Establishment of cell culture model and humanized mouse model of chronic hepatitis B virus infection

IMPORTANCE

Approximately 257 million people worldwide have been infected with hepatitis B virus (HBV), and HBV infection can cause chronic hepatitis, cirrhosis, and even liver cancer. The lack of suitable and effective infection models has greatly limited research in HBV-related fields for a long time, and it is still not possible to discover a method to completely and effectively remove the HBV genome. We have constructed a hepatocellular carcinoma cell line, HLCZ01, that can support the complete life cycle of HBV. This model can mimic the long-term stable infection of HBV in the natural state and can replace primary human hepatocytes for the development of human liver chimeric mice. This model will be a powerful tool for research in the field of HBV.

Distinct motifs in the E protein are required for SARS-CoV-2 virus particle formation and lysosomal deacidification in host cells

IMPORTANCE

Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), the virus responsible for coronavirus disease 2019 (COVID-19), has caused a global public health crisis. The E protein, a structural protein found in this virus particle, is also known to be a viroporin. As such, it forms oligomeric ion channels or pores in the host cell membrane. However, the relationship between these two functions is poorly understood. In this study, we showed that the roles of E protein in virus particle and viroporin formation are distinct. This study contributes to the development of drugs that inhibit SARS-CoV-2 virus particle formation. Additionally, we designed a highly sensitive and high-throughput virus-like particle detection system using the HiBiT tag, which is a useful tool for studying the release of SARS-CoV-2.

Tracking of Human Parvovirus B19 Virus-Like Particles Using Short Peptide Tags Reveals a Membrane-Associated Extracellular Release of These Particles

B19 virus (B19V) is a pathogenic human parvovirus that infects erythroid progenitor cells. Because there are limited in vitro culture systems to propagate this virus, little is known about the molecular mechanisms by which it propagates in cells. In this study, we introduced a HiBiT peptide tag into various loops of VP2 located on the surface of B19V particles and evaluated their ability to form virus-like particles (VLPs). Three independent sites were identified as permissive sites for peptide tag insertion without affecting VLP formation. When the HiBiT tag was introduced into B19V clones (pB19-M20) and transfected into a semipermissive erythroleukemia cell line (UT7/Epo-S1), HiBiT-dependent luciferase activities (HiBiT activities) increased depending on helicase activity of viral NS1. Furthermore, we used a GFP11 tag-split system to visualize VLPs in the GFP1-10-expressing live cells. Time-lapse imaging of green fluorescent protein (GFP)-labeled VLPs revealed that nuclear VLPs were translocated into the cytoplasm only after cell division, suggesting that the breakdown of the nuclear envelope during mitosis contributes to VLP nuclear export. Moreover, HiBiT activities of culture supernatants were dependent on the presence of a detergent, and the released VLPs were associated with extracellular vesicles, as observed under electron microscopy. Treatment with an antimitotic agent (nocodazole) enhanced the release of VLPs. These results suggest that the virions accumulated in the cytoplasm are constitutively released from the cell as membrane-coated vesicles. These properties are likely responsible for viral escape from host immune responses and enhance membrane fusion-mediated transmission. IMPORTANCE Parvovirus particles are expected to be applied as nanoparticles in drug delivery systems. However, little is known about how nuclear-assembled B19 virus (B19V) virions are released from host cells. This study provides evidence of mitosis-dependent nuclear export of B19V and extracellular vesicle-mediated virion release. Moreover, this study provides methods for modifying particle surfaces with various exogenous factors and contributes to the development of fine nanoparticles with novel valuable functions. The pB19-M20 plasmid expressing HiBiT-tagged VP2 is a novel tool to easily quantify VP2 expression. Furthermore, this system can be applied in high-throughput screening of reagents that affect VP2 expression, which might be associated with viral propagation.

Inhibition of Hepatitis B Virus (HBV) by Tachyplesin, a Marine Antimicrobial Cell-Penetrating Peptide

We investigate the role of Tachyplesin (Tpl), a marine antimicrobial cell-penetrating peptide, as an anti-HBV agent. Our findings, using confocal microscopy and flow cytometry, demonstrate the internalization of FITC-Tpl in both Huh7 and HepG2 cell lines. Further, our results show that Tpl inhibits the expression of HBV proteins, including hepatitis B surface antigen (HBsAg) and hepatitis B 'e' antigen (HBeAg) in cell supernatants of human liver cell lines transfected with 1.3× pHBV. Interestingly Tpl also reduces levels of HBV pre-core RNA and HBV pregenomic RNA, suggesting that Tpl-mediated inhibition occurs at the early stages of HBV replication, including viral transcription. In addition, Tpl led to a significant reduction in levels of hepatitis B virion secretion. In sum, here we demonstrate the potent anti-HBV activity of Tpl at non-cytotoxic concentrations indicating the potential of Tpl to emerge as an effective therapeutic peptide against HBV.

Persistence of Hepatitis B Virus DNA and the Tempos between Virion Secretion and Genome Maturation in a Mouse Model

Chronic infection with human hepatitis B virus (HBV) could lead to cirrhosis and hepatoma. At present, there is no effective treatment to eradicate the virus from patients. HBV in chronic carriers does not exist as a single homogeneous population. The most frequent naturally occurring mutation in HBV core protein occurs at amino acid 97, changing an isoleucine to leucine (I97L). One dogma in the field is that only virions containing a mature genome are preferentially secreted into the medium. Here, we demonstrated that mutant I97L can secrete immature genome in mice. Although viral DNA of mutant I97L with immature genome is less persistent than wild-type HBV in time course experiments, viral DNA of mutant P130T with genome hypermaturation, surprisingly, is more persistent. Therefore, virion secretion regulated by genome maturity could influence viral persistence. It remains an open issue whether virion secretion could be a drug target for HBV therapy.

Hepatitis B virus (HBV) core protein (HBc) accumulates frequent mutations in natural infection. Wild-type HBV is known to secrete predominantly virions containing mature DNA genome. However, a frequent naturally occurring HBc variant, I97L, changing from an isoleucine to a leucine at amino acid 97, exhibited an immature secretion phenotype in culture, which preferentially secretes virions containing immature genomes. In contrast, mutant P130T, changing from a proline to a threonine at amino acid 130, exhibited a hypermaturation phenotype by accumulating an excessive amount of intracellular fully mature DNA genome. Using a hydrodynamic delivery mouse model, we studied the in vivo behaviors of these two mutants, I97L and P130T. We detected no naked core particles in all hydrodynamically injected mice. Mutant I97L in mice exhibited pleiotropic phenotypes: (i) excessive numbers of serum HBV virions containing immature genomes, (ii) significantly reduced numbers of intracellular relaxed-circle and single-stranded DNAs, and (iii) less persistent intrahepatic and secreted HBV DNAs than wild-type HBV. These pleiotropic phenotypes were observed in both immunocompetent and immunodeficient mice. Although mutant P130T also displayed a hypermaturation phenotype in vivo, it cannot efficiently rescue the immature virion secretion of mutant I97L. Unexpectedly, the single mutant P130T exhibited in vivo a novel phenotype in prolonging the persistence of HBV genome in hepatocytes. Taken together, our studies provide a plausible rationale for HBV to regulate envelopment morphogenesis and virion secretion via genome maturity, which is likely to play an important role in the persistence of viral DNA in this mouse model.

IMPORTANCE Chronic infection with human hepatitis B virus (HBV) could lead to cirrhosis and hepatoma. At present, there is no effective treatment to eradicate the virus from patients. HBV in chronic carriers does not exist as a single homogeneous population. The most frequent naturally occurring mutation in HBV core protein occurs at amino acid 97, changing an isoleucine to leucine (I97L). One dogma in the field is that only virions containing a mature genome are preferentially secreted into the medium. Here, we demonstrated that mutant I97L can secrete immature genome in mice. Although viral DNA of mutant I97L with immature genome is less persistent than wild-type HBV in time course experiments, viral DNA of mutant P130T with genome hypermaturation, surprisingly, is more persistent. Therefore, virion secretion regulated by genome maturity could influence viral persistence. It remains an open issue whether virion secretion could be a drug target for HBV therapy.

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.

The Envelope Gene of Hepatitis B Virus Is Implicated in Both Differential Virion Secretion and Genome Replication Capacities between Genotype B and Genotype C Isolates

Chronic infection by hepatitis B virus (HBV) genotype C is associated with a prolonged replicative phase and an increased risk of liver cancer, compared with genotype B infection. We previously found lower replication capacity but more efficient virion secretion by genotype C than genotype B isolates. Virion secretion requires interaction between core particles and ENVELOPE proteins. In the present study, chimeric constructs between genotype B and genotype C clones were generated to identify the structural basis for differential virion secretion. In addition to dimeric constructs, we also employed 1.1mer constructs, where the cytomegalovirus (CMV) promoter drove pregenomic RNA transcription. Through transient transfection experiments in Huh7 cells, we found that exchanging the entire envelope gene or just its S region could enhance virion secretion by genotype B clones while diminishing virion secretion by genotype C. Site-directed mutagenesis established the contribution of genotype-specific divergence at codons 108 and 115 in the preS1 region, as well as codon 126 in the S region, to differential virion secretion. Surprisingly, exchanging the envelope gene or just its S region, but not the core gene or 3′ S region, could markedly increase intracellular replicative DNA for genotype C clones but diminish that for genotype B, although the underlying mechanism remains to be clarified.