Regulation of multiple stages of hepadnavirus replication by the carboxyl-terminal domain of viral core protein in trans

SRPK2 Mediates HBV Core Protein Phosphorylation and Capsid Assembly via Docking Interaction

Members of the serine-arginine protein kinase (SRPK) family, SRPK1 and SRPK2, phosphorylate the hepatitis B core protein (Cp) and are crucial for pregenomic RNA encapsidation during viral nucleocapsid assembly. Among them, SRPK2 exhibits higher kinase activity toward Cp. In this study, we identified Cp sites that are phosphorylated by SRPK2 and demonstrated that the kinase utilizes an SRPK-specific docking groove to interact with and regulate the phosphorylation of the C-terminal arginine rich domain of Cp. We determined that direct interaction between the docking groove of SRPK2 and unphosphorylated Cp inhibited premature viral capsid assembly in vitro, whereas the phosphorylation of the viral protein reactivated the process. Pull-down assays together with the new cryo-electron microscopy structure of the HBV capsid in complex with SRPK2 revealed that the kinases decorate the surface of the viral capsid by interacting with the C-terminal domain of Cp, underscoring the importance of the docking interaction in regulating capsid assembly and pregenome packaging. Moreover, SRPK2-knockout in HepG2 cells suppressed Cp phosphorylation, indicating that SRPK2 is an important cellular kinase for HBV life cycle.

Antiviral activities of Polygonum perfoliatum L. extract and related phenolic acid constituents against hepatitis B virus

Chronic hepatitis B virus (HBV) infection is an important public health problem. Polygonum perfoliatum L. is a traditional medicinal herb and has been reported to have pharmacological activities such as anti-inflammatory, antibacterial, and antiviral. In this study, the antiviral activities and mechanisms of Polygonum perfoliatum L. extract against HBV and the effective components were investigated. The results showed that the total extract of Polygonum perfoliatum L. reduced the levels of HBV e antigen (HBeAg) secretion and the viral covalently closed circular DNA (CCC DNA) formation, but had little or no negative effects on viral capsid assembly and pregenomic RNA packaging. Further fractionation showed that the water extract (WE) fraction exerted comparable anti-HBV activities with the total extract, especially in inhibiting the CCC DNA formation and HBeAg production, indicating that the effective antiviral components are mainly distributed in this fraction. Further study showed that the phenolic acids constituents, protocatechuic acid, and gallic acid, but not ethyl caffeate, which is reported enriched in the WE fraction, showed strong anti-HBV activities in inhibiting viral core DNA synthesis, CCC DNA formation, and HBeAg production. These results suggested that the Polygonum perfoliatum L. total extract and the related phenolic acids like protocatechuic acid and gallic acid could inhibit HBV replication and also indicated the potential utility of Polygonum perfoliatum L. and related constituents as sources of novel antivirals against HBV.

Conserved Lysine Residues of Hepatitis B Virus Core Protein Are Not Required for Covalently Closed Circular DNA Formation

Hepatitis B virus (HBV) core protein (HBc), the building block of the viral capsid, plays a critical role throughout the HBV life cycle. There are two highly conserved lysine residues, namely, K7 and K96, on HBc, which have been proposed to function at various stages of viral replication, potentially through lysine-specific posttranslational modifications (PTMs). Here, we substituted K7 and K96 with alanine or arginine, which would also block potential PTMs on these two lysine residues, and tested the effects of these substitutions on HBV replication and infection. We found that the two lysine residues were dispensable for all intracellular steps of HBV replication. In particular, all mutants were competent to form the covalently closed circular DNA (cccDNA) via the intracellular amplification pathway, indicating that K7 and K96, or any PTMs of these residues, were not essential for nucleocapsid uncoating, a prerequisite for cccDNA formation. Furthermore, we found that K7A and K7R mutations did not affect de novo cccDNA formation and RNA transcription during infection, indicating that K7 or any PTMs of this residue were dispensable for HBV infection. In addition, we demonstrated that the HBc K7 coding sequence (AAA), as part of the HBV polyadenylation signal UAUAAA, was indispensable for viral RNA production, implicating this cis requirement at the RNA level, instead of any function of HBc-K7, likely constrains the identity of the 7th residue of HBc. In conclusion, our results provided novel insights regarding the roles of lysine residues on HBc, and their coding sequences, in the HBV life cycle. IMPORTANCE Hepatitis B virus (HBV) infection remains a public health burden that affects 296 million individuals worldwide. HBV core protein (HBc) is involved in almost all steps in the HBV life cycle. There are two conserved lysine residues on HBc. Here, we found that neither of them is essential for HBV intracellular replication, including the formation of covalently closed circular DNA (cccDNA), the molecular basis for establishing and sustaining the HBV infection. However, K96 is critical for virion morphogenesis, while the K7 coding sequence, but not HBc-K7 itself, is indispensable, as part of the RNA polyadenylation signal, for HBV RNA production from cccDNA. Our results provide novel insights regarding the role of the conserved lysine residues on HBc, and their coding sequences, in viral replication, and should facilitate the development of antiviral drugs against the HBV capsid protein.

Host cell-dependent late entry step as determinant of hepatitis B virus infection

Hepatitis B virus (HBV) has a highly restricted host range and cell tropism. Other than the human sodium taurocholate cotransporting polypeptide (huNTCP), the HBV entry receptor, host determinants of HBV susceptibility are poorly understood. Woodchucks are naturally infected with woodchuck hepatitis virus (WHV), closely related to HBV, but not with HBV. Here, we investigated the capabilities of woodchuck hepatic and human non-hepatic cell lines to support HBV infection. DNA transfection assays indicated that all cells tested supported both HBV and WHV replication steps post entry, including the viral covalently closed circular DNA (cccDNA) formation, which is essential for establishing and sustaining infection. Ectopic expression of huNTCP rendered one, but not the other, woodchuck hepatic cell line and the non-hepatic human cell line competent to support productive HBV entry, defined here by cccDNA formation during de novo infection. All huNTCP-expressing cell lines tested became susceptible to infection with hepatitis D virus (HDV) that shares the same entry receptor and initial steps of entry with HBV, suggesting that a late entry/trafficking step(s) of HBV infection was defective in one of the two woodchuck cell lines. In addition, the non-susceptible woodchuck hepatic cell line became susceptible to HBV after fusion with human hepatic cells, suggesting the lack of a host cell-dependent factor(s) in these cells. Comparative transcriptomic analysis of the two woodchuck cell lines revealed widespread differences in gene expression in multiple biological processes that may contribute to HBV infection. In conclusion, other than huNTCP, neither human- nor hepatocyte-specific factors are essential for productive HBV entry. Furthermore, a late trafficking step(s) during HBV infection, following the shared entry steps with HDV and before cccDNA formation, is subject to host cell regulation and thus, a host determinant of HBV infection.

Fundamental studies on, and development of therapies against, chronic hepatitis B virus (HBV) infection, which inflicts hundreds of millions worldwide, are impeded by deficiencies in HBV-susceptible animal models. HBV displays a strict species and cell tropism that are not clearly understood. Here, by studying replication of HBV, and the related woodchuck hepatitis virus, in human and woodchuck hepatic or non-hepatic cells, we found that non-hepatic human cells and some woodchuck hepatic cells could support productive HBV entry after expression of the human cell receptor for HBV. Moreover, by studying the infection of hepatitis D virus, which shares the same entry receptor and initial steps of entry with HBV, we could narrow down a host determinant of HBV infection operating at a late entry/trafficking step(s). Our study thus provides new insights into determinants of HBV host tropism and facilitates the development of HBV-susceptible animal models.

Phosphoproteomics Unravel HBV Triggered Rewiring of Host Phosphosignaling Events

Hepatitis B virus (HBV) infection persists as a major global health problem despite the availability of HBV vaccines for disease prevention. However, vaccination rates remains low in some regions of the world, driving the need for novel strategies to minimise infections and prevent disease progression. Thus, understanding of perturbed molecular signaling events during early phases of HBV infection is required. Phosphosignaling is known to be involved in the HBV infection processes, yet systems-level changes in phosphosignaling pathways in the host during infection remain unclear. To this end, we performed phosphoproteome profiling on HBV-infected HepG2-NTCP cells. Our results showed that HBV infection drastically altered the host phosphoproteome and its associated proteins, including kinases. Computational analysis of this phosphoproteome revealed dysregulation of the pathways involved in immune responses, cell cycle processes, and RNA processing during HBV infection. Kinase Substrate Enrichment Analysis (KSEA) identified the dysregulated activities of important kinases, including those from CMGC (CDK, MAPK, GSK, and CLK), AGC (protein kinase A, G, and C), and TK (Tyrosine Kinase) families. Of note, the inhibition of CLKs significantly reduced HBV infection in HepG2-NTCP cells. In all, our study unravelled the aberrated phosphosignaling pathways and the associated kinases, presenting potential entry points for developing novel therapeutic strategies for HBV treatment.

Region-Specific Hepatitis B Virus Genome Exposure from Nucleocapsid Modulated by Capsid Linker Sequence and Inhibitor: Implications for Uncoating

Hepatitis B virus (HBV) contains a partially double-stranded, relaxed circular (RC) DNA genome synthesized within a nucleocapsid (NC) in the host cell cytoplasm. The release of RC DNA from the NC, in an ill-defined process called uncoating, to the nucleus is required for its conversion to the covalently closed circular (CCC) DNA, the viral episome serving as the transcriptional template for all viral RNAs necessary for replication and, thus, essential for establishing and sustaining viral infection. In efforts to better understand uncoating, we analyzed HBV core (HBc) mutants that show various levels of nuclear CCC DNA but little to no cytoplasmic RC DNA. We found that RC DNA could be synthesized by these mutants outside the cell, but in contrast to the wild type (wt), the mutant NCs were unable to protect RC DNA from digestion by the endogenous nuclease(s) in cellular lysates or exogenous DNase. Subcellular fractionation suggested that the major RC DNA-degrading activity was membrane associated. Digestion with sequence-specific and nonspecific DNases revealed the exposure of specific regions of RC DNA from the mutant NC. Similarly, treatment of wt NCs with a core inhibitor known to increase CCC DNA by affecting uncoating also led to region-specific exposure of RC DNA. Furthermore, a subpopulation of untreated wild type (wt) mature NCs showed site-specific exposure of RC DNA as well. Competition between RC DNA degradation and its conversion to CCC DNA during NC uncoating thus likely plays an important role in the establishment and persistence of HBV infection and has implications for the development of capsid-targeted antivirals. IMPORTANCE Disassembly of the hepatitis B virus (HBV) nucleocapsid (NC) to release its genomic DNA, in an ill-understood process called uncoating, is required to form the viral nuclear episome in the host cell nucleus, a viral DNA essential for establishing and sustaining HBV infection. The elimination of the HBV nuclear episome remains the holy grail for the development of an HBV cure. We report here that the HBV genomic DNA is exposed in a region-specific manner during uncoating, which is enhanced by mutations of the capsid protein and a capsid-targeted antiviral compound. The exposure of the viral genome can result in its rapid degradation or, alternatively, can enhance the formation of the nuclear episome, thus having a major impact on HBV infection and persistence. These results are thus important for understanding fundamental mechanisms of HBV replication and persistence and for the ongoing pursuit of an HBV cure.

Inhibitory Activities of Ranunculus japonicus Thunb. Ethanol Extract against Hepatitis B Virus

The chronic hepatitis B virus (HBV) infection is a worldwide public health problem, which cannot be cured by current therapeutics due to the persistence of viral CCC DNA in the infected hepatocytes. Screening from medicinal herbs for anti-HBV activities showed that the ethanol extract from Ranunculus japonicus Thunb. could decrease the production of HBV e antigen (HBeAg). Further study showed that the extract had no effect on core protein expression but significantly reduced the efficiency of viral capsid assembly. The levels of viral pgRNA and total core DNA were not affected significantly. However, the ratio of RC DNA/SS DNA decreased, indicating that the conversion of RC DNA from SS DNA was delayed by the extract. More interestingly, though similar levels of RC DNA were accumulated, the CCC DNA level and its formation efficiency were reduced significantly, which was also consistent with the decreased level of HBeAg, indicating that Ranunculus japonicus Thunb. extract could inhibit the CCC DNA formation. Together, this study found that Ranunculus japonicus Thunb. extract could inhibit HBV replication at multiple steps, especially showed significant inhibitory effects on capsid assembly and CCC DNA formation. This article is protected by copyright. All rights reserved.

Research progress in hepatitis B virus covalently closed circular DNA

Hepatitis B virus (HBV) infections are a global public health issue. HBV covalently closed circular DNA (cccDNA), the template for the transcription of viral RNAs, is a key factor in the HBV replication cycle. Notably, many host factors involved in HBV cccDNA epigenetic modulation promote the development of hepatocellular carcinoma (HCC). The HBV cccDNA minichromosome is a clinical obstacle that cannot be efficiently eliminated. In this review, we provide an update on the advances in research on HBV cccDNA and further discuss factors affecting the modulation of HBV cccDNA. Hepatitis B virus X protein (HBx) contributes to HBV cccDNA transcription and the development of hepatocarcinogenesis through modulating host epigenetic regulatory factors, thus linking the cccDNA to hepatocarcinogenesis. The measurable serological biomarkers of continued transcription of cccDNA, the effects of anti-HBV drugs on cccDNA, and potential therapeutic strategies targeting cccDNA are discussed in detail. Thus, this review describes new insights into HBV cccDNA mechanisms and therapeutic strategies for cleaning cccDNA, which will benefit patients with liver diseases.

PP2A Phosphatase as an Emerging Viral Host Factor

Protein phosphatase 2A (PP2A) is one of the most ubiquitous cellular proteins and is responsible for the vast majority of Ser/Thr phosphatase activity in eukaryotes. PP2A is a heterotrimer, and its assembly, intracellular localization, enzymatic activity, and substrate specificity are subject to dynamic regulation. Each of its subunits can be targeted by viral proteins to hijack and modulate its activity and downstream signaling to the advantage of the virus. Binding to PP2A is known to be essential to the life cycle of many viruses and seems to play a particularly crucial role for oncogenic viruses, which utilize PP2A to transform infected cells through controlling the cell cycle and apoptosis. Here we summarise the latest developments in the field of PP2A viral targeting; in particular recent discoveries of PP2A hijacking through molecular mimicry of a B56-specific motif by several different viruses. We also discuss the potential as well as shortcomings for therapeutic intervention in the face of our current understanding of viral PP2A targeting.

Regulation of Hepatitis B Virus Replication by Cyclin Docking Motifs in Core Protein

Hepatitis B virus (HBV) capsid or core protein (HBc) consists of an N-terminal domain (NTD) and a C-terminal domain (CTD) connected by a short linker peptide. Dynamic phosphorylation and dephosphorylation of HBc regulate its multiple functions in capsid assembly and viral replication. The cellular cyclin-dependent kinase 2 (CDK2) plays a major role in HBc phosphorylation and, furthermore, is incorporated into the viral capsid, accounting for most of the "endogenous kinase" activity associated with the capsid. The packaged CDK2 is thought to play a role in phosphorylating HBc to trigger nucleocapsid disassembly (uncoating), an essential step during viral infection. However, little is currently known on how CDK2 is recruited and packaged into the capsid. We have now identified three RXL motifs in the HBc NTD known as cyclin docking motifs (CDMs), which mediate the interactions of various CDK substrates/regulators with CDK/cyclin complexes. Mutations of the CDMs in the HBc NTD reduced CTD phosphorylation and diminished CDK2 packaging into the capsid. Also, the CDM mutations showed little effects on capsid assembly and pregenomic RNA (pgRNA) packaging but impaired the integrity of mature nucleocapsids. Furthermore, the CDM mutations blocked covalently closed circular DNA (CCC DNA) formation during infection while having no effect on or enhancing CCC DNA formation via intracellular amplification. These results indicate that the HBc NTD CDMs play a role in CDK2 recruitment and packaging, which, in turn, is important for productive infection.IMPORTANCE Hepatitis B virus (HBV) is an important global human pathogen and persistently infects hundreds of millions of people, who are at high risk of cirrhosis and liver cancer. HBV capsid packages a host cell protein kinase, the cyclin-dependent kinase 2 (CDK2), which is thought to be required to trigger disassembly of the viral nucleocapsid during infection by phosphorylating the capsid protein, a prerequisite for successful infection. We have identified docking sites on the capsid protein for recruiting CDK2, in complex with its cyclin partner, to facilitate capsid protein phosphorylation and CDK2 packaging. Mutations of these docking sites reduced capsid protein phosphorylation, impaired CDK2 packaging into HBV capsids, and blocked HBV infection. These results provide novel insights regarding CDK2 packaging into HBV capsids and the role of CDK2 in HBV infection and should facilitate the development of antiviral drugs that target the HBV capsid protein.

In vitro expression of precore proteins of hepatitis B virus subgenotype A1 is affected by HBcAg, and can affect HBsAg secretion

HBeAg, a non-particulate protein of hepatitis B virus (HBV), is translated from the precore/core region as a precursor, which is post-translationally modified. Subgenotype A1 of HBV, which is a risk factor for hepatocellular carcinoma (HCC), has unique molecular characteristics in the basic core promoter/precore regions. Carriers of A1 exhibit early HBeAg loss. We sought to further characterize the precore proteins of A1 in vitro. HuH-7 cells were transfected with subgenomic constructs expressing individual precore proteins. Western blot analysis using DAKO anti-core antibody showed the expected sizes and a 1 kDa larger band for P22, P20 and P17. Using confocal microscopy, a cytoplasmic accumulation of HBeAg and precursors was observed with P25-expressing plasmid, whereas P22 localized both in the cytoplasm and nucleus. P20 and P17, which lack the carboxy end of P22 showed strong nuclear accumulation, implicating a nuclear localization signal in the N-terminal 10 amino acids. G1862T, unique to subgenotype A1, is frequently found in HBV from HCC patients. P25 with G1862T showed delayed and reduced HBeAg expression/secretion. Knock-out of core in the replication competent clones led to precore protein accumulation in the cytoplasm/perinuclear region, and decreased HBeAg secretion. Knock-out of precore proteins increased HBsAg secretion but intracellular HBsAg expression was unaffected. Over-expression of precore proteins in trans led to decreased HBsAg expression and secretion. Intracellular trafficking of HBV A1 precore proteins was followed. This was unaffected by the CMV promoter and different cell types. In the viral context, precore protein expression was affected by absence of core, and affected HBsAg expression, suggesting an interrelationship between precore proteins, HBcAg and HBsAg. This modulatory role of HBeAg and its precursors may be important in viral persistence and ultimate development of HCC.

Multiple roles of PP2A binding motif in hepatitis B virus core linker and PP2A in regulating core phosphorylation state and viral replication

Hepatitis B virus (HBV) capsid or core protein (HBc) contains an N-terminal domain (NTD) and a C-terminal domain (CTD) connected by a short linker peptide. HBc plays a critical role in virtually every step of viral replication, which is further modulated by dynamic phosphorylation and dephosphorylation of its CTD. While several cellular kinases have been identified that mediate HBc CTD phosphorylation, there is little information on the cellular phosphatases that mediate CTD dephosphorylation. Herein, a consensus binding motif for the protein phosphatase 2A (PP2A) regulatory subunit B56 was recognized within the HBc linker peptide. Mutations within this motif designed to block or enhance B56 binding showed pleiotropic effects on CTD phosphorylation state as well as on viral RNA packaging, reverse transcription, and virion secretion. Furthermore, linker mutations affected the HBV nuclear episome (the covalently closed circular or CCC DNA) differentially during intracellular amplification vs. infection. The effects of linker mutations on CTD phosphorylation state varied with different phosphorylation sites and were only partially consistent with the linker motif serving to recruit PP2A-B56, specifically, to dephosphorylate CTD, suggesting that multiple phosphatases and/or kinases may be recruited to modulate CTD (de)phosphorylation. Furthermore, pharmacological inhibition of PP2A could decrease HBc CTD dephosphorylation and increase the nuclear HBV episome. These results thus strongly implicate the HBc linker in recruiting PP2A and other host factors to regulate multiple stages of HBV replication.

Hepatitis B virus (HBV) causes acute and chronic viral hepatitis, liver fibrosis, cirrhosis and cancer. The dynamic phosphorylation and dephosphorylation of the viral capsid protein (HBc), which are controlled by host cell protein kinases and phosphatases, play a critical role in regulating multiple stages of HBV replication. While a number of cellular kinases have been identified that mediate HBc phosphorylation, there is little information on cellular phosphatases that mediate its dephosphorylation. Herein we have identified a consensus binding motif in HBc for one of the major cellular phosphatases, the protein phosphatase 2A (PP2A). Genetic analysis of this motif revealed that it played multiple roles in regulating CTD phosphorylation state, as well as viral RNA packaging, reverse transcription, virion secretion, and formation of the nuclear HBV episome responsible for viral persistence. Furthermore, pharmacological inhibition of PP2A decreased HBc dephosphorylation and increased the nuclear episome, further supporting a role of PP2A in HBc dephosphorylation and HBV persistence. These results thus suggest that HBc recruits PP2A, among other host factors, to regulate HBc phosphorylation and dephosphorylation dynamics and HBV replication and persistence.

Hepatitis B virus Core protein nuclear interactome identifies SRSF10 as a host RNA-binding protein restricting HBV RNA production

Despite the existence of a preventive vaccine, chronic infection with Hepatitis B virus (HBV) affects more than 250 million people and represents a major global cause of hepatocellular carcinoma (HCC) worldwide. Current clinical treatments, in most of cases, do not eliminate viral genome that persists as a DNA episome in the nucleus of hepatocytes and constitutes a stable template for the continuous expression of viral genes. Several studies suggest that, among viral factors, the HBV core protein (HBc), well-known for its structural role in the cytoplasm, could have critical regulatory functions in the nucleus of infected hepatocytes. To elucidate these functions, we performed a proteomic analysis of HBc-interacting host-factors in the nucleus of differentiated HepaRG, a surrogate model of human hepatocytes. The HBc interactome was found to consist primarily of RNA-binding proteins (RBPs), which are involved in various aspects of mRNA metabolism. Among them, we focused our studies on SRSF10, a RBP that was previously shown to regulate alternative splicing (AS) in a phosphorylation-dependent manner and to control stress and DNA damage responses, as well as viral replication. Functional studies combining SRSF10 knockdown and a pharmacological inhibitor of SRSF10 phosphorylation (1C8) showed that SRSF10 behaves as a restriction factor that regulates HBV RNAs levels and that its dephosphorylated form is likely responsible for the anti-viral effect. Surprisingly, neither SRSF10 knock-down nor 1C8 treatment modified the splicing of HBV RNAs but rather modulated the level of nascent HBV RNA. Altogether, our work suggests that in the nucleus of infected cells HBc interacts with multiple RBPs that regulate viral RNA metabolism. Our identification of SRSF10 as a new anti-HBV restriction factor offers new perspectives for the development of new host-targeted antiviral strategies.

Chronic infection with Hepatitis B virus (HBV) affects more than 250 million of people world-wide and is a major global cause of liver cancer. Current treatments lead to a significant reduction of viremia in patients. However, viral clearance is rarely obtained and the persistence of the HBV genome in the hepatocyte’s nucleus generates a stable source of viral RNAs and subsequently proteins which play important roles in immune escape mechanisms and liver disease progression. Therapies aiming at efficiently and durably eliminating viral gene expression are still required. In this study, we identified the nuclear partners of the HBV Core protein (HBc) to understand how this structural protein, responsible for capsid assembly in the cytoplasm, could also regulate viral gene expression. The HBc interactome was found to consist primarily of RNA-binding proteins (RBPs). One of these RBPs, SRSF10, was demonstrated to restrict HBV RNA levels and a drug, able to alter its phosphorylation, behaved as an antiviral compound capable of reducing viral gene expression. Altogether, this study sheds new light on novel regulatory functions of HBc and provides information relevant for the development of antiviral strategies aiming at preventing viral gene expression.

Intracellular Trafficking of HBV Particles

The human hepatitis B virus (HBV), that is causative for more than 240 million cases of chronic liver inflammation (hepatitis), is an enveloped virus with a partially double-stranded DNA genome. After virion uptake by receptor-mediated endocytosis, the viral nucleocapsid is transported towards the nuclear pore complex. In the nuclear basket, the nucleocapsid disassembles. The viral genome that is covalently linked to the viral polymerase, which harbors a bipartite NLS, is imported into the nucleus. Here, the partially double-stranded DNA genome is converted in a minichromosome-like structure, the covalently closed circular DNA (cccDNA). The DNA virus HBV replicates via a pregenomic RNA (pgRNA)-intermediate that is reverse transcribed into DNA. HBV-infected cells release apart from the infectious viral parrticle two forms of non-infectious subviral particles (spheres and filaments), which are assembled by the surface proteins but lack any capsid and nucleic acid. In addition, naked capsids are released by HBV replicating cells. Infectious viral particles and filaments are released via multivesicular bodies; spheres are secreted by the classic constitutive secretory pathway. The release of naked capsids is still not fully understood, autophagosomal processes are discussed. This review describes intracellular trafficking pathways involved in virus entry, morphogenesis and release of (sub)viral particles.

Hepatitis B virus cccDNA: Formation, regulation and therapeutic potential

Hepatitis B virus (HBV) infection remains a major public health concern worldwide with about 257 million individuals chronically infected. Current therapies can effectively control HBV replication and slow down disease progress, but cannot cure HBV infection. Upon infection, HBV establishes a pool of covalently closed circular DNA (cccDNA) in the nucleus of infected hepatocytes. The cccDNA exists as a minichromosome and resists to antivirals, thus a therapeutic eradication of cccDNA from the infected cells remains unattainable. In this review, we summarize the state of knowledge on the mechanisms underlying cccDNA formation and regulation, and discuss the possible strategies that may contribute to the eradication of HBV through targeting cccDNA.

Role of Hepatitis B virus capsid phosphorylation in nucleocapsid disassembly and covalently closed circular DNA formation

Hepatitis B virus (HBV) delivers a partially double-stranded, relaxed circular (RC) DNA genome in complete virions to the host cell nucleus for conversion to the covalently closed circular (CCC) DNA, which establishes and sustains viral infection. An overlength pregenomic RNA (pgRNA) is then transcribed from CCC DNA and packaged into immature nucleocapsids (NCs) by the viral core (HBc) protein. pgRNA is reverse transcribed to produce RC DNA in mature NCs, which are then enveloped and secreted as complete virions, or delivered to the nucleus to replenish the nuclear CCC DNA pool. RC DNA, whether originating from extracellular virions or intracellular mature NCs, must be released upon NC disassembly (uncoating) for CCC DNA formation. HBc is known to undergo dynamic phosphorylation and dephosphorylation at its C-terminal domain (CTD) to facilitate pgRNA packaging and reverse transcription. Here, two putative phosphorylation sites in the HBc N-terminal domain (NTD), S44 and S49, were targeted for genetic and biochemical analysis to assess their potential roles in viral replication. The NTD mutant that mimics the non-phosphorylated state (N2A) was competent in all steps of viral replication tested from capsid assembly, pgRNA packaging, reverse transcription, to virion secretion, except for a decrease in CCC DNA formation. On the other hand, the phosphor-mimetic mutant N2E showed a defect in the early step of pgRNA packaging but enhanced the late step of mature NC uncoating and consequently, increased CCC DNA formation. N2E also enhanced phosphorylation in CTD and possibly elsewhere in HBc. Furthermore, inhibition of the cyclin-dependent kinase 2 (CDK2), which is packaged into viral capsids, could block CCC DNA formation. These results prompted us to propose a model whereby rephosphorylation of HBc at both NTD and CTD by the packaged CDK2, following CTD dephosphorylation during NC maturation, facilitates uncoating and CCC DNA formation by destabilizing mature NCs.

Revisiting Hepatitis B Virus: Challenges of Curative Therapies

With a yearly death toll of 880,000, hepatitis B virus (HBV) remains a major health problem worldwide, despite an effective prophylactic vaccine and well-tolerated, effective antivirals. HBV causes chronic hepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. The viral genome persists in infected hepatocytes even after long-term antiviral therapy, and its integration, though no longer able to support viral replication, destabilizes the host genome. HBV is a DNA virus that utilizes a virus-encoded reverse transcriptase to convert an RNA intermediate, termed pregenomic RNA, into the relaxed circular DNA genome, which is subsequently converted into a covalently closed circular DNA (cccDNA) in the host cell nucleus. cccDNA is maintained in the nucleus of the infected hepatocyte as a stable minichromosome and functions as the viral transcriptional template for the production of all viral gene products, and thus, it is the molecular basis of HBV persistence. The nuclear cccDNA pool can be replenished through recycling of newly synthesized, DNA-containing HBV capsids. Licensed antivirals target the HBV reverse transcriptase activity but fail to eliminate cccDNA, which would be required to cure HBV infection. Elimination of HBV cccDNA is so far only achieved by antiviral immune responses. Thus, this review will focus on possible curative strategies aimed at eliminating or crippling the viral cccDNA. Newer insights into the HBV life cycle and host immune response provide novel, potentially curative therapeutic opportunities and targets.

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.

Secretion of empty or complete hepatitis B virions: envelopment of empty capsids versus mature nucleocapsids

HBV replicates its DNA genome, a partially double-stranded, relaxed circular DNA, via reverse transcription of an RNA intermediate called pre-genomic RNA by its reverse transcriptase. A major characteristic of HBV replication is the selective envelopment and secretion of relaxed circular DNA-containing mature capsids and empty capsids with no DNA or RNA, but not those containing pre-genomic RNA or the single-stranded DNA replication intermediate. In this review, the potential mechanisms of HBV virion morphogenesis will be discussed, with a focus on key determinants of both the capsid and envelope proteins for the selective secretion of complete and empty virions.

Common and Distinct Capsid and Surface Protein Requirements for Secretion of Complete and Genome-Free Hepatitis B Virions

During the morphogenesis of hepatitis B virus (HBV), an enveloped virus, two types of virions are secreted: (i) a minor population of complete virions containing a mature nucleocapsid with the characteristic, partially double-stranded, relaxed circular DNA genome and (ii) a major population containing an empty capsid with no DNA or RNA (empty virions). Secretion of both types of virions requires interactions between the HBV capsid or core protein (HBc) and the viral surface or envelope proteins. We have studied the requirements from both HBc and envelope proteins for empty virion secretion in comparison with those for secretion of complete virions. Substitutions within the N-terminal domain of HBc that block secretion of DNA-containing virions reduced but did not prevent secretion of empty virions. The HBc C-terminal domain was not essential for empty virion secretion. Among the three viral envelope proteins, the smallest, S, alone was sufficient for empty virion secretion at a basal level. The largest protein, L, essential for complete virion secretion, was not required but could stimulate empty virion secretion. Also, substitutions in L that eliminated secretion of complete virions reduced but did not eliminate empty virion secretion. S mutations that blocked secretion of the hepatitis D virus (HDV), an HBV satellite, did not block secretion of either empty or complete HBV virions. Together, these results indicate that both common and distinct signals on empty capsids and mature nucleocapsids interact with the S and L proteins during the formation of complete and empty virions.IMPORTANCE Hepatitis B virus (HBV) is a major cause of severe liver diseases, including cirrhosis and cancer. In addition to the complete infectious virion particle, which contains an outer envelope layer and an interior capsid that, in turn, encloses a DNA genome, HBV-infected cells also secrete noninfectious, incomplete viral particles in large excess over the number of complete virions. In particular, the empty (or genome-free) virion shares with the complete virion the outer envelope and interior capsid but contains no genome. We have carried out a comparative study on the capsid and envelope requirements for the secretion of these two types of virion particles and uncovered both shared and distinct determinants on the capsid and envelope for their secretion. These results provide new information on HBV morphogenesis and have implications for efforts to develop empty HBV virions as novel biomarkers and a new generation of HBV vaccine.

Multiple roles of core protein linker in hepatitis B virus replication

Hepatitis B virus (HBV) core protein (HBc) contains an N-terminal domain (NTD, assembly domain) and a C-terminal domain (CTD), which are linked by a flexible linker region. HBc plays multiple essential roles in viral replication, including capsid assembly, packaging of the viral pregenomic RNA (pgRNA) into nucleocapsids, viral reverse transcription that converts pgRNA to the genomic DNA, and secretion of DNA-containing (complete) virions or genome-free (empty) virions. The HBc linker is generally assumed to act merely as a spacer between NTD and CTD but some results suggest that the linker may affect NTD assembly. To determine its role in viral replication, we have made a number of deletion and substitution mutants in the linker region, in either the presence or absence of CTD, and tested their abilities to support capsid assembly and viral replication in human cells. Our results indicate that the linker could indeed impede NTD assembly in the absence of CTD, which could be partially relieved by partial linker deletion. In contrast, when CTD was present, the linker deletions or substitutions did not affect capsid assembly. Deletion of the entire linker or its C-terminal part resulted in a partial defect in pgRNA packaging and severely impaired viral DNA synthesis. In contrast, deletion of the N-terminal part of the linker, or substitutions of the linker sequence, had little to no effect on RNA packaging or first-strand DNA synthesis. However, the N-terminal linker deletion and two linker substitution mutants were defective in the production of mature double-stranded viral DNA. Secretion of empty virions was blocked by all the linker deletions and substitutions tested. In particular, a conservative linker substitution that allowed mature viral DNA synthesis and secretion of complete virions severely impaired the secretion of empty virions, thus increasing the ratio of complete to empty virions that were secreted. Together, these results demonstrate that the HBc linker region plays critical and complex roles at multiple stages of HBV replication.

The hepatitis B virus (HBV) is a major human pathogen that infects hundreds of millions of people worldwide and represents a major cause of viral hepatitis, liver cirrhosis, and liver cancer. The HBV capsid protein (HBc) plays multiple roles in the viral life cycle and has emerged recently as a major target for developing antiviral therapies against HBV infection. HBc is divided into three separate regions, an N-terminal domain (NTD) responsible for capsid assembly, a C-terminal domain (CTD) that plays critical roles in the specific packaging of the viral pregenomic RNA (pgRNA) into replication-competent nucleocapsids and the subsequent reverse transcription of the pgRNA into the viral genomic DNA, and a linker region between the NTD and CTD. In contrast to the prevailing assumption that the linker merely serves to connect the NTD and CTD, we have discovered here that it plays a critical role in almost every stage of HBV replication. The linker likely exerted its pleiotropic effects via affecting the NTD and CTD as well as via direct interactions with other viral factors independent of the NTD or CTD. Our results thus not only deepen understanding of HBc structure and functions but also implicate the linker as a potential novel target for antiviral development against HBV infection.

Mechanisms and Effects on HBV Replication of the Interaction between HBV Core Protein and Cellular Filamin B

Hepatitis B virus (HBV) infection is one of the major problems that threatens global health. There have been many studies on HBV, but the relationship between HBV and host factors is largely unexplored and more studies are needed to clarify these interactions. Filamin B is an actin-binding protein that acts as a cytoskeleton protein, and it is involved in cell development and several signaling pathways. In this study, we showed that filamin B interacted with HBV core protein, and the interaction promoted HBV replication. The interaction between filamin B and core protein was observed in HEK 293T, Huh7 and HepG2 cell lines by co-immunoprecipitation and co-localization immnofluoresence. Overexpression of filamin B increased the levels of HBV total RNAs and pre-genome RNA (pgRNA), and improved the secretion level of hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg). In contrast, filamin B knockdown inhibited HBV replication, decreased the level of HBV total RNAs and pgRNA, and reduced the secretion level of HBsAg and HBeAg. In addition, we found that filamin B and core protein may interact with each other via four blocks of argentine residues at the C-terminus of core protein. In conclusion, we identify filamin B as a novel host factor that can interact with core protein to promote HBV replication in hepatocytes. Our study provides new insights into the relationship between HBV and host factors and may provide new strategies for the treatment of HBV infection.

Hepatitis B virus core protein allosteric modulators can distort and disrupt intact capsids

Defining mechanisms of direct-acting antivirals facilitates drug development and our understanding of virus function. Heteroaryldihydropyrimidines (HAPs) inappropriately activate assembly of hepatitis B virus (HBV) core protein (Cp), suppressing formation of virions. We examined a fluorophore-labeled HAP, HAP-TAMRA. HAP-TAMRA induced Cp assembly and also bound pre-assembled capsids. Kinetic and spectroscopic studies imply that HAP-binding sites are usually not available but are bound cooperatively. Using cryo-EM, we observed that HAP-TAMRA asymmetrically deformed capsids, creating a heterogeneous array of sharp angles, flat regions, and outright breaks. To achieve high resolution reconstruction (<4 Å), we introduced a disulfide crosslink that rescued particle symmetry. We deduced that HAP-TAMRA caused quasi-sixfold vertices to become flatter and fivefold more angular. This transition led to asymmetric faceting. That a disordered crosslink could rescue symmetry implies that capsids have tensegrity properties. Capsid distortion and disruption is a new mechanism by which molecules like the HAPs can block HBV infection.

Viruses are simple structures formed of genetic information wrapped inside a shell. For the hepatitis B virus, this casing looks like a soccer ball. It is composed of 240 copies of the same protein, arranged in a pattern of pentagons and hexagons. These proteins form a protective shield for the virus’ genetic information: they also interact with the cells of the host during key events of the virus’ life cycle.

When the hepatitis B virus infects a cell, it hijacks the cellular machinery to replicate. New shell proteins are produced and assemble within the cell. A type of potential antiviral drug called a CpAM disrupts this process: it causes the shell to assemble too early and inaccurately, which impairs the life cycle of the virus. However, a CpAM can bind to the shell even after it has already assembled. How this binding affects the virus is still unclear.

Here, Schlicksup et al. attach a fluorescent molecule to a CpAM, and use a cutting-edge microscopy method to look at the structures at the atomic level. This makes it possible to examine in detail how the CpAM attaches to a correctly formed virus shell. Schlicksup et al. show that when the CpAM binds to the shell, it disrupts and sometimes even breaks the soccer-like pattern of the shell: the hexagons flatten, and the pentagons buckle.

These misshaped shells could prevent the virus from interacting with the cellular structures necessary for infection or prevent it from releasing the virus’ genetic information. This is a new antiviral mechanism for a CpAM. By acting both before and after the shell has assembled, the CpAM targets the virus at different points of its life cycle.

Hepatitis B affects over 240 million people worldwide. While a vaccine exists, there is still no cure for it. A better understanding of the physics of the virus’ shell and the mode of action of CpAMs could lead to better drugs against the disease.

Host-regulated Hepatitis B Virus Capsid Assembly in a Mammalian Cell-free System

The hepatitis B virus (HBV) is an important global human pathogen and represents a major cause of hepatitis, liver cirrhosis and liver cancer. The HBV capsid is composed of multiple copies of a single viral protein, the capsid or core protein (HBc), plays multiple roles in the viral life cycle, and has emerged recently as a major target for developing antiviral therapies against HBV infection. Although several systems have been developed to study HBV capsid assembly, including heterologous overexpression systems like bacteria and insect cells, in vitro assembly using purified protein, and mammalian cell culture systems, the requirement for non-physiological concentrations of HBc and salts and the difficulty in manipulating host regulators of assembly presents major limitations for detailed studies on capsid assembly under physiologically relevant conditions. We have recently developed a mammalian cell-free system based on the rabbit reticulocyte lysate (RRL), in which HBc is expressed at physiological concentrations and assembles into capsids under near-physiological conditions. This system has already revealed HBc assembly requirements that are not anticipated based on previous assembly systems. Furthermore, capsid assembly in this system is regulated by endogenous host factors that can be readily manipulated. Here we present a detailed protocol for this cell-free capsid assembly system, including an illustration on how to manipulate host factors that regulate assembly.

Polo-like-kinase 1 is a proviral host factor for hepatitis B virus replication

Chronic hepatitis B virus (HBV) infection is a major risk factor for hepatocellular carcinoma (HCC) and current treatments for chronic hepatitis B and HCC are suboptimal. Herein, we identified cellular serine/threonine Polo-like-kinase 1 (PLK1) as a positive effector of HBV replication. The aim of this study was to demonstrate the proviral role of PLK1 in HBV biosynthesis and validate PLK1 inhibition a potential antiviral strategy. To this end, we employed physiologically relevant HBV infection models of primary human hepatocytes (PHHs) and differentiated HepaRG cells in conjunction with pharmacologic PLK1 inhibitors, small interfering RNA (siRNA)-mediated knockdown, and overexpression of constitutively active PLK1 (PLK1CA ). In addition, a humanized liver Fah-/- /Rag2-/- /Il2rg-/- (FRG) mouse model was used to determine the antiviral effect of PLK1 inhibitor BI-2536 on HBV infection in vivo. Finally, in vitro PLK1 kinase assays and site-directed mutagenesis were employed to demonstrate that HBV core protein (HBc) is a PLK1 substrate. We demonstrated that HBV infection activated cellular PLK1 in PHHs and differentiated HepaRG cells. PLK1 inhibition by BI-2536 or siRNA-mediated knockdown suppressed HBV DNA biosynthesis, whereas overexpression of PLK1CA increased it, suggesting that the PLK1 effects on viral biosynthesis are specific and that PLK1 is a proviral cellular factor. Significantly, BI-2536 administration to HBV-infected humanized liver FRG mice strongly inhibited HBV infection, validating PLK1 as an antiviral target in vivo. The proviral action of PLK1 is associated with the biogenesis of the nucleocapsid, as BI-2536 leads to its decreased intracellular formation/accumulation. In this respect, our studies identified HBc as a PLK1 substrate in vitro, and mapped PLK1 phosphorylation sites on this protein.

CONCLUSION

PLK1 is a proviral host factor that could be envisaged as a target for combined antiviral and antitumoral strategies against HBV infection and HBV-mediated carcinogenesis. (Hepatology 2017;66:1750-1765).

The diverse functions of the hepatitis B core/capsid protein (HBc) in the viral life cycle: Implications for the development of HBc-targeting antivirals

Virally encoded proteins have evolved to perform multiple functions, and the core protein (HBc) of the hepatitis B virus (HBV) is a perfect example. While HBc is the structural component of the viral nucleocapsid, additional novel functions for the nucleus-localized HBc have recently been described. These results extend for HBc, beyond its structural role, a regulatory function in the viral life cycle and potentially a role in pathogenesis. In this article, we review the diverse roles of HBc in HBV replication and pathogenesis, emphasizing how the unique structure of this protein is key to its various functions. We focus in particular on recent advances in understanding the significance of HBc phosphorylations, its interaction with host proteins and the role of HBc in regulating the transcription of host genes. We also briefly allude to the emerging niche for new direct-acting antivirals targeting HBc, known as Core (protein) Allosteric Modulators (CAMs).

Capsid Phosphorylation State and Hepadnavirus Virion Secretion

The C-terminal domain (CTD) of hepadnavirus core protein is involved in multiple steps of viral replication. In particular, the CTD is initially phosphorylated at multiple sites to facilitate viral RNA packaging into immature nucleocapsids (NCs) and the early stage of viral DNA synthesis. For the avian hepadnavirus duck hepatitis B virus (DHBV), CTD is dephosphorylated subsequently to facilitate the late stage of viral DNA synthesis and to stabilize NCs containing mature viral DNA. The role of CTD phosphorylation in virion secretion, if any, has remained unclear. Here, the CTD from the human hepatitis B virus (HBV) was found to be dephosphorylated in association with NC maturation and secretion of DNA-containing virions, as in DHBV. In contrast, the CTD in empty HBV virions (i.e., enveloped capsids with no RNA or DNA) was found to be phosphorylated. The potential role of CTD dephosphorylation in virion secretion was analyzed through mutagenesis. For secretion of empty HBV virions, which is independent of either viral RNA packaging or DNA synthesis, multiple substitutions in the CTD to mimic either phosphorylation or dephosphorylation showed little detrimental effect. Similarly, phospho-mimetic substitutions in the DHBV CTD did not block the secretion of DNA-containing virions. These results indicate that CTD dephosphorylation, though associated with NC maturation in both HBV and DHBV, is not essential for the subsequent NC-envelope interaction to secrete DNA-containing virions, and the CTD state of phosphorylation also does not play an essential role in the interaction between empty capsids and the envelope for secretion of empty virions.IMPORTANCE The phosphorylation state of the C-terminal domain (CTD) of hepatitis B virus (HBV) core or capsid protein is highly dynamic and plays multiple roles in the viral life cycle. To study the potential role of the state of phosphorylation of CTD in virion secretion, we have analyzed the CTD phosphorylation state in complete (containing the genomic DNA) versus empty (genome-free) HBV virions. Whereas CTD is unphosphorylated in complete virions, it is phosphorylated in empty virions. Mutational analyses indicate that neither phosphorylation nor dephosphorylation of CTD is required for virion secretion. These results demonstrate that while CTD dephosphorylation is associated with HBV DNA synthesis, the CTD state of phosphorylation may not regulate virion secretion.

Nuclear Import of Hepatitis B Virus Capsids and Genome

Hepatitis B virus (HBV) is an enveloped pararetrovirus with a DNA genome, which is found in an up to 36 nm-measuring capsid. Replication of the genome occurs via an RNA intermediate, which is synthesized in the nucleus. The virus must have thus ways of transporting its DNA genome into this compartment. This review summarizes the data on hepatitis B virus genome transport and correlates the finding to those from other viruses.

Inhibitory effect of CDK9 inhibitor FIT-039 on hepatitis B virus propagation

Current therapies for hepatitis B virus (HBV) cannot completely eliminate the HBV genome because of the stable population of covalently closed circular DNA (cccDNA) and so on. FIT-039, which is a cyclin-dependent kinase (CDK) 9 inhibitor, is known to suppress the replication of several DNA viruses including HSV, HPV and human adenovirus. In this study, we investigated the antiviral effect of FIT-039 on HBV infection. HepG2 cells expressing human sodium taurocholate cotransporting polypeptide (HepG2/NTCP cells) were infected with HBV in the presence of FIT-039. FIT-039 dose-dependently reduced intracellular viral RNA, nucleocapsid-associated viral DNA, and supernatant viral antigens without cytotoxicity in the infected cells (IC50 = 0.33 μM, CC50 > 50 μM). The antiviral activity of FIT-039 was prominent at an early phase of viral infection, although the compound did not inhibit preS1-binding to HepG2/NTCP cells. FIT-039 reduced cccDNA in HBV-replicating or HBV-infected cells. Furthermore, the antiviral activity of entecavir was significantly enhanced by the combination with FIT-039 in the chimeric mice having human hepatocytes infected with HBV. None of the mice had significant drug-related body weight or serum human-albumin concentration changes. These data suggest that CDK9 inhibitor FIT-039 is a promising antiviral candidate for HBV infection.

Importin β Can Bind Hepatitis B Virus Core Protein and Empty Core-Like Particles and Induce Structural Changes

Hepatitis B virus (HBV) capsids are found in many forms: immature single-stranded RNA-filled cores, single-stranded DNA-filled replication intermediates, mature cores with relaxed circular double-stranded DNA, and empty capsids. A capsid, the protein shell of the core, is a complex of 240 copies of core protein. Mature cores are transported to the nucleus by a complex that includes both importin α and importin β (Impα and Impβ), which bind to the core protein's C-terminal domains (CTDs). Here we have investigated the interactions of HBV core protein with importins in vitro. Strikingly, empty capsids and free core protein can bind Impβ without Impα. Cryo-EM image reconstructions show that the CTDs, which are located inside the capsid, can extrude through the capsid to be bound by Impβ. Impβ density localized on the capsid exterior near the quasi-sixfold vertices, suggested a maximum of 30 Impβ per capsid. However, examination of complexes using single molecule charge-detection mass spectrometry indicate that some complexes include over 90 Impβ molecules. Cryo-EM of capsids incubated with excess Impβ shows a population of damaged particles and a population of "dark" particles with internal density, suggesting that Impβ is effectively swallowed by the capsids, which implies that the capsids transiently open and close and can be destabilized by Impβ. Though the in vitro complexes with great excess of Impβ are not biological, these results have implications for trafficking of empty capsids and free core protein; activities that affect the basis of chronic HBV infection.

Cell-Free Hepatitis B Virus Capsid Assembly Dependent on the Core Protein C-Terminal Domain and Regulated by Phosphorylation

Multiple subunits of the hepatitis B virus (HBV) core protein (HBc) assemble into an icosahedral capsid that packages the viral pregenomic RNA (pgRNA). The N-terminal domain (NTD) of HBc is sufficient for capsid assembly, in the absence of pgRNA or any other viral or host factors, under conditions of high HBc and/or salt concentrations. The C-terminal domain (CTD) is deemed dispensable for capsid assembly although it is essential for pgRNA packaging. We report here that HBc expressed in a mammalian cell lysate, rabbit reticulocyte lysate (RRL), was able to assemble into capsids when (low-nanomolar) HBc concentrations mimicked those achieved under conditions of viral replication in vivo and were far below those used previously for capsid assembly in vitro Furthermore, at physiologically low HBc concentrations in RRL, the NTD was insufficient for capsid assembly and the CTD was also required. The CTD likely facilitated assembly under these conditions via RNA binding and protein-protein interactions. Moreover, the CTD underwent phosphorylation and dephosphorylation events in RRL similar to those seen in vivo which regulated capsid assembly. Importantly, the NTD alone also failed to accumulate in mammalian cells, likely resulting from its failure to assemble efficiently. Coexpression of the full-length HBc rescued NTD assembly in RRL as well as NTD expression and assembly in mammalian cells, resulting in the formation of mosaic capsids containing both full-length HBc and the NTD. These results have important implications for HBV assembly during replication and provide a facile cell-free system to study capsid assembly under physiologically relevant conditions, including its modulation by host factors.

IMPORTANCE

Hepatitis B virus (HBV) is an important global human pathogen and the main cause of liver cancer worldwide. An essential component of HBV is the spherical capsid composed of multiple copies of a single protein, the core protein (HBc). We have developed a mammalian cell-free system in which HBc is expressed at physiological (low) concentrations and assembles into capsids under near-physiological conditions. In this cell-free system, as in mammalian cells, capsid assembly depends on the C-terminal domain (CTD) of HBc, in contrast to other assembly systems in which HBc assembles into capsids independently of the CTD under conditions of nonphysiological protein and salt concentrations. Furthermore, the phosphorylation state of the CTD regulates capsid assembly and RNA encapsidation in the cell-free system in a manner similar to that seen in mammalian cells. This system will facilitate detailed studies on capsid assembly and RNA encapsidation under physiological conditions and identification of antiviral agents that target HBc.

Intracellular transport and egress of hepatitis B virus

Hepatitis B virus (HBV) replicates its genomic information in the nucleus via transcription and therefore has to deliver its partially double stranded DNA genome into the nucleus. Like other viruses with a nuclear replication phase, HBV genomes are transported inside the viral capsids first through the cytoplasm towards the nuclear envelope. Following the arrival at the nuclear pore, the capsids are transported through, using classical cellular nuclear import pathways. The arrest of nuclear import at the nucleoplasmic side of the nuclear pore is unique, however, and is where the capsids efficiently disassemble leading to genome release. In the latter phase of the infection, newly formed nucleocapsids in the cytosol have to move to budding sites at intracellular membranes carrying the three viral envelope proteins. Capsids containing single stranded nucleic acid are not enveloped, in contrast to empty and double stranded DNA containing capsids. A small linear domain in the large envelope protein and two areas on the capsid surface have been mapped, where point mutations strongly block nucleocapsid envelopment. It is possible that these domains are involved in the envelope--with capsid interactions driving the budding process. Like other enveloped viruses, HBV also uses the cellular endosomal sorting complexes required for transport (ESCRT) machinery for catalyzing budding through the membrane and away from the cytosol.

Viral DNA-Dependent Induction of Innate Immune Response to Hepatitis B Virus in Immortalized Mouse Hepatocytes

Hepatitis B virus (HBV) infects hundreds of millions of people worldwide and causes acute and chronic hepatitis, cirrhosis, and hepatocellular carcinoma. HBV is an enveloped virus with a relaxed circular (RC) DNA genome. In the nuclei of infected human hepatocytes, conversion of RC DNA from the incoming virion or cytoplasmic mature nucleocapsid (NC) to the covalently closed circular (CCC) DNA, which serves as the template for producing all viral transcripts, is essential to establish and sustain viral replication. A prerequisite for CCC DNA formation is the uncoating (disassembly) of NCs to expose their RC DNA content for conversion to CCC DNA. We report here that in an immortalized mouse hepatocyte cell line, AML12HBV10, in which RC DNA exposure is enhanced, the exposed viral DNA could trigger an innate immune response that was able to modulate viral gene expression and replication. When viral gene expression and replication were low, the innate response initially stimulated these processes but subsequently acted to shut off viral gene expression and replication after they reached peak levels. Inhibition of viral DNA synthesis or cellular DNA sensing and innate immune signaling diminished the innate response. These results indicate that HBV DNA, when exposed in the host cell cytoplasm, can function to trigger an innate immune response that, in turn, modulates viral gene expression and replication.

IMPORTANCE

Chronic infection by hepatitis B virus (HBV) afflicts hundreds of millions worldwide and is sustained by the episomal covalently closed circular (CCC) DNA in the nuclei of infected hepatocytes. Release of viral genomic DNA from cytoplasmic nucleocapsids (NCs) (NC disassembly or uncoating) is a prerequisite for its conversion to CCC DNA, which can also potentially expose the viral DNA to host DNA sensors and trigger an innate immune response. We have found that in an immortalized mouse hepatocyte cell line in which efficient CCC DNA formation was associated with enhanced exposure of nucleocapsid-associated DNA, the exposed viral DNA indeed triggered host cytoplasmic DNA sensing and an innate immune response that was able to modulate HBV gene expression and replication. Thus, HBV can, under select conditions, be recognized by the host innate immune response through exposed viral DNA, which may be exploited therapeutically to clear viral persistence.

Alteration of Mature Nucleocapsid and Enhancement of Covalently Closed Circular DNA Formation by Hepatitis B Virus Core Mutants Defective in Complete-Virion Formation

Assembly of hepatitis B virus (HBV) begins with packaging of the pregenomic RNA (pgRNA) into immature nucleocapsids (NC), which are converted to mature NCs containing the genomic relaxed circular (RC) DNA as a result of reverse transcription. Mature NCs have two alternative fates: (i) envelopment by viral envelope proteins, leading to secretion extracellularly as virions, or (ii) disassembly (uncoating) to deliver their RC DNA content into the host cell nucleus for conversion to the covalently closed circular (CCC) DNA, the template for viral transcription. How these two alternative fates are regulated remains to be better understood. The NC shell is composed of multiple copies of a single viral protein, the HBV core (HBc) protein. HBc mutations located on the surface of NC have been identified that allow NC maturation but block its envelopment. The potential effects of some of these mutations on NC uncoating and CCC DNA formation have been analyzed by transfecting HBV replication constructs into hepatoma cells. All envelopment-defective HBc mutations tested were competent for CCC DNA formation, indicating that core functions in envelopment and uncoating/nuclear delivery of RC DNA were genetically separable. Some of the envelopment-defective HBc mutations were found to alter specifically the integrity of mature, but not immature, NCs such that RC DNA became susceptible to nuclease digestion. Furthermore, CCC DNA formation could be enhanced by NC surface mutations that did or did not significantly affect mature NC integrity, indicating that the NC surface residues may be closely involved in NC uncoating and/or nuclear delivery of RC DNA.

IMPORTANCE

Hepatitis B virus (HBV) infection is a major health issue worldwide. HBV assembly begins with the packaging into immature nucleocapsids (NCs) of a viral RNA pregenome, which is converted to the DNA genome in mature NCs. Mature NCs are then selected for envelopment and secretion as complete-virion particles or, alternatively, can deliver their DNA to the host cell nucleus to maintain the viral genome as nuclear episomes, which are the basis for virus persistence. Previous studies have identified mutations on the capsid surface that selectively block NC envelopment without affecting NC maturation. We have now discovered that some of the same mutations result in preferential alteration of mature NCs and increased viral nuclear episomes. These findings provide important new insights into the regulation of the two alternative fates of mature NCs and suggest new ways to perturb viral persistence by manipulating levels of viral nuclear episomes.

Core protein: A pleiotropic keystone in the HBV lifecycle

Hepatitis B Virus (HBV) is a small virus whose genome has only four open reading frames. We argue that the simplicity of the virion correlates with a complexity of functions for viral proteins. We focus on the HBV core protein (Cp), a small (183 residue) protein that self-assembles to form the viral capsid. However, its functions are a little more complicated than that. In an infected cell Cp modulates almost every step of the viral lifecycle. Cp is bound to nuclear viral DNA and affects its epigenetics. Cp correlates with RNA specificity. Cp assembles specifically on a reverse transcriptase-viral RNA complex or, apparently, nothing at all. Indeed Cp has been one of the model systems for investigation of virus self-assembly. Cp participates in regulation of reverse transcription. Cp signals completion of reverse transcription to support virus secretion. Cp carries both nuclear localization signals and HBV surface antigen (HBsAg) binding sites; both of these functions appear to be regulated by contents of the capsid. Cp can be targeted by antivirals - while self-assembly is the most accessible of Cp activities, we argue that it makes sense to engage the broader spectrum of Cp function. This article forms part of a symposium in Antiviral Research on "From the discovery of the Australia antigen to the development of new curative therapies for hepatitis B: an unfinished story."

Hepatitis B Virus Covalently Closed Circular DNA Formation in Immortalized Mouse Hepatocytes Associated with Nucleocapsid Destabilization

Hepatitis B virus (HBV) infects hundreds of millions of people worldwide and causes acute and chronic hepatitis, cirrhosis, and hepatocellular carcinoma. HBV is an enveloped virus with a relaxed circular (RC) DNA genome. In the nuclei of infected human hepatocytes, conversion of RC DNA from the incoming virion or cytoplasmic mature nucleocapsid (NC) to the covalently closed circular (CCC) DNA, which serves as the template for producing all viral transcripts, is essential to establish and sustain viral replication. For reasons yet to be understood, HBV is apparently unable to make CCC DNA in normal mouse hepatocytes in the liver. We report here that HBV CCC DNA was formed efficiently in an immortalized mouse hepatocyte cell line, AML12HBV10, and this is associated with destabilization of mature NCs in these cells. These results suggest that destabilization of mature HBV NCs in AML12HBV10 cells facilitates efficient NC uncoating and subsequent CCC DNA formation. They further implicate NC uncoating as an important step in CCC DNA formation that is subject to host regulation and potentially a critical determinant of host range and/or cell tropism of HBV. IMPORTANCE Persistent infection by hepatitis B virus (HBV), afflicting hundreds of millions worldwide, is sustained by the episomal viral covalently closed circular (CCC) DNA in the nuclei of infected hepatocytes. CCC DNA is converted from the viral genomic (precursor) DNA contained in cytoplasmic viral nucleocapsids. The conversion process remains ill defined, but host cell factors are thought to play an essential role. In particular, HBV fails to make CCC DNA in normal mouse hepatocytes despite the presence of large amounts of nucleocapsids containing the precursor viral DNA. We have found that in an immortalized mouse hepatocyte cell line, HBV is able to make abundant amounts of CCC DNA. This ability correlates with increased instability of viral nucleocapsids in these cells, which likely facilitates nucleocapsid disassembly (uncoating) to release the genomic DNA for conversion to CCC DNA. Our studies have thus revealed a novel mechanism of controlling viral persistence via regulating nucleocapsid disassembly.