Grass Carp Laboratory of Genetics and Physiology 2 Serves As a Negative Regulator in Retinoic Acid-Inducible Gene I- and Melanoma Differentiation-Associated Gene 5-Mediated Antiviral Signaling in Resting State and Early Stage of Grass Carp Reovirus Infection

LGP2 Facilitates Bacterial Escape through Binding Peptidoglycan via EEK Motif and Suppressing NOD2-RIP2 Axis in Cyprinidae and Xenocyprididae Families

RIG-I-like receptors and NOD-like receptors play pivotal roles in recognizing microbe-associated molecular patterns and initiating immune responses. The LGP2 and NOD2 proteins are important members of the RIG-I-like receptor and NOD-like receptor families, recognizing viral RNA and bacterial peptidoglycan (PGN), respectively. However, in some instances bacterial infections can induce LPG2 expression via a mechanism that remains largely unknown. In the current study, we found that LGP2 can compete with NOD2 for PGN binding and inhibit antibacterial immunity by suppressing the NOD2-RIP2 axis. Recombinant CiLGP2 (Ctenopharyngodon idella LGP2) produced using either prokaryotic or eukaryotic expression platform can bind PGN and bacteria in pull-down and ELISA assays. Comparative protein structure models and intermolecular interaction prediction calculations as well as pull-down and colocalization experiments indicated that CiLGP2 binds PGN via its EEK motif with species and structural specificity. EEK deletion abolished PGN binding of CiLGP2, but insertion of the CiLGP2 EEK motif into zebrafish and mouse LGP2 did not confer PGN binding activity. CiLGP2 also facilitates bacterial replication by interacting with CiNOD2 to suppress expression of NOD2-RIP2 pathway genes. Sequence analysis and experimental verification demonstrated that LGP2 having EEK motif that can negatively regulate antibacterial immune function is present in Cyprinidae and Xenocyprididae families. These results show that LGP2 containing EEK motif competes with NOD2 for PGN binding and suppresses antibacterial immunity by inhibiting the NOD2-RIP2 axis, indicating that LGP2 plays a crucial negative role in antibacterial response beyond its classical regulatory function in antiviral immunity.

TLR7 neo-functionalizes to sense dsRNA and trigger antiviral and antibacterial immunity in non-tetrapod vertebrates

TLR7 plays a crucial role in sensing viral ssRNA and initiating immune responses. Piscine TLR7 also responds to dsRNA challenge. dsRNA exists in almost all the viruses at specific stages. However, the mechanism on sensing dsRNA by TLR7 remains unknown. In the present study, we employed Ctenopharyngodon idella TLR7 (CiTLR7) to systematically explore the immune functions and mechanisms in teleost. CiTLR7 can directly bind not only ssRNA but also dsRNA at different patches in lysosome, recruit MyD88 as adaptor, and activate the downstream IFN pathway via SLC15A4/TASLa/TASLb/IRF5/IRF7 complex for antiviral and antibacterial infections and AP-1 pathway for pro-inflammatory cytokines. The key binding sites for dsRNA are L29 and L811 in CiTLR7. Further, we found that the function on recognizing dsRNA by TLR7 emerges in pisciformes and loses in tetrapods in evolution. This is the first report on sensing both ssRNA and dsRNA by a TLR member.

Transcriptome analysis identifies LGP2 as an MDA5-mediated signaling activator following spring viremia of carp virus infection in common carp (Cyprinus carpio L.)

The common carp (Cyprinus carpio L.) is an important farmed species worldwide. Mucosal-associated lymphoid tissues play an essential role in the fight against pathogen infection. Spring viremia of carp virus (SVCV) poses a serious threat to the common carp aquaculture industry. Understanding the molecular mechanisms driving mucosal immune responses to SVCV infection is critical. In this study, the mucosal tissues (gills, foregut and hindgut) were collected from normal and infected fishes for transcriptome analysis. A total of 932,378,600 clean reads were obtained, of which approximately 80% were successfully mapped to the common carp genome. 577, 1,054 and 1,014 differential expressed genes (DEGs) were identified in the gills, foregut and hindgut, respectively. A quantitative polymerase chain reaction assay indicated that the DEGs expression in the foregut following SVCV infection was consistent with the transcriptome results. Among them, two key genes of the retinoic acid-inducible gene I (RIG-I)-like receptor family, melanoma-differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) (i.e., CcMDA5 and CcLGP2), underwent further analysis. Overexpression of CcMDA5 or CcLGP2 increased phosphorylation of TANK-binding kinase 1 and interferon regulatory factor 3 and the expression of interferon-1 (ifn-1), myxovirus resistance (mx), viperin and interferon-stimulated gene 15 (isg15), and inhibited SVCV replication in epithelioma papulosum cyprini cells. Furthermore, CcLGP2 significantly upregulated the CcMDA5-induced ifn-1 mRNA expression and the activation of the ifn-1 promoter. Finally, confocal microscopy and coimmunoprecipitation experiments revealed that CcLGP2 colocalizes and interacts with CcMDA5 via the C-terminal regulatory domain. This study provides essential gene resources for understanding the fish immune response to SVCV infection and sheds light on the potential role of fish LGP2 in the MDA5 regulation.

Function conservation and disparities of zebrafish and human LGP2 genes in fish and mammalian cells responsive to poly(I:C)

Retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) are viral RNA sensors that regulate host interferon (IFN)-mediated antiviral signaling. LGP2 (laboratory genetics and physiology 2) lacks the N-terminal caspase activation and recruitment domains (CARDs) responsible for signaling transduction in the other two RLR proteins, RIG-I and melanoma differentiation associated gene-5 (MDA5). How LGP2 regulates IFN signaling is controversial, and inconsistent results have often been obtained in overexpression assays when performed in fish cells and mammalian cells. Here we report that the differential sensitivity of fish cells and mammalian cells to poly(I:C) transfection conceals the function conservation of zebrafish and human LGP2. In fish cells, overexpression of zebrafish or human LGP2 initially activates IFN signaling in a dose-dependent manner, followed by inhibition at a critical threshold of LGP2 expression. A similar trend exists for LGP2-dependent IFN induction in response to stimulation by low and high concentrations of poly(I:C). In contrast, overexpression of zebrafish or human LGP2 alone in mammalian cells does not activate IFN signaling, but co-stimulation with very low or very high concentrations of poly(I:C) shows LGP2-dependent enhancement or inhibition of IFN signaling, respectively. Titration assays show that LGP2 promotes MDA5 signaling in mammalian cells mainly under low concentration of poly(I:C) and inhibits RIG-I/MDA5 signaling mainly under high concentration of poly(I:C). Our results suggest that fish and human LGP2s switch regulatory roles from a positive one to a negative one in increasing concentrations of poly(I:C)-triggered IFN response.

Grass Carp Reovirus Nonstructural Proteins Avoid Host Antiviral Immune Response by Targeting the RLR Signaling Pathway

Grass carp reovirus (GCRV) is a highly virulent RNA virus that mainly infects grass carp and causes hemorrhagic disease. The roles of nonstructural proteins NS38 and NS80 of GCRV-873 in the viral replication cycle and viral inclusion bodies have been established. However, the strategies that NS38 and NS80 used to avoid host antiviral immune response are still unknown. In this study, we report the negative regulations of NS38 and NS80 on the RIG-I-like receptors (RLRs) antiviral signaling pathway and the production of IFNs and IFN-stimulated genes. First, both in the case of overexpression and GCRV infection, NS38 and NS80 inhibited the IFN promoter activation induced by RIG-I, MDA5, MAVS, TBK1, IRF3, and IRF7 and mRNA abundance of key antiviral genes involved in the RLR-mediated signaling. Second, both in the case of overexpression and GCRV infection, NS38 interacted with piscine TBK1 and IRF3, but not with piscine RIG-I, MDA5, MAVS, and TNF receptor-associated factor (TRAF) 3. Whereas NS80 interacted with piscine MAVS, TRAF3, and TBK1, but not with piscine RIG-I, MDA5, and IRF3. Finally, both in the case of overexpression and GCRV infection, NS38 inhibited the formation of the TBK1-IRF3 complex, but NS80 inhibited the formation of the TBK1-TRAF3 complex. Most importantly, NS38 and NS80 could hijack piscine TBK1 and IRF3 into the cytoplasmic viral inclusion bodies and inhibit the translocation of IRF3 into the nucleus. Collectively, all of these data demonstrate that GCRV nonstructural proteins can avoid host antiviral immune response by targeting the RLR signaling pathway, which prevents IFN-stimulated gene production and facilitates GCRV replication.

Grass Carp Reovirus VP56 Allies VP4, Recruits, Blocks, and Degrades RIG-I to More Effectively Attenuate IFN Responses and Facilitate Viral Evasion

Grass carp reovirus (GCRV), the most virulent aquareovirus, causes epidemic hemorrhagic disease and tremendous economic loss in freshwater aquaculture industry. VP56, a putative fibrin inlaying the outer surface of GCRV-II and GCRV-III, is involved in cell attachment. In the present study, we found that VP56 localizes at the early endosome, lysosome, and endoplasmic reticulum, recruits the cytoplasmic viral RNA sensor retinoic acid-inducible gene I (RIG-I) and binds to it. The interaction between VP56 and RIG-I was detected by endogenous coimmunoprecipitation (co-IP), glutathione S-transferase (GST) pulldown, and subsequent liquid chromatography-tandem mass spectrometry (LC-MS/MS) and was then confirmed by traditional co-IPs and a novel far-red mNeptune-based bimolecular fluorescence complementation system. VP56 binds to the helicase domain of RIG-I. VP56 enhances K48-linked ubiquitination of RIG-I to degrade it by the proteasomal pathway. Thus, VP56 impedes the initial immune function of RIG-I by dual mechanisms (blockade and degradation) and attenuates signaling from RIG-I recognizing viral RNA, subsequently weakening downstream signaling transduction and interferon (IFN) responses. Accordingly, host antiviral effectors are reduced, and cytopathic effects are increased. These findings were corroborated by RNA sequencing (RNA-seq) and VP56 knockdown. Finally, we found that VP56 and the major outer capsid protein VP4 bind together in the cytosol to enhance the degradation of RIG-I and more efficiently facilitate viral replication. Collectively, the results indicated that VP56 allies VP4, recruits, blocks, and degrades RIG-I, thereby attenuating IFNs and antiviral effectors to facilitate viral evasion more effectively. This study reveals a virus attacking target and an escaping strategy from host antiviral immunity for GCRV and will help understand mechanisms of infection of reoviruses. IMPORTANCE Grass carp reovirus (GCRV) fibrin VP56 and major outer capsid protein VP4 inlay and locate on the outer surface of GCRV-II and GCRV-III, which causes tremendous loss in grass carp and black carp industries. Fibrin is involved in cell attachment and plays an important role in reovirus infection. The present study identified the interaction proteins of VP56 and found that VP56 and VP4 bind to the different domains of the viral RNA sensor retinoic acid-inducible gene I (RIG-I) in grass carp to block RIG-I sensing of viral RNA and induce RIG-I degradation by the proteasomal pathway to attenuate signaling transduction, thereby suppressing interferons (IFNs) and antiviral effectors, facilitating viral replication. VP56 and VP4 bind together in the cytosol to more efficiently facilitate viral evasion. This study reveals a virus attacking a target and an escaping strategy from host antiviral immunity for GCRV and will be helpful in understanding the mechanisms of infection of reoviruses.

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.

Teleost-Specific MxG, a Traitor in the Mx Family, Negatively Regulates Antiviral Responses by Targeting IPS-1 for Proteasomal Degradation and STING for Lysosomal Degradation

IFN-β promoter stimulator-1 (IPS-1)- and stimulator of IFN genes (STING)-mediated type I IFNs play a critical role in antiviral responses. Myxovirus resistance (Mx) proteins are pivotal components of the antiviral effectors induced by IFNs in many species. An unprecedented expansion of Mx genes has occurred in fish. However, the functions and mechanisms of Mx family members remain largely unknown in fish. In this study, we found that grass carp (Ctenopharyngodon idella) MxG, a teleost-specific Mx protein, is induced by IFNs and viruses, and it negatively regulates both IPS-1- and STING-mediated antiviral responses to facilitate grass carp reovirus, spring viremia of carp virus, and cyprinid herpesvirus-2 replication. MxG binds and degrades IPS-1 via the proteasomal pathway and STING through the lysosomal pathway, thereby negatively regulating IFN1 antiviral responses and NF-κB proinflammatory cytokines. MxG also suppresses the phosphorylation of STING IFN regulatory factor 3/7, and it subsequently downregulates IFN1 and NF-κB1 at the promoter, transcription, and protein levels. GTPase and GTPase effector domains of MxG contribute to the negative regulatory function. On the contrary, MxG knockdown weakens virus replication and cytopathic effect. Therefore, MxG can be an ISG molecule induced by IFNs and viruses, and degrade IPS-1 and STING proteins in a negative feedback manner to maintain homeostasis and avoid excessive immune responses after virus infection. To our knowledge, this is the first identification of a negative regulator in the Mx family, and our findings clarify a novel mechanism by which the IFN response is regulated.

Type II Grass Carp Reovirus Infects Leukocytes but Not Erythrocytes and Thrombocytes in Grass Carp (Ctenopharyngodon idella)

Grass carp reovirus (GCRV) causes serious losses to the grass carp industry. At present, infectious tissues of GCRV have been studied, but target cells remain unclear. In this study, peripheral blood cells were isolated, cultured, and infected with GCRV. Using quantitative real-time polymerase chain reaction (qRT-PCR), Western Blot, indirect immunofluorescence, flow cytometry, and transmission electron microscopy observation, a model of GCRV infected blood cells in vitro was established. The experimental results showed GCRV could be detectable in leukocytes only, while erythrocytes and thrombocytes could not. The virus particles in leukocytes are wrapped by empty membrane vesicles that resemble phagocytic vesicles. The empty membrane vesicles of leukocytes are different from virus inclusion bodies in C. idella kidney (CIK) cells. Meanwhile, the expression levels of IFN1, IL-1β, Mx2, TNFα were significantly up-regulated in leukocytes, indicating that GCRV could cause the production of the related immune responses. Therefore, GCRV can infect leukocytes in vitro, but not infect erythrocytes and thrombocytes. Leukocytes are target cells in blood cells of GCRV infections. This study lays a theoretical foundation for the study of the GCRV infection mechanism and anti-GCRV immunity.

Deciphering the Fine-Tuning of the Retinoic Acid-Inducible Gene-I Pathway in Teleost Fish and Beyond

Interferons are the first lines of defense against viral pathogen invasion during the early stages of infection. Their synthesis is tightly regulated to prevent excessive immune responses and possible deleterious effects on the host organism itself. The RIG-I-like receptor signaling cascade is one of the major pathways leading to the production of interferons. This pathway amplifies danger signals and mounts an appropriate innate response but also needs to be finely regulated to allow a rapid return to immune homeostasis. Recent advances have characterized different cellular factors involved in the control of the RIG-I pathway. This has been most extensively studied in mammalian species; however, some inconsistencies remain to be resolved. The IFN system is remarkably well conserved in vertebrates and teleost fish possess all functional orthologs of mammalian RIG-I-like receptors as well as most downstream signaling molecules. Orthologs of almost all mammalian regulatory components described to date exist in teleost fish, such as the widely used zebrafish, making fish attractive and powerful models to study in detail the regulation and evolution of the RIG-I pathway.

A novel role of Zebrafish TMEM33 in negative regulation of interferon production by two distinct mechanisms

The transmembrane protein 33 (TMEM33) was originally identified as an endoplasmic reticulum (ER) protein that influences the tubular structure of the ER and modulates intracellular calcium homeostasis. However, the role of TMEM33 in antiviral immunity in vertebrates has not been elucidated. In this article, we demonstrate that zebrafish TMEM33 is a negative regulator of virus-triggered interferon (IFN) induction via two mechanisms: mitochondrial antiviral signaling protein (MAVS) ubiquitination and a decrease in the kinase activity of TANK binding kinase 1 (TBK1). Upon stimulation with viral components, tmem33 was remarkably upregulated in the zebrafish liver cell line. The IFNφ1 promoter (IFNφ1pro) activity and mRNA level induced by retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) were significantly inhibited by TMEM33. Knockdown of TMEM33 increased host ifn transcription. Subsequently, we found that TMEM33 was colocalized in the ER and interacted with the RLR cascades, whereas MAVS was degraded by TMEM33 during the K48-linked ubiquitination. On the other hand, TMEM33 reduced the phosphorylation of mediator of IFN regulatory factor 3 (IRF3) activation (MITA)/IRF3 by acting as a decoy substrate of TBK1, which was also phosphorylated. A functional domain assay revealed that the N-terminal transmembrane domain 1 (TM1) and TM2 regions of TMEM33 were necessary for IFN suppression. Finally, TMEM33 significantly attenuated the host cellular antiviral capacity by blocking the IFN response. Taken together, our findings provide insight into the different mechanisms employed by TMEM33 in cellular IFN-mediated antiviral process.

Grass Carp (Ctenopharyngodon idellus) NIMA-Related Kinase 6 Blocks dsRNA-Induced IFN I Response by Targeting IRF3

Accumulating evidence indicates that mammalian NIMA (never in mitosis, gene A)-related kinase 6 (NEK6) plays potential roles during the course of tumorigenesis, but little is known about NEK6 in lower vertebrates. Herein, we reported a mammalian ortholog of NEK6 in grass carp (Ctenopharyngodon idellus) (CiNEK6). Multiple alignment of amino acid sequences and phylogenetic analysis showed that CiNEK6 shares a high level of sequence similarity with its counterparts in birds. CiNEK6 was ubiquitously expressed in all tested tissues, and its expression level was increased under treatment with GCRV (dsRNA virus) or poly I:C (dsRNA analog). Q-PCR and dual-luciferase assays suggested that CiNEK6 overexpression suppressed IFN I activity in CIK cells treated with poly I:C. Knockdown of CiNEK6 resulted in a higher level of IFN I expression in CIK cells treated with poly I:C compared to those which received PBS. Interestingly, analysis of subcellular localization demonstrated that CiNEK6 protein scattered throughout the cytoplasm is gradually congregated together at the edges of karyotheca upon stimulation with poly I:C. Co-IP and co-localization assays suggested that CiNEK6 interacts with CiIRF3 after poly I:C challenge. In poly I:C-treated cells, the phosphorylation of CiIRF3 was increased by CiNEK6 knockdown, but was suppressed by CiNEK6 overexpression, suggesting that CiNEK6 decreases IFN I expression through inhibiting CiIRF3 activity. Cell viability assay, crystal violet staining, and detection of Vp5 also showed that CiNEK6 plays an inhibitory role in IRF3-mediated antiviral responses.

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.

Hamp Type-1 Promotes Antimicrobial Defense via Direct Microbial Killing and Regulating Iron Metabolism in Grass Carp (Ctenopharyngodon idella)

Hepcidin is an antimicrobial peptide and regulator of iron homeostasis which has two isoforms in most fishes and some mammals. Previous studies have reported that the two hepcidin isoforms have different roles. Hamp type-1 plays a regulatory role in iron metabolism and hamp type-2 mostly performs an antimicrobial role. In this study, we found that Ctenopharyngodon idella (C. idella) have only one hepcidin isoform (hamp type-1), which showed both broad-spectrum antibacterial and iron regulatory functions. C. idella hepcidin mature peptide (hepcidin-25) and truncated peptide (hepcidin-20) exhibited bactericidal activities against both Gram-positive and Gram-negative bacteria in a dose-dependent manner in part through membrane rupture and binding to bacterial genomic DNA. The data from challenge tests demonstrated that the administration of hepcidin-25 significantly reduced mortality rates of C. idella by A. hydrophila infection, probably due to direct bactericidal activities of the peptide and a reduction of iron content in the fish serum. In addition, a comparison between hepcidin-20 and -25 suggests that the N terminal 5 amino acids play a critical role in reducing iron content in fish serum. Our findings revealed an important role of hamp type-1 in maintaining iron homeostasis and fighting against bacterial infections, suggesting the hepcidin has implications for the prevention and control of bacterial infection in aquaculture.

Grass Carp Reovirus Major Outer Capsid Protein VP4 Interacts with RNA Sensor RIG-I to Suppress Interferon Response

Diseases caused by viruses threaten the production industry and food safety of aquaculture which is a great animal protein source. Grass carp reovirus (GCRV) has caused tremendous loss, and the molecular function of viral proteins during infection needs further research, as for most aquatic viruses. In this study, interaction between GCRV major outer capsid protein VP4 and RIG-I, a critical viral RNA sensor, was screened out by GST pull-down, endogenous immunoprecipitation and subsequent LC-MS/MS, and then verified by co-IP and an advanced far-red fluorescence complementation system. VP4 was proved to bind to the CARD and RD domains of RIG-I and promoted K48-linked ubiquitination of RIG-I to degrade RIG-I. VP4 reduced mRNA and promoter activities of key genes of RLR pathway and sequential IFN production. As a consequence, antiviral effectors were suppressed and GCRV replication increased, resulting in intensified cytopathic effect. Furthermore, results of transcriptome sequencing of VP4 stably expressed CIK (C. idella kidney) cells indicated that VP4 activated the MyD88-dependent TLR pathway. Knockdown of VP4 obtained opposite effects. These results collectively revealed that VP4 interacts with RIG-I to restrain interferon response and assist GCRV invasion. This study lays the foundation for anti-dsRNA virus molecular function research in teleost and provides a novel insight into the strategy of immune evasion for aquatic virus.

Thoroughly Remold the Localization and Signaling Pathway of TLR22

TLR22 exists in nearly all the poikilothermic vertebrates and plays a central role in the initiation of innate immunity and activation of adaptive immunity. TLR22 signaling pathway has been characterized in detail in fugu (Takifugu rubripes). Here, we thoroughly remold the localization and signaling pathways of TLR22. We characterized TLR22a and TLR22b in grass carp (Ctenopharyngodon idella), designated as CiTLR22a and CiTLR22b, and explored the ligand(s), adaptor(s), and signaling pathway(s). Results show that both CiTLR22a and CiTLR22b localize to lysosome, acidic compartment. Correspondingly, CiTLR22a and CiTLR22b directly bind and respond to dsRNA analog poly(I:C) at pH 5, but not at pH 7.4, the physiological pH. Moreover, CiTLR22a and CiTLR22b exhibit antagonistic function in signal transmission, wherein CiTLR22a facilitates the protein and phosphorylation levels of IRF7 and enhances the promoter activities of major IFNs and NF-κBs, while CiTLR22b downregulates IRF7 phosphorylation and IRF3 protein level and suppresses the IFN and NF-κB pathways. Further investigations revealed that CiTLR22a restrains grass carp reovirus (GCRV) replication and protects cells from GCRV infection, whereas CiTLR22b plays a negative role in response to GCRV infection. This is the first time to systematically clarify the signaling pathways of two isotype TLR22s; especially, subcellular localization and adaptor are different from previous TLR22 report, which results from technical limitations. The results will serve the antiviral immune mechanisms in poikilothermic vertebrates and evolutionary immunology.

Pattern Recognition by Melanoma Differentiation-Associated Gene 5 (Mda5) in Teleost Fish: A Review

Teleost fish, as with other vertebrates, rely on their innate immune system as a first line of defense against invading pathogens. A very important characteristic of the innate immune response is its ability to recognize conserved molecular structures, such as viral dsRNA and ssRNA. Mda5 is one of the three pattern recognition receptors (PRRs) that recognize cytoplasmic viral ligands. Teleost Mda5 is widely conserved among several fish species and possesses the same structural domains as those seen in their mammalian counterparts. Fish Mda5 has been shown to be capable of initiating an inflammatory response both in vitro (in different fish cell lines) and in vivo using synthetic viral analogs or virus. The interferon (IFN) pathway is triggered as a result of Mda5 activation, leading to the expression of type I IFNs, IFN- stimulated genes and pro-inflammatory cytokines. Although it is known that Mda5 acts as a receptor for virally-produced ligands, it has been shown more recently that it can also initiate an immune response against bacterial challenges. This review discusses recent advances in the characterization of teleost Mda5 and its potential role in antiviral and antibacterial immunity in teleost fish.

Knockdown of IGF-1R Triggers Viral RNA Sensor MDA5- and RIG-I-Mediated Mitochondrial Apoptosis in Colonic Cancer Cells

The important role of insulin-like growth factor-1 receptor (IGF-1R) in tumorigenesis has been well established. The classical model involves IGF-1R binding to IGF-1/2, the following activation of PI3K-Akt-signaling cascades, driving cell proliferation and apoptosis inhibition. Here we report a new signal transduction pathway of IGF-1R in the intestinal epithelium. Using heterozygous knockout mice (Igf1r^+/-), we analyzed the expressions of viral RNA sensors MDA5 and RIG-I in the intestinal epithelium. Igf1r^+/- mice exhibited higher MDA5 and RIG-I than wild-type (WT) mice, indicating that knockdown of IGF-1R could trigger MDA5 and RIG-I. IGF-1R knockdown-triggered MDA5 and RIG-I were further investigated in human colonic cancer cells. Increased MDA5 and RIG-I were clearly seen in the cytoplasm in cancer cells as well as normal human colonic cells with silenced IGF-1R. Notably, the upregulations of MDA5 and RIG-I was not affected by blockage of the PI3K-Akt pathway with LY294002. These results suggested a new signal transduction pathway of IGF-1R. Importantly, IGF-1R knockdown-triggered MDA5 and RIG-I resulted in colorectal cancer apoptosis through activation of the mitochondrial pathway. These in vitro observations were evidenced in the azoxymethane (AOM)-dextran sulfate sodium (DSS) colorectal cancer model of mice. In conclusion, knockdown of IGF-1R triggers viral RNA sensor MDA5- and RIG-I-mediated mitochondrial apoptosis in cancer cells.

ROS-induced HSP70 promotes cytoplasmic translocation of high-mobility group box 1b and stimulates antiviral autophagy in grass carp kidney cells

Autophagy plays many physiological and pathophysiological roles. However, the roles and the regulatory mechanisms of autophagy in response to viral infections are poorly defined in teleost fish, such as grass carp (Ctenopharyngodon idella), which is one of the most important aquaculture species in China. In this study, we found that both grass carp reovirus (GCRV) infection and hydrogen peroxide (H₂O₂) treatment induced the accumulation of reactive oxygen species (ROS) in C. idella kidney cells and stimulate autophagy. Suppressing ROS accumulation with N-acetyl-l-cysteine significantly inhibited GCRV-induced autophagy activation and enhanced GCRV replication. Although ROS-induced autophagy, in turn, restricted GCRV replication, further investigation revealed that the multifunctional cellular protein high-mobility group box 1b (HMGB1b) serves as a heat shock protein 70 (HSP70)-dependent, pro-autophagic protein in grass carp. Upon H₂O₂ treatment, cytoplasmic HSP70 translocated to the nucleus, where it interacted with HMGB1b and promoted cytoplasmic translocation of HMGB1b. Overexpression and siRNA-mediated knockdown assays indicated that HSP70 and HMGB1b synergistically enhance ROS-induced autophagic activation in the cytoplasm. Moreover, HSP70 reinforced an association of HMGB1b with the C. idella ortholog of Beclin 1 (a mammalian ortholog of the autophagy-associated yeast protein ATG6) by directly interacting with C. idella Beclin 1. In summary, this study highlights the antiviral function of ROS-induced autophagy in response to GCRV infection and reveals the positive role of HSP70 in HMGB1b-mediated autophagy initiation in teleost fish.

Alternative Splicing Transcripts of Zebrafish LGP2 Gene Differentially Contribute to IFN Antiviral Response

In mammals, RIG-I like receptors (RLRs) RIG-I and melanoma differentiation-associated gene 5 (MDA5) sense cytosolic viral RNA, leading to IFN antiviral response; however, LGP2 exhibits controversial functions. The same happens to fish LGP2. In this study we report that three zebrafish LGP2 splicing transcripts, a full-length LGP2, and two truncating variants, LGP2v1 and LGP2v2, play distinct roles during IFN antiviral response. Overexpression of the full-length LGP2 not only potentiates IFN response through the RLR pathway, in the absence or presence of poly(I:C) at limited concentrations, but also inhibits IFN response by relative high concentrations of poly(I:C) through functional attenuation of signaling factors involved in the RLR pathway; however, LGP2v1 and LGP2v2 only retain the inhibitory role. Consistently, LGP2 but not LGP2v1 and LGP2v2 confers protection on fish cells against spring viremia of carp virus (SVCV) infection and at limited expression levels, LGP2 exerts more significant protection than either RIG-I or MDA5. Further data suggest that in the early phase of SVCV infection, LGP2 functions as a positive regulator but along with SVCV replicating in cells up to a certain titer, which leads to a far more robust expression of IFN, LGP2 switches to a negative role. These in vitro results suggest an ingenious mechanism where the three zebrafish LGP2 splicing transcripts work cooperatively to shape IFN antiviral responses.

Pattern recognition receptors in grass carp Ctenopharyngodon idella: I. Organization and expression analysis of TLRs and RLRs

Pattern recognition receptors (PRRs) play indispensable roles in the immune responses against invading pathogens. In the present study, we systematically identified and characterized Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) as well as their adaptors in grass carp (Ctenopharyngodon idella). A comprehensive analysis of BLAST and other bioinformatics methods showed that C. idella TLR family consist of 21 members and their adaptors contain four members. Phylogenetic analyses confirmed the existence of six TLR subfamilies (TLR1, 3, 4, 5, 7 and 11 subfamily) in C. idella and revealed their homologous relationships with other species. Most C. idella TLRs possess three typical structural features of TLR protein family: LRR, TM and TIR domains. Meanwhile, RLR family consist of three conserved members (RIG-I, MDA5 and LGP2) as well as two adaptors (IPS-1 and STING) in C. idella. mRNA expression analyses of TLRs, RLRs and their adaptors indicated that most members are sustainably expressed in multiple tissues before and after grass carp reovirus (GCRV) or Aeromonas hydrophila infection, while TLR9, TLR20a/b, TLR25, TIRAP, SARM1 and STING are transiently expressed in specific tissues. TLRs are transmembrane receptors with few introns, while RLRs are cytoplasmic receptors with plenty of introns. TLRs and RLRs interact with adaptors to perform their functions via various signaling pathways. In conclusion, this study systematically explores the characteristics of TLRs and RLRs in C. idella and provides evidence for the response patterns after viral and/or bacterial infection in vivo. These results contribute to studying the regulation mechanisms of TLR and RLR signaling pathways, and deeply understanding fish immune responses against pathogen infection.

Alternative Pre-mRNA Splicing in Mammals and Teleost Fish: A Effective Strategy for the Regulation of Immune Responses Against Pathogen Infection

Pre-mRNA splicing is the process by which introns are removed and the protein coding elements assembled into mature mRNAs. Alternative pre-mRNA splicing provides an important source of transcriptome and proteome complexity through selectively joining different coding elements to form mRNAs, which encode proteins with similar or distinct functions. In mammals, previous studies have shown the role of alternative splicing in regulating the function of the immune system, especially in the regulation of T-cell activation and function. As lower vertebrates, teleost fish mainly rely on a large family of pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) from various invading pathogens. In this review, we summarize recent advances in our understanding of alternative splicing of piscine PRRs including peptidoglycan recognition proteins (PGRPs), nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) and their downstream signaling molecules, compared to splicing in mammals. We also discuss what is known and unknown about the function of splicing isoforms in the innate immune responses against pathogens infection in mammals and teleost fish. Finally, we highlight the consequences of alternative splicing in the innate immune system and give our view of important directions for future studies.