Molecular cloning, characterization, expression and functional analysis of Japanese flounder Paralichthys olivaceus Fas ligand

A novel Fas ligand plays an important role in cell apoptosis of Crassostrea hongkongensis: molecular cloning, expression profiles and functional identification of ChFasL

Background

Apoptosis regulates normal development, homeostasis, immune tolerance and response to environmental stress by eliminating unwanted or diseased cells, and plays a key role in non-specific immunity of invertebrates. The exogenous pathway mediated by death receptors and death ligands is a very important pathway for cell apoptosis. Death ligands are mainly members of the tumour necrosis factor (TNF) family, of which FasL is an important member. The deep involvement of FasL in vertebrates cell apoptosis and immunity has been reported many times, but there is limited research on the FasL gene in shellfish, and its functional importance in oyster cell apoptosis and immunity remains unclear.

Methods

The full length of ChFasL was identified and cloned based on the genome of Crassostrea hongkongensis. Quantitative PCR was used to detect the relative expression of ChFasL in different developmental stages and tissues, as well as the changes of relative expression in hemocytes after bacterial infection. The expression position of ChFasL in HEK293T cells was also located by subcellular localization, and the effect of increased recombinant protein content on the activity of reporter genes p53 and p21 was studied by dual-fluorescence reporter gene. Finally, the changes of apoptosis rate in hemocytes after ChFasL silencing was identified by RNA interference technology.

Results

We identified a novel FasL gene from C. hongkongensis and named it ChFasL. We found that ChFasL has potential N-linked glycosylation site, a transmembrane domain and a TNF region, which was a typical characteristics of TNF family. ChFasL was expressed in all developmental stages of larvae and in all tissues of oysters. After stimulation by V. alginolyticus or S. haemolyticus, its relative expression in hemocytes increased significantly, suggesting that ChFasL was deeply engaged in the immune response process of C. hongkongensis to external microbial stimulation. The results of subcellular localization showed that ChFasL was mainly distributed in the cytoplasm of HEK293T cells. With the overexpression of the recombinant protein pcDNA3 1- ChFasL, the activity of p53 and p21 significantly increased, showing a positive regulatory effect. Moreover, after dsRNA successfully reduced the relative expression of ChFasL, the apoptosis rate of hemocytes was significantly lower than that the dsGFP group.

Conclusion

These results comprehensively confirmed the important role of ChFasL in the apoptosis process of C. hongkongensis, which provided the basis and premise for the in-depth understanding of the immune function of apoptosis in molluscs, and also contributed to the research on the pathogenic death mechanism and disease resistance breeding of marine bivalves.

TroTNFSF6, a tumor necrosis factor ligand superfamily member, promotes antibacterial immune response of golden pompano, Trachinotus ovatus

Tumor necrosis factor ligand superfamily member 6 (TNFSF6), also known as FasL/CD95L, is essential for maintaining the body's immune homeostasis. However, the current reports on TNFSF6 in fish are relatively scarce. In the present study, we conducted functional analyses of a TNFSF6 (TroTNFSF6) from the teleost fish golden pompano (Trachinotus ovatus). TroTNFSF6 is composed of 228 amino acids and has a low similarity with other species (9.65%-58.79%). TroTNFSF6 was expressed in the 11 tissues tested and was significantly up-regulated after Edwardsiella tarda infection. In vivo, overexpression of TroTNFSF6 effectively stimulated the AKP and ACP activities, and reduced bacterial infection in fish tissues. Correspondingly, knockdown of TroTNFSF6 expression resulted in increasing bacterial dissemination and colonization in fish tissues. In vitro, recombinant TroTNFSF6 protein promoted the proliferation of T. ovatus head kidney lymphocytes (HKLs), and promoted the apoptosis of murine liver cancer cells (Hepa1-6). The results indicated that TroTNFSF6 plays an important role in the T. ovatus antibacterial immunity. These observations will facilitate the future in-depth study of teleost TNFSF6.

Structures, evolutionary relationships and expression profiles of the tumour necrosis factor superfamily and their receptors in black rockfish (Sebastes schlegelii)

Members of tumour necrosis factor superfamily (TNFSF) and TNF receptor superfamily (TNFRSF) have crucial roles in many important biological processes such as cell proliferation, cell death, development, survival, immunity, and various diseases. The human TNFSF consists of 19 ligands and 29 receptors. Compared with those in human, fish have most of the TNFSF and receptors that have been found in mammals, while some of the homologues are specific or lost in fish. Especially, no systematic report on the identification of TNFSF ligands and their receptors in S. schlegelii. Therefore, to investigate the characterization and molecular evolution of TNFSF and TNFRSF genes in Sebastes schlegelii, we performed a genome-wide survey and identified 14 TNFSFs and 24 TNFRSFs from S. schlegelii. In S. schlegelii, we found duplication events occurred in TNFSF2, TNFSF6, TNFSF10, TNFSF13, TNFSF14, TNFRSF5, TNFRSF6, TNFRSF6B, TNFRSF10B, TNFRSF16, and TNFRSF19 genes. Among which, the tandem duplications events occurred in TNFSF13 and TNFRSF6, and the whole genome duplications events occurred in the remaining TNFSF and TNFRSF genes. Based on the molecular phylogenetic analysis, 14 TNFSFs were divided into three different clusters and 24 TNFRSFs were classed as three distinct subgroups, respectively. Meanwhile, protein domains and motifs analysis revealed that TNFSF contain homology domain (THD), and TNFRSF have typical cysteine-rich domains (CRDs). Synteny results indicates that the TNFSFs and TNFRSFs neighborhood genes have taken place great changes compared to those in human, fugu and zebrafish. Meanwhile, qRT-PCR results demonstrated that most TNFSFs and TNFSRSFs were significantly differentially expressed in gill, skin and intestine after E. tarda infection with time-dependent manners. In addition, protein-protein interaction network (PPI) analysis indicated that the most related genes connecting to TNFSF and TNFRSFs were TNFSF ligands and receptors. In summary, this study provided a new understanding for characterization and evolution of the TNFSF genes and their receptors in S. schlegelii.

Fas/FasL of pacific cod mediated apoptosis

Fas and Fas ligand (FasL) pathway plays important roles in virus defense and cell apoptosis. In our previous work, nervous necrosis virus (NNV) was discovered in Pacific cod (Gadus macrocephalus), and the Fas ligand (PcFasL) was up-regulated when NNV outbreak, however, signal transmission of Fas/FasL in fish are still unclear. In the present study, Pacific cod Fas (PcFas), PcFasL and Fas-associating protein with a novel death domain (PcFADD) were characterized. The predicted protein of PcFas, PcFasL and PcFADD includes 333 aa, 90 aa and 93 aa, separately. 3-D models of PcFas, PcFasL and PcFADD were well constructed based on reported templates, respectively, even though the sequence homology with other fish is very low. The transcript levels of PcFas increased gradually from 15 day-post hatching (dph) to 75dph. PcFas was significantly up-regulated when cod larvae had NNV symptoms at 24dph, 37dph, 46dph, 69dph, and 77dph. Subcellular localization revealed that PcFasL was located in the cytoplasm, while PcFas was mainly located in the cell membrane. Exogenous expressed PcFasL of 900 μg/mL could kill the Epithelioma papulosum cyprinid (EPC) cells by MTT test, but low concentration has no effect on the cells. qPCR analysis showed that overexpression of PcFas could significantly up-regulate the expression of genes related to Fas/FasL signaling pathway, including bcl-2, bax, and RIP3, while overexpression of PcFasL significantly up-regulate the expression of caspase-3, caspase-9, and MLKL. Overexpression of PcFas or PcFasL could induce EPC apoptosis significantly by flow cytometry, which was consistent with the results of caspase-3 mRNA level increasing. The results indicated that NNV could induce apoptosis through Fas/FasL signal pathway.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

T Cells in Fish

Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4⁺ and CD8α⁺ T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4⁺ helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α⁺, CD4⁺ T-cells and sIgM⁺ cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.

Molecular cloning, functional identification and expressional analyses of FasL in Tilapia, Oreochromis niloticus

FasL is the most extensively studied apoptosis ligand. In 2000, tilapia FasL was identified using anti-human FasL monoclonal antibody by Evans's research group. Recently, a tilapia FasL-like protein of smaller molecule weight was predicted in Genbank (XM_003445156.2). Based on several clues drawn from previous studies, we cast doubt on the authenticity of the formerly identified tilapia FasL. Conversely, using reverse transcription polymerase chain reaction (RT-PCR), the existence of the predicted FasL-like was verified at the mRNA level (The Genbank accession number of the FasL mRNA sequence we cloned is KM008610). Through multiple alignments, this FasL-like protein was found to be highly similar to the FasL of the Japanese flounder. Moreover, we artificially expressed the functional region of the predicted protein and later confirmed its apoptosis-inducing activity using a methyl thiazolyl tetrazolium (MTT) assay, Annexin-V/Propidium iodide (PI) double staining, and DNA fragment detection. Supported by these evidences, we suggest that the predicted protein is the authentic tilapia FasL. To advance this research further, tilapia FasL mRNA and its protein across different tissues were quantified. High expression levels were identified in the tilapia immune system and sites where active cell turnover conservatively occurs. In this regard, FasL may assume an active role in the immune system and cell homeostasis maintenance in tilapia, similar to that shown in other species. In addition, because the distribution pattern of FasL mRNA did not synchronize with that of the protein, post-transcriptional expression regulation is suggested. Such regulation may be dominated by potential adenylate- and uridylate-rich elements (AREs) featuring AUUUA repeats found in the 3' untranslated region (UTR) of tilapia FasL mRNA.

Teleost T and NK cell immunity

The main function of the immune system is to maintain the organism's homeostasis when invaded by foreign material or organisms. Prior to successful elimination of the invader it is crucial to distinguish self from non-self. Most pathogens and altered cells can be recognized by immune cells through expressed pathogen- or danger-associated molecular patterns (PAMPS or DAMPS, respectively), through non-self (e.g. allogenic or xenogenic cells) or missing major histocompatibility (MHC) class I molecules (some virus-infected target cells), and by presenting foreign non-self peptides of intracellular (through MHC class I-e.g. virus-infected target cells) or extracellular (through MHC class II-e.g. from bacteria) origin. In order to eliminate invaders directly or by destroying their ability to replicate (e.g. virus-infected cells) specialized immune cells of the innate and adaptive responses appeared during evolution. The first line of defence is represented by the evolutionarily ancient macrophages and natural killer (NK) cells. These innate mechanisms are well developed in bony fish. Two types of NK cell homologues have been described in fish: non-specific cytotoxic cells and NK-like cells. Adaptive cell-mediated cytotoxicity (CMC) requires key molecules expressed on cytotoxic T lymphocytes (CTLs) and target cells. CTLs kill host cells harbouring intracellular pathogens by binding of their T cell receptor (TCR) and its co-receptor CD8 to a complex of MHC class I and bound peptide on the infected host cell. Alternatively, extracellular antigens are taken up by professional antigen presenting cells such as macrophages, dendritic cells and B cells to process those antigens and present the resulting peptides in association with MHC class II to CD4(+) T helper cells. During recent years, genes encoding MHC class I and II, TCR and its co-receptors CD8 and CD4 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. Functional assays for innate and adaptive lymphocyte responses have been developed in only a few fish species. This review summarizes and discusses recent results and developments in the field of T and NK cell responses with focus on economically important and experimental model fish species in the context of vaccination.

Temperature-dependent viral replication and antiviral apoptotic response in viral haemorrhagic septicaemia virus (VHSV)-infected olive flounder (Paralichthys olivaceus)

The olive flounder (Paralichthys olivaceus) shows a high rate of mortality to viral haemorrhagic septicaemia virus (VHSV) in the winter and spring but has zero mortality over 20 °C. In this experiment, we studied the effect of rearing temperature on viral replication, viral transcription and antiviral apoptotic immune response in VHSV-infected olive flounder by real-time polymerase chain reaction. Olive flounder were given intra-peritoneal injections of VHSV (10(7.8) TCID(50)/ml) and were reared at 15 °C or 20 °C. Five fish were randomly sampled for head kidney at 3, 6 and 12 h post-infection (hpi) and 1, 2, 4 and 7 days post-infection (dpi). Total RNA extracted from the tissue was reverse transcribed and used as template for real-time PCR. In the 15 °C group, the number of viral gRNA copies peaked after 2 dpi and remained high through 7 dpi, while in the 20 °C group, the copy number was at the highest at 1 dpi but drastically declined at later stages. Viral mRNA levels in the 15 °C group gradually increased starting at 3 hpi to reach their maximum value at 12 hpi and remained high until 2 dpi, whereas the other group showed much lower copy numbers that were undetectably low at 4 and 7 dpi. Type II IFN expression increased as the viral copies increased and the 20 °C group showed quicker and stronger expression than the 15 °C group. The MHC class I and CD8 expression was high in both the groups at early stage of infection (3-6 hpi) but at later stages (2-7 dpi) in 15 °C group expression reduced below control levels, while they expressed higher to control in 20 °C group. The expression of granzyme in 15 °C fish showed a single peak at 2 dpi, but was consistently expressing in 20 °C fish. Individuals expressed very high levels of perforin expressed very high levels of caspase 3. In 15 °C fish, TNFα, FasL and p53 expressed significantly higher than 20 °C only at initial stages of infection (3-6 hpi). Caspase 3 expression found to be low in 15 °C fish whereas it was significantly elevated in 20 °C group. Interestingly individual fish with high caspase 3 expression contained very low viral RNA. Thus, from our experiment, we can conclude that an effective apoptotic immune response in VHSV-infected olive flounder plays a crucial role in the survival of the host at higher temperatures.

Origin and evolution of TNF and TNF receptor superfamilies

The tumor necrosis factor superfamily (TNFSF) and the TNF receptor superfamily (TNFRSF) have an ancient evolutionary origin that can be traced back to single copy genes within Arthropods. In humans, 18 TNFSF and 29 TNFRSF genes have been identified. Evolutionary models account for the increase in gene number primarily through multiple whole genome duplication events as well as by lineage and/or species-specific tandem duplication and translocation. The identification and functional analyses of teleost ligands and receptors provide insight into the critical transition between invertebrates and higher vertebrates. Bioinformatic analyses of fish genomes and EST datasets identify 14 distinct ligand groups, some of which are novel to teleosts, while to date, only limited numbers of receptors have been characterized in fish. The most studied ligand is TNF of which teleost species possess between 1 and 3 copies as well as a receptor similar to TNFR1. Functional studies using zebrafish indicate a conserved role of this ligand-receptor system in the regulation of cell survival and resistance to infectious disease. The increasing interest and use of TNFSF and TNFRSF modulators in human and animal medicine underscores the need to understand the evolutionary origins as well as conserved and novel functions of these biologically important molecules.

Cytotoxic T cells in teleost fish

The presence of antigen-specific cytotoxic T cells has been suggested in a number of in vivo and in vitro studies in fish. Acute allograft rejection with an accelerated response on second-set grafts and the presence of graft-versus-host reaction (GVHR) has been reported in teleost. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ex vivo studies in ginbuna and rainbow trout. In addition, alloantigen-specific cytotoxic T cell clones have been produced in cultures initiated with peripheral blood leukocytes (PBL) from an alloantigen-immunized channel catfish. Over the last decade several fish genomes have been sequenced and genetic information is rapidly accumulating. Thanks to these genome data bases and EST analysis, mRNA expression of T cell surface marker genes in alloantigen- or virus-specific effector cells has been reported in some fish species, e.g. TCR α or β and CD8α in ginbuna and rainbow trout, and TCR α, β or γ in channel catfish. These findings suggest the presence of CD8(+) cytotoxic T lymphocyte (CTL) in fish similar to those of higher vertebrates. Recently, monoclonal antibodies against CD8α and CD4 antigens have been produced in some fish species. Investigation on the characteristics of CTL and cell-mediated immune mechanisms is now possible at defined T cell subsets, although identification of T cell subset is limited in a few fish species at present. In this review, we describe the recent progress in this field focusing on cells involved in antigen specific cytotoxicity.

Molecular cloning and characterisation of the rock bream, Oplegnathus fasciatus, Fas (CD95/APO-1), and its expression analysis in response to bacterial or viral infection

Fas belongs to the tumour necrosis factor (TNF) receptor superfamily and can transmit a death signal leading to apoptosis. In the present study, we isolated the full-length cDNA for rock bream (Oplegnathus fasciatus) Fas (RbFas). The full-length RbFas cDNA was 1770 bp long and contained an open reading frame of 957 bp that encoded 319 amino acid residues with a predicted molecular mass of 35.1 kDa. The 319 amino-acid predicted RbFas sequence is homologous to other Fas sequences, contains three cysteine-rich domains and a death domain (DD) and two potential N-glycosylation sites. Expression of RbFas mRNA was detected in nine different tissues from healthy rock bream and was the highest in red blood cells. In analyses of mitogen-stimulated RbFas expression in peripheral blood leucocytes, expression of RbFas mRNA was observed between 1 and 36 h after stimulation with LPS, and 1 and 3 h stimulation with poly I:C. In the case of bacterial injection, the RbFas transcript peaked 6 h after injection in both the kidney and the spleen. Otherwise, the RbFas transcript peaked after 1 h in spleen and 6 h in kidney following injection with RSIV.

Lymphocytes with T-cell-like properties express the Fas ligand in the Japanese flounder Paralichthys olivaceus

In this study, we aimed to identify the leukocyte population that expresses Fas ligand (FasL) in the Japanese flounder (Paralichthys olivaceus). The transcriptional activity of FasL was examined for the first time in the fish leukocytes. Transcription of the FasL gene in flounder leukocytes was significantly increased by phytohemagglutinin (PHA) treatment. All the leukocyte populations we tested possessed binding activity for PHA, but this was especially high in the lymphocyte population. However, the lymphocytes consisted of two subsets showing heterogeneity with respect to PHA binding, with the high-binding subset being surface IgM-negative. We also found that only the lymphocyte population showed a significant increase in the expression of the FasL gene after stimulation with PHA. In addition, only the lymphocyte subset showing high binding to PHA showed conspicuous expression of the FasL gene. This subset also had a CD3γ/δ+, CD8α+ and IgM heavy-chain (-) phenotype. These results suggested that lymphocytes with T-cell-like properties are FasL-expressing cells in the Japanese flounder.

Perforin-dependent cytotoxic mechanism in killing by CD8 positive T cells in ginbuna crucian carp, Carassius auratus langsdorfii

T cell-mediated cytotoxicity occurs via pathways based on perforin or Fas mechanisms. Perforin is a protein present in the cytoplasmic granules of CD8(+) cytotoxic T lymphocytes and is secreted to form pores on target cell membranes. In fish, although the involvement of perforin in cytotoxicity have been suggested for several species, perforin-mediated cytotoxicity of CD8α(+) lymphocyte in conjunction with expression of the perforin gene has not been reported. In order to investigate the killing mechanism of CD8α(+) lymphocytes by perforin-mediated pathway in fish, we measured apoptosis of target cells triggered by CD8α(+) lymphocytes, performed cytotoxic assays in the presence or absence of perforin inhibitor; concanamycin A and EGTA, and analysed the expression of perforin1, perforin2 and perforin3 isotypic genes in ginbuna crucian carp. In the present study, we found that CTLs attached with target cells. CTL should have direct contact with target cells to kill them. Approximately 50% of target cells were positive for annexin V after co-cultured with CD8α(+) lymphocytes, indicating the induction of apoptotic cell death. Concanamycin A, which induces depolymerization of perforin resulting in lytic function, suppressed the cytotoxicity of CD8α(+) cells in a dose-dependent manner. In addition, cytotoxicity mediated by CD8α(+) lymphocytes were significantly suppressed by the addition of the Ca(2+)-chelating agents EGTA or EGTA-Mg(2+), and the addition of Ca(2+) restored the killing mechanism of target cells. We further found enhanced expression of perforin1 but not perforin2 or perforin3 in CTLs from allo-sensitized fish. The present study has demonstrated that ginbuna CTLs kill target cells through perforin-mediated pathway, suggesting that perforin-mediated pathway is conserved throughout vertebrate.

Kinetics of CD4+ and CD8alpha+ T-cell subsets in graft-versus-host reaction (GVHR) in ginbuna crucian carp Carassius auratus langsdorfii

We have previously demonstrated the presence of graft-versus-host reaction (GVHR) in fish employing a model system of clonal triploid ginbuna and tetraploid ginbuna-goldfish hybrids. To elucidate the role of CD8alpha+ T cells in the induction of GVHR, we investigate the kinetics of CD4+ and CD8+ T-cell subsets in GVHR along with the pathological changes associated with GVH disease (GVHD) in ginbuna. GVHR was not induced with a leukocyte fraction lacking CD8alpha+ T cells separated by magnetic cell sorting. Ploidy and immunofluorescence analysis revealed that CD4+ and CD8alpha+ T cells from sensitized donors greatly increased in the host trunk kidney, constituting more than 80% of total cells 1-2 weeks after donor cell injection, while those from non-sensitized donors constituted less than 50% of cells present. The increase of CD4+ T cells was greater and more rapid than that of CD8alpha+ T cells. The number of donor CD4+ and CD8alpha+ T cells was highest in trunk kidney followed by spleen. Increases in donor CD4+ and CD8alpha+ T cells were also found in liver and PBL, although the percentages were not as high. Pathologic changes similar to those in human and murine acute GVHD were observed in the lymphoid organs as well as target organs such as skin, liver and intestine, including the destruction of cells and tissues and massive leukocyte infiltration. The pathologic changes became more severe with the increase of CD8alpha+ T cells. These results suggest that donor-derived CD8alpha+ T cells play essential roles for the induction of acute GVHR/D in teleosts as in mammals.

A novel Fas ligand in mollusk abalone: molecular characterization, immune responses and biological activity of the recombinant protein

Fas ligand is a member of the TNF superfamily that plays an important role by inducing apoptosis and homeostasis of immune responses. The gene encoding Fas ligand was isolated from a disk abalone (Haliotis discus discus) cDNA library, denoted as the AbFas ligand. It contains an 1832bp transcript with a 945bp open reading frame, encoding 315 amino acids. The AbFas ligand showed characteristic transmembrane and TNF family signature domains. The deduced amino acid comparison showed that the AbFas ligand exhibits 22.0, 16.1 and 14.5% identities to human Fas ligand, TNF-alpha, and lymphotoxin (LT-alpha), respectively. Phylogenetic analysis indicates that the AbFas ligand belongs to the invertebrate TNF family and it is closely related to vertebrate Fas ligand counterparts. Quantitative real-time PCR analysis results showed that the AbFas ligand transcripts were constitutively expressed in abalone hemocytes, gills, mantle, muscle, digestive tract and digestive gland in a tissue-specific manner. By immune stimulation, AbFas ligand mRNA was significantly (p<0.05) up-regulated after infection with a mixture of bacteria (Vibrio alginolyticus, Vibrio parahemolyticus, and Listeria monocytogenes), viral haemorrhagic septicaemia virus (VHSV), and lipopolysaccharide (LPS) in abalone gills. The recombinant AbFas ligand was over-expressed in Escherichia coli (E. coli) and purified using a pMAL protein fusion system. This recombinant AbFas ligand showed its biological activity by inducing both superoxide anion (O(2-) and H(2)O(2) in human THP-1 cells in concentration-dependant manner. Correlating the AbFas ligand transcriptional up-regulation against bacteria, virus and LPS with the biological activity of its recombinant protein, we could suggest that the abalone Fas ligand may control microbial infection by inducing O(2-), H(2)O(2) and other ROS.

Fish as model systems for the study of vertebrate apoptosis

Apoptosis is a process of pivotal importance for multi-cellular organisms and due to its implication in the development of cancer and degenerative disease it is intensively studied in humans and mammalian model systems. Invertebrate models of apoptosis have been well-studied, especially in C. elegans and D. melanogaster, but as these are evolutionarily distant from mammals the relevance of findings for human research is sometimes limited. Presently, a non-mammalian vertebrate model for studying apoptosis is missing. However, in the past few years an increasing number of studies on cell death in fish have been published and thus new model systems may emerge. This review aims at highlighting the most important of these findings, showing similarities and dissimilarities between fish and mammals, and will suggest topics for future research. In addition, the outstanding usefulness of fish as research models will be pointed out, hoping to spark future research on this exciting, often underrated group of vertebrates.

Molecular cloning, expression, and functional analysis of caspase-10 from Japanese flounder Paralichthys olivaceus

We isolated and sequenced caspase-10 cDNA and gene from Japanese flounder, Paralichthys olivaceus. The Japanese flounder (JF)-caspase-10 cDNA consisted of 2282 bp and encoded 495 amino acid residues. The characteristic death effector domains (DEDs) of caspases were observed in JF-caspase-10 as well as the three aspartic acid residues (D-186, -382 and -392), which are potential cleavage sites for the large and small subunit structures. The amino acid residue (His-325) and pentapeptide (QACQG), which are involved in catalytic activity, were absolutely conserved in Japanese flounder-caspase-10. JF-caspase-10 gene has a length of 6.6 kb and consists of 11 exons and 10 introns similar to that of human. The strong expression of JF-caspase-10 mRNA was detected in the gills, peripheral blood leukocytes, spleen and posterior kidney, while the weak expression was observed in the head kidney, heart, intestine, skin and stomach. The over-expression analysis of JF-caspase-10 in Japanese flounder cell line HINAE was shown to induce apoptosis 24h post-transfection using TUNEL assay.

Early diversification of the TNF superfamily in teleosts: genomic characterization and expression analysis

The TNF superfamily (TNFSF) of proteins are cytokines involved in diverse immunological and developmental pathways. Little is known about their evolution or expression in lower vertebrate species. Bioinformatic searches of Zebrafish, Tetraodon, and Fugu genome and other teleost expressed sequence tag databases identified 44 novel gene sequences containing a TNF homology domain. This work reveals the following: 1) teleosts possess orthologs of BAFF, APRIL, EDA, TWEAK, 4-1BBL, Fas ligand, LIGHT, CD40L, RANKL, and possibly TL1A; 2) the BAFF-APRIL subfamily is enriched by a third member, BALM, unique to fish; 3) orthologs of lymphotoxins alpha and beta were not clearly identified in teleosts and are substituted by a related ligand, TNF-New; 4) as many as four TRAIL-like genes are present in teleosts, as compared with only one in mammals; and 5) T cell activation ligands OX40L, CD27L, CD30L, and GITRL were not identified in any fish species. Finally, we characterize mRNA expression of TNFSF members CD40L, LIGHT, BALM, APRIL, Fas ligand, RANKL, TRAIL-like, and TNF-New in rainbow trout, Oncorhynchus mykiss, immune and nonimmune tissues. In conclusion, we identified a total of 14 distinct TNFSF members in fishes, indicating expansion of this superfamily before the divergence of bony fish and tetrapods, approximately 360-450 million years ago. Based on these findings, we extend a model of TNFSF evolution and the co-emergence of the vertebrate adaptive immune system.

The evolutionary conservation of the core components necessary for the extrinsic apoptotic signaling pathway, in Medaka fish

Background

Death receptors on the cell surface and the interacting cytosolic molecules, adaptors and initiator caspases, are essential as core components of the extrinsic apoptotic signaling pathway. While the apoptotic machinery governing the extrinsic signaling pathway is well characterized in mammals, it is not fully understood in fish.

Results

We identified and characterized orthologs of mammalian Fas, FADD and caspase-8 that correspond to the death receptor, adaptor and initiator caspase, from the Medaka fish (Oryzias latipes). Medaka Fas, caspase-8 and FADD exhibited protein structures similar to that of their mammalian counterparts, containing a death domain (DD), a death effector domain (DED) or both. Functional analyses indicated that these molecules possess killing activity in mammalian cell lines upon overexpression or following activation by apoptotic stimuli, suggesting similar pro-apoptotic functions in the extrinsic pathway as those in mammals. Genomic sequence analysis revealed that the Medaka fas (tnfrsf6), fadd and caspase-8 (casp8) genes are organized in a similar genomic structure as the mammalian genes. Database search and phylogenetic analysis revealed that the fas gene, but not the fadd and casp8 genes, appear to be present only in vertebrates.

Conclusion

Our results indicate that the core components necessary for the extrinsic apoptotic pathway are evolutionarily conserved in function and structure across vertebrate species. Based on these results, we presume the mechanism of apoptosis induction via death receptors was evolutionarily established during the appearance of vertebrates.