A soluble form of the avian hepatitis B virus receptor. Biochemical characterization and functional analysis of the receptor ligand complex

Serum levels of preS antigen (HBpreSAg) in chronic hepatitis B virus infected patients

Background

Hepatitis B virus (HBV) infection is a serious health problem worldwide. Treatment recommendation and response are mainly indicated by viral load, e antigen (HBeAg) seroconversion, and ALT levels. The S antigen (HBsAg) seroconversion is much less frequent. Since HBeAg can be negative in the presence of high viral replication, preS antigen (HBpreSAg) might be a useful indicator in management of chronic HBV infection.

Results

A new assay of double antibody sandwich ELISA was established to detect preS antigens. Sera of 104 HBeAg-negative and 50 HBeAg-positive chronic hepatitis B patients have been studied and 23 HBeAg-positive patients were enrolled in a treatment follow-up study. 70% of the HBeAg-positive patients and 47% of the HBeAg-negative patients showed HBpreSAg positive. Particularly, in the HBeAg-negative patients, 30 out of 47 HBpreSAg positive patients showed no evidence of viral replication based on HBV DNA copies. A comparison with HBV DNA copies demonstrated that the overall accuracy of the HBpreSAg test could reach 72% for active HBV replication. HBpreSAg changes were well correlated with changes of HBsAg, HBV DNA and ALT levels during the course of IFN-α treatment and follow-up. HBeAg positive patients responded well to treatment when reduction of HBpreSAg levels was more pronounced.

Conclusion

Our results suggested that HBpreSAg could be detected effectively, and well correlated with HBsAg and HBV DNA copies. The reduction of HBpreSAg levels in conjunction with the HBV DNA copies appears to be an improved predictor of treatment outcome.

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.

A hydrophobic domain in the large envelope protein is essential for fusion of duck hepatitis B virus at the late endosome

The duck hepatitis B virus (DHBV) envelope is comprised of two transmembrane (TM) proteins, the large (L) and the small (S), that assemble into virions and subviral particles. Secondary-structure predictions indicate that L and S have three alpha-helical, membrane-spanning domains, with TM1 predicted to act as the fusion peptide following endocytosis of DHBV into the hepatocyte. We used bafilomycin A1 during infection of primary duck hepatocytes to show that DHBV must be trafficked from the early to the late endosome for fusion to occur. Alanine substitution mutations in TM1 of L and S, which lowered TM1 hydrophobicity, were used to examine the role of TM1 in infectivity. The high hydrophobicity of the TM1 domain of L, but not of S, was shown to be essential for virus infection at a step downstream of receptor binding and virus internalization. Using wild-type and mutant synthetic peptides, we demonstrate that the hydrophobicity of this domain is required for the aggregation and the lipid mixing of phospholipid vesicles, supporting the role of TM1 as the fusion peptide. While lipid mixing occurred at pH 7, the kinetics of insertion of the fusion peptide was increased at pH 5, consistent with the location of DHBV in the late-endosome compartment and previous studies of the nonessential role of low pH for infectivity. Exchange of the TM1 of DHBV with that of hepatitis B virus yielded functional, infectious DHBV particles, suggesting that TM1 of all of the hepadnaviruses act similarly in the fusion mechanism.

Mapping of the hepatitis B virus pre-S1 domain involved in receptor recognition

Hepatitis B virus (HBV) and woolly monkey hepatitis B virus (WMHBV) are primate hepadnaviruses that display restricted tissue and host tropisms. Hepatitis D virus (HDV) particles pseudotyped with HBV and WMHBV envelopes (HBV-HDV and WM-HDV) preferentially infect human and spider monkey hepatocytes, respectively, thereby confirming host range bias in vitro. The analysis of chimeric HBV and WMHBV large (L) envelope proteins suggests that the pre-S1 domain may comprise two regions that affect infectivity: one within the amino-terminal 40 amino acids of pre-S1 and one downstream of this region. In the present study, we further characterized the role of the amino terminus of pre-S1 in infectivity by examining the ability of synthetic peptides to competitively block HDV infection of primary human and spider monkey hepatocytes. A synthetic peptide representing the first 45 residues of the pre-S1 domain of the HBV L protein blocked infectivity of HBV-HDV and WM-HDV, with a requirement for myristylation of the amino terminal residue. Competition studies with truncated peptides suggested that pre-S1 residues 5 to 20 represent the minimal domain for inhibition of HDV infection and, thus, presumably represent the residues involved in virus-host receptor interaction. Recombinant pre-S1 proteins expressed in insect cells blocked infection with HBV-HDV and WM-HDV at a concentration of 1 nanomolar. The ability of short pre-S1 peptides to efficiently inhibit HDV infection suggests that they represent suitable ligands for identification of the HBV receptor and that a pre-S1 mimetic may represent a rational therapy for the treatment of HBV infection.

Palmitoylation of carboxypeptidase D. Implications for intracellular trafficking

Covalent lipid modifications mediate protein-membrane and protein-protein interactions and are often essential for function. The purposes of this study were to examine the Cys residues of the transmembrane domain of metallocarboxypeptidase D (CPD) that could be a target for palmitoylation and to clarify the function of this modification. CPD is an integral membrane protein that cycles between the trans Golgi network and the plasma membrane. We constructed AtT-20 cells stably expressing various constructs carrying a reporter protein (albumin) fused to a transmembrane domain and the CPD cytoplasmic tail. Some of the constructs contained the three Cys residues present in the CPD transmembrane region, while other constructs contained Ala in place of the Cys. Constructs carrying Cys residues were palmitoylated, while those constructs lacking the Cys residues were not. Because palmitoylation of several proteins affects their association with cholesterol and sphingolipid-rich membrane domains or caveolae, we tested endogenous CPD and several of the reporter constructs for resistance to extraction with Triton X-100. A construct containing the Cys residues of the CPD transmembrane domain was soluble in Triton X-100 as was endogenous palmitoylated CPD, indicating that palmitoylation does not target CPD to detergent-resistant membrane rafts. Interestingly, constructs of CPD that lack palmitoylation sites have an increased half-life, a slightly more diffuse steady-state localization, and a slower rate of exit from the Golgi as compared with constructs containing palmitoylation sites. Thus, the covalent attachment of palmitic acid to the Cys residues of CPD has a functional significance in the trafficking of the protein.

Inhibition of duck hepatitis B virus infection by a myristoylated pre-S peptide of the large viral surface protein

We have used the duck hepatitis B virus (DHBV) model to study the interference with infection by a myristoylated peptide representing an N-terminal pre-S subdomain of the large viral envelope protein. Although lacking the essential part of the carboxypeptidase D (formerly called gp180) receptor binding site, the peptide binds hepatocytes and subsequently blocks DHBV infection. Since its activity requires an amino acid sequence involved in host discrimination between DHBV and the related heron HBV (T. Ishikawa and D. Ganem, Proc. Natl. Acad. Sci. USA 92:6259-6263, 1995), we suggest that it is related to the postulated host-discriminating cofactor of infection.

Envelope protein-mediated down-regulation of hepatitis B virus receptor in infected hepatocytes

Entry of duck hepatitis B virus (DHBV) is initiated by specific interaction of its large envelope protein (L) with a cellular entry receptor, recently identified as carboxypeptidase D (CPD; historically gp180). In this report, we present evidence demonstrating that this receptor is down-regulated as a result of DHBV infection: (i) receptor levels determined by Western blot were much reduced in DHBV-infected duck livers and undetectable by immunostaining in infected cultured hepatocytes; (ii) results from metabolic labeling experiments indicate enhanced receptor protein turnover; (iii) the kinetics of receptor loss from newly infected cells correlated with the accumulation of newly synthesized viral protein; (iv) expression of DHBV L protein, transduced from a recombinant adenovirus, was sufficient to eliminate gp180/CPD from the Golgi compartment, its normal predominant location; (v) gp180/CPD remained absent from the Golgi compartment in infected hepatocytes, even after overexpression from a recombinant adenovirus, while residual amounts subsequently became detectable in a perinuclear compartment, containing DHBV L protein; (vi) expression of DHBV L protein in a HepG2 cell line, stably expressing gp180/CPD, leads to incomplete receptor maturation and induces its degradation. Taken together, these data are consistent with a model in which the virus receptor interacts early in the biosynthetic pathway with the viral L protein, leading to its retention in a pre-Golgi compartment and to subsequent degradation, thus preventing receptor interference with the export of DHBV via the secretory pathway which it shares with its receptor. Accordingly, and analogously with receptor down-regulation in retroviral systems, DHBV receptor down-modulation may account for the much-reduced efficiency of DHBV superinfection of preinfected hepatocytes.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Receptor recognition by a hepatitis B virus reveals a novel mode of high affinity virus-receptor interaction

The duck hepatitis B virus model system was used to elucidate the characteristics of receptor (carboxypeptidase D, gp180) interaction with polypeptides representing the receptor binding site in the preS part of the large viral surface protein. We demonstrate the pivotal role of carboxypeptidase D for virus entry and show its C-domain represents the virus attachment site, which binds preS with extraordinary affinity. Combining results from surface plasmon resonance spectroscopy and two-dimensional NMR analysis we resolved the contribution of preS sequence elements to complex stability and show that receptor binding potentially occurs in two steps. Initially, a short alpha-helix in the C-terminus of the receptor binding domain facilitates formation of a primary complex. This complex is stabilized sequentially, involving approximately 60 most randomly structured amino acids preceding the helix. Thus, hepadnaviruses exhibit a novel mechanism of high affinity receptor interaction by conserving the potential to adapt structure during binding rather than to preserve it per se. We propose that this process represents an alternative strategy to escape immune surveillance and the evolutionary pressure inherent in the compact hepadnaviral genome organization.

Cellular receptor traffic is essential for productive duck hepatitis B virus infection

We have investigated the mechanism of duck hepatitis B virus (DHBV) entry into susceptible primary duck hepatocytes (PDHs), using mutants of carboxypeptidase D (gp180), a transmembrane protein shown to act as the primary cellular receptor for avian hepatitis B virus uptake. The variant proteins were abundantly produced from recombinant adenoviruses and tested for the potential to functionally outcompete the endogenous wild-type receptor. Overexpression of wild-type gp180 significantly enhanced the efficiency of DHBV infection in PDHs but did not affect ongoing DHBV replication, an observation further supporting gp180 receptor function. A gp180 mutant deficient for endocytosis abolished DHBV infection, indicating endocytosis to be the route of hepadnaviral entry. With further gp180 variants, carrying mutations in the cytoplasmic domain and characterized by an accelerated turnover, the ability of gp180 to function as a DHBV receptor was found to depend on a wild-type-like sorting phenotype which largely avoids transport toward the endolysosomal compartment. Based on these data, we propose a model in which a distinct intracellular DHBV traffic to the endosome, but not beyond, is a prerequisite for completion of viral entry, i.e., for fusion and capsid release. Furthermore, the deletion of the two enzymatically active carboxypeptidase domains of gp180 did not lead to a loss of receptor function.

Avian hepatitis B virus infection is initiated by the interaction of a distinct pre-S subdomain with the cellular receptor gp180

Functionally relevant hepadnavirus-cell surface interactions were investigated with the duck hepatitis B virus (DHBV) animal model by using an in vitro infection competition assay. Recombinant DHBV pre-S polypeptides, produced in Escherichia coli, were shown to inhibit DHBV infection in a dose-dependent manner, indicating that monomeric pre-S chains were capable of interfering with virus-receptor interaction. Particle-associated pre-S was, however, 30-fold more active, suggesting that cooperative interactions enhance particle binding. An 85-amino-acid pre-S sequence, spanning about half of the DHBV pre-S chain, was characterized by deletion analysis as essential for maximal inhibition. Pre-S polypeptides from heron hepatitis B virus (HHBV) competed DHBV infection equally well despite a 50% difference in amino acid sequence and a much-reduced infectivity of HHBV for duck hepatocytes. These observations are taken to indicate (i) that the functionality of the DHBV pre-S subdomain, which interacts with the cellular receptor, is determined predominantly by a defined three-dimensional structure rather than by primary sequence elements; (ii) that cellular uptake of hepadnaviruses is a multistep process involving more than a single cellular receptor component; and (iii) that gp180, a cellular receptor candidate unable to discriminate between DHBV and HHBV, is a common component of the cellular receptor complex for avian hepadnaviruses.

Carboxypeptidase D (gp180), a Golgi-resident protein, functions in the attachment and entry of avian hepatitis B viruses

Carboxypeptidase D (gp180), one of many candidate receptors proposed for hepatitis B viruses (HBVs), was examined and found to be the actual cellular receptor for avian HBVs. This conclusion was based on the following observations: (i) gp180 was the only host protein that bound with high affinity to the pre-S ectodomain of the large duck hepatitis B virus (DHBV) envelope protein, which is known to be essential for virus infection; (ii) a pre-S subdomain which determines physical binding to gp180 was found to coincide with a domain functionally defined in infection competition experiments as a receptor binding domain; (iii) soluble gp180, lacking the membrane anchor, efficiently inhibited DHBV infection; (iv) efficient interspecies gp180-pre-S interaction was limited to the natural hosts of avian hepadnaviruses; and (v) expression of gp180 in a heterologous hepatoma cell line mediated cellular attachment and subsequent internalization of fluorescently labeled viral particles into vesicular structures. However, gp180 expression did not render transfected heterologous cells permissive for productive infection, suggesting that a species-specific coreceptor is required for fusion to complete viral entry. In contrast to the case for known virus receptors, gp180 was not detected on the hepatocyte cell surface but was found to be concentrated in the Golgi apparatus, from where it functions by cycling to and from the plasma membrane.