Stress granules (SGs) are dynamic cytoplasmic aggregates of translationally silenced mRNAs that assemble in response to environmental stress. SGs appear to play an important role in antiviral innate immunity and many viruses have evolved to block or subvert SGs components for their own benefit. Here, we demonstrate that intracellular Ebola virus (EBOV) replication and transcription-competent virus like particles (trVLP) infection does not lead to SG assembly but leads to a blockade to Arsenite-induced SG assembly. Moreover we show that EBOV VP35 represses the assembly of canonical and non-canonical SGs induced by a variety of pharmacological stresses. This SG blockade requires, at least in part, the C-terminal domain of VP35. Furthermore, results from our co-immunoprecipitation studies indicate that VP35 interacts with multiple SG components, including G3BP1, eIF3 and eEF2 through a stress- and RNA-independent mechanism. These data suggest a novel function for EBOV VP35 in the repression of SG assembly.
Ebola virus (EBOV) is a filovirus that causes a severe and rapidly progressing hemorrhagic syndrome whose recent epidemic enlightened the urgent need of novel therapeutic agents, since no drug is currently approved. A key contribution to the high lethality observed during EBOV outbreaks comes from viral evasion of the host antiviral innate immune response in which the viral protein VP35 plays a crucial role, blocking the interferon type I production, firstly by masking the viral dsRNA and preventing its detection by the pattern recognition receptor RIG-I. Aiming to identify inhibitors of VP35 interaction with the viral dsRNA, counteracting the VP35 viral innate immune evasion, we established a new methodology for high-yield recombinant VP35 (rVP35) expression and purification, and a novel and robust fluorescence-based rVP35-RNA interaction assay (Z'-factor of 0.69). Taking advantage of such newlyestablished methods, we screened a small library of Sardinian natural extracts finding Limonium morisianum as the most potent inhibitor extract. A bio-guided fractionation led to the identification of myricetin as the component able to inhibit rVP35-dsRNA interaction with an IC 50 value of 2.7 µM.Molecular docking studies showed that myricetin interacts with the highly conserved region of the VP35 RNA binding domain, laying the basis for further structural optimization of potent VP35-dsRNA inhibitors.
During Ebola virus (EBOV) infection, the type I interferon α/β (IFN-α/β) innate immune response is suppressed by EBOV viral protein 35 (VP35), a validated drug target. Identification of EBOV VP35 inhibitors requires a cellular system able to assess the VP35-based inhibitory functions of viral double-stranded RNA (dsRNA) IFN-β induction. We established a miniaturized luciferase gene reporter assay in A549 cells that measures IFN-β induction by viral dsRNA and is dose-dependently inhibited by VP35 expression. When compared to influenza A virus NS1 protein, EBOV VP35 showed improved inhibition of viral dsRNA-based IFN-β induction. This assay can be used to screen for EBOV VP35 inhibitors.
Ebola virus high lethality relies on its ability to efficiently bypass the host innate antiviral response, which senses the viral dsRNA through the RIG-I receptor and induces type I interferon a/b production. In the bypassing action, the Ebola virus protein VP35 plays a pivotal role at multiple levels of the RIG-I cascade, masking the viral 5 0-triphosphorylated dsRNA from RIG-I, and interacting with other cascade components. The VP35 type I interferon inhibition is exerted by the C-terminal domain, while the N-terminal domain, containing a coiled-coil region, is primarily required for oligomerization. However, mutations at key VP35 residues L90/93/107A (VP35-3m) in the coiled-coil region were reported to affect oligomerization and reduce type I interferon antagonism, indicating a possible but unclear role of homo-oligomerization on VP35 interaction with the RIG-I pathway components. In this work, we investigated the VP35 dimerization thermodynamics and its contribution to type I interferon antagonism by computational and biological methods. Focusing on the coiled-coil region, we combined coarse-grained and all-atom simulations on wild type VP35 and VP35-3m homo-dimerization. According to our results, wild type VP35 coiled-coil is able to self-assemble into dimers, while VP35-3m coiled-coil shows poor propensity to even dimerize. Free-energy calculations confirmed the key role of L90, L93 and L107 in stabilizing the coiled-coil homo-dimeric structure. In vitro type I interferon antagonism studies, using full-length wild type VP35 and VP35-3m, revealed that VP35 homo-dimerization is an essential preliminary step for dsRNA binding, which appears to be the main factor of the VP35 RIG-I cascade inhibition, while it is not essential to block the other steps.
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