Hepatitis B virus RNA element that facilitates accumulation of surface gene transcripts in the cytoplasm

The Post-Transcriptional Regulatory Element of Hepatitis B Virus: From Discovery to Therapy

The post-transcriptional regulatory element (PRE) is present in all HBV mRNAs and plays a major role in their stability, nuclear export, and enhancement of viral gene expression. Understanding PRE's structure, function, and mode of action is essential to leverage its potential as a therapeutic target. A wide range of PRE-based reagents and tools have been developed and assessed in preclinical and clinical settings for therapeutic and biotechnology applications. This manuscript aims to provide a systematic review of the characteristics and mechanism of action of PRE, as well as elucidating its current applications in basic and clinical research. Finally, we discuss the promising opportunities that PRE may provide to antiviral development, viral biology, and potentially beyond.

Hepatitis B virus RNAs co-opt ELAVL1 for stabilization and CRM1-dependent nuclear export

Hepatitis B virus (HBV) chronically infects 296 million people worldwide, posing a major global health threat. Export of HBV RNAs from the nucleus to the cytoplasm is indispensable for viral protein translation and genome replication, however the mechanisms regulating this critical process remain largely elusive. Here, we identify a key host factor embryonic lethal, abnormal vision, Drosophila-like 1 (ELAVL1) that binds HBV RNAs and controls their nuclear export. Using an unbiased quantitative proteomics screen, we demonstrate direct binding of ELAVL1 to the HBV pregenomic RNA (pgRNA). ELAVL1 knockdown inhibits HBV RNAs posttranscriptional regulation and suppresses viral replication. Further mechanistic studies reveal ELAVL1 recruits the nuclear export receptor CRM1 through ANP32A and ANP32B to transport HBV RNAs to the cytoplasm via specific AU-rich elements, which can be targeted by a compound CMLD-2. Moreover, ELAVL1 protects HBV RNAs from DIS3+RRP6+ RNA exosome mediated nuclear RNA degradation. Notably, we find HBV core protein is dispensable for HBV RNA-CRM1 interaction and nuclear export. Our results unveil ELAVL1 as a crucial host factor that regulates HBV RNAs stability and trafficking. By orchestrating viral RNA nuclear export, ELAVL1 is indispensable for the HBV life cycle. Our study highlights a virus-host interaction that may be exploited as a new therapeutic target against chronic hepatitis B.

Pangenomic antiviral effect of REP 2139 in CRISPR/Cas9 engineered cell lines expressing hepatitis B virus surface antigen

Chronic hepatitis B remains a global health problem with 296 million people living with chronic HBV infection and being at risk of developing cirrhosis and hepatocellular carcinoma. Non-infectious subviral particles (SVP) are produced in large excess over infectious Dane particles in patients and are the major source of Hepatitis B surface antigen (HBsAg). They are thought to exhaust the immune system, and it is generally considered that functional cure requires the clearance of HBsAg from blood of patient. Nucleic acid polymers (NAPs) antiviral activity lead to the inhibition of HBsAg release, resulting in rapid clearance of HBsAg from circulation in vivo. However, their efficacy has only been demonstrated in limited genotypes in small scale clinical trials. HBV exists as nine main genotypes (A to I). In this study, the HBsAg ORFs from the most prevalent genotypes (A, B, C, D, E, G), which account for over 96% of human cases, were inserted into the AAVS1 safe-harbor of HepG2 cells using CRISPR/Cas9 knock-in. A cell line producing the D144A vaccine escape mutant was also engineered. The secretion of HBsAg was confirmed into these new genotype cell lines (GCLs) and the antiviral activity of the NAP REP 2139 was then assessed. The results demonstrate that REP 2139 exerts an antiviral effect in all genotypes and serotypes tested in this study, including the vaccine escape mutant, suggesting a pangenomic effect of the NAPs.

Hepatitis B virus virion secretion is a CRM1-spike-mediated late event

Background

Hepatitis B virus (HBV) is a major human pathogen worldwide. To date, there is no curative treatment for chronic hepatitis B. The mechanism of virion secretion remains to be investigated. Previously, we found that nuclear export of HBc particles can be facilitated via two CRM1-specific nuclear export signals (NES) at the spike tip.

Methods

In this study, we used site-directed mutagenesis at the CRM1 NES, as well as treatment with CRM1 inhibitors at a low concentration, or CRM1-specific shRNA knockdown, in HBV-producing cell culture, and measured the secretion of various HBV viral and subviral particles via a native agarose gel electrophoresis assay. Separated HBV particles were characterized by Western blot analysis, and their genomic DNA contents were measured by Southern blot analysis. Secreted extracellular particles were compared with intracellular HBc capsids for DNA synthesis and capsid formation. Virion secretion and the in vivo interactions among HBc capsids, CRM1 and microtubules, were examined by proximity ligation assay, immunofluorescence microscopy, and nocodazole treatment.

Results

We report here that the tip of spike of HBV core (HBc) particles (capsids) contains a complex sensor for secretion of both HBV virions and naked capsids. HBV virion secretion is closely associated with HBc nuclear export in a CRM1-dependent manner. At the conformationally flexible spike tips of HBc particles, NES motifs overlap extensively with motifs important for secretion of HBV virions and naked capsids.

Conclusions

We provided experimental evidence that virions and naked capsids can egress via two distinct, yet overlapping, pathways. Unlike the secretion of naked capsids, HBV virion secretion is highly CRM1- and microtubule-dependent. CRM1 is well known for its involvement in nuclear transport in literature. To our knowledge, this is the first report that CRM1 is required for virion secretion. CRM1 inhibitors could be a promising therapeutic candidate for chronic HBV patients in clinical medicine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00827-w.

Host Poly(A) Polymerases PAPD5 and PAPD7 Provide Two Layers of Protection That Ensure the Integrity and Stability of Hepatitis B Virus RNA

Noncanonical poly(A) polymerases PAPD5 and PAPD7 (PAPD5/7) stabilize hepatitis B virus (HBV) RNA via the interaction with the viral posttranscriptional regulatory element (PRE), representing new antiviral targets to control HBV RNA metabolism, hepatitis B surface antigen (HBsAg) production, and viral replication. Inhibitors targeting these proteins are being developed as antiviral therapies; therefore, it is important to understand how PAPD5/7 coordinate to stabilize HBV RNA. Here, we utilized a potent small-molecule AB-452 as a chemical probe, along with genetic analyses to dissect the individual roles of PAPD5/7 in HBV RNA stability. AB-452 inhibits PAPD5/7 enzymatic activities and reduces HBsAg both in vitro (50% effective concentration [EC₅₀] ranged from 1.4 to 6.8 nM) and in vivo by 0.94 log₁₀. Our genetic studies demonstrate that the stem-loop alpha sequence within PRE is essential for both maintaining HBV poly(A) tail integrity and determining sensitivity toward the inhibitory effect of AB-452. Although neither single knockout (KO) of PAPD5 nor PAPD7 reduces HBsAg RNA and protein production, PAPD5 KO does impair poly(A) tail integrity and confers partial resistance to AB-452. In contrast, PAPD7 KO did not result in any measurable changes within the HBV poly(A) tails, but cells with both PAPD5 and PAPD7 KO show reduced HBsAg production and conferred complete resistance to AB-452 treatment. Our results indicate that PAPD5 plays a dominant role in stabilizing viral RNA by protecting the integrity of its poly(A) tail, while PAPD7 serves as a second line of protection. These findings inform PAPD5-targeted therapeutic strategies and open avenues for further investigating PAPD5/7 in HBV replication.

IMPORTANCE Chronic hepatitis B affects more than 250 million patients and is a major public health concern worldwide. HBsAg plays a central role in maintaining HBV persistence, and as such, therapies that aim at reducing HBsAg through destabilizing or degrading HBV RNA have been extensively investigated. Besides directly degrading HBV transcripts through antisense oligonucleotides or RNA silencing technologies, small-molecule compounds targeting host factors such as the noncanonical poly(A) polymerase PAPD5 and PAPD7 have been reported to interfere with HBV RNA metabolism. Herein, our antiviral and genetic studies using relevant HBV infection and replication models further characterize the interplays between the cis element within the viral sequence and the trans elements from the host factors. PAPD5/7-targeting inhibitors, with oral bioavailability, thus represent an opportunity to reduce HBsAg through destabilizing HBV RNA.

Genetic Constructs for the Control of Astrocytes' Activity

In the current review, we aim to discuss the principles and the perspectives of using the genetic constructs based on AAV vectors to regulate astrocytes’ activity. Practical applications of optogenetic approaches utilizing different genetically encoded opsins to control astroglia activity were evaluated. The diversity of astrocytic cell-types complicates the rational design of an ideal viral vector for particular experimental goals. Therefore, efficient and sufficient targeting of astrocytes is a multiparametric process that requires a combination of specific AAV serotypes naturally predisposed to transduce astroglia with astrocyte-specific promoters in the AAV cassette. Inadequate combinations may result in off-target neuronal transduction to different degrees. Potentially, these constraints may be bypassed with the latest strategies of generating novel synthetic AAV serotypes with specified properties by rational engineering of AAV capsids or using directed evolution approach by searching within a more specific promoter or its replacement with the unique enhancer sequences characterized using modern molecular techniques (ChIP-seq, scATAC-seq, snATAC-seq) to drive the selective transgene expression in the target population of cells or desired brain regions. Realizing these strategies to restrict expression and to efficiently target astrocytic populations in specific brain regions or across the brain has great potential to enable future studies.

Lost Small Envelope Protein Expression from Naturally Occurring PreS1 Deletion Mutants of Hepatitis B Virus Is Often Accompanied by Increased HBx and Core Protein Expression as Well as Genome Replication

Hepatitis B virus (HBV) transcribes coterminal mRNAs of 0.7 to 3.5 kb from the 3.2-kb covalently closed circular DNA, with the 2.1-kb RNA being most abundant. The 0.7-kb RNA produces HBx protein, a transcriptional transactivator, while the 3.5-kb pregenomic RNA (pgRNA) drives core and P protein translation as well as genome replication. The large (L) and small (S) envelope proteins are translated from the 2.4-kb and 2.1-kb RNAs, respectively, with the majority of the S protein being secreted as noninfectious subviral particles and detected as hepatitis B surface antigen (HBsAg). pgRNA transcription could inhibit transcription of subgenomic RNAs. The present study characterized naturally occurring in-frame deletions in the 3' preS1 region, which not only codes for L protein but also serves as the promoter for 2.1-kb RNA. The human hepatoma cell line Huh7 was transiently transfected with subgenomic expression constructs for envelope (and HBx) proteins, dimeric constructs, or constructs mimicking covalently closed circular DNA. The results confirmed lost 2.1-kb RNA transcription and HBsAg production from many deletion mutants, accompanied by increases in other (especially 2.4-kb) RNAs, intracellular HBx and core proteins, and replicative DNA but impaired virion and L protein secretion. The highest intracellular L protein levels were achieved by mutants that had residual S protein expression or retained the matrix domain in L protein. Site-directed mutagenesis of a high replicating deletion mutant suggested that increased HBx protein expression and blocked virion secretion both contributed to the high replication phenotype. Our findings could help explain why such deletions are selected at a late stage of chronic HBV infection and how they contribute to viral pathogenesis. IMPORTANCE Expression of hepatitis B e antigen (HBeAg) and overproduction of HBsAg by wild-type HBV are implicated in the induction of immune tolerance to achieve chronic infection. How HBV survives the subsequent immune clearance phase remains incompletely understood. Our previous characterization of core promoter mutations to reduce HBeAg production revealed the ability of the 3.5-kb pgRNA to diminish transcription of coterminal RNAs of 2.4 kb, 2.1 kb, and 0.7 kb. The later stage of chronic HBV infection often selects for in-frame deletions in the preS region. Here, we found that many 3' preS1 deletions prevented transcription of the 2.1-kb RNA for HBsAg production, which was often accompanied by increases in intracellular 3.5-, 0.7-, and especially 2.4-kb RNAs, HBx and core proteins, and replicative DNA but lost virion secretion. These findings established the biological consequences of preS1 deletions, thus shedding light on why they are selected and how they contribute to hepatocarcinogenesis.

The Dihydroquinolizinone Compound RG7834 Inhibits the Polyadenylase Function of PAPD5 and PAPD7 and Accelerates the Degradation of Matured Hepatitis B Virus Surface Protein mRNA

Hepatitis B virus (HBV) mRNA metabolism is dependent upon host proteins PAPD5 and PAPD7 (PAPD5/7). PAPD5/7 are cellular, noncanonical, poly(A) polymerases (PAPs) whose main function is to oligoadenylate the 3' end of noncoding RNA (ncRNA) for exosome degradation. HBV seems to exploit these two ncRNA quality-control factors for viral mRNA stabilization, rather than degradation. RG7834 is a small-molecule compound that binds PAPD5/7 and inhibits HBV gene production in both tissue culture and animal study. We reported that RG7834 was able to destabilize multiple HBV mRNA species, ranging from the 3.5-kb pregenomic/precore mRNAs to the 2.4/2.1-kb hepatitis B virus surface protein (HBs) mRNAs, except for the smallest 0.7-kb X protein (HBx) mRNA. Compound-induced HBV mRNA destabilization was initiated by a shortening of the poly(A) tail, followed by an accelerated degradation process in both the nucleus and cytoplasm. In cells expressing HBV mRNA, both PAPD5/7 were found to be physically associated with the viral RNA, and the polyadenylating activities of PAPD5/7 were susceptible to RG7834 repression in a biochemical assay. Moreover, in PAPD5/7 double-knockout cells, viral transcripts with a regular length of the poly(A) sequence could be initially synthesized but became shortened in hours, suggesting that participation of PAPD5/7 in RNA 3' end processing, either during adenosine oligomerization or afterward, is crucial for RNA stabilization.

A Genome-wide CRISPR Screen Identifies ZCCHC14 as a Host Factor Required for Hepatitis B Surface Antigen Production

A hallmark of chronic hepatitis B (CHB) virus infection is the presence of high circulating levels of non-infectious small lipid HBV surface antigen (HBsAg) vesicles. Although rare, sustained HBsAg loss is the idealized endpoint of any CHB therapy. A small molecule, RG7834, has been previously reported to inhibit HBsAg expression by targeting terminal nucleotidyltransferase proteins 4A and 4B (TENT4A and TENT4B). In this study, we describe a genome-wide CRISPR screen to identify other potential host factors required for HBsAg expression and to gain further insights into the mechanism of RG7834. We report more than 60 genes involved in regulating HBsAg and identify additional factors involved in RG7834 activity, including a zinc finger CCHC-type containing 14 (ZCCHC14) protein. We show that ZCCHC14, together with TENT4A/B, stabilizes HBsAg expression through HBV RNA tailing, providing a potential new therapeutic target to achieve functional cure in CHB patients.

Expression Level of Small Envelope Protein in Addition to Sequence Divergence inside Its Major Hydrophilic Region Contributes to More Efficient Surface Antigen Secretion by Hepatitis B Virus Subgenotype D2 than Subgenotype A2

Hepatitis B surface antigen (HBsAg) promotes persistent hepatitis B virus (HBV) infection. It primarily corresponds to small (S) envelope protein secreted as subviral particles. We previously found that genotype D clones expressed less S protein than genotype A clones but showed higher extracellular/intracellular ratio of HBsAg suggesting more efficient secretion. The current study aimed to characterize the underlying mechanism(s) by comparing a subgenotype A2 clone (geno5.4) with a subgenotype D2 clone (geno1.2). Five types of full-length or subgenomic constructs were transfected to Huh7 cells at different dosage. HBsAg was quantified by enzyme linked immunosorbent assay while envelope proteins were detected by Western blot. We found that ratio of extracellular/intracellular HBsAg decreased at increasing amounts of DNA transfected. Conflicting findings from two types of subgenomic construct confirmed stronger secretion inhibitory effect of the genotype D-derived large envelope protein. Chimeric constructs followed by site-directed mutagenesis revealed geno1.2 specific V118/T127 and F161/A168 in the S protein as promoting and inhibitory of HBsAg secretion, respectively. In conclusion, more efficient HBsAg secretion by subgenotype D2 than subgenotype A2 is attributed to lower level of S protein expression in addition to V118 and T127 in S protein, although its F161 and A168 sequences rather reduce HBsAg secretion.

In vitro inhibition effects of hepatitis B virus by dandelion and taraxasterol

Hepatitis B virus (HBV) causes hepatitis, which progresses to fatal liver diseases and remains a global health problem. Current treatments for chronic hepatitis B are unable to cure hepatitis. Thus, new antiviral drugs must be developed. In this study, the viral inhibition effects of dandelion and taraxasterol were assessed in HepG2.2.15 cell line. Taraxacum officinale F.H.Wigg. (compositae) with English name dandelion is used as a traditional herb for liver disorders and as a common antiviral agent. Taraxasterol is one of the active compounds of dandelion. The secretion of HBV DNA and HBV surface antigen (HBsAg) and HBeAg was detected using fluorescence quantitative PCR (qPCR) and ELISA, respectively. Intracellular HBsAg was detected by immunofluorescence. In order to demonstrate the potential mechanism of anti-viral activity, the expression levels of host factors polypyrimidine tract binding protein 1 (PTBP1) and sirtuin 1 (SIRT1) were detected with Western blotting and qPCR. Dandelion and taraxasterol effectively reduced the secretion of HBsAg, HBeAg and the HBV DNA in cell supernatants, and significantly reduced the intracellular HBsAg as indicated by immunofluorescence results. Taraxasterol may be one of the main effective components of dandelion. It significantly decreased the protein expression levels of PTBP1 and SIRT1. The present study revealed that dandelion and its component taraxasterol could inhibit HBV and may be a potential anti-HBV drug, whose potential targets were the host factors PTBP1 and SIRT1.

Viral hijacking of the TENT4-ZCCHC14 complex protects viral RNAs via mixed tailing

TENT4 enzymes generate 'mixed tails' of diverse nucleotides at 3' ends of RNAs via nontemplated nucleotide addition to protect messenger RNAs from deadenylation. Here we discover extensive mixed tailing in transcripts of hepatitis B virus (HBV) and human cytomegalovirus (HCMV), generated via a similar mechanism exploiting the TENT4-ZCCHC14 complex. TAIL-seq on HBV and HCMV RNAs revealed that TENT4A and TENT4B are responsible for mixed tailing and protection of viral poly(A) tails. We find that the HBV post-transcriptional regulatory element (PRE), specifically the CNGGN-type pentaloop, is critical for TENT4-dependent regulation. HCMV uses a similar pentaloop, an interesting example of convergent evolution. This pentaloop is recognized by the sterile alpha motif domain-containing ZCCHC14 protein, which in turn recruits TENT4. Overall, our study reveals the mechanism of action of PRE, which has been widely used to enhance gene expression, and identifies the TENT4-ZCCHC14 complex as a potential target for antiviral therapeutics.

Pushing the Right Buttons: Improving Efficacy of Therapeutic DNA Vectors

Gene therapy represents a potent therapeutical application for regenerative medicine. So far, viral and nonviral approaches suffer from major drawbacks hindering efficient gene therapeutic applicability: the immunogenicity of viral systems on the one hand, and the low gene transfer efficiency of nonviral systems on the other hand. Therefore, there is a high demand for improvements of therapeutical systems at several levels. This review summarizes different DNA vector modifications to enhance biological efficacy and efficiency of therapeutical vectors, aiming for low toxicity, high specificity, and biological efficacy-the cornerstones for successful translation of gene therapy into the clinic. We aim to provide a step-by-step instruction to optimize their vectors to achieve the desired outcome of gene therapy. Our review provides the means to either construct a potent gene therapeutic vector de novo or to specifically address a bottleneck in the chain of events mandatory for therapeutic success. Although most of the introduced techniques can be translated into different areas, this review primarily addresses improvements for applications in transient gene therapy in the field of tissue engineering.

HBsAg mRNA degradation induced by a dihydroquinolizinone compound depends on the HBV posttranscriptional regulatory element

In pursuit of novel therapeutics targeting the hepatitis B virus (HBV) infection, we evaluated a dihydroquinolizinone compound (DHQ-1) that in the nanomolar range reduced the production of virion and surface protein (HBsAg) in tissue culture. This compound also showed broad HBV genotype coverage, but was inactive against a panel of DNA and RNA viruses of other species. Oral administration of DHQ-1 in the AAV-HBV mouse model resulted in a significant reduction of serum HBsAg as soon as 4 days following the commencement of treatment. Reduction of HBV markers in both in vitro and in vivo experiments was related to the reduced amount of viral RNA including pre-genomic RNA (pgRNA) and 2.4/2.1 kb HBsAg mRNA. Nuclear run-on and subcellular fractionation experiments indicated that DHQ-1 mediated HBV RNA reduction was the result of accelerated viral RNA degradation in the nucleus, rather than the consequence of inhibition of transcription initiation. Through mutagenesis of HBsAg gene sequences, we found induction of HBsAg mRNA decay by DHQ-1 required the presence of the HBV posttranscriptional regulatory element (HPRE), with a 109 nucleotides sequence within the central region of the HPRE alpha sub-element being the most critical. Taken together, the current study shows that a small molecule can reduce the overall levels of HBV RNA, especially the HBsAg mRNA, and viral surface proteins. This may shed light on the development of a new class of HBV therapeutics.

Diverse activities of viral cis-acting RNA regulatory elements revealed using multicolor, long-term, single-cell imaging

Cis-acting RNA structural elements govern crucial aspects of viral gene expression. How these structures and other posttranscriptional signals affect RNA trafficking and translation in the context of single cells is poorly understood. Herein we describe a multicolor, long-term (>24 h) imaging strategy for measuring integrated aspects of viral RNA regulatory control in individual cells. We apply this strategy to demonstrate differential mRNA trafficking behaviors governed by RNA elements derived from three retroviruses (HIV-1, murine leukemia virus, and Mason-Pfizer monkey virus), two hepadnaviruses (hepatitis B virus and woodchuck hepatitis virus), and an intron-retaining transcript encoded by the cellular NXF1 gene. Striking behaviors include "burst" RNA nuclear export dynamics regulated by HIV-1's Rev response element and the viral Rev protein; transient aggregations of RNAs into discrete foci at or near the nuclear membrane triggered by multiple elements; and a novel, pulsiform RNA export activity regulated by the hepadnaviral posttranscriptional regulatory element. We incorporate single-cell tracking and a data-mining algorithm into our approach to obtain RNA element-specific, high-resolution gene expression signatures. Together these imaging assays constitute a tractable, systems-based platform for studying otherwise difficult to access spatiotemporal features of viral and cellular gene regulation.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Molecular biology of hepatitis B virus infection

Human hepatitis B virus (HBV) is the prototype of a family of small DNA viruses that productively infect hepatocytes, the major cell of the liver, and replicate by reverse transcription of a terminally redundant viral RNA, the pregenome. Upon infection, the circular, partially double-stranded virion DNA is converted in the nucleus to a covalently closed circular DNA (cccDNA) that assembles into a minichromosome, the template for viral mRNA synthesis. Infection of hepatocytes is non-cytopathic. Infection of the liver may be either transient (<6 months) or chronic and lifelong, depending on the ability of the host immune response to clear the infection. Chronic infections can cause immune-mediated liver damage progressing to cirrhosis and hepatocellular carcinoma (HCC). The mechanisms of carcinogenesis are unclear. Antiviral therapies with nucleoside analog inhibitors of viral DNA synthesis delay sequelae, but cannot cure HBV infections due to the persistence of cccDNA in hepatocytes.

Human immunodeficiency virus coinfection with hepatitis B virus leads to a decrease in extracellular and intracellular hepatitis B antigen

Chronic hepatitis B virus (HBV) infection could cause severe liver disease including cirrhosis, hepatocellular carcinoma, and end-stage liver failure in HIV-positive individuals. The available data from clinical studies suggest that HIV infection modulates the HBV-specific T cell response. However, the virological and molecular aspects of HIV-HBV coinfection are currently poorly understood due to the lack of appropriate model systems. In this study, the effect of HIV infection on the life cycle of HBV was explored using an in vitro model system. The present data show that the extracellular and intracellular hepatitis B surface antigen (HBsAg) and e antigen (HBeAg) decrease significantly in HepG2 cells cotransfected with HIV NL4-3 and pHBV1.3 as compared to those cells transfected only with pHBV1.3. Moreover, a significant decrease in HBV DNA and mRNA expression was also observed in the cotransfected cells. HIV Rev protein, an RNA-bound regulatory protein, could significantly decrease the expression levels of extracellular and intracellular HBsAg and HBeAg by mediating the expression of HBV mRNA in cells cotransfected with plasmids containing HIV-1 Rev and pHBV1.3. Further experiments demonstrate that HIV Rev manipulated neither the promoters of HBV nor the nuclear export of HBV mRNA. These results from the in vitro model system might provide clues to further understand the rapid progression of liver disease in HIV-HBV-coinfected patients.

Viral expression cassette elements to enhance transgene target specificity and expression in gene therapy

Over the last five years, the number of clinical trials involving AAV (adeno-associated virus) and lentiviral vectors continue to increase by about 150 trials each year. For continued success, AAV and lentiviral expression cassettes need to be designed to meet each disease's specific needs. This review discusses how viral vector expression cassettes can be engineered with elements to enhance target specificity and increase transgene expression. The key differences relating to target specificity between ubiquitous and tissue-specific promoters are discussed, as well as how endogenous miRNAs and their target sequences have been used to restrict transgene expression. Specifically, relevant studies indicating how cis-acting elements such as introns, WPRE, polyadenylation signals, and the CMV enhancer are highlighted to show their utility for enhancing transgene expression in gene therapy applications. All discussion bears in mind that expression cassettes have space constraints. In conclusion, this review can serve as a menu of vector genome design elements and their cost in terms of space to thoughtfully engineer viral vectors for gene therapy.

Nuclear export of human hepatitis B virus core protein and pregenomic RNA depends on the cellular NXF1-p15 machinery

Hepatitis B virus (HBV) core protein (HBc) can shuttle between nucleus and cytoplasm. Cytoplasm-predominant HBc is clinically associated with severe liver inflammation. Previously, we found that HBc arginine-rich domain (ARD) can associate with a host factor NXF1 (TAP) by coimmunoprecipitation. It is well known that NXF1-p15 heterodimer can serve as a major export receptor of nuclear mRNA as a ribonucleoprotein complex (RNP). In the NXF1-p15 pathway, TREX (transcription/export) complex plays an important role in coupling nuclear pre-mRNA processing with mRNA export in mammalian cells. Here, we tested the hypothesis whether HBc and HBV specific RNA can be exported via the TREX and NXF1-p15 mediated pathway. We demonstrated here that HBc can physically and specifically associate with TREX components, and the NXF1-p15 export receptor by coimmunoprecipitation. Accumulation of HBc protein in the nucleus can be induced by the interference with TREX and NXF1-p15 mediated RNA export machinery. HBV transcripts encodes a non-spliced 3.5 kb pregenomic RNA (pgRNA) which can serve as a template for reverse transcription. Cytoplasmic HBV pgRNA appeared to be reduced by siRNA treatment specific for the NXF1-p15 complex by quantitative RT-qPCR and Northern blot analyses. This result suggests that the pgRNA was also exported via the NXF1-p15 machinery. We entertain the hypothesis that HBc protein can be exported as an RNP cargo via the mRNA export pathway by hijacking the TREX and NXF1-p15 complex. In our current and previous studies, HBc is not required for pgRNA accumulation in the cytoplasm. Furthermore, HBc ARD can mediate nuclear export of a chimeric protein containing HBc ARD in a pgRNA-independent manner. Taken together, it suggests that while both pgRNA and HBc protein exports are dependent on NXF1-p15, they are using the same export machinery in a manner independent of each other.

A rational study for identification of highly effective siRNAs against hepatitis B virus

RNA interference (RNAi) is a powerful gene knockdown technique used for study gene function. It also potentially provides effective agents for inhibiting infectious and genetic diseases. Most of RNAi studies employ a single siRNA designing program and then require large-scale screening experiments to identify functional siRNAs. In this study, we demonstrate that an assembly of results generated from different siRNA designing programs could provide clusters of predicting sites that aided selection of potent siRNAs. Based on the clusters, three siRNA target sites were selected on a conserved RNA region of hepatitis B virus (HBV), known as HBV post-transcriptional regulatory element (HBV PRE) at nucleotide positions 1317-1337, 1357-1377 and 1644-1664. All three chosen siRNAs driven by H1 promoter were highly effective and could drastically decrease expression of HBV transcripts (core, surface and X) and surface protein without induction of interferon response and cell cytotoxicity in liver cancer cell line (HepG2). Based on prediction of secondary structures, the silencing effects of siRNAs were less effective against a loop sequence of the mRNA target with hairpin structure. In summary, we demonstrate an effectual approach for identification of functional siRNAs. Moreover, highly potent siRNAs identified here may serve as novel agents for development of nucleic acid-based HBV therapy.

A Sub-Element in PRE enhances nuclear export of intronless mRNAs by recruiting the TREX complex via ZC3H18

Viral RNA elements that facilitate mRNA export are useful tools for identifying cellular RNA export factors. Here we show that hepatitis B virus post-transcriptional element (PRE) is one such element, and using PRE several new cellular mRNA export factors were identified. We found that PRE drastically enhances the cytoplasmic accumulation of cDNA transcripts independent of any viral protein. Systematic deletion analysis revealed the existence of a 116 nt functional Sub-Element of PRE (SEP1). The RNP that forms on the SEP1 RNA was affinity purified, in which TREX components as well as several other proteins were identified. TREX components and the SEP1-associating protein ZC3H18 are required for SEP1-mediated mRNA export. Significantly, ZC3H18 directly binds to the SEP1 RNA, interacts with TREX and is required for stable association of TREX with the SEP1-containing mRNA. Requirements for SEP1-mediated mRNA export are similar to those for splicing-dependent mRNA export. Consistent with these similarities, several SEP1-interacting proteins, including ZC3H18, ARS2, Acinus and Brr2, are required for efficient nuclear export of polyA RNAs. Together, our data indicate that SEP1 enhances mRNA export by recruiting TREX via ZC3H18. The new mRNA export factors that we identified might be involved in cap- and splicing-dependent TREX recruitment to cellular mRNAs.

c-Myc-mediated overexpression of miR-17-92 suppresses replication of hepatitis B virus in human hepatoma cells

MicroRNAs (miRNAs) regulate post-transcriptional gene expression in various physiological and pathological processes, including viral infections. The miR-17-92 cluster encodes six miRNAs (miR-17-5p, miR-18a, miR-19a, miR-19b, miR-20a, and miR-92a-1) which are transactivated by c-Myc. Because hepatitis B virus transactivates c-Myc, the interaction between the miR-17-92 cluster and HBV replication was examined in this study. Inducing HBV replication in a human hepatoma cell line increased miR-17-5p, miR-20a and miR-92a-1 expression. HBV-induced overexpression of miR-17-92 was reversed by c-Myc knockdown. Antisense peptide nucleic acids against miR-20a and miR-92a-1 augmented HBV replication. A computational analysis yielded potential binding sites for miR-20a and miR-92a-1 in the HBV genome. The direct interaction between these two miRNAs and target regions in HBV transcripts was confirmed by luciferase reporter analysis. These results demonstrated negative feedback suppression of HBV replication by the miR-17-92 polycistron.

The G1613A mutation in the HBV genome affects HBeAg expression and viral replication through altered core promoter activity

Infection of hepatitis B virus (HBV) causes acute and chronic hepatitis and is closely associated with the development of cirrhosis and hepatocellular carcinoma (HCC). Previously, we demonstrated that the G1613A mutation in the HBV negative regulatory element (NRE) is a hotspot mutation in HCC patients. In this study, we further investigated the functional consequences of this mutation in the context of the full length HBV genome and its replication. We showed that the G1613A mutation significantly suppresses the secretion of e antigen (HBeAg) and enhances the synthesis of viral DNA, which is in consistence to our clinical result that the G1613A mutation associates with high viral load in chronic HBV carriers. To further investigate the molecular mechanism of the mutation, we performed the electrophoretic mobility shift assay with the recombinant RFX1 protein, a trans-activator that was shown to interact with the NRE of HBV. Intriguingly, RFX1 binds to the G1613A mutant with higher affinity than the wild-type sequence, indicating that the mutation possesses the trans-activating effect to the core promoter via NRE. The trans-activating effect was further validated by the enhancement of the core promoter activity after overexpression of RFX1 in liver cell line. In summary, our results suggest the functional consequences of the hotspot G1613A mutation found in HBV. We also provide a possible molecular mechanism of this hotspot mutation to the increased viral load of HBV carriers, which increases the risk to HCC.

Identification of an effective siRNA target site and functional regulatory elements, within the hepatitis B virus posttranscriptional regulatory element

Background

Infection with hepatitis B virus (HBV) is major public health concern. The limitations of available antiviral drugs require development of novel approaches to inhibit HBV replication. This study was conducted to identify functional elements and new siRNA target sites within the highly conserved regions of the 533 base post-transcriptional regulatory element (PRE) of HBV RNAs.

Results

Computational analysis of the PRE sequence revealed several conserved regulatory elements that are predicted to form local secondary structures some of these within known regulatory regions. A deletion analysis showed that sub-elements of the PRE have different effects on the reporter activity suggesting that the PRE contains multiple regulatory elements. Conserved siRNA targets at nucleotide position 1317-1337 and 1329-1349 were predicted. Although the siRNA at the position 1329-1349 had no effect on the expression of reporter gene, the siRNA target site at the position 1317-1337 was observed to significantly decrease expression of the reporter protein. This siRNA also specifically reduced the level of cccDNA in transiently HBV infected cells.

Conclusion

The HBV PRE is likely to contain multiple regulatory elements. A conserved target within this region at 1317-1337 is an effective siRNA target.

Inhibition of hepatitis B virus replication by MyD88 involves accelerated degradation of pregenomic RNA and nuclear retention of pre-S/S RNAs

Myeloid differentiation primary response protein 88 (MyD88), which can be induced by alpha interferon (IFN-alpha), has an antiviral activity against the hepatitis B virus (HBV). The mechanism of this antiviral activity remains poorly understood. Here, we report that MyD88 inhibited HBV replication in HepG2.2.15 cells and in a mouse model. The knockdown of MyD88 expression weakened the IFN-alpha-induced inhibition of HBV replication. Furthermore, MyD88 posttranscriptionally reduced the levels of viral RNA. Remarkably, MyD88 accelerated the decay of viral pregenomic RNA in the cytoplasm. Mapping analysis showed that the RNA sequence located in the 5'-proximal region of the pregenomic RNA was critical for the decay. In addition, MyD88 inhibited the nuclear export of pre-S/S RNAs via the posttranscriptional regulatory element (PRE). The retained pre-S/S RNAs were shown to degrade in the nucleus. Finally, we found that MyD88 inhibited the expression of polypyrimidine tract-binding protein (PTB), a key nuclear export factor for PRE-containing RNA. Taken together, our results define a novel antiviral mechanism against HBV mediated by MyD88.

Recombinant respiratory syncytial virus F protein expression is hindered by inefficient nuclear export and mRNA processing

Studies of the fusion activity of respiratory syncytial virus (RSV) F protein are significantly hindered by low recombinant expression levels. While infection produces F protein levels detectable by western blot, recombinant expression produces undetectable to low levels of F protein. Identifying the obstacles that hinder recombinant F protein expression may lead to improved expression and facilitate the study of F protein function. We hypothesized that nuclear localization and/or inefficient RNA polymerase II-mediated transcription contribute to poor recombinant F protein expression. This study shows a combination of stalled nuclear export, premature polyadenylation, and low mRNA abundance all contribute to low recombinant F protein expression levels. In addition, this study provides an expression optimization strategy that results in greater F protein expression levels than observed by codon-optimization of the F protein gene, which will be useful for future studies of F protein function.

Role of glyceraldehyde-3-phosphate dehydrogenase binding to hepatitis B virus posttranscriptional regulatory element in regulating expression of HBV surface antigen

The hepatitis B virus (HBV) posttranscriptional regulatory element (HPRE) has been demonstrated to facilitate the cytoplasmic localization of unspliced transcripts. One cellular protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is known to combine with this element. However, its function on HPRE remains unclear. Here, we show that recombinant GAPDH protein binds to HPRE RNA in vitro in streptavidin pull-down assays. Functional analysis demonstrated that GAPDH inhibited HPRE function in a pDM138-HPRE chloramphenicol acetyltransferase reporter assay system. Overexpression of GAPDH depressed the expression of HBs antigen, as detected both in cells transiently expressing HBs-HPRE and in HepG2.2.15 cells. These data indicate that GAPDH may be involved in the posttranscriptional regulation of HBV, which is critical for the life cycle of HBV.

Occult hepatitis B infection: an evolutionary scenario

Background

Occult or latent hepatitis B virus (HBV) infection is defined as infection with detectable HBV DNA and undetectable surface antigen (HBsAg) in patients' blood. The cause of an overt HBV infection becoming an occult one is unknown. To gain insight into the mechanism of the development of occult infection, we compared the full-length HBV genome from a blood donor carrying an occult infection (d4) with global genotype D genomes.

Results

The phylogenetic analysis of polymerase, core and X protein sequences did not distinguish d4 from other genotype D strains. Yet, d4 surface protein formed the evolutionary outgroup relative to all other genotype D strains. Its evolutionary branch was the only one where accumulation of substitutions suggests positive selection (dN/dS = 1.3787). Many of these substitutiions accumulated specifically in regions encoding the core/surface protein interface, as revealed in a 3D-modeled protein complex. We identified a novel RNA splicing event (deleting nucleotides 2986-202) that abolishes surface protein gene expression without affecting polymerase, core and X-protein related functions. Genotype D strains differ in their ability to perform this 2986-202 splicing. Strains prone to 2986-202 splicing constitute a separate clade in a phylogenetic tree of genotype D HBVs. A single substitution (G173T) that is associated with clade membership alters the local RNA secondary structure and is proposed to affect splicing efficiency at the 202 acceptor site.

Conclusion

We propose an evolutionary scenario for occult HBV infection, in which 2986-202 splicing generates intracellular virus particles devoid of surface protein, which subsequently accumulates mutations due to relaxation of coding constraints. Such viruses are deficient of autonomous propagation and cannot leave the host cell until it is lysed.

Hepatitis B pregenomic RNA splicing--the products, the regulatory mechanisms and its biological significance

Chronic hepatitis B infection is a major cause of morbidity and mortality worldwide. Despite effective vaccine and treatments, often unavailable in highly prevalent areas in Asia and Africa, the disease and economic burdens of the condition remain very high. There is as yet no cure for those who have already developed chronic infection, in part due to poor understanding of its pathogenesis. Here, we review the literature on the discovery, production, and regulation of hepatitis B virus pregenomic RNA splicing mechanism, and their effects on viral replication and viral protein expression of the wild-type. The splice variants are found in high numbers in many chronically infected patients, implicating a role in viral persistence. Recently a novel protein produced by a singly spliced viral genome which was detected in vivo, containing well-defined epitopes, was shown to induce specific T-cell responses in peripheral blood mononuclear cells from infected patients. We also highlight some of the major unresolved issues and controversies between the data from experimental and clinical studies.

Solution structure of stem-loop alpha of the hepatitis B virus post-transcriptional regulatory element

Chronic hepatitis B virus (HBV) infections may lead to severe diseases like liver cirrhosis or hepatocellular carcinoma (HCC). The HBV post-transcriptional regulatory element (HPRE) facilitates the nuclear export of unspliced viral mRNAs, contains a splicing regulatory element and resides in the 3′-region of all viral transcripts. The HPRE consists of three sub-elements α (nucleotides 1151–1346), β1 (nucleotides 1347–1457) and β2 (nucleotides 1458–1582), which confer together full export competence. Here, we present the NMR solution structure (pdb 2JYM) of the stem-loop α (SLα, nucleotides 1292–1321) located in the sub-element α. The SLα contains a CAGGC pentaloop highly conserved in hepatoviruses, which essentially adopts a CUNG-like tetraloop conformation. Furthermore, the SLα harbours a single bulged G residue flanked by A-helical regions. The structure is highly suggestive of serving two functions in the context of export of unspliced viral RNA: binding sterile alpha motif (SAM-) domain containing proteins and/or preventing the utilization of a 3′-splice site contained within SLα.

The Genetic Engineering of Hematopoietic Stem Cells: the Rise of Lentiviral Vectors, the Conundrum of the LTR, and the Promise of Lineage-restricted Vectors

Recent studies on the integration patterns of different categories of retroviral vectors, the genotoxicity of long-terminal repeats (LTRs) and other genetic elements, the rise of lentiviral technology and the emergence of regulated vector systems providing tissue-restricted transgene expression and RNA interference, are profoundly changing the landscape of stem cell-based therapies. New developments in vector design and an increasing understanding of the mechanisms underlying insertional oncogenesis are ushering in a new phase in hematopoietic stem cell (HSC) engineering, thus bringing the hitherto exclusive reliance on LTR-driven, gamma-retroviral vectors to an end. Based on their ability to transduce non-dividing cells and their genomic stability, lentiviral vectors offer new prospects for the manipulation of HSCs. Tissue-specific vectors, as exemplified by globin vectors, not only provide therapeutic efficacy, but may also enhance safety, insofar that they restrict transgene expression in stem cells, progenitor cells and blood cells in all but the transcriptionally targeted lineage. This review provides a survey of these advances as well as several remaining challenges, focusing in particular on the importance of achieving adequate levels of protein expression from a limited number of vector copies per cell-ideally one to two.

The hepatitis B virus PRE contains a splicing regulatory element

The posttranscriptional regulatory element (PRE) is considered to enhance hepatitis B virus (HBV) gene expression by facilitating the nuclear export of intronless viral subgenomic RNAs. Its role in the RNA metabolism of the viral pregenomic RNA (pgRNA) is currently unknown. We identified a positively cis-acting splicing regulatory element (SRE-1) and present two lines of evidence for its functionality. Firstly, in a heterologous context SRE-1 functionally substitutes for a retroviral bidirectional exonic splicing enhancer (ESE). As expected, SRE-1 is a splicing enhancer also in its natural viral sequence context, since deletion of SRE-1 reduces splicing of pgRNA in cell culture experiments. Secondly, we show that stimulation of HBV RNA splicing by the splicing factor PSF was repressed by the PRE. Analysis of a variety of PSF mutants indicated that RNA-binding and protein-protein interaction were required to enhance splicing. In addition, we show that the PRE contributed to pgRNA stability, but has little influence on its nuclear export. Herein, we report for the first time that the PRE harbors splicing stimulating and inhibiting regulatory elements controlling processing of the viral pregenome. We discuss a model in which the regulation of pgRNA splicing depends on cellular factors interacting with the PRE.

Functional analysis of hepatitis B virus reactivating in hepatitis B surface antigen-negative individuals

The biological properties of latent or occult hepatitis B virus (HBV) have been poorly characterized as a result of the extremely low virus concentration. This report describes the phenotype of HBV reactivating in two patients after an HBsAg-negative latency period. One patient had latent HBV infection for at least 12 years without detectable viremia and symptoms of liver disease. Several full-length HBV genomes were cloned at reactivation, sequenced, and functionally tested by transfection into HuH7 cells. Genomes from both patients showed a low replication phenotype. It was caused at the level of RNA encapsidation or HBV DNA synthesis, but was not attributable to uncommon mutations in the terminal protein domain of P protein. A substantial subpopulation ( approximately 50%) of genomes from one patient did not express pre-S2/S mRNA and HBsAg. Site-directed mutagenesis identified a single G-A mutation within the S gene (position 458) to be responsible for this effect. The G458A mutation was also effective if the S gene was placed under control of a heterologous promoter. Furthermore, nuclear run-on transcription showed that the G458A mutation acts at the posttranscriptional level. The mutation affected a 5' splice site and prevented splicing of the pre-S2/S mRNA from position 458 to 1305. In conclusion, HBV latency may be characterized by viruses with reduced replication competence and antigen expression. In one patient, HBsAg expression was terminated by an as yet undescribed posttranscriptional mechanism. A single mutation inactivated a 5' splice site that is obviously essential for pre-S2/S mRNA accumulation. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Translation of stable hepadnaviral mRNA cleavage fragments induced by the action of phosphorothioate-modified antisense oligodeoxynucleotides

Phosphorothioate-modified antisense oligodeoxynucleotides (ASOs) are used to suppress gene expression by inducing RNase H-mediated cleavage with subsequent degradation of the target mRNA. However, previous observations suggest that ASO/RNase H can also result in the generation of stable mRNA cleavage fragments and expression of truncated proteins. Here, we addressed the underlying translational mechanisms in more detail using hepadnavirus-transfected hepatoma cells as a model system of antisense therapy. Generation of stable mRNA cleavage fragments was restricted to the ASO/RNase H pathway and not observed upon cotransfection of isosequential small interfering RNA or RNase H-incompetent oligonucleotides. Furthermore, direct evidence for translation of mRNA fragments was established by polysome analysis. Polysome-associated RNA contained cleavage fragments devoid of a 5′ cap structure indicating that translation was, at least in part, cap-independent. Further analysis of the uncapped cleavage fragments revealed that their 5′ terminus and initiation codon were only separated by a few nucleotides suggesting a 5′ end-dependent mode of translation, whereas internal initiation could be ruled out. However, the efficiency of translation was moderate compared to uncleaved mRNA and amounted to 13–24% depending on the ASO used. These findings provide a rationale for understanding the translation of mRNA fragments generated by ASO/RNase H mechanistically.

Characterization of nonconventional hepatitis B viruses lacking the core promoter

The core gene (C-gene) promoter and regulatory sequences play a central role in the hepatitis B virus (HBV) life cycle. They are essential for the synthesis of the pregenomic and precore mRNA. The pregenomic RNA is the template required for replication and also the template for the synthesis of the core protein and polymerase. Here, we report the in vivo existence and functional characterization of HBV variants that lack the C-gene promoter region and the regulatory sequences located therein. HBV promoter fragments were isolated by PCR from sera of chronic carriers and characterized. Truncated promoter elements were identified, and then tested in the context of wild-type genomes in the HuH-7 cell line. The expression of the recombinant HBV genome resulted in the synthesis of surface proteins, and low level of core protein as well as a transcript pattern similar to, but smaller in size to wild-type virus. The recombinant HBV genome with the truncated promoter region produced pregenomic RNA-like transcripts. These transcripts were encapsidated and reverse transcribed when complemented by sufficient core and polymerase protein. These date provide an explanation as to why such deletion mutants of HBV can be produced at all, they highlight the functional potentials of viral sequences activated by mutations and may be of relevance for viral evolution and persistence.

Retroviral vectors for high-level transgene expression in T lymphocytes

Efficient expression of genes transferred by retroviral vectors is a prerequisite for gene therapy, especially when the biological effect depends on the amount of transgene product. High-level gene expression is desirable for several gene therapy approaches involving T lymphocytes. We evaluated standard retroviral vectors with cis-regulatory control elements of the Moloney murine leukemia virus (Mo-MLV) with or without the human T cell-specific CD2 enhancer. For comparison, vectors containing the long terminal repeat (LTR) of myeloproliferative sarcoma virus (MPSV) and an improved 5' untranslated region were used (MP71 vectors), with or without the woodchuck hepatitis virus posttranscriptional regulatory element (PRE). All vectors expressed the enhanced green fluorescent protein (GFP) to measure transgene expression. In mouse T cells MP71 vectors with and without the PRE yielded an up to 10-fold higher expression level compared with the Mo-MLV-based vectors currently used for gene transfer into T lymphocytes. A high multiplicity of infection (MOI) of standard Mo-MLV vectors could not reach expression levels obtained with a low MOI of MP71 vector. Ex vivo-transduced mouse T lymphocytes maintained the vector-dependent differences in level of transgene expression in Rag-1-deficient mice when adoptively transferred. In four human T cell lines and human primary T lymphocytes MP71 vectors yielded an up to 75-fold higher GFP expression level in comparison with the standard Mo-MLV vector. In contrast to mouse T cells, the integration of the PRE into MP71 vectors induced in human T cells a further significant increase in transgene expression level. Southern blot analysis of CEM T cells revealed that the superior performance of MP71 vectors was not due to a higher rate of viral integration. In summary, MP71 vectors are useful tools for stable, high-level gene expression in T lymphocytes, for example, in the expression of T cell receptor genes.

A long HBV transcript encoding pX is inefficiently exported from the nucleus

The longest hepatitis B virus transcript is a 3.9-kb mRNA whose function remained unclear. In this study, we wished to identify the translation products and physiological role of this viral transcript. This transcript initiates from the X promoter region ignoring the inefficient and noncanonical viral polyadenylation signal at the first round of transcription. However, an HBV mutant with canonical polyadenylation signal continues, though with lower efficiency, to program the synthesis of this long transcript, indicating that the deviated HBV polyadenylation signal is important but not essential to enable transcription of the 3.9-kb species. The 3.9-kb RNA contains two times the X open reading frame (ORF). The X ORF at the 5'-end is positioned upstream of the CORE gene. By generating an HBV DNA mutant in which the X and Core ORFs are fused, we demonstrated the production of a 40-kDa X-Core fusion protein that must be encoded by the 3.9-kb transcript. Mutagenesis studies revealed that the production of this protein depends on the 5' X ORF ATG, suggesting that the 3.9-kb RNA is active in translation of the X ORF. Based on these features, the 3.9-kb transcript was designated lxRNA for long X RNA. Unlike other HBV transcripts, lxRNA harbors two copies of PRE, the posttranscriptional regulatory element that controls the nuclear export of HBV mRNAs. Unexpectedly, despite the presence of PRE sequences, RNA fractionation analysis revealed that lxRNA barely accumulates in the cytoplasm, suggesting that nuclear export of lxRNA is poor. Collectively, our data suggest that two distinct HBV mRNA species encode pX and that the HBV transcripts are differentially regulated at the level of nuclear export.

Overexpression of hepatitis B virus surface antigens including the preS1 region in a serum-free Chinese hamster ovary cell line

Current hepatitis B virus (HBV) vaccines consist of preparations of recombinant HBV major surface antigen (sAg) and are protective in about 90-95% of vaccinated subjects. In improved vaccines, the frequency of nonresponders to the classical vaccine could be reduced by including additional epitopes from the preS-domains of the middle and large surface antigens. In this report, the development and characterization of a CHO cell line for HBsAg, expressing major, middle, and large antigens are described. Despite the previously reported retention of secreted proteins by the preS1 domain, cell lines could be amplified that secreted large amounts of the complete set of antigens. A producer line was established that expressed 1mg HBsAg per 100ml suspension culture per week during exponential growth. The productivity per cell increased further by at least threefold when the culture reached the stationary phase at high cell densities. In the production cell line, several hundred copies of the HBV vector were integrated at two adjacent sites into chromosome 2. The cell line was adapted to growth in a defined protein-free medium minimizing the risk of adventitious agents introduced by animal derived supplements. The cell line stably produced antigen over several months. In the candidate vaccine, both preS2 and preS1 domains were present at ratios similar to HBsAg from human sera. In summary, a production cell line for an improved HBV vaccine is presented with properties such as high productivity, long term stability of expression, and growth in protein-free media.

Real-time quantitative reverse transcriptase-polymerase chain reaction as a method for determining lentiviral vector titers and measuring transgene expression

The use of lentiviral vectors for basic research and potential future clinical applications requires methodologies that can accurately determine lentiviral titers and monitor viral transgene expression within target cells, beyond the context of reporter genes typically used for this purpose. Here we describe a quantitative RT-PCR (qRT-PCR) method that achieves both goals using primer sequences that are specific for the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), an enhancer contained in many retroviral vectors and that is incorporated in the 3' UTR of nascent transgene transcripts. Quantitation of titers of three recombinant lentiviruses, genetically identical except for the transgene, demonstrated consistent differences in titer that were likely due to transgene-associated toxicity in producer cells and target cells. Viruses encoding the tumor-associated antigens tyrosinase and neo-poly(A) polymerase yielded reproducibly lower titers than a virus encoding enhanced green fluorescent protein (GFP) at the viral RNA, integrated DNA, and transgene mRNA levels, as measured by WPRE qPCR. Furthermore, the magnitude of differences in expression of the three transgenes in transduced target cells could not have been predicted by measuring vector DNA integration events. Since transgene expression in target cells is the most common goal of lentiviral transduction, and since methods to quantify transgene expression on the protein level are not always readily available, qRT-PCR based on a nucleotide sequence included in the transcript provides a useful tool for titering novel recombinant lentiviruses.

Immunotherapy with a posttranscriptionally modified DNA vaccine induces complete protection against metastatic neuroblastoma

The successful induction of a T-cell-mediated tumor-protective immunity against poorly immunogenic malignancies remains a major challenge for cancer immunotherapy. We achieved this by immunization with a tyrosine hydroxylase (mTH)-based DNA vaccine, enhanced with the posttranscriptional regulatory acting RNA element (WPRE), derived from woodchuck hepatitis virus in combination with an antibody-cytokine fusion protein (ch14.18-IL-2) that targets interleukin-2 (IL-2) to the tumor microenvironment. This DNA vaccine mTH-WPRE was carried by attenuated Salmonella typhimurium and applied by oral gavage in a mouse model of neuroblastoma. Mice immunized with the mTH-WPRE vaccine, and which additionally received a boost with suboptimal doses of ch14.18-IL-2, were completely protected against hepatic neuroblastoma metastases. In contrast, all controls presented with disseminated metastases. Both T-cell and natural killer (NK) cell-dependent mechanisms were involved in the induction of a systemic tumor-protective immunity. Thus, up-regulation of interferon-gamma (IFN-gamma) expression in CD8(+) T cells occurred only in those animals that received the mTH-WPRE vaccine plus the ch14.18-IL-2 boost. Up-regulation of this proinflammatory cytokine was not observed in mice immunized with mTH-WPRE vaccine alone. A role for NK cells was indicated by the complete abrogation of systemic tumor-protective immunity in all animals that were depleted of NK cells in vivo. Taken together, these data demonstrate that immunization with a posttranscriptionally enhanced DNA vaccine encoding the WPRE sequence, combined with a boost of the ch14.18-IL-2 fusion protein, completely protects against hepatic metastases in a murine model of neuroblastoma and therefore may lead to a new strategy for immunotherapy and prevention of metastatic neuroblastoma.

Lentiviral vectors for efficient transduction of isolated primary quiescent hepatocytes

BACKGROUND/AIMS

Lentiviral vectors were designed to obtain efficient transduction of primary quiescent hepatocytes.

METHODS

A hepatitis B virus (HBV) fragment containing enhancers and posttranscriptional regulatory element was used to increase expression levels. The human immunodeficiency virus (HIV) central polypurine tract (PPT) was used to increase transduction of quiescent cells. HBV elements were incorporated downstream and the HIV PPT was incorporated upstream of green fluorescent protein expression cassettes in third generation self inactivating lentiviral vectors.

RESULTS

The HBV fragment increased mean fluorescence of transduced HepG2 hepatoma cells 4.3+/-1.7-fold and 2.3-6.0-fold in various other cell types. A role of HBV x protein in the function of the HBV element was excluded. The HBV element increased the number of transducing units per pg of HIV p24 twofold. The unmodified lentiviral vector transduced 5+/-1% of cultured quiescent primary rat hepatocytes, HBV elements increased transduction to 54+/-13% and increased fluorescence 2.8+/-0.6-fold. The PPT increased transduction to 47+/-11% and increased fluorescence 2.3+/-0.4-fold. The vector with PPT and HBV elements transduced 68+/-10% of hepatocytes and increased fluorescence synergistically, 17+/-6 fold.

CONCLUSIONS

This study shows that HBV elements or HIV PPT are required for efficient transduction of primary hepatocytes.

Characterization of the nuclear export signal of polypyrimidine tract-binding protein

The polypyrimidine tract-binding protein (PTB) is a nuclear protein that regulates alternative splicing. In addition, it plays a role in the cytoplasm during infection by some viruses and functions as a positive effector of hepatitis B virus RNA export. Thus, it presumably contains a nuclear export signal (NES). Using a heterokaryon export assay in transfected cultured cells, we have shown that the N-terminal 25 amino acid residues of PTB function as an autonomous NES, with residues 11-16 being important for its activity. Unlike the heteronuclear ribonucleoprotein A1 NES, this NES is separable from the nuclear localization signal, which spans the entire N-terminal 60 residues of PTB. The PTB NES cannot be shown to bind to CAS or Crm1, cellular receptors known to export proteins from the nucleus, and it functions in the presence of leptomycin B, a specific inhibitor of Crm1-dependent export. PTB deleted of its NES, unlike wild type PTB, does not stimulate the export of hepatitis B virus RNA. Therefore, the PTB NES is a functionally important domain of this multifunctional protein that utilizes an unknown export receptor.

Antiviral activity of interferon-alpha against hepatitis B virus can be studied in non-hepatic cells and Is independent of MxA

It is well established that interferon-alpha can induce non-cytotoxic intracellular suppression of hepatitis B virus replication, but the mechanisms involved are unclear. Cell culture studies to characterize these mechanisms are restricted, in part because hepatitis B virus replicates almost exclusively in liver-derived cells. To overcome this limitation we used a cytomegalovirus promoter-controlled hepatitis B virus expression system, which leads to intracellular viral replication even in non-hepatic cell lines. In this experimental system interferon-alpha treatment specifically suppressed viral replication demonstrating that antiviral activities against hepatitis B virus are not restricted to hepatic cells. Furthermore, the interferon-inducible MxA protein was recently reported to play a key role in the antiviral action of interferon-alpha against hepatitis B virus. Our data demonstrate that interferon-alpha also suppresses hepatitis B virus replication in MxA-deficient HEp2 cells, indicating that MxA is not essential for these activities. Taken together, our data imply that the experimental approach presented can also be adapted to established cell lines which are deficient in parts of the signal transduction pathway or other elements located further downstream, providing important insights into mechanisms specifically suppressing hepatitis B virus.

Role of polypyrimidine tract binding protein in the function of the hepatitis B virus posttranscriptional regulatory element

The hepatitis B virus posttranscriptional regulatory element (PRE) is an RNA element that increases the expression of unspliced mRNAs, apparently by facilitating their export from the nucleus. We have identified a cellular protein that binds to the PRE as the polypyrimidine tract binding protein (PTB), which shuttles rapidly between the nucleus and the cytoplasm. Mutants of the PRE with mutations in PTB binding sites show markedly decreased activity, while cells that stably overexpress PTB show increased PRE-dependent gene expression. Export of PTB from the nucleus, like PRE function, is blocked by a mutant form of Ran binding protein 1 but not by leptomycin B. Therefore, PTB is important for PRE activity and appears to function as an export factor for PRE-containing mRNAs.

Identification of a novel posttranscriptional regulatory element by using a rev- and RRE-mutated human immunodeficiency virus type 1 DNA proviral clone as a molecular trap

Human immunodeficiency virus (HIV) and all other lentiviruses utilize the essential viral protein Rev, which binds to RRE RNA, to export their unspliced and partially spliced mRNAs from the nucleus. We used a rev- and RRE-defective HIV type 1 (HIV-1) molecular clone in complementation experiments to establish a method for the rapid isolation of posttranscriptional regulatory elements from the mammalian genome by selecting for rescue of virus replication. Viruses rescued by this method contained a novel element with homology to rodent intracisternal A-particle (IAP) retroelements. A functional element was contained within a 247-nucleotide fragment named RNA transport element (RTE), which was able to promote replication of the Rev- and RRE-defective HIV-1 in both human lymphoid cell lines and primary lymphocytes, demonstrating its potent posttranscriptional function. RTE was functional in many cell types, indicating that the cellular factors that recognize RTE are widely expressed and evolutionarily conserved. RTE also promoted RNA export from Xenopus oocyte nuclei. RTE-mediated RNA transport was CRM1 independent, and RTE did not show high affinity for binding to mRNA export factor TAP/NXF1. Since CRM1 and TAP/NXF1 are critical export receptors associated with the two recognized mRNA export pathways, these results suggest that RTE functions via a distinct export mechanism. Taken together, our results identify a novel posttranscriptional control element that uses a conserved cellular export mechanism.

In vivo liver-directed gene transfer in rats and pigs with large anionic multilamellar liposomes: routes of administration and effects of surgical manipulations on transfection efficiency

Conventional large (500-800 nm) multilamellar liposomes encapsulating DNA have been used in vivo as gene vectors into rat and pig liver. By using the intraportal vein route, high dose DNA (10 mg/kg) provided low efficiency and transient luciferase gene expression in the liver. This gene expression was, however, increased by liver resection (> 50%), ischemia (20 min) or orthotopic transplantation. As evidenced by histochemical analysis of beta-galactosidase expression, the gene transfection mainly ensued in Kupffer cells, but spleen and lung were contaminated. In comparison, injection into the bile duct of even 25-fold lower dose of liposome-encapsulated DNA (0.4 mg/kg) produced higher (100-fold) and long-lasting (during 6 days, at least) luciferase expression in rat liver. The gene expression was restricted to the liver and enhanced by liver resection. By this route, transgene-expressing cells were mainly hepatocytes. A treatment with colchicine prior to the administration of the vector allowed the persistence of relative high gene expression for at least 7 days. In pigs, qualitatively similar, but quantitatively less efficient gene expression was obtained by either the portal vein or the bile duct route. These results indicate that intrabile duct route might render large non-viral vectors applicable to gene transfer into the hepatocytes. The efficiency of liposome-mediated gene transfer into the liver can be increased by liver resection, ischemia or transplantation performed before DNA injection.