Biological parameters, immune enzymes, and histological alterations in the livers of grass carp infected with Aeromonas hydrophila

Effects of Aeromonas infection on the immune system, physical barriers and microflora structure in the intestine of juvenile grass carp (Ctenopharyngodon idella)

Grass carp (Ctenopharyngodon idella) is an intensively cultured and economically important herbivorous fish species in China, but its culture is often impacted by Aeromonas pathogens such as Aeromonas hydrophila and Aeromonas veronii. In this study, healthy grass carp were separately infected with A. hydrophila or A. veronii for 12, 24, 48 or 72 h. The results showed that the mRNA expression levels of intestinal inflammatory factors (tnf-α, il-1β and il-8), complement factors (c3 and c4), antimicrobial peptides (hepcidin, nk-lysin and β-defensin-1), immunoglobulins (igm and igt), and immune pathway-related signaling molecules (tlr1, tlr2, tlr4, myd88, irak4, irak1, traf6, nf-κb p65 and ap-1) were differentially upregulated in response to A. hydrophila and A. veronii challenge. Additionally, the expression levels of the intestinal pro-apoptotic genes tnfr1, tnfr2, tradd, caspase-8, caspase-3 and bax were significantly increased, whereas the expression of the inhibitory factor bcl-2 was significantly downregulated, indicating that Aeromonas infection significantly induced apoptosis in the intestine of grass carp. Moreover, the expression of intestinal tight junction proteins (occludin, zo-1, claudin b and claudin c) was significantly decreased after infection with Aeromonas. Histopathological analysis indicated the Aeromonas challenge caused severe damage to the intestinal villi with adhesions and detachment of intestinal villi accompanied by severe inflammatory cell infiltration at 12 h and 72 h. The 16S rRNA sequencing results showed that Aeromonas infection significantly altered the structure of the intestinal microflora of the grass carp at the phylum (Proteobacteria, Fusobacteria, Bacteroidetes and Firmicutes) and genus (Proteus, Cetobacterium, Bacteroides, and Aeromonas) levels. Take together, the findings of this study revealed that Aeromonas infection induces an intestinal immune response, triggers cell apoptosis, destroys physical barriers and alters microflora structure in the intestine of juvenile grass carp; the results will help to reveal the pathogenesis of intestinal bacterial diseases in grass carp.

Nonspecific immune, histology and accumulation of marine worm, Urechis unicinctus in response to bisphenol A (BPA)

Bisphenol A (BPA) is one of the environmental endocrine disruptors, due to its chemical stability it exists in abundant concentrations in water and soil consequently accumulating in the food chain and causing many endocrine-related health problems. So far, studies on the effects of BPA on marine invertebrates have focused on acute toxicity, endocrine regulation, reproduction, and development. However, fewer studies have been conducted on marine benthos. The current study aimed to detect the accumulation of BPA and its impact on tissue structure, antioxidant capacity, and immune indexes in marine worm, Urechis unicinctus. U. unicinctus, as a common marine benthic animal, were exposed to different concentrations of BPA. Blood cells and intestinal tract were taken for tissue structure inspection, and supernatant of the coelomic fluid was collected for oxidative and antioxidant biomarkers. Results showed that the accumulation of BPA in muscles of U. unicinctus tended to increase with exposure time. BPA induced a rise in H2O2 and MDA content, and altered the activities of CAT, T-SOD, GST, LSZ and ACP, weaken the immune system functions. Moreover, pathological observation showed that BPA caused severe histopathology in the respiratory intestine, stomach, and midgut. These results will be helpful to understand the response mechanism of U. unicinctus under BPA exposure and provide a reference for controlling the aquaculture conditions and marine water quality of U. unicinctus.

The oral protective efficacy of magnolol against Aeromonas hydrophila and A. veronii infection via enhancing anti-inflammatory ability in goldfish (Carassius auratus)

Aeromonas hydrophila and A. veronii are widespread and important critical pathogenic bacteria in the aquaculture industry and cause severe economic damage. At present, magnolol has been proved to be a broad-spectrum antibacterial activity, such as A. hydrophila, Staphylococcus aureus and Streptococcus mutans. In order to explore the cause of in vivo disease resistance of magnolol and promote its safe application in aquaculture, the pathological detection and changes in immune indicators of fish after feeding with magnolol were conducted in this paper. Results showed that the diets supplemented with magnolol (3 g magnolol/kg commercial feed) significantly increase the expression level of anti-inflammatory cytokines (IL-10, TGF-β and IL-4) in the liver of goldfish (p < .05). Additionally, the expression levels of proinflammatory cytokines (IL-1β, IL-8 and IFN-γ) did not increase significantly. Subsequently, this study investigated the resistance of goldfish to A. hydrophila and A. veronii infection after feeding with magnolol. The results showed that the survival rates of treatment groups fed 3 g magnolol/kg commercial feed daily increased by 23.1% and 38.5% after 10 days post A. hydrophila and A. veronii (p = .0351) infection, respectively. Meanwhile, growth performance (body weight and length), major internal organs (liver, spleen, kidney and intestine) and the serum biochemistry indicators (ATL and AST) all exhibited no significant adverse effects after the goldfish fed with magnolol for 30 days. TP showed an increasing concentration in the treatment group (p < .05). Results of the mRNA expression of stress response indicated that the expression level of cyp1a and hsp70 was significantly down-regulated after a 30-day treatment (p < .05), and the two genes recovered to the similar level as the control group after a commercial feed diet. In brief, the diets supplemented with magnolol protected the host from the excessive immune response caused by A. hydrophila and A. veronii via enhancing its anti-inflammatory capacity and had no adverse effects with feeding.

Gill and hepatic histological alterations in Sciades herzbergii resulting from trace element contamination in the Port of São Luiz, Brazil

The objective of this study was to evaluate, through changes in the gills and livers of Sciades herzbergii, the environmental contamination to which estuarine organisms are exposed in two areas in São Marcos Bay - MA. Two collection areas located in São Marcos Bay were selected for this study: A1, an area close to the Port Complex, and A2, an area on Caranguejos Island (included in the Environmental Protection Area of Baixada Maranhense). Collections were carried out during rainy and dry periods. Sediments (for trace element analyses), surface water (for physico-chemical analyses), and specimens of S. herzbergii (for biometric measurements and identification of branchial and hepatic histopathology) were collected. Physico-chemical parameters (pH, dissolved oxygen, temperature and salinity) were within limits established by Brazilian legislation. Arsenic (in A1) and nickel (in A1 and A2) were above the legal standards in both periods. The highest percentage of histological alterations in the gills (aneurysms, lamellar fusion and detachment of the epithelium) occurred in the port area, in the rainy (93%) and dry (74%) periods. Liver alterations (melanomacrophage centers and necrosis) occurred only in specimens from the same area, in the rainy (41%) and dry (36%) periods. The highest histological indices of gill and liver changes were recorded in A1. This result was further supported by the total HI value of the lesions, which was higher in the port area compared to A2 (less impacted area), suggesting that the environmental conditions in that location are less favorable for the well-being of these organisms. Permanent environmental monitoring of the area is necessary to control environmental impacts efficiently.

Effects of Starvation and Refeeding on Glucose Metabolism and Immune Responses in Macrobrachium rosenbergii

Starvation is a common challenge for aquatic animals in both natural and cultured environments. To investigate the effects of starvation and refeeding on glucose metabolism and immunity in Macrobrachium rosenbergii, prawns were starved for 14 days and then refed for 7 days. Results showed that both glucose and trehalose levels decreased significantly at the beginning of starvation, followed by a significant decrease in glycogen content in the hepatopancreas and muscle. Triglyceride and total protein reserves were also mobilized under starvation, with a slightly quicker response from triglycerides. The mRNA levels of glycolysis (glucokinase) and anabolism-related enzymes (glycogen branching enzyme, diacylglycerol acyltransferase, and transpeptidase) decreased during starvation, while gluconeogenic potential was induced, as indicated by up-regulated transcriptional levels of gluconeogenic enzymes (phosphoenolpyruvate carboxykinase) and catabolism-related enzymes (glycogen debranching enzyme, adipose triglyceride lipase, and cathepsin B). Starvation also stimulated the expression of the crustacean hyperglycemic hormone and inhibited insulin-like peptide expression, indicating their potential role in glucose metabolism regulation. In addition, starvation increased the mRNA levels of superoxide dismutase and prophenoloxidase, indicating an influence on the immune system. After bacterial infection, starved prawns showed enhanced activity of non-specific immunological parameters and reduced mortality. Refeeding for 7 days led to a recovery of physiological and biochemical indices and transcriptional levels of metabolism/immune-related genes. Our findings provide a better understanding of the mechanisms underlying energy utilization, metabolic adaptation, and immune response to starvation in M. rosenbergii.

Isolation and identification of major bacteria from three Ethiopian rift valley lakes live and processed fish, and water samples: implications in sanitary system of fish products

Bacterial pathogens are a great threat to fish production. Gram-negative bacteria are among the major bacterial fish pathogens and are zoonotic with the potential to infect humans. This cross-sectional study was conducted to isolate and identify major gram-negative bacteria from live and processed fish, and water samples from Lakes Hawassa, Langanoo and Ziway. A total of 674 different types of samples: 630 tissue samples (210 samples for each intestine, Kkidney and liver collected from 210 live fish (Oreochromis niloticus, Cyprinus carpio and Clarias gariepinus), 20 processed fish samples from lake Ziway fish processing center and 24 lake water samples were included in the study from each lake. The mean values of pH, temperature, dissolved oxygen and nitrate in all water samples were within the normal range at which most freshwater fish species become non-stressed. Of a total of 674 samples included in the study, bacteria were isolated from 154(22.8%) samples with significant difference (P < 0.05) observed in some isolates with respect to sample origin. Of these 154 isolates, 103(66.8%) isolates were gram-negative bacteria consisting of 15 species based on morphology and a range of biochemical tests. From live fish samples, Escherichia coli was the dominant species with 15 isolates followed by Edwardsiella tarda (12), Salmonella Paratyphi (10), Salmonella Typhi (9),  Shigella dysenteriae (7), Shigella flexneri (7), Klebsiella pneumonia (7), Enterobacter aerogenes (6), Enterobacter cloacae (5), Pseudomonas aeruginosa (5), Vibrio parahemolyticus (5), Aeromonas sobria (4), Citrobacter freundii (4), Citrobacter koseri (4) and Plesiomonas shigelloides (3). The detection of the common fecal coliforms (E. coli, K. pneumoniae and E. aerogenes) and Salmonella spp. in processed fish indicates the potential danger of passage of pathogenic bacteria and/or their poisons to humans via infected and/or contaminated fish products. Human infection by pathogenic fish bacteria and food poisoning is possible through contamination of fish product in fish production chain due to inadequate handling, poor hygiene and contact with contaminated water. Therefore, producers, consumers and all other stakeholders need to be cautious during handling, processing and consumption of fish harvested from the study lakes.

Bacteriophages in the Control of Aeromonas sp. in Aquaculture Systems: An Integrative View

Aeromonas species often cause disease in farmed fish and are responsible for causing significant economic losses worldwide. Although vaccination is the ideal method to prevent infectious diseases, there are still very few vaccines commercially available in the aquaculture field. Currently, aquaculture production relies heavily on antibiotics, contributing to the global issue of the emergence of antimicrobial-resistant bacteria and resistance genes. Therefore, it is essential to develop effective alternatives to antibiotics to reduce their use in aquaculture systems. Bacteriophage (or phage) therapy is a promising approach to control pathogenic bacteria in farmed fish that requires a heavy understanding of certain factors such as the selection of phages, the multiplicity of infection that produces the best bacterial inactivation, bacterial resistance, safety, the host’s immune response, administration route, phage stability and influence. This review focuses on the need to advance phage therapy research in aquaculture, its efficiency as an antimicrobial strategy and the critical aspects to successfully apply this therapy to control Aeromonas infection in fish.

VasH Contributes to Virulence of Aeromonas hydrophila and Is Necessary to the T6SS-mediated Bactericidal Effect

Aeromonas hydrophila is a Gram-negative bacterium that is commonly distributed in aquatic surroundings and has been considered as a pathogen of fish, amphibians, reptiles, and mammals. In this study, a virulent strain A. hydrophila GD18, isolated from grass carp (Ctenopharyngodon idella), was characterized to belong to a new sequence type ST656. Whole-genome sequencing and phylogenetic analysis showed that GD18 was closer to environmental isolates, however distantly away from the epidemic ST251 clonal group. The type VI secretion system (T6SS) was known to target both eukaryotic and prokaryotic cells by delivering various effector proteins in diverse niches by Gram-negative bacteria. Genome-wide searching and hemolysin co-regulated protein (Hcp) expression test showed that GD18 possessed a functional T6SS and is conditionally regulated. Further analysis revealed that VasH, a σ54-transcriptional activator, was strictly required for the functionality of T6SS in A. hydrophila GD18. Mutation of vasH gene by homologous recombination significantly abolished the bactericidal property. Then the virulence contribution of VasH was characterized in both in vitro and in vivo models. The results supported that VasH not only contributed to the bacterial cytotoxicity and resistance against host immune cleaning, but also was required for virulence and systemic dissemination of A. hydrophila GD18. Taken together, these findings provide a perspective for understanding the VasH-mediated regulation mechanism and T6SS-mediated virulence and bactericidal effect of A. hydrophila.

The evolutionary analysis of complement component C5 and the gene co-expression network and putative interaction between C5a and C5a anaphylatoxin receptor (C5AR/CD88) in human and two Cyprinid fish

The complement system is a complex network of soluble and membrane-associated serum proteins that regulate immune response. Activation of the complement C5 generates C5a and C5b which generate chemoattractive effect on myeloid cells and initiate the membrane attack complex (MAC) assembly. However, the study of evolutionary process and systematic function of C5 are still limited. In this study, we performed an evolutionary analysis of C5. Phylogeny analysis indicated that C5 sequences underwent complete divergence in fish and non-fish vertebrate. It was found that codon usage bias improved and provided evolution evidence of C5 in species. Notably, the codon usage bias of grass carp was evolutionarily closer to the zebrafish genome compared with humans and stickleback. This suggested that the zebrafish cell line may provide an alternative environment for heterologous protein expression of grass carp. Sequence comparison showed a higher similarity between human and mouse, grass carp, and zebrafish. Moreover, selective pressure analysis revealed that the C5 genes in fish and non-fish vertebrates exhibited different evolutionary patterns. To study the function of C5, gene co-expression networks of human and zebrafish were built which revealed the complexity of C5 function networks in different species. The protein structure simulation of C5 indicated that grass carp and zebrafish are more similar than to human, however, differences between species in C5a proteins are extremely smaller. Spatial conformations of C5a-C5AR (CD88) protein complex were constructed, which showed that possible interaction may exist between C5a and CD88 proteins. Furthermore, the protein docking sites/residues were measured and calculated according to the minimum distance for all atoms from C5a and CD88 proteins. In summary, this study provides insights into the evolutionary history, function and potential regulatory mechanism of C5 in fish immune responses.

Effect of starvation and refeeding on growth, gut microbiota and non-specific immunity in hybrid grouper (Epinephelus fuscoguttatus♀×E. lanceolatus♂)

Environmental changes can lead to food deprivation among aquatic animals. The main objective of this present research was to assess the effect of starvation and refeeding on growth, gut microbiota and non-specific immunity in a hybrid grouper (Epinephelus fuscoguttatus♀×E. lanceolatus♂). A total of 120 fish with an average weight of 74.16 ± 12.08 g were randomly divided into two groups (control group and fasted-refed group). The control group was fed until satiation for 60 days, while the fasted-refed group was fasted for 30 days and then fed to satiation for 30 days. The results showed that starvation led to a significantly decreased growth performance parameters [weight gain rate (WGR) and specific weight gain rate (SGR), while the feeding rate (FR) ] increased during the refeeding, non-specific immunity was significantly improved (p < 0.05) during the first 15 days of starvation, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-PX), lysozyme (LYM) and catalase (CAT). However, non-specific immunity decreased at 30 days of starvation, the expression of genes related to immunity, such as TNF-α, was upregulated (p < 0.05) during starvation, while the expression levels of IL-17 and IFN-γ was reduced (p < 0.05). The expression of IFN-γ and IL-1β peaked during refeeding. Starvation led to significantly decreased abundance and diversity of intestinal microflora, with a higher abundance of Vibrio and a lower abundance of Brevibacillus, Bifidobacterium, Alloprevotella in the fasted-refed group during refeeding than in the control group. The above results reveal that starvation stimulates changes in growth, non-specific immunity, and the gut microbiota, providing new insights for the study of fish habitat selection and adaptability to environmental changes.

Condensed tannins decreased the growth performance and impaired intestinal immune function in on-growing grass carp (Ctenopharyngodon idella)

The present study investigated the effects of condensed tannins (CT) on intestinal immune function in on-growing grass carp (Ctenopharyngodon idella). A total of 540 healthy grass carp were fed six diets containing different levels of CT (0, 10·00, 20·00, 30·00, 40·00 and 50·00 g/kg diet) for 70 d and then challenged with Aeromonas hydrophila for 14 d. The results showed that, compared with the control group, dietary CT (1) induced intestinal histopathological lesions and aggravated enteritis; (2) decreased lysozyme and acid phosphatase activities, complement 3 (C3), C4 and IgM contents and down-regulated the Hepcidin, liver-expressed antimicrobial peptide (LEAP)-2A, LEAP-2B, Mucin2 and β-defensin-1 mRNA levels in the proximal intestine (PI), mid intestine (MI) and distal intestine (DI) (P < 0·05); (3) down-regulated the mRNA levels of anti-inflammatory cytokines transforming growth factor (TGF)-β1, TGF-β2 (not in MI and DI), IL-4/13A (not IL-4/13B), IL-10 and IL-11 partly correlated with target of rapamycin (TOR) signalling; and (4) up-regulated the mRNA levels of pro-inflammatory cytokines interferon-γ2, IL-1β, IL-6, IL-8 (not in PI), IL-12p35, IL-12p40, IL-15 and IL-17D partly related to NF-κB signalling in the intestine of on-growing grass carp. Overall, the results indicated that CT could impair the intestinal immune function, and its potential regulation mechanisms were partly associated with the TOR and NF-κB signalling pathways. Finally, based on the percentage weight gain and enteritis morbidity, the maximum allowable levels of CT for on-growing grass carp (232·22-890·11 g) were estimated to be 18·6 and 17·4 g/kg diet, respectively.

Dietary addition of rutin impairs inflammatory response and protects muscle of silver catfish (Rhamdia quelen) from apoptosis and oxidative stress in Aeromonas hydrophila-induced infection

This research aimed to assess the influence of dietary addition of rutin on inflammation, apoptosis and antioxidative responses in muscle of silver catfish (Rhamdia quelen) challenged with Aeromonas hydrophila (A. hydrophila). Fish were split into four groups as follows: control, 0.15% rutin, A. hydrophila, 0.15% rutin + A. hydrophila. After 2 weeks of feeding with standard or rutin diets, fish were challenged or not with A. hydrophila for 1 week. Rutin-added diet abrogates A. hydrophila induced-hemorrhage and inflammatory infiltration. It decreases A. hydrophila induced-apoptosis through decreasing the ratio of Bax to Bcl-2 and increasing phospho-Akt to Akt ratio. It diminishes the A. hydrophila induced-rise in nitric oxide and superoxide anion levels and reestablishes superoxide dismutase activity as well. Although such diet is unable to recover the levels of reduced glutathione (GSH), cysteine and glutamate cysteine ligase, which are depleted as a result of A. hydrophila infection, it diminishes the oxidized glutathione (GSSG) content, thus decreasing GSSG to GSH ratio. It increases the levels of cysteine residues of proteins and diminishes those of thiol-protein mixed disulfides, which were changed after A. hydrophila challenge. Finally, it reduces A. hydrophila induced-lipid peroxidation, markedly elevates ascorbic acid and thus reestablishes total antioxidant capacity, whose levels were decreased after A. hydrophila challenge. In conclusion, the dietary addition of rutin at 0.15% impairs A. hydrophila-induced inflammatory response, inhibits A. hydrophila-induced apoptosis and promotes cell survival. It also reduces the A. hydrophila-induced oxidative stress and stimulates the antioxidative responses in muscle of A. hydrophila-infected silver catfish.

Isolation, Identification and Characteristics of Aeromonas veronii From Diseased Crucian Carp (Carassius auratus gibelio)

Aeromonas species often cause disease in farmed fish. In the present study, dominant bacteria were isolated from diseased crucian carp (Carassius auratus gibelio). Based on this, a bacterial isolate was tentatively named CFJY-623. This isolate was identified as Aeromonas veronii based on analysis of its morphological, physiological, and biochemical features, as well as 16S rRNA and gyrB gene sequences. Six virulence genes related to pathogenicity including aerolysin, cytotonic enterotoxins, elastase, glycerophospholipid: cholesterol acyltransferase, lipase, and serine protease were identified in this A. veronii isolate. The median lethal dosage (LD50) of the CFJY-623 isolate for crucian carp was determined as 1.31 × 107 CFU/mL. Artificial experimental infection showed that the CFJY-623 isolate caused considerable histological lesions in the fish, including tissue cell degeneration, necrosis, and inflammatory cell infiltrating. Drug sensitivity testing showed that the isolate was susceptible to aminoglycosides, carbapenemes, and nitrofurans. Exploring its growing features showed that this isolate exhibited a high level of environmental adaptability. These results provided a scientific basis for the identification of A. veronii and treatment for fish infected by this pathogen.

Supplementation exogenous bile acid improved growth and intestinal immune function associated with NF-κB and TOR signalling pathways in on-growing grass carp (Ctenopharyngodon idella): Enhancement the effect of protein-sparing by dietary lipid

This study investigated the effects of bile acid (BA) supplementation on growth performance, intestinal immune function and the mRNA expression of the related signalling molecules in on-growing grass carp (Ctenopharyngodon idella). A total of 540 healthy grass carp (mean weight 179.85 ± 1.34 g) were fed a normal protein and lipid (NPNL) diet containing 29% crude protein (CP) and 5% ether extract (EE), and five low-protein and high-lipid (LPHL) diets (26% CP, 6% EE) with graded levels of BA (0-320 mg/kg diet) for 50 days. The fish were then challenged with Aeromonas hydrophila for 14 days. The results indicated that compared with the NPNL diet, the LPHL diet (unsupplemented BA) suppressed the growth performance, intestinal development and enteritis resistance capability and impaired the partial intestinal immune function of on-growing grass carp. Whereas in the LPHL diet, optimal BA supplementation significantly improved fish growth performance (percent weight gain, specific growth rate, feed intake and feed efficiency) and intestinal growth and function (intestine weight, intestine length and intestosomatic index), increased beneficial bacteria Lactobacillus and Bifidobacterium amounts, decreased harmful bacteria Aeromonas and Escherichia coli amounts, elevated lysozyme and acid phosphatase activities, increased complement (C3 and C4) and immunoglobulin M contents, and upregulated β-defensin-1, hepcidin, liver expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, Mucin2, interleukin 10 (IL-10), IL-11, transforming growth factor (TGF)-β1, TGF-β2, IL-4/13A (not IL-4/13B), TOR, S6K1 and inhibitor of κBα (IκBα) mRNA levels. In addition, optimal BA supplementation in the LPHL diet downregulated tumour necrosis factor α (TNF-α), interferon γ2 (IFN-γ2), IL-1β, IL-6, IL-8, IL-15, IL-17D, IL-12p35, IL-12p40 (rather than proximal intestine (PI) or mid intestine (MI), nuclear factor kappa B p65 (NF-κB p65) (except NF-κB p52), c-Rel, IκB kinase β (IKKβ), IKKγ (except IKKα), eIF4E-binding proteins (4E-BP)1 and 4E-BP2 mRNA levels in all three intestinal segments of on-growing grass carp (P < 0.05). These findings suggest that BA supplementation in the LPHL diet improves growth and intestinal immune function of fish. Furthermore, 240 mg/kg BA supplementation in the LPHL diet was superior to the NPNL diet in improving growth and enhancing intestinal immune function of fish. Finally, based on percent weight gain, feed intake, protecting fish against enteritis, lysozyme activity in MI and acid phosphatase activity in distal intestine (DI), the optimal BA supplementation for on-growing grass carp were estimated to be 168.98, 170.23, 166.67, 176.50 and 191.97 mg/kg diet, respectively.

Complement component 3 (C3): An important role in grass carp (Ctenopharyngodon idella) experimentally exposed to Aeromonas hydrophila

Complement is traditionally recognized as part of the innate immune system, defending the host against the invasion of foreign pathogens. In complement system, C3 (complement component 3) is a central component. Therefore, research into C3 can help us better understand the functions of fish complement system. In this study, we detected the grass carp C3 (gcC3) mRNA expression in all sample tissues from healthy grass carp, which was highest in the liver, followed by the heart and the spleen, and lowest in the muscle, head kidney, trunk kidney, blood and intestine. After infection with Aeromonas hydrophila, gcC3 mRNA expression levels were significantly upregulated in the gill, liver, spleen, intestine, trunk kidney and head kidney. Interestingly, C3 protein levels were downregulated and subsequently upregulated in the liver and serum. Histologically, C3 protein at 24 h pi was over expressed in necrotic liver sites, and the liver index (LI) at this point was significantly higher than that of the control. These findings are indicated that C3 plays an important role in the immune response of grass carp after A. hydrophila infection, and C3 protein may play an assistant role in repairing liver tissues from A. hydrophila injury.

Identification and functional characterization of grass carp (Ctenopharyngodon idella) tumor necrosis factor receptor 2 and its soluble form with potentiality for targeting inflammation

Tumor necrosis factor-alpha (TNF-α) signals through two distinct cell surface receptors, TNFR1 and TNFR2 in mammals. In the present study, grass carp Tnfr2 (gcTnfr2) was isolated and characterized. Sequence alignment and phylogenetic analysis suggested that gcTnfr2 was a homolog of goldfish and zebrafish Tnfr2. Tissue distribution assay showed gctnfr2 transcripts were expressed in all examined tissues similar to gctnfr1. To functionally characterize the newly cloned molecule, gcTnfr2 was overexpressed in COS7 cell lines and it showed the ability to mediate the recombinant grass carp Tnf (rgcTnf)-α-triggered NF-κΒ activity and gcil1b promoter activity, clarifying its role in mediating Tnf-α signaling. The recombinant soluble form of gcTnfr2 (rgcsTnfr2) was prepared and it was able to interact with rgcTnf-α with higher affinity than that of rgcsTnfr1. Moreover, grass carp soluble Tnfr2 (gcsTnfr2) were detected in the culture medium of grass carp head kidney leukocytes (HKLs) and heat-inactivated A. hydrophila challenge significantly induced its production, indicating involvement of gcsTnfr2 in inflammation response. In agreement with this notion, rgcsTnfr2 effectively antagonized the effect of rgcTnf-α on il1b mRNA expression in HKLs, suggesting anti-Tnf-α property of gcsTnfr2. To strengthen the anti-inflammatory role of soluble Tnfr2, bacteria were injected intraperitoneally in grass carp followed by rgcsTnfr2. Hematoxylin-eosin (HE) staining of head kidney, spleen and intestine showed that rgcsTnfr2 could significantly improve infection-induced histopathological changes. These results functionally identified gcTnfr2 and its soluble form, particularly highlighting the role of gcsTnfr2 against Tnf-α-triggered inflammatory signaling. In this line, rgcsTnfr2 displayed anti-inflammatory potentiality during infection, thereby providing a powerful mediator of inflammation control in fish.

Dietary magnesium deficiency impaired intestinal structural integrity in grass carp (Ctenopharyngodon idella)

Grass carp (223.85–757.33 g) were fed diets supplemented with magnesium (73.54–1054.53 mg/kg) for 60 days to explore the impacts of magnesium deficiency on the growth and intestinal structural integrity of the fish. The results demonstrated that magnesium deficiency suppressed the growth and damaged the intestinal structural integrity of the fish. We first demonstrated that magnesium is partly involved in (1) attenuating antioxidant ability by suppressing Nrf2 signalling to decrease antioxidant enzyme mRNA levels and activities (except CuZnSOD mRNA levels and activities); (2) aggravating apoptosis by activating JNK (not p38MAPK) signalling to upregulate proapoptotic protein (Apaf-1, Bax and FasL) and caspase-2, -3, -7, -8 and -9 gene expression but downregulate antiapoptotic protein (Bcl-2, IAP and Mcl-1b) gene expression; (3) weakening the function of tight junctional complexes (TJs) by promoting myosin light chain kinase (MLCK) signalling to downregulate TJ gene expression [except claudin-7, ZO-2b and claudin-15 gene expression]. Additionally, based on percent weight gain (PWG), against reactive oxygen species (ROS), against caspase-9 and claudin-3c in grass carp, the optimal dietary magnesium levels were calculated to be 770.38, 839.86, 856.79 and 811.49 mg/kg, respectively.

Selenium deficiency impaired immune function of the immune organs in young grass carp (Ctenopharyngodon idella)

This study aimed to investigate the effects of dietary selenium on resistance to skin haemorrhages and lesions and on immune function as well as the underlying mechanisms of those effects in the head kidney, spleen and skin of young grass carp (Ctenopharyngodon idella). A total of 540 healthy grass carp with initial body weight (226.48 ± 0.68 g) were randomly divided into six groups and fed six separate diets with graded dietary levels of selenium (0.025, 0.216, 0.387, 0.579, 0.795 and 1.049 mg/kg diet) for 80 days. After the feeding period, an immunization trial was performed by infection with Aeromonas hydrophila for 14 days. The results showed that, compared with the optimal selenium level, (1) selenium deficiency impaired the production of antibacterial compounds and immunoglobulins and down-regulated the transcript abundances of antimicrobial peptides and selenoproteins; (2) selenium deficiency aggravated inflammatory responses in part by up-regulating pro-inflammatory cytokines and down-regulating anti-inflammatory cytokines mRNA levels, which were partially related to [IKKα, β, γ/IκBα/NF-κB] signalling and [TOR/(S6K1, 4E-BP1)] signalling, respectively. Interestingly, selenium deficiency had no effect on the expression of TGF-β2, IL-4/13B, IL-10, IL-12p35, IL-15 (skin only) or 4E-BP2 in the head kidney, spleen and skin of young grass carp. Finally, based on the percent weight gain (PWG), the morbidity of skin haemorrhages and lesions, the ACP activity in the head kidney and the lysozyme activity in spleen, the optimal dietary selenium requirements for young grass carp were estimated to be 0.546-0.604 mg/kg diet. In summary, selenium deficiency decreased the growth performance and impaired the immune function in the head kidney, spleen and skin of young grass carp.