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

Characterization of Host Cell Potential Proteins Interacting with OsHV-1 Membrane Proteins

The interaction between viral membrane associate proteins and host cellular surface molecules should facilitate the attachment and entry of OsHV-1 into host cells. Thus, blocking the putative membrane proteins ORF25 and ORF72 of OsHV-1 with antibodies that have previously been reported to subdue OsHV-1 replication in host cells, especially ORF25. In this study, prey proteins in host hemocytes were screened by pull-down assay with recombinant baits ORF25 and ORF72, respectively. Gene Ontology (GO) analysis of these prey proteins revealed that most of them were mainly associated with binding, structural molecule activity and transport activity in the molecular function category. The protein–protein interaction (PPI) network of the prey proteins was constructed by STRING and clustered via K-means. For both ORF25 and ORF72, three clusters of these prey proteins were distinguished that were mainly associated with cytoskeleton assembly, energy metabolism and nucleic acid processing. ORF25 tended to function in synergy with actins, while ORF72 functioned mainly with tubulins. The above results suggest that these two putative membrane proteins, ORF25 and ORF72, might serve a role in the transport of viral particles with the aid of a cytoskeleton inside cells.

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

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

Natural resistance-associated macrophage protein is a cellular receptor for sindbis virus in both insect and mammalian hosts

Alphaviruses, including several emerging human pathogens, are a large family of mosquito-borne viruses with Sindbis virus being a prototypical member of the genus. The host factor requirements and receptors for entry of this class of viruses remain obscure. Using a Drosophila system, we identified the divalent metal ion transporter natural resistance-associated macrophage protein (NRAMP) as a host cell surface molecule required for Sindbis virus binding and entry into Drosophila cells. Consequently, flies mutant for dNRAMP were protected from virus infection. NRAMP2, the ubiquitously expressed vertebrate homolog, mediated binding and infection of Sindbis virus into mammalian cells, and murine cells deficient for NRAMP2 were nonpermissive to infection. Alphavirus glycoprotein chimeras demonstrated that the requirement for NRAMP2 is at the level of Sindbis virus entry. Given the conserved structure of alphavirus glycoproteins, and the widespread use of transporters for viral entry, other alphaviruses may use conserved multipass membrane proteins for infection.

Initiation of duck hepatitis B virus infection requires cleavage by a furin-like protease

The entry mechanism of hepatitis B virus (HBV) has not been defined, and this impedes development of antiviral therapies aimed at an early step in the viral life cycle. HBV infection has both host and tissue specificities. For the related duck hepatitis B virus (DHBV), duck carboxypeptidase D (DCPD) has been proposed as the species-specific docking receptor, while glycine decarboxylase (DGD) may serve as a tissue-specific cofactor or secondary receptor. DGD binds to several truncated versions of the viral large envelope protein but not to the full-length protein, suggesting a need for proteolytic cleavage of the envelope protein by a furin-like proprotein convertase. In the present study, we found that transfected DCPD could confer DHBV binding to non-duck cell lines but that this was followed by rapid virus release from cells. Coexpression of furin led to DCPD cleavage and increased virus retention. Treatment of DHBV particles with endosome prepared from duck liver led to cleavage of the large envelope protein, and such viral preparation could generate a small amount of covalently closed circular DNA in LMH cells, a chicken hepatoma cell line resistant to DHBV infection. A furin inhibitor composed of decanoyl-RVKR-chloromethylketone blocked endosomal cleavage of the large envelope protein in vitro and suppressed DHBV infection of primary duck hepatocytes in vivo. These findings suggest that furin or a furin-like proprotein convertase facilitates DHBV infection by cleaving both the docking receptor and the viral large envelope protein.

Enveloped viral fusion: insights into the fusion of hepatitis B viruses

Viral fusion, the mechanism by which viruses gain entry into the host cell, is a key step in the replication cycle and an important new target in antiviral therapy and vaccine strategies owing to the conservation of the envelope domains involved and their resistance to immune pressure. The fusion domains of HIV-1 have been studied intensively resulting in the potent antiviral agent T20 and the identification of broadly neutralizing antibody epitopes for vaccine development. Another chronic disease-causing virus, HBV, requires the identification of new antiviral agents to deal with the disease burden of 350 million chronically-infected individuals worldwide, 20% of whom will develop liver cancer. The aim of this review is to bring together basic knowledge on the envelope signatures, mechanisms and strategies for the study of viral fusion and how that knowledge has been applied to the study of hepadnaviral fusion.

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.

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.

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.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

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.

Glycoprotein D receptor-dependent, low-pH-independent endocytic entry of herpes simplex virus type 1

Two herpes simplex virus type 1 (HSV-1) entry pathways have been described: direct fusion between the virion envelope and the plasma membrane, as seen on Vero cells, and low-pH-dependent endocytosis, as seen on CHO nectin-1 and HeLa cells. In this paper, we studied HSV entry into C10 murine melanoma cells and identified a third entry pathway for this virus. During entry into C10 cells, virion envelope glycoproteins rapidly became protected from the membrane-impermeable chemical cross-linker BS3 and from proteinase K. Protection was gD receptor dependent, and the time taken to detect protected protein was proportional to the rate of virus entry. Ultrastructural examination revealed that virions attached to the surface of C10 cells were localized to membrane invaginations, whereas those on the surface of receptor-negative B78 cells were peripherally attached. Virus entry into C10 cells was energy dependent, and intracellular enveloped virions were seen within membrane-bound vesicles consistent with endocytic entry. Entry was not inhibited by bafilomycin A1 or ammonium chloride, showing that passage of the virion through a low-pH environment was not required for infection. Resistance to similar reagents should therefore not be taken as proof of HSV entry by a nonendosomal pathway. These data define a novel gD receptor-dependent acid-independent endocytic entry pathway for HSV.

NACA as a potential cellular target of hepatitis B virus preS1 protein

The mechanisms of the attachment and penetration of hepatitis B virus remain obscure. It has been demonstrated that the preS1 region is essential for viral assembly and infectivity, however, as its cellular receptor has still not been identified unequivocally, we used a yeast two-hybrid system to screen the cellular proteins that can interact with preS1 protein. The protein recovered from a human liver cDNA library was nascent polypeptide-associated complex alpha polypeptide. The interaction between preS1 and nascent polypeptide-associated complex alpha polypeptide was verified by mating experiment and coimmunoprecipitation of COS7 cell lysates expressing both proteins. Based on these results, we speculate that nascent polypeptide-associated complex alpha polypeptide is a functional target of hepatitis B virus preS1 protein in cells.

Superinfection exclusion in duck hepatitis B virus infection is mediated by the large surface antigen

Superinfection exclusion is the phenomenon whereby a virus prevents the subsequent infection of an already infected host cell. The Pekin duck hepatitis B virus (DHBV) model was used to investigate superinfection exclusion in hepadnavirus infections. Superinfection exclusion was shown to occur both in vivo and in vitro with a genetically marked DHBV, DHBV-ClaI, which was unable to establish an infection in either DHBV-infected ducklings or DHBV-infected primary duck hepatocytes (PDHs). In addition, exclusion occurred in vivo even when the second virus had a replicative advantage. Superinfection exclusion appears to be restricted to DHBV, as adenovirus, herpes simplex virus type 1, and vesicular stomatitis virus were all capable of efficiently infecting DHBV-infected PDHs. Exclusion was dependent on gene expression by the original infecting virus, since UV-irradiated DHBV was unable to mediate the exclusion of DHBV-ClaI. Using recombinant adenoviruses expressing DHBV proteins, we determined that the large surface antigen mediated exclusion. The large surface antigen is known to cause down-regulation of a DHBV receptor, carboxypeptidase D (CPD). Receptor down-regulation is a mechanism of superinfection exclusion seen in other viral infections, and so it was investigated as a possible mechanism of DHBV-mediated exclusion. However, a mutant large surface antigen which did not down-regulate CPD was still capable of inhibiting DHBV infection of PDHs. In addition, exclusion of DHBV-ClaI did not correlate with a decrease in CPD levels. Finally, virus binding assays and confocal microscopy analysis of infected PDHs indicated that the block in infection occurs after internalization of the second virus. We suggest that superinfection exclusion may result from the role of the L surface antigen as a regulator of intracellular trafficking.

Study of the early steps of the Hepatitis B Virus life cycle

Hepatitis B virus (HBV) is a human pathogen, causing the serious liver disease. Despite considerable advances in the understanding of the natural history of HBV disease, most of the early steps in the virus life cycle remain unclear. Virus attachment to permissive cells, fusion and penetration through cell membranes and subsequent genome release, are largely a mystery. Current knowledge on the early steps of HBV life cycle has mostly come from molecular cloning, expression of individual genes and studies of the infection of duck hepatitis B virus (DHBV) with duck primary duck hepatocytes. However, considering of the difference of the surface protein of HBV and DHBV both in the composition and sequence, the degree to which information from DHBV applies to human HBV attachment and entry may be limited. A major obstacle to the study HBV infection is the lack of a reliable and sensitive in vitro infection system. We have found that the digestion of HBV and woodchuck hepatitis virus (WHBV) by protease V8 led to the infection of HepG2 cell, a cell line generally is refractory for their infection [Lu et al. J Virol. 1996. 70. 2277-2285 . Lu et al. Virus Research. 2001. 73(1): 27-4].. Further studies showed that a serine protease inhibitor Kazal (SPIK) was over expressed in the HepG2 cells. Therefore, it is possible that to silence the over expressed SPIK and thus to reinstate the activity of indispensable cellular proteases can result in the restoration of the susceptibility of HepG2 cells for HBV infection. The establishing a stable cell line for study of the early steps of HBV life cycle by silencing of SPIK is discussed.

Insider information: what viruses tell us about endocytosis

Viruses have long served as tools in molecular and cellular biology to study a variety of complex cellular processes. Currently, there is a revived interest in virus entry into animal cells because it is evident that incoming viruses make use of numerous endocytic pathways that are otherwise difficult to study. Besides the classical clathrin-mediated uptake route, viruses use caveolae-mediated endocytosis, lipid-raft-mediated endocytic pathways, and macropinocytosis. Some of these are subject to regulation, involve novel endocytic organelles, and some of them connect organelles that were previously not known to communicate by membrane traffic.

The earliest steps in hepatitis B virus infection

The early steps in hepatitis B virus (HBV) infection, a human hepadnavirus, initiates from cell attachment followed by entry and delivery of the genetic information to the nucleus. Despite the fact that these steps determine the virus-related pathogenesis, their molecular basis is poorly understood. Cumulative data suggest that this process can be divided to cell attachment, endocytosis, membrane fusion and post-fusion consecutive steps. These steps are likely to be regulated by the viral envelope proteins and by the cellular membrane, receptors and extracellular matrix. In the absence of animal model for HBV, the duck hepadnavirus DHBV turned out to be a fruitful animal model. Therefore data concerning the early, post-attachment steps in hepadnaviral entry are largely based on studies performed with DHBV in primary duck liver hepatocytes. These studies are now starting to illuminate the mechanisms of hepadnavirus route of cell entry and to provide some new insights on the molecular basis of the strict species specificity of hepadnavirus infection.

Interaction of hepatitis B virus with cells

Virus infection is initiated by recognition and attachment of the virus to the cell surface. Despite the fact that this interaction determines the virus-related pathogenesis, its molecular basis remained obscure for HBV. This process is mediated primarily by the viral envelope and the cellular receptors. HBV infection is not exceptional in this regard but its putative receptors have not been identified yet. The recent development of protocols to establish HBV susceptible cell lines and unique tools to measure HBV-cell attachment at a single cell resolution set the stage for the study of HBV-host cell interaction. These studies revealed that the QLDPAF epitope of the HBV surface antigen large protein (LHBsAg) plays a major role in this process. Quantitative measurements suggested the presence of a second player in this process and both act synergistically to improve cell attachment. As the step of virus-cell attachment is potentially susceptible to specific inhibitors, understanding the molecular basis of virus-cell attachment can be expected to have therapeutic impacts.

Hepatitis B virus large envelope protein interacts with gamma2-adaptin, a clathrin adaptor-related protein

For the outcome of a hepatitis B virus (HBV) infection, the viral L envelope protein with its pre-S domain performs pivotal functions by mediating attachment of HBV to liver cells, envelopment of viral capsids, release of (sub)viral particles, regulation of supercoiled DNA amplification, and transcriptional transactivation. To assess its multiple functions and host-protein assistance involved, we initiated a two-hybrid screen using the L-specific pre-S1 domain as bait. With this approach, we have identified gamma2-adaptin, a putative member of the clathrin adaptor proteins responsible for protein sorting and trafficking, as a specific binding partner of L protein. Evidence for a physical interaction between L protein and gamma2-adaptin was also demonstrated by affinity chromatography and coimmunoprecipitation, and the binding sites were mapped to the L-specific pre-S1 domain and the gamma2-adaptin-specific ear domain. The specificity of the interaction was further sustained by the failure of gamma1-adaptin, a closely related gamma2-adaptin homologue, to associate with L protein. Analysis of an L mutant protein indicates that the L-gamma2-adaptin interaction strictly depends on the pre-S1 domain of transmembrane L protein oriented to the cytosol and thus appears to occur in the cytosolic environment. Interestingly, coexpression of the two interacting partners in transfected cells resulted in recruitment of gamma2-adaptin by L protein onto cis-Golgi-like structures, strongly indicating that the association is physiologically relevant. Together, the results suggest a role for gamma2-adaptin in L-mediated processes of viral biogenesis and/or pathogenesis, such as facilitating and guiding HBV assembly.

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.

A metastable form of the large envelope protein of duck hepatitis B virus: low-pH release results in a transition to a hydrophobic, potentially fusogenic conformation

We have examined the structure and fusion potential of the duck hepatitis B virus (DHBV) envelope proteins by treating subviral particles with deforming agents known to release envelope proteins of viruses from a metastable to a fusion-active state. Exposure of DHBV particles to low pH triggered a major structural change in the large envelope protein (L), resulting in exposure of trypsin sites within its S domain but without affecting the same region in the small surface protein (S) subunits. This conformational change was associated with increased hydrophobicity of the particle surface, most likely arising from surface exposure of the hydrophobic first transmembrane domain (TM1). In the hydrophobic conformation, DHBV particles were able to bind to liposomes and intact cells, while in their absence these particles aggregated, resulting in viral inactivation. These results suggests that some L molecules are in a spring-loaded metastable state which, when released, exposes a previously hidden hydrophobic domain, a transition potentially representing the fusion-active state of the envelope.

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.