Dengue Virus Reporter Replicon is a Valuable Tool for Antiviral Drug Discovery and Analysis of Virus Replication Mechanisms

Functional assessments of SARS-CoV-2 single-round infectious particles with variant-specific spike proteins on infectivity, drug sensitivity, and antibody neutralization

Working with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is restricted to biosafety level III (BSL-3) laboratory. The study used a trans-complementation system consisting of virus-like particles (VLPs) and DNA-launched replicons to generate SARS-CoV-2 single-round infectious particles (SRIPs) with variant-specific spike (S) proteins. S gene of Wuhan-Hu-1 strain (SWH1) or Omicron BA.1 variant (SBA.1), along with the envelope (E) and membrane (M) genes, were cloned into a tricistronic vector, co-expressed in the cells to produce variant-specific S-VLPs. Additionally, the replicon of the WH1-like strain without S, E, M and accessory genes, was engineered under the control by a CMV promoter to produce self-replicating RNAs within VLP-producing cells, led to create SWH1- and SBA.1-based SARS-CoV-2 SRIPs. The SBA.1-based SRIP showed lower virus yield, replication, N protein expression, fusogenicity, and infectivity compared to SWH1-based SRIPs. SBA.1-based SRIP also exhibited intermediate resistance to neutralizing antibodies produced by SWH1-based vaccines, but were effective at infecting cells with low ACE2 expression. Importantly, both S-based SRIPs responded similarly to remdesivir and GC376, with EC50 values ranging from 0.17 to 1.46 μM, respectively. The study demonstrated that this trans-complementation system is a reliable and efficient tool for generating SARS-CoV-2 SRIPs with variant-specific S proteins. SARS-CoV-2 SRIPs, mimicking authentic live viruses, facilitate comprehensive analysis of variant-specific virological characteristics, including antibody neutralization, and drug sensitivity in non-BSL-3 laboratories.

Dengue hemorrhagic fever: a growing global menace

Dengue virus is an arthropod-borne virus, transmitted by Aedes aegypti among humans. In this review, we discussed the epidemiology of dengue hemorrhagic fever (DHF) as well as the disease's natural history, cycles of transmission, clinical diagnosis, aetiology, prevention, therapy, and management. A systematic literature search was done by databases such as PubMed and Google Scholar using search terms, 'dengue fever', 'symptoms and causes of dengue fever', 'dengue virus transmission', and 'strategies to control dengue'. We reviewed relevant literature to identify hazards related to DHF and the most recent recommendations for its management and prevention. Clinical signs and symptoms of dengue infection range from mild dengue fever (DF) to potentially lethal conditions like DHF or dengue shock syndrome (DSS). Acute-onset high fever, muscle and joint pain, myalgia, a rash on the skin, hemorrhagic episodes, and circulatory shock are among the most common symptoms. An early diagnosis is vital to lower mortality. As dengue virus infections are self-limiting, but in tropical and subtropical areas, dengue infection has become a public health concern. Hence, developing and executing long-term control policies that can reduce the global burden of DHF is a major issue for public health specialists everywhere.

Cell type dependent stability and virulence of a recombinant SARS-CoV-2, and engineering of a propagation deficient RNA replicon to analyze virus RNA synthesis

Engineering of reverse genetics systems for newly emerged viruses allows viral genome manipulation, being an essential tool for the study of virus life cycle, virus-host interactions and pathogenesis, as well as for the development of effective antiviral strategies. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emergent human coronavirus that has caused the coronavirus disease (COVID-19) pandemic. The engineering of a full-length infectious cDNA clone and a fluorescent replicon of SARS-CoV-2 Wuhan-Hu-1, using a bacterial artificial chromosome, is reported. Viral growth and genetic stability in eleven cell lines were analyzed, showing that both VeroE6 cells overexpressing transmembrane serin protease 2 (TMPRSS2) and human lung derived cells resulted in the optimization of a cell system to preserve SARS-CoV-2 genetic stability. The recombinant SARS-CoV-2 virus and a point mutant expressing the D614G spike protein variant were virulent in a mouse model. The RNA replicon was propagation-defective, allowing its use in BSL-2 conditions to analyze viral RNA synthesis. The SARS-CoV-2 reverse genetics systems developed constitute a useful tool for studying the molecular biology of the virus, the development of genetically defined vaccines and to establish systems for antiviral compounds screening.

Antiviral Activity of an Indole-Type Compound Derived from Natural Products, Identified by Virtual Screening by Interaction on Dengue Virus NS5 Protein

Dengue is an acute febrile illness caused by the Dengue virus (DENV), with a high number of cases worldwide. There is no available treatment that directly affects the virus or the viral cycle. The objective of this study was to identify a compound derived from natural products that interacts with the NS5 protein of the dengue virus through virtual screening and evaluate its in vitro antiviral effect on DENV-2. Molecular docking was performed on NS5 using AutoDock Vina software, and compounds with physicochemical and pharmacological properties of interest were selected. The preliminary antiviral effect was evaluated by the expression of the NS1 protein. The effect on viral genome replication and/or translation was determined by NS5 production using DENV-2 Huh-7 replicon through ELISA and viral RNA quantification using RT-qPCR. The in silico strategy proved effective in finding a compound (M78) with an indole-like structure and with an effect on the replication cycle of DENV-2. Treatment at 50 µM reduced the expression of the NS5 protein by 70% and decreased viral RNA by 1.7 times. M78 is involved in the replication and/or translation of the viral genome.

Dengue virus infection - a review of pathogenesis, vaccines, diagnosis and therapy

The transmission of dengue virus (DENV) from an infected Aedes mosquito to a human, causes illness ranging from mild dengue fever to fatal dengue shock syndrome. The similar conserved structure and sequence among distinct DENV serotypes or different flaviviruses has resulted in the occurrence of cross reaction followed by antibody-dependent enhancement (ADE). Thus far, the vaccine which can provide effective protection against infection by different DENV serotypes remains the biggest hurdle to overcome. Therefore, deep investigation is crucial for the potent and effective therapeutic drugs development. In addition, the cross-reactivity of flaviviruses that leads to false diagnosis in clinical settings could result to delay proper intervention management. Thus, the accurate diagnostic with high specificity and sensitivity is highly required to provide prompt diagnosis in respect to render early treatment for DENV infected individuals. In this review, the recent development of neutralizing antibodies, antiviral agents, and vaccine candidates in therapeutic platform for DENV infection will be discussed. Moreover, the discovery of antigenic cryptic epitopes, principle of molecular mimicry, and application of single-chain or single-domain antibodies towards DENV will also be presented.

Establishment of Vero cell lines persistently harboring a yellow fever virus 17D subgenomic replicon

Yellow fever virus (YFV), a member of the genus Flavivirus, family Flaviviridae, is the etiological agent for an acute viral hemorrhagic disease, yellow fever. Although effective live attenuated vaccines based on the strain YFV 17D are currently available, no specific antiviral drug is available, and the disease remains a major public health concern. Hence, the discovery and development of antiviral drugs should lead to great benefits in controlling the disease. To provide a screening platform for antiviral agents targeting YFV RNA translation/replication, we have established and characterized two Vero cell lines that persistently harbor a subgenomic replicon derived from YFV 17D-204 (referred to as replicon cells). The replicon carries YFV nucleotides (1 - 176 and 2382-10,862) and a green fluorescent protein (GFP)-Zeocin resistance fusion gene as a selection marker and indicator of persistent replication. Immunofluorescence analysis revealed that both replicon cells and YFV 17D-infected cells showed similar distribution patterns of viral NS4B protein and replication intermediate, double-stranded RNA. Sequencing analysis of persistent replicons from the two replicon cell lines suggested that their nucleotide sequences did not vary greatly following multiple passages. We examined the effect of five agents, the antiviral cytokines interferon-β and -γ, the nucleoside analog ribavirin, the squalene synthase inhibitor zaragozic acid A, and the antibiotic rifapentine, a recently reported entry and replication inhibitor against YFV, on the persistent replication in the two replicon cell lines. These agents were selected because they inhibited both production of YFV 17D and transient replication of a luciferase-expressing replicon in Vero cells, without greatly affecting cell viability. We found that each of the agents decreased GFP fluorescence in the replicon cells, albeit to varying degrees. The agents other than rifapentine also showed a decrease in viral RNA levels in the replicon cells comparable to that seen for GFP fluorescence. These results indicate that persistent replication is susceptible to each of these five agents, although their mechanisms of action may differ. Taken together, these results provide evidence that translation/replication of the replicon in the replicon cells mimics that of the viral genome upon YFV 17D infection, indicating that the replicon cell lines can serve as a useful tool for high-throughput antiviral drug screening.

A DNA-based non-infectious replicon system to study SARS-CoV-2 RNA synthesis

The coronavirus disease-2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has seriously affected public health around the world. In-depth studies on the pathogenic mechanisms of SARS-CoV-2 is urgently necessary for pandemic prevention. However, most laboratory studies on SARS-CoV-2 have to be carried out in bio-safety level 3 (BSL-3) laboratories, greatly restricting the progress of relevant experiments. In this study, we used a bacterial artificial chromosome (BAC) method to assemble a SARS-CoV-2 replication and transcription system in Vero E6 cells without virion envelope formation, thus avoiding the risk of coronavirus exposure. Furthermore, an improved real-time quantitative reverse transcription PCR (RT-qPCR) approach was used to distinguish the replication of full-length replicon RNAs and transcription of subgenomic RNAs (sgRNAs). Using the SARS-CoV-2 replicon, we demonstrated that the nucleocapsid (N) protein of SARS-CoV-2 facilitates the transcription of sgRNAs in the discontinuous synthesis process. Moreover, two high-frequency mutants of N protein, R203K and S194L, can obviously enhance the transcription level of the replicon, hinting that these mutations likely allow SARS-CoV-2 to spread and reproduce more quickly. In addition, remdesivir and chloroquine, two well-known drugs demonstrated to be effective against coronavirus in previous studies, also inhibited the transcription of our replicon, indicating the potential applications of this system in antiviral drug discovery. Overall, we developed a bio-safe and valuable replicon system of SARS-CoV-2 that is useful to study the mechanisms of viral RNA synthesis and has potential in novel antiviral drug screening.

Construction of Genomic and Sub-Genomic Dengue Infectious Replicons

Dengue replicons are powerful tools used in studying virus biology as well as in high-throughput screening of drug candidates. Replicon constructs are developed as genomic (consists of all the viral protein genes) or sub-genomic (consists of only nonstructural protein genes) and are used to study various aspects of the virus life cycle such as genome replication and virus assembly. In addition, a replicon usually includes a reporter gene used in monitoring virus replication. In this chapter, we provide methods to develop both genomic and sub-genomic dengue replicons with a luciferase reporter and describe different assays to utilize these systems.

Antiviral Activity of CD437 Against Mumps Virus

Many efforts have been dedicated to the discovery of antiviral drug candidates against the mumps virus (MuV); however, no specific drug has yet been approved. The development of efficient screening methods is a key factor for the discovery of antiviral candidates. In this study, we evaluated a screening method using an Aequorea coerulescens green fluorescent protein-expressing MuV infectious molecular clone. The application of this system to screen for active compounds against MuV replication revealed that CD437, a retinoid acid receptor agonist, has anti-MuV activity. The point of antiviral action was a late step(s) in the MuV life cycle. The replication of other paramyxoviruses was also inhibited by CD437. The induction of retinoic acid-inducible gene (RIG)-I expression is a reported mechanism for the antiviral activity of retinoids, but our results indicated that CD437 did not stimulate RIG-I expression. Indeed, we observed antiviral activity despite the absence of RIG-I, suggesting that CD437 antiviral activity does not require RIG-I induction.

Carrot mottle virus ORF4 movement protein targets plasmodesmata by interacting with the host cell SUMOylation system

Plant virus movement proteins (MPs) facilitate virus spread in their plant hosts, and some of them are known to target plasmodesmata (PD). However, how the MPs target PD is still largely unknown. Carrot mottle virus (CMoV) encodes the ORF3 and ORF4 proteins, which are involved in CMoV movement. In this study, we used CMoV as a model to study the PD targeting of a plant virus MP. We showed that the CMoV ORF4 protein, but not the ORF3 protein, modified PD and led to the virus movement. We found that the CMoV ORF4 protein interacts with the host cell small ubiquitin-like modifier (SUMO) 1, 2 and the SUMO-conjugating enzyme SCE1, resulting in the ORF4 protein SUMOylation. Downregulation of mRNAs for NbSCE1 and NbSUMO impaired CMoV infection. The SUMO-interacting motifs (SIMs) LVIVF, VIWV, and a lysine residue at position 78 (K78) are required for the ORF4 protein SUMOylation. The mutation of these motifs prevented the protein to efficiently target PD, and further slowed or completely abolished CMoV systemic movement. Finally, we found that some of these motifs are highly conserved among umbraviruses. Our data suggest that the CMoV ORF4 protein targets PD by interacting with the host cell SUMOylation system.

Enhancing the Antiviral Efficacy of RNA-Dependent RNA Polymerase Inhibition by Combination with Modulators of Pyrimidine Metabolism

Genome-wide analysis of the mode of action of GSK983, a potent antiviral agent, led to the identification of dihydroorotate dehydrogenase as its target along with the discovery that genetic knockdown of pyrimidine salvage sensitized cells to GSK983. Because GSK983 is an ineffective antiviral in the presence of physiological uridine concentrations, we explored combining GSK983 with pyrimidine salvage inhibitors. We synthesized and evaluated analogs of cyclopentenyl uracil (CPU), an inhibitor of uridine salvage. We found that CPU was converted into its triphosphate in cells. When combined with GSK983, CPU resulted in large drops in cellular UTP and CTP pools. Consequently, CPU-GSK983 suppressed dengue virus replication in the presence of physiological concentrations of uridine. In addition, the CPU-GSK983 combination markedly enhanced the effect of RNA-dependent RNA polymerase (RdRp) inhibition on viral infection. Our findings highlight a new host-targeting strategy for potentiating the antiviral activity of RdRp inhibitors.

Development, Characterization, and Application of Two Reporter-Expressing Recombinant Zika Viruses

Zika virus (ZIKV), a mosquito-borne transplacentally transmissible flavivirus, is an enveloped virus with an ~10.8 kb plus-strand RNA genome that can cause neurological disease. To facilitate the identification of potential antivirals, we developed two reporter-expressing ZIKVs, each capable of expressing an enhanced green fluorescent protein or an improved luminescent NanoLuc luciferase. First, a full-length functional ZIKV cDNA clone was engineered as a bacterial artificial chromosome, with each reporter gene under the cap-independent translational control of a cardiovirus-derived internal ribosome entry site inserted downstream of the single open reading frame of the viral genome. Two reporter-expressing ZIKVs were then generated by transfection of ZIKV-susceptible BHK-21 cells with infectious RNAs derived by in vitro run-off transcription from the respective cDNAs. As compared to the parental virus, the two reporter-expressing ZIKVs grew to lower titers with slower growth kinetics and formed smaller foci; however, they displayed a genome-wide viral protein expression profile identical to that of the parental virus, except for two previously unrecognized larger forms of the C and NS1 proteins. We then used the NanoLuc-expressing ZIKV to assess the in vitro antiviral activity of three inhibitors (T-705, NITD-008, and ribavirin). Altogether, our reporter-expressing ZIKVs represent an excellent molecular tool for the discovery of novel antivirals.

Model System for the Formation of Tick-Borne Encephalitis Virus Replication Compartments without Viral RNA Replication

TBEV infection causes a broad spectrum of symptoms, ranging from mild fever to severe encephalitis. Similar to other flaviviruses, TBEV exploits intracellular membranes to build RCs for viral replication. The viral NS proteins have been suggested to be involved in this process; however, the mechanism of RC formation and the roles of individual NS proteins remain unclear. To study how TBEV induces membrane remodeling, we developed an inducible stable cell system expressing the TBEV NS polyprotein in the absence of viral RNA replication. Using this system, we were able to reproduce RC-like vesicles that resembled the RCs formed in flavivirus-infected cells, in terms of morphology and size. This cell system is a robust tool to facilitate studies of flavivirus RC formation and is an ideal model for the screening of antiviral agents at a lower biosafety level.

Flavivirus is a positive-sense, single-stranded RNA viral genus, with members causing severe diseases in humans such as tick-borne encephalitis, yellow fever, and dengue fever. Flaviviruses are known to cause remodeling of intracellular membranes into small cavities, where replication of the viral RNA takes place. Nonstructural (NS) proteins are not part of the virus coat and are thought to participate in the formation of these viral replication compartments (RCs). Here, we used tick-borne encephalitis virus (TBEV) as a model for the flaviviruses and developed a stable human cell line in which the expression of NS proteins can be induced without viral RNA replication. The model system described provides a novel and benign tool for studies of the viral components under controlled expression levels. We show that the expression of six NS proteins is sufficient to induce infection-like dilation of the endoplasmic reticulum (ER) and the formation of RC-like membrane invaginations. The NS proteins form a membrane-associated complex in the ER, and electron tomography reveals that the dilated areas of the ER are closely associated with lipid droplets and mitochondria. We propose that the NS proteins drive the remodeling of ER membranes and that viral RNA, RNA replication, viral polymerase, and TBEV structural proteins are not required.

IMPORTANCE TBEV infection causes a broad spectrum of symptoms, ranging from mild fever to severe encephalitis. Similar to other flaviviruses, TBEV exploits intracellular membranes to build RCs for viral replication. The viral NS proteins have been suggested to be involved in this process; however, the mechanism of RC formation and the roles of individual NS proteins remain unclear. To study how TBEV induces membrane remodeling, we developed an inducible stable cell system expressing the TBEV NS polyprotein in the absence of viral RNA replication. Using this system, we were able to reproduce RC-like vesicles that resembled the RCs formed in flavivirus-infected cells, in terms of morphology and size. This cell system is a robust tool to facilitate studies of flavivirus RC formation and is an ideal model for the screening of antiviral agents at a lower biosafety level.

De novo design approaches targeting an envelope protein pocket to identify small molecules against dengue virus

Dengue fever is a mosquito-borne viral disease that has become a major public health concern worldwide. This disease presents with a wide range of clinical manifestations, from a mild cold-like illness to the more serious hemorrhagic dengue fever and dengue shock syndrome. Currently, neither an approved drug nor an effective vaccine for the treatment are available to fight the disease. The envelope protein (E) is a major component of the virion surface. This protein plays a key role during the viral entry process, constituting an attractive target for the development of antiviral drugs. The crystal structure of the E protein reveals the existence of a hydrophobic pocket occupied by the detergent n-octyl-β-d-glucoside (β-OG). This pocket lies at the hinge region between domains I and II and is important for the low pH-triggered conformational rearrangement required for the fusion of the virion with the host's cell. Aiming at the design of novel molecules which bind to E and act as virus entry inhibitors, we undertook a de novo design approach by "growing" molecules inside the hydrophobic site (β-OG). From more than 240000 small-molecules generated, the 2,4 pyrimidine scaffold was selected as the best candidate, from which one synthesized compound displayed micromolar activity. Molecular dynamics-based optimization was performed on this hit, and thirty derivatives were designed in silico, synthesized and evaluated on their capacity to inhibit dengue virus entry into the host cell. Four compounds were found to be potent antiviral compounds in the low-micromolar range. The assessment of drug-like physicochemical and in vitro pharmacokinetic properties revealed that compounds 3e and 3h presented acceptable solubility values and were stable in mouse plasma, simulated gastric fluid, simulated intestinal fluid, and phosphate buffered saline solution.

Genetically stable reporter virus, subgenomic replicon and packaging system of duck Tembusu virus based on a reverse genetics system

Duck Tembusu virus (DTMUV) is a novel flavivirus that has caused an outbreak of severe duck egg-drop syndrome since 2010. It has spread rapidly to other avian species, causing enormous economic loss. In the present study, we generated a reporter virus expressing NanoLuc luciferase, which was stable after 10 rounds of continuous propagation without reporter gene deletion. Moreover, we generated two types of replicons driven by the T7 promoter or CMV promoter, both of which worked well in BHK21 cells. Furthermore, we developed the first packaging system for DTMUV by co-transfection into BHK21 cells of a replicon (containing mature C) and a plasmid encoding C₁₆-prM-E, which resulted in the production of single round infectious particles (SRIPs). We also generated a packaging cell line for DTMUV to produce SRIPs. We believe that these multicomponent platform tools are important for DTMUV pathogenesis research and novel vaccine development.

Stearoyl-CoA desaturase-1 is required for flavivirus RNA replication

Dengue virus (DENV) is the most prevalent human arthropod-borne virus and causes severe problems worldwide, mainly in tropical and sub-tropical regions. However, there is no specific antiviral drug against DENV infection. We and others recently reported that stearoyl-CoA desaturase-1 (SCD1) inhibitor showed potent suppression of hepatitis C virus replication. In this study, we examined the impact of SCD1 on DENV replication. We found that SCD1 inhibitors (MK8245 and #1716) dramatically suppressed DENV replication in a dose-dependent manner without cytotoxicity. This anti-DENV efficacy was observed against all four DENV serotypes and other flaviviruses, including Zika virus and Japanese encephalitis virus. A subgenomic replicon system of DENV was used to confirm that SCD1 inhibitor suppressed viral RNA replication. Interestingly, exogenous supplementation of unsaturated fatty acids resulted in recovery of the DENV titer even in the presence of SCD1 inhibitor, suggesting that fatty acid biosynthesis contributes to DENV genome replication. These findings indicate that SCD1 is a novel host factor required for DENV replication, and SCD1 inhibitor is a potential candidate for treating dengue fever.

Reverse Genetic Approaches for the Generation of Recombinant Zika Virus

Zika virus (ZIKV) is an emergent mosquito-borne member of the Flaviviridae family that was responsible for a recent epidemic in the Americas. ZIKV has been associated with severe clinical complications, including neurological disorder such as Guillain-Barré syndrome in adults and severe fetal abnormalities and microcephaly in newborn infants. Given the significance of these clinical manifestations, the development of tools and reagents to study the pathogenesis of ZIKV and to develop new therapeutic options are urgently needed. In this respect, the implementation of reverse genetic techniques has allowed the direct manipulation of the viral genome to generate recombinant (r)ZIKVs, which have provided investigators with powerful systems to answer important questions about the biology of ZIKV, including virus-host interactions, the mechanism of transmission and pathogenesis or the function of viral proteins. In this review, we will summarize the different reverse genetic strategies that have been implemented, to date, for the generation of rZIKVs and the applications of these platforms for the development of replicon systems or reporter-expressing viruses.

The antiparasitic drug niclosamide inhibits dengue virus infection by interfering with endosomal acidification independent of mTOR

Background

The antiparasitic agent niclosamide has been demonstrated to inhibit the arthropod-borne Zika virus. Here, we investigated the antiviral capacity of niclosamide against dengue virus (DENV) serotype 2 infection in vitro and in vivo.

Principle finding

Niclosamide effectively retarded DENV-induced infection in vitro in human adenocarcinoma cells (A549), mouse neuroblastoma cells (Neuro-2a), and baby hamster kidney fibroblasts (BHK-21). Treatment with niclosamide did not retard the endocytosis of DENV while niclosamide was unable to enhance the antiviral type I interferon response. Furthermore, niclosamide did not cause a direct effect on viral replicon-based expression. Niclosamide has been reported to competitively inhibit the mTOR (mammalian target of rapamycin), STAT3 (signal transducer and activator of transcription 3), and NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) signaling pathways; however, selective inhibitors of those pathways did not reduce DENV infection. Similar to the vacuolar-type H⁺-ATPase inhibitor bafilomycin A1, both niclosamide and other protonophores, such as CCCP (carbonyl cyanide m-chlorophenyl hydrazone), and FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone), effectively reduced endosomal acidification and viral dsRNA replication. Co-administration of a single dose of niclosamide partially decreased viral replication, viral encephalitis, and mortality in DENV-infected ICR suckling mice.

Significance

These results demonstrate that niclosamide diminishes viral infection by hindering endosomal acidification.

Dengue and severe dengue cause global health concerns annually. Without antiviral drugs, supportive care is the only treatment option for patients with DENV infection. A current vaccine has been approved for protection against DENV infection; however, the potential risks and challenges associated with the immunopathogenesis of DENV remain unresolved. For anti-dengue therapy, the repurposing of drugs with antimicrobial and anticancer properties is a possible pharmacological strategy. In this study, we evaluated the potential antiviral effects of the antiparasitic drug niclosamide, considering its current pharmacological efficacy against arthropod-borne Zika virus infection. Using in vitro and in vivo models of DENV infection, we demonstrated that one of the therapeutic effects of niclosamide is to significantly target endosomal acidification. Following safety screening, repurposing niclosamide treatment may facilitate the development of anti-dengue drugs in the near future.

Polyketides and Alkaloids from the Marine-Derived Fungus Dichotomomyces cejpii F31-1 and the Antiviral Activity of Scequinadoline A against Dengue Virus

In our continuous chemical investigation on the marine-derived fungus Dichotomomyces cejpii F31-1, two new polyketides dichocetides B-C (1, 2), two new alkaloids dichotomocejs E-F (3, 4), and three known fumiquinozalines: scequinadoline A (5), quinadoline A (6), and scequinadoline E (7) were discovered from the culture broth and the mycelium in the culture medium, by the addition of l-tryptophan and l-phenylalanine. Their chemical structures were established by one dimensional (1D), two dimensional (2D) nuclear magnetic resonance (NMR) and high resolution mass spectrometry (HR-MS) data. Among them, scequinadoline A (5) exhibited significant inhibitory activity against dengue virus serotype 2 production by standard plaque assay, equivalent to the positive control andrographlide. Scequinadoline A (5) possesses the potential for further development as a dengue virus inhibitor.

Posaconazole inhibits dengue virus replication by targeting oxysterol-binding protein

Dengue virus (DENV) is associated with an estimated 390 million infections per year, occurring across approximately 100 countries in tropical and sub-tropical regions. To date, there are no antiviral drugs or specific therapies to treat DENV infection. Posaconazole and itraconazole are potent antifungal drugs that inhibit ergosterol biosynthesis in fungal cells, but also target a number of human proteins. Here, we show that itraconazole and posaconazole have antiviral activity against DENV. Posaconazole inhibited replication of multiple serotypes of DENV and the related flavivirus Zika virus, and reduced viral RNA replication, but not translation of the viral genome. We used a combination of knockdown and drug sensitization assays to define the molecular target of posaconazole that mediates its antiviral activity. We found that knockdown of oxysterol-binding protein (OSBP) inhibited DENV replication. Moreover, knockdown of OSBP, but not other known targets of posaconazole, enhanced the inhibitory effect of posaconazole. Our findings imply OSBP as a potential target for the development of antiviral compounds against DENV.

Establishment of a reverse genetics system for duck Tembusu virus to study virulence and screen antiviral genes

Recently, a newly emerged avian flavivirus, duck Tembusu virus (TMUV), was identified as the causative agent of a serious duck viral disease in Asia. Its rapid spread and expanded host range have raised substantial concerns regarding its potential threat to non-avian hosts, including humans. In this study, we report an infectious cDNA clone for a clinical strain CQW1 isolated from Southwest China, which is representative of the disease outbreak in the Chinese mainland. We generated a full-length cDNA clone pACYC FL-TMUV, which is infectious, and this cDNA clone-derived recombinant TMUV (rTMUV) showed comparative growth kinetics in both BHK21 cells and DEF cells compared with parental TMUV (pTMUV). In addition, rTMUV also showed the same high virulence in 9-day-old duck embryos as that in pTMUV, suggesting that rTMUV possessed similar properties to the natural virus both in vitro and in vivo. Based on the cDNA-clone, we first generated a reporter TMUV (TMUV-RLuc) carrying a Renilla luciferase (RLuc) gene. The luciferase kinetics of TMUV-RLuc were determined both in BHK21 and DEF cells. It seems that TMUV-RLuc grew well in vitro; however, the insertion of the RLuc gene attenuated viral replication in vitro. The higher viral titres of TMUV-RLuc were observed in BHK21 compared with that in DEF cells. The antiviral effects of exogenous-expressed duck RIG-I, MDA5, STING, MAVS, TBK1, IFNα and IFNγ were studied in vitro by using TMUV-RLuc. Our reverse genetics system will provide a multicomponent platform for the pathogenesis study of duck TMUV and the development of molecular countermeasures against duck TMUV infection.

Development of a replicon cell line-based high throughput antiviral assay for screening inhibitors of Zika virus

Zika virus (ZIKV) is an important emerging human pathogen associated with microcephaly, Guillain-Barré syndrome and meningoencephalitis. Developing rapid and reliable HTS assay is important for ZIKV drug discovery. Here, we constructed a dicistronic ZIKV replicon (ZIKV-Pac-Rluc-Rep) that contained the Renilla luciferase (Rluc) reporter gene separated from the puromycin N-acetyl-transferase (Pac) selectable marker by a short peptide cleavage site. A clonal replicon cell line stably expressing high level of ZIKV replicon was established by selection with puromycin. By optimizing cell number, compound concentration and incubation time, a robust replicon cell-based HTS assay was developed with a calculated Z' value of >0.5. The fully optimized assay was further validated using several known flavivirus replication inhibitors. Altogether, the replicon cell-based HTS assay developed in this study will facilitate the discovery of antiviral compounds against ZIKV.

Hirsutine, an Indole Alkaloid of Uncaria rhynchophylla, Inhibits Late Step in Dengue Virus Lifecycle

Dengue virus (DENV) is transmitted to humans by Aedes mosquitoes and is a public health issue worldwide. No antiviral drugs specific for treating dengue infection are currently available. To identify novel DENV inhibitors, we analyzed a library of 95 compounds and 120 extracts derived from crude drugs (herbal medicines). In the primary screening, A549 cells infected with DENV-1 were cultured in the presence of each compound and extract at a final concentration of 10 μM (compound) and 100 μg/mL (extract), and reduction of viral focus formation was assessed. Next, we eliminated compounds and extracts which were cytotoxic using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Hirsutine, an indole alkaloid of Uncaria rhynchophylla, was identified as a potent anti-DENV compound exhibiting high efficacy and low cytotoxicity. Hirsutine showed antiviral activity against all DENV serotypes. Time-of-drug-addition and time-of-drug-elimination assays indicated that hirsutine inhibits the viral particle assembly, budding, or release step but not the viral translation and replication steps in the DENV lifecycle. A subgenomic replicon system was used to confirm that hirsutine does not restrict viral genome RNA replication. Hirsutine is a novel DENV inhibitor and potential candidate for treating dengue fever.

A highly pathogenic porcine reproductive and respiratory syndrome virus candidate vaccine based on Japanese encephalitis virus replicon system

In the swine industry, porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease which causes heavy economic losses worldwide. Effective prevention and disease control is an important issue. In this study, we described the construction of a Japanese encephalitis virus (JEV) DNA-based replicon with a cytomegalovirus (CMV) promoter based on the genome of Japanese encephalitis live vaccine virus SA14-14-2, which is capable of offering a potentially novel way to develop and produce vaccines against a major pathogen of global health. This JEV DNA-based replicon contains a large deletion in the structural genes (C-prM-E). A PRRSV GP5/M was inserted into the deletion position of JEV DNA-based replicons to develop a chimeric replicon vaccine candidate for PRRSV. The results showed that BALB/c mice models with the replicon vaccines pJEV-REP-G-2A-M-IRES and pJEV-REP-G-2A-M stimulated antibody responses and induced a cellular immune response. Analysis of ELSA data showed that vaccination with the replicon vaccine expressing GP5/M induced a better antibodies response than traditional DNA vaccines. Therefore, the results suggested that this ectopic expression system based on JEV DNA-based replicons may represent a useful molecular platform for various biological applications, and the JEV DNA-based replicons expressing GP5/M can be further developed into a novel, safe vaccine candidate for PRRS.

Pseudo-typed Semliki Forest virus delivers EGFP into neurons

Semliki Forest virus (SFV), a neurotropic virus, has been used to deliver heterologous genes into cells in vitro and in vivo. In this study, we constructed a reporter SFV4-FL-EGFP and found that it can deliver EGFP into neurons located at the injection site without disseminating throughout the brain. Lacking of the capsid gene of SFV4-FL-EGFP does not block its life cycle, while forming replication-competent virus-like particles (VLPs). These VLPs hold subviral genome by using the packaging sequence (PS) located within the nsP2 gene, and can transfer their genome into cells. In addition, we found that the G protein of vesicular stomatitis virus (VSVG) can package SFV subviral genome, which is consistent with the previous reports. The G protein of rabies virus (RVG) could also package SFV subviral genome. These pseudo-typed SFV can deliver EGFP gene into neurons. Taken together, these findings may be used to construct various SFV-based delivery systems for virological studies, gene therapy, and neural circuit labeling.