Hepatitis C virus glycoproteins mediate low pH-dependent membrane fusion with liposomes

Respiratory syncytial virus fuses with plasma membrane to infect primary cultures of bronchial epithelial cells

Background

Respiratory syncytial virus (RSV) is a common cause of bronchiolitis in children under the age of five. RSV infection proceeds by fusion of the viral envelope with the target cell membrane, but it is unclear whether fusion occurs with plasma or endosomal membranes.

Methods

Entry and/or infection was studied in undifferentiated primary cultures of human bronchial epithelial cells. Synchronization of viral entry or infection was achieved by attaching the virus to the plasma membrane at temperatures of 4°C or 22°C. Cells in which entry events had occurred were identified by the enzymatic action of beta-lactamase M (BlaM) fused to the RSV P protein (BlaM-P) carried by rgRSV virions. BlaM cleaves the beta-lactam ring of CCF2 loaded into the cells, disrupting FRET and allowing blue light to be emitted. Green fluorescent protein (GFP) expression, encoded by the rgRSV genome, was used to identify infected cells.

Results

We found that adsorption of RSV at 4°C favors entry via endocytosis, whereas binding of the virus to the membrane at 22°C favors RSV entry via the plasma membrane. The induction of endocytosis by synchronization at 4°C is, therefore, an artifact. In addition, we found that all drugs that interfered with RSV infection reduced cell membrane deformations such as filopodia and lamellipodia, suggesting a mechanism by which they may interfere with RSV fusion with the cell membrane.

Discussion

In conclusion, RSV enters the cell by direct fusion of its envelope with the plasma membrane.

Low-density hepatitis C virus infectious particles are protected from oxidation by secreted cellular proteins

IMPORTANCE

Assessments of viral stability on surfaces or in body fluids under different environmental conditions and/or temperatures are often performed, as they are key to understanding the routes and parameters of viral transmission and to providing clues on the epidemiology of infections. However, for most viruses, the mechanisms of inactivation vs stability of viral particles remain poorly defined. Although they are structurally diverse, with different compositions, sizes, and shapes, enveloped viruses are generally less stable than non-enveloped viruses, pointing out the role of envelopes themselves in virus lability. In this report, we investigated the properties of hepatitis C virus (HCV) particles with regards to their stability. We found that, compared to alternative enveloped viruses such as Dengue virus (DENV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), hepatitis delta virus (HDV), and Crimean-Congo hemorrhagic fever virus (CCHFV) that infect the liver, HCV particles are intrinsically labile. We determined the mechanisms that drastically alter their specific infectivity through oxidation of their lipids, and we highlighted that they are protected from lipid oxidation by secreted cellular proteins, which can protect their membrane fusion capacity and overall infectivity.

Harnessing the Therapeutic Potential and Biological Activity of Antiviral Peptides

Peptides are ideal candidates for the development of antiviral therapeutics due to their specificity, chemical diversity and potential for highly potent, safe, molecular interventions. By restricting conformational freedom and flexibility, cyclic peptides frequently increase peptide stability. Viral targets are often very challenging as their evasive strategies for infectivity can preclude standard therapies. In recent years, several peptides from natural sources mitigated an array of viral infections. In parallel, short peptides derived from key viral proteins, modified with chemical groups such as lipids and cell-penetrating sequences, led to highly effective antiviral inhibitor designs. These strategies have been further developed during the recent COVID-19 pandemic caused by the novel coronavirus SARS-CoV-2. Several anti-SARS-CoV-2 peptides are gaining ground in pre-clinical development. Overall, peptides are strong contenders for lead compounds against many life-threatening viruses and may prove to be the key to future efforts revealing viral mechanisms of action and alleviating their effects.

Viral Hepatitis, Cholesterol Metabolism, and Cholesterol-Lowering Natural Compounds

Hepatitis is defined as inflammation of the liver; it can be acute or chronic. In chronic cases, the prolonged inflammation gradually damages the liver, resulting in liver fibrosis, cirrhosis, and sometimes liver failure or cancer. Hepatitis is often caused by viral infections. The most common causes of viral hepatitis are the five hepatitis viruses—hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), and hepatitis E virus (HEV). While HAV and HEV rarely (or do not) cause chronic hepatitis, a considerable proportion of acute hepatitis cases caused by HBV (sometimes co-infected with HDV) and HCV infections become chronic. Thus, many medical researchers have focused on the treatment of HBV and HCV. It has been documented that host lipid metabolism, particularly cholesterol metabolism, is required for the hepatitis viral infection and life cycle. Thus, manipulating host cholesterol metabolism-related genes and proteins is a strategy used in fighting the viral infections. Efforts have been made to evaluate the efficacy of cholesterol-lowering drugs, particularly 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, in the treatment of hepatitis viral infections; promising results have been obtained. This review provides information on the relationships between hepatitis viruses and host cholesterol metabolism/homeostasis, as well as the discovery/development of cholesterol-lowering natural phytochemicals that could potentially be applied in the treatment of viral hepatitis.

A fusion peptide in preS1 and the human protein disulfide isomerase ERp57 are involved in hepatitis B virus membrane fusion process

Cell entry of enveloped viruses relies on the fusion between the viral and plasma or endosomal membranes, through a mechanism that is triggered by a cellular signal. Here we used a combination of computational and experimental approaches to unravel the main determinants of hepatitis B virus (HBV) membrane fusion process. We discovered that ERp57 is a host factor critically involved in triggering HBV fusion and infection. Then, through modeling approaches, we uncovered a putative allosteric cross-strand disulfide (CSD) bond in the HBV S glycoprotein and we demonstrate that its stabilization could prevent membrane fusion. Finally, we identified and characterized a potential fusion peptide in the preS1 domain of the HBV L glycoprotein. These results underscore a membrane fusion mechanism that could be triggered by ERp57, allowing a thiol/disulfide exchange reaction to occur and regulate isomerization of a critical CSD, which ultimately leads to the exposition of the fusion peptide.

Chlorcyclizine Inhibits Viral Fusion of Hepatitis C Virus Entry by Directly Targeting HCV Envelope Glycoprotein 1

Chlorcyclizine (CCZ) is a potent hepatitis C virus (HCV) entry inhibitor, but its molecular mechanism is unknown. Here, we show that CCZ directly targets the fusion peptide of HCV E1 and interferes with the fusion process. Generation of CCZ resistance-associated substitutions of HCV in vitro revealed six missense mutations in the HCV E1 protein, five being in the putative fusion peptide. A viral fusion assay demonstrated that CCZ blocked HCV entry at the membrane fusion step and that the mutant viruses acquired resistance to CCZ's action in blocking membrane fusion. UV cross-linking of photoactivatable CCZ-diazirine-biotin in both HCV-infected cells and recombinant HCV E1/E2 protein demonstrated direct binding to HCV E1 glycoprotein. Mass spectrometry analysis revealed that CCZ cross-linked to an E1 sequence adjacent to the putative fusion peptide. Docking simulations demonstrate a putative binding model, wherein CCZ binds to a hydrophobic pocket of HCV E1 and forms extensive interactions with the fusion peptide.

Hepatitis C Virus Entry: Protein Interactions and Fusion Determinants Governing Productive Hepatocyte Invasion

Hepatitis C virus (HCV) entry is among the best-studied uptake processes for human pathogenic viruses. Uptake follows a spatially and temporally tightly controlled program. Numerous host factors including proteins, lipids, and glycans promote productive uptake of HCV particles into human liver cells. The virus initially attaches to surface proteoglycans, lipid receptors such as the scavenger receptor BI (SR-BI), and to the tetraspanin CD81. After lateral translocation of virions to tight junctions, claudin-1 (CLDN1) and occludin (OCLN) are essential for entry. Clathrin-mediated endocytosis engulfs HCV particles, which fuse with endosomal membranes after pH drop. Uncoating of the viral RNA genome in the cytoplasm completes the entry process. Here we systematically review and classify HCV entry factors by their mechanistic role, relevance, and level of evidence. Finally, we report on more recent knowledge on determinants of membrane fusion and close with an outlook on future implications of HCV entry research.

Innovative particle standards and long-lived imaging for 2D and 3D dSTORM

Direct stochastic optical reconstruction microscopy (dSTORM), developed in the last decade, has revolutionised optical microscopy by enabling scientists to visualise objects beyond the resolution provided by conventional microscopy (200 nm). We developed an innovative method based on blinking particle standards and conditions for long-lived imaging over several weeks. Stable localisation precisions within the 10 nm-range were achieved for single virions and in cellulo 2D imaging of centrosomes, as well as their reliable reconstruction in 3D dSTORM.

A central hydrophobic E1 region controls the pH range of hepatitis C virus membrane fusion and susceptibility to fusion inhibitors

BACKGROUND & AIMS

Hepatitis C virus (HCV) infection causes chronic liver disease. Antivirals have been developed and cure infection. However, resistance can emerge and salvage therapies with alternative modes of action could be useful. Several licensed drugs have emerged as HCV entry inhibitors and are thus candidates for drug repurposing. We aimed to dissect their mode of action, identify improved derivatives and determine their viral targets.

METHODS

HCV entry inhibition was tested for a panel of structurally related compounds, using chimeric viruses representing diverse genotypes, in addition to viruses containing previously determined resistance mutations. Chemical modeling and synthesis identified improved derivatives, while generation of susceptible and non-susceptible chimeric viruses pinpointed E1 determinants of compound sensitivity.

RESULTS

Molecules of the diphenylpiperazine, diphenylpiperidine, phenothiazine, thioxanthene, and cycloheptenepiperidine chemotypes inhibit HCV infection by interfering with membrane fusion. These molecules and a novel p-methoxy-flunarizine derivative with improved efficacy preferentially inhibit genotype 2 viral strains. Viral residues within a central hydrophobic region of E1 (residues 290-312) control susceptibility. At the same time, viral features in this region also govern pH-dependence of viral membrane fusion.

CONCLUSIONS

Small molecules from different chemotypes related to flunarizine preferentially inhibit HCV genotype 2 membrane fusion. A hydrophobic region proximal to the putative fusion loop controls sensitivity to these drugs and the pH range of membrane fusion. An algorithm considering viral features in this region predicts viral sensitivity to membrane fusion inhibitors. Resistance to flunarizine correlates with more relaxed pH requirements for fusion.

LAY SUMMARY

This study describes diverse compounds that act as HCV membrane fusion inhibitors. It defines viral properties that determine sensitivity to these molecules and thus provides information to identify patients that may benefit from treatment with membrane fusion inhibitors.

Functional expression and characterization of the envelope glycoprotein E1E2 heterodimer of hepatitis C virus

Hepatitis C virus (HCV) is a member of Hepacivirus and belongs to the family of Flaviviridae. HCV infects millions of people worldwide and may lead to cirrhosis and hepatocellular carcinoma. HCV envelope proteins, E1 and E2, play critical roles in viral cell entry and act as major epitopes for neutralizing antibodies. However, unlike other known flaviviruses, it has been challenging to study HCV envelope proteins E1E2 in the past decades as the in vitro expressed E1E2 heterodimers are usually of poor quality, making the structural and functional characterization difficult. Here we express the ectodomains of HCV E1E2 heterodimer with either an Fc-tag or a de novo designed heterodimeric tag and are able to isolate soluble E1E2 heterodimer suitable for functional and structural studies. Then we characterize the E1E2 heterodimer by electron microscopy and model the structure by the coevolution based modeling strategy with Rosetta, revealing the potential interactions between E1 and E2. Moreover, the E1E2 heterodimer is applied to examine the interactions with the known HCV receptors, neutralizing antibodies as well as the inhibition of HCV infection, confirming the functionality of the E1E2 heterodimer and the binding profiles of E1E2 with the cellular receptors. Therefore, the expressed E1E2 heterodimer would be a valuable target for both viral studies and vaccination against HCV.

Hepatitis C virus (HCV) is an enveloped virus that infects millions of people worldwide and may lead to cirrhosis and hepatocellular carcinoma. HCV has two envelope proteins, E1 and E2, which form heterodimers on viral surface and are critical for HCV cell entry. However, current studies of HCV E1E2 are often limited by the poor quality of the in vitro expressed E1E2 heterodimers. Here we express the ectodomains of HCV E1E2 with different tags, and are able to isolate soluble E1E2 ectodomains suitable for structural and functional studies. Then we generate the 3D reconstruction of E1E2 heterodimer by electron microscopy and also model the E1E2 structure by the coevolution based strategy with Rosetta, showing the potential interactions between E1 and E2. Moreover, the E1E2 heterodimer is applied to examine the interactions with the HCV cellular receptors, neutralizing antibodies as well as the inhibition of HCV infection. These results suggest that the expressed E1E2 heterodimer would be a promising target for both viral studies and vaccination against HCV.

Membrane Fusion Assays for Studying Entry Hepatitis C Virus into Cells

The membrane fusion properties of HCV envelope glycoproteins can be evaluated using several assays. Fusion assays generally require contacts between glycoproteins expressed on a donor membrane, such as those from a cell or a viral particle, and an acceptor membrane that may or may not express cognate viral receptor, such as those from an indicator cell or a liposome. In this chapter, we describe three well-established methods in the field that use either cell surface expression of glycoproteins, HCV pseudoparticles (HCVpp), or cell culture-grown HCV (HCVcc) particles for donor membrane and cells or liposomes as acceptor membrane in which specific components can be included to monitor and quantify fusion. We provide details of cell-cell fusion assay, virus-liposome fusion assay, and finally virus-plasma membrane fusion assay. We also describe inhibitors that can block HCV envelope membrane fusion.

In vitro Membrane Interaction and Liposome Fusion Assays Using Recombinant Hepatitis C Virus Envelope Protein E2

In order to study the mechanism underlying the Hepatitis C Virus (HCV) fusion process we have performed assays using phospholipid liposomes and a truncated form of E2 protein, E2661 (amino acids 384-661 of the HCV polyprotein) lacking the transmembrane region. E2661 has been previously generated by using the baculovirus expression system. This form has been used in lipid-protein interaction studies with different model vesicles at different pHs, and monitored using a variety of fluorescent assays. After the analysis of the results, we observed that E2661 is able to insert into lipid bilayers and to induce vesicle aggregation, lipid mixing and liposome leakage, showing higher values of membrane destabilization for negatively charged phospholipids at acidic pH. This is indicative of the role of E2 glycoprotein in the HCV initial infective steps, interacting with the target membranes and producing their destabilization.

A protein coevolution method uncovers critical features of the Hepatitis C Virus fusion mechanism

Amino-acid coevolution can be referred to mutational compensatory patterns preserving the function of a protein. Viral envelope glycoproteins, which mediate entry of enveloped viruses into their host cells, are shaped by coevolution signals that confer to viruses the plasticity to evade neutralizing antibodies without altering viral entry mechanisms. The functions and structures of the two envelope glycoproteins of the Hepatitis C Virus (HCV), E1 and E2, are poorly described. Especially, how these two proteins mediate the HCV fusion process between the viral and the cell membrane remains elusive. Here, as a proof of concept, we aimed to take advantage of an original coevolution method recently developed to shed light on the HCV fusion mechanism. When first applied to the well-characterized Dengue Virus (DENV) envelope glycoproteins, coevolution analysis was able to predict important structural features and rearrangements of these viral protein complexes. When applied to HCV E1E2, computational coevolution analysis predicted that E1 and E2 refold interdependently during fusion through rearrangements of the E2 Back Layer (BL). Consistently, a soluble BL-derived polypeptide inhibited HCV infection of hepatoma cell lines, primary human hepatocytes and humanized liver mice. We showed that this polypeptide specifically inhibited HCV fusogenic rearrangements, hence supporting the critical role of this domain during HCV fusion. By combining coevolution analysis and in vitro assays, we also uncovered functionally-significant coevolving signals between E1 and E2 BL/Stem regions that govern HCV fusion, demonstrating the accuracy of our coevolution predictions. Altogether, our work shed light on important structural features of the HCV fusion mechanism and contributes to advance our functional understanding of this process. This study also provides an important proof of concept that coevolution can be employed to explore viral protein mediated-processes, and can guide the development of innovative translational strategies against challenging human-tropic viruses.

Several virus-mediated molecular processes remain poorly described, which dampen the development of potent anti-viral therapies. Hence, new experimental strategies need to be undertaken to improve and accelerate our understanding of these processes. Here, as a proof of concept, we employ amino-acid coevolution as a tool to gain insights into the structural rearrangements of Hepatitis C Virus (HCV) envelope glycoproteins E1 and E2 during virus fusion with the cell membrane, and provide a basis for the inhibition of this process. Our coevolution analysis predicted that a specific domain of E2, the Back Layer (BL) is involved into significant conformational changes with E1 during the fusion of the HCV membrane with the cellular membrane. Consistently, a recombinant, soluble form of the BL was able to inhibit E1E2 fusogenic rearrangements and HCV infection. Moreover, predicted coevolution networks involving E1 and BL residues, as well as E1 and BL-adjacent residues, were found to modulate virus fusion. Our data shows that coevolution analysis is a powerful and underused approach that can provide significant insights into the functions and structural rearrangements of viral proteins. Importantly, this approach can also provide structural and molecular basis for the design of effective anti-viral drugs, and opens new perspectives to rapidly identify effective antiviral strategies against emerging and re-emerging viral pathogens.

Functional Analysis of Hepatitis C Virus (HCV) Envelope Protein E1 Using a trans-Complementation System Reveals a Dual Role of a Putative Fusion Peptide of E1 in both HCV Entry and Morphogenesis

Hepatitis C virus (HCV) is an enveloped RNA virus belonging to the Flaviviridae family. It infects mainly human hepatocytes and causes chronic liver diseases, including cirrhosis and cancer. HCV encodes two envelope proteins, E1 and E2, that form a heterodimer and mediate virus entry. While E2 has been extensively studied, less has been done so for E1, and its role in the HCV life cycle still needs to be elucidated. Here we developed a new cell culture model for HCV infection based on the trans-complementation of E1. Virus production of the HCV genome lacking the E1-encoding sequence can be efficiently rescued by the ectopic expression of E1 in trans The resulting virus, designated HCVΔE1, can propagate in packaging cells expressing E1 but results in only single-cycle infection in naive cells. By using the HCVΔE1 system, we explored the role of a putative fusion peptide (FP) of E1 in HCV infection. Interestingly, we found that the FP not only contributes to HCV entry, as previously reported, but also may be involved in virus morphogenesis. Finally, we identified amino acid residues in FP that are critical for biological functions of E1. In summary, our work not only provides a new cell culture model for studying HCV but also provides some insights into understanding the role of E1 in the HCV life cycle.IMPORTANCE Hepatitis C virus (HCV), an enveloped RNA virus, encodes two envelope proteins, E1 and E2, that form a heterodimeric complex to mediate virus entry. Compared to E2, the biological functions of E1 in the virus life cycle are not adequately investigated. Here we developed a new cell culture model for single-cycle HCV infection based on the trans-complementation of E1. The HCV genome lacking the E1-encoding sequence can be efficiently rescued for virus production by the ectopic expression of E1 in trans This new model renders a unique system to dissect functional domains and motifs in E1. Using this system, we found that a putative fusion peptide in E1 is a multifunctional structural element contributing to both HCV entry and morphogenesis. Our work has provided a new cell culture model to study HCV and provides insights into understanding the biological roles of E1 in the HCV life cycle.

Claudins in viral infection: from entry to spread

Tight junctions are critically important for many physiological functions, including the maintenance of cell polarity, regulation of paracellular permeability, and involvement in signal transduction pathways to regulate integral cellular processes. Furthermore, tight junctions enable epithelial cells to form physical barriers, which act as an innate immune mechanism that can impede viral infection. Viruses, in turn, have evolved mechanisms to exploit tight junction proteins to gain access to cells or spread through tissues in an infected host. Claudin family proteins are integral components of tight junctions and are thought to play crucial roles in regulating their permeability. Claudins have been implicated in the infection process of several medically important human pathogens, including hepatitis C virus, dengue virus, West Nile virus, and human immunodeficiency virus, among others. In this review, we summarize the role of claudins in viral infections and discuss their potential as novel antiviral targets. A better understanding of claudins during viral infection may provide insight into physiological roles of claudins and uncover novel therapeutic antiviral strategies.

The viral capping enzyme nsP1: a novel target for the inhibition of chikungunya virus infection

The chikungunya virus (CHIKV) has become a substantial global health threat due to its massive re-emergence, the considerable disease burden and the lack of vaccines or therapeutics. We discovered a novel class of small molecules ([1,2,3]triazolo[4,5-d]pyrimidin-7(6H)-ones) with potent in vitro activity against CHIKV isolates from different geographical regions. Drug-resistant variants were selected and these carried a P34S substitution in non-structural protein 1 (nsP1), the main enzyme involved in alphavirus RNA capping. Biochemical assays using nsP1 of the related Venezuelan equine encephalitis virus revealed that the compounds specifically inhibit the guanylylation of nsP1. This is, to the best of our knowledge, the first report demonstrating that the alphavirus capping machinery is an excellent antiviral drug target. Considering the lack of options to treat CHIKV infections, this series of compounds with their unique (alphavirus-specific) target offers promise for the development of therapy for CHIKV infections.

Far-Red Fluorescent Lipid-Polymer Probes for an Efficient Labeling of Enveloped Viruses

Far-red emitting fluorescent lipid probes are desirable to label enveloped viruses, for their efficient tracking by optical microscopy inside autofluorescent cells. Most used probes are rapidly released from membranes, leading to fluorescence signal decay and loss of contrast. Here, water-soluble lipid-polymer probes are synthesized harboring hydrophilic or hydrophobic far-red emitting dyes, and exhibiting enhanced brightness. They efficiently label Hepatitis C Virus pseudotyped particles (HCVpp), more stably and reproducibly than commercial probes, and a strong fluorescence signal is observed with a high contrast. Labeling with such probes do not alter virion morphology, integrity, nor infectivity. Finally, it is shown by fluorescence microscopy that these probes enable efficient tracking of labeled HCVpp inside hepatocarcinoma cells used as model hepatocytes, in spite of their autofluorescence up to 700 nm. These novel fluorescent lipid-polymer probes should therefore enable a better characterization of early stages of infection of autofluorescent cells by enveloped viruses.

The deletion of residues 268-292 of E1 impairs the ability of HCV envelope proteins to induce pore formation

We have obtained a chimeric protein containing the ectodomains of hepatitis C virus (HCV) envelope proteins but lacking the region 268-292 of E1. All its structural properties are coincident with those of the corresponding full length chimera. The deleted and entire chimeras were compared in terms of their membrane destabilizing properties. No differences were found in their ability to induce vesicle aggregation and lipid mixing but the deleted chimera showed a reduced capacity to promote leakage. The role of the deletion was also studied by obtaining HCV pseudoparticles (HCVpp). Both E1 and E2, and also the E1 deleted mutant, were incorporated into HCVpp to a similar level. However, HCVpp containing the E1 deleted protein are almost unable to infect Huh7 cells. These results point to the involvement of the region 268-292 in the formation of pores in the membrane necessary for the complete fusion of the membranes.

Glutathione peroxidase 4 is reversibly induced by HCV to control lipid peroxidation and to increase virion infectivity

OBJECTIVE

Inflammation and oxidative stress drive disease progression in chronic hepatitis C (CHC) towards hepatocellular carcinoma. HCV is known to increase intracellular levels of reactive oxygen species (ROS), but how it eliminates ROS is less well known. The role of the ROS scavenger glutathione peroxidase 4 (GPx4), induced by HCV, in the viral life cycle was analysed.

DESIGN

The study was performed using a replicative in vitro HCV infection model and liver biopsies derived from two different CHC patient cohorts.

RESULTS

A screen for HCV-induced peroxide scavengers identified GPx4 as a host factor required for HCV infection. The physiological role of GPx4 is the elimination of lipid peroxides from membranes or lipoproteins. GPx4-silencing reduced the specific infectivity of HCV by up to 10-fold. Loss of infectivity correlated with 70% reduced fusogenic activity of virions in liposome fusion assays. NS5A was identified as the protein that mediates GPx4 induction in a phosphatidylinositol-3-kinase-dependent manner. Levels of GPx4 mRNA were found increased in vitro and in CHC compared with control liver biopsies. Upon successful viral eradication, GPx4 transcript levels returned to baseline in vitro and also in the liver of patients.

CONCLUSIONS

HCV induces oxidative stress but controls it tightly by inducing ROS scavengers. Among these, GPx4 plays an essential role in the HCV life cycle. Modulating oxidative stress in CHC by specifically targeting GPx4 may lower specific infectivity of virions and prevent hepatocarcinogenesis, especially in patients who remain difficult to be treated in the new era of interferon-free regimens.

Targeting Viral Entry for Treatment of Hepatitis B and C Virus Infections

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections remain major health problems worldwide, with 400-500 million chronically infected people worldwide. Chronic infection results in liver cirrhosis and hepatocellular carcinoma, the second leading cause of cancer death. Current treatments for HBV limit viral replication without efficiently curing infection. HCV treatment has markedly progressed with the licensing of direct-acting antivirals (DAAs) for HCV cure, yet limited access for the majority of patients is a major challenge. Preventative and curative treatment strategies, aimed at novel targets, are needed for both viruses. Viral entry represents one such target, although detailed knowledge of the entry mechanisms is a prerequisite. For HBV, the recent discovery of the NTCP cell entry factor enabled the establishment of an HBV cell culture model and showed that cyclosporin A and Myrcludex B are NTCP-targeting entry inhibitors. Advances in the understanding of HCV entry revealed it to be a complex process involving many factors, offering several antiviral targets. These include viral envelope proteins E1 and E2, virion-associated lipoprotein ApoE, and cellular factors CD81, SRBI, EGFR, claudin-1, occludin, and the cholesterol transporter NPC1L1. Small molecules targeting SR-BI, EGFR, and NPC1L1 have entered clinical trials, whereas other viral- and host-targeted small molecules, peptides, and antibodies show promise in preclinical models. This review summarizes the current understanding of HBV and HCV entry and describes novel antiviral targets and compounds in different stages of clinical development. Overall, proof-of-concept studies indicate that entry inhibitors are a promising class of antivirals to prevent and treat HBV and HCV infections.

Hepatitis C Virus Envelope Glycoprotein E1 Forms Trimers at the Surface of the Virion

In hepatitis C virus (HCV)-infected cells, the envelope glycoproteins E1 and E2 assemble as a heterodimer. To investigate potential changes in the oligomerization of virion-associated envelope proteins, we performed SDS-PAGE under reducing conditions but without thermal denaturation. This revealed the presence of SDS-resistant trimers of E1 in the context of cell-cultured HCV (HCVcc) as well as in the context of HCV pseudoparticles (HCVpp). The formation of E1 trimers was found to depend on the coexpression of E2. To further understand the origin of E1 trimer formation, we coexpressed in bacteria the transmembrane (TM) domains of E1 (TME1) and E2 (TME2) fused to reporter proteins and analyzed the fusion proteins by SDS-PAGE and Western blotting. As expected for strongly interacting TM domains, TME1-TME2 heterodimers resistant to SDS were observed. These analyses also revealed homodimers and homotrimers of TME1, indicating that such complexes are stable species. The N-terminal segment of TME1 exhibits a highly conserved GxxxG sequence, a motif that is well documented to be involved in intramembrane protein-protein interactions. Single or double mutations of the glycine residues (Gly354 and Gly358) in this motif markedly decreased or abrogated the formation of TME1 homotrimers in bacteria, as well as homotrimers of E1 in both HCVpp and HCVcc systems. A concomitant loss of infectivity was observed, indicating that the trimeric form of E1 is essential for virus infectivity. Taken together, these results indicate that E1E2 heterodimers form trimers on HCV particles, and they support the hypothesis that E1 could be a fusion protein.

IMPORTANCE

HCV glycoproteins E1 and E2 play an essential role in virus entry into liver cells as well as in virion morphogenesis. In infected cells, these two proteins form a complex in which E2 interacts with cellular receptors, whereas the function of E1 remains poorly understood. However, recent structural data suggest that E1 could be the protein responsible for the process of fusion between viral and cellular membranes. Here we investigated the oligomeric state of HCV envelope glycoproteins. We demonstrate that E1 forms functional trimers after virion assembly and that in addition to the requirement for E2, a determinant for this oligomerization is present in a conserved GxxxG motif located within the E1 transmembrane domain. Taken together, these results indicate that a rearrangement of E1E2 heterodimer complexes likely occurs during the assembly of HCV particles to yield a trimeric form of the E1E2 heterodimer. Gaining structural information on this trimer will be helpful for the design of an anti-HCV vaccine.

Human hepatic stellate cells are not permissive for hepatitis C virus entry and replication

BACKGROUND

Chronic HCV infection is associated with the development of hepatic fibrosis. The direct role of HCV in the fibrogenic process is unknown. Specifically, whether HCV is able to infect hepatic stellate cells (HSCs) is debated.

OBJECTIVE

To assess whether human HSCs are susceptible to HCV infection.

DESIGN

We combined a set of original HCV models, including the infectious genotype 2a JFH1 model (HCVcc), retroviral pseudoparticles expressing the folded HCV genotype 1b envelope glycoproteins (HCVpp) and a subgenomic genotype 1b HCV replicon, and two relevant cellular models, primary human HSCs from different patients and the LX-2 cell line, to assess whether HCV can infect/replicate in HSCs.

RESULTS

In contrast with the hepatocyte cell line Huh-7, neither infectious HCVcc nor HCVpp infected primary human HSCs or LX-2 cells. The cellular expression of host cellular factors required for HCV entry was high in Huh-7 cells but low in HSCs and LX-2 cells, with the exception of CD81. Finally, replication of a genotype 2a full-length RNA genome and a genotype 1b subgenomic replicon was impaired in primary human HSCs and LX-2 cells, which expressed low levels of cellular factors known to play a key role in the HCV life-cycle, suggesting that human HSCs are not permissive for HCV replication.

CONCLUSIONS

Human HSCs are refractory to HCV infection. Both HCV entry and replication are deficient in these cells, regardless of the HCV genotype and origin of the cells. Thus, HCV infection of HSCs does not play a role in liver fibrosis. These results do not rule out a direct role of HCV infection of hepatocytes in the fibrogenic process.

Host cell kinases and the hepatitis C virus life cycle

Hepatitis C virus (HCV) infection relies on virus-host interactions with human hepatocytes, a context in which host cell kinases play critical roles in every step of the HCV life cycle. During viral entry, cellular kinases, including EGFR, EphA2 and PKA, regulate the localization of host HCV entry factors and induce receptor complex assembly. Following virion internalization, viral genomes replicate on endoplasmic reticulum-derived membranous webs. The formation of membranous webs depends on interactions between the HCV NS5a protein and PI4KIIIα. The phosphorylation status of NS5a, regulated by PI4KIIIα, CKI and other kinases, also acts as a molecular switch to virion assembly, which takes place on lipid droplets. The formation of lipid droplets is enhanced by HCV activation of IKKα. In view of the multiple crucial steps in the viral life cycle that are mediated by host cell kinases, these enzymes also represent complementary targets for antiviral therapy. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

Two conserved histidines (His490 and His621) on the E2 glycoprotein of hepatitis C virus are critical for CD81-mediated cell entry

Hepatitis C virus (HCV) entry is a sequential and multi-step process that includes receptor interactions followed by pH-dependent membrane fusion. Specific and conserved histidine residues on the viral envelope proteins are involved in most pH-induced virus entries. In the case of HCV, some conserved histidines on the E1 and E2 proteins have been investigated in HCV pseudotype particle (HCVpp) systems. However, the roles of these histidines in cell-culture-derived HCV particle (HCVcc) systems remain unclear due to the different aspects of the viral life cycle emphasized by the two systems. In this study, the role of two conserved histidines (His490 and His621, located in domains II and III of E2, respectively) in HCV infection was evaluated in the context of JFH-1-based HCVcc using alanine substitutions. The infectivity of the H490A mutant decreased in spite of comparable initial RNA replication, protein expression and assembly efficiency as WT virus. The H621A mutant did not affect viral protein expression, but exhibited no obvious infectivity; there were fewer core proteins in the culture supernatant compared with WT virus, indicating the partially deficient virus assembly. The HCV receptor CD81-binding ability of the two mutant E2s was assessed further using enzyme immunoassays. The CD81-binding activity of H490A-E2 was reduced, and H621A-E2 was unable to bind to CD81. These data revealed the crucial role played by His490 and His621 in HCV infection, particularly during CD81 binding in cell entry. These results also contributed to the mechanical identification of the histidines involved in pH-dependent HCV entry.

The mechanism of HCV entry into host cells

Hepatitis C virus (HCV) is an enveloped, positive strand RNA virus classified within the Flaviviridae family and is a major cause of liver disease worldwide. HCV life cycle and propagation are tightly linked to several aspects of lipid metabolism. HCV propagation depends on and also shapes several aspects of lipid metabolism such as cholesterol uptake and efflux through different lipoprotein receptors during its entry into cells, lipid metabolism modulating HCV genome replication, lipid droplets acting as a platform for recruitment of viral components, and very low density lipoprotein assembly pathway resulting in incorporation of neutral lipids and apolipoproteins into viral particles. During the first steps of infection, HCV enters hepatocytes through a multistep and slow process. The initial capture of HCV particles by glycosaminoglycans and/or lipoprotein receptors is followed by coordinated interactions with the scavenger receptor class B type I, a major receptor of high-density lipoprotein, the CD81 tetraspanin, and the tight junction proteins Claudin-1 and Occludin. This tight concert of receptor interactions ultimately leads to uptake and cellular internalization of HCV through a process of clathrin-dependent endocytosis. Over the years, the identification of the HCV entry receptors and cofactors has led to a better understanding of HCV entry and of the narrow tropism of HCV for the liver. Yet, the role of the two HCV envelope glycoproteins, E1 and E2, remains ill-defined, particularly concerning their involvement in the membrane fusion process. Here, we review the current knowledge and advances addressing the mechanism of HCV cell entry within hepatocytes and we highlight the challenges that remain to be addressed.

Impact of lipids and lipoproteins on hepatitis C virus infection and virus neutralization

Hepatitis C virus (HCV) infections represent a major global health problem. End-stage liver disease caused by chronic HCV infection is a major indication for liver transplantation. However, after transplantation the engrafted liver inevitably becomes infected by the circulating virus. Direct acting antivirals are not yet approved for use in liver transplant patients, and limited efficacy and severe side effects hamper the use of pegylated interferon combined with ribavirin in a post-transplant setting. Therefore, alternative therapeutic options need to be explored. Viral entry represents an attractive target for such therapeutic intervention. Understanding the mechanisms of viral entry is essential to define the viral and cellular factors involved. The HCV life cycle is dependent of and associated with lipoprotein physiology and the presence of lipoproteins has been correlated with altered antiviral efficacy of entry inhibitors. In this review, we summarise the current knowledge on how lipoprotein physiology influences the HCV life cycle. We focus especially on the influence of lipoproteins on antibodies that target HCV envelope proteins or antibodies that target the cellular receptors of the virus. This information can be particularly relevant for the prevention of HCV re-infection after liver transplantation.

Benzhydrylpiperazine compounds inhibit cholesterol-dependent cellular entry of hepatitis C virus

Hepatitis C virus (HCV) remains a serious global health problem that lacks an effective cure. Although the introduction of protease inhibitors to the current standard-of-care interferon/ribavirin therapy for HCV infection has improved sustained virological response of genotype 1-infected patients, these inhibitors exacerbate already problematic side effects. Thus, new HCV antivirals are urgently needed. Using a cell-protection screen previously developed in our laboratory, we evaluated 30,426 compounds for inhibitors of potentially any stage of the HCV life cycle and identified 49 new HCV inhibitors. The two most potent hits, hydroxyzine and chlorcyclizine, belong to the family of benzhydrylpiperazines and were found to inhibit the entry of cell culture-produced HCV with IC50 values of 19 and 2.3 nM, respectively, and therapeutic indices of >500 and >6500. Both compounds block HCV entry at a late stage of entry prior to viral fusion and their inhibitory activities are highly dependent on the host's cholesterol content. Both compounds are currently used in the clinic for treating allergy-related disorders and the reported peak plasma level (160 nM) and estimated liver concentration (1.7 μM) of hydroxyzine in humans are much higher than the molecule's anti-HCV IC90 in cell culture (64 nM). Further studies are therefore justified to evaluate the use of these molecules in an anti-HCV therapeutic regimen.

Unfolded protein response in hepatitis C virus infection

Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus of clinical importance. The virus establishes a chronic infection and can progress from chronic hepatitis, steatosis to fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). The mechanisms of viral persistence and pathogenesis are poorly understood. Recently the unfolded protein response (UPR), a cellular homeostatic response to endoplasmic reticulum (ER) stress, has emerged to be a major contributing factor in many human diseases. It is also evident that viruses interact with the host UPR in many different ways and the outcome could be pro-viral, anti-viral or pathogenic, depending on the particular type of infection. Here we present evidence for the elicitation of chronic ER stress in HCV infection. We analyze the UPR signaling pathways involved in HCV infection, the various levels of UPR regulation by different viral proteins and finally, we propose several mechanisms by which the virus provokes the UPR.

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.

The stability of secreted, acid-labile H77/JFH-1 hepatitis C virus (HCV) particles is altered by patient isolate genotype 1a p7 sequences

Secreted infectious particles generated by the genotype 2a JFH-1 hepatitis C virus infectious clone are resistant to acidic pH, whereas intracellular virions remain acid-labile. Thus, JFH-1 particles are thought to undergo pH maturation as they are secreted from the cell. Here, we demonstrate that both infectious intracellular and secreted genotype 1a (H77)/JFH-1 chimaeric particles display enhanced acid sensitivity compared with JFH-1, although pH maturation still occurs upon release. Introduction of p7 sequences from genotype 1a infected HCV patients into the H77/JFH-1 background yielded variable effects on infectious particle production and sensitivity to small molecule inhibitors. However, two selected patient p7 sequences increased the acid stability of secreted, but not intracellular H77/JFH-1 particles, suggesting that p7 directly influences particle pH maturation via an as yet undefined mechanism. We propose that HCV particles vary in acid stability, and that this may be dictated by variations in both canonical structural proteins and p7.

Silibinin inhibits hepatitis C virus entry into hepatocytes by hindering clathrin-dependent trafficking

Hepatitis C virus (HCV) is a global health concern infecting 170 million people worldwide. Previous studies indicate that the extract from milk thistle known as silymarin and its main component silibinin inhibit HCV infection. Here we investigated the mechanism of anti-HCV action of silymarin-derived compounds at the molecular level. By using live-cell confocal imaging, single particle tracking, transmission electron microscopy and biochemical approaches on HCV-infected human hepatoma cells and primary hepatocytes, we show that silibinin potently inhibits HCV infection and hinders HCV entry by slowing down trafficking through clathrin-coated pits and vesicles. Detailed analyses revealed that silibinin altered the formation of both clathrin-coated pits and vesicles in cells and caused abnormal uptake and trafficking of transferrin, a well-known cargo of the clathrin endocytic pathway. Silibinin also inhibited infection by other viruses that enter cells by clathrin-mediated endocytosis including reovirus, vesicular stomatitis and influenza viruses. Our study demonstrates that silibinin inhibits HCV early steps of infection by affecting endosomal trafficking of virions. It provides new insights into the molecular mechanisms of action of silibinin against HCV entry and also suggests that silibinin is a potential broad-spectrum antiviral therapy.

Analysis of serine codon conservation reveals diverse phenotypic constraints on hepatitis C virus glycoprotein evolution

Serine is encoded by two divergent codon types, UCN and AGY, which are not interchangeable by a single nucleotide substitution. Switching between codon types therefore occurs via intermediates (threonine or cysteine) or via simultaneous tandem substitutions. Hepatitis C virus (HCV) chronically infects 2 to 3% of the global population. The highly variable glycoproteins E1 and E2 decorate the surface of the viral envelope, facilitate cellular entry, and are targets for host immunity. Comparative sequence analysis of globally sampled E1E2 genes, coupled with phylogenetic analysis, reveals the signatures of multiple archaic codon-switching events at seven highly conserved serine residues. Limited detection of intermediate phenotypes indicates that associated fitness costs restrict their fixation in divergent HCV lineages. Mutational pathways underlying codon switching were probed via reverse genetics, assessing glycoprotein functionality using multiple in vitro systems. These data demonstrate selection against intermediate phenotypes can act at the structural/functional level, with some intermediates displaying impaired virion assembly and/or decreased capacity for target cell entry. These effects act in residue/isolate-specific manner. Selection against intermediates is also provided by humoral targeting, with some intermediates exhibiting increased epitope exposure and enhanced neutralization sensitivity, despite maintaining a capacity for target cell entry. Thus, purifying selection against intermediates limits their frequencies in globally sampled strains, with divergent functional constraints at the protein level restricting the fixation of deleterious mutations. Overall our study provides an experimental framework for identification of barriers limiting viral substitutional evolution and indicates that serine codon-switching represents a genomic "fossil record" of historical purifying selection against E1E2 intermediate phenotypes.

Hepatitis C virus entry

Hepatitis C virus (HCV) is a hepatotropic virus and a major cause of chronic hepatitis and liver disease worldwide. Initial interactions between HCV virions and hepatocytes are required for productive viral infection and initiation of the viral life cycle. Furthermore, HCV entry contributes to the tissue tropism and species specificity of this virus. The elucidation of these interactions is critical, not only to understand the pathogenesis of HCV infection, but also to design efficient antiviral strategies and vaccines. This review summarizes our current knowledge of the host factors required for the HCV-host interactions during HCV binding and entry, our understanding of the molecular mechanisms underlying HCV entry into target cells, and the relevance of HCV entry for the pathogenesis of liver disease, antiviral therapy, and vaccine development.

Structural and antigenic definition of hepatitis C virus E2 glycoprotein epitopes targeted by monoclonal antibodies

Hepatitis C virus (HCV) is the major cause of chronic liver disease as well as the major indication for liver transplantation worldwide. Current standard of care is not completely effective, not administrable in grafted patients, and burdened by several side effects. This incomplete effectiveness is mainly due to the high propensity of the virus to continually mutate under the selective pressure exerted by the host immune response as well as currently administered antiviral drugs. The E2 envelope surface glycoprotein of HCV (HCV/E2) is the main target of the host humoral immune response and for this reason one of the major variable viral proteins. However, broadly cross-neutralizing monoclonal antibodies (mAbs) directed against HCV/E2 represent a promising tool for the study of virus-host interplay as well as for the development of effective prophylactic and therapeutic approaches. In the last few years many anti-HCV/E2 mAbs have been evaluated in preclinical and clinical trials as possible candidate antivirals, particularly for administration in pre- and post-transplant settings. In this review we summarize the antigenic and structural characteristics of HCV/E2 determined through the use of anti-HCV/E2 mAbs, which, given the absence of a crystal structure of this glycoprotein, represent currently the best tool available.

New insights in recurrent HCV infection after liver transplantation

Hepatitis C virus (HCV) is a small-enveloped RNA virus belonging to the Flaviviridae family. Since first identified in 1989, HCV has been estimated to infect 170 million people worldwide. Mostly chronic hepatitis C virus has a uniform natural history, from liver cirrhosis to the development of hepatocellular carcinoma. The current therapy for HCV infection consists of a combination of Pegylated interferon and ribavirin. On the other hand, HCV-related liver disease is also the leading indication for liver transplantation. However, posttransplant HCV re-infection of the graft has been reported to be universal. Furthermore, the graft after HCV re-infection often results in accelerated progression to liver failure. In addition, treatment of recurrent HCV infection after liver transplantation is often compromised by enhanced adverse effects and limited efficacy of interferon-based therapies. Taken together, poor outcome after HCV re-infection, regardless of grafts or recipients, poses a major issue for the hepatologists and transplant surgeons. The aim of this paper is to review several specific aspects regarding HCV re-infection after transplant: risk factors, current therapeutics for HCV in different stages of liver transplantation, cellular function of HCV proteins, and molecular mechanisms of HCV entry. Hopefully, this paper will inspire new strategies and novel inhibitors against recurrent HCV infection after liver transplantation and greatly improve its overall outcome.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Phenothiazines inhibit hepatitis C virus entry, likely by increasing the fluidity of cholesterol-rich membranes

Despite recent progress in the development of direct-acting antiviral agents against hepatitis C virus (HCV), more effective therapies are still urgently needed. We and others previously identified three phenothiazine compounds as potent HCV entry inhibitors. In this study, we show that phenothiazines inhibit HCV entry at the step of virus-host cell fusion, by intercalating into cholesterol-rich domains of the target membrane and increasing membrane fluidity. Perturbation of the alignment/packing of cholesterol in lipid membranes likely increases the energy barrier needed for virus-host fusion. A screening assay based on the ability of molecules to selectively increase the fluidity of cholesterol-rich membranes was subsequently developed. One compound that emerged from the library screen, topotecan, is able to very potently inhibit the fusion of liposomes with cell culture-derived HCV (HCVcc). These results yield new insights into HCV infection and provide a platform for the identification of new HCV inhibitors.

Host-targeting agents for prevention and treatment of chronic hepatitis C - perspectives and challenges

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease and hepatocellular carcinoma worldwide. Furthermore, HCV-induced liver disease is a major indication of liver transplantation. In the past years, direct-acting antivirals (DAAs) targeting HCV enzymes have been developed. DAAs increase the virologic response to anti-HCV therapy but may lead to selection of drug-resistant variants and treatment failure. To date, strategies to prevent HCV infection are still lacking and antiviral therapy in immunocompromised patients, patients with advanced liver disease and HIV/HCV-co-infection remains limited. Alternative or complementary approaches addressing the limitations of current antiviral therapies are to boost the host's innate immunity or interfere with host factors required for pathogenesis. Host-targeting agents (HTAs) provide an interesting perspective for novel antiviral strategies against viral hepatitis since they have (i) a high genetic barrier to resistance, (ii) a pan-genotypic antiviral activity, and (iii) complementary mechanisms of action to DAAs and might therefore act in a synergistic manner with current standard of care or DAAs in clinical development. This review highlights HTAs against HCV infection that have potential as novel antivirals and are in preclinical or clinical development.

Cell culture systems for hepatitis C virus

Due to the obligatory intracellular lifestyle of viruses, cell culture systems for efficient viral propagation are crucial to obtain a detailed understanding of the virus-host cell interaction. For hepatitis C virus (HCV) the development of permissive and authentic culture models continues to be a challenging task. The first efforts to culture HCV had limited success and range back to before the virus was molecularly cloned in 1989. Since then several major breakthroughs have gradually overcome limitations in culturing the virus and sequentially permitted analysis of viral RNA replication, cell entry, and ultimately the complete replication cycle in cultured cells in 2005. Until today, basic and applied HCV research greatly benefit from these tremendous efforts which spurred multiple complementary cell-based model systems for distinct steps of the HCV replication cycle. When used in combination they now permit deep insights into the fascinating biology of HCV and its interplay with the host cell. In fact, drug development has been much facilitated and our understanding of the molecular determinants of HCV replication has grown in parallel to these advances. Building on this groundwork and further refining our cellular models to better mimic the architecture, polarization and differentiation of natural hepatocytes should reveal novel unique aspects of HCV replication. Ultimately, models to culture primary HCV isolates across all genotypes may teach us important new lessons about viral functional adaptations that have evolved in exchange with its human host and that may explain the variable natural course of hepatitis C.

Entry and replication of recombinant hepatitis C viruses in cell culture

Hepatitis C virus (HCV) is a positive-strand enveloped RNA virus and belongs to the Flaviviridae family. The heavy health burden associated with the virus infection in humans and the intriguing peculiarities of the interaction between the HCV replication cycle and the hepatocyte host cell have stimulated a flourishing research field. The present review aims at recapitulating the different viral and cellular systems modelling HCV entry and replication, and in particular at gathering the tools available to dissect the HCV entry pathway.

Characterization of hepatitis C virus particle subpopulations reveals multiple usage of the scavenger receptor BI for entry steps

Hepatitis C virus (HCV) particles assemble along the very low density lipoprotein pathway and are released from hepatocytes as entities varying in their degree of lipid and apolipoprotein (apo) association as well as buoyant densities. Little is known about the cell entry pathway of these different HCV particle subpopulations, which likely occurs by regulated spatiotemporal processes involving several cell surface molecules. One of these molecules is the scavenger receptor BI (SR-BI), a receptor for high density lipoprotein that can bind to the HCV glycoprotein E2. By studying the entry properties of infectious virus subpopulations differing in their buoyant densities, we show that these HCV particles utilize SR-BI in a manifold manner. First, SR-BI mediates primary attachment of HCV particles of intermediate density to cells. These initial interactions involve apolipoproteins, such as apolipoprotein E, present on the surface of HCV particles, but not the E2 glycoprotein, suggesting that lipoprotein components in the virion act as host-derived ligands for important entry factors such as SR-BI. Second, we found that in contrast to this initial attachment, SR-BI mediates entry of HCV particles independent of their buoyant density. This function of SR-BI does not depend on E2/SR-BI interaction but relies on the lipid transfer activity of SR-BI, probably by facilitating entry steps along with other HCV entry co-factors. Finally, our results underscore a third function of SR-BI governed by specific residues in hypervariable region 1 of E2 leading to enhanced cell entry and depending on SR-BI ability to bind to E2.

New hepatitis C virus drug discovery strategies and model systems

INTRODUCTION

Hepatitis C virus (HCV) is a major cause of liver disease worldwide and the leading indication for liver transplantation in the United States. Current treatment options are expensive, not effective in all patients and are associated with serious side effects. Although preclinical, anti-HCV drug screening is still hampered by the lack of readily infectable small animal models, the development of cell culture HCV experimental model systems has driven a promising new wave of HCV antiviral drug discovery.

AREAS COVERED

This review contains a concise overview of current HCV treatment options and limitations with a subsequent in-depth focus on the available experimental models and novel strategies that have, and continue to enable, important advances in HCV drug development.

EXPERT OPINION

With a large cohort of chronically HCV-infected patients progressively developing liver disease that puts them at risk for hepatocellular carcinoma and hepatic decompensation, there is an urgent need to develop effective therapeutics that are well tolerated and effective in all patients and against all HCV genotypes. Significant advances in HCV experimental model development have expedited drug discovery; however, additional progress is needed. Importantly, the current trends and momentum in the field suggests that we will continue to overcome critical experimental challenges to reach this end goal.

Lipids: a key for hepatitis C virus entry and a potential target for antiviral strategies

Viruses have evolved to complex relationship with their host cells. Many viruses modulate the lipid composition, lipid synthesis and signaling of their host cell. Lipids are also an essential part of the life cycle of the hepatitis C virus (HCV). HCV is a major human pathogen, persistently infecting 170 million people worldwide, with no currently effective treatment available for all patients. HCV appears to make use of the host lipid metabolism and one common feature of chronic hepatitis C is the steatosis, characterized by excessive accumulation of triglycerides and lipid content in the liver. Thus, HCV lifecycle appears to be closely connected to host cell lipid metabolism, from cell entry, through viral RNA replication to viral particle production and formation/assembly. HCV particles have a unique lipid composition, certainly distinct from other viruses. In the blood of chronically-infected patients, viral particles are bound to serum lipoproteins and are thus called lipo-viro-particles. The density of these circulating viral particles is heterogeneous. Specific infectivity and fusion of low density particles are greater than those of high density particles. Lipids and association to lipoproteins therefore play a key role in HCV life cycle. The purpose of this review is to make a state of the art on recent findings on the contribution of lipids in cell entry and membrane fusion of HCV. The influence of lipids as chemically-defined entities will be analyzed, as well as the role played by cholesterol transporters and lipoprotein receptors in HCV entry and fusion. Since viral entry would constitute a key target for antiviral strategies, inhibitor molecules interacting with viral and/or cellular membranes or interfering with the function of lipid metabolism regulators of HCV entry could offer strong antiviral potential. This will be lastly discussed in this review.

Small-molecule inhibitors of NMO-IgG binding to aquaporin-4 reduce astrocyte cytotoxicity in neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of spinal cord and optic nerve caused by pathogenic autoantibodies (NMO-IgG) against astrocyte aquaporin-4 (AQP4). We developed a high-throughput screen to identify blockers of NMO-IgG binding to human AQP4 using a human recombinant monoclonal NMO-IgG and transfected Fisher rat thyroid cells stably expressing human M23-AQP4. Screening of ∼60,000 compounds yielded the antiviral arbidol, the flavonoid tamarixetin, and several plant-derived berbamine alkaloids, each of which blocked NMO-IgG binding to AQP4 without affecting AQP4 expression, array assembly, or water permeability. The compounds inhibited NMO-IgG binding to AQP4 in NMO patient sera and blocked NMO-IgG-dependent complement- and cell-mediated cytotoxicity with IC(50) down to ∼5 μM. Docking computations identified putative sites of blocker binding at the extracellular surface of AQP4. The blockers did not affect complement-dependent cytotoxicity caused by anti-GD3 antibody binding to ganglioside GD3. The blockers reduced by >80% the severity of NMO lesions in an ex vivo spinal cord slice culture model of NMO and in mice in vivo. Our results provide proof of concept for a small-molecule blocker strategy to reduce NMO pathology. Small-molecule blockers may also be useful for other autoimmune diseases caused by binding of pathogenic autoantibodies to defined targets.

Distinct roles in folding, CD81 receptor binding and viral entry for conserved histidine residues of hepatitis C virus glycoprotein E1 and E2

The protonation of histidine in acidic environments underpins its role in regulating the function of pH-sensitive proteins. For pH-sensitive viral fusion proteins, histidine protonation in the endosome leads to the activation of their membrane fusion function. The HCV (hepatitis C virus) glycoprotein E1–E2 heterodimer mediates membrane fusion within the endosome, but the roles of conserved histidine residues in the formation of a functional heterodimer and in sensing pH changes is unknown. We examined the functional roles of conserved histidine residues located within E1 and E2. The E1 mutations, H222A/R, H298R and H352A, disrupted E1–E2 heterodimerization and reduced virus entry. A total of five out of six histidine residues located within the E2 RBD (receptor-binding domain) were important for the E2 fold, and their substitution with arginine or alanine caused aberrant heterodimerization and/or CD81 binding. Distinct roles in E1–E2 heterodimerization and in virus entry were identified for His691 and His693 respectively within the membrane-proximal stem region. Viral entry and cell–cell fusion at neutral and low pH values were enhanced with H445R, indicating that the protonation state of His445 is a key regulator of HCV fusion. However, H445R did not overcome the block to virus entry induced by bafilomycin A1, indicating a requirement for an endosomal activation trigger in addition to acidic pH.

Benzophenone-containing fatty acids and their related photosensitive fluorescent new probes: design, physico-chemical properties and preliminary functional investigations

Hydrophobic photoaffinity labeling is a powerful strategy to identify hydrophobic segments within molecules, in particular membrane proteins. Here we report the design and synthesis of a novel family of fluorescent and photosensitive lipid tools, which have a common amino acid scaffold functionalized by three groups: (i) a first fatty acid chain grafted with a photoactivatable benzophenone moiety (Fatty Acid BenzoPhenone, FABP), (ii) a second fatty acid chain to ensure anchoring into a half-bilayer or hydrophobic environment, and (iii) a fluorescent carboxytetramethylrhodamine headgroup (CTMR) to detect the photolabeled compound. We present data of the synthesis and characterization of three lipid tools whose benzophenone ring is situated at various distances from the central scaffold. We could therefore establish structure/properties relationships dependent upon the depth of insertion of benzophenone into the membrane. Our lipid tools were extensively characterized both physico- and bio-chemically, and we assessed their functionality in vitro using bacterioRhodopsin (bR). We thus provide the scientific community with novel and reliable tools for the identification and study of hydrophobic regions in proteins.

Hepatitis C virus (HCV) envelope glycoproteins E1 and E2 contain reduced cysteine residues essential for virus entry

The HCV envelope glycoproteins E1 and E2 contain eight and 18 highly conserved cysteine residues, respectively. Here, we examined the oxidation state of E1E2 heterodimers incorporated into retroviral pseudotyped particles (HCVpp) and investigated the significance of free sulfhydryl groups in cell culture-derived HCV (HCVcc) and HCVpp entry. Alkylation of free sulfhydryl groups on HCVcc/pp with a membrane-impermeable sulfhydryl-alkylating reagent 4-(N-maleimido)benzyl-α-trimethylammonium iodide (M135) prior to virus attachment to cells abolished infectivity in a dose-dependent manner. Labeling of HCVpp envelope proteins with EZ-Link maleimide-PEG2-biotin (maleimide-biotin) detected free thiol groups in both E1 and E2. Unlike retroviruses that employ disulfide reduction to facilitate virus entry, the infectivity of alkylated HCVcc could not be rescued by addition of exogenous reducing agents. Furthermore, the infectivity of HCVcc bound to target cells was not affected by addition of M135 indicative of a change in glycoprotein oxidation state from reduced to oxidized following virus attachment to cells. By contrast, HCVpp entry was reduced by 61% when treated with M135 immediately following attachment to cells, suggesting that the two model systems might demonstrate variations in oxidation kinetics. Glycoprotein oxidation was not altered following binding of HCVpp incorporated E1E2 to soluble heparin or recombinant CD81. These results suggest that HCV entry is dependent on the presence of free thiol groups in E1 and E2 prior to cellular attachment and reveals a new essential component of the HCV entry process.