Infectious cDNA clone of the hepatitis C virus genotype 1 prototype sequence

A full-length cDNA clone of the hepatitis C virus (HCV) genotype 1 prototype (subtype 1a) sequence was constructed. Synthetic RNA produced from the initial cDNA clone was not infectious following intrahepatic inoculation of a chimpanzee. A consensus clone was prepared by comparison with multiple full-length HCV sequences of genotypes 1, 2 and 3. A total of 11 non-consensus amino acid residues were altered by mutagenesis. Synthetic RNA from the repaired clone initiated a typical, acute-resolving HCV infection following intrahepatic inoculation of a chimpanzee. In addition, at least one of three chimeric cDNA clones constructed between the HCV-1 and H77 genotype 1a strains of HCV was infectious in a chimpanzee. This is the first example of an infectious chimeric HCV clone. An infectious cDNA clone of HCV-1 will be of particular value, since it is the prototype HCV sequence and many commonly used reagents are based on this sequence.

Protective immune response to hepatitis C virus in chimpanzees rechallenged following clearance of primary infection

Hepatitis C virus (HCV) infections were evaluated in chimpanzees that had previously cleared HCV and were rechallenged. Animals that had previously cleared HCV infection rapidly cleared homologous and heterologous virus upon rechallenge, indicative of a strong protective immunity. In one animal, sterilizing immunity was observed with regard to viremia, although viral RNA was transiently detected in the liver. Accelerated viral clearance following rechallenge with HCV was observed in animals that had not been exposed to HCV for over 16 years, suggesting that long-lasting protective immunity may be possible. The ability of peripheral blood mononuclear cells (PBMC) to recognize HCV proteins was evaluated during the course of the rechallenge experiments. A very early and strong in vitro recall response to HCV nonstructural proteins appeared to be associated with viral clearance. In contrast, proliferative responses to HCV proteins were not observed in 4 persistently infected chimpanzees, and a weak proliferative response was observed in 1 of 2 animals during acute resolving infection. The results suggest that a strong T-cell proliferative response is induced upon rechallenge of chimpanzees with HCV and that this response is associated with rapid viral clearance. The antibody response to HCV proteins increased by over 1,000-fold in all animals following rechallenge as well. A more complete understanding of the role of the cellular immune response in the clearance of HCV and the nature of the protective immune response following viral clearance may aid in the generation of therapies and vaccines.

Interaction between hepatitis B virus core protein and reverse transcriptase

Previous mutagenesis studies with hepatitis B virus (HBV) suggest that continued interactions with core are required for several steps in genomic replication. To examine core-polymerase (Pol) interactions, insect cells were coinfected with baculovirus constructs that independently expressed core and Pol. The results demonstrated several features with implications that core plays an interactive role with HBV Pol: (i) core coprecipitated with constructs expressing full-length Pol as well as the terminal protein (TP), reverse transcriptase (RT) and RNase H domains of Pol, independently; (ii) coprecipitation of core was not dependent on the presence of an epsilon stem-loop sequence; and (iii) core-Pol complexes migrated as intact capsid particles, as detected by sucrose gradient analysis. To analyze the structural and sequence requirements of core in recognition of Pol, a series of core mutants with two- to four-amino-acid insertions or carboxy-terminal deletions were assessed for Pol interaction. The results indicated that capsid formation is required but not sufficient for interaction with Pol and that the TP and RT domains of Pol have different requirements for interaction with core. To map the core binding sites on Pol, a panel of amino- and carboxy-terminal deletion mutants of the TP and RT domains of Pol were analyzed for interaction with core. At least three separate core binding sites on Pol were detected. This analysis begins to define basic requirements for core-Pol interactions, but further study is necessary to delineate the effects of these interactions on encapsidation and genome replication.

Development of a primary tamarin hepatocyte culture system for GB virus-B: a surrogate model for hepatitis C virus

GB virus-B (GBV-B) causes an acute hepatitis in tamarins characterized by increased alanine transaminase levels that quickly return to normal as the virus is cleared. Phylogenetically, GBV-B is the closest relative to hepatitis C virus (HCV), and thus GBV-B infection of tamarins represents a powerful surrogate model system for the study of HCV. In this study, the course of infection of GBV-B in tamarins was followed using a real-time 5' exonuclease (TaqMan) reverse transcription-PCR assay to determine the level of GBV-B in the serum. Peak viremia levels exceeded 10(9) genome equivalents/ml, followed by viral clearance within 14 to 16 weeks. Rechallenge of animals that had cleared infection resulted in viremia that was limited to 1 week, suggestive of a strong protective immune response. A robust tissue culture system for GBV-B was developed using primary cultures of tamarin hepatocytes. Hepatocytes obtained from a GBV-B-infected animal maintained high levels of cell-associated viral RNA and virion secretion for 42 days of culture. In vitro infection of normal hepatocytes resulted in rapid amplification of cell-associated viral RNA and secretion of up to 10(7) genome equivalents/ml of culture supernatant. In addition, infection could be monitored by immunofluorescence staining for GBV-B nonstructural NS3 protein. This model system overcomes many of the current obstacles to HCV research, including low levels of viral replication, lack of a small primate animal model, and lack of a reproducible tissue culture system.

Human monoclonal antibodies that inhibit binding of hepatitis C virus E2 protein to CD81 and recognize conserved conformational epitopes

The intrinsic variability of hepatitis C virus (HCV) envelope proteins E1 and E2 complicates the identification of protective antibodies. In an attempt to identify antibodies to E2 proteins from divergent HCV isolates, we produced HCV E2 recombinant proteins from individuals infected with HCV genotypes 1a, 1b, 2a, and 2b. These proteins were then used to characterize 10 human monoclonal antibodies (HMAbs) produced from peripheral B cells isolated from an individual infected with HCV genotype 1b. Nine of the antibodies recognize conformational epitopes within HCV E2. Six HMAbs identify epitopes shared among HCV genotypes 1a, 1b, 2a, and 2b. Six, including five broadly reactive HMAbs, could inhibit binding of HCV E2 of genotypes 1a, 1b, 2a, and 2b to human CD81 when E2 and the antibody were simultaneously exposed to CD81. Surprisingly, all of the antibodies that inhibited the binding of E2 to CD81 retained the ability to recognize preformed CD81-E2 complexes generated with some of the same recombinant E2 proteins. Two antibodies that did not recognize preformed complexes of HCV 1a E2 and CD81 also inhibited binding of HCV 1a virions to CD81. Thus, HCV-infected individuals can produce antibodies that recognize conserved conformational epitopes and inhibit the binding of HCV to CD81. The inhibition is mediated via antibody binding to epitopes outside of the CD81 binding site in E2, possibly by preventing conformational changes in E2 that are required for CD81 binding.

Hypervariable region 1 sequence stability during hepatitis C virus replication in chimpanzees

The putative envelope 2 (E2) gene of hepatitis C virus (HCV) contains a highly variable region referred to as hypervariable region 1 (HVR1). We hypothesized that this genetic variability is driven by immune selection pressure, rather than representing the accumulation of random mutations in a region with relatively little functional constraint. To test this hypothesis, we examined the E2 sequence of a human inoculum that was passaged through eight chimpanzees, which appear to have a replicative rate (opportunity for chance mutation) similar to that of humans. Acute-phase plasma samples from a human (the inoculum) and six of eight serially infected chimpanzees were studied. For each, 33 cloned cDNAs were examined by a combined heteroduplex-single-stranded conformational polymorphism assay to assess quasispecies complexity and optimize selection of clones with unique gel shift patterns (clonotypes) for sequencing. The sequence diversity of HCV was significantly lower in the chimpanzees than in the humans, and during eight serial passages there was no change in the sequence of the majority clonotype from each animal examined. Similarly, the rates of protein sequence altering (nonsynonymous) substitution were lower in the chimpanzees than in the humans. These findings demonstrate that nonsynonymous mutations indicate selection pressure rather than being an incidental result of HCV replication.

Hepadnavirus infection in captive gibbons

The recent isolation of a nonhuman primate hepadnavirus from woolly monkeys prompted an examination of other primates for potentially new hepadnaviruses. A serological analysis of 30 captive gibbons revealed that 47% were positive for at least one marker of ongoing or previous infection with a hepatitis B virus (HBV). The amino acid sequences of the core and surface genes of human and gibbon virus isolates were very similar. Phylogenetic analysis indicated that the gibbon isolates lie within the human HBV family, indicating that these HBV isolates most likely stem from infection of gibbons from a human source.

Molecular quantification of cell cycle-related gene expression at the protein level

BACKGROUND

Immunofluorescence cytometry of antigen and DNA content provides relative measurements of the cell cycle phase distribution of a specific epitope. Measurement of correlated expression of epitopes on signaling and regulatory proteins will be useful in the study of the complex pathways involved in cell cycle regulation and carcinogenesis. However, to formulate regulatory pathway models, measurements of molecules per cell would be more useful than relative measurements of intensity. Here, we report on a system in which the relationship between molecules and fluorescence is determined for a reference set of cell lines that are then used to directly calculate the number of molecules for unknowns. To demonstrate the process, we calculated the cell cycle phase distribution of SV40 large T antigen (Tag) in the reference cells.

METHODS

A set of cell line clones expressing different levels of Tag were isolated. Quantitative Western blots of these cells and purified, recombinant Tag were performed. Cells from the same sample were stained and analyzed by flow cytometry for Tag and DNA. The relationship between molecules and fluorescence was established and calculations were performed for the phase distributions of Tag.

RESULTS

The five cell lines had 0.11, 0.27, 1.06, 2.44, and 2.63 x 10(6) molecules of Tag per cell, determined by Western blot. The average coefficient of variation was 10.6%. The relationship of molecules to fluorescence fit a linear equation (r(2) = 0.96) over the range, 0.11 - 2.63 x 10(6) molecules, however, the same equation did not fit the relationship between 0 molecules, defined by isotype staining controls, and the lowest expressing cell line. To calculate the phase distributions of molecules in the lowest cell line, a second linear equation from 0 to 110,000 molecules was used.

CONCLUSIONS

This work describes a system where fixed cells expressing various levels of a target antigen quantified by Western blots can be used to standardize flow cytometric measurements of gene expression in absolute terms.

Effects of iron loading on pathogenicity in hepatitis C virus-infected chimpanzees

Elevated iron levels have been associated with raised serum alanine transaminase (ALT) levels in hepatitis C virus (HCV)-infected humans. However, it is not clear if HCV infection causes increased iron accumulation by the liver or if the severity of HCV infection is actually worsened by higher iron levels in the host. To better understand the relationship between iron and persistent HCV infections, we examined the effect of excess dietary iron on disease severity in HCV-infected chimpanzees. Iron was supplemented in the diets of four HCV-infected and two uninfected chimpanzees for 29 weeks to achieve iron loading. Iron loading was confirmed by increases in serum iron levels, percentages of transferrin saturation, ferritin levels, elevations in hepatic iron concentration (HIC), and by histological examination. The majority of HCV-infected chimpanzees had higher iron levels before iron feeding than the uninfected animals. Although various degrees of iron loading occurred in all chimpanzees, HCV-infected animals exhibited increased loading in comparison with uninfected animals. The effects of iron loading on HCV disease expression was determined by comparing disease parameters during an extended baseline period before iron loading with the period during iron loading and immediately following iron loading. Iron loading did not influence the viral load, but did exacerbate liver injury in HCV-infected chimpanzees, as evidenced by elevated ALT and histological changes. Because all chimpanzees on high iron diets experienced iron loading, but pathological effects were only observed in HCV-infected chimpanzees, HCV infection appears to increase the susceptibility of the liver to injury following iron loading. These results confirm and extend previous observations made in human populations and serve to further validate the chimpanzee model of chronic hepatitis C.

Properties of monoclonal antibodies directed against hepatitis B virus polymerase protein

Hepadnavirus polymerases are multifunctional enzymes that play critical roles during the viral life cycle but have been difficult to study due to a lack of a well-defined panel of monoclonal antibodies (MAbs). We have used recombinant human hepatitis B virus (HBV) polymerase (Pol) expressed in and purified from baculovirus-infected insect cells to generate a panel of six MAbs directed against HBV Pol protein. Such MAbs were subsequently characterized with respect to their isotypes and functions in analytical and preparative assays. Using these MAbs as probes together with various deletion mutants of Pol expressed in insect cells, we mapped the B-cell epitopes of Pol recognized by these MAbs to amino acids (aa) 8 to 20 and 20 to 30 in the terminal protein (TP) region of Pol, to aa 225 to 250 in the spacer region, and to aa 800 to 832 in the RNase H domain. Confocal microscopy and immunocytochemical studies using various Pol-specific MAbs revealed that the protein itself appears to be exclusively localized to the cytoplasm. Finally, MAbs specific for the TP domain, but not MAbs specific for the spacer or RNase H regions of Pol, appeared to inhibit Pol function in the in vitro priming assay, suggesting that antibody-mediated interference with TP may now be assessed in the context of HBV replication.

Generation of transmissible hepatitis C virions from a molecular clone in chimpanzees

Multiple alignments of hepatitis C virus (HCV) polyproteins from six different genotypes identified a total of 22 nonconsensus mutations in a clone derived from the Hutchinson (H77) isolate. These mutations, collectively, may have contributed to the failure in generating a "functionally correct" or "infectious" clone in earlier attempts. A consensus clone was constructed after systematic repair of these mutations, which yielded infectious virions in a chimpanzee after direct intrahepatic inoculation of in vitro transcribed RNAs. This RNA-infected chimpanzee has developed hepatitis and remained HCV positive for more than 11 months. To further verify this RNA-derived infectivity, a second naive chimpanzee was injected intravenously with serum collected from the first chimpanzee. Infectivity analysis of the second chimpanzee demonstrated that the HCV infection was successfully transmitted, which validated unequivocally the infectivity of our repaired molecular clone. Amino acid sequence comparisons revealed that our repaired infectious clone had 4 mismatches with the isogenic clone reported by Kolykhalov et al. (1997, Science 277, 570-574) and 8 mismatches with that reported by Yanagi et al. (1997, Proc. Natl. Acad. Sci. USA 94, 8738-8743). At the RNA level, more mismatches (43 and 67, respectively) were identified; most of them were synonymous substitutions. Further comparisons with 16 isolates from different genotypes demonstrated that our repaired clone shares greater consensus than the reported isogenic clones. This approach of generating infectious HCV RNA validates the importance of amino acid sequence consensus in relation to the biology of HCV.

Mapping of the hepatitis B virus reverse transcriptase TP and RT domains by transcomplementation for nucleotide priming and by protein-protein interaction

Hepadnavirus polymerases initiate reverse transcription in a protein-primed reaction. We previously described a complementation assay for analysis of the roles of the TP and RT domains of HBV reverse transcriptase (pol) in the priming reaction. Independently expressed TP and RT domains form a complex functional for in vitro priming reactions. To map the minimal functional TP and RT domains, we prepared baculoviruses expressing amino- and carboxyl-terminal deletions of both the TP and RT domains and analyzed the proteins for the ability to participate in transcomplementation for the priming reaction. The minimal TP domain spanned amino acids 20 to 175; however, very little activity was observed without a TP domain spanning amino acids 1 to 199. The minimal RT domain spanned amino acids 300 to 775; however, little activity was observed unless the carboxyl end of the RT domain extended to amino acid 800. Thus, most of the RNase H domain was required. In previous studies, we observed a TP inhibitory domain between amino acids 199 and 344. The current analysis narrowed this domain to residues 300 to 334, which is a portion of the minimal RT domain. In addition, the ability of TP and RT deletion mutants to form stable TP-RT complexes was examined in coimmunoprecipitation assays. The minimal TP and RT domains capable of protein-protein interaction were considerably smaller than the domains required for functional interaction in the transcomplementation assays, and unlike priming activity, TP-RT interaction did not require the epsilon RNA stem-loop. These studies help to further define the complex protein-protein interactions required in HBV genome replication.

Presecretory degradation of apolipoprotein [a] is mediated by the proteasome pathway

Plasma levels of atherogenic lipoprotein [a] (Lp[a]) vary over a 1000-fold range and are largely determined by the gene for its unique glycoprotein, apolipoprotein [a] (apo[a]). The apo[a] locus comprises more than 100 alleles, encoding proteins from <300 to >800 kDa. Using primary baboon hepatocyte cultures, we previously demonstrated that differences in the secretion efficiency of apo[a] allelic variants contribute to the variation in plasma Lp[a] levels. In the current study, we investigated the mechanism of apo[a] presecretory degradation. The proteasome inhibitors, acetyl-leucyl-leucyl-norleucinal and lactacystin, prevented apo[a] degradation and increased apo[a] secretion. Transfection with an HA-tagged ubiquitin construct demonstrated the accumulation of ubiquitinated apo[a] in the presence of lactacystin. These results suggest a role for the cytoplasmic proteasome in apo[a] proteolysis. Apo[a] that accumulated intracellularly in the presence of lactacystin remained sensitive to endo-B-N-glucosaminidase H, and apo[a] degradation was reversibly inhibited by brefeldin A, suggesting that transport to a post-endoplasmic reticulum (ER) pre-medial Golgi compartment is required for apo[a] degradation. Newly synthesized apo[a] bound to the ER chaperone calnexin and conditions that enhanced this interaction prevented apo[a] degradation, suggesting that calnexin can protect apo[a] from proteolysis. These studies provide further support for the role of the proteasome in endoplasmic reticulum quality control, and expand this role to one that influences plasma levels of the atherogenic lipoprotein Lp[a].-White, A. L., B. Guerra, J. Wang, and R. E. Lanford. Presecretory degradation of apolipoprotein[a] is mediated by the proteasome pathway.

Viral persistence, antibody to E1 and E2, and hypervariable region 1 sequence stability in hepatitis C virus-inoculated chimpanzees

The relationship of viral persistence, the immune response to hepatitis C virus (HCV) envelope proteins, and envelope sequence variability was examined in chimpanzees. Antibody reactivity to the HCV envelope proteins E1 or E2 was detected by enzyme-linked immunosorbent assay (ELISA) in more than 90% of a human serum panel. Although the ELISAs appeared to be sensitive indicators of HCV infection in human serum panels, the results of a cross-sectional study revealed that a low percentage of HCV-inoculated chimpanzees had detectable antibody to E1 (22%) and E2 (15%). Viral clearance, which was recognized in 28 (61%) of the chimpanzees, was not associated with an antibody response to E1 or E2. On the contrary, antibody to E2 was observed only in viremic chimpanzees. A longitudinal study of animals that cleared the viral infection or became chronically infected confirmed the low level of antibody to E1, E2, and the HVR-1. In 10 chronically infected animals, the sequence variation in the E2 hypervariable region (HVR-1) was minimal and did not coincide with antibody to E2 or to the HVR-1. In addition, low nucleotide and amino acid sequence variation was observed in the E1 and E2 regions from two chronically infected chimpanzees. These results suggest that mechanisms in addition to the emergence of HVR-1 antibody escape variants are involved in maintaining viral persistence. The significance of antibodies to E1 and E2 in the chimpanzee animal model is discussed.

Amplification of simian retroviral sequences from human recipients of baboon liver transplants

Investigations into the use of baboons as organ donors for human transplant recipients, a procedure called xenotransplantation, have raised the specter of transmitting baboon viruses to humans and possibly establishing new human infectious diseases. Retrospective analysis of tissues from two human transplant recipients with end-stage hepatic disease who died 70 and 27 days after the transplantation of baboon livers revealed the presence of two simian retroviruses of baboon origin, simian foamy virus (SFV) and baboon endogenous virus (BaEV), in multiple tissue compartments. The presence of baboon mitochondrial DNA was also detected in these same tissues, suggesting that xenogeneic "passenger leukocytes" harboring latent or active viral infections had migrated from the xenografts to distant sites within the human recipients. The persistence of SFV and BaEV in human recipients throughout the posttransplant period underscores the potential infectious risks associated with xenotransplantation.

Isolation of a hepadnavirus from the woolly monkey, a New World primate

Hepatitis B virus (HBV) infections are a major worldwide health problem with chronic infections leading to cirrhosis and liver cancer. Viruses related to human HBV have been isolated from birds and rodents, but despite efforts to find hepadnaviruses that infect species intermediate in evolution between rodents and humans, none have been described. We recently isolated a hepadnavirus from a woolly monkey (Lagothrix lagotricha) that was suffering from fulminant hepatitis. Phylogenetic analysis of the nucleotide sequences of the core and surface genes indicated that the virus was distinct from the human HBV family, and because it is basal (ancestral) to the human monophyletic group, it probably represents a progenitor of the human viruses. This virus was designated woolly monkey hepatitis B virus (WMHBV). Analysis of woolly monkey colonies at five zoos indicated that WMHBV infections occurred in most of the animals at the Louisville zoo but not at four other zoos in the United States. The host range of WMHBV was examined by inoculation of one chimpanzee and two black-handed spider monkeys (Ateles geoffroyi), the closest nonendangered relative of the woolly monkey. The data suggest that spider monkeys are susceptible to infection with WMHBV and that minimal replication was observed in a chimpanzee. Thus, we have isolated a hepadnavirus with a host intermediate between humans and rodents and establishes a new animal model for evaluation of antiviral therapies for treating HBV chronic infections.

Analysis of hepatitis C virus-inoculated chimpanzees reveals unexpected clinical profiles

The clinical course of hepatitis C virus (HCV) infections in a chimpanzee cohort was examined to better characterize the outcome of this valuable animal model. Results of a cross-sectional study revealed that a low percentage (39%) of HCV-inoculated chimpanzees were viremic based on reverse transcription (RT-PCR) analysis. A correlation was observed between viremia and the presence of anti-HCV antibodies. The pattern of antibodies was dissimilar among viremic chimpanzees and chimpanzees that cleared the virus. Viremic chimpanzees had a higher prevalence of antibody reactivity to NS3, NS4, and NS5. Since an unexpectedly low percentage of chimpanzees were persistently infected with HCV, a longitudinal analysis of the virological profile of a small panel of HCV-infected chimpanzees was performed to determine the kinetics of viral clearance and loss of antibody. This study also revealed that a low percentage (33%) of HCV-inoculated chimpanzees were persistently viremic. Analysis of serial bleeds from six HCV-infected animals revealed four different clinical profiles. Viral clearance with either gradual or rapid loss of anti-HCV antibody was observed in four animals within 5 months postinoculation. A chronic-carrier profile characterized by persistent HCV RNA and anti-HCV antibody was observed in two animals. One of these chimpanzees was RT-PCR positive, antibody negative for 5 years and thus represented a silent carrier. If extrapolated to the human population, these data would imply that a significant percentage of unrecognized HCV infections may occur and that silent carriers may represent potentially infectious blood donors.

In vitro infection and replication of hepatitis E virus in primary cynomolgus macaque hepatocytes

An in vitro model was developed to replicate hepatitis E virus (HEV) in normal primary cynomolgus macaque hepatocytes using a hormonally defined, serum-free medium formulation. Primary hepatocytes were infected in tissue culture following isolation by collagenase treatment of liver wedge biopsy material. Viral replication was monitored by a highly strand-specific reverse transcription-polymerase chain reaction (RT-PCR) assay, which could detect the positive- and negative-strands of HEV RNA independently in a sensitive and specific manner. Several infectious HEV (Burma strain) inocula were titered by this RT-PCR assay, and a minimum effective infectious dose was determined. Appearance of newly replicated virus was demonstrated by detection of both strands of HEV RNA in experimentally infected hepatocytes as well as the genomic positive-strand viral RNA in the culture medium. Infectivity of the virus particles present in the media was confirmed by serial passage and replication of the virus in culture. Using this in vitro infection system, a neutralization assay was developed to assess the ability of anti-HEV antibodies to block virus infection of liver cells. Results presented in this report represent the first in vitro demonstration of a neutralizing anti-HEV antibody directed against the ORF2-encoded putative capsid protein.

Transcomplementation of nucleotide priming and reverse transcription between independently expressed TP and RT domains of the hepatitis B virus reverse transcriptase

Hepadnavirus polymerases initiate reverse transcription in a protein-primed reaction that involves the covalent linkage of the first deoxyribonucleotide to the polymerase polypeptide. We recently expressed human hepatitis B virus (HBV) reverse transcriptase (pol) in insect cells by using the recombinant baculovirus system. The purified protein is active in nucleotide priming and reverse transcription reactions. In this report, we demonstrate that the tyrosine residue at amino acid number 63 within the TP (terminal protein) domain of the polymerase is the site of covalent linkage of the first nucleotide of minus-strand DNA. Analysis of pol polypeptides with mutations in the TP and RT (reverse transcriptase) domains indicated that both domains were required for in vitro nucleotide priming activity. Polymerase proteins with mutations in the TP and RT domains were not capable of complementing each other in the nucleotide priming reaction, suggesting that transcomplementation between full-length polypeptides was not possible. However, when the TP and RT domains were expressed as separate polypeptides, they formed a highly stable complex that was active in nucleotide priming and reverse transcription. The presence of an epsilon stem-loop dramatically increased the nucleotide priming activity in transcomplementation assays, even though full-length pol displayed similar activities in the absence and presence of epsilon. These data raise the possibility that in the transcomplementation assay, epsilon may play a role in the formation of a functional complex between TP and RT, rather than being required only as the template for nucleotide priming. The results indicate that using the baculovirus system, it is possible to dissect the protein-protein and protein-RNA interactions required for HBV genome replication.

Influence of allelic variation on apolipoprotein(a) folding in the endoplasmic reticulum

Plasma levels of lipoprotein(a) (Lp(a)) vary over 1000-fold between individuals and are determined by the gene for its unique apolipoprotein, apo(a), which has greater than 100 alleles. Using primary baboon hepatocyte cultures, we previously demonstrated that differences in the ability of apo(a) allelic variants to escape the endoplasmic reticulum (ER) are a major determinant of Lp(a) production rate. To examine the reason for these differences, the folding of newly synthesized apo(a) was analyzed in pulse-chase experiments. Samples were harvested in the presence of N-ethylmaleimide to preserve disulfide-bonded folding intermediates, and apo(a) was analyzed by immunoprecipitation and SDS-polyacrylamide gel electrophoresis. Apo(a) required a prolonged period (30-60 min) to reach its fully oxidized form. Multiple folding intermediates were resolved, including a disulfide-linked, apo(a)-containing complex. Unexpectedly, all allelic variants examined showed similar patterns and kinetics of folding. Even "null" apo(a) proteins, which are unable to exit the ER, appeared to fold normally. The ER glucosidase inhibitor, castanospermine, prevented apo(a) secretion, but did not inhibit folding. This suggests that an event which is dependent on trimming of N-linked glucoses, and which occurs after the folding events detectable in our assay, is required for apo(a) secretion. Differences in the ability to undergo this event may explain the variable efficiency with which apo(a) allelic variants exit the ER.

Neomycin inhibits secretion of apolipoprotein[a] by increasing retention on the hepatocyte cell surface

Neomycin therapy reduces plasma levels of low density lipoprotein and lipoprotein[a] (Lp[a]). To determine whether neomycin directly alters the biogenesis of Lp[a], we have examined the effect of neomycin on apolipoprotein[a] (apo[a]) synthesis and secretion in primary cultures of baboon hepatocytes. Using this system, we have previously shown that apo[a] is synthesized as a lower molecular weight precursor that upon maturation becomes associated with the cell surface before release into the culture medium. Treatment of hepatocytes with 10 mM neomycin reduced levels of apo[a] in the culture medium by as much as 12-fold. Although a portion of the reduced secretion could be accounted for by a reduction in total protein synthesis, the greatest effect of neomycin on apo[a] secretion was to decrease the release of mature apo[a] from the hepatocyte cell surface into the culture medium. Treatment of hepatocyte cultures with trypsin confirmed that mature apo[a] in neomycin-treated cells was still transported to the cell surface. Examination of related antibiotics demonstrated that inhibition of apo[a] secretion is a general property shared by the deoxystreptamine antibiotics. The mechanism by which neomycin affects the apo[a]-cell surface interaction is not known, but neomycin is known to perturb cell surface membranes, inhibit the interaction of some ligands with their cell surface receptors, and inhibit the metabolism of phosphatidylinositol 4,5 biphosphate. These studies suggest that cell surface association of apo[a] may play a role in Lp[a] biogenesis in vivo.

Genetic analysis of serum alanine transaminase activity in normal and hepatitis C virus-infected chimpanzees: an application of research-oriented genetic management

The hepatic enzyme alanine transaminase (ALT) is a diagnostic marker for liver damage but has a considerable degree of normal variation. We used complex segregation analysis to determine whether evidence exists for major genic determination of normal ALT values in an important animal model, the chimpanzee (Pan troglodytes). Normal ALT values were available for 212 chimpanzees. Available genealogical data allowed assignment of 165 animals to a total of 19 pedigrees; 47 animals were treated as independent. A major gene explaining 60% of the phenotypic variance in normal alanine transaminase (ALT) activity was detected by complex segregation analysis. The allele for high ALT activity had a frequency of 0.20. Polygenes accounted for an additional 20% of the variation. The observation that 80% of the total phenotypic variance is attributable to genetic factors has important implications for studies that use ALT activity in assessments. Genetic analysis of change in ALT activity after inoculation with hepatitis C virus (HCV) in a subset of animals indicated that approximately 30% of the variation in response may also be attributable to genetic factors and that the estimated major locus genotypes differ in their responses. This suggests that genetic components can exert substantial influences on experimental parameters in hepatitis research.

In vitro propagation and production of hepatitis E virus from in vivo-infected primary macaque hepatocytes

Hepatitis E virus (HEV) is responsible for sporadic cases as well as large epidemics of acute viral hepatitis in many developing countries. The nucleotide sequence of HEV appears to be unique among known viruses and thus may represent a prototype human pathogen in a novel class of single-stranded, positive-sense RNA viruses. To facilitate further studies of the biology of HEV, a tissue culture system using a serum-free medium formulation has been developed to propagate the virus in vitro. Hepatocytes were isolated from livers of cynomolgus macaques experimentally infected with a HEV (Burma strain) inoculum and maintained in long-term cultures. Using a highly strand-specific RT-PCR assay, both the positive-sense and the negative replicative strands of HEV RNA were detected in these hepatocytes throughout the course of the experiments. Positive-strand genomic RNA was also detected in the culture medium, suggesting the production and secretion of HEV virus particles. The virus particles were successfully concentrated 200-fold from the medium using ultrafiltration, and they could be observed by immunoelectron microscopy using anti-HEV-positive immune serum. These results demonstrate the capacity of this hepatocyte culture system to replicate HEV in vitro, thus providing an experimental means to study the replicative process of the virus.