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

Study of the antivirus function mediated by STING in Micropterus salmoides

Stimulator of interferon genes (STING) has been demonstrated as a critical mediator in the innate immune response to cytosolic DNA and RNA derived from different pathogens. While the role of Micropterus salmoides STING (MsSTING) in largemouth bass virus is still unknown. In this study, RT-qPCR assay and Western-blot assay showed that the expression levels of MsSTING and its downstream genes were up-regulated after LMBV infection. Pull down experiment proved that a small peptide called Fusion peptide (FP) that previously reported to target to marine and human STING as a selective inhibitor also interacted with MsSTING in vitro. Comparing with the RNA-seq of Largemouth bass infected with LMBV singly, 326 genes were significantly up-regulated and 379 genes were significantly down-regulated in the FP plus LMBV group in which Largemouth bass was treatment with FP before LMBV-challenged. KEGG analysis indicated that the differentially expressed genes (DEGs) were mainly related to signaling transduction, infectious disease viral, immune system and endocrine system. Besides, the survival rate of LMBV-infected largemouth bass was highly decreased following FP treatment. Taken together, our study showed that MsSTING played an important role in immune response against LMBV infection.

RING finger protein 122-like (RNF122L) negatively regulates antiviral immune response by targeting STING in common carp (Cyprinus carpio L.)

Stimulator of interferon genes (STING), as an imperative adaptor protein in innate immune, responds to nucleic acid from invading pathogens to build antiviral responses in host cells. Aberrant activation of STING may trigger tissue damage and autoimmune diseases. Given the decisive role in initiating innate immune response, the activity of STING is intricately governed by several posttranslational modifications, including phosphorylation and ubiquitination. Here, we cloned and characterized a novel RNF122 homolog from common carp (named CcRNF122L). Expression analysis disclosed that the expression of CcRNF122L is up-regulated under spring viremia of carp virus (SVCV) stimulation in vivo and in vitro. Overexpression of CcRNF122L hampers SVCV- or poly(I:C)-mediated the expression of IFN-1 and ISGs in a dose-dependent way. Mechanistically, CcRNF122L interacts with STING and promotes the polyubiquitylation of STING. This polyubiquitylation event inhibits the aggregation of STING and the subsequent recruitment of TBK1 and IRF3 to the signaling complex. Additionally, the deletion of the TM domain abolishes the negative regulatory function of CcRNF122L. Collectively, our discoveries unveil a mechanism that governs the STING function and the precise adjustment of the innate immune response in teleost.

The antiviral role of largemouth bass STING against iridovirus infection

Stimulator of interferon gene (STING) plays a crucial role in the innate immune response against viral and bacterial pathogens. However, its function in largemouth bass iridovirus (LMBV) infection remains uncertain. Here, a STING homolog (MsSTING) from largemouth bass (Micropterus salmoides) was cloned and characterized. MsSTING encoded a 407-amino-acid polypeptide, which shared 84.08% and 41.45% identity with golden perch (Perca flavescens) and human (Homo sapiens) homologs, respectively. MsSTING contained four transmembrane domains and a conserved C-terminal domain. The mRNA level of MsSTING was significantly increased in response to LMBV infection in vitro. Subcellular localization observation indicated that MsSTING encoded a cytoplasmic protein, which co-localized predominantly with endoplasmic reticulum (ER) and partially with mitochondria. Moreover, its accurate localization was dependent on the N-terminal transmembrane motif (TM) domains. MsSTING was able to activate interferon (IFN) response, evidenced by the activation of IFN1, IFN3 and ISRE promoters by its overexpression in vitro. Mutant analysis showed that both the N-terminal and C-terminal domain of MsSTING were essential for its activation on IFN response. In addition, overexpression of MsSTING inhibited the transcription and protein levels of viral core genes, indicating that MsSTING exerted antiviral action against LMBV. Consistently, the inhibitory effects were significantly attenuated when the N-terminal or C-terminal domains of MsSTING was deleted. Furthermore, MsSTING overexpression upregulated the transcriptions of interferon-related genes and pro-inflammatory factors, including TANK-binding kinase 1(TBK1), interferon regulatory factor 3 (IRF3), interferon regulatory factor 7 (IRF7), interferon stimulated exonuclease gene 20 (ISG20), interferon-induced transmembrane protein 1(IFITM1), interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6). Together, MsSTING exerted antiviral action upon LMBV infection through positive regulation the innate immune response.

Alternative splicing variants of stimulator of interferon genes (STING) from Asian seabass (Lates calcarifer) and their immune response against red spotted grouper nervous necrosis virus (RGNNV)

The Stimulator of Interferon Genes (STING, also known as MITA/ERYS/MPYS) is an adaptor molecule that plays a crucial role in the RLR pathway and responds to DNA and RNA viruses. In the present study, we have identified two novel isoforms of STING (the canonical form named as LcSTINGa and its alternative splicing isoform named as LcSTINGb) from teleost Lates calcarifer. LcSTINGa has an ORF of 1230 bp, encoding a 409 amino acid protein, while its alternative splicing variant, LcSTINGb, features an ORF of 987 bp, encoding 328 amino acids. LcSTINGa is predicted to contain four transmembrane helices, whereas LcSTINGb has only two. The Lates STING protein showed about 86.85% identity with Perca flavescens, 86.45% with Seriola and 39.51% with Homo sapiens. The tissue distribution studies revealed that the STING variants were constitutively expressed in all the tissues examined, with the highest expression in blood. In-vivo upregulation of LcSTINGa and LcSTINGb mRNA following immune challenge with poly (I:C), Red-spotted grouper nervous necrosis virus (RGNNV) and zymosan A suggests its significance in the immune response.

MiRNAs and lncRNAs in the regulation of innate immune signaling

The detection and defense against foreign agents and pathogens by the innate immune system is a crucial mechanism in the body. A comprehensive understanding of the signaling mechanisms involved in innate immunity is essential for developing effective diagnostic tools and therapies for infectious diseases. Innate immune response is a complex process involving recognition of pathogens through receptors, activation of signaling pathways, and cytokine production, which are all crucial for deploying appropriate countermeasures. Non-coding RNAs (ncRNAs) are vital regulators of the immune response during infections, mediating the body's defense mechanisms. However, an overactive immune response can lead to tissue damage, and maintaining immune homeostasis is a complex process in which ncRNAs play a significant role. Recent studies have identified microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) as key players in controlling gene expression in innate immune pathways, thereby participating in antiviral defenses, tumor immunity, and autoimmune diseases. MiRNAs act by regulating host defense mechanisms against viruses, bacteria, and fungi by targeting mRNA at the post-transcriptional level, while lncRNAs function as competing RNAs, blocking the binding of miRNAs to mRNA. This review provides an overview of the regulatory role of miRNAs and lncRNAs in innate immunity and its mechanisms, as well as highlights potential future research directions, including the expression and maturation of new ncRNAs and the conservation of ncRNAs in evolution.

Characterization of STING from common carp (Cyprinus carpio L.) involved in spring viremia of carp virus infection

Stimulator of interferon genes (STING) serve as an endoplasmic reticulum (ER) protein and modulates innate immune responses to viral contagion. Most investigations involving teleost STING antiviral immunity have examined DNA viruses. Therefore, fish STING signaling events against RNA viruses require additional exploration. Here, common carp STING (named CcSTING) was cloned and characterized. The bioinformatics analyses of CcSTING showed evolutionary conservations and were most closely related to other cyprinid STINGs. Immunofluorescence staining discovered that the CcSTING was chiefly placed in the cytoplasm, specifically within the ER. CcSTING was ubiquitously generated in all analyzed organs, with especially strong expression in the gills and head kidney. Spring viremia of carp virus (SVCV) stimulation and poly(I:C) infection induced the generation of CcSTING in immune-associated organs, as well as in peripheral blood leukocytes. Additional investigations revealed that CcSTING overexpression strongly suppressed SVCV replication in EPC cells. Mechanistically, CcSTING enhanced IFN-1 and ISGs expression following SVCV infection. CcSTING also substantially increased both IFN and NF-κB promoter luciferase activity via a dosage-dependent fashion. Lastly, CcSTING significantly up-regulated both TBK1 and p65 phosphorylation. Collectively, these findings demonstrated the critical role and underlying mechanism of fish STING in response to RNA virus.

Molecular cloning and functional analysis of polymeric immunoglobulin receptor, pIgR, gene in mandarin fish Siniperca chuatsi

Polymeric immunoglobulin receptor (pIgR) can bind and transport immunoglobulins (Igs), thus playing a role in mucosal immunity. In this study, pIgR gene was cloned in mandarin fish, Siniperca chuatsi, with the open reading frame (ORF) of 1011 bp, encoding 336 amino acids. The pIgR protein consists of a signal peptide, an extracellular domain, a transmembrane domain and an intracellular region, with the presence of two Ig-like domains (ILDs) in the extracellular domain, as reported in other species of fish. The pIgR gene was expressed in all organs/tissues of healthy mandarin fish, with higher level observed in liver and spleen. Following the immersion infection of Flavobacterium columnare, pIgR transcripts were detected in immune related, especially mucosal tissues, with significantly increased transcription during the first two days of infection. Through transfection of plasmids expressing pIgR, IgT and IgM, pIgR was found to be interacted with IgT and IgM as revealed by co-immunoprecipitation and immunofluorescence.

Stimulator of interferon genes from Asian swamp eel (MaSTING) is involved in host defense against bacterial infection

Stimulator of interferon genes (STING) is an endoplasmic reticulum (ER)-associated protein that plays critical roles in innate immunity and pathogenesis of various diseases. To date, teleost STING against viral stimulation has been identified, whereas STING signaling events in fish against bacteria are not well understood. In the present study, the open reading frame (ORF) of STING from Asian swamp eel (Monopterus albus) was cloned (named MaSTING) and its roles in bacterial infection were investigated. Amino acid sequence alignment and phylogenetic analysis revealed that MaSTING had conserved structures with mammalian STING and shared the closest relationship with mandarin fish STING. Subcellular localization analysis showed that MaSTING distributed in the whole cytoplasm and mainly co-localized with ER. Expression pattern analysis found that MaSTING was constitutively expressed in all the examined tissues with the highest expression in the liver and spleen. Post stimulation with bacteria and various PAMPs, the expression of MaSTING was induced at indicated time points in the immune-related organs and isolated peripheral blood leucocytes. Furthermore, the mechanism underlying MaSTING against bacterial infection was further studied. The qPCR analysis showed that MaSTING overexpression promoted 2'3'-cGAMP induced the expression of IFN-1, ISG15, Viperin, Mx, IL-1β and TNF-α. Western blotting assay suggested that MaSTING significantly enhanced the phosphorylation of TANK-binding kinase 1 (TBK1) and p65. MaSTING also significantly increased the luciferase activity of IFN-1 and NF-κB promoters. Taken together, MaSTING is involved in host defense against bacterial infection by inducing the inflammatory response.

Transcriptome profiling and differential expression analysis of altered immune-related genes in goldfish (Carassius auratus) infected with Aeromonas hydrophila

Goldfish (Carassius auratus) have been employed as a model organism to investigate the innate immune system and host-pathogen interactions. A Gram-negative bacterium called Aeromonas hydrophila has been found to cause mass mortality due to infection in a wide variety of fish species in the aquatic system. In this study, damages in Bowman's capsule, inflammatory tubular (proximal and distilled convoluted) structure, and glomerular necrosis were observed in A. hydrophila-infected head kidney of goldfish. To increase the better understanding of immune mechanisms of host defense against A. hydrophila, we performed a transcriptome analysis in head kidney of goldfish at 3 and 7 days of post-infection (dpi). Comparing to the control group, 4638 and 2580 differentially expressed genes (DEGs) were observed at 3 and 7 dpi, respectively. The DEGs were subsequently enriched in multiple immune-related pathways including Protein processing in endoplasmic reticulum, Insulin signaling pathway, and NOD-like receptor signaling pathway. The expression profile of immune-related genes such as TRAIL, CCL19, VDJ recombination-activating protein 1-like, Rag-1, and STING was validated by qRT-PCR. Furthermore, the levels of immune-related enzyme (LZM, AKP, SOD, and CAT) activities were examined at 3 and 7 dpi. The knowledge gained from the current study will be helpful for better understanding of early immune response in goldfish after A. hydrophila challenge, which will aid in future research on prevention strategies in teleost.

CRISPR/Cas9-Induced Knockout of Sting Increases Susceptibility of Zebrafish to Bacterial Infection

Stimulator of interferon genes (STING) is an adapter protein that is activated when cyclic dinucleotides (CDNs) are present. CDNs originate from the cytosolic DNA of both pathogens and hosts. STING activation promotes efficient immune responses against viral infections; however, its impact in bacterial infections is unclear. In this study, we investigated the role of Sting in bacterial infections by successfully creating a sting-deficient (sting^(-/-) with a 4-bp deletion) knockout zebrafish model using CRISPR/Cas9. The transcriptional modulation of genes downstream of cGAS (cyclic GMP-AMP synthase)-Sting pathway-related genes was analyzed in seven-day-old wild-type (WT) and sting^(-/-) embryos, as well as in four-day-old LPS-stimulated embryos. The expression of downstream genes was higher in sting^(-/-) than in healthy WT fish. The late response was observed in sting^(-/-) larvae following LPS treatment, demonstrating the importance of Sting-induced immunity during bacterial infection by activating the cGAS-STING pathway. Furthermore, adult sting^(-/-) fish had a high mortality rate and significantly downregulated cGAS-STING pathway-related genes during Edwardsiella piscicida (E. piscicida) infection. In addition, we assessed NF-κB pathway genes following E. piscicida infection. Our results show fluctuating patterns of interleukin-6 (il6) and tumor necrosis factor-α (tnfα) expression, which is likely due to the influence of other NF-κB pathway-related immune genes. In summary, this study demonstrates the important role of Sting against bacterial infection.

Spring Viremia of Carp Virus N Protein Negatively Regulates IFN Induction through Autophagy-Lysosome-Dependent Degradation of STING

Fish possess a powerful IFN system to defend against aquatic virus infections. Nevertheless, spring viremia of carp virus (SVCV) causes large-scale mortality in common carp and significant economic losses to aquaculture. Therefore, it is necessary to investigate the strategies used by SVCV to escape the IFN response. In this study, we show that the SVCV nucleoprotein (N protein) negatively regulates cellular IFN production by degrading stimulator of IFN genes (STING) via the autophagy-lysosome-dependent pathway. First, overexpression of N protein inhibited the IFN promoter activation induced by polyinosinic-polycytidylic acid and STING. Second, the N protein associated with STING and experiments using a dominant-negative STING mutant demonstrated that the N-terminal transmembrane domains of STING were indispensable for this interaction. Then, the N protein degraded STING in a dose-dependent and autophagy-lysosome-dependent manner. Intriguingly, in the absence of STING, individual N proteins could not elicit host autophagic flow. Furthermore, the autophagy factor Beclin1 was found to interact with the N protein to attenuate N protein-mediated STING degradation after beclin1 knockdown. Finally, the N protein remarkably weakened STING-enhanced cellular antiviral responses. These findings reveal that SVCV uses the host autophagic process to achieve immune escape, thus broadening our understanding of aquatic virus pathogenesis.

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.

Molecular and functional characterization of zinc finger aspartate-histidine-histidine-cysteine (DHHC)-type containing 1, ZDHHC1 in Chinese perch Siniperca chuatsi

In the present study, the zinc finger aspartate-histidine-histidine-cysteine (DHHC)-type containing 1 (ZDHHC1) gene was identified in a commercial fish, the Chinese perch Siniperca chuatsi. The ZDHHC1 has five putative transmembrane motifs and conserved DHHC domain, showing high amino-acid identity with other teleost fish, and vertebrate ZDHHC1 loci are conserved from fish to human. In vivo expression analysis indicated that ZDHHC1 gene was constitutively transcribed in all the examined organs/tissues, and was induced following infectious spleen and kidney necrosis virus (ISKNV) infection. It is further observed that ZDHHC1 interacts with MITA and the overexpression of ZDHHC1 in cells resulted in the upregulated expression of ISGs, such as Mx, RSAD2, IRF3 and type I IFNs such as IFNh and IFNc, exhibiting its antiviral function in fish as reported in mammals.

The dynamic immune responses of Mandarin fish (Siniperca chuatsi) to ISKNV in early infection based on full-length transcriptome analysis and weighted gene co-expression network analysis

Mandarin fish (Siniperca chuatsi) been seriously harmed by infectious spleen and kidney necrosis virus (ISKNV) in recent years, but the early immune response mechanism of infection is still unknown. Here, we performed RNA sequencing on the spleens of mandarin fish infected with ISKNV at 0, 12, 24, 48, and 72 h post-infection (hpi) using short-read Illumina RNA sequencing and long-read Pacific Biosciences isoform sequencing to generate a full-length transcriptome. The immune responses of mandarin fish infected with ISKNV at the molecular level were characterized by RNA-seq analysis and weighted gene co-expression network analysis (WGCNA). A total of 26,528 full-length transcript sequences were obtained. There were 2,729 (1,680 up-regulated and 1,112 down-regulated), 1,874 (1,136 up-regulated and 738 down-regulated), 2,032 (1,158 up-regulated and 847 down-regulated), and 4,176 (2,233 up-regulated and 1,943 down-regulated) differentially expressed genes (DEGs) in mandarin fish at 12, 24, 48, and 72 hpi, compared with uninfected fish, respectively. A total of four modules of co-expressed DEGs identified by WGCNA were significantly positively correlated to the four time points after infection, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the immune-related DEGs in all these modules were mainly enriched in Phagosome, Endocytosis, Herpes simplex infection, and Cytokine-cytokine receptor interaction pathways. Further analysis showed that oher signaling pathways, including CAMs, NOD-like receptor and ER protein processing, Intestinal immune network for IgA production, TLR pathway, and Apoptosis significantly enriched in four modules corresponding to 12, 24, 48, and 72 hpi respectively, had specifically participated in the immune response. Hub genes identified based on the high-degree nodes in the WGCN, including CAM3, IL-8, CCL21, STING, SNX1, PFR and TBK1, and some DEGs such as MHCI, MHCII, TfR, STING, TNF α, TBK1, IRF1, and NF-kB, BCR, IgA and Bcl-XL had involved in dynamic molecular response of mandarin fish to ISKNV infection. In sum, this study provides a set of full-length transcriptome of the spleen tissue of mandarin fish for the first time and revealed a group of immune genes and pathways involved in different temporal responses to ISKNV infection, which has implications for resource conservation and aiding the development of strategies to prevent virus early infection for mandarin fish.