Host factors involved in hepatitis B virus maturation, assembly, and egress

Biogenesis of serum HBV RNA and clinical phenomena of serum HBV RNA in chronic hepatitis B patients before and after receiving nucleos(t)ide analogues therapy

There are estimated 300 million people afflicted with chronic hepatitis B (CHB) worldwide. The risk of liver cirrhosis and hepatocellular carcinoma (HCC) increases considerably with chronic hepatitis B infection. While current therapeutics are effective in controlling hepatitis B virus (HBV) infection and disease progression, a cure for HBV infection remains unattainable due to an intranuclear replicative intermediate known as covalently closed circular DNA (cccDNA). It has recently been shown that serum HBV RNA is a non-invasive biomarker that reflects cccDNA transcriptional activity. This review provides a comprehensive overview and the latest updates on the molecular characteristics and clinical significance of serum HBV RNA, such as species of serum HBV RNA, forms of serum HBV RNA carriers and predictive value for relapses in CHB patients after nucleos(t)ide analogues (NAs) discontinuation and development of liver fibrosis and HCC. Furthermore, we summarize standardized assays for testing serum HBV RNA, the dynamic changes of serum HBV RNA levels in treatment-naïve CHB patients and those under NAs therapy, as well as the host and viral influencing factors of serum HBV RNA levels. Finally, we discuss the future perspectives in studies of serum HBV RNA.

Hepatitis B surface antigen expression impairs endoplasmic reticulum stress-related autophagic flux by decreasing LAMP2

Background & Aims

Hepatitis B surface antigen (HBsAg) drives hepatocarcinogenesis. Factors and mechanisms involved in this progression remain poorly defined, hindering the development of effective therapeutic strategies. Therefore, the mechanisms involved in the HBsAg-induced transformation of normal liver into hepatocellular carcinoma (HCC) were investigated.

Methods

Hemizygous Tg(Alb1HBV)44Bri/J mice were examined for HBsAg-induced carcinogenic events. Gene set-enrichment analysis identified significant signatures in HBsAg-transgenic mice that correlated with endoplasmic reticulum (ER) stress, unfolded protein response, autophagy and proliferation. These events were investigated by western blotting, immunohistochemical and immunocytochemical staining in 2-, 8- and 12-month-old HBsAg-transgenic mice. The results were verified in HBsAg-overexpressing Hepa1-6 cells and validated in human HBV-related HCC samples.

Results

Increased BiP expression in HBsAg-transgenic mice indicated induction of the unfolded protein response. In addition, early-phase autophagy was enhanced (increased BECN1 and LC3B) and late-phase autophagy blocked (increased p62) in HBsAg-transgenic mice. Finally, HBsAg altered lysosomal acidification via ATF4- and ATF6-mediated downregulation of lysosome-associated membrane protein 2 (LAMP2) expression. In patients, HBV-related HCC and adjacent tissues showed increased BiP, p62 and downregulated LAMP2 compared to uninfected controls. In vitro, the use of ER stress inhibitors reversed the HBsAg-related suppression of LAMP2. Furthermore, HBsAg promoted hepatocellular proliferation as indicated by Ki67, cleaved caspase-3 and AFP staining in paraffin-embedded liver sections from HBsAg-transgenic mice. These results were further verified by colony formation assays in HBsAg-expressing Hepa1-6 cells. Interestingly, inhibition of ER stress in HBsAg-overexpressing Hepa1-6 cells suppressed HBsAg-mediated cell proliferation.

Conclusions

These data showed that HBsAg directly induces ER stress, impairs autophagy and promotes proliferation, thereby driving hepatocarcinogenesis. In addition, this study expanded the understanding of HBsAg-mediated intracellular events in carcinogenesis.

Impact and implications

Factors and mechanisms involved in hepatocarcinogenesis driven by hepatitis B surface antigen (HBsAg) are poorly defined, hindering the development of effective therapeutic strategies. This study showed that HBsAg-induced endoplasmic reticulum stress suppressed LAMP2, thereby mediating autophagic injury. The present data suggest that restoring LAMP2 function in chronic HBV infection may have both antiviral and anti-cancer effects. This study has provided insights into the role of HBsAg-mediated intracellular events in carcinogenesis and thereby has relevance for future drug development.

Interconnection of cellular autophagy and endosomal vesicle trafficking and its role in hepatitis B virus replication and release

Hepatitis B virus (HBV) produces and releases various particle types, including complete virions, subviral particles with envelope proteins, and naked capsids. Recent studies demonstrate that HBV exploits distinct intracellular membrane trafficking pathways, including the endosomal vesicle trafficking and autophagy pathway, to assemble and release viral and subviral particles. Herein, we summarize the findings about the distinct roles of autophagy and endosomal membrane trafficking and the interaction of both pathways in HBV replication, assembly, and release.

SCD1 inhibits HBV replication by regulating autophagy under high lipid conditions

Chronic hepatitis B virus (HBV) infection remains a significant public health concern worldwide. Several metabolic processes regulate HBV DNA replication, including autophagy and lipid metabolism. In this study, we clarified the effect of lipids on HBV replication and elucidated possible mechanisms. We discovered that lipid metabolic gene expression levels were negatively correlated with the HBV DNA in plasma. Our data showed that fatty acid stimulation significantly reduced HBV DNA, hepatitis B surface antigen (HBsAg), and hepatitis B e antigen (HBeAg) levels in HepG2.2.15 cells, which are human hepatoma cell cultures transfected with HBV DNA. The Stearoyl coenzyme A desaturase 1 (SCD1)-autophagy pathway has also been implicated in inhibiting HBV replication by fatty acids stimulation. SCD1 knockdown deregulates the inhibitory effect of fatty acids on HBV by enhancing autophagy. When 3 methyladenine (3MA) was added, the inhibitory effects of specific autophagy inhibitors eliminated the positive effects of SCD1 knockdown on HBV replication. Our results indicate that SCD1 participates in the regulation of inhibition of HBV replication by fatty acids stimulation through regulating autophagy.

Conditional replication and secretion of hepatitis B virus genome uncover the truncated 3' terminus of encapsidated viral pregenomic RNA

IMPORTANCE

The biogenesis and clinical application of serum HBV pgRNA have been a research hotspot in recent years. This study further characterized the heterogeneity of the 3' terminus of capsid RNA by utilizing a variety of experimental systems conditionally supporting HBV genome replication and secretion, and reveal that the 3' truncation of capsid pgRNA is catalyzed by cellular ribonuclease(s) and viral RNaseH at positions after and before 3' DR1, respectively, indicating the 3' DR1 as a boundary between the encapsidated portion of pgRNA for reverse transcription and the 3' unprotected terminus, which is independent of pgRNA length and the 3' terminal sequence. Thus, our study provides new insights into the mechanism of pgRNA encapsidation and reverse transcription, as well as the optimization of serum HBV RNA diagnostics.

Pre- and Post-Transcriptional Control of HBV Gene Expression: The Road Traveled towards the New Paradigm of HBx, Its Isoforms, and Their Diverse Functions

Hepatitis B virus (HBV) is an enveloped DNA human virus belonging to the Hepadnaviridae family. Perhaps its main distinguishable characteristic is the replication of its genome through a reverse transcription process. The HBV circular genome encodes only four overlapping reading frames, encoding for the main canonical proteins named core, P, surface, and X (or HBx protein). However, pre- and post-transcriptional gene regulation diversifies the full HBV proteome into diverse isoform proteins. In line with this, hepatitis B virus X protein (HBx) is a viral multifunctional and regulatory protein of 16.5 kDa, whose canonical reading frame presents two phylogenetically conserved internal in-frame translational initiation codons, and which results as well in the expression of two divergent N-terminal smaller isoforms of 8.6 and 5.8 kDa, during translation. The canonical HBx, as well as the smaller isoform proteins, displays different roles during viral replication and subcellular localizations. In this article, we reviewed the different mechanisms of pre- and post-transcriptional regulation of protein expression that take place during viral replication. We also investigated all the past and recent evidence about HBV HBx gene regulation and its divergent N-terminal isoform proteins. Evidence has been collected for over 30 years. The accumulated evidence simply strengthens the concept of a new paradigm of the canonical HBx, and its smaller divergent N-terminal isoform proteins, not only during viral replication, but also throughout cell pathogenesis.

Live Cell Imaging Reveals HBV Capsid Translocation from the Nucleus To the Cytoplasm Enabled by Cell Division

Hepatitis B virus (HBV) capsid assembly is traditionally thought to occur predominantly in the cytoplasm, where the virus gains access to the virion egress pathway. To better define sites of HBV capsid assembly, we carried out single cell imaging of HBV Core protein (Cp) subcellular trafficking over time under conditions supporting genome packaging and reverse transcription in Huh7 hepatocellular carcinoma cells. Time-course analyses including live cell imaging of fluorescently tagged Cp derivatives showed Cp to accumulate in the nucleus at early time points (~24 h), followed by a marked re-distribution to the cytoplasm at 48 to 72 h. Nucleus-associated Cp was confirmed to be capsid and/or high-order assemblages using a novel dual label immunofluorescence strategy. Nuclear-to-cytoplasmic re-localization of Cp occurred predominantly during nuclear envelope breakdown in conjunction with cell division, followed by strong cytoplasmic retention of Cp. Blocking cell division resulted in strong nuclear entrapment of high-order assemblages. A Cp mutant, Cp-V124W, predicted to exhibit enhanced assembly kinetics, also first trafficked to the nucleus to accumulate at nucleoli, consistent with the hypothesis that Cp's transit to the nucleus is a strong and constitutive process. Taken together, these results provide support for the nucleus as an early-stage site of HBV capsid assembly, and provide the first dynamic evidence of cytoplasmic retention after cell division as a mechanism underpinning capsid nucleus-to-cytoplasm relocalization. IMPORTANCE Hepatitis B virus (HBV) is an enveloped, reverse-transcribing DNA virus that is a major cause of liver disease and hepatocellular carcinoma. Subcellular trafficking events underpinning HBV capsid assembly and virion egress remain poorly characterized. Here, we developed a combination of fixed and long-term (>24 h) live cell imaging technologies to study the single cell trafficking dynamics of the HBV Core Protein (Cp). We demonstrate that Cp first accumulates in the nucleus, and forms high-order structures consistent with capsids, with the predominant route of nuclear egress being relocalization to the cytoplasm during cell division in conjunction with nuclear membrane breakdown. Single cell video microscopy demonstrated unequivocally that Cp's localization to the nucleus is constitutive. This study represents a pioneering application of live cell imaging to study HBV subcellular transport, and demonstrates links between HBV Cp and the cell cycle.

Sera of Individuals Chronically Infected with Hepatitis B Virus (HBV) Contain Diverse RNA Types Produced by HBV Replication or Derived from Integrated HBV DNA

This study aimed to better characterize the repertoire of serum hepatitis B virus (HBV) RNAs during chronic HBV infection in humans, which remains understudied. Using reverse transcription-PCR (RT-PCR), real-time quantitative PCR (RT-qPCR), RNA-sequencing, and immunoprecipitation, we found that (i) >50% of serum samples bore different amounts of HBV replication-derived RNAs (rd-RNAs); (ii) a few samples contained RNAs transcribed from integrated HBV DNA, including 5'-HBV-human-3' RNAs (integrant-derived RNAs [id-RNAs]) and 5'-human-HBV-3' transcripts, as a minority of serum HBV RNAs; (iii) spliced HBV RNAs were abundant in <50% of analyzed samples; (iv) most serum rd-RNAs were polyadenylated via conventional HBV polyadenylation signal; (v) pregenomic RNA (pgRNA) was the major component of the pool of serum RNAs; (vi) the area of HBV positions 1531 to 1739 had very high RNA read coverage and thus should be used as a target for detecting serum HBV RNAs; (vii) the vast majority of rd-RNAs and pgRNA were associated with HBV virions but not with unenveloped capsids, exosomes, classic microvesicles, or apoptotic vesicles and bodies; (viii) considerable rd-RNAs presence in the circulating immune complexes was found in a few samples; and (ix) serum relaxed circular DNA (rcDNA) and rd-RNAs should be quantified simultaneously to evaluate HBV replication status and efficacy of anti-HBV therapy with nucleos(t)ide analogs. In summary, sera contain various HBV RNA types of different origin, which are likely secreted via different mechanisms. In addition, since we previously showed that id-RNAs were abundant or predominant HBV RNAs in many of liver and hepatocellular carcinoma tissues as compared to rd-RNAs, there is likely a mechanism favoring the egress of replication-derived RNAs. IMPORTANCE The presence of integrant-derived RNAs (id-RNAs) and 5'-human-HBV-3' transcripts derived from integrated hepatitis B virus (HBV) DNA in sera was demonstrated for the first time. Thus, sera of individuals chronically infected with HBV contained both replication-derived and integrant-transcribed HBV RNAs. The majority of serum HBV RNAs were the transcripts produced by HBV genome replication, which were associated with HBV virions and not with other types of extracellular vesicles. These and other above-mentioned findings advanced our understanding of the HBV life cycle. In addition, the study suggested a promising target area on the HBV genome to increase sensitivity of the detection of serum HBV RNAs and supported the idea that simultaneous detection of replication-derived RNAs (rd-RNAs) and relaxed circular DNA (rcDNA) in serum provides more adequate evaluation of (i) the HBV genome replication status and (ii) the durability and efficiency of the therapy with anti-HBV nucleos(t)ide analogs, which could be useful for improvement of the diagnostics and treatment of HBV-infected individuals.

Many Ways to Communicate-Crosstalk between the HBV-Infected Cell and Its Environment

Chronic infection with the hepatitis B virus (HBV) affects an estimated 257 million people worldwide and can lead to liver diseases such as cirrhosis and liver cancer. Viral replication is generally considered not to be cytopathic, and although some HBV proteins may have direct carcinogenic effects, the majority of HBV infection-related disease is related to chronic inflammation resulting from disrupted antiviral responses and aberrant innate immune reactions. Like all cells, healthy and HBV-infected cells communicate with each other, as well as with other cell types, such as innate and adaptive immune cells. They do so by both interacting directly and by secreting factors into their environment. Such factors may be small molecules, such as metabolites, single viral proteins or host proteins, but can also be more complex, such as virions, protein complexes, and extracellular vesicles. The latter are small, membrane-enclosed vesicles that are exchanged between cells, and have recently gained a lot of attention for their potential to mediate complex communication and their potential for therapeutic repurposing. Here, we review how HBV infection affects the communication between HBV-infected cells and cells in their environment. We discuss the impact of these interactions on viral persistence in chronic infection, as well as their relation to HBV infection-related pathology.

Sac1 phosphatidylinositol 4-phosphate phosphatase is a novel host cell factor regulating hepatitis B virus particles assembly and release

The life-cycle of the Hepatitis B Virus (HBV), an enveloped DNA virus affecting the lives of more than 296 million chronicallyinfected people, is tightly dependent on the lipid metabolism of the host cell. Fatty acids and cholesterol are among the lipid factors with documented roles in regulating HBV replication and infection, respectively, but little is known about the phosphoinositide metabolism in these processes. In this study, we investigated the role of Sac1, a highly conserved phosphatidylinositol-4-phosphate (PI4P) phosphatase, with essential functions in phospholipid metabolism, in HBV assembly, and release. PI4P is one of the most abundant cellular phosphoinositide with complex functions at the level of the secretory pathway. Owing to the highly specific phosphatase activity toward PI4P, Sac1 controls the levels and restricts the localization of this lipid particularly at the trans-Golgi network, where it regulates sphingolipid synthesis, proteins sorting, and vesicles budding, by recruiting specific adaptor proteins. As a complete loss of Sac1 function compromises cell viability, in this work, we first developed and characterized several HBV replication-permissive cellular models with a moderate, transient, or stable downregulation of Sac1 expression. Our results show that Sac1 depletion in hepatic cells results in increased levels and redistribution of intracellular PI4P pools and impaired trafficking of the HBV envelope proteins to the endosomal vesicular network. Importantly, virus envelopment and release from these cells are significantly inhibited, revealing novel roles for Sac1, as a key host cell factor regulating morphogenesis of a DNA virus.

Autophagic membranes participate in hepatitis B virus nucleocapsid assembly, precore and core protein trafficking, and viral release

Hepatitis B virus (HBV) DNA replication takes place inside the viral core particle and is dependent on autophagy. Here we show that HBV core particles are associated with autophagosomes and phagophores in cells that productively replicate HBV. These autophagic membrane-associated core particles contain almost entirely the hypophosphorylated core protein and are DNA replication competent. As the hyperphosphorylated core protein can be localized to phagophores and the dephosphorylation of the core protein is associated with the packaging of viral pregenomic RNA (pgRNA), these results are in support of the model that phagophores can serve as the sites for the packaging of pgRNA. In contrast, in cells that replicate HBV, the precore protein derivatives, which are related to the core protein, are associated with autophagosomes but not with phagophores via a pathway that is independent of its signal peptide. Interestingly, when the core protein is expressed by itself, it is associated with phagophores but not with autophagosomes. These observations indicate that autophagic membranes are differentially involved in the trafficking of precore and core proteins. HBV induces the fusion of autophagosomes and multivesicular bodies and the silencing of Rab11, a regulator of this fusion, is associated with the reduction of release of mature HBV particles. Our studies thus indicate that autophagic membranes participate in the assembly of HBV nucleocapsids, the trafficking of HBV precore and core proteins, and likely also the egress of HBV particles.

Hepatitis B virus virion secretion is a CRM1-spike-mediated late event

Background

Hepatitis B virus (HBV) is a major human pathogen worldwide. To date, there is no curative treatment for chronic hepatitis B. The mechanism of virion secretion remains to be investigated. Previously, we found that nuclear export of HBc particles can be facilitated via two CRM1-specific nuclear export signals (NES) at the spike tip.

Methods

In this study, we used site-directed mutagenesis at the CRM1 NES, as well as treatment with CRM1 inhibitors at a low concentration, or CRM1-specific shRNA knockdown, in HBV-producing cell culture, and measured the secretion of various HBV viral and subviral particles via a native agarose gel electrophoresis assay. Separated HBV particles were characterized by Western blot analysis, and their genomic DNA contents were measured by Southern blot analysis. Secreted extracellular particles were compared with intracellular HBc capsids for DNA synthesis and capsid formation. Virion secretion and the in vivo interactions among HBc capsids, CRM1 and microtubules, were examined by proximity ligation assay, immunofluorescence microscopy, and nocodazole treatment.

Results

We report here that the tip of spike of HBV core (HBc) particles (capsids) contains a complex sensor for secretion of both HBV virions and naked capsids. HBV virion secretion is closely associated with HBc nuclear export in a CRM1-dependent manner. At the conformationally flexible spike tips of HBc particles, NES motifs overlap extensively with motifs important for secretion of HBV virions and naked capsids.

Conclusions

We provided experimental evidence that virions and naked capsids can egress via two distinct, yet overlapping, pathways. Unlike the secretion of naked capsids, HBV virion secretion is highly CRM1- and microtubule-dependent. CRM1 is well known for its involvement in nuclear transport in literature. To our knowledge, this is the first report that CRM1 is required for virion secretion. CRM1 inhibitors could be a promising therapeutic candidate for chronic HBV patients in clinical medicine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00827-w.

The Role of NEDD4 E3 Ubiquitin–Protein Ligases in Parkinson’s Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disease that causes a great clinical burden. However, its exact molecular pathologies are not fully understood. Whilst there are a number of avenues for research into slowing, halting, or reversing PD, one central idea is to enhance the clearance of the proposed aetiological protein, oligomeric α-synuclein. Oligomeric α-synuclein is the main constituent protein in Lewy bodies and neurites and is considered neurotoxic. Multiple E3 ubiquitin-protein ligases, including the NEDD4 (neural precursor cell expressed developmentally downregulated protein 4) family, parkin, SIAH (mammalian homologues of Drosophila seven in absentia), CHIP (carboxy-terminus of Hsc70 interacting protein), and SCFFXBL5 SCF ubiquitin ligase assembled by the S-phase kinase-associated protein (SKP1), cullin-1 (Cul1), a zinc-binding RING finger protein, and the F-box domain/Leucine-rich repeat protein 5-containing protein FBXL5), have been shown to be able to ubiquitinate α-synuclein, influencing its subsequent degradation via the proteasome or lysosome. Here, we explore the link between NEDD4 ligases and PD, which is not only via α-synuclein but further strengthened by several additional substrates and interaction partners. Some members of the NEDD4 family of ligases are thought to crosstalk even with PD-related genes and proteins found to be mutated in familial forms of PD. Mutations in NEDD4 family genes have not been observed in PD patients, most likely because of their essential survival function during development. Following further in vivo studies, it has been thought that NEDD4 ligases may be viable therapeutic targets in PD. NEDD4 family members could clear toxic proteins, enhancing cell survival and slowing disease progression, or might diminish beneficial proteins, reducing cell survival and accelerating disease progression. Here, we review studies to date on the expression and function of NEDD4 ubiquitin ligases in the brain and their possible impact on PD pathology.

Phosphorylation of the Hepatitis B Virus Large Envelope Protein

We here establish the phosphorylation sites in the human hepatitis B virus (HBV) large envelope protein (L). L is involved in several functionally important interactions in the viral life cycle, including with the HBV cellular receptor, HBV capsid, Hsc70 chaperone, and cellular membranes during fusion. We have recently shown that cell-free synthesis of the homologous L protein of duck HBV in wheat germ extract results in very similar phosphorylation events to those previously observed in animal cells. Here, we used mass spectrometry and NMR to establish the phosphorylation patterns of human HBV L protein produced by both in vitro cell-free synthesis and in E. coli with the co-expression of the human MAPK14 kinase. While in the avian virus the phosphorylation of L has been shown to be dispensable for infectivity, the identified locations in the human virus protein, both in the PreS1 and PreS2 domains, raise the intriguing possibility that they might play a functional role, since they are found at strategic sites predicted to be involved in L interactions. This would warrant the further investigation of a possible function in virion formation or cell entry.

Endoplasmic reticulum stress promotes HBV production by enhancing use of the autophagosome/multivesicular body axis

BACKGROUND AND AIMS

HBV infection has been reported to trigger endoplasmic reticulum (ER) stress and initiate autophagy. However, how ER stress and autophagy influence HBV production remains elusive. Here, we studied the effect of tunicamycin (TM), an N-glycosylation inhibitor and ER stress inducer, on HBV replication and secretion and examined the underlying mechanisms.

APPROACH AND RESULTS

Protein disulfide isomerase (an ER marker), microtubule-associated protein 1 light chain 3 beta (an autophagosome [AP] marker), and sequestosome-1 (a typical cargo for autophagic degradation) expression were tested in liver tissues of patients with chronic HBV infection and hepatoma cell lines. The role of TM treatment in HBV production and trafficking was examined in hepatoma cell lines. TM treatment that mimics HBV infection triggered ER stress and increased AP formation, resulting in enhanced HBV replication and secretion of subviral particles (SVPs) and naked capsids. Additionally, TM reduced the number of early endosomes and HBsAg localization in this compartment, causing HBsAg/SVPs to accumulate in the ER. Thus, TM-induced AP formation serves as an alternative pathway for HBsAg/SVP trafficking. Importantly, TM inhibited AP-lysosome fusion, accompanied by enhanced AP/late endosome (LE)/multivesicular body fusion, to release HBsAg/SVPs through, or along with, exosome release. Notably, TM treatment inhibited HBsAg glycosylation, resulting in impairment of HBV virions' envelopment and secretion, but it was not critical for HBsAg/SVP trafficking in our cell systems.

CONCLUSIONS

TM-induced ER stress and autophagic flux promoted HBV replication and the release of SVPs and naked capsids through the AP-LE/MVB axis.

Pathogenicity and virulence of Hepatitis B virus

Hepatitis B virus (HBV) is a hepatotropic virus and an important human pathogen. There are an estimated 296 million people in the world that are chronically infected by this virus, and many of them will develop severe liver diseases including hepatitis, cirrhosis and hepatocellular carcinoma (HCC). HBV is a small DNA virus that replicates via the reverse transcription pathway. In this review, we summarize the molecular pathways that govern the replication of HBV and its interactions with host cells. We also discuss viral and non-viral factors that are associated with HBV-induced carcinogenesis and pathogenesis, as well as the role of host immune responses in HBV persistence and liver pathogenesis.

A nuanced role of the small loop of hepatitis B virus small envelope protein in virion morphogenesis and secretion

BACKGROUND

The virion secretion mechanism of human hepatitis B virus (HBV) remains to be investigated. In our current study, we characterized a reverse transcriptase mutant, which changed from the YMDD motif to YMHA. We noted that this mutant YMHA secreted no virions in the medium. Because of the overlapping open reading frame (ORF) between the polymerase and the envelope genes, the lack of virion secretion is likely due to corresponding concurrent mutations in a small loop of the envelope protein (HBsAg, HBV surface antigen). In literature, small loop mutations are thought to affect virion secretion of hepatitis delta virus (HDV), but not HBV.

METHODS

Here, we revisited the relationship between the small loop and virion secretion by site-directed mutagenesis and native agarose gel electrophoresis.

RESULTS

A proline substitution at residue 196 or 198 in the small loop blocked both HBV genome-containing and genome-free virion secretion, but not the secretion of 22-nm HBsAg subviral particles. Surprisingly, a leucine substitution at residue 196 enhanced genome-containing virion secretion. It is also intriguing that a proline-197, sandwiched by residue 196 and 198, exhibited no apparent defect in secreted virions, with or without containing an HBV genome. By complementation assay, we demonstrated that the wild type small envelope protein alone is sufficient to rescue the virion secretion defect of a small loop mutant M198P.

CONCLUSIONS

The effect of the small loop mutation of HBV small envelope protein on virion secretion is position-dependent. It warrants further investigation how the small loop of HBsAg plays a subtle role in HBV morphogenesis and secretion of virions with or without containing an HBV genome.

Core Protein-Directed Antivirals and Importin β Can Synergistically Disrupt HBV Capsids

Viral structural proteins can have multiple activities. Antivirals that target structural proteins have potential to exhibit multiple antiviral mechanisms. Hepatitis B Virus (HBV) core protein (Cp) is involved in most stages of the viral lifecycle: it assembles into capsids, packages viral RNA, is a metabolic compartment for reverse transcription, interacts with nuclear trafficking machinery, and disassembles to release the viral genome into the nucleus. During nuclear localization, HBV capsids bind to host importins (e.g. Impβ) via Cp's C-terminal domain (CTD); the CTD is localized to the interior of the capsid and is transiently exposed on the exterior. We used HAP12 as a representative Cp Allosteric Modulators (CpAMs), a class of antivirals that inappropriately stimulates and misdirects HBV assembly and deforms capsids. CpAM impact on other aspects of the HBV lifecycle is poorly understood. We investigated how HAP12 influenced the interactions between empty or RNA-filled capsids with Impβ and trypsin in vitro. We showed that HAP12 can modulate CTD accessibility and capsid stability, depending on the saturation of HAP12-binding sites. We demonstrated that Impβ synergistically contributes to capsid disruption at high levels of HAP12 saturation, using electron microscopy to visualize disruption and rearrangement of Cp dimers into aberrant complexes. However, RNA-filled capsids resisted the destabilizing effects of HAP12 and Impβ. In summary, we show host protein-induced catalysis of capsid disruption, an unexpected additional mechanism of action for CpAMs. Potentially, untimely capsid disassembly can hamper the HBV lifecycle and also cause the virus to become vulnerable to host innate immune responses. IMPORTANCE The HBV core, an icosahedral complex of 120 copies of the homodimeric core (capsid) protein with or without packaged nucleic acid, is transported to the host nucleus by its interaction with host importin proteins. Importin-core interaction requires the core protein C-terminal domain, which is inside the capsid, to "flip" to the capsid exterior. Core-protein directed drugs that affect capsid assembly and stability have been developed recently. We show that these molecules can, synergistically with importins, disrupt capsids. This mechanism of action, synergism with host protein, has potential to disrupt the virus lifecycle and activate the innate immune system.

PDIA3: Structure, functions and its potential role in viral infections

The catalysis of disulphide (SS) bonds is the most important characteristic of protein disulphide isomerase (PDI) family. Catalysis occurs in the endoplasmic reticulum, which contains many proteins, most of which are secretory in nature and that have at least one s-s bond. Protein disulphide isomerase A3 (PDIA3) is a member of the PDI family that acts as a chaperone. PDIA3 is highly expressed in response to cellular stress, and also intercept the apoptotic cellular death related to endoplasmic reticulum (ER) stress, and protein misfolding. PDIA3 expression is elevated in almost 70% of cancers and its expression has been linked with overall low cell invasiveness, survival and metastasis. Viral diseases present a significant public health threat. The presence of PDIA3 on the cell surface helps different viruses to enter the cells and also helps in replication. Therefore, inhibitors of PDIA3 have great potential to interfere with viral infections. In this review, we summarize what is known about the basic structure, functions and role of PDIA3 in viral infections. The review will inspire studies of pathogenic mechanisms and drug targeting to counter viral diseases.

CCDC88A/GIV promotes HBV replication and progeny secretion via enhancing endosomal trafficking and blocking autophagic degradation

Hepatitis B virus (HBV) particles are thought to be secreted from hepatocytes through multivesicular bodies (MVBs); however, the cellular trafficking mechanisms prior to this process remain elusive. It has been reported that CCDC88A/GIV expression, which is involved in multiple aspects of vesicular trafficking, changes dynamically at different phases of chronic HBV infection. In this study, we focused on the role of CCDC88A/GIV in HBV replication. In the liver tissues of chronically HBV-infected patients, HBV infection significantly enhanced CCDC88A/GIV expression, and increased endoplasmic reticulum (ER) stress and autophagosome formation without changing endosome formation. Additionally, colocalization of SHBsAg with early endosomes (~30.2%) far exceeded that with autophagosomes (~3.2%). In hepatoma cells, CCDC88A/GIV and its downstream proteins, DNM2 (dynamin 2; a CCDC88A/GIV effector), CLTC and RAB5A significantly enhanced HBV replication and endosome formation but inhibited autophagosome formation. Blocking endocytosis disrupted HBsAg trafficking to endosomes and caused its accumulation in the ER lumen, which triggered ER stress to initiate the unfolded protein response (UPR). Therefore, HBsAg trafficking into autophagosomes was increased, and the lysosomal activity and maturation, which was inhibited by HBV infection, were restored. Meanwhile, core particles were prevented from entering MVBs. CCDC88A/GIV and its other effector, GNAI3, decreased autophagic flux by enhancing the insulin-induced AKT-MTOR pathway, thereby inhibiting HBV antigens autophagic degradation. In conclusion, CCDC88A/GIV enhanced HBV replication by increasing endosomal trafficking and reducing autophagic degradation of HBV antigens, suggesting that CCDC88A/GIV-mediated endosomal trafficking plays an important role in HBV replication and progeny secretion.Abbreviations: ACTB: actin beta; AO: acridine orange; ATF6: activating transcription factor 6; CCDC88A/GIV: coiled-coil domain containing 88A; CLTC: clathrin heavy chain; CQ: chloroquine; DAPI: 4ʹ,6-diamidino-2-phenylindole; DNM2: dynamin 2; ER: endoplasmic reticulum; ERN1: endoplasmic reticulum to nucleus signaling 1; EIF2A: eukaryotic translation initiation factor 2A; FBS: fetal bovine serum; GNAI3: G protein subunit alpha i3; HBV: hepatitis B virus; HBV RIs: HBV replication intermediates; HBcAg: HBV core protein; HBsAg: HBV surface antigen; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MVBs: multivesicular bodies; MTOR: mechanistic target of rapamycin kinase; PDI: protein disulfide isomerase; PHH: primary human hepatocyte; pSM2: a HBV replication-competent plasmid; HSPA5/BIP: heat shock protein family A (Hsp70) member 5; SQSTM1/p62: sequestosome 1; siRNA: small interfering RNA; SEM: standard error of the mean; UPR: unfolded protein response

Envelope Proteins of Hepatitis B Virus: Molecular Biology and Involvement in Carcinogenesis

The envelope of hepatitis B virus (HBV), which is required for the entry to hepatocytes, consists of a lipid bilayer derived from hepatocyte and HBV envelope proteins, large/middle/small hepatitis B surface antigen (L/M/SHBs). The mechanisms and host factors for the envelope formation in the hepatocytes are being revealed. HBV-infected hepatocytes release a large amount of subviral particles (SVPs) containing L/M/SHBs that facilitate escape from the immune system. Recently, novel drugs inhibiting the functions of the viral envelope and those inhibiting the release of SVPs have been reported. LHBs that accumulate in ER is considered to promote carcinogenesis and, especially, deletion mutants in the preS1/S2 domain have been reported to be associated with the development of hepatocellular carcinoma (HCC). In this review, we summarize recent reports on the findings regarding the biological characteristics of HBV envelope proteins, their involvement in HCC development and new agents targeting the envelope.

Understanding the antiviral effects of RNAi-based therapy in HBeAg-positive chronic hepatitis B infection

The RNA interference (RNAi) drug ARC-520 was shown to be effective in reducing serum hepatitis B virus (HBV) DNA, hepatitis B e antigen (HBeAg) and hepatitis B surface antigen (HBsAg) in HBeAg-positive patients treated with a single dose of ARC-520 and daily nucleosidic analogue (entecavir). To provide insights into HBV dynamics under ARC-520 treatment and its efficacy in blocking HBV DNA, HBsAg, and HBeAg production we developed a multi-compartmental pharmacokinetic-pharamacodynamic model and calibrated it with frequent measured HBV kinetic data. We showed that the time-dependent single dose ARC-520 efficacies in blocking HBsAg and HBeAg are more than 96% effective around day 1, and slowly wane to 50% in 1-4 months. The combined single dose ARC-520 and entecavir effect on HBV DNA was constant over time, with efficacy of more than 99.8%. The observed continuous HBV DNA decline is entecavir mediated, the strong but transient HBsAg and HBeAg decays are ARC-520 mediated. The modeling framework may help assess ongoing RNAi drug development for hepatitis B virus infection.

Virus Budding

Enveloped viruses exit producer cells and acquire their external lipid envelopes by budding through limiting cellular membranes. Most viruses encode multifunctional structural proteins that coordinate the processes of virion assembly, membrane envelopment, budding, and maturation. In many cases, the cellular ESCRT pathway is recruited to facilitate the membrane fission step of budding, but alternative strategies are also employed. Recently, many viruses previously considered to be non-enveloped have been shown to exit cells non-lytically within vesicles, adding further complexity to the intricacies of virus budding and egress.

Virion Secretion of Hepatitis B Virus Naturally Occurring Core Antigen Variants

In natural infection, hepatitis B virus (HBV) core protein (HBc) accumulates frequent mutations. The most frequent HBc variant in chronic hepatitis B patients is mutant 97L, changing from an isoleucine or phenylalanine to a leucine (L) at HBc amino acid 97. One dogma in the HBV research field is that wild type HBV secretes predominantly virions containing mature double-stranded DNA genomes. Immature genomes, containing single-stranded RNA or DNA, do not get efficiently secreted until reaching genome maturity. Interestingly, HBc variant 97L does not follow this dogma in virion secretion. Instead, it exhibits an immature secretion phenotype, which preferentially secretes virions containing immature genomes. Other aberrant behaviors in virion secretion were also observed in different naturally occurring HBc variants. A hydrophobic pocket around amino acid 97 was identified by bioinformatics, genetic analysis, and cryo-EM. We postulated that this hydrophobic pocket could mediate the transduction of the genome maturation signal for envelopment from the capsid interior to its surface. Virion morphogenesis must involve interactions between HBc, envelope proteins (HBsAg) and host factors, such as components of ESCRT (endosomal sorting complex required for transport). Immature secretion can be offset by compensatory mutations, occurring at other positions in HBc or HBsAg. Recently, we demonstrated in mice that the persistence of intrahepatic HBV DNA is related to virion secretion regulated by HBV genome maturity. HBV virion secretion could be an antiviral drug target.

An Intracellular Model of Hepatitis B Viral Infection: An In Silico Platform for Comparing Therapeutic Strategies

Hepatitis B virus (HBV) is a major focus of antiviral research worldwide. The International Coalition to Eliminate HBV, together with the World Health Organisation (WHO), have prioritised the search for a cure, with the goal of eliminating deaths from viral hepatitis by 2030. We present here a comprehensive model of intracellular HBV infection dynamics that includes all molecular processes currently targeted by drugs and agrees well with the observed outcomes of several clinical studies. The model reveals previously unsuspected kinetic behaviour in the formation of sub-viral particles, which could lead to a better understanding of the immune responses to infection. It also enables rapid comparative assessment of the impact of different treatment options and their potential synergies as combination therapies. A comparison of available and currently developed treatment options reveals that combinations of multiple capsid assembly inhibitors perform best.

Virus spread in the liver: mechanisms, commonalities, and unanswered questions

The liver is home to five known human hepatitis viruses (hepatitis A virus-hepatitis E virus). Despite being phylogenetically unrelated, these viruses replicate and spread in the liver without causing apparent cytopathic effects, and all have evolved strategies to counteract antibody-mediated inhibition of virus spread. In this review, we discuss the current understanding regarding the spread mechanisms for these viruses with an attempt to extract common principles and identify key questions for future studies.

Biogenesis and molecular characteristics of serum hepatitis B virus RNA

HBV is an enveloped DNA virus that replicates its DNA genome via reverse transcription of a pregenomic (pg) RNA intermediate in hepatocytes. Interestingly, HBV RNA can be detected in virus-like particles in chronic hepatitis B (CHB) patient serum and has been utilized as a biomarker for intrahepatic cccDNA activity in treated patients. However, the biogenesis and molecular characteristics of serum HBV RNA remain to be fully defined. In this study, we found that the encapsidated serum HBV RNA predominately consists of pgRNA, which are detergent- and ribonuclease-resistant. Through blocking HBV DNA replication without affecting pgRNA encapsidation by using the priming-defective HBV mutant Y63D or 3TC treatment, we demonstrated that the cell culture supernatant contains a large amount of pgRNA-containing nonenveloped capsids and a minor population of pgRNA-containing virions. The formation of pgRNA-virion requires both capsid assembly and viral envelope proteins, which can be inhibited by capsid assembly modulators and an envelope-knockout mutant, respectively. Furthermore, the pgRNA-virion utilizes the multivesicular body pathway for egress, in a similar way as DNA-virion morphogenesis. Northern blotting, RT-PCR, and 3' RACE assays revealed that serum/supernatant HBV pgRNA are mainly spliced and devoid of the 3'-terminal sequences. Furthermore, pgRNA-virion collected from cells treated with a reversible HBV priming inhibitor L-FMAU was unable to establish infection in HepG2-NTCP cells. In summary, serum HBV RNA is secreted in noninfectious virion-like particle as spliced and poly(A)-free pgRNA. Our study will shed light on the molecular biology of serum HBV RNA in HBV life cycle, and aid the development of serum HBV RNA as a novel biomarker for CHB diagnosis and treatment prognosis.

Interplay between Cellular Autophagy and Hepatitis B Virus Replication: A Systematic Review

Autophagy, a conserved process in which cells break down and destroy old, damaged, or abnormal proteins and other substances in the cytoplasm through lysosomal degradation, occurs via autophagosome formation and aids in the maintenance of intracellular homeostasis. Autophagy is closely associated with hepatitis B virus (HBV) replication and assembly. Currently, HBV infection is still one of the most serious public health issues worldwide. The unavailability of satisfactory therapeutic strategies for chronic HBV infection indicates an urgent need to elucidate the mechanisms underlying the pathogenesis of HBV infection. Increasing evidence has shown that HBV not only possesses the ability to induce incomplete autophagy but also evades autophagic degradation, indicating that HBV utilizes or hijacks the autophagy machinery for its own replication. Therefore, autophagy might be a crucial target pathway for controlling HBV infection. The definite molecular mechanisms underlying the association between cellular autophagy and HBV replication require further clarification. In this review, we have summarized and discussed the latest findings on the interplay between autophagy and HBV replication.

Expression Level of Small Envelope Protein in Addition to Sequence Divergence inside Its Major Hydrophilic Region Contributes to More Efficient Surface Antigen Secretion by Hepatitis B Virus Subgenotype D2 than Subgenotype A2

Hepatitis B surface antigen (HBsAg) promotes persistent hepatitis B virus (HBV) infection. It primarily corresponds to small (S) envelope protein secreted as subviral particles. We previously found that genotype D clones expressed less S protein than genotype A clones but showed higher extracellular/intracellular ratio of HBsAg suggesting more efficient secretion. The current study aimed to characterize the underlying mechanism(s) by comparing a subgenotype A2 clone (geno5.4) with a subgenotype D2 clone (geno1.2). Five types of full-length or subgenomic constructs were transfected to Huh7 cells at different dosage. HBsAg was quantified by enzyme linked immunosorbent assay while envelope proteins were detected by Western blot. We found that ratio of extracellular/intracellular HBsAg decreased at increasing amounts of DNA transfected. Conflicting findings from two types of subgenomic construct confirmed stronger secretion inhibitory effect of the genotype D-derived large envelope protein. Chimeric constructs followed by site-directed mutagenesis revealed geno1.2 specific V118/T127 and F161/A168 in the S protein as promoting and inhibitory of HBsAg secretion, respectively. In conclusion, more efficient HBsAg secretion by subgenotype D2 than subgenotype A2 is attributed to lower level of S protein expression in addition to V118 and T127 in S protein, although its F161 and A168 sequences rather reduce HBsAg secretion.

Hepatitis B Virus Exploits ERGIC-53 in Conjunction with COPII to Exit Cells

Several decades after its discovery, the hepatitis B virus (HBV) still displays one of the most successful pathogens in human populations worldwide. The identification and characterization of interactions between cellular and pathogenic components are essential for the development of antiviral treatments. Due to its small-sized genome, HBV highly depends on cellular functions to produce and export progeny particles. Deploying biochemical-silencing methods and molecular interaction studies in HBV-expressing liver cells, we herein identified the cellular ERGIC-53, a high-mannose-specific lectin, and distinct components of the endoplasmic reticulum (ER) export machinery COPII as crucial factors of viral trafficking and egress. Whereas the COPII subunits Sec24A, Sec23B and Sar1 are needed for both viral and subviral HBV particle exit, ERGIC-53 appears as an exclusive element of viral particle propagation, therefore interacting with the N146-glycan of the HBV envelope in a productive manner. Cell-imaging studies pointed to ER-derived, subcellular compartments where HBV assembly initiates. Moreover, our findings provide evidence that HBV exploits the functions of ERGIC-53 and Sec24A after the envelopment of nucleocapsids at these compartments in conjunction with endosomal sorting complexes required for transport (ESCRT) components. These data reveal novel insights into HBV assembly and trafficking, illustrating therapeutic prospects for intervening with the viral life cycle.

The Hepatitis B Virus Envelope Proteins: Molecular Gymnastics Throughout the Viral Life Cycle

New hepatitis B virions released from infected hepatocytes are the result of an intricate maturation process that starts with the formation of the nucleocapsid providing a confined space where the viral DNA genome is synthesized via reverse transcription. Virion assembly is finalized by the enclosure of the icosahedral nucleocapsid within a heterogeneous envelope. The latter contains integral membrane proteins of three sizes, collectively known as hepatitis B surface antigen, and adopts multiple conformations in the course of the viral life cycle. The nucleocapsid conformation depends on the reverse transcription status of the genome, which in turn controls nucleocapsid interaction with the envelope proteins for virus exit. In addition, after secretion the virions undergo a distinct maturation step during which a topological switch of the large envelope protein confers infectivity. Here we review molecular determinants for envelopment and models that postulate molecular signals encoded in the capsid scaffold conducive or adverse to the recruitment of envelope proteins.

Characterization of the antiviral effects of REP 2139 on the HBV lifecycle in vitro

During hepatitis B virus (HBV) infection, HBV subviral particles (SVP) are produced in large excess in comparison to infectious virions and account for the major source of HBV surface antigen (HBsAg) in the blood. This abundant circulating HBsAg has been postulated to promote HBV chronicity by inducing immune exhaustion against HBsAg. Nucleic acid polymers (NAPs) such as REP 2139 display promising antiviral activity against both HBV and hepatitis Delta virus (HDV) in clinical trials. REP 2139 is accompanied by clearance of HBsAg from blood with concomitant reappearance of anti-HBsAg antibodies. To decipher the mechanism-of-action of NAPs, a recently developed cell-based assay in human HepG2.2.15 cells was used (Blanchet et al., 2019). This assay recapitulates the HBsAg secretion inhibition observed in treated patients. In the present study, we analysed the antiviral effect of REP 2139 on the HBV lifecycle. Importantly, we confirm here the potent inhibitory activity of the compound on HBsAg secretion, and report minor or no effect on other viral markers such as intracellular DNA and RNA, and HBeAg or Dane particle secretion. Notably, intracellular HBsAg accumulation is prevented by proteasomal and lysosomal degradation.

Hepatitis B Virus Core Protein Domains Essential for Viral Capsid Assembly in a Cellular Context

Hepatitis B virus (HBV) core protein (HBc) is essential to the formation of the HBV capsid. HBc contains two domains: the N-terminal domain corresponding to residues 1-140 essential to form the icosahedral shell and the C-terminal domain corresponding to a basic and phosphorylated peptide, and required for DNA replication. The role of these two domains for HBV capsid assembly was essentially studied in vitro with HBc purified from mammalian or non-mammalian cell lysates, but their respective role in living cells remains to be clarified. We therefore investigated the assembly of the HBV capsid in Huh7 cells by combining fluorescence lifetime imaging microscopy/Förster's resonance energy transfer, fluorescence correlation spectroscopy and transmission electron microscopy approaches. We found that wild-type HBc forms oligomers early after transfection and at a sub-micromolar concentration. These oligomers are homogeneously diffused throughout the cell. We quantified a stoichiometry ranging from ~170 to ~230 HBc proteins per oligomer, consistent with the visualization of eGFP-containingHBV capsid shaped as native capsid particles by transmission electron microscopy. In contrast, no assembly was observed when HBc-N-terminal domain was expressed. This highlights the essential role of the C-terminal domain to form capsid in mammalian cells. Deletion of either the third helix or of the 124-135 residues of HBc had a dramatic impact on the assembly of the HBV capsid, inducing the formation of mis-assembled oligomers and monomers, respectively. This study shows that our approach using fluorescent derivatives of HBc is an innovative method to investigate HBV capsid formation.

Hepatitis B subviral envelope particles use the COPII machinery for intracellular transport via selective exploitation of Sec24A and Sec23B

Hepatitis B virus (HBV) is a leading cause of liver disease. Its success as a human pathogen is related to the immense production of subviral envelope particles (SVPs) contributing to viral persistence by interfering with immune functions. To explore cellular pathways involved in SVP formation and egress, we investigated host-pathogen interactions. Yeast-based proteomics revealed Sec24A, a component of the coat protein complex II (COPII), as an interaction partner of the HBV envelope S domain. To understand how HBV co-opts COPII as a proviral machinery, we studied roles of key Sec proteins in HBV-expressing liver cells. Silencing of Sar1, Sec23, and Sec24, which promote COPII assembly concomitant with cargo loading, strongly diminished endoplasmic reticulum (ER) envelope export and SVP secretion. By analysing Sec paralog specificities, we unexpectedly found that the HBV envelope is a selective interaction partner of Sec24A and Sec23B whose functions could not be substituted by their related isoforms. In support, we found that HBV replication upregulated Sec24A and Sec23B transcription. Furthermore, HBV encountered the Sec24A/Sec23B complex via an interaction that involved the N-terminal half of Sec24A and a di-arginine motif of its S domain, mirroring a novel ER export code. Accordingly, an interference with the COPII/HBV cross-talk might display a tool to effectively inhibit SVP release.

Impact of the Interaction of Hepatitis B Virus with Mitochondria and Associated Proteins

Around 350 million people are living with hepatitis B virus (HBV), which can lead to death due to liver cirrhosis and hepatocellular carcinoma (HCC). Various antiviral drugs/nucleot(s)ide analogues are currently used to reduce or arrest the replication of this virus. However, many studies have reported that nucleot(s)ide analogue-resistant HBV is circulating. Cellular signaling pathways could be one of the targets against the viral replication. Several studies reported that viral proteins interacted with mitochondrial proteins and localized in the mitochondria, the powerhouse of the cell. And a recent study showed that mitochondrial turnover induced by thyroid hormones protected hepatocytes from hepatocarcinogenesis mediated by HBV. Strong downregulation of numerous cellular signaling pathways has also been reported to be accompanied by profound mitochondrial alteration, as confirmed by transcriptome profiling of HBV-specific CD8 T cells from chronic and acute HBV patients. In this review, we summarize the ongoing research into mitochondrial proteins and/or signaling involved with HBV proteins, which will continue to provide insight into the relationship between mitochondria and HBV and ultimately lead to advances in viral pathobiology and mitochondria-targeted antiviral therapy.

Modeling the dynamics of hepatitis B infection, immunity, and drug therapy

Hepatitis B virus infection is the cause of liver diseases such as cirrhosis and liver cancer. Understanding the host-virus mechanisms that mediate virus pathogenesis can help design better preventive measures for disease control. Mathematical models have been used alongside experimental data to provide insight into the role of immune responses during the acute and chronic hepatitis B infections as well as virus dynamics following administration of combined drug therapy. In this paper, we review several modeling studies on virus-host interactions during acute infection, the virus-host characteristics responsible for transition to chronic disease, and the efficacy and optimal control measures of drug therapy. We conclude by presenting our opinion on the future directions of the field.

Serum Hepatitis B Virus RNA: A New Potential Biomarker for Chronic Hepatitis B Virus Infection

Chronic hepatitis B infection is one of the major etiological causes of liver failure, cirrhosis, and hepatocellular carcinoma (HCC) worldwide. This condition cannot be completely cured by currently available drugs due to the persistent existence of hepatitis B virus (HBV) covalently closed circular DNA (cccDNA), the bona fide transcription template for HBV RNAs, in infected hepatocytes. Because quantifying cccDNA per se requires an invasive procedure, serum biomarkers reflecting intrahepatic cccDNA activity are warranted. Recently, a growing body of research suggests that the circulating HBV RNA may serve as a serum biomarker for HBV infection, treatment, and prognosis. In order to delineate the molecular and clinical characteristics of serum HBV RNA, we systematically reviewed the available literature on serum HBV RNA dating back to the early 1990s. In this review, we summarize the reported serum HBV RNA quantification methods and discuss the potential HBV RNA species in patient serum. We also compare the reported correlations of serum HBV RNA with other serological markers, including HBV DNA, hepatitis B surface antigen, e antigen, and core-related antigen, as well as their correlations with intrahepatic cccDNA, to assess their potential in clinical applications. Future directions for serum HBV RNA research are also discussed.

Down-regulation of hepatitis delta virus super-infection in the woodchuck model

Mechanisms mediating clearance of hepatitis delta virus (HDV) are poorly understood. This study analyzed in detail profound down-regulation of HDV infection in the woodchuck model. Super-infection with HDV of woodchucks chronically infected with HBV-related woodchuck hepatitis virus produced two patterns. In the first, HDV viremia had a sharp peak followed by a considerable decline, and initial rise of HDV virions' infectivity followed by abrupt infectivity loss. In the second, HDV titer rose and later displayed plateau-like profile with high HDV levels; and HDV infectivity became persistently high when HDV titer reached the plateau. The infectivity loss was not due to defects in the virions' envelope, binding to anti-envelope antibodies, or mutations in HDV genome, but it correlated with profound reduction of the replication capacity of virion-associated HDV genomes. Subsequent finding that in virions with reduced infectivity most HDV RNAs were not full-length genomes suggests possible HDV clearance via RNA fragmentation.

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.

Extracellular Hepatitis B Virus RNAs Are Heterogeneous in Length and Circulate as Capsid-Antibody Complexes in Addition to Virions in Chronic Hepatitis B Patients

Extracellular HBV RNA has been detected in both HBV-replicating cell culture media and sera from chronic hepatitis B (CHB) patients, but its exact origin and composition remain controversial. Here, we demonstrated that extracellular HBV RNA species were of heterogeneous lengths, ranging from the length of pregenomic RNA to a few hundred nucleotides. In cell models, these RNAs were predominantly associated with naked capsids, although virions also harbored a minority of them. Moreover, HBV RNAs in hepatitis B patients' blood circulation were localized in unenveloped capsids in the form of capsid-antibody complexes (CACs) and in virions. Furthermore, we showed that extracellular HBV RNAs could serve as the template for viral DNA synthesis. In conclusion, extracellular HBV RNAs mainly consist of pgRNA or the pgRNA species degraded by the RNase H domain of the polymerase in the process of viral DNA synthesis and circulate as CACs and virions. Their presence in blood circulation of CHB patients may be exploited to develop novel biomarkers for HBV persistence.IMPORTANCE Although increasing evidence suggests the presence of extracellular HBV RNA species, their origin and molecular forms are still under debate. In addition to the infectious virions, HBV is known to secrete several species of incomplete viral particles, including hepatitis B surface antigen (HBsAg) particles, naked capsids, and empty virions, during its replication cycle. Here, we demonstrated that extracellular HBV RNAs were associated with naked capsids and virions in HepAD38 cells. Interestingly, we found that unenveloped capsids circulate in the blood of hepatitis B patients in the form of CACs and, together with virions, serve as vehicles carrying these RNA molecules. Moreover, extracellular HBV RNAs are heterogeneous in length and represent either pregenomic RNA (pgRNA) or products of incomplete reverse transcription during viral replication. These findings provide a conceptual basis for further application of extracellular RNA species as novel biomarkers for HBV persistence.

Hepatitis B virus core protein phosphorylation: Identification of the SRPK1 target sites and impact of their occupancy on RNA binding and capsid structure

Hepatitis B virus (HBV) replicates its 3 kb DNA genome through capsid-internal reverse transcription, initiated by assembly of 120 core protein (HBc) dimers around a complex of viral pregenomic (pg) RNA and polymerase. Following synthesis of relaxed circular (RC) DNA capsids can be enveloped and secreted as stable virions. Upon infection of a new cell, however, the capsid disintegrates to release the RC-DNA into the nucleus for conversion into covalently closed circular (ccc) DNA. HBc´s interactions with nucleic acids are mediated by an arginine-rich C terminal domain (CTD) with intrinsically strong non-specific RNA binding activity. Adaptation to the changing demands for nucleic acid binding during the viral life cycle is thought to involve dynamic phosphorylation / dephosphorylation events. However, neither the relevant enzymes nor their target sites in HBc are firmly established. Here we developed a bacterial coexpression system enabling access to definably phosphorylated HBc. Combining Phos-tag gel electrophoresis, mass spectrometry and mutagenesis we identified seven of the eight hydroxy amino acids in the CTD as target sites for serine-arginine rich protein kinase 1 (SRPK1); fewer sites were phosphorylated by PKA and PKC. Phosphorylation of all seven sites reduced nonspecific RNA encapsidation as drastically as deletion of the entire CTD and altered CTD surface accessibility, without major structure changes in the capsid shell. The bulk of capsids from human hepatoma cells was similarly highly, yet non-identically, phosphorylated as by SRPK1. While not proving SRPK1 as the infection-relevant HBc kinase the data suggest a mechanism whereby high-level HBc phosphorylation principally suppresses RNA binding whereas one or few strategic dephosphorylation events enable selective packaging of the pgRNA/polymerase complex. The tools developed in this study should greatly facilitate the further deciphering of the role of HBc phosphorylation in HBV infection and its evaluation as a potential new therapeutic target.

Exosomes Exploit the Virus Entry Machinery and Pathway To Transmit Alpha Interferon-Induced Antiviral Activity

Alpha interferon (IFN-α) induces the transfer of resistance to hepatitis B virus (HBV) from liver nonparenchymal cells (LNPCs) to hepatocytes via exosomes. However, little is known about the entry machinery and pathway involved in the transmission of IFN-α-induced antiviral activity. In this study, we found that macrophage exosomes uniquely depend on T cell immunoglobulin and mucin receptor 1 (TIM-1), a hepatitis A virus (HAV) receptor, to enter hepatocytes for delivering IFN-α-induced anti-HBV activity. Moreover, two primary endocytic routes for virus infection, clathrin-mediated endocytosis (CME) and macropinocytosis, collaborate to permit exosome entry and anti-HBV activity transfer. Subsequently, lysobisphosphatidic acid (LBPA), an anionic lipid closely related to endosome penetration of virus, facilitates membrane fusion of exosomes in late endosomes/multivesicular bodies (LEs/MVBs) and the accompanying exosomal cargo uncoating. Together, our findings provide comprehensive insights into the transmission route of macrophage exosomes to efficiently deliver IFN-α-induced antiviral substances and highlight the similarities between the entry mechanisms of exosomes and virus.IMPORTANCE Our previous study showed that LNPC-derived exosomes could transmit IFN-α-induced antiviral activity to HBV replicating hepatocytes, but the concrete transmission mechanisms, which include exosome entry and exosomal cargo release, remain unclear. In this study, we found that virus entry machinery and pathway were also applied to exosome-mediated cell-to-cell antiviral activity transfer. Macrophage-derived exosomes distinctively exploit hepatitis A virus receptor for access to hepatocytes. Later, CME and macropinocytosis are utilized by exosomes, followed by exosome-endosome fusion for efficient transfer of IFN-α-induced anti-HBV activity. We believe that understanding the cellular entry pathway of exosomes will be beneficial to designing exosomes as efficient vehicles for antiviral therapy.

Host Cell Rab GTPases in Hepatitis B Virus Infection

Hepatitis B virus (HBV) is a leading cause of liver disease and is presently estimated to infect more than 250 million humans. The extremely successful spread of this virus among the human population is explained by its effective transmission strategies and its manifold particle types, including virions, empty envelopes and naked capsids. Due to its tiny genome, HBV depends on cellular machineries to thrive in infected hepatocytes. To enter, traverse and exit the cell, HBV exploits host membrane trafficking pathways, including intracellular highways directed by Rab GTPases. Here, we review recent discoveries focused on how HBV co-opts and perturbs host Rab GTPase functions with an emphasis on Rab7A- and Rab33B-mediated trafficking pathways. Rab7A plays bidirectional roles in the viral life cycle, as it promotes the endocytic uptake of HBV in early stages, but restricts exocytic virion release in late stages. In intermediate stages of HBV propagation, Rab33B is needed to guide the assembly of replicative progeny nucleocapsids. Rab33B acts together with its Atg5-12/16L1 effector, a protein complex required for autophagosome formation, suggesting the concept that HBV exploits this Rab/effector complex as an assembly scaffold and machine. We also discuss whether Rab-directed trafficking pathways engaged by HBV may be applicable to other virus families. Identification of overlapping Rab functions may offer new chances to develop broad-spectrum host-targeted antiviral strategies.

Hepatitis B virus: The challenge of an ancient virus with multiple faces and a remarkable replication strategy

The hepatitis B virus (HBV) is the prototype member of the Hepadnaviridae, an ancient family of hepatotropic DNA viruses, which may have originated from 360 to 430 million years ago and with evidence of endogenization in reptilian genomes >200 million years ago. The virus is currently estimated to infect more than 250 million humans. The extremely successful spread of this pathogen among the human population is explained by its multiple particulate forms, effective transmission strategies (particularly perinatal transmission), long induction period and low associated mortality. These characteristics confer selective advantages, enabling the virus to persist in small, disperse populations and spread worldwide, with high prevalence rates in many countries. The HBV replication strategy is remarkably complex and includes a multiplicity of particulate structures. In addition to the common virions containing DNA in a relaxed circular (rcDNA) or double-stranded linear (dslDNA) forms, the viral population includes virion-like particles containing RNA or "empty" (viral envelopes and capsids without genomes), subviral particles (only an envelope) and even naked capsids. Consequently, several forms of the genome coexist in a single infection: (i) the "traveler" forms found in serum, including rcDNA and dslDNA, which originate from retrotranscription of a messenger RNA (the pregenomic RNA, another form of the viral genome itself) and (ii) forms confined to the host cell nucleus, including covalently closed circular DNA (cccDNA), which leads to a minichromosome form associated with histones and viral proteins, and double-stranded DNA integrated into the host genome. This complex composition lends HBV a kind of "multiple personality". Are these additional particles and genomic forms simple intermediaries/artifacts or do they play a role in the viral life cycle?

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.

Domains of the Hepatitis B Virus Small Surface Protein S Mediating Oligomerization

During hepatitis B virus (HBV) infections, subviral particles (SVP) consisting only of viral envelope proteins and lipids are secreted. Heterologous expression of the small envelope protein S in mammalian cells is sufficient for SVP generation. S is synthesized as a transmembrane protein with N-terminal (TM1), central (TM2), and hydrophobic C-terminal (HCR) transmembrane domains. The loops between TM1 and TM2 (the cytosolic loop [CL]) and between TM2 and the HCR (the luminal loop [LL]) are located in the cytosol and the endoplasmic reticulum (ER) lumen, respectively. To define the domains of S mediating oligomerization during SVP morphogenesis, S mutants were characterized by expression in transiently transfected cells. Mutation of 12 out of 15 amino acids of TM1 to alanines, as well as the deletion of HCR, still allowed SVP formation, demonstrating that these two domains are not essential for contacts between S proteins. Furthermore, the oligomerization of S was measured with a fluorescence-activated cell sorter (FACS)-based Förster resonance energy transfer (FRET) assay. This approach demonstrated that the CL, TM2, and the LL independently contributed to S oligomerization, while TM1 and the HCR played minor roles. Apparently, intermolecular homo-oligomerization of the CL, TM2, and the LL drives S protein aggregation. Detailed analyses revealed that the point mutation C65S in the CL, the mutation of 13 out of 19 amino acids of TM2 to alanine residues, and the simultaneous replacement of all 8 cysteine residues in the LL by serine residues blocked the abilities of these domains to support S protein interactions. Altogether, specific domains and residues in the HBV S protein that are required for oligomerization and SVP generation were defined.IMPORTANCE The small hepatitis B virus envelope protein S has the intrinsic ability to direct the morphogenesis of spherical 20-nm subviral lipoprotein particles. Such particles expressed in yeast or mammalian cells represent the antigenic component of current hepatitis B vaccines. Our knowledge about the steps leading from the initial, monomeric, transmembrane translation product of S to SVP is very limited, as is our information on the structure of the complex main epitope of SVP that induces the formation of protective antibodies after vaccination. This study contributes to our understanding of the oligomerization process of S chains during SVP formation and shows that the cytoplasmic loop, one membrane-embedded domain, and the luminal loop of S independently drive S-S oligomerization.

Hepatitis B Virus Subverts the Autophagy Elongation Complex Atg5-12/16L1 and Does Not Require Atg8/LC3 Lipidation for Viral Maturation

Previous studies indicated that hepatitis B virus (HBV) stimulates autophagy to favor its production. To understand how HBV co-opts autophagy as a proviral machinery, we studied the roles of key autophagy proteins in HBV-replicating liver cell cultures. RNA interference-mediated silencing of Atg5, Atg12, and Atg16L1, which promote autophagophore expansion and LC3 membrane conjugation, interfered with viral core/nucleocapsid (NC) formation/stability and strongly diminished virus yields. Concomitantly, the core/NC membrane association and their sorting to envelope-positive compartments were perturbed. A close inspection of the HBV/autophagy cross talk revealed that the virus depended on Atg12 covalently conjugated to Atg5. In support of this finding, HBV required the E2-like enzymes Atg10 and Atg3, which catalyze or facilitate Atg5-12 conjugation, respectively. Atg10 and Atg3 knockdowns decreased HBV production, while Atg3 overexpression increased virus yields. Mapping analyses demonstrated that the HBV core protein encountered the Atg5-12/16L1 complex via interaction with the intrinsically disordered region of the Atg12 moiety that is dispensable for autophagy function. The role of Atg12 in HBV replication was confirmed by its incorporation into virions. Although the Atg5-12/16L1 complex and Atg3 are essential for LC3 lipidation and, thus, for autophagosome maturation and closure, HBV propagation did not require LC3. Silencing of LC3B, the most abundant LC3 isoform, did not inhibit but rather augmented virus production. Similar augmenting effects were obtained upon overexpression of a dominant negative mutant of Atg4B that blocked the lipid conjugation of the LC3 isoforms and their GABARAP paralogues. Together, our data indicate that HBV subverts early, nondegradative autophagy components as assembly scaffolds, thereby concurrently avoiding autophagosomal destruction.IMPORTANCE Infections with the hepatitis B virus (HBV), an enveloped pararetrovirus, cause about 1 million deaths per year, as current therapies rarely achieve a cure. Understanding the HBV life cycle and concomitant host cell interactions is instrumental to develop new antiviral concepts. Here, we proceeded to dissect the roles of the autophagy machinery in virus propagation. By using RNA interference and overexpression studies in HBV-replicating cell lines, we identified the autophagic Atg5-12/16L1 elongation complex along with Atg10 and Atg3 to be an essential scaffold for HBV nucleocapsid assembly/stability. Deficits in Atg5-12/16L1 and Atg10/Atg3, which normally drive autophagophore membrane expansion, strongly impaired progeny virus yields. HBV gained access to Atg5-12/16L1 via interaction of its core protein with the Atg12 moiety of the complex. In contrast, subsequent autophagosome maturation and closure events were unnecessary for HBV replication, as evidenced by inhibition of Atg8/LC3 conjugation. Interfering with the HBV/Atg12 cross talk may be a tool for virus control.

Detection of Hepatitis B Virus Particles Released from Cultured Cells by Particle Gel Assay

The culture fluid of HBV replicating cells contains a mixture of viral particles with different structural and genetic components, including enveloped infectious virions, genome-free virion, envelope-only subviral particles, and nonenveloped naked capsids. Based on their different physical and chemical properties, the enveloped and nonenveloped particles can be separated by the native agarose gel electrophoresis and transferred onto a positively charged microporous membrane, then the virus particle-associated protein components and nucleic acid content can be detected by antibody-based enzyme immunoassay (EIA) and hybridization, respectively. Such convenient experimental procedure is called HBV particle assay and described in detail in this chapter. The particle gel assay can be used to study viral and host regulations of HBV virus morphogenesis and egress, and for antiviral assessment of HBV inhibitors as well.