RNA interference (RNAi) is originally regarded as a mechanism of eukaryotic posttranscriptional gene regulation mediated by small interfering RNA (siRNA)-induced sequence-specific RNA degradation (1). It is also well known to exert as an important antiviral defense mechanism in a wide range of organisms, from plants to invertebrates (2). During the virus infection, the virus-derived long double-stranded RNA (dsRNA) is cleaved by RNAIII-like endonuclease (named Dicer) into approximately 21-to 23-nucleotide (nt) siRNA, which is incorporated into the RNA-induced silencing complex (RISC) and activates the antiviral RNAi for viral RNA degradation. In mammalian cells, although the activation of RNAi by synthetic siRNA or short hairpin RNA (shRNA) is widely used as a tool for gene knockdown and antiviral treatment, the RNAi-mediated antiviral mechanism has been debated for a long time (3), because the interferon (IFN) response of the innate immune system is well known as the dominant antiviral mechanism (4). However, more and more evidence has provided strong support for the existence of a natural RNAi-mediated antiviral response in mammals (5). Moreover, recent studies showed that in undifferentiated cells and immature mice, the RNAi-mediated antiviral response is essential (6-8).To overcome the RNAi-mediated antiviral defense, viruses have evolved to encode a viral suppressor of RNA silencing (VSR) (9, 10). For example, in plant viruses, rice hoja blancavirus NS3, tombusvirus P19, and tomato aspermy virus 2b bind to long dsRNA or siRNA to block RNAi (11-13). Turnip crinkle virus P38 and cauliflower mosaic virus P6 disrupt the components of RNAi machinery (14,15). In insect viruses, flock house virus (FHV) B2 blocks RNAi by dsRNA binding (16,17), and Wuhan nodavirus (WhNV) B2 was identified as a VSR by targeting both dsRNAs and 19). Although the majority of VSRs have been identified in plant and invertebrate viruses, several mammalian viruses were shown to encode VSRs. For instance, Ebola virus VP35, influenza A virus NS1, vaccinia virus E3L, and Nodamura virus (NoV) B2 act as VSRs by binding dsRNA (20-23). Hepatitis C virus core and HIV-1 Tat block RNAi by inhibiting the activity of

The outbreak of coronavirus disease 2019 (COVID-19) has resulted in a global pandemic due to the rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). At the time of this manuscript’s publication, remdesivir is the only COVID-19 treatment approved by the United States Food and Drug Administration. However, its effectiveness is still under question due to the results of the large Solidarity Trial conducted by the World Health Organization. Herein, we report that the parent nucleoside of remdesivir, GS-441524, potently inhibits the replication of SARS-CoV-2 in Vero E6 and other cell lines. Challenge studies in both an AAV-hACE2 mouse model of SARS-CoV-2 and in mice infected with murine hepatitis virus, a closely related coronavirus, showed that GS-441524 was highly efficacious in reducing the viral titers in CoV-infected organs without notable toxicity. Our results support that GS-441524 is a promising and inexpensive drug candidate for treating of COVID-19 and other CoV diseases.

SARS-CoV-2 causes the pandemic of COVID-19 and no effective drugs for this disease are available thus far. Due to the high infectivity and pathogenicity of this virus, all studies on the live virus are strictly confined in the biosafety level 3 (BSL3) laboratory but this would hinder the basic research and antiviral drug development of SARS-CoV-2 because the BSL3 facility is not commonly available and the work in the containment is costly and laborious. In this study, we constructed a reverse genetics system of SARS-CoV-2 by assembling the viral cDNA in a bacterial artificial chromosome (BAC) vector with deletion of the spike (S) gene. Transfection of the cDNA into cells results in the production of an RNA replicon that keeps the capability of genome or subgenome replication but is deficient in virion assembly and infection due to the absence of S protein. Therefore, such a replicon system is not infectious and can be used in ordinary biological laboratories. We confirmed the efficient replication of the replicon by demonstrating the expression of the subgenomic RNAs which have similar profiles to the wild-type virus. By mutational analysis of nsp12 and nsp14, we showed that the RNA polymerase, exonuclease, and cap N7 methyltransferase play essential roles in genome replication and sgRNA production. We also created a SARS-CoV-2 replicon carrying a luciferase reporter gene and this system was validated by the inhibition assays with known anti-SARS-CoV-2 inhibitors. Thus, such a one-plasmid system is biosafe and convenient to use, which will benefit both fundamental research and development of antiviral drugs.

Interferon-inducible p200 family protein IFI204 was reported to be involved in DNA sensing, and subsequently induces the production of type I interferons and proinflammatory mediators. However, its function in the regulation of antiviral innate immune signaling pathway remains unclear. Here we reported a novel role of IFI204 that specifically inhibits the IRF7-mediated type I interferons response during viral infection. IFI204 and other p200 family proteins are highly expressed in mouse hepatitis coronavirus-infected bone marrow-derived dendritic cells. The abundant IFI204 could significantly interact with IRF7 in nucleus by its HIN domain and prevent the binding of IRF7 with its corresponding promoter. Moreover, other p200 family proteins that possess HIN domain could also inhibit the IRF7-mediated type I interferons. These results reveal that, besides the positive regulation function in type I interferon response at the early stage of DNA virus infection, the interferon-inducible p200 family proteins such as IFI204 could also negatively regulate the IRF7-mediated type I interferon response after RNA virus infection to avoid unnecessary host damage from hyper-inflammatory responses.