endoplasmic reticulum (ER) is not well understood. We recently discovered a novel ER molecular chaperone termed Cosmc, which is essential for folding and normal activity of the Golgi enzyme T-synthase. Cosmc, a type II single-pass transmembrane protein, lacks any known ER retrieval/retention motifs. To explore specific ER localization determinants in Cosmc we generated a series of Cosmc mutants along with chimeras of Cosmc with a non-ER resident type II protein, the human transferrin receptor. Here we show that the 18 amino acid transmembrane domain (TMD) of Cosmc is essential for ER localization and confers ER retention to select chimeras. Moreover, mutations of a single Cys residue within the TMD of Cosmc prevent formation of disulfide-bonded dimers of Cosmc and eliminate ER retention. These studies reveal that Cosmc has a unique ER-retention motif within its TMD and provide new insights into the molecular mechanisms by which TMDs of resident ER proteins contribute to ER localization.

Characterization of antibody-mediated neutralization directed against the hypervariable region 1 of hepatitis C virus E2 glycoprotein

The hypervariable region 1 (HVR1) comprising the first 27 aa of E2 glycoprotein is a target for neutralizing antibodies against hepatitis C virus (HCV), but the mechanisms of this neutralization in the cell-culture-infectious genotype 2a strain JFH1 HCV virus (HCVcc) system are unknown. Two rabbit polyclonal sera, R1020 and R140, recognizing the HVR1 of the genotype 1a isolates H77c and Glasgow (Gla), respectively, and a Gla HVR1-specific mouse mAb AP213 have been described previously. However, attempts to generate of antibodies to the JFH1 HVR1 were unsuccessful. Therefore, this study produced chimeric JFH1 HCVcc viruses harbouring the H77c or Gla HVR1 to assess the reactivity of antibodies to this region and their effects on virus infectivity. The inter-genotypic HVR1 swap did not significantly affect virus infectivity. The genotype 1a HVR1-specific antibodies neutralized chimeric viruses in an isolate-dependent manner, underlining the role of HVR1 in HCV infection. The neutralizing antibodies reacted mainly with the C-terminal portion of HVR1, and detailed mapping identified A17, F20 and Q21 in the Gla HVR1 sequence and T21 (and possibly L20) in the corresponding H77c sequence as key epitope residues for AP213 and R140, and R1020, respectively. Importantly, none of the antibodies inhibited in vitro binding of viral envelope glycoproteins to the best-characterized HCV receptor, CD81, or to the glycosaminoglycan attachment factors. However, the HVR1 antibodies were capable of post-attachment neutralization. Overall, this study emphasizes the role of HVR1 in HCVcc entry and provides new tools to study this region further in the context of complete virions.

Interacting regions of CD81 and two of its partners, EWI-2 and EWI-2wint, and their effect on hepatitis C virus infection

CD81 is a tetraspanin protein that is involved in several essential cellular functions, as well as in the hepatitis C virus (HCV) infection. CD81 interacts with a high stoichiometry with its partner proteins EWI-2, EWI-2wint, and EWI-F. These latter proteins modify the functions of CD81 and can thereby potentially inhibit infection or modulate cell migration. Here, we characterized the cleavage of EWI-2 leading to the production of EWI-2wint, which has been shown to inhibit HCV infection. We determined the regions of EWI-2/EWI-2wint and CD81 that are important for their interaction and their functionality. More precisely, we identified a glycine zipper motif in the transmembrane domain of EWI-2/EWI-2wint that is essential for the interaction with CD81. In addition, we found that palmitoylation on two juxtamembranous cysteines in the cytosolic tail of EWI-2/EWI-2wint is required for their interaction with CD81 as well as with CD9, another tetraspanin. Thus, we have shown that palmitoylation of a tetraspanin partner protein can influence the interaction with a tetraspanin. We therefore propose that palmitoylation not only of tetraspanins, but also of their partner proteins is important in regulating the composition of complexes in tetraspanin networks. Finally, we identified the regions in CD81 that are necessary for its functionality in HCV entry and we demonstrated that EWI-2wint needs to interact with CD81 to exert its inhibitory effect on HCV infection.