鱼类和哺乳类RLR介导的抗病毒免疫反应的泛素化修饰调控

RLR[retinoic acid-inducible gene Ⅰ(RIG-Ⅰ)-like Receptors]是一类表达在胞浆中的模式识别受体, 在识别细胞质中经病毒复制产生的病毒RNA后, 启动一系列信号级联反应, 以诱导机体Ⅰ型干扰素及干扰素诱导的抗病毒基因的表达, 最后达到清除机体病毒感染的目的。由于在病毒感染时机体干扰素反应必须迅速启动, 当病毒清除后干扰素反应又需要立即恢复到正常本底水平, 因此RLR激活的信号转导途径受到了严格的调控, 其中就包括由E3泛素连接酶参与的泛素化修饰调控和由去泛素化酶参与的去泛素化修饰调控。自2003年成功鉴定出鱼类干扰素基因以来, 鱼类也被发现具有保守的RLR信号转导途径诱导干扰素抗病毒免疫反应, 该信号途径同样受到泛素化修饰的调控。文章总结了近年来泛素化修饰在哺乳类和鱼类RLR介导的抗病毒免疫应答通路中的调节机制。     

RACK1 attenuates RLR antiviral signaling by targeting VISA-TRAF complexes

Virus-induced signaling adaptor (VISA), which mediates the production of type I interferon, is crucial for the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling pathway. Upon viral infection, RIG-I recognizes double-stranded viral RNA and interacts with VISA to mediate antiviral innate immunity. However, the mechanisms underlying RIG/VISA-mediated antiviral regulation remain unclear. In this study, we confirmed that receptor for activated C kinase 1 (RACK1) interacts with VISA and attenuates the RIG/VISA-mediated antiviral innate immune signaling pathway. Overexpression of RACK1 inhibited the interferon-β (IFN-β) promoter; interferon-stimulated response element (ISRE); nuclear factor kappa B (NF-κB) activation; and dimerization of interferon regulatory factor 3 (IRF3) mediated by RIG-I, VISA, and TANK-binding kinase 1 (TBK1). A reduction in RACK1 expression level upon small interfering RNA knockdown increased RIG/VISA-mediated antiviral transduction. Additionally, RACK1 disrupted formation of the VISA-tumor necrosis factor receptor-associated factor 2 (TRAF2), VISA-TRAF3, and VISA-TRAF6 complexes during RIG-I/VISA-mediated signal transduction. Additionally, RACK1 enhanced K48-linked ubiquitination of VISA, attenuated its K63-linked ubiquitination, and decreased VISA-mediated antiviral signal transduction. Together, these results indicate that RACK1 interacts with VISA to repress downstream signaling and downregulates virus-induced IFN-β production in the RIG-I/VISA signaling pathway.     

Identification of DreI as an antiviral factor regulated by RLR signaling pathway

Background

Retinoic acid-inducible gene I (RIG-I)–like receptors (RLRs) had been demonstrated to prime interferon (IFN) response against viral infection via the conserved RLR signaling in fish, and a novel fish-specific gene, the grass carp reovirus (GCRV)-induced gene 2 (Gig2), had been suggested to play important role in host antiviral response.

Methodology/Principal Findings

In this study, we cloned and characterized zebrafish Gig2 homolog (named Danio rerio Gig2-I, DreI), and revealed its antiviral role and expressional regulation signaling pathway. RT-PCR, Western blot and promoter activity assay indicate that DreI can be induced by poly I:C, spring viremia of carp virus (SVCV) and recombinant IFN (rIFN), showing that DreI is a typical ISG. Using the pivotal signaling molecules of RLR pathway, including RIG-I, MDA5 and IRF3 from crucian carp, it is found that DreI expression is regulated by RLR cascade and IRF3 plays an important role in this regulation. Furthermore, promoter mutation assay confirms that the IFN-stimulated regulatory elements (ISRE) in the 5′ flanking region of DreI is essential for its induction. Finally, overexpression of DreI leads to establish a strong antiviral state against SVCV and Rana grylio virus (RGV) infection in EPC (Epithelioma papulosum cyprinid) cells.

Conclusions/Significance

These data indicate that DreI is an antiviral protein, which is regulated by RLR signaling pathway.     

Herpes simplex virus 1 tegument protein US11 downmodulates the RLR signaling pathway via direct interaction with RIG-I and MDA-5

The interferon (IFN)-mediated antiviral response is a major defense of the host immune system. In order to complete their life cycle, viruses must modulate host IFN-mediated immune responses. Herpes simplex virus 1 (HSV-1) is a large DNA virus containing more than 80 genes, many of which encode proteins that are involved in virus-host interactions and show immune modulatory capabilities. In this study, we demonstrate that the US11 protein, an RNA binding tegument protein of HSV-1, is a novel antagonist of the beta IFN (IFN-β) pathway. US11 significantly inhibited Sendai virus (SeV)-induced IFN-β production, and its double-stranded RNA (dsRNA) binding domain was indispensable for this inhibition activity. Additionally, wild-type HSV-1 coinfection showed stronger inhibition than US11 mutant HSV-1 in SeV-induced IFN-β production. Coimmunoprecipitation analysis demonstrated that the US11 protein in HSV-1-infected cells interacts with endogenous RIG-I and MDA-5 through its C-terminal RNA-binding domain, which was RNA independent. Expression of US11 in both transfected and HSV-1-infected cells interferes with the interaction between MAVS and RIG-I or MDA-5. Finally, US11 dampens SeV-mediated IRF3 activation. Taken together, the combined data indicate that HSV-1 US11 binds to RIG-I and MDA-5 and inhibits their downstream signaling pathway, preventing the production of IFN-β, which may contribute to the pathogenesis of HSV-1 infection.     

Subcellular localization and functional characterization of a fish IRF9 from crucian carp Carassius auratus

Mammalian interferon (IFN) regulatory factor 9 (IRF-9) has long been recognized as the DNA sequence recognition subunit of IFN-stimulated gene factor 3 (ISGF3) complex, which is critical for type I IFN to induce the expression of IFN-stimulated genes (ISGs) against viral infection. Recent studies have shown that fish IFN exerts antiviral effects by induction of a number of ISGs and also of itself; however, little is known about the role of fish IRF9 in IFN signaling. Here we identify a fish IRF9 orthologue (CaIRF9) from IFN-producing cell line, crucian carp Carassius auratus blastulae embryonic (CAB) cells. Analysis of subcellular distribution of CaIRF9-green fluorescent protein indicates that CaIRF9 is constitutively present in the nucleus, which is driven by two nuclear localization signals (NLS), one locating within DNA-binding domain (DBD) of CaIRF9 and the other immediately behind DBD, although human IRF9 contains only one NLS analogous to the former of CaIRF9. Overexpression of CaIRF9 together with CaSTAT2 not only activates ISRE-containing promoter but also upregulates the expression of fish ISGs. Strikingly, CaIRF9 together with CaSTAT2 also exhibits an ability to activate crucian carp IFN promoter, and blockade of cellular CaIRF9 attenuates IFN itself-induced activation of crucian carp IFN promoter. Taken together, these data suggest that crucian carp IFN induces the expression of ISGs and also of itself possibly by the JAK-STAT signaling pathway that is conserved from fish to mammals.     

IFN regulatory factor family members differentially regulate the expression of type III IFN (IFN-lambda) genes

Virus replication induces the expression of antiviral type I (IFN-alphabeta) and type III (IFN-lambda1-3 or IL-28A/B and IL-29) IFN genes via TLR-dependent and -independent pathways. Although type III IFNs differ genetically from type I IFNs, their similar biological antiviral functions suggest that their expression is regulated in a similar fashion. Structural and functional characterization of the IFN-lambda1 and IFN-lambda3 gene promoters revealed them to be similar to IFN-beta and IFN-alpha genes, respectively. Both of these promoters had functional IFN-stimulated response element and NF-kappaB binding sites. The binding of IFN regulatory factors (IRF) to type III IFN promoter IFN-stimulated response element sites was the most important event regulating the expression of these genes. Ectopic expression of the components of TLR7 (MyD88 plus IRF1/IRF7), TLR3 (Toll/IL-1R domain-containing adapter-inducing factor), or retinoic acid-inducible gene I (RIG-I) signal transduction pathways induced the activation of IFN-lambda1 promoter, whereas the IFN-lambda3 promoter was efficiently activated only by overexpression of MyD88 and IRF7. The ectopic expression of Pin1, a recently identified suppressor for IRF3-dependent antiviral response, decreased the IFN promoter activation induced by any of these three signal transduction pathways, including the MyD88-dependent one. To conclude, the data suggest that the IFN-lambda1 gene is regulated by virus-activated IRF3 and IRF7, thus resembling that of the IFN-beta gene, whereas IFN-lambda2/3 gene expression is mainly controlled by IRF7, thus resembling those of IFN-alpha genes.     

Knock-in of the duck retinoic acid-inducible gene I (RIG-I) into the Mx gene in DF-1 cells enables both stable and immune response-dependent RIG-I expression

Waterfowls, such as ducks, are natural hosts of avian influenza virus (AIV) and can genetically limit the pathogenicity. On the other hand, some AIV strains cause severe pathogenicity in chickens. It is suggested that differences in the pathogenicity of AIV infection between waterfowls and chickens are related to the expression of retinoic acid-inducible gene I (RIG-I), a pattern recognition receptor that chickens evolutionally lack. Here, we knocked-in the duck RIG-I bearing the T2A peptide sequence at the 3′ region of the Mx, an interferon-stimulated gene (ISG), in chicken embryo fibroblast cells (DF-1) using the precise integration into target chromosome (PITCh) system to control the duck RIG-I expression in chickens. The expression patterns of the duck RIG-I were then analyzed using qPCR. The knocked-in DF-1 cells expressed RIG-I via the stimulation of IFN-β and poly(I:C) in a dose-dependent manner. Moreover, poly(I:C) stimulation in the knocked-in DF-1 cells upregulated RIG-I-like receptor (RLR) family signaling pathway-related genes IFN-β, OASL, and IRF7. The IFN-β-dependent expression of RIG-I and upregulation of IFN-β in the poly(I:C) stimulation demonstrated a positive-feedback loop via RIG-I, usually evident in ducks. Overall, this novel strategy established RIG-I-dependent immune response in chickens without overexpression of RIG-I and disruption of the host genes.     

Characterization of a PIAS4 homologue from zebrafish: insights into its conserved negative regulatory mechanism in the TRIF, MAVS, and IFN signaling pathways during vertebrate evolution

Members of the protein inhibitor of activated STAT (PIAS) family are key regulators of various human and mammalian signaling pathways, but data on their occurrence and functions in ancient vertebrates are limited. This study characterizes for the first time to our knowledge a PIAS4 homologue (PIAS4a) from zebrafish. Structurally, this zebrafish PIAS4a (zfPIAS4a) shares a number of conserved functional domains with mammalian PIAS4 proteins, including the scaffold attachment factor A/B/acinus/PIAS box, PINIT, and RING-finger-like zinc-binding domains and a highly acidic domain in the C-terminal region. Subcellular localization analysis shows that zfPIAS4a is a nuclear-localized protein and that the C terminus of the molecule harbors strict nuclear localization signals. Functionally, zfPIAS4a expression can be dramatically induced by the stimulation of polyinosinic-polycytidylic acid and zebrafish IFN1. It acts as a critical negative regulator of the TIR domain-containing adapter inducing IFN-β, mitochondrial antiviral signaling (MAVS), and IFN signaling pathways, and it is the first PIAS protein that plays a role in the MAVS-mediated pathway to be identified. The structure and functionality of PIAS4 seem highly conserved from zebrafish to mammals, making zebrafish an attractive model for screens designed to uncover genes involved in IFN- and inflammatory cytokine-induced signaling pathways. This study provides preliminary evidence that the PIAS regulatory mechanism already existed in fish during vertebrate evolution. It presents valuable clues for improving the understanding of not only the negative regulation of cytokine signaling in fish but also the evolutionary history of the PIAS family from fish to mammals as a whole.     

Rotavirus NSP1 inhibits expression of type I interferon by antagonizing the function of interferon regulatory factors IRF3, IRF5, and IRF7

Secretion of interferon (IFN) by virus-infected cells is essential for activating autocrine and paracrine pathways that promote cellular transition to an antiviral state. In most mammalian cells, IFN production is initiated by the activation of constitutively expressed IFN regulatory factor 3, IRF3, which in turn leads to the induction of IRF7, the "master regulator" of IFN type I synthesis (alpha/beta IFN). Previous studies established that rotavirus NSP1 antagonizes IFN signaling by inducing IRF3 degradation. In the present study, we have determined that, in comparison to wild-type rotaviruses, rotaviruses encoding defective NSP1 grow to lower titers in some cell lines and that this poor growth phenotype is due to their failure to suppress IFN expression. Furthermore, we provide evidence that rotaviruses encoding wild-type NSP1 subvert IFN signaling by inducing the degradation of not only IRF3, but also IRF7, with both events occurring through proteasome-dependent processes that proceed with similar efficiencies. The capacity of NSP1 to induce IRF7 degradation may allow rotavirus to move across the gut barrier by enabling the virus to replicate in specialized trafficking cells (dendritic cells and macrophages) that constitutively express IRF7. Along with IRF3 and IRF7, NSP1 was found to induce the degradation of IRF5, a factor that upregulates IFN expression and that is involved in triggering apoptosis during viral infection. Our analysis suggests that NSP1 mediates the degradation of IRF3, IRF5, and IRF7 by recognizing a common element of IRF proteins, thereby allowing NSP1 to act as a broad-spectrum antagonist of IRF function.     

Identification of nucleoporin 93 (Nup93) that mediates antiviral innate immune responses

RIG-I-like receptors (RLRs) are cytoplasmic sensors for viral RNA that elicit antiviral innate immune responses. RLR signaling culminates in the activation of the protein kinase TBK1, which mediates phosphorylation and nuclear translocation of IRF3 that regulates expression of type I interferon genes. Here, we found that Nucleoporin 93 (Nup93), components of nuclear pore complex (NPC), plays an important role in RLR-mediated antiviral responses. Nup93-deficient RAW264.7 macrophage cells exhibited decreased expression of Ifnb1 and Cxcl10 genes after treatment with a synthetic RLR agonist stimulation as well as Newcastle Disease Virus infection. Silencing Nup93 in murine primary macrophages and embryonic fibroblasts also resulted in reduced expression of these genes. IRF3 nuclear translocation during RLR signaling was impaired in Nup93-deficient RAW264.7 cells. Notably, the activation of TBK1 during RLR signaling was also decreased in Nup93-deficient cells. We found that Nup93 formed a complex with TBK1, and Nup93 overexpression enhanced TBK1-mediated IFNβ promoter activation. Taken together, our findings suggest that Nup93 regulates antiviral innate immunity by enhancing TBK1 activity and IRF3 nuclear translocation.