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.

Identification and functional characterization of complement regulatory protein CD59 in golden pompano (Trachinotus ovatus)

CD59, one of the essential inhibitors of the complement membrane attack complex (MAC), plays a crucial role in regulation of complement activation. In this study, we cloned and identified the CD59 gene (named ToCD59) of golden pompano (Trachinotus ovatus). The ORF sequence of ToCD59 is 357 bp long encoding 118 amino acids with a molecular weight of 13.09 kDa. Prediction of protein domains showed that ToCD59 contained an Lu domain and a C-terminal glycosylphosphatidylinositol (GPI) partial anchor. Homology comparisons indicated that ToCD59 shared the high sequence similarity with other fish CD59. RT-qPCR analysis showed that ToCD59 was expressed in all tested healthy tissues of golden pompano, with the highest level of expression in the brain. After stimulation with bacteria, ToCD59 expression levels were significantly up-regulated in head kidney, liver, gill and brain, but down-regulated in spleen. Subcellular localization results showed that ToCD59 localized to the cytoplasm of A549 cells. The hemolytic activity analysis showed that rToCD59 might have complement inhibitory activity through the alternative complement pathway. In addition, antibacterial test showed that rToCD59 had antibacterial ability against S. agalactiae and V. alginolyticus in vitro. These results suggest that ToCD59 might play an important role in the immune response against pathogens, which would provide basic information for elucidating the functional evolutionary history of complement system in teleost.

Bactericidal permeability-increasing protein/LPS-binding protein (BPI/LBP) enhances resistance of golden pompano Trachinotus ovatus against bacterial infection

Antimicrobial peptides are crucial components of innate immunity against microbial invasions. As a kind of antimicrobial peptides, bactericidal permeability-increasing protein (BPI)/lipopolysaccharide-binding protein (LBP) play vital roles in defending the host against gram-negative bacteria. In the current study, a novel BPI/LBP from Trachinotus ovatus (TroBPI/LBP) was characterized. The full length of TroBPI/LBP cDNA sequence is 1434 bp, which contained 477 amino acids. Multiple amino acid alignments of TroBPI/LBP shows 34.07%-84.49% identity with other fish BPI/LBP. Similar to other BPI/LBP, TroBPI/LBP also possesses an N-terminal signal peptide, a BPI/LBP/CETP N-terminal domain, and a BPI/LBP/CETP C-terminal domain. In vitro, the recombinant protein of TroBPI/LBP showed effective bacterial depression activity and binding activity to gram-negative bacteria. In vivo, TroBPI/LBP was constitutively expressed in tested tissues, and the highest expression level was in liver. Following Vibrio alginolyticus stimulation, the mRNA expression of TroBPI/LBP was significantly upregulated in immune-related tissues, and peaked at 12 h post-infection, which confirmed that TroBPI/LBP was highly sensitive to V. alginolyticus stimuli. Furthermore, functional analyses showed that the overexpression of TroBPI/LBP could enhance the ability of fish to against V. alginolyticus infection, and the knockdown of TroBPI/LBP significantly diminished bacterial clearance capacity post-infection. Therefore, these results suggest that TroBPI/LBP may play an important role in host defense against bacterial infection.

Diet supplementation of astaxanthin mitigates cadmium induced negative effects on oxidative, inflammatory and non-specific immune responses, and the intestinal morphology in Trachinotus ovatus

Knowledge about additive on alleviating the negative effects of fish exposed to cadmium (Cd) needs to be more identified in the aquaculture. Therefore, the present study aims to investigate whether diet supplemented with astaxanthin could mitigate adverse effects on hepatic Cd deposition, oxidative, inflammatory and non-specific immune responses, and intestinal morphology of Trachinotus ovatus exposed to dietary Cd treatment. Three isonitrogen-isolipid experimental diets supplemented with/without CdCl₂/Astaxanthin were formulated to feed the T. ovatus for 30 days. Results demonstrated that considerably higher hepatic Cd level was found in two Cd supplementing groups compared to the control group (p < 0.05), while no statistical difference of hepatic Cd concentration between these two Cd supplementing groups (p > 0.05). The activity of hepatic total superoxide dismutase, RNA expression levels of hepatic Nrf2-keap1 pathway genes, pro-inflammatory genes, and non-specific immune genes were no statistical differences between the control group and the dietary Cd supplementing group (p > 0.05), while these data in the dietary Cd with astaxanthin group showed significantly higher than that in the dietary Cd without astaxanthin group (p < 0.05). On the contrary, hepatic malondialdehyde content in the dietary Cd group showed significantly higher than that in the control group and dietary Cd with astaxanthin group (p < 0.05). Significantly lower mid-intestine morphology parameters were obtained in the dietary Cd group than the control group (p < 0.05), while significantly higher data were found in the dietary Cd with astaxanthin group compared to the dietary Cd group (p < 0.05). These results indicated that astaxanthin could mitigate the inhibitory effects of Cd on the oxidative, inflammatory and non-specific immune responses, and intestinal morphology of T. ovatus while not reduce the hepatic Cd deposition.

ROS Induced by Streptococcus agalactiae Activate Inflammatory Responses via the TNF-α/NF-κB Signaling Pathway in Golden Pompano Trachinotus ovatus (Linnaeus, 1758)

Streptococcus agalactiae is common pathogenic bacteria in aquaculture and can cause mass mortality after fish infection. This study aimed to investigate the effects of S. agalactiae infection on the immune and antioxidant regulatory mechanisms of golden pompano (Trachinotus ovatus). Serum and liver samples were obtained at 0, 6, 12, 24, 48, 96, and 120 h after golden pompano infection with S. agalactiae for enzyme activity and gene expression analyses. After infection with S. agalactiae, the content of reactive oxygen species (ROS) in serum was significantly increased (p < 0.05). Serum levels of glucose (GLU), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and malondialdehyde (MDA) increased and then decreased (p < 0.05), reaching a maximum at 6 h. Serum antioxidant enzyme (LZM) activity increased significantly (p < 0.05) and reached a maximum at 120 h. In addition, the mRNA expression levels of antioxidant genes (SOD, CAT, and GPx) in the liver increased and then decreased, reaching the maximum at 24 h, 48 h, and 24 h, respectively. During the experimental period, the mRNA expression levels of NF-κB-related genes of the inflammatory signaling pathway inhibitory κB (IκB) showed an overall decreasing trend (p < 0.05) and the lowest expression at 120 h, whereas the mRNA expression levels of tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), IκB kinase (IKK), and nuclear factor NF-κB increased significantly (p < 0.05) and the highest expression was at 120 h. In conclusion, these results showed that S. agalactiae could activate internal regulatory signaling in the liver of golden pompano to induce defense and immune responses. This study is expected to lay a foundation to develop the healthy aquaculture of golden pompano and promote a more comprehensive understanding of its disease resistance mechanisms.

Effects of High Voltage Atmospheric Cold Plasma Treatment on the Number of Microorganisms and the Quality of Trachinotus ovatus during Refrigerator Storage

In order to investigate the effects of high voltage atmospheric cold plasma (HVACP) treatment on the number of microorganisms in and the quality of Trachinotus ovatus during refrigerator storage, fresh fish was packaged with gases CO₂:O₂:N₂ (80%:10%:10%) and treated by HVACP at 75 kV for 3 min; then, the samples were stored at 4 ± 1 °C for nine days. The microbial numbers, water content, color value, texture, pH value, thiobarbituric acid reactive substance (TBARS), and total volatile base nitrogen (TVB-N) values of the fish were analyzed during storage. The results showed the growth of the total viable bacteria (TVB), psychrophilic bacteria, Pseudomonas spp., H₂S-producing bacteria, yeast, and lactic acid bacteria in the treated samples was limited, and they were 1.11, 1.01, 1.04, 1.13, 0.77, and 0.80 log CFU/g⁻¹ lower than those in the control group after nine days of storage, respectively. The hardness, springiness, and chewiness of the treated fish decreased slowly as the storage time extended, and no significant changes in either pH or water content were found. The lightness (L*) value increased and the yellowness (b*) value decreased after treatment, while no changes in the redness (a*) value were found. The TBARS and TVB-N of the treated samples increased to 0.79 mg/kg and 21.99 mg/100 g, respectively, after nine days of refrigerator storage. In conclusion, HVACP can limit the growth of the main microorganisms in fish samples effectively during nine days of refrigerator storage with no significant negative impact on their quality. Therefore, HVACP is a useful nonthermal technology to extend the refrigerator shelf-life of Trachinotus ovatus.

Cathepsin C (CTSC) contributes to the antibacterial immunity in golden pompano (Trachinotus ovatus)

Cathepsins, as a class of protein hydrolases, are widely found in the lysosomes of many tissues and play an essential role in various physiological activities. Cathepsin C (CTSC), a lysosomal cysteine protease, is an essential component of the lysosomal hydrolase family. In this study, we identified a CTSC from Trachinotus ovatus (TroCTSC) and analyzed its function. TroCTSC contained an ORF of 1368 bp and encoded 455 amino acids, which included three conserved catalytically active sites (Cys²⁵¹, His³⁹⁷, and Asn⁴¹⁹). It shares high homology (69.47%-90.77%) with the other known CTSC sequences of teleosts, which was most closely related to Seriola dumerili. TroCTSC was most abundant in the muscle, liver, and head kidney. After Vibrio harveyi infection, the expression levels of TroCTSC in liver, spleen, and head kidney were significantly up-regulated. TroCTSC was found in the cytoplasm with some of which were co-located with the lysosome. After V. harveyi stimulation, TroCTSC was translocated to nucleus in golden pompano snout (GPS) cells. In vitro, results revealed that the optimal hydrolase activity of the recombinant protein, rTroCTSC, was at 40 °C and pH 5.5. The activity of rTroCTSC was promoted by Zn²⁺ and Ca²⁺ but inhibited by Fe²⁺ and Cu²⁺. However, three mutant proteins, rTroCTSC-C251A, rTroCTSC-H397A, rTroCTSC-N419A, were dramatically reduced the proteolytic activity. Furthermore, in vivo results showed that overexpression of TroCTSC could significantly enhance body's ability to resist V. harveyi and promote the expression of proinflammatory cytokines, including interleukin 1-beta (IL-1β), IL-6, IL-8, interferon-gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α). In contrast, the interference of TroCTSC expression induced a significant increase in the number of bacteria after V. harveyi infection. Our results suggested that TroCTSC was an essential effector of the innate immune system and played a pivotal role in antibacterial immunity.

Skin transcriptomic analysis and immune-related gene expression of golden pompano (Trachinotus ovatus) after Amyloodinium ocellatum infection

Amyloodiniosis is a severe disease of marine and brackish water fish caused by Amyloodinium ocellatum. Golden pompano (Trachinotus ovatus) is often repeatedly infected by A. ocellatum, leading to extensive mortality. However, little is known about the immune response mechanisms of the T. ovatus following reinfection with A. ocellatum. In this study, an extensive analysis at the transcriptome level of T. ovatus skin was carried out at 24 h post-infection by A. ocellatum. During the transcriptomic analysis, 1367 differentially expressed genes (DEGs) in the skin of T. ovatus under A. ocellatum infection and control conditions were obtained. In Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotated analyses, the DEGs were significantly enriched in the immune-related pathways. To better understand the immune-related gene expression dynamics, a quantitative reverse transcription-polymerase chain reaction (RT-qPCR) was used to assess the primary and secondary infection groups of T. ovatus at different stages (3 h, 12 h, 24 h, 48 h and, 72 h post-infection) of infection with A.ocellatum. The results showed that innate immunity-related genes [interleukin (IL-8), chemokine ligand 3 (CCL3), toll-like receptor 7 (TLR7), and G-type lysosome (LZM g)] and adaptive immunity-related gene [major histocompatibility complex (MHC) alpha antigen I and MHC alpha antigen II] expression levels in the primary and secondary infection groups were significantly increased compared to the control group. The expression of MHC I and MHC II was more rapidly upregulated in the secondary infection group compared with the primary infection group after A.ocellatum infection. However, no significant differences of A.ocellatum load were observed in primary and secondary infection groups. In addition, the serum of the primary infection group had significantly higher concentrations of triglyceride (TG), higher alanine transaminase (ALT), aspartate transaminase (AST), and lactate dehydrogenase (LDH) activities than the control group. This study contributes to understanding the defense mechanisms in fish skin against ectoparasite infection.

Molecular characterization and expression analysis of TRIF, TRAF6, and TBK1 of golden pompano (Trachinotus ovatus)

Toll/IL-1R domain-containing adaptor-inducing IFN-β (TRIF), tumor necrosis factor receptor-associated factor 6 (TRAF6) and TANK-binding kinase 1 (TBK1) are critical signal transducers in toll-like receptors (TLRs) signaling pathway. In the present study, TRIF, TRAF6 and TBK1 were characterized from golden pompano (Trachinotus ovatus), named as TroTRIF, TroTRAF6 and TroTBK1, respectively. The full cDNA length of TroTRIF, TroTRAF6 and TroTBK1 was 2297 bp, 2293 bp, and 2482 bp, which respectively encoded 589, 573 and 723 amino acids. The deduced amino acids sequences of TroTRIF, TroTRAF6 and TroTBK1 contained conserved motifs, similar to their counterparts in other vertebrates. Phylogenetic tree analysis revealed that TroTRIF, TroTRAF6 and TroTBK1 were well clustered with their counterparts in other fish species. Quantitative Real-Time PCR (qPCR) analysis showed that TroTRIF, TroTBK1 and TroTRAF6 were detected in all examined tissues of healthy fish, but shared distinct transcript levels. Moreover, the expressions of TroTRIF, TroTBK1 and TroTRAF6 were generally induced by polyriboinosinic-polyribocytidylic acid (polyI:C), lipopolysaccharide (LPS), and Vibrio alginolyticus stimulation in vivo, indicating their critical roles in the immune defense of golden pompano against pathogen invasion. Our results provide valuable information for understanding the functions of these genes in golden pompano.

Macrophage migration inhibitory factor (MIF) of golden pompano (Trachinotus ovatus) is involved in the antibacterial immune response

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine with a unique structure involved in immune regulation and inflammation. In the present study, we identified a MIF from Trachinotus ovatus (golden pompano) and analyzed its function. TroMIF shares high homology (58.26%-94.78%) with the other known MIF sequences of vertebrates. TroMIF is most closely related to large yellow croaker (Larimichthys crocea). The expression of TroMIF was most abundant in the liver and head kidney, and was significantly up-regulated after Edwardsiella tarda infection. The subcellular localization of TroMIF was mostly distributed in the cytoplasm. In vitro results revealed that the recombinant protein rTroMIF could inhibit the migration of head kidney lymphocytes (HKLs) and macrophages (HKMs) and enhance the phagocytic activity of HKMs. As a pro-inflammatory cytokine, rTroMIF could increase the expression levels of some pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-α), interleukin 1-beta (IL-1β), IL-6, IL-8, and interferon-gamma (IFN-γ) and decrease the expression of IL-10. The rTroMIF was proved to have enzymatic redox activity in vitro. Furthermore, overexpression of TroMIF in the head kidney cell line of golden pompano could significantly enhance its ability to resist E. tarda infection from 1 h to 4 h. The knockdown of TroMIF expression induced a significant increase in the number of bacteria after E. tarda infection at 1, 2, and 4 hpi. Our results suggest that TroMIF is an essential effector of the innate immune system and plays a pivotal role in antibacterial immunity.

Antimicrobial activity and mechanisms of a derived antimicrobial peptide TroNKL-27 from golden pompano (Trachinotus ovatus) NK-lysin

NK-lysin, a homologue of granulysin among human, is predominantly found in natural killer cells and cytotoxic T-lymphocytes, which plays a pivotal part in innate immune responses against diverse pathogenic bacteria. Nonetheless, in teleosts, the research on antimicrobial activity and mechanisms of NK-lysin are seldom reported. In this study, we determined the antimicrobial activity of the truncated peptide TroNKL-27 that derived from golden pompano (Trachinotus ovatus) NK-lysin, and investigated its antimicrobial mechanisms. The results showed that TroNKL-27 had considerable antimicrobial potency against both Gram-positive (Staphylococcus aureus, Streptococcus agalactiae) and Gram-negative bacteria (Vibrio harveyi, V. alginolyticus, Escherichia coli, Edwardsiella tarda). Cytoplasmic membrane depolarization and propidium iodide (PI) uptake assay manifested that TroNKL-27 could induce the bacterial membrane depolarization and change its membrane permeability, respectively. In the light of scanning electron microscopy (SEM) observation, TroNKL-27 was capable of altering morphological structures of bacteria and leading to leakage of cellular contents. Moreover, the results of gel retardation assay indicated TroNKL-27 had the ability to induce the degradation of bacterial genomic DNA. As regards in vivo assay, TroNKL-27 could reduce the replication of V. harveyi in tissues of golden pompano, protect the tissue from pathological changes. Moreover, TroNKL-27 in vivo could significantly increase the expression of the immune genes (such as IL1β, TNFα, IFN-γ, C3 and Mx) in presence or absence of V. harveyi infection. All of these results suggest that TroNKL-27 is a novel antimicrobial peptide possessing antibacterial and immunoregulatory function in vivo and in vitro, and the observed effects of TroNKL-27 will lay a solid foundation for the development of new antimicrobial agents used in aquaculture.

Effects of Modified Atmosphere Packaging with Different Gas Ratios on the Quality Changes of Golden Pompano (Trachinotus ovatus) Fillets during Superchilling Storage

The quality changes of golden pompano fillets in air packaging (AP) and modified atmosphere packaging (MAP) with 30% CO2/70% N2, 50% CO2/50% N2, and 70% CO2/30% N2 were evaluated under superchilling (−3 °C). The results showed that the whiteness of fillets decreased during storage. The rate of pH increase of MAP was significantly slower than in AP groups, in which MAP with 70% CO2/30% N2 effectively suppressed the PH. Interestingly, the hardness decreased on day five following the treatments, followed by a relatively stationary trend. MAP could greatly suppress the increase of total volatile basic nitrogen (TVB-N) contents of fillets compared to fillets packed in AP. All MAP groups of fillets maintained first-grade freshness throughout storage, while the AP samples decreased to second-grade freshness on about the 25th day. MAP with 70% CO2/30% N2 and MAP with 50% CO2/50% N2 had the best results in inhibiting protein degeneration and explanation. Unexpectedly, drip loss of fillets in MAP far exceeded the AP group during storage, which causes sensory discomfort. Anaerobic plate count (APC) of fillets in AP exceeded the consumption limit of 6.7 log CFU/g on day 26 (6.75 log CFU/g on the 26th day), whereas the MAP was still microbiologically acceptable after 30 days of storage (6.43, 6.41, 6.22 log CFU/g, respectively). Considering physicochemical and microbiological parameters, the shelf life of fillets packed in AP was 25 days. MAP treatments could prolong the shelf life of fillets by ~4−5 days compared to AP. Overall, MAP with 70% CO2/30% N2 gas ratio was best for inhibiting the quality deterioration of fillets. Furthermore, principal component analysis (PCA) was performed to evaluate the critical indicators of quality deterioration of the fillets. Two principal components were determined by dimensionality reduction, in which the contribution of the first principal component was centrifugal loss > hardness > TVB-N > APC > CO2 solubility > TBARs > drip loss > pH, which mainly reflected the degree of microbial proliferation, protein hydrolysis, and oxidation. The contribution of the second principal component was pH > TBRAs > drip loss > APC > CO2 solubility > TVB-N > hardness > centrifugal loss, indicating a high correlation between lipid oxidation and microbial proliferation index.

Determination of the geographical origin of Trachinotus ovatus and Pampus argenteus in China by multi-element and stable isotope analysis

This study aimed to determine whether Trachinotus ovatus and Pampus argenteus could be differentiated by multi-element composition or stable isotope ratio analysis of δ¹³C and δ¹⁵N. The multi-element analysis values of a total of 60 Pampus argenteus samples from three sites and 60 Trachinotus ovatus samples from four sites around China were determined, and the other 60 samples of Pampus argenteus and 51 samples of Trachinotus ovatus were tested by principal component analysis (PCA) to estimate the accuracy of origin identification. The results showed that the cross-validation accuracy rate is 92.2% for the Pampus argenteus and 98.3% for the Trachinotus ovatus. Stable isotope analysis of δ¹³C and δ¹⁵N also can distinguish Trachinotus ovatus from different geographical origins. These results showed that the usefulness of multi-element and stable isotope analysis as indicators for authenticating the geographical origin of two pomfrets in China.

Dietary calcium pyruvate could improve growth performance and reduce excessive lipid deposition in juvenile golden pompano (Trachinotus ovatus) fed a high fat diet

Excessive lipid deposition in farmed fish is a challenge in the aquaculture industry. To study the effect of dietary calcium pyruvate (CaP) on lipid accumulation in fish, we used a high fat diet (HFD) to establish a lipid accumulation model in juvenile golden pompano (Trachinotus ovatus) and supplemented with 0%, 0.25%, 0.50%, 0.75% and 1.0% CaP (diets D0-D4, respectively). After 8-week feeding in floating cages, dietary CaP significantly improved growth performance, which peaked in fish fed diet D3. Supplementation of CaP significantly decreased whole body lipid content in fish fed D2-D4 and hepatosomatic index and liver lipid content in fish fed D3 and D4. Serum and hepatic antioxidant indices, including glutathione, catalase and superoxide dismutase, showed generally increasing trends in fish fed diets with CaP. In addition, increasing dietary CaP increasingly reduced hepatic activities of hexokinase, phosphofructokinase and pyruvate kinase involved in glycolysis, and increased glycogen contents of the liver and muscle. Dietary CaP up-regulated the liver mRNA expression of pparα, cpt1, hsl and fabp1, but down-regulated expression of srebp-1, fas and acc. In conclusion, 0.75% CaP improved growth performance and reduced excessive lipid deposition by affecting fatty acid synthesis and lipolysis in juvenile T. ovatus fed HFD.

Preparation of chitosan grafted caffeic acid coating and its effect on pompano (Trachinotus ovatus) preservation

BACKGROUND

The present study aimed to investigate the preservative effect of chitosan-caffeic acid grafts coating (CS-g-CA) on the quality and microbial characteristics of pompano (Trachinotus ovatus) during iced storage. CS-g-CA was prepared by a 1-(3-dimethylaminopropyl)-3-ethylcarbodiimidehydro/N-hydroxysuccinimide coupling reaction. The grafting of CS-g-CA was confirmed by UV-visible and Fourier-transform infrared spectra. Samples were treated with distilled water (control), chitosan (CS), caffeic acid (CA) and CS-g-CA for 10 min, respectively. Microbiological [total viable count (TVC), H₂ S-producing bacteria count, Pseudomonas bacteria count], physicochemical indicators [water holding capacity (WHC), cooking loss, pH, total volatile basic nitrogen (TVB-N), thiobarbituric acid (TBA), texture profile analysis, free amino acids] and sensory evaluation were investigated during ice storage at 4 °C for up to 27 days.

RESULTS

The results showed that the antioxidant and antibacterial activities of CS could be improved by grafting CA onto CS. CS-g-CA coating could greatly slow down the speed of water loss and maintain WHC. Furthermore, CS-g-CA coating showed superior antibacterial activities by inhibiting the growth of TVC, delayed the decline of flavor amino acids and reduced sensory change. In addition, CS-g-CA coating reduced lipid oxidation and protein degradation as indicated by the decrease in TBA and TVB-N, possibly as a result of the addition of CA into CS membrane significantly improving the antioxidant activity of CS.

CONCLUSION

Compared with the control group, CS-g-CA coating had the optimal effect and could enhance the shelf-life of Trachinotus ovatus for at least another 9 days. © 2021 Society of Chemical Industry.

Multi-omics reveals the regulatory mechanisms of zinc exposure on the intestine-liver axis of golden pompano Trachinotus ovatus

Metal zinc (Zn) has been the focus of many environmental toxicological studies, but there are limited studies on its potential dietary molecular toxicity and physiology. The present study was the first to use multi-omics-based approaches to explore the fish intestine-liver axis under dietary Zn exposure. Golden pompano Trachinotus ovatus were exposed to different dietary concentrations (78.4, 134.6, and 161.4 mg/kg as the control, low-dose Zn, and high-dose Zn groups, respectively) of Zn for 4-week. Low-dose Zn exposure significantly promoted the fish growth, whereas the high-dose Zn exposure reduced the fish growth. Co-analysis of 16S diversity, metagenome and transcriptome showed that the low-dose Zn enriched the intestinal microflora and changed the dominant microflora abundances (Proteobacteria, Fusobacteria, Firmicutes and Bacteroidetes), as well as activated the growth hormone metabolism in the liver. Meanwhile, the high-dose of Zn caused the intestinal microbiota dysbiosis, activated the Type VI secretion systems (T6SSs), and further triggered the oxidative stress response, immunity, and antiviral function of the liver. Multi-omics revealed the interference of long-term Zn dietary exposure on the intestine-liver axis. There was an apparent homeostasis of Zn accumulation in the fish tissues, but the window of dietary Zn nutritional requirements versus toxicity appeared to be narrow for the golden pompano. These results provided new insight into the adverse effects and regulatory mechanisms of dietary Zn requirements and toxicity in marine fish.

Candidate sex-associated gene identification in Trachinotus ovatus (Carangidae) using an integrated SLAF-seq and bulked segregant analysis approach

It is difficult to distinguish the sexes of Trachinotus ovatus based on appearance, and little data about sex-determining genes are available for this species. Here, we generated 200 F2 individuals using the parents R404 and R403. DNA samples were collected from 50 individuals of each sex and aggregated into sex-specific DNA pools. Specific-locus amplified fragment sequencing was integrated with bulked segregant analysis to detect candidate sex-associated genes. Approximately 3,153,153 high-quality single-nucleotide polymorphism (SNP) markers and 135,363 high-quality insertion-deletion (Indel) markers were generated. Six candidate regions within chromosome 14, encompassing 132 candidate genes, were identified as closely related to sex. Based on annotations, six genes (EVM0019817, EVM0004192, EVM0001445, EVM0005260, EVM0014734, and EVM0009626) were predicted to be closely associated with sex. These results present an efficient genetic mapping approach that lays a foundation for molecular sex discrimination in T. ovatus.

Identification and functional characterization of gonadotropin -releasing hormone in pompano (Trachinotus ovatus)

Gonadotropin-releasing hormone (GnRH) is an important neuropeptide in the reproductive system. Although GnRH analogues have been used to artificially spawn pompano (Trachinotus sp.), the native forms of GnRH have not been described in this species. In this study three GnRH subtypes [sea bream GnRH (sbGnRH), chicken GnRH-Ⅱ (cGnRH-Ⅱ) and salmon GnRH (sGnRH)] were identified in pompano (Trachinotus ovatus). cgnrh-Ⅱ and sgnrh were mainly expressed in the brain of male and female fish, showing a tissue-specific expression pattern, while sbgnrh was expressed at different transcriptional levels in all tested tissues. In vivo injection experiment showed that sbGnRH significantly increased fsh and lh genes expression in a dose-dependent manner, but a high concentration of sbGnRH could desensitize the expression of lh. High concentrations of cGnRH-Ⅱ and sGnRH could induce the expression of fsh and lh. In addition, the results of in vitro incubation experiments showed that the high concentration of sbGnRH peptide could induce the expression of fsh and lh, while cGnRH-Ⅱ and sGnRH peptides could only induce the expression of fsh. 17β-estradiol (E₂) and 17α-methyltestosterone (MT) significantly inhibited sbgnrh mRNA expression in a dose-dependent manner, but did not affect the expression of cgnrh-Ⅱ and sgnrh mRNA. sbGnRH is the main GnRH subtype in pompano. E₂ and MT can play a negative role in the regulation of sbgnrh. This study provides a theoretical basis for the reproductive endocrinology of pompano.

Physiological Effects and Transcriptomic Analysis of sbGnRH on the Liver in Pompano (Trachinotus ovatus)

Pompano (Trachinotus ovatus) is one of the important economic marine fishes in the south coast of China. At present, the research on the basic biology of pompano is relatively weak, which has seriously affected the development of this economic important fish. The liver is an important digestive and metabolic organ of fish which plays an important regulatory role in its growth and development. It is necessary to clarify the effects of sea bream gonadotropin releasing hormone (sbGnRH) on liver physiology and metabolic enzyme activity. The effects of sbGnRH peptides (10 ng/gbw) on the physiological and biochemical indices and metabolic enzyme activities of pompano liver were studied. It was found that after injection of 10 ng/gbw sbGnRH peptides, the contents of albumin, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, glucose, creatine kinase, iron, magnesium, aspartate aminotransferase, alanine aminotransferase and creatinine increased, while of cholesterol and calcium contents decreased. The activities of amylase, lipase, pyruvate kinase, acyl CoA oxidase, superoxide dismutase, phospholipid hydroperoxide glutathione peroxidase, catalase, glucose-6-phosphate dehydrogenase, fatty acid synthase and lipoprotein lipase increased, while the activities of malic enzyme, carnitine acyl, carnitine translocation, acetyl CoA carboxylase and malondialdehyde decreased. Three hours after the injection of different concentrations of sbGnRH peptides (0 and 10 ng/gbw), the transcriptome sequences of the two groups of livers were sequenced. After quality control and removal of some low-quality data, clean reads of 21,283,647、19,427,359、21,873,990、21,732,174、23,660,062 and 21,592,338 were obtained respectively. In this study, 99 genes were screened and identified as differentially expressed genes, including 77 up-regulated genes and 22 down-regulated genes. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathway analyses, these pathways and the typical genes involved can be divided into cellular processes, environmental information processing, genetic information processing, diseases, metabolism and organismal systems. The results from this study provide a the oretical basis for studying the effects of sbGnRH on the physiology, biochemistry and metabolic enzyme activities of liver in pompano.

Transcriptome analysis of the influence of high plant protein based diet on Trachinotus ovatus liver

Seeking out fish meal (FM) alternatives is an important requirement for aquaculture all over the world. And most practitioners believe that the plant protein is most potential for FM surrenal. While high plant protein feed caused some common problems in aquatic livestock: the absorption rate and growth rate are decreased, and even caused digestive tract inflammation. In present study, the inflence of high plant protien feed in Trachinotus ovatus was investigated using illumina HiSeqTM2000 based RNA-Seq. By comparing the two groups of cDNA libraries developed from high plant protien based diet or FM based diet fed T. ovatus livers, 836 unigenes were significantly upregulated, and 345 were significantly down regulated. KEGG analysis indicated that the differentially expressed genes (DEGs) are mainly metabolic-related genes. It was found that more than 28 DGEs beloned to the protein metabolism and absorption, lipid biosynthesis or other metabolic pathways. It indicated that high plant protein based diet had broad effects on metabolism on T. ovatus. There were also more DEGs belong to immune-related signaling pathways, include genes were involved in pathpathogen resistance and genes related to immunity system. These DEGs provided useful clues to explore the mechanisms that high plant protein based diet caused side effects on T. ovatus. These results improved our current understanding of the response of high plant protein based diet in T. ovatus, and outstanding the reasons of the side effect caused by high protein based diet.

Functional characterization of cathepsin B and its role in the antimicrobial immune responses in golden pompano (Trachinotus ovatus)

Cathepsin B (CTSB) is one of the typical representatives of cysteine protease family. It has the activity of both exopeptidase and endopeptidase. It plays an important role in antigen presentation, degradation, apoptosis, inflammatory response and physiological process of many diseases. In this study, CTSB of Trachinotus ovatus (TroCTSB) was cloned, and its structure and function were analyzed. The results showed that the coding region of TroCTSB was 993 bp, encoding 330 amino acid residues. The homology analysis showed that the amino acid sequence of TroCTSB was similar to that in other teleosts and mammals (68.69%-88.48%). Under normal physiological conditions, TroCTSB was widely distributed in various tissues with the highest expression level in stomach, followed by liver, and the lowest expression level in blood. The optimal pH and temperature of purified recombinant protein rTroCTSB were 5.5 and 40 °C, respectively. The toxicity test of metal ions showed that Fe²⁺, Cu²⁺, Ca²⁺ and Zn²⁺ could all inhibit the activity of TroCTSB, with Zn²⁺ ranking the first. In addition, after Edwardsiella tarda infection, the expression of TroCTSB was significantly up-regulated in liver, spleen and head kidney. The overexpression of TroCTSB significantly inhibited the infection of E. tarda in golden pompano tissues, and the knockdown of TroCTSB remarkably promoted the reproduction of E. tarda in golden pompano tissues in vivo. This study suggests that TroCTSB was involved in the antibacterial immune response of T. ovatus, and provided a reference for further research in elucidating the resistance mechanism of TroCTSB.

Characterization, expression and function analysis of the TLR3 gene in golden pompano (Trachinotus ovatus)

Toll-like receptors (TLRs)are pattern recognition receptors (PRRs) that are important in invertebrate innate immunity for the recognition and elimination of pathogens. Although they were reported in many fishes, Toll-like receptors subfamily contain a large number of members with different functions that need to research in deep. In the present study, the full-length cDNA of TLR3 from the golden pompano, Trachinotus ovatus, was cloned and characterized. The full length of ToTLR3 cDNA was 3710 bp including an open reading frame of 2760 bp encoding a peptide of 919 amino acids. The derived amino acids sequence comprised of 14 leucine-rich repeats (LRR), capped with LRRCT followed by transmembrane domain and cytoplasmic Toll/IL-1R domain (TIR). Multiple sequence alignment and phylogenetic analysis revealed that ToTLR3 shared the highest similarity to the teleost fish and suggested ToTLR3 is fairly conservative in evolution process. Tissues distribution analysis indicated that ToTLR3 showed a tissue-specific variation with high expression in blood and liver. After the fish were stimulated by poly(I:C), flagellin and LPS, ToTLR3 expression in the liver, intestine, blood, kidney, skin and muscle was significantly upregulated in a time-depended manner, especially in immune related tissues such as liver, blood and kidney. Binding assay revealed the specificity of rToTLR3 for pathogen-associated molecular patterns (PAMPs) and bacteria that included Vibrio harveyi, V. vulnificus, V. anguillarum, Photobacterium damselae, Escherichia coli, Aeromonas hydrophila, Staphylococcus aureus and PolyI:C, LPS, Flagellin, and PGN. In addition, a luciferase reporter assay showed that overexpression ToTLR3 significantly increased NF-κB activity. Collectively, our results suggested that ToTLR3 might play an important role as a pattern recognition receptor (PRR) in the immune response towards pathogen infections, and transmiss the danger signal to downstream signaling pathways.

Interferon regulatory factor 7 contributes to the host response during Vibrio harveyi infection in the golden pompano Trachinotus ovatus

Vibrio harveyi is regarded as serious pathogen for marine fishes. However, host defense mechanisms involved in V. harveyi infection remain incompletely defined. The transcription factor IFN regulatory factor 7 (IRF7) is largely associated with host defense against viral infections, and the role of IRF7 during V. harveyi infection in fish has not been well illuminated previously. In this study, IRF7 from golden pompano (Trachinotus ovatus) was characterized (TroIRF7). The TroIRF7 gene is 1323 bp, which encodes 440 amino acid residues. Multiple amino acid alignments of TroIRF7 shows 30.37%-80.18% identity with other fish IRF7s, including Epinephelus coioides (80.18%), Larimichthys crocea (79.72%), Collichthys lucidus (79.26%), Miichthys miiuy (79.26%), Channa argus (78.77%), Cynoglossus semilaevis (72.67%), and Gadus morhua (65.23%). Like other IRF7s, TroIRF7 also contains 3 conserved domains: an N-terminal DNA-binding domain (DBD), an IRF association domain (IAD), and a C-terminal serine-rich domain (SRD). In the DBD, 4-5 conserved tryptophans were observed, which is a characteristic unique to all fish IRF7 members. TroIRF7 was constitutively expressed, with high levels in gill, head kidney, spleen, skin, and intestine. V. harveyi infection-induced TroIRF7 transcripts significantly up-regulation and translocation to the nucleus. TroIRF7 overexpression promote the fish to inhibit the replication of V. harveyi. And TroIRF7 knockdown led to decreased bacterial clearance in fish tissue. Furthermore, over-expression of TroIRF7 resulted in an increased production of interferon a3 and IFN signaling molecule in the spleen, suggesting that V. harveyi activates the IRF7- IFN pathway. These results suggest that TroIRF7 is an important component of immune responses against V. harveyi infection.

PPARγ regulates fabp4 expression to increase DHA content in golden pompano (Trachinotus ovatus) hepatocytes

N-3 long-chain (≥C20) PUFA (LC-PUFA) are vital fatty acids for fish and humans. As a main source of n-3 LC-PUFA for human consumers, the n-3 LC-PUFA content of farmed fish is important. Previously, we identified fatty acid-binding protein (fabp)-4 as a candidate gene for regulating the n-3 LC-PUFA content. Herein, we further assessed the role of fabp4 in this process. First, a 2059 bp promoter sequence of fabp4 in Trachinotus ovatus was cloned and, using progressive deletion, determined -2006 bp to -1521 bp to be the core promoter sequence. The PPAR-γ binding sites were predicted to occur in this region. A luciferase reporter assay showed that the promoter activity of fabp4 decreased following mutation of the PPARγ binding site and that PPARγ increased the fabp4 promoter activity in a dose-dependent manner, implying that T. ovatus fabp4 is a target of PPARγ. The overexpression of fabp4 or PPARγ increased the DHA content in hepatocytes, whereas suppression of their expression diminished this effect, suggesting that both fabp4 and PPARγ play an active role in regulating DHA content. Moreover, the inhibition of fabp4 attenuated the increase in PPARγ-mediated DHA content, and the overexpression of fabp4 alleviated this effect. Collectively, our findings indicated that fabp4, which is controlled by PPARγ, plays an important role in DHA content regulation. The new regulation axis can be considered a promising novel target for increasing the n-3 LC-PUFA content in T. ovatus.

Dual-Function Analysis of Astaxanthin on Golden Pompano (Trachinotus ovatus) and Its Role in the Regulation of Gastrointestinal Immunity and Retinal Mitochondrial Dysfunction Under Hypoxia Conditions

The present study investigated the potential mechanisms of astaxanthin in the regulation of gastrointestinal immunity and retinal mitochondrial function of golden pompano (Trachinotus ovatus). Triplicate groups of juvenile T. ovatus (mean initial weight: 6.03 ± 0.01 g) were fed one of six diets (D1, D2, D3, D4, D5, and D6) for 8 weeks, with each diet containing various concentrations of astaxanthin (0, 0.0005, 0.001, 0.005, 0.01, or 0.1%, respectively). Growth performance of fish fed the D2–D5 diets was higher than that of fish fed the D1 diet; however, growth performance and survival of fish deteriorated sharply in fish fed the D6 diet. Gut villus in fish fed the D2–D5 diets were significantly longer and wider than that of fish fed the D6 diet. Feeding with D2–D5 diets led to increased abundance of Bacillus, Pseudomonas, Oceanobacillus, Lactococcus, Halomonas, Lactobacillus, and Psychrobacter while abundance of Vibrio and Bacterium decreased. Additionally, feeding with the D6 diet resulted in a sharp decline in Pseudomonas and Lactobacillus abundance and a sharp increase in Vibrio abundance. A low dissolved oxygen environment (DO, 1.08 mg/L) was conducted for 10 h after the rearing trial. No fish mortality was observed for any of the diet treatments. Lysozyme (LZY) activity in fish fed the D6 diet decreased sharply and was significantly lower than that in other groups. ROS production also decreased sharply in fish fed the D6 diet. Moreover, the conjunctiva and sclera in the fish fed the D6 diet were indistinguishable. Suitable dietary astaxanthin supplementation levels (0.005–0.1%) exerting a neuroprotective effect from low dissolved oxygen environments is due to up-regulated expression of anti-apoptotic factors, such as phosphorylated Bcl-2-associated death promoter (pBAD), phosphorylated glycogen synthase kinase-3β (pGSK-3β), Bcl-2 extra large (Bcl-xL), and down-regulated expression of Bcl-2-associated X protein (Bax) pro-apoptotic factor in retinas. Furthermore, suitable dietary astaxanthin levels (0.0005–0.01%) suppressed up-regulation of critical mitochondrial components, such as peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), mitochondrial transcription factor A (TFAM), and mitochondrial DNA (mtDNA), while excessive astaxanthin supplementation produces the opposite effect. In brief, high-dose astaxanthin arouses and aggravates low dissolved oxygen-induced inflammation, oxidative stress, intestinal disorder, retinal apoptosis, and retinal mitochondrial dysfunction in T. ovatus. Second-degree polynomial regression of WG indicated that the optimum dietary astaxanthin for juvenile T. ovatus is 0.049%.