SARS-CoV-2 ORF10 suppresses the antiviral innate immune response by degrading MAVS through mitophagy

The global coronavirus disease 2019(COVID-19)pandemic caused by severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)has caused severe morbidity and mortality in humans.It is urgent to understand the function of viral genes.However,the function of open reading frame 10(ORF10),which is uniquely expressed by SARS-CoV-2,remains unclear.In this study,we showed that overexpression of ORF10 markedly suppressed the expression of type I interferon(IFN-I)genes and IFN-stimulated genes.Then,mitochondrial antiviral signaling protein(MAVS)was identified as the target via which ORF10 suppresses the IFN-I signaling pathway,and MAVS was found to be degraded through the ORF10-induced autophagy pathway.Furthermore,overexpression of ORF10 promoted the accumulation of LC3 in mitochondria and induced mitophagy.Mechanistically,ORF10 was translocated to mitochondria by interacting with the mitophagy receptor Nip3-like protein X(NIX)and induced mitophagy through its interaction with both NIX and LC3B.Moreover,knockdown of NIX expression blocked mitophagy activation,MAVS degradation,and IFN-I signaling pathway inhibition by ORF10.Consistent with our observations,in the context of SARS-CoV-2 infection,ORF10 inhibited MAVS expression and facilitated viral replication.In brief,our results reveal a novel mechanism by which SARS-CoV-2 inhibits the innate immune response;that is,ORF10 induces mitophagy-mediated MAVS degradation by binding to NIX.     

Regulation of mitochondrial antiviral signaling pathways

Mitochondrial antiviral immunity involves the detection of viral RNA by intracellular pattern-recognition receptors (PRRs) belonging to the RIG-I-like helicase family. The convergence of these and other signaling molecules to the outer mitochondrial membrane results in the rapid induction of antiviral cytokines including type-1 interferon. Here, we discuss recent studies describing new molecules implicated in the regulation of this antiviral response.     

MicroRNA-33/33* inhibit the activation of MAVS through AMPK in antiviral innate immunity

Innate immunity plays a prominent role in the host defense against pathogens and must be precisely regulated. As vital orchestrators in cholesterol homeostasis, microRNA-33/33* have been widely investigated in cellular metabolism. However, their role in antiviral innate immunity is largely unknown. Here, we report that VSV stimulation decreased the expression of miR-33/33* through an IFNAR-dependent manner in macrophages. Overexpression of miR-33/33* resulted in impaired RIG-I signaling, enhancing viral load and lethality whereas attenuating type I interferon production both in vitro and in vivo. In addition, miR-33/33* specifically prevented the mitochondrial adaptor mitochondrial antiviral-signaling protein (MAVS) from forming activated aggregates by targeting adenosine monophosphate activated protein kinase (AMPK), subsequently impeding the mitophagy-mediated elimination of damaged mitochondria and disturbing mitochondrial homeostasis which is indispensable for efficient MAVS activation. Our findings establish miR-33/33* as negative modulators of the RNA virus-triggered innate immune response and identify a previously unknown regulatory mechanism linking mitochondrial homeostasis with antiviral signaling pathways.     

Molecular regulation of interferon antiviral response in fish

Interferon (IFN) response is the first line of host defense against virus infection. The recent years have witnessed tremendous progress in understanding of fish IFN antiviral response. Varied number of IFN genes has been identified in different fish species but obviously, they do not show a one-to-one orthologous relationship with mammalian IFN homologs. These genes are divided into two groups with different abilities to induce downstream gene expression through binding to different receptor complexes. Consistently, some fish IFN-stimulated genes such as Mx and PKR have been confirmed for their antiviral effects. In this review, we focus on how fish cells respond to IFNs and how fish IFNs are triggered through TLR pathway and RLR pathway. We highlight the roles of IRF3 and IRF7 in activation of fish IFN response. In addition, the unique mechanisms underlying IRF3/7-dependent fish IFN response and auto-regulation of fish IFN gene expression are discussed.     

Molecular cloning and functional characterization of duck mitochondrial antiviral-signaling protein (MAVS)

Mitochondrial antiviral-signaling protein (MAVS), also called IPS-1/VISA/Cardif, is an important molecule involved in host defense and triggers a signal for producing type I IFN. Currently the function of MAVS in ducks (duMAVS) remains largely unclear while significant progress has been made in mammals. In this study, the full-length duMAVS cDNA was cloned from duck embryo fibroblasts (DEFs) for the first time. Tissue specificity analysis showed duMAVS was universally expressed in all detected tissues. DEFs transfected with duMAVS were able to induce interferon-β (IFN-β) expression through activating interferon regulatory factor 1 (IRF1) and nuclear factor kappa B (NF-κB). Both the CARD-like domain and transmembrane domain were required for duMAVS signaling via deletion mutant analysis. In addition, poly(I:C)- or Sendai virus (SeV)-induced IFN-β expression in DEFs were significantly decreased by knock-down of duMAVS with siRNA. Altogether, these results indicate that MAVS is a critical immunoregulator in duck innate immune system.     

The interferon in TLR signaling: more than just antiviral

The Toll-like receptor (TLR) system is responsible for the recognition of infectious agents leading to initiation of the primary innate, and later adaptive, immune response. Genetic technologies have enabled the discovery of new factors involved in these systems, their genetic manipulation and the global analyses of their effects on gene expression. Furthermore, this increased understanding has resulted in the need to reassess our preconceptions about the functions of well-known molecules. For example, type I interferons (IFNs), which were discovered as antiviral proteins, are now known to be produced in response to TLR activation by many pathogens, including bacteria. Should we be surprised? Has the inflammatory response unexpectedly highjacked the body's antiviral system? Or are we too easily blinkered by preconceptions from how a compound was discovered?     

Inhibition of the interferon antiviral response by hepatitis C virus

Hepatitis C virus (HCV) causes acute and chronic hepatitis by targeting the liver hepatocyte for infection and destruction. The standard treatment for chronic HCV infection is pegylated interferon plus ribavirin. Unfortunately, the sustained response rate and associated toxicity with this treatment are far from ideal; more effective and less toxic treatment regimens are needed. With more than 170 million people infected worldwide, there is an unmet medical need for new effective treatments. Recent advances in the understanding of the signaling pathways leading to the host antiviral response to HCV, the mechanisms used by HCV to evade the immune response, the development of cell culture models of HCV infection and the development of small molecule inhibitors of HCV have generated optimism that novel therapeutic approaches to control HCV will soon be available.     

Effect of hyaluronidase on cell response to the antiviral and interferon inducing activity of poly(rI) · poly(rC)

The effect of hyaluronidase on cell response to poly(rI) . poly(rC) [poly(IC)] was investigated in rabbit kidney (RK) cells. For this purpose, bovine testicular and Staphylococcal hyaluronidase preparations at various degrees of purity were used. These enzyme preparations were employed at the maximal nontoxic dose for 2 hours before poly(IC) treatment. This enzymatic pretreatment of the cells strongly inhibited the antiviral activity of poly(IC), determined by using both Herpes simplex virus type-1 and vesicular stomatitis virus. It also remarkably reduced the poly(IC)-induced interferon production. This later effect could account for the diminished antiviral activity of poly(IC) in the hyaluronidase-treated cells.     

线粒体抗病毒信号蛋白真核表达载体的构建及鉴定

目的 构建线粒体抗病毒信号蛋白真核表达载体.方法 通过聚合酶链反应(PCR)的方法扩增获得线粒体抗病毒信号蛋白基因的完整序列,进而纯化回收后连接到pMDTM 18-T载体上,将pcDNA6/myc-HisA载体和带有目的片段的pMDTM 18-T载体同时进行双酶切,酶切产物过夜连接转化至大肠杆菌JM109,挑取阳性克隆pcDNA6/myc-HisA-MA VS扩增后,提取重组质粒;利用PCR和核酸测序验证线粒体抗病毒信号蛋白真核表达载体构建的正确性;应用Western blotting的方法检测融合蛋白的表达.结果 线粒体抗病毒信号蛋白真核表达载体构建成功,并且该载体能够在OL细胞中表达融合蛋白.结论 成功构建线粒体抗病毒信号蛋白真核表达载体,为进一步研究该蛋白的功能和作用机制奠定基础.     

Constitutive type I interferon modulates homeostatic balance through tonic signaling

Interferons (IFNs) were discovered as cytokines induced during and protecting from viral infection. They have been documented to play essential roles in numerous physiological processes beyond antiviral and antimicrobial defense, including immunomodulation, cell cycle regulation, cell survival, and cell differentiation. Recent data have also uncovered a potentially darker side to IFN, including roles in inflammatory diseases, such as autoimmunity and diabetes. IFN can have effects in the absence of acute infection, highlighting a physiologic role for constitutive IFN. Type I IFNs are constitutively produced at vanishingly low quantities and yet exert profound effects, mediated in part through modulation of signaling intermediates required for responses to diverse cytokines. We review evidence for a yin-yang of IFN function through its role in modulating crosstalk between multiple cytokines by both feedforward and feedback regulation of common signaling intermediates and postulate a homeostatic role for IFN through tonic signaling in the absence of acute infection.     

Mitoxosome: a mitochondrial platform for cross-talk between cellular stress and antiviral signaling

Evidence is accumulating that the mitochondria form an integral platform from which innate signaling takes place. Recent studies revealed that the mitochondria are shaping the innate response to intracellular pathogens, and mitochondrial function is modulating and being modulated by innate immune signaling. Further, cell biologic analyses have uncovered the dynamic relocalization of key components involved in cytosolic viral recognition and signaling to the mitochondria, as well as the mobilization of mitochondria to the sites of viral replication. In this review, we provide an integrated view of how cellular stress and signals following cytosolic viral recognition are intimately linked and coordinated at the mitochondria. We incorporate recent findings into our current understanding of the role of mitochondrial function in antiviral immunity and suggest the existence of a 'mitoxosome', a mitochondrial oxidative signalosome where multiple pathways of viral recognition and cellular stress converge on the surface of the mitochondria to facilitate a coordinated antiviral response.     

Human leukocyte interferon and the antiviral factor (AVF) from virus-infected plants stimulate plant tissues to produce nucleotides with antiviral activity

Leaves of Nicotiana glutinosa and tobacco, as well as tobacco callus cultures, were treated with the plant antiviral factor AVF or with a purified subspecies of human leukocyte interferon. After incubation, a fraction containing short oligonucleotides was extracted directly from the plant tissue. In addition, a synthetase preparation was fractionated from treated tissues, which polymerized ATP in the presence of polyinosinic, polycytidylic acid (poly(I:C)). The various plant nucleotides were applied to tobacco mosaic virus (TMV)-infected leaf disks, the TMV content of which were determined by an enzyme-linked immunosorbent assay. The nucleotide fraction extracted directly from TMV-infected leaves exhibited a considerable antiviral activity, whereas similar fractions from AVF- or interferon-treated leaves did not, even though an antiviral state was induced in the tissue by these agents. However, when the synthetase fraction from TMV-infected, AVF-treated, or interferon-treated tissues was incubated with ATP and poly(I:C), the resultant, heat-stable, acid-soluble, polymerized ATP markedly inhibited TMV multiplication. It is concluded that the presence of double-stranded RNA is obligatory for the formation of the antiviral nucleotides. The analogy to interferon-induced resistance in animal tissues is discussed.     

Lack of correlation between thymidine kinase activity and the antiviral or antiproliferative response to interferon

Murine L cells lacking thymidine kinase activity (Ltk-) do not respond to the antiviral or antiproliferative activity of interferon, whereas human tk- cells show a normal response. Furthermore, a clone of tk+ cells derived from Ltk- cells by DNA-mediated transfer of the tk herpes gene does not respond to interferon. Cells of this clone and Ltk- cells do not produce interferon when infected with Newcastle disease virus or treated with poly(I) X poly(C) and DEAE-dextran. Possible reasons for this inability to produce interferon and the lack of response to interferon are discussed.     

Subregional localization of the gene(s) governing the human interferon induced antiviral state in man.

A dosage effect of chromosomal translocation was used to locate the gene(s) which codes for the human interferon induced antiviral state on the long arm of chromosome 21.     

TIRAP: an adapter molecule in the Toll signaling pathway

Mammalian Toll-like receptors (TLRs) recognize conserved products of microbial metabolism and activate NF-kappa B and other signaling pathways through the adapter protein MyD88. Although some cellular responses are completely abolished in MyD88-deficient mice, TLR4, but not TLR9, can activate NF-kappa B and mitogen-activated protein kinases and induce dendritic cell maturation in the absence of MyD88. These differences suggest that another adapter must exist that can mediate MyD88-independent signaling in response to TLR4 ligation. We have identified and characterized a Toll-interleukin 1 receptor (TIR) domain-containing adapter protein (TIRAP) and have shown that it controls activation of MyD88-independent signaling pathways downstream of TLR4. We have also shown that the double-stranded RNA-binding protein kinase PKR is a component of both the TIRAP- and MyD88-dependent signaling pathways.     

An intensification of antiviral and interferon effects of ridostin (an interferon inducer) by cationic liposomes in vitro and in intranasal administration

Combined application of ridostine with catonic liposomes was shown to essentially enhance the interferon-inducing and antiviral activity of the former in experiments with cell cultures L-929, which is apparently related with an improved efficiency of intracellular delivery of dsRNA. A comparative study demonstrated that ridostine, when combined with liposomes, is needed by 10(3)-10(4) times less as when it is used alone. A pretreatment of the cellular monolayer by cationic liposomes contributes also to enhancing the activity of ridostine, which can be explained by an enhanced permeability of cells for dsRNA holding on-for as long as 30 minutes after the removal of liposomes from the liquid culture. A separate successive administration of, first, liposomes and, then, of ridostine in BALB/c mice (20 mg/kg) leads to a more intensified induction of interferon in the upper respiratory tract tissues as compared with the administration of ridostine alone.     

Constitutive overexpressed type I interferon induced downregulation of antiviral activity in medaka fish (Oryzias latipes)

In fish, as well as vertebrates, type I interferons (IFNs) are important cytokines that help to provide innate, antiviral immunity. Although low amounts of IFN are constitutively secreted under normal physiological conditions, long-term and excessive IFN stimulation leads to reduced sensitivity to the IFN signal. This provides a negative feedback mechanism that prevents inappropriate responses and autoimmunity. At present, however, neither IFN desensitization nor the normal physiological role of constitutive IFN are well characterized in fish. The objective here was therefore to produce and characterize a transgenic medaka fish (Oryzias latipes), designated IFNd-Tg, that constitutively overexpressed the IFNd gene. A dual promoter expression vector was constructed for overexpression of IFNd under an EF1α promoter and a DsRed reporter gene under control of a γF-crystaline promoter. The phenotype of the IFNd-Tg fish had a lower response to poly(I:C) and increased susceptibility to nervous necrosis virus (NNV) infection compared to wild-type (WT). Furthermore, transduction of IFN signals for STAT1b, STAT2 and IRF9 were down-regulated in the IFNd-Tg fish, and expression levels of RLR signal molecules (MDA5, MITA, IRF1 and IRF3) were lower than in WT. The constitutive overexpression of IFNd resulted in desensitization of IFN-stimulation, apparently due to downregulation of IFN signal transduction, and this caused increased susceptibility to NNV.