In situ hybridization of a marine fish virus, Singapore grouper iridovirus with a nucleic acid probe of major capsid protein

Polysaccharides derived from Spirulina platensis inhibited Singapore grouper iridovirus by impeding the entry of viral particles

Attributable to the rapid dissemination and high lethality of Singapore grouper iridovirus (SGIV), it has caused significant economic losses for marine fish aquaculture in China and Southeast Asian nations. Hence, there is an urgent need to find antiviral drugs that are both safe and effective. In this study, a novel heteropolysaccharide named Spirulina platensis polysaccharides (SPP) was purified and characterized from S. platensis. The molecular weight of SPP is 276 kDa and it mainly consists of Glc and Rha, followed by minor components such as Gal, Xyl, and Fuc. The backbone of SPP was determined to be →2) -β-Rhap-(1 → 4) -α-Fucp-(1 → [2) -α-Rhap-(1] 2[→6)-α-Glcp-(1] 4[→ 4) -α-Glcp-(1] 8[→ 4) -β-Glcp-(1]2→, with branches of β-Galp, α-Xylp and α-Glcp. SPP significantly inhibited SGIV-induced cytopathic effects (CPEs), viral gene replication and viral protein expression. The antiviral mechanism of SPP was associated with the disruption of SGIV entry to host cells. Furthermore, it was not observed that SPP made statistically significant impact on the expression of interferon-related cytokines. Our results offered novel insights into the potential utilization of spirulina polysaccharides for combating aquatic animal viruses.

Grouper Atg14 promotes Singapore grouper iridovirus (SGIV) replication by inhibiting the host innate immune response

As one of the important members of the autophagy-related protein family, Atg14 plays a key role in the formation and maturation of autophagosomes. However, little is known about the potential roles of fish Atg14 and its roles in virus infection. In the present study, the homolog of Atg14 (EcAtg14) from the orange-spotted grouper (Epinephelus coioides) was cloned and characterized. The open reading frame (ORF) of EcAtg14 consists of 1530 nucleotides, encoding 509 amino acids, with a predicted molecular weight of 56.9 kDa. EcAtg14 was distributed in all tested tissues, with higher expression in liver, blood and spleen. The expression of EcAtg14 was increased in grouper spleen (GS) cells after Singapore grouper iridovirus (SGIV) infection. EcAtg14 was distributed in the cytoplasm of GS cells. Overexpression of EcAtg14 promoted SGIV replication in GS cells and inhibited IFN3, ISRE and NF-κB promoter activities. Co-immunoprecipitation results showed that there was an interaction between EcAtg14 and EcBeclin. EcAtg14 also promoted the synthesis of LC3-II in GS cells. These findings provide a basis for understanding the innate immune mechanism of grouper against viral infection.

Curcumin Alleviates Singapore Grouper Iridovirus-Induced Intestine Injury in Orange-Spotted Grouper (Epinephelus coioides)

Singapore grouper iridovirus (SGIV) is a new ranavirus species in the Iridoviridae family, whose high lethality and rapid spread have resulted in enormous economic losses for the aquaculture industry. Curcumin, a polyphenolic compound, has been proven to possess multiple biological activities, including antibacterial, antioxidant, and antiviral properties. This study was conducted to determine whether curcumin protected orange-spotted grouper (Epinephelus coioides) from SGIV-induced intestinal damage by affecting the inflammatory response, cell apoptosis, oxidative stress, and intestinal microbiota. Random distribution of healthy orange-spotted groupers (8.0 ± 1.0 cm and 9.0 ± 1.0 g) into six experimental groups (each group with 90 groupers): Control, DMSO, curcumin, SGIV, DMSO + SGIV, and curcumin + SGIV. The fish administered gavage received DMSO dilution solution or 640 mg/L curcumin every day for 15 days and then were injected intraperitoneally with SGIV 24 h after the last gavage. When more than half of the groupers in the SGIV group perished, samples from each group were collected for intestinal health evaluation. Our results showed that curcumin significantly alleviated intestine damage and repaired intestinal barrier dysfunction, which was identified by decreased intestine permeability and serum diamine oxidase (DAO) activity and increased expressions of tight junction proteins during SGIV infection. Moreover, curcumin treatment suppressed intestinal cells apoptosis and inflammatory response caused by SGIV and protected intestinal cells from oxidative injury by enhancing the activity of antioxidant enzymes, which was related to the activation of nuclear factor erythroid 2-related factor 2 (Nrf2) signaling. Moreover, we found that curcumin treatment restored the disruption of the intestinal microbiota caused by SGIV infection. Our study provided a theoretical basis for the functional development of curcumin in aquaculture by highlighting the protective effect of curcumin against SGIV-induced intestinal injury.

Megalocytivirus and Other Members of the Family Iridoviridae in Finfish: A Review of the Etiology, Epidemiology, Diagnosis, Prevention and Control

Aquaculture has expanded to become the fastest growing food-producing sector in the world. However, its expansion has come under threat due to an increase in diseases caused by pathogens such as iridoviruses commonly found in aquatic environments used for fish farming. Of the seven members belonging to the family Iridoviridae, the three genera causing diseases in fish comprise ranaviruses, lymphocystiviruses and megalocytiviruses. These three genera are serious impediments to the expansion of global aquaculture because of their tropism for a wide range of farmed-fish species in which they cause high mortality. As economic losses caused by these iridoviruses in aquaculture continue to rise, the urgent need for effective control strategies increases. As a consequence, these viruses have attracted a lot of research interest in recent years. The functional role of some of the genes that form the structure of iridoviruses has not been elucidated. There is a lack of information on the predisposing factors leading to iridovirus infections in fish, an absence of information on the risk factors leading to disease outbreaks, and a lack of data on the chemical and physical properties of iridoviruses needed for the implementation of biosecurity control measures. Thus, the synopsis put forth herein provides an update of knowledge gathered from studies carried out so far aimed at addressing the aforesaid informational gaps. In summary, this review provides an update on the etiology of different iridoviruses infecting finfish and epidemiological factors leading to the occurrence of disease outbreaks. In addition, the review provides an update on the cell lines developed for virus isolation and culture, the diagnostic tools used for virus detection and characterization, the current advances in vaccine development and the use of biosecurity in the control of iridoviruses in aquaculture. Overall, we envision that the information put forth in this review will contribute to developing effective control strategies against iridovirus infections in aquaculture.

Characterization and function analysis of Epinephelus coioides Hsp40 response to Vibrio alginolyticus and SGIV infection

Heat shock protein 40 (Hsp40), a member of Heat shock proteins (Hsps) family, plays a crucial role in regulation of cell proliferation, survival and apoptosis in mammals. In this study, Hsp40, EcHsp40, was identified from Epinephelus coioides, an economically important marine-cultured fish in China and Southeast Asian counties. The full length of EcHsp40 was 2236 bp in length containing a 1026 bp open reading frame (ORF) encoding 341 amino acids, with a molecular mass of 37.88 kDa and a theoretical pI of 9.09. EcHsp40 has two conserved domains DnaJ and DnaJ_C. EcHsp40 mRNA was detected in all tissues examined, and the expression was significantly up-regulated response to challenged with Vibrio alginolyticus or Singapore grouper iridovirus (SGIV), one of the important pathogens of marine fish. EcHsp40 was distributed in both the cytoplasm and nucleus, over-expression of EcHsp40 can inhibit the activity of nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), significantly promote SGIV-induced apoptosis, intracellular caspase-3 activity and viral replication, suggesting that the EcHsp40 may play an important role in pathogenic stimulation.

Molecular cloning, inducible expression with SGIV and Vibrio alginolyticus challenge, and function analysis of Epinephelus coioides PDCD4

Programmed cell death 4 (PDCD4) in mammals, a gene closely associated with apoptosis, is involved in many biological processes, such as cell aging, differentiation, regulation of cell cycle, and inflammatory response. In this study, grouper Epinephelus coioides PDCD4, EcPDCD4-1 and EcPDCD4-2, were obtained. The open reading frame (ORF) of EcPDCD4-1 is 1413 bp encoding 470 amino acids with a molecular mass of 52.39 kDa and a theoretical pI of 5.33. The ORF of EcPDCD4-2 is 1410 bp encoding 469 amino acids with a molecular mass of 52.29 kDa and a theoretical pI of 5.29. Both EcPDCD4-1 and EcPDCD4-2 proteins contain two conserved MA3 domains, and their mRNA were detected in all eight tissues of E. coioides by quantitative real-time PCR (qRT-PCR) with the highest expression in liver. The expressions of two EcPDCD4s were significantly up-regulated after Singapore grouper iridovirus (SGIV) or Vibrio alginolyticus infection. In addition, over-expression of EcPDCD4-1 or EcPDCD4-2 can inhibit the activity of the nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), and regulate SGIV-induced apoptosis. The results demonstrated that EcPDCD4s might play important roles in E. coioides tissues during pathogen-caused inflammation.

Grouper Atg12 negatively regulates the antiviral immune response against Singapore grouper iridovirus (SGIV) infection

Autophagy is an evolutionarily conserved, multi-step lysosomal degradation process used to maintain cell survival and homeostasis. A series of autophagy-related genes (Atgs) are involved in the autophagic pathway. In mammals, a growing number of studies have attributed functions to some Atgs that are distinct from their classical role in autophagosome biogenesis, such as resistance to pathogens. However, little is known about the functions of fish Atgs. In this study, we cloned and characterized an atg12 homolog from orange spotted grouper (Epinephelus coioides) (Ecatg12). Ecatg12 encodes a 117 amino acid protein that shares 94.0% and 76.8% identity with gourami (Anabas_testudineus) and humans (Homo sapiens), respectively. The transcription level of Ecatg12 was lower in cells infected with Singapore grouper iridovirus (SGIV) than in non-infected cells. Fluorescence microscopy revealed that EcAtg12 localized in the cytoplasm and nucleus in grouper spleen cells. Overexpression of EcAtg12 significantly increased the replication of SGIV, as evidenced by increased severity of the cytopathic effect, transcription levels of viral genes, levels of viral proteins, and progeny virus yield. Further studies showed that EcAtg12 overexpression decreased the expression levels of interferon (IFN) related molecules and pro-inflammatory factors and inhibited the promoter activity of IFN-3, interferon-stimulated response element, and nuclear factor-κB. Together, these results demonstrate that EcAtg12 plays crucial roles in SGIV replication by downregulating antiviral immune responses.

Grouper TRADD Mediates Innate Antiviral Immune Responses and Apoptosis Induced by Singapore Grouper Iridovirus (SGIV) Infection

Tumor necrosis factor (TNF) receptor type 1-associated DEATH domain protein (TRADD) is a TNFR1-associated signal transducer and an essential component of the TNFR1 complex that is involved in activating both apoptotic and nuclear factor (NF)-κB pathways as an adaptor. It also is required for TNFR-1-initiated neuronal apoptosis following in vitro infection with virus as an essential component of the antiviral response. To date, few studies have investigated the function of TRADD in lower vertebrates and its antiviral response to DNA virus infection. In the present study, a TRADD gene (named as EcTRADD) from the orange-spotted grouper (Epinephelus coioides) was cloned and characterized. The full-length cDNA of EcTRADD consists of 1,370 base pairs (bp) and contains a 44 bp 5′-terminal untranslated region (UTR), a 450 bp 3′-UTR including a poly (A) tail, and an 876 bp open reading frame encoding a putative 291 amino acid protein. EcTRADD has two conserved domains of N-terminal domain (TRADD-N) and a death domain (DD). EcTRADD was detected in all examined tissues. EcTRADD was up-regulated in the spleen after infection with Singapore grouper iridovirus (SGIV). Subcellular localization analysis revealed that EcTRADD and EcTRADD-DD exhibited a clear pattern of discrete and interconnecting cytoplasmic filaments resembling the death-effector filaments, while EcTRADD-N was observed in the cytoplasm. After infection with SGIV, EcTRADD, and EcTRADD-DD were transferred to the nucleus. Overexpression of EcTRADD and its domains inhibited replication of SGIV in vitro. Both EcTRADD and EcTRADD-DD induced the caspase-dependent apoptosis in control and infected cells, while EcTRADD-N inhibited the apoptosis. Additionally, EcTRADD and EcTRADD-DD significantly promoted activation of NF-κB and reporter gene p53, whereas EcTRADD-N had no significant effect on p53. The results may provide new insights into the role of fish TRADD in fish virus infection.

Bax inhibitor-1 from orange spotted grouper, Epinephelus coioides involved in viral infection

Bax inhibitor-1 (BI-1) is a conserved anti-apoptotic protein that suppresses endoplasmic reticulum (ER) stress-induced cell death. However, the function of fish BI-1 is not quite clear. In the present study, a bi-1 homolog (Ecbi-1) from orange spotted grouper, Epinephelus coioides was identified and its roles in viral infection were investigated. EcBI-1 encoded 237 amino acids protein, contained six transmembrane regions and a conservative C-terminus motif. Ecbi-1 predominantly expressed in kidney and spleen of healthy grouper. After SGIV stimulation, Ecbi-1 transcript was significantly increased in vitro. Subcellular localization analysis revealed that EcBI-1 was localized throughout the cytoplasm and co-localized with ER. Furthermore, overexpression of EcBI-1 suppressed SGIV infection induced cell death, caspase-3 activity and viral genes transcription. And C-terminus motif was critical for regulation roles of EcBI-1 during SGIV infection. In addition, EcBI-1 could interact with EcBNIP3 in vitro. Together, our data firstly demonstrated that fish BI-1 play important roles in response to viral infection.

Molecular cloning and characterization of FADD from the orange-spotted grouper (Epinephelus coioides)

Fas-associated protein with death domain (FADD) is the key adaptor protein that transmits apoptotic signals mediated by the main death receptors. Besides being an essential instrument in cell death, FADD is also implicated in proliferation, cell cycle progression, tumor development, inflammation, innate immunity, and autophagy. In the present study, a FADD homologue (EcFADD) from the orange-spotted grouper (Epinephelus coioides) was cloned and its possible role in fish immunity was analyzed. The full length cDNA of EcFADD contains 808 base pairs (bp), including a 573 bp open reading frame that encodes a 190 amino acid protein with a predicted molecular mass of 21.81 kDa. Quantitative real-time polymerase chain reaction analysis indicated that EcFADD was distributed in all examined tissues. The expression of EcFADD in the spleen of E. coioides was differentially up-regulated when challenged with Singapore grouper iridovirus (SGIV) or polyinosine-polycytidylic acid(poly[I:C]). EcFADD was abundantly distributed in both the cytoplasm and nucleus in grouper spleen (GS) and fathead minnow (FHM) epithelial cells. Over-expression of EcFADD inhibited SGIV infection and replication and SGIV-induced apoptosis. To achieve antiviral and anti-apoptosis activities, FADD promoted the activation of interferon-stimulated response element (ISRE) and type I interferon (IFN) genes in the antiviral IFN signaling pathway and inhibited activation of apoptosis-related transcription factors p53. Our results not only characterize FADD but also reveal new immune functions and the molecular mechanisms by which FADD responds to virus infection and virus-induced apoptosis.

Antiviral function of Tachyplesin I against iridovirus and nodavirus

Antimicrobial peptides (AMPs) are ubiquitously found in living organisms and are an important component in innate immune response. Tachyplesin I is a potent antimicrobial peptide isolated from the hemocytes of the horseshoe crab, Tachypleus tridentatus. Previous studies have shown that the 17-residue peptide exhibits a wide spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria, fungi, protozoa, and viruses. However, the efficiencies and defense mechanisms of the Tachyplesin I against fish viruses are still unknown. In this study, Tachyplesin I showed a key role in inhibiting the infection and replication of two kinds of newly emerging marine fish viruses, an enveloped DNA virus of Singapore grouper iridovirus (SGIV), and a non-enveloped RNA virus of viral nervous necrosis virus (RGNNV). Synthetic peptides of Tachyplesin I incubated with virus or cells before infection reduced the viral infectivity. Synthetic peptides of Tachyplesin I drastically decreased SGIV and RGNNV titers and viral gene expression. Grouper spleen (GS) and brain (GB) cells over-expressing Tachyplesin I (GS/pcDNA3.1-flag-Tac I and GB/pcDNA3.1-flag-Tac I) support the inhibition of viral infection. Tachyplesin I activated type I IFN and Interferon-sensitive response element (ISRE) in vitro. The promoter activity of IFN-β and ISRE were significantly up-regulated in cells transfected with pcDNA3.1-flag-Tac I after infection with SGIV and VNNV. These results suggest that Tachyplesin I is importantly involved in host immune responses to invasion of viral pathogens.

JNK1 Derived from Orange-Spotted Grouper, Epinephelus coioides, Involving in the Evasion and Infection of Singapore Grouper Iridovirus (SGIV)

c-Jun N-terminal kinase (JNK) regulates cellular responses to various extracellular stimuli, environmental stresses, pathogen infections, and apoptotic agents. Here, a JNK1, Ec-JNK1, was identified from orange-spotted grouper, Epinephelus coioides. Ec-JNK1 has been found involving in the immune response to pathogen challenges in vivo, and the infection of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis in vitro. SGIV infection activated Ec-JNK1, of which phosphorylation of motif TPY is crucial for its activity. Over-expressing Ec-JNK1 phosphorylated transcription factors c-Jun and promoted the infection and replication of SGIV, while partial inhibition of the phosphorylation of Ec-JNK1 showed the opposite effects by over-expressing the dominant-negative EcJNK1-Δ183-185 mutant. Interestingly, SGIV enhanced the viral infectivity by activating Ec-JNK1 which in turn drastically inhibited the antiviral responses of type 1 IFN, indicating that Ec-JNK1 could be involved in blocking IFN signaling during SGIV infection. In addition, Ec-JNK1 enhanced the activation of AP-1, p53, and NF-κB, and resulted in increasing the levels of SGIV-induced cell death. The caspase 3-dependent activation correlated with the phosphorylation of Ec-JNK1 and contributed to SGIV-induced apoptosis. Taken together, SGIV modulated the phosphorylation of Ec-JNK1 to inactivate the antiviral signaling, enhance the SGIV-induced apoptosis and activate transcription factors for efficient infection and replication. The “positive cooperativity” molecular mechanism mediated by Ec-JNK1 contributes to the successful evasion and infection of iridovirus pathogenesis.

Examination of the early infection stages of koi herpesvirus (KHV) in experimentally infected carp, Cyprinus carpio L. using in situ hybridization

Koi herpesvirus (KHV) causes a highly infectious disease afflicting common carp and koi, Cyprinus carpio L. Various molecular and antibody-based detection methods have been used to elucidate the rapid attachment and dissemination of the virus throughout carp tissues, facilitating ongoing development of effective diagnostic approaches. In situ hybridization (ISH) was used here to determine the target tissues of KHV during very early infection, after infecting carp with a highly virulent KHV isolate. Analysis of paraffin-embedded tissues (i.e. gills, skin, spleen, kidney, gut, liver and brain) during the first 8 h and following 10 days post-infection (hpi; dpi) revealed positive signals in skin mucus, gills and gut sections after only 1 hpi. Respiratory epithelial cells were positive as early as 2 hpi. Viral DNA was also detected within blood vessels of various tissues early in the infection. Notable increases in signal abundance were observed in the gills and kidney between 5 and 10 dpi, and viral DNA was detected in all tissues except brain. This study suggests that the gills and gut play an important role in the early pathogenesis of this Alloherpesvirus, in addition to skin, and demonstrates ISH as a useful diagnostic tool for confirmation of acutely infected carp.

Development and characterization of two monoclonal antibodies against grouper iridovirus 55L and 97L proteins

Grouper iridovirus (GIV) is one of the most important viral pathogens in grouper, particularly at the fry and fingerling stages. The study of GIV pathogenicity has been hampered by the lack of proper immunological reagents to study the expression and function of viral proteins in the infected cells. In this study, two mouse monoclonal antibodies (mAbs) against GIV 55L and 97L proteins were produced. Enzyme-linked immunosorbent assay (ELISA) and Western blotting were used to screen these hybridomas, resulting in the identification of two high-affinity mAbs named GIV55L-mAb-2 and GIV97L-mAb-3, respectively. Both mAbs belong to the IgG1 isotype and were effective in detecting their respective target viral protein. Reverse-transcription polymerase chain reaction (RT-PCR) and Western blot analyses of GIV-infected GK cells revealed that GIV 97L is an immediate early gene, whereas GIV 55L a late one. The localization of 55L and 97L in GIV-infected cells was further characterized by immunofluorescence microscopy with the mAbs. The 55L protein mainly aggregated in the cytoplasm while 97L distributed in both the nucleus and cytoplasm of the infected cells. These studies demonstrate the validity of the two mAbs as immunodiagnostic and research reagents.

Immunogenicity and protective effects of inactivated Singapore grouper iridovirus (SGIV) vaccines in orange-spotted grouper, Epinephelus coioides

Vaccination is one of the best methods against viral diseases. In this study, experimental inactivated Singapore grouper iridovirus (SGIV) vaccines were prepared, and immunogenicity and protection against virus infection of the vaccines were investigated in orange-spotted grouper, Epinephelus coioides. Two kinds of vaccines, including β-propiolactone (BPL) inactivated virus at 4°C for 12 h and formalin inactivated virus at 4°C for 12 d, was highly protective against the challenge at 30-day post-vaccination and produced relative percent of survival rates of 91.7% and 100%, respectively. These effective vaccinations induced potent innate immune responses mediated by pro-inflammatory cytokines and type I interferon (IFN)-stimulated genes (ISGs). It is noteworthy that ISGs, such as Mx and ISG15, were up-regulated only in the effective vaccine groups, which suggested that type I IFN system may be the functional basis of early anti-viral immunity. Moreover, effective vaccination also significantly up-regulated of the expression of MHC class I gene and produced substantial amount of specific serum antibody at 4 weeks post-vaccination. Taken together, our results clearly demonstrated that effective vaccination in grouper induced an early, nonspecific antiviral immunity, and later, a specific immune response involving both humoral and cell-mediated immunity.

Antiviral effects of β-defensin derived from orange-spotted grouper (Epinephelus coioides)

Defensins are a group of small antimicrobial peptides playing an important role in innate host defense. In this study, a β-defensin cloned from liver of orange-spotted grouper, Epinephelus coioides, EcDefensin, showed a key role in inhibiting the infection and replication of two kinds of newly emerging marine fish viruses, an enveloped DNA virus of Singapore grouper iridovirus (SGIV), and a non-enveloped RNA virus of viral nervous necrosis virus (VNNV). The expression profiles of EcDefensin were significantly (P < 0.001) up-regulated after challenging with Lipopolysaccharide (LPS), SGIV and Polyriboinosinic Polyribocytidylic Acid (polyI:C) in vivo. Immunofluorescence staining observed its intracellular innate immune response to viral infection of SGIV and VNNV. EcDefensin was found to possess dual antiviral activity, inhibiting the infection and replication of SGIV and VNNV and inducting a type I interferon-related response in vitro. Synthetic peptide of EcDefensin (Ec-defensin) incubated with virus or cells before infection reduced the viral infectivity. Ec-defensin drastically decreased SGIV and VNNV titers, viral gene expression and structural protein accumulation. Grouper spleen cells over-expressing EcDefensin (GS/pcDNA-EcDefensin) support the inhibition of viral infection and the upregulation of the expression of host immune-related genes, such as antiviral protein Mx and pro-inflammatory cytokine IL-1β. EcDefensin activated type I IFN and Interferon-sensitive response element (ISRE) in vitro. Reporter genes of IFN-Luc and ISRE-Luc were significantly up-regulated in cells transfected with pcDNA-EcDefenisn after infection with SGIV and VNNV. These results suggest that EcDefensin is importantly involved in host immune responses to invasion of viral pathogens, and open the new avenues for design of antiviral agents in fisheries industry.

Application of in situ detection techniques to determine the systemic condition of lymphocystis disease virus infection in cultured gilt-head seabream, Sparus aurata L

Immunohistochemistry (IHC) and in situ hybridization (ISH) techniques have been used for the detection of lymphocystis disease virus (LCDV) in formalin-fixed, paraffin-embedded tissues from gilt-head seabream, Sparus aurata L. Diseased and recovered fish from the same population were analysed. IHC was performed with a polyclonal antibody against a 60-kDa viral protein. A specific digoxigenin-labelled probe, obtained by PCR amplification of a 270-bp fragment of the gene coding the LCDV major capsid protein, was used for ISH. LCDV was detected in skin dermis and gill lamellae, as well as in several internal organs such as the intestine, liver, spleen and kidney using both techniques. Fibroblasts, hepatocytes and macrophages seem to be target cells for virus replication. The presence of lymphocystis cells in the dermis of the skin and caudal fin, and necrotic changes in the epithelium of proximal renal tubules were the only histological alterations observed in fish showing signs of the disease.

Whole-genome transcriptional profiles of a novel marine fish iridovirus, Singapore grouper iridovirus (SGIV) in virus-infected grouper spleen cell cultures and in orange-spotted grouper, Epinephulus coioides

A DNA microarray containing all Singapore grouper iridovirus (SGIV) open reading frames (ORFs) was constructed to map the viral gene transcriptional profiles in virus-infected grouper spleen (GS) cells and in spleen tissues of virus-infected grouper. The results showed that viral genes started to be transcribed as early as 1 h postinfection (p.i.), and followed by a rapid increasing gene expression along with virus infection in cell cultures. The three temporal kinetic classes (15 immediate-early, 89 early and 53 late transcripts) were classified during an in vitro infection by their dependence on de novo protein synthesis and viral DNA replication inhibitors. In SGIV-infected grouper, Epinephulus coioides, most of the viral genes were expressed between 1 and 4 d p.i., and the number and expression levels started to decrease after 5 d p.i. These data were confirmed by real-time RT-PCR. This study provides an experimental basis for investigation of virus-host interactions and the development of control strategies against SGIV infection.

Rapid and sensitive detection of Singapore grouper iridovirus by loop-mediated isothermal amplification

AIMS

The aim of this paper was to develop a loop-mediated isothermal amplification (LAMP) method for rapid, sensitive and inexpensive detection of Singapore grouper iridovirus (SGIV) in grouper (GP), Epinephelus sp.

METHODS AND RESULTS

A set of six specific primers was designed by targeting the SGIV ORF-014L. With Bst DNA polymerase large fragment, the target DNA can be amplified as early as 20 min at 65 degrees C in a simple water bath. The detection limit is about 0.02 fg (equivalent to 6.3 copies) of plasmid ORF-014L. LAMP products could be judged with three different methods. There were no cross-reactions with seven other aquatic animal viruses indicating high specificity of the LAMP. The LAMP method was applied to detect SGIV in virus-infected GP cells and GP tissues effectively.

CONCLUSIONS

The LAMP described in this study is a cheap, sensitive, specific and rapid protocol for the detection of SGIV in cells and in GP tissues.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed LAMP method can be simply applied both in field condition and in laboratory operation for specific detection of SGIV infection.

Transcriptional profile of red seabream iridovirus in a fish model as revealed by viral DNA microarrays

Red seabream iridovirus (RSIV) disease is a serious disease of many marine fish species in Japan and elsewhere. For a better understanding of the molecular pathogenic mechanism, we examined the transcriptional profile of RSIV in infected fish using a DNA microarray. Expression of RSIV open reading frames (ORFs) was first detected at about 5 days post-infection (d.p.i.), and accounted for about 45% of total ORFs. Almost all the ORFs (97-99%) were expressed at their maximum levels during 7-9 d.p.i. The expression levels and the number of expressed ORFs started to decrease at 10 d.p.i. These results suggest that pathogenesis of RSIV infection began at around day 5, and continued with high levels of viral multiplication until viral clearance by host antiviral defenses starting from around 10 d.p.i. A comparison of viral gene expressions in the spleen and kidney over the course of the infection suggests that RSIV preferentially targets the spleen. The spleen may thus be the most susceptible organ for diagnosis of iridoviral disease.

Transcription program of red sea bream iridovirus as revealed by DNA microarrays

Red sea bream iridovirus (RSIV) has been identified as the causative agent of a serious disease in red sea bream and at least 30 other marine fish species. We developed a viral DNA microarray containing 92 putative open reading frames of RSIV to monitor the viral gene transcription program over the time course of an in vitro infection and to classify RSIV transcripts into temporal kinetic expression classes. The microarray analysis showed that viral genes commenced expression as early as 3 h postinfection (p.i.) and this was followed by a rapid escalation of gene expression from 8 h p.i. onwards. Based on the expression of some enzymes associated with viral DNA replication, the DNA replication of RSIV appeared to begin at around 8 h p.i. in infected cells in vitro. Using a de novo protein synthesis inhibitor (cycloheximide) and a viral DNA replication inhibitor (phosphonoacetic acid), 87 RSIV transcripts could be classified into three temporal kinetic classes: nine immediate-early (IE), 40 early (E), and 38 late (L) transcripts. The gene expression of RSIV occurred in a temporal kinetic cascade with three stages (IE, E, and L). Although the three classes of transcripts were distributed throughout the RSIV genome, E transcripts appeared to cluster in at least six discrete regions and L transcripts appeared to originate from seven discrete regions. The microarray data were statistically confirmed by using a t test, and were also clustered into groups based on similarity in the gene expression patterns by using a cluster program.

Development and characterization of a new tropical marine fish cell line from grouper, Epinephelus coioides susceptible to iridovirus and nodavirus

The development and characterization of a new tropical marine fish cell line (GS), derived from the spleen of orange spotted grouper, Epinephelus coioides is described. The GS cells grow well in Leibovitz's L-15 medium supplemented with 10% foetal bovine serum, and have been subcultured more than 200 times. The optimal growth temperature was 27 degrees C. The GS cell culture consisted of mostly fibroblastic cells. The modal diploid chromosome number was 48. GS cell cultures showed advanced cytopathic effects after infection with a pathogenic grouper iridovirus (Singapore grouper iridovirus, SGIV) or with a grouper nodavirus (Epinephelus tauvina nervous necrosis virus, ETNNV). Analysis by transmission electron microscopy showed a large number of SGIV and ETNNV particles in the cytoplasm of virus-infected cells, respectively, indicative of high sensitivity to these two viruses. Immunofluorescence microscopy showed that iridovirus-infected GS cells reacted strongly with monoclonal antibody against the grouper iridovirus. It is suggested that the GS cell line has good potential as a diagnostic tool for isolation and propagation of iridovirus and nodavirus. When the GS cells were transfected with pEGFP vector DNA, significant fluorescent signals were observed suggesting that the GS cell line can be used as a useful tool for transgenic and genetic manipulation studies.

Quantitative detection of a marine fish iridovirus isolated from large yellow croaker, Pseudosciaena crocea, using a molecular beacon

A real-time polymerase chain reaction (PCR) assay utilizing a molecular beacon for the quantitative detection of a marine fish iridovirus isolated from large yellow croaker, Pseudosciaena crocea (LYCIV), was developed, which involved the amplification of a 122bp DNA fragment from a conserved region of LYCIV ATPase gene. The specific probe consisting of two short arm and a central loop sequences complementary to the target amplicon was characterized with respect to its efficiency of quenching (E(ff)), and signal to background ratio by spectrofluorometric analysis of its hybridization with the complementary oligonucleotide target. The positive control plasmid pFHT-ATPase containing the target sequence was quantified to make the standard curve for sample detection after serial 10-fold dilution. Linear coefficient correlations between cycle threshold (C(T)) value and logarithmic positive plasmid concentration were close to one (r(2)=0.998) and the detection limit of the assay was 70 copies of positive plasmid/assay. The specificity of this real-time PCR was also demonstrated by using the genomic DNA templates from the healthy fish, white spot syndrome baculovirus (WSSV), and epizootic heamatopietic necrosis virus (EHNV), respectively. The coefficient of variation (CV) of the assay ranged from 1.16 to 4.42%, depending on the concentration of the positive plasmid. The quantitative detection of different tissues from LYCIV-infected fish showed that the spleen and kidney contained the largest number of viral particles (6.86 x 10(6) and 4.62 x 10(6) viral genome copies/mg tissue, respectively) while no viral DNA was detected in the muscular tissue. These results suggested that the real-time PCR assay reported here could be used for rapid, sensitive, and quantitative detection of LYCIV infection.