Carbonyl J acid derivatives block protein priming of hepadnaviral P protein and DNA-dependent DNA synthesis activity of hepadnaviral nucleocapsids

The combination of Schisandrin C and Luteolin synergistically attenuates hepatitis B virus infection via repressing HBV replication and promoting cGAS-STING pathway activation in macrophages

BACKGROUND

HBV infection can result in severe liver diseases and is one of the primary causes of liver cell carcinoma-related mortality. Liuwei Wuling tablet (LWWL) is a traditional Chinese medicine formula, with a protecting liver and decreasing enzyme activity, usually used to treat chronic hepatitis B with NAs in clinic. However, its main active ingredients and mechanism of action have not been fully investigated. Hence, we aimed to screen the active ingredient and effective ingredient combinations from Liuwei Wuling tablet to explore the anti-herpatitis B virus activity and mechanism.

METHODS

Analysis and screening of effective antiviral components in LWWL by network pharmacology, luteolin (Lut) may be a compound with significant antiviral activity. The mechanism of antiviral action of Lut was also found by real-time PCR detection and western blotting. Meanwhile, we established a co-culture model to investigate the antiviral mechanism of Schisandrin C (SC), one of the main active components of Schisandra chinensis fructus (the sovereign drug of LWWL). Next, HBV-infected mice were established by tail vein injection of pAAV-HBV1.2 plasmid and administered continuously for 20 days. And their antiviral capacity was evaluated by checking serum levels of HBsAg, HBeAg, levels of HBV DNA, and liver levels of HBcAg.

RESULTS

In this study, we conducted network pharmacology analysis on LWWL, and through in vitro experimental validation and data analysis, we found that luteolin (Lut) possessed obviously anti-HBV activity, inhibiting HBV replication by downregulating hepatocyte nuclear factor 4α (HNF4α) via the ERK pathway. Additionally, we established a co-culture system and proved that SC promoted activation of cGAS-STINIG pathway and IFN-β production in THP-1 cells to inhibit HBV replication in HepG2.2.15 cells. Moreover, we found the combination of SC and Lut shows a greater effect in inhibiting HBV compared to SC or Lut alone in HBV-infected mice.

CONCLUSION

Taken together, our study suggests that combination of SC and Lut may be potential candidate drug for the prevention and treatment of chronic hepatitis B.

Advancing beyond reverse transcriptase inhibitors: The new era of hepatitis B polymerase inhibitors

Hepatitis B virus (HBV) is a genetically diverse blood-borne virus responsible for chronic hepatitis B. The HBV polymerase plays a key role in viral genome replication within the human body and has been identified as a potential drug target for chronic hepatitis B therapeutics. However, available nucleotide reverse transcriptase inhibitors only target the reverse transcriptase domain of the HBV polymerase; they also pose resistance issues and require lifelong treatment that can burden patients financially. In this study, various chemical classes are reviewed that have been developed to target different domains of the HBV polymerase: Terminal protein, which plays a vital role in the formation of the viral DNA; Reverse transcriptase, which is responsible for the synthesis of the viral DNA from RNA, and; Ribonuclease H, which is responsible for degrading the RNA strand in the RNA-DNA duplex formed during the reverse transcription process. Host factors that interact with the HBV polymerase to achieve HBV replication are also reviewed; these host factors can be targeted by inhibitors to indirectly inhibit polymerase functionality. A detailed analysis of the scope and limitations of these inhibitors from a medicinal chemistry perspective is provided. The structure-activity relationship of these inhibitors and the factors that may affect their potency and selectivity are also examined. This analysis will be useful in supporting the further development of these inhibitors and in designing new inhibitors that can inhibit HBV replication more efficiently.

A 33-residue peptide tag increases solubility and stability of Escherichia coli produced single-chain antibody fragments

Single-chain variable fragments (scFvs), composed of variable domains of heavy and light chains of an antibody joined by a linker, share antigen binding capacity with their parental antibody. Due to intrinsically low solubility and stability, only two Escherichia coli-produced scFvs have been approved for therapy. Here we report that a 33-residue peptide, termed P17 tag, increases the solubility of multiple scFvs produced in Escherichia coli SHuffle strain by up to 11.6 fold. Hydrophilic sequence, especially charged residues, but not the predicted α-helical secondary structure of P17 tag, contribute to the solubility enhancement. Notably, the P17 tag elevates the thermostability of scFv as efficiently as intra-domain disulfide bonds. Moreover, a P17-tagged scFv targeting hepatitis B virus surface proteins shows over two-fold higher antigen-binding affinity and virus-neutralizing activity than the untagged version. These data strongly suggest a type I intramolecular chaperone-like activity of the P17 tag. Hence, the P17 tag could benefit the research, production, and application of scFv.

Low solubility and stability of Escherichia coli produced single chain variable fragments (scFvs) restrict their applications. Here the authors report a 33-residue peptide tag which simultaneously increases the solubility and thermostability of multiple scFvs produced in Escherichia coli SHuffle strain.

Relaxing the restricted structural dynamics in the human hepatitis B virus RNA encapsidation signal enables replication initiation in vitro

Hepadnaviruses, including hepatitis B virus (HBV) as a major human pathogen, replicate their tiny 3 kb DNA genomes by capsid-internal protein-primed reverse transcription of a pregenomic (pg) RNA. Initiation requires productive binding of the viral polymerase, P protein, to a 5´ proximal bipartite stem-loop, the RNA encapsidation signal ε. Then a residue in the central ε bulge directs the covalent linkage of a complementary dNMP to a Tyr sidechain in P protein´s Terminal Protein (TP) domain. After elongation by two or three nucleotides (nt) the TP-linked DNA oligo is transferred to a 3´ proximal acceptor, enabling full-length minus-strand DNA synthesis. No direct structural data are available on hepadnaviral initiation complexes but their cell-free reconstitution with P protein and ε RNA (Dε) from duck HBV (DHBV) provided crucial mechanistic insights, including on a major conformational rearrangement in the apical Dε part. Analogous cell-free systems for human HBV led at most to P—ε binding but no detectable priming. Here we demonstrate that local relaxation of the highly basepaired ε upper stem, by mutation or via synthetic split RNAs, enables ε-dependent in vitro priming with full-length P protein from eukaryotic translation extract yet also, and without additional macromolecules, with truncated HBV miniP proteins expressed in bacteria. Using selective 2-hydroxyl acylation analyzed by primer extension (SHAPE) we confirm that upper stem destabilization correlates with in vitro priming competence and show that the supposed bulge-closing basepairs are largely unpaired even in wild-type ε. We define the two 3´ proximal nt of this extended bulge as main initiation sites and provide evidence for a Dε-like opening of the apical ε part upon P protein binding. Beyond new HBV-specific basic aspects our novel in vitro priming systems should facilitate the development of high-throughput screens for priming inhibitors targeting this highly virus-specific process.

Chronic hepatitis B virus (HBV) infection puts >250 million people at an increased risk for severe liver disease. Current treatments can control but rarely cure infection. HBV features a 3,200 bp DNA genome, generated by reverse transcription of a pregenomic (pg) RNA. To initiate DNA synthesis the viral polymerase, P protein, employs a stem-loop on pgRNA, ε, to covalently link a defined first nucleotide to its Terminal Protein (TP) domain. This protein-priming is highly virus-specific yet poorly understood. More is known for duck HBV (DHBV) where, different from HBV, protein-priming was successfully reconstituted in vitro years ago. One insight was that gaining priming-competence involves opening of the apical stem in DHBV ε RNA (Dε); in HBV ε the more extensive basepairing might restrict such dynamics. Here we relaxed these constraints by identifying functional but less stably folded, including split, HBV ε variants. Several such variants supported in vitro priming, including in a simple two-component-system employing a shortened recombinant P protein. Amongst other data the new cell-free systems yielded a first view on a major conformational change in HBV ε RNA bound to P protein, highlighting the importance of RNA dynamics for the human virus. Beyond furthering basic understanding our data should facilitate screening for protein-priming inhibitors as new anti-HBV agents.

Establishment of a system for finding inhibitors of ε RNA binding with the HBV polymerase

Although several nucleo(s)tide analogs are available for treatment of HBV infection, long‐term treatment with these drugs can lead to the emergence of drug‐resistant viruses. Recent HIV‐1 studies suggest that combination therapies using nucleo(s)tide reverse transcriptase inhibitors (NRTIs) and non‐nucleo(s)tide reverse transcriptase inhibitors (NNRTIs) could drastically inhibit the viral genome replication of NRTI‐resistant viruses. In order to carry out such combinational therapy against HBV, several new NRTIs and NNRTIs should be developed. Here, we aimed to identify novel NNRTIs targeting the HBV polymerase terminal protein (TP)‐reverse transcriptase (RT) (TP‐RT) domain, which is a critical domain for HBV replication. We expressed and purified the HBV TP‐RT with high purity using an Escherichia coli expression system and established an in vitro ε RNA‐binding assay system. Then, we used TP‐RT in cell‐free assays to screen candidate inhibitors from a chemical compound library, and identified two compounds, 6‐hydroxy‐DL‐DOPA and N‐oleoyldopamine, which inhibited the binding of ε RNA with the HBV polymerase. Furthermore, these drugs reduced HBV DNA levels in cell‐based assays as well by inhibiting packaging of pregenome RNA into capsids. The novel screening system developed herein should open a new pathway the discovery of drugs targeting the HBV TP‐RT domain to treat HBV infection.

Interferon-inducible MX2 is a host restriction factor of hepatitis B virus replication

BACKGROUND & AIMS

Non-cytolytic cure of HBV-infected hepatocytes by cytokines, including type I interferons (IFNs), is of importance for resolving acute and chronic infection. However, as IFNs stimulate hundreds of genes, those most relevant for HBV suppression remain largely unknown. Amongst them are the large myxovirus resistance (Mx) GTPases. Human MX1 (or MxA) is active against many RNA viruses, while MX2 (or MxB) was recently found to restrict HIV-1, HCV, and herpesviruses. Herein, we investigated the anti-HBV activity of MX2.

METHODS

The potential anti-HBV activity of MX2 and functional variants were assessed in transfected and HBV-infected hepatoma cells and primary human hepatocytes, employing multiple assays to analyze the synthesis and decay of HBV nucleic acids. The specific roles of MX2 in IFN-α-driven inhibition of HBV transcription and replication were assessed by MX2-specific shRNA interference (RNAi).

RESULTS

Both MX2 alone and IFN-α substantially inhibited HBV replication, due to significant deceleration of the synthesis and slight acceleration of the turnover of viral RNA. RNAi knockdown of MX2 significantly reduced the inhibitory effects of IFN-α. Strikingly, MX2 inhibited HBV infection by reducing covalently closed circular DNA (cccDNA), most likely by indirectly impairing the conversion of relaxed circular DNA to cccDNA rather than by destabilizing existing cccDNA. Various mutations affecting the GTPase activity and oligomerization status reduced MX2's anti-HBV activity.

CONCLUSION

MX2 is an important IFN-α inducible effector that decreases HBV RNA levels but can also potently inhibit HBV infection by indirectly impairing cccDNA formation. MX2 likely has the potential for therapeutic applications aimed at curing HBV infection by eliminating cccDNA.

LAY SUMMARY

This study shows that the protein MX2, which is induced by interferon-α, has important anti-hepatitis B virus (HBV) effector functions. MX2 can reduce the amount of covalently closed circular DNA, which is the form of DNA that HBV uses to maintain viral persistence within hepatocytes. MX2 also reduces HBV RNA levels by downregulating synthesis of viral RNA. MX2 likely represents a novel intrinsic HBV inhibitor that could have therapeutic potential, as well as being useful for improving our understanding of the complex biology of HBV and the antiviral mechanisms of interferon-α.

Identification and characterization of a novel hepatitis B virus pregenomic RNA encapsidation inhibitor

Currently, therapies to treat chronic hepatitis B (CHB) infection are based on the use of interferon-α or nucleos(t)ide analogs (NAs) to prevent viral DNA synthesis by inhibiting the reverse transcriptase activity of the hepatitis B virus (HBV) polymerase (Pol). However, these therapies are not curative; thus, the development of novel anti-HBV agents is needed. In accordance with this unmet medical need, we devised a new target- and cell-based, high-throughput screening assay to identify novel small molecules that block the initial interaction of the HBV Pol with its replication template the viral pregenomic RNA (pgRNA). We screened approximately 110,000 small molecules for the ability to prevent HBV Pol recognition of the pgRNA 5' epsilon (ε) stem-loop structure, identifying (Z)-2-(allylamino)-4-amino-N'-cyanothiazole-5-carboximidamide (AACC). Viral nucleocapsid-captured quantitative RT-PCR and Western blot results revealed that AACC significantly decreased encapsidated pgRNA levels and blocked capsid assembly without affecting core protein expression in stable HBV-replicating cells. As a result, both intra- and extracellular accumulation of viral DNA was strongly reduced. AACC treatment of HepG2-sodium taurocholate transporting polypeptide (NTCP) cells and primary human hepatocytes infected with cell culture- or patient-derived HBV isolates showed both time- and dose-dependent inhibition of infectious viral progeny and rcDNA production. Furthermore, AACC showed cross-genotypic activity against genotypes B, C, and D. Of note, AACC inhibited the viral replication of lamivudine and a capsid inhibitor-resistant HBV, and showed synergistic effects with NAs and a capsid inhibitor. In conclusion, we identified a novel class of compounds specifically targeting the ε-Pol interaction and thereby preventing the encapsidation of pgRNAs into viral capsids. This promising new HBV inhibitor class potently inhibits HBV amplification with distinct characteristics from existing NAs and other drugs currently under development, promising to add value to existing therapies for CHB.

Rosmarinic acid is a novel inhibitor for Hepatitis B virus replication targeting viral epsilon RNA-polymerase interaction

Current therapeutics for hepatitis B virus (HBV) patients such as nucleoside analogs (NAs) are effective; however, new antiviral drugs against HBV are still desired. Since the interaction between the epsilon (ε) sequence of HBV pregenomic RNA and viral polymerase (Pol) is a key step in the HBV replication cycle, we aimed to identify small compounds for its inhibition, and established a pull-down assay system for the detection of ε-RNA-binding-Pol. Screening showed that 5 out of 3,965 compounds inhibited ε-Pol binding, and we identified rosmarinic acid, which exhibited specificity, as a potential antiviral agent. In order to examine the anti-HBV effects of rosmarinic acid, HBV-infected primary human hepatocytes from a humanized mouse liver were treated with rosmarinic acid. The rosmarinic acid treatment decreased HBV components including the amounts of extracellular HBV DNA with negligible cytotoxicity. We also investigated the combined effects of rosmarinic acid and the NA, lamivudine. rosmarinic acid slightly enhanced the anti-HBV activity of lamivudine, suggesting that the HBV replication step targeted by rosmarinic acid is distinct from that of NA. We analyzed an additional 25 rosmarinic acid derivatives, and found that 5 also inhibited ε-Pol. Structural comparisons between these derivatives implied that the "two phenolic hydroxyl groups at both ends" and the "caffeic acid-like structure" of rosmarinic acid are critical for the inhibition of ε-Pol binding. Collectively, our results demonstrate that rosmarinic acid inhibits HBV replication in HBV-infected cells by specifically targeting ε-Pol binding.

Animal models of HBV infection

The mechanisms determining hepatitis B virus (HBV) persistence and pathogenesis are not fully elucidated, but appear to be multi-factorial. Current medication to repress viral replication is available; however, the unique replication strategies employed by HBV enable the virus to persist within the infected hepatocytes. Consequently, cure is rarely achieved. Progresses in HBV research and preclinical testing of antiviral agents have been limited by the narrow species- and tissue-tropism of the virus, the paucity of infection models available and the restrictions imposed by the use of chimpanzees, the only animals fully susceptible to HBV infection. Mice are not HBV permissive but major efforts have focused on the development of mouse models of HBV replication and infection, such as the generation of humanized mice. By presenting the different animal models available, this review will highlight the most important and clinically relevant findings that have been retrieved from the respective systems.

Hepatitis B virus reverse transcriptase - Target of current antiviral therapy and future drug development

Hepatitis B virus (HBV) infections rely on the proper functioning of the viral polymerase enzyme, a specialized reverse transcriptase (RT) with multiple activities. All currently approved antiviral drugs for the treatment of chronic HBV infection, except for interferon, target the RT and belong to the same chemical class - they are all nucleoside analogs. Viral DNA synthesis is carried out by the RT enzyme in several different steps, each with distinct RT conformational requirements. In principle, each stage may be targeted by distinct antiviral drugs. In particular, the HBV RT has the unique ability to initiate viral DNA synthesis using itself as a protein primer in a novel protein priming reaction. In order to help identify RT inhibitors and study their mechanisms of action, a number of experimental systems have been developed, each varying in its ability to dissect the protein priming stage and subsequent stages of viral DNA synthesis at the molecular level. Two of the most effective drugs to date, entecavir and tenofovir, can inhibit both the protein priming and the subsequent DNA elongation stages of HBV DNA synthesis. Interestingly, clevudine, a thymidine analog, can inhibit both protein priming and DNA elongation in a non-competitive manner and without being incorporated into the viral DNA. Thus, a nucleoside RT inhibitor (NRTI) can functionally mimic a non-NRTI (NNRTI) in its inhibition of the HBV RT. Therefore, novel NRTIs as well as NNRTIs may be developed to inhibit the DNA synthesis activity of the HBV RT. Furthermore, additional activities of the RT that are also essential to HBV replication, including specific recognition of the viral RNA and its packaging into viral nucleocapsids, may be exploited for antiviral development. To achieve a more potent inhibition of viral replication and ultimately cure chronic HBV infection, the next generation of anti-HBV therapies will likely need to include NRTIs, NNRTIs, and other agents that target the viral RT as well as other viral and host factors in various combinations. This article forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for hepatitis B."

Inhibition of duck hepatitis B virus replication by mimic peptides in vitro

The aim of the present study was to investigate the inhibitory effect of specific mimic peptides targeting duck hepatitis B virus polymerase (DHBVP) on duck hepatitis B virus (DHBV) replication in primary duck hepatocytes. Phage display technology (PDT) was used to screen for mimic peptides specifically targeting DHBVP and the associated coding sequences were determined using DNA sequencing. The selected mimic peptides were then used to treat primary duck hepatocytes infected with DHBV in vitro. Infected hepatocytes expressing the mimic peptides intracellularly were also prepared. The cells were divided into mimic peptide groups (EXP groups), an entecavir-treated group (positive control) and a negative control group. The medium was changed every 48 h. Following a 10-day incubation, the cell supernatants were collected. DHBV-DNA in the cellular nucleus, cytoplasm and culture supernatant was analyzed by quantitative polymerase chain reaction (qPCR). Eight mimic peptides were selected following three PDT screening rounds for investigation in the DHBV-infected primary duck hepatocytes. The qPCR results showed that following direct treatment with mimic peptide 2 or 7, intracellular expression of mimic peptide 2 or 7, or treatment with entecavir, the DHBV-DNA levels in the culture supernatant and cytoplasm of duck hepatocytes were significantly lower than those in the negative control (P<0.05). The cytoplasmic DHBV-DNA content of the cells treated with mimic peptide 7 was lower than that in the other groups (P<0.05). In addition, the DHBV-DNA content of the nuclear fractions following the intracellular expression of mimic peptide 7 was significantly lower than that in the other groups (P<0.05). Mimic peptides specifically targeting DHBVP, administered directly or expressed intracellularly, can significantly inhibit DHBV replication in vitro.

New insights into hepatitis B virus biology and implications for novel antiviral strategies

Hepatitis B virus (HBV), a small DNA virus with a unique replication mode, can cause chronic hepatitis (CHB), which is characterized by the persistence of the viral covalently closed circular DNA that serves as the template for HBV replication and the production of large amounts of secreted HBV surface antigen (HBsAg) that is present in excess of the levels of infectious virus. Despite the success of currently approved antiviral treatments for CHB patients, including interferon and nucleotide analogs, which suppress HBV replication and reduce the risk of CHB-related liver diseases, these therapies fail to eradicate the virus in most of the patients. With the development of the cell and animal models for HBV study, a better understanding of the HBV life cycle has been achieved and a series of novel antiviral strategies that target different stages of HBV replication have been designed to overcome the viral factors that contribute to HBV persistence. Such basic HBV research advancements and therapeutic developments are the subject of this review.

Unveiling the roles of HBV polymerase for new antiviral strategies

Infection with HBV is common worldwide, with over 350 million chronic carriers. Chronic HBV infection is associated with cirrhosis and hepatocellular carcinoma. All currently available oral antivirals are directed against the HBV polymerase enzyme, a reverse transcriptase. HBV polymerase contains several important domains and motifs which define its functions and reveal ways to further target it. This enzyme executes many functions required for the HBV replication cycle, including viral RNA binding, RNA packaging, protein priming, template switching, DNA synthesis and RNA degradation. In addition, HBV polymerase must interact with host proteins for its functions. Future therapeutics may inhibit not only the DNA synthesis steps which are carried out by the reverse transcriptase domain (as all current antivirals do) but other domains, functions and interactions which are essential to the HBV replication cycle.

Sequences in the terminal protein and reverse transcriptase domains of the hepatitis B virus polymerase contribute to RNA binding and encapsidation

Hepatitis B virus (HBV) antiviral therapy is plagued by limited efficacy and resistance to most nucleos(t)ide analog drugs. We have proposed that the complex RNA binding mechanism of the HBV reverse transcriptase (P) may be a novel target for antivirals. We previously found that RNA binds to the duck HBV (DHBV) P through interactions with the T3 and RT1 motifs in the viral terminal protein and reverse transcriptase domains, respectively. Here, we extended these studies to HBV P. HBV T3 and RT1 synthetic peptides bound RNA in a similar manner as did analogous DHBV peptides. The HBV T3 motif could partially substitute for DHBV T3 during RNA binding and DNA priming by DHBV P, whereas replacing RT1 supported substantial RNA binding but not priming. Substituting both the HBV T3 and RT1 motifs restored near wild-type levels of RNA binding but supported very little priming. Alanine-scanning mutations to the HBV T3 and RT1 motifs blocked HBV ε RNA binding in vitro and pgRNA encapsidation in cells. These data indicate that both the HBV T3 and RT1 motifs contain sequences essential for HBV ε RNA binding and encapsidation of the RNA pregenome, which is similar to their functions in DHBV. Small molecules that bind to T3 and/or RT1 would therefore inhibit encapsidation of the viral RNA and block genomic replication. Such drugs would target a novel viral function and would be good candidates for use in combination with the nucleoside analogs to improve efficacy of antiviral therapy.

Nucleoside/nucleotide analog inhibitors of hepatitis B virus polymerase: mechanism of action and resistance

Hepatitis B virus (HBV) polymerase and human immunodeficiency virus (HIV) reverse transcriptase are structurally related. However, the HBV enzyme has a protein priming activity absent in the HIV enzyme. Approved nucleoside/nucleotide inhibitors of the HBV polymerase include lamivudine, adefovir, telbivudine, entecavir and tenofovir. Although most of them target DNA elongation, guanosine and adenosine analogs (e.g. entecavir and tenofovir, respectively) also impair protein priming. Major mutational patterns conferring nucleoside/nucleotide analog resistance include the combinations rtL180M/rtM204(I/V) (for lamivudine, entecavir, telbivudine and clevudine) and rtA181V/rtN236T (for adefovir and tenofovir). However, development of drug resistance is very slow for entecavir and tenofovir. Novel nucleoside/nucleotide analogs in advanced clinical trials include phosphonates similar to adefovir or tenofovir, and new tenofovir derivatives with improved pharmacological properties.

Large-scale production and structural and biophysical characterizations of the human hepatitis B virus polymerase

Hepatitis B virus (HBV) is a major human pathogen that causes serious liver disease and 600,000 deaths annually. Approved therapies for treating chronic HBV infections usually target the multifunctional viral polymerase (hPOL). Unfortunately, these therapies--broad-spectrum antivirals--are not general cures, have side effects, and cause viral resistance. While hPOL remains an attractive therapeutic target, it is notoriously difficult to express and purify in a soluble form at yields appropriate for structural studies. Thus, no empirical structural data exist for hPOL, and this impedes medicinal chemistry and rational lead discovery efforts targeting HBV. Here, we present an efficient strategy to overexpress recombinant hPOL domains in Escherichia coli, purifying them at high yield and solving their known aggregation tendencies. This allowed us to perform the first structural and biophysical characterizations of hPOL domains. Apo-hPOL domains adopt mainly α-helical structures with small amounts of β-sheet structures. Our recombinant material exhibited metal-dependent, reverse transcriptase activity in vitro, with metal binding modulating the hPOL structure. Calcomine orange 2RS, a small molecule that inhibits duck HBV POL activity, also inhibited the in vitro priming activity of recombinant hPOL. Our work paves the way for structural and biophysical characterizations of hPOL and should facilitate high-throughput lead discovery for HBV.

IMPORTANCE

The viral polymerase from human hepatitis B virus (hPOL) is a well-validated therapeutic target. However, recombinant hPOL has a well-deserved reputation for being extremely difficult to express in a soluble, active form in yields appropriate to the structural studies that usually play an important role in drug discovery programs. This has hindered the development of much-needed new antivirals for HBV. However, we have solved this problem and report here procedures for expressing recombinant hPOL domains in Escherichia coli and also methods for purifying them in soluble forms that have activity in vitro. We also present the first structural and biophysical characterizations of hPOL. Our work paves the way for new insights into hPOL structure and function, which should assist the discovery of novel antivirals for HBV.

An efficient antiviral strategy for targeting hepatitis B virus genome using transcription activator-like effector nucleases

The hepatitis B virus (HBV) is a DNA virus that can cause chronic hepatitis B (CHB) in humans. Current therapies for CHB infection are limited in efficacy and do not target the pre-existing viral genomic DNA, which are present in the nucleus as a covalently closed circular DNA (cccDNA) form. The transcription activator-like (TAL) effector nucleases (TALENs) are newly developed enzymes that can cleave sequence-specific DNA targets. Here, TALENs targeting the conserved regions of the viral genomic DNA among different HBV genotypes were constructed. The expression of TALENs in Huh7 cells transfected with monomeric linear full-length HBV DNA significantly reduced the viral production of HBeAg, HBsAg, HBcAg, and pgRNA, resulted in a decreased cccDNA level and misrepaired cccDNAs without apparent cytotoxic effects. The anti-HBV effect of TALENs was further demonstrated in a hydrodynamic injection-based mouse model. In addition, an enhanced antiviral effect with combinations of TALENs and interferon-α (IFN-α) treatment was observed and expression of TALENs restored HBV suppressed IFN-stimulated response element-directed transcription. Taken together, these data indicate that TALENs can specifically target and successfully inactivate the HBV genome and are potently synergistic with IFN-α, thus providing a potential therapeutic strategy for treating CHB infection.

Hepatitis B virus reverse transcriptase: diverse functions as classical and emerging targets for antiviral intervention

Hepatitis B virus (HBV) infection remains a global health problem with over 350 million chronically infected, causing an increased risk of cirrhosis and hepatocellular carcinoma. Current antiviral chemotherapy for HBV infection include five nucleos(t)ide analog reverse transcriptase inhibitors (NRTIs) that all target one enzymatic activity, DNA strand elongation, of the HBV polymerase (HP), a specialized reverse transcriptase (RT). NRTIs are not curative and long-term treatment is associated with toxicity and the emergence of drug resistant viral mutations, which can also result in vaccine escape. Recent studies on the multiple functions of HP have provided important mechanistic insights into its diverse roles during different stages of viral replication, including interactions with viral pregenomic RNA, RNA packaging into nucleocapsids, protein priming, minus- and plus-strand viral DNA synthesis, RNase H-mediated degradation of viral RNA, as well as critical host interactions that regulate the multiple HP functions. These diverse functions provide ample opportunities to develop novel HP-targeted antiviral treatments that should contribute to curing chronic HBV infection.

Evidence for multiple distinct interactions between hepatitis B virus P protein and its cognate RNA encapsidation signal during initiation of reverse transcription

Replication of hepatitis B virus (HBV) via protein-primed reverse transcription is initiated by binding of the viral P protein to the conserved ε stem-loop on the pregenomic (pg) RNA. This triggers encapsidation of the complex and the ε-templated synthesis of a short P protein-linked DNA oligonucleotide (priming) for subsequent minus-strand DNA extension. ε consists of a lower and upper stem, a bulge containing the priming template, and an apical loop. The nonhelical subelements are considered important for DNA synthesis and pgRNA packaging whereas the role of the upper stem is not well characterized. Priming itself could until recently not be addressed because in vitro generated HBV P - ε complexes showed no activity. Focussing on the four A residues at the base and tip of the upper ε stem and the two U residues in the loop we first investigated the impact of 24 mutations on viral DNA accumulation in transfected cells. While surprisingly many mutations were tolerated, further analyzing the negatively acting mutations, including in a new cell-free priming system, revealed divergent position-related impacts on pgRNA packaging, priming activity and possibly initiation site selection. This genetic separability implies that the ε RNA undergoes multiple distinct interactions with P protein as pgRNA encapsidation and replication initiation progress, and that the strict conservation of ε in nature may reflect its optimal adaptation to comply with all of them. The data further define the most attractive mutants for future studies, including as decoys for interference with HBV replication.