Hepatitis B virus maturation is affected by the incorporation of core proteins having a C-terminal substitution of arginine or lysine stretches

Testing an electrostatic interaction hypothesis of hepatitis B virus capsid stability by using an in vitro capsid disassembly/reassembly system

To test a previously coined "charge balance hypothesis" of human hepatitis B virus (HBV) capsid stability, we established an in vitro disassembly and reassembly system using bacterially expressed HBV capsids. Capsid disassembly can be induced by micrococcal nuclease digestion of encapsidated RNA. HBV core protein (HBc) mutants containing various amounts of arginine were constructed by serial truncations at the C terminus. Capsids containing smaller amounts of arginine (HBc 149, 154, and 157) remained intact after micrococcal nuclease digestion by native gel electrophoresis. Capsids containing larger amounts of arginine (HBc 159, 164, 169, and 171) exhibited reduced and more diffuse banding intensity and slightly upshifted mobility (HBc 159 and 164). Capsids containing the largest amounts of arginine (HBc 173, 175, and 183), as well as HBc 167, exhibited no detectable banding signal, indicating loss of capsid integrity or stability. Interestingly, capsid reassembly can be induced by polyanions, including oligonucleotides, poly-glutamic acid, and nonbiological polymer (polyacrylic acid). In contrast, polycations (polylysine and polyethylenimine) and low-molecular-weight anions (inositol triphosphate) induced no capsid reassembly. Results obtained by gel assay were confirmed by electron microscopy. Reassembled capsids comigrated with undigested parental capsids on agarose gels and cosedimented with undigested capsids by sucrose gradient ultracentrifugation. Taken together, the results indicate that HBV capsid assembly and integrity depend on polyanions, which probably can help minimize intersubunit charge repulsion caused mainly by arginine-rich domain III or IV in close contact. The exact structure of polyanions is not important for in vitro capsid reassembly. A large amount of independent experimental evidence for this newly coined "electrostatic interaction hypothesis" is discussed.

A novel system for efficient gene transfer into primary human hepatocytes via cell-permeable hepatitis B virus-like particle

Protein transduction domains (PTDs) have been used to deliver a variety of biologically active cargo across cellular membranes. However the potential of PTDs to mediate transport of nanoparticular structures into the cytoplasm bypassing the endosomal compartment remains unclear. Cell-permeable virus-like particles (VLPs) harboring a marker gene based on hepatitis B virus nucleocaspids were established. Cell permeability was achieved by fusion with translocation motif (TLM)-PTD. Electron and confocal microscopy revealed that these VLPs translocate as complete particles across the plasma membrane and transverse the cytoplasm toward the nucleus. Inhibition of endocytosis did not affect translocation of these VLPs into the cytoplasm. Based on these particles, a gene transfer system was developed. To this end the particles were loaded with DNA-encoding small hepatitis B virus surface antigen (SHBs) or green fluorescence protein (eGFP) that served as marker genes. Although the DNA-packaging efficiency was very low, applying the appropriate number of VLPs to primary human hepatocytes a gene transfer efficiency of approximately 95% was observed. In conclusion, the TLM-PTD has the potential to mediate efficient transfer of assembled particles and its cargo, nucleic acids, into primary human hepatocytes. This provides the basis for development of novel transducible therapeutic or diagnostic particles.

The alkylated imino sugar, n-(n-Nonyl)-deoxygalactonojirimycin, reduces the amount of hepatitis B virus nucleocapsid in tissue culture

n-(n-Nonyl)-deoxygalactonojirimycin (n,n-DGJ), an alkylated imino sugar, reduces the amount of HBV DNA produced within the stably transfected HBV-producing HepG2.2.15 line in culture and is under consideration for development as a human therapeutic. n,n-DGJ does not appear to inhibit HBV DNA polymerase activity or envelop antigen production (A. Mehta, S. Carrouee, B. Conyers, R. Jordan, T. Butters, R. A. Dwek, and T. M. Block, Hepatology 33:1488-1495, 2001), and the mechanism of antiviral action is unknown. In this study, the step in the virus life cycle affected by n,n-DGJ was explored. Using Northern analysis and immunoprecipitation with anti-HBc antibody, we found that, under conditions in which cell viability was not affected but viral DNA production was substantially reduced, neither the amount of HBV transcription products nor the core polypeptide was detectably reduced. However, the pregenomic RNA, endogenous polymerase activity, and core polypeptide sedimenting in sucrose gradients with a density consistent with that of assembled nucleocapsids were significantly less in the HepG2.2.15 cells incubated with n,n-DGJ. These data suggest that n,n-DGJ either prevents the maturation of HBV nucleocapsids or destabilizes the formed nucleocapsids. Although the cellular and viral mediators of this inhibition are not known, depletion of nucleocapsid has been attributed to some other compounds as well as interferon's mechanism of anti-HBV action. The similarities and differences between this alkylated imino sugar and these other mediators are discussed.

The role of inducible nitric oxide synthase in a murine acute hepatitis B virus (HBV) infection model induced by hydrodynamics-based in vivo transfection of HBV-DNA

BACKGROUND/AIMS

Inducible nitric oxide synthase (iNOS) is found to have antiviral activity. Its role is evaluated using a murine acute hepatitis B virus (HBV) infection model.

METHODS

pHBV3.6 plasmid containing HBV genome was injected into mice by hydrodynamics-based in vivo transfection. HBV antigenemia and serum HBV-DNA were detected by enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction, respectively. HBV replication in liver was analyzed by Northern and Southern blot. Intrahepatic leukocytes were isolated and analyzed with flow cytometry.

RESULTS

After hydrodynamics injection of pHBV3.6, HBV genome was synthesized in the liver and HBV-DNA, as well as hepatitis B surface antigen and hepatitis B e antigen were secreted into the blood. Anti-HBV antibody responses developed afterward. A murine acute HBV infection model was established with hydrodynamics injection of non-transponase based HBV-DNA. Using this protocol in iNOS deficient or wild type B6 mice, the level of HBV transcript, replicative intermediate, and antigenemia were higher in iNOS(-/-) than in B6 mice. The intrahepatic leukocytes in iNOS(-/-) mice were also affected after transfection.

CONCLUSIONS

Our data suggests that the iNOS expression not only affects the HBV clearance, but also modulates the infiltrating leukocytes response to HBV antigens.

Nuclear import of hepatitis B virus capsids and release of the viral genome

While studying the import of the hepatitis B virus genome into the nucleus of permeabilized tissue culture cells, we found that viral capsids were imported in intact form through the nuclear pore into the nuclear basket. Import depended on phosphorylation of the capsid protein and was mediated by the cellular transport receptors importin alpha and beta. Virus-derived capsids that contained the mature viral genome were able to release the viral DNA and capsid protein into the nucleoplasm. The uncoating reaction was independent of Ran, a GTP-binding enzyme responsible for dissociating other imported cargo from the inner face of the nuclear pore. Immature capsids that did not contain the mature viral genome reached the basket but did not release capsid proteins nor immature genomes into the nucleoplasm. The different fate of mature and immature capsids after passing the nuclear pore indicates that the outcome of a nuclear import event may be regulated within the nuclear basket.

Basic residues of the helix six domain of influenza virus M1 involved in nuclear translocation of M1 can be replaced by PTAP and YPDL late assembly domain motifs

Influenza type A virus matrix (M1) protein possesses multiple functional motifs in the helix 6 (H6) domain (amino acids 91 to 105), including nuclear localization signal (NLS) (101-RKLKR-105) involved in translocating M1 from the cytoplasm into the nucleus. To determine the role of the NLS motif in the influenza virus life cycle, we mutated these and the neighboring sequences by site-directed mutagenesis, and influenza virus mutants were generated by reverse genetics. Our results show that infectious viruses were rescued by reverse genetics from all single alanine mutations of amino acids in the H6 domain and the neighboring region except in three positions (K104A and R105A within the NLS motif and E106A in loop 6 outside the NLS motif). Among the rescued mutant viruses, R101A and R105K exhibited reduced growth and small-plaque morphology, and all other mutant viruses showed the wild-type phenotype. On the other hand, three single mutations (K104A, K105A, and E106A) and three double mutations (R101A/K102A, K104A/K105A, and K102A/R105A) failed to generate infectious virus. Deletion (Delta YRKL) or mutation (4A) of YRKL also abolished generation of infectious virus. However, replacement of the YRKL motif with PTAP or YPDL as well as insertion of PTAP after 4A mutation yielded infectious viruses with the wild-type phenotype. Furthermore, mutant M1 proteins (R101A/K102A, Delta YRKL, 4A, PTAP, 4A+PTAP, and YPDL) when expressed alone from cloned cDNAs were only cytoplasmic, whereas the wild-type M1 expressed alone was both nuclear and cytoplasmic as expected. These results show that the nuclear translocation function provided by the positively charged residues within the NLS motif does not play a critical role in influenza virus replication. Furthermore, these sequences of H6 domain can be replaced by late (L) domain motifs and therefore may provide a function similar to that of the L domains of other negative-strand RNA and retroviruses.

Identification of a cellular protein specifically interacting with the precursor of the hepatitis B e antigen

In hepatitis B virus (HBV) the precore gene encodes a protein from which derives P22, the precursor of the mature secreted hepatitis B virus e antigen (HBeAg). Circumstantial evidences suggest that HBeAg and/or its precursor P22 are important for establishing persistent infection. Although P22 is essentially present in the secretory pathway, a substantial fraction has been found in the cytosol. In order to get new insights into the biological function of P22, we looked for cellular proteins which could strongly associate with this protein. Using immunoprecipitation studies on human cell extracts, we found that a non-secreted cellular protein of about 32 kDa (P32) bound with a high specificity to P22. P32 associated neither with HBeAg nor with the viral core protein P21 which exhibits the same amino acids sequence as P22 but is N-terminally shorter by 10 residues. We also demonstrated that this interaction depended on the presence of the P22 C-terminal domain. Our data argues for a potential biological function of P22.

Hepatitis B viral core proteins with an N-terminal extension can assemble into core-like particles but cannot be enveloped

The structure of hepatitis B virus (HBV) nucleocapsids has been revealed in great detail by cryoelectron microscopy. How nucleocapsids interact with surface antigens to form enveloped virions remains unknown. In this study, core mutants with N-terminal additions were created to address two questions: (1) can these mutant core proteins still form nucleocapsids and (2) if so, can the mutant nucleocapsids interact with surface antigens to form virion-like particles. One plasmid encoding an extra stretch of 23 aa, including six histidine residues, fused to the N terminus of the core protein (designated HisC183) was expressed in Escherichia coli and detected by Western blot. CsCl gradient and electron microscopy analyses indicated that HisC183 could self-assemble into nucleocapsids. When HisC183 or another similar N-terminal fusion core protein (designated FlagC183) was co-expressed with a core-negative plasmid in human hepatoma cells, both mutant core proteins self-assembled into nucleocapsids. These particles also retained kinase activity. Using an endogenous polymerase assay, a fill-in HBV DNA labelled with isotope was obtained from intracellular nucleocapsids formed by mutant cores. In contrast, no such signal was detected from the transfection medium, which was consistent with PCR and Southern blot analyses. Results indicate that core mutants with N-terminal extensions can form nucleocapsids, but are blocked during the envelopment process and cannot form secreted virions. The mutant nucleocapsids generated from this work should facilitate further study on how nucleocapsids interact with surface antigens.