Construction of dengue virus protease expression plasmid and in vitro protease assay for screening antiviral inhibitors

Inhibition of dengue virus by curcuminoids

The dengue virus is considered to be a globally important human pathogen prevalent in tropical and subtropical regions of the world. According to a recent estimate, the disease burden due to DENV infections is ∼390 million infections per year globally in ∼100 countries including the southern US, Puerto Rico and Hawaii, resulting in nearly ∼25,000 deaths mostly among children. Despite the significant morbidity and mortality that results from DENV infections, there is currently no effective chemotherapeutic treatment for DENV infections. We identified curcumin as an inhibitor of DENV2 NS2B/NS3protease in a previous high-throughput screening (HTS) campaign. We synthesized four analogues of curcumin (curcuminoids) and tested the in vitro protease inhibition activity and inhibition of replication by cell-based assays. The results revealed that curcumin is a weak inhibitor of the viral protease. However, the analogues exhibited more potent inhibition of DENV infectivity in plaque assays suggesting that the cellular pathway(s) required for viral replication and/or assembly are targeted by these compounds. Further analysis shows that inhibition of genes involved in lipid biosynthesis, and of actin polymerization by curcuminoids, are likely to be involved as their mode of action in DENV2-infected cells. Three of the curcumin derivatives possess good selectivity indices (SI) (>10) when compared to the parent curcumin.

High-throughput screening for the identification of small-molecule inhibitors of the flaviviral protease

The mosquito-borne dengue virus serotypes 1-4 (DENV1-4) and West Nile virus (WNV) cause serious illnesses worldwide associated with considerable morbidity and mortality. According to the World Health Organization (WHO) estimates, there are about 390 million infections every year leading to ∼500,000 dengue haemorrhagic fever (DHF) cases and ∼25,000 deaths, mostly among children. Antiviral therapies could reduce the morbidity and mortality associated with flaviviral infections, but currently there are no drugs available for treatment. In this study, a high-throughput screening assay for the Dengue protease was employed to screen ∼120,000 small molecule compounds for identification of inhibitors. Eight of these inhibitors have been extensively analyzed for inhibition of the viral protease in vitro and cell-based viral replication using Renilla luciferase reporter replicon, infectivity (plaque) and cytotoxicity assays. Three of these compounds were identified as potent inhibitors of DENV and WNV proteases, and viral replication of DENV2 replicon and infectious RNA. Fluorescence quenching, kinetic analysis and molecular modeling of these inhibitors into the structure of NS2B-NS3 protease suggest a mode of inhibition for three compounds that they bind to the substrate binding pocket.

Live Cell Reporter Systems for Positive-Sense Single Strand RNA Viruses

Cell-based reporter systems have facilitated studies of viral replication and pathogenesis, virus detection, and drug susceptibility testing. There are three types of cell-based reporter systems that express certain reporter protein for positive-sense single strand RNA virus infections. The first type is classical reporter system, which relies on recombinant virus, reporter virus particle, or subgenomic replicon. During infection with the recombinant virus or reporter virus particle, the reporter protein is expressed and can be detected in real time in a dose-dependent manner. Using subgenomic replicon, which are genetically engineered viral RNA molecules that are capable of replication but incapable of producing virions, the translation and replication of the replicon could be tracked by the accumulation of reporter protein. The second type of reporter system involves genetically engineered cells bearing virus-specific protease cleavage sequences, which can sense the incoming viral protease. The third type is based on viral replicase, which can report the specific virus infection via detection of the incoming viral replicase. This review specifically focuses on the major technical breakthroughs in the design of cell-based reporter systems and the application of these systems to the further understanding and control of viruses over the past few decades.

The flavivirus NS2B-NS3 protease-helicase as a target for antiviral drug development

The flavivirus NS3 protein is associated with the endoplasmic reticulum membrane via its close interaction with the central hydrophilic region of the NS2B integral membrane protein. The multiple roles played by the NS2B-NS3 protein in the virus life cycle makes it an attractive target for antiviral drug discovery. The N-terminal region of NS3 and its cofactor NS2B constitute the protease that cleaves the viral polyprotein. The NS3 C-terminal domain possesses RNA helicase, nucleoside and RNA triphosphatase activities and is involved both in viral RNA replication and virus particle formation. In addition, NS2B-NS3 serves as a hub for the assembly of the flavivirus replication complex and also modulates viral pathogenesis and the host immune response. Here, we review biochemical and structural advances on the NS2B-NS3 protein, including the network of interactions it forms with NS5 and NS4B and highlight recent drug development efforts targeting this protein. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.