Inducible TAP1 Negatively Regulates the Antiviral Innate Immune Response by Targeting the TAK1 Complex

Zinc-finger CCHC-type containing protein 8 promotes RNA virus replication by suppressing the type-I interferon responses

Host cells have evolved an intricate regulatory network to fine tune the type-I interferon responses. However, the full picture of this regulatory network remains to be depicted. In this study, we found that knock out of zinc-finger CCHC-type containing protein 8 (ZCCHC8) impairs the replication of influenza A virus (IAV), Sendai virus (Sev), Japanese encephalitis virus (JEV), and vesicular stomatitis virus (VSV). Further investigation unveiled that ZCCHC8 suppresses the type-I interferon responses by targeting the interferon regulatory factor 3 (IRF3) signaling pathway. Mechanistically, ZCCHC8 associates with phosphorylated IRF3 and disrupts the interaction of IRF3 with the co-activator CREB-binding protein (CBP). Additionally, the direct binding of ZCCHC8 with the IFN-stimulated response element (ISRE) impairs the ISRE-binding of IRF3. Our study contributes to the comprehensive understanding for the negative regulatory network of the type-I interferon responses and provides valuable insights for the control of multiple viruses from a host-centric perspective.IMPORTANCEThe innate immune responses serve as the initial line of defense against invading pathogens and harmful substances. Negative regulation of the innate immune responses plays an essential role in avoiding auto-immune diseases and over-activated immune responses. Hence, the comprehensive understanding of the negative regulation network for innate immune responses could provide novel therapeutic insights for the control of viral infections and immune dysfunction. In this study, we report that ZCCHC8 negatively regulates the type-I interferon responses. We illustrate that ZCCHC8 impedes the IRF3-CBP association by interacting with phosphorylated IRF3 and competes with IRF3 for binding to ISRE. Our study demonstrates the role of ZCCHC8 in the replication of multiple RNA viruses and contributes to a deeper understanding of the negative regulation system for the type-I interferon responses.

NSUN2-mediated M5c methylation of IRF3 mRNA negatively regulates type I interferon responses during various viral infections

5-Methylcytosine (m⁵C) is a widespread post-transcriptional RNA modification and is reported to be involved in manifold cellular responses and biological processes through regulating RNA metabolism. However, its regulatory role in antiviral innate immunity has not yet been elucidated. Here, we report that NSUN2, a typical m⁵C methyltransferase, negatively regulates type I interferon responses during various viral infections, including SARS-CoV-2. NSUN2 specifically mediates m⁵C methylation of IRF3 mRNA and accelerates its degradation, resulting in low levels of IRF3 and downstream IFN-β production. Knockout or knockdown of NSUN2 enhanced type I interferon and downstream ISGs during various viral infection in vitro. And in vivo, the antiviral innate response is more dramatically enhanced in Nsun2^+/- mice than in Nsun2^+/+ mice. The highly m⁵C methylated cytosines in IRF3 mRNA were identified, and their mutation enhanced cellular IRF3 mRNA levels. Moreover, infection with Sendai virus (SeV), vesicular stomatitis virus (VSV), herpes simplex virus 1 (HSV-1), or Zika virus (ZIKV) resulted in a reduction of endogenous NSUN2 levels. Especially, SARS-CoV-2 infection (WT strain and BA.1 omicron variant) also decreased endogenous levels of NSUN2 in COVID-19 patients and K18-hACE2 KI mice, further increasing type I interferon and downstream ISGs. Together, our findings reveal that NSUN2 serves as a negative regulator of interferon response by accelerating the fast turnover of IRF3 mRNA, while endogenous NSUN2 levels decrease during SARS-CoV-2 and various viral infections to boost antiviral responses for effective elimination of viruses.

The influence of SARS-CoV-2 infection on expression of drug-metabolizing enzymes and transporters in a hACE2 murine model

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the resulting Coronavirus disease 2019 emerged in late 2019 and is responsible for significant morbidity and mortality worldwide. A hallmark of severe COVID-19 is exaggerated systemic inflammation, regarded as a "cytokine storm," which contributes to the damage of various organs, primarily the lungs. The inflammation associated with some viral illnesses is known to alter the expression of drug-metabolizing enzymes and transporters. These alterations can lead to modifications in drug exposure and the processing of various endogenous compounds. Here, we provide evidence to support changes in the mitochondrial ribonucleic acid expression of a subset of drug transporters (84 transporters) in the liver, kidneys, and lungs and metabolizing enzymes (84 enzymes) in the liver in a humanized angiotensin-converting enzyme 2 receptor mouse model. Specifically, three drug transporters (Abca3, Slc7a8, Tap1) and the pro-inflammatory cytokine IL-6 were upregulated in the lungs of SARS-CoV-2 infected mice. We also found significant downregulation of drug transporters responsible for the movement of xenobiotics in the liver and kidney. Additionally, expression of cytochrome P-450 2f2 which is known to metabolize some pulmonary toxicants, was significantly decreased in the liver of infected mice. The significance of these findings requires further exploration. Our results suggest that further research should emphasize altered drug disposition when investigating therapeutic compounds, whether re-purposed or new chemical entities, in other animal models and ultimately in individuals infected with SARS-CoV-2. Moreover, the influence and impact of these changes on the processing of endogenous compounds also require further investigation.

Essential Oils and Their Compounds as Potential Anti-Influenza Agents

Essential oils (EOs) are chemical substances, mostly produced by aromatic plants in response to stress, that have a history of medicinal use for many diseases. In the last few decades, EOs have continued to gain more attention because of their proven therapeutic applications against the flu and other infectious diseases. Influenza (flu) is an infectious zoonotic disease that affects the lungs and their associated organs. It is a public health problem with a huge health burden, causing a seasonal outbreak every year. Occasionally, it comes as a disease pandemic with unprecedentedly high hospitalization and mortality. Currently, influenza is managed by vaccination and antiviral drugs such as Amantadine, Rimantadine, Oseltamivir, Peramivir, Zanamivir, and Baloxavir. However, the adverse side effects of these drugs, the rapid and unlimited variabilities of influenza viruses, and the emerging resistance of new virus strains to the currently used vaccines and drugs have necessitated the need to obtain more effective anti-influenza agents. In this review, essential oils are discussed in terms of their chemistry, ethnomedicinal values against flu-related illnesses, biological potential as anti-influenza agents, and mechanisms of action. In addition, the structure-activity relationships of lead anti-influenza EO compounds are also examined. This is all to identify leading agents that can be optimized as drug candidates for the management of influenza. Eucalyptol, germacrone, caryophyllene derivatives, eugenol, terpin-4-ol, bisabolene derivatives, and camphecene are among the promising EO compounds identified, based on their reported anti-influenza activities and plausible molecular actions, while nanotechnology may be a new strategy to achieve the efficient delivery of these therapeutically active EOs to the active virus site.

The Influence of Temperature on the Antiviral Response of mIgM+ B Lymphocytes Against Hirame Novirhabdovirus in Flounder (Paralichthys olivaceus)

Hirame novirhabdovirus (HIRRV) is an ongoing threat to the aquaculture industry. The water temperature for the onset of HIRRV is below 15°C, the peak is about 10°C, but no mortality is observed over 20°C. Previous studies found the positive signal of matrix protein of HIRRV (HIRRV-M) was detected in the peripheral blood leukocytes of viral-infected flounder. Flow cytometry and indirect immunofluorescence assay showed that HIRRV-M was detected in mIgM⁺ B lymphocytes in viral-infected flounder maintained at 10°C and 20°C, and 22% mIgM⁺ B lymphocytes are infected at 10°C while 13% are infected at 20°C, indicating that HIRRV could invade into mIgM⁺ B lymphocytes. Absolute quantitative RT-PCR showed that the viral copies in mIgM⁺ B lymphocytes were significantly increased at 24 h post infection (hpi) both at 10°C and 20°C, but the viral copies in 10°C infection group were significantly higher than that in 20°C infection group at 72 hpi and 96 hpi. Furthermore, the B lymphocytes were sorted from HIRRV-infected flounder maintained at 10°C and 20°C for RNA-seq. The results showed that the differentially expression genes in mIgM⁺ B lymphocyte of healthy flounder at 10°C and 20°C were mainly enriched in metabolic pathways. Lipid metabolism and Amino acid metabolism were enhanced at 10°C, while Glucose metabolism was enhanced at 20°C. In contrast, HIRRV infection at 10°C induced the up-regulation of the Complement and coagulation cascades, FcγR-mediated phagocytosis, Platelets activation, Leukocyte transendothelial migration and Natural killer cell mediated cytotoxicity pathways at 72 hpi. HIRRV infection at 20°C induced the up-regulation of the Antigen processing and presentation pathway at 72 hpi. Subsequently, the temporal expression patterns of 16 genes involved in Antigen processing and presentation pathway were investigated by qRT-PCR, and results showed that the pathway was significantly activated by HIRRV infection at 20°C but inhibited at 10°C. In conclusion, HIRRV could invade into mIgM⁺ B lymphocytes and elicit differential immune response under 10°C and 20°C, which provide a deep insight into the antiviral response in mIgM⁺ B lymphocytes.

Characterising proteolysis during SARS-CoV-2 infection identifies viral cleavage sites and cellular targets with therapeutic potential

SARS-CoV-2 is the causative agent behind the COVID-19 pandemic, responsible for over 170 million infections, and over 3.7 million deaths worldwide. Efforts to test, treat and vaccinate against this pathogen all benefit from an improved understanding of the basic biology of SARS-CoV-2. Both viral and cellular proteases play a crucial role in SARS-CoV-2 replication. Here, we study proteolytic cleavage of viral and cellular proteins in two cell line models of SARS-CoV-2 replication using mass spectrometry to identify protein neo-N-termini generated through protease activity. We identify previously unknown cleavage sites in multiple viral proteins, including major antigens S and N: the main targets for vaccine and antibody testing efforts. We discover significant increases in cellular cleavage events consistent with cleavage by SARS-CoV-2 main protease, and identify 14 potential high-confidence substrates of the main and papain-like proteases. We show that siRNA depletion of these cellular proteins inhibits SARS-CoV-2 replication, and that drugs targeting two of these proteins: the tyrosine kinase SRC and Ser/Thr kinase MYLK, show a dose-dependent reduction in SARS-CoV-2 titres. Overall, our study provides a powerful resource to understand proteolysis in the context of viral infection, and to inform the development of targeted strategies to inhibit SARS-CoV-2 and treat COVID-19.

Inducible ATP1B1 Upregulates Antiviral Innate Immune Responses by the Ubiquitination of TRAF3 and TRAF6

The antiviral innate immune responses are crucial steps during host defense and must be strictly regulated, but the molecular mechanisms of control remain unclear. In this study, we report increased expression of human ATPase Na⁺/K⁺ transporting subunit β 1(ATP1B1) after DNA and RNA virus infections. We found that the expression of ATP1B1 can inhibit viral replication and increase the levels of IFNs, IFN-stimulated genes, and inflammatory cytokines. Knockdown of ATP1B1 by specific short hairpin RNA had the opposite effects. Upon viral infection, ATP1B1 was induced, interacted with TRAF3 and TRAF6, and potentiated the ubiquitination of these proteins, leading to increased phosphorylation of downstream molecules, including TGF-β-activated kinase 1 (TAK1) and TANK-binding kinase 1 (TBK1). These results reveal a previously unrecognized role of ATP1B1 in antiviral innate immunity and suggest a novel mechanism for the induction of IFNs and proinflammatory cytokines during viral infection.

Broadly Antiviral Activities of TAP1 through Activating the TBK1-IRF3-Mediated Type I Interferon Production

Deeply understanding the virus-host interaction is a prerequisite for developing effective anti-viral strategies. Traditionally, the transporter associated with antigen processing type 1 (TAP1) is critical for antigen presentation to regulate adaptive immunity. However, its role in controlling viral infections through modulating innate immune signaling is not yet fully understood. In the present study, we reported that TAP1, as a product of interferon-stimulated genes (ISGs), had broadly antiviral activity against various viruses such as herpes simplex virus 1 (HSV-1), adenoviruses (AdV), vesicular stomatitis virus (VSV), dengue virus (DENV), Zika virus (ZIKV), and influenza virus (PR8) etc. This antiviral activity by TAP1 was further confirmed by series of loss-of-function and gain-of-function experiments. Our further investigation revealed that TAP1 significantly promoted the interferon (IFN)-β production through activating the TANK binding kinase-1 (TBK1) and the interferon regulatory factor 3 (IRF3) signaling transduction. Our work highlighted the broadly anti-viral function of TAP1 by modulating innate immunity, which is independent of its well-known function of antigen presentation. This study will provide insights into developing novel vaccination and immunotherapy strategies against emerging infectious diseases.

Hepatitis B Surface Antigen Suppresses the Activation of Nuclear Factor Kappa B Pathway via Interaction With the TAK1-TAB2 Complex

Chronic hepatitis B is a major health problem worldwide, with more than 250 million chronic carriers. Hepatitis B virus interferes with the host innate immune system so as to evade elimination via almost all of its constituent proteins; nevertheless, the function of HBsAg with respect to immune escape remains unclear. This study aimed to determine the role HBsAg plays in assisting HBV to escape from immune responses. We found that HBsAg suppressed the activation of the nuclear factor kappa B (NF-кB) pathway, leading to downregulation of innate immune responses. HBsAg interacted with TAK1 and TAB2 specifically, inhibiting the phosphorylation and polyubiquitination of TAK1 and the K63-linked polyubiquitination of TAB2. Autophagy is a major catabolic process participating in many cellular processes, including the life cycle of HBV. We found that HBsAg promoted the autophagic degradation of TAK1 and TAB2 via the formation of complexes with TAK1 and TAB2, resulting in suppression of the NF-κB pathway. The expression of TAK1, TAB2, and the translocation of NF-κB inversely correlated with HBsAg levels in clinical liver tissues. Taken together, our findings suggest a novel mechanism by which HBsAg interacts with TAK1-TAB2 complex and suppresses the activation of NF-κB signaling pathway via reduction of the post-translational modifications and autophagic degradation.

Inducible Guanylate-Binding Protein 7 Facilitates Influenza A Virus Replication by Suppressing Innate Immunity via NF-κB and JAK-STAT Signaling Pathways

Guanylate-binding protein 7 (GBP7) belongs to the GBP family, which plays key roles in mediating innate immune responses to intracellular pathogens. Thus far, GBP7 has been reported to be a critical cellular factor against bacterial infection. However, the relationship between GBP7 and influenza A virus (IAV) replication is unknown. Here, we showed that GBP7 expression was significantly upregulated in the lungs of mice, human peripheral blood mononuclear cells (PBMCs), and A549 cells during IAV infection. Using the CRISPR-Cas9 system and overexpression approaches, it was found that GBP7 knockout inhibited IAV replication by enhancing the expression of IAV-induced type I interferon (IFN), type III IFN, and proinflammatory cytokines. Conversely, overexpression of GBP7 facilitated IAV replication by suppressing the expression of those factors. Furthermore, GBP7 knockout enhanced IAV-induced nuclear factor-κB (NF-κB) activation and phosphorylation of stat1 and stat2; overexpression of GBP7 had the opposite effect. Our data indicated that GBP7 suppresses innate immune responses to IAV infection via NF-κB and JAK-STAT signaling pathways. Taken together, upon IAV infection, the induced GBP7 facilitated IAV replication by suppressing innate immune responses to IAV infection, which suggested that GBP7 serves as a therapeutic target for controlling IAV infection.IMPORTANCE So far, few studies have mentioned the distinct function of guanylate-binding protein 7 (GBP7) on virus infection. Here, we reported that GBP7 expression was significantly upregulated in the lungs of mice, human PBMCs, and A549 cells during IAV infection. GBP7 facilitated IAV replication by suppressing the expression of type I interferon (IFN), type III IFN, and proinflammatory cytokines. Furthermore, it was indicated that GBP7 suppresses innate immune responses to IAV infection via NF-κB and JAK-STAT signaling pathways. Taken together, our results elucidate a critical role of GBP7 in the host immune system during IAV infection.

Role of p38 mitogen-activated protein kinase signalling in virus replication and potential for developing broad spectrum antiviral drugs

Mitogen-activated protein kinases (MAPKs) play a key role in complex cellular processes such as proliferation, development, differentiation, transformation and apoptosis. Mammals express at least four distinctly regulated groups of MAPKs which include extracellular signal-related kinases (ERK)-1/2, p38 proteins, Jun amino-terminal kinases (JNK1/2/3) and ERK5. p38 MAPK is activated by a wide range of cellular stresses and modulates activity of several downstream kinases and transcription factors which are involved in regulating cytoskeleton remodeling, cell cycle modulation, inflammation, antiviral response and apoptosis. In viral infections, activation of cell signalling pathways is part of the cellular defense mechanism with the basic aim of inducing an antiviral state. However, viruses can exploit enhanced cell signalling activities to support various stages of their replication cycles. Kinase activity can be inhibited by small molecule chemical inhibitors, so one strategy to develop antiviral drugs is to target these cellular signalling pathways. In this review, we provide an overview on the current understanding of various cellular and viral events regulated by the p38 signalling pathway, with a special emphasis on targeting these events for antiviral drug development which might identify candidates with broad spectrum activity.

Eukaryotic Translation Elongation Factor 1 Delta Inhibits the Nuclear Import of the Nucleoprotein and PA-PB1 Heterodimer of Influenza A Virus

The viral ribonucleoprotein (vRNP) of the influenza A virus (IAV) is responsible for the viral RNA transcription and replication in the nucleus, and its functions rely on host factors. Previous studies have indicated that eukaryotic translation elongation factor 1 delta (eEF1D) may associate with RNP subunits, but its roles in IAV replication are unclear. Herein, we showed that eEF1D was an inhibitor of IAV replication because knockout of eEF1D resulted in a significant increase in virus yield. eEF1D interacted with RNP subunits polymerase acidic protein (PA), polymerase basic 1 (PB1), polymerase basic 2 (PB2), and also with nucleoprotein (NP) in an RNA-dependent manner. Further studies revealed that eEF1D impeded the nuclear import of NP and PA-PB1 heterodimer of IAV, thereby suppressing the vRNP assembly, viral polymerase activity, and viral RNA synthesis. Together, our studies demonstrate eEF1D negatively regulating the IAV replication by inhibition of the nuclear import of RNP subunits, which not only uncovers a novel role of eEF1D in IAV replication but also provides new insights into the mechanisms of nuclear import of vRNP proteins.IMPORTANCE Influenza A virus is the major cause of influenza, a respiratory disease in humans and animals. Different from most other RNA viruses, the transcription and replication of IAV occur in the cell nucleus. Therefore, the vRNPs must be imported into the nucleus for viral transcription and replication, which requires participation of host proteins. However, the mechanisms of the IAV-host interactions involved in nuclear import remain poorly understood. Here, we identified eEF1D as a novel inhibitor for the influenza virus life cycle. Importantly, eEF1D impaired the interaction between NP and importin α5 and the interaction between PB1 and RanBP5, which impeded the nuclear import of vRNP. Our studies not only reveal the molecular mechanisms of the nuclear import of IAV vRNP but also provide potential anti-influenza targets for antiviral development.

Insulin Potentiates JAK/STAT Signaling to Broadly Inhibit Flavivirus Replication in Insect Vectors

The World Health Organization estimates that more than half of the world’s population is at risk for vector-borne diseases, including arboviruses. Because many arboviruses are mosquito borne, investigation of the insect immune response will help identify targets to reduce the spread of arboviruses. Here, we use a genetic screening approach to identify an insulin-like receptor as a component of the immune response to arboviral infection. We determine that vertebrate insulin reduces West Nile virus (WNV) replication in Drosophila melanogaster as well as WNV, Zika, and dengue virus titers in mosquito cells. Mechanistically, we show that insulin signaling activates the JAK/STAT, but not RNAi, pathway via ERK to control infection in Drosophila cells and Culex mosquitoes through an integrated immune response. Finally, we validate that insulin priming of adult female Culex mosquitoes through a blood meal reduces WNV infection, demonstrating an essential role for insulin signaling in insect antiviral responses to human pathogens.

The world’s population is at risk for infection with several flaviviruses. Ahlers et al. use a living library of insects to determine that an insulin-like receptor controls West Nile virus infection. Insulin signaling is antiviral via the JAK/STAT pathway in both fly and mosquito models and against a range of flaviviruses.

Innate Immune Sensing of Influenza A Virus

Influenza virus infection triggers host innate immune response by stimulating various pattern recognition receptors (PRRs). Activation of these PRRs leads to the activation of a plethora of signaling pathways, resulting in the production of interferon (IFN) and proinflammatory cytokines, followed by the expression of interferon-stimulated genes (ISGs), the recruitment of innate immune cells, or the activation of programmed cell death. All these antiviral approaches collectively restrict viral replication inside the host. However, influenza virus also engages in multiple mechanisms to subvert the innate immune responses. In this review, we discuss the role of PRRs such as Toll-like receptors (TLRs), Retinoic acid-inducible gene I (RIG-I), NOD-, LRR-, pyrin domain-containing protein 3 (NLRP3), and Z-DNA binding protein 1 (ZBP1) in sensing and restricting influenza viral infection. Further, we also discuss the mechanisms influenza virus utilizes, especially the role of viral non-structure proteins NS1, PB1-F2, and PA-X, to evade the host innate immune responses.

Influenza A virus protein PB1-F2 impairs innate immunity by inducing mitophagy

Influenza A virus (IAV) infection induces mitophagy, which is essential for the clearance of damaged mitochondria. Dysfunctional mitochondria can be selectively targeted by PINK1, which recruits PRKN/PARK2 and leads to subsequent mitochondrial sequestration within autophagosomes. The IAV PB1-F2 protein translocates to mitochondria, accelerates the mitochondrial fragmentation and impairs the innate immunity. However, whether PB1-F2 mediates IAV-induced mitophagy and the relation between mitophagy and PB1-F2-attenuated innate immunity remain obscure. Here, we showed that PB1-F2 translocated to mitochondria by interacting and colocalizing with TUFM (Tu translation elongation factor, mitochondrial). Further studies revealed that PB1-F2 induced complete mitophagy, which required the interactions of PB1-F2 with both TUFM and MAP1LC3B/LC3B that mediated the autophagosome formation. PB1-F2-induced mitophagy was critical for the MAVS (mitochondrial antiviral signaling protein) degradation and led to its suppression of the type I IFN production. Importantly, the C-terminal LIR motif of PB1-F2 protein was demonstrated to be essential for its mitophagy induction and attenuated innate immunity. In conclusion, PB1-F2-induced mitophagy strongly correlates with impaired cellular innate immunity, revealing it is a potential therapeutic target.Abbreviations: BCL2L13: BCL2 like 13; BECN1: beclin 1; BNIP3L/Nix: BCL2 interacting protein 3 like; CQ: chloroquine; DDX58: DExD/H-box helicase 58; eGFP: enhanced green fluorescent protein; hpi: hours post infection; IAV: influenza A virus; IFN: interferon; IP: immunoprecipitation; LIR: LC3-interacting region; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAVS: mitochondrial antiviral signaling protein; MMP: mitochondrial membrane potential; MOI, multiplicity of infection; mRFP: monomeric red fluorescent protein; NBR1: NBR1 autophagy cargo receptor; NC: negative control; NLRP3: NLR family pyrin domain containing 3; PINK1: PTEN induced kinase 1; PRKN/PARK2: parkin RBR E3 ubiquitin protein ligase; RLR: RIG-I-like-receptor; ROS: reactive oxygen species; SEV: sendai virus; SQSTM1/p62: sequestosome 1; TAX1BP1: Tax1 binding protein 1; TM: transmembrane; TOMM20/40: translocase of outer mitochondrial membrane 20/40; TUFM: Tu translation elongation factor, mitochondrial.

Anti-H1N1 viral activity of three main active ingredients from zedoary oil

Influenza virus is one of the most widespread infectious diseases in the world. It poses a serious public health threat to humans. With the emergence of drug-resistant virus strains, antiviral drugs are urgently needed to control virus transmission and disease progression. In this study, three main active substances-curcumol, curdione and germacrone-were isolated from the traditional Chinese medicine zedoary. They inhibited the replication of influenza A (H1N1) virus in a dose-dependent manner. After treatment with these compounds, the expression of viral protein and RNA synthesis were inhibited. In vivo, these compounds also reduced H1N1-induced lung damage and the load of virus in serum as well as whole blood cells. In a proteomic analysis, after treatment with germacrone, the expression of antiviral protein and the amount of intracellular virus were significantly reduced, further proving that germacrone can inhibit viral replication. Our experiments have shown that curcumol, curdione and germacrone can inhibit the replication of H1N1 virus; in particular, germacrone shows potential both in vitro and in vivo as a therapeutic drug.

LYAR Suppresses Beta Interferon Induction by Targeting Phosphorylated Interferon Regulatory Factor 3

The innate immune response is vital for host defense and must be tightly controlled, but the mechanisms responsible for its negative regulation are not fully understood. The cell growth-regulating nucleolar protein LYAR was found to promote replication of multiple viruses in our previous study. Here, we report that LYAR acts as a negative regulator of innate immune responses. We found that LYAR expression is induced by beta interferon (IFN-β) during virus infection. Further studies showed that LYAR interacts with phosphorylated IFN regulatory factor 3 (IRF3) to impede the DNA binding capacity of IRF3, thereby suppressing the transcription of IFN-β and downstream IFN-stimulated genes (ISGs). In addition, LYAR inhibits nuclear factor-κB (NF-κB)-mediated expression of proinflammatory cytokines. In summary, our study reveals the mechanism of LYAR in modulating IFN-β-mediated innate immune responses by targeting phosphorylated IRF3, which not only helps us to better understand the mechanisms of LYAR-regulated virus replication but also uncovers a novel role of LYAR in host innate immunity.IMPORTANCE Type I interferon (IFN-I) plays a critical role in the antiviral innate immune responses that protect the host against virus infection. The negative regulators of IFN-I are important not only for fine-tuning the antiviral responses to pathogens but also for preventing excessive inflammation. Identification of negative regulators and study of their modulation in innate immune responses will lead to new strategies for the control of both viral and inflammatory diseases. Here, we report for the first time that the cell growth-regulating nucleolar protein LYAR behaves as a repressor of host innate immune responses. We demonstrate that LYAR negatively regulates IFN-β-mediated immune responses by inhibiting the DNA binding ability of IFN regulatory factor 3 (IRF3). Our study reveals a common mechanism of LYAR in promoting different virus replication events and improves our knowledge of host negative regulation of innate immune responses.

Inducible LGALS3BP/90K activates antiviral innate immune responses by targeting TRAF6 and TRAF3 complex

The galectin 3 binding protein (LGALS3BP, also known as 90K) is a ubiquitous multifunctional secreted glycoprotein originally identified in cancer progression. It remains unclear how 90K functions in innate immunity during viral infections. In this study, we found that viral infections resulted in elevated levels of 90K. Further studies demonstrated that 90K expression suppressed virus replication by inducing IFN and pro-inflammatory cytokine production. Upon investigating the mechanisms behind this event, we found that 90K functions as a scaffold/adaptor protein to interact with TRAF6, TRAF3, TAK1 and TBK1. Furthermore, 90K enhanced TRAF6 and TRAF3 ubiquitination and served as a specific ubiquitination substrate of TRAF6, leading to transcription factor NF-κB, IRF3 and IRF7 translocation from the cytoplasm to the nucleus. Conclusions: 90K is a virus-induced protein capable of binding with the TRAF6 and TRAF3 complex, leading to IFN and pro-inflammatory production.

The innate immune system detects the presence of viruses through germline-encoded pattern-recognition receptors (PRRs) and leads to the production of proinflammatory cytokines and interferons (IFNs) as the first line of defense against viral infections. Here, we identified a host protein, LGALS3BP, as a positive regulator of PRR-mediated signal transduction pathways by interacting with TRAF6-TAK1 and TRAF3-TBK1 axes, enhancing their recruitment and promoting the ubiquitination of TRAF6 and TRAF3. LGALS3BP exhibited antiviral activity toward a broad range of viral infections. LGALS3BP^-/- mice are highly susceptible to lethal influenza A virus infection with increasing pulmonary viral load, morbidity and mortality. Thus, our study highlight the importance of LGALS3BP in host antiviral innate immune responses.

The Nucleolar Protein LYAR Facilitates Ribonucleoprotein Assembly of Influenza A Virus

Influenza A viral ribonucleoprotein (vRNP) is responsible for transcription and replication of the viral genome in infected cells and depends on host factors for its functions. Identification of the host factors interacting with vRNP not only improves understanding of virus-host interactions but also provides insights into novel mechanisms of viral pathogenicity and the development of new antiviral strategies. Here, we have identified 80 host factors that copurified with vRNP using affinity purification followed by mass spectrometry. LYAR, a cell growth-regulating nucleolar protein, has been shown to be important for influenza A virus replication. During influenza A virus infection, LYAR expression is increased and partly translocates from the nucleolus to the nucleoplasm and cytoplasm. Furthermore, LYAR interacts with RNP subunits, resulting in enhancing viral RNP assembly, thereby facilitating viral RNA synthesis. Taken together, our studies identify a novel vRNP binding host partner important for influenza A virus replication and further reveal the mechanism of LYAR regulating influenza A viral RNA synthesis by facilitating viral RNP assembly.IMPORTANCE Influenza A virus (IAV) must utilize the host cell machinery to replicate, but many of the mechanisms of IAV-host interaction remain poorly understood. Improved understanding of interactions between host factors and vRNP not only increases our basic knowledge of the molecular mechanisms of virus replication and pathogenicity but also provides insights into possible novel antiviral targets that are necessary due to the widespread emergence of drug-resistant IAV strains. Here, we have identified LYAR, a cell growth-regulating nucleolar protein, which interacts with viral RNP components and is important for efficient replication of IAVs and whose role in the IAV life cycle has never been reported. In addition, we further reveal the role of LYAR in viral RNA synthesis. Our results extend and improve current knowledge on the mechanisms of IAV transcription and replication.