Grouper DDX41 exerts antiviral activity against fish iridovirus and nodavirus infection

Discovery and Characterization of the ddx41 Gene in Atlantic Salmon: Evolutionary Implications, Structural Functions, and Innate Immune Responses to Piscirickettsia salmonis and Renibacterium salmoninarum Infections

The innate immune response in Salmo salar, mediated by pattern recognition receptors (PRRs), is crucial for defending against pathogens. This study examined DDX41 protein functions as a cytosolic/nuclear sensor for cyclic dinucleotides, RNA, and DNA from invasive intracellular bacteria. The investigation determined the existence, conservation, and functional expression of the ddx41 gene in S. salar. In silico predictions and experimental validations identified a single ddx41 gene on chromosome 5 in S. salar, showing 83.92% homology with its human counterpart. Transcriptomic analysis in salmon head kidney confirmed gene transcriptional integrity. Proteomic identification through mass spectrometry characterized three unique peptides with 99.99% statistical confidence. Phylogenetic analysis demonstrated significant evolutionary conservation across species. Functional gene expression analysis in SHK-1 cells infected by Piscirickettsia salmonis and Renibacterium salmoninarum indicated significant upregulation of DDX41, correlated with increased proinflammatory cytokine levels and activation of irf3 and interferon signaling pathways. In vivo studies corroborated DDX41 activation in immune responses, particularly when S. salar was challenged with P. salmonis, underscoring its potential in enhancing disease resistance. This is the first study to identify the DDX41 pathway as a key component in S. salar innate immune response to invading pathogens, establishing a basis for future research in salmonid disease resistance.

Amphiprion clarkii DDX41 modulates fish immune responses: Characterization by expression profiling, antiviral assay, and macrophage polarization analysis

DDX41, a member of the DEAD-box helicase family, serves as a vital cytosolic DNA sensor and plays a pivotal role in controlling the activation of type I interferon responses in mammals. However, the functional aspects of fish DDX41 remain relatively unexplored. In this study, we identified and characterized the DDX41 gene in Amphiprion clarkii transcriptomes and designated the gene as AcDDX41. The complete open reading frame of AcDDX41 encoded a putative protein comprising 617 amino acids. Notably, the predicted AcDDX41 protein shared several structural features that are conserved in DDX41, including DEXDc, HELICc, and zinc finger domains, as well as conserved sequence "Asp-Glu-Ala-Asp (D-E-A-D)." AcDDX41 exhibited the highest sequence homology (99.68 % similarity) with DDX41 from Acanthochromis polyacanthus. Phylogenetic analysis revealed that DDX41s from fish formed a branch distinct from that in other animals. All investigated tissues were shown to express AcDDX41 constitutively, with blood showing the highest expression levels, followed by the brain. Furthermore, AcDDX41 expression was significantly induced upon stimulation with poly I:C, lipopolysaccharide, and Vibrio harveyi, indicating its responsiveness to immune stimuli. We confirmed the antiviral function of AcDDX41 by analyzing gene expression and viral replication during viral hemorrhagic septicemia virus infection. Additionally, using a luciferase reporter assay, we validated the ability of AcDDX41 to activate the NF-κB signaling pathway upon stimulation with poly I:C. Finally, AcDDX41 influenced cytokine gene expression and played a regulatory role in macrophage M1 polarization in RAW 264.7 cells. Collectively, these results highlight the significance of AcDDX41 as an immune-related gene that contributes substantially to antiviral defense and regulation of NF-κB activity.

DDX41: exploring the roles of a versatile helicase

DDX41 is a DEAD-box helicase and is conserved across species. Mutations in DDX41 have been associated with myeloid neoplasms, including myelodysplastic syndrome and acute myeloid leukemia. Though its pathogenesis is not completely known, DDX41 has been shown to have many cellular roles, including in pre-mRNA splicing, innate immune sensing, ribosome biogenesis, translational regulation, and R-loop metabolism. In this review, we will summarize the latest understandings regarding the various roles of DDX41, as well as highlight challenges associated with drug development to target DDX41. Overall, understanding the molecular and cellular mechanisms of DDX41 could help develop novel therapeutic options for DDX41 mutation-related hematologic malignancies.

Singapore grouper iridovirus infection counteracts poly I:C induced antiviral immune response in vitro

Groupers are important mariculture fish in South China and Southeast Asian countries. However, the increasing frequency of infectious disease outbreaks has caused great economic losses in the grouper industry. Among these pathogens, Singapore grouper iridovirus (SGIV) infection causes high mortality in larval and juvenile stages of grouper. However, the mechanism underlying the action of viral manipulation on cellular immune response still remained largely uncertain. Here, using RNA-seq technology, we investigated the regulatory roles of SGIV infection on synthetic RNA duplex poly I:C induced immune response in vitro. Using reporter gene assays, we found that SGIV infection decreased poly I:C induced interferon promoter activation. Transcriptomic analysis showed that the mRNA expression levels of 2238 genes were up-regulated, while 1247 genes were down-regulated in poly I:C transfected grouper spleen (GS) cells. Interestingly, SGIV infection decreased the expression of 1479 up-regulated genes and increased the expression of 297 down-regulated genes in poly I:C transfected cells. The differentially expressed genes (DEGs) down-regulated by SGIV were directly related to immune, inflammation and viral infection, and JUN, STAT1, NFKB1, MAPK14A, TGFB1 and MX were the 6 top hub genes in the down-regulated DEGs' protein-protein interaction (PPI) network. Furthermore, quantitative real-time PCR (qPCR) analysis confirmed that the interferon signaling and inflammatory-related genes, including cGAS, STING, TBK1, MAVS, TNF, IRAK4 and NOD2 were up-regulated by poly I:C stimulation, but all significantly down-regulated after SGIV infection. Thus, we speculated that SGIV infection counteracted poly I:C induced antiviral immune response and this ability helped itself to escape host immune surveillance. Together, our data will contribute greatly to understanding the potential immune evasion mechanism of iridovirus infection in vitro.

The Interaction of Mandarin Fish DDX41 with STING Evokes type I Interferon Responses Inhibiting Ranavirus Replication

DDX41 is an intracellular DNA sensor that evokes type I interferon (IFN-I) production via the adaptor stimulator of interferon gene (STING), triggering innate immune responses against viral infection. However, the regulatory mechanism of the DDX41-STING pathway in teleost fish remains unclear. The mandarin fish (Siniperca chuatsi) is a cultured freshwater fish species that is popular in China because of its high market value. With the development of a high-density cultural mode in mandarin fish, viral diseases have increased and seriously restricted the development of aquaculture, such as ranavirus and rhabdovirus. Herein, the role of mandarin fish DDX41 (scDDX41) and its DEAD and HELIC domains in the antiviral innate immune response were investigated. The level of scDDX41 expression was up-regulated following treatment with poly(dA:dT) or Mandarin fish ranavirus (MRV), suggesting that scDDX41 might be involved in fish innate immunity. The overexpression of scDDX41 significantly increased the expression levels of IFN-I, ISGs, and pro-inflammatory cytokine genes. Co-immunoprecipitation and pull-down assays showed that the DEAD domain of scDDX41 recognized the IFN stimulatory DNA and interacted with STING to activate IFN-I signaling pathway. Interestingly, the HELIC domain of scDDX41 could directly interact with the N-terminal of STING to induce the expression levels of IFN-I and ISGs genes. Furthermore, the scDDX41 could enhance the scSTING-induced IFN-I immune response and significantly inhibit MRV replication. Our work would be beneficial to understand the roles of teleost fish DDX41 in the antiviral innate immune response.

DExH/D-box helicases at the frontline of intrinsic and innate immunity against viral infections

DExH/D-box helicases are essential nucleic acid and ribonucleoprotein remodelers involved in all aspects of nucleic acid metabolism including replication, gene expression and post-transcriptional modifications. In parallel to their importance in basic cellular functions, DExH/D-box helicases play multiple roles in viral life cycles, with some of them highjacked by viruses or negatively regulating innate immune activation. However, other DExH/D-box helicases have recurrently been highlighted as direct antiviral effectors or as positive regulators of innate immune activation. Innate immunity relies on the ability of Pathogen Recognition Receptors to recognize viral signatures and trigger the production of interferons (IFNs) and pro-inflammatory cytokines. Secreted IFNs interact with their receptors to establish antiviral cellular reprogramming via expression regulation of the interferon-stimulated genes (ISGs). Several DExH/D-box helicases have been reported to act as viral sensors (DDX3, DDX41, DHX9, DDX1/DDX21/DHX36 complex), and others to play roles in innate immune activation (DDX60, DDX60L, DDX23). In contrast, the DDX39A, DDX46, DDX5 and DDX24 helicases act as negative regulators and impede IFN production upon viral infection. Beyond their role in viral sensing, the ISGs DDX60 and DDX60L act as viral inhibitors. Interestingly, the constitutively expressed DEAD-box helicases DDX56, DDX17, DDX42 intrinsically restrict viral replication. Hence, DExH/D-box helicases appear to form a multilayer network of primary and secondary factors involved in both intrinsic and innate antiviral immunity. In this review, we highlight recent findings on the extent of antiviral defences played by helicases and emphasize the need to better understand their immune functions as well as their complex interplay.

Conserved function of crucian carp cGAS in the MITA-mediated interferon signaling

Mammalian cyclic GMP-AMP synthase (cGAS) is pivotal for cytosolic DNA-triggered interferon (IFN) response. However, the function of cGAS in fish IFN response remains unclear. Our recent study has reported that cGAS from crucian and grass carps downregulates the IFN response by attenuating the K63-linked ubiquitination of retinoic acid-inducible gene-I (RIG-I) and its interaction with mitochondrial antiviral signaling protein (MAVS). Here, the function of crucian carp cGAS was further investigated. We found that crucian carp cGAS directly binds to poly deoxyadenylic-deoxythymidylic acid (poly (dA:dT)) and exhibits mediator of IFN regulatory factor 3 (IRF3) activation (MITA)-dependent activation of the IFN response, indicating a conserved function of crucian carp cGAS in the MITA-mediated IFN signaling. However, crucian carp cGAS could suppress the IFN activation stimulated by polyinosinic: polycytidylic acid (poly (I:C)) in time- and dose-dependent manners. These data collectively suggest complicated functions of crucian carp cGAS in the IFN antiviral response.

IFP35 Is a Relevant Factor in Innate Immunity, Multiple Sclerosis, and Other Chronic Inflammatory Diseases: A Review

Simple Summary

In this review, we focused on the emerging role of IFP35, a highly conserved leucine zipper protein from fish to humans, with a still unknown biological function. The considered literature indicates this protein as a key-pleiotropic factor reflecting JAK-STAT and DAMPs pathways activation in innate immunity-dependent inflammation, as well as in the physiology and general pathology of a wide range of phylogenetically distant organisms. These findings also indicate IFP35 as a biologically relevant molecule in human demyelinating diseases of the central nervous system, including Multiple Sclerosis, and other organ-specific chronic inflammatory disorders.

Abstract

Discovered in 1993 by Bange et al., the 35-kDa interferon-induced protein (IFP35) is a highly conserved cytosolic interferon-induced leucine zipper protein with a 17q12-21 coding gene and unknown function. Belonging to interferon stimulated genes (ISG), the IFP35 reflects the type I interferon (IFN) activity induced through the JAK-STAT phosphorylation, and it can homodimerize with N-myc-interactor (NMI) and basic leucine zipper transcription factor (BATF), resulting in nuclear translocation and a functional expression. Casein kinase 2-interacting protein-1 (CKIP-1), retinoic acid-inducible gene I (RIG-I), and laboratory of genetics and physiology 2 Epinephelus coioides (EcLGP2) are thought to regulate IFP35, via the innate immunity pathway. Several in vitro and in vivo studies on fish and mammals have confirmed the IFP35 as an ISG factor with antiviral and antiproliferative functions. However, in a mice model of sepsis, IFP35 was found working as a damage associated molecular pattern (DAMP) molecule, which enhances inflammation by acting in the innate immune-mediated way. In human pathology, the IFP35 expression level predicts disease outcome and response to therapy in Multiple Sclerosis (MS), reflecting IFN activity. Specifically, IFP35 was upregulated in Lupus Nephritis (LN), Rheumatoid Arthritis (RA), and untreated MS. However, it normalized in the MS patients undergoing therapy. The considered data indicate IFP35 as a pleiotropic factor, suggesting it as biologically relevant in the innate immunity, general pathology, and human demyelinating diseases of the central nervous system.

Molecular characterization and expressional analysis of two poly (ADP-ribose) polymerase (PARP) domain-containing Gig2 isoforms in rockfish (Sebastes schlegelii) and their antiviral activity against viral hemorrhagic septicemia virus

Remedies toward sustainable aquaculture rely upon research that unveils the molecular mechanisms behind host immunity and their interactions with pathogens. Antiviral defense is a major innate immune response in fish. The antiviral protein GCHV-induced gene-2 (Gig2), a member of the interferon-stimulated gene (ISG), was identified and characterized from rockfish (Sebastes schlegelii). Gig2 exists in two isoforms, namely, SsGig2-I1 and SsGig2-I2, in rockfish with lengths of 163 and 223 bp, respectively. Bioinformatic analysis indicated the availability of poly (ADP-ribose) polymerase domain in both proteins, and 51.3% identity and 71.3% similarity between both isoforms were observed. The basal expression pattern revealed the highest tissue-specific expression in rockfish gills for both isoforms. The immune challenge experiment disclosed a distinctive and strong expression of each transcript in the presence of poly I:C. Both isoforms are localized in the endoplasmic reticulum. Interferon (IFN) pathway gene analysis revealed no significant upregulation of IFN related genes. Viral hemorrhagic septicemia virus (VHSV) gene expression analysis revealed strong downregulation of viral transcripts after 48 h of infection in the presence of Gig2 isoforms. Collectively, these results indicate the protective role of Gig2 in rockfish against VHSV infection and help broaden our understanding of the innate immunity of fish.

Efficient RNA virus targeting via CRISPR-CasRx in fish

The emergence of the CRISPR-Cas system as a technology has transformed our ability to modify nucleic acids, and the CRISPR-Cas13 system has been used to target RNA. CasRx is a small sized type VI-D effector (Cas13d) with RNA knockdown efficiency that may have an interference effect on RNA viruses. However, the RNA virus-targeting activity of CasRx still needs to be verified in vivo in vertebrates. In this study, we successfully engineered a highly effective CasRx system for fish virus interference. We designed synthetic mRNA coding for CasRx and used CRISPR RNAs to guide it to target the grouper nervous necrosis virus (RGNNV). This technique resulted in significant interference with virus infections both in vitro and in vivo. These results indicate that CRISPR/CasRx can be used to engineer interference against RNA viruses in fish, which provides a potential novel mechanism for RNA-guided immunity against other RNA viruses in vertebrates. Importance RNA viruses are most important viral pathogens infecting vertebrates and mammals. RNA virus populations are highly dynamic due to short generation times, large population sizes, and high mutation frequencies. Therefore, it is difficult to find a widely effective ways to inhibit RNA viruses. Therefore, we urgently need to develop effective antiviral methods. CasRx is a small sized type VI-D effector (Cas13d) with RNA knockdown efficiency that can have an interference effect on RNA viruses. Nervous necrosis virus (NNV), a non-enveloped positive-strand RNA virus, is one of the most serious viral pathogens infecting more than 40 cultured fish species resulting in huge economic losses worldwide. Here, we establish a novel efective CasRx system for RNA virus interference using NNV and grouper (Epinephelus coioices) as model. Our data show that CasRx have the most robust for RNA virus interference applications in fish and demonstrate its suitability for studying key questions relating to virus biology.

DDX41: a multifunctional DEAD-box protein involved in pre-mRNA splicing and innate immunity

DEAD-box helicases participate in nearly all steps of an RNA's life. In recent years, increasing evidence has shown that several family members are multitasking enzymes. They are often involved in different processes, which may be typical for RNA helicases, such as RNA export and translation, or atypical, e.g., acting as nucleic acid sensors that activate downstream innate immune signaling. This review focuses on the DEAD-box protein DDX41 and summarizes our current understanding of its roles as an innate immunity sensor in the cytosol and in pre-mRNA splicing in the nucleus and discusses DDX41's involvement in disease.

Grass carp Mre11A activates IFN 1 response by targeting STING to defend against GCRV infection

Mre11A is considered as a cytosolic DNA receptor in mammals. However, it is rarely known about Mre11A in other vertebrates. Recently, a mammalian ortholog of Mre11A has been identified in grass carp (Ctenopharyngodon idellus) in our lab. Phylogenetic-tree analysis provided evidence for a close genetic relationship between C.idellus Mre11A and Carassius auratus Mre11A. The tissue expression profile of CiMre11A was detected, with a relatively higher level of expression in kidney, intestines, liver and spleen than that in other tissues after grass carp reovirus (GCRV) infection. Similarly, CiMre11A was also up-regulated in CIK cells after treatment with GCRV. Q-PCR and dual-luciferase assays indicated that the transcription levels of IFN1 and ISG15 were inhibited by CiMre11A knockdown, but were gradually augmented after CIK cells were transfected with increasing amounts of CiMre11A. Subcellular localization assays showed that a part of CiMre11A was translocated from the nucleus to the cytoplasm. Co-immunoprecipitation and co-localization assays demonstrated that CiMre11A interacts with CiSTING in response to GCRV infection. In CIK cells, the expressions of both IFN1 and ISG15 were acutely up-regulated by CiMre11A overexpression, as well as by co-overexpression of CiMre11A and CiSTING. CiMre11A and CiSTING induced the phosphorylation and cytoplasmic-to-nuclear translocation of IRF7 in CIK cells. The multiplication of GCRV in CIK cells was inhibited by the overexpression of CiMre11A and CiSTING.

Hepatitis B Virus Might Be Sensed by STING-Dependent DNA Sensors and Attenuates the Response of STING-Dependent DNA Sensing Pathway in Humans with Acute and Chronic Hepatitis B Virus Infection

DNA-dependent activator of interferon regulatory factors (DAIs), interferon gamma inducible protein 16 (IFI16), DEAD-box polypeptide 41 (DDX41), DNA-dependent protein kinase (DNA-PK), meiotic recombination 11 homolog A (MRE11), and cyclic GMP-AMP synthase (cGAS) have been identified as intracellular STING-dependent DNA sensors in recent years. Studies have shown that the DNA sensor-STING-interferon (IFN)-β pathway plays an important role in the defense against intracellular invasion of many DNA viruses. However, the intracellular recognition of hepatitis B virus (HBV) DNA by DNA sensors is still largely unclear. In this study, we aimed to determine whether the DNA sensor-STING pathway in peripheral blood mononuclear cells (PBMCs) can be activated by acute and chronic HBV infections in humans. We first evaluated the expression of these DNA sensors in PBMCs of acute and chronic HBV-infected patients by quantitative real-time polymerase chain reaction. We next compared the expression of the upregulated DNA sensor between monocytes and nonmonocytes to find its cellular source. Finally, by in vitro stimulation, we analyzed the IFN-β response of the DNA sensor-STING pathway in PBMCs and monocytes from chronic HBV-infected patients. The results showed that IFI16, DDX41, MRE11, and the adaptor STING were upregulated in chronic HBV-infected patients, whereas only IFI16 was upregulated in acute HBV-infected patients. However, IFN-β expression was not changed in PBMCs from acute and chronic HBV-infected patients. We next found IFI16 was mainly expressed in monocytes of acute and chronic hepatitis B patients. Finally, by stimulation of monocytes with VACV ds 70mer, a ligand for IFI16, we confirmed the attenuated response of the IFI16-STING pathway. Taken together, our results suggest that HBV might be sensed by DNA sensors in PBMCs of acute and chronic HBV-infected patients, and meanwhile HBV infection attenuates the response of the DNA sensor-STING pathway in PBMCs and monocytes, which may facilitate the persistence of HBV infection.

Ctenopharyngodon idellus DDX41 initiates IFN I and ISG15 expression in response to GCRV infection

As a member of DExD/H-box helicase family, DDX41 (DEAD box polypeptide 41) acts as an intracellular DNA sensor that induces type I IFN expression in mammals. Fish DDX41 shares some similar properties with the mammalian counterparts. In this study, a DDX41 orthologous gene from grass carp (Ctenopharyngodon idellus) (CiDDX41) was cloned and characterized. The ORF of CiDDX41 encodes a polypeptide of 614 amino acids. Multiple alignments showed that DDX41 is highly conserved among different species. Phylogenetic tree analysis revealed that CiDDX41 shares a high degree of homology with Sinocyclocheilus rhinocerous DDX41. CiDDX41 is highly expressed in kidney, intestines, liver and spleen. Their expressions are up-regulated more obviously after the treatment with GCRV. Over-expression of CiDDX41 in CIK cells increases the transcription level of grass carp IFN I and ISG15. On the contrary, knockdown of CiDDX41 inhibits the IFN I and ISG15 transcription. Moreover, a part of CiDDX41 translocates from the nuclear to cytoplasm to interact with grass carp STING (CiSTING). In CIK cells, overexpression of CiDDX41 and CiSTING can promote the phosphorylation and nuclear-cytoplasm translocation of grass carp IRF7 (CiIRF7) and then acutely up-regulate the IFN I and ISG15 expression. However, the knockdown of CiDDX41 inhibits the phosphorylation IRF7. Taken together, all these results above suggested that CiDDX41 performs as an activator for innate immune through STING-IRF7 mediated signaling pathway.

Cytosolic Sensors for Pathogenic Viral and Bacterial Nucleic Acids in Fish

Recognition of the non-self signature of invading pathogens is a crucial step for the initiation of the innate immune mechanisms of the host. The host response to viral and bacterial infection involves sets of pattern recognition receptors (PRRs), which bind evolutionarily conserved pathogen structures, known as pathogen-associated molecular patterns (PAMPs). Recent advances in the identification of different types of PRRs in teleost fish revealed a number of cytosolic sensors for recognition of viral and bacterial nucleic acids. These are DExD/H-box RNA helicases including a group of well-characterized retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) and non-RLR DExD/H-box RNA helicases (e.g., DDX1, DDX3, DHX9, DDX21, DHX36 and DDX41) both involved in recognition of viral RNAs. Another group of PRRs includes cytosolic DNA sensors (CDSs), such as cGAS and LSm14A involved in recognition of viral and intracellular bacterial dsDNAs. Moreover, dsRNA-sensing protein kinase R (PKR), which has a role in antiviral immune responses in higher vertebrates, has been identified in fish. Additionally, fish possess a novel PKR-like protein kinase containing Z-DNA binding domain, known as PKZ. Here, we review the current knowledge concerning cytosolic sensors for recognition of viral and bacterial nucleic acids in teleosts.

Strain-specific disruption of interferon-stimulated N-myc and STAT interactor (NMI) function by Toxoplasma gondii type I ROP18 in human cells

Toxoplasma gondii rhoptry protein TgROP18 is a polymorphic virulence effector that targets immunity-related GTPases (IRGs) in rodents. Given that IRGs are uniquely diversified in rodents and not in other T. gondii intermediate hosts, the role of TgROP18 in manipulating non-rodent cells is unclear. Here we show that in human cells TgROP18I interacts with the interferon-gamma-inducible protein N-myc and STAT interactor (NMI) and that this is a property that is unique to the type I TgROP18 allele. Specifically, when expressed ectopically in mammalian cells only TgROP18I co-immunoprecipitates with NMI in IFN-γ-treated cells, while TgROP18II does not. In parasites expressing TgROP18I or TgROP18II, NMI only co-immunoprecipitates with TgROP18I and this is associated with allele-specific immunolocalization of NMI on the parasitophorous vacuolar membrane (PVM). We also found that TgROP18I reduces NMI association with IFN-γ-activated sequences (GAS) in the IRF1 gene promoter. Finally, we determined that polymorphisms in the C-terminal kinase domain of TgROP18I are required for allele-specific effects on NMI. Together, these data further define new host pathway targeted by TgROP18I and provide the first function driven by allelic differences in the highly polymorphic ROP18 locus.

The roles of mandarin fish STING in innate immune defense against Infectious spleen and kidney necrosis virus infections

The mandarin fish Siniperca chuatsi is a cultured freshwater fish species that is popular in China because of its high market value. With the development of high-density cultural mode in mandarin fish, viral diseases such as Infectious spleen and kidney necrosis virus (ISKNV) are becoming increasingly serious. Stimulator of interferon genes (STING) is a central component in the innate immune response to cytosolic DNA and RNA derived from different pathogens. However, the roles of STING in innate immune response of mandarin fish remain unknown. In the present study, S. chuatsi STING (scSTING)-mediated host immune response against ISKNV infection was investigated. ScSTING transcription level increased remarkably in response to ISKNV infection, LPS, PMA, or poly (I:C) stimulation in mandarin fish fry (MFF-1) cells. Immunofluorescence results showed that scSTING localized majorly in the endoplasmic reticulum. scSTING overexpression remarkably increased the expression levels of scIFN-h, scMx, scISG15, scPKR, scViperin, scIL-1β, scIL-18, and scTNF-α genes. IFN-β-luciferase report assay results showed that the relative expressions of luciferin were remarkably increased in MFF-1 cells. Site mutation of serine (S) on C-terminus of scSTING showed that both S388 and S396 were important for mediated signaling. Furthermore, scSTING overexpression inhibited ISKNV infection, and knockdown of scSTING promoted ISKNV infection, indicating that scSTING could suppress ISKNV infection in MFF-1 cells. These observations suggested that the scSTING played an important role in innate immune against ISKNV infection. Our work would help elucidate the roles of teleost fish STING in innate immunity.

Molecular and functional characterization of tilapia DDX41 in IFN regulation

DEAD-box helicase 41 (DDX41) is a key cytosolic DNA sensor playing critical roles in the regulation of type I IFN responses, and their functions have been well-characterized in mammals. However, little information is available regarding the function of fish DDX41. In this study, a DDX41 gene, named On-DDX41, was identified in Nile tilapia, Oreochromis niloticus. The predicted protein of On-DDX41 contains several structural features known in DDX41, including conserved DEADc and HELICc domains, and a conserved sequence "Asp-Glu-Ala-Asp (D-E-A-D)". On-DDX41 gene was constitutively expressed in all tissues examined, with the highest expression level observed in liver and muscle, and was inducible after poly(I:C) stimulation. Moreover, the overexpression of On-DDX41 can elicit a strong activation of both zebrafish IFN1 and IFN3 promoter in fish cells treated with poly(dA:dT). The present study thus contributes to a better understanding of the functional properties of DDX41 in fish.

A Cytosolic Sensor, PmDDX41, Binds Double Stranded-DNA and Triggers the Activation of an Innate Antiviral Response in the Shrimp Penaeus monodon via the STING-Dependent Signaling Pathway

Helicase DDX41 is a cytosolic sensor capable of detecting double-stranded DNA in mammals. However, the function of DDX41 remains poorly understood in invertebrates. In a previous study, we identified the first DDX41 sensor in the black tiger shrimp Penaeus monodon (PmDDX41) and showed that it played a role in anti-viral response. In the present study, we demonstrated that PmDDX41 was localized in the cytoplasm of shrimp hemocytes. However, PmDDX41 was localized in both the cytoplasm and nucleus of hemocytes in the presence of white spot syndrome virus (WSSV) infection or when stimulated by the nucleic acid mimics, poly(dA:dT) and poly(I:C). Similar results were observed when PmDDX41 was transfected into human embryonic kidney 293T (HEK293T) cells. Immunoprecipitation further demonstrated that PmDDX41 bound to biotin-labeled poly(dA:dT) but not poly(I:C). The overexpression of shrimp PmDDX41 and mouse stimulator of interferon gene (MmSTING) in HEK293T cells synergistically promoted IFN-β and NF-κB promoter activity via the DEADc domain. Co-immunoprecipitation (Co-IP) also confirmed that there was an interaction between PmDDX41 and STING after stimulation with poly(dA:dT) but not poly(I:C). Our study is the first to demonstrate that PmDDX41 acts as a cytosolic DNA sensor that interacts with STING via its DEADc domain to trigger the IFN-β and NF-κB signaling pathways, thus activating antiviral innate immune responses.