Biochemical and morphological properties of hepatitis C virus particles and determination of their lipidome

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.

Human apolipoprotein E peptides inhibit hepatitis C virus entry by blocking virus binding

Hepatitis C virus (HCV) entry is a multiple-step process involving a number of host factors and hence represents a promising target for new antiviral drug development. In search of novel inhibitors of HCV infection, we found that a human apolipoprotein E (apoE) peptide, hEP, containing both a receptor binding fragment and a lipid binding fragment of apoE specifically blocked the entry of cell culture grown HCV (HCVcc) at submicromolar concentrations. hEP caused little cytotoxicity in vitro and remained active even if left 24 hours in cell culture. Interestingly, hEP inhibited neither human immunodeficiency virus (HIV)-HCV pseudotypes (HCVpp) nor HIV and Dengue virus (DENV) infection. Further characterization mapped the anti-HCV activity to a 32-residue region that harbors the receptor binding domain of apoE, but this fragment must contain a cysteine residue at the N-terminus to mediate dimer formation. The anti-HCV activity of the peptide appears to be dependent on both its length and sequence and correlates with its ability to bind lipids. Finally, we demonstrated that the apoE-derived peptides directly blocked the binding of both HCVcc and patient serum-derived virus to hepatoma cells as well as primary human hepatocytes.

CONCLUSION

apoE peptides potently inhibit HCV infection and suggest a direct role of apoE in mediating HCV entry. Our findings also highlight the potential of developing apoE mimetic peptides as novel HCV entry inhibitors by targeting HCV-host interactions.

MAP-kinase regulated cytosolic phospholipase A2 activity is essential for production of infectious hepatitis C virus particles

Hepatitis C virus (HCV) has infected around 160 million individuals. Current therapies have limited efficacy and are fraught with side effects. To identify cellular HCV dependency factors, possible therapeutic targets, we manipulated signaling cascades with pathway-specific inhibitors. Using this approach we identified the MAPK/ERK regulated, cytosolic, calcium-dependent, group IVA phospholipase A2 (PLA2G4A) as a novel HCV dependency factor. Inhibition of PLA2G4A activity reduced core protein abundance at lipid droplets, core envelopment and secretion of particles. Moreover, released particles displayed aberrant protein composition and were 100-fold less infectious. Exogenous addition of arachidonic acid, the cleavage product of PLA2G4A-catalyzed lipolysis, but not other related poly-unsaturated fatty acids restored infectivity. Strikingly, production of infectious Dengue virus, a relative of HCV, was also dependent on PLA2G4A. These results highlight previously unrecognized parallels in the assembly pathways of these human pathogens, and define PLA2G4A-dependent lipolysis as crucial prerequisite for production of highly infectious viral progeny.

The human genome encodes more than 30 phospholipase A2s. These enzymes cleave fatty acids at the C2 atom of phosphoglycerides and thus modulate membrane properties. Among all PLA2s only PLA2G4A, which is recruited to perinuclear membranes by Ca²⁺ and activated by extracellular stimuli via the mitogen activated protein kinase pathway, specifically cleaves lipids with arachidonic acid. Metabolism of arachidonic acid yields prostaglandins and leukotriens, important lipid mediators of inflammation. We show that inhibition of PLA2G4A produces aberrant HCV particles and that infectivity is rescued by addition of arachidonic acid. Our results suggest that a specific lipid (arachidonic acid) is essential for production of highly infectious HCV progeny, likely by creating a membrane environment conducive for efficient incorporation of crucial host and viral factors into the lipid envelope of nascent particles. Strikingly, PLA2G4A is also essential for production of highly infectious Dengue Virus (DENV) particles but not for vesicular stomatitis virus (VSV). These observations argue that HCV and DENV which unlike VSV produce particles at intracellular membranes usurp a common host factor (PLA2G4A) for assembly of highly infectious progeny. These findings open new perspectives for antiviral intervention and highlight thus far unrecognized parallels in the assembly pathway of HCV and DENV.

Hepatitis C virus: a new class of virus associated with particles derived from very low-density lipoproteins

Hepatitis C virus (HCV) infects 3% of the world population and is the leading cause of liver failure in the United States. A unique feature of HCV is that the viral particles are integral to very low-density lipoprotein (VLDL)-derived lipoprotein particles. The virus is assembled into VLDL in hepatocytes and released out of the cells together with VLDL. The virus then infects more hepatocytes by entering the cells through the low-density lipoprotein receptor, which mediates uptake of majorities of VLDL-derived lipoprotein particles. These observations suggest that HCV may belong to a novel class of viruses that is associated with VLDL. Understanding the relationship between HCV and VLDL metabolism may reveal new strategies to treat HCV infection.

High plasma level of nucleocapsid-free envelope glycoprotein-positive lipoproteins in hepatitis C patients

Hepatitis C virus (HCV) particles associate viral and lipoprotein moieties to form hybrid lipoviral particles (LVPs). Cell culture-produced HCV (HCVcc) and ex vivo-characterized LVPs primarily differ by their apolipoprotein (apo) B content, which is low for HCVcc, but high for LVPs. Recombinant nucleocapsid-free subviral LVPs are assembled and secreted by apoB-producing cell lines. To determine whether such subviral particles circulate in HCV-infected individuals, LVPs complexed with immunoglobulin were precipitated with protein A from low-density plasma fractions of 36 hepatitis C patients, and their lipid content, apolipoprotein profile, and viral composition were determined. HCV RNA in LVPs was quantified and molar ratios of apoB and HCV genome copy number were calculated. LVPs lipidome from four patients was determined via electrospray ionization/tandem mass spectrometry. Protein A-purified LVPs contained at least the envelope glycoprotein E2 and E2-specific antibodies. LVPs were present in every patient and were characterized by high lipid content, presence of apolipoproteins characteristic of triglyceride-rich lipoproteins (TRLs), HCV RNA, and viral glycoprotein. Importantly, save for four patients, LVPs fractions contained large amounts of apoB, with on average more than 1 × 10(6) apoB molecules per HCV RNA genome. Because there is one apoB molecule per TRL, this ratio suggested that most LVPs are nucleocapsid-free, envelope glycoprotein-containing subviral particles. LVPs and TRLs had similar composition of triacylglycerol and phospholipid classes.

CONCLUSION

LVPs are a mixed population of particles, comprising predominantly subviral particles that represent a distinct class of modified lipoproteins within the TRL family.

Apolipoprotein-E and hepatitis C lipoviral particles in genotype 1 infection: evidence for an association with interferon sensitivity

BACKGROUND & AIMS

Hepatitis C virus (HCV) interacts with apolipoproteins B (apoB) and E (apoE) to form infectious lipoviral particles (LVP). Response to peginterferon is influenced by interferon-stimulated genes (ISGs) and IL28B genotype. LDL cholesterol (LDL-C) also predicts interferon response, therefore we hypothesised that LVP may also be associated with interferon sensitivity.

METHODS

LVP (HCV RNA density ≤1.07 g/ml) and 'non-LVP' (d >1.07 g/ml) were measured in 72 fasted HCV-G1 patients by iodixanol density gradient ultracentrifugation and the LVP ratio (LVP/LVP+non-LVP) was calculated. Fasting lipid profiles and apolipoproteins B and E were measured. Interferon-gamma-inducible protein 10 kDa (IP10), a marker of ISGs, was measured by ELISA.

RESULTS

Complete early virological response (EVR) was associated with lower apoE (23.9±7.7 vs. 36.1±15.3 mg/L, p=0.013), higher LDL-C (p=0.039) and lower LVP ratios (p=0.022) compared to null responders. In multivariate linear regression analysis, apoE was independently associated with LVP (R(2) 19.5%, p=0.003) and LVP ratio (p=0.042), and negatively with LDL-C (p<0.001). IP10 was significantly associated with ApoB (p=0.001) and liver stiffness (p=0.032). IL28B rs12979860 CC was associated with complete EVR (p=0.044), low apoE (CC 28±11 vs. CT/TT 35±13 mg/L, p=0.048) and higher non-LVP (p=0.008). Logistic regression analysis indicated that patients with high LVP ratios were less likely to have EVR (odds ratio 0.01, p=0.018).

CONCLUSIONS

In HCV-G1, interferon sensitivity is characterised by low LVP ratios and low apoE levels in addition to higher LDL-C and IL28B rs12979860 CC. Null-response is associated with increased LVP ratio. The association of apoE and LVP with peginterferon treatment response suggests that lipid modulation is a potential target to modify interferon sensitivity.

Role of phosphatidylinositol 4-phosphate (PI4P) and its binding protein GOLPH3 in hepatitis C virus secretion

Hepatitis C virus (HCV) RNA replicates within the ribonucleoprotein complex, assembled on the endoplasmic reticulum (ER)-derived membranous structures closely juxtaposed to the lipid droplets that facilitate the post-replicative events of virion assembly and maturation. It is widely believed that the assembled virions piggy-back onto the very low density lipoprotein particles for secretion. Lipid phosphoinositides are important modulators of intracellular trafficking. Golgi-localized phosphatidylinositol 4-phosphate (PI4P) recruits proteins involved in Golgi trafficking to the Golgi membrane and promotes anterograde transport of secretory proteins. Here, we sought to investigate the role of Golgi-localized PI4P in the HCV secretion process. Depletion of the Golgi-specific PI4P pool by Golgi-targeted PI4P phosphatase hSac1 K2A led to significant reduction in HCV secretion without any effect on replication. We then examined the functional role of a newly identified PI4P binding protein GOLPH3 in the viral secretion process. GOLPH3 is shown to maintain a tensile force on the Golgi, required for vesicle budding via its interaction with an unconventional myosin, MYO18A. Silencing GOLPH3 led to a dramatic reduction in HCV virion secretion, as did the silencing of MYO18A. The reduction in virion secretion was accompanied by a concomitant accumulation of intracellular virions, suggesting a stall in virion egress. HCV-infected cells displayed a fragmented and dispersed Golgi pattern, implicating involvement in virion morphogenesis. These studies establish the role of PI4P and its interacting protein GOLPH3 in HCV secretion and strengthen the significance of the Golgi secretory pathway in this process.

Translational research in infectious disease: current paradigms and challenges ahead

In recent years, the biomedical community has witnessed a rapid scientific and technologic evolution after the development and refinement of high-throughput methodologies. Concurrently and consequentially, the scientific perspective has changed from the reductionist approach of meticulously analyzing the fine details of a single component of biology to the "holistic" approach of broadmindedly examining the globally interacting elements of biological systems. The emergence of this new way of thinking has brought about a scientific revolution in which genomics, proteomics, metabolomics, and other "omics" have become the predominant tools by which large amounts of data are amassed, analyzed, and applied to complex questions of biology that were previously unsolvable. This enormous transformation of basic science research and the ensuing plethora of promising data, especially in the realm of human health and disease, have unfortunately not been followed by a parallel increase in the clinical application of this information. On the contrary, the number of new potential drugs in development has been decreasing steadily, suggesting the existence of roadblocks that prevent the translation of promising research into medically relevant therapeutic or diagnostic application. In this article, we will review, in a noninclusive fashion, several recent scientific advancements in the field of translational research, with a specific focus on how they relate to infectious disease. We will also present a current picture of the limitations and challenges that exist for translational research, as well as ways that have been proposed by the National Institutes of Health to improve the state of this field.

Anti-hepatitis C virus drugs in development

Development of robust cell culture models for hepatitis C viral infection has greatly increased our understanding of this virus and its life cycle. This knowledge has led to the development of many drugs that target specific elements of viral replication, including viral proteins and host factors required for replication. The NS3/4A serine protease inhibitors were the first of these to be used in the clinic, and reagents that target other elements of the viral lifecycle are in advanced stages of clinical development. These include new NS3/4A protease inhibitors, NS5B RNA-dependent RNA polymerase inhibitors, NS5A inhibitors, and host-directed antivirals, such as cyclophilin inhibitors. Alternative interferons with possibly improved tolerability, specifically interferon-λ1 (interleukin-29), are also under development. These new reagents against hepatitis C virus should lead to highly effective, well-tolerated, and likely interferon-sparing therapies in the next several years.

Hepatitis C virus attachment mediated by apolipoprotein E binding to cell surface heparan sulfate

Viruses are known to use virally encoded envelope proteins for cell attachment, which is the very first step of virus infection. In the present study, we have obtained substantial evidence demonstrating that hepatitis C virus (HCV) uses the cellular protein apolipoprotein E (apoE) for its attachment to cells. An apoE-specific monoclonal antibody was able to efficiently block HCV attachment to the hepatoma cell line Huh-7.5 as well as primary human hepatocytes. After HCV bound to cells, however, anti-apoE antibody was unable to inhibit virus infection. Conversely, the HCV E2-specific monoclonal antibody CBH5 did not affect HCV attachment but potently inhibited HCV entry. Similarly, small interfering RNA-mediated knockdown of the key HCV receptor/coreceptor molecules CD81, claudin-1, low-density lipoprotein receptor (LDLr), occludin, and SR-BI did not affect HCV attachment but efficiently suppressed HCV infection, suggesting their important roles in HCV infection at postattachment steps. Strikingly, removal of heparan sulfate from the cell surface by treatment with heparinase blocked HCV attachment. Likewise, substitutions of the positively charged amino acids with neutral or negatively charged residues in the receptor-binding region of apoE resulted in a reduction of apoE-mediating HCV infection. More importantly, mutations of the arginine and lysine to alanine or glutamic acid in the receptor-binding region ablated the heparin-binding activity of apoE, as determined by an in vitro heparin pulldown assay. HCV attachment could also be inhibited by a synthetic peptide derived from the apoE receptor-binding region. Collectively, these findings demonstrate that apoE mediates HCV attachment through specific interactions with cell surface heparan sulfate.

Modulation of hepatitis C virus release by the interferon-induced protein BST-2/tetherin

Hepatitis C virus is a leading cause of chronic hepatitis and liver cancer. Little information exists on the interplay between innate defense mechanisms and viral antagonists that promote viral egress. Herein, the effects of Tetherin/BST-2 on HCV release were investigated. In Huh-7.5 hepatocytes, low expression levels of BST-2 were detected. Treatment of Huh-7.5 cells with IFNα, elevated BST-2 expression levels. However, HCV could not alter the expression of IFNα-induced BST-2, nor of stably over-expressed BST-2. Significantly, over expressed BST-2 moderately blocked HCV production and release from Huh-7.5 cells. Functional analysis of BST-2, confirmed its ability to inhibit the release of HIV delta-Vpu from Huh-7.5-BST-2 cells. HIV-Vpu antagonized BST-2 activity and rescued HIV delta-Vpu release from Huh-7.5-BST-2 cells. However, vpu slightly rescued HCV release and production from Huh-7.5-BST-2. We conclude that BST-2 moderately restricts HCV production and release from Huh-7.5 hepatocytes, while the virus lacks mechanisms to counteract this restriction.

Exploitation of lipid components by viral and host proteins for hepatitis C virus infection

Hepatitis C virus (HCV), which is a major causative agent of blood-borne hepatitis, has chronically infected about 170 million individuals worldwide and leads to chronic infection, resulting in development of steatosis, cirrhosis, and eventually hepatocellular carcinoma. Hepatocellular carcinoma associated with HCV infection is not only caused by chronic inflammation, but also by the biological activity of HCV proteins. HCV core protein is known as a main component of the viral nucleocapsid. It cooperates with host factors and possesses biological activity causing lipid alteration, oxidative stress, and progression of cell growth, while other viral proteins also interact with host proteins including molecular chaperones, membrane-anchoring proteins, and enzymes associated with lipid metabolism to maintain the efficiency of viral replication and production. HCV core protein is localized on the surface of lipid droplets in infected cells. However, the role of lipid droplets in HCV infection has not yet been elucidated. Several groups recently reported that other viral proteins also support viral infection by regulation of lipid droplets and core localization in infected cells. Furthermore, lipid components are required for modification of host factors and the intracellular membrane to maintain or up-regulate viral replication. In this review, we summarize the current status of knowledge regarding the exploitation of lipid components by viral and host proteins in HCV infection.

Exiting from uncharted territory: hepatitis C virus assembles in mouse cell lines

BACKGROUND & AIMS

Hepatitis C virus (HCV) has a high propensity to establish persistence; better understanding of this process requires the development of a fully permissive and immunocompetent small animal model. Mouse cells can be engineered to express the human orthologs of the entry molecules CD81 and occludin to allow entry of HCV. However, RNA replication is poor in mouse cells, and it is not clear whether they support assembly and release of infectious HCV particles. We used a trans-complementation-based system to demonstrate HCV assembly competence of mouse liver cell lines.

METHODS

A panel of 3 mouse hepatoma cell lines that contain a stable subgenomic HCV replicon was used for ectopic expression of the HCV structural proteins, p7, nonstructural protein 2, and/or apolipoprotein E (ApoE). Assembly and release of infectious HCV particles was determined by measuring viral RNA, proteins, and infectivity of virus released into the culture supernatant.

RESULTS

Mouse replicon cells released low amounts of HCV particles, but ectopic expression of apoE increased release of infectious HCV to levels observed in the human hepatoma cell line Huh7.5. ApoE is the limiting factor for assembly of HCV in mouse hepatoma cells but probably not in primary mouse hepatocytes. Products of all 3 human alleles of apoE and mouse apoE support HCV assembly with comparable efficiency. Mouse and human cell-derived HCV particles have similar biophysical properties, dependency on entry factors, and levels of association with ApoE. CONCLUSIONS: Mouse hepatic cells permit HCV assembly and might be developed to create an immunocompetent and fully permissive mouse model of HCV infection.

CD59 incorporation protects hepatitis C virus against complement-mediated destruction

Several enveloped viruses including human immunodeficiency virus type 1 (HIV-1), cytomegalovirus (CMV), herpes simplex virus 1 (HSV-1), Ebola virus, vaccinia virus, and influenza virus have been found to incorporate host regulators of complement activation (RCA) into their viral envelopes and, as a result, escape antibody-dependent complement-mediated lysis (ADCML). Hepatitis C virus (HCV) is an enveloped virus of the family Flaviviridae and incorporates more than 10 host lipoproteins. Patients chronically infected with HCV develop high-titer and crossreactive neutralizing antibodies (nAbs), yet fail to clear the virus, raising the possibility that HCV may also use the similar strategy of RCA incorporation to escape ADCML. The current study was therefore undertaken to determine whether HCV virions incorporate biologically functional CD59, a key member of RCA. Our experiments provided several lines of evidence demonstrating that CD59 was associated with the external membrane of HCV particles derived from either Huh7.5.1 cells or plasma samples from HCV-infected patients. First, HCV particles were captured by CD59-specific Abs. Second, CD59 was detected in purified HCV particles by immunoblot analysis and in the cell-free supernatant from HCV-infected Huh7.5.1 cells, but not from uninfected or adenovirus serotype 5 (Ad5) (a nonenveloped cytolytic virus)-infected Huh7.5.1 cells by enzyme-linked immunosorbent assay. Last, abrogation of CD59 function with its blockers increased the sensitivity of HCV virions to ADCML, resulting in a significant reduction of HCV infectivity. Additionally, direct addition of CD59 blockers into plasma samples from HCV-infected patients increased autologous virolysis.

CONCLUSION

Our study, for the first time, demonstrates that CD59 is incorporated into both cell line-derived and plasma primary HCV virions at levels that protect against ADCML. This is also the first report to show that direct addition of RCA blockers into plasma from HCV-infected patients renders endogenous plasma virions sensitive to ADCML.

Molecular determinants and dynamics of hepatitis C virus secretion

The current model of hepatitis C virus (HCV) production involves the assembly of virions on or near the surface of lipid droplets, envelopment at the ER in association with components of VLDL synthesis, and egress via the secretory pathway. However, the cellular requirements for and a mechanistic understanding of HCV secretion are incomplete at best. We combined an RNA interference (RNAi) analysis of host factors for infectious HCV secretion with the development of live cell imaging of HCV core trafficking to gain a detailed understanding of HCV egress. RNAi studies identified multiple components of the secretory pathway, including ER to Golgi trafficking, lipid and protein kinases that regulate budding from the trans-Golgi network (TGN), VAMP1 vesicles and adaptor proteins, and the recycling endosome. Our results support a model wherein HCV is infectious upon envelopment at the ER and exits the cell via the secretory pathway. We next constructed infectious HCV with a tetracysteine (TC) tag insertion in core (TC-core) to monitor the dynamics of HCV core trafficking in association with its cellular cofactors. In order to isolate core protein movements associated with infectious HCV secretion, only trafficking events that required the essential HCV assembly factor NS2 were quantified. TC-core traffics to the cell periphery along microtubules and this movement can be inhibited by nocodazole. Sub-populations of TC-core localize to the Golgi and co-traffic with components of the recycling endosome. Silencing of the recycling endosome component Rab11a results in the accumulation of HCV core at the Golgi. The majority of dynamic core traffics in association with apolipoprotein E (ApoE) and VAMP1 vesicles. This study identifies many new host cofactors of HCV egress, while presenting dynamic studies of HCV core trafficking in infected cells.

The current model of HCV egress is that virions assemble at lipid droplets, envelope at the ER and then likely exit the hepatocyte via the secretory pathway in association with apolipoproteins. To gain a more detailed insight into infectious HCV release, we combined an RNAi analysis of host factors that are required for infectious HCV secretion with live cell imaging of HCV core trafficking. Using this approach, we identified numerous components of the secretory pathway that are both required for infectious HCV release and co-traffic with HCV core. The dynamics of HCV core trafficking, both in terms of frequency of transport, particle velocity, and the corresponding run lengths were quantified. We observe that dynamic core movements in the periphery require NS2, a viral protein required for virion assembly. Core co-traffics with multiple components of the secretory pathway, including the Golgi, recycling endosome, microtubules, VAMP1 secretory vesicles, and ApoE. This study identifies new molecular determinants of HCV secretion and describes the dynamics of their movements with HCV core in real time.

Hepatitis C virus entry into the hepatocyte

Hepatitis C virus (HCV) is a small enveloped virus with a positive stranded RNA genome belonging to the Flaviviridae family. The virion has the unique ability of forming a complex with lipoproteins, which is known as the lipoviroparticle. Lipoprotein components as well as the envelope proteins, E1 and E2, play a key role in virus entry into the hepatocyte. HCV entry is a complex multistep process involving sequential interactions with several cell surface proteins. The virus relies on glycosaminoglycans and possibly the low-density lipoprotein receptors to attach to cells. Furthermore, four specific entry factors are involved in the following steps which lead to virus internalization and fusion in early endosomes. These molecules are the scavenger receptor SRB1, tetraspanin CD81 and two tight junction proteins, Claudin-1 and Occludin. Although they are essential to HCV entry, the precise role of these molecules is not completely understood. Finally, hepatocytes are highly polarized cells and which likely affects the entry process. Our current knowledge on HCV entry is summarized in this review.

Hepatitis C virus evasion mechanisms from neutralizing antibodies

Hepatitis C virus (HCV) represents a major public health problem, affecting 3% of the world's population. The majority of infected individuals develop chronic hepatitis, which can progress to cirrhosis and hepatocellular carcinoma. To date, a vaccine is not available and current therapy is limited by resistance, adverse effects and high costs. Although it is very well established that cell-mediated immunity is necessary for viral clearance, the importance of host antibodies in clearing HCV infection is being increasingly recognized. Indeed, recent studies indicate that neutralizing antibodies are induced in the early phase of infection by patients who subsequently clear viral infection. Conversely, patients who do not clear the virus develop high titers of neutralizing antibodies during the chronic stage. Surprisingly, these antibodies are not able to control HCV infection. HCV has therefore developed mechanisms to evade immune elimination, allowing it to persist in the majority of infected individuals. A detailed understanding of the mechanisms by which the virus escapes immune surveillance is therefore necessary if novel preventive and therapeutic treatments have to be designed. This review summarizes the current knowledge of the mechanisms used by HCV to evade host neutralizing antibodies.

Hepatitis C virus assembly imaging

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

Role of lipids in virus replication

Viruses intricately interact with and modulate cellular membranes at several stages of their replication, but much less is known about the role of viral lipids compared to proteins and nucleic acids. All animal viruses have to cross membranes for cell entry and exit, which occurs by membrane fusion (in enveloped viruses), by transient local disruption of membrane integrity, or by cell lysis. Furthermore, many viruses interact with cellular membrane compartments during their replication and often induce cytoplasmic membrane structures, in which genome replication and assembly occurs. Recent studies revealed details of membrane interaction, membrane bending, fission, and fusion for a number of viruses and unraveled the lipid composition of raft-dependent and -independent viruses. Alterations of membrane lipid composition can block viral release and entry, and certain lipids act as fusion inhibitors, suggesting a potential as antiviral drugs. Here, we review viral interactions with cellular membranes important for virus entry, cytoplasmic genome replication, and virus egress.

HepG2 cells expressing microRNA miR-122 support the entire hepatitis C virus life cycle

The liver-specific microRNA miR-122 is required for efficient hepatitis C virus (HCV) RNA replication both in cell culture and in vivo. In addition, nonhepatic cells have been rendered more efficient at supporting this stage of the HCV life cycle by miR-122 expression. This study investigated how miR-122 influences HCV replication in the miR-122-deficient HepG2 cell line. Expression of this microRNA in HepG2 cells permitted efficient HCV RNA replication and infectious virion production. When a missing HCV receptor is also expressed, these cells efficiently support viral entry and thus the entire HCV life cycle.

Mouse hepatic cells support assembly of infectious hepatitis C virus particles

BACKGROUND & AIMS

Hepatitis C virus (HCV) has a high propensity to establish persistence; better understanding of this process requires the development of a fully permissive and immunocompetent small animal model. Mouse cells can be engineered to express the human orthologs of the entry molecules CD81 and occludin to allow entry of HCV. However, RNA replication is poor in mouse cells, and it is not clear whether they support assembly and release of infectious HCV particles. We used a trans-complementation-based system to demonstrate HCV assembly competence of mouse liver cell lines.

METHODS

A panel of 3 mouse hepatoma cell lines that contain a stable subgenomic HCV replicon was used for ectopic expression of the HCV structural proteins, p7, nonstructural protein 2, and/or apolipoprotein E (apoE). Assembly and release of infectious HCV particles was determined by measuring viral RNA, proteins, and infectivity of virus released into the culture supernatant.

RESULTS

Mouse replicon cells released low amounts of HCV particles, but ectopic expression of apoE increased release of infectious HCV to levels observed in the human hepatoma cell line Huh7.5. Thus, apoE is the limiting factor for assembly of HCV in mouse hepatoma cells but probably not in primary mouse hepatocytes. Products of all 3 human alleles of apoE and mouse apoE support HCV assembly with comparable efficiency. Mouse and human cell-derived HCV particles have similar biophysical properties, dependency on entry factors, and levels of association with apoE.

CONCLUSIONS

Mouse hepatic cells permit HCV assembly and might be developed to create an immunocompetent and fully permissive mouse model of HCV infection.

Impact of the autophagy machinery on hepatitis C virus infection

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

HCV replication and assembly: a play in one act

HCV represents a serious public health problem worldwide. The current therapy for this virus is only partially effective and new antiviral therapies are urgently needed. Therefore, HCV assembly emerges as a potential therapeutic target. The HCV morphogenesis process presents the peculiarity of the double role of the nonstructural proteins in both the replication and assembly processes. Recently, the cross-talk between structural and nonstructural proteins for virion morphogenesis has been under investigation. We aim to review genetic, cell biology and biochemical data in order to reach a working model for the collaboration of all HCV proteins in the assembly process.

Phenotypic profiling of the human genome reveals gene products involved in plasma membrane targeting of SRC kinases

SRC proteins are non-receptor tyrosine kinases that play key roles in regulating signal transduction by a diverse set of cell surface receptors. They contain N-terminal SH4 domains that are modified by fatty acylation and are functioning as membrane anchors. Acylated SH4 domains are both necessary and sufficient to mediate specific targeting of SRC kinases to the inner leaflet of plasma membranes. Intracellular transport of SRC kinases to the plasma membrane depends on microdomains into which SRC kinases partition upon palmitoylation. In the present study, we established a live-cell imaging screening system to identify gene products involved in plasma membrane targeting of SRC kinases. Based on siRNA arrays and a human model cell line expressing two kinds of SH4 reporter molecules, we conducted a genome-wide analysis of SH4-dependent protein targeting using an automated microscopy platform. We identified and validated 54 gene products whose down-regulation causes intracellular retention of SH4 reporter molecules. To detect and quantify this phenotype, we developed a software-based image analysis tool. Among the identified gene products, we found factors involved in lipid metabolism, intracellular transport, and cellular signaling processes. Furthermore, we identified proteins that are either associated with SRC kinases or are related to various known functions of SRC kinases such as other kinases and phosphatases potentially involved in SRC-mediated signal transduction. Finally, we identified gene products whose function is less defined or entirely unknown. Our findings provide a major resource for future studies unraveling the molecular mechanisms that underlie proper targeting of SRC kinases to the inner leaflet of plasma membranes.

Unique ties between hepatitis C virus replication and intracellular lipids

Hepatitis C virus (HCV) infects approximately 3% of the world's population, establishing a lifelong infection in the majority of cases. The life cycle of HCV is closely tied to the lipid metabolism of liver cells, and lipid droplets have emerged as crucial intracellular organelles that support persistent propagation of viral infection. In this review, we examine recent advances in our understanding of how HCV usurps intracellular lipids to propagate, and highlight unique opportunities for therapeutic intervention.

Characterization and purification of recombinant bovine viral diarrhea virus particles with epitope-tagged envelope proteins

Bovine viral diarrhea virus (BVDV) belongs to the genus Pestivirus within the family Flaviviridae. The lipid membrane of the virions is supposed to contain the three glycosylated envelope proteins Erns, E1 and E2, but detailed studies of virus assembly are complicated because no efficient purification method for pestiviruses has been described so far. In this study, we generated infectious BVDV with N-terminally FLAG-tagged Erns or E2 proteins, respectively. The expression of the epitope-tagged Erns and E2 proteins could be shown by immunofluorescence and Western blot experiments. Furthermore, an affinity tag purification protocol for the isolation and concentration of infectious BVDV was established. In the preparation with a titre of 108.75 TCID50 ml−1, spherical particles with a diameter of 43–58 nm (mean diameter: 48 nm) could be detected by negative staining electron microscopy, and immunogold labelling located both Erns and E2 proteins at the virus membrane.

Structural requirements of virion-associated cholesterol for infectivity, buoyant density and apolipoprotein association of hepatitis C virus

Our earlier study has demonstrated that hepatitis C virus (HCV)-associated cholesterol plays a key role in virus infectivity. In this study, the structural requirement of sterols for infectivity, buoyant density and apolipoprotein association of HCV was investigated further. We removed cholesterol from virions with methyl β-cyclodextrin, followed by replenishment with 10 exogenous cholesterol analogues. Among the sterols tested, dihydrocholesterol and coprostanol maintained the buoyant density of HCV and its infectivity, and 7-dehydrocholesterol restored the physical appearance of HCV, but suppressed its infectivity. Other sterol variants with a 3β-hydroxyl group or with an aliphatic side chain did not restore density or infectivity. We also provide evidence that virion-associated cholesterol contributes to the interaction between HCV particles and apolipoprotein E. The molecular basis for the effects of different sterols on HCV infectivity is discussed.

Multifaceted roles for lipids in viral infection

Viruses have evolved complex and dynamic interactions with their host cell. In recent years we have gained insight into the expanding roles for host lipids in the virus life cycle. In particular, viruses target lipid signaling, synthesis, and metabolism to remodel their host cells into an optimal environment for their replication. This review highlights examples from different viruses that illustrate the importance of these diverse virus–lipid interactions.

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

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

Protein kinase D negatively regulates hepatitis C virus secretion through phosphorylation of oxysterol-binding protein and ceramide transfer protein

Hepatitis C virus (HCV) RNA replicates its genome on specialized endoplasmic reticulum modified membranes termed membranous web and utilizes lipid droplets for initiating the viral nucleocapsid assembly. HCV maturation and/or the egress pathway requires host sphingolipid synthesis, which occur in the Golgi. Ceramide transfer protein (CERT) and oxysterol-binding protein (OSBP) play a crucial role in sphingolipid biosynthesis. Protein kinase D (PKD), a serine/threonine kinase, is recruited to the trans-Golgi network where it influences vesicular trafficking to the plasma membrane by regulation of several important mediators via phosphorylation. PKD attenuates the function of both CERT and OSBP by phosphorylation at their respective Ser(132) and Ser(240) residues (phosphorylation inhibition). Here, we investigated the functional role of PKD in HCV secretion. Our studies show that HCV gene expression down-regulated PKD activation. PKD depletion by shRNA or inhibition by pharmacological inhibitor Gö6976 enhanced HCV secretion. Overexpression of a constitutively active form of PKD suppressed HCV secretion. The suppression by PKD was subverted by the ectopic expression of nonphosphorylatable serine mutant CERT S132A or OSBP S240A. These observations imply that PKD negatively regulates HCV secretion/release by attenuating OSBP and CERT functions by phosphorylation inhibition. This study identifies the key role of the Golgi components in the HCV maturation process.