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The question of whether RNA interference (RNAi) acts as an antiviral mechanism in mammalian cells remains controversial. The antiviral interferon (IFN) response cannot easily be distinguished from a possible antiviral RNAi pathway owing to the involvement of double-stranded RNA (dsRNA) as a common inducer molecule. The non-structural protein 3 (NS3) protein of rice hoja blanca virus (RHBV) is an RNA silencing suppressor (RSS) that exclusively binds to small dsRNA molecules. Here, we show that this plant viral RSS lacks IFN antagonistic activity, yet it is able to substitute the RSS function of the Tat protein of human immunodeficiency virus type 1. An NS3 mutant that is deficient in RNA binding and its associated RSS activity is inactive in this complementation assay. This cross-kingdom suppression of RNAi in mammalian cells by a plant viral RSS indicates the significance of the antiviral RNAi response in mammalian cells and the usefulness of well-defined RSS proteins. [ABSTRACT FROM AUTHOR]

The article discusses the highlights of the first European Science Foundation (ESF)-European Molecular Biology Organisation (EMBO) symposium on applications of anti-viral ribonucleic acid interference (RNAi) held in San Feliu de Guixols at the Costa Brava in Spain from April 5 to 10, 2008. The topics tackled included RNAi technology, the mechanism of RNAi in plants and the interaction of adenovirus with the RNAi/microRNA (miRNA) pathways. Mark Kay, Thomas Hohn and Goran Akusjarvi were among the lecturers featured.

RNA interference (RNAi) is a conserved sequence-specific, gene-silencing mechanism that is induced by double-stranded RNA. RNAi holds great promise as a novel nucleic acid–based therapeutic against a wide variety of diseases, including cancer, infectious diseases and genetic disorders. Antiviral RNAi strategies have received much attention and several compounds are currently being tested in clinical trials. Although induced RNAi is able to trigger profound and specific inhibition of virus replication, it is becoming clear that RNAi therapeutics are not as straightforward as we had initially hoped. Difficulties concerning toxicity and delivery to the right cells that earlier hampered the development of antisense-based therapeutics may also apply to RNAi. In addition, there are indications that viruses have evolved ways to escape from RNAi. Proper consideration of all of these issues will be necessary in the design of RNAi-based therapeutics for successful clinical intervention of human pathogenic viruses. [ABSTRACT FROM AUTHOR]

RNA silencing or interference (RNAi) is a gene regulation mechanism in eukaryotes that controls cell differentiation and developmental processes via expression of microRNAs. RNAi also serves as an innate antiviral defence response in plants, nematodes, and insects. This antiviral response is triggered by virus-specific double-stranded RNA molecules (dsRNAs) that are produced during infection. To overcome antiviral RNAi responses, many plant and insect viruses encode RNA silencing suppressors (RSSs) that enable them to replicate at higher titers. Recently, several human viruses were shown to encode RSSs, suggesting that RNAi also serves as an innate defence response in mammals. Here, we demonstrate that the Ebola virus VP35 protein is a suppressor of RNAi in mammalian cells and that its RSS activity is functionally equivalent to that of the HIV-1 Tat protein. We show that VP35 can replace HIV-1 Tat and thereby support the replication of a Tat-minus HIV-1 variant. The VP35 dsRNA-binding domain is required for this RSS activity. Vaccinia virus E3L protein and influenza A virus NS1 protein are also capable of replacing the HIV-1 Tat RSS function. These findings support the hypothesis that RNAi is part of the innate antiviral response in mammalian cells. Moreover, the results indicate that RSSs play a critical role in mammalian virus replication. [ABSTRACT FROM AUTHOR]

Background: Human immunodeficiency virus type 1 (HIV-1) can be inhibited by means of RNA silencing or interference (RNAi) using synthetic short interfering RNAs (siRNAs) or gene constructs encoding short hairpin RNAs (shRNAs) or long hairpin RNAs (lhRNAs). The use of siRNA and shRNA as antiviral therapeutic is limited because of the emergence of viral escape mutants. This problem is theoretically prevented by intracellular expression of lhRNAs generating multiple siRNAs that target the virus simultaneously, thus reducing the chance of viral escape. However, gene constructs encoding lhRNA molecules face problems with delivery to the right cells in an infected individual. In order to solve this problem, we constructed an HIV-1 variant with a 300 bp long hairpin structure in the 3' part of the genome corresponding to the Nef gene (HIVlhNef). Results: Intriguingly, HIV-lhNef potently inhibited wild-type HIV-1 production in trans. However, HIV-lhNef demonstrated a severe production and replication defect, which we were able to solve by selecting spontaneous virus variants with truncated hairpin structures. Although these escape variants lost the ability to trans-inhibit HIV-1, they effectively outgrew the wild-type virus in competition experiments in SupT1 cells. Conclusion: Expression of the lhNef hairpin within the HIV-1 genome results in potent transinhibition of wild-type HIV-1. Although the mechanism of trans-inhibition is currently unknown, it remains of interest to study the molecular details because the observed effect is extremely potent. This may have implications for the development of virus strains to be used as live-attenuated virus vaccines. [ABSTRACT FROM AUTHOR]

Abstract: RNA interference (RNAi) plays a pivotal role in the regulation of gene expression to control cell development and differentiation. In plants, insects and nematodes RNAi also functions as an innate defence response against viruses. Similarly, there is accumulating evidence that RNAi functions as an antiviral defence mechanism in mammalian cells. Viruses have evolved highly sophisticated mechanisms for interacting with the host cell machinery, and recent evidence indicates that this also involves RNAi pathways. The cellular RNAi machinery can inhibit virus replication, but viruses may also exploit the RNAi machinery for their own replication. In addition, viruses can encode proteins or RNA molecules that suppress existing RNAi pathways or trigger the silencing of specific host genes. Besides the natural interplay between RNAi and viruses, induced RNAi provides an attractive therapy approach for the fight against human pathogenic viruses. Here, we summarize the latest news on virus–RNAi interactions and RNAi based antiviral therapy. [Copyright &y& Elsevier]