A targeted functional RNA interference screen uncovers glypican 5 as an entry factor for hepatitis B and D viruses

Combined small molecule and loss-of-function screen uncovers estrogen receptor alpha and CAD as host factors for HDV infection and antiviral targets

OBJECTIVE

Hepatitis D virus (HDV) is a circular RNA virus coinfecting hepatocytes with hepatitis B virus. Chronic hepatitis D results in severe liver disease and an increased risk of liver cancer. Efficient therapeutic approaches against HDV are absent.

DESIGN

Here, we combined an RNAi loss-of-function and small molecule screen to uncover host-dependency factors for HDV infection.

RESULTS

Functional screening unravelled the hypoxia-inducible factor (HIF)-signalling and insulin-resistance pathways, RNA polymerase II, glycosaminoglycan biosynthesis and the pyrimidine metabolism as virus-hepatocyte dependency networks. Validation studies in primary human hepatocytes identified the carbamoyl-phosphatesynthetase 2, aspartate transcarbamylase and dihydroorotase (CAD) enzyme and estrogen receptor alpha (encoded by ESR1) as key host factors for HDV life cycle. Mechanistic studies revealed that the two host factors are required for viral replication. Inhibition studies using N-(phosphonoacetyl)-L-aspartic acid and fulvestrant, specific CAD and ESR1 inhibitors, respectively, uncovered their impact as antiviral targets.

CONCLUSION

The discovery of HDV host-dependency factors elucidates the pathogenesis of viral disease biology and opens therapeutic strategies for HDV cure.

Exosomes in Hepatitis B Virus Transmission and Related Immune Response

The chronicity of Hepatitis B virus (HBV) infection relates to both viral factors and host factors. HBV could result in persistent infection and even serious liver disease, including chronic hepatitis B (CHB), cirrhosis and hepatocellular carcinoma (HCC). Although the HBV vaccine can effectively prevent HBV infection, chronic HBV infection still endangers human health and results in a large social burden. Moreover, the mechanisms underlying the HBV-mediated imbalance of the immune response and persistent infection are not fully understood. Exosomes are extracellular vesicles (EVs) 40-160 nm in size that are released from many cells and transfer specific functional RNAs, proteins, lipids and viral components from donor to recipient cells. These exosome nanovesicles are associated with various biological processes, such as cellular homeostasis, immune response and cancer progression. Besides, previous studies on exosomes have shown that they take part in viral pathogenicity due to the similarity in structure and function between exosomes and enveloped viruses. Moreover, exosome as a novel immunomodulatory carrier plays a significant role in viral immunology. In this review, we focus on the latest progress in understanding the role of exosomes in HBV transmission as well as their vital roles in immune regulation during HBV infection. Furthermore, we discuss the potential clinical applications of exosomes in hepatitis B infection, including the use of exosomes in the auxiliary diagnosis and treatment of hepatitis B.

Optimization-by-design of hepatotropic lipid nanoparticles targeting the sodium-taurocholate cotransporting polypeptide

Active targeting and specific drug delivery to parenchymal liver cells is a promising strategy to treat various liver disorders. Here, we modified synthetic lipid-based nanoparticles with targeting peptides derived from the hepatitis B virus large envelope protein (HBVpreS) to specifically target the sodium-taurocholate cotransporting polypeptide (NTCP; SLC10A1) on the sinusoidal membrane of hepatocytes. Physicochemical properties of targeted nanoparticles were optimized and NTCP-specific, ligand-dependent binding and internalization was confirmed in vitro. The pharmacokinetics and targeting capacity of selected lead formulations was investigated in vivo using the emerging zebrafish screening model. Liposomal nanoparticles modified with 0.25 mol% of a short myristoylated HBV derived peptide, that is Myr-HBVpreS2-31, showed an optimal balance between systemic circulation, avoidance of blood clearance, and targeting capacity. Pronounced liver enrichment, active NTCP-mediated targeting of hepatocytes and efficient cellular internalization were confirmed in mice by ¹¹¹In gamma scintigraphy and fluorescence microscopy demonstrating the potential use of our hepatotropic, ligand-modified nanoparticles.

Phosphodiesterase-induced cAMP degradation restricts hepatitis B virus infection

Hepatitis B virus (HBV) entry into hepatocytes is mediated via a high-affinity interaction between the preS1 glycoprotein and sodium/bile acid cotransporting polypeptide (NTCP). To date, in vitro model systems rely on high multiplicities of infection to achieve infection of cell lines overexpressing human NTCP. This study investigates a novel regulatory pathway for NTCP trafficking to the cell surface, induced by DMSO-mediated cellular differentiation. DMSO rapidly induces high cell surface expression of NTCP and results in increased susceptibility of cells to HBV infection. Additionally, DMSO treatment induces actin, as well as Tubulin reshaping within the cells. We show that direct disruption of the actin and Tubulin network directly enhances NTCP expression and the subsequent susceptibility of cells to HBV infection. DMSO induces these changes via alterations in the levels of cyclic (c)AMP, which participates in the observed actin rearrangements. Blocking of phosphodiesterases (PDEs), which degrade accumulated cAMP, had the same effect as DMSO differentiation and demonstrates that DMSO prevents phosphodiesterase-mediated cAMP degradation. This identifies adenylate cyclase as a novel target for blocking the entry of HBV via targeting the cell surface accumulation of NTCP. This article is part of the theme issue 'Silent cancer agents: multi-disciplinary modelling of human DNA oncoviruses'.

Recapitulation of HDV infection in a fully permissive hepatoma cell line allows efficient drug evaluation

Hepatitis delta virus (HDV) depends on the helper function of hepatitis B virus (HBV), which provides the envelope proteins for progeny virus secretion. Current infection-competent cell culture models do not support assembly and secretion of HDV. By stably transducing HepG2 cells with genes encoding the NTCP-receptor and the HBV envelope proteins we produce a cell line (HepNB2.7) that allows continuous secretion of infectious progeny HDV following primary infection. Evaluation of antiviral drugs shows that the entry inhibitor Myrcludex B (IC₅₀: 1.4 nM) and interferon-α (IC₅₀: 28 IU/ml, but max. 60–80% inhibition) interfere with primary infection. Lonafarnib inhibits virus secretion (IC₅₀: 36 nM) but leads to a substantial intracellular accumulation of large hepatitis delta antigen and replicative intermediates, accompanied by the induction of innate immune responses. This work provides a cell line that supports the complete HDV replication cycle and presents a convenient tool for antiviral drug evaluation.

Hepatitis delta virus (HDV) depends on the envelope proteins of hepatitis B virus (HBV) for virion production. Here, Lempp et al. produce a cell line expressing HBV envelope proteins and their receptor, which allows continuous secretion of infectious progeny HDV and testing of antiviral drugs.

Virological Basis for the Cure of Chronic Hepatitis B

Hepatitis B virus (HBV) has infected one-third of world population, and 240 million people are chronic carriers, to whom a curative therapy is still not available. Similar to other viruses, persistent HBV infection relies on the virus to exploit host cell functions to support its replication and efficiently evade host innate and adaptive antiviral immunity. Understanding HBV replication and concomitant host cell interactions is thus instrumental for development of therapeutics to disrupt the virus-host interactions critical for its persistence and cure chronic hepatitis B. Although the currently available cell culture systems of HBV infection are refractory to genome-wide high throughput screening of key host cellular factors essential for and/or regulating HBV replication, classic one-gene (or pathway)-at-a-time studies in the last several decades have already revealed many aspects of HBV-host interactions. An overview of the landscape of HBV-hepatocyte interaction indicates that, in addition to more tightly suppressing viral replication by directly targeting viral proteins, disruption of key viral-host cell interactions to eliminate or inactivate the covalently closed circular (ccc) DNA, the most stable HBV replication intermediate that exists as an episomal minichromosome in the nucleus of infected hepatocyte, is essential to achieve a functional cure of chronic hepatitis B. Moreover, therapeutic targeting of integrated HBV DNA and their transcripts may also be required to induce hepatitis B virus surface antigen (HBsAg) seroclearance and prevent liver carcinogenesis.

Enveloped viruses distinct from HBV induce dissemination of hepatitis D virus in vivo

Hepatitis D virus (HDV) doesn't encode envelope proteins for packaging of its ribonucleoprotein (RNP) and typically relies on the surface glycoproteins (GPs) from hepatitis B virus (HBV) for virion assembly, envelopment and cellular transmission. HDV RNA genome can efficiently replicate in different tissues and species, raising the possibility that it evolved, and/or is still able to transmit, independently of HBV. Here we show that alternative, HBV-unrelated viruses can act as helper viruses for HDV. In vitro, envelope GPs from several virus genera, including vesiculovirus, flavivirus and hepacivirus, can package HDV RNPs, allowing efficient egress of HDV particles in the extracellular milieu of co-infected cells and subsequent entry into cells expressing the relevant receptors. Furthermore, HCV can propagate HDV infection in the liver of co-infected humanized mice for several months. Further work is necessary to evaluate whether HDV is currently transmitted by HBV-unrelated viruses in humans.

A review on hepatitis D: From virology to new therapies

Hepatitis delta virus (HDV) is a defective virus that requires the hepatitis B virus (HBV) to complete its life cycle in human hepatocytes. HDV virions contain an envelope incorporating HBV surface antigen protein and a ribonucleoprotein containing the viral circular single-stranded RNA genome associated with both forms of hepatitis delta antigen, the only viral encoded protein. Replication is mediated by the host cell DNA-dependent RNA polymerases. HDV infects up to72 million people worldwide and is associated with an increased risk of severe and rapidly progressive liver disease. Pegylated interferon-alpha is still the only available treatment for chronic hepatitis D, with poor tolerance and dismal success rate. Although the development of antivirals inhibiting the viral replication is challenging, as HDV does not possess its own polymerase, several antiviral molecules targeting other steps of the viral life cycle are currently under clinical development: Myrcludex B, which blocks HDV entry into hepatocytes, lonafarnib, a prenylation inhibitor that prevents virion assembly, and finally REP 2139, which is thought to inhibit HBsAg release from hepatocytes and interact with hepatitis delta antigen. This review updates the epidemiology, virology and management of HDV infection.

Decellularized human liver scaffold-based three-dimensional culture system facilitate hepatitis B virus infection

Hepatitis B virus (HBV) study is hampered by lacking of idea cell model which support effective HBV infection and meanwhile recapitulate hepatocyte biology function in vivo. In this study, we developed decellularized human liver scaffolds for cell culture and further applied for HBV infection. As a result, primary human hepatocytes (PHHs) engrafted into liver scaffolds and maintained differentiation with stable albumin secretion and liver-specific gene expression. Comparing to mono-layer cell culture, scaffold-based three-dimensional (3D) culture system significantly augment HBV DNA (including cccDNA), RNA level as well as HBsAg secretion. Moreover, HepG2-NTCP cells cultured on 3D system exhibited higher infection efficiency and longer infection period in vitro. In addition, HBV DNA level was suppressed when anti-HBV medicine Entecavir (ETV) introduced into HepG2-NTCP 3D system. Herein, we evaluated the potential of decellularized human liver scaffold-based in 3D cell culture and disclosed that scaffold-based 3D culture system can facilitate HBV infection in vitro. This 3D culture system could be further applied in HBV-related study. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1744-1753, 2019.

A hepatitis B virus-derived human hepatic cell-specific heparin-binding peptide: identification and application to a drug delivery system

Viruses are naturally evolved nanocarriers that can evade host immune systems, attach specifically to the surfaces of target cells, enter the cells through endocytosis, escape from endosomes efficiently, and then transfer their genomes to host cells. Hepatitis B virus (HBV) is a ∼42 nm enveloped DNA virus that can specifically infect human hepatic cells. To utilize the HBV-derived early infection machinery in synthetic nanocarriers, the human hepatic cell-binding site (i.e., the sodium taurocholate co-transporting polypeptide (NTCP)-binding site, with myristoylated pre-S1(2-47)) and the low pH-dependent fusogenic domain (pre-S1(9-24)) are indispensable for targeting and endosomal escape, respectively. However, cell-surface NTCP has recently been shown not to be involved in the initial attachment of HBV. In this study, we identified a novel heparin-binding site (pre-S1(30-42)) in the N-terminal half of the pre-S1 region, which presumably interacts with cell-surface heparan sulfate proteoglycan (HSPG) and plays a pivotal role in the initial attachment of HBV to human hepatic cells. The evolutionarily conserved amino acid residues Asp-31, Trp-32, and Asp-33 are indispensable for the heparin-binding activity. Liposomes (LPs) displaying the peptide were endocytosed by human hepatic cells in a cell-surface heparin-dependent manner and delivered doxorubicin to human hepatic cells more efficiently than myristoylated pre-S1(2-47)-displaying LPs. These results demonstrated that the pre-S1(30-42) peptide is the most promising HBV-derived targeting peptide for synthetic nanocarriers, and that this peptide exhibits high specificity for human hepatic cells and efficiently induces endocytosis.

Evolution of Hepatitis B Virus Receptor NTCP Reveals Differential Pathogenicities and Species Specificities of Hepadnaviruses in Primates, Rodents, and Bats

Human hepatitis B virus (HBV) is a global health problem, affecting more than 250 million people worldwide. HBV-like viruses, named orthohepadnaviruses, also naturally infect nonhuman primates, rodents, and bats, but their pathogenicity and evolutionary history are unclear. Here, we determined the evolutionary history of the HBV receptors NTCP and GPC5 over millions of years of primate, rodent, and bat evolution. We use this as a proxy to understand the pathogenicity of orthohepadnaviruses in mammalian hosts and to determine the implications for species specificity. We found that NTCP, but not GPC5, has evolved under positive selection in primates (27 species), rodents (18 species), and bats (21 species) although at distinct residues. Notably, the positively selected codons map to the HBV-binding sites in primate NTCP, suggesting past genetic "arms races" with pathogenic orthohepadnaviruses. In rodents, the positively selected codons fall outside and within the presumed HBV-binding sites, which may contribute to the restricted circulation of rodent orthohepadnaviruses. In contrast, the presumed HBV-binding motifs in bat NTCP are conserved, and none of the positively selected codons map to this region. This suggests that orthohepadnaviruses may bind to different surfaces in bat NTCP. Alternatively, the patterns may reflect adaptive changes associated with metabolism rather than pathogens. Overall, our findings further point to NTCP as a naturally occurring genetic barrier for cross-species transmissions in primates, which may contribute to the narrow host range of HBV. In contrast, this constraint seems less important in bats, which may correspond to greater orthohepadnavirus circulation and diversity.IMPORTANCE Chronic infection with hepatitis B virus (HBV) is a major cause of liver disease and cancer in humans. Mammalian HBV-like viruses are also found in nonhuman primates, rodents, and bats. As for most viruses, HBV requires a successful interaction with a host receptor for replication. Cellular receptors are thus key determinants of host susceptibility as well as specificity. One hallmark of pathogenic virus-host relationships is the reciprocal evolution of host receptor and viral envelope proteins, as a result of their antagonistic interaction over time. The dynamics of these so-called "evolutionary arms races" can leave signatures of adaptive selection, which in turn reveal the evolutionary history of the virus-host interaction as well as viral pathogenicity and the genetic determinants of species specificity. Here, we show how HBV-like viruses have shaped the evolutionary history of their mammalian host receptor, as a result of their ancient pathogenicity, and decipher the genetic determinants of cross-species transmissions.

Cell and Animal Models for Studying Hepatitis B Virus Infection and Drug Development

Many cell culture and animal models have been used to study hepatitis B virus (HBV) replication and its effects in the liver; these have facilitated development of strategies to control and clear chronic HBV infection. We discuss the advantages and limitations of systems for studying HBV and developing antiviral agents, along with recent advances. New and improved model systems are needed. Cell culture systems should be convenient, support efficient HBV infection, and reproduce responses of hepatocytes in the human body. We also need animals that are fully permissive to HBV infection, convenient for study, and recapitulate human immune responses to HBV and effects in the liver. High-throughput screening technologies could facilitate drug development based on findings from cell and animal models.

Development of Direct-acting Antiviral and Host-targeting Agents for Treatment of Hepatitis B Virus Infection

Hepatitis B virus (HBV) infection affects approximately 300 million people worldwide. Although antiviral therapies have improved the long-term outcomes, patients often require life-long treatment and there is no cure for HBV infection. New technologies can help us learn more about the pathogenesis of HBV infection and develop therapeutic agents to reduce its burden. We review recent advances in development of direct-acting antiviral and host-targeting agents, some of which have entered clinical trials. We also discuss strategies for unbiased high-throughput screens to identify compounds that inhibit HBV and for repurposing existing drugs.

Hepatitis B virus genome recycling and de novo secondary infection events maintain stable cccDNA levels

BACKGROUND & AIMS

Several steps in the HBV life cycle remain obscure because of a lack of robust in vitro infection models. These steps include particle entry, formation and maintenance of covalently closed circular (ccc) DNA, kinetics of gene expression and viral transmission routes. This study aimed to investigate infection kinetics and cccDNA dynamics during long-term culture.

METHODS

We selected a highly permissive HepG2-NTCP-K7 cell clone engineered to express sodium taurocholate co-transporting polypeptide (NTCP) that supports the full HBV life cycle. We characterized the replication kinetics and dynamics of HBV over six weeks of infection.

RESULTS

HBV infection kinetics showed a slow infection process. Nuclear cccDNA was only detected 24 h post-infection and increased until 3 days post-infection (dpi). Viral RNAs increased from 3 dpi reaching a plateau at 6 dpi. HBV protein levels followed similar kinetics with HBx levels reaching a plateau first. cccDNA levels modestly increased throughout the 45-day study period with 5-12 copies per infected cell. Newly produced relaxed circular DNA within capsids was reimported into the nucleus and replenished the cccDNA pool. In addition to intracellular recycling of HBV genomes, secondary de novo infection events resulted in cccDNA formation. Inhibition of relaxed circular DNA formation by nucleoside analogue treatment of infected cells enabled us to measure cccDNA dynamics. HBV cccDNA decayed slowly with a half-life of about 40 days.

CONCLUSIONS

After a slow infection process, HBV maintains a stable cccDNA pool by intracellular recycling of HBV genomes and via secondary infection. Our results provide important insights into the dynamics of HBV infection and support the future design and evaluation of new antiviral agents.

LAY SUMMARY

Using a unique hepatocellular model system designed to support viral growth, we demonstrate that hepatitis B virus (HBV) has remarkably slow infection kinetics. Establishment of the episomal transcription template and the persistent form of the virus, so called covalently closed circular DNA, as well as viral transcription and protein expression all take a long time. Once established, HBV maintains a stable pool of covalently closed circular DNA via intracellular recycling of HBV genomes and through infection of naïve cells by newly formed virions.

Hepatitis B Virus Evasion From Cyclic Guanosine Monophosphate-Adenosine Monophosphate Synthase Sensing in Human Hepatocytes

Chronic hepatitis B virus (HBV) infection is a major cause of chronic liver disease and cancer worldwide. The mechanisms of viral genome sensing and the evasion of innate immune responses by HBV infection are still poorly understood. Recently, the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) was identified as a DNA sensor. In this study, we investigated the functional role of cGAS in sensing HBV infection and elucidate the mechanisms of viral evasion. We performed functional studies including loss-of-function and gain-of-function experiments combined with cGAS effector gene expression profiling in an infectious cell culture model, primary human hepatocytes, and HBV-infected human liver chimeric mice. Here, we show that cGAS is expressed in the human liver, primary human hepatocytes, and human liver chimeric mice. While naked relaxed-circular HBV DNA is sensed in a cGAS-dependent manner in hepatoma cell lines and primary human hepatocytes, host cell recognition of viral nucleic acids is abolished during HBV infection, suggesting escape from sensing, likely during packaging of the genome into the viral capsid. While the hepatocyte cGAS pathway is functionally active, as shown by reduction of viral covalently closed circular DNA levels in gain-of-function studies, HBV infection suppressed cGAS expression and function in cell culture models and humanized mice. Conclusion: HBV exploits multiple strategies to evade sensing and antiviral activity of cGAS and its effector pathways.

The functional role of sodium taurocholate cotransporting polypeptide NTCP in the life cycle of hepatitis B, C and D viruses

Chronic hepatitis B, C and D virus (HBV, HCV and HDV) infections are a major cause of liver disease and cancer worldwide. Despite employing distinct replication strategies, the three viruses are exclusively hepatotropic, and therefore depend on hepatocyte-specific host factors. The sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes that mediates the transport of bile acids, plays a key role in HBV and HDV entry into hepatocytes. Recently, NTCP has been shown to modulate HCV infection of hepatocytes by regulating innate antiviral immune responses in the liver. Here, we review the current knowledge of the functional role and the molecular and cellular biology of NTCP in the life cycle of the three major hepatotropic viruses, highlight the impact of NTCP as an antiviral target and discuss future avenues of research.

Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

The Epidemiology of Hepatitis D Virus in North Africa: A Systematic Review and Meta-Analysis

BACKGROUND

Hepatitis D virus (HDV) infection has been considered a serious neglected pandemic, particularly in developing countries. The virus causes a more severe disease than mono infection with hepatitis B virus (HBV). The epidemiology of HDV is not well documented in North Africa, which is known to be endemic for HBV. In this study, we explored the prevalence of HDV infection and also attempted to identify factors associated with hepatitis D positive status among chronic hepatitis B patients in North Africa.

METHODS

The electronic databases PubMed, Embase, Scopus, Science Direct, Web of Science, and Google Scholar were comprehensively searched for all papers published between January 1, 1998, and December 31, 2017, using appropriate strategies containing all related keywords, including North Africa, names of countries in the region, and all permutations of hepatitis D virus. The estimated prevalence of HDV in North Africa was calculated as an average of the pooled infection prevalence in each country weighted by the ratio of the country's hepatitis D virus population to the study's sample size in the survey data analysis.

FINDINGS

A total of 312 studies were identified and 32 were included in this study, with a total sample of 4907 individuals screened for HDV. There was considerable variability in the prevalence estimates of HDV within the countries of the region. The overall prevalence of HDV in the general population of North Africa was 5·01% (95% CI: 1·25-8·27) and in liver disease patients it was 20.7% (95% CI:9.87-44.53). Genotype-1 was the most prominent genotype reported in five published studies. Ten studies reported on HDV RNA in participants who were seropositive for HDV, and four studies highlighted the impact of demographic factors (sex and age). No study showed the impact of risk factors on the prevalence of HDV in North Africa.

INTERPRETATION

This review provides a comprehensive assessment of the burden of HDV in Northern Africa. There were significant differences in seroprevalence, study population, and diagnostic testing between the countries in the region. The results presented here will alert health professionals to implement clear policies based on evidence to diminish the burden of HDV infection. Such measures may include but are not restricted to improving the laboratory diagnostic tests and initiating patient data registries and blood screening. Further epidemiological and research studies are needed to explore the risk factors, coinfections, and approaches to increase testing for HDV, particularly in high-risk subpopulations, such as intravenous drug users and immigrants, and to define the consequences of HDV infection in North Africa.

Recapitulation of hepatitis B virus-host interactions in liver organoids from human induced pluripotent stem cells

Therapies against hepatitis B virus (HBV) have improved in recent decades; however, the development of individualized treatments has been limited by the lack of individualized infection models. In this study, we used human induced pluripotent stem cell (hiPSC) to generate a functional liver organoid (LO) that inherited the genetic background of the donor, and evaluated its application in modeling HBV infection and exploring virus–host interactions. To establish a functional hiPSC-LO, we cultured hiPSC-derived endodermal, mesenchymal, and endothelial cells with a chemically defined medium in a three-dimensional microwell culture system. Based on cell-cell interactions, these cells could organize themselves and gradually differentiate into a functional organoid, which exhibited stronger hepatic functions than hiPSC derived hepatic like cell (HLC). Moreover, the functional LO demonstrated more susceptibility to HBV infection than hiPSC-HLC, and could maintain HBV propagation and produce infectious virus for a prolonged duration. Furthermore, we found that virus infection could cause hepatic dysfunction of hiPSC-LOs, with down-regulation of hepatic gene expression, induced release of early acute liver failure markers, and altered hepatic ultrastructure. Therefore, our study demonstrated that HBV infection in hiPSC-LOs could recapitulate virus life cycle and virus induced hepatic dysfunction, suggesting that hiPSC-LOs may provide a promising individualized infection model for the development of individualized treatment for hepatitis.

Bona fide receptor for hepatitis B and D viral infections: Mechanism, research models and molecular drug targets

Hepatitis B infections have become a serious public health issue globally, and the current first-line antiviral treatment for this disease is not a true cure. Recently, sodium taurocholate cotransporting polypeptide (NTCP), a liver-specific bile acid transporter, was identified as a bona fide receptor for hepatitis B virus (HBV) and its satellite virus, hepatitis delta virus (HDV). Identification of the HBV receptor has led to the development of robust cell cultures and provides a potential target for new treatments. This review summarizes the process by which NTCP was discovered and describes its clinical significance as the receptor for HBV and HDV entry.

miR-135a-5p-mediated downregulation of protein tyrosine phosphatase receptor delta is a candidate driver of HCV-associated hepatocarcinogenesis

BACKGROUND AND AIMS

HCV infection is a leading risk factor of hepatocellular carcinoma (HCC). However, even after viral clearance, HCC risk remains elevated. HCV perturbs host cell signalling to maintain infection, and derailed signalling circuitry is a key driver of carcinogenesis. Since protein phosphatases are regulators of signalling events, we aimed to identify phosphatases that respond to HCV infection with relevance for hepatocarcinogenesis.

METHODS

We assessed mRNA and microRNA (miRNA) expression profiles in primary human hepatocytes, liver biopsies and resections of patients with HCC, and analysed microarray and RNA-seq data from paired liver biopsies of patients with HCC. We revealed changes in transcriptional networks through gene set enrichment analysis and correlated phosphatase expression levels to patient survival and tumour recurrence.

RESULTS

We demonstrate that tumour suppressor protein tyrosine phosphatase receptor delta (PTPRD) is impaired by HCV infection in vivo and in HCC lesions of paired liver biopsies independent from tissue inflammation or fibrosis. In liver tissue adjacent to tumour, high PTPRD levels are associated with a dampened transcriptional activity of STAT3, an increase of patient survival from HCC and reduced tumour recurrence after surgical resection. We identified miR-135a-5p as a mechanistic regulator of hepatic PTPRD expression in patients with HCV.

CONCLUSIONS

We previously demonstrated that STAT3 is required for HCV infection. We conclude that HCV promotes a STAT3 transcriptional programme in the liver of patients by suppressing its regulator PTPRD via upregulation of miR-135a-5p. Our results show the existence of a perturbed PTPRD-STAT3 axis potentially driving malignant progression of HCV-associated liver disease.

Nucleic Acid Polymers Are Active against Hepatitis Delta Virus Infection In Vitro

In this study, an in vitro infection model for the hepatitis delta virus (HDV) was used to evaluate the antiviral effects of phosphorothioate nucleic acid polymers (NAPs) and investigate their mechanism of action. The results show that NAPs inhibit HDV infection at concentrations less than 4 μM in cultures of differentiated human hepatoma cells. NAPs were shown to be active at viral entry but inactive postentry on HDV RNA replication. Inhibition was independent of the NAP nucleotide sequence but dependent on both size and amphipathicity of the polymer. NAP antiviral activity was effective against HDV virions bearing the main hepatitis B virus (HBV) immune escape substitutions (D144A and G145R) and was pangenomic with regard to HBV envelope proteins. Furthermore, similar to immobilized heparin, immobilized NAPs could bind HDV particles, suggesting that entry inhibition was due, at least in part, to preventing attachment of the virus to cell surface glycosaminoglycans. The results document NAPs as a novel class of antiviral compounds that can prevent HDV propagation.IMPORTANCE HDV infection causes the most severe form of viral hepatitis in humans and one of the most difficult to cure. Currently, treatments are limited to long-term administration of interferon at high doses, which provide only partial efficacy. There is thus an urgent need for innovative approaches to identify new antiviral against HDV. The significance of our study is in demonstrating that nucleic acid polymers (NAPs) are active against HDV by targeting the envelope of HDV virions. In an in vitro infection assay, NAP activity was recorded at concentrations less than 4 μM in the absence of cell toxicity. Furthermore, the fact that NAPs could block HDV at viral entry suggests their potential to control the spread of HDV in a chronically HBV-infected liver. In addition, NAP anti-HDV activity was pangenomic with regard to HBV envelope proteins and not circumvented by HBsAg substitutions associated with HBV immune escape.

Upcoming pharmacological developments in chronic hepatitis B: can we glimpse a cure on the horizon?

Background

Hepatitis B virus (HBV) chronic infection affects up to 240 million people in the world and it is a common cause of cirrhosis and hepatocellular carcinoma (HCC). HBV covalently closed circular DNA (cccDNA) plays an essential role in HBV persistence and replication. Current pharmacological treatment with nucleos(t)ide analogues (NA) may suppress HBV replication with little or no impact on cccDNA, hence lifelong treatment is required in the vast majority of patients. Clearances of intrahepatic cccDNA and/or HBsAg are critical endpoints for future antiviral therapy in chronic HBV. Recent promising developments targeting different molecular HBV life cycle steps are being pre-clinically tested or have moved forward in early clinical trials.

Methods

We review the current state of the art of these pharmacological developments, mainly focusing on efficacy and safety results, which are expected to lay the ground for future HBV eradication. An inclusive literature search on new treatments of HBV using the following electronic databases: Pubmed/MEDLINE, AMED, CINAHL and the Cochrane Central Register of Controlled Trials. Full-text manuscripts and abstracts published over the last 12 years, from 2005 to March 2011 were reviewed for relevance and reference lists were crosschecked for additional applicable studies regarding new HBV antiviral treatment.

Results

HBV entry inhibitors, HBV core inhibitors, HBV cccDNA transcripts RNA interference, HBV cell apoptosis inducers, HBV RNA, viral proteins and DNA knock down agents, HBV release inhibitors, anti-sense nucleosides, exogenous interferon stimulation, interferon response stimulation and HBV therapeutic vaccines were reviewed.

Conclusion

This review will provide readers with an updated vision of current and foreseeable therapeutic developments in chronic hepatitis B.

Solute Carrier NTCP Regulates Innate Antiviral Immune Responses Targeting Hepatitis C Virus Infection of Hepatocytes

Chronic hepatitis B, C, and D virus (HBV, HCV, and HDV) infections are the leading causes of liver disease and cancer worldwide. Recently, the solute carrier and sodium taurocholate co-transporter NTCP has been identified as a receptor for HBV and HDV. Here, we uncover NTCP as a host factor regulating HCV infection. Using gain- and loss-of-function studies, we show that NTCP mediates HCV infection of hepatocytes and is relevant for cell-to-cell transmission. NTCP regulates HCV infection by augmenting the bile-acid-mediated repression of interferon-stimulated genes (ISGs), including IFITM3. In conclusion, our results uncover NTCP as a mediator of innate antiviral immune responses in the liver, and they establish a role for NTCP in the infection process of multiple viruses via distinct mechanisms. Collectively, our findings suggest a role for solute carriers in the regulation of innate antiviral responses, and they have potential implications for virus-host interactions and antiviral therapies.

•

NTCP is involved in hepatocyte infection by multiple viruses via distinct mechanisms

•

NTCP facilitates HCV infection by modulating innate antiviral responses

•

Solute carrier NTCP is a regulator of antiviral immune responses in the liver

•

This function is relevant for infection and therapies for hepatotropic viruses

Hepatitis B virus: virology, molecular biology, life cycle and intrahepatic spread

Hepatitis B virus is a member of the Hepadnaviridae family and responsible for causing acute and chronic hepatitis in humans. The current estimates of people chronically infected with the virus are put at 250 million worldwide. Immune-mediated liver damage in these individuals may lead to the development of cirrhosis and hepatocellular carcinoma later in life. This review deals with our current understanding of the virology, molecular biology, life cycle and cell-to-cell spread of this very important pathogen, all of which are considered essential for current and future approaches to antiviral treatment.

Current Progress of Virus-mimicking Nanocarriers for Drug Delivery

Nanomedicines often involve the use of nanocarriers as a delivery system for drugs or genes for maximizing the therapeutic effect and/or minimizing the adverse effect. From drug administration to therapeutic activity, nanocarriers must evade the host's immune system, specifically and efficiently target and enter the cell, and release their payload into the cell cytoplasm by endosomal escape. These processes constitute the early infection stage of viruses. Viruses are a powerful natural nanomaterial for the efficient delivery of genetic information by sophisticated mechanisms. Over the past two decades, many virus-inspired nanocarriers have been generated to permit successful drug and gene delivery. In this review, we summarize the early infection machineries of viruses, of which the part has so far been utilized for delivery systems. Furthermore, we describe basics and applications of the bio-nanocapsule, which is a hepatitis B virus-mimicking nanoparticle harboring nearly all activities involved in the early infection machineries (i.e., stealth activity, targeting activity, cell entry activity, endosomal escaping activity).

Upregulation of sodium taurocholate cotransporter polypeptide during hepatogenic differentiation of umbilical cord matrix mesenchymal stem cells facilitates hepatitis B entry

BACKGROUND

Hepatitis B virus (HBV) carriers worldwide number approximately 240 million people and around 780,000 people die every year from HBV infection. HBV entry and uptake are functionally linked to the presence of the human sodium-taurocholate cotransporting peptide (hNTCP) receptor. Recently, our group demonstrated that human umbilical cord matrix stem cells (UCMSCs) become susceptible to HBV after in-vitro hepatogenic differentiation (D-UCMSCs).

METHODS

In the present study, we examined the involvement of hNTCP in governing D-UCMSC susceptibility to HBV infection by characterizing the modulation of this transporter expression during hepatogenic differentiation and by appreciating the inhibition of its activity on infection efficacy.

RESULTS

We show here that in-vitro hepatogenic differentiation upregulated hNTCP mRNA and protein expression as well as its activity in D-UCMSCs. Pre-treatment of D-UCMSCs with taurocholate, a specific NTCP substrate, blocked their infection by HBV which supports the crucial involvement of this transporter in the early steps of the virus entry.

CONCLUSION

Altogether, our data support the usefulness of D-UCMSCs as a unique human and non-transformed in-vitro model to study the early stages of HBV infection thanks to its ability to endogenously regulate the expression of hNTCP.

An Effective Antiviral Approach Targeting Hepatitis B Virus with NJK14047, a Novel and Selective Biphenyl Amide p38 Mitogen-Activated Protein Kinase Inhibitor

Despite recent advances in therapeutic strategies against hepatitis B virus (HBV) infection, chronic hepatitis B remains a major global health burden. Recent studies have shown that targeting host factors instead of viral factors can be an effective antiviral strategy with low risk of the development of resistance. Efforts to identify host factors affecting viral replication have identified p38 mitogen-activated protein kinase (MAPK) as a possible target for antiviral strategies against various viruses, including HBV. Here, a series of biphenyl amides were synthesized as novel p38 MAPK selective inhibitors and assessed for their anti-HBV activities. The suppression of HBV surface antigen (HBsAg) production by these compounds was positively correlated with p38 MAPK-inhibitory activity. The selected compound NJK14047 displayed significant anti-HBV activity, as determined by HBsAg production, HBeAg secretion, and HBV production. NJK14047 efficiently suppressed the secretion of HBV antigens and HBV particles from HBV genome-transfected cells and HBV-infected sodium taurocholate cotransporting polypeptide-expressing human hepatoma cells. Furthermore, NJK14047 treatment resulted in a significant decrease of pregenomic RNA and covalently closed circular DNA (cccDNA) of HBV in HBV-harboring cells, indicating its ability to inhibit HBV replication. Considering that suppression of HBsAg secretion and elimination of cccDNA of HBV are the major aims of anti-HBV therapeutic strategies, the results suggested the potential use of these compounds as a novel class of anti-HBV agents targeting host factors critical for viral infection.

Hepatitis Delta Virus: Replication Strategy and Upcoming Therapeutic Options for a Neglected Human Pathogen

The human Hepatitis Delta Virus (HDV) is unique among all viral pathogens. Encoding only one protein (Hepatitis Delta Antigen; HDAg) within its viroid-like self-complementary RNA, HDV constitutes the smallest known virus in the animal kingdom. To disseminate in its host, HDV depends on a helper virus, the human Hepatitis B virus (HBV), which provides the envelope proteins required for HDV assembly. HDV affects an estimated 15-20 million out of the 240 million chronic HBV-carriers and disperses unequally in disparate geographical regions of the world. The disease it causes (chronic Hepatitis D) presents as the most severe form of viral hepatitis, leading to accelerated progression of liver dysfunction including cirrhosis and hepatocellular carcinoma and a high mortality rate. The lack of approved drugs interfering with specific steps of HDV replication poses a high burden for gaining insights into the molecular biology of the virus and, consequently, the development of specific novel medications that resiliently control HDV replication or, in the best case, functionally cure HDV infection or HBV/HDV co-infection. This review summarizes our current knowledge of HBV molecular biology, presents an update on novel cell culture and animal models to study the virus and provides updates on the clinical development of the three developmental drugs Lonafarnib, REP2139-Ca and Myrcludex B.

A Role for the Host DNA Damage Response in Hepatitis B Virus cccDNA Formation-and Beyond?

Chronic hepatitis B virus (HBV) infection puts more than 250 million people at a greatly increased risk to develop end-stage liver disease. Like all hepadnaviruses, HBV replicates via protein-primed reverse transcription of a pregenomic (pg) RNA, yielding an unusually structured, viral polymerase-linked relaxed-circular (RC) DNA as genome in infectious particles. Upon infection, RC-DNA is converted into nuclear covalently closed circular (ccc) DNA. Associating with cellular proteins into an episomal minichromosome, cccDNA acts as template for new viral RNAs, ensuring formation of progeny virions. Hence, cccDNA represents the viral persistence reservoir that is not directly targeted by current anti-HBV therapeutics. Eliminating cccDNA will thus be at the heart of a cure for chronic hepatitis B. The low production of HBV cccDNA in most experimental models and the associated problems in reliable cccDNA quantitation have long hampered a deeper understanding of cccDNA molecular biology. Recent advancements including cccDNA-dependent cell culture systems have begun to identify select host DNA repair enzymes that HBV usurps for RC-DNA to cccDNA conversion. While this list is bound to grow, it may represent just one facet of a broader interaction with the cellular DNA damage response (DDR), a network of pathways that sense and repair aberrant DNA structures and in the process profoundly affect the cell cycle, up to inducing cell death if repair fails. Given the divergent interactions between other viruses and the DDR it will be intriguing to see how HBV copes with this multipronged host system.

Hepatitis delta and HIV infection

Viral liver diseases are frequent comorbidities and major contributors to death in HIV-positive individuals on antiretroviral therapy. Although cure of hepatitis C and control of hepatitis B with antivirals avert liver disease progression in most HIV-coinfected patients, the lack of satisfactory treatment for hepatitis delta virus (HDV) infection remains a major threat for developing cirrhosis and liver cancer in this population. In the European Union (EU) and North America, sexual contact has replaced injection drug use that has been the major transmission route for HDV in HIV-positive persons. PegIFNα is the only approved HDV therapy; however, sustained HDV-RNA clearance is achieved by less than 25%. The recent discovery of sodium taurocholate cotransporting polypeptide as the key hepatitis B virus (HBV) and HDV cell entry receptor has opened the door to a new therapeutic era. Indeed, promising results have been released using Myrcludex-B, a sodium taurocholate cotransporting polypeptide inhibitor. More encouraging are data with new classes of HDV blockers, such as prenylation inhibitors (i.e. lonafarnib) and nucleic acid polymers. At this time, sustained suppression of HDV replication is the primary goal of HDV therapy, as it is associated with normalization of liver enzymes and histological improvement. Of note, the use of specific antivirals for HDV must be given along with anti-HBV agents to prevent HBV rebounds following removal of viral interference. The lack of persistent forms of HDV-RNA could provide a unique opportunity for curing hepatitis delta, even without eliminating HBV circular covalently closed DNA. Ultimately, suppression of HDV replication along with hepatitis B surface antigen clearance once drugs are off would be the best reflect of hepatitis delta cure.

N-Glycosylation of the Na+-Taurocholate Cotransporting Polypeptide (NTCP) Determines Its Trafficking and Stability and Is Required for Hepatitis B Virus Infection

The sodium/bile acid cotransporter NTCP was recently identified as a receptor for hepatitis B virus (HBV). NTCP is glycosylated and the role of glycans in protein trafficking or viral receptor activity is not known. NTCP contains two N-linked glycosylation sites and asparagine amino acid residues N5 and N11 were mutated to a glutamine to generate NTCP with a single glycan (NTCP-N5Q or NTCP- N11Q) or no glycans (NTCP- N5,11Q). HepG2 cells expressing NTCP with a single glycan supported HBV infection at a comparable level to NTCP-WT. The physiological function of NTCP, the uptake of bile acids, was also not affected in cells expressing these single glycosylation variants, consistent with their trafficking to the plasma membrane. However, glycosylation-deficient NTCP (NTCP-N5,11Q) failed to support HBV infection, showed minimal cellular expression and was degraded in the lysosome. This affected the physiological bile acid transporter function of NTCP-N5,11Q in a similar fashion. In conclusion, N-glycosylation is required for efficient NTCP localization at the plasma membrane and subsequent HBV infection and these characteristics are preserved in NTCP carrying a single carbohydrate moiety.

The current status and future directions of hepatitis B antiviral drug discovery

INTRODUCTION

The current standard care of chronic hepatitis B fails to induce a durable off-drug control of hepatitis B virus (HBV) replication in the majority of treated patients. The primary reasons are its inability to eliminate the covalently closed circular (ccc) DNA, the nuclear form of HBV genome, and restoration of the dysfunctional host antiviral immune response against the virus. Accordingly, discovery and development of therapeutics to completely stop HBV replication, eliminate or functionally inactivate cccDNA as well as activate a functional antiviral immune response against HBV are the primary efforts for finding a cure for chronic hepatitis B. Area covered: Herein, the authors highlight the current efforts of HBV drug discovery and offer their opinions for the future directions of this research. Expert opinion: The authors believe that through a consecutive or overlapping three-stage antiviral and immunotherapy program to: (i) completely stop HBV replication and cccDNA amplification; (ii) reduce viral antigen load and induce HBV surface antigen (HBsAg) seroclearance through eradication or inactivation of cccDNA and RNA interference-mediated degradation of viral mRNA and (iii) activate a functional antiviral immune response against HBV through therapeutic immunization or immunotherapy, a functional cure of chronic HBV infection can be achieved in the majority of chronic HBV carriers.

Antiviral therapy of hepatitis delta virus infection - progress and challenges towards cure

Hepatitis B-/D-virus co-infection causes the most severe form of viral hepatitis, frequently leading to liver cirrhosis, hepatic decompensation and consecutive liver-related mortality. Treatment options for hepatitis delta are limited. The only recommended therapy is pegylated interferon alpha which leads to virological responses in about 25-30% of patients. However, interferon therapy is associated with frequent side-effects and late HDV RNA relapses have been described during long-term follow even in patients who were HDV RNA negative 24 weeks after the end of therapy. Thus, alternative treatment options are urgently needed. Clinical studies have been performed exploring prenylation inhibitors, viral entry inhibitors and nucleic acid polymers to block particle release. We here summarize the progress and challenges towards cure of HDV infection.

Aiming for cure in HBV and HDV infection

Chronic hepatitis B virus (HBV) infection continues to be a major health burden worldwide. Currently available antiviral treatment options for chronic hepatitis B include pegylated interferon alpha2a (PegIFN) or nucleos(t)ide analogues (NAs). The major advantages of NAs are good tolerance and potent antiviral activity associated with high rates of sustained on-treatment response to therapy. The advantages of PegIFN include a finite course of treatment, the absence of drug resistance, and an opportunity to obtain a durable post-treatment response to therapy. Furthermore, PegIFN is the only approved agent known to be active against hepatitis D virus (HDV). The use of these two antiviral agents with different mechanisms of action in combination against hepatitis B is theoretically an attractive approach for treatment. Although several studies have confirmed certain virological advantages of combination therapies, data supporting a long-term clinical benefit for patients are lacking and monotherapy with PegIFN or NAs remains the therapy of choice. Moreover, with the current treatment approaches, only a limited number of patients achieve hepatitis B surface antigen (HBsAg) loss. HBsAg loss is considered a "functional cure", but does not mean viral eradication. There is a need for novel therapeutic approaches that enable not only suppression of viral replication, but resolution of HBV infection. A key challenge is to target covalently closed circular DNA (cccDNA) in the nucleus of infected hepatocytes. The recent development and availability of innovative in vitro and in vivo systems and sensitive molecular techniques has opened new possibilities to study the complex network of interactions that HBV establishes with the host in the course of infection and to define new targets for antiviral strategies. Several new antiviral or immunomodulatory compounds have reached preclinical or clinical testing with the aim of silencing or eradicating cccDNA to achieve functional cure. Many of these strategies may also be effective for the treatment of HDV, which is dependent on HBsAg for its life cycle. This Clinical Trial Watch summarizes the most recent therapeutic strategies designed to directly target the viruses B and D or to improve immune responses during chronic HBV infection.

Hepatitis delta virus: insights into a peculiar pathogen and novel treatment options

Chronic hepatitis D is the most severe form of viral hepatitis, affecting ∼20 million HBV-infected people worldwide. The causative agent, hepatitis delta virus (HDV), is a unique human pathogen: it is the smallest known virus; it depends on HBV to disseminate its viroid-like RNA; it encodes only one protein (HDAg), which has both structural and regulatory functions; and it replicates using predominantly host proteins. The failure of HBV-specific nucleoside analogues to suppress the HBV helper function, and the limitations of experimental systems to study the HDV life cycle, have impeded the development of HDV-specific drugs. Thus, the only clinical regimen for HDV is IFNα, which shows some efficacy but long-term virological responses are rare. Insights into the receptor-mediated entry of HDV, and the observation that HDV assembly requires farnesyltransferase, have enabled novel therapeutic strategies to be developed. Interference with entry, for example through blockade of the HBV-HDV-specific receptor sodium/taurocholate cotransporting polypeptide NTCP by Myrcludex B, and inhibition of assembly by blockade of farnesyltransferase using lonafarnib or nucleic acid polymers such as REP 2139-Ca, have shown promising results in phase II studies. In this Review, we summarize our knowledge of HDV epidemiology, pathogenesis and molecular biology, with a particular emphasis on possible future developments.

Cell Culture Models for the Investigation of Hepatitis B and D Virus Infection

Chronic hepatitis B virus (HBV) and hepatitis D virus (HDV) infections are major causes of liver disease and hepatocellular carcinoma worldwide. Despite the presence of an efficient preventive vaccine, more than 250 million patients are chronically infected with HBV. Current antivirals effectively control but only rarely cure chronic infection. While the molecular biology of the two viruses has been characterized in great detail, the absence of robust cell culture models for HBV and/or HDV infection has limited the investigation of virus-host interactions. Native hepatoma cell lines do not allow viral infection, and the culture of primary hepatocytes, the natural host cell for the viruses, implies a series of constraints restricting the possibilities of analyzing virus-host interactions. Recently, the discovery of the sodium taurocholate co-transporting polypeptide (NTCP) as a key HBV/HDV cell entry factor has opened the door to a new era of investigation, as NTCP-overexpressing hepatoma cells acquire susceptibility to HBV and HDV infections. In this review, we summarize the major cell culture models for HBV and HDV infection, discuss their advantages and limitations and highlight perspectives for future developments.

Modeling Viral Spread

The way in which a viral infection spreads within a host is a complex process that is not well understood. Different viruses, such as human immunodeficiency virus type 1 and hepatitis C virus, have evolved different strategies, including direct cell-to-cell transmission and cell-free transmission, to spread within a host. To what extent these two modes of transmission are exploited in vivo is still unknown. Mathematical modeling has been an essential tool to get a better systematic and quantitative understanding of viral processes that are difficult to discern through strictly experimental approaches. In this review, we discuss recent attempts that combine experimental data and mathematical modeling in order to determine and quantify viral transmission modes. We also discuss the current challenges for a systems-level understanding of viral spread, and we highlight the promises and challenges that novel experimental techniques and data will bring to the field.

Unusual Features of Sodium Taurocholate Cotransporting Polypeptide as a Hepatitis B Virus Receptor

Cell culture (cc)-derived hepatitis B virus (HBV) can infect differentiated HepaRG cells, but efficient infection requires addition of polyethylene glycol (PEG) during inoculation. Identification of sodium taurocholate cotransporting polypeptide (NTCP) as an HBV receptor enabled ccHBV infection of NTCP reconstituted HepG2 cells, although very little hepatitis B surface antigen (HBsAg) is produced. We found infection by patient serum-derived HBV (sHBV), which required purification of viral particles through ultracentrifugation or PEG precipitation, was PEG independent and much more efficient in HepaRG cells than in HepG2/NTCP cells. In contrast to hepatitis B e antigen (HBeAg), HBsAg was not a reliable marker of productive sHBV infection at early time points. A low HBsAg/HBeAg ratio by ccHBV-infected HepG2/NTCP cells was attributable to dimethyl sulfoxide (DMSO) in culture medium, NTCP overexpression, and HBV genotype D. HepG2/NTCP cells released more viral antigens than HepG2 cells after HBV genome delivery by adeno-associated virus, and stable expression of NTCP in a ccHBV producing cell line increased viral mRNAs, proteins, replicative DNA, and covalently closed circular DNA. NTCP protein expression in HepG2/NTCP cells, despite being driven by the cytomegalovirus promoter, was markedly increased by DMSO treatment. This at least partly explains ability of DMSO to promote ccHBV infection in such cell lines. In conclusion, NTCP appeared inefficient to mediate infection by serum-derived HBV. It could promote HBV RNA transcription while inhibiting HBsAg secretion. Efficient PEG-independent sHBV infection of HepaRG cells permits comparative studies of diverse clinical HBV isolates and will help identify additional factors on virion surface promoting attachment to hepatocytes.

IMPORTANCE

Currently in vitro infection with hepatitis B virus (HBV) depends on cell culture-derived HBV inoculated in the presence of polyethylene glycol. We found patient serum-derived HBV could efficiently infect differentiated HepaRG cells independent of polyethylene glycol, which represents a more physiological infection system. Serum-derived HBV has poor infectivity in HepG2 cells reconstituted with sodium taurocholate cotransporting polypeptide (NTCP), the currently accepted HBV receptor. Moreover, HepG2/NTCP cells secreted very little hepatitis B surface antigen after infection with cell culture-derived HBV, which was attributed to NTCP overexpression, genotype D virus, and dimethyl sulfoxide added to culture medium. NTCP could promote HBV RNA transcription, protein expression, and DNA replication in HepG2 cells stably transfected with HBV DNA, while dimethyl sulfoxide could increase NTCP protein level despite transcriptional control by a cytomegalovirus promoter. Therefore, this study revealed several unusual features of NTCP as an HBV receptor and established conditions for efficient serum virus infection in vitro.

HBV cure: why, how, when?

Current HBV treatments control replication and liver disease progression in the vast majority of treated patients. However, HBV patients often require lifelong therapies due to the persistence of transcriptionally active viral cccDNA mini-chromosome in the nucleus, which is not directly targeted by current antiviral therapies. A true complete cure of HBV would require clearance of intranuclear cccDNA from all infected hepatocytes. An intermediate but still relevant step forward that would allow treatment cessation would be reaching a functional cure, equivalent to resolved acute infection, with a durable HBsAg loss±anti-HBs seroconversion, undetectable serum DNA and persistence of cccDNA in a transcriptionally inactive status. Recent advances in technologies and pharmaceutical sciences, including the cloning of the mayor HBV receptor (i.e. the NTCP transporter) and the development in vitro HBV infection models, have heralded a new horizon of innovative antiviral and immune-therapeutic approaches.

Cellular uptake of hepatitis B virus envelope L particles is independent of sodium taurocholate cotransporting polypeptide, but dependent on heparan sulfate proteoglycan

Sodium taurocholate cotransporting polypeptide (NTCP) was recently discovered as a hepatitis B virus (HBV) receptor, however, the detailed mechanism of HBV entry is not yet fully understood. We investigated the cellular entry pathway of HBV using recombinant HBV surface antigen L protein particles (bio-nanocapsules, BNCs). After the modification of L protein in BNCs with myristoyl group, myristoylated BNCs (Myr-BNCs) were found to bind to NTCP in vitro, and inhibit in vitro HBV infection competitively, suggesting that Myr-BNCs share NTCP-dependent infection machinery with HBV. Nevertheless, the cellular entry rates of Myr-BNCs and plasma-derived HBV surface antigen (HBsAg) particles were the same as those of BNCs in NTCP-overexpressing HepG2 cells. Moreover, the cellular entry of these particles was mainly driven by heparan sulfate proteoglycan-mediated endocytosis regardless of NTCP expression. Taken together, cell-surface NTCP may not be involved in the cellular uptake of HBV, while presumably intracellular NTCP plays a critical role.

Hepatitis B virus molecular biology and pathogenesis

As obligate intracellular parasites, viruses need a host cell to provide a milieu favorable to viral replication. Consequently, viruses often adopt mechanisms to subvert host cellular signaling processes. While beneficial for the viral replication cycle, virus-induced deregulation of host cellular signaling processes can be detrimental to host cell physiology and can lead to virus-associated pathogenesis, including, for oncogenic viruses, cell transformation and cancer progression. Included among these oncogenic viruses is the hepatitis B virus (HBV). Despite the availability of an HBV vaccine, 350-500 million people worldwide are chronically infected with HBV, and a significant number of these chronically infected individuals will develop hepatocellular carcinoma (HCC). Epidemiological studies indicate that chronic infection with HBV is the leading risk factor for the development of HCC. Globally, HCC is the second highest cause of cancer-associated deaths, underscoring the need for understanding mechanisms that regulate HBV replication and the development of HBV-associated HCC. HBV is the prototype member of the Hepadnaviridae family; members of this family of viruses have a narrow host range and predominately infect hepatocytes in their respective hosts. The extremely small and compact hepadnaviral genome, the unique arrangement of open reading frames, and a replication strategy utilizing reverse transcription of an RNA intermediate to generate the DNA genome are distinguishing features of the Hepadnaviridae. In this review, we provide a comprehensive description of HBV biology, summarize the model systems used for studying HBV infections, and highlight potential mechanisms that link a chronic HBV-infection to the development of HCC. For example, the HBV X protein (HBx), a key regulatory HBV protein that is important for HBV replication, is thought to play a cofactor role in the development of HBV-induced HCC, and we highlight the functions of HBx that may contribute to the development of HBV-associated HCC.

Visualization of hepatitis B virus entry - novel tools and approaches to directly follow virus entry into hepatocytes

Hepatitis B virus (HBV) is a widespread human pathogen, responsible for chronic infections of ca. 240 million people worldwide. Until recently, the entry pathway of HBV into hepatocytes was only partially understood. The identification of human sodium taurocholate cotransporting polypeptide (NTCP) as a bona fide receptor of HBV has provided us with new tools to investigate this pathway in more details. Combined with advances in virus visualization techniques, approaches to directly visualize HBV cell attachment, NTCP interaction, virion internalization and intracellular transport are now becoming feasible. This review summarizes our current understanding of how HBV specifically enters hepatocytes, and describes possible visualization strategies applicable for a deeper understanding of the underlying cell biological processes.

Entry of hepatitis B and hepatitis D virus into hepatocytes: Basic insights and clinical implications

For almost three decades following the discovery of the human Hepatitis B Virus (HBV) the early events of virus infection (attachment to hepatocytes, specific binding to a receptor on hepatocytes) remained enigmatic. The gradual improvement of tissue culture systems for HBV has enabled the identification of viral determinants for viral infectivity and facilitated the discovery of the human sodium taurocholate co-transporting polypeptide (hNTCP) as a liver specific receptor of HBV and its satellite, the human Hepatitis Delta Virus (HDV). These findings are currently leading basic and clinical research activities in new directions. (1) Stable hNTCP-expressing cell lines have become a valuable platform to study the full HBV replication cycle from its native template, the cccDNA. (2) The suitability of NTCP complemented cell culture systems for high throughput screening approaches will facilitate identification of novel host factors involved in HBV replication (including those that determine the peculiar host specificity of HBV infection) and will enable identification and development of novel drug candidates for improved therapeutics. (3) Since NTCP is a major host-specific restriction factor for HBV and HDV, hNTCP-expressing animals provide the basis for future susceptible in vivo models. (4) The concept obtained with the entry inhibitor Myrcludex B demonstrates that NTCP is a suitable target for clinical interference with viral entry. This will foster further clinical approaches aiming at curative combination therapies.