Grouper Mx confers resistance to nodavirus and interacts with coat protein

Functional analysis of an orange-spotted grouper (Epinephelus coioides) interferon gene and characterisation of its expression in response to nodavirus infection

We cloned and sequenced 2C I-IFN, a two-cysteine containing type I interferon (I-IFN) gene, in orange-spotted grouper (Epinephelus coioides). The cDNA has 769 base pairs, the protein has 172 amino acids, and the predicted signal peptide has 18 amino acids with two cysteines. This gene is similar to I-FNs from sea bass and other teleosts. 2C I-IFN has 5 exons and 4 introns, also similar to other teleost I-IFNs. Immunohistochemical (IHC) analysis indicated that expression is predominantly membrane-localized in healthy grouper, but has a zonal distribution in nodavirus-infected grouper. Grouper infected with nodavirus had elevated levels of 2C I-IFN at 72 h and Mx at days 6-7. Recombinant 2C I-IFN activated grouper Mx, leading to upregulated antiviral activity. The grouper Mx promoter was highly induced after treatment with recombinant 2C I-IFN. The present results suggest that expression of grouper 2C I-IFN may participate in the immunologic barrier function against nodavirus.

Cloning and characterization of the Mx1, Mx2 and Mx3 promoters from gilthead seabream (Sparus aurata)

Mx proteins are main effectors of the antiviral innate immune response mediated by type I interferon (IFN I). Actually, diverse Mx proteins from fish proved highly active against fish viruses, standing out among them the Mx1, Mx2 and Mx3 from gilthead seabream (Sparus aurata), a species exhibiting a natural resistance to viral diseases. In this study, the structure and functional activity of their corresponding promoters (pMx1, pMx2 and pMx3) have been assessed. The three promoters present an identical 3' region of 157 bp, exhibiting a single canonical interferon-stimulated response element (ISRE), which is indispensible for the poli:IC induction of pMx1 and pMx3, while not for that of pMx2. In the remaining part of the three promoters other regulatory motifs were identified, as gamma IFN activated sites in variable number (1, 4 and 2 in pMx1, pMx2 and pMx3, respectively), as well as several independent GAAA elements or ISRE core sequences (13, 15 and 12 in pMx1, pMx2 and pMx3, respectively). The structural dissimilarities shown by the three promoters parallels with the differences observed in their response profiles, in terms of the time course of the induction, and basal and induced expression levels of each promoter. Altogether, these findings indicate that the expression of Mx1, Mx2 and Mx3 genes from the gilthead seabream might be specifically regulated, in accordance with the functional role of each Mx protein in the successful antiviral response shown by this species.

Mx proteins: antiviral gatekeepers that restrain the uninvited

Fifty years after the discovery of the mouse Mx1 gene, researchers are still trying to understand the molecular details of the antiviral mechanisms mediated by Mx proteins. Mx proteins are evolutionarily conserved dynamin-like large GTPases, and GTPase activity is required for their antiviral activity. The expression of Mx genes is controlled by type I and type III interferons. A phylogenetic analysis revealed that Mx genes are present in almost all vertebrates, usually in one to three copies. Mx proteins are best known for inhibiting negative-stranded RNA viruses, but they also inhibit other virus families. Recent structural analyses provide hints about the antiviral mechanisms of Mx proteins, but it is not known how they can suppress such a wide variety of viruses lacking an obvious common molecular pattern. Perhaps they interact with a (partially) symmetrical invading oligomeric structure, such as a viral ribonucleoprotein complex. Such an interaction may be of a fairly low affinity, in line with the broad target specificity of Mx proteins, yet it would be strong enough to instigate Mx oligomerization and ring assembly. Such a model is compatible with the broad "substrate" specificity of Mx proteins: depending on the size of the invading viral ribonucleoprotein complexes that need to be wrapped, the assembly process would consume the necessary amount of Mx precursor molecules. These Mx ring structures might then act as energy-consuming wrenches to disassemble the viral target structure.

Immunity to betanodavirus infections of marine fish

Betanodaviruses cause viral nervous necrosis in numerous fish species, but some species are resistant to infection by these viruses. It is essential to fully characterize the immune responses that underlie this protective response. Complete characterization of the immune responses against nodaviruses may allow the development of methods that stimulate fish immunity and of an effective betanodavirus vaccine. Such strategies could include stimulation of specific immune system responses or blockage of factors that decrease the immune response. The innate immune system clearly provides a front-line defense, and this includes the production of interferons and other cytokines. Interferons that are released inside infected cells and that suppress viral replication may be the most ancient form of innate immunity. This review focuses on the immune responses of fish to betanodavirus infection.

EcVig, a novel grouper immune-gene associated with antiviral activity against NNV infection

VHSV-induced genes (VIGs) were first identified in rainbow trout (Oncorhynchus mykiss) and subsequently isolated in a variety of fish. Recent studies have shown that most VIGs have immunological functions against pathogenic infections. However, most research has focused on Vig1, such that our present understanding of these genes in other fish species remains limited. This study isolated a homologue of the uncharacterized O. mykiss Vig-B319 (EcVig) from orange-spotted grouper (Epinephelus coioides). Genomic organization suggests that four EcVig isoforms (EcVig A-D), are generated through alternative splicing. Due to the encoding of 2 immunoglobulin (Ig) domains, the EcVig protein can be considered a member of the immunoglobulin superfamily. The expression of EcVig increased 3 days after hatching (dph) and peaked at 9 dph. This pattern is similar to that displayed by EcMx, an important grouper antiviral gene. Additionally, a tissue tropism assay revealed that EcVig A is the major EcVig isoform present in the tissues considered by this study, with the expression of EcVig A exceeding that of EcVig B. We subsequently investigated whether EcVig expression was induced by the viral pathogen nervous necrosis virus (NNV) or the bacterial pathogen Vibrio anguillarum. Following injection with NNV, the expression levels of EcVig showed significant up-regulation. Conversely, a significant reduction was observed in EcVig expression in brain samples collected from V. anguillarum injected grouper. The overexpression of EcVig A suppressed the replication of NNV in grouper GF-1 cell lines, suggesting that EcVig is an important antiviral factor in the grouper immune responses.

The biofunction of orange-spotted grouper (Epinephelus coioides) CC chemokine ligand 4 (CCL4) in innate and adaptive immunity

CC chemokine (motif) ligand 4 (CCL4) is indispensable to the chemoattraction of macrophages, natural killer cells, and lymphocytes in mammals; however, it has only been cloned in a limited number of fish species and information related to its biofunction remains ambiguous with regard to teleosts. To explore the role of teleost CCL4, we first evaluated the mRNA expression of the Epinephelus coioides CCL4 (gCCL4) gene in various organs under LPS and poly (I:C) stimulated; secondary, we evaluated the immune-related genes expression of fish under the recombinant gCCL4 protein stimulated. Our results revealed an increase in the mRNA of gCCL4 in immune organs immediately following stimulation by poly (I:C); however, in LPS stimulated fish, the expression did not increase until nearly 24 h after induction. In biofunction assays, recombinant gCCL4 was found to induce chemotactic activity in the peripheral blood leukocytes of groupers and up-regulate the gene expressions of grouper TNFA1 (TNF-α1), TNFA2 (TNF-α2), IFNG (IFN-γ), MX, TBX21 (T-bet), CD8 (α and β chain). These findings indicate that grouper CCL4 attracts leukocytes, induces an inflammatory response, and drives lymphocyte differentiation into the Th1 pathway.

Pathogen-driven adaptive evolution of myxovirus resistance (Mx) genes in fishes

Myxovirus resistance (Mx) proteins, which belong to the dynamin super-family, are known to inhibit RNA viral replication in a wide range of taxonomic groups, including fishes. Given their crucial role in host immune defense, the key amino acid residues in the GTP effector domain (GED) near the C-terminus are expected to evolve adaptively in order to protect the host against invading viral pathogens. The present study reveals the role of recombination and positive selection in the evolution of Mx proteins in fishes. While the GTP-binding domain in the N-terminal domain has experienced purifying selection, several amino acid residues in GED have evolved under positive selection, thus indicating adaptive evolution. Given the antiviral activity of GED, the adaptive evolutionary changes that were observed in this region are therefore predicted to be pathogen-driven.

Antiviral specificity of the Solea senegalensis Mx protein constitutively expressed in CHSE-214 cells

Interferons play a key role in fish resistance to viral infections by inducing the expression of antiviral proteins, such as Mx. The aim of the present study was to test the antiviral activity of the Senegalese sole Mx protein (SsMx) against RNA and DNA viruses pathogenic to fish, i.e. the infectious pancreatic necrosis virus (IPNV, dsRNA), the viral haemorrhagic septicaemia virus (VHSV, ssRNA), and the European sheatfish virus (ESV, dsDNA), using a CHSE-214 cell clone expressing this antiviral protein. A strong inhibition of IPNV and VHSV replication was recorded in SsMx-expressing cells, as has been shown by the virus yield reduction and the decrease in the synthesis of the viral RNA encoding the polyprotein (for IPNV) and the nucleoprotein (for VHSV). The titres of these viruses replicating on SsMx-expressing cells were 100 times lower than those recorded on non-transfected cells. In contrast, SsMx did not inhibit ESV replication since no significant differences were observed regarding the virus yield or the major capsid protein gene transcription in transfected and non-transfected cells.

Establishing a cell line from Atlantic cod as a novel tool for in vitro studies

The present work describes the generation of a cell line from newly hatched Atlantic cod (Gadus morhua) larvae (ACL cells). Primary cultures were initiated by explant outgrowth from partially minced tissues and subcultured cells were exposed to UV radiation. After a substantial period of growth lag, cells started to proliferate and different growth conditions were tested to establish the cell line. At present, the ACL cell line has been subcultured for more than 100 passages. ACL cells had a polygonal shape and the morphology appeared homogenous with epithelial-like cells. Cell growth was dependent on the presence of foetal bovine serum and cells proliferated in a wide temperature range with optimal growth at 15 °C. By exposure to a viral dsRNA mimic (poly I:C) the cells expressed high levels of a repertoire of genes comprising both inflammatory mediators and interferon stimulated genes. Infection studies with two different viruses showed that infectious pancreatic necrosis virus (IPNV) propagated efficiently, and induced low level expression of genes of both pathways before the cells rapidly died. No productive infection was obtained with nervous necrosis virus (NNV), but a transient increase in the viral RNA level, followed by a high increase in expression of selected ISGs, suggests that the virus enters the cells but is unable to complete its replication cycle. To our knowledge, ACL cells are at the moment the only existing cell line from Atlantic cod. Our results demonstrate that ACL cells can be a useful research tool for further exploration of host-pathogen interactions and it is believed that this cell line will serve as a valuable tool also for studies within other research areas.

Protective roles of grass carp Ctenopharyngodon idella Mx isoforms against grass carp reovirus

Background

Myxovirus resistance (Mx) proteins are crucial effectors of the innate antiviral response against a wide range of viruses, mediated by the type I interferon (IFN-I) signaling pathway. However, the antiviral activity of Mx proteins is diverse and complicated in different species.

Methodology/Principal Findings

In the current study, two novel Mx genes (CiMx1 and CiMx3) were identified in grass carp (Ctenopharyngodon idella). CiMx1 and CiMx3 proteins exhibit high sequence identity (92.1%), and low identity with CiMx2 (49.2% and 49.5%, respectively) from the GenBank database. The predicted three-dimensional (3D) structures are distinct among the three isoforms. mRNA instability motifs also display significant differences in the three genes. The spatial and temporal expression profiles of three C. idella Mx genes and the IFN-I gene were investigated by real-time fluorescence quantitative RT-PCR (qRT-PCR) following infection with grass carp reovirus (GCRV) in vivo and in vitro. The results demonstrated that all the four genes were implicated in the anti-GCRV immune response, that mRNA expression of Mx genes might be independent of IFN-I, and that CIK cells are suitable for antiviral studies. By comparing expression patterns following GCRV challenge or poly(I:C) treatment, it was observed that GCRV blocks mRNA expression of the four genes. To determine the functions of Mx genes, three CiMx cDNAs were cloned into expression vectors and utilized for transfection of CIK cells. The protection conferred by each recombinant CiMx protein against GCRV infection was evaluated. Antiviral activity against GCRV was demonstrated by reduced cytopathic effect, lower virus titer and lower levels of expressed viral transcripts. The transcription of IFN-I gene was also monitored.

Conclusions/Significance

The results indicate all three Mx genes can suppress replication of grass carp reovirus and over-expression of Mx genes mediate feedback inhibition of the IFN-I gene.

Atlantic halibut experimentally infected with nodavirus shows increased levels of T-cell marker and IFNγ transcripts

The transcript levels of viral RNAs, selected T-cell marker and cytokine genes, toll like receptor (TLR) 7, and two interferon stimulated genes (ISG) were analysed in sexually immature adult Atlantic halibut (Hippoglossus hippoglossus L.) experimentally infected with nodavirus. The expression of the T-cell markers, TLR7 and the cytokine genes was further explored in in vitro stimulated anterior kidney leucocytes (AK leucocytes) isolated from the experiment fish and from additional untreated non-injected fish. The levels of viral RNA1 and RNA2 were increasing in brain and eye at around 4 and 8weeks post injection (wpi), respectively, and still increasing at the end of the experiment, especially in eye. Immuno-positive cells and signs of vacuolisation in both brain and eye were seen at 14wpi. Increased transcript levels of TCRβ, CD4-2, CD4, CD8α, and Lck in brain and eye of the experimentally infected halibut suggested an involvement of halibut T-cells in the immune response against nodavirus. Interestingly, a similar expression pattern of TCRβ, CD4 and Lck was seen in both brain and eye. However, compared to brain that showed elevated transcript levels of TCRβ, CD4 and Lck mainly at 10 and 14wpi, the increase appeared earlier between 3 and 4wpi in the eye. Yet, an increase in the transcript level of IFNγ was seen at 10 and 14wpi in both organs. Moreover, elevated levels of TLR7, IL-1β, IL-6, ISG15 and Mx were detected in vivo. The in vitro experiments, stimulating AK leucocytes with ConA-PMA, imiquimod or nodavirus, further supported an involvement of IL-6 and IFNγ in the immune response against nodavirus and the involvement of CD8β(+) cells. Results from the present study thus indicate an importance of T-cells, IFNγ and the analysed ISGs in the immune response against nodavirus in Atlantic halibut, and would be of great help in future vaccination trials giving the possibility to monitor the immune response rather than mortality during post-vaccination challenge experiments.

Grouper (Epinephelus coioides) CXCR4 is expressed in response to pathogens infection and early stage of development

Chemokine (C-X-C motif) receptor 4 (CXCR4) from orange-spotted grouper (Epinephelus coioides) was identified and characterized in this study. gCXCR4 shared common features in protein sequence and predicted structure of CXCR4 family. This suggested that gCXCR4 is a member of G protein-coupled receptors with seven transmembrane domains. The expression patterns revealed that gCXCR4 may play a key role in early development of grouper. Furthermore, overexpression of gCXCR4-GFP for 48 h had significant effects on the GF-1 cell viability. gCXCR4 protein was mainly expressed in the marginal zone of head kidney and on the surface of intestinal villi. gCXCR4 expression can be detected in all the examined tissues and significantly up-regulated in eye and brain, which are the main targets for nervous necrosis virus (NNV) infection and replication. gCXCR4 gene expression can be induced in the spleen and eye by lipopolysaccharide and NNV, respectively. Our data suggested that gCXCR4 may not only play a role in the early immune response to microbial infection but also restrain to the immune system and central nervous system.

Cloning of crystallin from orange-spotted grouper and characterization of its activity as potential protective agent

Oxidative stress associated with nodavirus infection is poorly understood, especially pertaining to infection-mediated brain injury. Indirect evidence indicates that infection increases cellular abundance of reactive oxygen species (ROS) with consequent increase in cellular dityrosine production. The detection of dityrosine in nodavirus-infected grouper was demonstrated using immunohistochemical (IHC) staining. Proteomic analyses with eye tissues of healthy grouper revealed more abundant expression of crystallin protein in the eye than in various tissues, which was confirmed by real-time polymerase chain reaction and IHC analyses. Grouper crystallin belongs to a small heat shock protein family with chaperone-like function that prevents heat-induced and oxidative stress-induced protein aggregation. Recombinant crystallin induced nitric oxide (NO) production in RAW 264.7 cells after treatment. The results provide new insight into the pathogenesis of nodavirus and demonstrate an experimental rationale for antioxidant therapy research.

Molecular cloning and functional analysis of an orange-spotted grouper (Epinephelus coioides) secreted protein acidic and rich in cysteine (SPARC) and characterization of its expression response to nodavirus

Mammalian secreted protein acidic and rich in cysteine (SPARC) is the primary regulator of cell shape and cell adhesion to fibronectin. We, for the first time, report the complete sequencing of SPARC cDNA from orange-spotted grouper. Despite the difference in the lengths of the SPARC transcripts, all of the SPARC molecules encoded a signal peptide, follistain-like copper binding sequence (KGHK) domain, and extracellular domain. The grouper SPARC gene was differentially expressed in vivo and contributed differently to high-level expression of SPARC in muscle. Immunohistochemical staining demonstrated a decreased level of SPARC in nodavirus-infected grouper compared with healthy grouper. Comparative real-time polymerase chain reaction analyses of eye tissues of viral nervous necrosis grouper and healthy grouper were performed. Recombinant SPARC produced changes in grouper cell shape 24 h after treatment. The results provide new insight into the pathogenesis of nodavirus, and demonstrate an experimental rationale for SPARC characterization in nodavirus-infected grouper.

A gene delivery system based on the N-terminal domain of human topoisomerase I

The N-terminal 200 amino acid residues of topoisomerase I (TopoN) is highly positive in charge and has DNA binding activity, without DNA sequence and topological specificity. Here, a fusion protein (6 x His-PTD-TopoN) containing a hexahistidine (6 x His) tag, a membrane penetration domain and TopoN (amino acid 3-200) was designed and developed. The protein can bind to different sizes (3.0-8.0 kb) and forms (circular and linear) of DNA and translocates the bound DNA to the nucleus. The protein also showed low cytotoxicity to GF-1 grouper fish fin cells that were previously very sensitive and difficult to transfect in vitro. Maintaining the hexahistidine tag increased the protein's transfection efficiency in COS7 African green monkey kidney cells and simplified the purification process. The plasmid pEGFP-N1 was delivered into COS7 cells by the protein in ATP- and temperature-dependent manners. The results indicate that the binding ability of TopoN is very useful for DNA delivery and the carrier protein can be expressed in Escherichia coli without removal of the hexahistidine tag.

The fight between the teleost fish immune response and aquatic viruses

Teleost fish represent a transition point on the phylogenetic spectrum between invertebrates that depend only on innate immunity and mammals that heavily depend on adaptive immunity. The major mechanisms of the teleost fish innate immune response are suggested to be similar to mammals, although fine details of the process require further studies. Within the innate immune response the type I interferon (IFN) system is an essential innate antiviral component that protects fish from some virus infections. The current progress of cloning and functional characterization of fish antiviral genes is promising in further elucidation of the fish antiviral response. The adaptive immune system of fish utilizes cellular components more or less similar to mammals. Teleost fish produce IgM as a primary antibody response and lack isotype switching to mount virus-specific antibodies during the infection process. Despite this, the development of successful fish rhabdoviral vaccines suggest that vaccination may prove to be an effective way of promoting fish adaptive immune responses to viruses. This paper reviews the bony fish antiviral response with specific discussion on the evolutionary mechanisms that allow aquatic viruses to co-exist with their host. Detailed aspects of the teleost type I IFN system are also addressed.

Impact of asymptomatic nodavirus carrier state and intraperitoneal viral mimic injection on brain transcript expression in Atlantic cod (Gadus morhua)

Nodaviruses and other RNA viruses have a profoundly negative impact on the global aquaculture industry. Nodaviruses target nervous tissue causing viral nervous necrosis, a disease characterized by neurological damage, swimming abnormalities, and morbidity. This study used functional genomic techniques to study the Atlantic cod (Gadus morhua) brain transcript expression responses to asymptomatic high nodavirus carrier state and intraperitoneal injection of polyriboinosinic polyribocytidylic acid (pIC). Reciprocal suppression subtractive hybridization (SSH) cDNA libraries enriched for virus-responsive brain transcripts were constructed and characterized. We generated 1,938 expressed sequence tags (ESTs) from a forward brain SSH library (enriched for transcripts upregulated by nodavirus and/or pIC) and 1,980 ESTs from a reverse brain SSH library (enriched for transcripts downregulated by nodavirus and/or pIC). To examine the effect of nodavirus carrier state on individual brain gene expression in asymptomatic cod, 27 transcripts of interest were selected for quantitative reverse transcription-polymerase chain reaction (QPCR) studies. Transcripts found to be >10-fold upregulated in individuals with a high nodavirus carrier state relative to those in a no/low nodavirus carrier state were identified as ISG15, IL8, DHX58 (alias LGP2), ZNFX1, RSAD2 (alias viperin), and SACS (sacsin, alias spastic ataxia of Charlevoix-Saguenay). These and other SSH-identified transcripts were also found by QPCR to be significantly (P < 0.05) upregulated by pIC compared with saline-injected controls within 72 h of injection. Several transcripts identified in the reverse SSH library, including two putative ubiquitination pathway members (HERC4 and SUMO2), were found to be significantly (P < 0.05) downregulated in individuals with a high nodavirus carrier state. Our data shows that Atlantic cod brains have a strong interferon pathway response to asymptomatic high nodavirus carrier state and that many interferon pathway and other immune relevant transcripts are significantly induced in brain by both nodavirus and pIC.

Cloning of an orange-spotted grouper Epinephelus coioides heat shock protein 90AB (HSP90AB) and characterization of its expression in response to nodavirus

The heat shock proteins (HSPs) family which consists of HSP90, HSP70, and low molecular mass HSPs are involved in chaperone activity. Here, we report the cloning and characterization of HSP90AB gene from orange-spotted grouper, Epinephelus coioides. The full-length of grouper HSP90AB was 727 amino acids and possessed an ATPase domain as well as an evolutionarily conserved molecular chaperone. The HSP90AB-green fluorescent protein fusion protein was evenly distributed in the cytoplasm. Immunohistochemistry (IHC) and real-time polymerase chain reaction (PCR) analyses indicated that the expression of grouper HSP90AB was marginally increased following nodavirus infection. Grouper E. coioides that received HSP90 inhibitor geldanamycin (GA) showed an increase in HSP90AB expression and growth of nodavirus supporting nodavirus replication.

Nodavirus encephalopathy in turbot (Scophthalmus maximus): inflammation, nitric oxide production and effect of anti-inflammatory compounds

Nodaviruses are the etiological agents of one of the most serious viral diseases affecting marine fish aquaculture. Nodavirus infection produces an abnormal swimming behaviour and causes encephalopathy and retinopathy associated to important mortalities. The expression of TNF-alpha, IRF-1 and Mx was increased in turbot after nodavirus infection. A significant increase in the production of nitrogen radicals was also observed in experimentally infected turbot. Several anti-inflammatory compounds (the antioxidant N-acetylcysteine, cortisone, dexamethasone, prednisolone and aminoguanidine) were assayed to determine the role of inflammation on nodavirus infection. Cortisone and aminoguanidine were able to accelerate the mortality onset associated to nodavirus infection, modulating the gene expression of TNF-alpha and, in addition, modifying the arrival time of nodavirus to the brain. These results suggest the importance of early inflammatory processes to overcome the infection.