MDA5

Abstract: Melanoma differentiation-associated gene 5 (MDA5) is an important intracellular receptor that recognizes long molecules of viral double-stranded RNA in innate immunity. To understand the mechanism of duck MDA5-mediated innate immunity, we cloned the MDA5 cDNA from the Muscovy duck (Cairina moschata). Quantitative real-time PCR analysis indicates that duck MDA5 mRNA was constitutively expressed in all sampled tissues. A significant increase of MDA5 mRNA was detected in the brain, spleen and lungs of ducks after infection with an H5N1 highly pathogenic avian influenza virus (HPAIV). We investigated the role of the predicted functional domains of MDA5. The results indicate the caspase activation and recruitment domain (CARD) of duck MDA5 had a signal transmission function through IRF-7-dependent signaling pathway. Overexpression of the CARD strongly activated the chicken IFN-β promoter and upregulated the mRNA expression of antiviral molecules (such as OAS, PKR and Mx), proinflammatory cytokines (such as IL-2, IL-6, IFN-α and IFN-γ, but not IL-1β and IL-8) and retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) (RIG-I and LGP2) without exogenous stimulation. We also demonstrate the NS1 of the H5N1 HPAIV inhibited the duck MDA5-mediated signaling pathway in vitro. These results suggest that duck MDA5 is an important receptor for inducing antiviral activity in the host immune response of ducks.

Abstract: Upon recognition of specific molecular patterns on microbes, host cells trigger an innate immune response, which culminates in the production of type I interferons, proinflammatory cytokines and chemokines, and restricts pathogen replication and spread within the host. At each stage of this response, there are stimulatory and inhibitory signals that regulate the magnitude, quality, and character of the response. Positive regulation promotes an antiviral state to control and eventually clear infection, whereas negative regulation dampens inflammation and prevents immune-mediated tissue damage. An overexuberant innate response can lead to cell and tissue destruction, and the development of spontaneous autoimmunity. The retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), RIG-I and melanoma differentiation-associated gene 5 (MDA5), belong to a family of cytosolic host RNA helicases that recognize distinct nonself RNA signatures and trigger innate immune responses against several RNA viruses by signaling through the essential adaptor protein mitochondrial antiviral signaling (MAVS). The RLR signaling pathway is tightly regulated to maximize antiviral immunity and minimize immune-mediated pathology. This review highlights contemporary findings on negative regulators of the RLR signaling pathway, with specific focus on the proteins and biological processes that directly regulate RIG-I, MDA5 and MAVS signaling function.

Abstract: Type I interferon (IFN) is an important component of antiviral innate immune signaling mediated by viral DNA and RNA recognition by the DNA sensor cGAS and RNA sensors RIG-I and MDA5. Activation of these DNA and RNA sensors leads to the recruitment of STING and MAVS, respectively, and converges on TANK-binding kinase 1 (TBK1) signaling for subsequent phosphorylation of IFN regulatory factor 3 (IRF3). However, the mechanisms that control TBK1 activation are still poorly defined. Here, we identify tripartite motif 9 short isoform (TRIM9s) as a positive regulator in type I IFN signaling. Upon viral infection, TRIM9s undergoes Lys-63-linked auto-polyubiquitination and serves as a platform to bridge GSK3β to TBK1, leading to the activation of IRF3 signaling. Interestingly, we found that TRIM9s selectively inhibits the production of pro-inflammatory cytokines, but enhances the expression of type I IFNs as well as IFN-stimulated genes, in response to viral infection. Our findings reveal novel dual functions of TRIM9s in antiviral immunity, which serve to balance pro-inflammatory response and production of type I IFNs.

Abstract: The human cytomegalovirus (HCMV) TRS1 and IRS1 genes rescue replication of vaccinia virus (VV) that has a deletion of the double-stranded RNA binding protein gene E3L (VVΔE3L). Like E3L, these HCMV genes block the activation of key interferon-induced, double-stranded RNA (dsRNA)-activated antiviral pathways. We investigated the hypothesis that the products of these HCMV genes act by binding to dsRNA. pTRS1 expressed by cell-free translation or by infection of mammalian cells with HCMV or recombinant VV bound to dsRNA. Competition experiments revealed that pTRS1 preferentially bound to dsRNA compared to double-stranded DNA or single-stranded RNA. 5′- and 3′-end deletion analyses mapped the TRS1 dsRNA-binding domain to amino acids 74 through 248, a region of identity to pIRS1 that contains no homology to known dsRNA-binding proteins. Deletion of the majority of this region (Δ86-246) completely abrogated dsRNA binding. To determine the role of the dsRNA-binding domain in the rescue of VVΔE3L replication, wild-type or deletion mutants of TRS1 were transfected into HeLa cells, which were then infected with VVΔE3L. While full-length TRS1 rescued VVΔE3L replication, deletion mutants affecting a carboxy-terminal region of TRS1 that is not required for dsRNA binding failed to rescue VVΔE3L. Analyses of stable cell lines revealed that the carboxy-terminal domain is necessary to prevent the shutoff of protein synthesis and the phosphorylation of eIF2α after VVΔE3L infection. Thus, pTRS1 contains an unconventional dsRNA-binding domain at its amino terminus, but a second function involving the carboxy terminus is also required for countering host cell antiviral responses.