The quaternary structure, antigenicity, and aggregational behavior of the secretory core protein of human hepatitis B virus are determined by its signal sequence

rt269I Type of Hepatitis B Virus (HBV) Polymerase versus rt269L Is More Prone to Mutations within HBV Genome in Chronic Patients Infected with Genotype C2: Evidence from Analysis of Full HBV Genotype C2 Genome

Recently, it has been reported that the rt269I type of hepatitis B virus (HBV) polymerase (Pol) versus the rt269L type is more significantly related to lower viral replication and HBeAg negative infections in chronic hepatitis B (CHB) patients of genotype C2. In this study, we compared mutation rates within HBV genomes between rt269L and rt269I using a total of 234 HBV genotype C2 full genome sequences randomly selected from the HBV database (115 of rt269L and 119 of rt269I type). When we applied the Benjamini and Hochberg procedure for multiple comparisons, two parameters, dN and d, at the amino acids level in the Pol region were significantly higher in the rt269I type than in the rt269L type. Although it could not reach statistical significance from the Benjamini and Hochberg procedure, nonsynonymous (NS) mutations in the major hydrophilic region (MHR) or “a” determinant in the surface antigens (HBsAg ORF) related to host immune escape or vaccine escape are more frequently generated in rt269I strains than in rt269L. We also found that there are a total of 19 signature single nucleotide polymorphisms (SNPs), of which 2 and 17 nonsynonymous mutation types were specific to rt269L and rt269I, respectively: Of these, most are HBeAg negative infections (preC-W28*, X-V5M and V131I), lowered HBV DNA or virion production (C-I97F/L, rtM204I/V) or preexisting nucleot(s)ide analog resistance (NAr) (rtN139K/H, rtM204I/V and rtI224V) or disease severity (preC-W28*, C-I97F/L, C-Q182K/*, preS2-F141L, S-L213I/S, V/L5M, T36P/S/A, V131I, rtN139K/H, rtM204I/V and rtI224V). In conclusion, our data showed that rt269I types versus rt269L types are more prone to overall genome mutations, particularly in the Pol region and in the MHR or “a” determinant in genotype C2 infections and are more prevalent in signature NS mutations related to lowered HBV DNA replication, HBsAg and HBeAg secretion and potential NAr variants and hepatocellular carcinoma (HCC), possibly via type I interferon (IFN-I)-mediated enhanced inflammation. Our data suggest that rt269L types could contribute to liver disease progression via the generation of immune escape or enhanced persistent infection in chronic patients of genotype C2.

Hepatitis B core antigen can regulate NLRP3 inflammasome pathway in HepG2 cells

Hepatitis B virus (HBV) has four open reading frames (ORFs) of which ORF C is consists of the pre Core and Core genes encodes the Hepatitis B core antigen (HBcAg) and Hepatitis B e antigen (HBeAg). Studies have shown that HBeAg significantly inhibits the NLRP3 inflammasome activation and interleukin-1β (IL-1β) production. However, the role of HBcAg and ORF C proteins (in this paper, ORF C proteins = HBcAg + HBeAg) were remain unclear. Our study aims to assess whether HBcAg and ORF C proteins can affect the NLRP3 inflammasome pathway. Vectors expressing ORF C proteins and HBcAg were designed and transfected into HepG2 cells. And then, cells were stimulated with lipopolysaccharide (LPS). Activation of the NLRP3 inflammasome and the levels of IL-1β and IL-18 were evaluated by Western blot analysis, quantitative reverse-transcription polymerase chain reaction, enzyme-linked immunosorbent assay, and immunofluorescence. The expression of NLRP3 and IL-1β peaked when HepG2 cells were stimulated with 1000 ng/mL LPS for 18 to 24 hours. HBcAg, but not ORF C proteins, promoted LPS-induced NLRP3 inflammasome activation and IL-1β production. These findings provide a novel mechanism on how the HBV causes liver inflammation and may provide insights into the search for new therapeutic strategies.

Natural history of chronic hepatitis B virus infection from infancy to adult life - the mechanism of inflammation triggering and long-term impacts

Chronic hepatitis B virus (HBV) infection in endemic areas usually starts since infancy and early childhood and persists lifelong. The clinical course varies among different chronic infected subjects. Majority of chronic HBV infected children present with immune-tolerant status initially, experience the immune clearance phase with various degree of liver injury during or beyond puberty, and then enter the inactive phase after hepatitis B e antigen (HBeAg) seroconversion. Part of them may have HBV DNA titers elevation with hepatitis flare after HBeAg seroconversion, the so call HBeAg-negative hepatitis flare. Liver cirrhosis, and even hepatocellular carcinoma may develop afterward.

The complex course of chronic HBV infection is associated with the age/route of viral acquisition, host factors such as immune and endocrine factors, viral factors, and host-viral interactions. The adrenarche and puberty onset modulate the start of immune clearance and the severity of liver inflammation in chronic HBV infected children. The genotype and phenotype of human cytokines, innate immunity, and human leukocyte antigens are also associated with the onset of immune clearance of HBV and severity of inflammation. Immune escape HBV mutant strains, emerged during the immune clearance phase under host immune surveillance, may cause different impacts on viral biosynthesis, host immune responses, and clinical course.

Early events in childhood during chronic HBV infection may serve as important predictors for the later outcome in adulthood. Understanding the mechanisms triggering liver inflammation and their long-term impacts may enhance the development of better and earlier therapeutic strategies for patients with chronic HBV infection.

The inhibitory effect of the hepatitis B virus singly-spliced RNA-encoded p21.5 protein on HBV nucleocapsid formation

Hepatitis B virus (HBV) is the smallest DNA virus and the major cause of acute and chronic hepatitis. The 3.2 kb HBV viral genome generates four major species of unspliced viral transcript as well as several alternatively spliced RNAs. A 2.2 kb singly-spliced RNA is the most abundant spliced RNA and is widely expressed among all HBV genotypes. The expression of the singly-spliced RNA, as well as that of its encoded protein HBSP, is strongly associated with hepatopathology during HBV infection. Here, we report a novel inhibitory role of a p21.5 protein, which is encoded by a 2.2 kb singly-spliced RNA, in the modulation of HBV replication. We show that overexpression of the singly-spliced RNA is able to efficiently inhibit HBV replication. Furthermore, a mutation in the ATG start codon of the precore region completely abolishes the inhibitory effect of the singly-spliced RNA, indicating that a viral protein (p21.5) derived from the singly-spliced RNA is the mediator of the inhibition. Furthermore, p21.5 is able to form a homodimer that interacts with core dimers forming hybrid viral assembly components. Sucrose gradient fractionation revealed that co-expression of p21.5 resulted in a spread distribution pattern of core proteins ranging from low to high sucrose densities. When compared with p22, p21.5 is almost ten times more efficient at destabilizing HBV nucleocapsid assembly in Huh7 cells overexpressing either p21.5 or p22 protein. Moreover, in vivo expression of p21.5 protein by tail vein injection was found to decrease the amount of nucleocapsid in the livers of HBV-expressing BALB/c mice. In conclusion, our study reveals that the HBV 2.2 kb singly-spliced RNA encodes a 21.5 kDa viral protein that significantly interferes with the assembly of nucleocapsids during HBV nucleocapsid formation. These findings provide a possible strategy for elimination of HBV particles inside cells.

The impact of hepatitis B virus precore/core gene carboxyl terminal mutations on viral biosynthesis and the host immune response

BACKGROUND

 We aimed to elucidate the impact of hepatitis B virus (HBV) precore/core gene mutations on spontaneous hepatitis B e antigen (HBeAg) seroconversion, HBV biosynthesis, and the human immune responses in chronic HBV-infected patients.

METHODS

 We analyzed the HBV precore/core gene sequences by cloning method in 33 chronic HBV-infected patients during the inflammatory phase before spontaneous HBeAg seroconversion. The impact of the most prevalent mutant on HBeAg biosynthesis was assessed by Western blotting and native agarose gel analysis in Huh7 cells, and the human immune responses were assessed by in vitro stimulation of CD3+ CD8+ T lymphocytes of chronic HBV-infected subjects.

RESULTS

 The P135Q and G1896A were the most prevalent mutants before HBeAg seroconversion, acting as markers of HBeAg seroconversion (hazard ratios = 2.75 and 4.50; P = .01 and <.001, respectively). The P135Q mutants displayed decreased HBeAg secretion and HBV capsid molecular weight, while showing increased 22 kD HBeAg proprotein accumulation in Huh7 cells. The P135Q mutant peptide induced less interferon-γ expression in CD3+ CD8+ T lymphocytes in HBeAg-negative subjects compared to the wild-type peptide (P = .03).

CONCLUSIONS

 The HBV P135Q mutant emergence during the inflammatory phase was associated with HBeAg seroconversion. It was associated with altered HBV capsid assembly, HBeAg biosynthesis, and reduced human immune responses following HBeAg seroconversion.

Human interleukin-10 genotypes are associated with different precore/core gene mutation patterns in children with chronic hepatitis B virus infection

OBJECTIVE

To elucidate the association between human interleukin-10 (IL-10) genotypes and hepatitis B virus (HBV) precore/core gene mutation in children with chronic HBV infection.

STUDY DESIGN

The study group comprised of 21 children with chronic HBV infection with spontaneous hepatitis B e antigen (HBeAg) seroconversion who were followed for more than 10 years. Another nine children without HBeAg seroconversion served as the control subjects. Sera at the immune tolerance and inflammatory phase (alanine aminotransferase, >80 IU/L) were subjected to HBV precore/core sequence analysis. IL-10 -1082 polymorphism was also determined.

RESULTS

HBV precore/core gene mutation increased significantly more in the inflammatory phase than in the tolerance phase (G1896A, 76.2% versus 4.8%; C1913A, 33.3% versus 0%; C2189A, 28.6% versus 4.8%; G2304A, 52.4% versus 14.3%) in study group (n = 21) but not the control group (n = 9). Subjects with the G/G genotype at the IL-10-1082 polymorphism site had higher C2189A mutation rate than the A allele carriers (P = .02). C2189A mutation carriers are associated with more viral load decrement from tolerance to inflammatory phase (P = .01) and earlier spontaneous HBeAg seroconversion (P = .01).

CONCLUSIONS

The G/G genotype at the IL-10 -1082 polymorphism is associated with higher C2189A mutations, lower HBV viral load at immune inflammatory phase, and earlier spontaneous HBeAg seroconversion than A allele carriers.

Characterization of genotype-specific carboxyl-terminal cleavage sites of hepatitis B virus e antigen precursor and identification of furin as the candidate enzyme

Hepatitis B e antigen (HBeAg) is a secreted version of hepatitis B virus (HBV) core protein that promotes immune tolerance and persistent infection. It is derived from a translation product of the precore/core gene by two proteolytic cleavage events: removal of the amino-terminal signal peptide and removal of the carboxyl-terminal arginine-rich sequence. Four RXXR motifs are present at the carboxyl terminus of the HBeAg precursor, with the first two fused as (151)RRGRSPR(157). Genotype A possesses two extra amino acids at the first motif ((151)RRDRGRSPR(159)), which weakens the first motif and separates it from the second one. Western blot analysis of patient sera revealed a single HBeAg form for genotypes B to D but two additional forms of larger sizes for genotype A. Site-directed mutagenesis and transfection experiments with human hepatoma cell lines indicated that HBeAg of genotype B is derived from cleavage at the first ((151)RRGR(154)) motif. The major HBeAg form of genotype A corresponds to cleavage at the second ((156)RSPR(159)) motif, and the other two forms are cleavage products of the first ((151)RRDR(154)) and third ((166)RRRR(169)) motifs, respectively. Only the cleavage product of the third motif of genotype A was observed in furin-deficient LoVo cells, and an inhibitor of furin-like proprotein convertases blocked cleavage of the first and second motifs in human hepatoma cells. In conclusion, our study reveals genotypic differences in HBeAg processing and implicates furin as the major enzyme involved in the cleavage of the first and second RXXR motifs.

A valine to phenylalanine mutation in the precore region of hepatitis B virus causes intracellular retention and impaired secretion of HBe-antigen

AIM

Hepatitis B virus (HBV) e antigen (HBeAg) is translated from precore mRNA as a precore/core protein, which is post-translationally modified to give rise to the protein that is secreted into the serum. The G1862T mutation in HBV occurs in the bulge of the encapsidation signal within the pregenomic RNA. When the precore mRNA is translated, this mutation results in a valine to phenylalanine substitution at the -3 position to the signal peptide cleavage site at the amino end of the precursor protein. The aim of this study was to determine whether this mutation could affect HBV replication and/or HBeAg expression.

METHODS

Following transfection of Huh 7 cells, HBV replication was followed using real time polymerase reaction (PCR) and expression of HBeAg expression was monitored using confocal microscopy.

RESULTS

HBV replication was reduced when this mutation was introduced into genotype D but not into genotype A replication-competent constructs. Using mutant HBeAg-expressing plasmids, we demonstrated a 54% reduction in HBeAg secretion relative to the wild type. Confocal microscopy demonstrated that the mutant HBeAg accumulated in the endoplasmic reticulum, endoplasmic reticulum intermediate compartment and Golgi. These aggregates of mutant protein increased in size following treatment of the cells with a proteasome inhibitor, MG132, and had the hallmark features of aggresomes. They attracted ubiquitin, heat shock proteins and proteasomes and were isolated from the cytosol by the intermediate filaments, vimentin and cytokeratin.

CONCLUSION

The formation of aggresomes, as a result of the G1862T mutation, may play a contributory role in HBV-induced liver disease.

Hepatitis B virus DNA-negative dane particles lack core protein but contain a 22-kDa precore protein without C-terminal arginine-rich domain

DNA-negative Dane particles have been observed in hepatitis B virus (HBV)-infected sera. The capsids of the empty particles are thought to be composed of core protein but have not been studied in detail. In the present study, the protein composition of the particles was examined using new enzyme immunoassays for the HBV core antigen (HBcAg) and for the HBV precore/core proteins (core-related antigens, HBcrAg). HBcrAg were abundant in fractions slightly less dense than HBcAg and HBV DNA. Three times more Dane-like particles were observed in the HBcrAg-rich fraction than in the HBV DNA-rich fraction by electron microscopy. Western blots and mass spectrometry identified the HBcrAg as a 22-kDa precore protein (p22cr) containing the uncleaved signal peptide and lacking the arginine-rich domain that is involved in binding the RNA pregenome or the DNA genome. In sera from 30 HBV-infected patients, HBcAg represented only a median 10.5% of the precore/core proteins in enveloped particles. These data suggest that most of the Dane particles lack viral DNA and core capsid but contain p22cr. This study provides a model for the formation of the DNA-negative Dane particles. The precore proteins, which lack the arginine-rich nucleotide-binding domain, form viral RNA/DNA-negative capsid-like particles and are enveloped and released as empty particles.

Hepatitis B core antigen is a potent inductor of interleukin-18 in peripheral blood mononuclear cells of healthy controls and patients with hepatitis B infection

Clearance of hepatitis B virus infection (HBV) infection implies a polyclonal vigorous T-helper 1 (Th1) and cytotoxic T-lymphocyte (CTL) response. Interleukin-18 (IL-18), a monokine that shares functional abilities with IL-12, is a potent inductor of interferon-gamma (IFN-gamma) by Th1 and natural killer (NK) cells. However, the role and regulation in HBV infection of IFN-gamma have not been defined. This study therefore sought to determine hepatitis B core antigen (HBcAg)-mediated regulation of IL-18 production in peripheral blood mononuclear cells (PBMCs) from healthy controls (HC) and patients with chronic hepatitis B (CHB) or acute hepatitis B (AHB); 31 HC, 27 patients with CHB and 8 patients with AHB infection were included in the study. HBcAg-mediated induction of IL-18 was determined by quantitative reverse transcription-polymerase chain reaction (RT-PCR) and specific enzyme-linked immunosorbent assay (ELISA). HBcAg induced IL-18 gene transcription and dose-dependent secretion of mature IL-18 protein in HC, CHB, and AHB. HBcAg-dependent IL-18 levels were abrogated by inhibition of Caspase-1, but not by blockade of CD40-CD154 interaction. Serum levels of IFN-gamma correlated inversely with viremia in patients with CHB (rho = - 0.54, P < 0.05), but not with serum levels of IL-12 or IL-18. Interestingly, in PBMCs of HBeAg-negative patients, HBcAg induced significantly higher amounts of IL-18 than in those of HBeAg-positive patients. A variant, lacking the histone-like arginine-rich domain, did not induce IL-18 in either HC or CHB in vitro. Taken together, these results indicate that HBcAg induces IL-18 secretion by induction of Caspase-1. Differential regulation in HBeAg-negative and positive patients suggests an important role of IL-18 in CHB infection.

Identification of the authentic varicella-zoster virus gB (gene 31) initiating methionine overlapping the 3' end of gene 30

The varicella-zoster virus (VZV) gB sequence was re-examined in light of recent knowledge about unusually long gB signal peptides in other herpesviral gB homologs. Through mutational analysis, the discovery was made that the authentic initiating methionine for VZV gB is a codon beginning at genome nucleotide 56,819. The total length for the VZV gB primary translation product was 931 amino acids (aa) with a 71-aa signal sequence. Considering the likely signal sequence cleavage site to be located between Ser 71 and Val 72, the length of the mature VZV gB polypeptide would then be 860 amino acids prior to further internal endoproteolytic cleavage between amino acids Arg 494 and Ser 495. In this report, we also produced a full-length gB and demonstrated its association with VZV gE, suggesting a possible gE-gB interaction during gB trafficking before its cleavage in the Golgi.

Molecular, immunological and clinical properties of mutated hepatitis B viruses

Hepatitis B virus (HBV) is at the origin of severe liver diseases like chronic active hepatitis, liver cirrhosis and hepatocellular carcinoma. There are some groups of patients with high risk of generation of HBV mutants: infected infants, immunosuppressed individuals (including hemodialysis patients), patients treated with interferon and lamivudine for chronic HBV infection. These groups are the target for molecular investigations reviewed in this paper. The emergence of lamivudine- or other antiviral-resistant variants, rises concern regarding long term use of these drugs. Infection or immunization with one HBV subtype confers immunity to all subtypes. However, reinfection or reactivation of latent HBV infection with HBV mutants have been reported in patients undergoing transplant and those infected with HIV. Mutations of the viral genome which are not replicative incompetent can be selected in further course of infection or under prolonged antiviral treatment and might maintain the liver disease. Four open reading frames (ORF) which are called S-gene, C-gene, X-gene and P-gene were identified within the HBV genome. Mutations may affect each of the ORFs. Mutated S-genes were described to be responsible for HBV-infections in successfully vaccinated persons, mutated C-genes were found to provoke severe chronic liver diseases, mutated X-genes could cause serious medical problems in blood donors by escaping the conventional test systems and mutated P-genes were considered to be the reason for chemotherapeutic drug resistance. This paper reviews molecular, immunological and clinical aspects of the HBV mutants.

Formation of HIV-1 envelope-hepatitis B core antigen hybrids with high affinity for CD4

We have identified an acceptor site on HIV gp120, where foreign protein sequences can be inserted while retaining the native conformation of gp120. The resulting hybrids showed dual antigenicity, normal glycosylation, and high affinity binding of the CD4 receptor. This site allows insertion of highly immunogenic proteins such as core antigen of hepatitis B virus. By combining the immunogenicity of the carrier protein with the antigenicity of gp120, these hybrids may lead to modified HIV-1 antigens with enhanced immunogenicity.

Presence of bovine viral diarrhea virus (BVDV) E2 glycoprotein in VSV recombinant particles and induction of neutralizing BVDV antibodies in mice

We generated a recombinant vesicular stomatitis virus (VSV-E2) encoding the bovine viral diarrhea virus (BVDV) E2 glycoprotein with the VSV-G protein signal peptide. Infection of BHK21 cells with VSV-E2 induced the synthesis of a recombinant E2 (rE2) that comigrated with authentic BVDV-E2 in PAGE-SDS gels. Non-reducing immunoblots showed that rE2 is a disulfide bond-linked homodimer with at least 10-fold higher avidity for conformation-dependent anti-BVDV-E2 antibodies than its reduced monomeric counterpart. Immunofluorescence microscopy also showed that rE2 was transported to the plasma membrane of infected cells and analysis of purified particles demonstrated that dimeric rE2 was incorporated into VSV-E2 virions in approximately 1:10 ratio with respect to the G glycoprotein. BALB/c mice inoculated intranasally with VSV-E2 doses of up to 10(7) plaque forming units (pfu) showed no symptoms of viral-induced disease and developed a specific BVDV neutralizing response that lasted for at least 180 days post inoculation.

Differential antigen-processing pathways of the hepatitis B virus e and core proteins

BACKGROUND & AIMS

Hepatitis B e antigen (HBeAg) and hepatitis B core antigen (HBcAg) seem to play different roles in the induction and regulation of the antiviral immune response, although the two antigens share all major CD4(+) T-cell epitopes, and these epitopes can be processed from both antigens via the exogenous antigen-presenting pathway. The aim of this study was to test the ability of antigen-presenting cells to present epitopes from endogenously synthesized HBcAg/HBeAg on HLA class II molecules.

METHODS

Lymphoblastoid cell lines infected with recombinant vaccinia viruses containing various HBcAg or HBeAg constructs and stable transfectants were tested for their ability to stimulate HBcAg/HBeAg-specific CD4(+) T-cell clones.

RESULTS

Only antigen-presenting cells infected with HBeAg constructs but not those infected with HBcAg constructs were able to stimulate HBcAg/HBeAg-specific CD4(+) T-cell clones. T-cell activation by HBeAg constructs was completely inhibited by brefeldin A but not affected by chloroquin. In contrast, T-cell activation by exogenous, recombinant HBcAg was inhibited by chloroquin but not by brefeldin A.

CONCLUSIONS

The findings indicate that processing and HLA class II-associated presentation of endogenously synthesized HBeAg in virus-infected cells, including hepatocytes, may occur. This mechanism may be involved in the regulation of the CD4(+) T-cell response to HBcAg/HBeAg.

Signal sequences: more than just greasy peptides

Export signal sequences target newly synthesized proteins to the endoplasmic reticulum of eukaryotic cells and the plasma membrane of bacteria. All signal sequences contain a hydrophobic core region, but, despite this, they show great variation in both overall length and amino acid sequence. Recently, it has become clear that this variation allows signal sequences to specify different modes of targeting and membrane insertion and even to perform functions after being cleaved from the parent protein. This review argues that signal sequences are not simply greasy peptides but sophisticated, multipurpose peptides containing a wealth of functional information.

Major histocompatibility complex class I viral antigen processing in the secretory pathway defined by the trans-Golgi network protease furin

Classical antigen presentation by major histocompatibility complex class I molecules involves cytosolic processing of endogenously synthesized antigens by proteasomes and translocation of processed peptides into the endoplasmic reticulum (ER) by transporters associated with antigen presentation (TAP). Alternative pathways for processing of endogenous antigens, generally involving the ER, have been suggested but not fully proved. We analyzed the potential for class I presentation of proteolytic maturation of secretory antigens in the exocytic pathway. We found that hepatitis B (HB) virus secretory core protein HBe can efficiently deliver COOH-terminally located antigenic peptides for endogenous class I loading in the absence of TAP. Antigen presentation to specific cytotoxic T lymphocytes correlates with protein maturation at the COOH terminus, since modification of maturation and transport of HBe through the secretory pathway alters antigen presentation. Both maturation and a necessary processing step occur in the Golgi or post-Golgi compartment. Antigen presentation is independent of proteasome activity, but inhibitors of the trans-Golgi network resident protease furin inhibit both HBe maturation and antigen presentation. These results define a new antigen processing pathway located in the secretory route, with a central role for proteolytic maturation mediated by the subtilisin protease family member furin as an efficient source for antigen presentation.

The C terminus of the hepatitis B virus e antigen precursor is required for a tunicamycin-sensitive step that promotes efficient secretion of the antigen

The Hepatitis B virus encodes the secreted e antigen (HBe) whose function in the viral life cycle is unknown. HBe derives from a 25-kDa precursor that is directed to the secretory pathway. After cleavage of the signal sequence, the resulting 22-kDa protein (P22) is processed in a post-endoplasmic reticulum compartment to mature HBe by removal of the 34-amino acid C-terminal domain. The efficiency of HBe secretion is specifically decreased in cells grown in the presence of tunicamycin, an inhibitor of N-glycosylation. Inasmuch as HBe precursor is not N-glycosylated, our data suggest that a cellular tunicamycin-sensitive protein increases the intracellular transport through the HBe secretory pathway. The study of the secretion of HBe derived from C-terminal-truncated precursors demonstrates that the tunicamycin-sensitive secretion absolutely requires a part of the C-terminal region that is removed to form mature HBe, indicating that the cellular tunicamycin-sensitive protein increases the efficiency of the intracellular transport of P22. We have also shown that the Escherichia coli beta-galactosidase can be secreted when fused to the HBe precursor signal sequence and that the P22 C-terminal domain renders the secretion of this reporter protein also tunicamycin-sensitive.

Hepatitis B virus capsid: localization of the putative immunodominant loop (residues 78 to 83) on the capsid surface, and implications for the distinction between c and e-antigens

Hepatitis B virus capsid protein comprises a 149 residue "assembly" domain that polymerizes into icosahedral particles, and a 34 residue RNA-binding "protamine" domain. Recently, the capsid structure has been studied to resolutions below 10 A by cryo-electron microscopy, revealing much of its alpha-helical substructure and that it appears to have a novel fold for a capsid protein; however, the resolution is still too low for chain-tracing by conventional criteria. Aiming to establish a fiducial marker to aid in the process of chain-tracing, we have used cryo-microscopy to pinpoint the binding site of a monoclonal antibody that recognizes the peptide from residues 78 to 83. This epitope resides on the outer rim of the 30 A long spikes that protrude from the capsid shell. These spikes are four-helix bundles formed by the pairing of helix-turn-helix motifs from two subunits; by means of a tilting experiment, we have determined that this bundle is right-handed. Variants of the same protein present two clinically important and non-crossreactive antigens: core antigen (HBcAg), which appears early in infection as assembled capsids; and the sentinel e-antigen (HBeAg), a non-particulate form. Knowledge of the binding site of our anti-HBcAg antibody bears on the molecular basis of the distinction between the two antigens, which appears to reflect conformational differences between the assembled and unassembled states of the capsid protein dimer, in addition to epitope masking in capsids.

Molecular biology of hepatitis B virus e antigen

Hepatitis B virus (HBV) e antigen (HBeAg) was discovered in 1972 as one of the serological markers of HBV infection. Although 25 years have passed since its initial discovery, the function of this antigen in the life cycle of HBV has remained elusive. Mutations in the HBV genome that prevent the expression of HBeAg do not abolish the replication of HBV, indicating that this antigen is not essential for HBV replication. In contrast, the conservation of the HBeAg gene in the genomes of related animal viruses, including the distantly related duck HBV, argues for an important function of this antigen. The purpose of the present article is to review the molecular biology of HBeAg and to examine its possible functions in the life cycle of HBV.

Long-term expression of the hepatitis B virus core-e- and X-proteins does not cause pathologic changes in transgenic mice

BACKGROUND/AIMS

Chronic infections with the human hepatitis B virus can result in liver cirrhosis and primary hepatocellular carcinoma. The reasons for these long-term effects are unclear. The aim of this study was to generate transgenic mice expressing the HBV X- and c/e-gene under authentic and foreign promoter control and to test whether the respective gene products can cause pathologic effects during the lifespan of a mouse. Moreover, the temporal and the tissue-specific regulation of the crucial HBV c/e-gene promoter was analyzed.

METHODS

Eight transgenic mouse lines were generated. Four contained the c/e- and X-gene and two contained only the X-gene under authentic promoter control. Two lines expressed only the X-gene under control of the rat insulin promoter/enhancer. Gene expression was tested by protein and mRNA analyses. During an observation period of 2 years, mice were sacrificed and organs subjected to histologic examination. Mice expressing the X-gene in pancreatic beta cells were tested for the development of diabetes.

RESULTS

In the liver, slight histopathologic alterations but no neoplastic changes could be observed in mice expressing the X-gene. Activity of the c/e-gene promoter/enhancer was age dependent and was not restricted to hepatocytes.

CONCLUSION

No evidence was obtained that long-term expression of the HBV c/e- and X-gene products can cause neoplasia during the lifespan of a mouse.

Posttranscriptional regulation of hepatitis B virus replication by the precore protein

Hepadnaviruses encode two core-related open reading frames. One directs the synthesis of the p21 core protein, which subsequently becomes a structural component of the viral nucleocapsid. The other produces a p25 precore protein that is targeted by a signal peptide to a cell secretory pathway where N-terminal processing will create a p22 species. This molecule will be further modified at the C-terminal region to generate p17, and the truncated protein is secreted from the cell as hepatitis B e antigen (HBeAg). The function of the precore gene in the biology of hepadnaviruses is unknown. We found that ablation of the precore gene resulted in the generation of a hepatitis B virus (HBV) species with a high-replication-level phenotype. More important, expression in trans of physiologic levels of p25 restored viral replication to wild-type levels. Moreover, transient or stable overexpression of the precore gene resulted in striking inhibition of HBV replication. The molecular species responsible for this viral inhibitory effect was identified as the p22 nonsecreted HBeAg precursor protein. By sucrose gradient sedimentation analysis, we determined that expression of p22 leads to the formation of nucleocapsids similar to those made with wild-type p21 core protein. Immunoprecipitation experiments revealed that the p21 and p22 physically interact and form hybrid nucleocapsid structures devoid of pregenomic viral RNA. These experiments suggest that expression of the precore gene may be important in the regulation of HBV replication and describe a possible molecular mechanism(s) for this effect.

The hepatitis B virus (HBV) precore protein inhibits HBV replication in transgenic mice

In this study, we examined the ability of the hepatitis B virus (HBV) precore, envelope, and X gene products to modulate HBV replication in the livers of transgenic mice that replicate the virus. Hepatic HBV replication was not affected by overexpression of the envelope or X gene products when these animals were crossed with transgenic mice that express the corresponding viral genes in the hepatocyte. Overexpression of the precore protein, however, eliminated nucleocapsid particles from the cytoplasm of the hepatocytes and abolished HBV replication without affecting the hepatic steady-state content of pregenomic HBV RNA. These observations suggest that the precore protein can exert a dominant negative effect on HBV replication, presumably at the level of nucleocapsid particle maturation or stability, suggesting an important role for this enigmatic viral protein in the HBV life cycle.

Comparison of three different recombinant hepatitis B virus core particles expressed in Escherichia coli

The properties of three different recombinant hepatitis B virus core proteins expressed in Escherichia coli were compared: an N-terminal fusion protein, a C-terminally truncated protein and a sequence-authentic protein. All three proteins assembled into capsid-like particles with typical HBc-antigenicity, sedimentation behavior and distinctive electron microscopical images. Apart from this, however, variant HBc proteins displayed properties different from sequence-authentic HBc protein p21.4. Unlike p21.4, the particles of the N-terminal fusion protein p22.2 were sensitive to proteolytic attack by trypsin at variable sites within its arginine-rich C-terminus but not in its extended N-terminus. We therefore conclude that the C-terminal region is located on the surface of the p22.2 particle. These particles also showed increased HBe-antigenicity, as did the C-terminally truncated core particles p17.6, and to an even greater extent p18* particles which were derived from p22.2 by tryptic digestion. This might be interpreted as evidence for an--albeit minor--structural change. All variant core particles were less stable and contained less RNA. Electron microscopic indication for DNA binding of C-terminal deleted p17.6 particles was obtained using an aqueous spreading technique.

Hepatitis B virus preC mutants in human hepatocellular carcinoma tissues

A comparative analysis of preC sequences of hepatitis B virus (HBV) in human hepatoma (hepatocellular carcinoma; HCC) tissues and non-tumoral liver samples from HCC patients was performed. Ten out of 17 HCC tissue samples exhibited an amino acid substitution at the level of the distal cysteine residue of the HBV preC region, while generation of a TAG translational stop codon was observed in 4 of these samples. Interestingly, substitution of the distal cysteine residue was not observed in non-tumoral liver (available from 8 of the 17 patients), thus suggesting either that a selection among different HBV variants occurs in HCC cells, or that modifications to the conformation and stability of the HBV capsid protein may play a role in the process of selection and escape of transformed liver cells.

An intramolecular disulfide bridge between Cys-7 and Cys61 determines the structure of the secretory core gene product (e antigen) of hepatitis B virus

Hepatitis B virus, the prototypic member of the Hepadnaviridae, is a small enveloped DNA virus that replicates via reverse transcription. Efficient usage of its compact 3.2-kb genome is exemplified by the pre-C/C gene from which two proteins with largely overlapping primary sequences but distinctly different properties are synthesized: the self-assembling core protein p21c (hepatitis B core antigen [HbcAg]) and the secretory, nonparticulate protein p17e (hepatitis B e antigen [HbeAg]). Mature p17e carries a 10-amino-acid N-terminal extension with a Cys residue (Cys-7). Using transient transfection of a human liver cell line with constructs expressing wild-type p17 or a series of Cys mutants of p17, we show that Cys-7 forms an intramolecular S-S bond to Cys61, which in assembly-competent core proteins is available for intermolecular disulfide bonds between two neighboring subunits. Removal of the Cys-7/Cys61 bond by mutating either residue has differential effects: in the absence of Cys-7, secretion is relatively efficient and independent of Cys61; however, the molecules are exported as homodimers exhibiting both HBe and HBc antigenicity. In the absence of Cys61, the nonpaired Cys-7 interferes with secretion efficiency. The amino acid sequence flanking Cys-7 also contributes to the formation of the proper intramolecular S-S bond. These results suggest that the Cys-7/Cys61 bond imposes on p17e a conformation that is critical for its secretion and distinct biophysical and antigenic properties. This mechanism adds selective disulfide formation to the repertoire of hepatitis B virus for efficient use of its tiny genome.

Expression of hepatitis B virus core gene products with specific immunoreactivity for e antigen (HBeAg) in Saccharomyces cerevisiae

The e antigen of the hepatitis B virus (HBeAg) was expressed in S. cerevisiae. Yeast-derived HBeAg exhibits high HBe antigenicity while lacking any HBc antigenicity. The production yield of HBeAg expressed in yeast is dependent on the host strains and the nature of the leader sequences used in the plasmid constructions. The recombinant antigen is not secreted into the medium, independent from the leader sequences which are used. A simple extraction procedure was developed, enabling the isolation of HBeAg from the cells without killing them. Recombinant HBeAg derived from yeast can replace plasma-derived antigen in ELISA for determining antibodies to HBeAg.

Relevance of cysteine residues for biosynthesis and antigenicity of human hepatitis B virus e protein

The mature form of the secretory core protein (HBe protein) of human hepatitis B virus contains four cysteines which are located at amino acid positions -7, 48, 61, and 107 relative to the HBc start methionine. In addition, there is a cysteine, Cys-183, located in the C-terminal domain of the HBe precursor, which is cleaved during HBe maturation. Here, the significance of these cysteines for biosynthesis and antigenicity of the HBe protein was examined. The cysteines at positions -7 and 61 were found to be crucial for HBe biosynthesis. As has already been described, if the Cys at position -7 is mutated, disulfide-linked HBe homodimers which have both HBe antigenicity and HBc antigenicity are expressed. Here we show that these dimers are due to Cys-61-Cys-61 disulfide bridges which are formed only if the Cys at position -7 is not present. In the wild-type protein, this dimerization appears to be inhibited by formation of intramolecular disulfide bridges between the Cys at -7 and one of the internal cysteines. Moreover, Cys-61 is important for HBe biosynthesis in general since mutation of this amino acid results in production of HBe proteins which are either only poorly secreted or possess a different antigenicity.

Molecular basis of the diversity of hepatitis B virus core-gene products

All hepatitis B viruses examined to date code for at least two different core-gene products which are referred to as the c- and the e-protein. In the case of the human hepatitis B virus, they are known as the HBcAg and the HBeAg. Although these proteins share most of their primary amino acid sequence, they exhibit quite distinct properties. The e-protein is located in the cytoplasm and the nucleus of infected cells and very efficiently assembles into nucleocapsids. By contrast, the e-protein does not form particles. It enters the secretory pathway and is actively secreted by the cells. Here we describe the biosynthetic pathways by which the c- and e-proteins are expressed and summarize recent data from our laboratory showing that the antigenic and biophysical properties which distinguish the HBeAg from the HBcAg are primarily due to the 10 amino acid long portion of the HBeAg leader sequence that remains attached to the HBeAg after cleavage.

A cysteine and a hydrophobic sequence in the noncleaved portion of the pre-C leader peptide determine the biophysical properties of the secretory core protein (HBe protein) of human hepatitis B virus

The molecular basis of the biophysical and antigenic differences between the cellular core protein (HBc protein) and the secreted core protein (HBe protein) of human hepatitis B virus was examined. The data show that the properties which distinguish the HBe protein from the HBc protein are due mostly to the 10-amino-acid portion of the HBe leader sequence which remains attached to the HBe protein after cleavage. A cysteine located within this region determines the quaternary structure and the antigenicity of the HBe protein. If this cysteine is lacking, the HBe protein, which is predominantly a monomer with only HBe antigenicity, is expressed as a disulfide-linked homodimer showing both HBe and HBc antigenicity. However, dimerization of the HBe protein was found to be neither sufficient nor required for particle formation. In fact, aggregation of the HBe protein was found to be inhibited by the strongly hydrophobic tripeptide Trp-Leu-Trp, which is also located in the noncleaved portion of the signal sequence. If this tripeptide was converted into either Asp-Asn-Asn or Ala-Asp-Leu, the HBe protein assembled into particles, independent of the presence of the cysteine.

Sequence analysis of the hepatitis B virus pre-C region in hepatocellular carcinoma [HCC] and nontumoral liver tissues from HCC patients

We investigated whether replication-competent pre-C/C defective mutants of hepatitis B virus (HBV) are detectable in primary human hepatocellular carcinoma (HCC) tissues from patients of a geographic area endemic for such mutants. DNAs extracted from formalin-fixed paraffin-embedded HCC samples were checked for the presence of specific HBV DNA sequences using the polymerase chain reaction (PCR). Amplified pre-C regions from nine HCC samples were directly sequenced as were samples of nontumoral liver tissues from five of these patients. The data show that hypervariable distal pre-C sequences were present in all nine HCC samples; this high variability was dependent on point mutations, which led to amino acid substitutions in nearly all cases. Interestingly, seven of the nine HBV DNA-positive samples from HCC tissues (but not samples from peritumoral liver tissue) showed mutations leading to amino acid substitution at the level of a distal cysteine residue. No mutation generating a translationally defective pre-C/C region was detectable in the tumor samples. Otherwise, in four of the six nontumoral liver tissues available from the same patients, a pre-C sequence with an in-frame TAG stop codon was detectable, although in three cases as a component of mixed population.

Frequent and rapid emergence of mutated pre-C sequences in HBV from e-antigen positive carriers who seroconvert to anti-HBe during interferon treatment

Hepatitis B virus (HBV) variants which cannot express e-antigen (HBeAg) are characteristic for many viremic anti-HBe positive chronic carriers who often have particularly severe and fluctuating hepatitis. Whether such variants are selected for and are less amenable to interferon treatment is under dispute. Therefore, by DNA amplification and direct sequencing we have investigated the emergence of HBV pre-C sequence variants in nine e-antigen positive chronic carriers, all of whom seroconverted to anti-HBe or lost HBeAg during interferon treatment, and in three of whom no viral DNA was detectable after interferon treatment. In most, but not all of the patients we found newly emerging pre-C sequences in a subpopulation of the viral genomes that included silent point mutations, amino acid changes, start and stop codon and frameshift mutations. The emergence of these mutations was paralleled by a drastic decrease of viremia during treatment. The observed mutations appeared most frequently during interferon treatment. Some of the mutations appeared or disappeared late after interferon treatment concomitant with anti-HBe antibody development. The appearance or lack of mutations in the pre-C region of a subpopulation of HBV of these patients was independent of successful virus elimination. These data indicate that interferon treatment is frequently associated with the simultaneous fall in titer of viral DNA by several orders of magnitude and the emergence of novel pre-C sequences, some of them preventing HBeAg expression. However, the presumably immune-mediated selection for pre-C mutant viruses and decrease in viremia under interferon treatment appears not to be prognostic for successful or unsuccessful virus elimination.