Infections of nervous necrosis virus in wild and cage-reared marine fish from South China Sea with unexpected wide host ranges

Development and characterization of brain cell line from Trachinotus blochii and its application in virological and gene expression studies

A permanent cell line, SPB (Snubnose pompano brain) was established from Trachinotus blochii by the explant culture method. It has been sub-cultured more than 75 passages and showed optimal growth at 28°C using L-15 medium supplemented with 15% to 20% FBS. The SPB cells were cryopreserved at different passage levels for various applications. SPB cells were composed of fibroblastic and epithelial-like cells. The SPB cells were tested for mycoplasma contamination which was found to be negative. The origin of the SPB cell line from T. blochii was confirmed by amplification of the mitochondrial cytochrome oxidase I (COI) gene. The transfection efficiency of SPB cell line is 15% assessed by expression of green fluorescent protein using pEGFP-N1 plasmid. In addition, two CMV promotor plasmids pFNCPE42-DNA and pcDNAVP28 were transfected to SPB cells and it shows high expression levels of FNCP of fish nodavirus and VP28 protein of white spot syndrome virus by immunostaining. The SPB cells showed susceptibility to SJNNV and the infection was confirmed by RT-PCR, Western blot, ELISA, TCID50 and RT-qPCR. Experimental infection was carried out in T. blochii using SJNNV propagated in SPB cell line and found 100% mortality with clinical signs. The infection was confirmed by RT-PCR. The SPB cell line can be used for propagation of fish viral pathogens and production of the recombinant proteins.

Detection of different Betanodavirus genotypes in wild fish from Spanish Atlantic coastal waters (Galicia, northwestern Spain)

OBJECTIVE

The nervous necrosis virus (NNV; genus Betanodavirus) is an aquatic pathogen that is responsible for a neurological disease affecting marine fish. Despite its almost worldwide distribution, global warming could favor the spread of NNV to new areas, highlighting the importance of conducting epidemiological surveys on both wild and farmed marine fish species. In this study, we assessed NNV prevalence in wild fish caught along the Galician Atlantic coast.

METHODS

In total, 1277 fish were analyzed by reverse transcription real-time polymerase chain reaction.

RESULT

Twenty two (1.72%) of those fish tested positive for NNV, including two species in which the pathogen had not yet been reported.

CONCLUSION

The reassortant RGNNV/SJNNV (red-spotted grouper NNV/striped jack NNV) was detected in 55% of NNV-positive individuals, while the remaining 45% harbored the SJNNV-type genome. Moreover, from European Pilchard Sardina pilchardus and Atlantic Mackerel Scomber scombrus, we isolated four reassortant strains that carried amino acid mutations at key sites related to NNV-host interaction.

Cells and Fugu Response to Capsid of BFNNV Genotype

The nervous necrosis virus (NNV) of the BFNNV genotype is the causative agent of viral encephalopathy and retinopathy (VER) in cold water fishes. Similar to the RGNNV genotype, BFNNV is also considered a highly destructive virus. In the present study, the RNA2 of the BFNNV genotype was modified and expressed in the EPC cell line. The subcellular localization results showed that the capsid and N-terminal (1-414) were located in the nucleus, while the C-terminal (415-1014) of the capsid was located in the cytoplasm. Meanwhile, cell mortality obviously increased after expression of the capsid in EPC. EPC cells were transfected with pEGFP-CP and sampled at 12 h, 24 h and 48 h for transcriptome sequencing. There are 254, 2997 and 229 up-regulated genes and 387, 1611, and 649 down-regulated genes post-transfection, respectively. The ubiquitin-activating enzyme and ubiquitin-conjugating enzyme were up-regulated in the DEGs, indicating that cell death evoked by capsid transfection may be related to ubiquitination. The qPCR results showed that heat stock protein 70 (HSP70) is extremely up-regulated after expression of BFNNV capsid in EPC, and N-terminal is the key region to evoke the high expression. For further study, the immunoregulation of the capsid in fish pcDNA-3.1-CP was constructed and injected into the Takifugu rubripes muscle. pcDNA-3.1-CP can be detected in gills, muscle and head kidney, and lasted for more than 70 d post-injection. The transcripts of IgM and interferon inducible gene Mx were up-regulated after being immunized in different tissues, and immune factors, such as IFN-γ and C3, were also up-regulated in serum, while C4 was down-regulated one week after injection. It was suggested that pcDNA-3.1-CP can be a potential DNA vaccine in stimulating the immune system of T. rubripes; however, NNV challenge needs to be conducted in the following experiments.

Construction of Attenuated Strains for Red-Spotted Grouper Nervous Necrosis Virus (RGNNV) via Reverse Genetic System

The nervous necrosis virus (NNV) mainly attacks the central nervous system of fish to cause viral nervous necrosis, which is an acute and serious prevalent disease in fish. Among different genotypes of NNV, red-spotted grouper nervous necrosis virus (RGNNV) is the most widely reported, with the highest number of susceptible species. To better understand the pathogenicity of RGNNV, we first developed a reverse genetic system for recombinant RGNNV rescue using B7GG and striped snakehead (SSN-1) cells. Furthermore, we constructed attenuated RGNNV strains rRGNNV-B2-M1 and rRGNNV-B2-M2 with the loss of B2 protein expression, which grew slower and induced less Mx1 expression than that of wild-type RGNNV. Moreover, rRGNNV-B2-M1 and rRGNNV-B2-M2 were less virulent than the wild-type RGNNV. Our study provides a potential tool for further research on the viral protein function, virulence pathogenesis, and vaccine development of RGNNV, which is also a template for the rescue of other fish viruses.

Investigation of betanodavirus in sea bass (Dicentrarchus labrax) at all production stages in all hatcheries and on selected farms in Turkey

Viral nervous necrosis (VNN) is one of the most important problems in sea bass culture. Although there have been many studies on detection and molecular characterization of betanodavirus, the causative agent of VNN, there has been little focus on understanding its prevalence to create epidemiological maps. The purpose of this study was to investigate the prevalence of betanodavirus in active sea bass hatcheries and on selected farms in Turkey by RT-qPCR. A total of 2460 samples, including fertilized eggs, prelarvae, postlarvae, fry, and fingerlings, were collected from 16 hatcheries to cover all production stages. A total of 600 sea bass were also collected from 20 farms. Betanodavirus was detected in one hatchery (1/16) in fingerling-sized sea bass, and the prevalence of betanodavirus at the hatchery level was calculated to be 6.25%. Betanodavirus was also detected on one farm (1/20) in fingerling-sized sea bass, and the prevalence of betanodavirus at the farm level was calculated to be 5%. Virus isolation initially could not be achieved in E-11 cells, but later, SSN-1 cells were used successfully. Partial genome sequence analysis of the RNA1 and RNA2 segments of the viruses revealed that they were of the red-spotted grouper nervous necrosis virus genotype, which is endemic in the Mediterranean basin. The absence of mortality related to VNN in the hatcheries and on the farms, the healthy appearance of the sea bass, the low viral load detected, and the results of retrospective epidemiological studies indicated that the infection was subclinical. Not detecting betanodavirus in other age groups where biosecurity was implemented indicates that there was no active infection. In light of these findings, it can be concluded that there was no betanodavirus circulating in hatcheries, and the virus might have been of seawater origin.

Molecular Monitoring and Phylogenetic Analysis of Betanodavirus in Groupers (Epinephelus spp.) and Asian Sea bass (Lates calcarifer) of Iranian Northern Waters of the Persian Gulf

The emergence of diseases has caused much health and economic damage. Viral Nervous Necrosis (VNN) is considered as one of the most important threats to aquatic ecosystems. VNN can cause severe mortality and economic loss in fish farms. The high water temperatures in southern Iran and the observed incidences of fish mortality in the Persian Gulf led to the hypothesis of the possible emergence of VNN. Therefore, this study aimed to monitor two species of fish susceptible to VNN using PCR, and Nested PCR methods and comparing the sensitivity of these methods to the identification of Betanodavirus infection in apparently healthy and symptomatic fish. About 850 Grouper (Epinephelus spp.) and Asian Sea bass (Lates calcarifer) fish of the Persian Gulf were collected randomly and examined. Molecular methods were used to identify NNV in visibly healthy and symptomatic fish of the Persian Gulf of Iran. The results of the PCR showed no positive cases, but the Nested PCR revealed some positive results. Then, the phylogenetic analysis of the virus sequence was performed. The nucleotide sequence of Nested PCR products revealed a 98-100% homology with Red Spotted Grouper Viral Nervous Necrosis (RGNNV). This is the first report on VNN tracing and detection as well as phylogenetic analysis of the virus from the Persian Gulf of Iran. Therefore, considering the importance of emerging viral diseases and the irreparable damage they cause, continuous monitoring and epidemiological studies of VNN were recommended by authorized organizations.

Prevalence of Infectious Spleen and Kidney Necrosis Virus (ISKNV), Nervous Necrosis Virus (NNV) and Ectoparasites in Juvenile Epinephelus spp. Farmed in Aceh, Indonesia

A cross-sectional survey was used to estimate the prevalence of infections with the Infectious spleen and kidney necrosis virus (ISKNV, Megalocytivirus), nervous necrosis virus (NNV, Betanodavirus), and infestations with ectoparasites during the rainy season in juvenile grouper (Epinephelus spp.) farmed in Aceh, Indonesia. The survey was intended to detect aquatic pathogens present at 10% prevalence with 95% confidence, assuming 100% sensitivity and specificity using a sample size of 30 for each diagnostic test. Eight populations of grouper from seven farms were sampled. Additional targeted sampling was conducted for populations experiencing high mortality. Infection with NNV was detected at all farms with seven of the eight populations being positive. The apparent prevalence for NNV ranged from 0% (95% CI: 0–12) to 73% (95% CI: 54–88). All of the fish tested from the targeted samples (Populations 9 and 10) were positive for NNV and all had vacuolation of the brain and retina consistent with viral nervous necrosis (VNN). Coinfections with ISKNV were detected in five populations, with the highest apparent prevalence being 13% (95% CI: 4–31%). Trichodina sp., Cryptocaryonirritans and Gyrodactylus sp. were detected at three farms, with 66% to 100% of fish being infested. Hybrid grouper sourced from a hatchery were 5.4 and 24.9 times more likely to have a NNV infection and a higher parasite load compared to orange-spotted grouper collected from the wild (p < 0.001). This study found that VNN remains a high-impact disease in grouper nurseries in Aceh, Indonesia.

Characterization of Novel Aptamers Specifically Directed to Red-Spotted Grouper Nervous Necrosis Virus (RGNNV)-Infected Cells for Mediating Targeted siRNA Delivery

Nervous necrosis virus (NNV) causes viral nervous necrosis, the most devastating disease in more than 50 fish species worldwide, with massive mortality rates up to 100%, resulting in great economic losses to mariculture. However, few methods are available for the efficient diagnosis and treatment of viral nervous necrosis. Aptamers are molecular recognition ligands characterized by their remarkably high specificity and affinity, great stability, and ease of synthesis, and have been widely studied in application of disease diagnosis and therapies. In this study, we generated three aptamers against red-spotted grouper nervous necrosis virus (RGNNV)-infected grouper brain (GB) cells using the Cell-SELEX (cell based-systematic evolution of ligands by exponential enrichment) technology. The selected aptamers formed stable stem-loop structures, and could specifically recognize RGNNV-infected GB cells, with calculated dissociation constants (K_d) of 27.96, 29.3, and 59.5 nM for aptamers GBN2, GBN10, and GBN34, respectively. They also recognized RGNNV-infected brain tissues. The three aptamers were non-toxic and showed antiviral activities both in vitro and in vivo. Fluorescence microscopy and flow cytometry also demonstrated that aptamer GBN34 could be efficiently and specifically internalized into RGNNV-infected GB cells. The targeted cellular delivery of aptamer-small interfering RNA (siRNA) conjugates remarkably inhibited RGNNV infection in GB cells. The efficiency of the aptamer-based targeted delivery system was about 75% reduction in infection after 48 h, which was similar to that of transfection. These aptamers have great potential utility in the rapid diagnosis and inhibition of RGNNV infection in mariculture.

Red grouper nervous necrosis virus (RGNNV) induces autophagy to promote viral replication

Autophagy is an evolutionarily conserved cellular degradation process that is essential for homeostasis. As a cell steward, autophagy is thought to be a process that may have evolved to combat intracellular pathogens. However, some virus can subvert or utilize autophagy-related membrane structures to increase viral replication. The red-spotted grouper nervous necrosis virus (RGNNV) is a fish pathogen which leads to disastrous viral nervous necrosis in larvae and juvenile groupers and other marine fishes. To better comprehend the pathogenesis and replication mechanism of RGNNV, we investigated the relationship between RGNNV and autophagy. Here, we demonstrated that RGNNV induced autophagy in grouper spleen (GS) cells, as the significant increase in ultrastructural autophagosome-like vesicles, fluorescent punctate pattern of microtubule-associated protein 1 light chain 3 (LC3), and the conversion of LC3-I to LC3-II. Additionally, ultraviolet-inactivated RGNNV and the capsid protein also triggered autophagy. Enhancement of autophagy contributed to RGNNV replication, whereas blocked autophagy decreased RGNNV replication. Moreover, impeded fusion of autophagosomes and lysosomes also reduced RGNNV replication, indicating that RGNNV utilized the different steps of autophagy pathway to facilitate viral replication. The further study showed that RGNNV induced autophagy through activating the phosphorylation of eIF2α and inhibiting the phosphorylation of mTOR. These results will assist the search for novel drugs targets and vaccine design against RGNNV from the perspective of downregulating autophagy.

Betanodavirus and VER Disease: A 30-year Research Review

The outbreaks of viral encephalopathy and retinopathy (VER), caused by nervous necrosis virus (NNV), represent one of the main infectious threats for marine aquaculture worldwide. Since the first description of the disease at the end of the 1980s, a considerable amount of research has gone into understanding the mechanisms involved in fish infection, developing reliable diagnostic methods, and control measures, and several comprehensive reviews have been published to date. This review focuses on host–virus interaction and epidemiological aspects, comprising viral distribution and transmission as well as the continuously increasing host range (177 susceptible marine species and epizootic outbreaks reported in 62 of them), with special emphasis on genotypes and the effect of global warming on NNV infection, but also including the latest findings in the NNV life cycle and virulence as well as diagnostic methods and VER disease control.

Characterization of ssDNA aptamers specifically directed against Trachinotus ovatus NNV (GTONNV)-infected cells with antiviral activities

Nervous necrosis virus (NNV), is one of the most fatal viruses in marine fish aquaculture, and is capable of infecting over 50 different fish species. Trachinotus ovatus NNV (GTONNV) was isolated from diseased golden pompano. This T. ovatus strain was isolated from Guangxi, China. Single-stranded DNA (ssDNA) aptamers with high specificity for GTONNV-infected T. ovatus cerebellum cells (TOCC) were produced by Systematic Evolution of Ligands by Exponential Enrichment (SELEX). The characterization of these aptamers was performed using flow cytometry and laser scanning confocal microscopy. The selected aptamers showed significant specificity for GTONNV-infected cells. Based on MFOLD prediction, aptamers formed distinct stem-loop structures that could form the basis for the aptamers' specific binding to their cellular targets. Protease treatment results revealed that the target molecules for aptamers TNA1, TNA4 and TNA19 within GTONNV-infected cells may be membrane proteins that were trypsin-sensitive. Specific endocytosis of aptamer TNA1, TNA4 and TNA19 into GTONNV-infected cells was also shown. The selected aptamers demonstrated antiviral effects against GTONNV both in vitro and in vivo. This is the first time that aptamers targeting GTONNV-infected T. ovatus cells have been selected and characterized. These aptamers hold promise as rapid diagnostic reagents or targeted therapeutic drugs against GTONNV.

Detection and molecular characterization of betanodaviruses retrieved from bivalve molluscs

Betanodaviruses are small ssRNA viruses responsible for viral encephalopathy and retinopathy, otherwise known as viral nervous necrosis, in marine fish worldwide. These viruses can be either horizontally or vertically transmitted and have been sporadically detected in invertebrates, which seem to be one of the possible viral sources. Twenty-eight new betanodavirus strains were retrieved in three molluscs species collected from different European countries between 2008 and 2015. The phylogenetic analyses revealed that strains retrieved from bivalve molluscs are closely related to viruses detected in finfish in Southern Europe in the period 2000-2009. Nevertheless, a new betanodavirus strain, markedly different from the other members of the RGNNV genotype, was detected. Such a massive and varied presence of betanodaviruses in bivalve molluscs greatly stresses the risks of transmission previously feared for other invertebrates. Bivalve molluscs reared in the same area as farmed and wild finfish could act as a reservoir of the virus. Furthermore, current European regulations allow relaying activities and the sale of live bivalve molluscs, which could pose a real risk of spreading betanodaviruses across different geographic regions. To our knowledge, this is the first study, which focuses on the detection and genetic characterization of betanodaviruses in bivalve molluscs.

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.

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.

Generation and characterization of novel DNA aptamers against coat protein of grouper nervous necrosis virus (GNNV) with antiviral activities and delivery potential in grouper cells

Nervous necrosis virus (NNV) infected larvae and juveniles of more than 50 fish species, resulting in mortality rates of greater than 95%. However, there is no efficient method to control NNV infections. Aptamers generated by selective evolution of ligands by exponential enrichment (SELEX) are short, single-stranded nucleic acid oligomers. They display a high degree of affinity and specificity for many targets, such as viruses and viral proteins. In this study, three novel DNA aptamers (A5, A10, and B11) that specifically target the coat protein (CP) of grouper nervous necrosis virus (GNNV) were selected using SELEX. Secondary structures and minimum free energy (ΔG) predictions indicated that these aptamers could form stable, secondary stem-loop structures. Electrophoretic mobility shift assays, enzyme-linked immunosorbent assays, Kd measurements, the co-localization of tetramethylrhodamine (TAMRA) labeled-aptamers with the CP and flow cytometry analysis revealed that these aptamers could specifically bind the CP with high (nanomolar) affinities. In addition, competition analysis suggested the aptamers shared some common CP binding sites with the anti-CP antibody. Moreover, all three aptamers did not show any cytotoxic effects in vitro or in vivo, and anti-viral analysis indicated the selected aptamers could inhibit NNV infection in vitro and in vivo. Compared with controls, mortality of GNNV-infected fish decreased by 40% and 80% after 10 days infection, when the GNNV was pre-incubated with the 1000 nM A10 and B11, respectively. TAMRA-labeled aptamers could bind to NNV virions and directly enter NNV-infected cells, suggesting they could be used as tracers to study the mechanism of viral infection, as well as for targeted therapy. This is the first time that aptamers targeting a viral protein of marine fish have been generated and characterized. These aptamers hold promise as diagnostic, therapeutic, and targeted drug delivery agents for controlling NNV infections.

Epidemiological characterization of VNNV in hatchery-reared and wild marine fish on Hainan Island, China, and experimental infection of golden pompano (Trachinotus ovatus) juveniles

The current epidemiological situation of viral nervous necrosis virus (VNNV) on Hainan Island was investigated. A total of 490 hatchery-reared fish and 652 wild fish were sampled for VNNV detection from March 2013 to May 2014. Positive detection rates of 84.53% (153/181) and 0.97 % (3/309) were obtained in diseased and healthy hatchery-reared samples, respectively, by conventional RT-PCR. However, using more-sensitive nested RT-PCR, the positive detection rates in healthy hatchery-reared fish reached up to 64.08% (198/309), suggesting that asymptomatic VNNV carriers commonly exist among larvae and juveniles breeding on Hainan Island. In wild-fish samples, 2.6% (17/652) and 34.2% (223/652) positive detection rates were observed using RT-PCR and nested RT-PCR, respectively, indicating that wild fish may be a potential reservoir for VNNV. Phylogenetic analysis showed that all 52 VNNV isolates from cultured fish belong to the RGNNV genotype, but 2 out of 48 VNNV isolates from wild fish samples were found to be of the SJNNV genotype. This study is the first to confirm the existence of SJNNV-genotype VNNV in China. Golden pompano, an important fish species for culture, was selected as a fish model to investigate the optimal conditions for RGNNV disease progression in artificial infection experiments. The effects of temperature, salinity, and fish size were evaluated. Results showed that 28 °C and 20 ‰ are the optimal infection temperature and salinity, respectively, and golden pompano juveniles with small body sizes are more susceptible to RGNNV. These findings are highly consistent with those conditions involved in the natural outbreak of RGNNV.