Inflammation refers to the response of the immune system to viral, bacterial, and fungal infections, or other foreign particles in the body, which can involve the production of a wide array of soluble inflammatory mediators. It is important for the development of many RNA virus-infected diseases. The primary factors through which the infection becomes inflammation involve inflammasome. Inflammasomes are proteins complex that the activation is responsive to specific pathogens, host cell damage, and other environmental stimuli. Inflammasomes bring about the maturation of various pro-inflammatory cytokines such as IL-18 and IL-1β in order to mediate the innate immune defense mechanisms. Many RNA viruses and their components, such as encephalomyocarditis virus (EMCV) 2B viroporin, the viral RNA of hepatitis C virus, the influenza virus M2 viroporin, the respiratory syncytial virus (RSV) small hydrophobic (SH) viroporin, and the human rhinovirus (HRV) 2B viroporin can activate the Nod-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) inflammasome to influence the inflammatory response. On the other hand, several viruses use virus-encoded proteins to suppress inflammation activation, such as the influenza virus NS1 protein and the measles virus (MV) V protein. In this review, we summarize how RNA virus infection leads to the activation or inhibition of the NLRP3 inflammasome.

Nucleolin (NCL), a stress-responsive RNA-binding protein, has been implicated in the translation of internal ribosome entry site (IRES)-containing mRNAs, which encode proteins involved in cell proliferation, carcinogenesis, and viral infection (Type I IRESs). However, the details of the mechanisms by which NCL participates in IRES-driven translation have not hitherto been described. Here, we identified NCL as a protein that interacts with the IRES of foot-and-mouth disease virus (FMDV), which is a Type II IRES. We also mapped the interactive regions within FMDV IRES and NCL in vitro. We found that NCL serves as a substantial regulator of FMDV IRES-driven translation but not of bulk cellular or vesicular stomatitis virus cap-dependent translation. NCL also modulates the translation of and infection by Seneca Valley virus (Type III-like IRES) and classical swine fever virus (Type III IRES), which suggests that its function is conserved in unrelated IRES-containing viruses. We also show that NCL affects viral replication by directly regulating the production of viral proteins and indirectly regulating FMDV RNA synthesis. Importantly, we observed that the cytoplasmic relocalization of NCL during FMDV infection is a substantial step for viral IRES-driven translation, and that NCL specifically promotes the initiation phase of the translation process by recruiting translation initiation complexes to viral IRES. Finally, the functional importance of NCL in FMDV pathogenicity was confirmed in vivo. Taken together, our findings demonstrate a specific function for NCL in selective mRNA translation, and identify a target for the development of a broad-spectrum class of antiviral interventions. IMPORTANCE FMDV usurps the cellular translation machinery to initiate viral protein synthesis via a mechanism driven by IRES elements. It allows the virus to shut down bulk cellular translation, while providing an advantage for its own gene expression. With limited coding capacity in its own genome, FMDV has evolved a mechanism to hijack host proteins to promote the recruitment of the host translation machinery, a process that is still not well understood. Here, we identified nucleolin (NCL) as a positive regulator of the IRES-driven translation of FMDV. Our study supports a model in which NCL relocalizes from the nucleus to the cytoplasm during the course of FMDV infection, where the cytoplasmic NCL promotes FMDV IRES-driven translation by bridging the translation initiation complexes with viral IRES. Our study demonstrates a previously uncharacterized role of NCL in the translation initiation of IRES-containing viruses, with important implications for the development of broad antiviral interventions.

In cells, the contributions of DEAD-box helicases (DDXs), without which cellular life is impossible, are of utmost importance. The extremely diverse roles of the nucleolar helicase DDX21, ranging from fundamental cellular processes such as cell growth, ribosome biogenesis, protein translation, protein–protein interaction, mediating and sensing transcription, and gene regulation to viral manipulation, drew our attention. We designed this project to study virus–host interactions and viral pathogenesis. A pulldown assay was used to investigate the association between foot-and-mouth disease virus (FMDV) and DDX21. Further insight into the DDX21–FMDV interaction was obtained through dual-luciferase, knockdown, overexpression, qPCR, and confocal microscopy assays. Our results highlight the antagonistic feature of DDX21 against FMDV, as it progressively inhibited FMDV internal ribosome entry site (IRES) -dependent translation through association with FMDV IRES domains 2, 3, and 4. To subvert this host helicase antagonism, FMDV degraded DDX21 through its non-structural proteins 2B, 2C, and 3C protease (3Cpro). Our results suggest that DDX21 is degraded during 2B and 2C overexpression and FMDV infection through the caspase pathway; however, DDX21 is degraded through the lysosomal pathway during 3Cpro overexpression. Further investigation showed that DDX21 enhanced interferon-beta and interleukin-8 production to restrict viral replication. Together, our results demonstrate that DDX21 is a novel FMDV IRES trans-acting factor, which negatively regulates FMDV IRES-dependent translation and replication.