Molecular characterization and expression analysis of septin gene family and phagocytic function of recombinant septin 2, 3 and 8 of starry flounder (Platichthys stellatus)

Septin is an evolutionarily conserved family of GTP-binding proteins. Septins are known to be involved in a variety of cellular processes, including cell division, chromosome separation, cell polarity, motility, membrane dynamics, exocytosis, apoptosis, phagocytosis, DNA damage responses, and other immune responses. In this study, the sequences of the septin gene family of starry flounder were obtained using NGS sequencing, and the integrity of the sequences was verified through cloning and sequencing. At first, the amino acid sequence was annotated using the cDNA sequence, and then, the gene sequence was verified through multiple sequence alignment and phylogenetic analyses using the related conserved sequences. The septin gene family was classified into three subgroups based on the phylogenetic analysis. High conservation within the domain and homology between the genes reported in different species were confirmed. The expression level of septin gene family mRNA in each tissue of healthy starry flounder was evaluated to confirm the tissue- and gene-specific expression levels. Additionally, as a result of the analysis of mRNA expression after simulated pathogen infection, significant expression changes and characteristics were confirmed upon infection with bacteria (Streptococcus parauberis PH0710) and virus (VHSV). Based on the current results and that of previous studies, to confirm the immunological function, Septin 2, 3, and 8 were produced as recombinant proteins based on the amino acid sequences, and their role in phagocytosis was further investigated. The results of this study indicate that septin gene family plays a complex and crucial role in the host immune response to pathogens of starry flounder.

First report of eosinophil peroxidase in starry flounder (Platichthys stellatus): Gene identification and gene expression profiling

Eosinophils are granular leukocytes that are evolutionarily preserved in the innate immune system of some invertebrates and vertebrates, and these cells can directly remove invading microorganisms and secrete various cytokines, and are also involved in homeostasis. These eosinophils are made up of specific granular proteins that can be differentiated from other cells, and eosinophil peroxidase (EPX) is a peroxidase released only from eosinophils that plays an important role in maintaining the main function and homeostasis of eosinophils. We obtained the sequence information of EPX for the first time from the starry flounder (Platichthys stellatus), and predicted it by amino acid sequencing to confirm sequence alignment and phylogenetic characteristics with other species. Based on analysis of the expression characteristics of PsEPX mRNA in healthy P. stellatus, it was expressed at the highest level in peripheral blood lymphocytes (PBLs) and was also expressed at a relatively high level in the head kidney and intestine, which are immune-related tissues. After artificial infection with Streptococcus parauberis and viral haemorrhagic septicaemia virus, which are the causes of major pathogenic diseases, the expression level of PsEPX was significantly regulated, which showed specific characteristics of pathogens or tissues. These results suggest that PsEPX is an important component of the immune system of P. stellatus and is considered a basic research case for the study of the immunological function of eosinophils in fish.

Functional characterization and gene expression profile of perforin-2 in starry flounder (Platichthys stellatus)

The membrane attack complex/perforin (MACPF) superfamily consists of multifunctional proteins that form pores on the membrane surface of microorganisms to induce their death and have various immune-related functions. PFN2 is a perforin-like protein with an MACPF domain, and humans with deficient PFN2 levels have increased susceptibility to bacterial infection, which can lead to fatal consequences for some patients. Therefore, in this study, we confirmed the antimicrobial function of PFN2 in starry flounder (Platichthys stellatus). The molecular properties were confirmed based on the verified amino acid sequence of PsPFN2. In addition, the expression characteristics of tissue-specific and pathogen-specific PsPFN2 mRNA were also confirmed. The recombinant protein was produced using Escherichia coli, and the antimicrobial activity was then confirmed. The coding sequence of PFN2 (PsPFN2) in P. stellatus consists of 710 residues. The MACPF domain was conserved throughout evolution, as shown by multiple sequence alignment and phylogenetic analysis. PsPFN2 mRNA is abundantly distributed in immune-related organs such as the spleen and gills of healthy starry flounder, and significant expression changes were confirmed after artificial infection by bacteria or viruses. We cloned the MACPF domain region of PFN2 to produce a recombinant protein (rPFN2) and confirmed its antibacterial effect against a wide range of bacterial species and the parasite (Miamiensis avidus).

Presence of viral haemorrhagic septicaemia virus (VHSV) in the environment of virus-contaminated fish farms and processing plants

After the first outbreak of viral haemorrhagic septicaemia virus (VHSV) in Finnish brackish water rainbow trout Oncorhynchus mykiss farms, infection spread rapidly between the farms. The infrastructure of fish farming did not take into account spreading of infectious fish diseases. To show the presence of VHSV in the environment, we tested seawater, sediment and wild blue mussels Mytilus edulis from VHSV-infected fish farms, and liquid waste from a processing plant that handled infected rainbow trout. Additionally, blue mussels were bath-challenged with VHSV (exposed to cultivated virus or naturally infected rainbow trout). To detect VHSV, virus isolation in cell culture and real-time reverse transcriptase polymerase chain reaction (qRT-PCR) were used. The virus or viral RNA was detected in sea water and in liquid waste from processing plants during wintertime when water temperature is close to 0°C and sunlight is sparse. VHSV did not appear to replicate in blue mussels in our study. Therefore, blue mussels were not considered relevant carriers of VHSV. However, traces of viral RNA were detected up to 29 d post challenge in mussels. Contact with water from processing plants handling VHSV-infected fish populations increases the risk of the disease spreading to susceptible fish populations, especially during cold and dark times of the year.

Comparative susceptibilities and immune reactions of wild and cultured populations of Caspian trout Salmo trutta caspius to VHSV

Caspian trout Salmo trutta caspius is an endangered subspecies of brown trout Salmo trutta which is native to the Caspian Sea. Restocking programmes have been established, but recent introduction of the rhabdovirus viral haemorrhagic septicaemia virus (VHSV) into Iranian rainbow trout farms connected to waterbodies supporting wild Caspian trout may represent an additional threat to the declining stock. The susceptibility of wild and cultured populations of this endemic subspecies was demonstrated by performing controlled VHSV infection experiments (both by bath and injection challenges). Subsequently, VHSV infection in exposed fish was confirmed (CPE and quantitative PCR), virus levels were measured, and regulation of immune genes in exposed fish was investigated with a focus on the genes encoding IL-8, IFNγ, TGFβ, TNFα, SAA, C3-4, CD8α, IgM, MHC I, MHC II, iNOS and IGF-1. The presence of IgM-, CD8α- and MHC II-positive cells in host organs was visualized by immunohistochemistry. Both wild and cultured trout strains proved to be VHSV-susceptible following experimental challenge, but the mortality curves and associated regulation of immune-related genes differed between the 2 trout types. Implications of the results for future management of Caspian trout populations are discussed.

Virulence marker candidates in N-protein of viral haemorrhagic septicaemia virus (VHSV): virulence variability within VHSV Ib clones

Four major genotypes of viral haemorrhagic septicaemia virus (VHSV), which have been isolated from many marine and freshwater fish species, are known to differ in virulence. While fast and low-cost genotyping systems based on monoclonal antibodies (MAbs) have been developed for typing of VHSV virulence, there is a need for supplementing the knowledge. In particular, 2 field isolates from viral haemorrhagic septicaemia (VHS) outbreaks in sea-reared rainbow trout Oncorhynchus mykiss in Sweden, SE-SVA-14 and SE-SVA-1033 (both genotype Ib), have yielded contradictory reactions. In the present study, upon cloning by limited dilution, both isolates appeared to be heterogeneous in terms of reactivity with nucleo (N)-protein-specific MAbs as well their gene sequences. Infection trials in rainbow trout further revealed differences in the virulence of these virus clones derived from the same primary isolate. Based on a comparative analysis of the entire genome of the clones tested, we suggest that the differences in virulence are tentatively linked to substitutions of amino acids (aa) in the N-protein region covered by aa 43-46 and aa position 168, or a combination of the two. The fact that such minor naturally occurring genetic differences affect the virulence implies that even low-virulent VHSV isolates in the marine environment should be considered as a potential threat for the trout farming industry. The described MAbs can represent useful tools for initial risk assessment of disease outbreaks in farmed trout by marine VHSV isolates.

Viral haemorrhagic septicaemia virus (VHSV) remains viable for several days but at low levels in the water flea Moina macrocopa

Viral haemorrhagic septicaemia virus (VHSV) Genotype IVb has been isolated from amphipods belonging to the genus Diporeia, but it has yet to be established whether crustacean zooplankton act as vectors of this virus for fish species. Therefore, we evaluated the viability of infectious VHSV in the water flea Moina macrocopa. VHSV was re-isolated from replicate groups of M. macrocopa that had been immersed with 108.0, 107.0, and 105.0 TCID50 ml-1 of VHSV (DK-3592B, Genotype Ia). Furthermore, 40 M. macrocopa that had been immersed with 108.0 TCID50 ml-1 of VHSV for 72 h had VHSV titers of 102.7-104.3 TCID50. Thus, VHSV was clearly taken up by M. macrocopa and remained viable in this crustacean for several days. However, no mortality was observed over a 28 d period in rainbow trout Oncorhynchus mykiss that were fed VHSV-contaminated M. macrocopa for 14 d, and we found that the virus titer significantly decreased after a 4 h incubation with pyloric caecal extracts from rainbow trout, indicating that passage through the gut is likely to result in a significant decrease in viral titer. This may explain why consumption of prey containing low levels of VHSV did not result in clinical VHS.

Prediction of G gene epitopes of viral hemorrhagic septicemia virus and eukaryotic expression of major antigen determinant sequence

This study aims to express fish Viral hemorrhagic septicemia virus (VHSV) G main antigen domain by using Bac-to-bac expression system. Using bioinformatics tools, B cell epitope of VHSV G gene was predicted, and G main antigen domain was optimized. GM gene was inserted into pFastBac1 vector, then transferred recombinant plasmid into DH10Bac to get recombinant rBacmid-GM. Obtained shuttle plasmid rBacmid-GM was transfected into sf9 cells. GM expression was examined using by PCR and western-blot. Results indicated that G main antigen domain gene of VHSV was successfully cloned and sequenced which contains 1209 bp. PCR proved that shuttle plasmid rBacmid-GM was constructed correctly. SDS-PAGE electrophoresis analysis detected a band of protein about 45kD in expression product of G gene. Obtained recombinant G protein reacted with VHSV-positive serum that was substantiated by western-blot analysis. In conclusion, the main antigen domain of VHSV G was successfully expressed in the Bac-to-Bac baculovirus system.

Differentially expressed genes after viral haemorrhagic septicaemia virus infection in olive flounder (Paralichthys olivaceus)

A strain of viral haemorrhagic septicaemia virus (VHSV) was isolated from cultured olive flounder (Paralichthys olivaceus) during epizootics in South Korean. This strain showed high mortality to olive flounder in in vivo challenge experiment. The complete genomic RNA sequences were determined and phylogenetic analysis of the amino acid sequences of glycoprotein revealed that this isolate was grouped into genotype IVa of genus Novirhabdovirus. Expression profile of genes in olive flounder was analyzed at day 1 and day3 after infection with this VHSV isolate by using cDNA microarray containing olive flounder 13K cDNA clones. Microarray analysis revealed 785 up-regulated genes and 641 down-regulated genes by at least two-fold in virus-infected fish compared to healthy control groups. Among 785 up-regulated genes, we identified seven immune response-associated genes, including the interferon (IFN)-induced 56-kDa protein (IFI56), suppressor of cytokine signaling 1 (SOCS1), interleukin 8 (IL-8), cluster of differentiation 83 (CD83), α-globin (HBA), VHSV-induced protein-6 (VHSV6), and cluster of differentiation antigen 9 (CD9). Our results confirm previous reports that even virulent strain of VHSV induces expression of genes involved in protective immunity against VHSV.

In vivo virulence of viral haemorrhagic septicaemia virus (VHSV) in rainbow trout Oncorhynchus mykiss correlates inversely with in vitro Mx gene expression

The in vitro replication of viral haemorrhagic septicaemia virus (VHSV) isolates from each VHSV genotype and the associated cellular host Mx gene expression were analysed. All the isolates were able to infect RTG-2 cells and induce increased Mx gene expression (generic assay detecting isoforms 1 and 3 [Mx1/3]). A trout pathogenic, genotype Ia isolate (J167), showing high replication in RTG-2 cells (by infective titre and N gene expression) induced lower Mx1/3 gene expression than observed in VHSV isolates known to be non-pathogenic to rainbow trout: 96-43/8, 96-43/10 (Ib); 1p49, 1p53 (II); and MI03 (IVb). Paired co-inoculation assays were analysed using equal number of plaque forming units per ml (PFU) of J167 (Ia genotype) with other less pathogenic VHSV genotypes. In these co-inoculations, the Mx1/3 gene expression was significantly lower than for the non-pathogenic isolate alone. Of the three rainbow trout Mx isoforms, J167 did not induce Mx1 up-regulation in RTG-2 or RTgill-W1 cells. Co-inoculating isolates resulted in greater inhibition of Mx in both rainbow trout cell lines studied. Up-regulation of sea bream Mx in SAF-1 cells induced by 96-43/8 was also lower in co-inoculation assays with J167. The RTG-P1 cell line, expressing luciferase under the control of the interferon-induced Mx rainbow trout gene promoter, showed low luciferase activity when inoculated with pathogenic strains: J167, DK-5131 (Ic), NO-A-163/68 (Id), TR-206239-1, TR-22207111 (Ie), 99-292 (IVa), and CA-NB00-01 (IVc). Co-inoculation assays showed a J167-dose dependent inhibition of the luciferase activity. The data suggest that virulent VHSV isolates may interfere in the interferon pathways, potentially determining higher pathogenicity.