Anti-viral mechanism of barramundi Mx against betanodavirus involves the inhibition of viral RNA synthesis through the interference of RdRp

Combined Dietary Administration of Chlorella fusca and Ethanol-Inactivated Vibrio proteolyticus Modulates Intestinal Microbiota and Gene Expression in Chelon labrosus

The use of functional feeds in aquaculture is currently increasing. This study aimed to assess the combined impact of dietary green microalgae Chlorella fusca and ethanol-inactivated Vibrio proteolyticus DCF12.2 (CVP diet) on thick-lipped grey mullet (Chelon labrosus) juvenile fish. The effects on intestinal microbiota and the transcription of genes related to metabolism, stress, and the immune system were investigated after 90 days of feeding. Additionally, the fish were challenged with Aeromonas hydrophila and polyinosinic-polycytidylic acid (poly I:C) to evaluate the immune response. Microbiota analysis revealed no significant differences in alpha and beta diversity between the anterior and posterior intestinal sections of fish fed the control (CT) and CVP diets. The dominant genera varied between the groups; Pseudomonas and Brevinema were most abundant in the CVP group, whereas Brevinema, Cetobacterium, and Pseudomonas were predominant in the CT group. However, microbial functionality remained unaltered. Gene expression analysis indicated notable changes in hif3α, mhcII, abcb1, mx, and tnfα genes in different fish organs on the CVP diet. In the head kidney, gene expression variations were observed following challenges with A. hydrophila or poly I:C, with higher peak values seen in fish injected with poly I:C. Moreover, c3 mRNA levels were significantly up-regulated in the CVP group 72 h post-A. hydrophila challenge. To conclude, incorporating C. fusca with V. proteolyticus in C. labrosus diet affected the microbial species composition in the intestine while preserving its functionality. In terms of gene expression, the combined diet effectively regulated the transcription of stress and immune-related genes, suggesting potential enhancement of fish resistance against stress and infections.

RNAi-Based Therapy: Combating Shrimp Viral Diseases

Shrimp aquaculture has become a vital industry, meeting the growing global demand for seafood. Shrimp viral diseases have posed significant challenges to the aquaculture industry, causing major economic losses worldwide. Conventional treatment methods have proven to be ineffective in controlling these diseases. However, recent advances in RNA interference (RNAi) technology have opened new possibilities for combating shrimp viral diseases. This cutting-edge technology uses cellular machinery to silence specific viral genes, preventing viral replication and spread. Numerous studies have shown the effectiveness of RNAi-based therapies in various model organisms, paving the way for their use in shrimp health. By precisely targeting viral pathogens, RNAi has the potential to provide a sustainable and environmentally friendly solution to combat viral diseases in shrimp aquaculture. This review paper provides an overview of RNAi-based therapy and its potential as a game-changer for shrimp viral diseases. We discuss the principles of RNAi, its application in combating viral infections, and the current progress made in RNAi-based therapy for shrimp viral diseases. We also address the challenges and prospects of this innovative approach.

Proteomic and Phosphoproteomic Analysis Reveals Differential Immune Response to Hirame Novirhabdovirus (HIRRV) Infection in the Flounder (Paralichthys olivaceus) under Different Temperature

Hirame novirhabdovirus (HIRRV) is one of most serious viral pathogens causing significant economic losses to the flounder (Paralichthys olivaceus)-farming industry. Previous studies have shown that the outbreak of HIRRV is highly temperature-dependent, and revealed the viral replication was significantly affected by the antiviral response of flounders under different temperatures. In the present study, the proteome and phosphoproteome was used to analyze the different antiviral responses in the HIRRV-infected flounder under 10 °C and 20 °C. Post viral infection, 472 differentially expressed proteins (DEPs) were identified in the spleen of flounder under 10 °C, which related to NOD-like receptor signaling pathway, RIG-I-like receptor signaling pathway, RNA transport and so on. Under 20 °C, 652 DEPs were identified and involved in focal adhesion, regulation of actin cytoskeleton, phagosome, NOD-like receptor signaling pathway and RIG-I-like receptor signaling pathway. Phosphoproteome analysis showed that 675 differentially expressed phosphoproteins (DEPPs) were identified in the viral infected spleen under 10 °C and significantly enriched in Spliceosome, signaling pathway, necroptosis and RNA transport. Under 20 °C, 1304 DEPPs were identified and significantly enriched to Proteasome, VEGF signaling pathway, apoptosis, Spliceosome, mTOR signaling pathway, mRNA surveillance pathway, and RNA transport. To be noted, the proteins and phosphoproteins involved in interferon production and signaling showed significant upregulations in the viral infected flounder under 20 °C compared with that under 10 °C. Furthermore, the temporal expression profiles of eight selected antiviral-related mRNA including IRF3, IRF7, IKKβ, TBK1, IFIT1, IFI44, MX1 and ISG15 were detected by qRT-PCR, which showed a significantly stronger response at early infection under 20 °C. These results provided fundamental resources for subsequent in-depth research on the HIRRV infection mechanism and the antiviral immunity of flounder, and also gives evidences for the high mortality of HIRRV-infected flounder under low temperature.

Cytokine-like activity of European sea bass ISG15 protein on RGNNV-infected E-11 cells

IFN-I generates an antiviral state by inducing the expression of numerous genes, called IFN-stimulated genes, ISGs, including ISG15, which is the only ISG with cytokine-like activity. In a previous study, we developed the Dl_ISG15_E11 cell line, which consisted of E11 cells able to express and secrete sea bass ISG15. The current study is a step forward, analysing the effect of secreted sea bass ISG15 on RGNNV replication in E11 cells, and looking into its immunomodulatory activity in order to corroborate its cytokine-like activity. The medium from ISG15-produccing cells compromised RGNNV replication, as it has been demonstrated both, by reduction in the viral genome synthesis and, specially, in the yield of infective viral particles. The implication of sea bass ISG15 in this protection has been demonstrated by ISG15 removal, which decreased the percentage of surviving cells upon viral infection, and by incubation of RGNNV-infected cells with a recombinant sea bass ISG15 protein, which resulted in almost full protection. Furthermore, the immunomodulatory activity of extracellular sea bass ISG15 has been demonstrated, which reaffirms a cytokine-like role for this protein.

The extensive transgenerational transcriptomic effects of ocean acidification on the olfactory epithelium of a marine fish are associated with a better viral resistance

Background

Progressive CO₂-induced ocean acidification (OA) impacts marine life in ways that are difficult to predict but are likely to become exacerbated over generations. Although marine fishes can balance acid–base homeostasis efficiently, indirect ionic regulation that alter neurosensory systems can result in behavioural abnormalities. In marine invertebrates, OA can also affect immune system function, but whether this is the case in marine fishes is not fully understood. Farmed fish are highly susceptible to disease outbreak, yet strategies for overcoming such threats in the wake of OA are wanting. Here, we exposed two generations of the European sea bass (Dicentrarchus labrax) to end-of-century predicted pH levels (IPCC RCP8.5), with parents (F1) being exposed for four years and their offspring (F2) for 18 months. Our design included a transcriptomic analysis of the olfactory rosette (collected from the F2) and a viral challenge (exposing F2 to betanodavirus) where we assessed survival rates.

Results

We discovered transcriptomic trade-offs in both sensory and immune systems after long-term transgenerational exposure to OA. Specifically, RNA-Seq analysis of the olfactory rosette, the peripheral olfactory organ, from 18-months-old F2 revealed extensive regulation in genes involved in ion transport and neuronal signalling, including GABAergic signalling. We also detected OA-induced up-regulation of genes associated with odour transduction, synaptic plasticity, neuron excitability and wiring and down-regulation of genes involved in energy metabolism. Furthermore, OA-exposure induced up-regulation of genes involved in innate antiviral immunity (pathogen recognition receptors and interferon-stimulated genes) in combination with down-regulation of the protein biosynthetic machinery. Consistently, OA-exposed F2 challenged with betanodavirus, which causes damage to the nervous system of marine fish, had acquired improved resistance.

Conclusion

F2 exposed to long-term transgenerational OA acclimation showed superior viral resistance, though as their metabolic and odour transduction programs were altered, odour-mediated behaviours might be consequently impacted. Although it is difficult to unveil how long-term OA impacts propagated between generations, our results reveal that, across generations, trade-offs in plastic responses is a core feature of the olfactory epithelium transcriptome in OA-exposed F2 offspring, and will have important consequences for how cultured and wild fish interacts with its environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08647-w.

Establishing an effective gene knockdown system using cultured cells of the model fish medaka (Oryzias latipes)

In spite of the growing attention given to medaka (Oryzias latipes) as an excellent vertebrate model, an effective gene knockdown system has not yet been established using cultured cells of this fish species. In this study, a gene knockdown system using short interfering RNA (siRNA) in medaka cell lines was established through the optimization of transfection conditions. By extensive screening of several medaka cell lines and transfection reagents, OLHNI-2 cells and X-tremeGENE siRNA Transfection Reagent were selected as the best combination to achieve high transfection efficiency of siRNA without cytotoxic effect. Knockdown conditions were then refined using the endogenous heat shock protein 90 (Hsp90) genes as the siRNA targets. Among the parameters tested, cell density, serum concentration in the culture medium, and duration of transfection improved knockdown efficiency, where the target mRNA in cells transfected with each of the siRNAs was reduced from 12.0% to 26.7% of the control level. Our results indicate that the established knockdown system using siRNA is a promising tool for functional analysis of medaka genes in vitro.

Early Immune Modulation in European Seabass (Dicentrarchus labrax) Juveniles in Response to Betanodavirus Infection

The early host–pathogen interaction between European seabass (Dicentrarchus labrax) and Betanodavirus was examined by using juvenile fish infected intramuscularly with RGNNV (red-spotted grouper nervous necrosis virus). The time course selected for sampling (0–144 h post-infection (hpi)) covered the early stages of infection, with hematological, antioxidant and immunological responses examined. Early activation of the host’s immune system was seen in the first few hours post-infection (6 to 9 hpi), as evidenced by an increase in tnfα, cd28 and c3 expression in the head kidney of infected fish. Most hematological parameters that were examined showed significant differences between sampling times, including differences in the number of thrombocytes and various leukocyte populations. The plasma lysozyme concentration decreased significantly over the course of the trial, and most antioxidant parameters examined in the liver showed significant differences over the infection period. At 144 hpi, peak expression of tnfα and il-1β coincided with the appearance of disease symptoms, peak levels of virus in the brain and high levels of fish mortality. The results of the study show the importance of analyzing the early interactions between European seabass and Betanodavirus to establish early indicators of infection to prevent more severe outcomes of the infection from occurring.

Nervous Necrosis Virus-like Particle (VLP) Vaccine Stimulates European Sea Bass Innate and Adaptive Immune Responses and Induces Long-Term Protection against Disease

The rapidly increasing Mediterranean aquaculture production of European sea bass is compromised by outbreaks of viral nervous necrosis, which can be recurrent and detrimental. In this study, we evaluated the duration of protection and immune response in sea bass given a single dose of a virus-like particle (VLP)-based vaccine. Examinations included experimental challenge with nervous necrosis virus (NNV), serological assays for NNV-specific antibody reactivity, and immune gene expression analysis. VLP-vaccinated fish showed high and superior survival in challenge both 3 and 7.5 months (1800 and 4500 dd) post-vaccination (RPS 87 and 88, OR (surviving) = 16.5 and 31.5, respectively, p < 0.01). Although not providing sterile immunity, VLP vaccination seemed to control the viral infection, as indicated by low prevalence of virus in the VLP-vaccinated survivors. High titers of neutralizing and specific antibodies were produced in VLP-vaccinated fish and persisted for at least ~9 months post-vaccination as well as after challenge. However, failure of immune sera to protect recipient fish in a passive immunization trial suggested that other immune mechanisms were important for protection. Accordingly, gene expression analysis revealed that VLP-vaccination induced a mechanistically broad immune response including upregulation of both innate and adaptive humoral and cellular components (mx, isg12, mhc I, mhc II, igm, and igt). No clinical side effects of the VLP vaccination at either tissue or performance levels were observed. The results altogether suggested the VLP-based vaccine to be suitable for clinical testing under farming conditions.

Both TGF-β1 and Smad4 regulate type I collagen expression in the muscle of grass carp, Ctenopharyngodon idella

Type I collagen is proven to make an important contribution to fish muscle quality. Our previous study has shown the Smad4-dependent regulation of type I collagen expression in the muscle of crisp grass carp fed with faba bean. However, the regulatory roles of TGF-β1 or TGF-β1/Smad4 on type I collagen remain unclear in ordinary grass carp fed with normal diets or in other fish species. To clarify this point, the effect of TGF-β1 and Smad4 over-expression and RNAi knockdown on type I collagen (COL1-α1 and COL1-α2) expression were tested in vitro (zebrafish ZF4 cells) and in vivo (grass carp) along with the TGF-β1/Smad4 co-expression and co-knockdown. The mRNA levels of TGF-β1, Smad4, and type I collagen were upregulated in the groups with over-expressed TGF-β1 and Smad4 and downregulated in the groups of TGF-β1 and Smad4 RNAi in comparison to controls in vitro (P < 0.05). Similarly, in the in vivo experiment, the mRNA abundance of TGF-β1, Smad4, and type I collagen of over-expression group was higher than the controls at 36 h (P < 0.05). Co-injection of TGF-β1/Smad4 over-expression and RNAi vectors generally showed the higher efficacy. This study revealed that TGF-β1 and Smad4 genes regulated type I collagen expression in grass carp muscle and zebrafish. These findings will provide references for the collagen regulation of other freshwater fishes.

Functional characterization of a group II interferon, IFNc in the perciform fish, Nile tilapia (Oreochromis niloticus)

Interferons are a family of class II α-helical cytokines playing vital roles in antiviral immune response, and little information is available to date regarding the interferon system of tilapia. In this study, a type I IFN gene, named On-IFNc, was identified in Nile tilapia, Oreochromis niloticus. The predicted protein of On-IFNc contains several structural features known in type I IFNs, and On-IFNc was clustered together with the known IFNc in fish into a separated clade in the phylogenetic tree. On-IFNc gene was constitutively expressed in all tissues examined, with the highest expression level observed in liver, and was rapidly induced in all organs/tissues tested following the stimulation of poly(I:C). In addition, recombinant On-IFNc has been proven to markedly induce the expression of the antiviral effectors, Mx and viperin, the signalling components, STAT1, STAT2, and IRF9, and the transcription factors, IRF3 and IRF7, as well as the tyrosine phosphorylation of STAT1 and STAT2 in fish cells. Furthermore, recombinant On-IFNc has been proven to possess antiviral activity against ISKNV. The present study thus contributes to a better understanding of the functional properties of the type I IFN system in tilapia.

The influence of a GT repeat polymorphism on poly(I:C) induction of the grouper MxII gene promoter in GK cells

Interferons play an important role in the fish innate immune system against viral infection by inducing the interferon stimulated genes, such as Mx gene. We cloned the MxII gene promoter from orange-spotted grouper and found three MxII gene promoters. All of them contained two interferon stimulated response elements (ISREs), and three dinucleotide repeat sequences located at 5' end of ISREs. Interestingly, there is a polymorphic GT repeat element located upstream of these ISREs. The three MxII gene promoters respectively contained 27, 29, and 31 GT repeats, namely EcMx_27, EcMx_29, and EcMx_31. To determine whether GT repeat element influence the MxII gene expression, the MxII gene promoters were subcloned into promoterless reporter plasmid and transfected into grouper kidney (GK) cells. The results showed that a significant induction by poly(I:C) was detected in GK cells transfected with pEcMx_31 (2.65 folds) whereas there was no induction in GK cells transfected with pEcMx_27 and pEcMx_29. However, the significant induction by nervous necrosis virus (NNV) infection was found in GK cells separately transfected with three reporter plasmids. These results suggest that the GT repeat element plays an important role in modulation of MxII gene expression and the induction by poly(I:C) and NNV may be mediated through different signal transduction pathways.

Characterization, Expression Pattern and Antiviral Activities of Mx Gene in Chinese Giant Salamander, Andrias davidianus

Mx, Myxovirus resistance is an important interferon-stimulated protein that mediates antiviral responses. In this study, the expression and activities of Chinese giant salamander, Andrias davidianus Mx gene, AdMx, were investigated. The AdMx cDNA sequence contains an open reading frame (ORF) of 2112 nucleotides, encoding a putative protein of 703 aa. Meanwhile, AdMx possesses the conserved tripartite GTP binding motif and a dynamin family signature. qRT-PCR analysis revealed a broad expression of AdMx in vivo, with the highest expression levels in brain, kidney and spleen. The AdMx expression level in kidney, spleen and muscle significantly increased at 6 h after Chinese giant salamander iridovirus (GSIV) infection and peaked at 48 h, while that in muscle cell line (GSM) was not noticeably up-regulated until 72 h post infection. Additionally, a plasmid expressing AdMx was constructed and transfected into the Chinese giant salamander GSM cells. The virus load and gene copies in AdMx over-expressed cells were significantly reduced compared with those in the control cells. Moreover, compared to the control cells, a lower level of virus major capsid protein (MCP) synthesis in AdMx over-expressed cells was confirmed by Western blot. These results collectively suggest that Mx plays an important antiviral role in the immune responses against GSIV in Chinese giant salamander.

An interferon-induced GTP-binding protein, Mx, from the redlip mullet, Liza haematocheila: Deciphering its structural features and immune function

The interferon-induced GTP-binding protein Mx is responsible for a specific antiviral state against a broad spectrum of viral infections that are induced by type-I interferons (IFN α/β) in different vertebrates. In this study, the Mx gene was isolated from the constructed mullet cDNA database. Structural features of mullet Mx (MuMx) were analyzed using different in-silico tools. The pairwise comparison revealed that the MuMx sequence was related to Stegastes partitus Mx with an 83.7% sequence identity, whereas MuMx was clustered into the teleost category in the phylogentic analysis. Sequence alignment showed that the dynamin-type guanine nucleotide-binding domain (G_DYNAMIN_2), central interactive domain (CID), and GTPase effector domain (GED) were conserved among Mx counterparts. The transcriptional expression of MuMx was the highest in blood cells from unchallenged fish. The temporal mRNA profile showed that MuMx expression was significantly elevated in all tissues, including blood, spleen, head kidney, liver, and gills after the injection of polyinosinic-polycytidylic acid (poly I:C) at many time points. Moreover, MuMx expression increased slightly, in the blood, spleen, and head kidney at a few time points after the injection of lipopolysaccharide (LPS) and Lactococcus garvieae (L. garvieae). Results of the subcellular localization analysis confirmed that the MuMx protein was highly expressed in the cytoplasm. The analysis of the gene expression of the viral hemorrhagic septicemia virus (VHSV) under conditions of MuMx overexpression confirmed the significant inhibition of viral transcripts. The cell viability (MTT) assay and VHSV titer quantification with the presence of MuMx indicated a significant reduction in virus replication. Collectively, these findings suggest that Mx is a specific immune-related gene that elicits crucial antiviral functions against viral antigens in the mullet fish.

Application of Virus Targeting Nanocarrier Drug Delivery System in Virus-Induced Central Nervous System Disease Treatment

Virus-induced central nervous system (CNS) diseases represent a significant burden to animal health worldwide. The difficulty in treating these diseases is mainly attributable to the elaborate barrier system, which limits the transport of drugs to the infected sites. Therefore, it is necessary to develop smart delivery technologies for treatment of these diseases. In the study, viral nervous necrosis disease was studied as a model to evaluate the feasibility of multiwalled carbon nanotubes (MWCNTs) conjugated with virus-specific nanobody and antiviral drug for targeted therapy of virus-induced CNS diseases. The virus (named as PGNNV) was isolated, identified and purified from diseased grouper. A naïve phage-displayed alpaca nanobody library was constructed, and the purified PGNNV was used for biopanning of PGNNV-specific nanobody from the library. The targeted delivery system based on MWCNTs conjugated with polyethylenimine, ribavirin, and PGNNV-specific nanobody was constructed and designated as MWCNTs-PEI-R-Nb. Targeting ability and treatment effects of the MWCNTs-PEI-R-Nb were checked both in vitro and in vivo. MWCNTs-PEI-R-Nb showed an increasing distribution in PGNNV-infected cells, and an obvious accumulation in the brain of PGNNV-infected zebrafish larvae. MWCNTs-PEI-R-Nb also showed a strong anti-PGNNV ability both in vitro and in vivo. The mortality of larvae treated with MWCNTs-PEI-R-Nb (equivalent to 100 mg/L ribavirin) was 27% during 10 days post infection, whereas it was 100% for the control group. The results so far indicate that MWCNTs conjugated with antiviral drugs and viral-specific antibody are effective means for virus-induced CNS disease targeted therapy.

Global profiling of megalocytivirus-induced proteins in tongue sole (Cynoglossus semilaevis) spleen identifies cellular processes essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among farmed fish including tongue sole (Cynoglossus semilaevis). In this study, label-free proteomics was performed to examine protein expression in tongue sole spleen induced by megalocytivirus at 8 and 12 days post infection (dpi). Compared to uninfected control fish, virus-infected fish displayed 315 up-regulated proteins and 111 down-regulated proteins at 8 dpi, and 48 up-regulated proteins and 43 down-regulated proteins at 12 dpi. The expressions of five differentially expressed proteins were confirmed by Western blot. The differentially expressed proteins were enriched in the pathways and processes associated with innate immune response and viral infection. Interference with the expression of two up-regulated proteins of the ubiquitin proteasome system (UPS), i.e. proteasome assembly chaperone 2 and proteasome maturation protein, significantly reduced viral propagation in fish, whereas overexpression of these two proteins significantly enhanced viral propagation. Consistently, inhibition of the functioning of proteasome significantly impaired viral replication in vivo. This study provided the first global protein profile responsive to megalocytivirus in tongue sole, and revealed an essential role of UPS in viral infection.

Analysis of the Atlantic salmon genome reveals a cluster of Mx genes that respond more strongly to IFN gamma than to type I IFN

Mx proteins are antiviral GTPases, which are induced by type I IFN and virus infection. Analysis of the Atlantic salmon genome revealed the presence of 9 Mx genes localized to three chromosomes. A cluster of three Mx genes (SsaMx1 - SsaMx3), which includes previously cloned Mx genes, is present on chromosome (Chr) 12. A cluster of five Mx genes (SsaMx4-SsaMx8) is present on Chr25 while one Mx gene (SsaMx9) is present on Chr9. Phylogenetic and gene synteny analyses showed that SsaMx1-SsaMx3 are most closely related to the main group of teleost Mx proteins. In contrast, SsaMx 4-SsaMx9 formed a separate group together with zebrafish MxD and MxG and eel MxB. The Mx cluster in Chr25 showed gene synteny similar to a Mx gene cluster in the gar genome. Expression of Mx genes in cell lines stimulated with recombinant IFNs showed that Mx genes in Chr12 responded more strongly to type I IFN than to type II IFN (IFN gamma) whilst Mx genes in Chr25 responded more strongly to IFN gamma than to type I IFNs. SsaMx9 showed no response to the IFNs.

Investigation of nervous necrosis virus (NNV) replication in vitro using RNA in situ hybridization

Nervous necrosis virus (NNV) belongs to the genus Betanodavirus of family Nodaviridae. Its genome consists of two RNA segments, RNA1 and RNA2. Several studies have investigated NNV detection by in situ hybridization (ISH), but these have typically focused on the detection of the RNA2 gene. In this study, we localized both RNA1 and RNA2 NNV segments in viral-infected cells by ISH, using labeled RNA probes (RNA-ISH). Also, immunocytochemistry (ICC) assay was carried out for localization of viral particle by targeting the coat protein. Further, viral quantification assays were performed by quantitative RT-PCR and viral infectivity (TCID₅₀) in SSN-1 cells. Viral segments were observed by RNA-ISH at 6 h post infection (hpi), while NNV particles were detected at 24 hpi by ICC. Use of double labeling RNA-ISH revealed the co-expression of the two viral segments in the same area of the cells, while RNA1 was also detected separately. Comparison of the level of viral genomic segments and viral infectivity revealed significantly more copies of RNA1 at each time points than copies of RNA2 and greater NNV titers. The results suggest that RNA1 might be expressed in the early stages of replication, with RNA2 expressed later. The virions then assemble through initially expressed viral genomic segments. Even though infectious particles displayed very efficient packaging, the RNA1 segment was still over-produced.

Smad4-dependent regulation of type I collagen expression in the muscle of grass carp fed with faba bean

Smad4 is the key regulator in the transforming growth factor β1 (TGF-β1)/Smads signal pathway, and is also the crux of the regulation of type I collagen expression in mammals. In fish, however, the relationship between Smad4 and type I collagen is still unknown. Given the widely accepted importance of type I collagen in fish muscle hardness, we seek to explore this issue by analyzing the expressions of the TGF-β1/Smads pathway molecules and type I collagen in the muscle of crisp grass carp fed with faba bean, which shows increased muscle hardness. The study found that (1) in the process of feeding the grass carp with faba bean, the mRNA and protein expressions of TGF-β1, Smad2 and Smad4 all increased along with the increase of type I collagen expression (Col1α1 and Col1α2); (2) one day after the injection of Smad4 over-expression vector, both mRNA and protein expressions of Col1α1 and Col1α2 significantly increased, reaching the maximum on the 2nd and 5th day, respectively; (3) one day after the injection of Smad4 RNAi interference vector, the mRNA and protein expressions of Col1α1 and Col1α2 decreased, reaching the minimum on the 5th day. These results revealed that Smad4 is the major regulator of type I collagen in the muscle of grass carp fed with faba bean. This study would provide an important mechanistic basis for nutritional regulation of type I collagen in the muscle of fish.

Molecular characterization and expression analysis of interferon-gamma in black seabream Acanthopagrus schlegelii

Interferon gamma (IFN-γ) is a major component in immunological signaling and plays a key role in resisting viral infection. In this study, we identified and characterized an IFN-γ gene (AsIFN-γ) in the marine fish black seabream (Acanthopagrus schlegelii). We cloned AsIFN-γ genomic sequence, which comprises four exons, three introns and an upstream promoter including several conserved regulatory elements. The complete cDNA of AsIFN-γ was 816 bp in length and encoded a putative 194 amino acids (aa) protein with a 22 aa signal peptide, six α-helices and one nuclear localization signal (NLS). Multiple alignment showed that AsIFN-γ protein shared 31-60% identity with IFN-γ of other fish but low identity with fish IFN-γrel and IFN-γ of other vertebrates. AsIFN-γ was constitutively expressed in all examined tissues with the highest expression level in immune organs, such as spleen, gill and kidney. In black seabream infected by red spotted nervous necrosis virus (RGNNV), the expression of AsIFN-γ was significantly up-regulated in most tissues, and RGNNV infection in vitro also induced significant up-regulation of AsIFN-γ, indicating that AsIFN-γ was involved in immune response to RGNNV infection. Overexpression of AsIFN-γ in cultured Acanthopagrus schlegelii brain (AsB) cells rapidly and transiently stimulated the expression of JAK-STAT signaling pathway related genes including STAT1, STAT2 and IRF9, as well as the downstream antiviral genes MX1 and ISG15. Furthermore, overexpression of AsIFN-γ was able to significantly inhibit RGNNV replication and virus production in AsB cells. In summary, we identified a conserved IFN-γ gene of black seabream, and demonstrated the rapid and strong antiviral activities of AsIFN-γ against RGNNV in black seabream.

Gene expression profiles associated with lymphocystis disease virus (LCDV) in experimentally infected Senegalese sole (Solea senegalensis)

In the present study, the pathogenesis of lymphocystis disease virus (LCDV) and the immune gene expression patterns associated with this viral infection were determined in the flatfish Senegalese sole. The results indicate that LCDV spreads rapidly from the peritoneal cavity through the bloodstream to reach target organs such as kidney, gut, liver, and skin/fin. The viral load was highest in kidney and reduced progressively thorough the experiment in spite of the viral major capsid protein gene was transcribed. The LCDV injection activated a similar set of differentially expressed transcripts in kidney and intestine although with some differences in the intensity and time-course response. This set included antiviral-related transcripts (including the mx and interferon-related factors irf1, irf2, irf3, irf7, irf8, irf9, irf10), cytokines (il1b, il6, il8, il12 and tnfa) and their receptors (il1r, il8r, il10r, il15ra, il17r), chemokines (CXC-type, CC-type and IL-8), prostaglandins (cox-2), g-type lysozymes, hepcidin, complement fractions (c2, c4-1 and c4-2) and the antigen differentiation factors cd4, cd8a, and cd8b. The expression profile observed indicated that the host triggered a systemic defensive response including inflammation able to cope with the viral challenge.

Humoral and cytokine responses in giant groupers after vaccination and challenge with betanodavirus

Giant groupers were immunized with two dosages (V_high and V_low) of inactivated nervous necrosis virus (NNV) and subsequently challenged with NNV at 4 weeks post vaccination (wpv). Several indicators were used to analyze the protective effects of the NNV vaccine. The neutralizing antibody titer of fish serum mostly corresponded to the survival rate of immunized fish in the NNV challenge test. Extravascular IgM⁺ cells were detected in the brains of both NNV-infected and noninfected groupers. After NNV infection, CD8α and IgM gene expression increased in the brains, indicating CD8α⁺ and IgM⁺ lymphocyte infiltration. Moreover, the NNV load was not the highest in dead grouper brains, suggesting that this load in the brain was not the key factor for the death of groupers. However, the brains of dead fish showed the highest expression of the interleukin (IL)-1β gene, a neurotoxic factor in the brain. Therefore, IL-1β overexpression is likely to be associated with the death of NNV-infected groupers.

Nervous necrosis virus capsid protein exploits nucleolar phosphoprotein Nucleophosmin (B23) function for viral replication

Nucleolar proteins facilitate the replication of certain human and animal viruses through interaction with viral proteins. In this study, an interaction between nervous necrosis virus capsid protein and nucleolar phosphoprotein B23 was identified using in vitro experimental approaches. The capsid protein binds to B23 early during the viral infection and accumulates in the nucleus, particularly in the nucleolus. However, over the course of the infection B23 is redistributed from the nucleoli to the nucleoplasm. siRNA-mediated knockdown of B23 reduced viral replication and cytopathic effect. Thus, B23 targets capsid protein to the nucleus and facilitates NNV replication. The results provide the first demonstration that nucleolar protein B23 has a direct role in the nodavirus replication process.

Fish interferon-stimulated genes: The antiviral effectors

Type I interferons (IFN) are the cornerstone cytokine of innate antiviral immunity. In response to a viral infection, IFN signaling results in the expression of a diverse group of genes known as interferon-stimulated genes (ISGs). These ISGs are responsible for interfering with viral replication and infectivity, helping to limit viral infection within a cell. In mammals, many antiviral effector ISGs have been identified and the antiviral mechanisms are at least partially elucidated. In fish fewer ISGs have been identified and while there is evidence they limit viral infection, almost nothing is known of their respective antiviral mechanisms. This review discusses seven ISGs common to mammals and fish and three ISGs that are unique to fish. The lack of understanding regarding fish ISG's antiviral effector functions is highlighted and draws attention to the need for research in this aspect of aquatic innate immunity.

Characterization and viral susceptibility of a brain cell line from brown-marbled grouper Epinephelus fuscoguttatus (Forsskål) with persistent betanodavirus infection

A continuous cell line designated BMGB (brown-marbled grouper brain) was established from the brain tissues of the brown-marbled grouper Epinephelus fuscoguttatus and characterized. BMGB cells were identified as astroglial progenitor cells because they expressed glial fibrillary acidic protein and keratin and were persistently infected by betanodavirus, as confirmed through immunocytochemistry, polymerase chain reaction and immunoblot analyses. Because few intact virions were present in the BMGB cell culture fluid, the cytopathic effect (CPE) was not observed when the culture fluid was inoculated with GBC1 cells. However, BMGB cells displayed typical CPE after infection with additional betanodavirus, megalocytivirus and chum salmon reovirus. BMGB cells showed low myxovirus resistance (Mx) protein expression, which increased following betanodavirus and reovirus infection. Because the cells contained several unusual or degraded viral proteins, the persistent infection of betanodavirus in the BMGB cells may have resulted from a mechanism that destroys the viral proteins rather than the result of Mx protein expression. Despite the persistent betanodavirus infection, BMGB cells proliferated in a manner similar to other normal tropic fish cells and supported the propagation of several piscine viruses; however, the yield was lower than that of normal cells. The BMGB cells will be useful for investigating virus and host cell interaction.

Mx1 of black carp functions importantly in the antiviral innate immune response

Mx (myxovirus resistance) is an important antiviral protein in the innate immune responses of vertebrates to microbial pathogens. In this study, we cloned and characterized Mx1 of black carp (Mylopharyngodon piceus). The full-length cDNA of black carp Mx1 (bcMx1) consists of 2781 nucleotides and the predicted bcMx1 protein contains 631 amino acids. bcMx1 contains a GTPase domain at the N-terminnus, a "central interactive domain" in the middle and a GTPase effector domain at the C-terminus. bcMx1 mRNA was constitutively transcribed in all tissues tested, including the heart, liver, spleen, kidney, intestine, muscle, skin and gill; and bcMx1 mRNA levels increased in all but the gill after grass carp reovirus (GCRV) or viraemia of carp virus (SVCV) infection. Quantitative PCR analysis of Mylopharyngodon piceus fin (MPF) cells indicated that bcMx1 mRNA levels increased after GCRV or SVCV infection at different multiplicities of infection (MOI). Western blotting demonstrated that the molecular weight of bcMx1 is ∼75 kDa and immunofluorescent staining data of both HeLa cells and EPC cells showed that bcMx1 is a cytosolic protein. EPC cells transfected with plasmid expressing bcMx1 showed increased antiviral activity against SVCV and GCRV. All our data suggest that bcMx1 is an antiviral protein in the innate immune response of the black carp.

Autochthonous Bacterial Isolates Successfully Stimulate In vitro Peripheral Blood Leukocytes of the European Sea Bass (Dicentrarchus labrax)

Commercially available probiotics are routinely administered as feed supplements in aquaculture important species. Among them, the European sea bass (Dicentrarchus labrax) is the most widely reared fish in the Mediterranean, whose rearing systems are highly variable between countries, affecting at some level the sustainability of production. After random isolation of autochthonous gut bacteria of the sea bass, their identification and pathogenicity testing, we have selected three potentially probiotic isolates; Pseudoalteromonas sp., Alteromonas sp., and Enterovibrio coralii. Selected isolates were tested and their immunostimulative efficiency was compared with a commercially available Lactobacillus casei isolate, inferring inflammatory, apoptotic and anti-pathogen response of sea bass’ peripheral blood leukocytes. Phagocytic activity, respiratory burst, and expression of lysozyme, Mx protein, caspase 3, TNF-α, IL-10 genes was measured 1, 3, 5, and 12 h post-stimulation by four bacterial isolates to evaluate early kinetics of the responses. Best immunostimulative properties were observed in Pseudoalteromonas-stimulated leukocytes, followed by Alteromonas sp. and L. casei, while Enterovibrio coralii failed to induce significant stimulation. Based on such in vitro assay intestinal autochthonous bacterial isolates showed to have better immunostimulative effect in sea bass compared to aquaculture-widely used L. casei, and further steps need to engage tank and field feeding trials to evaluate long-term prophylactic suitability of the chosen isolates. A panel of biomarkers that represent pro-/anti-inflammatory, pro-/anti-apoptotic, and anti-bacteria/viral responses of the fish should be taken into consideration when evaluating the usefulness of the potential probiotic in aquaculture.

Effects of hirami lemon, Citrus depressa Hayata, leaf meal in diets on the immune response and disease resistance of juvenile barramundi, Lates calcarifer (bloch), against Aeromonas hydrophila

The present study was conducted to evaluate the dietary supplementation of leaf meal from Citrus depressa Hayata on the growth, innate immune response, and disease resistance of juvenile barramundi, Lates calcarifer. Four diets were formulated to contain 0% (control), 1% (C1), 3% (C3), and 5% (C5) leaf meal, respectively. During a 56 d feeding trial, fish survival, growth performance, and feed efficiency were not significantly different among all groups. For immune response, respiratory burst, superoxide dismutase and lysozyme activities were not significantly different among all groups. However, fish fed the C5 diet for 56 d had significantly higher phagocytic activity. Also, fish fed C3 and C5 diets had significantly higher Mx gene expressions in spleens and head kidneys with nerve necrosis virus injections after 24 h. Disease resistance against Aeromonas hydrophila was increased by the C5 diet. In this study, barramundi fed on a diet containing 5% C. depressa Hayata leaf meal had significantly better innate immune response and disease resistance against A. hydrophila.

Understanding the interaction between Betanodavirus and its host for the development of prophylactic measures for viral encephalopathy and retinopathy

Over the last three decades, the causative agent of viral encephalopathy and retinopathy (VER) disease has become a serious problem of marine finfish aquaculture, and more recently the disease has also been associated with farmed freshwater fish. The virus has been classified as a Betanodavirus within the family Nodaviridae, and the fact that Betanodaviruses are known to affect more than 120 different farmed and wild fish and invertebrate species, highlights the risk that Betanodaviruses pose to global aquaculture production. Betanodaviruses have been clustered into four genotypes, based on the RNA sequence of the T4 variable region of their capsid protein, and are named after the fish species from which they were first derived i.e. Striped Jack nervous necrosis virus (SJNNV), Tiger puffer nervous necrosis virus (TPNNV), Barfin flounder nervous necrosis virus (BFNNV) and Red-spotted grouper nervous necrosis virus (RGNNV), while an additional genotype turbot betanodavirus strain (TNV) has also been proposed. However, these genotypes tend to be associated with a particular water temperature range rather than being species-specific. Larvae and juvenile fish are especially susceptible to VER, with up to 100% mortality resulting in these age groups during disease episodes, with vertical transmission of the virus increasing the disease problem in smaller fish. A number of vaccine preparations have been tested in the laboratory and in the field e.g. inactivated virus, recombinant proteins, virus-like particles and DNA based vaccines, and their efficacy, based on relative percentage survival, has ranged from medium to high levels of protection to little or no protection. Ultimately a combination of effective prophylactic measures, including vaccination, is needed to control VER, and should also target larvae and broodstock stages of production to help the industry deal with the problem of vertical transmission. As yet there are no commercial vaccines for VER and the aquaculture industry eagerly awaits such a product. In this review we provide an overview on the current state of knowledge of the disease, the pathogen, and interactions between betanodavirus and its host, to provide a greater understanding of the multiple factors involved in the disease process. Such knowledge is needed to develop effective methods for controlling VER in the field, to protect the various aquaculture species farmed globally from the different Betanodavirus genotypes to which they are susceptible.

Endogenous grouper and barramundi Mx proteins facilitated the clearance of betanodavirus RNA-dependent RNA polymerase

This study confirmed that the infection of nervous necrosis virus (NNV), belonging to the betanodavirus, can induce the expression of endogenous Mx in grouper fin-3 (GF-3), grouper brain (cGB), and barramundi brain (cBB) cells, but not in grouper fin-1 (GF-1) cells. In a co-sedimentation assay, RdRp appeared in the mitochondrial pellet of GF-1 cells without endogenous Mx expression. However, in GF-3, cGB, and cBB cells, RdRp was detected in the nuclear pellet accompanied by endogenous Mx. By immunostaining, RdRp was found to colocalize with not only endogenous Mx but also lysosomes and monodansylcadaverine (MDC)-labeled autophagic vacuoles. In GF-1 cells, the RdRp level continuously increased during 24-72 h post infection (hpi). When endogenous Mx expressed during 24-72 hpi in virus-infected GF-3, cGB, and cBB cells, the RdRp level peaked at 24 hpi but decreased at 48-72 hpi. The degradation of RdRp could be suppressed by treatment with 3-methyladenine (3MA), NH4Cl, and Mx-specific siRNA respectively. After poly I:C transfection, the endogenous Mx level peaked at 3 days post transfection (dpt) and then spontaneously decreased at 5-7 dpt. The poly I:C-indued Mx also colocalized with MDC-labeled autophagic vacuoles at 3 dpt, and its degradation could be inhibited by 3MA or NH4Cl treatments. Therefore, the anti-NNV mechanism of endogenous grouper and barramundi Mx is suggested to sequester RdRp for degradation through autophagy and lysosomes.

SJNNV down-regulates RGNNV replication in European sea bass by the induction of the type I interferon system

European sea bass is highly susceptible to the betanodavirus RGNNV genotype, although the SJNNV genotype has also been detected in this fish species. The coexistence of both genotypes may affect the replication of both viruses by viral interaction or by stimulation of the host antiviral defense system in which the IFN I system plays a key role. IFN I triggers the transcription of interferon-stimulated genes, including Mx genes, whose expression has been used as a reporter of IFN I activity. The present study evaluated the effect of a primary exposure to an SJNNV isolate on a subsequent RGNNV infection and analyzed the role of the IFN I system in controlling VNNV infections in sea bass using different in vivo approaches. VNNV infection and Mx transcription were comparatively evaluated after single infections, superinfection (SJ+RG) and co-infection (poly I:C+RG). The single RGNNV infection resulted in a 24% survival rate, whereas the previous SJNNV or poly I:C inoculation increased the survival rate up to 96 and 100%, respectively. RGNNV replication in superinfection was reduced compared with RGNNV replication after a single inoculation. Mx transcription analysis shows differential induction of the IFN I system by both isolates. SJNNV was a potent Mx inducer, whereas RGNNV induced lower Mx transcription and did not interfere with the IFN I system triggered by SJNNV or poly I:C. This study demonstrates that an antiviral state exists after SJNNV and poly I:C injection, suggesting that the IFN I system plays an important role against VNNV infections in sea bass.

Role of the IFN I system against the VHSV infection in juvenile Senegalese sole (Solea senegalensis)

Senegalese sole is susceptible to marine VHSV isolates but is not affected by freshwater isolates, which may indicate differences regarding virus-host immune system interaction. IFN I induces an antiviral state in fish, stimulating the expression of genes encoding antiviral proteins (ISG). In this study, the stimulation of the Senegalese sole IFN I by VHSV infections has been evaluated by the relative quantification of the transcription of several ISG (Mx, Isg15 and Pkr) after inoculation with marine (pathogenic) and freshwater (non-pathogenic) VHSV isolates. Compared to marine VHSV, lower levels of RNA of the freshwater VHSV induced transcription of ISG to similar levels, with the Isg15 showing the highest fold induction. The protective role of the IFN I system was evaluated in poly I:C-inoculated animals subsequently challenged with VHSV isolates. The cumulative mortality caused by the marine isolate in the control group was 68%, whereas in the poly I:C-stimulated group was 5%. The freshwater VHSV isolate did not cause any mortality. Furthermore, viral RNA fold change and viral titers were lower in animals from the poly I:C + VHSV groups than in the controls. The implication of the IFN I system in the protection observed was confirmed by the transcription of the ISG in animals from the poly I:C + VHSV groups. However, the marine VHSV isolate exerts a negative effect on the ISG transcription at 3 and 6 h post-inoculation (hpi), which is not observed for the freshwater isolate. This difference might be partly responsible for the virulence shown by the marine isolate.

Grouper voltage-dependent anion selective channel protein 2 is required for nervous necrosis virus infection

Nervous necrosis virus (NNV) is a non-enveloped virus with 2 segmented positive-sense single-stranded RNAs. NNV-induced mass mortality has occurred worldwide in many species of cultured marine fish and resulted in substantial economic losses. In our previous study, we cloned the gene of voltage-dependent anion selective channel protein 2 (GVDAC2), and the NNV RNA2 expression level decreased in GVDAC2-knockdown GF-1 cells 24 h after infection. Here, we investigated the role of GVDAC2 in the NNV infection in GF-1 cells. NNV infection did not considerably affect GVDAC2 gene expression. After performing immunostaining, we detected GVDAC2 at the mitochondria and GVDAC2 was colocalized with NNV-RNA-dependent RNA polymerase. However, these 2 proteins did not interact with each other in immunoprecipitation assay. The cellular ATP level in GVDAC2-downregulated cells was lower than that in control cells, and NNV-induced apoptosis was delayed in GVDAC2-siRNA-transfected cells. Therefore, we suggest that GVDAC2 is required for NNV infection for maintaining the cellular ATP level and has positive impact on virus-induced apoptosis.

Improvement in non-specific immunity and disease resistance of barramundi, Lates calcarifer (Bloch), by diets containing Daphnia similis meal

A 42-day study was conducted with barramundi, Lates calcarifer, to evaluate the effects of Daphnia meal derived from Daphnia similis on fish growth, immune response, and disease resistance to Aeromonas hydrophila. Three isonitrogenous (45%) and isolipid (10%) experimental diets were formulated to contain 0% (control), 5% (D5), and 10% (D10) Daphnia meal. Growth was depressed when fish were fed with the D10 diet for 42 days compared to control. However, the growth in fish fed with control and D5 diets for 42 days was not significantly different. By day 42, the leukocyte phagocytic activity and respiratory burst activity were significantly increased in D5 and D10 groups compared to control. Mx gene expression in the spleen and head kidney of fish after being injected with nerve necrosis virus was also significantly up-regulated in both groups compared to control. In an increased immune response, D5 and D10 fish had significantly higher survival rates than control after being challenged by A. hydrophila. Therefore, we suggest that a 5% Daphnia-meal diet could improve the barramundi immune response and disease resistance without a negative impact on growth.

GHSC70 is involved in the cellular entry of nervous necrosis virus

Nervous necrosis virus (NNV) is a devastating pathogen of cultured marine fish and has affected more than 40 fish species. NNV belongs to the betanodaviruses of Nodaviridae and is a nonenveloped icosahedral particle with 2 single-stranded positive-sense RNAs. To date, knowledge regarding NNV entry into the host cell remains limited, and no NNV-specific receptor protein has been published. Using grouper fin cell line GF-1 and purified NNV capsid protein in a virus overlay protein binding assay (VOPBA), grouper heat shock cognate protein 70 (GHSC70) and grouper voltage-dependent anion selective channel protein 2 (GVDAC2) were investigated as NNV receptor protein candidates. We cloned and sequenced the genes for GHSC70 and GVDAC2 and expressed them in Escherichia coli for antiserum preparation. Knockdown of the expression of GHSC70 and GVDAC2 genes with specific short interfering RNAs (siRNAs) significantly downregulated viral RNA expression in NNV-infected GF-1 cells. By performing an immunoprecipitation assay, we confirmed that GHSC70 interacted with NNV capsid protein, while VDAC2 did not. Immunofluorescence staining and flow cytometry analysis revealed the presence of the GHSC70 protein on the cell surface. After a blocking assay, we detected the NNV RNA2 levels after 1 h of adsorption to GF-1 cells; the level was significantly lower in the cells pretreated with the GHSC70 antiserum than in nontreated cells. Therefore, we suggest that GHSC70 participates in the NNV entry of GF-1 cells, likely functioning as an NNV receptor or coreceptor protein.

IMPORTANCE

Fish nodavirus has caused mass mortality of more than 40 fish species worldwide and resulted in huge economic losses in the past 20 years. Among the four genotypes of fish nodaviruses, the red-spotted grouper nervous necrosis virus (RGNNV) genotype exhibits the widest host range. In our previous study, we developed monoclonal antibodies with high neutralizing efficiency against grouper NNV in GF-1 cells, indicating that NNV-specific receptor(s) may exist on the GF-1 cell membrane. However, no NNV receptor protein has been published. In this study, we found GHSC70 to be an NNV receptor (or coreceptor) candidate through VOBPA and provided several lines of evidence demonstrating that GHSC70 protein has a role in the NNV entry step of GF-1 cells. To the best of our knowledge, this is the first report identifying grouper HSC70 and its role in NNV entry into GF-1 cells.

Immune gene expressions in grouper larvae (Epinephelus coioides) induced by bath and oral vaccinations with inactivated betanodavirus

Nervous necrosis virus (NNV) has caused mass mortality in many mariculture fish species. Bath vaccination of inactivated NNV and oral immunization of recombinant NNV coat protein are reported to protect grouper larvae against NNV infection. However, the information of immune gene expression in grouper larvae (Epinephelus coioides) after bath and oral immunizations is still limited. In this study, grouper larvae were respectively bath- and orally immunized with binary ethylenimine (BEI)-inactivated NNV, and the expression levels of immune genes were analyzed. Significant gene expressions of IL-1β, Mx, MHC-I, MHC-II, CD8α, IgM and IgT were observed in bath- and orally immunized fish 1-4 weeks post immunization (wpi). Particularly, the up-regulation of IL-1β and Mx gene expression lasted for 4 weeks. The IgT gene expression in gill was only induced by bath immunization, while that in gut was only stimulated by oral immunization. Both immunizations elicited MHC-I and CD8α gene expression relative to cellular immunity. Furthermore, NNV RNA genome, which was detected in inactivated NNV, could induce Mx gene expression in grouper brain (GB) cells, indicating that NNV RNA genome could be recognized by pathogen-recognition receptors (PRRs). In summary, bath and oral vaccinations with BEI-inactivated NNV triggered the gene expression of not only humoral immunity but also cellular immunity.

RNA interference technology used for the study of aquatic virus infections

Aquaculture is one of the most important economic activities in Asia and is presently the fastest growing sector of food production in the world. Explosive increases in global fish farming have been accompanied by an increase in viral diseases. Viral infections are responsible for huge economic losses in fish farming, and control of these viral diseases in aquaculture remains a serious challenge. Recent advances in biotechnology have had a significant impact on disease reduction in aquaculture. RNAi is one of the most important technological breakthroughs in modern biology, allowing us to directly observe the effects of the loss of specific genes in living systems. RNA interference technology has emerged as a powerful tool for manipulating gene expression in the laboratory. This technology represents a new therapeutic approach for treating aquatic diseases, including viral infections. RNAi technology is based on a naturally occurring post-transcriptional gene silencing process mediated by the formation of dsRNA. RNAi has been proven widely effective for gene knockdown in mammalian cultured cells, but its utility in fish remains unexplored. This review aims to highlight the RNAi technology that has made significant contributions toward the improvement of aquatic animal health and will also summarize the current status and future strategies concerning the therapeutic applications of RNAi to combat viral disease in aquacultured organisms.

Inhibition of porcine reproductive and respiratory syndrome virus by specific siRNA targeting Nsp9 gene

To screen siRNAs for effectively inhibiting the replication of porcine reproductive and respiratory syndrome virus (PRRSV). Four pairs of siRNA targeting Nsp9 gene of PRRSV and one non-efficient pair used as control were designed, synthesized and cloned into pSilencer4.1-CMV neo, designated as pSi-294, pSi-367, pSi-409, pSi-1488, pSi-Ctr. The recombinant plasmids were transfected into Marc-145 cells and infected with PRRSV 24h post transfection. Subsequently, IFA, real-time PCR, TCID50 and western blot were used for evaluating the inhibitory effect of the siRNA. IFA and western-blot results showed that pSi-294, pSi-1488 can effectively inhibit the expression of Nsp9 and M protein of PRRSV, real-time PCR result showed that the expression of Nsp9 gene were decreased from 86.56% to 93.66% compared to the negative control. siRNAs can be used as candidates for basic research of PRRSV.

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.

Persistently betanodavirus-infected barramundi (Lates calcarifer) exhibit resistances to red sea bream iridovirus infection

Nervous necrosis virus (NNV) and red sea bream iridovirus (RSIV) are two important pathogens that have caused acute, highly contagious, and widespread diseases among wild and cultured fish, especially at larval and juvenile stages. We discovered that the pathogenicity of NNV to the 80 days post-hatch (dph) barramundi is lower than that to the 14 dph barramundi. Following NNV challenge, no mortality occurred in the 80 dph barramundi, but NNV RNA2 and barramundi Mx (BMx) gene expression was detected in the brain and liver. The 80 dph barramundi pre-challenged with NNV became more resistant to the following RSIV challenge (mortality: 62%) compared to the NNV-free barramundi challenged with RSIV (mortality: 100%). A similar phenomenon was revealed in the cell culture system that RSIV proliferated less progeny in the barramundi brain (BB) cell line which exhibit persistent NNV infection than in NNV-free cured BB (cBB) cell line. The potential factors involved in the resistance of the persistently NNV-infected barramundi and BB cells to the secondary RSIV infection were examined in this study. We prove that barramundi anti-NNV polyclonal antibodies do not cross-neutralize RSIV, and NNV infection does not interfere with RSIV replication. However, the interferon (IFN) response and BMx gene expression in cBB cells suppresses the RSIV proliferation. Our study suggests that the NNV-induced IFN response and BMx expression are responsible for the resistance of barramundi to RSIV infection.

Identification of the STAT1 gene and the characterisation of its immune response to immunostimulants, including nervous necrosis virus (NNV) infection, in Malabar grouper (Epinephelus malabaricus)

Signal Transducer and Activator of Transcription (STAT)-1 is an indispensable signal transduction protein that is involved in the interferon pathway. STAT-1 plays an important role in the innate immune response. The full-length cDNA of Malabar grouper (Epinephelus malabaricus) STAT-1, MgSTAT1, was cloned. Phylogenetic analysis was performed based on the amino acid sequence. Our results indicate that STAT1 is highly conserved with other vertebrates. We also report the expression of MgSTAT1 in different tissues treated with immune stimulants, including LPS, CpG ODN, and poly (I:C), in vivo. The expression of MgSTAT1 was significantly induced in the head kidney upon treatment with poly (I:C) compared to the control. Moreover, the results indicate that MgSTAT1 is up-regulated during nervous necrosis virus (NNV) infection. This study reveals that similar to the mammalian antiviral response, MgSTAT1 mediates the immune response in Malabar grouper.

Expression kinetics of key genes in the early innate immune response to Great Lakes viral hemorrhagic septicemia virus IVb infection in yellow perch (Perca flavescens)

The recently discovered strain of viral hemorrhagic septicemia virus, VHSV-IVb, represents an example of the introduction of an extremely pathogenic rhabdovirus capable of infecting a wide variety of new fish species in a new host-environment. The goal of the present study was to delineate the expression kinetics of key genes in the innate immune response relative to the very early stages of VHSV-IVb infection using the yellow perch (Perca flavescens) as a model. Administration of VHSV-IVb by IP-injection into juvenile yellow perch resulted in 84% cumulative mortality, indicating their high susceptibility to this disease. In fish sampled in the very early stages of infection, a significant up-regulation of Mx gene expression in the liver, as well as IL-1β and SAA activation in the head kidney, spleen, and liver was directly correlated to viral load. The potential down-regulation of Mx in the hematopoietic tissues, head kidney and spleen, may represent a strategy utilized by the virus to increase replication.

Exploring RNAi as a therapeutic strategy for controlling disease in aquaculture

Aquatic animal diseases are one of the most significant constraints to the development and management of aquaculture worldwide. As a result, measures to combat diseases of fish and shellfish have assumed a high priority in many aquaculture-producing countries. RNA interference (RNAi), a natural mechanism for post-transcriptional silencing of homologous genes by double-stranded RNA (dsRNA), has emerged as a powerful tool not only to investigate the function of specific genes, but also to suppress infection or replication of many pathogens that cause severe economic losses in aquaculture. However, despite the enormous potential as a novel therapeutical approach, many obstacles must still be overcome before RNAi therapy finds practical application in aquaculture, largely due to the potential for off-target effects and the difficulties in providing safe and effective delivery of RNAi molecules in vivo. In the present review, we discuss the current knowledge of RNAi as an experimental tool, as well as the concerns and challenges ahead for the application of such technology to combat infectious disease of farmed aquatic animals.

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.