Essential role of IPS-1 in innate immune responses against RNA viruses

RIG-I-driven CDKN1A stabilization reinforces cellular senescence

The innate immune signaling network follows a canonical format for signal transmission. The innate immune pathway is crucial for defense against pathogens, yet its mechanistic crosstalk with aging processes remains largely unexplored. Retinoic acid-inducible gene-I (RIG-I), a key mediator of antiviral immunity within this pathway, has an enigmatic role in stem cell senescence. Our study reveals that RIG-I levels increase in human genetic and physiological cellular aging models, and its accumulation drives cellular senescence. Conversely, CRISPR/Cas9-mediated RIG-I deletion or pharmacological inhibition in human mesenchymal stem cells (hMSCs) confers resistance to senescence. Mechanistically, RIG-I binds to endogenous mRNAs, with CDKN1A mRNA being a prominent target. Specifically, RIG-I stabilizes CDKN1A mRNA, resulting in elevated CDKN1A transcript levels and increased p21Cip1 protein expression, which precipitates senescence. Collectively, our findings establish RIG-I as a post-transcriptional regulator of senescence and suggest potential targets for the mitigation of aging-related diseases.

Quercetin-3-Methyl Ether Induces Early Apoptosis to Overcome HRV1B Immune Evasion, Suppress Viral Replication, and Mitigate Inflammatory Pathogenesis

Human rhinovirus (HRV) causes the common cold and exacerbates chronic respiratory diseases, such as asthma and chronic obstructive pulmonary disease. Despite its significant impact on public health, there are currently no approved vaccines or antiviral treatments for HRV infection. Apoptosis is the process through which cells eliminate themselves through the systematic activation of intrinsic death pathways in response to various stimuli. It plays an important role in viral infections and serves as a key immune defense mechanism in the interactions between viruses and the host. In the present study, we investigated the antiviral effects of quercetin-3-methyl ether, a flavonoid isolated from Serratula coronata, on human rhinovirus 1B (HRV1B). Quercetin-3-methyl ether significantly inhibited HRV1B replication in HeLa cells in a concentration-dependent manner, thereby reducing cytopathic effects and viral RNA levels. Time-course and time-of-addition analyses confirmed that quercetin-3-methyl ether exhibited antiviral activity during the early stages of viral infection, potentially targeting the replication and translation phases. Gene expression analysis using microarrays revealed that pro-apoptotic genes were upregulated in quercetin-3-methyl ether-treated cells, suggesting that quercetin-3-methyl ether enhances early apoptosis to counteract HRV1B-induced immune evasion. In vivo administration of quercetin-3-methyl ether to HRV1B-infected mice significantly reduced viral RNA levels and inflammatory cytokine production in the lung tissues. Our findings demonstrated the potential of quercetin-3-methyl ether as a novel antiviral agent against HRV1B, thereby providing a promising therapeutic strategy for the management of HRV1B infections and related complications.

Microparticle and nanoparticle-based influenza vaccines

Influenza infections are a health public problem worldwide every year with the potential to become the next pandemic. Vaccination is the most effective strategy to prevent future influenza outbreaks, however, influenza vaccines need to be reformulated each year to provide protection due to viral antigenic drift and shift. As more efficient influenza vaccines are needed, it is relevant to recapitulate strategies to improve the immunogenicity and broad reactivity of the current vaccines. Here, we review the current approved vaccines in the U.S. market and the platform used for their production. We discuss the different approaches to develop a broadly reactive vaccine as well as reviewing the adjuvant systems that are under study for being potentially included in future influenza vaccine formulations. The main components of the immune system involved in achieving a protective immune response are reviewed and how they participate in the trafficking of particles systemically and in the mucosa. Finally, we describe and classify, according to their physicochemical properties, some of the potential micro and nano-particulate platforms that can be used as delivery systems for parenteral and mucosal vaccinations.

STAT6 promotes innate immunity against BEFV and VSV by inhibiting STUB1 and NIX-mediated MAVS degradation

Signal transducers and activators of transcription 6 (STAT6), an essential member of the STAT protein family, plays vital roles in innate immunity, however, its function in regulating innate immunity through the degradation of MAVS has not been described. In this study, we found that STAT6 suppresses the replication of both bovine ephemeral fever virus (BEFV) and vesicular stomatitis virus (VSV). Further investigations revealed that STAT6 promotes the type I IFN (IFN-I) signaling pathway in the context of BEFV and VSV infection. Moreover, the knockout of STAT6 leads to the degradation of MAVS through both the ubiquitin-proteasome and autophagolysosomal pathways. Mechanistically, STAT6 results in the downregulation of E3 ubiquitin ligase STIP1 homology and Ubox-containing protein 1 (STUB1), inhibits the interaction between STUB1 and MAVS, and reduces STUB1- mediated K48-linked MAVS ubiquitination, thereby inhibiting the MAVS degradation through the ubiquitin-proteasome pathway. Furthermore, STAT6 also suppresses MAVS degradation through the autophagy receptor Bcl2 interacting protein 3 like (NIX)-mediated autophagy pathway. Taken together, our study unveils a novel mechanism by which STAT6 acts as a positive regulator of the type I IFN signaling pathway during BEFV and VSV infection, predominantly by inhibiting MAVS degradation and ultimately suppressing BEFV and VSV infection. These findings provide valuable insights into the regulation of MAVS degradation by STAT6, which may serve as a basis for the design of novel antiviral agents.

Fetal MAVS and type I IFN signaling pathways control ZIKV infection in the placenta and maternal decidua

The contribution of placental immune responses to congenital Zika virus (ZIKV) syndrome remains poorly understood. Here, we leveraged a mouse model of ZIKV infection to identify mechanisms of innate immune restriction exclusively in the fetal compartment of the placenta. ZIKV principally infected mononuclear trophoblasts in the junctional zone, which was limited by mitochondrial antiviral-signaling protein (MAVS) and type I interferon (IFN) signaling mechanisms. Single nuclear RNA sequencing revealed MAVS-dependent expression of IFN-stimulated genes (ISGs) in spongiotrophoblasts but not in other placental cells that use alternate pathways to induce ISGs. ZIKV infection of Ifnar1-/- or Mavs-/- placentas was associated with greater infection of the adjacent immunocompetent decidua, and heterozygous Mavs+/- or Ifnar1+/- dams carrying immunodeficient fetuses sustained greater maternal viremia and tissue infection than dams carrying wild-type fetuses. Thus, MAVS-IFN signaling in the fetus restricts ZIKV infection in junctional zone trophoblasts, which modulates dissemination and outcome for both the fetus and the pregnant mother.

HuR (ELAVL1) regulates the CCHFV minigenome and HAZV replication by associating with viral genomic RNA

Crimean-Congo Hemorrhagic Fever virus (CCHFV) is a tick-borne pathogen that causes severe acute fever disease in humans and requires a biosafety level 4 laboratory for handling. Hazara virus (HAZV), belonging to the same virus genus as CCHFV, does not exhibit pathogenesis in humans. To investigate host RNA-binding proteins (RBPs) that regulate CCHFV replication, we generated a series of mutant RAW264.7 cells by CRISPR/Cas9 system and these cells were infected with HAZV. The viral titers in the supernatant of these cells was investigated, and HuR (ELAVL1) was identified. HuR KO RAW264.7 cells reduced HAZV replication. HuR is an RBP that enhances mRNA stability by binding to adenyl-uridine (AU)-rich regions in their 3' non-coding region (NCR). HuR regulates innate immune response by binding to host mRNAs of signaling molecules. The expression of cytokine genes such as Ifnb, Il6, and Tnf was reduced in HuR KO cells after HAZV infection. Although HuR supports the innate immune response during HAZV infection, we found that innate immune activation by HAZV infection did not affect its replication. We then investigated whether HuR regulates HAZV genome RNA stability. HAZV RNA genome was precipitated with an anti-HuR antibody, and HAZV genome RNA stability was lowered in HuR KO cells. We found that HuR associated with HAZV RNA and stabilized it to enhance HAZV replication. Furthermore, HuR-deficiency reduced CCHFV minigenome replication. CCHFV is a negative-strand RNA virus and positive-strand RNA is produced during replication. HuR was associated with positive-strand RNA rather than negative-strand RNA, and AU-rich region in 3'-NCR of S segment was responsible for immunoprecipitation with anti-HuR antibody and minigenome replication. Additionally, HuR inhibitor treatment reduced CCHFV minigenome replication. Our results indicate that HuR aids replication of the CCHFV minigenome by associating with the AU-rich region in the 3'-NCR.

A closer look at mammalian antiviral condensates

Several biomolecular condensates assemble in mammalian cells in response to viral infection. The most studied of these are stress granules (SGs), which have been proposed to promote antiviral innate immune signaling pathways, including the RLR-MAVS, the protein kinase R (PKR), and the OAS-RNase L pathways. However, recent studies have demonstrated that SGs either negatively regulate or do not impact antiviral signaling. Instead, the SG-nucleating protein, G3BP1, may function to perturb viral RNA biology by condensing viral RNA into viral-aggregated RNA condensates, thus explaining why viruses often antagonize G3BP1 or hijack its RNA condensing function. However, a recently identified condensate, termed double-stranded RNA-induced foci, promotes the activation of the PKR and OAS-RNase L antiviral pathways. In addition, SG-like condensates known as an RNase L-induced bodies (RLBs) have been observed during many viral infections, including SARS-CoV-2 and several flaviviruses. RLBs may function in promoting decay of cellular and viral RNA, as well as promoting ribosome-associated signaling pathways. Herein, we review these recent advances in the field of antiviral biomolecular condensates, and we provide perspective on the role of canonical SGs and G3BP1 during the antiviral response.

MAVS disruption impairs downstream signaling and results in higher virus replication levels of salmonid alphavirus subtype 3 but not infectious pancreatic necrosis virus in vitro

The mitochondrial anti-viral signaling (MAVS) protein is an intermediary adaptor protein of retinoic acid-inducible gene-1 (RIG-I) like receptor (RLR) signaling, which activates the transcription factor interferon (IFN) regulatory factor 3 (IRF3) and NF-kB to produce type I IFNs. MAVS expression has been reported in different fish species, but few studies have shown its functional role in anti-viral responses to fish viruses. In this study, we used the transcription activator-like effector nuclease (TALEN) as a gene editing tool to disrupt the function of MAVS in Chinook salmon (Oncorhynchus tshawytscha) embryonic cells (CHSE) to understand its role in induction of interferon I responses to infections with the (+) RNA virus salmonid alphavirus subtype 3 (SAV-3), and the dsRNA virus infectious pancreatic necrosis virus (IPNV) infection. A MAVS-disrupted CHSE clone with a 7-aa polypeptide (GVFVSRV) deletion mutation at the N-terminal of the CARD domain infected with SAV-3 resulted in significantly lower expression of IRF3, IFNa, and ISGs and increased viral titer (1.5 log10) compared to wild-type. In contrast, the IPNV titer in MAVS-disrupted cells was not different from the wild-type. Furthermore, overexpression of salmon MAVS in MAVS-disrupted CHSE cells rescued the impaired type I IFN-mediated anti-viral effect against SAV-3.

ADAM9 promotes type I interferon-mediated innate immunity during encephalomyocarditis virus infection

Viral myocarditis, an inflammatory disease of the heart, causes significant morbidity and mortality. Type I interferon (IFN)-mediated antiviral responses protect against myocarditis, but the mechanisms are poorly understood. We previously identified A Disintegrin And Metalloproteinase domain 9 (ADAM9) as an important factor in viral pathogenesis. ADAM9 is implicated in a range of human diseases, including inflammatory diseases; however, its role in viral infection is unknown. Here, we demonstrate that mice lacking ADAM9 are more susceptible to encephalomyocarditis virus (EMCV)-induced death and fail to mount a characteristic type I IFN response. This defect in type I IFN induction is specific to positive-sense, single-stranded RNA (+ ssRNA) viruses and involves melanoma differentiation-associated protein 5 (MDA5)-a key receptor for +ssRNA viruses. Mechanistically, ADAM9 binds to MDA5 and promotes its oligomerization and thereby downstream mitochondrial antiviral-signaling protein (MAVS) activation in response to EMCV RNA stimulation. Our findings identify a role for ADAM9 in the innate antiviral response, specifically MDA5-mediated IFN production, which protects against virus-induced cardiac damage, and provide a potential therapeutic target for treatment of viral myocarditis.

Aberrant RNA sensing in regulatory T cells causes systemic autoimmunity

Chronic and aberrant nucleic acid sensing causes type I IFN-driven autoimmune diseases, designated type I interferonopathies. We found a significant reduction of regulatory T cells (Tregs) in patients with type I interferonopathies caused by mutations in ADAR1 or IFIH1 (encoding MDA5). We analyzed the underlying mechanisms using murine models and found that Treg-specific deletion of Adar1 caused peripheral Treg loss and scurfy-like lethal autoimmune disorders. Similarly, knock-in mice with Treg-specific expression of an MDA5 gain-of-function mutant caused apoptosis of peripheral Tregs and severe autoimmunity. Moreover, the impact of ADAR1 deficiency on Tregs is multifaceted, involving both MDA5 and PKR sensing. Together, our results highlight the dysregulation of Treg homeostasis by intrinsic aberrant RNA sensing as a potential determinant for type I interferonopathies.

Lipid Nanoparticle Encapsulation Empowers Poly(I:C) to Activate Cytoplasmic RLRs and Thereby Increases Its Adjuvanticity

Poly(I:C) is a synthetic analogue of dsRNA capable of activating both TLR3 and RLRs, such as MDA-5 and RIG-I, as pathogen recognition receptors. While poly(I:C) is known to provoke a robust type I IFN, type III IFN, and Th1 cytokine response, its therapeutic use as a vaccine adjuvant is limited due to its vulnerability to nucleases and poor uptake by immune cells. is encapsulated poly(I:C) into lipid nanoparticles (LNPs) containing an ionizable cationic lipid that can electrostatically interact with poly(I:C). LNP-formulated poly(I:C) triggered both lysosomal TLR3 and cytoplasmic RLRs, in vitro and in vivo, whereas poly(I:C) in an unformulated soluble form only triggered endosomal-localized TLR3. Administration of LNP-formulated poly(I:C) in mouse models led to efficient translocation to lymphoid tissue and concurrent innate immune activation following intramuscular (IM) administration, resulting in a significant increase in innate immune activation compared to unformulated soluble poly(I:C). When used as an adjuvant for recombinant full-length SARS-CoV-2 spike protein, LNP-formulated poly(I:C) elicited potent anti-spike antibody titers, surpassing those of unformulated soluble poly(I:C) by orders of magnitude and offered complete protection against a SARS-CoV-2 viral challenge in vivo, and serum from these mice are capable of significantly reducing viral infection in vitro.

Chicken IFI6 inhibits avian reovirus replication and affects related innate immune signaling pathways

Interferon-alpha inducible protein 6 (IFI6) is an important interferon-stimulated gene. To date, research on IFI6 has mainly focused on human malignant tumors, virus-related diseases and autoimmune diseases. Previous studies have shown that IFI6 plays an important role in antiviral, antiapoptotic and tumor-promoting cellular functions, but few studies have focused on the structure or function of avian IFI6. Avian reovirus (ARV) is an important virus that can exert immunosuppressive effects on poultry. Preliminary studies have shown that IFI6 expression is upregulated in various tissues and organs of specific-pathogen-free chickens infected with ARV, suggesting that IFI6 plays an important role in ARV infection. To analyze the function of avian IFI6, particularly in ARV infection, the chicken IFI6 gene was cloned, a bioinformatics analysis was conducted, and the roles of IFI6 in ARV replication and the innate immune response were investigated after the overexpression or knockdown of IFI6 in vitro. The results indicated that the molecular weight of the chicken IFI6 protein was approximately 11 kDa and that its structure was similar to that of the human IFI27L1 protein. A phylogenetic tree analysis of the IFI6 amino acid sequence revealed that the evolution of mammals and birds was clearly divided into two branches. The evolutionary history and homology of chickens are similar to those of other birds. Avian IFI6 localized to the cytoplasm and was abundantly expressed in the chicken lung, intestine, pancreas, liver, spleen, glandular stomach, thymus, bursa of Fabricius and trachea. Further studies demonstrated that IFI6 overexpression in DF-1 cells inhibited ARV replication and that the inhibition of IFI6 expression promoted ARV replication. After ARV infection, IFI6 modulated the expression of various innate immunity-related factors. Notably, the expression patterns of MAVS and IFI6 were similar, and the expression patterns of IRF1 and IFN-β were opposite to those of IFI6. The results of this study further advance the research on avian IFI6 and provide a theoretical basis for further research on the role of IFI6 in avian virus infection and innate immunity.

Pevonedistat, a first-in-class NEDD8-activating enzyme inhibitor, sensitizes cancer cells to VSVΔ51 oncolytic virotherapy

The clinical efficacy of VSVΔ51 oncolytic virotherapy has been limited by tumor resistance to viral infection, so strategies to transiently repress antiviral defenses are warranted. Pevonedistat is a first-in-class NEDD8-activating enzyme (NAE) inhibitor currently being tested in clinical trials for its antitumor potential. In this study, we demonstrate that pevonedistat sensitizes human and murine cancer cells to increase oncolytic VSVΔ51 infection, increase tumor cell death, and improve therapeutic outcomes in resistant syngeneic murine cancer models. Increased VSVΔ51 infectivity was also observed in clinical human tumor samples. We further identify the mechanism of this effect to operate via blockade of the type 1 interferon (IFN-1) response through neddylation-dependent interferon-stimulated growth factor 3 (ISGF3) repression and neddylation-independent inhibition of NF-κB nuclear translocation. Together, our results identify a role for neddylation in regulating the innate immune response and demonstrate that pevonedistat can improve the therapeutic outcomes of strategies using oncolytic virotherapy.

Immune responses to Tilapia lake virus infection: what we know and what we don't know

Tilapia lake virus (TiLV) is a novel contagious pathogen associated with a lethal disease affecting and decimating tilapia populations on several continents across the globe. Fish viral diseases, such as Tilapia lake virus disease (TiLVD), represent a serious threat to tilapia aquaculture. Therefore, a better understanding of the innate immune responses involved in establishing an antiviral state can help shed light on TiLV disease pathogenesis. Moreover, understanding the adaptive immune mechanisms involved in mounting protection against TiLV could greatly assist in the development of vaccination strategies aimed at controlling TiLVD. This review summarizes the current state of knowledge on the immune responses following TiLV infection. After describing the main pathological findings associated with TiLVD, both the innate and adaptive immune responses and mechanisms to TiLV infection are discussed, in both disease infection models and in vitro studies. In addition, our work, highlights research questions, knowledge gaps and research areas in the immunology of TiLV infection where further studies are needed to better understand how disease protection against TiLV is established.

MAVS Antagonizes Human Stem Cell Senescence as a Mitochondrial Stabilizer

Mitochondrial dysfunction is a hallmark feature of cellular senescence and organ aging. Here, we asked whether the mitochondrial antiviral signaling protein (MAVS), which is essential for driving antiviral response, also regulates human stem cell senescence. To answer this question, we used CRISPR/Cas9-mediated gene editing and directed differentiation techniques to generate various MAVS-knockout human stem cell models. We found that human mesenchymal stem cells (hMSCs) were sensitive to MAVS deficiency, as manifested by accelerated senescence phenotypes. We uncovered that the role of MAVS in maintaining mitochondrial structural integrity and functional homeostasis depends on its interaction with the guanosine triphosphatase optic atrophy type 1 (OPA1). Depletion of MAVS or OPA1 led to the dysfunction of mitochondria and cellular senescence, whereas replenishment of MAVS or OPA1 in MAVS-knockout hMSCs alleviated mitochondrial defects and premature senescence phenotypes. Taken together, our data underscore an uncanonical role of MAVS in safeguarding mitochondrial homeostasis and antagonizing human stem cell senescence.

Regulation of ribonucleoprotein condensates by RNase L during viral infection

In response to viral infection, mammalian cells activate several innate immune pathways to antagonize viral gene expression. Upon recognition of viral double-stranded RNA, protein kinase R (PKR) phosphorylates the alpha subunit of eukaryotic initiation factor 2 (eIF2α) on serine 51. This inhibits canonical translation initiation, which broadly antagonizes viral protein synthesis. It also promotes the assembly of cytoplasmic ribonucleoprotein complexes termed stress granules (SGs). SGs are widely thought to promote cell survival and antiviral signaling. However, co-activation of the OAS/RNase L antiviral pathway inhibits the assembly of SGs and promotes the assembly of an alternative ribonucleoprotein complex termed an RNase L-dependent body (RLB). The formation of RLBs has been observed in response to double-stranded RNA, dengue virus infection, or SARS-CoV-2 infection. Herein, we review the distinct biogenesis pathways and properties of SGs and RLBs, and we provide perspective on their potential functions during the antiviral response. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Turnover and Surveillance > Regulation of RNA Stability RNA Export and Localization > RNA Localization.

Enterovirus D68 VP3 Targets the Interferon Regulatory Factor 7 To Inhibit Type I Interferon Response

Enterovirus D68 (EV-D68) is a globally emerging pathogen causing severe respiratory illnesses mainly in children. The protease from EV-D68 could impair type I interferon (IFN-I) production. However, the role of the EV-D68 structural protein in antagonizing host antiviral responses remains largely unknown. We showed that the EV-D68 structural protein VP3 interacted with IFN regulatory factor 7 (IRF7), and this interaction suppressed the phosphorylation and nuclear translocation of IRF7 and then repressed the transcription of IFN. Furthermore, VP3 inhibited the TNF receptor associated factor 6 (TRAF6)-induced ubiquitination of IRF7 by competitive interaction with IRF7. IRF7Δ305-503 showed much weaker interaction ability to VP3, and VP3Δ41-50 performed weaker interaction ability with IRF7. The VP3 from enterovirus A71 (EV-A71) and coxsackievirus A16 (CV-A16) was also found to interact with the IRF7 protein. These results indicate that the enterovirus structural protein VP3 plays a pivotal role in subverting host innate immune responses and may be a potential target for antiviral drug research. IMPORTANCE EV-D68 is a globally emerging pathogen that causes severe respiratory illnesses. Here, we report that EV-D68 inhibits innate immune responses by targeting IRF7. Further investigations revealed that the structural protein VP3 inhibited the TRAF6-induced ubiquitination of IRF7 by competitive interaction with IRF7. These results indicate that the control of IRF7 by VP3 may be a mechanism by which EV-D68 represses IFN-I production.

Impaired Mitochondrial Function in T-Lymphocytes as a Result of Exposure to HIV and ART

Mitochondrial dysfunction is a described phenomenon for a number of chronic and infectious diseases. At the same time, the question remains open: is this condition a consequence or a cause of the progression of the disease? In this review, we consider the role of the development of mitochondrial dysfunction in the progression of HIV (human immunodeficiency viruses) infection and the onset of AIDS (acquired immunodeficiency syndrome), as well as the direct impact of HIV on mitochondria. In addition, we will touch upon such an important issue as the effect of ART (Antiretroviral Therapy) drugs on mitochondria, since ART is currently the only effective way to curb the progression of HIV in infected patients, and because the identification of potential side effects can help to more consciously approach the development of new drugs in the treatment of HIV infection.

Retinoic acid-inducible gene 1 (RIG-1) and IFN-β promoter stimulator-1 (IPS-1) significantly down-regulated in the severe coronavirus disease 2019 (COVID-19)

INTRODUCTION

Retinoic acid-inducible gene 1 (RIG-1) and melanoma differentiation-associated protein 5 (MDA5) are the well-known cytoplasmic sensors that recognize microbial DNA or RNA and active down-stream molecules, including IFN-β promoter stimulator-1 (IPS-1) and receptor interacting protein 1 (RIP1). The roles played by the networked molecules on the infection with SARS-CoV-2 needs more investigations.

MATERIAL AND METHOD

In this project MDA5, RIG-1, IPS-1 and RIP1 mRNA levels were evaluated in 45 hospitalized patients suffering from coronavirus disease of 2019 (COVID-19) and 45 healthy subjects using Real Time-qPCR technique.

RESULT

The results showed significant decreased RIG-1 and IPS-1 in the SARS-CoV-2 infected patients when compared to healthy cases. MDA5 and RIP1 did not change when compared two groups. Male patients had similar expression of MDA5, RIG-1, IPS-1 and RIP1 when compared to female patients.

CONCLUSION

Based on the results, it seems that RIG-1 and its signaling molecule, IPS-1, play key roles in the peripheral blood immune cells against SARS-CoV-2 and, their down-regulation may be induced by the virus to escape from immune responses.

Non-coding RNAs targeting NF-κB pathways in aquatic animals: A review

Nuclear factor-kappa B (NF-κB) pathways have a close relationship with many diseases, especially in terms of the regulation of inflammation and the immune response. Non-coding RNAs (ncRNAs) are a heterogeneous subset of endogenous RNAs that directly affect cellular function in the absence of proteins or peptide products; these include microRNAs (miRNAs), long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), etc. Studies on the roles of ncRNAs in targeting the NF-κB pathways in aquatic animals are scarce. A few research studies have confirmed detailed regulatory mechanisms among ncRNAs and the NF-κB pathways in aquatic animals. This comprehensive review is presented concerning ncRNAs targeting the NF-κB pathway in aquatic animals and provides new insights into NF-κB pathways regulatory mechanisms of aquatic animals. The review discusses new possibilities for developing non-coding-RNA-based antiviral applications in fisheries.

RNA-binding proteins and translation control in angiogenesis

Tissue vascularization through the process of angiogenesis ensures adequate oxygen and nutrient supply during development and regeneration. The complex morphogenetic events involved in new blood vessel formation are orchestrated by a tightly regulated crosstalk between extra and intracellular factors. In this context, RNA-binding protein (RBP) activity and protein translation play fundamental roles during the cellular responses triggered by particular environmental cues. A solid body of work has demonstrated that key RBPs (such as HuR, TIS11 proteins, hnRNPs, NF90, QKIs and YB1) are implicated in both physiological and pathological angiogenesis. These RBPs are critical for the metabolism of messenger (m)RNAs encoding angiogenic modulators and, importantly, strong evidence suggests that RBP-mRNA interactions can be altered in disease. Lesser known, but not less important, the mechanistic aspects of protein synthesis can also regulate the generation of new vessels. In this review, we outline the key findings demonstrating the implications of RBP-mediated RNA regulation and translation control in angiogenesis. Furthermore, we highlight how these mechanisms of post-transcriptional control of gene expression have led to promising therapeutic strategies aimed at targeting undesired blood vessel formation.

Molecular mechanism of RIPK1 and caspase-8 in homeostatic type I interferon production and regulation

Type I interferons (IFNs) are essential innate immune proteins that maintain tissue homeostasis through tonic expression and can be upregulated to drive antiviral resistance and inflammation upon stimulation. However, the mechanisms that inhibit aberrant IFN upregulation in homeostasis and the impacts of tonic IFN production on health and disease remain enigmatic. Here, we report that caspase-8 negatively regulates type I IFN production by inhibiting the RIPK1-TBK1 axis during homeostasis across multiple cell types and tissues. When caspase-8 is deleted or inhibited, RIPK1 interacts with TBK1 to drive elevated IFN production, leading to heightened resistance to norovirus infection in macrophages but also early onset lymphadenopathy in mice. Combined deletion of caspase-8 and RIPK1 reduces the type I IFN signaling and lymphadenopathy, highlighting the critical role of RIPK1 in this process. Overall, our study identifies a mechanism to constrain tonic type I IFN during homeostasis which could be targeted for infectious and inflammatory diseases.

Wang et al. report the mechanistic regulation of homeostatic type I IFN production by caspase-8 through the RIPK1-TBK1 axis. Hyper-activation of this pathway due to loss of caspase-8 has profound physiological impacts on natural resistance to viral infection and the pathogenesis of lymphadenopathy.

The protein arginine methyltransferase PRMT9 attenuates MAVS activation through arginine methylation

The signaling adaptor MAVS forms prion-like aggregates to activate the innate antiviral immune response after viral infection. However, spontaneous aggregation of MAVS can lead to autoimmune diseases. The molecular mechanism that prevents MAVS from spontaneous aggregation in resting cells has been enigmatic. Here we report that protein arginine methyltransferase 9 targets MAVS directly and catalyzes the arginine methylation of MAVS at the Arg41 and Arg43. In the resting state, this modification inhibits MAVS aggregation and autoactivation of MAVS. Upon virus infection, PRMT9 dissociates from the mitochondria, leading to the aggregation and activation of MAVS. Our study implicates a form of post-translational modification on MAVS, which can keep MAVS inactive in physiological conditions to maintain innate immune homeostasis.

Bta-miR-677 contribute to interferon pathway affecting the proliferation of caprine parainfluenza virus type 3

Caprine parainfluenza virus type 3 (CPIV3), a new strain of virus, was isolated from the goats in 2014 in China. Studies have shown that viral infection can induce changes in the expression profile of host miRNAs, which modulate natural immune responses and viral infection. In this study, we report that bta-miR-677 suppressed CPIV3 replication in Madin-Darby bovine kidney (MDBK) cells and guinea pigs. Bta-miR-677 overexpression promoted type I interferon (IFN-I) and IFN-stimulated genes (ISGs) production, thereby inhibiting CPIV3 replication, while bta-miR-677 inhibitor suppressed the antiviral innate immune response to promoted viral replication in MDBK cells. We showed that bta-miR-677 suppresses CPIV3 replication via directly targeted the 3'-untranslated region (3'-UTR) of mitochondrial antiviral signaling protein (MAVS) thus enhancing IFN pathway in MDBK cells. We also demonstrated that bta-miR-677 agomir could inhibit CPIV3 proliferation in guinea pigs, with much lower viral RNA levels in lung and trachea. Guinea pigs showed no obvious pathological changes and less severe lung lesions in bta-miR-677 agomir treated group at 7 dpi. This study contributes to our understanding of the molecular mechanisms underlying CPIV3 pathogenesis.

Long non-coding RNA LTCONS7822 positively regulates innate immunity by targeting MITA in miiuy croaker (Miichthys miiuy)

Accumulated studies have shown that long non-coding RNA (lncRNA) is considered a critical regulatory factor in mammals, with a length greater than 200 nucleotides, and it can participate in gene imprinting, dose compensation, transcription enhancement, and antisense regulation. Most of the above studies are carried out in mammals, and there are very few studies on lncRNA of lower vertebrates. Here, we report a novel lncRNA, LTCONS7822, which can play a positive regulatory effect on antiviral immunity in miiuy croaker, Miichthys miiuy. Our results show that the levels of lncRNA LTCONS7822 were significantly increased after poly (I:C) stimulation. Further study, we found that lncRNA LTCONS7822 could positively regulate MITA at the post-transcriptional level. In addition, the dual-luciferase reporter assay analysis showed that the positive regulatory effect of lncRNA LTCONS7822 on NF-κB and IRF3 signaling pathways presented the dose and time-dependent manner. Western blotting experiments proved that lncRNA LTCONS7822 has a positive regulatory effect on MITA. Collectively, our study provided new information to enrich the immune regulation network of lncRNA in teleost fish.

Immune Regulator Retinoic Acid-Inducible Gene I (RIG-I) in the Pathogenesis of Cardiovascular Disease

Retinoic acid-inducible gene I (RIG-I) is a cytosolic pattern recognition receptor that contains two CARD domains, an RNA helicase domain, and a C-terminal domain. RIG-I initiates antiviral innate immunity by recognizing exogenous viral RNAs/DNAs. However, some studies have reported that RIG-I activation leads to damage in various organs and tissues in diverse circumstances. Recent studies have shown that RIG-I is involved in cancer, lupus nephritis, immunoglobulin A nephropathy, Crohn's disease, and atherosclerosis. These reports indicate that RIG-I not only participates in antiviral signaling pathways but also exerts an influence on non-viral infectious diseases. RIG-I is widely expressed in immune and non-immune cells including smooth muscle cells, endothelial cells, and cardiomyocytes. A succinct overview of RIG-I and its signaling pathways, with respect to the cardiovascular system, will aid in the development of novel therapeutics for cardiovascular diseases. In this review, we summarize the structure, activation, signaling pathways, and role of RIG-I in cardiovascular diseases.

The gut microbiota prime systemic antiviral immunity via the cGAS-STING-IFN-I axis

The microbiota are vital for immune homeostasis and provide a competitive barrier to bacterial and fungal pathogens. Here, we investigated how gut commensals modulate systemic immunity and response to viral infection. Antibiotic suppression of the gut microbiota reduced systemic tonic type I interferon (IFN-I) and antiviral priming. The microbiota-driven tonic IFN-I-response was dependent on cGAS-STING but not on TLR signaling or direct host-bacteria interactions. Instead, membrane vesicles (MVs) from extracellular bacteria activated the cGAS-STING-IFN-I axis by delivering bacterial DNA into distal host cells. DNA-containing MVs from the gut microbiota were found in circulation and promoted the clearance of both DNA (herpes simplex virus type 1) and RNA (vesicular stomatitis virus) viruses in a cGAS-dependent manner. In summary, this study establishes an important role for the microbiota in peripheral cGAS-STING activation, which promotes host resistance to systemic viral infections. Moreover, it uncovers an underappreciated risk of antibiotic use during viral infections.

Genome-wide association study of disease resilience traits from a natural polymicrobial disease challenge model in pigs identifies the importance of the major histocompatibility complex region

Infectious diseases cause tremendous financial losses in the pork industry, emphasizing the importance of disease resilience, which is the ability of an animal to maintain performance under disease. Previously, a natural polymicrobial disease challenge model was established, in which pigs were challenged in the late nursery phase by multiple pathogens to maximize expression of genetic differences in disease resilience. Genetic analysis found that performance traits in this model, including growth rate, feed and water intake, and carcass traits, as well as clinical disease phenotypes, were heritable and could be selected for to increase disease resilience of pigs. The objectives of the current study were to identify genomic regions that are associated with disease resilience in this model, using genome-wide association studies and fine-mapping methods, and to use gene set enrichment analyses to determine whether genomic regions associated with disease resilience are enriched for previously published quantitative trait loci, functional pathways, and differentially expressed genes subject to physiological states. Multiple quantitative trait loci were detected for all recorded performance and clinical disease traits. The major histocompatibility complex region was found to explain substantial genetic variance for multiple traits, including for growth rate in the late nursery (12.8%) and finisher (2.7%), for several clinical disease traits (up to 2.7%), and for several feeding and drinking traits (up to 4%). Further fine mapping identified 4 quantitative trait loci in the major histocompatibility complex region for growth rate in the late nursery that spanned the subregions for class I, II, and III, with 1 single-nucleotide polymorphism in the major histocompatibility complex class I subregion capturing the largest effects, explaining 0.8–27.1% of genetic variance for growth rate and for multiple clinical disease traits. This single-nucleotide polymorphism was located in the enhancer of TRIM39 gene, which is involved in innate immune response. The major histocompatibility complex region was pleiotropic for growth rate in the late nursery and finisher, and for treatment and mortality rates. Growth rate in the late nursery showed strong negative genetic correlations in the major histocompatibility complex region with treatment or mortality rates (−0.62 to −0.85) and a strong positive genetic correlation with growth rate in the finisher (0.79). Gene set enrichment analyses found genomic regions associated with resilience phenotypes to be enriched for previously identified disease susceptibility and immune capacity quantitative trait loci, for genes that were differentially expressed following bacterial or virus infection and immune response, and for gene ontology terms related to immune and inflammatory response. In conclusion, the major histocompatibility complex and other quantitative trait loci that harbor immune-related genes were identified to be associated with disease resilience traits in a large-scale natural polymicrobial disease challenge. The major histocompatibility complex region was pleiotropic for growth rate under challenge and for clinical disease traits. Four quantitative trait loci were identified across the class I, II, and III subregions of the major histocompatibility complex for nursery growth rate under challenge, with 1 single-nucleotide polymorphism in the major histocompatibility complex class I subregion capturing the largest effects. The major histocompatibility complex and other quantitative trait loci identified play an important role in host response to infectious diseases and can be incorporated in selection to improve disease resilience, in particular the identified single-nucleotide polymorphism in the major histocompatibility complex class I subregion.

Pathogenic insights from genetic causes of autoinflammatory inflammasomopathies and interferonopathies

A number of systemic autoinflammatory diseases arise from gain-of-function mutations in genes encoding IL-1-activating inflammasomes or cytoplasmic nucleic acid sensors including the receptor and sensor STING and result in increased IL-1 and type I interferon production, respectively. Blocking these pathways in human diseases has provided proof-of-concept, confirming the prominent roles of these cytokines in disease pathogenesis. Recent insights into the multilayered regulation of these sensor pathways and insights into their role in amplifying the disease pathogenesis of monogenic and complex genetic diseases spurred new drug development targeting the sensors. This review provides insights into the pathogenesis and genetic causes of these "prototypic" diseases caused by gain-of function mutations in IL-1-activating inflammasomes (inflammasomopathies) and in interferon-activating pathways (interferonopathies) including STING-associated vasculopathy with onset in infancy, Aicardi-Goutieres syndrome, and proteasome-associated autoinflammatory syndromes that link activation of the viral sensors STING, "self" nucleic acid metabolism, and the ubiquitin-proteasome system to "type I interferon production" and human diseases. Clinical responses and biomarker changes to Janus kinase inhibitors confirm a role of interferons, and a growing number of diseases with "interferon signatures" unveil extensive cross-talk between major inflammatory pathways. Understanding these interactions promises new tools in tackling the significant clinical challenges in treating patients with these conditions.

Type I interferons and MAVS signaling are necessary for tissue resident memory CD8+ T cell responses to RSV infection

Respiratory syncytial virus (RSV) can cause bronchiolitis and viral pneumonia in young children and the elderly. Lack of vaccines and recurrence of RSV infection indicate the difficulty in eliciting protective memory immune responses. Tissue resident memory T cells (TRM) can confer protection from pathogen re-infection and, in human experimental RSV infection, the presence of lung CD8+ TRM cells correlates with a better outcome. However, the requirements for generating and maintaining lung TRM cells during RSV infection are not fully understood. Here, we use mouse models to assess the impact of innate immune response determinants in the generation and subsequent expansion of the TRM cell pool during RSV infection. We show that CD8+ TRM cells expand independently from systemic CD8+ T cells after RSV re-infection. Re-infected MAVS and MyD88/TRIF deficient mice, lacking key components involved in innate immune recognition of RSV and induction of type I interferons (IFN-α/β), display impaired expansion of CD8+ TRM cells and reduction in antigen specific production of granzyme B and IFN-γ. IFN-α treatment of MAVS deficient mice during primary RSV infection restored TRM cell expansion upon re-challenge but failed to recover TRM cell functionality. Our data reveal how innate immunity, including the axis controlling type I IFN induction, instructs and regulates CD8+ TRM cell responses to RSV infection, suggesting possible mechanisms for therapeutic intervention.

Shaping the Innate Immune Response Through Post-Transcriptional Regulation of Gene Expression Mediated by RNA-Binding Proteins

Innate immunity is the frontline of defense against infections and tissue damage. It is a fast and semi-specific response involving a myriad of processes essential for protecting the organism. These reactions promote the clearance of danger by activating, among others, an inflammatory response, the complement cascade and by recruiting the adaptive immunity. Any disequilibrium in this functional balance can lead to either inflammation-mediated tissue damage or defense inefficiency. A dynamic and coordinated gene expression program lies at the heart of the innate immune response. This expression program varies depending on the cell-type and the specific danger signal encountered by the cell and involves multiple layers of regulation. While these are achieved mainly via transcriptional control of gene expression, numerous post-transcriptional regulatory pathways involving RNA-binding proteins (RBPs) and other effectors play a critical role in its fine-tuning. Alternative splicing, translational control and mRNA stability have been shown to be tightly regulated during the innate immune response and participate in modulating gene expression in a global or gene specific manner. More recently, microRNAs assisting RBPs and post-transcriptional modification of RNA bases are also emerging as essential players of the innate immune process. In this review, we highlight the numerous roles played by specific RNA-binding effectors in mediating post-transcriptional control of gene expression to shape innate immunity.

The Triangle Relationship Between Long Noncoding RNA, RIG-I-like Receptor Signaling Pathway, and Glycolysis

Long noncoding RNA (LncRNA), a noncoding RNA over 200nt in length, can regulate glycolysis through metabolic pathways, glucose metabolizing enzymes, and epigenetic reprogramming. Upon viral infection, increased aerobic glycolysis providzes material and energy for viral replication. Mitochondrial antiviral signaling protein (MAVS) is the only protein-specified downstream of retinoic acid-inducible gene I (RIG-I) that bridges the gap between antiviral immunity and glycolysis. MAVS binding to RIG-I inhibits MAVS binding to Hexokinase (HK2), thereby impairing glycolysis, while excess lactate production inhibits MAVS and the downstream antiviral immune response, facilitating viral replication. LncRNAs can also regulate antiviral innate immunity by interacting with RIG-I and downstream signaling pathways and by regulating the expression of interferons and interferon-stimulated genes (ISGs). Altogether, we summarize the relationship between glycolysis, antiviral immunity, and lncRNAs and propose that lncRNAs interact with glycolysis and antiviral pathways, providing a new perspective for the future treatment against virus infection, including SARS-CoV-2.

ADAR1 restricts ZBP1-mediated immune response and PANoptosis to promote tumorigenesis

Cell death provides host defense and maintains homeostasis. Zα-containing molecules are essential for these processes. Z-DNA binding protein 1 (ZBP1) activates inflammatory cell death, PANoptosis, whereas adenosine deaminase acting on RNA 1 (ADAR1) serves as an RNA editor to maintain homeostasis. Here, we identify and characterize ADAR1's interaction with ZBP1, defining its role in cell death regulation and tumorigenesis. Combining interferons (IFNs) and nuclear export inhibitors (NEIs) activates ZBP1-dependent PANoptosis. ADAR1 suppresses this PANoptosis by interacting with the Zα2 domain of ZBP1 to limit ZBP1 and RIPK3 interactions. Adar1fl/flLysMcre mice are resistant to development of colorectal cancer and melanoma, but deletion of the ZBP1 Zα2 domain restores tumorigenesis in these mice. In addition, treating wild-type mice with IFN-γ and the NEI KPT-330 regresses melanoma in a ZBP1-dependent manner. Our findings suggest that ADAR1 suppresses ZBP1-mediated PANoptosis, promoting tumorigenesis. Defining the functions of ADAR1 and ZBP1 in cell death is fundamental to informing therapeutic strategies for cancer and other diseases.

Avian Metapneumovirus Subgroup C Induces Mitochondrial Antiviral Signaling Protein Degradation through the Ubiquitin-Proteasome Pathway

The mitochondrial antiviral signaling (MAVS) protein, a critical adapter, links the upstream recognition of viral RNA to downstream antiviral signal transduction. However, the interaction mechanism between avian metapneumovirus subgroup C (aMPV/C) infection and MAVS remains unclear. Here, we confirmed that aMPV/C infection induced a reduction in MAVS expression in Vero cells in a dose-dependent manner, and active aMPV/C replication was required for MAVS decrease. We also found that the reduction in MAVS occurred at the post-translational level rather than at the transcriptional level. Different inhibitors were used to examine the effect of proteasome or autophagy on the regulation of MAVS. Treatment with a proteasome inhibitor MG132 effectively blocked MAVS degradation. Moreover, we demonstrated that MAVS mainly underwent K48-linked ubiquitination in the presence of MG132 in aMPV/C-infected cells, with amino acids 363, 462, and 501 of MAVS being pivotal sites in the formation of polyubiquitin chains. Finally, E3 ubiquitin ligases for MAVS degradation were screened and identified and RNF5 targeting MAVS at Lysine 363 and 462 was shown to involve in MAVS degradation in aMPV/C-infected Vero cells. Overall, these results reveal the molecular mechanism underlying aMPV/C infection-induced MAVS degradation by the ubiquitin-proteasome pathway.

AIM2 forms a complex with pyrin and ZBP1 to drive PANoptosis and host defence

Inflammasomes are important sentinels of innate immune defense, sensing pathogens and inducing cell death in infected cells¹. There are several inflammasome sensors that each detect and respond to specific pathogen- and damage-associated molecular patterns (PAMPs and DAMPs)¹. In contrast to this one PAMP/DAMP to one sensor specificity, during infection, live pathogens can induce the release of multiple PAMPs and DAMPs, which could contemporaneously engage multiple inflammasome sensors^2–5. Here we discovered that AIM2 regulated the innate immune sensors Pyrin and ZBP1 to drive inflammatory signaling and inflammatory cell death, PANoptosis, and provide host protection during infections with herpes simplex virus 1 (HSV1) and Francisella novicida. We also observed that AIM2, Pyrin and ZBP1 were members of a large multi-protein complex along with ASC, caspase-1, caspase-8, RIPK3, RIPK1 and FADD that drove inflammatory cell death. Collectively, our findings define a previously unknown regulatory connection and molecular interaction among AIM2, Pyrin and ZBP1 that drives assembly of an AIM2-mediated multi-protein complex that involves multiple inflammasome sensors and cell death regulators. These results represent a new paradigm in understanding the functions of these molecules in innate immunity and inflammatory cell death, suggesting new therapeutic targets for AIM2-, ZBP1- and Pyrin-mediated diseases.

Black carp TUFM collaborates with NLRX1 to inhibit MAVS-mediated antiviral signaling pathway

TUFM is a mitochondrial protein and serves as a regulator of antiviral signaling; nevertheless, the character of TUFM in teleosts remains unidentified. In this study, TUFM homologue of black carp (Mylopharyngodon piceus) has been characterized and its role in innate immunity has been explored. Black carp TUFM (bcTUFM) comprises 447 amino acids and shows the high similarity to human TUFM. bcTUFM was about 50 kDa in the Western blot assay and was determined as a cytosolic protein by immunofluorescent microscopy. Knockdown of bcTUFM by shRNA enhanced the antiviral ability of the host cells. The induction fold of interferon promoter transcription in the cells co-expressing bcTUFM and bcMAVS was much lower than that of the cells expressing bcMAVS alone. Our previous study has identified that bcNLRX1 interacted with bcMAVS and functioned as an inhibitor of bcMAVS. The interaction between bcTUFM and bcNLRX1, but not bcTUFM and bcMAVS, was detected through co-immunoprecipitation. The subsequent reporter assay and plaque assay demonstrated that the inhibition of bcMAVS-mediated interferon production and antiviral activity by bcNLRX1 was enhanced by co-expressed bcTUFM. Thus, our data suggests that bcTUFM cooperates with bcNLRX1 to inhibit bcMAVS-mediated antiviral signaling during host antiviral innate immune response against SVCV.

Type I interferon receptor-independent interferon-α induction upon infection with a variety of negative-strand RNA viruses

Type I interferons (IFNs) are a first line of defence against viral infections. Upon infection, a first small wave of early type I IFN, mainly IFN-β and particularly IFN-α₄, are induced and bind to the type I IFN receptor (IFNAR) to amplify the IFN response. It was shown for several viruses that robust type I IFN responses require this positive feedback loop via the IFNAR. Recently, we showed that infection of IFNAR knockout mice with the orthomyxovirus Thogoto virus lacking the ML open reading frame (THOV(ML-)) results in the expression of unexpected high amounts of type I IFN. To investigate if IFNAR-independent IFN responses are unique for THOV(ML-), we performed infection experiments with several negative-strand RNA viruses using different routes and dosages for infection. A variety of these viruses induced type I IFN responses IFNAR-independently when using the intraperitoneal (i.p.) route for infection. In vitro studies demonstrated that myeloid dendritic cells (mDC) are capable of producing IFNAR-independent IFN-α responses that are dependent on the expression of the adaptor protein mitochondrial antiviral-signalling protein (MAVS) whereas pDC where entirely depending on the IFNAR feedback loop in vitro. Thus, depending on dose and route of infection, the IFNAR feedback loop is not strictly necessary for robust type I IFN expression and an IFNAR-independent type I IFN production might be the rule rather than the exception for infections with numerous negative-strand RNA viruses.

Autophagy receptors as viral targets

Activation of autophagy is part of the innate immune response during viral infections. Autophagy involves the sequestration of endogenous or foreign components from the cytosol within double-membraned vesicles and the delivery of their content to the lysosomes for degradation. As part of innate immune responses, this autophagic elimination of foreign components is selective and requires specialized cargo receptors that function as links between a tagged foreign component and the autophagic machinery. Pathogens have evolved ways to evade their autophagic degradation to promote their replication, and recent research has shown autophagic receptors to be an important and perhaps previously overlooked target of viral autophagy inhibition. This is a brief summary of the recent progress in knowledge of virus-host interaction in the context of autophagy receptors.

TLR2 senses the SARS-CoV-2 envelope protein to produce inflammatory cytokines

The innate immune response is critical for recognizing and controlling infections through the release of cytokines and chemokines. However, severe pathology during some infections, including SARS-CoV-2, is driven by hyperactive cytokine release, or cytokine storm. The innate sensors that activate production of pro-inflammatory cytokines and chemokines during COVID-19 remain poorly characterized. Here we show that both TLR2 and MYD88 expression were associated with COVID-19 disease severity. Mechanistically, TLR2 and MyD88 were required for β-coronavirus–induced inflammatory responses, and TLR2-dependent signaling induced the production of pro-inflammatory cytokines during coronavirus infection independent of viral entry. TLR2 sensed the SARS-CoV-2 envelope protein as its ligand. Additionally, blocking TLR2 signaling in vivo provided protection against the pathogenesis of SARS-CoV-2 infection. Overall, our study provides a critical understanding of the molecular mechanism of β-coronavirus sensing and inflammatory cytokine production, which opens new avenues for therapeutic strategies to counteract the ongoing COVID-19 pandemic.

Transcriptional and Non-Transcriptional Activation, Posttranslational Modifications, and Antiviral Functions of Interferon Regulatory Factor 3 and Viral Antagonism by the SARS-Coronavirus

The immune system defends against invading pathogens through the rapid activation of innate immune signaling pathways. Interferon regulatory factor 3 (IRF3) is a key transcription factor activated in response to virus infection and is largely responsible for establishing an antiviral state in the infected host. Studies in Irf3^−/− mice have demonstrated the absence of IRF3 imparts a high degree of susceptibility to a wide range of viral infections. Virus infection causes the activation of IRF3 to transcribe type-I interferon (e.g., IFNβ), which is responsible for inducing the interferon-stimulated genes (ISGs), which act at specific stages to limit virus replication. In addition to its transcriptional function, IRF3 is also activated to trigger apoptosis of virus-infected cells, as a mechanism to restrict virus spread within the host, in a pathway called RIG-I-like receptor-induced IRF3 mediated pathway of apoptosis (RIPA). These dual functions of IRF3 work in concert to mediate protective immunity against virus infection. These two pathways are activated differentially by the posttranslational modifications (PTMs) of IRF3. Moreover, PTMs regulate not only IRF3 activation and function, but also protein stability. Consequently, many viruses utilize viral proteins or hijack cellular enzymes to inhibit IRF3 functions. This review will describe the PTMs that regulate IRF3′s RIPA and transcriptional activities and use coronavirus as a model virus capable of antagonizing IRF3-mediated innate immune responses. A thorough understanding of the cellular control of IRF3 and the mechanisms that viruses use to subvert this system is critical for developing novel therapies for virus-induced pathologies.

Circular RNA circPIKfyve acts as a sponge of miR-21-3p to enhance antiviral immunity through regulating MAVS in teleost fish

Circular RNAs (circRNAs) are a class of widespread and diverse covalently closed circular endogenous RNAs that exert crucial functions in regulating gene expression in mammals. However, the function and regulation mechanism of circRNAs in lower vertebrates are still unknown. Here, we discovered a novel circRNA derived from PIKfyve, named circPIKfyve, that is related to the antiviral responses in teleost fish. The results showed that circPIKfyve plays essential roles in host antiviral immunity and inhibition of SCRV replication. Moreover, we also found that the antiviral effect inhibited by miR-21-3p could be reversed with the addition of circPIKfyve. In mechanism, our data revealed that circPIKfyve is a competitive endogenous RNA (ceRNA) of MAVS by sponging miR-21-3p, leading to activation of NF-κB/IRF3 pathway, which then enhance the innate antiviral responses. In addition, we firstly found that RNA binding protein QKI is involved in the formation and regulation of circPIKfyve. Our results provided a strong basis that circRNAs to play a regulatory role in antiviral immune responses in teleost fish.Importance: Here, we identified a novel circRNA, namely, circPIKfyve, that can act as a key regulator of the innate immune response in teleost fish. circPIKfyve acts as a molecular sponge by competitive adsorbing of miR-21-3p, thereby increasing the abundance of MAVS and activating the downstream NF-κB/IRF3 pathway to enhance the antiviral response. In addition, this study was the first to find that QKI protein is involved in regulating the formation of circPIKfyve in fish. The overall results of this study suggest that circPIKfyve plays an active regulatory role in the antiviral immune response of teleost fish.

Role of Mitochondria in Viral Infections

Viral diseases account for an increasing proportion of deaths worldwide. Viruses maneuver host cell machinery in an attempt to subvert the intracellular environment favorable for their replication. The mitochondrial network is highly susceptible to physiological and environmental insults, including viral infections. Viruses affect mitochondrial functions and impact mitochondrial metabolism, and innate immune signaling. Resurgence of host-virus interactions in recent literature emphasizes the key role of mitochondria and host metabolism on viral life processes. Mitochondrial dysfunction leads to damage of mitochondria that generate toxic compounds, importantly mitochondrial DNA, inducing systemic toxicity, leading to damage of multiple organs in the body. Mitochondrial dynamics and mitophagy are essential for the maintenance of mitochondrial quality control and homeostasis. Therefore, metabolic antagonists may be essential to gain a better understanding of viral diseases and develop effective antiviral therapeutics. This review briefly discusses how viruses exploit mitochondrial dynamics for virus proliferation and induce associated diseases.

Picornaviruses: A View from 3A

Picornaviruses are comprised of a positive-sense RNA genome surrounded by a protein shell (or capsid). They are ubiquitous in vertebrates and cause a wide range of important human and animal diseases. The genome encodes a single large polyprotein that is processed to structural (capsid) and non-structural proteins. The non-structural proteins have key functions within the viral replication complex. Some, such as 3D^pol (the RNA dependent RNA polymerase) have conserved functions and participate directly in replicating the viral genome, whereas others, such as 3A, have accessory roles. The 3A proteins are highly divergent across the Picornaviridae and have specific roles both within and outside of the replication complex, which differ between the different genera. These roles include subverting host proteins to generate replication organelles and inhibition of cellular functions (such as protein secretion) to influence virus replication efficiency and the host response to infection. In addition, 3A proteins are associated with the determination of host range. However, recent observations have challenged some of the roles assigned to 3A and suggest that other viral proteins may carry them out. In this review, we revisit the roles of 3A in the picornavirus life cycle. The 3AB precursor and mature 3A have distinct functions during viral replication and, therefore, we have also included discussion of some of the roles assigned to 3AB.

m6A methylation potentiates cytosolic dsDNA recognition in a sequence-specific manner

Nucleic acid sensing through pattern recognition receptors is critical for immune recognition of microbial infections. Microbial DNA is frequently methylated at the N⁶ position of adenines (m6A), a modification that is rare in mammalian host DNA. We show here how that m6A methylation of 5′-GATC-3′ motifs augments the immunogenicity of synthetic double-stranded (ds)DNA in murine macrophages and dendritic cells. Transfection with m6A-methylated DNA increased the expression of the activation markers CD69 and CD86, and of Ifnβ, iNos and Cxcl10 mRNA. Similar to unmethylated cytosolic dsDNA, recognition of m6A DNA occurs independently of TLR and RIG-I signalling, but requires the two key mediators of cytosolic DNA sensing, STING and cGAS. Intriguingly, the response to m6A DNA is sequence-specific. m6A is immunostimulatory in some motifs, but immunosuppressive in others, a feature that is conserved between mouse and human macrophages. In conclusion, epigenetic alterations of DNA depend on the context of the sequence and are differentially perceived by innate cells, a feature that could potentially be used for the design of immune-modulating therapeutics.

Regulation of RIG-I-like receptor-mediated signaling: interaction between host and viral factors

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are RNA sensor molecules that play essential roles in innate antiviral immunity. Among the three RLRs encoded by the human genome, RIG-I and melanoma differentiation-associated gene 5, which contain N-terminal caspase recruitment domains, are activated upon the detection of viral RNAs in the cytoplasm of virus-infected cells. Activated RLRs induce downstream signaling via their interactions with mitochondrial antiviral signaling proteins and activate the production of type I and III interferons and inflammatory cytokines. Recent studies have shown that RLR-mediated signaling is regulated by interactions with endogenous RNAs and host proteins, such as those involved in stress responses and posttranslational modifications. Since RLR-mediated cytokine production is also involved in the regulation of acquired immunity, the deregulation of RLR-mediated signaling is associated with autoimmune and autoinflammatory disorders. Moreover, RLR-mediated signaling might be involved in the aberrant cytokine production observed in coronavirus disease 2019. Since the discovery of RLRs in 2004, significant progress has been made in understanding the mechanisms underlying the activation and regulation of RLR-mediated signaling pathways. Here, we review the recent advances in the understanding of regulated RNA recognition and signal activation by RLRs, focusing on the interactions between various host and viral factors.

DDIT3 Targets Innate Immunity via the DDIT3-OTUD1-MAVS Pathway To Promote Bovine Viral Diarrhea Virus Replication

DNA damage-inducible transcript 3 (DDIT3) plays important roles in endoplasmic reticulum (ER) stress-induced apoptosis and autophagy, but its role in innate immunity is not clear. Here, we report that DDIT3 inhibits the antiviral immune response during bovine viral diarrhea virus (BVDV) infection by targeting mitochondrial antiviral signaling (MAVS) in Madin-Darby bovine kidney (MDBK) cells and in mice. BVDV infection induced high DDIT3 mRNA and protein expression. DDIT3 overexpression inhibited type I interferon (IFN-I) and IFN-stimulated gene production, thereby promoting BVDV replication, while DDIT3 knockdown promoted the antiviral innate immune response to suppress viral replication. DDIT3 promoted NF-κB-dependent ovarian tumor (OTU) deubiquitinase 1 (OTUD1) expression. Furthermore, OTUD1 induced upregulation of the E3 ubiquitin ligase Smurf1 by deubiquitinating Smurf1, and Smurf1 degraded MAVS in MDBK cells in a ubiquitination-dependent manner, ultimately inhibiting IFN-I production. Moreover, knocking out DDIT3 promoted the antiviral innate immune response to reduce BVDV replication and pathological changes in mice. These findings provide direct insights into the molecular mechanisms by which DDIT3 inhibits IFN-I production by regulating MAVS degradation.IMPORTANCE Extensive studies have demonstrated roles of DDIT3 in apoptosis and autophagy during viral infection. However, the role of DDIT3 in innate immunity remains largely unknown. Here, we show that DDIT3 is positively regulated in bovine viral diarrhea virus (BVDV)-infected Madin-Darby bovine kidney (MDBK) cells and could significantly enhance BVDV replication. Importantly, DDIT3 induced OTU deubiquitinase 1 (OTUD1) expression by activating the NF-κB signaling pathway, thus increasing intracellular Smurf1 protein levels to degrade MAVS and inhibit IFN-I production during BVDV infection. Together, these results indicate that DDIT3 plays critical roles in host innate immunity repression and viral infection facilitation.

Role of Polyinosinic:Polycytidylic Acid-Induced Maternal Immune Activation and Subsequent Immune Challenge in the Behaviour and Microglial Cell Trajectory in Adult Offspring: A Study of the Neurodevelopmental Model of Schizophrenia

Multiple lines of evidence support the pathogenic role of maternal immune activation (MIA) in the occurrence of the schizophrenia-like disturbances in offspring. While in the brain the homeostatic role of neuron-microglia protein systems is well documented, the participation of the CX3CL1-CX3CR1 and CD200-CD200R dyads in the adverse impact of MIA often goes under-recognized. Therefore, in the present study, we examined the effect of MIA induced by polyinosinic:polycytidylic acid (Poly I:C) on the CX3CL1-CX3CR1 and CD200-CD200R axes, microglial trajectory (MhcII, Cd40, iNos, Il-1β, Tnf-α, Il-6, Arg1, Igf-1, Tgf-β and Il-4), and schizophrenia-like behaviour in adult male offspring of Sprague-Dawley rats. Additionally, according to the “two-hit” hypothesis of schizophrenia, we evaluated the influence of acute challenge with Poly I:C in adult prenatally MIA-exposed animals on the above parameters. In the present study, MIA evoked by Poly I:C injection in the late period of gestation led to the appearance of schizophrenia-like disturbances in adult offspring. Our results revealed the deficits manifested as a diminished number of aggressive interactions, presence of depressive-like episodes, and increase of exploratory activity, as well as a dichotomy in the sensorimotor gating in the prepulse inhibition (PPI) test expressed as two behavioural phenotypes (MIA_PPI-low and MIA_PPI-high). Furthermore, in the offspring rats subjected to a prenatal challenge (i.e., MIA) we noticed the lack of modulation of behavioural changes after the additional acute immune stimulus (Poly I:C) in adulthood. The important finding reported in this article is that MIA affects the expression and levels of the neuron-microglia proteins in the frontal cortex and hippocampus of adult offspring. We found that the changes in the CX3CL1-CX3CR1 axis could affect microglial trajectory, including decreased hippocampal mRNA level of MhcII and elevated cortical expression of Igf-1 in the MIA_PPI-high animals and/or could cause the up-regulation of an inflammatory response (Il-6, Tnf-α, iNos) after the “second hit” in both examined brain regions and, at least in part, might differentiate behavioural disturbances in adult offspring. Consequently, the future effort to identify the biological background of these interactions in the Poly I:C-induced MIA model in Sprague-Dawley rats is desirable to unequivocally clarify this issue.

Disruption of interferon-β production by the Npro of atypical porcine pestivirus

Atypical porcine pestivirus (APPV) is an emerging porcine virus that threatens global swine production. Pestiviruses can prevent interferon (IFN) production to avoid the host innate immune response, and the N^pro viral protein can play a critical role. Knowledge of the host immune response to APPV infection is limited. Here, we showed that the IFN-β production was suppressed by APPV-N^pro and the IFN regulatory factor 3 (IRF3) promoter activity stimulated by adaptor molecules of the IFN-β signaling pathway was also inhibited in the APPV-N^pro-expressed cells. The APPV-N^pro was able to interact with IRF3 and interfere the phosphorylation of IRF3, indicated that the IFN-β antagonism of APPV-N^pro mainly depended on blocking IRF3 activity. To identify the functional region of APPV-N^pro, a panel of truncated APPV-N^pro was constructed, and its influence on the IRF3 activation was investigated. The results showed that the N-terminal 31–51 amino acids of APPV-N^pro were mainly associated with inhibition of the IFN-β response. Taken together, this is the first study focusing on elucidating the function of APPV protein by revealing a novel mechanism of N^pro in disruption of host IFN-β production, which will enlighten future study in addressing APPV pathogenesis and immune evasion.

Viral RNA recognition by LGP2 and MDA5, and activation of signaling through step-by-step conformational changes

Cytoplasmic RIG-I-like receptor (RLR) proteins in mammalian cells recognize viral RNA and initiate an antiviral response that results in IFN-β induction. Melanoma differentiation-associated protein 5 (MDA5) forms fibers along viral dsRNA and propagates an antiviral response via a signaling domain, the tandem CARD. The most enigmatic RLR, laboratory of genetics and physiology (LGP2), lacks the signaling domain but functions in viral sensing through cooperation with MDA5. However, it remains unclear how LGP2 coordinates fiber formation and subsequent MDA5 activation. We utilized biochemical and biophysical approaches to observe fiber formation and the conformation of MDA5. LGP2 facilitated MDA5 fiber assembly. LGP2 was incorporated into the fibers with an average inter-molecular distance of 32 nm, suggesting the formation of hetero-oligomers with MDA5. Furthermore, limited protease digestion revealed that LGP2 induces significant conformational changes on MDA5, promoting exposure of its CARDs. Although the fibers were efficiently dissociated by ATP hydrolysis, MDA5 maintained its active conformation to participate in downstream signaling. Our study demonstrated the coordinated actions of LGP2 and MDA5, where LGP2 acts as an MDA5 nucleator and requisite partner in the conversion of MDA5 to an active conformation. We revealed a mechanistic basis for LGP2-mediated regulation of MDA5 antiviral innate immune responses.

Innate Immune Sensing of Viruses and Its Consequences for the Central Nervous System

Viral infections remain a global public health concern and cause a severe societal and economic burden. At the organismal level, the innate immune system is essential for the detection of viruses and constitutes the first line of defense. Viral components are sensed by host pattern recognition receptors (PRRs). PRRs can be further classified based on their localization into Toll-like receptors (TLRs), C-type lectin receptors (CLR), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors (NLRs) and cytosolic DNA sensors (CDS). TLR and RLR signaling results in production of type I interferons (IFNα and -β) and pro-inflammatory cytokines in a cell-specific manner, whereas NLR signaling leads to the production of interleukin-1 family proteins. On the other hand, CLRs are capable of sensing glycans present in viral pathogens, which can induce phagocytic, endocytic, antimicrobial, and pro- inflammatory responses. Peripheral immune sensing of viruses and the ensuing cytokine response can significantly affect the central nervous system (CNS). But viruses can also directly enter the CNS via a multitude of routes, such as the nasal epithelium, along nerve fibers connecting to the periphery and as cargo of infiltrating infected cells passing through the blood brain barrier, triggering innate immune sensing and cytokine responses directly in the CNS. Here, we review mechanisms of viral immune sensing and currently recognized consequences for the CNS of innate immune responses to viruses.