Establishment of a hydrodynamic delivery system in ducks

Chronic hepatitis B virus (HBV) poses a significant global health challenge as it can lead to acute or chronic liver disease and hepatocellular carcinoma (HCC). To establish a safety experimental model, a homolog of HBV-duck HBV (DHBV) is often used for HBV research. Hydrodynamic-based gene delivery (HGD) is an efficient method to introduce exogenous genes into the liver, making it suitable for basic research. In this study, a duck HGD system was first constructed by injecting the reporter plasmid pLIVE-SEAP via the ankle vein. The highest expression of SEAP occurred when ducks were injected with 5 µg/mL plasmid pLIVE-SEAP in 10% bodyweight volume of physiological saline for 6 s. To verify the distribution and expression of exogenous genes in multiple tissues, the relative level of foreign gene DNA and β-galactosidase staining of LacZ were evaluated, which showed the plasmids and their products were located mainly in the liver. Additionally, β-galactosidase staining and fluorescence imaging indicated the delivered exogenous genes could be expressed in a short time. Further, the application of the duck HGD model on DHBV treatment was investigated by transferring representative anti-HBV genes IFNα and IFNγ into DHBV-infected ducks. Delivery of plasmids expressing IFNα and IFNγ inhibited DHBV infection and we established a novel efficient HGD method in ducks, which could be useful for drug screening of new genes, mRNAs and proteins for anti-HBV treatment.

An in vivo duck hepatitis B virus model recapitulates key aspects of nucleic acid polymer treatment outcomes in chronic hepatitis B patients

Nucleic acid polymers (NAPs) are an attractive treatment modality for chronic hepatitis B (CHB), with REP2139 and REP2165 having shown efficacy in CHB patients. A subset of patients achieve functional cure, whereas the others exhibit a moderate response or are non-responders. NAP efficacy has been difficult to recapitulate in animal models, with the duck hepatitis B virus (DHBV) model showing some promise but remaining underexplored for NAP efficacy testing. Here we report on an optimized in vivo DHBV duck model and explore several characteristics of NAP treatment. REP2139 was efficacious in reducing DHBV DNA and DHBsAg levels in approximately half of the treated ducks, whether administered intraperitoneally or subcutaneously. Intrahepatic or serum NAP concentrations did not correlate with efficacy, nor did the appearance of anti-DHBsAg antibodies. Furthermore, NAP efficacy was only observed in experimentally infected ducks, not in endogenously infected ducks (vertical transmission). REP2139 add-on to entecavir treatment induced a deeper and more sustained virological response compared to entecavir monotherapy. Destabilized REP2165 showed a different activity profile with a more homogenous antiviral response followed by a faster rebound. In conclusion, subcutaneous administration of NAPs in the DHBV duck model provides a useful tool for in vivo evaluation of NAPs. It recapitulates many aspects of this class of compound's efficacy in CHB patients, most notably the clear division between responders and non-responders.

Genetic characterization of duck hepatitis B viruses from Anhui Province, China

Duck hepatitis B virus (DHBV) infection model was frequently used as the experimental model for human hepatitis B virus (HBV) research. In order to decipher the genetic characteristics of DHBVs from Anhui province of China, 120 duck liver tissue samples were collected and subjected to PCR screening, and 28 samples were detected as DHBV positive. Subsequently, five DHBV-positive samples were selected for genome-wide amplification and a comprehensive analysis. Comparative analysis of complete genome sequences using the MegAlign program showed that five strains of DHBVs shared 94.5-96.3% with each other and 93.2-98.7% with other reference strains in GenBank. The phylogenetic analysis showed that all five DHBV strains belonged to the evolutionary branch of "Chinese DHBV" isolates or DHBV-2. Importantly, three potential intra-genotypic recombination events, between strains AAU-6 and Guilin, strains AAU-1 and GD3, and strains AAU-6 and AAU-1, were respectively found using the RDP and SimPlot softwares and considered the first report in avihepadnaviruses. These results not only improve our understanding for molecular prevalence status of DHBV among ducks, but also provide a reference for recombination mechanism of HBV.

Identification of avihepadnaviruses and circoviruses in an unexplained death event in farmed ducks

Research into disease pathogens can greatly benefit from viral metagenomics technology. Using this technique, we investigated potential disease pathogens that resulted in the death of many ducks on a duck farm. Two duck circoviruses (DuCV) and one duck hepatitis B virus (DHBV) were detected and identified, and all three strains were closely related to avian-associated viruses. Two duck circoviruses had 81.64%-97.65% genome-wide sequence identity to some reference strains, and duck hepatitis B virus shared 75.85%-98.92% identity with other strains. Clinical characteristics of the diseased ducks, including ruffled feathers, lethargy, and weight loss, were comparable to those observed in cases of DuCV infection. Further research is needed to determine whether coinfection with DHBV leads to liver damage and exacerbation of the disease.

Activities of endogenous APOBEC3s and uracil-DNA-glycosylase affect the hypermutation frequency of hepatitis B virus cccDNA

The covalently closed circular DNA (cccDNA) of hepatitis B virus (HBV) plays a key role in the persistence of viral infection. We have previously shown that overexpression of an antiviral factor APOBEC3G (A3G) induces hypermutation in duck HBV (DHBV) cccDNA, whereas uracil-DNA-glycosylase (UNG) reduces these mutations. In this study, using cell-culture systems, we examined whether endogenous A3s and UNG affect HBV cccDNA mutation frequency. IFNγ stimulation induced a significant increase in endogenous A3G expression and cccDNA hypermutation. UNG inhibition enhanced the IFNγ-mediated hypermutation frequency. Transfection of reconstructed cccDNA revealed that this enhanced hypermutation caused a reduction in viral replication. These results suggest that the balance of endogenous A3s and UNG activities affects HBV cccDNA mutation and replication competency.

Relaxing the restricted structural dynamics in the human hepatitis B virus RNA encapsidation signal enables replication initiation in vitro

Hepadnaviruses, including hepatitis B virus (HBV) as a major human pathogen, replicate their tiny 3 kb DNA genomes by capsid-internal protein-primed reverse transcription of a pregenomic (pg) RNA. Initiation requires productive binding of the viral polymerase, P protein, to a 5´ proximal bipartite stem-loop, the RNA encapsidation signal ε. Then a residue in the central ε bulge directs the covalent linkage of a complementary dNMP to a Tyr sidechain in P protein´s Terminal Protein (TP) domain. After elongation by two or three nucleotides (nt) the TP-linked DNA oligo is transferred to a 3´ proximal acceptor, enabling full-length minus-strand DNA synthesis. No direct structural data are available on hepadnaviral initiation complexes but their cell-free reconstitution with P protein and ε RNA (Dε) from duck HBV (DHBV) provided crucial mechanistic insights, including on a major conformational rearrangement in the apical Dε part. Analogous cell-free systems for human HBV led at most to P—ε binding but no detectable priming. Here we demonstrate that local relaxation of the highly basepaired ε upper stem, by mutation or via synthetic split RNAs, enables ε-dependent in vitro priming with full-length P protein from eukaryotic translation extract yet also, and without additional macromolecules, with truncated HBV miniP proteins expressed in bacteria. Using selective 2-hydroxyl acylation analyzed by primer extension (SHAPE) we confirm that upper stem destabilization correlates with in vitro priming competence and show that the supposed bulge-closing basepairs are largely unpaired even in wild-type ε. We define the two 3´ proximal nt of this extended bulge as main initiation sites and provide evidence for a Dε-like opening of the apical ε part upon P protein binding. Beyond new HBV-specific basic aspects our novel in vitro priming systems should facilitate the development of high-throughput screens for priming inhibitors targeting this highly virus-specific process.

Chronic hepatitis B virus (HBV) infection puts >250 million people at an increased risk for severe liver disease. Current treatments can control but rarely cure infection. HBV features a 3,200 bp DNA genome, generated by reverse transcription of a pregenomic (pg) RNA. To initiate DNA synthesis the viral polymerase, P protein, employs a stem-loop on pgRNA, ε, to covalently link a defined first nucleotide to its Terminal Protein (TP) domain. This protein-priming is highly virus-specific yet poorly understood. More is known for duck HBV (DHBV) where, different from HBV, protein-priming was successfully reconstituted in vitro years ago. One insight was that gaining priming-competence involves opening of the apical stem in DHBV ε RNA (Dε); in HBV ε the more extensive basepairing might restrict such dynamics. Here we relaxed these constraints by identifying functional but less stably folded, including split, HBV ε variants. Several such variants supported in vitro priming, including in a simple two-component-system employing a shortened recombinant P protein. Amongst other data the new cell-free systems yielded a first view on a major conformational change in HBV ε RNA bound to P protein, highlighting the importance of RNA dynamics for the human virus. Beyond furthering basic understanding our data should facilitate screening for protein-priming inhibitors as new anti-HBV agents.

Slowly folding surface extension in the prototypic avian hepatitis B virus capsid governs stability

Hepatitis B virus (HBV) is an important but difficult to study human pathogen. Most basics of the hepadnaviral life-cycle were unraveled using duck HBV (DHBV) as a model although DHBV has a capsid protein (CP) comprising ~260 rather than ~180 amino acids. Here we present high-resolution structures of several DHBV capsid-like particles (CLPs) determined by electron cryo-microscopy. As for HBV, DHBV CLPs consist of a dimeric α-helical frame-work with protruding spikes at the dimer interface. A fundamental new feature is a ~ 45 amino acid proline-rich extension in each monomer replacing the tip of the spikes in HBV CP. In vitro, folding of the extension takes months, implying a catalyzed process in vivo. DHBc variants lacking a folding-proficient extension produced regular CLPs in bacteria but failed to form stable nucleocapsids in hepatoma cells. We propose that the extension domain acts as a conformational switch with differential response options during viral infection.

Effects of traditional Chinese medicine formula Le-Cao-Shi on hepatitis B: In vivo and in vitro studies

ETHNOPHARMACOLOGICAL RELEVANCE

Formula Le-Cao-Shi (LCS) is a traditional Chinese medicine (TCM), which has long been used as a folk remedy against hepatitis B in China. The present study was conducted to evaluate the anti-hepatitis B effects of aqueous extract of LCS in vivo and in vitro.

MATERIALS AND METHOD

we investigated the anti-HBV effects of LCS in vivo and in vitro with duck hepatitis B model and HepG2.2.15 cell line model, respectively. The serologic and cellular biomarkers and the histopathological changes were examined.

RESULTS

By a duck hepatitis B model, the extract of LCS was found to restrain the expressions of duck hepatitis B surface antigen (DHBsAg), hepatitis B e antigen (DHBeAg), and HBV-DNA (DHBV-DNA). Moreover, LCS could decrease the levels of aspartate and alanine aminotransferases (AST and ALT) and ameliorate duck liver histological lesions. Correspondingly, in a HepG2.2.15 cellular model, LCS could also significantly inhibit the secretions of HBsAg and HBeAg.

CONCLUSION

LCS exerted potent anti-hepatitis effects against the infection of HBV. The above results demonstrated the first-hand experimental evidences for the anti-hepatitis B efficiency of LCS. Our study provides a basis for further exploration and development of this promising compound prescription to treat hepatitis B disease.

Display of malaria transmission-blocking antigens on chimeric duck hepatitis B virus-derived virus-like particles produced in Hansenula polymorpha

BACKGROUND

Malaria caused by Plasmodium falciparum is one of the major threats to human health globally. Despite huge efforts in malaria control and eradication, highly effective vaccines are urgently needed, including vaccines that can block malaria transmission. Chimeric virus-like particles (VLP) have emerged as a promising strategy to develop new malaria vaccine candidates.

METHODS

We developed yeast cell lines and processes for the expression of malaria transmission-blocking vaccine candidates Pfs25 and Pfs230 as VLP and VLP were analyzed for purity, size, protein incorporation rate and expression of malaria antigens.

RESULTS

In this study, a novel platform for the display of Plasmodium falciparum antigens on chimeric VLP is presented. Leading transmission-blocking vaccine candidates Pfs25 and Pfs230 were genetically fused to the small surface protein (dS) of the duck hepatitis B virus (DHBV). The resulting fusion proteins were co-expressed in recombinant Hansenula polymorpha (syn. Pichia angusta, Ogataea polymorpha) strains along with the wild-type dS as the VLP scaffold protein. Through this strategy, chimeric VLP containing Pfs25 or the Pfs230-derived fragments Pfs230c or Pfs230D1M were purified. Up to 100 mg chimeric VLP were isolated from 100 g dry cell weight with a maximum protein purity of 90% on the protein level. Expression of the Pfs230D1M construct was more efficient than Pfs230c and enabled VLP with higher purity. VLP showed reactivity with transmission-blocking antibodies and supported the surface display of the malaria antigens on the native VLP.

CONCLUSION

The incorporation of leading Plasmodium falciparum transmission-blocking antigens into the dS-based VLP scaffold is a promising novel strategy for their display on nano-scaled particles. Competitive processes for efficient production and purification were established in this study.

Distribution of Hepatitis B Virus Nuclear DNA

Chronic hepatitis B affects over 300 million people who are at risk of developing liver cancer. The basis for the persistence of hepatitis B virus (HBV) in hepatocytes, even in the presence of available antiviral therapies, lies in the accumulation of covalently closed circular DNA (cccDNA) in nuclei of infected cells. While methods for cccDNA quantification from liver biopsy specimens and cell lines expressing the virus are known, information about cccDNA formation, stability, and turnover is lacking. In particular, little is known about the fate of cccDNA during cell division. To fill the gaps in knowledge concerning cccDNA biology, we have developed a fluorescence imaging in situ hybridization (FISH)-based assay for the detection of duck hepatitis B virus (DHBV) cccDNA and HBV nuclear DNA in established cell lines. Using FISH, we determined the distribution of cccDNA under conditions mimicking chronic infections with and without antiviral therapy, which prevents de novo viral replication. Our results showed that the copy numbers of viral nuclear DNA can vary by as much as 1.8 orders of magnitude among individual cells and that antiviral therapy leads to a reduction in nuclear DNA in a manner consistent with symmetrical distribution of viral DNA to daughter cells.IMPORTANCE A mechanistic understanding of the stability of HBV cccDNA in the presence of antiviral therapy and during cell division induced by immune-mediated lysis of infected hepatocytes will be critical for the future design of curative antiviral therapies against chronic hepatitis B. Current knowledge about cccDNA stability was largely derived from quantitative analyses of cccDNA levels present in liver samples, and little was known about the fate of cccDNA in individual cells. The development of a FISH-based assay for cccDNA tracking provided the first insights into the fate of DHBV cccDNA and nuclear HBV DNA under conditions mimicking antiviral therapy.

Efficient inhibition of duck hepatitis B virus DNA by the CRISPR/Cas9 system

Current therapeutic strategies cannot eradicate hepatitis B virus covalently closed circular DNA (HBV cccDNA), which accounts for the persistence of HBV infection. Very recently, the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR‑associated protein 9 (Cas9) system has been used as an efficient and powerful tool for viral genome editing. Given that the primary duck hepatocyte (PDH) infected with duck hepatitis B virus (DHBV) has been widely used to study human HBV infection in vitro, the present study aimed to demonstrate the targeted inhibition of DHBV DNA, especially cccDNA, by the CRISPR/Cas9 system using this model. We designed six single‑guide RNAs (sgRNA1‑6) targeting the DHBV genome. The sgRNA/Cas9 plasmid was transfected into DHBV‑infected PDHs, and then DHBV total DNA (in culture medium and PDHs) and cccDNA were quantified by reverse transcription‑quantitative polymerase chain reaction. The combined inhibition of CRISPR/Cas9 system and entecavir (ETV) was also assessed. Two sgRNAs, sgRNA4 and sgRNA6, exhibited efficient inhibition on DHBV total DNA (77.23 and 86.51%, respectively), cccDNA (75.67 and 85.34%, respectively) in PDHs, as well as DHBV total DNA in the culture medium (62.17 and 59.52%, respectively). The inhibition remained or enhanced from day 5 to day 9 following transfection. The combination of the CRISPR/Cas9 system and ETV further increased the inhibitory effect on DHBV total DNA in PDHs and culture medium, but not cccDNA. The CRISPR/Cas9 system has the potential to be a useful tool for the suppression of DHBV DNA.

Duck Hepatitis B Virus cccDNA Amplification Efficiency in Natural Infection Is Regulated by Virus Secretion Efficiency

Previous mutation based studies showed that ablating synthesis of viral envelope proteins led to elevated hepadnaviral covalently closed circular DNA (cccDNA) amplification, but it remains unknown how cccDNA amplification is regulated in natural hepadnaviral infection because of a lack of research system. In this study we report a simple procedure to prepare two identical duck hepatitis B virus inocula, but they possess 10-100-fold difference in cccDNA amplification in infected cell culture. We demonstrate that the infected cells with higher cccDNA amplification significantly reduce the virus secretion efficiency that results in higher accumulation of relaxed circular DNA (rcDNA) and DHBsAg in the cells. The infected cells with lower cccDNA amplification significantly increase the virus secretion efficiency that leads to lower intracellular rcDNA and DHBsAg accumulation. In contrast with the findings generated in the mutation based experimental system, the regulation of cccDNA amplification in natural hepadnaviral infection bypasses direct regulation of the cellular envelope proteins concentration, instead it modulates virus secretion efficiency that ultimately impacts the intracellular rcDNA concentration, an important factor determining the destination of the synthesized rcDNA in infected cells.

Does Tyrosyl DNA Phosphodiesterase-2 Play a Role in Hepatitis B Virus Genome Repair?

Hepatitis B virus (HBV) replication and persistence are sustained by a nuclear episome, the covalently closed circular (CCC) DNA, which serves as the transcriptional template for all viral RNAs. CCC DNA is converted from a relaxed circular (RC) DNA in the virion early during infection as well as from RC DNA in intracellular progeny nucleocapsids via an intracellular amplification pathway. Current antiviral therapies suppress viral replication but cannot eliminate CCC DNA. Thus, persistence of CCC DNA remains an obstacle toward curing chronic HBV infection. Unfortunately, very little is known about how CCC DNA is formed. CCC DNA formation requires removal of the virally encoded reverse transcriptase (RT) protein from the 5' end of the minus strand of RC DNA. Tyrosyl DNA phosphodiesterase-2 (Tdp2) was recently identified as the enzyme responsible for cleavage of tyrosyl-5' DNA linkages formed between topoisomerase II and cellular DNA. Because the RT-DNA linkage is also a 5' DNA-phosphotyrosyl bond, it has been hypothesized that Tdp2 might be one of several elusive host factors required for CCC DNA formation. Therefore, we examined the role of Tdp2 in RC DNA deproteination and CCC DNA formation. We demonstrated Tdp2 can cleave the tyrosyl-minus strand DNA linkage using authentic HBV RC DNA isolated from nucleocapsids and using RT covalently linked to short minus strand DNA produced in vitro. On the other hand, our results showed that Tdp2 gene knockout did not block CCC DNA formation during HBV infection of permissive human hepatoma cells and did not prevent intracellular amplification of duck hepatitis B virus CCC DNA. These results indicate that although Tdp2 can remove the RT covalently linked to the 5' end of the HBV minus strand DNA in vitro, this protein might not be required for CCC DNA formation in vivo.

[Establishment of a method to detect duck hepatitis B virus covalently closed circular DNA based on rolling circle amplification]

Rolling circle amplification (RCA) is a newly developed experimental technique that can specific ally amplify circular DNA. Since 2008, RCA has been extensively used in hepatitis B virus (HBV) research, such as the amplification of the full-length sequence of the HBV genome, and the analysis of the drug-resistant mutations of HBV covalently closed circular DNA (cccDNA), amongst others. To create an easy assay for the analysis of duck hepatitis B virus (DHBV) cccDNA, this study established an RCA-based method. DHBV cccDNA was amplified from the DHBV DNA samples of duck liver with four pairs of sulfur-modified primers, which were designed according to the highly conserved sequence of DHBV using sera DHBV DNA as the negative control. DHBV cccDNA was detected in the obtained RCA products by the sequencing of RCA amplicons that were amplified with primer pairs on both sides of the gap of DH BV relaxed circular DNA, rather than by digesting RCA products with a restriction enzyme. The liver and sera DHBV DNA samples of 39 ducks infected with DHBV were examined with the RCA-based DHBV cccDNA detection method, and the results showed that while DHBV cccDNA was detected from all 39 liver DHBV DNA samples, no DHBV cccDNA was found in any of the sera DHBV DNA samples. These results suggest that the method established in the study is highly specific and sensitive for the detection of DHBV cccDNA. The establishment of this RCA-based DHBV method for cccDNA detection lays the groundwork for using a DHBV model to study the role of cccDNA in the pathogenesis of hepatitis B and to evaluate the effect of anti-virus therapies.

Evaluation of hepatocyteprotective and anti-hepatitis B virus properties of Cichoric acid from Cichorium intybus leaves in cell culture

Hepatitis B is the most common serious liver infection in the world. To date, there is still no complete cure for chronic hepatitis B. Natural caffeic acid analogues possess prominent antiviral activity, especially anti-hepatitis B virus (HBV) and anti-human immunodeficiency virus effects. Cichoric acid is a caffeic acid derivative from Cichorium intybus. In the study, the anti-hepatitis B property of cichoric acid was evaluated by the D-galactosamine (D-GalN)-induced normal human HL-7702 hepatocyte injury model, the duck hepatitis B virus (DHBV)-infected duck fetal hepatocytes and the HBV-transfected cell line HepG2.2.15 cells, respectively. The results showed that cichoric acid attenuated significantly D-GalN-induced HL-7702 hepatocyte injury at 10-100 µg/mL and produced a maximum protection rate of 56.26%. Moreover, cichoric acid at 1-100 µg/mL inhibited markedly DHBV DNA replication in infected duck fetal hepatocytes. Also, cichoric acid at 10-100 µg/mL reduced significantly the hepatitis B surface and envelope antigen levels in HepG2.2.15 cells and produced the maximum inhibition rates of 79.94% and 76.41%, respectively. Meanwhile, test compound at 50-100 µg/mL inhibited markedly HBV DNA replication. In conclusion, this study verifies the anti-hepatitis B effect of cichoric acid from Cichorium intybus leaves. In addition, cichoric acid could be used to design the antiviral agents.

Molecular characterization of duck hepatitis B virus isolated from Hubei brown ducks

The objective of this study was to characterize the genome structure of duck hepatitis B virus (DHBV) isolated from Hubei brown ducks. The natural carrier rate of DHBV in adult ducks from Hubei area was investigated and the DHBV DNA-positive serum screened out. The complete genome of a DHBV strain was amplified by polymerase chain reaction (PCR) and cloned into T vector and sequenced. The results showed that the carrier rate of DHBV in Hubei brown ducks was 10 %. This strain (GenBank accession number DQ276978) had a genome of 3024 nucleotides with three overlapping open reading frames encoding the surface, core and polymerase proteins respectively. Comparison of the strain with 17 DHBV strains registered in GenBank revealed a homology from 89.3 % to 93.5 % at the nucleotide level. The sequences of the structural and functional domains of these proteins were highly conserved. The strain was found to share more signature amino acids in the polymerase genes with the "Chinese" DHBV strains than those of the "Western" country strains. This finding was also corroborated by a phylogenetic tree analysis. Therefore, the DQ276978 might belong to a subtype of the Chinese DHBV strains.

Development and application of a universal Taqman real-time PCR for quantitation of duck hepatitis B virus DNA

To develop a quantitative assay for universal detection of duck hepatitis B virus (DHBV) DNA, a Taqman real-time fluorescent quantitative polymerase chain reaction (FQ-PCR) assay was developed using primers and probes based on genomic sequences located at nucleotide 241-414 of the DHBV Core region which possesses the highest homology among the 44 DHBV genomes available in Genbank. The DHBV Core gene cloned in pGEM-T was used to generate DHBV DNA standard. The assay had a lowest detection limit of 10(3) copies/ml and a good linear standard curve (Y=-3.989X+49.086, r(2)=0.9993) over a wide range of input DHBV DNA (10(3) to 10(10) copies/ml). The standard deviation of intra- and inter-assay was 0.01-0.06 and 0.05-0.16, respectively, and the coefficient of variation was 1.3-1.8%. The specificity of the assay was validated using duck hepatitis virus type 1, hepatitis B virus, and E. coli DNA. Comparison of ABI 7300 and Bio-Rad iQ5 PCR instruments yielded highly consistent results. The assay showed a positive rate of 63.8% (51/80) DHBV DNA in peripheral blood and liver tissue from ducks from Xi'an, China. The FQ-PCR developed is highly sensitive, specific, reproducible and versatile, and may be used to universally detect DHBV DNA of different DHBV strains.

[Cloning and sequence analysis of the DHBV genome of the brown ducks in Guilin region and establishment of the quantitative method for detecting DHBV]

Brown ducks carrying DHBV were widely used as hepatitis B animal model in the research of the activity and toxicity of anti-HBV dugs. Studies showed that the ratio of DHBV carriers in the brown ducks in Guilin region was relatively high. Nevertheless, the characters of the DHBV genome of Guilin brown duck remain unknown. Here we report the cloning of the genome of Guilin brown duck DHBV and the sequence analysis of the genome. The full length of the DHBV genome of Guilin brown duck was 3 027bp. Analysis using ORF finder found that there was an ORF for an unknown peptide other than S-ORF, PORF and C-ORF in the genome of the DHBV. Vector NTI 8. 0 analysis revealed that the unknown peptide contained a motif which binded to HLA * 0201. Aligning with the DHBV sequences from different countries and regions indicated that there were no obvious differences of regional distribution among the sequences. A fluorescence quantitative PCR for detecting DHBV was establishment based on the recombinant plasmid pGEM-DHBV-S constructed. This study laid the groundwork for using Guilin brown duck as a hepatitis B animal model.

Potent inhibition of late stages of hepadnavirus replication by a modified cell penetrating peptide

Cationic cell-penetrating peptides (CPPs) and their lipid domain-conjugates (CatLip) are agents for the delivery of (uncharged) biologically active molecules into the cell. Using infection and transfection assays we surprisingly discovered that CatLip peptides were able to inhibit replication of Duck Hepatitis B Virus (DHBV), a reference model for human HBV. Amongst twelve CatLip peptides we identified Deca-(Arg)₈ having a particularly potent antiviral activity, leading to a drastic inhibition of viral particle secretion without detectable toxicity. Inhibition of virion secretion was correlated with a dose-dependent increase in intracellular viral DNA. Deca-(Arg)₈ peptide did neither interfere with DHBV entry, nor with formation of mature nucleocapsids nor with their travelling to the nucleus. Instead, Deca-(Arg)₈ caused envelope protein accumulation in large clusters as revealed by confocal laser scanning microscopy indicating severe structural changes of preS/S. Sucrose gradient analysis of supernatants from Deca-(Arg)₈-treated cells showed unaffected naked viral nucleocapsids release, which was concomitant with a complete arrest of virion and surface protein-containing subviral particle secretion. This is the first report showing that a CPP is able to drastically block hepadnaviral release from infected cells by altering late stages of viral morphogenesis via interference with enveloped particle formation, without affecting naked nucleocapsid egress, thus giving a view inside the mode of inhibition. Deca-(Arg)₈ may be a useful tool for elucidating the hepadnaviral secretory pathway, which is not yet fully understood. Moreover we provide the first evidence that a modified CPP displays a novel antiviral mechanism targeting another step of viral life cycle compared to what has been so far described for other enveloped viruses.

[Research on the gene structure of duck hepatitis B virus and its encoding proteins]

Duck hepatitis B virus (DHBV) belongs to the Avihepadnavirus genus of the Hepadnaviridae, and it not only has the same replication pattern, but also has the similar genomic and antigenic structures to Hepatitis B virus (HBV). The genome of DHBV is a partially double-stranded closed circular DNA. The genome consists of three distinct open reading frames (ORFs): ORF-PreS/S, ORF-PreC/C and ORF-P, which all locate on the negative DNA strand and encode four separate proteins. The ORF-PreS/S encodes envelope proteins L and S, and the ORF-PreC/C and ORF-P encode capsid proteins C and polymerase proteins P, respectively. The characteristics of genome structure,viral proteins features and functions were described in this review in order to provide useful information for the further study of DHBV and the duck model infected by DHBV.

[Construction of a duck hepatitis B virus YMDD mutant and identification of its resistance phenotype]

OBJECTIVE

To construct a lamivudine-resistant plasmid containing 1.2 unit genome of duck hepatitis B virus and identify its replication and drug-resistance in avian LMH hepatica cells.

METHODS

The recombinant plasmid PBS-DHBV1.2 was constructed using the 1.2-genome length DHBV DNA sequence from a dimer DHBV genome with pcDNA3.1 as the template. With site-directed mutagenesis, we obtained PBS-DHBV1.2-M512V plasmids with polymerase gene mutation from PBS-DHBV1.2. Two constructed plasmids were transiently transfected into LMH cells using FuGENETM6 transfection reagent and cultured in the medium containing different concentrations of lamivudine. Southern blot hybridization was performed to detect DHBV replication intermediates.

RESULTS

PCR amplification, restriction digestion and plasmid sequencing all confirmed successful construction of PBS-DHBV1.2-M512V recombinant plasmid. Southern blot analysis identified the presence of all the expected DHBV replication intermediates in LMH cells. The replication capacity of the mutant plasmid was decreased by 2.7 times compared with that of the wild plasmid. The IC(50) of lamivudine was 37.12∓8.81 ng/ml for the mutant, greater than that of the wild plasmid (10.90∓4.80 ng/ml).

CONCLUSION

Compared with the wild plasmid, the mutant plasmid has a lower replication capacity and sensitivity to lamivudine in vitro.

[Dynamic detection of duck hepatitis B virus cccDNA in serum of ducks with liver injury]

OBJECTIVE

To confirm whether DHBV cccDNA could be detected in serum of DHBV-infected ducks after liver injury.

METHODS

Twenty four ducks with persistent DHBV infection were divided into 4 groups with the following treatments: A, D-galactosamine (D-GalN, 2.2 g/kg) and lipopolysaccharide (LPS, 100 µg/kg); B, 10 mg/kg of carbon tetrachloride (CCl4) every 12 h twice following D-GalN and LPS; C, 15 mg/kg of CCl4 every 12 h twice following D-GalN and LPS; D, normal saline as normal control (NC). At 0 h, 24 h, 36 h and 48 h post-treatment, sera were collected from each duck for determination of serum DHBV load, DHBV cccDNA and alanine aminotransferase (ALT). And ducks were eventually sacrificed to obtain liver specimens for pathological assessment of liver lesions and determination of intrahepatic total DHBV DNA and DHBV cccDNA.

RESULTS

(1) No obvious pathological change was observed in the liver of ducks from NC group while the indices of liver injury were significantly different between Groups A, B and C; (2) DHBV cccDNA was undetectable in the sera of ducks from NC and A group at all time points. In contrast, DHBV cccDNA, varying from 3.17 × 10(3) copies/ml to 1.72 × 10(4) copies/ml, was detected in the sera of 2 ducks from Group B and 3 ducks from Group C at 36 h post-treatment. The occurrence of DHBV cccDNA in serum was significantly correlated with the degree of liver injury while no significant association with serum ALT level and DHBV load as well as with the level of intrahepatic total DHBV DNA and DHBV cccDNA was observed.

CONCLUSION

DHBV cccDNA may be detected in the serum when the liver of duck is seriously damaged. The incidence of DHBV cccDNA occurrence in the serum is significantly associated with the severity of liver injury.

Generation of covalently closed circular DNA of hepatitis B viruses via intracellular recycling is regulated in a virus specific manner

Persistence of hepatitis B virus (HBV) infection requires covalently closed circular (ccc)DNA formation and amplification, which can occur via intracellular recycling of the viral polymerase-linked relaxed circular (rc) DNA genomes present in virions. Here we reveal a fundamental difference between HBV and the related duck hepatitis B virus (DHBV) in the recycling mechanism. Direct comparison of HBV and DHBV cccDNA amplification in cross-species transfection experiments showed that, in the same human cell background, DHBV but not HBV rcDNA converts efficiently into cccDNA. By characterizing the distinct forms of HBV and DHBV rcDNA accumulating in the cells we find that nuclear import, complete versus partial release from the capsid and complete versus partial removal of the covalently bound polymerase contribute to limiting HBV cccDNA formation; particularly, we identify genome region-selectively opened nuclear capsids as a putative novel HBV uncoating intermediate. However, the presence in the nucleus of around 40% of completely uncoated rcDNA that lacks most if not all of the covalently bound protein strongly suggests a major block further downstream that operates in the HBV but not DHBV recycling pathway. In summary, our results uncover an unexpected contribution of the virus to cccDNA formation that might help to better understand the persistence of HBV infection. Moreover, efficient DHBV cccDNA formation in human hepatoma cells should greatly facilitate experimental identification, and possibly inhibition, of the human cell factors involved in the process.

Persistent infection with hepatitis B virus (HBV) causes chronic hepatitis B which frequently progresses to hepatocellular carcinoma, a leading cause of cancer-mediated mortality worldwide. Persistence requires formation and amplification of covalently closed circular (ccc)DNA, an episomal form of the viral genome that is not targeted by current drugs and thus is responsible for the notorious difficulties in therapeutic elimination of infection. Initial generation of cccDNA occurs upon nuclear import of the virion-borne relaxed circular (rc) DNA to which the viral polymerase is covalently linked; amplification occurs via intracellular recycling. The underlying molecular pathway is poorly understood. Because HBV infects only primates, in vivo studies are extremely restricted; in vitro, select hepatoma cell lines transfected with HBV support viral replication, however with little if any cccDNA formation. Here, we compared intracellular recycling of HBV and DHBV, a model hepatitis B virus from ducks, in cross-species transfections. Surprisingly, the major contribution to cccDNA formation comes from the virus rather than the cell as DHBV but not HBV rcDNA converted efficiently into cccDNA in the same human cell background. This unexpected difference might help to better understand persistence of HBV infection; efficient DHBV cccDNA formation in human cells provides a new tool to facilitate identification, and possibly targeting, of the human cell factors involved.

[Effects of hyperin on the cccDNA of duck hepatitis B virus and its immunological regulation]

The aim of this study is to investigate the effect of hyperin on the cccDNA of duck hepatitis B virus and its immunological regulation. Duck hepatitis B virus (DHBV) infection model and normal mouse spleen lymphocyte were used to evaluate the anti-HBV and immunoregulation effects. The DHBV-DNA of serum was detected at different time points by using serum DOT-BLOT hybridization. Polymerase chain reaction (PCR) was used for the determination of nuclear covalent closed circular DNA (cccDNA). Cytokine secretion was determined by ELISA method. DHBV-DNA were inhibited by hyperin (25 or 50 mg x kg(-1)), while cccDNA of liver could be eliminated efficiently by hyperin (25 or 50 mg x kg(-1), P < 0.05, P < 0.01). The T helper 1 effector cytokine was markedly enhanced by hyperin (25 or 50 microg x mL(-1), P < 0.01). In conclusion, hyperin has anti-HBV activity via multiple targets and pathways, and cccDNA may be one of the important targets.

[An undamaged bulge in epsilon is essential for initiating priming of DHBV reverse transcriptase]

Previously, we have established an epsilon library and selected out a series of RNA aptamers with higher affinity to P protein based on the in vitro Systematic Evolution of Ligands by Exponential Enrichment (SELEX) in duck hepatitis B virus (DHBV) system. In order to study the structural elements within the epsilon that is essential for initiating priming of HBV reverse transcriptase (P protein), all selected aptamers were subjected to in vitro priming assay and RNA secondary structure probing. We found that all those aptamers supporting priming had an undamaged bulge, while those lacking of the bulge no more support priming. Our results suggest an undamaged bulge within Depsilon is indispensable for initiating priming of P protein.

An interdomain RNA binding site on the hepadnaviral polymerase that is essential for reverse transcription

The T3 motif on the duck hepatitis B virus reverse transcriptase (P) is proposed to be a binding site essential for viral replication, but its ligand and roles in DNA synthesis are unknown. Here, we found that T3 is needed for P to bind the viral RNA, the first step in DNA synthesis. A second motif, RT-1, was predicted to assist T3. T3 and RT-1 appear to form a composite RNA binding site because mutating T3 and RT-1 had similar effects on RNA binding, exposure of antibody epitopes on P, and DNA synthesis. The T3 and RT-1 motifs bound RNA non-specifically, yet they were essential for specific interactions between P and the viral RNA. This implies that specificity for the viral RNA is provided by a post-binding step. The T3:RT-1 motifs are conserved with the human hepatitis B virus and may be an attractive target for novel antiviral drug development.

[Establishment of an in vivo model for duck hepatitis B virus infection using Hubei duckling]

OBJECTIVE

To develop a standard duck hepatitis B virus (DHBV) animal model using a local Hubei species of duck, Ma Ya, and use it as an in vivo experimental system to study antiviral strategies against hepatitis B.

METHODS

Two-day-old Ma Ya ducklings were experimentally infected via intraperitoneal injection with the DHBV inocula which was collected from the transfected culture supernatant of 1.5-fold-overlength genome recombinant plasmid. Blood samples were taken twice or thrice a week during post-inoculation for 50 days. Viremia was quantified by serum real-time PCR to show the peak. Antiviral treatment of the DHBV-infected ducklings was started 3 d post-inoculation. The animals received oral administration of lamivudine (3TC) at a dose of 25 mg/kg/d for 5 d, followed by a maintenance therapy thrice weekly for 3 more weeks. Serum was quantified to show the viremia peak and liver biopsy specimens were analysed by Southern blotting and in-situ hybridization at the end of antiviral drug treatment.

RESULTS

The experimental infection rate of 2-day-old ducklings was 87.5%. Viremia started to be detectable on day 7 and reached a peak on day 11 post-inoculation, followed by a decrease and fluctuations. Four weeks of oral administration of 3TC led to a significant decrease in viremia peak during. This effect was not sustained, as a rebound in viremia was observed after drug withdrawal. Similarly, the analysis of liver biopsies at the end of 3TC treatment showed a marked decrease in DHBV DNA. However, after drug withdrawal a rebound of intrahepatic DHBV DNA was observed in duck livers.

CONCLUSION

The Hubei duck model with experimental DHBV infection of transfected supernatant is more suitable for the hepadnavirus biologic research due to its stability and practicability.

Chaperones activate hepadnavirus reverse transcriptase by transiently exposing a C-proximal region in the terminal protein domain that contributes to epsilon RNA binding

All hepatitis B viruses replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase, P protein, that carries a unique terminal protein (TP) domain. To initiate reverse transcription, P protein must bind to a stem-loop, epsilon, on the pregenomic RNA template. TP then provides a Y residue for covalent attachment of the first nucleotide of an epsilon-templated DNA oligonucleotide (priming reaction) that serves to initiate full-length minus-strand DNA synthesis. epsilon binding requires the chaperone-dependent conversion of inactive P protein into an activated, metastable form designated P*. However, how P* differs structurally from P protein is not known. Here we used an in vitro reconstitution system for active duck hepatitis B virus P combined with limited proteolysis, site-specific antibodies, and defined P mutants to structurally compare nonactivated versus chaperone-activated versus primed P protein. The data show that Hsp70 action, under conditions identical to those required for functional activation, transiently exposes the C proximal TP region which is, probably directly, involved in epsilon RNA binding. Notably, after priming and epsilon RNA removal, a very similar new conformation appears stable without further chaperone activity; hence, the activation of P protein is triggered by energy-consuming chaperone action but may be completed by template RNA binding.

Antiviral effects of PNA in duck hepatitis B virus infection model

AIM

To study the efficacy of antiviral treatment with PNA for the duck model of HBV (DHBV)-infected ducks. PNA is a 2-amine-9-(2,3-dideoxy-2,3-dihydro-beta-D-arabinofuranosyl)-6-methoxy-9H-purine.

METHODS

The Sichuan Mallard ducklings in the hepatitis B virus model were treated with PNA, a new antiviral agent. DHBV DNA from the blood serum and liver tissues were measured at 0, 5, and 10 d during the treatment and at 3 d withdrawal by real-time PCR. The duck hepatitis B surface antigen (DHBsAg) in the liver cells was observed by Immunohistochemistry (IHC). Pathological changes in the liver tissues were also observed. Control group I was administered with distilled water and control group II was administered with 3-thiacytidine. Treatment group I was administered with PNA at a dose of 40 mg/kg and treatment group II was administered perorally (po) with PNA at a dose of 80 mg/kg. Treatment group III was administered with PNA at a dose of 20 mg/kg and treatment group IV was intravenously administered with PNA at a dose of 40 mg/kg. Each group contained 15 ducklings.

RESULTS

PNA can significantly lower the DHBV replication levels in serum and liver. Compared with control group II, there were no significant differences in inhibiting efficacy in treatment groups I and III (P>0.05) and there were significant differences in inhibiting efficacy in treatment groups II and IV (P<0.05). Interestingly, significant differences were observed at 3 d withdrawal. The DHBV replication levels in each group slightly increased at 3 d withdrawal, but rebounded slightly in the PNA treatment groups than in control group II (P<0.05). The DHBV replication levels in the treatment groups were lower than in control group I. The DHBV replication levels in sera had a positive relationship with that in the liver, but the DHBV replication levels in the liver was lower than that in sera. Pathological changes in the treatment groups were obviously improved and the changes were associated with liver viral DNA levels.

CONCLUSION

The results demonstrate that PNA is a strong inhibitor of DHBV replication in the DHBV-infected duck model.

A structural model for duck hepatitis B virus core protein derived by extensive mutagenesis

Duck hepatitis B virus (DHBV) shares many fundamental features with human HBV. However, the DHBV core protein (DHBc), forming the nucleocapsid shell, is much larger than that of HBV (HBc) and, in contrast to HBc, there is little direct information on its structure. Here we applied an efficient expression system for recombinant DHBc particles to the biochemical analysis of a large panel of mutant DHBc proteins. By combining these data with primary sequence alignments, secondary structure prediction, and three-dimensional modeling, we propose a model for the fold of DHBc. Its major features are a HBc-like two-domain structure with an assembly domain comprising the first about 185 amino acids and a C-terminal nucleic acid binding domain (CTD), connected by a morphogenic linker region that is longer than in HBc and extends into the CTD. The assembly domain shares with HBc a framework of four major alpha-helices but is decorated at its tip with an extra element that contains at least one helix and that is made up only in part by the previously predicted insertion sequence. All subelements are interconnected, such that structural changes at one site are transmitted to others, resulting in an unexpected variability of particle morphologies. Key features of the model are independently supported by the accompanying epitope mapping study. These data should be valuable for functional studies on the impact of core protein structure on virus replication, and some of the mutant proteins may be particularly suitable for higher-resolution structural investigations.

Assembly and budding of a hepatitis B virus is mediated by a novel type of intracellular vesicles

Formation of enveloped viruses involves assembly and budding at cellular membranes. In this study, we elucidated the morphogenesis of hepadnaviruses on the ultrastructural and biochemical level using duck hepatitis B virus (DHBV) as a model system. Formation of virus progeny initiates at the endoplasmic reticulum (ER) and is conserved both in vitro and in vivo. The morphogenesis proceeds via membrane-surrounded vesicles containing both virions and subviral particles, indicating a common morphogenetic pathway. The virus particle-containing vesicles (VCVs) are generated and maintained by reorganization of endomembranes accompanied by a striking disorganization of the rough ER (rER). VCVs are novel organelles with unique identity and properties of ER, intermediate compartment, endosomes, and multivesicular bodies. VCVs are dynamic structures whose size and shape are regulated by both membrane fusion and fission.

CONCLUSION

Our data indicate a strong reorganization of endomembranes during DHBV infection, resulting in the biogenesis of novel organelles serving as multifunctional platforms for assembly and budding of virus progeny.

High level of genetic heterogeneity in S and P genes of genotype D hepatitis B virus

The genetic heterogeneity of hepatitis B virus (HBV) genotypes and subgenotypes was investigated by directly sequencing amplified PreS, S and P genes of HBV isolates obtained from the plasma of 99 subjects with chronic HBV infection. Genotype D showed the greatest intragenotypic and intrasubgenotypic divergence: in particular, the a determinant was mutated in 58.2% of the genotype D patients, two of whom showed prototypic vaccine-induced escape mutants at codon 145. Moreover, five sites under significant positive selection were found in the S protein of the D isolates: one in the a determinant and four in the highly hydrophobic C terminal. Our results suggest that careful surveillance of vaccine-induced escape mutants should be considered in populations with highly frequent genotype D infections, and raise questions concerning the possible relationship between the genetic heterogeneity, host immunity and pathogenicity of this HBV genotype.

Thermodynamics and NMR studies on Duck, Heron and Human HBV encapsidation signals

Hepatitis B virus (HBV) replication is initiated by binding of its reverse transcriptase (P) to the apical stem-loop (AL) and primer loop (PL) of epsilon, a highly conserved RNA element at the 5'-end of the RNA pregenome. Mutation studies on duck/heron and human in vitro systems have shown similarities but also differences between their P-epsilon interaction. Here, NMR and UV thermodynamic data on AL (and PL) from these three species are presented. The stabilities of the duck and heron ALs were found to be similar, and much lower than that of human. NMR data show that this low stability stems from an 11-nt internal bulge destabilizing the stem of heron AL. In duck, although structured at low temperature, this region also forms a weak point as its imino resonances broaden to disappearance between 30 and 35 degrees C well below the overall AL melting temperature. Surprisingly, the duck- and heron ALs were both found to be capped by a stable well-structured UGUU tetraloop. All avian ALs are expected to adhere to this because of their conserved sequence. Duck PL is stable and structured and, in view of sequence similarities, the same is expected for heron - and human PL.

Base pairing between cis-acting sequences contributes to template switching during plus-strand DNA synthesis in human hepatitis B virus

Hepadnaviruses utilize two template switches (primer translocation and circularization) during synthesis of plus-strand DNA to generate a relaxed-circular (RC) DNA genome. In duck hepatitis B virus (DHBV) three cis-acting sequences, 3E, M, and 5E, contribute to both template switches through base pairing, 3E with the 3' portion of M and 5E with the 5' portion of M. Human hepatitis B virus (HBV) also contains multiple cis-acting sequences that contribute to the accumulation of RC DNA, but the mechanisms through which these sequences contribute were previously unknown. Three of the HBV cis-acting sequences (h3E, hM, and h5E) occupy positions equivalent to those of the DHBV 3E, M, and 5E. We present evidence that h3E and hM contribute to the synthesis of RC DNA through base pairing during both primer translocation and circularization. Mutations that disrupt predicted base pairing inhibit both template switches while mutations that restore the predicted base pairing restore function. Therefore, the h3E-hM base pairing appears to be a conserved requirement for template switching during plus-strand DNA synthesis of HBV and DHBV. Also, we show that base pairing is not sufficient to explain the mechanism of h3E and hM, as mutating sequences adjacent to the base pairing regions inhibited both template switches. Finally, we did not identify predicted base pairing between h5E and the hM region, indicating a possible difference between HBV and DHBV. The significance of these similarities and differences between HBV and DHBV will be discussed.

Entry of duck hepatitis B virus into primary duck liver and kidney cells after discovery of a fusogenic region within the large surface protein

Hepatitis B viruses exhibit a narrow host range specificity that is believed to be mediated by a domain of the large surface protein, designated L. For duck hepatitis B virus, it has been shown that the pre-S domain of L binds to carboxypeptidase D, a cellular receptor present in many species on a wide variety of cell types. Nonetheless, only hepatocytes become infected. It has remained vague which viral features determine host range specificity and organotropicity. By using chymotrypsin to treat duck hepatitis B virus, we addressed the question of whether a putative fusogenic region within the amino-terminal end of the small surface protein may participate in viral entry and possibly constitute one of the determinants of the host range of the virus. Addition of the enzyme to virions resulted in increased infectivity. Remarkably, even remnants of enzyme-treated subviral particles proved to be inhibitory to infection. A noninfectious deletion mutant devoid of the binding region for carboxypeptidase D could be rendered infectious for primary duck hepatocytes by treatment with chymotrypsin. Although because of the protease treatment mutant and wild-type viruses may have become infectious in an unspecific and receptor-independent manner, their host range specificity was not affected, as shown by the inability of the virus to replicate in different hepatoma cell lines, as well as primary chicken hepatocytes. Instead, the organotropicity of the virus could be reduced, which was demonstrated by infection of primary duck kidney cells.

Hepatitis B virus replication

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cell-free systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.

Avian hepatitis B viruses: Molecular and cellular biology, phylogenesis, and host tropism

The human hepatitis B virus (HBV) and the duck hepatitis B virus (DHBV) share several fundamental features. Both viruses have a partially double-stranded DNA genome that is replicated via a RNA intermediate and the coding open reading frames (ORFs) overlap extensively. In addition, the genomic and structural organization, as well as replication and biological characteristics, are very similar in both viruses. Most of the key features of hepadnaviral infection were first discovered in the DHBV model system and subsequently confirmed for HBV. There are, however, several differences between human HBV and DHBV. This review will focus on the molecular and cellular biology, evolution, and host adaptation of the avian hepatitis B viruses with particular emphasis on DHBV as a model system.

Viral and cellular determinants involved in hepadnaviral entry

Hepadnaviridae is a family of hepatotropic DNA viruses that is divided into the genera orthohepadnavirus of mammals and avihepadnavirus of birds. All members of this family can cause acute and chronic hepatic infection, which in the case of human hepatitis B virus (HBV) constitutes a major global health problem. Although our knowledge about the molecular biology of these highly liver-specific viruses has profoundly increased in the last two decades, the mechanisms of attachment and productive entrance into the differentiated host hepatocytes are still enigmatic. The difficulties in studying hepadnaviral entry were primarily caused by the lack of easily accessible in vitro infection systems. Thus, for more than twenty years, differentiated primary hepatocytes from the respective species were the only in vitro models for both orthohepadnaviruses (e.g. HBV) and avihepadnaviruses (e.g. duck hepatitis B virus [DHBV]). Two important discoveries have been made recently regarding HBV: (1) primary hepatocytes from tree-shrews; i.e., Tupaia belangeri, can be substituted for primary human hepatocytes, and (2) a human hepatoma cell line (HepaRG) was established that gains susceptibility for HBV infection upon induction of differentiation in vitro. A number of potential HBV receptor candidates have been described in the past, but none of them have been confirmed to function as a receptor. For DHBV and probably all other avian hepadnaviruses, carboxypeptidase D (CPD) has been shown to be indispensable for infection, although the exact role of this molecule is still under debate. While still restricted to the use of primary duck hepatocytes (PDH), investigations performed with DHBV provided important general concepts on the first steps of hepadnaviral infection. However, with emerging data obtained from the new HBV infection systems, the hope that DHBV utilizes the same mechanism as HBV only partially held true. Nevertheless, both HBV and DHBV in vitro infection systems will help to: (1) functionally dissect the hepadnaviral entry pathways, (2) perform reverse genetics (e.g. test the fitness of escape mutants), (3) titrate and map neutralizing antibodies, (4) improve current vaccines to combat acute and chronic infections of hepatitis B, and (5) develop entry inhibitors for future clinical applications.

Regulation of hepadnavirus reverse transcription by dynamic nucleocapsid phosphorylation

Reverse transcription, an essential step in the life cycle of all retroelements, is a complex, multistep process whose regulation is not yet clearly understood. We have recently shown that reverse transcription in the pararetrovirus duck hepatitis B virus is associated with complete dephosphorylation of the viral core protein, which forms the nucleocapsid wherein reverse transcription takes place. Here we present a genetic study of the role of this dynamic nucleocapsid phosphorylation in regulating viral reverse transcription. Detailed analyses of the reverse transcription products synthesized within nucleocapsids composed of core phosphorylation site mutants revealed that alanine substitutions, mimicking the nonphosphorylated state, completely blocked reverse transcription at a very early stage. In contrast, aspartate substitutions, mimicking the phosphorylated state, allowed complete first-strand DNA synthesis but were severely defective in accumulating mature double-stranded DNA. The latter defect was due to a combination of mutant nucleocapsid instability during maturation and a block in mature second-strand DNA synthesis. Thus, the reversible phosphorylation of the nucleocapsids regulates the ordered progression of reverse transcription.

Sequence identity of the direct repeats, DR1 and DR2, contributes to the discrimination between primer translocation and in situ priming during replication of the duck hepatitis B virus

There are two mutually exclusive pathways for plus-strand DNA synthesis in hepadnavirus reverse transcription. The predominant pathway gives rise to relaxed circular DNA, while the other pathway yields duplex linear DNA. At the completion of minus-strand DNA synthesis, the final RNase H cleavage generates the plus-strand primer at direct repeat 1 (DR1). A small fraction of viruses make duplex linear DNA after initiating plus-strand DNA synthesis from this site, a process called in situ priming. To make relaxed circular DNA, a template switch is necessary for the RNA primer generated at DR1 to initiate plus-strand DNA synthesis from the direct repeat 2 (DR2) located near the opposite end of the minus-strand DNA, a process called primer translocation. We are interested in understanding the mechanism that discriminates between these two processes. Previously, we showed that a small DNA hairpin forms at DR1 in the avihepadnaviruses and acts as an inhibitor of in situ priming. Here, using genetic approaches, we show that sequence identity between DR1 and DR2 is necessary, but not sufficient for primer translocation in the duck hepatitis B virus. The discrimination between in situ priming and primer translocation depends upon suppression of in situ priming, a process that is dependent upon both sequence identity between DR1 and DR2, and the presence of the hairpin at DR1. Finally, our analysis indicates the entire RNA primer can contribute to primer translocation and is translocated to DR2 before initiation of plus-strand DNA synthesis from that site.

pH-independent entry and sequential endosomal sorting are major determinants of hepadnaviral infection in primary hepatocytes

Entry and intracellular transport of hepatitis B viruses have several unusual, largely unknown aspects. In this study, we explored the mode of virus entry using the duck hepatitis B virus (DHBV) and the primary hepatocyte infection model. Upon internalization, viral particles were enriched in an endosomal compartment, as revealed by biochemical and ultrastructural analysis. Virus-containing vesicles harbored early endosome markers. Kinetic analysis revealed time-dependent partial translocation of viral DNA from endosomes into the cytosol. This was strongly reduced by inhibition of vacuolar ATPase; (vATPase) activity with bafilomycin A1 and resulted in abortive infection and prevention of cccDNA formation. Inactivation of vATPase induced accumulation and stabilization of incoming viral particles in endosomes, presumably by blocking endosomal carrier vesicle-mediated cargo transport and sorting. Although neutralization of the endomembrane organelles alone led to stabilization of incoming viral particles, it did not inhibit virus infection. In line with this, a pH-dependent ectopic virus fusion at the plasma membrane could not be artificially induced. This provided further evidence for a pH-neutral translocation mechanism. Endosomal membrane potential was required for viral infection because cotreatment of cells with monensin partially overcame the inhibitory effect of bafilomycin A1. In conclusion, hepatitis B viral infection is mediated by a novel cellular entry mechanism with features different from that of all other known viruses.

Antiviral effect of Phyllanthus nanus ethanolic extract against hepatitis B virus (HBV) by expression microarray analysis

Ethanolic extract of Phyllanthus nanus (P. nanus) treatment exhibited potent antiviral activity against Hepatitis B virus (HBV). The effects of these extracts on HBV in the HBV genome integrated cell lines--Alexander cells and HepG2 2.2.15 cells were examined. Experimental results showed that the ethanolic extract of P. nanus produced suppressive effect on HBsAg secretion and HBsAg mRNA expression. The extract also inhibited HBV replication as measured by HBV DNA level in vitro. In addition, using a duck HBV (DHBV) primary culture model, the P. nanus ethanolic extract suppressed viral replication of DHBV in DHBV infected primary duck hepatocytes. The gene expression pattern in Alexander cells that had been treated with the ethanolic extract of P. nanus was also revealed by microarray techniques. The microarray results indicated that there was up-regulation of expression of several genes, including annexin A7 (Axn7). The subcellular localization of Axn7 and anti-HBV effect of Axn7 over-expression in Alexander cells were also investigated. Results showed that expression of Axn7-GFP fusion protein are localized around the secretory vesicles and could cause a decrease in HBsAg secretion in Alexander cells. Axn7 protein might play an important role in the medicinal effect of the active principle(s) of P. nanus.

Effects of liver growth factors on hepadnavirus replication in chronically infected duck hepatocytes

BACKGROUND/AIMS

Duck hepatitis B virus (DHBV) replication is up-regulated by cell cycle during the early infection of primary duck but the effect of cell cycle on DHBV replication in chronically infected hepatocyte is not known.

METHODS

Hepatocytes obtained from DHBV congenitally infected embryos were used. Cell proliferation was controlled by addition of liver growth factors and the impact on viral replication analyzed.

RESULTS

EGF induced cell proliferation is associated with a slight increase in CCC DNA synthesis and a decrease in viral transcription. Conversely, TGFbeta blocked cell cycle progression, diminished CCC DNA synthesis but increased viral transcription.

CONCLUSIONS

Cell proliferation decreases DHBV transcription but this effect seems to be compensated by an opposite effect on the synthesis of CCC DNA resulting in a global moderate effect on viral replication. Our results also indicate that after division of chronically infected hepatocytes both daughter cells are infected, confirming that liver regeneration is not sufficient to induce CCC DNA eradication as suggested by the lack of effect of some long term anti-HBV therapies.

In vitro characterization of the anti-hepatitis B virus activity and cross-resistance profile of 2',3'-dideoxy-3'-fluoroguanosine

The fluorinated guanosine analog 2',3'-dideoxy-3'-fluoroguanosine (FLG) was shown to inhibit wild-type (wt) hepatitis B virus (HBV) replication in a human hepatoma cell line permanently expressing HBV. Experiments performed in the duck model of HBV infection also showed its in vivo antiviral activity. In this study, we investigated the mechanism of inhibition of FLG on HBV replication and its profile of antiviral activity against different HBV or duck hepatitis B virus (DHBV) drug-resistant mutants. We found that FLG-triphosphate inhibits weakly the priming of the reverse transcription compared to adefovir-diphosphate in a cell-free system assay allowing the expression of an enzymatically active DHBV reverse transcriptase. It inhibits more potently wt DHBV minus-strand DNA synthesis compared to lamivudine-triphosphate and shows a similar activity compared to adefovir-diphosphate. FLG-triphosphate was most likely a competitive inhibitor of dGTP incorporation and a DNA chain terminator. In Huh7 cells transiently transfected with different HBV constructs, FLG inhibited similarly the replication of wt, lamivudine-resistant, adefovir-resistant, and lamivudine-plus-adefovir-resistant HBV mutants. These results were consistent with those obtained in the DHBV polymerase assay using the same drug-resistant polymerase mutants. In conclusion, our data provide new insights in the mechanism of action of FLG-triphosphate on HBV replication and demonstrate its inhibitory activity on drug-resistant mutant reverse transcriptases in vitro. Furthermore, our results provide the rationale for further clinical evaluation of FLG in the treatment of drug-resistant virus infection and in the setting of combination therapy to prevent or delay drug resistance.

DNA vaccines expressing the duck hepatitis B virus surface proteins lead to reduced numbers of infected hepatocytes and protect ducks against the development of chronic infection in a virus dose-dependent manner

We tested the efficacy of DNA vaccines expressing the duck hepatitis B virus (DHBV) pre-surface (pre-S/S) and surface (S) proteins in modifying the outcome of infection in 14-day-old ducks. In two experiments, Pekin Aylesbury ducks were vaccinated on days 4 and 14 of age with plasmid DNA vaccines expressing either the DHBV pre-S/S or S proteins, or the control plasmid vector, pcDNA1.1Amp. All ducks were then challenged intravenously on day 14 of age with 5 x 10(7) or 5 x 10(8) DHBV genomes. Levels of initial DHBV infection were assessed using liver biopsy tissue collected at day 4 post-challenge (p.c.) followed and immunostained for DHBV surface antigen to determine the percentage of infected hepatocytes. All vector vaccinated ducks challenged with 5 x 10(7) and 5 x 10(8) DHBV genomes had an average of 3.21% and 20.1% of DHBV-positive hepatocytes respectively at day 4 p.c. and 16 out of 16 ducks developed chronic DHBV infection. In contrast, pre-S/S and S vaccinated ducks challenged with 5 x 10(7) DHBV genomes had reduced levels of initial infection with an average of 1.38% and 1.93% of DHBV-positive hepatocytes at day 4 p.c. respectively and 10 of 18 ducks were protected against chronic infection. The pre-S/S and the S DNA vaccinated ducks challenged with 5 x 10(8) DHBV genomes had an average of 31.5% and 9.2% of DHBV-positive hepatocytes on day 4 p.c. respectively and only 4 of the 18 vaccinated ducks were protected against chronic infection. There was no statistically significant difference in the efficacy of the DHBV pre-S/S or S DNA vaccines. In conclusion, vaccination of young ducks with DNA vaccines expressing the DHBV pre-S/S and S proteins induced rapid immune responses that reduced the extent of initial DHBV infection in the liver and prevented the development of chronic infection in a virus dose-dependent manner.

Identification of a structural motif crucial for infectivity of hepatitis B viruses

Infectious entry of hepatitis B viruses (HBV) has nonconventional facets. Here we analyzed whether a cell-permeable peptide [translocation motif (TLM)] identified within the surface protein of human HBV is a general feature of all hepadnaviruses and plays a role in the viral life cycle. Surface proteins of all hepadnaviruses contain conserved functional TLMs. Genetic inactivation of the duck HBV TLMs does not interfere with viral morphogenesis; however, these mutants are noninfectious. TLM mutant viruses bind to cells and are taken up into the endosomal compartment, but they cannot escape from endosomes. Processing of surface protein by endosomal proteases induces their exposure on the virus surface. This unmasking of TLMs mediates translocation of viral particles across the endosomal membrane into the cytosol, a prerequisite for productive infection. The ability of unmasked TLMs to translocate processed HBV particles across cellular membranes was shown by confocal immunofluorescence microscopy and by infection of nonpermissive cell lines with HBV processed in vitro with endosomal lysate. Based on these data, we propose an infectious entry mechanism unique for hepadnaviruses that involves virus internalization by receptor-mediated endocytosis followed by processing of surface protein in endosomes. This processing activates the function of TLMs that are essential for viral particle translocation through the endosomal membrane into the cytosol and productive infection.

Characterization of a hepatitis B and hepatitis delta virus receptor binding site

Insights into the early infection events of the human hepatitis B (HBV) and hepatitis delta virus (HDV) have been limited because of the lack of a cell culture system supporting the full replication cycle for these important pathogens. The human hepatoma cell line HepaRG allows the experimental induction of a differentiated state, thereby gaining susceptibility toward HBV and HDV infection. We recently identified HBV envelope protein-derived lipopeptides comprising amino acids 2 though 48 of the preS-domain of the L-surface protein, which block infection already at picomolar concentrations. To map the responsible sequence for the peptides' activity we describe an Escherichia coli expression system that permits myristoylation and investigated recombinant HBVpreS-GST fusion proteins with deletion- and point-mutations for their ability to prevent HBV and HDV infection. We found that (1) a myristoylated HBVpreS/2-48-GST fusion protein efficiently interferes with HBV infection of HepaRG cells; (2) deletions and point mutations in the highly conserved preS1 sequence between amino acids 11 through 21 result in the loss of infection inhibition activity; (3) hepatitis B viruses carrying single amino acid exchanges within this region lose infectivity; and (4) HDV infection of HepaRG cells can be inhibited by myristoylated HBVpreS peptides with the same specificity. In conclusion, HBV and HDV use at least one common step to enter hepatocytes and require a highly conserved preS1-sequence within the L-protein. This step is exceptionally sensitive toward inactivation by acylated HBVpreS1 peptides, which therefore represent a novel group of entry inhibitors that could be used for the treatment of hepatitis B and D.

Suppression of mRNA accumulation by the duck hepatitis B virus reverse transcriptase

Hepadnaviruses establish chronic liver infections, but the mechanisms of persistence and immune evasion are poorly understood. We previously found that the duck hepatitis B virus (DHBV) and hepatitis B virus reverse transcriptases (P protein) unexpectedly accumulate in the cytoplasm where they could affect function(s) beyond viral DNA synthesis, such as gene expression. Therefore, we measured effects of DHBV P on gene expression from reporter constructs and the viral genome. P reduced reporter expression at the mRNA level to approximately 30-40%, independent of reporter tested. Accumulation of the viral pregenomic RNA from its native promoter was suppressed three-to four-fold by P, and accumulation of the capsid protein and intracellular core particles was similarly suppressed because the pregenomic RNA encodes the capsid protein. Therefore, suppression of the pregenomic RNA by DHBV P creates a negative feedback loop to limit viral antigen accumulation and replication, possibly contributing to maintenance of chronic infection.

Vaccination of ducks with a whole-cell vaccine expressing duck hepatitis B virus core antigen elicits antiviral immune responses that enable rapid resolution of de novo infection

As a first step in developing immuno-therapeutic vaccines for patients with chronic hepatitis B virus infection, we examined the ability of a whole-cell vaccine, expressing the duck hepatitis B virus (DHBV) core antigen (DHBcAg), to target infected cells leading to the resolution of de novo DHBV infections. Three separate experiments were performed. In each experiment, ducks were vaccinated at 7 and 14 days of age with primary duck embryonic fibroblasts (PDEF) that had been transfected 48 h earlier with plasmid DNA expressing DHBcAg with and without the addition of anti-DHBcAg (anti-DHBc) antibodies. Control ducks were injected with either 0.7% NaCl or non-transfected PDEF. The ducks were then challenged at 18 days of age by intravenous inoculation with DHBV (5 x 10(8) viral genome equivalents). Liver biopsies obtained on day 4 post-challenge demonstrated that vaccination did not prevent infection of the liver as similar numbers of infected hepatocytes were detected in all vaccinated and control ducks. However, analysis of liver tissue obtained 9 or more days post-challenge revealed that 9 out of 11 of the PDEF-DHBcAg vaccinated ducks and 8 out of 11 ducks vaccinated with PDEF-DHBcAg plus anti-DHBc antibodies had rapidly resolved the DHBV infection with clearance of infected cells. In contrast, 10 out of 11 of the control unvaccinated ducks developed chronic DHBV infection. In conclusion, vaccination of ducks with a whole-cell PDEF vaccine expressing DHBcAg elicited immune responses that induced a rapid resolution of DHBV infection. The results establish that chronic infection can be prevented via the vaccine-mediated induction of a core-antigen-specific immune response.