CD81 is an entry coreceptor for hepatitis C virus

Know your enemy: translating insights about the molecular biology of hepatitis C virus into novel therapeutic approaches

Identified in 1989 as the cause of what was then known as hepatitis non-A non-B, the hepatitis C virus (HCV) continues to be a significant global public health threat, given that an estimated 123 million individuals are chronically infected and, thus, at risk for cirrhosis and hepatocellular carcinoma. After 20 years of basic and clinical research into HCV infection, the backbone of therapy has remained interferon, a drug that - in a different formulation - was already being employed before HCV was even identified. Nonetheless, research has overcome many obstacles that stood in the way of studying this pre-eminent human pathogen. Hard-won insights into its molecular biology have identified promising therapeutic targets, and we are now on the verge of an era where rationally designed therapeutics, also referred to as specifically targeted antiviral therapy for HCV, will reshape the treatment of hepatitis C. This article describes recent insights on the molecular biology of HCV and the efforts to translate them into clinical applications.

Role of scavenger receptor class B type I in hepatitis C virus entry: kinetics and molecular determinants

Scavenger receptor class B type I (SR-BI) is an essential receptor for hepatitis C virus (HCV) and a cell surface high-density-lipoprotein (HDL) receptor. The mechanism of SR-BI-mediated HCV entry, however, is not clearly understood, and the specific protein determinants required for the recognition of the virus envelope are not known. HCV infection is strictly linked to lipoprotein metabolism, and HCV virions may initially interact with SR-BI through associated lipoproteins before subsequent direct interactions of the viral glycoproteins with SR-BI occur. The kinetics of inhibition of cell culture-derived HCV (HCVcc) infection with an anti-SR-BI monoclonal antibody imply that the recognition of SR-BI by HCV is an early event of the infection process. Swapping and single-substitution mutants between mouse and human SR-BI sequences showed reduced binding to the recombinant soluble E2 (sE2) envelope glycoprotein, thus suggesting that the SR-BI interaction with the HCV envelope is likely to involve species-specific protein elements. Most importantly, SR-BI mutants defective for sE2 binding, although retaining wild-type activity for receptor oligomerization and binding to the physiological ligand HDL, were impaired in their ability to fully restore HCVcc infectivity when transduced into an SR-BI-knocked-down Huh-7.5 cell line. These findings suggest a specific and direct role for the identified residues in binding HCV and mediating virus entry. Moreover, the observation that different regions of SR-BI are involved in HCV and HDL binding supports the hypothesis that new therapeutic strategies aimed at interfering with virus/SR-BI recognition are feasible.

Novel function of CD81 in controlling hepatitis C virus replication

The mechanisms of hepatitis C virus (HCV) replication remain poorly understood, and the cellular factors required for HCV replication are yet to be completely defined. CD81 is known to mediate HCV entry. Our study uncovered an unexpected novel function of CD81 in the HCV life cycle that is important for HCV RNA replication. HCV replication occurred efficiently in infected cells with high levels of CD81 expression. In HCV-infected or RNA-transfected cells with low levels of CD81 expression, initial viral protein synthesis occurred normally, but efficient replication failed to proceed. The aborted replication could be restored by the transient transfection of a CD81 expression plasmid. CD81-dependent replication was demonstrated with both an HCV infectious cell culture and HCV replicon cells of genotypes 1b and 2a. We also showed that CD81 expression is positively correlated with the kinetics of HCV RNA synthesis but inversely related to the kinetics of viral protein production, suggesting that CD81 may control viral replication by directing viral RNA template function to RNA replication. Thus, CD81 may be necessary for the efficient replication of the HCV genome in addition to its role in viral entry.

The cell biology of hepatitis C virus

Hepatitis C virus infects 3% of the world's population and has a variable disease course with potentially sever outcomes, liver failure and hepatocellular carcinoma. The influence of HCV the biology of infected hepatocytes is now just becoming known. This review will focus on effect of HCV on host cells.

Hepatitis C virus replication and potential targets for direct-acting agents

We finally stand at the brink of novel, oral, direct-acting antivirals for the treatment of hepatitis C virus (HCV) infection. Basic science research has lead to a greater understanding of the viral life cycle and identified numerous potential targets for therapy. Early compounds were plagued by inconsistent in vivo activity and side effects that led to discontinuation of investigational efforts. However, several agents have now progressed to phase 2 human studies and two protease inhibitors have completed enrolment for their phase 3 clinical trials and look promising. Thus, while it appears that protease inhibitors will likely be the next available drugs for the treatment of HCV infection, the quest for additional therapeutic agents will continue. The future of HCV therapy lies in multidrug cocktails of several agents targeted against a variety of targets. In the near future these agents will be added to the current standard therapy consisting of pegylated interferon and ribavirin; however, the ultimate and probably realistic goal will be to develop multidrug oral regiments to replace the need for interferon.

Lamiridosins, hepatitis C virus entry inhibitors from Lamium album

Phytochemical study of the aqueous extract of the flowering tops of Lamium album led to identification of the antiviral iridoid isomers lamiridosins A and B (1, 2). These compounds were found to significantly inhibit hepatitis C virus entry (IC(50) 2.31 muM) in vitro. Studies of 14 iridoid analogues showed that, while the parent iridoid glucosides demonstrated no anti-HCV entry activity, the aglycones of shanzhiside methyl ester (4), loganin (5), loganic acid (6), geniposide (10), verbenalin (12), eurostoside (15), and picroside II (17) exhibited significant anti-HCV entry and anti-infectivity activities.

Mouse-specific residues of claudin-1 limit hepatitis C virus genotype 2a infection in a human hepatocyte cell line

Recently, claudin-1 (CLDN1) was identified as a host protein essential for hepatitis C virus (HCV) infection. To evaluate CLDN1 function during virus entry, we searched for hepatocyte cell lines permissive for HCV RNA replication but with limiting endogenous CLDN1 expression, thus permitting receptor complementation assays. These criteria were met by the human hepatoblastoma cell line HuH6, which (i) displays low endogenous CLDN1 levels, (ii) efficiently replicates HCV RNA, and (iii) produces HCV particles with properties similar to those of particles generated in Huh-7.5 cells. Importantly, naïve cells are resistant to HCV genotype 2a infection unless CLDN1 is expressed. Interestingly, complementation of HCV entry by human, rat, or hamster CLDN1 was highly efficient, while mouse CLDN1 (mCLDN1) supported HCV genotype 2a infection with only moderate efficiency. These differences were observed irrespective of whether cells were infected with HCV pseudoparticles (HCVpp) or cell culture-derived HCV (HCVcc). Comparatively low entry function of mCLDN1 was observed in HuH6 but not 293T cells, suggesting that species-specific usage of CLDN1 is cell type dependent. Moreover, it was linked to three mouse-specific residues in the second extracellular loop (L152, I155) and the fourth transmembrane helix (V180) of the protein. These determinants could modulate the exposure or affinity of a putative viral binding site on CLDN1 or prevent optimal interaction of CLDN1 with other human cofactors, thus precluding highly efficient infection. HuH6 cells represent a valuable model for analysis of the complete HCV replication cycle in vitro and in particular for analysis of CLDN1 function in HCV cell entry.

Human liver transplantation as a model to study hepatitis C virus pathogenesis

Hepatitis C is a leading etiology of liver cancer and a leading reason for liver transplantation. Although new therapies have improved the rates of sustained response, a large proportion of patients (approximately 50%) fail to respond to antiviral treatment, thus remaining at risk for disease progression. Although chimpanzees have been used to study hepatitis C virus biology and treatments, their cost is quite high, and their use is strictly regulated; indeed, the National Institutes of Health no longer supports the breeding of chimpanzees for study. The development of hepatitis C virus therapies has been hindered by the relative paucity of small animal models for studying hepatitis C virus pathogenesis. This review presents the strengths of human liver transplantation and highlights the advances derived from this model, including insights into viral kinetics and quasispecies, viral receptor binding and entry, and innate and adaptive immunity. Moreover, consideration is given to current and emerging antiviral therapeutic approaches based on translational research results.

293 cells over-expressing human ADI1 and CD81 are permissive for serum-derived hepatitis C virus infection

Human aci-reductone dioxygenase 1 (ADI1) is a member of the Cupin superfamily. It binds to and inhibits the activities of membrane-type 1 matrix metalloproteinase, a protein known to interact with the tight junction protein, claudin-1. Previously, a variant protein, named submergence-induced protein-like factor (Sip-L), consisting of ADI1 amino acids 64-179, was found to support hepatitis C virus (HCV) infection and replication in 293 cells. In the present study, it was discovered that over-expression of human ADI1 in 293 cells (293-ADI1 cells) also supported HCV infection and replication. Using serum-derived HCV as an infectious source, enhanced cell uptake of HCV to a Northern blot detectable level was found in 293 cells over-expressing both CD81 and ADI1 (293-ADI1-CD81 cells). The enhanced cell entry was confirmed by the use of the vesicular stomatitis virus-based HCV pseudotype particles. However, transfection of HCV replicon RNA by electroporation into naïve 293 and 293-ADI1 cells revealed no difference in replication efficiency. Using the infectious J6/JFH chimera as an infectious source, the infectivity was compared between 293-ADI1-CD81 and Huh-7.5 cells. More infection foci were formed in the 293-ADI1-CD81 cells in the first round of infection. In conclusion, human ADI1 over-expression in 293 cells enhances cell entry but not replication of HCV. 293-ADI1-CD81 cells are permissive for serum-derived HCV infection.

Screening of small-molecule compounds as inhibitors of HCV entry

The hepatitis C virus (HCV) has infected some 170 million people worldwide, and is expected to pose a significant medical problem for the foreseeable future. No vaccine is presently available, and the current antiviral therapies (pegylated interferon-alpha and ribavirin) are characterized by limited efficacy, high costs, and substantial side effects. Initiation of infection requires attachment of the HCV virus to the cell surface followed by viral entry and represents a critical determinant of tissue tropism and pathogenesis. Small molecules that inhibit the virus at the stage of viral entry, for example, by blocking the interactions between viral envelope glycoprotein and cellular receptor or coreceptor or by inhibiting the viral fusion process, would serve as attractive antiviral drugs. Recent development of HCV pseudoparticles (HCVpp), displaying unmodified and functional HCV glycoprotein on the surface of retroviral core particles, has greatly facilitated studies of HCV entry and provides an essential tool for the identification and characterization of molecules that block HCV entry. We have adapted the HCVpp infection assay with HCVpp harboring a luciferase reporter to a 96-well format and screened a small-molecule compound library to identify inhibitors of HCV entry. Such active viral entry inhibitors have the potential to be first-in-class antiviral drugs that can be incorporated into combinations of multiple drugs with different targets for the treatment of chronic HCV infection.

Amphipathic DNA polymers inhibit hepatitis C virus infection by blocking viral entry

BACKGROUND & AIMS

Hepatitis C virus (HCV) gains entry into susceptible cells by interacting with cell surface receptor(s). Viral entry is an attractive target for antiviral development because of the highly conserved mechanism.

METHODS

HCV culture systems were used to study the effects of phosphorothioate oligonucleotides (PS-ONs), as amphipathic DNA polymers (APs), on HCV infection. The in vivo effects of APs were tested in urokinase plasminogen activator (uPA)/severe combined immunodeficient (SCID) mice engrafted with human hepatocytes.

RESULTS

We show the sequence-independent inhibitory effects of APs on HCV infection. APs were shown to potently inhibit HCV infection at submicromolar concentrations. APs exhibited a size-dependent antiviral activity and were equally active against HCV pseudoparticles of various genotypes. Control phosphodiester oligonucleotide (PO-ON) polymer without the amphipathic structure was inactive. APs had no effect on viral replication in the HCV replicon system or binding of HCV to cells but inhibited viral internalization, indicating that the target of inhibition is at the postbinding, cell entry step. In uPA/SCID mice engrafted with human hepatocytes, APs efficiently blocked de novo HCV infection.

CONCLUSIONS

Our results demonstrate that APs are a novel class of antiviral compounds that hold promise as a drug to inhibit HCV entry.

Hepatitis C Virus entry: the early steps in the viral replication cycle

Approximately 170 million are infected with the hepatitis C virus (HCV) world wide and an estimated 2.7 million are HCV RNA positive in the United States alone. The acute phase of the HCV infection, in majority of individuals, is asymptomatic. A large percentage of those infected with HCV are unable to clear the virus and become chronically infected. The study of the HCV replication cycle was hampered due to difficulties in growing and propagating the virus in an in vitro setting. The advent of the HCV pseudo particle (HCVpp) and HCV cell culture (HCVcc) systems have made possible the study of the HCV replication cycle, in vitro. Studies utilizing the HCVpp and HCVcc systems have increased our insight into the early steps of the viral replication cycle of HCV, such as the identification of cellular co-receptors for binding and entry. The aim of this article is to provide a review of the outstanding literature on HCV entry, specifically looking at cellular co-receptors involved and putting the data in the context of the systems used (purified viral envelope proteins, HCVpp system, HCVcc system and/or patient sera) and to also give a brief description of the cellular co-receptors themselves.

Kinases required in hepatitis C virus entry and replication highlighted by small interference RNA screening

The entry pathway of the hepatitis C virus (HCV), a major human pathogen, into the cell is incompletely defined. To better characterize this viral life cycle stage, we screened a small interfering RNA library dedicated to the membrane trafficking and remodeling with the infection model of Huh-7.5.1 cells by HCV pseudoparticles (HCVpp). Results showed that the down-regulation of different factors implied in clathrin-mediated endocytosis (CME) inhibits HCVpp cell infection. In addition, knockdown of the phosphatidylinositol 4-kinase type III-alpha (PI4KIIIalpha) prevented infection by HCVpp or by cell-culture grown JFH-1-based HCV. Moreover, the replication activity of an HCV replicon was also affected by the PI4KIIIalpha knockdown. Additional investigations on the different members of the PI4K family revealed that the presence of PI4KIIIbeta in the host cells influenced their susceptibility to HCVpp infection and their capacity to sustain the HCV replication. The PI4KIII involvement during the HCV life cycle seemed to occur by other ways than the control of the CME or of the membranous expression of HCV receptors. Finally, our library screening completed data on the CME-dependant entry route of HCV and identified 2 kinases, PI4KIIIalpha and beta, as relevant potential therapeutic targets.

Cellular models for the screening and development of anti-hepatitis C virus agents

Investigations on the biology of hepatitis C virus (HCV) have been hampered by the lack of small animal models. Efforts have therefore been directed to designing practical and robust cellular models of human origin able to support HCV replication and production in a reproducible, reliable and consistent manner. Many different models based on different forms of virions and hepatoma or other cell types have been described including virus-like particles, pseudotyped particles, subgenomic and full length replicons, virion productive replicons, immortalised hepatocytes, fetal and adult primary human hepatocytes. This review focuses on these different cellular models, their advantages and disadvantages at the biological and experimental levels, and their respective use for evaluating the effect of antiviral molecules on different steps of HCV biology including virus entry, replication, particles generation and excretion, as well as on the modulation by the virus of the host cell response to infection.

Polarization restricts hepatitis C virus entry into HepG2 hepatoma cells

The primary reservoir for hepatitis C virus (HCV) replication is believed to be hepatocytes, which are highly polarized with tight junctions (TJ) separating their basolateral and apical domains. HepG2 cells develop polarity over time, resulting in the formation and remodeling of bile canalicular (BC) structures. HepG2 cells expressing CD81 provide a model system to study the effects of hepatic polarity on HCV infection. We found an inverse association between HepG2-CD81 polarization and HCV pseudoparticle entry. As HepG2 cells polarize, discrete pools of claudin-1 (CLDN1) at the TJ and basal/lateral membranes develop, consistent with the pattern of receptor staining observed in liver tissue. The TJ and nonjunctional pools of CLDN1 show an altered association with CD81 and localization in response to the PKA antagonist Rp-8-Br-cyclic AMPs (cAMPs). Rp-8-Br-cAMPs reduced CLDN1 expression at the basal membrane and inhibited HCV infection, supporting a model where the nonjunctional pools of CLDN1 have a role in HCV entry. Treatment of HepG2 cells with proinflammatory cytokines, tumor necrosis factor alpha and gamma interferon, perturbed TJ integrity but had minimal effect(s) on cellular polarity and HCV infection, suggesting that TJ integrity does not limit HCV entry into polarized HepG2 cells. In contrast, activation of PKC with phorbol ester reduced TJ integrity, ablated HepG2 polarity, and stimulated HCV entry. Overall, these data show that complex hepatocyte-like polarity alters CLDN1 localization and limits HCV entry, suggesting that agents which disrupt hepatocyte polarity may promote HCV infection and transmission within the liver.

Progress towards recombinant anti-infective antibodies

The global market for monoclonal antibody therapeutics reached a total of $11.2 billion in 2004, with an impressive 42% growth rate over the previous five years and is expected to reach approximately $34 billion by 2010. Coupled with this growth are stream-lined product development, production scale-up and regulatory approval processes for the highly conserved antibody structure. While only one of the 21 current FDA-approved antibodies, and one of the 38 products in advanced clinical trials target infectious diseases, there is increasing academic, government and commercial interest in this area. Synagis, an antibody neutralizing respiratory syncitial virus (RSV), garnered impressive sales of $1.1 billion in 2006 in spite of its high cost and undocumented effects on viral titres in human patients. The success of anti-RSV passive immunization has motivated the continued development of anti-infectives to treat a number of other infectious diseases, including those mediated by viruses, toxins and bacterial/ fungal cells. Concurrently, advances in antibody technology suggest that cocktails of several monoclonal antibodies with unique epitope specificity or single monoclonal antibodies with broad serotype specificity may be the most successful format. Recent patents and patent applications in these areas will be discussed as predictors of future anti-infective therapeutics.

Differential effects of hepatitis C virus JFH1 on human myeloid and plasmacytoid dendritic cells

Dendritic cells (DCs) are reported to be functionally deficient during chronic hepatitis C virus (HCV) infection. Differing results have been reported on direct effects of intact replicative-form HCV on DC function. To better understand the effect of HCV on DC function, we treated freshly purified human myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) with HCV JFH1. We found that HCV upregulated mDC maturation marker (CD83, CD86, and CD40) expression and did not inhibit Toll-like receptor 3 (TLR3) ligand [poly(I:C)]-induced mDC maturation, a finding consistent with the phenotype of DCs from HCV-infected subjects. At the same time, HCV JFH1 inhibited the ability of poly(I:C)-treated mDCs to activate naive CD4 T cells. In contrast, although there was no direct effect of virus on pDC maturation, HCV JFH1 inhibited TLR7 ligand (R848)-induced pDC CD40 expression, and this was associated with impaired ability to activate naive CD4 T cells. Parallel experiments with recombinant HCV proteins indicated HCV core protein may be responsible for a portion of the activity. Furthermore, HCV-mediated mDC maturation was dependent upon CD81-E2 interaction and, in part, TLR2. Using UV-treated HCV, we show that HCV-mediated mDC and pDC maturation is virus replication independent and, using strand specific PCR, we found no evidence for HCV replication within DCs. Because these effects of HCV on DC subset maturation and function in part recapitulate direct ex vivo analysis of DCs in chronic HCV infection, the mechanisms described here likely account for a portion of the DC subset defects observed in vivo.

Residues in a highly conserved claudin-1 motif are required for hepatitis C virus entry and mediate the formation of cell-cell contacts

Claudin-1, a component of tight junctions between liver hepatocytes, is a hepatitis C virus (HCV) late-stage entry cofactor. To investigate the structural and functional roles of various claudin-1 domains in HCV entry, we applied a mutagenesis strategy. Putative functional intracellular claudin-1 domains were not important. However, we identified seven novel residues in the first extracellular loop that are critical for entry of HCV isolates drawn from six different subtypes. Most of the critical residues belong to the highly conserved claudin motif W(30)-GLW(51)-C(54)-C(64). Alanine substitutions of these residues did not impair claudin-1 cell surface expression or lateral protein interactions within the plasma membrane, including claudin-1-claudin-1 and claudin-1-CD81 interactions. However, these mutants no longer localized to cell-cell contacts. Based on our observations, we propose that cell-cell contacts formed by claudin-1 may generate specialized membrane domains that are amenable to HCV entry.

Hepatitis C virus cell entry: role of lipoproteins and cellular receptors

Hepatitis C virus (HCV), a major cause of chronic liver disease, is a single-stranded positive sense virus of the family Flaviviridae. HCV cell entry is a multi-step process, involving several viral and cellular factors that trigger virus uptake into the hepatocyte. Tetraspanin CD81, human scavenger receptor SR-BI, and tight junction molecules Claudin-1 and occludin are the main receptors that mediate HCV entry. In addition, the virus may use glycosaminoglycans and/or low density receptors on host cells as initial attachment factors. A unique feature of HCV is the dependence of virus replication and assembly on host cell lipid metabolism. Most notably, during HCV assembly and release from the infected cells, virus particles associate with lipids and very-low-density lipoproteins. Thus, infectious virus circulates in patient sera in the form of triglyceride-rich particles. Consequently, lipoproteins and lipoprotein receptors play an essential role in virus uptake and the initiation of infection. This review summarizes the current knowledge about HCV receptors, mechanisms of HCV cell entry and the role of lipoproteins in this process.

CD81 is dispensable for hepatitis C virus cell-to-cell transmission in hepatoma cells

Hepatitis C virus (HCV) infects cells by the direct uptake of cell-free virus following virus engagement with specific cell receptors such as CD81. Recent data have shown that HCV is also capable of direct cell-to-cell transmission, although the role of CD81 in this process is disputed. Here, we generated cell culture infectious strain JFH1 HCV (HCVcc) genomes carrying an alanine substitution of E2 residues W529 or D535 that are critical for binding to CD81 and infectivity. Co-cultivation of these cells with naïve cells expressing enhanced green fluorescent protein (EGFP) resulted in a small number of cells co-expressing both EGFP and HCV NS5A, showing that the HCVcc mutants are capable of cell-to-cell spread. In contrast, no cell-to-cell transmission from JFH1_ΔE1E2-transfected cells occurred, indicating that the HCV glycoproteins are essential for this process. The frequency of cell-to-cell transmission of JFH1_W529A was unaffected by the presence of neutralizing antibodies that inhibit E2–CD81 interactions. By using cell lines that expressed little or no CD81 and that were refractive to infection with cell-free virus, we showed that the occurrence of viral cell-to-cell transmission is not influenced by the levels of CD81 on either donor or recipient cells. Thus, our results show that CD81 plays no role in the cell-to-cell spread of HCVcc and that this mode of transmission is shielded from neutralizing antibodies. These data suggest that therapeutic interventions targeting the entry of cell-free HCV may not be sufficient in controlling an ongoing chronic infection, but need to be complemented by additional strategies aimed at disrupting direct cell-to-cell viral transmission.

Analysis of a conserved RGE/RGD motif in HCV E2 in mediating entry

BACKGROUND

Hepatitis C virus (HCV) encodes two transmembrane glycoproteins E1 and E2 which form a heterodimer. E1 is believed to mediate fusion while E2 has been shown to bind cellular receptors. It is clear that HCV uses a multi-receptor complex to gain entry into susceptible cells, however key elements of this complex remain elusive. In this study, the role of a highly conserved RGE/RGD motif of HCV E2 glycoprotein in viral entry was examined. The effect of each substitution mutation in this motif was tested by challenging susceptible cell lines with mutant HCV E1E2 pseudotyped viruses generated using a lentiviral system (HCVpp). In addition to assaying infectivity, producer cell expression and HCVpp incorporation of HCV E2 proteins, CD81 binding profiles, and conformation of mutants were examined.

RESULTS

Based on these characteristics, mutants either displayed wt characteristics (high infectivity [> or = 90% of wt HCVpp], CD81 binding, E1E2 expression, and incorporation into viral particles and proper conformation) or very low infectivity (< or = 20% of wt HCVpp). Only amino acid substitutions of the 3rd position (D or E) resulted in wt characteristics as long as the negative charge was maintained or a neutral alanine was introduced. A change in charge to a positive lysine, disrupted HCVpp infectivity at this position.

CONCLUSION

Although most amino acid substitutions within this conserved motif displayed greatly reduced HCVpp infectivity, they retained soluble CD81 binding, proper E2 conformation, and incorporation into HCVpp. Our results suggest that although RGE/D is a well-defined integrin binding motif, in this case the role of these three hyperconserved amino acids does not appear to be integrin binding. As the extent of conservation of this region extends well beyond these three amino acids, we speculate that this region may play an important role in the structure of HCV E2 or in mediating the interaction with other factor(s) during viral entry.

Interaction of hepatitis C virus envelope glycoprotein E2 with the large extracellular loop of tupaia CD81

AIM: To further analyze the interaction of tupaia CD81 with hepatitis C virus (HCV) envelope protein E2.

METHODS: A tupaia CD81 large extracellular loop (CD81 LEL), which binds to HCV E2 protein, was cloned and expressed as a GST-fusion protein, and interaction of HCV E2 protein with a tupaia CD81 LEL was evaluated by enzyme-linked immunosorbent assay (EIA).

RESULTS: Although tupaia and human CD81 LEL differed in 6 amino acid changes, tupaia CD81 LEL was strongly recognized by anti-CD81 antibodies against human CD81 LEL conformation-dependent epitopes. Investigating LEL CD81-E2 interactions by EIA, we demonstrated that binding of tupaia CD81 LEL GST fusion protein to recombinant HCV E2 protein was markedly reduced compared to binding of human CD81 LEL GST fusion protein to recombinant HCV E2 protein.

CONCLUSION: These data suggest that the structural differences in-between the tupaia and human CD81 may alter the interaction of the large extracellular loop with HCV envelope glycoprotein E2. These findings may be important for the understanding of the mechanisms of binding and entry of HCV to PTHs.

Rapid synchronization of hepatitis C virus infection by magnetic adsorption

Hepatitis C virus (HCV) entry into target cells is thought to be a multistep process involving several cellular factors. However, their precise role during virus entry is unclear. Investigation of the mechanisms of HCV entry, such as the order of intervention by the cellular receptors, requires synchronizing infections. This study describes a new method involving magnetic adsorption of virus to nanoparticles to synchronize infection, which can be adapted to both HCV pseudoparticles and cell culture infectious HCV. By combining these particles with negatively or positively charged magnetic nanoparticles it was possible to adsorb them onto target cells under a magnetic field in only 2min. This resulted in greater efficiency of virus adsorption to cells, and increased the infectivity of cell culture infectious virus, as compared to the standard protocol involving incubation of the virus with cells at 4 degrees C for 1h, or to a standard infection at 37 degrees C. Furthermore, magnetic adsorption respected the natural entry route of the virus, making this system suitable to study the early stages of HCV infection.

Hepatitis C virus entry: molecular mechanisms and targets for antiviral therapy

With an estimated 170 million infected individuals, hepatitis C virus (HCV) has a major impact on public health. The liver is the primary target organ of HCV, and the hepatocyte is its primary target cell. Attachment of the virus to the cell surface followed by viral entry is the first step in a cascade of interactions between the virus and the target cell that is required for successful entry into the cell and initiation of infection. Using recombinant HCV envelope glycoproteins and HCV pseudotype particles, several cell surface molecules have been identified interacting with HCV during viral binding and entry. These include CD81, highly sulfated heparan sulfate, the low-density lipoprotein receptor, scavenger receptor class B type I and claudin-1. Treatment options for chronic HCV infection are limited and a vaccine to prevent HCV infection is not available. Interfering with HCV entry holds promise for drug design and discovery as the understanding of molecular mechanisms underlying HCV interaction with the host cell is advancing. The complexity of the virus entry process offers several therapeutic targets.

Association of functional polymorphism of ApoB promoter with hepatitis C virus infection

BACKGROUND

Hepatitis C virus (HCV) binds to apolipoprotein B (ApoB) and low-density lipoprotein (LDL) before entering into hepatocytes. ApoB promoter polymorphisms influence the levels of ApoB and LDL in blood. We investigated the correlations between ApoB promoter polymorphism and HCV infection.

METHODS

ApoB promoter polymorphism was analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 243 HCV-infected patients and 346 healthy individuals. Serum lipids concentrations were measured in all subjects.

RESULTS

ApoB polymorphisms on -516 position in patients were statistically different from healthy individuals (p<0.0001). CC genotype was higher in patients (58.7%) than in healthy individuals (43.1%) (p<0.0001). TT genotype was lower in patients (6.2%) than in healthy group (16.1%) (p<0.0001). C allele frequency was higher (76.3%) in patients than in healthy individuals (63.4%) (p<0.0001). T allele was lower in patients (23.7%) than in controls (36.6%) (p<0.0001). CC genotype was higher in HCV RNA positive patients (63.5%) than in negative ones (45.9%) (p<0.0001). TT genotype was significantly higher in HCV RNA negative patients (19.2%) than in positive individuals (2.6%) (p<0.0001).

CONCLUSIONS

CC genotype of ApoB promoter at -516 position may increase susceptibility of HCV infection and TT genotype may be associated with viral clearance.

Channel catfish, Ictalurus punctatus Rafinesque 1818, tetraspanin membrane protein family: characterization and expression analysis of CD81 cDNA

CD81, also known as the target of an antiproliferative antibody 1 (TAPA-1) in human, is a member of tetraspanin integral membrane protein family. This protein plays many important roles in immune and other physiological functions. In this report, we characterized and analyzed expression of the channel catfish CD81 transcript. The full-length of channel catfish CD81 cDNA comprised of 1130 nucleotides, including an open reading frame which appears to encode a putative peptide of 234 amino acid residues. By comparison with the human counterpart, the channel catfish CD81 peptide could be divided into domains, including four transmembrane domains, three intracellular domains, and one of each small and large extracellular loops. The degree of conservation of the channel catfish CD81 amino acid sequence to that of mammalian counterparts ranged from 65% to 67%. The large extracellular domain shows the least conservation between fish and mammals. However, the characteristic Cys(159)-Cys(160)-Gly(161) motif and Cys(176/188) in this domain were conserved. The channel catfish CD81 transcript was detected by RT-PCR in spleen, head kidney, liver, intestine, skin and gill. This result provides important information for further elucidating CD81 functions in channel catfish.

Entry of pseudotyped hepatitis C virus into primary human hepatocytes depends on the scavenger class B type I receptor

Entry of the hepatitis C virus (HCV) into the cell seems to be a complex multi-step process involving several cellular factors such as the scavenger class B type I receptor (SRBI). Until now, all investigations conducted to assess the involvement of SRBI have been based on in vitro infection models which use human hepatoma-derived cell lines. However, the HCV entry pathway may be altered in these types of cells because of the impairment of some hepatic characteristics. In this study, we showed that SRBI also plays an essential role in HCV entry into primary human hepatocytes with two distinct approaches: gene extinction and antibodies neutralization assays.

Anti-CD81 antibodies can prevent a hepatitis C virus infection in vivo

The viral life cycle of the hepatitis C virus (HCV) has been studied mainly using different in vitro cell culture models. Studies using pseudoviral particles (HCVpp) and more recently cell culture-derived virus (HCVcc) suggest that at least three host cell molecules are important for HCV entry in vitro: the tetraspanin CD81, the scavenger receptor class B member I, and the tight junction protein Claudin-1. Whether these receptors are equally important for an in vivo infection remains to be demonstrated. We show that CD81 is indispensable for an authentic in vivo HCV infection. Prophylactic treatment with anti-CD81 antibodies completely protected human liver-uPA-SCID mice from a subsequent challenge with HCV consensus strains of different genotypes. Administration of anti-CD81 antibodies after viral challenge had no effect.

CONCLUSION

Our experiments provide evidence for the critical role of CD81 in a genuine HCV infection in vivo and open new perspectives for the prevention of allograft reinfection after orthotopic liver transplantation in chronically infected HCV patients.

Characterization of a CD4-independent clinical HIV-1 that can efficiently infect human hepatocytes through chemokine (C-X-C motif) receptor 4

OBJECTIVE

HIV-1 isolates are prominently CD4-dependent and, to date, only a few laboratory-adapted CD4-independent strains have been reported. Therefore, whether CD4-independent viruses may exist in HIV-1-infected patients has remained unclear. Here, we report the successful isolation of a CD4-independent clinical HIV-1 strain, designated SDA-1, from the viral quasispecies of a therapy-naive HIV-1 and Pneumocystis jirovecii pneumonia patient in the late-stage of AIDS with extremely low CD4 cell count (CD4 = 1/microl). We characterized this virus and further explored whether it could infect or induce pathological effects in human hepatocytes.

DESIGN AND METHODS

To determine coreceptor usage and CD4-independent infection, the HIV-1 envelope (Env)-pseudotypes and Env-chimeric viruses were used.

RESULTS

SDA-1 was able to infect CD4 cell lines through either chemokine (C-X-C motif) receptor 4 or CCR5. It still maintained the ability to infect CD4 cells through multiple coreceptors of chemokine (C-X-C motif) receptor 4, chemokine (C-C motif) receptor 5, chemokine (C-C motif) receptor 3 and chemokine (C-C motif) receptor 8. Productive infection by SDA-1 was noted in both CD4-negative hepatoma cells and primary cultured human hepatocytes. Moreover, we demonstrated that SDA-1 could efficiently infect human hepatocytes on both static and mitotic phases through chemokine (C-X-C motif) receptor 4, without inducing apoptotic cell death.

CONCLUSION

The present study provides evidence that emergence of CD4-independent HIV-1 virus in vivo may occur in HIV-1-infected patients. In addition, these results shed light on the mechanisms involved in liver damage in HIV-1-infected individuals, which could have important implications concerning the range of mutability and the pathogenesis of AIDS.

Cutting the gordian knot-development and biological relevance of hepatitis C virus cell culture systems

Worldwide approximately 180 million people are chronically infected with hepatitis C virus (HCV). HCV isolates exhibit extensive genetic heterogeneity and have been grouped in six genotypes and various subtypes. Additionally, several naturally occurring intergenotypic recombinants have been described. Research on the viral life cycle, efficient therapeutics, and a vaccine has been hampered by the absence of suitable cell culture systems. The first system permitting studies of the full viral life cycle was intrahepatic transfection of RNA transcripts of HCV consensus complementary DNA (cDNA) clones into chimpanzees. However, such full-length clones were not infectious in vitro. The development of the replicon system and HCV pseudo-particles allowed in vitro studies of certain aspects of the viral life cycle, RNA replication, and viral entry, respectively. Identification of the genotype 2 isolate JFH1, which for unknown reasons showed an exceptional replication capability and resulted in formation of infectious viral particles in the human hepatoma cell line Huh7, led in 2005 to the development of the first full viral life cycle in vitro systems. JFH1-based systems now enable in vitro studies of the function of viral proteins, their interaction with each other and host proteins, new antivirals, and neutralizing antibodies in the context of the full viral life cycle. However, several challenges remain, including development of cell culture systems for all major HCV genotypes and identification of other susceptible cell lines.

CD81 is a central regulator of cellular events required for hepatitis C virus infection of human hepatocytes

Infection with hepatitis C virus (HCV) is still a major public health problem, and the events leading to hepatocyte infection are not yet fully understood. Combining confocal microscopy with biochemical analysis and studies of infection requirements using pharmacological inhibitors and small interfering RNAs, we show here that engagement of CD81 activates the Rho GTPase family members Rac, Rho, and Cdc42 and that the block of these signaling pathways drastically reduces HCV infectivity. Activation of Rho GTPases mediates actin-dependent relocalization of the HCV E2/CD81 complex to cell-cell contact areas where CD81 comes into contact with the tight-junction proteins occludin, ZO-1, and claudin-1, which was recently described as an HCV coreceptor. Finally, we show that CD81 engagement activates the Raf/MEK/ERK signaling cascade and that this pathway affects postentry events of the virus life cycle. In conclusion, we describe a range of cellular events that are manipulated by HCV to coordinate interactions with its multiple coreceptors and to establish productive infections and find that CD81 is a central regulator of these events.

Cellular proteins in influenza virus particles

Virions are thought to contain all the essential proteins that govern virus egress from the host cell and initiation of replication in the target cell. It has been known for some time that influenza virions contain nine viral proteins; however, analyses of other enveloped viruses have revealed that proteins from the host cell can also be detected in virions. To address whether the same is true for influenza virus, we used two complementary mass spectrometry approaches to perform a comprehensive proteomic analysis of purified influenza virus particles. In addition to the aforementioned nine virus-encoded proteins, we detected the presence of 36 host-encoded proteins. These include both cytoplasmic and membrane-bound proteins that can be grouped into several functional categories, such as cytoskeletal proteins, annexins, glycolytic enzymes, and tetraspanins. Interestingly, a significant number of these have also been reported to be present in virions of other virus families. Protease treatment of virions combined with immunoblot analysis was used to verify the presence of the cellular protein and also to determine whether it is located in the core of the influenza virus particle. Immunogold labeling confirmed the presence of membrane-bound host proteins on the influenza virus envelope. The identification of cellular constituents of influenza virions has important implications for understanding the interactions of influenza virus with its host and brings us a step closer to defining the cellular requirements for influenza virus replication. While not all of the host proteins are necessarily incorporated specifically, those that are and are found to have an essential role represent novel targets for antiviral drugs and for attenuation of viruses for vaccine purposes.

Viruses are released from infected cells in the form of virions, which contain all the essential factors necessary for initiating infection in a new target cell. For influenza virus, it is known that virions contain the viral genome, a lipid envelope, and at least nine viral proteins. We performed a detailed proteomic analysis of purified influenza virus particles using mass spectrometry and database searching for protein identification, and in addition to the nine viral proteins, we identified 36 host proteins. These host proteins are present both inside the influenza virus particle and on the viral envelope. All viruses require host cell factors to complete their replication cycles, and they also have to contend with the antiviral defense mechanisms of the host. Virus–host interactions may therefore provide the key to understanding viral pathogenesis and may also present us with new targets for the design of antiviral drugs. For influenza virus, information on the requirement of cellular factors is limited, but the description of these 36 host proteins that are packaged into the virion provides a foundation for further analysis into the involvement of these cellular pathways in the influenza virus life cycle.

Overcoming hurdles in hepatitis C virus research: efficient production of infectious virus in cell culture

Hepatitis C virus is a flavivirus that infects nearly 2% of the world population. There is no vaccine available and current therapy with interferon and ribavirin is expensive, not well tolerated and effective in only 60% of patients. HCV research has been hampered by the lack of a robust tissue culture system, but recent advances have made virus growth in culture possible. Here we review the current state-of-the-art and the molecular hurdles that have been met and those that still need to be overcome.

Centrifugal enhancement of hepatitis C virus infection of human hepatocytes

Hepatitis C virus (HCV) is a human pathogen associated with chronic liver disease. Recently, the cell culture systems supporting complete replication and production of HCV genotype 2a (JFH1) have been established. This study investigated the effect of low-speed centrifugation on HCV JFH1 infection of human hepatocytes (Huh7.5.1). Higher levels of HCV RNA expression were observed in Huh7.5.1 cells infected with centrifugal inoculation of HCV JFH1 than those in the control cells. This increased HCV RNA expression was associated with the elevated expression of HCV NS3 protein in the hepatocytes. The centrifugal enhancement of HCV infection was time and speed dependent. However, the enhancement was not observed when centrifugation was performed before or after HCV infection. In addition, there was no association between centrifugal enhancement and the expression of HCV entry receptors (CD81 and claudin-1) and intracellular IFN-alpha in the hepatocytes. These data indicate that centrifugal inoculation is a useful tool for increasing the efficiency of HCV infection and replication in the target cells in vitro.

Hepatitis C virus infection in mouse hepatoma cells co-expressing human CD81 and Sip-L

Although human CD81 has been shown to be essential for hepatitis C virus (HCV) infection, non-hepatic cells or transgenic animals expressing human CD81 alone did not support HCV replication. Co-expression of other cofactors was thus necessary for HCV replication. Previously, a hepatic factor named Sip-L was found to support HCV replication in an otherwise non-permissive cell line. To understand the species specificity of hepatic factors required for HCV replication, mouse hepatoma cells co-expressing human CD81 and Sip-L (Hepa1-6-CD81-Sip-L cells) were subjected for HCV infection assay. It was discovered that Hepa1-6-CD81-Sip-L cells were permissive for HCV infection and replication. An animal model was thus established by subcutaneous injection of the permissive cells into nude mice to generate tumors. Viral passages could be achieved in these animals. The antiviral effects of interferon and sodium stibogluconate administrated as a single agent or in combination were demonstrated in this animal model.

Recent advances in our understanding of receptor binding, viral fusion and cell entry of hepatitis C virus: new targets for the design of antiviral agents

Improvements to antiviral therapies for the treatment of hepatitis C virus (HCV) infections will require the use of multiple drugs that target viral proteins essential for replication. The discovery of anti-HCV compounds has been severely hampered by the lack of cell culture replication systems. Since the late 1990s, the advent of sub-genomic replicons that model the intracellular events leading to HCV genome replication have enabled the discovery of HCV protease and polymerase inhibitors, but did not allow the study of HCV entry or entry inhibitors. More recently, retroviral pseudotyping of the viral glycoproteins and the development of a cell culture-based system that recapitulates the entire HCV replication cycle were achieved. These new experimental systems have enabled a rapid advance in our knowledge of how HCV glycoproteins, E1 and E2, mediate receptor binding and viral entry. These systems have facilitated the discovery of a range of viral receptors. Evidence is emerging that CD81, scavenger receptor class B type I, claudin-1 and the low-density lipoprotein receptor are involved in viral entry. In addition, DC-SIGN and L-SIGN may function to internalize virus into dendritic or endothelial cells, facilitating the transport of virions to sites of infection such as the liver. This review focuses on the interaction between the HCV glycoproteins and cellular receptors, and our current understanding of the viral entry pathway. In addition, key questions on the role that these receptors play in viral entry are raised and potential avenues for the discovery of new antiviral agents are highlighted.

CD81 and claudin 1 coreceptor association: role in hepatitis C virus entry

Hepatitis C virus (HCV) is an enveloped positive-stranded RNA hepatotropic virus. HCV pseudoparticles infect liver-derived cells, supporting a model in which liver-specific molecules define HCV internalization. Three host cell molecules have been reported to be important entry factors or receptors for HCV internalization: scavenger receptor BI, the tetraspanin CD81, and the tight junction protein claudin-1 (CLDN1). None of the receptors are uniquely expressed within the liver, leading us to hypothesize that their organization within hepatocytes may explain receptor activity. Since CD81 and CLDN1 act as coreceptors during late stages in the entry process, we investigated their association in a variety of cell lines and human liver tissue. Imaging techniques that take advantage of fluorescence resonance energy transfer (FRET) to study protein-protein interactions have been developed. Aequorea coerulescens green fluorescent protein- and Discosoma sp. red-monomer fluorescent protein-tagged forms of CD81 and CLDN1 colocalized, and FRET occurred between the tagged coreceptors at comparable frequencies in permissive and nonpermissive cells, consistent with the formation of coreceptor complexes. FRET occurred between antibodies specific for CD81 and CLDN1 bound to human liver tissue, suggesting the presence of coreceptor complexes in liver tissue. HCV infection and treatment of Huh-7.5 cells with recombinant HCV E1-E2 glycoproteins and anti-CD81 monoclonal antibody modulated homotypic (CD81-CD81) and heterotypic (CD81-CLDN1) coreceptor protein association(s) at specific cellular locations, suggesting distinct roles in the viral entry process.

The CD81 partner EWI-2wint inhibits hepatitis C virus entry

Two to three percent of the world's population is chronically infected with hepatitis C virus (HCV) and thus at risk of developing liver cancer. Although precise mechanisms regulating HCV entry into hepatic cells are still unknown, several cell surface proteins have been identified as entry factors for this virus. Among these molecules, the tetraspanin CD81 is essential for HCV entry. Here, we have identified a partner of CD81, EWI-2wint, which is expressed in several cell lines but not in hepatocytes. Ectopic expression of EWI-2wint in a hepatoma cell line susceptible to HCV infection blocked viral entry by inhibiting the interaction between the HCV envelope glycoproteins and CD81. This finding suggests that, in addition to the presence of specific entry factors in the hepatocytes, the lack of a specific inhibitor can contribute to the hepatotropism of HCV. This is the first example of a pathogen gaining entry into host cells that lack a specific inhibitory factor.

Dissecting the role of putative CD81 binding regions of E2 in mediating HCV entry: putative CD81 binding region 1 is not involved in CD81 binding

Background

Hepatitis C virus (HCV) encodes two transmembrane glycoproteins E1 and E2 which form a heterodimer. E1 is believed to mediate fusion while E2 has been shown to bind cellular receptors including CD81. In this study, alanine substitutions in E2 were generated within putative CD81 binding regions to define residues critical for viral entry. The effect of each mutation was tested by challenging susceptible cell lines with mutant HCV E1E2 pseudotyped viruses generated using a lentiviral system (HCVpp). In addition to assaying infectivity, producer cell expression and HCVpp incorporation of HCV E1 and E2 proteins, CD81 binding profiles, and E1E2 association of mutants were examined.

Results

Based on these characteristics, mutants either displayed wt characteristics (high infectivity [≥ 50% of wt HCVpp], CD81 binding, E1E2 expression, association, and incorporation into viral particles and proper conformation) or segregated into 4 distinct low infectivity (≤ 50% of wt HCVpp) mutant phenotypes: (I) CD81 binding deficient (despite wt E1E2 expression, incorporation and association and proper conformation); (II) CD81 binding competent, but lack of E1 detection on the viral particle, (despite adequate E1E2 expression in producer cell lysates and proper conformation); (III) CD81 binding competent, with adequate E1E2 expression, incorporation, association, and proper E2 conformation (i.e. no defect identified to explain the reduced infectivity observed); (IV) CD81 binding deficient due to disruption of E2 mutant protein conformation.

Conclusion

Although most alanine substitutions within the putative CD81 binding region 1 (amino acids 474–492) displayed greatly reduced HCVpp infectivity, they retained soluble CD81 binding, proper E2 conformation, E1E2 association and incorporation into HCVpp suggesting that region 1 of E2 does not mediate binding to CD81. In contrast, conformationally correct E2 mutants (Y527 and W529) within the second putative CD81 binding region (amino acids 522–551) disrupted binding of E2 to CD81-GST, suggesting that region 2 is critical to CD81 binding. Likewise, all conformationally intact mutants within the third putative CD81 binding region (amino acids 612–619), except L615A, were important for E2 binding to CD81-GST. This region is highly conserved across genotypes, underlining its importance in mediating viral entry.

The tight junction proteins claudin-1, -6, and -9 are entry cofactors for hepatitis C virus

Hepatitis C virus (HCV) is a major cause of liver disease in humans. The CD81 tetraspanin is necessary but not sufficient for HCV penetration into hepatocytes, and it was recently reported that the tight junction protein claudin-1 is a critical HCV entry cofactor. Here, we confirm the role of claudin-1 in HCV entry. In addition, we show that claudin-6 and claudin-9 expressed in CD81(+) cells also enable the entry of HCV pseudoparticles derived from six of the major genotypes. Whereas claudin-1, -6, and -9 function equally well as entry cofactors in endothelial cells, claudin-1 is more efficient in hepatoma cells. This suggests that additional cellular factors modulate the ability of claudins to function as HCV entry cofactors. Our work has generated novel and essential means to investigate the mechanism of HCV penetration into hepatocytes and the role of the claudin protein family in HCV dissemination, replication, and pathogenesis.

Correlation of the tight junction-like distribution of Claudin-1 to the cellular tropism of hepatitis C virus

Claudin-1 (CLDN1), a tight junction (TJ) protein, has recently been identified as an entry co-receptor for hepatitis C virus (HCV). Ectopic expression of CLDN1 rendered several non-hepatic cell lines permissive to HCV infection. However, little is known about the mechanism by which CLDN1 mediates HCV entry. It is believed that an additional entry receptor(s) is required because ectopic expression of CLDN1 in both HeLa and NIH3T3 cells failed to confer susceptibility to viral infection. Here we found that CLDN1 was co-immunoprecipitated with both HCV envelope proteins when expressed in 293T cells. Results from biomolecular fluorescence complementation assay showed that overexpressed CLDN1 also formed complexes with CD81 and low density lipoprotein receptor. Subsequent imaging analysis revealed that CLDN1 was highly enriched at sites of cell-cell contact in permissive cell lines, co-localizing with the TJ marker, ZO-1. However, in both HeLa and NIH3T3 cells the ectopically expressed CLDN1 appeared to reside predominantly in intracellular vesicles. The CLDN1-CD81 complex formed in HeLa cells was also exclusively distributed intracellularly, co-localizing with EEA1, an early endosomal marker. Correspondingly, transepithelial electric resistance, obtained from the naturally susceptible human liver cell line, Huh7, was much higher than that of the HeLa-CLDN1 cell line, suggesting that Huh7 is likely to form functional tight junctions. Finally, the disruption of TJ-enriched CLDN1 by tumor necrosis factor-alpha treatment markedly reduced the susceptibility of Huh7.5.1 cells to HCV infection. Our results suggest that the specific localization pattern of CLDN1 may be crucial in the regulation of HCV cellular tropism.

The transferrin receptor and the tetraspanin web molecules CD9, CD81, and CD9P-1 are differentially sorted into exosomes after TPA treatment of K562 cells

Here we show that treatment of K562 cells with the phorbol ester TPA induces the down-modulation of various surface antigens. Among them, the transferrin receptor (TfR), the tetraspanin CD81, and a CD81-associated protein, CD9P-1, were unique in that their expression levels were lower after 24 h incubation than after 3 h. We demonstrated that like the TfR, CD81 was internalized at early times, and was less synthesized at latter times. Despite the association of a fraction of the TfR with CD81, these two molecules were subjected to different fates. TPA increased targeting of CD81 and CD9P-1 into exosomes but strongly reduced the localization of the TfR in these vesicles. Using this model we have shown that a fraction of CD81 and CD9P-1 in exosomes comes from a surface pool and that these molecules remain associated in exosomes. However, CD9P-1 could be targeted to exosomes in the absence of CD81 and of another tetraspanin, CD9. The targeting of CD9 into exosomes did not require palmitoylation of the protein. J. Cell. Biochem. 102: 650-664, 2007. (c) 2007 Wiley-Liss, Inc.

Ceramide enrichment of the plasma membrane induces CD81 internalization and inhibits hepatitis C virus entry

Virus entry is a major step in which host-cell lipids can play an essential role. In this report, we investigated the importance of sphingolipids in hepatitis C virus (HCV) entry. For this purpose, sphingomyelin present in the plasma membrane of target cells was hydrolysed into ceramide by sphingomyelinase treatment. Interestingly, ceramide enrichment of the plasma membrane strongly inhibited HCV entry. To understand how ceramide affected HCV entry, we analysed the effect of ceramide enrichment of the plasma membrane on three cell-surface molecules identified as entry factors for HCV: CD81 tetraspanin, scavenger receptor BI and Claudin-1. These proteins, which we found to be mainly associated with detergent-soluble membranes in Huh-7 cells, were not relocated in detergent-resistant microdomains after sphingomyelin hydrolysis into ceramide. Importantly, ceramide enrichment of the plasma membrane led to a 50% decrease in cell-surface CD81, which was due to its ATP-independent internalization. Our results strongly suggest that the ceramide-induced internalization of CD81 is responsible for the inhibitory effect of ceramide on HCV entry. Together, these data indicate that some specific lipids of the plasma membrane are essential for HCV entry and highlight plasma membrane lipids as potential targets to block HCV entry.

Serum-derived hepatitis C virus infection of primary human hepatocytes is tetraspanin CD81 dependent

Hepatitis C virus-positive serum (HCVser, genotypes 1a to 3a) or HCV cell culture (JFH1/HCVcc) infection of primary normal human hepatocytes was assessed by measuring intracellular HCV RNA strands. Anti-CD81 antibodies and siRNA-CD81 silencing markedly inhibited (>90%) HCVser infection irrespective of HCV genotype, viral load, or liver donor, while hCD81-large intracellular loop (LEL) had no effect. However, JFH1/HCVcc infection of hepatocytes was modestly inhibited (40 to 60%) by both hCD81-LEL and anti-CD81 antibodies. In conclusion, CD81 is involved in HCVser infection of human hepatocytes, and comparative studies of HCVser versus JFH1/HCVcc infection of human hepatocytes and Huh-7.5 cells revealed that the cell-virion combination is determinant of the entry process.

Hepatitis C viral life cycle

Hepatitis C virus (HCV) has been recognized as a major cause of chronic liver diseases worldwide. Molecular studies of the virus became possible with the successful cloning of its genome in 1989. Although much work remains to be done regarding early and late stages of the HCV life cycle, significant progress has been made with respect to the molecular biology of HCV, especially the viral protein processing and the genome replication. This review summarizes our current understanding of genomic organization of HCV, features of the viral protein characteristics, and the viral life cycle.

Reliance of host cholesterol metabolic pathways for the life cycle of hepatitis C virus

Hepatitis C virus (HCV), a single-stranded positive-sense RNA virus of the Flaviviridae family, infects more than 170 million people worldwide and is the leading cause of liver failure in the United States. A unique feature of HCV is that the viral life cycle depends on cholesterol metabolism in host cells. This review summarizes the cholesterol metabolic pathways that are required for the replication, secretion, and entry of HCV. The potential application of drugs that alter host cholesterol metabolism in treating HCV infection is also discussed.

Claudin-6 and claudin-9 function as additional coreceptors for hepatitis C virus

Hepatitis C virus (HCV) is a global challenge to public health. Several factors have been proven to be critical for HCV entry, including the newly identified claudin-1 (CLDN1). However, the mechanism of HCV entry is still obscure. Presently, among the 20 members of the claudin family identified in humans so far, CLDN1 has been the only member shown to be necessary for HCV entry. Recently, we discovered that Bel7402, an HCV-permissive cell line, does not express CLDN1 but expresses other members of claudin family. Among these claudins, CLDN9 was able to mediate HCV entry just as efficiently as CLDN1. We then examined if other members of the claudin family could mediate entry. We show that CLDN6 and CLDN9, but not CLDN2, CLDN3, CLDN4, CLDN7, CLDN11, CLDN12, CLDN15, CLDN17, and CLDN23, were able to mediate the entry of HCV into target cells. We found that CLDN6 and CLDN9 are expressed in the liver, the primary site of HCV replication. We also showed that CLDN6 and CLDN9, but not CLDN1, are expressed in peripheral blood mononuclear cells, an additional site of HCV replication. Through sequence comparison and mutagenesis studies, we show that residues N38 and V45 in the first extracellular loop (EL1) of CLDN9 are necessary for HCV entry.