Experimental horizontal transmission of viral hemorrhagic septicemia virus (VHSV) in Japanese flounder Paralichthys olivaceus

Infection by viral hemorrhagic septicemia virus (VHSV) has recently occurred among wild and farmed Japanese flounder Paralichthys olivaceus in Japan. In the present study, horizontal transmission of VHSV among Japanese flounder was experimentally demonstrated by immersion challenge. Exposure to a flounder isolate (Obama25) of VHSV revealed a dose-response, with higher mortality (81 and 70%) at the 2 higher exposure levels (6.0 and 4.0 log10 TCID50 ml(-1)). In a second experiment, high titers of VHSV were expressed from moribund and dead flounder based on virus detection in holding-tank waters 2 to 3 d prior to death of the fish and 1 d after death. The virus could not be detected in tank waters 2 d after death. Finally, a third cohabitation experiment in small tanks demonstrated horizontal transmission of VHSV from experimentally infected to uninfected fish.     

An outbreak of VHSV (viral hemorrhagic septicemia virus) infection in farmed Japanese flounder Paralichthys olivaceus in Japan.

A rhabdoviral disease occurred in farmed populations of market sized Japanese flounder (hirame) Paralichthys olivaceus in the Seto Inland Sea of Japan in 1996. The causative agent was identified as viral hemorrhagic septicemia virus (VHSV) based on morphological, immunological, and genetic analyses. Diseased fish that were artificially injected with a representative virus isolate showed the same pathological signs and high mortality as observed in the natural outbreak. This is the first report of an outbreak of VHSV infection in cultured fish in Japan. Clinical signs of diseased fish included dark body coloration, an expanded abdomen due to ascites, congested liver, splenomegaly, and a swollen kidney. Myocardial necrosis was most prominent and accompanied by inflammatory reactions. Necrotic lesions also occurred in the liver, spleen and hematopoietic tissue, and were accompanied by circulatory disturbances due to cardiac failure. Hemorrhagic lesions did not always appear in the lateral musculature. Transmission electron microscopy revealed many rhabdovirus particles and associated inclusion bodies containing nucleocapsids in the necrotized myocardium. The histopathological findings indicated that the necrotizing myocarditis could be considered a pathognomonic sign of VHSV infection in Japanese flounder.     

Oral immunization of olive flounder (Paralichthys olivaceus) with recombinant live viral hemorrhagic septicemia virus (VHSV) induces protection against VHSV infection

A recombinant viral hemorrhagic septicemia virus (rVHSV-ΔNV-EGFP) that has enhanced green fluorescent protein (EGFP) gene instead of NV gene was previously generated using reverse genetics technology. In this study, potential of the rVHSV-ΔNV-EGFP to be used as a live oral vaccine candidate was assessed. The presence of the recombinant virus in internal organs of orally administered olive flounder (Paralichthys olivaceus) was analyzed by semi-quantitative RT-PCR. Although the recombinant VHSV-specific band was detected only when the number of PCR cycle was increased to 35, the band was detected from internal organs, such as kidney, spleen, and liver of fish that were reared at either 15 °C or 20 °C till even 20 days, suggesting that a few orally administered rVHSV-ΔNV-EGFP might be transported to internal organs, and might keep weak replication ability in the organs. VHSV-neutralizing activity was induced by oral immunization of olive flounder with the NV gene knock-out recombinant VHSV not only in skin and intestinal mucus but also in serum, suggesting that mucosal and systemic adaptive immune responses were elicited by oral immunization. In challenge experiment, groups of fish immunized with 10?, 10?, and 2 × 10? PFU of rVHSV-ΔNV-EGFP/fish showed 25%, 50%, and 70% of relative percent survival (RPS), respectively. The RPSs were elevated to 60%, 75%, and 90% by a boost immunization in fish boost immunized with 10?, 10?, and 2 × 10? PFU of rVHSV-ΔNV-EGFP, respectively. The cumulative mortality of fish in the control groups was 100%. Conclusionly, the present results demonstrate that the NV gene knock-out recombinant VHSV administered orally to olive flounder can induce dose- and boosting-dependent VHSV-neutralizing antibody in mucus and serum, and can provide a high protection in olive flounder against a virulent VHSV challenge.     

Cloning and characterisation of a cDNA encoding Japanese flounder Paralichthys olivaceus IgD

A cDNA containing the gene for Japanese flounder IgD consisted of 3240 bp encoding 998 amino acid residues. The amino acid sequence of the constant region of Japanese flounder IgD shares 38-80% identity with the sequences of previously reported teleost IgDs. The structure of the constant region of Japanese flounder IgD, which contains the micro1, delta1, delta2, delta3, delta4, delta5, delta6, delta7, and TM regions, is similar to the structures of the constant regions of the IgDs of channel catfish and Atlantic salmon. Southern blot hybridisation showed that the Japanese flounder IgD gene exists as a single locus. The Japanese flounder IgD gene was mainly detected in peripheral blood leucocytes (PBLs) and small amounts were detected in the spleen, head and trunk kidney, although IgM mRNA was detected in similar amounts in PBLs, the head kidney, and spleen. The copy number of IgM mRNA in Japanese flounder PBL was 56-fold higher than that of IgD.     

Difference in Japanese flounder, Paralichthys olivaceus gene expression profile following hirame rhabdovirus (HIRRV) G and N protein DNA vaccination

The glycoprotein (G protein) gene, but not other genes, of fish rhabdoviruses, when used as a DNA vaccine was previously shown to be highly effective in inducing a protective immune response. In this study we used a DNA microarray to examine differential gene expression in Japanese flounder (Paralichthys olivaceus) in response to a DNA vaccine made from the genes of hirame rhabdovirus (Rhabdovirus olivaceus) (HIRRV) G protein (pHRV-G) and nucleocapsid (N) protein (pHRV-N). A high level of protection against HIRRV infection was observed following vaccination with the pHRV-G but no protection was observed following vaccination with the pHRV-N. Microarray analyses showed that the set of genes induced by pHRV-G was different from the set induced by pHRV-N. Specifically, five genes (Interferon-stimulated gene, 15kDa (ISG15), Interferon-stimulated gene, 56kDa (ISG56), Mx and two unknown genes) were strongly induced after injection by the pHRV-G but not pHRV-N and three of these genes are known as type I IFN-inducible genes. Poly I:C, a known inducer of type I interferon that elicits immune response similar to that elicited by a virus infection, also induced these five genes in kidney cells. These results suggest that in order to be effective and confer protection, vaccines against HIRRV and probably fish rhabdoviruses may need to stimulate the type I IFN system.     

Japanese flounder (Paralichthys olivaceus) embryos are difficult to cryopreserve by vitrification

The first successful cryopreservation of fish embryos was reported in the Japanese flounder by vitrification [Chen and Tian, Theriogenology, 63, 1207-1219, 2005]. Since very high concentrations of cryoprotectants are needed for vitrification and fish embryos have a large volume, Japanese flounder embryos must have low sensitivity to cryoprotectant toxicity and high permeability to water and cryoprotectants. So, we investigated the sensitivity and the permeability of Japanese flounder embryos. In addition, we assessed the survival of flounder embryos after vitrification with solutions containing methanol and propylene glycol, following Chen and Tian's report. The embryos were relatively insensitive to the toxicity of individual cryoprotectants at lower concentrations, especially methanol and propylene glycol as their report. Although their permeability to water and cryoprotectants could not be measured from volume changes in cryoprotectant solutions, the embryos appeared to be permeable to methanol but less permeable to DMSO, ethylene glycol, and propylene glycol. Although vitrification solutions containing methanol and propylene glycol, which were used in Chen and Tian's report, were toxic to embryos, a small proportion of embryos did survived. However, when vitrified with the vitrification solutions, no embryos survived after warming. The embryos became opaque during cooling with liquid nitrogen, indicating the formation of intracellular ice during cooling. When embryos had been kept in vitrification solutions for 60 min after being treated with the vitrification solution, some remained transparent during cooling, but became opaque during warming. This suggests that dehydration and/or permeation by cryoprotectants were insufficient for vitrification of the embryos even after they had been over-treated with the vitrification solutions. Thus, Chen and Tian's cryopreservation method lacks general application to Japanese flounder embryos.     

Cloning and expression of partial Japanese flounder (Paralichthys olivaceus) IgD

The cDNA sequence of the Japanese flounder (Paralychthys olivaceus) IgD has been previously reported (GenBank accession no. AB052658) and this was followed by the detection of IgD mRNA expression in some flounder organ tissues. However, it has not been determined whether the flounder IgD gene is virtually expressed into IgD protein. To characterize the flounder immunoglobulins utilized in elucidating the mechanism, evolution and diversity of the flounder immune system, antibodies specific to IgD and IgM were necessary. In the present study, partial flounder recombinant IgD (rIgD), IgM (rIgM) and the conserved regions of IgD and IgM (rCIg) were produced by cloning the cDNA sequence using isotype specific primers which were designed to produce unique fragments of IgD and IgM specific amino acid sequences. The production of recombinant Igs was ascertained by SDS-gel electrophoresis and immunoblot analysis using anti-T7 d Taq antibody. The produced recombinant Igs were purified using affinity columns, and used as immunogens. Antibodies specific to the isotype of flounder Igs were generated by immunizing rabbits with rfIgs and the antibodies produced were identified by enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Specificities of the generated antibodies were evaluated by testing cross-reactivity between recombinant IgM and IgD. By ELISA, rabbit antibodies against the rfIgD fragment (anti-rfIgD) failed to recognize any kind of flounder serum Igs, whereas respective antibodies against rfCIg (anti-rfCIg) and rfIgM fragments (anti-rfIgM) reacted with serum Igs. Likewise, in immunoblot assays, though anti-rfIgD did not, both anti-rfCIg and anti-rfIgM bound with the ~85 kd flounder IgM heavy chain. By flow cytometry analysis, anti-rfCIg, anti-rfIgD and anti-rfIgM reacted with 6%, 3% and 6.5% of cells, respectively, suggesting that flounder IgD is not secreted in serum but expressed on flounder B-like cell surfaces as in mammals. Antibodies produced against recombinant flounder Igs could be used to develop sandwich assay systems for detecting flounder Igs and for further investigating the flounder immune system.     

Viral hemorrhagic septicemia (VHS) of nonsalmonids

Viral hemorrhagic septicemia (VHS) is a viscerotropic disease of fish that can cause enormous losses in European rainbow trout populations. Although previously thought to be species specific, recent reports have indicated that other nonsalmonid fish species, such as pike, whitefish, grayling, turbot, and herring, can become mortally infected with VHS. This paper reviews several cases of natural VHS-outbreaks in nonsalmonids. Pike, whitefish, grayling, turbot and Pacific herring infected with VHS displayed symptoms typical of hemorrhagic septicemia. The isolation and serological identification of the viruses from all of the infected fish species revealed a close relation to the Egtved-virus strain F1. The virus isolates from diseased pike and rainbow trout were capable of infecting their respective host fish. Experimental infections of pike, whitefish, and grayling fry with Egtved virus strain F1 resulted in high mortalities, with symptoms typical of VHS. Although experiments with older fish suggested an age-dependent decrease in susceptibility, the virus could be isolated from most of the infected fish after several months of experimental infection (carrier status). Histological and electron microscopical findings were comparable to those seen in VHS-infected rainbow trout. The susceptibility of nonsalmonid species to VHS infection is disconcerting with respect to the potential losses of the economically important turbot, Pacific herring, and whitefish, as well as the ecologically valuable grayling which is threatened by extinction. If the VHS susceptible nonsalmonid species can also support the propagation of the Egtved-virus, it could acquire carrier-status and, as a result, represent a “natural” reservoir for the virus.     

牙鲆变态过程中的细胞凋亡

利用整体的原位TUNEL方法检测了牙鲆(Paralichthysolivaceus)变态过程中身体各器官细胞凋亡的分布及变化情况。结果如下:(1)与眼睛移动相关的脑颅骨骼的细胞凋亡右侧眼睛移动开始之后,在额骨、中筛软骨和犁骨软骨中出现细胞凋亡,并保持到眼睛移动结束;(2)中枢神经和感觉器官的细胞凋亡在眼睛移动开始之前,脊髓和脊髓鞘出现细胞凋亡,在眼睛移动开始之后,脊髓和脊髓鞘细胞凋亡停止,而在脑、眼睛和内耳出现细胞凋亡,并一直持续到眼睛移动结束;(3)与游泳、捕食和消化等功能相关的器官的细胞凋亡在眼睛移动开始后,冠状幼鳍的基部出现凋亡;在变态中后期,尾鳍基部出现细胞凋亡;下颌骨、鳃弓以及肝脏在眼睛移动开始之后,出现细胞凋亡,也一直持续到眼睛移动结束。细胞凋亡通过有序地去除多余的细胞来参与器官形态建立和重组,本研究的结果表明,在牙鲆器官功能变化过程中,细胞凋亡在与其相适应的的器官形态重塑中起着重要作用[动物学报52(2):355-361,2006]。     

Characterization of Japanese flounder (Paralichthys olivaceus) NK-lysin, an antimicrobial peptide

The NK-lysin cDNA of Japanese flounder, Paralichthys olivaceus, consists of 657bp, containing an open reading frame (ORF) of 444bp, which encodes 147 amino acid residues. The amino acid sequence of Japanese flounder NK-lysin has 21% identity to porcine NK-lysin and bovine NK-lysin, 23% to equine NK-lysin, and 46% to zebrafish NK-lysin-like protein. Multiple alignments of Japanese flounder NK-lysin and other known saposin-like proteins revealed that the six cysteine residues important for structural folding are completely conserved. The Japanese flounder NK-lysin gene is approximately 2kb and consists of five exons and four introns. Japanese flounder NK-lysin mRNA constitutive expression was mainly detected in gills, heart, head kidney, intestines, peripheral blood leukocytes (PBLs), spleen and trunk kidney, and was detected at low levels in liver, muscle and ovary. However, expression was not detected in brain, skin and stomach of apparently healthy Japanese flounder. Gene expression of Japanese flounder NK-lysin was not inducible by lipopolysaccharide (LPS) treatment. A synthesized NK-lysin peptide, consisting of 27 amino acid residues, showed antimicrobial activity against Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Photobacterium damselae subsp. piscicida.     

Neuromast formation in the prehatching embryos of the Japanese flounder (Paralichthys olivaceus)

The present paper clarifies the initial development of the lateral line organs in the embryonic Japanese flounder, Paralichthys olivaceus. The first appearances of lateral line primordia, and the proliferation, distribution and morphological development of the free neuromasts, including nerve ending formation: establishment of hair cell innervations via the formation of synapses, were examined by light microscopy, scanning and transmission electron microscopy. The first pair of neuromast primordia appeared in the otic region ≈ 30 h prior to hatching and subsequently differentiated into free neuromasts, otic neuromasts, after ≈ 8 h. At hatching, a pair of free neuromasts and three pairs of neuromast primordia were present on the head, and three pairs of neuromast primordia were present on the trunk. The hair cell polarity of the otic neuromast until just prior to hatching was radial, but not bi‐directional. The typical afferent and efferent nerve endings in the otic neuromasts had formed by the time of hatching, suggesting that the otic neuromasts are functional prior to hatching. The three neuromast primordia located on each side of the trunk were derived from a long, narrow ectodermal cell cluster and erupted through the epidermis after hatching.     

Effects of NV gene knock-out recombinant viral hemorrhagic septicemia virus (VHSV) on Mx gene expression in Epithelioma papulosum cyprini (EPC) cells and olive flounder (Paralichthys olivaceus)

To determine whether the NV gene of viral hemorrhagic septicemia virus (VHSV) is related to the type I interferon response of hosts, expression of Mx gene in Epithelioma papulosum cyprini (EPC) cells and in olive flounder (Paralichthys olivaceus) in response to infection with either wild-type VHSV or recombinant VHSVs (rVHSV-ΔNV-EGFP and rVHSV-wild) was investigated. A reporter vector was constructed for measuring Mx gene expression using olive flounder Mx promoter, in which the reporter Metridia luciferase was designed to be excreted to culture medium to facilitate measurement. The highest increase of luciferase activity was detected from supernatant of cells infected with rVHSV-ΔNV-EGFP. In contrast cells infected with wild-type VHSV showed a slight increase of the luciferase activity. Interestingly, cells infected with rVHSV-wild that has artificially changed nucleotides just before and after the NV gene ORF, also showed highly increased luciferase activity, but the increased amplitude was lower than that by rVHSV-ΔNV-EGFP. These results strongly suggest that the NV protein of VHSV plays an important role in suppressing interferon response in host cells, which provides a condition for the viruses to efficiently proliferate in host cells. In an in vivo experiment, the Mx gene expression in olive flounder challenged with the rVHSV-ΔNV-EGFP was clearly higher than fish challenged with rVHSV-wild or wild-type VHSV, suggesting that lacking of the NV gene in the genome of rVHSV-ΔNV-EGFP brought to strong interferon response that subsequently inhibit viral replication in fish.     

cDNA cloning and phylogenetic analysis of pancreatic serine proteases from Japanese flounder,Paralichthys olivaceus

To better understand the digestive physiology and phylogeny of the pancreatic serine proteases of teleosts, we cloned trypsin, chymotrypsin and elastase from flounder (Paralichthys olivaceus). Fifty phage plaques randomly chosen from a flounder pancreatic cDNA library were found to contain three species of trypsin, two species of chymotrypsin and four species of elastase. cDNAs of two species of carboxypeptidase A, one carboxypeptidase B and lipase were also obtained. In total, 23 out of 24 digestive enzyme cDNAs were those of proteolytic enzymes. Such a high ratio of proteolytic enzyme cDNA in the pancreas may reflect the carnivorous feeding habits of flounder. A phylogenetic comparison of the peptide sequences of flounder enzymes with those of other teleosts and mammals suggested that duplication of trypsin, chymotrypsin and elastase occurred before the divergence of the ray finned fish. It is also hypothesized that functional descendants of both duplicated genes of elastase exist in the teleosts and mammals, whereas only one of the genes of trypsin and chymotrypsin gave rise to the functional descendants in the teleosts but not in the mammals.     

In vivo and in vitro analysis of the resistance against viral haemorrhagic septicaemia virus in Japanese flounder (Paralichthys olivaceus) precedingly infected with aquabirnavirus

The resistance of Japanese flounder (Paralichthys olivaceus Temminck et Schlegel) against a viral haemorrhagic septicaemia virus (VHSV) challenge induced by a preceding non-lethal aquabirnavirus (ABV) challenge was investigated through experimental dual-infections with different intervals between the two challenges. The non-specific protection conferred by the primary ABV infection against the secondary VHSV infection commenced at Day 3 and persisted up to Day 14 but vanished at Day 21 post-ABV challenge. The in vitro assay using HINAE (hirame natural embryo) cells demonstrated anti-VHSV activity in the serum of ABV-challenged flounder from Day 1 to Day 14 but not at Day 21 post-ABV challenge. A high expression of a Mx gene, a molecular marker of type I interferon(s) (IFN) occurred in the head kidneys of ABV-challenged flounder from Day 1 to Day 7. These results suggest that the non-specific protection against the secondary VHSV infection in flounder was due to IFN(s) induced by the primary ABV infection.     

Digestive capacity, growth and social stress in newly-metamorphosed Japanese flounder (Paralichthys olivaceus)

Hierarchies readily appear when rearing flounders through metamorphosis in space-limited conditions. In this experiment, subordinate fish were stressed, as suggested by their elevated cortisol level compared to dominant fish. Subordinate fish, although of smaller size than the dominant fish, showed no suppressed digestive capacity. By separating the two hierarchical groups into different tanks at low density, the cortisol level of the subordinate fish substantially decreased. The removal of the stressor also resulted in an increase in digestive function and improved coloration. Also, compensatory growth in length but not in weight was observed in the subordinate group, suggesting that the subordinate fish devoted their energy to increase their length. These results imply that the climax of flounder metamorphosis (i.e. the settlement stage) is a highly sensitive period, where social interactions may induce high levels of stress. However, Japanese flounder early juveniles prove to have a high recovery capacity from stress and show to allocate energy preferentially to growth in length. This seems to be an adaptation to diminish the probability of death by predation.     

Poly(lactic-co-glycolic acid) nanoparticles as candidate DNA vaccine carrier for oral immunization of Japanese flounder (Paralichthys olivaceus) against lymphocystis disease virus

In order to protect DNA vaccine against degradation in alimentary tract of fish, poly(lactic-co-glycolic acid) (PLGA) nanoparticles encapsulating vaccine were prepared using W/O/W emulsification combined with spray drying technique in our laboratory. The characteristics of PLGA nanoparticles were described as follows: (1) shape, spherical; (2) size, <500 nm; (3) yield, ～96.2%; loading percentage, ～0.5%; encapsulation efficiency, ～63.7%; supercoiled conformation percentage, ～65%; (4) release dynamics, gradual release. In vitro transfection in SISK cells showed that PLGA nanoparticles could be utilized to transfect eukaryotes. After oral administration, FITC-labeled PLGA nanoparticles were detected in blood of fish, and RNA containing major capsid protein (MCP) gene information existed in various tissues of fish 10-90 days. In addition, the analysis of immune parameters in sera of treatment fish showed that: (1) infection rate of LCDV post-challenge, ～16.7%; (2) prophenoloxidase, superoxide dismutase, respiratory burst, lysozyme and antibody levels, increased significantly (p<0.05); (3) activities of serum complement, changed a little (p>0.05). Pearson's correlation displayed that correlation of immune factors mentioned above (not including serum complement) were all positive for fish vaccinated. The data in this study suggested that PLGA nanoparticles were promising carriers for plasmid DNA vaccine and might be used to vaccinate fish by oral approach.