Identification and molecular analysis of a ferritin subunit from red drum (Sciaenops ocellatus)

Plasma and tissue transferrin and ferritin, and gene expression of ferritin, transferrin, and transferrin receptors I and II in channel catfish Ictalurus punctatus fed diets with different concentrations of inorganic or organic iron

Ferritin, transferrin, and transferrin receptors I and II play a vital role in iron metabolism, health, and indication of iron deficiency anaemia in fish. To evaluate the use of high-iron diets to prevent or reverse channel catfish (Ictalurus punctatus) anaemia of unknown causes, we investigated the expression of these iron-regulatory genes and proteins in channel catfish fed plant-based diets. Catfish fingerlings were fed five diets supplemented with 0 (basal), 125, and 250 mg/kg of either inorganic iron or organic iron for 2 weeks. Ferritin, transferrin, and transferrin receptor I and II mRNA and protein expression levels in fish tissues (liver, intestine, trunk kidney, and head kidney) and plasma were determined. Transferrin (iron transporter) and TfR (I and II) genes were generally highly expressed in fish fed the basal diet compared to those fed the iron-supplemented diets. In contrast, ferritin (iron storage) genes were more expressed in the trunk kidney of fish fed the iron-supplemented diets than in those fed the basal diet. Our results demonstrate that supplementing channel catfish plant-based diets with iron from either organic or inorganic iron sources affected the expression of the iron-regulatory genes and increased body iron status in the fish.

Teleost-specific TLR23 in Takifugu rubripes recruits MyD88 to trigger ERK pathway and promotes antibacterial defense

Takifugu rubripes is a highly valued cultured fish in Asia, while pathogen infections can result in severe diseases and lead to substantial economic losses. Toll-like receptors (TLRs), as pattern recognition receptors, play a crucial role on recognition pathogens and initiation innate immune response. However, the immunological properties of teleost-specific TLR23 remain largely unknown. In this study, we investigated the biological functions of TLR23 (TrTLR23) from T. rubripes, found that TrTLR23 existed in various organs. Following bacterial pathogen challenge, the expression levels of TrTLR23 were significantly increased in immune related organs. TrTLR23 located on the cellular membrane and specifically recognized pathogenic microorganism. Co-immunoprecipitation and antibody blocking analysis revealed that TrTLR23 recruited myeloid differentiation primary response protein (MyD88), thereby mediating the activation of the ERK signaling pathway. Furthermore, in vivo showed that, when TrTLR23 is overexpressed in T. rubripes, bacterial replication in fish tissues is significantly inhibited. Consistently, when TrTLR23 expression in T. rubripes is knocked down, bacterial replication is significantly enhanced. In conclusion, these findings suggested that TrTLR23 played a critical role on mediation TLR23-MyD88-ERK axis against bacterial infection. This study revealed that TLR23 involved in the innate immune mechanism, and provided the foundation for development disease control strategies in teleost.

Identification and functional regulation of three alternative splicing isoforms of the fthl27 gene in miiuy croaker, Miichthys miiuy

Alternative splicing is an important basic mechanism for eukaryotes to control gene expression. Different forms of alternative splicing may lead to the production of protein subtypes with different functions, leading to the expansion of protein diversity in organisms, affecting cell production and metabolism, and is even related to the occurrence of many diseases. Many studies have shown that ferritin is usually associated with inflammation, vascular proliferation, and tumors, which is the focus of immunological research. It not only plays a role in iron metabolism and storage in the body, but also plays an important regulatory role in pathways related to immune and inflammatory regulation. However, there are few studies on alternative splicing events of the ferritin gene nowadays. Therefore, this study identified three different splicing isoforms in its ferritin gene fthl27 of Miichthys miiuy through Sanger sequencing, qRT-PCR, and other experimental techniques, and we found that three different splicing isoforms of the ferritin gene fthl27 in M. Miiuy cells showed an upregulation trend after being stimulated by Lipopolysaccharide (LPS) and poly (I: C). The experiment also found that the three isoforms may have different regulatory effects on the expression of inflammatory factors and antiviral immune factors, playing an important role in the innate immune response of fish.

IL8 of Takifugu rubripes is a chemokine that interacts with peripheral blood leukocytes and promotes antibacterial defense

Interleukin 8 (IL8) is a CXC chemokine that plays a crucial role on promoting inflammatory response and immune regulation. In teleost, IL8 can induce the migration and activation of immune cells. However, the biological functions of IL8 are still unknown in Takifugu rubripes. In this study, we examined the biological characteristics of TrIL8 in T. rubripes. TrIL8 is composed of 98 residues and contained a chemokine CXC domain. We found that the TrIL8 expression was detected in diverse organs and significantly increased by Vibrio harveyi or Edwardsiella tarda challenge. The recombinant TrIL8 (rTrIL8) exhibited significantly the binding capacities to 8 tested bacteria. In addition, rTrIL8 could bind to peripheral blood leukocytes (PBL), and increased the expression of immune gene, resistance to bacterial infection, respiratory burst, acid phosphatase activity, chemotactic activity, and phagocytic activity of PBL. In the presence of rTrIL8, T. rubripes was enhanced the resistance to V. harveyi infection. These results indicated that TrIL8 is a chemokine and involved in the activation of immune cells against bacterial infection in teleost.

A novel C1qDC (PoC1qDC) with a collagen domain in Paralichthys olivaceus mediates complement activation and against bacterial infection

Complement C1q domain containing protein (C1qDC) is a vital recognition molecule and has an important effect on immunity. The C1qDCs exhibit opsonic activity in fish, while the mechanisms of C1qDCs in activation complement still remain unclear. This study explored immunological characteristics of a C1qDC from Japanese flounder (Paralichthys olivaceus) (PoC1qDC). PoC1qDC consists of 296 amino acid residues, possessing a collagen domain and a C1q domain. According to our results, PoC1qDC was expressed in 9 diverse tissue samples and showed up-regulation after bacterial challenge. Recombinant PoC1qDC (rPoC1qDC) activated normal serum bactericidal and hemolytic activities by interaction with Japanese flounder IgM, but not enhanced the complement activity of C3-depeleted serum. rPoC1qDC was significantly bound to various bacterial species and agglutination activity against Edwardsiella piscicida and Streptococcus iniae. Furthermore, rPoC1qDC showed direct interaction with peripheral blood leucocytes while enhancing phagocytic and chemotactic activity. When PoC1qDC was overexpressed in Japanese flounder before E. piscicida infection, bacterial replication was significantly inhibited in fish tissues. Consistently, when PoC1qDC expression in Japanese flounder was knocked down, bacterial replication was significantly enhanced. The above findings first suggested the role of PoC1qDC in teleost in mediating complement activation by interaction with IgM, which can positively influence bacterial infection.

Exposure to Deepwater Horizon crude oil increases free cholesterol in larval red drum (Sciaenops ocellatus)

The 2010 Deepwater Horizon oil spill impacted over 2100 km of shoreline along the northern Gulf of Mexico, which coincided with the spawning season of many coastal species, including red drum (Sciaenops ocellatus). Red drum develop rapidly and are sensitive to crude oil exposure during the embryonic and larval periods. This study investigates the predictions from recent transcriptomic studies that cholesterol biosynthetic processes are impacted by oil exposure in fish early life stages. We found that red drum larvae exposed for 72 h to ΣPAH₅₀ 3.55-15.45 µg L^-1 exhibited significantly increased pericardial area, a cardiotoxicity metric, but the expression of several genes targeted in the cholesterol synthesis pathway was not affected. However, whole-mount staining revealed significant increases in free cholesterol throughout the larval body (ΣPAH₅₀ 4.71-16.15 µg L^-1), and total cholesterol followed an increasing trend (ΣPAH₅₀ 3.55-15.45 µg L^-1). Cholesterol plays a critical role in fish embryo development and ion channel function. Therefore, the disruption of cholesterol homeostasis, as observed here, could play a role in the oil toxicity phenotype observed across many fish species.

Molecular characterisation and biological activity of an antiparasitic peptide from Sciaenops ocellatus and its immune response to Cryptocaryon irritans

Cryptocaryon irritans, a holotrichous ciliate parasitic protozoan, can trigger marine white spot disease and cause substantial economic losses in mariculture. However, methods of preventing and curing the disease have negatively affect fish, human, other organisms, and the natural environment. The antiparasitic activity of some antimicrobial peptides (AMPs) has garnered extensive attention of scholars. In this study, we identified and characterised a novel antiparasitic peptide, named So-pis, from Sciaenops ocellatus. The sequence analysis, structural features, and tissue distribution suggested that So-pis is genetically related to the piscidins family. However, So-pis showed a relatively low overall conservation compared with other known piscidins. So-pis is abound in glycine residues (22.7 %) and it has a neutral isoelectric point, weak amphipathicity, relatively long α-helix, and high hydrophobicity. These key elements are responsible for its biological activity. Quantitative real-time polymerase chain reaction (qRT-PCR) data indicated that So-pis is a typically gill-expressed peptide. The expression of So-pis in the gill, skin, spleen, and head kidney could be regulated during C. irritans infection, thereby implicating a role of So-pis in immune defence against C. irritans. The synthetic So-pis had limited or no antimicrobial activity against bacterial and yeasts but exhibited potent antiparasitic activity against C. irritans in vitro. The activity of synthetic So-pis against erythrocytes was less potent than its antiparasitic activity against C. irritans. These results indicated that So-pis might be one of the crucial defence cytokines against C. irritans in the red drum. Cumulatively, our data suggested that So-pis might be a potential candidate for developing a novel, effective, and safe therapeutic agent against marine white spot disease.

Characterization of TCR+ and CD8+ head kidney leucocytes in Japanese flounder (Paralichthys olivaceus) with antisera against TCRα and CD8α

T lymphocytes play an important role in cellular and adaptive immunity in vertebrates. The mechanisms of the fish immune system are little studied because of the lack of population-specific antibodies. This study examined the expression of two T lymphocyte markers, TCRα (PoTCRα) and CD8α (PoCD8α) in the Japanese flounder (Paralichthys olivaceus). The expression of PoTCRα and PoCD8α was mainly detected in immune/mucosal tissues. Recombinant PoTCRα and PoCD8α were expressed in pET32a and pET259, respectively. Then, rabbit anti-PoTCRα serum and rat anti-PoCD8α serum were prepared. Using serum, the characteristics of TCR+ and CD8+ head kidney leucocytes (HKLs) were investigated. The results of laser scanning confocal microscopy (LSCM) demonstrated that TCRα and CD8α were transmembrane proteins localized on the cell surface. The populations of CD8α^- , CD8α⁺ , TCRα^- , and TCRα⁺ were sorted by flow cytometry (FCM) and analysed using qRT-PCR. The results demonstrated that all TCRα⁺ /TCRα^- or CD8α⁺ /CD8α^- HKLs expressed IFN-γ. The CD4-1 and IgM transcripts were detected only in TCRα^- and CD8α^- cells. Furthermore, HKL mitogenesis was induced with concanavalin A (ConA) stimulation. Taken together, the results from LSCM and FCM analyses showed that mammalian and P. olivaceus TCR⁺ and CD8⁺ leucocytes share basic characteristics.

A deep-sea pathogenic Bacillus subtilis isolate employs different strategies to escape the killing of teleost and murine complements

Bacillus subtilis subsp. subtilis G7 was isolated from a deep-sea hydrothermal vent and is pathogenic to pathogenic to fish (Japanese flounder) and mice. G7 is able to survive in host sera and phagocytes. In this study, we investigated the underlying mechanism of G7 serum resistance. We found that (i) the remaining complement activity was very low in G7-incubated flounder serum but high in G7-incubated mouse serum; (ii) cleaved C3 and C5 components were detected on flounder serum-incubated G7 but not on mouse serum-incubated G7; (iii) abundant uncleaved C5 was localized in G7-incubated mouse, but not flounder, serum; (iv) G7-incubated flounder, but not mouse, serum exhibited strong chemotactic activity; (v) pre-treatment with low-dose lysozyme abolished the serum resistance of G7. Hence, G7 activates flounder complement but is protected from complement-mediated destruction by its cell wall structure, while G7 prevents the activation of mouse complement. These results indicate that G7 employs different mechanisms to avoid the complement killing of different hosts.

Recent insights into anti-WSSV immunity in crayfish

White spot syndrome virus (WSSV) is currently the most severely viral pathogen for farmed crustaceans such as shrimp and crayfish, which has been causing huge economic losses for crustaceans farming worldwide every year. Unfortunately, study on the molecular mechanisms of WSSV has been restricted by the lack of crustacean cell lines for WSSV propagation as well as the incompletely annotated genomes for host species, resulting in limited elucidation for WSSV pathogenesis at present. In addition to the findings of anti-WSSV response in shrimp, some of novel cellular events involved in WSSV infection have been recently revealed in crayfish, including endocytosis and intracellular transport of WSSV, innate immune pathways in response to WSSV infection, and regulation of viral gene expression by host genes. Despite these advances, many fundamental gaps in WSSV pathogenesis are still remaining, for example, how WSSV genome enters into nucleus and how the progeny virions are fully assembled in the host cell nucleus. In this review, recent findings in WSSV infection mechanism and the antiviral immunity against WSSV in crayfish are summarized and discussed, which may provide us a better understanding of the WSSV pathogenesis as well as new ideas for the target design of antiviral drugs against WSSV in crustaceans farming.

Antibacterial potential and mechanism of action of dithiocyano-methane as a bactericidal agent for farm disinfection

AIMS

This study aimed to investigate the antibacterial ability and action mechanism of dithiocyano-methane against Aeromonas hydrophila, so as to provide a reference for its application in farm disinfection.

METHODS AND RESULTS

After exposing the bacteria to dithiocyano-methane, the minimum inhibitory concentration (MIC), minimum bactericide concentration (MBC), activities of alkaline phosphatase, aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase and electric conductivity in bacterial suspensions were determined, transmission electron microscope images on cellular structure and SDS-PAGE profile of bacterial proteins were analysed and the expression of genes related to the above experimental observations was confirmed by real-time quantitative PCR. The MIC and MBC of dithiocyano-methane against three tested strains was 1·46 and 2·93 mg l^-1 respectively. The results showed that dithiocyano-methane significantly damaged bacterial cell structure, inhibited the biosynthesis of bacterial proteins and changed the integrity and permeability of bacterial cell wall and cell membrane.

CONCLUSIONS

Dithiocyano-methane showed remarkable antibacterial ability against three tested strains, indicating it is a potential effective bactericidal agent for preventing animal diseases resulted from Aer. hydrophila.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our best knowledge, this is the first report to examine the antibacterial ability and action mechanism of dithiocyano-methane against bacteria. The results demonstrate the great potential of dithiocyano-methane as a disinfectant against Aer. hydrophila in settings such as aquaculture ponds and livestock farms.

Expression of four new ferritins from grooved carpet shell clam Ruditapes decussatus challenged with Perkinsus olseni and metals (Cd, Cu and Zn)

Iron has a fundamental role in life and in its biochemical reactions but, when in excess, it can promote the formation of free radicals which can lead to cell death. Therefore, managing the levels of iron is essential to regulate the production of oxidative stress related to iron, and ferritins are one of the main protein families involved in this process. Ferritins are ≈480 kDa multimeric proteins composed by 24 subunits, each with 19-26 kDa, which can accumulate up to 4500 iron atoms. Besides their role in managing iron bioavailability, they have also developed a role in organism immunity and defence present throughout evolution. In this work, we identified and characterized, for the first time, four different ferritin subunits in the clam Ruditapes decussatus, a bivalve commercially and ecologically important along the south Atlantic coast and in the Mediterranean basin, which is a major target of the parasitic protozoa Perkinsus olseni, considered one of the main causes of high levels of clam mortality. Following phylogenetic annotation, the four ferritins subunits identified were subdivided into two cytosolic and two secreted forms. All four subunits maintain the canonical ferritin structure with four main helices α (A-D) and a small helix (E), but the secreted ferritins present an additional helix in their N-terminal region (F), located after the signal peptide and with possible antimicrobial properties. Additionally, we identified in ferritin 4 an extra helix α (G) located between helices B and C. These alpha helix domains revealed high degree of similarity with antimicrobial peptides associated with antibacterial and antifungal activities. Analysis of the expression of these subunits showed that ferritins 1 and 2 are ubiquitously expressed while ferritins 3 and 4 are present mainly in visceral mass. Ferritin 1 lacked a putative functional iron response element (IRE) and appeared to be under a tight regulation. Ferritins 2 and 3 showed a strong response to infection by parasite Perkinsus olseni in contrast to ferritin 4, whose main response was related to exposure to a combination of metals. The synergistic effect between metals and infection promoted a general upregulation of the four ferritins. In conclusion, our results suggest that ferritins, besides their function in iron and metals detoxification, may play a determinant role in clam immune response.

Effect of dithiocyano-methane on hexose monophosphate pathway in the respiratory metabolism of Escherichia coli

This paper studied the inhibitory effects of dithiocyano-methane (DM) on the glucose decomposition pathway in the respiratory metabolism of Escherichia coli. We investigated the effects of DM on the activities of key enzymes (ATPase and glucose-6-phosphate dehydrogenase, G6PDH), the levels of key product (nicotinamide adenosine denucleotide hydro-phosphoric acid, NADPH), and gene expression in the hexose monophosphate pathway (HMP). The results showed that the minimum inhibitory concentration (MIC) and the minimum bactericide concentration (MBC) of DM against the tested strains were 5.86 mg/L and 11.72 mg/L, respectively. Bacteria exposed to DM at MIC demonstrated an increase in bacterial ATPase and G6PDH activities, NADPH levels, and gene expression in the HMP pathway compared to bacteria in the control group, which could be interpreted as a behavioral response to stress introduced by DM. However, DM at a lethal concentration of 10 × MIC affected glucose decomposition by inhibiting mainly the HMP pathway in E. coli.

Transcriptome analysis reveals seven key immune pathways of Japanese flounder (Paralichthys olivaceus) involved in megalocytivirus infection

Megalocytivirus is a serious viral pathogen to many farmed fish including Japanese flounder (Paralichthys olivaceus). In this study, in order to systematically identify host immune genes induced by megalocytivirus infection, we examined the transcription profiles of flounder infected by megalocytivirus for 2, 6, and 8 days. Compared with uninfected fish, virus-infected fish exhibited 1242 differentially expressed genes (DEGs), with 225, 275, and 877 DEGs occurring at 2, 6, and 8 days post infection, respectively. Of these DEGs, 728 were upregulated and 659 were downregulated. The majority of DEGs were time-specific and formed four distinct expression profiles well correlated with the time of infection. The DEGs were classified into diverse Gene Ontology (GO) functional terms and enriched in 27 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, approximately one third of which were related to immunity. Weighted co-expression network analysis (WGCNA) was used to identify 16 key immune DEGs belonging to seven immune pathways (RIG-I-like receptor signaling pathway, JAK-STAT signaling pathway, TLR signaling pathway, cytokine-cytokine receptor interaction, phagosome, apoptosis, and p53 signaling pathway). These pathways interacted extensively and formed complicated networks. This study provided a global picture of megalocytivirus-induced gene expression profiles of flounder at the transcriptome level and uncovered a set of key immune genes and pathways closely linked to megalocytivirus infection. These results provided a set of targets for future delineation of the key factors implicated in the anti-megalocytivirus immunity of flounder.

The novel fish miRNA pol-miR-novel_171 and its target gene FAM49B play a critical role in apoptosis and bacterial infection

MicroRNAs (miRNAs) are a type of small, non-coding RNAs that participate in many cellular and biological processes by regulating mRNA stability. In a previous study, we identified 96 Japanese flounder (Paralichthys olivaceus) miRNAs responsive to the infection of Edwardsiella tarda, a bacterial pathogen to fish as well as humans. In the current study, we examined the regulation and function of one novel miRNA, i.e., pol-miR-novel_171, from the above 96 miRNA pool. We found that pol-miR-novel_171 expression was regulated by E. tarda and megalocytivirus in a pathogen-specific manner, and that pol-miR-novel_171 targeted the gene of FAM49B (family with sequence similarity 49 member B) of flounder (named PoFAM49B) by negative interaction with the 3'-UTR of PoFAM49B. To date, the function fish FAM49B is unknown. We found that PoFAM49B expressed in multiple tissues of flounder, and recombinant PoFAM49B interacted with and inhibited the growth of Gram-negative bacterial pathogens. Interference with PoFAM49B expression in flounder cells promoted E. tarda infection. Similar effects on E. tarda infection were observed with pol-miR-novel_171 overexpression. Consistently, in vivo knockdown of PoFAM49B in flounder enhanced E. tarda dissemination in fish tissues. Furthermore, interference with PoFAM49B expression, or overexpression of pol-miR-novel_171, promoted apoptosis of flounder cells, while in vitro and in vivo knockdown of PoFAM49B augmented the expressions of key apoptosis-associated genes. These results revealed for the first time the immune function of fish FAM49B and the regulatory mechanism of a novel fish miRNA by demonstrating that pol-miR-novel_171, via PoFAM49B, played a critical role in apoptosis and anti-bacterial immunity.

The trypsin-like serine protease domain of Paralichthys olivaceus complement factor I regulates complement activation and inhibits bacterial growth

In mammals, complement factor I (CFI) is a serine protease in serum and plays a pivotal role in the regulation of complement activation. In the presence of cofactor, CFI cleaves C3b to iC3b, and further degrades iC3b to C3c and C3d. In teleost, the function of CFI is poorly understood. In this study, we examined the immunological property of CFI from Japanese flounder (Paralichthys olivaceus) (PoCFI), a teleost species with important economic value. PoCFI is composed of 597 amino acid residues and possesses a trypsin-like serine protease (Tryp) domain. We found that PoCFI expressions occurred in nine different tissues and were upregulated by bacterial challenge. Recombinant PoCFI-Tryp (rPoCFI-Tryp) inhibited complement activation and degraded C3b in serum. rPoCFI-Tryp exhibited apparent binding capacities to a board-spectrum of bacteria and inhibited bacterial growth. These results provide the first evidence to indicate that CFI in teleost negatively regulates complement activation via degradation C3b, and probably plays a role in host immune defense against bacterial infection.

A novel ferritin gene from Procambarus clarkii involved in the immune defense against Aeromonas hydrophila infection and inhibits WSSV replication

Ferritin is a protein related to the storage of iron and widely distributed in animals. It participates in many biological process, including antioxidation, cell activation, angiogenesis, regulation of iron metabolic balance and immune defense. In the present study, a novel ferritin gene was identified from red swamp crayfish Procambarus clarkii, with a cDNA sequence encoding a predicted 221 amino-acid residues. The ferritin protein contains a 19-residue signal peptide and 145-residue classic ferritin domain. The NJ phylogenetic analysis showed PcFer clustered with other crustacean peptides. The recombinant PcFer protein was produced and purified in E. coli, and the anti-rabbit polyclonal antibody was obtained. The rPcFer exhibited iron binding activity at a dose-dependent effect. The qPCR and western blot analysis revealed that PcFer was highly expressed in hemocytes, hepatopancreas, and gills. After challenged with WSSV and Aeromonas hydrophila, the mRNA and protein expression patterns of PcFer were significantly up-regulated in hemocytes and hepatopancreas. dsRNA interfering technique was utilized to silence the expression of PcFer gene. The WSSV copy number in PcFer silenced shrimp was much higher than that in the control group. The present study indicated that PcFer was involved in the immune defense against WSSV and Aeromonas hydrophila, and might inhibit WSSV replication in P. clarkii. These results will provide important data support for further study of the functional role of the ferritin gene.

PcLys-i3, an invertebrate lysozyme, is involved in the antibacterial immunity of the red swamp crayfish, Procambarus clarkii

Antimicrobial peptides (AMPs) play important roles in innate immunity against pathogens and lysozymes are a particularly type of AMP. Lysozymes are hydrolytic enzymes that are characterized by their ability to cleave the beta-(1,4)-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan, which is the major bacterial cell wall polymer. In this work, a lysozyme was identified from Procambarus clarkii and designated PcLys-i3. Quantitative RT-PCR was used to analyze the tissue distribution and expression profiles of PcLys-i3. PcLys-i3 was present in all tested tissues and had high expression levels in gills, stomach and intestine. The expression levels of PcLys-i3 were up-regulated in gills and intestine after challenge with Vibrio parahaemolyticus, Staphylococcus aureus and Aeromonas hydrophila. PcLys-i3 and PcFer proteins can enhance the bacterial elimination in crayfish, whereas the bacterial elimination was weakened when the expression level of PcLys-i3 or PcFer RNAs was suppressed by RNAi. Recombinant PcLys-i3 and PcFer significantly reduced the mortality of crayfish with bacterial infections. Further study found that PcLys-i3 could interact with PcFer in vitro. Finally, the PcLys-i3 and PcFer proteins could bind to bacteria and inhibit bacterial replication. These results suggest that both PcLys-i3 and PcFer play important roles in the antibacterial immunity of red swamp crayfish.

First characterization of two C-type lectins of the tubeworm Alaysia sp. from a deep-sea hydrothermal vent

C-type lectins (CTLs) play an important role in innate immune defense. In this study, we identified and characterized two CTLs (Lec1 and Lec2) from the tubeworm Alaysia sp. collected from a hydrothermal vent in Pacmanus. Lec1 and Lec2 possess the typical CTL domain but share low sequence identities (10.8%-20.4%) with known CTLs. Recombinant (r) of Lec1 and Lec2 bound to various PAMPs and a wide arrange of bacteria from neritic and deep-sea environments in a Ca²⁺-independent manner, but only rLec1 caused agglutination of the bound bacteria. The activities of rLec1 and rLec2 were most stable and highest at 4 °C, the ambient temperature of the hydrothermal vent, and decreased at higher temperatures. Both lectins inhibited bacterial growth in a highly selective manner and agglutinated the erythrocytes of fish, rabbit, and chicken in a Ca²⁺-dependent manner. These results provided the first insights into the functional properties of CTLs in deep-sea Alaysia sp.

A CqFerritin protein inhibits white spot syndrome virus infection via regulating iron ions in red claw crayfish Cherax quadricarinatus

It is well known that iron is an essential element for all living organism. The intracellular iron availability is also important for the host's innate immune response to various pathogens, in which the iron homeostasis can be regulated by ferritin due to its iron storage property. In this study, a full-length cDNA sequence of ferritin (named as CqFerritin) was identified with 1410 bp from red claw crayfish Cherax quadricarinatus, which contained an open reading frame of 513 bp, encoding 170 amino acids with a conserved ferritin domain. Tissue distribution analysis demonstrated that CqFerritin was widely expressed in various tissues with high presence in haemocyte, haematopoietic tissue (Hpt) and heart, while lowest expression in hepatopancreas. In addition, loss-of-function of CqFerritin by gene silencing resulted in significantly higher expression of an envelope protein VP28 of white spot syndrome virus (WSSV) in red claw crayfish Hpt cell cultures, indicating the potential antiviral response of CqFerritin. To further explore the effect on WSSV replication by CqFerritin, recombinant CqFerritin protein (rCqFerritin) was transfected into Hpt cells followed by WSSV infection. Importantly, the replication of WSSV was obviously decreased in Hpt cells if transfected with rCqFerritin protein, suggesting that CqFerritin had clearly negative effect on WSSV infection. Furthermore, intracellular accumulation of iron ions was found to promote the WSSV replication in a dose-dependent manner, illustrating that the iron level regulated by CqFerritin was likely to be vital for WSSV infection in red claw crayfish. Taken together, these data suggest that CqFerritin plays an important role in immune defense against WSSV infection in a crustacean C. quadricarinatus.

A teleost CD46 is involved in the regulation of complement activation and pathogen infection

In mammals, CD46 is involved in the inactivation of complement by factor I (FI). In teleost, study on the function of CD46 is very limited. In this study, we examined the immunological property of a CD46 molecule (CsCD46) from tongue sole, a teleost species with important economic value. We found that recombinant CsCD46 (rCsCD46) interacted with FI and inhibited complement activation in an FI-dependent manner. rCsCD46 also interacted with bacterial pathogens via a different mechanism to that responsible for the FI interaction, involving different rCsCD46 sites. Cellular study showed that CsCD46 was expressed on peripheral blood leukocytes (PBL) and protected the cells against the killing effect of complement. When the CsCD46 on PBL was blocked by antibody before incubation of the cells with bacterial pathogens, cellular infection was significantly reduced. Consistently, when tongue sole were infected with bacterial pathogens in the presence of rCsCD46, tissue dissemination and survival of the pathogens were significantly inhibited. These results provide the first evidence to indicate that CD46 in teleosts negatively regulates complement activation via FI and protects host cells from complement-induced damage, and that CD46 is required for optimal bacterial infection probably by serving as a receptor for the bacteria.

Molecular Characterization and Functional Analysis of a Ferritin Heavy Chain Subunit from the Eri-Silkworm, Samia cynthia ricini

Ferritins are conserved iron-binding proteins that are primarily involved in iron storage, detoxification and the immune response. Despite the importance of ferritin in organisms, little is known about their roles in the eri-silkworm (Samia cynthia ricini). We previously identified a ferritin heavy chain subunit named ScFerHCH in the S. c. ricini transcriptome database. The full-length S. c. ricini ferritin heavy chain subunit (ScFerHCH) was 1863 bp and encoded a protein of 231 amino acids with a deduced molecular weight of 25.89 kDa. Phylogenetic analysis revealed that ScFerHCH shared a high amino acid identity with the Bombyx mori and Danaus plexippus heavy chain subunits. Higher ScFerHCH expression levels were found in the silk gland, fat body and midgut of S. c. ricini by reverse transcription quantitative polymerase chain reaction (RT-qPCR) and Western blotting. Injection of Staphylococcus aureus and Pseudomonas aeruginosa was associated with an upregulation of ScFerHCH in the midgut, fat body and hemolymph, indicating that ScFerHCH may contribute to the host’s defense against invading pathogens. In addition, the anti-oxidation activity and iron-binding capacity of recombinant ScFerHCH protein were examined. Taken together, our results suggest that the ferritin heavy chain subunit from eri-silkworm may play critical roles not only in innate immune defense, but also in organismic iron homeostasis.

Comparative analysis of two ferritin subunits from blunt snout bream (Megalobrama amblycephala): Characterization, expression, iron depriving and bacteriostatic activity

Iron is an essential microelement for almost all living organisms, while an excess of iron is toxic, thus maintenance of iron homeostasis is vital. As iron storage protein, ferritin plays an important role in iron metabolism. In the present study, we cloned and characterized the ferritin H subunit from Megalobrama amblycephala, termed as MamFerH. An iron-responsive element (IRE) was predicted in the 5' untranslated region (UTR) of MamFerH, while its bulge structural was different from that of the reported ferritin M subunit (MamFerM). The MamFerH and MamFerM genes exhibited similar expression patterns during early development with specifically high expression post hatching, whereas their tissue expression patterns were different. Specifically, MamFerM was highly expressed in the spleen, liver and kidney, while MamFerH was predominantly expressed in the blood and brain, indicating their different functions. In addition, the expression of the two genes was induced upon Aeromonas hydrophila infection at both transcriptional and translational levels, and MamFerH was more efficient. Immunohistochemistry and immunofluorescence analysis confirmed their significant changes at protein level and distribution in the liver post infection, indicating their participation in host immune response. Furthermore, bacteriostatic experiment revealed that recombinant MamFerH displayed more significant inhibitory effect on the growth of A. hydrophila.

Molecular characterization and gene expression of ferritin in blunt snout bream (Megalobrama amblycephala)

Ferritins are conserved iron storage proteins that exist in most living organisms and play an essential role in iron homeostasis. In this study, we reported the identification and analysis of a ferritin middle-chain (M) subunit, MaFerM, from blunt snout bream, Megalobrama amblycephala. The full length cDNA of MaFerM contains a 5'-untranslated region (UTR) of 152 bp, an open reading frame (ORF) of 522 bp and a 3'-UTR of 270 bp. The ORF encodes a putative protein of 174 amino acids, which shares extensive sequence identities with the M ferritins of several fish species. In silico analysis identified both the ferroxidase center of mammalian heavy-chain (H) ferritins and the iron nucleation site of mammalian light-chain (L) ferritins in MaFerM. Quantitative real-time reverse transcription polymerase chain reaction analysis indicated that MaFerM expression was highest in the liver and lowest in the heart and responded positively to experimental challenges with Aeromonas hydrophila. The exposure of cultured M. amblycephala to treatment with stress inducers (iron and H2O2) significantly up-regulated the expression of MaFerM in a dose-dependent manner. Iron chelation analysis showed that recombinant MaFerM purified from Escherichia coli exhibited apparent iron binding activity. These results suggest that MaFerM is a functional M ferritin and is likely to play a role in iron sequestration and protection against oxidative stress and immune stimulus.

CsPTX1, a pentraxin of Cynoglossus semilaevis, is an innate immunity factor with antibacterial effects

Pentraxin 1 (PTX1) is a member of the pentraxin protein family, which plays important roles in the innate immunity of vertebrates. In fish, the biological function of PTX1 is essentially unknown. In this study, we examined the expression and function of a PTX homologue (CsPTX1) from the tongue sole, Cynoglossus semilaevis. CsPTX1 contains 223 amino acids and shares 49.3%-38.8% overall sequence identity with other known fish pentraxins. CsPTX1 is expressed in multiple tissues and is upregulated by bacterial and viral infection. CsPTX1 contains a pentraxin domain, which is known to bind extracellular antigens, and recombinant CsPTX1 (rCsPTX1) bound a wide range of Gram-positive and Gram-negative bacteria. rCsPTX1 also agglutinated all the bacteria tested in a Ca(2+)-dependent manner and the agglutinating capacity of rCsPTX1 was abolished in the absence of calcium. As well as its ability to agglutinate bacterial cells, rCsPTX1 displayed apparent bacteriostatic activity against Pseudomonas fluorescens in vitro by influencing the permeability of the microbial envelope. When introduced in vivo, rCsPTX1 enhanced the host's resistance to bacterial infection. These results indicate that CsPTX1 is a classic pattern recognition molecule that defends C. semilaevis against bacterial infection.

Pseudomonas fluorescens Filamentous Hemagglutinin, an Iron-Regulated Protein, Is an Important Virulence Factor that Modulates Bacterial Pathogenicity

Pseudomonas fluorescens is a common bacterial pathogen to a wide range of aquaculture animals including various species of fish. In this study, we employed proteomic analysis and identified filamentous hemagglutinin (FHA) as an iron-responsive protein secreted by TSS, a pathogenic P. fluorescens isolate. In vitro study showed that compared to the wild type, the fha mutant TSSfha (i) exhibited a largely similar vegetative growth profile but significantly retarded in the ability of biofilm growth and producing extracellular matrix, (ii) displayed no apparent flagella and motility, (iii) was defective in the attachment to host cells and unable to form self-aggregation, (iv) displayed markedly reduced capacity of hemagglutination and surviving in host serum. In vivo infection analysis revealed that TSSfha was significantly attenuated in the ability of dissemination in fish tissues and inducing host mortality, and that antibody blocking of the natural FHA produced by the wild type TSS impaired the infectivity of the pathogen. Furthermore, when introduced into turbot as a subunit vaccine, recombinant FHA elicited a significant protection against lethal TSS challenge. Taken together, these results indicate for the first time that P. fluorescens FHA is a key virulence factor essential to multiple biological processes associated with pathogenicity.

Identification and characterization of a ferritin gene involved in the immune defense response of scallop Chlamys farreri

Scallop Chlamys farreri is an important aquaculture species in northern China. However, its mass mortality caused by several pathogens can result in great economic loss and negative impacts to the sustainable development of the scallop industry. Thus, improving the overall understanding of immune response mechanisms involved in host-pathogen interactions is necessary. Ferritins are conserved molecules in organisms that are involved in diverse biological processes, such as mediating host-pathogen responses. In this study, we report a novel ferritin gene from C. farreri (denoted as CfFER). The full length of CfFER is 848 bp and contains a 5'-UTR of 113 bp, a 3'-UTR of 219 bp, and a complete open reading frame (ORF) of 516 bp. The ORF encodes a polypeptide of 171 amino acid residues with a molecular weight of approximately 19.95 kDa and an isoelectric point of 5.07. The CfFER protein exhibited typical ferritin structures, namely, a ferroxidase diiron center, a ferrihydrite nucleation center, and an iron-binding response signature. Phylogenetic analysis revealed that CfFER was closely related to other mollusk ferritin proteins. Expression of CfFER in different tissues was analyzed by quantitative real-time PCR, and results showed that CfFER was ubiquitously expressed in all examined tissues. The highest and lowest expression levels of CfFER were measured in the muscle and hemocyte, respectively. The relative mRNA expression of CfFER in response to bacterial (Vibrio anguillarum) and viral (acute viral necrobiotic virus) challenges sharply increased by ca. 5-fold about12 h post-infection (hpi) and then normalized at 48 hpi. Western blot analysis with polyclonal antibodies generated from the recombinant product of CfFER also demonstrated the presence of ferritin protein in hemocytes. These findings strongly suggest that CfFER is involved in the immune response of C. farreri and protection against pathogen challenge.

Identification and involvement of ferritin in the response to pathogen challenge in the abalone, Haliotis diversicolor

Accumulating data has demonstrated that ferritin plays an important role in host defense responses against infection by pathogens in many organisms. In this study, ultracentrifugation was used to isolate ferritin from abalone, Haliotis diversicolor, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis revealed that this ferritin consisted of two subunits (designated as HdFer1 and HdFer2). There are no disulfide bonds between the HdFer1 and HdFer2 subunits; however, these subunits co-assemble to form heteropolymers. A novel ferritin subunit (HdFer2) was cloned from H. diversicolor by 5' and 3' RACE (rapid amplification of cDNA ends) approach. The full-length HdFer2 cDNA sequence consists of 878 bp with an open reading frame of 513 bp that encodes a protein that is 170 amino acids in length. Quantitative real-time PCR analysis revealed that HdFer1 and HdFer2 were transcribed in various tissues, such as the mantle, gill and hepatopancreas, with the highest levels of expression in the hepatopancreas. Following a challenge with the pathogen, Vibrio harveyi, the expression of HdFer1 and HdFer2 were markedly induced at different times. This study has identified a novel ferritin subunit in H. diversicolor which will contribute to further exploration of the role of ferritin in mollusk innate immune defense against invading pathogens.

Toll-like receptor 2 of tongue sole Cynoglossus semilaevis: Signaling pathway and involvement in bacterial infection

Toll-like receptor (TLR) 2 is a member of the TLR family that plays a pivotal role in innate immunity. In mammals, TLR2 is known to recognize specific microbial structures and trigger MyD88-dependent signaling to induce various cytokine responses. In this study, we examined the expression and function of the tongue sole Cynoglossus semilaevis TLR2, CsTLR2. CsTLR2 is composed of 898 amino acid residues and shares 25.6%-27.3% overall sequence identities with known teleost TLR2. CsTLR2 is a transmembrane protein with a toll/interleukin-1 receptor domain and eight leucine-rich repeats. Expression of CsTLR2 occurred in multiple tissues and was upregulated during bacterial infection. Stimulation of the CsTLR2 pathway led to enhanced expression of MyD88-dependent signaling molecules. Recombinant CsTLR2 (rCsTLR2) corresponding to the extracellular region was able to bind to a wide range of bacteria. Under both in vitro and in vivo conditions, rCsTLR2 significantly reduced bacterial infection. These observations add new insights into the signaling and function of teleost TLR2.

First comparative characterization of three distinct ferritin subunits from a teleost: Evidence for immune-responsive mRNA expression and iron depriving activity of seahorse (Hippocampus abdominalis) ferritins

Ferritins play an indispensable role in iron homeostasis through their iron-withholding function in living beings. In the current study, cDNA sequences of three distinct ferritin subunits, including a ferritin H, a ferritin M, and a ferritin L, were identified from big belly seahorse, Hippocampus abdominalis, and molecularly characterized. Complete coding sequences (CDS) of seahorse ferritin H (HaFerH), ferritin M (HaFerM), and ferritin L (HaFerL) subunits were comprised of 531, 528, and 522 base pairs (bp), respectively, which encode polypeptides of 177, 176, and 174 amino acids, respectively, with molecular masses of ∼20-21 kDa. Our in silico analyses demonstrate that these three ferritin subunits exhibit the typical characteristics of ferritin superfamily members including iron regulatory elements, domain signatures, and reactive centers. The coding sequences of HaFerH, M, and L were cloned and the corresponding proteins were overexpressed in a bacterial system. Recombinantly expressed HaFer proteins demonstrated detectable in vivo iron sequestrating (ferroxidase) activity, consistent with their putative iron binding capability. Quantification of the basal expression of these three HaFer sequences in selected tissues demonstrated a gene-specific ubiquitous spatial distribution pattern, with abundance of mRNA in HaFerM in the liver and predominant expression of HaFerH and HaFerL in blood. Interestingly, the basal expression of all three ferritin genes was found to be significantly modulated against pathogenic stress mounted by lipopolysaccharides (LPS), poly I:C, Streptococcus iniae, and Edwardsiella tarda. Collectively, our findings suggest that the three HaFer subunits may be involved in iron (II) homeostasis in big belly seahorse and that they are important in its host defense mechanisms.

CsTNF1, a teleost tumor necrosis factor that promotes antibacterial and antiviral immune defense in a manner that depends on the conserved receptor binding site

Tumor necrosis factor (TNF) is one of the most important cytokines involved in inflammation, apoptosis, cell proliferation, and stimulation of the immune system. The TNF gene has been cloned in teleost fish; however, the in vivo function of fish TNF is essentially unknown. In this study, we report the identification of a TNF homologue, CsTNF1, from tongue sole (Cynoglossus semilaevis) and analysis of its expression and biological effect. CsTNF1 is composed of 242 amino acid residues and possesses a TNF domain and conserved receptor binding sites. Expression of CsTNF1 was detected in a wide range of tissues and up-regulated in a time-dependent manner by experimental challenge with bacterial and viral pathogens. Bacterial infection of peripheral blood leukocytes (PBL) caused extracellular secretion of CsTNF1. Purified recombinant CsTNF1 (rCsTNF1) was able to bind to PBL and stimulate the respiratory burst activity of PBL. In contrast, rCsTNF1M1 and rCsTNF1M2, the mutant CsTNF1 bearing substitutions at the receptor binding site, failed to activate PBL. Fish administered with rCsTNF1, but not with rCsTNF1M1 and rCsTNF1M2, exhibited enhanced expression of IL-1, IL-6, IL-8, IL-27, TLR9 and G3BP in a time-dependent manner and augmented resistance against bacterial and viral infection. These results provide the first evidence that the receptor binding sites are essential to a fish TNF, and that CsTNF1 is involved in the innate immune defense of fish against microbial pathogens.

Tongue sole (Cynoglossus semilaevis) prothymosin alpha: Cytokine-like activities associated with the intact protein and the C-terminal region that lead to antiviral immunity via Myd88-dependent and -independent pathways respectively

Prothymosin alpha (ProTα) is a small protein that in mammals is known to participate in diverse biological processes including immunomodulation. In teleost, the immunological function of ProTα is unknown. In the current study, we investigated the expression and function of the ProTα (named CsProTα) from the teleost fish tongue sole (Cynoglossus semilaevis). We found that CsProTα expression was abundant in immune relevant tissues and upregulated by megalocytivirus infection. Immunoblot detected secretion of CsProTα by peripheral blood leukocytes. Recombinant CsProTα (rCsProTα) as well as the C-terminal 11-residue (Ct11) were able to bind head kidney monocytes (HKM) and induce immune gene expression; however, the induction patterns caused by rCsProTα and Ct11 differed considerably. When introduced in vivo, rCsProTα and Ct11 significantly reduced megalocytivirus infection in fish tissues, whereas rCsProTα antibody significantly promoted viral replication. Blocking of Myd88 activity abolished the virus-inhibitory effect of rCsProTα but not Ct11. Taken together, these results demonstrate for the first time that both the intact protein and the C-terminal segment of a teleost ProTα can act like cytokines and induce antiviral immunity via, however, distinct signaling pathways that differ in the requirement of Myd88.

Ferritin M of Paralichthys olivaceus possesses antimicrobial and antioxidative properties

Ferritin is an evolutionarily conserved protein that plays a vital role in maintaining iron homeostasis. In this study, we identified a ferritin M (PoFerM) from Japanese flounder (Paralichthys olivaceus) and analyzed its biological property. PoFerM is composed of 176 amino acid residues and contains the conserved ferroxidase diiron center and the ferrihydrite nucleation center typical of M ferritins. Expression of PoFerM occurred in multiple tissues and was most abundant in blood. Bacterial infection upregulated PoFerM expression in head kidney, spleen, and liver in a time-dependent manner. Recombinant PoFerM (rPoFerM) purified from Escherichia coli exhibited iron-chelating activity and inhibited bacterial growth, whereas rPoFerMM, the mutant protein that bears alanine substitution at two conserved residues of the ferroxidase center and the ferrihydrite nucleation center, failed to do so. Oxidative protection analysis showed that rPoFerM, but not rPoFerMM, was able to alleviate the deleterious effect of H2O2-induced free radicals on plasmid DNA and primary flounder cells. Together these results indicate that PoFerM is an iron chelator with antimicrobial and antioxidative properties, all which depend on the conserved ferroxidase center and the ferrihydrite nucleation site.

ORF75 of megalocytivirus RBIV-C1: A global transcription regulator and an effective vaccine candidate

Megalocytivirus, a DNA virus belonging to the Iridoviridae family, is a severe pathogen to a wide range of marine and freshwater fish. In this study, using turbot (Scophthalmus maximus) as a host model, we examined the immunoprotective property of one megalocytivirus gene, ORF75, in the form of DNA vaccine (named pORF75). Immunofluorescence microscopy and RT-PCR analysis showed that P444, the protein encoded by ORF75, was naturally produced in the tissues of turbot during megalocytivirus infection, and that the vaccine gene in pORF75 was expressed in fish cells transfected with pORF75 and in the tissues of turbot immunized with pORF75. Following vaccination of turbot with pORF75, a high level of survival (73%) was observed against a lethal megalocytivirus challenge. Consistently, viral replication in the vaccinated fish was significantly inhibited. Immune response analysis showed that pORF75-vaccinated fish (i) exhibited upregulated expression of the genes involved in innate and adaptive immunity, (ii) possessed specific memory immune cells that showed significant response to secondary antigen stimulation, and (iii) produced specific serum antibodies which, when co-introduced into turbot with megalocytivirus, blocked viral replication. Furthermore, whole-genome transcriptome analysis revealed that ORF75 knockdown altered the transcription of 43 viral genes. Taken together, these results indicate that ORF75 encoded a highly protective immunogen that is also a global transcription regulator of megalocytivirus.

CD83 is required for the induction of protective immunity by a DNA vaccine in a teleost model

In mammals, CD83 is a surface marker on mature dendritic cells and vital to lymphocyte activation. In teleost, studies on the function of CD83 are very limited. In this study, we examined the potential involvement of turbot (Scophthalmus maximus) CD83, SmCD83, in vaccine-induced immunity. For this purpose, turbot were immunized with pORF75, a DNA vaccine against megalocytivirus, in the presence or absence of pSmCD83, a plasmid that constitutively expresses SmCD83. Immune response and protection analysis showed that the presence of pSmCD83 significantly (i) enhanced the activation of head kidney macrophages (HKM) and immune gene expression, (ii) inhibited viral replication in fish tissues following megalocytivirus challenge and increased the survival of the vaccinated fish, and (iii) stimulated production of specific serum antibody and the cytotoxicity of peripheral blood leukocytes. To further examine the effect of SmCD83, pORF75 was administered into turbot in which SmCD83 was knocked down. Subsequent analysis showed that in fish with SmCD83 knockdown, vaccine-induced HKM activation and antibody production were severely reduced, and, consistently, the protectivity of pORF75 was drastically decreased. Taken together, these results indicate for the first time that teleost CD83 is required for the induction of protective immune response by DNA vaccine.

Pseudomonas fluorescens: identification of Fur-regulated proteins and evaluation of their contribution to pathogenesis

Pseudomonas fluorescens is a Gram-negative bacterium and a common pathogen to a wide range of farmed fish. In a previous study, we found that the ferric uptake regulator gene (fur) is essential to the infectivity of a pathogenic fish isolate of P. fluorescens (wild-type strain TSS). In the present work, we conducted comparative proteomic analysis to examine the global protein profiles of TSS and the P. fluorescens fur knockout mutant TFM. Twenty-eight differentially produced proteins were identified, which belong to different functional categories. Four of these proteins, viz. TssP (a type VI secretion protein), PspA (a serine protease), OprF (an outer membrane porin), and ClpP (the proteolytic subunit of an ATP-dependent Clp protease), were assessed for virulence participation in a model of turbot Scophthalmus maximus. The results showed that the oprF and clpP knockouts exhibited significantly reduced capacities in (1) resistance against the bactericidal effect of host serum, (2) dissemination into and colonization of host tissues, and (3) inducing host mortality. In contrast, mutation of tssP and pspA had no apparent effect on the pathogenicity of TSS. Purified recombinant OprF, when used as a subunit vaccine, induced production of specific serum antibodies in immunized fish and elicited significant protection against lethal TSS challenge. Antibody blocking of the OprF in TSS significantly impaired the ability of the bacteria to invade host tissues. Taken together, these results indicate for the first time that in pathogenic P. fluorescens, Fur regulates the expression of diverse proteins, some of which are required for optimal infection.

Molecular characterization of the cathepsin B of turbot (Scophthalmus maximus)

Cathepsin B is an enzymatic protein belonging to the peptidase C1 family. It is involved in diverse physiological and pathological functions that include immune response. In this study, we identified and characterized a cathepsin B homolog (SmCatB) from turbot (Scophthalmus maximus). SmCatB is composed of 330 amino acid residues and possesses typical domain architecture of cathepsin B, which contains a propeptide region and a cysteine protease domain, and the latter processes four conserved residues (Q101, C107, H277, and N297) in the active site. SmCatB shares 80.6-87.6% overall sequence identities with the cathepsin B of a number of teleost. SmCatB expression was detected in a wide range of tissues and upregulated by bacterial infection in a time-dependent manner. Recombinant SmCatB (rSmCatB-WT) purified from Escherichia coli exhibited apparent protease activity, which was optimal at 50 °C and pH 5.5. Compared to rSmCatB-WT, the mutant proteins rSmCatB-C107S, rSmCatB-H277A, and rSmCatB-N297A, which bear C107S, H277A, and N297A mutations, respectively, were significantly reduced in protease activity, with the highest reduction observed with rSmCatB-N297A. These results indicate that SmCatB is a bioactive protease that depends on the conserved structural features and that SmCatB is involved in pathogen-induced immune response.

P247 and p523: two in vivo-expressed megalocytivirus proteins that induce protective immunity and are essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among teleost fish. Although the complete genome sequences of a number of megalocytivirus isolates have been reported, the functions of most of the genes of this virus are unknown. In this study, we selected two megalocytivirus immunogens, P247 and P523, which were expressed during host infection and, when in the form of DNA vaccines (pCN247 and pCN523 respectively), elicited strong protectivity against lethal megalocytivirus challenge in a turbot (Scophthalmus maximus) model. Compared to control fish, fish vaccinated with pCN247 and pCN523 exhibited drastically reduced viral loads in tissues and high levels of survival rates. Immune response analysis showed that pCN247 and pCN523 (i) induced production of specific serum antibodies, (ii) caused generation of cytotoxic immune cells and specific memory immune cells that responded to secondary antigen stimulation, and (iii) upregulated the expression of genes involved in innate and adaptive immunity. To examine the potential role of P247 and P523 in viral infection, the expression of P247 and P523 was knocked down by siRNA. Subsequent in vivo infection study showed that P247 and P523 knockdown significantly impaired viral replication. Furthermore, whole-genome transcriptome analysis revealed that P247 and P523 knockdown altered the expression profiles of 26 and 41 viral genes, respectively, putatively participating in diverse aspects of viral infection. Taken together, these results indicate that P247 and P523 induce protective immunity in teleost and play fundamental roles essential to viral replication. These observations provide the first evidence that suggests a likely link between the protectivity of viral immunogens and their biological significance in viral replication.

Ferritin protect shrimp Litopenaeus vannamei from WSSV infection by inhibiting virus replication

Iron is considered as an essential element for all living organisms. Therefore, limiting iron availability may be key part of the host's innate immune response to various pathogens. Ferritin is a major iron storage protein in living cells and plays an important role in iron homeostasis. One way the host can transiently reduce iron bioavailability is by ferritin over expression. In invertebrates, ferritin was found to be up-regulated after pathogens challenge and is considered to be an important element in the innate immune system. This study was designed to investigate the involvement of ferritin in shrimp Litopenaeus vannamei defense against WSSV. We discovered that the viral load of shrimp injected with recombinant ferritin protein was lower than that of control group. The suppression of ferritin by dsRNA increased susceptibility to WSSV with 3-fold high viral copies. The present study documented that ferritin protected shrimp L. vannamei from WSSV by inhibiting virus replication. We presume that ferritin reduce iron availability, leading to inhibit the activity of ribonucleotide reductase and delay the replication of virus genome. This study provided new insights into the understanding of molecular responses and defense mechanisms in shrimp against WSSV.

Toxicological responses of the hard clam Meretrix meretrix exposed to excess dissolved iron or challenged by Vibrio parahaemolyticus

The responses of genes encoding defense components such as ferritin, the lipopolysaccharide-induced tumor necrosis factor-alpha factor (LITAF), the inhibitor of nuclear factor-κB (IκB), metallothionein, and glutathione peroxidase were assessed at the transcriptional level in order to investigate the toxicological and immune mechanism of the hard clam Meretrix meretrix (HCMM) following challenge with iron or a bacterium (Vibrio parahaemolyticus). Fe dissolved in natural seawater led to an increase of Fe content in both the hepatopancreas and gill tissue of HCMM between 4 and 15 days of exposure. The ferritin gene responded both transcriptionally as indicated by real-time quantitative PCR and translationally as shown by western blotting results to iron exposure and both transcriptional and translational ferritin expression in the hepatopancreas had a positive correlation with the concentration of dissolved iron in seawater. Both iron and V. parahaemolyticus exposure triggered immune responses with similar trends in clam tissues. There was a significant post-challenge mRNA expression of LITAF and IκB at 3h, ferritin at 24h, and metallothionein and glutathione peroxidase at 48h. This behavior might be linked to their specific functions in physiological processes. These results suggested that similar signaling pathways were triggered during both iron and V. parahaemolyticus challenges. Here, we indicated that the ferritin of Meretrix meretrix was an intermediate in the pathway of iron homeostasis and in its innate immune defense mechanism.

Junctional adhesion molecule A of red drum (Sciaenops ocellatus): a possible immunomodulator and a target for bacterial immune evasion

Junctional adhesion molecules (JAMs) are a family of type I cell surface receptors with two immunoglobulin (Ig) domains in the extracellular region. The family contains three classical members, i.e., JAM-A, -B, and -C. To date very little is known about the function of JAMs in teleost. In this work, we identified a JAM-A homologue (named SoJAMa) from red drum (Sciaenops ocellatus) and examined its expression and biological property. SoJAMa is composed of 347 amino acid residues and was predicted to be a transmembrane protein with a large extracellular region that contains two Ig domains. SoJAMa expression occurred in multiple tissues, in particular immune relevant organs. SoJAMa expression was downregulated by experimental challenge with an extracellular pathogen but upregulated by challenge with an intracellular pathogen that is known to be capable of immune evasion. Likewise, cellular study showed that infection of peripheral blood leukocytes (PBL) with intracellular pathogen induced significantly higher expression of SoJAMa. Immunofluorescence microscopy showed that SoJAMa was localized on the surface of PBL and recognized by antibodies against recombinant SoJAMa. Blockage of the SoJAMa on PBL with antibodies resulted in augmented respiratory burst activity. Consistently, antibody-treated PBL exhibited enhanced resistance against bacterial infection. Taken together, these results suggest for the first time that a teleost JAM-A likely possesses immunoregulatory property in a negative manner, and that this property may be taken advantage of by intracellular pathogens as an invasion strategy.

Identification and analysis of three virulence-associated TonB-dependent outer membrane receptors of Pseudomonas fluorescens

Pseudomonas fluorescens is a Gram-negative bacterium that can infect a wide range of farmed fish. However, very little is known about the virulence mechanism of P. fluorescens as a fish pathogen. In this study, we identified and analyzed 3 TonB-dependent outer membrane receptors (TDRs) from a pathogenic P. fluorescens strain isolated from fish. In silico analysis revealed that all 3 proteins (named Tdr1 to 3) possess structural domains typical of TDRs. Quantitative real time RT-PCR analysis showed that tdr1, tdr2, and tdr3 expressions were upregulated under iron-depleted conditions. Compared to the wild type, mutants defective in tdr1, tdr2, and tdr3 were retarded in growth to different extents. Infection in a turbot Scophthalmus maximus model showed that all 3 mutants were impaired in their ability to desseminate into and colonize host tissues. In addition, the tdr1 and tdr3 mutants exhibited significantly reduced virulence. When used as subunit vaccines, purified recombinant proteins of Tdr1, Tdr2, and, in particular, Tdr3 elicited significant protection in turbot against lethal P. fluorescens challenge. The vaccinated fish produced specific serum antibodies, which, when incubated with P. fluorescens, blocked infection of P. fluorescens in fish cells. Together these results indicate that Tdr1, Tdr2, and Tdr3 are iron-regulated factors that participate in bacterial virulence and induce protective immunity as subunit vaccines.

Rock bream (Oplegnathus fasciatus) IL-12p40: identification, expression, and effect on bacterial infection

IL-12p40, also called IL-12β, is a subunit of the proinflammatory cytokines interleukin (IL)-12 and IL-23. In teleost, IL-12p40 homologues have been identified in several species, however, the biological function of fish IL-12p40 is essentially unknown. In this work, we reported the identification and analysis of an IL-12p40, OfIL-12p40, from rock bream (Oplegnathus fasciatus). OfIL-12p40 is composed of 361 amino acids and possesses a conserved IL-12p40 domain and a WSxWS signature motif characteristic of known IL-12p40. Constitutive expression of OfIL-12p40 occurred in multiple tissues and was highest in kidney. Experimental infection with bacterial pathogen upregulated the expression of OfIL-12p40 in kidney and spleen in a time-dependent manner. Purified recombinant OfIL-12p40 (rOfIL-12p40) stimulated the respiratory burst activity of peripheral blood leukocytes in a dose-dependent manner. rOfIL-12p40 also enhanced the resistance of rock bream against bacterial infection and upregulated the expression of innate immune genes in kidney. Taken together, these results indicate that OfIL-12p40 possesses cytokine-like property and plays a role in immune defense against bacterial infection.

Cloning and characterisation of multiple ferritin isoforms in the Atlantic salmon (Salmo salar)

Ferritin is a highly-conserved iron-storage protein that has also been identified as an acute phase protein within the innate immune system. The iron-storage function is mediated through complementary roles played by heavy (H)-chain subunit as well as the light (L) in mammals or middle (M)-chain in teleosts, respectively. In this study, we report the identification of five ferritin subunits (H1, H2, M1, M2, M3) in the Atlantic salmon that were supported by the presence of iron-regulatory regions, gene structure, conserved domains and phylogenetic analysis. Tissue distribution analysis across eight different tissues showed that each of these isoforms is differentially expressed. We also examined the expression of the ferritin isoforms in the liver and kidney of juvenile Atlantic salmon that was challenged with Aeromonas salmonicida as well as in muscle cell culture stimulated with interleukin-1β. We found that each isoform displayed unique expression profiles, and in certain conditions the expressions between the isoforms were completely diametrical to each other. Our study is the first report of multiple ferritin isoforms from both the H- and M-chains in a vertebrate species, as well as ferritin isoforms that showed decreased expression in response to infection. Taken together, the results of our study suggest the possibility of functional differences between the H- and M-chain isoforms in terms of tissue localisation, transcriptional response to bacterial exposure and stimulation by specific immune factors.

Molecular cloning and expression analysis of ferritin, heavy polypeptide 1 gene from duck (Anas platyrhynchos)

H-ferritin is a core subunit of the iron storage protein ferritin, and is related to the pathogenesis of malignant diseases. A differential expressed sequence tag of the ferritin, heavy polypeptide 1 gene (FTH1) was obtained from our previously constructed suppression subtractive cDNA library from 3-day-old ducklings challenged with duck hepatitis virus type I (DHV-1). The expression and function of FTH1 in immune defense against infection remains largely unknown in ducks. In this study, the full-length duFTH1 cDNA was obtained using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. It consisted of 153 basepairs (bp) 5'untranslated region (UTR), 183 bp 3'UTR, and 546 bp open reading frame that encodes a single protein of 181 amino acid residues. duFTH1 shares high similarity with FTH1 genes from other vertebrates. The amino acid sequence possesses the conserved domain of typical ferritin H subunits, including seven metal ligands in the ferroxidase center, one iron binding region signature, and a potential bio-mineralization residue (Thy(29)). Moreover, in agreement with a previously reported ferritin H subunit, we identified an iron response element in the 5'UTR. RT-PCR analyses revealed duFTH1 mRNA is widely expressed in various tissues. Real-time quantitative polymerase chain reaction analyses suggested that duFTH1 mRNA is significantly up-regulated in the liver after DHV-1 injection or polyriboinosinic polyribocytidylic acid (polyI:C) treatment, reaching a peak 4 h post-infection, and dropping progressively and returning to normal after 24 h. Our findings suggest that duFTH1 functions as an iron chelating protein subunit in duck and contributes to the innate immune responses against viral infections.

Sil: a Streptococcus iniae bacteriocin with dual role as an antimicrobial and an immunomodulator that inhibits innate immune response and promotes S. iniae infection

Streptococcus iniae is a Gram-positive bacterium and a severe pathogen to a wide range of economically important fish species. In addition, S. iniae is also a zoonotic pathogen and can cause serious infections in humans. In this study, we identified from a pathogenic S. iniae strain a putative bacteriocin, Sil, and examined its biological activity. Sil is composed of 101 amino acid residues and shares 35.6% overall sequence identity with the lactococcin 972 of Lactococcus lactis. Immunoblot analysis showed that Sil was secreted by S. iniae into the extracellular milieu. Purified recombinant Sil (rSil) exhibited a dose-dependent inhibitory effect on the growth of Bacillus subtilis but had no impact on the growths of other 16 Gram-positive bacteria and 10 Gram-negative bacteria representing 23 different bacterial species. Treatment of rSil by heating at 50°C abolished the activity of rSil. rSil bound to the surface of B. subtilis but induced no killing of the target cells. Cellular study revealed that rSil interacted with turbot (Scophthalmus maximus) head kidney monocytes and inhibited the innate immune response of the cells, which led to enhanced cellular infection of S. iniae. Antibody blocking of the extracellular Sil produced by S. iniae significantly attenuated the infectivity of S. iniae. Consistent with these in vitro observations, in vivo study showed that administration of turbot with rSil prior to S. iniae infection significantly increased bacterial dissemination and colonization in fish tissues. Taken together, these results indicate that Sil is a novel virulence-associated bacteriostatic and an immunoregulator that promotes S. iniae infection by impairing the immune defense of host fish.