Establishment and characterization of a fish-cell line from the brain of Japanese flounder Paralichthys olivaceus

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

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

Molecular cytogenetic analysis of the olive flounder embryonic cell line FGBC8 and its applicability to biotechnology

We explored the biotechnological applicability of a previously established olive flounder (Paralichthys olivaceus) embryonic cell line (FGBC8). FGBC8 was transfected with pEGFP-c1 and pluripotency-related genes, then infected with viral hemorrhagic septicemia virus (VHSV), and the expression of immune-related genes was observed through quantitative real-time polymerase chain reaction. Transfected cells showed strong green fluorescence 48 h after transfection, and pluripotency-related genes were successfully transfected. In addition, FGBC8 cells were highly susceptible to VHSV and the expression of immune-related genes was induced during infection. Our results demonstrate that FGBC8 cells are valuable research tools for assessing host-pathogen interactions and biotechnological applications.

Establishment and Characterization of Continuous Satellite Muscle Cells from Olive Flounder (Paralichthys olivaceus): Isolation, Culture Conditions, and Myogenic Protein Expression

Olive flounder (Paralichthys olivaceus) muscle satellite cells (OFMCs) were obtained by enzymatic primary cell isolation and the explant method. Enzymatic isolation yielded cells that reached 80% confluence within 8 days, compared to 15 days for the explant method. Optimal OFMC growth was observed in 20% fetal bovine serum at 28 °C with 0.8 mM CaCl2 and the basic fibroblast growth factor (BFGF) to enhance cell growth. OFMCs have become permanent cell lines through the spontaneous immortalization crisis at the 20th passage. Olive flounder skeletal muscle myoblasts were induced into a mitogen-poor medium containing 2% horse serum for differentiation; they fused to form multinucleate myotubes. The results indicated complete differentiation of myoblasts into myotubes; we also detected the expression of the myogenic regulatory factors myoD, myogenin, and desmin. Upregulation (Myogenin, desmin) and downregulation (MyoD) of muscle regulation factors confirmed the differentiation in OFMCs.

Establishment and application of four long-term culture cell lines of the olive flounder Paralichthys olivaceus blastocysts

Four new embryonic cell lines derived from blastocysts of the olive flounder Paralichthys olivaceus, an important commercial marine fish, were established and characterized. They were designated as PoEFCI, PoEFCII, PoEFCIII, and PoEFCIV and were all fibroblastic cells. The cells were cultured in DMEM/F-12 medium supplemented with antibiotics, FBS, and growth factors at temperature of 25 °C and subcultured for >100 passages over 18 months. The origin of the cell lines was confirmed by examining the partial sequences of the cytochrome oxidase c subunit I (COI) gene of the flounder mitochondrial DNA (mtDNA). The four cell lines showed different growth curve patterns. According to the results of gene and protein expression and enzyme activity, the cell lines PoEFCI, PoEFCII, and PoEFC III could be pluripotent. The cells of all four cell lines were also successfully transfected with the green fluorescent protein (GFP) reporter gene, suggesting that they could be used to study gene function in the flounder or other fish. More importantly, PoEFCI-III were sensitive to chromium (Cr) and red sea bream Pagrus major iridovirus (RSIV), so they could be used as a powerful tool for the study of the toxicological investigation of heavy metals and RSIV in fish. Therefore, these cell lines would be useful for biotechnological and toxicological research on marine fish as an in vitro biological system.

A New Cell Line Derived from the Spleen of the Japanese Flounder (Paralichthys olivaceus) and Its Application in Viral Study

A new cell line Japanese flounder spleen (JFSP) derived from the spleen of Japanese flounder (Paralichthys olivaceus) was established and characterized in this study. The JFSP cells grew rapidly at 29 °C, and the optimum fetal bovine serum concentration in the L-15 medium was 15%. Cells were subcultured for more than 80 passages. The JFSP cells have a diploid chromosome number of 2n = 68, which differs from the chromosome number of normal diploid Japanese flounder. The established cells were susceptible to Bohle virus (BIV), Viral hemorrhagic septicemia virus (VHSV), Hirame rhabdovirus (HIRRV), Infectious hematopoietic necrosis virus (IHNV), and Lymphocystis disease virus (LCDV), as evidenced by varying degrees of cytopathic effects (CPE). Replication of the virus in JFSP cells was confirmed by qRT-PCR and transmission electron microscopy. In addition, the expression of four immune-related genes, TRAF3, IL-1β, TNF-α, and TLR2, was differentially altered following viral infection. The results indicated that the cells underwent an antiviral immune response. JFSP cell line is an ideal tool in vitro for virology. The use of fish cell lines to study the immune genes and immune mechanism of fish and to clarify the immune mechanism of fish has important theoretical significance and practical application value for the fundamental prevention and treatment of fish diseases.

Characterization of three novel cell lines derived from the brain of spotted sea bass: Focusing on cell markers and susceptibility toward iridoviruses

Despite tens of cell lines originating from fish brain tissue have been constructed, little is known about the definite cell types they belong to. Whether fish cell lines derived from the brain shares similar characteristics is not well-answered yet. Here, we constructed three cell lines designated as LMB-S, LMB-M, LMB-L using brain tissue of spotted sea bass (Lateolabrax maculatus). Among them, LMB-L was identified as astroglia-like cells considering the high expression of GFAP, DCX, PTX, S100b, which are regarded as astrocyte-specific or astrocyte-associated cell markers. LMB-M exhibited smooth muscle-like features showing strong expression of LMOD1, SLAMP, M-cadherin, MGP, which are confirmed as muscle-restricted or myogenesis-involved cell markers. Although LMB-S was not definitely identified, it appeared an activation of WNT/β-catenin pathway. Besides the distinct expression profiles of cell markers, the three cell lines also presented differences in transfection efficiency and susceptibility to iridovirus infection. Relying on the established cell lines, a novel megalocytivirus, named LMIV (Lateolabrax maculatus iridovirus), was first isolated from diseased spotted sea bass. Genetic analysis of major capsid protein (MCP) and adenosine triphosphatase (ATPase) manifested that LMIV was clearly distinguishable from other representative teleost iridoviruses. Further investigations revealed that LMIV could replicate most efficiently in LMB-L cells obtaining the highest viral load (2.16 × 10¹⁰ copy/mL). By contrast, LMB-S cells gave rise to the highest viral load up to 3.86 × 10⁸ copy/mL, when the three cell lines were infected with MRV, a newly emerged ranavirus. Moreover, LMIV infection caused lots of cells to be detached from monolayers, generating adherent and non-adherent cells. An opposite expression profiling of type I IFN pathway-related genes (JAK1, STAT1, STAT2, IRF9, Mx1) was found between adherent and non-adherent cells. Combined with the analysis of MCP gene expression, it is speculated that inhibiting type I IFN pathway in non-adherent cells allowed the facilitation of virus duplication. Taken together, the present study broadens our understanding about the diversity of cell lines derived from fish brain tissue and screening cells more susceptible to virus is not only meaningful for the development of vaccine, but also provide clues for further clarification of cell-iridovirus interactions.

Establishment of an astrocyte-like cell line from the brain of tilapia (Oreochromis niloticus) for virus pathogenesis and a vitro model of the blood-brain barrier

A new cell line was established from the brain of a cultured fish, tilapia (Oreochromis niloticus), designated as TA-02 (Tilapia Astrocyte clone 02 cell line). The TA-02 cells are grown for 300 days in an L-15 medium supplemented with 10% fetal bovine serum (FBS). This cell line showed excellent proliferative capacity and expressed various neuroglial cell markers, including SOX2, SOX10, Hes1, Notch1, Occludin, E-cadherin, and GFAP. In addition, TA-02 cells were susceptible to Tilapia Lake Virus (TiLV) as demonstrated by the presence of a severe cytopathic effect (CPE), virus particle in a transmission electron microscope (TEM), and PCR positive signal. Bacterial cytotoxicity studies showed that Streptococcus agalactiae was toxic to TA-02 cells. When co-culture with trans-well, TA-02 exhibited prominent barrier properties, manifested by tight intercellular junctions and increased trans-endothelial electrical resistance (TEER). In addition, the barrier is effective against Escherichia coli (non-meningitis pathogenic bacteria). In contrast, S. agalactiae (meningitis pathogenic bacteria) can pass through the membrane comprising the cells in the trans-well insert. The newly established TA-02 cell line provided a valuable tool for virus pathogenesis and a vitro model of the fish blood-brain barrier.

Establishment, characterization, and transfection potential of a new continuous fish cell line (CAM) derived from the muscle tissue of grass goldfish (Carassius auratus)

A new continuous fish cell line (CAM) has been successfully derived from the muscle tissues of grass goldfish, Carassius auratus. The primary cell cultures were initiated by incomplete trypsinization first and then explant culture in a Leibovitz-15 medium supplemented with 15% fetal bovine serum and 10% fish muscle extract. It was found that the CAM cells were very sensitive to trypsinization and needed to be sub-cultured at a low trypsin concentration of 0.0625% to be able to go through the crisis of spontaneous immortalization transformation, and afterward a total of five derivative cell strains were isolated from the original CAM cell line. This spontaneous immortalization transformation event was recorded successively at passages 44-47, beginning with a large-scale apoptosis and senescence and followed by mitosis arrest and re-activation, thus designated as cell strain CAM-44A, 44B, 45A, 44B, and 47A. Now both the CAM cell line and strains had been sub-cultured for more than 89 times and could be well cryopreserved in the growth medium containing 5% dimethylsulfoxide. Chromosome analysis and COI gene analysis had confirmed the grass goldfish origin of these CAM cells. Transfection potential analysis indicated that Lipofectamine LTX and Xfect were two suitable transfection reagents to be used in the gene delivery of CAM cells with a transfection efficiencies up to 11±6% and 8±3% in the CAM cell lines, respectively. Among the five cell strains, CAM-47A showed the highest transfection potential with a transfection efficiency up to 28 ± 5%. This work will provide a useful cell source for works on the cell-based artificial fish meat production and functional studies of fish myogenesis-related genes.

Establishment and characterization of a cold-sensitive neural cell line from the brain of tilapia (Oreochromis niloticus)

The aquaculture of tilapia (Oreochromis sp.) is adversely affected by the sensitivity to cold stress. A large number of genes in tilapia were found to be regulated by cold stress, but their functions and mechanisms in cold tolerance remain largely unknown, partially due to the lack of a suitable in vitro model. An immortal neural cell line designated as tilapia brain neural (TBN) was established from brain tissue of the genetically improved farmed tilapia strain of Nile tilapia (Oreochromis niloticus). The TBN cells show a neuron-like morphology at low density and form a fibroblast-like monolayer at high density. Transcriptome profiling through RNA-sequencing revealed that a total of 15,011 genes were expressed in the TBN cells. The TBN cells express a wide array of marker genes for neural cells. A comparative analysis of the featured genes among the 17 cell clusters isolated from the subventricular zone of mouse brain revealed the highest transcriptome similarity between the TBN cells and the transient amplifying progenitors (TAPs). The TBN cells tolerate relatively high culture temperatures, and the highest growth rate was observed for the cells cultured at 32°C compared with those at 30°C, 28°C and 26°C. Nonetheless, this cell line is cold sensitive. Exposure of the cells to 16°C or lower temperatures significantly decreased cell confluences and induced apoptosis. The TBN cells were more sensitive to cold stress than the ZF4 cells (embryonic zebrafish fibroblasts). Moreover, the TBN cells can be efficiently transfected through electroporation. This study provides an invaluable research tool to understand the nature of cold sensitivity of tilapia and to dissect the function and mechanism of genes in regulating cold tolerance of fish.

Establishment of a brain cell line (SaB-1) from gilthead seabream and its application to fish virology

Gilthead seabream (Sparus aurata) is among the most important cultured fish species in the Mediterranean area and pathogen diseases one of the bottlenecks to the aquaculture sector. For this reason, generation of laboratory tools for diagnostic and research applications would be beneficial to improve the seabream aquaculture. In this sense, we aimed to generate a seabream cell line for biological studies. Thus, we have obtained a brain-derived cell line (SaB-1) that is continuously growing for more than 4 years. Cellular characterization of the SaB-1 cells shows that they express both neural and glial cell markers, suggesting they are neural-stem cells, have a neuron-like morphology and show a rapid growth in culture. We evaluated their susceptibility to the main fish viruses: nervous necrosis virus (NNV), spring viremia carp virus (SVCV), infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicaemia virus (VHSV). SaB-1 cells are susceptible to all the tested viruses. In addition, the transcription of genes related to the type I interferon (IFN) is greatly up-regulated by the NNV infection whilst the viral infection with SVCV, IPNV or VHSV failed to do so. These data demonstrate that the seabream SaB-1 cell line is continuous, stable and could be useful, at least, for fish virology and immunity applications.

Establishment of Conditions for Long-Term Maintenance of Primary Embryonic Cell Cultures from Olive Flounder Paralichthys olivaceus

Primary cell culture is a sufficient method frequently used to study the cellular properties and mechanisms of isolated cells in a controlled environment. In this study, an embryonic cell line (FGBC8) derived from the blastula stages of embryos of olive flounder Paralichthys olivaceus was developed. Furthermore, conditions for optimal long-term maintenance of this primary embryonic cell culture were investigated. Morphologically, FGBC8 cells were composed primarily of epithelial-like cells. FGBC8 cells were subcultured for >160 passages over ~830 days. The doubling time of FGBC8 cells was 73.8 h, and the modal diploid chromosome number was 48. FGBC8 cells transfected with green fluorescence protein (GFP)-expression plasmid exhibited a strong signal 48 h after transfection. Consequently, we demonstrated that fish serum is a crucial supplement for the long-term survival and maintenance of comparable morphology in these primary embryonic cells. Our results can be used as a guide for primary embryonic cell cultures for other fish species and may be useful for cell biotechnological applications.

Growth differentiation factor 9 (gdf9) and bone morphogenetic protein 15 (bmp15) are potential intraovarian regulators of steroidogenesis in Japanese flounder (Paralichthys olivaceus)

Members of transforming growth factor-β (TGF-β) superfamily are vital regulators during the development of fish ovary. However, its intraovarian functions in teleost are still unclear. As members of the TGF-β superfamily, gdf9 and bmp15 are necessary for follicle formation and granulosa cell proliferation. Here in Japanese flounder, quantitative real-time polymerase chain reaction (qRT-PCR) and in situ hybridization (ISH) analysis showed that gdf9 and bmp15 were mainly expressed in oogonia and oocytes, whereas weakly expressed in non-ovarian tissues. Overexpression of single gdf9 and the co-overexpression with bmp15 could up-regulate the expression of most steroidogenic genes, while the overexpression of single bmp15 could down-regulate the expression of most steroidogenic genes. These findings demonstrate that single gdf9 and the combination with bmp15 may act as "activator", while single bmp15 may act as "inhibitor" in the process of steroidogenesis in flounder. This was also verified in negative feedback regulation of gdf9 and bmp15 during hormone treatment. High concentration of human chorionic gonadotropin (hCG) could down-regulate gdf9 and up-regulate bmp15, which were beneficial for the homeostasis of hCG hormone. Besides, knockdown of either gdf9 or bmp15 could significantly down-regulate most steroidogenic genes. This indicated that heterodimer of GDF9:BMP15 might be the most bioactive ligand in gonad development of flounder. Taken together, our study provided a novel recognition that gdf9 and bmp15 could regulate steroidogenesis in teleost through mechanism different from that in mammals.

Establishment of a brain cell line (FuB-1) from mummichog (Fundulus heteroclitus) and its application to fish virology, immunity and nanoplastics toxicology

The marine fish mummichog (Fundulus heteroclitus), extensively used as research model, including in ecotoxicology, for over a century has been surpassed by other fish species. This fact may be associated with the lack of cell lines from this species, excellent models for the comprehension of fish physiology, immunology, toxicology and virology, that contribute to the reduction in the number of animals used in research. We have generated, for the first time, a brain-derived cell line from mummichog, FuB-1, and evaluated its application to the fields of fish virology, immunity and toxicology. First, FuB-1 cells show epithelial morphology and neural stem/astroglial origin. Secondly, FuB-1 cells effectively supports the replication of both spring viremia carp (SVCV) and infectious pancreatic necrosis (IPNV) viruses, but not nodavirus (NNV), indicating its potential use for fish virology. Related to this, FuB-1 cells infected with NNV up-regulate the transcription of genes related to the antiviral immune response, leading to cell resistance; while they are unaltered when infected with IPNV and SVCV, facilitating viral replication. Finally, FuB-1 cells were used for toxicological purposes and we demonstrated that exposure to either polystyrene nanoplastics (PS-100) or several human-usage pharmaceuticals are cytotoxic. Additionally, PS-100 particles increase the antioxidant catalase and glutathione S-transferase activities and decrease the total non-protein thiols in FuB-1 cells. However, PS-100 particles are able to reduce the cytotoxic effects induced by the pharmaceuticals. In conclusion, we have generated a cell line from mummichog, which might represent a valuable model for fish studies in the fields of virology, immunology and toxicology.

Development and Characterization of a New Cell Line from Olive Flounder Paralichthys olivaceus

A new embryonic cell line (OFEC-17FEN) derived from olive flounder Paralichthys olivaceus was developed. OFEC-17FEN cells were subcultured for <30 passages over ~200 days. OFEC-17FEN cells had a doubling time of 114.34 h and modal diploid chromosome number was 48. The pluripotency genes POU5f1 and NANOG were expressed in OFEC-17FEN cells. However, the lack of several pluripotency-related genes expression indicates that OFEC-17FEN cells are not stem cells. OFEC-17FEN cells transfected with plasmid pEGFP-c1 exhibited a strong green fluorescent signal at 48 h after transfection. Accordingly, OFEC-17FEN cells may be useful for both basic research and biotechnological application.

Two skin cell lines from wild-type and albino Japanese flounder (Paralichthys olivaceus): establishment, characterization, virus susceptibility, efficient transfection, and application to albinism study

In order to provide an applicable cell platform to study fish pathology and skin pigmentation, two cell lines derived from skin tissues of wild-type and albino Japanese flounder were established and named JFSK_wt and JFSK_alb, respectively. These two cell lines were cultured for 45 passages within approximately 300 days. JFSK_wt and JFSK_alb cells were maintained in Dulbecco's Modified Eagle's Medium and Ham's F-12 Nutrient Mixture (DMEM/F12) supplemented with antibiotics, fetal bovine serum (FBS), 2-mercaptoethanol (2-Me), N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), and basic fibroblast growth factor (bFGF). The optimal growth temperature for JFSK_wt and JFSK_alb cells was 24 °C, and microscopically, the two cell lines were composed of fibroblast-like cells. Chromosomal analysis revealed that JFSK_wt and JFSK_alb cells had an identical diploid karyotype with 2n = 48t. Results of viral inoculation assays revealed that both cell lines shared similar patterns of viral susceptibility to nervous necrosis virus (NNV). High transfection efficiency was observed in JFSK_wt and JFSK_alb cells transfected with a pEGFP-N3 reporter plasmid and Cy3-siRNA. The detection of dermal marker Dermo-1 showed that these two cells were both derived from the dermis. Finally, three genes involved in the melanogenesis pathway, including adenylate cyclase type 5 (adcy5), microphthalmia-associated transcription factor (mitf), and endothelin B receptor (ednrb), were downregulated in JFSK_alb versus JFSK_wt cells. Thus, the two cell lines, sampled from skin tissue of wild-type and albino Japanese flounder will be not only helpful for fish pathogen research but also beneficial for albinism-related gene function studies.

Establishment of a new teleost brain cell line (DLB-1) from the European sea bass and its use to study metal toxicology

In teleost fish, there are no commercial cell lines for the European sea bass (Dicentrarchus labrax). Thus, we have established the sea bass brain (DLB-1) cell line, using a fish retrovirus for immortalization, which resemble epithelial cells and express glial cells markers. Exposure to metals [Cd, methylmercury (MeHg), Pb or As] produces cytotoxicity and induction of reactive oxygen species (ROS) production. Interestingly, cell cycle analysis of DLB-1 cells shows that exposure to metals alters it significantly. Moreover, all the metals induce apoptosis as indicated by sub-Go/G1 population and annexin V binding. Finally, exposure of DLB-1 cells to metals also produces significant alterations at gene expression level, which confirm the above functional results. This is the first study in which metal cytotoxicity has been evaluated in a fish brain cell line and results seem to support that DLB-1 cells are suitable for toxicological studies.