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    • PRRSV is a single stranded positive sense enveloped RNA viru

    2019-10-22

    PRRSV is a single-stranded, positive-sense enveloped RNA virus in the family Arteriviridae of the order Nidovirales (Meng et al., 1994, 1996). The PRRSV Cyclophosphamide monohydrate is approximately 15 kb in length and contains eleven known open reading frames (ORFs) (Snijder et al., 2013). The 3′ end of the genome encodes structural proteins GP2a, GP3, GP4 GP5, GP5a, M and N. The replication-associated genes, ORF1a and ORF1b, are located at the 5′ end of the genome and encode two large polyproteins, pp1a and pp1b, respectively (Snijder et al., 2013; Fang and Snijder, 2010). A proteolytic cascade mediated by four proteinase domains encoded in the ORF1a processes the polyproteins into 14 nonstructural proteins (NSPs) (Fang and Snijder, 2010). Of the NSPs, NSP1α, NSP1β, and NSP2 have been shown to strongly antagonize the type I IFN response (Beura et al., 2010, 2012; Sun et al., 2012b), although the molecular mechanisms responsible for this inhibition are not completely understood (Sun et al., 2012a). Stress granules (SGs) are dynamic cytoplasmic protein-RNA structures that quickly form and dissolve in response to various stress conditions (Panas et al., 2016). Heat shock, oxidative stress, and viral infection have all been shown to induce robust SG formation in various cell types (Taniuchi et al., 2016). Cellular kinases respond to these stresses by phosphorylating the alpha subunit of eukaryotic initiation factor 2 (eIF2a). As a result, the available eIF2/tRNAiMet/GTP ternary complex is depleted. A reduction in this complex results in the reversible inhibition of translation and subsequent polysome disassembly accumulation of stalled 43S and 48S ribosomal pre-initiations complexes (PICs). Stalled PICs in turn recruit RNA-binding proteins (RBPs) such as G3BP1, G3BP2, TIAR and other proteins that are involved in SG nucleation (Panas et al., 2016; Kedersha et al., 1999). A hallmark feature of canonical, bona fide SGs is the presence of these protein factors along with mRNAs (Panas et al., 2015; Lloyd, 2012). SGs are thought to associate with another cellular mRNA repository known as a processing body (P-body). While SGs are temporary storage sites of mRNA during cellular stress, P-bodies are thought to specifically act as sites of mRNA degradation in cells. It is currently thought that mRNAs are transiently stored in SGs until they are either released for translation, or transferred to P-bodies for degradation (Panas et al., 2016; Mollet et al., 2008; Brengues et al., 2005). Formation of SGs and P-bodies in cells are directly involved in global repression of specific host mRNAs (Panas et al., 2016). PRRSV reportedly induced SGs during infection to regulate the antiviral response (Zhou et al., 2017), although the underlying mechanism is not fully understood. Here, we first definitively demonstrated that PRRSV infection induces bona fide SGs, although they differ from the canonical SGs in terms of size and distribution. These granules are closely associated with viral replication complexes (VRCs) and P-bodies, and their formation is coordinated with the phosphorylation of eIF2a and subsequent arrest in cellular translation. The PRRSV-induced SGs were disrupted upon cycloheximide treatment. Ablation of the PRRSV-induced granules through the shRNA silencing of G3BP1 and G3BP2 had no effect on viral replication. Collectively, our results indicate that PRRSV-induced SGs are involved in, but not required, for efficient viral replication. However, the role of SGs in regulating the immune response to PRRSV still warrants further investigation in the future.
    Materials and methods
    Results
    Discussion The main function of SGs is to prevent apoptosis and promote cellular survival during times of stress such as virus infections (Arimoto et al., 2008; McCormick and Khaperskyy, 2017). This is accomplished by regulating rates of host cellular translation. However, SGs have been recently determined to play alternative and unique roles during viral infection. In the case of certain RNA viruses, such as West Nile virus and Dengue virus, SG components play important proviral roles during viral infections. G3BP1 binds to and stabilizes viral RNAs to prevent degradation (Bidet et al., 2014; Li et al., 2002). Conversely, in the cases of poliovirus and hepatitis C virus, SGs have been shown to act as antiviral signaling platforms to initiate innate immune responses (Garaigorta et al., 2012; Onomoto et al., 2012). In this situation, proteases encoded by the viral genome cleave G3BP1 to prevent SG formation and subsequent antiviral signaling pathways (Garaigorta et al., 2012; Beckham and Parker, 2008). The mechanisms by which viruses subvert or hijack the SG machinery are still not fully understood. A more thorough understanding of the molecular mechanisms involving SG formation and dynamics during viral infection will help understand the mechanisms of viral pathogenesis.