European Sea Bass (Dicentrarchus labrax) Immune Status and Disease Resistance Are Impaired by Arginine Dietary Supplementation

Molecular characterization of Lernathropus kroyeri from intensive aquaculture and pathophysiology of infested sea bass

The copepod Lernathropus kroyeri constitutes one of the major parasites for the Mediterranean aquaculture, infesting the sea bass Dicentrarchus labrax causing thus disruptions of growth performance and occasionally mortalities. Despite the large spread and the high frequency of this parasite in mariculture farms of Eastern Mediterranean, L. kroyeri genetic profile from aquaculture as well as the pathophysiological response of D. labrax have not been studied so far. Keeping this in mind, in the present study we investigated the L. kroyeri infestation on D. labrax from two farms in Greece, examining both healthy and heavy parasitized individuals. Assays included histopathology, phylogenetic reconstruction of the parasite and physiological response of the fish by the means of antioxidant, inflammatory metabolic and stress related gene expression analysis at both mRNA and protein levels. Genetic analysis indicated that L. kroyeri composes a monophyletic group, highly phylogenetically distant from other congeneric groups. Heavy infested D. labrax witnessed a significantly increased immune response that further led to oxidative stress and metabolic alterations. Overall, our results demonstrate the, seasonally independent, high infestation of this parasitic copepods, which continue to affect Mediterranean intensive aquaculture systems.

Functional Additives in a Selected European Sea Bass (Dicentrarchus labrax) Genotype: Effects on the Stress Response and Gill Antioxidant Response to Hydrogen Peroxide (H2O2) Treatment

Functional ingredients have profiled as suitable candidates for reinforcing the fish antioxidant response and stress tolerance. In addition, selective breeding strategies have also demonstrated a correlation between fish growth performance and susceptibility to stressful culture conditions as a key component in species domestication processes. The aim of the present study is to evaluate the ability of a selected high-growth genotype of 300 days post-hatch European sea bass (Dicentrarchus labrax) juveniles to use different functional additives as endogenous antioxidant capacity and stress resistance boosters when supplemented in low fish meal (FM) and fish oil (FO) diets. Three isoenergetic and isonitrogenous diets (10% FM/6% FO) were supplemented with 200 ppm of a blend of garlic and Labiatae plant oils (PHYTO0.02), 1000 ppm of a mixture of citrus flavonoids and Asteraceae and Labiatae plant essential oils (PHYTO0.1) or 5000 ppm of galactomannan-oligosaccharides (GMOS0.5). A reference diet was void of supplementation. The fish were fed the experimental diets for 72 days and subjected to a H₂O₂ exposure oxidative stress challenge. The fish stress response was evaluated through measuring the circulating plasma cortisol levels and the fish gill antioxidant response by the relative gene expression analysis of nfΚβ2, il-1b, hif-1a, nd5, cyb, cox, sod, cat, gpx, tnf-1α and caspase 9. After the oxidative stress challenge, the genotype origin determined the capacity of the recovery of basal cortisol levels after an acute stress response, presenting GS fish with a better pattern of recovery. All functional diets induced a significant upregulation of cat gill gene expression levels compared to fish fed the control diet, regardless of the fish genotype. Altogether, suggesting an increased capacity of the growth selected European sea bass genotype to cope with the potential negative side-effects associated to an H₂O₂ bath exposure.

Total fishmeal replacement by defatted Tenebrio molitor larvae meal induces alterations in intermediary metabolism of European sea bass (Dicentrarchus labrax)

The replacement of fishmeal (FM) by insect meal (IM) in aquafeed formulation has been thoroughly studied lately, but little is known about their impact on nutrient metabolism of fish. This study evaluated the impact not only of partial but also total FM replacement by IM on intermediary metabolism of European sea bass (Dicentrarchus labrax). A fishmeal-based diet was used as a control (CTRL) and two other diets were formulated to include 20% and 40% of defatted Tenebrio molitor larvae meal (dTM), replacing 50% (TM50) and 100% (TM100) of fishmeal (FM), respectively. After a 16-week feeding trial, a multidisciplinary approach including assessment of histological, biochemical, molecular, and enzymatic parameters was adopted to investigate hepatic and plasmatic responses to the different dietary formulations. The results obtained demonstrated that dTM can be successfully used to replace 50% of FM in diets for European sea bass, without adversely affecting liver health or intermediary metabolism of nutrients. As for TM100, although no signs of steatosis were observed in the liver, the activity of glycolytic and lipogenic genes and enzymes increased when compared to CTRL diet (P < 0.05), resulting in higher levels of plasmatic non-esterified fatty acids and triacylglycerides (P < 0.05), which in the long-term may compromise fish health, thus precluding such a high degree of substitution for use in practical diets for European sea bass.

Genome-wide association study of host resistance to the ectoparasite Ichthyophthirius multifiliis in the Amazon fish Colossoma macropomum

BACKGROUND

Tambaqui, Colossoma macropomum, is the most important native fish species farmed in South America, particularly in Brazil, where its production is limited in the southern and southeastern regions due to disease outbreaks caused by the parasite Ichthyophthirius multifiliis. Therefore, genome level analysis to understand the genetic architecture of the host resistance against I. multifiliis is fundamental to improve this trait in tambaqui. The objective of the present study was to map QTL (quantitative trait loci) associated with resistance to I. multifiliis in tambaqui by GWAS (genome-wide association study).

METHODS AND RESULTS

Individuals belonging to seven families, which were previously submitted to an experimental challenge to assess the natural resistance to the parasite I. multifiliis, were used for genomic analysis. A total of 7717 SNPs were identified in this population by ddRAD (double digest restriction site associated DNA). GWAS revealed four SNPs significantly associated in the LGs (linkage groups) 2, 9, 11 and 20 for the traits time of death and parasite load. The SNPs explained a low proportion of the variance to I. multifiliis resistance for time of death and parasite load (about 0.622% and 0.375%, respectively). The SNPs were close to 11 genes related to the immune system: abcf3, znf830, ccr9, gli3, ackr4, tbata, ndr2, tgfbr3, nhej1, znf644b, and cldn10a.

CONCLUSIONS

In conclusion, the resistance to I. multifiliis is probably under polygenic control in tambaqui, in which different QTLs of low variance can be involved in the immune responses against this ectoparasite.

Dietary chlorpyrifos-methyl exposure impair transcription of immune-, detoxification- and redox signaling genes in leukocytes isolated from cod (Gadus morhua)

Inclusion of new environmental toxicants increase with the amount of plant ingredients substituting marine proteins and oils in feed for farmed Atlantic salmon (Salma salar). Agricultural pesticides like chlorpyrifos-methyl, present in commercial salmon feeds, may affect salmon immune and detoxification responses. Atlantic cod (Gadus morhua), surrounding the net pens, grazing on feces and uneaten pellets may be affected accordingly. The aim of this study was to analyze transcription responses in Atlantic cod head kidney tissue and isolated leukocytes following dietary chlorpyrifos-methyl inclusions and possible interactions with proinflammatory signals. Head kidney tissues and leukocytes were isolated from cod fed diets contaminated with chlorpyrifos-methyl (0.5 mg/kg, 2.4 mg/kg, 23.2 mg/kg) for 30 days. The isolated leukocytes were further challenged with bacteria (lipopolysaccharide (LPS), virus (polyinosinic acid:polycytidylic acid (PIC) mimic and l-arginine, an immuno-modulating amino acid, in vitro. The LPS-induced transcription of the interleukin genes il-1β, il-6, il-8 increased in leukocytes isolated from cod fed chlorpyrifos-methyl 23.2 mg/kg, compared to cod fed the control diet, indicating increased inflammation. Transcriptional levels of carnitine palmitoyl transferase (cpt1a), aryl hydrogen receptor (ahr) and catalase (cat) were all reduced by dietary inclusions of chlorpyrifos-methyl in the leukocytes. The findings suggests that dietary chlorpyrifos-methyl exposure impair inflammation, detoxification and redox signaling in cod leukocytes.

Chitosan Treatment of E-11 Cells Modulates Transcription of Nonspecific Immune Genes and Reduces Nodavirus Capsid Protein Gene Expression

This study explores whether crustacean products inhibit viral infections in aquaculture. Chitosan (CHT) was extracted from waste products of Parapenaeus longirostris. Biochemical composition, viscosity measurement, molecular weight, structure and cytotoxicity tests were used to characterize the extracted chitosan. Cultures of E-11 cells derived from snakehead Ophicephalus striatus were inoculated with 106.74 TCID50 of an isolate of betanodavirus genotype RGNNV (redspotted grouper nervous necrosis virus) after being treated with solutions of 0.3% CHT for 1 h at room temperature. The antiviral effect of CHT was assessed by comparing the ability of RGNVV to replicate and produce cytopathic effects on CHT-treated cell cultures. The change in RNA expression levels of the nodavirus capsid protein gene and three mediator genes in infected cells with or without CHT treatment was evaluated by qPCR. Changes in gene expression compared to control groups were monitored at 6, 24, 48 and 71 h post treatment in all target gene transcripts. The CCR3 expression in CHT treated cells showed a significant increase (p < 0.05) until day 3. On the other hand, the expression of TNF-α decreased significantly (p < 0.05) in CHT treated cells throughout the experimental period. Likewise, the expression of the IL-10 gene showed a significant downregulation in CHT treated cells at all time points (p ≤ 0.05). As further evidence of an antiviral effect, CHT treatment of cells produced a reduction in virus load as measured by a reduced expression of the viral capsid gene and the increase in RQ values from 406 ± 1.9 at hour 1 to 695 ± 3.27 at 72 h post inoculation. Statistical analysis showed that the expression of the viral capsid gene was significantly lower in cells treated with chitosan (p ≤ 0.05). These results improve our knowledge about the antiviral activity of this bioactive molecule and highlight its potential use in fish feed industry.

Arginine metabolism and its functions in growth, nutrient utilization, and immunonutrition of fish

Fish have limited ability in endogenous biosynthesis of arginine. Arginine is an indispensable amino acid for fish, and the arginine requirement varies with fish species and fish size. Recent studies on fish have demonstrated that arginine influences nutrient metabolism, stimulates insulin release, is involved in nonspecific immune responses and antioxidant responses, and elevates disease resistance. Specifically, arginine can regulate energy homeostasis via modulating the adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) pathway, and also regulate protein synthesis via activating the target of rapamycin (TOR) signaling pathway. The present article reviews pertinent knowledge of arginine in fish, including dietary quantitative requirements, endogenous anabolism and catabolism, regulation of the endocrine and metabolic systems, and immune-regulatory functions under pathogenic challenge. Our findings showed that further data about the distribution of arginine after intake into specific cells, its sub-cellular sensor to initiate downstream signaling pathways, and its effects on fish mucosal immunity, especially the adaptive immune response against pathogenic infection in different species, are urgently needed.

Methionine and Tryptophan Play Different Modulatory Roles in the European Seabass (Dicentrarchus labrax) Innate Immune Response and Apoptosis Signaling-An In Vitro Study

The range of metabolic pathways that are dependent on a proper supply of specific amino acids (AA) unveils their importance in the support of health. AA play central roles in key pathways vital for immune support and individual AA supplementation has shown to be able to modulate fish immunity. In vitro trials are important tools to evaluate the immunomodulatory role of AA, and the present study was conceived to evaluate methionine and tryptophan roles in immune-related mechanisms aiming to understand their effects in leucocyte functioning and AA pathways. For that purpose, head-kidney leucocytes were isolated and a primary cell culture established. The effect of methionine or tryptophan surplus on cell viability was assessed. Medium L-15 10% FBS without AA addition (0.5mM of L-methionine, 0.1 mM of L-tryptophan) was used as control. To that, L-methionine or L-tryptophan were supplemented at 1 and 2 times (M1x or M2x, and T1x or T2x). Nitric oxide, ATP, total antioxidant capacity, and immune-related genes were evaluated in response to lipopolysaccharides extracted from Photobacterium damselae subsp. piscicida or UV-inactivated bacteria). Moreover, caspase 3 activity and apoptosis-related genes were evaluated in response to the apoptosis-inducing protein, AIP56. Distinct roles in leucocytes' immune response were observed, with contrasting outcomes in the modulation of individual pathways. Methionine surplus improved cell viability, polyamine production, and methionine-related genes expression in response to an inflammatory agent. Also, methionine supplementation lowered signals of apoptosis by AIP56, presenting lower caspase 3 activity and higher il1β and nf-κb expression. Cells cultured in tryptophan supplemented medium presented signals of an attenuated inflammatory response, with decreased ATP and enhanced expression of anti-inflammatory and catabolism-related genes in macrophages. In response to AIP56, leucocytes cultured in a tryptophan-rich medium presented lower resilience to the toxin, higher caspase 3 activity and expression of caspase 8, and lower expression of several genes, including nf-κb and p65. This study showed the ability of methionine surplus to improve leucocytes' response to an inflammatory agent and to lower signals of apoptosis by AIP56 induction, while tryptophan attenuated several cellular signals of the inflammatory response to UV-inactivated bacteria and lowered leucocyte resilience to AIP56.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

The coordinated outcome of STIM1-Orai1 and superoxide signalling is crucial for headkidney macrophage apoptosis and clearance of Mycobacterium fortuitum

The mechanisms underlying M. fortuitum-induced pathogenesis remains elusive. Using headkidney macrophages (HKM) from Clarias gariepinus, we report that TLR-2-mediated internalization of M. fortuitum is imperative to the induction of pathogenic effects. Inhibiting TLR-2 signalling alleviated HKM apoptosis, thereby favouring bacterial survival. Additionally, TLR-2-mediated cytosolic calcium (Ca²⁺)_c elevation was instrumental for eliciting ER-stress in infected HKM. ER-stress triggered the activation of membrane-proximal calcium entry channels comprising stromal interaction molecule 1 (STIM1) and calcium-release activated calcium channel 1 (Orai1). RNAi studies suggested STIM1-Orai1 signalling initiate calpain-mediated cleavage of nitric oxide synthase interacting protein, prompting the release of pro-apoptotic nitric oxide. Inhibiting STIM1-Orai1 signalling attenuated superoxide production (O₂^•-) and vice versa. We conclude, TLR-2-induced ER-stress triggers STIM1/Orai1 expression and that the reciprocal association between STIM1-Orai1 signalling and oxidative stress is critical for sustaining (Ca²⁺)_c level, thereby prolonging ER-stress and maintenance of pro-oxidant rich environment to induce HKM apoptosis and bacterial clearance.

The alleviation of skin wound-induced intestinal barrier dysfunction via modulation of TLR signalling using arginine in gilthead seabream (Sparus aurata L)

The present study sought to investigate the effect of arginine on the involvement of toll-like receptors (TLRs) in skin wound-induced intestinal barrier dysfunction in gilthead seabream (Sparus aurata L.). Two replicates of fish (n = 8) were fed a commercial diet (CON, total 2.75% arginine), CON diet enriched with 1% arginine (ARG1, total 3.65% arginine) and 2% arginine (ARG2, total 4.53% arginine) for 30 days. Half of the fish were sampled, whereas the others were injured and sampled 7 days post-wounding. The intestinal histology results showed that a more intense infiltration of mixed leucocytes was evident in the wounded fish, which was remarkably reduced in fish that were fed the ARG1 diet. Serum IgM levels were significantly higher in the ARG1 group than levels in the CON group at 7 days post-wounding. Compared with the fish in the CON group after wounding, dietary administration of 1% arginine markedly downregulated the gene expression of TLRs (TLR2 and TLR5), MyD88, and proinflammatory cytokines (CSF1R, IL-1β, and TNFα), but significantly enhanced the gene expression of IκBα, the anti-inflammatory cytokine TGF-β1, and tight junction proteins (tricellulin and occludin) in wounded fish. Furthermore, the ARG2 diet demonstrated no additional benefits on intestinal cells, compared to both the ARG1 and the CON diets, and it even appeared to induce negative effects. In summary, dietary administration of 1% arginine significantly inhibited intestinal inflammatory response and tight junction disruption in skin-wounded gilthead seabream by modulating TLR signalling in the intestine.

Evaluating mucus exudation dynamics through isotopic enrichment and turnover of skin mucus fractions in a marine fish model

Fish skin mucus is composed of insoluble components, which form the physical barrier, and soluble components, which are key for interrelationship functions. Mucus is continuously secreted, but rates of production and exudation are still unknown, as are the underlying mechanisms. Using stable isotope analysis, here, we evaluate skin mucus turnover and renewal in gilthead sea bream, separating raw mucus and its soluble and insoluble fractions. Isotopic abundance analysis reveals no differences between mucus and white muscle, thus confirming mucus samples as reliable non-invasive biomarkers. Mucus production was evaluated using a single labelled meal packaged in a gelatine capsule, with both ¹³C and ¹⁵N, via a time-course trial. ¹³C was gradually allocated to skin mucus fractions over the first 12 h and was significantly (4-fold) higher in the soluble fraction, indicating a higher turnover of soluble mucus components that are continuously produced and supplied. ¹⁵N was also gradually allocated to mucus, indicating incorporation of new proteins containing the labelled dietary amino acids, but with no differences between fractions. When existent mucus was removed, dietary stable isotopes revealed stimulated mucus neoformation dependent on the components. All this is novel knowledge concerning skin mucus dynamics and turnover in fish and could offer interesting non-invasive approaches to the use of skin mucus production in ecological or applied biological studies such as climate change effects, human impact, alterations in trophic networks or habitat degradation, especially of wild-captured species or protected species.

Dietary arginine and citrulline supplementation modulates the immune condition and inflammatory response of European seabass

The present study was designed to determine the modulatory effects of arginine and citrulline dietary supplementation on the immune condition and inflammatory response of European seabass, Dicentrarchus labrax. Four diets were manufactured: a control diet (CTRL) was formulated to meet the indispensable amino acids profile established for seabass. Based on this formulation, three other diets were supplemented with l-arginine at two different levels (0.5% and 1%, ARG1 and ARG2, respectively) and l-citrulline at 0.5% (CIT). Fish were fed these diets for 2 or 4 weeks under controlled conditions. At the end of 4 weeks, fish from all dietary treatments were intraperitoneally-injected with Photobacterium damselae piscicida and sampled after 4, 24 our 48 h. Immune status was characterized by a lymphocyte time-dependent decrease regardless of dietary treatment, whereas peroxidase values dropped in time in fish fed ARG1 and ARG2 and was lower at 4 weeks in fish fed ARG1 than in fish fed CTRL. Up-regulation of several genes was more evident in ARG1-and CIT-fed fish, though pro-inflammatory cytokines were down-regulated by CIT dietary treatment. Following immune stimulation, seabass fed ARG1 showed a decrease in neutrophils and monocytes circulating numbers. On the other hand, expression of 17 selected immune and inflammatory responses genes was barely affected by dietary treatments. Based on the analyzed parameters, results suggest an active role of dietary arginine/citrulline supplementation in modulating immune defences that seem to translate into a suppressed immune repertoire, mostly at the cell response level. The observed changes due to citrulline dietary supplementation were in part similar to those caused by arginine, suggesting that citrulline might have been used by macrophages as an arginine precursor and then engaged in similar immune-impairment leading mechanisms.

Skin wound healing in gilthead seabream (Sparus aurata L.) fed diets supplemented with arginine

Dietary administration of arginine on the wound healing process of gilthead seabream was studied. Two replicates of fish (n = 8) were fed with either a commercial diet [control diet (CON), no arginine added] and the CON diet supplemented with 1% arginine (ARG1) or with 2% arginine (ARG2) for 30 days. Afterward, half of the fish were sampled while the other half were injured and continued to be fed the same diet for an extra week. Results by image analysis showed that the wound closure rate was significantly improved in fish that were fed the ARG1 diet, compared with those in the CON group. After seven days of wound healing, the aminotransferase and creatine kinase levels in the serum and the protease and peroxidase activities in the skin mucus were down-regulated, while the immunoglobulin M level in the skin mucus was up-regulated in the ARG1 group after wounding and in the CON group before wounding. Compared with the CON diet, the ARG1 diet remarkedly depressed the gene expression of mpo, il-8, and tnf-α, and enhanced the gene expression of tgf-β1, igf-1, pcna, krt2, mmp9, fn1α, and colIα and the antioxidant enzyme cat in the skin tissues after wounding. Furthermore, compared with both the ARG1 and the CON groups, negative effects of the ARG2 diet on wound healing were demonstrated. In conclusion, a 1% arginine supplementation facilitates skin wound healing and prevents a systemic inflammation reaction by alleviating the inflammatory response and enhancing the re-epithelialization and ECM biosynthesis in skin wound sites.

Short-Term Supplementation of Dietary Arginine and Citrulline Modulates Gilthead Seabream (Sparus aurata) Immune Status

Several amino acids (AA) are known to regulate key metabolic pathways that are crucial for immune responses. In particular, arginine (ARG) appears to have important roles regarding immune modulation since it is required for macrophage responses and lymphocyte development. Moreover, citrulline (CIT) is a precursor of arginine, and it was reported as an alternative to ARG for improving macrophage function in mammals. The present study aimed to explore the effects of dietary ARG and CIT supplementation on the gilthead seabream (Sparus aurata) immune status. Triplicate groups of fish (23.1 ± 0.4 g) were either fed a control diet (CTRL) with a balanced AA profile, or the CTRL diet supplemented with graded levels of ARG or CIT (i.e., 0.5 and 1% of feed; ARG1, CIT1, ARG2, and CIT2, respectively). After 2 and 4 weeks of feeding, fish were euthanized and blood was collected for blood smears, plasma for humoral immune parameters and shotgun proteomics, and head-kidney tissue for the measurement of health-related transcripts. A total of 94 proteins were identified in the plasma of all treatments. Among them, components of the complement system, apolipoproteins, as well as some glycoproteins were found to be highly abundant. After performing a PLS of the expressed proteins, differences between the two sampling points were observed. In this regard, component 1 (61%) was correlated with the effect of sampling time, whereas component 2 (18%) seemed associated to individual variability within diet. Gilthead seabream fed ARG2 and CIT2 at 4 weeks were more distant than fish fed all dietary treatments at 2 weeks and fish fed the CTRL diet at 4 weeks. Therefore, data suggest that the modulatory effects of AA supplementation at the proteome level were more effective after 4 weeks of feeding and at the higher inclusion level (i.e., 1% of feed). The bactericidal activity increased in fish fed the highest supplementation level of both AAs after 4 weeks. Peripheral monocyte numbers correlated positively with nitric oxide, which showed an increasing trend in a dose-dependent manner. The colony-stimulating factor 1 receptor tended to be up-regulated at the final sampling point regardless of dietary treatments. Data from this study point to an immunostimulatory effect of dietary ARG or CIT supplementation after 4 weeks of feeding in the gilthead seabream, particularly when supplemented at a 1% inclusion level.

Prebiotics and phytogenics functional additives in low fish meal and fish oil based diets for European sea bass (Dicentrarchus labrax): Effects on stress and immune responses

The use of terrestrial raw materials to replace fish meal (FM) and fish oil (FO) in marine fish diets may affect fish growth performance and health. In the last years functional additives have been profiled as good candidates to reduce the effects on health and disease resistance derived from this replacement, via reinforcement of the fish immune system. In the present study, three isoenergetic and isonitrogenous diets with low FM and FO (10% and 6% respectively) were tested based on supplementation either with 0.5% galactomannanoligosaccharides (GMOS diet) or 0.02% of a mixture of essential oils (PHYTO diet), a non-supplemented diet was defined as a control diet. Fish were fed the experimental diets in triplicate for 9 weeks and then they were subjected to a stress by confinement as a single challenge (C treatment) or combined with an experimental intestinal infection with Vibrio anguillarum (CI treatment). Along the challenge test, selected stress and immunological parameters were evaluated at 2, 24 and 168h after C or CI challenges. As stress indicators, circulating plasma cortisol and glucose concentrations were analyzed as well as the relative gene expression of cyp11b hydroxylase, hypoxia inducible factor, steroidogenic acute regulatory protein, heat shock protein 70 and heat shock protein 90 (cyp11b, hif-1α, StAR, hsp70 and hsp90). As immune markers, serum and skin mucus lysozyme, bactericidal and peroxidase activities were measured, as well as gene expression of Caspase-3 (casp-3) and interleukin 1β (il-1ß). The use of functional additives induced a significant (p < 0.05) reduction of circulating plasma cortisol concentration when confinement was the unique challenge test applied. Supplementation of PHYTO induced a down-regulation of cyp11b, hif-1α, casp-3 and il-1β gene expression 2h after stress test, whereas StAR expression was significantly (p < 0.05) up-regulated. However, when combination of confinement stress and infection was applied (CI treatment), the use of PHYTO significantly (p < 0.05) down-regulated StAR and casp-3 gene expression 2h after challenge test, denoting that PHYTO diet reinforced fish capacity of stress response via protection of head kidney leucocytes from stress-related apoptotic processes, with lower caspase-3 gene expression and a higher il-1β gene expression when an infection occurs. Additionally, dietary supplementation with GMOS and PHYTO compounds increased fish serum lysozyme after infection. Both functional additives entailed a better capability of the animals to cope with infection in European sea bass when fed low FM and FO diets.

Arginine, ornithine and citrulline supplementation in rainbow trout: Free amino acid dynamics and gene expression responses to bacterial infection

Supplementing the diet with functional ingredients is a key strategy to improve fish performance and health in aquaculture. The amino acids of the urea and nitric oxide (NO) cycles - arginine, ornithine and citrulline - perform crucial roles in the immune response through the generation of NO and the synthesis of polyamine used for tissue repair. We previously found that citrulline supplementation improves and maintains circulating free arginine levels in rainbow trout more effectively than arginine supplementation. Here, to test whether supplementation of urea cycle amino acids modulates the immune response in rainbow trout (Oncorhynchus mykiss), we supplemented a commercial diet with high levels (2% of total diet) of either arginine, ornithine or citrulline during a 7-week feeding trial, before challenging fish with the bacterium Aeromonas salmonicida. We carried out two separate experiments to investigate fish survival and 24 h post-infection to investigate the immediate response of free amino acid levels, and transcriptional changes in genes encoding urea cycle, NO cycle and polyamine synthesis enzymes. There were no differences in percentage fish mortality between diets, however there were numerous highly significant changes in free amino acid levels and gene expression to both dietary supplementation and infection. Out of 26 amino acids detected in blood plasma, 8 were significantly changed by infection and 9 by dietary supplementation of either arginine, ornithine or citrulline. Taurine, glycine and aspartic acid displayed the largest decreases in circulating levels in infected fish, while ornithine and isoleucine were the only amino acids that increased in concentration. We investigated transcriptional responses of the enzymes involved in arginine metabolism in liver and head kidney; transcripts for polyamine synthesis enzymes showed highly significant increases in both tissues across all diets following infection. The paralogous arginase-encoding genes, Arg1a, Arg1b, Arg2a and Arg2b, displayed complex responses across tissues and also due to diet and infection. Overall, these findings improve our understanding of amino acid metabolism following infection and suggests new potential amino acid targets for improving the immune response in salmonids.

Supplementation of arginine, ornithine and citrulline in rainbow trout (Oncorhynchus mykiss): Effects on growth, amino acid levels in plasma and gene expression responses in liver tissue

Functional amino acids (FAA) regulate metabolic pathways directly linked to health, survival, growth and development. Arginine is a FAA with crucial roles in protein deposition and the immune response. In mammals, supplementation of arginine's precursor amino acid, citrulline, is known to increase circulating arginine to levels beyond direct arginine supplementation, however, citrulline supplementation is poorly studied in fish. To address this knowledge gap, we supplemented the diet of rainbow trout with arginine and its precursor amino acids, ornithine and citrulline, at 3 levels (0.5%, 1% and 2% of the total diet) during a 14-week experiment. We sampled fish at 3 h and 24 h post-feeding to investigate immediate and steady-state effects, respectively. There were no differences in fish growth for any of the diets across a range of indicators. In blood plasma, out of 26 amino acids detected, 11 and 6 displayed significant changes 24 h and 3 h post-prandial, respectively. Arginine, ornithine and citrulline levels were all significantly increased by the citrulline supplemented diets. In muscle, 8 amino acids were significantly altered by supplemented diets, while there were no significant changes in liver. Arginine was increased by 2% citrulline supplementation in muscle tissue. We also investigated the transcriptional responses of urea cycle, nitric oxide cycle and rate-limiting polyamine synthesis enzymes, related to arginine's metabolism, in liver. At both time points, only 2 enzymes were significantly altered by the supplemented diets, however several significant changes were observed comparing 3 h and 24 h post-prandial expression levels. Of these, the paralogous polyamine synthesis enzyme encoding genes ODC1 and ODC2 displayed the largest increases in 3 h post-prandial fish. These findings demonstrate that endogenous synthesis of arginine is possible from a citrulline supplemented diet and improve our understanding of arginine metabolism in fish.

Creatine as a Neuroprotector: an Actor that Can Play Many Parts

Creatine is a nitrogenous organic acid that plays a central role as an energy buffer in high energy demanding systems, including the muscular and the central nervous system. It can be acquired from diet or synthesized endogenously, and its main destination is the system creatine/phosphocreatine that strengthens cellular energetics via a temporal and spatial energy buffer that can restore cellular ATP without a reliance on oxygen. This compound has been proposed to possess secondary roles, such as direct and indirect antioxidant, immunomodulatory agent, and possible neuromodulator. However, these effects may be associated with its bioenergetic role in the mitochondria. Given the fundamental roles that creatine plays in the CNS, several preclinical and clinical studies have tested the potential that creatine has to treat degenerative disorders. However, although in vitro and in vivo animal models are highly encouraging, most clinical trials fail to reproduce positive results suggesting that the prophylactic use for neuroprotection in at-risk populations or patients is the most promising field. Nonetheless, the only clearly positive data of the creatine supplementation in human beings are related to the (rare) creatine deficiency syndromes. It seems critical that future studies must establish the best dosage regime to increase brain creatine in a way that can relate to animal studies, provide new ways for creatine to reach the brain, and seek larger experimental groups with biomarkers for prediction of efficacy.

Skin mucus metabolites in response to physiological challenges: A valuable non-invasive method to study teleost marine species

Knowledge concerning the health and welfare of fish is important to conserve species diversity. Fish mucosal surfaces, and particularly the skin, are of utmost importance to protect the integrity and homeostasis of the body and to prevent skin infections by pathogens. We performed three trials simulating different environmental and anthropogenic challenges: fish capture (air exposure), bacterial infection and fasting, with the aim of evaluating epidermal mucus as a non-invasive target of studies in fish. In this initial approach, we selected three well-known marine species: meagre (Argyrosomus regius), European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) for our study. Mucus viscosity was measured in order to determine its rheological properties, and mucus metabolite (glucose, lactate, protein and cortisol) levels were analysed to establish their suitability as potential biomarkers. Skin mucus appeared as a viscous fluid exhibiting clearly non-Newtonian behaviour, with its viscosity being dependent on shear rate. The highest viscosity (p < 0.05) was observed in sea bream. Mucus metabolites composition responded to the different challenges. In particular, glucose increased significantly due to the air exposure challenge in meagre; and it decreased during food deprivation in sea bream by a half (p < 0.05). In contrast, mucus protein only decreased significantly after pathogenic bacterial infection in sea bass. In addition, mucus lactate immediately reflected changes closely related to an anaerobic condition; whereas cortisol was only modified by air exposure, doubling its mucus concentration (p < 0.05). The data provided herein demonstrate that mucus metabolites can be considered as good non-invasive biomarkers for evaluating fish physiological responses; with the glucose/protein ratio being the most valuable and reliable parameter. Determining these skin mucus metabolites and ratios will be very useful when studying the condition of critically threatened species whose conservation status prohibits the killing of specimens.

Under control: The innate immunity of fish from the inhibitors' perspective

The innate immune response involves a concerted network of induced gene products, preformed immune effectors, biochemical signalling cascades and specialised cells. However, the multifaceted activation of these defensive measures can derail or overshoot and, if left unchecked, overwhelm the host. A plenty of regulatory devices therefore mediate the fragile equilibrium between pathogen defence and pathophysiological manifestations. Over the past decade in particular, an almost complete set of teleostean sequences orthologous to mammalian immunoregulatory factors has been identified in various fish species, which prove the remarkable conservation of innate immune-control concepts among vertebrates. This review will present the current knowledge on more than 50 teleostean regulatory factors (plus additional fish-specific paralogs) that are of paramount importance for controlling the clotting cascade, the complement system, pattern-recognition pathways and cytokine-signalling networks. A special focus lies on those immunoregulatory features that have emerged as potential biomarker genes in transcriptome-wide research studies. Moreover, we report on the latest progress in elucidating control elements that act directly with immune-gene-encoding nucleic acids, such as transcription factors, hormone receptors and micro- and long noncoding RNAs. Investigations into the function of teleostean inhibitory factors are still mainly based on gene-expression profiling or overexpression studies. However, in support of structural and in-vitro analyses, evidence from in-vivo trials is also available and revealed many biochemical details on piscine immune regulation. The presence of multiple gene copies in fish adds a degree of complexity, as it is so far hardly understood if they might play distinct roles during inflammation. The present review addresses this and other open questions that should be tackled by fish immunologists in future.

Under control: how a dietary additive can restore the gut microbiome and proteomic profile, and improve disease resilience in a marine teleostean fish fed vegetable diets

BACKGROUND

The constant increase of aquaculture production and wealthy seafood consumption has forced the industry to explore alternative and more sustainable raw aquafeed materials, and plant ingredients have been used to replace marine feedstuffs in many farmed fish. The objective of the present study was to assess whether plant-based diets can induce changes in the intestinal mucus proteome, gut autochthonous microbiota and disease susceptibility of fish, and whether these changes could be reversed by the addition of sodium butyrate to the diets. Three different trials were performed using the teleostean gilthead sea bream (Sparus aurata) as model. In a first preliminary short-term trial, fish were fed with the additive (0.8%) supplementing a basal diet with low vegetable inclusion (D1) and then challenged with a bacteria to detect possible effects on survival. In a second trial, fish were fed with diets with greater vegetable inclusion levels (D2, D3) and the long-term effect of sodium butyrate at a lower dose (0.4%) added to D3 (D4 diet) was tested on the intestinal proteome and microbiome. In a third trial, the long-term effectiveness of sodium butyrate (D4) to prevent disease outcome after an intestinal parasite (Enteromyxum leei) challenge was tested.

RESULTS

The results showed that opposed forces were driven by dietary plant ingredients and sodium butyrate supplementation in fish diet. On the one hand, vegetable diets induced high parasite infection levels that provoked drops in growth performance, decreased intestinal microbiota diversity, induced the dominance of the Photobacterium genus, as well as altered the gut mucosal proteome suggesting detrimental effects on intestinal function. On the other hand, butyrate addition slightly decreased cumulative mortality after bacterial challenge, avoided growth retardation in parasitized fish, increased intestinal microbiota diversity with a higher representation of butyrate-producing bacteria and reversed most vegetable diet-induced changes in the gut proteome.

CONCLUSIONS

This integrative work gives insights on the pleiotropic effects of a dietary additive on the restoration of intestinal homeostasis and disease resilience, using a multifaceted approach.

Amino acids as modulators of the European seabass, Dicentrarchus labrax, innate immune response: an in vitro approach

Teleost innate immune system is a most developed and powerful system in which fish highly rely throughout their lives. Conditions in aquaculture farms are particularly prone to disease, thus, health and welfare ensuring strategies are an urgent call to which nutrition is gradually becoming a most regarded achievement tool. This study intended to evaluate different amino acids' effect on immune-related mechanisms as well as their potential as enhancers of European seabass, Dicentrarchus labrax, leucocyte functioning. To achieve these goals, primary cultures of head-kidney leucocytes were established and kept in amino acid (glutamine, arginine, tryptophan or methionine) supplemented culture media in two doses. The effects of amino acids treatments were then evaluated after stimulation with either Vibrio anguillarum or Vibrio anguillarum lipopolysaccharides by measuring nitric oxide production, extracellular respiratory burst, ATP and arginase activities, and expression of immune-related genes. Glutamine, arginine and tryptophan showed to be particularly relevant regarding cell energy dynamics; arginine and tryptophan supplementation also resulted in down-regulation of important immune-related genes. Immune responses in cells treated with methionine were generally enhanced but further studies, particularly those of enzymes activity, are essential to complement gene expression results and to better understand this nutrient's immune role in fish.

Neuroendocrine and Immune Responses Undertake Different Fates following Tryptophan or Methionine Dietary Treatment: Tales from a Teleost Model

Methionine and tryptophan appear to be fundamental in specific cellular pathways involved in the immune response mechanisms, including stimulation of T-regulatory cells by tryptophan metabolites or pro-inflammatory effects upon methionine supplementation. Thus, the aim of this study was to evaluate the immunomodulatory effect of these amino acids on the inflammatory and neuroendocrine responses in juveniles of European seabass, Dicentrarchus labrax. To achieve this, goal fish were fed for 14 days methionine and tryptophan-supplemented diets (MET and TRP, respectively, 2× dietary requirement level) or a control diet meeting the amino acids requirement levels (CTRL). Fish were sampled for immune status assessment and the remaining fish were challenged with intraperitoneally injected inactivated Photobacterium damselae subsp. piscicida and sampled either 4 or 24 h post-injection. Respiratory burst activity, brain monoamines, plasma cortisol, and immune-related gene expression showed distinct and sometimes opposite patterns regarding the effects of dietary amino acids. While neuroendocrine intermediates were not affected by any dietary treatment at the end of the feeding trial, both supplemented diets led to increased levels of plasma cortisol after the inflammatory insult, while brain monoamine content was higher in TRP-fed fish. Peripheral blood respiratory burst was higher in TRP-fed fish injected with the bacteria inoculum but only compared to those fed MET. However, no changes were detected in total antioxidant capacity. Complement factor 3 was upregulated in MET-fed fish but methionine seemed to poorly affect other genes expression patterns. In contrast, fish fed MET showed increased immune cells numbers both before and after immune challenge, suggesting a strong enhancing effect of methionine on immune cells proliferation. Differently, tryptophan effects on inflammatory transcripts suggested an inhibitory mode of action. This, together with a high production of brain monoamine and cortisol levels, suggests that tryptophan might mediate regulatory mechanisms of neuroendocrine and immune systems cooperation. Overall, more studies are needed to ascertain the role of methionine and tryptophan in modulating (stimulate or regulate) fish immune and neuroendocrine responses.

Administration of a polyphenol-enriched feed to farmed sea bass (Dicentrarchus labrax L.): Kidney melanomacrophages response

The reinforcement of the defense mechanism of fish, through the administration of immunostimulants, is considered as a promising alternative to vaccines. Natural immunostimulants such as polyphenols, flavanoids, pigments and essential oils can modulate the innate immune response. In lower vertebrates, melano-macrophage centres, i.e. clusters of pigment-containing cells forming the extracutaneous pigment system, are wide-spread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. In the present study, we evaluated the effect of a polyphenol-enriched diet on the health status of European sea bass (Dicentrarchus labrax L.). Farmed sea bass were administered a feed containing a phytocomplex, rich in catechins and epigallocatechins, which was obtained from the seeds of Canosina Nero di Troia Vitis vinifera and mixed with conventional feed at two different concentrations. The effects of such a diet were investigated in juvenile and commercial size samples, i.e. undergoing a short- and long-term period of diet, respectively, focusing on their extracutaneous pigmentary system and, in more detail, on the enzymatic activities leading to melanin biosynthesis. Our results show that prolonged dietary treatments with higher concentration of polyphenols might modulate tyrosinase activity and gene expression in commercial size fishes. An increase of melano-macrophage activity is correlated to a stimulation of cytoprotective functions against antigenic stimulants and pathogens, as an expression of a robust and protective adaptive immune response.

The European seabass (Dicentrarchus labrax) innate immunity and gut health are modulated by dietary plant-protein inclusion and prebiotic supplementation

Inclusion of prebiotics in aqua feeds, though a costly strategy, has increased as a means to improve growth. Still, its effects on health improvement are not fully disclosed. Regarding their immunestimulatory properties, research has focused on carbohydrates such as fructooligosaccharides and xylooligosaccharides demonstrating their modulatory effects on immune defences in higher vertebrates but few studies have been done on their impact on fish immunity. Replacing fish meal (FM) by plant protein (PP) sources is a current practice in the aquaculture business but their content in antinutrients is still a drawback in terms of gut well-functioning. This work intends to evaluate the short-term effect (7 or 15 days feeding the experimental diets) on juvenile European seabass (Dicentrarchus labrax) immune status of dietary i) replacement of FM by PP sources; ii) prebiotics supplementation. Six isoproteic (46%) and isolipidic (15%) diets were tested including a FM control diet (FMCTRL), a PP control diet (PPCTRL, 30 FM:70 PP) and four other diets based on either FM or PP to which short-chain fructooligosaccharides (scFOS) or xylooligosaccharides (XOS) were added at 1% (FMFOS, PPFOS, FMXOS, PPXOS). The replacement of FM by PP in the diets induced nitric oxide (NO) and lysozyme production, while immunoglobulins (Ig), monocytes percentage and gut interleukin 10 (IL10) gene expression were inhibited. Dietary scFOS supplementation inhibited total bactericidal activity and neutrophils relative percentage regardless protein source and increased plasma NO and thrombocytes percentage in fish fed FM-based diets, while monocytes percentage was increased in PPFOS-fed fish. XOS supplementation down-regulated immune gene expression in the gut while it partly enhanced systemic response. Inconsistency among results regarding FM replacement by PP-based ingredients exposes the need for further research considering both local and systemic responses. Distinct outcomes of prebiotic supplementation were highlighted reflecting sight-specific effects with no clear interaction with protein source.

Beyond muscles: The untapped potential of creatine

Creatine is widely used by both elite and recreational athletes as an ergogenic aid to enhance anaerobic exercise performance. Older individuals also use creatine to prevent sarcopenia and, accordingly, may have therapeutic benefits for muscle wasting diseases. Although the effect of creatine on the musculoskeletal system has been extensively studied, less attention has been paid to its potential effects on other physiological systems. Because there is a significant pool of creatine in the brain, the utility of creatine supplementation has been examined in vitro as well as in vivo in both animal models of neurological disorders and in humans. While the data are preliminary, there is evidence to suggest that individuals with certain neurological conditions may benefit from exogenous creatine supplementation if treatment protocols can be optimized. A small number of studies that have examined the impact of creatine on the immune system have shown an alteration in soluble mediator production and the expression of molecules involved in recognizing infections, specifically toll-like receptors. Future investigations evaluating the total impact of creatine supplementation are required to better understand the benefits and risks of creatine use, particularly since there is increasing evidence that creatine may have a regulatory impact on the immune system.