Molecular characterization and expression of the teleost cytosolic DNA sensor genes cGAS, LSm14A, DHX9, and DHX36 in Japanese medaka, Oryzias latipes

Grouper cGAS is a negative regulator of STING-mediated interferon response

Cyclic GMP-AMP synthase (cGAS) is one of the classical pattern recognition receptors that recognizes mainly intracytoplasmic DNA. cGAS induces type I IFN responses to the cGAS-STING signaling pathway. To investigate the roles of cGAS-STING signaling pathway in grouper, a cGAS homolog (named EccGAS) was cloned and identified from orange-spotted grouper (Epinephelus coioides). The open reading frame (ORF) of EccGAS is 1695 bp, encodes 575 amino acids, and contains a Mab-21 typical structural domain. EccGAS is homologous to Sebastes umbrosus and humans at 71.8% and 41.49%, respectively. EccGAS mRNA is abundant in the blood, skin, and gills. It is uniformly distributed in the cytoplasm and colocalized in the endoplasmic reticulum and mitochondria. Silencing of EccGAS inhibited the replication of Singapore grouper iridovirus (SGIV) in grouper spleen (GS) cells and enhanced the expression of interferon-related factors. Furthermore, EccGAS inhibited EcSTING-mediated interferon response and interacted with EcSTING, EcTAK1, EcTBK1, and EcIRF3. These results suggest that EccGAS may be a negative regulator of the cGAS-STING signaling pathway of fish.

The Emerging Role of STING in Insect Innate Immune Responses and Pathogen Evasion Strategies

Emerging evidence reveals that the stimulator of the interferon genes (STING) signaling pathway in insects and other animal cells helps them to sense and effectively respond to infection caused by numerous types of microbial pathogens. Recent studies have shown that genomic material from microbial pathogens induces the STING signaling pathway for the production of immune factors to attenuate infection. In contrast, microbial pathogens are equipped with various factors that assist them in evading the STING signaling cascade. Here we discuss the STING signaling pathway different animal groups compared to human and then focus on its crucial biological roles and application in the microbial infection of insects. In addition, we examine the negative and positive modulators of the STING signaling cascade. Finally, we describe the microbial pathogen strategies to evade this signaling cascade for successful invasion.

Anti-Inflammatory and Active Biological Properties of the Plant-Derived Bioactive Compounds Luteolin and Luteolin 7-Glucoside

Flavonoids are interesting molecules synthetized by plants. They can be found abundantly in seeds and fruits, determining the color, flavor, and other organoleptic characteristics, as well as contributing to important nutritional aspects. Beyond these characteristics, due to their biochemical properties and characteristics, they can be considered bioactive compounds. Several interesting studies have demonstrated their biological activity in different cellular and physiological processes in high-order organisms including humans. The flavonoid molecular structure confers the capability of reacting with and neutralizing reactive oxygen species (ROS), behaving as scavengers in all processes generating this class of molecules, such as UV irradiation, a process widely present in plant physiology. Importantly, the recent scientific literature has demonstrated that flavonoids, in human physiology, are active compounds acting not only as scavengers but also with the important role of counteracting the inflammation process. Among the wide variety of flavonoid molecules, significant results have been shown by investigating the role of the flavones luteolin and luteolin-7-O-glucoside (LUT-7G). For these compounds, experimental results demonstrated an interesting anti-inflammatory action, both in vitro and in vivo, in the interaction with JAK/STAT3, NF-κB, and other pathways described in this review. We also describe the effects in metabolic pathways connected with inflammation, such as cellular glycolysis, diabetes, lipid peroxidation, and effects in cancer cells. Moreover, the inhibition of inflammatory pathway in endothelial tissue, as well as the NLRP3 inflammasome assembly, demonstrates a key role in the progression of such phenomena. Since these micronutrient molecules can be obtained from food, their biochemical properties open new perspectives with respect to the long-term health status of healthy individuals, as well as their use as a coadjutant treatment in specific diseases.

DDX43 recruits TRIF or IPS-1 as an adaptor and activates the IFN-β pathway in Nile tilapia (Oreochromis niloticus)

DDX43 is one of the members of the DExD/H-box protein family, and emerging data suggest that it may play an important role in antiviral immunity across mammals. However, little is known about DDX43 in the fish immune response. In this study, we isolated the cDNA sequence of ddx43 in Nile tilapia (Oreochromis niloticus). The ddx43 gene was 2338 bp in length, contained an open reading frame (ORF) of 2064 bp and encoded a polypeptide of 687 amino acids. The predicted protein of OnDDX43 has three conserved domains, including the RNA binding domain KH, DEAD-like helicase superfamily DEXDc and C-terminal HELICc domain. In healthy Nile tilapia, the Onddx43 transcript was broadly expressed in all examined tissues, with the highest expression levels in the muscle and brain and the lowest in the liver. After challenge with Streptococcus agalactiae, lipopolysaccharides (LPS) and polyinosinic polycytidylic acid (Poly I:C), the expression level of Onddx43 mRNA was upregulated or downregulated in all of the tissues tested. Overexpression of OnDDX43 in 293 T cells showed that it has a positive regulatory effect on IFN-β. The subcellular localization showed that OnDDX43 was expressed in the cytoplasm. We performed further pull-down assays and found that OnDDX43 interacted with both interferon-β promoter stimulator1 (IPS-1) and TIR domain-containing adaptor inducing interferon-β (TRIF).

cGASa and cGASb from grass carp (Ctenopharyngodon idellus) play opposite roles in mediating type I interferon response

Cyclic GMP-AMP synthase (cGAS) is known as a DNA sensor for the initiation of innate immune responses in human and other mammals. However, the knowledge about fish cGAS is limited. In this study, we identified two paralogs of cGAS genes from grass carp (Ctenopharyngodon idellus), namely, CicGASa and CicGASb. Grass carp cGASa and cGASb share some conservative domains with mammalian cGASs; however, cGASb contains a unique transmembrane domain. Grass carp cGASa and cGASb responded to GCRV and poly (dA:dT) infection, but they played opposite roles in the regulation of type I IFN response, i.e. cGASa served as an activator for ISGs and NF-κB in a dose-dependent manner, while cGASb acted as an inhibitor. We found that cGASa and cGASb interacted with STING. Similarly, cGASa is an activator for IRF7, but cGASb inhibited IRF7 expression. Both cGASa and STING can protect cells from GCRV infection. Grass carp cGASb inhibited cGASa-induced type I IFN response by the competitive interaction with STING, suggesting that cGASb may be a negative regulator of cGASa-STING-IRF7 axis.

Identification of two interleukin 17 receptor C (IL-17RC) genes and their binding activities to three IL-17A/F ligands in the Japanese medaka, Oryzias latipes

In mammals, interleukin (IL)-17 receptor C (IL-17RC) and IL-17RA mediate IL-17A and IL-17F signaling to produce mucin, antimicrobial peptides, and maintain healthy intestinal flora. However, IL-17RC signaling in fish remains unclear. In this study, three il17rc transcripts (il17rca1, il17rca2, and il17rcb) from the Japanese medaka (Oryzias latipes) were cloned; il17rca1 and il17rca2 mRNAs were alternatively spliced from il17rca pre-mRNA as transcript variants. The il17rca and il17rcb genes were located on chromosomes 7 and 5, respectively. Teleost clades containing medaka il17rca and il17rcb clustered separately from the tetrapod clade. In adult tissues, il17rca1 expression was significantly higher than il17rca2 and il17rcb. Conversely, il17rcb expression was significantly higher in embryos and larvae. These expression patterns changed following infection with Edwardsiella piscicida and Aeromonas hydrophila. Furthermore, an immunoprecipitation assay using recombinant IL-17RCs and rIL-17A/Fs suggested that, in teleosts, three ligands could function in signaling through two IL-17RCs.

cGAS Is a Negative Regulator of RIG-I-Mediated IFN Response in Cyprinid Fish

In mammals, cyclic GMP-AMP synthase (cGAS) recognizes cytosolic dsDNA to induce the type I IFN response. However, the functional role of cGAS in the IFN response of fish remains unclear or controversial. In this study, we report that cGAS orthologs from crucian carp Carassius auratus (CacGAS) and grass carp Ctenopharyngodon idellus (CicGAS) target the dsRNA sensor retinoic acid-inducible gene I (RIG-I) for negative regulation of the IFN response. First, poly(deoxyadenylic-deoxythymidylic) acid-, polyinosinic-polycytidylic acid-, and spring viremia of carp virus-induced IFN responses were impaired by overexpression of CacGAS and CicGAS. Then, CacGAS and CicGAS interacted with CiRIG-I and CiMAVS and inhibited CiRIG-I- and CiMAVS-mediated IFN induction. Moreover, the K63-linked ubiquitination of CiRIG-I and the interaction between CiRIG-I and CiMAVS were attenuated by CacGAS and CicGAS. Finally, CacGAS and CicGAS decreased CiRIG-I-mediated the cellular antiviral response and facilitated viral replication. Taken together, data in this study identify CacGAS and CicGAS as negative regulators in RIG-I-like receptor signaling, which extends the current knowledge regarding the role of fish cGAS in the innate antiviral response.

Characterization and expression analysis of tandemly-replicated asc genes in the Japanese medaka, Oryzias latipes

ASC is a component of the inflammasome playing crucial roles in the inflammatory response. In mammals, ASC induces pyroptosis and inflammatory cytokine production. In this study, three asc genes (asc1, asc2, and asc3) from the Japanese medaka (Oryzias latipes) were identified and characterized. These asc genes were tandem replicates on chromosome 16, and their exon-intron structures differed between them. All three ASCs conserved the pyrin and caspase-recruitment domains, which are important for inflammasome formation. In phylogenetic analysis, all ASCs clustered with those of other teleosts. The asc1 expression levels were significantly higher in several organs than those of asc2 and asc3, suggesting that asc1 may act as a dominant asc in the Japanese medaka. Expression of the three asc genes showed different patterns during Aeromonas hydrophila and Edwardsiella piscicida infections. Furthermore, their expression was adequately down-regulated in the medaka fin-derived cells stimulated with ATP for 12 h, while asc2 expression was statistically up-regulated after nigericin stimulation for 24 h. Moreover, the expression of asc2 and asc3 was significantly higher in the skin of ASC-1-knockout medaka than in that of the wild type medaka during A. hydrophila infection.

The Current View on the Helicase Activity of RNA Helicase A and Its Role in Gene Expression

RNA helicase A (RHA) is a DExH-box helicase that plays regulatory roles in a variety of cellular processes, including transcription, translation, RNA splicing, editing, transport, and processing, microRNA genesis and maintenance of genomic stability. It is involved in virus replication, oncogenesis, and innate immune response. RHA can unwind nucleic acid duplex by nucleoside triphosphate hydrolysis. The insight into the molecular mechanism of helicase activity is fundamental to understanding the role of RHA in the cell. Herein, we reviewed the current advances on the helicase activity of RHA and its relevance to gene expression, particularly, to the genesis of circular RNA.

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.

The Central Role and Possible Mechanisms of Bacterial DNAs in Sepsis Development

The pathological roles of bacterial DNA have been documented many decades ago. Bacterial DNAs are different from mammalian DNAs; the latter are heavily methylated. Mammalian cells have sensors such as TLR-9 to sense the DNAs with nonmethylated CpGs and distinguish them from host DNAs with methylated CpGs. Further investigation has identified many other types of DNA sensors distributed in a variety of cellular compartments. These sensors not only sense foreign DNAs, including bacterial and viral DNAs, but also sense damaged DNAs from the host cells. The major downstream signalling pathways includeTLR-9-MyD88-IKKa-IRF-7/NF-κB pathways to increase IFN/proinflammatory cytokine production, STING-TBK1-IRF3 pathway to increase IFN-beta, and AIM2-ASC-caspas-1 pathway to release IL-1beta. The major outcome is to activate host immune response by inducing cytokine production. In this review, we focus on the roles and potential mechanisms of DNA sensors and downstream pathways in sepsis. Although bacterial DNAs play important roles in sepsis development, bacterial DNAs alone are unable to cause severe disease nor lead to death. Priming animals with bacterial DNAs facilitate other pathological factors, such as LPS and other virulent factors, to induce severe disease and lethality. We also discuss compartmental distribution of DNA sensors and pathological significance as well as the transport of extracellular DNAs into cells. Understanding the roles of DNA sensors and signal pathways will pave the way for novel therapeutic strategies in many diseases, particularly in sepsis.

Viral infection-induced changes in the expression profile of non-RLR DExD/H-box RNA helicases (DDX1, DDX3, DHX9, DDX21 and DHX36) in zebrafish and common carp

In mammals, several non-RLR DExD/H-box RNA helicases are involve in sensing of viral nucleic acids and activation of antiviral immune response, however their role in the immune defense of fish is much less known. In this study, the expression profile of non-RLR DExD/H-box RNA helicase genes: ddx1, ddx3, dhx9, ddx21 and dhx36, was studied in zebrafish (Danio rerio) and common carp (Cyprinus carpio L.) during infection with two RNA viruses: spring viremia of carp virus (SVCV) and Chum salmon reovirus (CSV). Bioinformatic analysis of the amino acid sequences of the core helicase of DDX1, DDX3, DHX9, DDX21 and DHX36 in zebrafish and common carp revealed presence of all conserved motifs found amongst all other species, with the exception of common carp DHX9 which do not possess motif V. The transcripts of studied DExD/H-box RNA helicases were found in zebrafish ZF4 cell line as well as in all studied organs from zebrafish and common carp. The expression study demonstrated the up-regulation of the expression of selected non-RLR DExD/H-box RNA helicases during viral infections in ZF4 cell line (in vitro study) and in zebrafish and common carp organs (in vivo study). DDX1 was the only DExD/H-box RNA helicase which expression was repetitively up-regulated during in vivo infections with SVCV and CSV in zebrafish and SVCV in common carp. In ZF4 cells and kidney of common carp, viral infection-induced up-regulation of DExD/H-box RNA helicases preceded the up-regulation of type I IFN gene. Our results suggest that studied non-RLR DExD/H-box RNA helicases might be involved in antiviral immune response in fish.

Grass carp cGASL negatively regulates fish IFN response by targeting MITA

Mammalian cyclic GMP-AMP synthase (cGAS) senses double-stranded (ds) DNA in the cytosol to activate the innate antiviral response. In the present study, a cGAS-like gene, namely cGASL, was cloned from grass carp Ctenopharyngodon idellus, and its role as a negative regulator of the IFN response was revealed. Phylogenetic analysis indicated that cGASL was evolutionarily closest to cGAS, but was not a true ortholog of cGAS. Overexpression of cGASL inhibited poly I:C-stimulated grass carp (gc)IFN1pro and ISRE activities. In addition, MITA-, but not TBK1-mediated activation of gcIFN1pro was impaired by cGASL. Co-immunoprecipitation and Western blot experiments indicated that cGASL interacted with MITA and TBK1, resulting in a reduction in the phosphorylation of MITA. Lastly, overexpression of cGASL reduced the transcriptional levels of several IFN-stimulated genes activated by MITA. Collectively, these data suggest that cGASL is a negative regulator of IFN response by targeting MITA in fish.