Ultrastructural analysis of hepatitis C virus particles. Proc Natl Acad Sci U S A. 2013;110:9505-9510. [PMID: 23690609 DOI: 10.1073/pnas.1307527110]

CD81 Receptor Regions outside the Large Extracellular Loop Determine Hepatitis C Virus Entry into Hepatoma Cells

Hepatitis C virus (HCV) enters human hepatocytes using four essential entry factors, one of which is human CD81 (hCD81). The tetraspanin hCD81 contains a large extracellular loop (LEL), which interacts with the E2 glycoprotein of HCV. The role of the non-LEL regions of hCD81 (intracellular tails, four transmembrane domains, small extracellular loop and intracellular loop) is poorly understood. Here, we studied the contribution of these domains to HCV susceptibility of hepatoma cells by generating chimeras of related tetraspanins with the hCD81 LEL. Our results show that non-LEL regions in addition to the LEL determine susceptibility of cells to HCV. While closely related tetraspanins (X. tropicalis CD81 and D. rerio CD81) functionally complement hCD81 non-LEL regions, distantly related tetraspanins (C. elegans TSP9 amd D. melanogaster TSP96F) do not and tetraspanins with intermediate homology (hCD9) show an intermediate phenotype. Tetraspanin homology and susceptibility to HCV correlate positively. For some chimeras, infectivity correlates with surface expression. In contrast, the hCD9 chimera is fully surface expressed, binds HCV E2 glycoprotein but is impaired in HCV receptor function. We demonstrate that a cholesterol-coordinating glutamate residue in CD81, which hCD9 lacks, promotes HCV infection. This work highlights the hCD81 non-LEL regions as additional HCV susceptibility-determining factors.

Protease inhibitor therapy for hepatitis C virus-infection

INTRODUCTION

The hepatitis C virus (HCV) has affected an estimated of 80 million individuals worldwide and is a strain on public health. Around 25-30% of patients in Europe and the US who are infected with HIV are coinfected with HCV. Prior to 2013, treatment modalities containing an NS3/4A protease inhibitor in combination with pegylated interferon and ribavirin improved sustained virological response (SVR) rates. However, rates of severe side effects were high. Nowadays, oral direct-acting antiviral (DAA) combination therapy offers excellent treatment efficacy, safety and tolerability.

AREAS COVERED

This review focuses on the current literature and clinical evidence and their impact regarding NS3/4A protease inhibitors. The pitfalls encountered in treating HIV- and HBV-coinfected patients are also discussed.

EXPERT OPINION

In the era of DAA treatment, third-generation pan-genotypic NS3/4A protease inhibitors (mainly glecaprevir and voxilaprevir) show high antiviral activity and a genetic resistance barrier with cure rates of over 95% when combined with an NS5A inhibitor, irrespective of baseline resistance associated variants (RASs) being present. These new key components of DAA combination therapy are impressive options to eradicate HCV in the so-called difficult-to-treat population (e.g. compensated cirrhosis, end-stage renal disease and patients who failed previous DAA treatment).

Quantitative Lipid Droplet Proteome Analysis Identifies Annexin A3 as a Cofactor for HCV Particle Production

Lipid droplets are vital to hepatitis C virus (HCV) infection as the putative sites of virion assembly, but morphogenesis and egress of virions remain ill defined. We performed quantitative lipid droplet proteome analysis of HCV-infected cells to identify co-factors of that process. Our results demonstrate that HCV disconnects lipid droplets from their metabolic function. Annexin A3 (ANXA3), a protein enriched in lipid droplet fractions, strongly impacted HCV replication and was characterized further: ANXA3 is recruited to lipid-rich fractions in HCV-infected cells by the viral core and NS5A proteins. ANXA3 knockdown does not affect HCV RNA replication but severely impairs virion production with lower specific infectivity and higher density of secreted virions. ANXA3 is essential for the interaction of viral envelope E2 with apolipoprotein E (ApoE) and for trafficking, but not lipidation, of ApoE in HCV-infected cells. Thus, we identified ANXA3 as a regulator of HCV maturation and egress.

Autoantibody to apolipoprotein A-1 in hepatitis C virus infection: a role in atherosclerosis?

Background/purpose

One to three per cent of the world’s population has hepatitis C virus (HCV) infection, which is not only a major cause of liver disease and cancer but also associated with an increased risk of atherosclerosis, despite an ostensibly favourable lipid profile. Autoantibodies are frequent in HCV infection and emerging evidence shows that autoantibodies could be valuable for cardiovascular disease (CVD) risk stratification. This study investigated a novel independent biomarker of CVD, autoantibodies to apolipoprotein A-1 (anti-apoA-1 IgG) and lipids in patients with chronic HCV before, during and after direct-acting anti-viral (DAA) therapy.

Methods

Eighty-nine blinded serum samples from 27 patients with advanced chronic HCV were assayed for lipids and anti-apoA-1 IgG by ELISA.

Results

Pre-treatment HCV viral load correlated with high-density lipoprotein cholesterol (HDL-C, r = 0.417; p = 0.042) and negatively with apolipoprotein (apo)B (r = − 0.497; p = 0.013) and markers of CVD risk, the apoB/apoA-1 ratio (r = − 0.490; p = 0.015) and triglyceride level (TG)/HDL-C ratio (r = − 0.450; p = 0.031). Fourteen (52%) of 27 patients had detectable anti-apoA-1 IgG autoantibodies pre-treatment; only two became undetectable with virological cure. Autoantibody-positive sera had lower apoA-1 (p = 0.012), HDL-C (p = 0.009) and total cholesterol (p = 0.006) levels.

Conclusions

This is the first report of the presence of an emerging biomarker for atherosclerosis, anti-apoA-1 IgG, in some patients with HCV infection. It may be induced by apoA-1 on the surface of HCV lipoviral particles. The autoantibodies inversely correlate with apoA-1 and HDL levels and may render HDL dysfunctional. Whether these hypothesis-generating findings have clinical implications in HCV patients requires further study.

Lysophosphatidylcholine acyltransferase 1 is downregulated by hepatitis C virus: impact on production of lipo-viro-particles

OBJECTIVE

HCV is intimately linked with the liver lipid metabolism, devoted to the efflux of triacylglycerols stored in lipid droplets (LDs) in the form of triacylglycerol-rich very-low-density lipoproteins (VLDLs): (i) the most infectious HCV particles are those of lowest density due to association with triacylglycerol-rich lipoproteins and (ii) HCV-infected patients frequently develop hepatic steatosis (increased triacylglycerol storage). The recent identification of lysophosphatidylcholine acyltransferase 1 (LPCAT1) as an LD phospholipid-remodelling enzyme prompted us to investigate its role in liver lipid metabolism and HCV infectious cycle.

DESIGN

Huh-7.5.1 cells and primary human hepatocytes (PHHs) were infected with JFH1-HCV. LPCAT1 depletion was achieved by RNA interference. Cells were monitored for LPCAT1 expression, lipid metabolism and HCV production and infectivity. The density of viral particles was assessed by isopycnic ultracentrifugation.

RESULTS

Upon HCV infection, both Huh-7.5.1 cells and PHH had decreased levels of LPCAT1 transcript and protein, consistent with transcriptional downregulation. LPCAT1 depletion in either naive or infected Huh-7.5.1 cells resulted in altered lipid metabolism characterised by LD remodelling, increased triacylglycerol storage and increased secretion of VLDL. In infected Huh-7.5.1 cells or PHH, LPCAT1 depletion increased production of the viral particles of lowest density and highest infectivity.

CONCLUSIONS

We have identified LPCAT1 as a modulator of liver lipid metabolism downregulated by HCV, which appears as a viral strategy to increase the triacylglycerol content and hence infectivity of viral particles. Targeting this metabolic pathway may represent an attractive therapeutic approach to reduce both the viral titre and hepatic steatosis.

The Rationale for a Preventative HCV Virus-Like Particle (VLP) Vaccine

HCV represents a global health problem with ~200 million individuals currently infected, worldwide. With the high cost of antiviral therapies, the global burden of chronic hepatitis C infection (CHCV) infection will be substantially reduced by the development of an effective vaccine for HCV. The field of HCV vaccines is generally divided into proponents of strategies to induce neutralizing antibodies (NAb) and those who propose to elicit cell mediated immunity (CMI). However, for a hepatitis C virus (HCV) vaccine to be effective in preventing infection, it must be capable of generating cross-reactive CD4+, CD8+ T cell, and NAb responses that will cover the major viral genotypes. Simulation models of hepatitis C have predicted that a vaccine of even modest efficacy and coverage will significantly reduce the incidence of hepatitis C. A HCV virus like particle (VLP) based vaccine would fulfill the requirement of delivering critical conformational neutralizing epitopes in addition to providing HCV specific CD4⁺ and CD8⁺ epitopes. Several approaches have been reported including insect cell-derived genotype 1b HCV VLPs; a human liver-derived quadrivalent genotype 1a, 1b, 2, and 3a vaccine; a genotype 1a HCV E1 and E2 glycoprotein/MLV Gag pseudotype VLP vaccine; and chimeric HBs-HCV VLP vaccines. All to result in the production of cross-NAb and/or T cell responses against HCV. This paper summarizes the evidence supporting the development of a HCV VLP based vaccine.

How have retrovirus pseudotypes contributed to our understanding of viral entry?

Study of virus entry into host cells is important for understanding viral tropism and pathogenesis. Studying the entry of in vitro cultured viruses is not always practicable. Study of highly pathogenic viruses, viruses that do not grow in culture, and viruses that rapidly change phenotype in vitro can all benefit from alternative models of entry. Retrovirus particles can be engineered to display the envelope proteins of heterologous enveloped viruses. This approach, broadly termed ‘pseudotyping’, is an important technique for interrogating virus entry. In this perspective we consider how retrovirus pseudotypes have addressed these challenges and improved our understanding of the entry pathways of diverse virus species, including Ebolavirus, human immunodeficiency virus and hepatitis C virus.

Efficient replication of blood-borne hepatitis C virus in human fetal liver stem cells

The development of pathogenic mechanisms, specific antiviral treatments and preventive vaccines for hepatitis C virus (HCV) infection has been limited due to lack of cell culture models that can naturally imitate the entire HCV life cycle. Here, we established an HCV cell culture model based on human fetal liver stem cells (hFLSCs) that supports the entire blood-borne hepatitis C virus (bbHCV) life cycle. More than 90% of cells remained infected by various genotypes. bbHCV was efficiently propagated, and progeny virus were infectious to hFLSCs. The virus could be passed efficiently between cells. The viral infectivity was partially blocked by specific antibodies or small interfering RNA against HCV entry factors, whereas HCV replication was inhibited by antiviral drugs. We observed viral particles of approximately 55 nm in diameter in both cell culture media and infected cells after bbHCV infection.

CONCLUSION

Our data show that the entire bbHCV life cycle could be naturally imitated in hFLSCs. This model is expected to provide a powerful tool for exploring the process and the mechanism of bbHCV infection at the cellular level and for evaluating the treatment and preventive strategies of bbHCV infection. (Hepatology 2017;66:1045-1057).

Cryo-EM: beyond the microscope

The pace at which cryo-EM is being adopted as a mainstream tool in structural biology has continued unabated over the past year. Initial successes in obtaining near-atomic resolution structures with cryo-EM were enabled to a large extent by advances in microscope and detector technology. Here, we review some of the complementary technical improvements that are helping sustain the cryo-EM revolution. We highlight advances in image processing that permit high resolution structure determination even in the presence of structural and conformational heterogeneity. We also review selected examples where biochemical strategies for membrane protein stabilization facilitate cryo-EM structure determination, and discuss emerging approaches for further improving the preparation of reliable plunge-frozen specimens.

Long Non-coding RNAs in Hepatitis C Virus-Infected Cells

Hepatitis C virus (HCV) often leads to a chronic infection in the liver that may progress to steatosis, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Several viral and cellular factors are required for a productive infection and for the development of liver disease. Some of these are long non-coding RNAs (lncRNAs) deregulated in infected cells. After HCV infection, the sequence and the structure of the viral RNA genome are sensed to activate interferon (IFN) synthesis and signaling pathways. These antiviral pathways regulate transcription of several cellular lncRNAs. Some of these are also deregulated in response to viral replication. Certain viral proteins and/or viral replication can activate transcription factors such as MYC, SP1, NRF2, or HIF1α that modulate the expression of additional cellular lncRNAs. Interestingly, several lncRNAs deregulated in HCV-infected cells described so far play proviral or antiviral functions by acting as positive or negative regulators of the IFN system, while others help in the development of liver cirrhosis and HCC. The study of the structure and mechanism of action of these lncRNAs may aid in the development of novel strategies to treat infectious and immune pathologies and liver diseases such as cirrhosis and HCC.

Hepatitis C Virus Strain-Dependent Usage of Apolipoprotein E Modulates Assembly Efficiency and Specific Infectivity of Secreted Virions

Hepatitis C virus (HCV) is extraordinarily diverse and uses entry factors in a strain-specific manner. Virus particles associate with lipoproteins, and apolipoprotein E (ApoE) is critical for HCV assembly and infectivity. However, whether ApoE dependency is common to all HCV genotypes remains unknown. Therefore, we compared the roles of ApoE utilizing 10 virus strains from genotypes 1 through 7. ApoA and ApoC also support HCV assembly, so they may contribute to virus production in a strain-dependent fashion. Transcriptome sequencing (RNA-seq) revealed abundant coexpression of ApoE, ApoB, ApoA1, ApoA2, ApoC1, ApoC2, and ApoC3 in primary hepatocytes and in Huh-7.5 cells. Virus production was examined in Huh-7.5 cells with and without ApoE expression and in 293T cells where individual apolipoproteins (ApoE1, -E2, -E3, -A1, -A2, -C1, and -C3) were provided in trans All strains were strictly ApoE dependent. However, ApoE involvement in virus production was strain and cell type specific, because some HCV strains poorly produced infectious virus in ApoE-expressing 293T cells and because ApoE knockout differentially affected virus production of HCV strains in Huh-7.5 cells. ApoE allelic isoforms (ApoE2, -E3, and -E4) complemented virus production of HCV strains to comparable degrees. All tested strains assembled infectious progeny with ApoE in preference to other exchangeable apolipoproteins (ApoA1, -A2, -C1, and -C3). The specific infectivity of HCV particles was similar for 293T- and Huh-7.5-derived particles for most strains; however, it differed by more than 100-fold in some viruses. Collectively, this study reveals strain-dependent and host cell-dependent use of ApoE during HCV assembly. These differences relate to the efficacy of virus production and also to the properties of released virus particles and therefore govern viral fitness at the level of assembly and cell entry.IMPORTANCE Chronic HCV infections are a major cause of liver disease. HCV is highly variable, and strain-specific determinants modulate the response to antiviral therapy, the natural course of infection, and cell entry factor usage. Here we explored whether host factor dependency of HCV in particle assembly is modulated by strain-dependent viral properties. We showed that all examined HCV strains, which represent all seven known genotypes, rely on ApoE expression for assembly of infectious progeny. However, the degree of ApoE dependence is modulated in a strain-specific and cell type-dependent manner. This indicates that HCV strains differ in their assembly properties and host factor usage during assembly of infectious progeny. Importantly, these differences relate not only to the efficiency of virus production and release but also to the infectiousness of virus particles. Thus, strain-dependent features of HCV modulate ApoE usage, with implications for virus fitness at the level of assembly and cell entry.

Maturation of secreted HCV particles by incorporation of secreted ApoE protects from antibodies by enhancing infectivity

BACKGROUND & AIMS

Hepatitis C virus (HCV) evades humoral immunity and establishes chronic infections. Virus particles circulate in complex with lipoproteins facilitating antibody escape. Apolipoprotein E (ApoE) is essential for intracellular HCV assembly and for HCV cell entry. We aimed to explore if ApoE released from non-infected cells interacts with and modulates secreted HCV particles.

METHODS

ApoE secreted from non-infected cells was incubated with HCV from primary human hepatocytes or Huh-7.5 cells. Co-immunoprecipitation, viral infectivity and neutralization experiments were conducted.

RESULTS

Physiological levels of secreted ApoE (10-60µg/ml) enhanced the infectivity of HCV up to 8-fold across all genotypes, which indirectly decreased virus neutralization by antibodies targeting E1 or E2 up to 10-fold. Infection enhancement was observed for particles produced in primary human hepatocytes and Huh-7.5 cells. Selective depletion of ApoE ablated infection enhancement. Addition of HA-tagged ApoE to HCV particles permitted co-precipitation of HCV virions. Serum ApoE levels ranged between 10-60µg/ml, which is ca 100-fold higher than in Huh-7.5 conditioned cell culture fluids. Serum-derived HCV particles carried much higher amounts of ApoE than cell culture-derived HCV particles. Serum ApoE levels correlated with efficiency of co-precipitation of HCV upon exogenous addition of HA-ApoE. ApoE-dependent infection enhancement was independent of the hypervariable region 1 and SR-B1, but was dependent on heparan sulfate proteoglycans (HSPGs).

CONCLUSIONS

Physiological quantities of secreted ApoE stimulate HCV infection and increase antibody escape, by incorporating into virus particles and enhancing particle interactions with cellular HSPGs. Thus, secreted particles undergo ApoE-dependent maturation to enhance infectivity and to facilitate evasion from neutralizing antibodies. Lay summary: This study shows that HCV particle infectivity is remodeled by secreted ApoE after particle release from cells. Fluctuation of the availability of ApoE likely influences HCV infectivity, antibody escape and transmission.

Ultrastructural organisation of HCV from the bloodstream of infected patients revealed by electron microscopy after specific immunocapture

OBJECTIVE

HCV particles are associated with very low-density lipoprotein components in chronically infected patients. These hybrid particles, or 'lipo-viro particles' (LVPs), are rich in triglycerides, and contain the viral RNA, the capsid protein, E1E2 envelope glycoproteins and apolipoproteins B and E. However, their specific ultrastructural organisation has yet to be determined. We developed a strategy for the preparation of any viral sample that preserves the native structure of the LVPs, facilitating their precise morphological characterisation.

DESIGN

Using a strategy based on the direct specific immunocapture of particles on transmission electron microscopy (TEM) grids, we characterised the precise morphology of the viral particle by TEM.

RESULTS

The LVP consists of a broad nucleocapsid surrounding an electron-dense centre, presumably containing the HCV genome. The nucleocapsid is surrounded by an irregular, detergent-sensitive crescent probably composed of lipids. Lipid content may determine particle size. These particles carry HCV E1E2, ApoB and ApoE, as shown in our immuno-EM analysis. Our results also suggest that these putative LVPs circulate in the serum of patients as part of a mixed population, including lipoprotein-like particles and complete viral particles.

CONCLUSIONS

Twenty-five years after the discovery of HCV, this study finally provides information about the precise morphological organisation of viral particles. It is truly remarkable that our TEM images fully confirm the ultrastructure of LVPs predicted by several authors, almost exclusively from the results of molecular biology studies.

HCV replication in gastrointestinal mucosa: Potential extra-hepatic viral reservoir and possible role in HCV infection recurrence after liver transplantation

PURPOSE

Hepatitis C virus (HCV) predominantly infects hepatocytes, although it is known that receptors for viral entry are distributed on a wide array of target cells. Chronic HCV infection is indeed characterized by multiple non-liver manifestations, suggesting a more complex HCV tropism extended to extrahepatic tissues and remains to be fully elucidated. In this study, we investigated the gastrointestinal mucosa (GIM) as a potential extrahepatic viral replication site and its contribution to HCV recurrence.

METHODS

We analyzed GIM biopsies from a cohort of 76 patients, 11 of which were HCV-negative and 65 HCV-positive. Of these, 54 biopsies were from liver-transplanted patients. In 29 cases, we were able to investigate gastrointestinal biopsies from the same patient before and after transplant. To evaluate the presence of HCV, we looked for viral antigens and genome RNA, whilst to assess viral replicative activity, we searched for the replicative intermediate minus-strand RNA. We studied the genetic diversity and the phylogenetic relationship of HCV quasispecies from plasma, liver and gastrointestinal mucosa of HCV-liver-transplanted patients in order to assess HCV compartmentalization and possible contribution of gastrointestinal variants to liver re-infection after transplantation.

RESULTS

Here we show that HCV infects and replicates in the cells of the GIM and that the favorite hosts were mostly enteroendocrine cells. Interestingly, we observed compartmentalization of the HCV quasispecies present in the gastrointestinal mucosa compared to other tissues of the same patient. Moreover, the phylogenetic analysis revealed a high similarity between HCV variants detected in gastrointestinal mucosa and those present in the re-infected graft.

CONCLUSIONS

Our results demonstrated that the gastrointestinal mucosa might be considered as an extrahepatic reservoir of HCV and that could contribute to viral recurrence. Moreover, the finding that HCV infects and replicates in neuroendocrine cells opens new perspectives on the role of these cells in the natural history of HCV infection.

Analysis of Hepatitis C Virus Particle Heterogeneity in Immunodeficient Human Liver Chimeric fah-/- Mice

BACKGROUND & AIMS

Hepatitis C virus (HCV) is a leading cause of chronic liver diseases and the most common indication for liver transplantation in the United States. HCV particles in the blood of infected patients are characterized by heterogeneous buoyant densities, likely owing to HCV association with lipoproteins. However, clinical isolates are not infectious in vitro and the relative infectivity of the particles with respect to their buoyant density therefore cannot be determined, pointing to the need for better in vivo model systems.

METHODS

To analyze the evolution of the buoyant density of in vivo-derived infectious HCV particles over time, we infected immunodeficient human liver chimeric fumaryl acetoacetate hydrolase-/- mice with J6/JFH1 and performed ultracentrifugation of infectious mouse sera on isopicnic iodixanol gradients. We also evaluated the impact of a high sucrose diet, which has been shown to increase very-low-density lipoprotein secretion by the liver in rodents, on lipoprotein and HCV particle characteristics.

RESULTS

Similar to the severe combined immunodeficiency disease/Albumin-urokinase plasminogen activator human liver chimeric mouse model, density fractionation of infectious mouse serum showed higher infectivity in the low-density fractions early after infection. However, over the course of the infection, viral particle heterogeneity increased and the overall in vitro infectivity diminished without loss of the human liver graft over time. In mice provided with a sucrose-rich diet we observed a minor shift in HCV infectivity toward lower density that correlated with a redistribution of triglycerides and cholesterol among lipoproteins.

CONCLUSIONS

Our work indicates that the heterogeneity in buoyant density of infectious HCV particles evolves over the course of infection and can be influenced by diet.

Hepatitis C Virus Lipoviroparticles Assemble in the Endoplasmic Reticulum (ER) and Bud off from the ER to the Golgi Compartment in COPII Vesicles

Hepatitis C virus (HCV) exists as a lipoprotein-virus hybrid lipoviroparticle (LVP). In vitro studies have demonstrated the importance of apolipoproteins in HCV secretion and infectivity, leading to the notion that HCV coopts the secretion of very-low-density lipoprotein (VLDL) for its egress. However, the mechanisms involved in virus particle assembly and egress are still elusive. The biogenesis of VLDL particles occurs in the endoplasmic reticulum (ER), followed by subsequent lipidation in the ER and Golgi compartment. The secretion of mature VLDL particles occurs through the Golgi secretory pathway. HCV virions are believed to latch onto or fuse with the nascent VLDL particle in either the ER or the Golgi compartment, resulting in the generation of LVPs. In our attempt to unravel the collaboration between HCV and VLDL secretion, we studied HCV particles budding from the ER en route to the Golgi compartment in COPII vesicles. Biophysical characterization of COPII vesicles fractionated on an iodixanol gradient revealed that HCV RNA is enriched in the highly buoyant COPII vesicle fractions and cofractionates with apolipoprotein B (ApoB), ApoE, and the HCV core and envelope proteins. Electron microscopy of immunogold-labeled microsections revealed that the HCV envelope and core proteins colocalize with apolipoproteins and HCV RNA in Sec31-coated COPII vesicles. Ultrastructural analysis also revealed the presence of HCV structural proteins, RNA, and apolipoproteins in the Golgi stacks. These findings support the hypothesis that HCV LVPs assemble in the ER and are transported to the Golgi compartment in COPII vesicles to embark on the Golgi secretory route.IMPORTANCE HCV assembly and release accompany the formation of LVPs that circulate in the sera of HCV patients and are also produced in an in vitro culture system. The pathway of HCV morphogenesis and secretion has not been fully understood. This study investigates the exact site where the association of HCV virions with host lipoproteins occurs. Using immunoprecipitation of COPII vesicles and immunogold electron microscopy (EM), we characterize the existence of LVPs that cofractionate with lipoproteins, viral proteins, RNA, and vesicular components. Our results show that this assembly occurs in the ER, and LVPs thus formed are carried through the Golgi network by vesicular transport. This work provides a unique insight into the HCV LVP assembly process within infected cells and offers opportunities for designing antiviral therapeutic cellular targets.

Extracellular Interactions between Hepatitis C Virus and Secreted Apolipoprotein E

Interactions between hepatitis C virus (HCV) and lipoproteins in humans play an important role in the efficient establishment of chronic infection. Apolipoprotein E (ApoE) on the HCV envelope mediates virus attachment to host cells as well as immune evasion. This interaction is thought to occur in hepatocytes, as ApoE plays dual functions in HCV assembly and maturation as well as cell attachment. In the present study, we found that secreted ApoE (sApoE) can also bind to viral particles via its C-terminal domain after HCV is released from the cell. Furthermore, the binding affinity of interactions between the sApoE N terminus and cell surface receptors affected HCV infectivity in a dose-dependent manner. The extracellular binding of sApoE to HCV is dependent on HCV envelope proteins, and recombinant HCV envelope proteins are also able to bind to sApoE. These results suggest that extracellular interactions between HCV and sApoE may potentially complicate vaccine development and studies of viral pathogenesis.IMPORTANCE End-stage liver disease caused by chronic HCV infection remains a clinical challenge, and there is an urgent need for a prophylactic method of controlling HCV infection. Because host immunity against HCV is poorly understood, additional investigations of host-virus interactions in the context of HCV are important. HCV is primarily transmitted through blood, which is rich in lipoproteins. Therefore, it is of interest to further determine how HCV interacts with lipoproteins in human blood. In this study, we found that secreted ApoE (sApoE), an exchangeable component found in lipoproteins, participates in extracellular interactions with HCV virions. More significantly, different variants of sApoE differentially affect HCV infection efficiency in a dose-dependent manner. These findings provide greater insight into HCV infection and host immunity and could help propel the development of new strategies for preventing HCV infection.

Hepatitis C virus-apolipoprotein interactions: molecular mechanisms and clinical impact

INTRODUCTION

Chronic hepatitis C virus (HCV) infection is a leading cause of cirrhosis, hepatocellular carcinoma and liver failure. Moreover, chronic HCV infection is associated with liver steatosis and metabolic disorders. With 130-150 million people chronically infected in the world, HCV infection represents a major public health problem. One hallmark on the virus is its close link with hepatic lipid and lipoprotein metabolism. Areas covered: HCV is associated with lipoprotein components such as apolipoproteins. These interactions play a key role in the viral life cycle, viral persistence and pathogenesis of liver disease. This review introduces first the role of apolipoproteins in lipoprotein metabolism, then highlights the molecular mechanisms of HCV-lipoprotein interactions and finally discusses their clinical impact. Expert commentary: While the study of virus-host interactions has resulted in a improvement of the understanding of the viral life cycle and the development of highly efficient therapies, major challenges remain: access to therapy is limited and an urgently needed HCV vaccine remains still elusive. Furthermore, the pathogenesis of disease biology is still only partially understood. The investigation of HCV-lipoproteins interactions offers new perspectives for novel therapeutic approaches, contribute to HCV vaccine design and understand virus-induced liver disease and cancer.

Viral effects on the content and function of extracellular vesicles

The release of membrane-bound vesicles from cells is being increasingly recognized as a mechanism of intercellular communication. Extracellular vesicles (EVs) or exosomes are produced by virus-infected cells and are thought to be involved in intercellular communication between infected and uninfected cells. Viruses, in particular oncogenic viruses and viruses that establish chronic infections, have been shown to modulate the production and content of EVs. Viral microRNAs, proteins and even entire virions can be incorporated into EVs, which can affect the immune recognition of viruses or modulate neighbouring cells. In this Review, we discuss the roles that EVs have during viral infection to either promote or restrict viral replication in target cells. We will also discuss our current understanding of the molecular mechanisms that underlie these roles, the potential consequences for the infected host and possible future diagnostic applications.

Interplay between hepatitis C virus and lipid metabolism during virus entry and assembly

Hepatitis C virus (HCV) infection is a major public health problem worldwide. In most cases, HCV infection becomes chronic, leading to the development of liver diseases that range from fibrosis to cirrhosis and hepatocellular carcinoma. Due to its medical importance, the HCV life cycle has been deeply characterized, and a unique feature of this virus is its interplay with lipids. Accordingly, all the steps of the virus life cycle are influenced by the host lipid metabolism. Indeed, due to their association with host lipoproteins, HCV particles have a unique lipid composition. Furthermore, the biogenesis pathway of very low density lipoproteins has been shown to be involved in HCV morphogenesis with apolipoprotein E being an essential element for the production of infectious HCV particles. Association of viral components with host cytoplasmic lipid droplets is also central to the HCV morphogenesis process. Finally, due to its close connection with host lipoproteins, HCV particle also uses several lipoprotein receptors to initiate its infectious cycle. In this review, we outline the way host lipoproteins participate to HCV particle composition, entry and assembly.

Attachment and Postattachment Receptors Important for Hepatitis C Virus Infection and Cell-to-Cell Transmission

Hepatitis C virus (HCV) requires multiple receptors for its attachment to and entry into cells. Our previous studies found that human syndecan-1 (SDC-1), SDC-2, and T cell immunoglobulin and mucin domain-containing protein 1 (TIM-1) are HCV attachment receptors. Other cell surface molecules, such as CD81, Claudin-1 (CLDN1), Occludin (OCLN), SR-BI, and low-density lipoprotein receptor (LDLR), function mainly at postattachment steps and are considered postattachment receptors. The underlying molecular mechanisms of different receptors in HCV cell-free and cell-to-cell transmission remain elusive. In the present study, we used a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 technology, gene-specific small interfering RNAs, and a newly developed luciferase-based reporter system to quantitatively determine the importance of individual receptors in HCV cell-free and cell-to-cell transmission. Knockouts of SDC-1 and SDC-2 resulted in remarkable reductions of HCV infection and cell attachment, whereas SDC-3 and SDC-4 knockouts did not affect HCV infection. Defective HCV attachment to SDC-1 and/or SDC-2 knockout cells was completely restored by SDC-1 and SDC-2 but not SDC-4 expression. Knockout of the attachment receptors SDC-1, SDC-2, and TIM-1 also modestly decreased HCV cell-to-cell transmission. In contrast, silencing and knockout of the postattachment receptors CD81, CLDN1, OCLN, SR-BI, and LDLR greatly impaired both HCV cell-free and cell-to-cell transmission. Additionally, apolipoprotein E was found to be important for HCV cell-to-cell spread, but very-low-density lipoprotein (VLDL)-containing mouse serum did not affect HCV cell-to-cell transmission, although it inhibited cell-free infection. These findings demonstrate that attachment receptors are essential for initial HCV binding and that postattachment receptors are important for both HCV cell-free and cell-to-cell transmission.IMPORTANCE The importance and underlying molecular mechanisms of cell surface receptors in HCV cell-free and cell-to-cell transmission are poorly understood. The role of some of the HCV attachment and postattachment receptors in HCV infection and cell-to-cell spread remains controversial. Using CRISPR-Cas9-mediated knockouts of specific cellular genes, we demonstrate that both SDC-1 and SDC-2, but not SDC-3 or SDC-4, are bona fide HCV attachment receptors. We also used a newly developed luciferase-based reporter system to quantitatively determine the importance of attachment and postattachment receptors in HCV cell-to-cell transmission. SDC-1, SDC-2, TIM-1, and SR-BI were found to modestly promote HCV cell-to-cell spread. CD81, CLDN1, OCLN, and LDLR play more important roles in HCV cell-to-cell transmission. Likewise, apolipoprotein E (apoE) is critically important for HCV cell-to-cell spread, unlike VLDL-containing mouse serum, which did not affect HCV cell-to-cell spread. These findings suggest that the mechanism(s) of HCV cell-to-cell spread differs from that of cell-free infection.

Conformational Flexibility in the Immunoglobulin-Like Domain of the Hepatitis C Virus Glycoprotein E2

The hepatitis C virus (HCV) glycoprotein E2 is the major target of neutralizing antibodies and is therefore highly relevant for vaccine design. Its structure features a central immunoglobulin (Ig)-like β-sandwich that contributes to the binding site for the cellular receptor CD81. We show that a synthetic peptide corresponding to a β-strand of this Ig-like domain forms an α-helix in complex with the anti-E2 antibody DAO5, demonstrating an inside-out flip of hydrophobic residues and a secondary structure change in the composite CD81 binding site. A detailed interaction analysis of DAO5 and cross-competing neutralizing antibodies with soluble E2 revealed that the Ig-like domain is trapped by different antibodies in at least two distinct conformations. DAO5 specifically captures retrovirus particles bearing HCV glycoproteins (HCVpp) and infectious cell culture-derived HCV particles (HCVcc). Infection of cells by DAO5-captured HCVpp can be blocked by a cross-competing neutralizing antibody, indicating that a single virus particle simultaneously displays E2 molecules in more than one conformation on its surface. Such conformational plasticity of the HCV E2 receptor binding site has important implications for immunogen design.IMPORTANCE Recent advances in the treatment of hepatitis C virus (HCV) infection with direct-acting antiviral drugs have enabled the control of this major human pathogen. However, due to their high costs and limited accessibility in combination with the lack of awareness of the mostly asymptomatic infection, there is an unchanged urgent need for an effective vaccine. The viral glycoprotein E2 contains regions that are crucial for virus entry into the host cell, and antibodies that bind to these regions can neutralize infection. One of the major targets of neutralizing antibodies is the central immunoglobulin (Ig)-like domain within E2. We show here that this Ig-like domain is conformationally flexible at the surface of infectious HCV particles and pseudoparticles. Our study provides novel insights into the interactions of HCV E2 with the humoral immune system that should aid future vaccine development.

Regulated Entry of Hepatitis C Virus into Hepatocytes

Hepatitis C virus (HCV) is a model for the study of virus–host interaction and host cell responses to infection. Virus entry into hepatocytes is the first step in the HCV life cycle, and this process requires multiple receptors working together. The scavenger receptor class B type I (SR-BI) and claudin-1 (CLDN1), together with human cluster of differentiation (CD) 81 and occludin (OCLN), constitute the minimal set of HCV entry receptors. Nevertheless, HCV entry is a complex process involving multiple host signaling pathways that form a systematic regulatory network; this network is centrally controlled by upstream regulators epidermal growth factor receptor (EGFR) and transforming growth factor β receptor (TGFβ-R). Further feedback regulation and cell-to-cell spread of the virus contribute to the chronic maintenance of HCV infection. A comprehensive and accurate disclosure of this critical process should provide insights into the viral entry mechanism, and offer new strategies for treatment regimens and targets for HCV therapeutics.

Role of cleavage at the core-E1 junction of hepatitis C virus polyprotein in viral morphogenesis

In hepatitis C virus (HCV) polyprotein sequence, core protein terminates with E1 envelope signal peptide. Cleavage by signal peptidase (SP) separates E1 from the complete form of core protein, anchored in the endoplasmic reticulum (ER) membrane by the signal peptide. Subsequent cleavage of the signal peptide by signal-peptide peptidase (SPP) releases the mature form of core protein, which preferentially relocates to lipid droplets. Both of these cleavages are required for the HCV infectious cycle, supporting the idea that HCV assembly begins at the surface of lipid droplets, yet SPP-catalyzed cleavage is dispensable for initiation of budding in the ER. Here we have addressed at what step(s) of the HCV infectious cycle SP-catalyzed cleavage at the core-E1 junction is required. Taking advantage of the sole system that has allowed visualization of HCV budding events in the ER lumen of mammalian cells, we showed that, unexpectedly, mutations abolishing this cleavage did not prevent but instead tended to promote the initiation of viral budding. Moreover, even though no viral particles were released from Huh-7 cells transfected with a full-length HCV genome bearing these mutations, intracellular viral particles containing core protein protected by a membrane envelope were formed. These were visualized by electron microscopy as capsid-containing particles with a diameter of about 70 nm and 40 nm before and after delipidation, respectively, comparable to intracellular wild-type particle precursors except that they were non-infectious. Thus, our results show that SP-catalyzed cleavage is dispensable for HCV budding per se, but is required for the viral particles to acquire their infectivity and secretion. These data support the idea that HCV assembly occurs in concert with budding at the ER membrane. Furthermore, capsid-containing particles did not accumulate in the absence of SP-catalyzed cleavage, suggesting the quality of newly formed viral particles is controlled before secretion.

Hepatitis C Virus-Genotype 3: Update on Current and Emergent Therapeutic Interventions

PURPOSE OF REVIEW

Direct-acting antiviral agents (DAAs) have markedly improved the prognosis of hepatitis C virus (HCV)-genotype 3 (GT3), a highly prevalent infection worldwide. However, in patients with hepatic fibrosis, cirrhosis, or hepatocellular carcinoma (HCC), GT3 infection presents a treatment challenge compared with other genotypes. The dependence of the HCV life cycle on host lipid metabolism suggests the possible utility of targeting host cellular factors for combination anti-HCV therapy. We discuss current and emergent DAA regimens for HCV-GT3 treatment. We then summarize recent research findings on the reliance of HCV entry, replication, and virion assembly on host lipid metabolism.

RECENT FINDINGS

Current HCV treatment guidelines recommend the use of daclatasvir plus sofosbuvir (DCV/SOF) or sofosbuvir plus velpatasvir (SOF/VEL) for the management of GT3 based upon clinical efficacy [≥88% overall sustained virological response (SVR)] and tolerability. Potential future DAA options, such as SOF/VEL co-formulated with GS-9857, also look promising in treating cirrhotic GT3 patients. However, HCV resistance to DAAs will likely continue to impact the therapeutic efficacy of interferon-free treatment regimens. Disruption of HCV entry by targeting required host cellular receptors shows potential in minimizing HCV resistance and broadening therapeutic options for certain subpopulations of GT3 patients. The use of cholesterol biosynthesis and transport inhibitors may also improve health outcomes for GT3 patients when used synergistically with DAAs. Due to the morbidity and mortality associated with HCV-GT3 infection compared to other genotypes, efforts should be made to address current limitations in the therapeutic prevention and management of HCV-GT3 infection.

Ultrastructural and biochemical basis for hepatitis C virus morphogenesis

Chronic infection with HCV is a leading cause of cirrhosis, hepatocellular carcinoma and liver failure. One of the least understood steps in the HCV life cycle is the morphogenesis of new viral particles. HCV infection alters the lipid metabolism and generates a variety of microenvironments in the cell cytoplasm that protect viral proteins and RNA promoting viral replication and assembly. Lipid droplets (LDs) have been proposed to link viral RNA synthesis and virion assembly by physically associating these viral processes. HCV assembly, envelopment, and maturation have been shown to take place at specialized detergent-resistant membranes in the ER, rich in cholesterol and sphingolipids, supporting the synthesis of luminal LDs-containing ApoE. HCV assembly involves a regulated allocation of viral and host factors to viral assembly sites. Then, virus budding takes place through encapsidation of the HCV genome and viral envelopment in the ER. Interaction of ApoE with envelope proteins supports the viral particle acquisition of lipids and maturation. HCV secretion has been suggested to entail the ion channel activity of viral p7, several components of the classical trafficking and autophagy pathways, ESCRT, and exosome-mediated export of viral RNA. Here, we review the most recent advances in virus morphogenesis and the interplay between viral and host factors required for the formation of HCV virions.

Computational Prediction of the Heterodimeric and Higher-Order Structure of gpE1/gpE2 Envelope Glycoproteins Encoded by Hepatitis C Virus

Despite the recent success of newly developed direct-acting antivirals against hepatitis C, the disease continues to be a global health threat due to the lack of diagnosis of most carriers and the high cost of treatment. The heterodimer formed by glycoproteins E1 and E2 within the hepatitis C virus (HCV) lipid envelope is a potential vaccine candidate and antiviral target. While the structure of E1/E2 has not yet been resolved, partial crystal structures of the E1 and E2 ectodomains have been determined. The unresolved parts of the structure are within the realm of what can be modeled with current computational modeling tools. Furthermore, a variety of additional experimental data is available to support computational predictions of E1/E2 structure, such as data from antibody binding studies, cryo-electron microscopy (cryo-EM), mutational analyses, peptide binding analysis, linker-scanning mutagenesis, and nuclear magnetic resonance (NMR) studies. In accordance with these rich experimental data, we have built an in silico model of the full-length E1/E2 heterodimer. Our model supports that E1/E2 assembles into a trimer, which was previously suggested from a study by Falson and coworkers (P. Falson, B. Bartosch, K. Alsaleh, B. A. Tews, A. Loquet, Y. Ciczora, L. Riva, C. Montigny, C. Montpellier, G. Duverlie, E. I. Pecheur, M. le Maire, F. L. Cosset, J. Dubuisson, and F. Penin, J. Virol. 89:10333-10346, 2015, https://doi.org/10.1128/JVI.00991-15). Size exclusion chromatography and Western blotting data obtained by using purified recombinant E1/E2 support our hypothesis. Our model suggests that during virus assembly, the trimer of E1/E2 may be further assembled into a pentamer, with 12 pentamers comprising a single HCV virion. We anticipate that this new model will provide a useful framework for HCV envelope structure and the development of antiviral strategies.IMPORTANCE One hundred fifty million people have been estimated to be infected with hepatitis C virus, and many more are at risk for infection. A better understanding of the structure of the HCV envelope, which is responsible for attachment and fusion, could aid in the development of a vaccine and/or new treatments for this disease. We draw upon computational techniques to predict a full-length model of the E1/E2 heterodimer based on the partial crystal structures of the envelope glycoproteins E1 and E2. E1/E2 has been widely studied experimentally, and this provides valuable data, which has assisted us in our modeling. Our proposed structure is used to suggest the organization of the HCV envelope. We also present new experimental data from size exclusion chromatography that support our computational prediction of a trimeric oligomeric state of E1/E2.

A Biologically-validated HCV E1E2 Heterodimer Structural Model

The design of vaccine strategies and the development of drugs targeting the early stages of Hepatitis C virus (HCV) infection are hampered by the lack of structural information about its surface glycoproteins E1 and E2, the two constituents of HCV entry machinery. Despite the recent crystal resolution of limited versions of both proteins in truncated form, a complete picture of the E1E2 complex is still missing. Here we combined deep computational analysis of E1E2 secondary, tertiary and quaternary structure with functional and immunological mutational analysis across E1E2 in order to propose an in silico model for the ectodomain of the E1E2 heterodimer. Our model describes E1-E2 ectodomain dimerization interfaces, provides a structural explanation of E1 and E2 immunogenicity and sheds light on the molecular processes and disulfide bridges isomerization underlying the conformational changes required for fusion. Comprehensive alanine mutational analysis across 553 residues of E1E2 also resulted in identifying the epitope maps of diverse mAbs and the disulfide connectivity underlying E1E2 native conformation. The predicted structure unveils E1 and E2 structures in complex, thus representing a step towards the rational design of immunogens and drugs inhibiting HCV entry.

Current therapy for chronic hepatitis C: The role of direct-acting antivirals

One of the most exciting developments in antiviral research has been the discovery of the direct-acting antivirals (DAAs) that effectively cure chronic hepatitis C virus (HCV) infections. Based on more than 100 clinical trials and real-world studies, we provide a comprehensive overview of FDA-approved therapies and newly discovered anti-HCV agents with a special focus on drug efficacy, mechanisms of action, and safety. We show that HCV drug development has advanced in multiple aspects: (i) interferon-based regimens were replaced by interferon-free regimens; (ii) genotype-specific drugs evolved to drugs for all HCV genotypes; (iii) therapies based upon multiple pills per day were simplified to a single pill per day; (iv) drug potency increased from moderate (∼60%) to high (>90%) levels of sustained virologic responses; (v) treatment durations were shortened from 48 to 12 or 8 weeks; and (vi) therapies could be administered orally regardless of prior treatment history and cirrhotic status. However, despite these remarkable achievements made in HCV drug discovery, challenges remain in the management of difficult-to-treat patients.

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HCV genotype-specific drugs evolve to pan-genotypic drugs.

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Drug potency increases from moderate (∼60%) to high (>90%) levels of sustained virologic response.

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Treatment durations are shortened from a 48-week to 12-week or 8-week period.

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HCV therapies based upon multiple pills per day are simplified to a single pill per day.

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HCV therapies are administered orally regardless of prior treatment history and cirrhotic status.

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.

Green fluorescent protein-tagged apolipoprotein E: A useful marker for the study of hepatic lipoprotein egress

Apolipoprotein E (ApoE), a component of very-low-density and high-density lipoproteins, participates in many aspects of lipid transport in the bloodstream. Underscoring its important functions, ApoE isoforms have been associated with metabolic and circulatory disease. ApoE is also incorporated into hepatitis C virus (HCV) particles, and promotes their production and infectivity. Live cell imaging analysis of ApoE behavior during secretion from producing cells thus has the potential to reveal important details regarding lipoprotein and HCV particle biogenesis and secretion from cells. However, this approach requires expression of fluorescently tagged ApoE constructs that need to faithfully reproduce known ApoE behaviors. Herein, we evaluate the usefulness of using an ApoE-GFP fusion protein in studying hepatocyte-derived, ApoE-containing lipoproteins and HCV particles. We show that while ApoE-GFP alone is not sufficient to support infectious HCV production, it nonetheless colocalizes intracellularly and associates with secreted untagged lipoprotein components. Furthermore, its rate of secretion from hepatic cells is indistinguishable from that of untagged ApoE. ApoE-GFP thus represents a useful marker for ApoE-containing hepatic lipoproteins.

Protein Interactions during the Flavivirus and Hepacivirus Life Cycle

Protein-protein interactions govern biological functions in cells, in the extracellular milieu, and at the border between cells and extracellular space. Viruses are small intracellular parasites and thus rely on protein interactions to produce progeny inside host cells and to spread from cell to cell. Usage of host proteins by viruses can have severe consequences e.g. apoptosis, metabolic disequilibria, or altered cell proliferation and mobility. Understanding protein interactions during virus infection can thus educate us on viral infection and pathogenesis mechanisms. Moreover, it has led to important clinical translations, including the development of new therapeutic and vaccination strategies. Here, we will discuss protein interactions of members of the Flaviviridae family, which are small enveloped RNA viruses. Dengue virus, Zika virus and hepatitis C virus belong to the most prominent human pathogenic Flaviviridae With a genome of roughly ten kilobases encoding only ten viral proteins, Flaviviridae display intricate mechanisms to engage the host cell machinery for their purpose. In this review, we will highlight how dengue virus, hepatitis C virus, Japanese encephalitis virus, tick-borne encephalitis virus, West Nile virus, yellow fever virus, and Zika virus proteins engage host proteins and how this knowledge helps elucidate Flaviviridae infection. We will specifically address the protein composition of the virus particle as well as the protein interactions during virus entry, replication, particle assembly, and release from the host cell. Finally, we will give a perspective on future challenges in Flaviviridae interaction proteomics and why we believe these challenges should be met.

Visualizing the Essential Role of Complete Virion Assembly Machinery in Efficient Hepatitis C Virus Cell-to-Cell Transmission by a Viral Infection-Activated Split-Intein-Mediated Reporter System

Hepatitis C virus (HCV) infects 2 to 3% of the world population and is a leading cause of liver diseases such as fibrosis, cirrhosis, and hepatocellular carcinoma. Many aspects of HCV study, ranging from molecular virology and antiviral drug development to drug resistance profiling, were supported by straightforward assays of HCV replication and infection. Among these assays, the HCV-dependent fluorescence relocalization (HDFR) system allowed live-cell visualization of infection without modifying the viral genome, but this strategy required careful recognition of the fluorescence relocalization pattern for its high fluorescence background in the cytoplasm. In this study, to achieve background-free visualization of HCV infection, a viral infection-activated split-intein-mediated reporter system (VISI) was devised. Uninfected Huh7.5.1-VISI cells show no background signal, while HCV infection specifically illuminates the nuclei of infected Huh7.5.1-VISI cells with either green fluorescent protein (GFP) or mCherry. Combining VISI-GFP and VISI-mCherry systems, we revisited HCV cell-to-cell transmission with clear-cut distinction of donor and recipient cells in a live-cell manner. Independently of virion assembly, exosomes have been reported to transfer HCV subgenomic RNA to initiate replication in uninfected cells, which suggested an assembly-free pathway. However, our data demonstrated that HCV structural genes and the p7 gene were essential for not only cell-free infectivity but also cell-to-cell transmission. Additionally, depletion of apolipoprotein E (ApoE) from donor cells but not from recipient cells significantly reduced HCV cell-to-cell transmission efficiency. In summary, we developed a background-free cell-based reporter system for convenient live-cell visualization of HCV infection, and our data indicate that complete HCV virion assembly machinery is essential for both cell-free and cell-to-cell transmission.

IMPORTANCE

Hepatitis C virus (HCV) infects hepatocytes via two pathways: cell-free infection and cell-to-cell transmission. Structural modules of the HCV genome are required for production of infectious cell-free virions; however, the role of specific genes within the structural module in cell-to-cell transmission is not clearly defined. Our data demonstrate that deletion of core, E1E2, and p7 genes individually results in no HCV cell-to-cell transmission and that ApoE knockdown from donor cells causes less-efficient cell-to-cell transmission. Thus, this work indicates that the complete HCV assembly machinery is required for HCV cell-to-cell transmission. At last, this work presents an optimized viral infection-activated split-intein-mediated reporter system for easy live-cell monitoring of HCV infection.

Native Folding of a Recombinant gpE1/gpE2 Heterodimer Vaccine Antigen from a Precursor Protein Fused with Fc IgG

A recombinant strain HCV1 (hepatitis C virus [HCV] genotype 1a) gpE1/gpE2 (E1E2) vaccine candidate was previously shown by our group to protect chimpanzees and generate broad cross-neutralizing antibodies in animals and humans. In addition, recent independent studies have highlighted the importance of conserved neutralizing epitopes in HCV vaccine development that map to antigenic clusters in E2 or the E1E2 heterodimer. E1E2 can be purified using Galanthis nivalis lectin agarose (GNA), but this technique is suboptimal for global production. Our goal was to investigate a high-affinity and scalable method for isolating E1E2. We generated an Fc tag-derived (Fc-d) E1E2 that was selectively captured by protein G Sepharose, with the tag being removed subsequently using PreScission protease. Surprisingly, despite the presence of the large Fc tag, Fc-d E1E2 formed heterodimers similar to those formed by GNA-purified wild-type (WT) E1E2 and exhibited nearly identical binding profiles to HCV monoclonal antibodies that target conserved neutralizing epitopes in E2 (HC33.4, HC84.26, and AR3B) and the E1E2 heterodimer (AR4A and AR5A). Antisera from immunized mice showed that Fc-d E1E2 elicited anti-E2 antibody titers and neutralization of HCV pseudotype viruses similar to those with WT E1E2. Competition enzyme-linked immunosorbent assays (ELISAs) showed that antisera from immunized mice inhibited monoclonal antibody binding to neutralizing epitopes. Antisera from Fc-d E1E2-immunized mice exhibited stronger competition for AR3B and AR5A than the WT, whereas the levels of competition for HC84.26 and AR4A were similar. We anticipate that Fc-d E1E2 will provide a scalable purification and manufacturing process using protein A/G-based chromatography.

IMPORTANCE

A prophylactic HCV vaccine is still needed to control this global disease despite the availability of direct-acting antivirals. Previously, we demonstrated that a recombinant envelope glycoprotein (E1E2) vaccine (genotype 1a) elicited cross-neutralizing antibodies from human volunteers. A challenge for isolating the E1E2 antigen is the reliance on GNA, which is unsuitable for large scale-up and global vaccine delivery. We have generated a novel Fc domain-tagged E1E2 antigen that forms functional heterodimers similar to those with native E1E2. Affinity purification and removal of the Fc tag from E1E2 resulted in an antigen with a nearly identical profile of cross-neutralizing epitopes. This antigen elicited anti-HCV antibodies that targeted conserved neutralizing epitopes of E1E2. Owing to the high selectivity and cost-effective binding capacity of affinity resins for capture of the Fc-tagged rE1E2, we anticipate that our method will provide a means for large-scale production of this HCV vaccine candidate.

Maximum levels of hepatitis C virus lipoviral particles are associated with early and persistent infection

BACKGROUND & AIMS

Hepatitis C virus (HCV) is bound to plasma lipoproteins and circulates as an infectious lipoviral particle (LVP). Experimental evidence indicates that LVPs have decreased susceptibility to antibody-mediated neutralisation and higher infectivity. This study tested the hypothesis that LVPs are required to establish persistent infection, and conversely, low levels of LVP in recent HCV infection increase the probability of spontaneous HCV clearance.

METHODS

LVP in non-fasting plasma was measured using the concentration of HCV RNA bound to large >100 nm sized lipoproteins after ex vivo addition of a lipid emulsion, that represented the maximum concentration of LVP (maxi-LVP). This method correlated with LVP in fasting plasma measured using iodixanol density gradient ultracentrifugation. Maxi-LVP was measured in a cohort of 180 HCV participants with recent HCV infection and detectable HCV RNA from the Australian Trial in Acute Hepatitis C (ATAHC) and Hepatitis C Incidence and Transmission Study in prison (HITS-p) cohorts.

RESULTS

Spontaneous clearance occurred in 15% (27 of 180) of individuals. In adjusted analyses, low plasma maxi-LVP level was independently associated with spontaneous HCV clearance (≤827 IU/ml; adjusted odds ratio 3.98, 95% CI: 1.02, 15.51, P = 0.047), after adjusting for interferon lambda-3 rs8099917 genotype, estimated duration of HCV infection and total HCV RNA level.

CONCLUSIONS

Maxi-LVP is a biomarker for the maximum concentration of LVP in non-fasting samples. Low maxi-LVP level is an independent predictor of spontaneous clearance of acute HCV.

Structure and Function of the Hepatitis C Virus Envelope Glycoproteins E1 and E2: Antiviral and Vaccine Targets

The hepatitis C virus (HCV) envelope glycoproteins E1 and E2 are critical in viral attachment and cell fusion, and studies of these proteins may provide valuable insights into their potential uses in vaccines and antiviral strategies. Progress has included elucidating the crystal structures of portions of their ectodomains, as well as many other studies of hypervariable regions, stem regions, glycosylation sites, and the participation of E1/E2 in viral fusion with the endosomal membrane. The available structural data have shed light on the binding sites of cross-neutralizing antibodies. A large amount of information has been discovered concerning heterodimerization, including the roles of transmembrane domains, disulfide bonding, and heptad repeat regions. The possible organization of higher order oligomers within the HCV virion has also been evaluated on the basis of experimental data. In this review, E1/E2 structure and function is discussed, and some important issues requiring further study are highlighted.

Neglected but Important Role of Apolipoprotein E Exchange in Hepatitis C Virus Infection

Hepatitis C virus (HCV) is a major cause of chronic liver disease, infecting approximately 170 million people worldwide. HCV assembly is tightly associated with the lipoprotein pathway. Exchangeable apolipoprotein E (apoE) is incorporated on infectious HCV virions and is important for infectious HCV virion morphogenesis and entry. Moreover, the virion apoE level is positively correlated with its ability to escape E2 antibody neutralization. However, the role of apoE exchange in the HCV life cycle is unclear. In this study, the relationship between apoE expression and cell permissiveness to HCV infection was assessed by infecting apoE knockdown and derived apoE rescue cell lines with HCV. Exchange of apoE between lipoproteins and HCV lipoviral particles (LVPs) was evaluated by immunoprecipitation, infectivity testing, and viral genome quantification. Cell and heparin column binding assays were applied to determine the attachment efficiency of LVPs with different levels of incorporated apoE. The results showed that cell permissiveness for HCV infection was determined by exogenous apoE-associated lipoproteins. Furthermore, apoE exchange did occur between HCV LVPs and lipoproteins, which was important to maintain a high apoE level on LVPs. Lipid-free apoE was capable of enhancing HCV infectivity for apoE knockdown cells but not apoE rescue cells. A higher apoE level on LVPs conferred more efficient LVP attachment to both the cell surface and heparin beads. This study revealed that exogenous apoE-incorporating lipoproteins from uninfected hepatocytes safeguarded the apoE level of LVPs for more efficient attachment during HCV infection.

IMPORTANCE

In this study, a neglected but important role of apoE exchange in HCV LVP infectivity after virus assembly and release was identified. The data indicated that apoE expression level in uninfected cells is important for high permissiveness to HCV infection. Secreted apoE-associated lipoprotein specifically enhances infection of HCV LVPs. apoE exchange between HCV LVP and lipoproteins is important to maintain an adequate apoE level on LVPs for their efficient attachment to cell surface. These data defined for the first time an extracellular role of exchangeable apoE in HCV infection and suggested that exchangeable apolipoproteins reach a natural equilibrium between HCV LVPs and lipoprotein particles, which provides a new perspective to the understanding of the heterogeneity of HCV LVPs in composition.

Future landscape of hepatitis C research - Basic, translational and clinical perspectives

With the latest all-oral interferon- and ribavirin-free regimens based on direct acting antivirals against the hepatitis C virus (HCV), sustained virological response rates of >90% are achieved, which is equivalent to cure. This has become possible for all genotypes and all subgroups of patients, including many of the most difficult-to-treat populations so far. Since a prophylactic HCV vaccine is not yet available, control of HCV infection will for the time being have to rely on the use of effective and safe antiviral treatments as well as their accessibility and affordability. Different approaches may apply to different parts of the world, eradication of HCV representing a major long-term goal. Whether hepatitis C becomes the first chronic viral infection to be eradicated without a prophylactic vaccine remains to be shown. Here, we briefly summarize advances in the molecular virology of hepatitis C, highlight lessons of biological relevance that were learned through the study of HCV, and its translational and clinical implications. We have also listed selected unsolved challenges, emphasizing that HCV is a unique model and that advances in this direction may yield knowledge of broad biological significance, novel technologies and insights into related important human pathogens.

Hepatitis C virus resistance to broadly neutralizing antibodies measured using replication-competent virus and pseudoparticles

A better understanding of natural variation in neutralization resistance and fitness of diverse hepatitis C virus (HCV) envelope (E1E2) variants will be critical to guide rational development of an HCV vaccine. This work has been hindered by inadequate genetic diversity in viral panels and by a lack of standardization of HCV entry assays. Neutralization assays generally use lentiviral pseudoparticles expressing HCV envelope proteins (HCVpp) or chimeric full-length viruses that are replication competent in cell culture (HCVcc). There have been few systematic comparisons of specific infectivities of E1E2-matched HCVcc and HCVpp, and to our knowledge, neutralization of E1E2-matched HCVpp and HCVcc has never been compared using a diverse panel of human broadly neutralizing monoclonal antibodies (bNAbs) targeting distinct epitopes. Here, we describe an efficient method for introduction of naturally occurring E1E2 genes into a full-length HCV genome, producing replication-competent chimeric HCVcc. We generated diverse panels of E1E2-matched HCVcc and HCVpp and measured the entry-mediating fitness of E1E2 variants using the two systems. We also compared neutralization of E1E2-matched HCVcc and HCVpp by a diverse panel of human bNAbs targeting epitopes across E1E2. We found no correlation between specific infectivities of E1E2-matched HCVcc versus HCVpp, but found a very strong positive correlation between relative neutralization resistance of these same E1E2-matched HCVcc and HCVpp variants. These results suggest that quantitative comparisons of neutralization resistance of E1E2 variants can be made with confidence using either HCVcc or HCVpp, allowing the use of either or both systems to maximize diversity of neutralization panels.

Characterization of a hepatitis C virus-like particle vaccine produced in a human hepatocyte-derived cell line

An effective immune response against hepatitis C virus (HCV) requires the early development of multi-specific class 1 CD8+ and class II CD4+ T-cells together with broad neutralizing antibody responses. We have produced mammalian-cell-derived HCV virus-like particles (VLPs) incorporating core, E1 and E2 of HCV genotype 1a to produce such immune responses. Here we describe the biochemical and morphological characterization of the HCV VLPs and study HCV core-specific T-cell responses to the particles. The E1 and E2 glycoproteins in HCV VLPs formed non-covalent heterodimers and together with core protein assembled into VLPs with a buoyant density of 1.22 to 1.28 g cm-3. The HCV VLPs could be immunoprecipited with anti-ApoE and anti-ApoC. On electron microscopy, the VLPs had a heterogeneous morphology and ranged in size from 40 to 80 nm. The HCV VLPs demonstrated dose-dependent binding to murine-derived dendritic cells and the entry of HCV VLPs into Huh7 cells was blocked by anti-CD81 antibody. Vaccination of BALB/c mice with HCV VLPs purified from iodixanol gradients resulted in the production of neutralizing antibody responses while vaccination of humanized MHC class I transgenic mice resulted in the prodution of HCV core-specific CD8+ T-cell responses. Furthermore, IgG purified from the sera of patients chronically infected with HCV genotypes 1a and 3a blocked the binding and entry of the HCV VLPs into Huh7 cells. These results show that our mammalian-cell-derived HCV VLPs induce humoral and HCV-specific CD8+ T-cell responses and will have important implications for the development of a preventative vaccine for HCV.

Release of Infectious Hepatitis C Virus from Huh7 Cells Occurs via a trans-Golgi Network-to-Endosome Pathway Independent of Very-Low-Density Lipoprotein Secretion

The release of infectious hepatitis C virus (HCV) particles from infected cells remains poorly characterized. We previously demonstrated that virus release is dependent on the endosomal sorting complex required for transport (ESCRT). Here, we show a critical role of trans-Golgi network (TGN)-endosome trafficking during the assembly, but principally the secretion, of infectious virus. This was demonstrated by both small interfering RNA (siRNA)-mediated silencing of TGN-associated adaptor proteins and a panel of dominant negative (DN) Rab GTPases involved in TGN-endosome trafficking steps. Importantly, interfering with factors critical for HCV release did not have a concomitant effect on secretion of triglycerides, ApoB, or ApoE, indicating that particles are likely released from Huh7 cells via pathways distinct from that of very-low-density lipoprotein (VLDL). Finally, we show that HCV NS2 perturbs TGN architecture, redistributing TGN membranes to closely associate with HCV core protein residing on lipid droplets. These findings support the notion that HCV hijacks TGN-endosome trafficking to facilitate particle assembly and release. Moreover, although essential for assembly and infectivity, the trafficking of mature virions is seemingly independent of host lipoproteins.

IMPORTANCE The mechanisms by which infectious hepatitis C virus particles are assembled and released from the cell are poorly understood. We show that the virus subverts host cell trafficking pathways to effect the release of virus particles and disrupts the structure of the Golgi apparatus, a key cellular organelle involved in secretion. In addition, we demonstrate that the mechanisms used by the virus to exit the cell are distinct from those used by the cell to release lipoproteins, suggesting that the virus effects a unique modification to cellular trafficking pathways.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Opportunistic intruders: how viruses orchestrate ER functions to infect cells

Viruses exploit the functions of the endoplasmic reticulum (ER) to promote both early and later stages of their life cycle, including entry, translation, replication, assembly, morphogenesis and egress. This observation reveals a shared principle that underlies virus–host cell relationships.

Viral entry often requires disassembly of the incoming virus particle. This is best exemplified in the case of polyomavirus entry, in which ER-associated machineries are hijacked to disassemble the virus and promote entry to the cytosol en route to the nucleus.

Many enveloped viruses, such as HIV and influenza virus, co-opt the ER-associated protein biosynthetic machinery to translate their genome and produce structural proteins that are necessary for the formation of virus particles and non-structural proteins that are essential during genome replication.

Replication of the viral genome, particularly for positive-sense RNA ((+)RNA) viruses including hepatitis C virus (HCV), dengue virus (DENV) and West Nile virus (WNV), occurs in virus-induced membranous structures that are most often derived from the ER. The formation of these structures requires morphological changes to the ER membrane, involving membrane rearrangements that are induced by viral non-structural proteins that are targeted to the ER.

As virus assembly is often coupled to genome replication, the assembly process frequently relies on the ER membrane. This strategy is seen for both RNA and DNA viruses.

Morphogenesis of assembled virus particles can also take advantage of the ER. This is best observed in the non-enveloped rotavirus, for which a transient enveloped intermediate is converted to the mature and infectious particle in the lumen of the ER.

After maturation in the ER, progeny virus particles egress the host through the ER-dependent secretory pathway, which provides a physical conduit to the extracellular environment.

The overall observations that the ER actively promotes all steps of viral infection have therapeutic implications. The development of chemical inhibitors of selective ER-associated components is emerging as a potential avenue of antiviral therapy, provided that these inhibitors have minimal toxicity to the host cell.

Many host structures are vital for viral infection and the endoplasmic reticulum (ER), in particular, is essential. In this Review, Tsai and colleagues highlight examples of subversion of the ER by diverse viruses to promote all stages of their life cycle, from entry to egress.

Viruses subvert the functions of their host cells to replicate and form new viral progeny. The endoplasmic reticulum (ER) has been identified as a central organelle that governs the intracellular interplay between viruses and hosts. In this Review, we analyse how viruses from vastly different families converge on this unique intracellular organelle during infection, co-opting some of the endogenous functions of the ER to promote distinct steps of the viral life cycle from entry and replication to assembly and egress. The ER can act as the common denominator during infection for diverse virus families, thereby providing a shared principle that underlies the apparent complexity of relationships between viruses and host cells. As a plethora of information illuminating the molecular and cellular basis of virus–ER interactions has become available, these insights may lead to the development of crucial therapeutic agents.

Extracellular Vesicles Exploit Viral Entry Routes for Cargo Delivery

Extracellular vesicles (EVs) have emerged as crucial mediators of intercellular communication, being involved in a wide array of key biological processes. Eukaryotic cells, and also bacteria, actively release heterogeneous subtypes of EVs into the extracellular space, where their contents reflect their (sub)cellular origin and the physiologic state of the parent cell. Within the past 20 years, presumed subtypes of EVs have been given a rather confusing diversity of names, including exosomes, microvesicles, ectosomes, microparticles, virosomes, virus-like particles, and oncosomes, and these names are variously defined by biogenesis, physical characteristics, or function. The latter category, functions, in particular the transmission of biological signals between cells in vivo and how EVs control biological processes, has garnered much interest. EVs have pathophysiological properties in cancer, neurodegenerative disorders, infectious disease, and cardiovascular disease, highlighting possibilities not only for minimally invasive diagnostic applications but also for therapeutic interventions, like macromolecular drug delivery. Yet, in order to pursue therapies involving EVs and delivering their cargo, a better grasp of EV targeting is needed. Here, we review recent progress in understanding the molecular mechanisms underpinning EV uptake by receptor-ligand interactions with recipient cells, highlighting once again the overlap of EVs and viruses. Despite their highly heterogeneous nature, EVs require common viral entry pathways, and an unanticipated specificity for cargo delivery is being revealed. We discuss the challenges ahead in delineating specific roles for EV-associated ligands and cellular receptors.

Roles of human apolipoprotein E in the infectivity and replication of hepatitis C virus genotype 2a

Hepatitis C virus (HCV) infection is associated with lipoproteins, and apolipoprotein E (apoE) plays an essential role in infectious HCV particles. Although the role of apoE in HCV infection is well known, its role in the replication of HCV remains unclear. The aims of this study were to determine the role of apoE in the RNA replication of major HCV genotypes 1b and 2a, and to determine whether this role is HCVgenotype-dependent using HCV genotype 1b replicon cells and HCV genotype 2a producing (HP) cells. HCV infection was blocked in Huh7.5 cells treated with low-density lipoproteins, very low-density lipoproteins, or apoE3. An apoE3-specific monoclonal antibody also efficiently neutralized HCV infectivity, and HCV infection was dramatically suppressed by the knockdown of apoE expression with an apoE-specific small interfering RNA, suggesting a requirement for apoE in infectious HCV particles. HCV RNA replication was not affected in HP cells treated with each apoE isoform or transfected with apoE-specific siRNAs. However, the knockdown of apoE expression suppressed RNA replication of HCV genotype 1b. The siRNA-mediated knockdown of apoE, apoA1, and apoB expression also suppressed the RNA replication of HCV genotype 1b, but not that of HCV genotype 2a. Taken together, these findings indicate that apoE plays an important role in HCV genotype 2a infection and in HCV genotype 1b RNA replication, but not in the replication of HCV genotype 2a. These results provide important information for the future development of HCV-genotypespecific anti-HCV agents.

Lipoprotein Receptors Redundantly Participate in Entry of Hepatitis C Virus

Scavenger receptor class B type 1 (SR-B1) and low-density lipoprotein receptor (LDLR) are known to be involved in entry of hepatitis C virus (HCV), but their precise roles and their interplay are not fully understood. In this study, deficiency of both SR-B1 and LDLR in Huh7 cells was shown to impair the entry of HCV more strongly than deficiency of either SR-B1 or LDLR alone. In addition, exogenous expression of not only SR-B1 and LDLR but also very low-density lipoprotein receptor (VLDLR) rescued HCV entry in the SR-B1 and LDLR double-knockout cells, suggesting that VLDLR has similar roles in HCV entry. VLDLR is a lipoprotein receptor, but the level of its hepatic expression was lower than those of SR-B1 and LDLR. Moreover, expression of mutant lipoprotein receptors incapable of binding to or uptake of lipid resulted in no or slight enhancement of HCV entry in the double-knockout cells, suggesting that binding and/or uptake activities of lipid by lipoprotein receptors are essential for HCV entry. In addition, rescue of infectivity in the double-knockout cells by the expression of the lipoprotein receptors was not observed following infection with pseudotype particles bearing HCV envelope proteins produced in non-hepatic cells, suggesting that lipoproteins associated with HCV particles participate in the entry through their interaction with lipoprotein receptors. Buoyant density gradient analysis revealed that HCV utilizes these lipoprotein receptors in a manner dependent on the lipoproteins associated with HCV particles. Collectively, these results suggest that lipoprotein receptors redundantly participate in the entry of HCV.

Hepatitis C virus (HCV) utilizes several receptors to enter hepatocytes, including scavenger receptor class B type 1 (SR-B1) receptor and low-density lipoprotein receptor (LDLR). HCV particles interact with lipoprotein and apolipoproteins to form complexes termed lipoviroparticles. Several reports have shown that SR-B1 and LDLR participate in the entry of lipoviroparticles through interaction with lipoproteins. However, the precise roles of SR-B1 and LDLR in HCV entry have not been fully clarified. In this study, we showed that SR-B1 and LDLR have a redundant role in HCV entry. In addition, we showed that very low-density lipoprotein receptor (VLDLR) played a role in HCV entry similar to the roles of SR-B1 and LDLR. Interestingly, VLDLR expression was low in the liver in contrast to the abundant expressions of SR-B1 and LDLR, but high in several extrahepatic tissues. Our data suggest that lipoprotein receptors participate in the entry of HCV particles associated with various lipoproteins.

The expanding toolbox for hepatitis C virus research

Hepatitis C virus is a major global health concern with 170 million people chronically infected. Despite the availability of potent antiviral agents targeting multiple HCV proteins and cure rates above 90%, global treatment availability, the likelihood of emerging drug-resistant viral variants and the unavailability of a protective vaccine underline the many unresolved questions remaining to be answered. Model systems allowing the dissection of individual HCV life cycle steps have previously been developed and span noninfectious and infectious means of assessing HCV entry and replication, multiple cellular systems enabling host/pathogen interaction studies as well as in vivo model systems for basic as well as translational HCV research. This review provides an overview of available systems and a comparative summary of assays and models.