Marine medaka heat shock protein 90ab1 is a receptor for red-spotted grouper nervous necrosis virus and promotes virus internalization through clathrin-mediated endocytosis.

Nervous necrosis virus (NNV) can infect many species of fish and causes serious acute or persistent infection. However, its pathogenic mechanism is still far from clear. Specific cellular surface receptors are crucial determinants of the species tropism of a virus and its pathogenesis. Here, the heat shock protein 90ab1 of marine model fish species marine medaka (MmHSP90ab1) was identified as a novel receptor of red-spotted grouper NNV (RGNNV). MmHSP90ab1 interacted directly with RGNNV capsid protein (CP). Specifically, MmHSP90ab1 bound to the linker region (LR) of CP through its NM domain. Inhibition of MmHSP90ab1 by HSP90-specific inhibitors or MmHSP90ab1 siRNA caused significant inhibition of viral binding and entry, whereas its overexpression led to the opposite effect. The binding of RGNNV to cultured marine medaka hMMES1 cells was inhibited by blocking cell surface-localized MmHSP90ab1 with anti-HSP90ß antibodies or pretreating virus with recombinant MmHSP90ab1 or MmHSP90ab1-NM protein, indicating MmHSP90ab1 was an attachment receptor for RGNNV. Furthermore, we found that MmHSP90ab1 formed a complex with CP and marine medaka heat shock cognate 70, a known NNV receptor. Exogenous expression of MmHSP90ab1 independently facilitated the internalization of RGNNV into RGNNV impenetrable cells (HEK293T), which was blocked by chlorpromazine, an inhibitor of clathrin-dependent endocytosis. Further study revealed that MmHSP90ab1 interacted with the marine medaka clathrin heavy chain. Collectively, these data suggest that MmHSP90ab1 is a functional part of the RGNNV receptor complex and involved in the internalization of RGNNV via the clathrin endocytosis pathway.

Screening for Antiviral Medaka Haploid Embryonic Stem Cells by Genome Wide Mutagenesis.

Nervous necrosis virus (NNV), one of the most prevalent fish pathogens, has caused significant losses in both yield and economy to the aquaculture. Host factors involved in NNV infection remain to be identified due to the lack of ideal model for the study of NNV and host interaction. Haploid stem cells have proven to be ideal materials in genetic screens. Here, we generated a cell line HX1G1 (simply named G1) with the activity against red-spotted grouper nervous necrosis virus (RGNNV) by N-ethyl-N-nitrosourea (ENU)-mediated whole genome random mutagenesis from the haploid embryonic stem cell HX1a, a cell clone from haploid cell line HX1 that we previously derived from the medaka fish. G1 cells retained the characteristics of haploidy and pluripotency as indicated by the EBs differentiation ability after genetic mutagenesis. Compared with HX1a cells, no typical cytopathic effects were observed, and the expression of RNA-dependent RNA polymerase (RDRP) was significantly reduced in G1 cells post RGNNV infection, indicating the enhanced anti-RGNNV activity of G1. Furthermore, we demonstrated that RGNNV entry into G1 cells was partially inhibited, and this inhibition might be relevant to the induced mutation of heat shock cognate protein 70 (HSC70) which was decisive for NNV entry. Interestingly, G1 cells were to some extent permissive to RGNNV infection, but RGNNV was spontaneously cleared in G1 cells during serial passage. In addition, we also found that the expression levels of interferon (IFN)-related genes were higher in G1 cells than those in HX1a cells, suggesting that viral clearance might be associated with the elevated expression of IFN-related genes in G1 cells.

Comparative biomarker responses in Japanese medaka (Oryzias latipes) exposed to benzo[a]pyrene and challenged with betanodavirus at three different life stages.

It is now well documented that several contaminants can modulate the fish immune system, leading to disrupted host resistance against pathogens and increased incidence of disease. Since fish are usually co-exposed to chemicals and pathogens in the natural environment, analysis of the immunotoxic effects of pollutants is particularly relevant. The authorities in the European Union have recommended the development of toxicity assays on cell cultures and embryos, as an alternative to testing in vertebrates. This is why in our study, a fish immune challenge assay was developed for the early life stages of Japanese medaka to evaluate and compare the relevance of new biomarkers. Fish were exposed to benzo[a]pyrene (BaP), a model pollutant, for 8days at the embryonic stage, or for 48h at the larvae and juvenile stages, and fish were infected with betanodavirus by bath-challenge of 106TCID50/mL. Biometric changes and induction of malformations were observed after embryonic exposure. DNA damage and induction of EROD activity were recorded at the end of all chemical exposures. Viral infection increased the mortality rate significantly and disturbed the behavior of fish after light stimulation. While BaP exposure increased swimming speed, betanodavirus infection slowed swimming activity. In larvae co-exposed to BaP and the virus, the viral titer in the whole body was higher than in fish infected only with the virus. This study highlighted the sensitivity and usefulness of the immune challenge assay on the early life stages of Japanese medaka to evaluate the toxic effects of pollutants.

Tributyltin exposure increases mortality of nodavirus infected Japanese medaka Oryzias latipes larvae.

We investigated the effect of combined exposure to nodavirus infection and TBT on medaka (Oryzias latipes). Medaka larvae were infected by immersion in medium containing nodavirus at titers of 102.5, 103.5, or 104.5 TCID50/mL. Infected fish then were exposed to TBT at 0, 0.17, 0.52, 1.6, or 4.7µg/L. Of the 12 groups exposed to both stressors, the mortalities of 6 (102.5 TCID50/mL+0.52, 1.6, or 4.7µg/L, 103.5 TCID50/mL+4.7µg/L and 104.5 TCID50/mL+1.6 or 4.7µg/L) were significantly higher than that of each TBT control. Specifically, mortality was 46±5.5% in the group exposed to both 102.5 TCID50/mL virus and 0.52µg/L TBT, which represent the lowest observed effective dose and concentration, respectively, among the 6 groups with increased mortalities. Our results suggest that combined exposure to both stressors suppresses antiviral mechanisms in the fish, thus increasing mortality.

TALENs-mediated gene disruption of myostatin produces a larger phenotype of medaka with an apparently compromised immune system.

Although myostatin, a suppressor of skeletal muscle development and growth, has been well studied in mammals, its function in fish remains unclear. In this study, we used a popular genome editing tool with high efficiency and target specificity (TALENs; transcription activator-like effector nucleases) to mutate the genome sequence of myostatin (MSTN) in medaka (Oryzias latipes). After the TALEN pair targeting OlMyostatin was injected into fertilized medaka eggs, mutant G0 fish carrying different TALENs-induced frameshifts in the OlMSTN coding sequence were mated together in order to transmit the mutant sequences to the F1 generation. Two F1 mutants with frameshifted myostatin alleles were then mated to produce the F2 generation, and these F2 OlMSTN null (MSTN(-/-)) medaka were evaluated for growth performance. The F2 fish showed significantly increased body length and weight compared to the wild type fish at the juvenile and post-juvenile stages. At the post-juvenile stage, the average body weight of the MSTN(-/-) medaka was ∼25% greater than the wild type. However, we also found that when the F3 generation were challenged with red spotted grouper nervous necrosis virus (RGNNV), the expression levels of the interferon-stimulated genes were lower than in the wild type, and the virus copy number was maintained at a high level. We therefore conclude that although the MSTN(-/-) medaka had a larger phenotype, their immune system appeared to be at least partially suppressed or undeveloped.

Expression and biological activity of two types of interferon genes in medaka (Oryzias latipes).

Type I interferon (IFN) is one of most important cytokines for antiviral responses in fish innate immunity, after the induction pathway following pattern recognition. In this study, 2 types of type I IFN mRNA from a medaka (Japanese rice fish; Oryzias latipes) were identified and classified (phylogenetic analysis) into subgroup-a and -d by (designated olIFNa and olIFNd, respectively). Both olIFNa and olIFNd (encoding 197 and 187 amino acid residues, respectively) contained 2 cysteines. Gene expression pattern of olIFNa, olIFNd and IFN-stimulated genes (ISGs) was assessed (quantitative real-time reverse transcriptase PCR, qRT-PCR) in various organs (i.e., whole kidney, liver and spleen) of medaka stimulated by polyI:C or infected with nervous necrosis virus (NNV). Expression of olIFNa, olIFNd and ISGs, especially the ISG15 gene, were significantly upregulated after NNV-infection. Furthermore, olIFNa, olIFNd and ISGs mRNAs were sufficiently induced in DIT cells (i.e., medaka hepatoma cell line) transfected with polyI:C or infected with NNV. In addition, in vitro biological activities of recombinant olIFNa and olIFNd (rolIFNa and rolIFNd) produced by mammalian cell line HEK293T were also characterized. Expression of GIG1a and ISG15 genes in kidney cells of adult medaka were induced by rolIFNa or rolIFNd. The olIFNs-overexpressing DIT cells had reduced viral titers following NNV infection. Therefore, we inferred that 2 type I IFNs were involved in innate immunity (antiviral response) in medaka fish.

Medaka haploid embryonic stem cells are susceptible to Singapore grouper iridovirus as well as to other viruses of aquaculture fish species.

Viral infection is a challenge in high-density aquaculture, as it leads to various diseases and causes massive or even complete loss. The identification and disruption of host factors that viruses utilize for infection offer a novel approach to generate viral-resistant seed stocks for cost-efficient and sustainable aquaculture. Genetic screening in haploid cell cultures represents an ideal tool for host factor identification. We have recently generated haploid embryonic stem (ES) cells in the laboratory fish medaka. Here, we report that HX1, one of the three established medaka haploid ES cell lines, was susceptible to the viruses tested and is thus suitable for genetic screening to identify host factors. HX1 cells displayed a cytopathic effect and massive death upon inoculation with three highly infectious and notifiable fish viruses, namely Singapore grouper iridovirus (SGIV), spring viremia of carp virus (SVCV) and red-spotted grouper nervous necrosis virus (RGNNV). Reverse transcription-PCR and Western blot analyses revealed the expression of virus genes. SGIV infection in HX1 cells elicited a host immune response and apoptosis. Viral replication kinetics were determined from a virus growth curve, and electron microscopy revealed propagation, assembly and release of infectious SGIV particles in HX1 cells. Our results demonstrate that medaka haploid ES cells are susceptible to SGIV, as well as to SVCV and RGNNV, offering a unique opportunity for the identification of host factors by genetic screening.

Transient and permanent gene transfer into the brain of the teleost fish medaka (Oryzias latipes) using human adenovirus and the Cre-loxP system.

In this study, we demonstrated that human type-5 adenovirus infected the brain of the teleost fish, medaka (Oryzias latipes), in vivo. Injection of adenoviral vector into the mesencephalic ventricle of medaka larvae induced the expression of reporter genes in some parts of the telencephalon, the periventricular area of the mesencephalon and diencephalon, and the cerebellum. Additionally, the Cre-loxP system works in medaka brains using transgenic medaka carrying a vector containing DsRed2, flanked by loxP sites under control of the beta-actin promoter and downstream promoterless enhanced green fluorescent protein (EGFP). We demonstrated that the presence of green fluorescence depended on injection of adenoviral vector expressing the Cre gene and confirmed that EGFP mRNA was transcribed in the virus-injected larvae.

Screening of freshwater fish species for their susceptibility to a betanodavirus.

Betanodaviruses, the causative agents of viral nervous necrosis in marine fish, have bipartite positive-sense RNA genomes. Because the genomes are the smallest and simplest among viruses, betanodaviruses have been well studied using a reversed genetics system as model viruses. However, studies of virus-host interactions have progressed slowly because permissive hosts for betanodaviruses (basically larvae and juveniles of marine fish) are only available for limited periods of the year and are not suitable for the construction of a genetic engineering system. To obtain a model fish species that are not subject to these problems, 21 freshwater fish species were injected intramuscularly with a betanodavirus (redspotted grouper nervous necrosis virus) and tested for their susceptibility to the virus. Based on their responses, the tested fish were classified into 3 groups: 4 susceptible fish, 10 less susceptible fish, and 7 resistant fish. The susceptible fish, celebes rainbowfish Telmatherina ladigesi, threadfin rainbowfish Iriatherina werneri, dwarf rainbowfish Melanotaenia praecox, and medaka Oryzias latipes, exhibited erratic swimming and eventually died within 10 d post-inoculation. The virus was specifically localized in the brains, spinal cords, and retinas of the infected fish, similar to the pattern of infection in naturally infected marine fish. We believe that these susceptible freshwater fish species could act as good host models for betanodavirus-fish interaction studies.

Betanodavirus infection in the freshwater model fish medaka (Oryzias latipes).

Betanodaviruses, the causal agents of viral nervous necrosis in marine fish, have bipartite, positive-sense RNA genomes. As their genomes are the smallest and simplest among viruses, betanodaviruses have been studied in detail as model viruses by using a genetic-engineering system, as has occurred with the insect alphanodaviruses, the other members of the family Nodaviridae. However, studies of virus-host interactions have been limited, as betanodaviruses basically infect marine fish at early developmental stages (larval and juvenile). These fish are only available for a few months of the year and are not suitable for the construction of a reverse-genetics system. To overcome these problems, several freshwater fish species were tested for their susceptibility to betanodaviruses. It was found that adult medaka (Oryzias latipes), a well-known model fish, was susceptible to both Striped jack nervous necrosis virus (the type species of the genus Betanodavirus) and Redspotted grouper nervous necrosis virus (RGNNV), which have different host specificities in marine fish species. Infected medaka exhibited erratic swimming and the viruses were localized specifically in the brain, spinal cord and retina of the infected fish, similar to the pattern of infection in naturally infected marine fish. Moreover, medaka were susceptible to RGNNV at the larval stage. This is the first report of a model virus-model host infection system in fish. This system should facilitate elucidation of the mechanisms underlying RNA virus infections in fish.

Pantropic retroviral vector integration, expression, and germline transmission in medaka (Oryzias latipes).

Pantropic retroviral vectors were used to introduce transgenes into Japanese medaka (Oryzias latipes). These vectors contain the long terminal repeat (LTR) sequence of Moloney murine leukemia virus (Mo-MLV) and a reporter gene (neo or lacZ) regulated by the LTR sequence of rous sarcoma virus (RSV). Because these pseudotyped retroviral vectors contain the vesicular stomatitis virus envelope glycoprotein (VSV-G), they have an extremely broad host cell range and can infect many no mammalian species. Newly fertilized medaka eggs (intact or dechorionated) were electroporated at different voltage settings in the presence of 4 x 10(4) cfu of pantropic retroviral vector. The survival rates of the pantropic retroviral vector-treated embryos ranged from 65% to 20% with increasing amplitude of electroporation. Dechorionation did not substantially affect the survival rate of embryos. PCR amplification demonstrated proviral sequences in up to 60% of the 2-month-old fish. The efficiency of gene transfer was enhanced by dechorionation. Furthermore, overnight incubation of dechorionated embryos with pantropic retroviral vectors without electroporation also resulted in proviral integration in 60% of the embryos without compromising survival rate. Southern blot analysis of DNA samples isolated from polymerase chain reaction (PCR) as positive F1 reaction animals confirmed the integration of a single copy of the provirus into the host genome. Three P1 transgenic females transmitted the proviral sequence to 50% of their F1 progeny in a back cross with wild-type males, suggesting that the entire germline of these P1 fish was transformed by the pantropic retroviral vector. Expression of the neomycin phosphotranferase transgene in F1 transgenic individuals was detected by reverse transcription (RT)-PCR amplification of the neo mRNA sequence. Furthermore, expression of a beta-galactosidase transgene was also observed in 4-day-old F1 transgenic individuals. Thus, pantropic retroviral vectors provide a convenient method to stably introduce and express foreign genes in medaka.