Dietary arginine affects growth, gut morphology, oxidation resistance and immunity of hybrid grouper ( Epinephelus fuscoguttatus ♀× Epinephelus lanceolatus ♂) juveniles

Effects of essential amino acids supplementation in a low-protein diet on growth performance, intestinal health and microbiota of juvenile blotched snakehead (Channa maculata)

Developing a low-protein feed is important for the sustainable advancement of aquaculture. The aim of this study was to investigate the effects of essential amino acid (EAA) supplementation in a low-protein diet on the growth, intestinal health, and microbiota of the juvenile blotched snakehead, Channa maculata in an 8-week trial conducted in a recirculating aquaculture system. Three isoenergetic diets were formulated to include a control group (48.66 % crude protein (CP), HP), a low protein group (42.54 % CP, LP), and a low protein supplementation EAA group (44.44 % CP, LP-AA). The results showed that significantly lower weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER), and feed efficiency ratio (FER) were observed in fish that were fed LP than in the HP and LP-AA groups (P < 0.05). The HP and LP-AA groups exhibited a significant increase in intestinal villus length, villus width, and muscular thickness compared to the LP group (P < 0.05). Additionally, the HP and LP-AA groups demonstrated significantly higher levels of intestinal total antioxidant capacity (T-AOC), catalase (CAT), and superoxide dismutase (SOD) and lower levels of malondialdehyde (MDA) compared to the LP group (P < 0.05). The apoptosis rate of intestinal cells in the LP group was significantly higher than those in the LP and HP groups (P < 0.05). The mRNA expression levels of superoxide dismutase (sod), nuclear factor kappa B p65 subunit (nfκb-p65), heat shock protein 70 (hsp70), and inhibitor of NF-κBα (iκba) in the intestine were significantly higher in the LP group than those in the HP and LP-AA groups (P < 0.05). The 16s RNA analysis indicated that EAA supplementation significantly increased the growth of Desulfovibrio and altered the intestinal microflora. The relative abundances of Firmicutes and Cyanobacteria were positively correlated with antioxidant parameters (CAT and T-AOC), whereas Desulfobacterota was negatively correlated with sod and T-AOC. The genera Bacillus, Bacteroides, and Rothia were associated with the favorable maintenance of gut health. In conclusion, dietary supplementation with EAAs to achieve a balanced amino acid profile could potentially reduce the dietary protein levels from 48.66 % to 44.44 % without adversely affecting the growth and intestinal health of juvenile blotched snakeheads.

Effects of Antimicrobial Peptides on Antioxidant Properties, Non-specific Immune Response and Gut Microbes of Tsinling Lenok Trout (Brachymystax lenok tsinlingensis)

Antimicrobial peptides (AMPs) are an important part of non-specific immunity and play a key role in the cellular host defense against pathogens and tissue injury infections. We investigated the effects of AMP supplementation on the antioxidant capacity, non-specific immunity, and gut microbiota of tsinling lenok trout. 240 fish were fed diets (CT, A120, A240 and A480) containing different amounts of AMP peptides (0, 120 mg kg-1, 240 mg kg-1, 480 mg kg-1) for 8 weeks. Our results showed that the activity of total antioxidant capacity (T-SOD) and glutathione peroxidase (GSH-Px), lysozyme (LZM), catalase (CAT) and acid phosphatase (ACP) in the A240 and A480 group were higher than that in the CT group (P < 0.05). The content of malondialdehyde (MDA) in AMP group was significantly lower than that in CT group (P < 0.05). Furthermore, we harvested the mid-gut and applied next-generation sequencing of 16S rDNA. The results showed that the abundance of Halomonas in AMP group was significantly lower than that in CT group. Functional analysis showed that the abundance of chloroalkane and chloroalkene degradation pathway increased significantly in AMP group. In conclusion, AMP enhanced the antioxidant capacity, non-specific immunity, and intestinal health of tsinling lenok trout.

Glutamine improves growth and intestinal health in juvenile hybrid groupers fed high-dose glycinin

Glutamine addition can improve immunity and intestinal development in fish. This study examined the protective roles of glutamine on growth suppression and enteritis induced by glycinin in juvenile hybrid groupers (female Epinephelus fuscoguttatus × male Epinephelus lanceolatus). The experiment set four isonitrogenous and isolipidic trial diets: a diet containing 10% glycinin (11S), 10% of 11S diet supplemented with 1% or 2% alanine-glutamine (1% or 2% Ala-Gln), and a diet containing neither 11S nor Ala-Gln (FM). A feeding trial was conducted in hybrid grouper for 8 weeks. Weight gain and specific growth rates in Groups 1% and 2% Ala-Gln were significantly higher than those of the 11S group but were similar to those of the FM group. The intestinal muscular layer thickness, plica height and width of the 2% Ala-Gln group were significantly higher than those of Group 11S. The enterocyte proliferation efficiency of the 11S group was significantly lower compared to other groups. Compared with the 11S group, Groups 1% and 2% Ala-Gln fish had increased intestinal lysozyme activities, complement 3 and immunoglobulin M as well as cathelicidin contents. The mRNA levels of tnf-α, il-1β, ifn-α, and hsp70 genes were more downregulated in Groups 1% and 2% Ala-Gln than in Group 11S. Compared with FM group, fish from the 11S group had significantly lower mRNA levels of myd88, ikkβ, and nf-κb p65 genes. These three values in the 2% Ala-Gln group were significantly lower than those in Group 11S but not significantly different from those of Group FM. The relative abundance of Vibrio in Group 11S was higher than that in Groups FM and 2% Ala-Gln. Intestinal glutamine, glutaminase, glutamic acid, α-ketoglutarate, malate dehydrogenase and ATP contents were higher in Groups 1% and 2% Ala-Gln than in Group 11S. These results suggest that glutamine is a useful feed additive to enhance growth and intestinal immunity, alleviate inflammation, and modulate gut microbiota in hybrid grouper fed high-dose glycinin.

The Optimum Dietary Phenylalanine Requirement of Hybrid Grouper (Epinephelusfuscoguttatus ♀ × Epinepheluslanceolatus ♂) Juveniles: Effects on Growth Performance, Gut Micromorphology, and Antioxidation

The optimum phenylalanine (Phe) requirement for hybrid grouper (Epinephelusfuscoguttatus ♀ × Epinepheluslanceolatus ♂) juveniles was determined through an 8-week growth trial. A total of seven isoenergetic (340 kcal per 100 g of dry matter), isonitrogenous, and isolipidic diets were made, containing 8.2 (Phe 8.2), 9.2 (Phe 9.2), 10.1 (Phe 10.1), 11.2 (Phe 11.2), 13.3 (Phe 13.3), 15.2 (Phe 15.2), and 17.3 g/kg (Phe 17.3), respectively. Triplicate tanks of juvenile fish (about 16.7 g/fish) were fed each experimental diet twice daily until apparent satiation. The results indicated that different dietary Phe levels significantly influenced weight gain percentage (WG), feed efficiency (FE), protein efficiency ratio (PER), as well as, productive protein value (PPV). Fish fed Phe 8.2 had the lowest WG or PPV among all experimental treatments. Furthermore, the optimal dietary Phe level increased fold height, width, enterocyte, and microvillus height of fish. The Phe 10.1 group exhibited higher growth hormone (GH) expression in the pituitary compared to other groups. Expression of hepatic insulin-like growth factor-1 (IGF-1) and growth hormone receptor 1 (GHR1) displayed a similar pattern of variation to that of GH. The Phe 13.3 group had lower expression of S6 kinase 1 (S6K1) and target of rapamycin (TOR) than other groups. In addition, fish fed Phe 10.1 had lower levels of nuclear factor erythroid 2 (Nrf2) and heat shock protein 70 (HSP70) in the head kidney, and Cu/Zn-superoxide (Cu/ZnSOD) dismutases in the midgut compared to fish fed other Phe levels. Generally, optimal Phe content in the diet of hybrid grouper was estimated to be 12.7 g/kg of dry matter (27.3 g/kg of dietary protein), and at this level, the feed utilization, gut micromorphology, and immunity of fish were also elevated.

Effects of Dietary Protein-to-Energy Ratios on Growth, Immune Response, Antioxidative Capacity, Liver and Intestinal Histology, and Growth-Related Gene Expression in Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × Pelteobagrus vachelli ♂)

This study is aimed at evaluating the effects of dietary protein-to-energy ratios on the growth, immunological response, antioxidative capacity, liver and intestinal histology, and growth-related gene expression of hybrid yellow catfish (Pelteobagrus fulvidraco ♀ × Pelteobagrus vachelli ♂). Eight diets were formulated to form different protein/energy ratios of 84, 88, 90, 93, 95, 96, 99, and 103 mg/kcal (P/E84, P/E88, P/E90, P/E93, P/E95, P/E96, P/E99, and P/E103), respectively. These diets contain different levels of gross energy (GE), ranging from 4.13 to 4.76 kcal g^-1. Seven hundred and twenty healthy fish (17.15 ± 0.02 g) were randomly dispersed into 24 rectangular fiberglass tanks with 8 treatments in triplicate groups. The fish fed a P/E ratio of 95 mg/kcal demonstrated the best growth and feed utilization. A significant (P < 0.05) increase in percent weight gain (WG%) and specific growth rate (SGR) was seen as the dietary P/E ratio ameliorated from P/E84 to P/E95, followed by a decreased pattern in these parameters. Feed conversion ratio (FCR) and daily feed intake (DFI) were significantly impacted by dietary P/E ratios (P < 0.05). Additionally, an optimum P/E ratio improved intestinal morphology. However, low or high P/E ratio diets can cause oxidative stress, impaired liver function, and significantly reduced nonspecific immunity. The expression of target of rapamycin (TOR) and insulin-like growth factor-1 (IGF1) genes in the liver was considerably influenced by dietary protein-to-energy ratios (P < 0.05). Based on the statistical analysis of WG% against the dietary P/E ratio, the optimal P/E ratio for the studied species was estimated to be 92.92 mg/kcal.

Transcriptomic Analysis of Liver in Silver sillago, Sillago sihama Fed with High-Level Low-Gossypol Cottonseed Meal in Replacement of Fishmeal Diet

Understanding the molecular mechanisms involved in adaptation to alternate diets has become a significant concern, as increasing amounts of fishmeal (FM) protein in aquafeeds are being substituted with plant protein. Thus, the goal of this study was to assess growth performance, quality, and liver function of juvenile Sillago sihama (S. sihama) through growth indices, whole-body composition, histology of the liver, and RNA-sequencing (RNA-seq), after they were fed a formulated diet with 64% low-gossypol cottonseed meal (LCSM) for 56 days, compared to those fed a traditional FM-based diet. Indicators of growth, including final body weight (FBW), weight gain rate (WGR), specific growth rate (SGR), protein efficiency ratio (PER), and condition factor (CF), were considerably lower in the 64% LCSM (R64) group than in the FM diet group. In the R64 diet, the whole crude lipid was significantly lower than in the FM diet. The hematoxylin–eosin section showed that dietary high levels of LCSM resulted in diffuse lipid vacuolation in the liver of S. sihama. According to a liver transcriptome analysis, high LCSM intake in the diet significantly impacted lipid synthesis and catabolism, elevated pathways for cholesterol synthesis, blocked several amino acid metabolic pathways, and adversely affected hepatic gluconeogenesis and glycolysis. The findings of this study indicate that feeding high levels of LCSM in S. sihama is harmful to the growth of the organism and can harm the liver’s structural integrity, as well as obstruct the normal metabolism of amino acids, lipids, and carbohydrates. Therefore, it is not recommended to substitute LCSM for high levels of FM in the diet of S. sihama.

Dietary valine improved growth, immunity, enzymatic activities and expression of TOR signaling cascade genes in rainbow trout, Oncorhynchus mykiss fingerlings

This study was conducted to determine the effects of dietary valine (Val) on growth, hemato-biochemical parameters, immunity, enzymatic activities, antioxidant status and expression of target of rapamycin (TOR) and 4E-BP genes in rainbow trout, Oncorhynchus mykiss (1.57 ± 0.03 g; 5.10 ± 0.34 cm). Six isonitrogenous (450 g kg⁻¹) and isoenergetic (20.90 kJ 100 g⁻¹_, gross energy) diets were designed to represent varied Val levels (10.5, 13.0, 15.5, 18.0, 20.5 and 23.0 g kg⁻¹ dry diet basis). Growth parameters improved significantly (P < 0.05) with the amelioration of dietary Val level up to 18.0 g kg⁻¹. Highest (P < 0.05) body protein content was noted at 18.0 g kg⁻¹ dietary Val. Significant differences in hematological, intestinal enzymatic activities and antioxidant parameters were noted. However, plasma variables did not show any significant differences except aspartate transaminase and uric acid. Total protein content increased significantly, while the albumin and globulin content did not show any significant (P > 0.05) difference. Moreover expression of TOR mRNA and elF4E-binding protein (4E-BP) was observed higher (P < 0.05) at 18.0 g kg⁻¹ Val. On the basis of results, optimum dietary Val requirement for maximal growth of rainbow trout was determined to be 18.19 g kg⁻¹ of dry diet, corresponding to 40.42 g kg⁻¹ of dietary protein.

Arginine Regulates TOR Signaling Pathway through SLC38A9 in Abalone Haliotis discus hannai

Arginine plays an important role in the regulation of the target of the rapamycin (TOR) signaling pathway, and Solute Carrier Family 38 Member 9 (SLC38A9) was identified to participate in the amino acid-dependent activation of TOR in humans. However, the regulations of arginine on the TOR signaling pathway in abalone are still unclear. In this study, slc38a9 of abalone was cloned, and the slc38a9 was knocked down and overexpressed to explore its function in the regulation of the TOR signaling pathway. The results showed that knockdown of slc38a9 decreased the expression of tor, ribosomal s6 protein kinase (s6k) and eukaryotic translation initiation factor 4e (eif4e) and inhibited the activation of the TOR signaling pathway by arginine. Overexpression of slc38a9 up-regulated the expression of TOR-related genes. In addition, hemocytes of abalone were treated with 0, 0.2, 0.5, 1, 2 and 4 mmol/L of arginine, and abalones were fed diets with 1.17%, 1.68% and 3.43% of arginine, respectively, for 120 days. Supplementation of arginine (0.5–4 mmol/L) increased the expressions of slc38a9, tor, s6k and eif4e in hemocytes, and abalone fed with 1.68% of dietary arginine showed higher mRNA levels of slc38a9, tor, s6k and eif4e and phosphorylation levels of TOR, S6 and 4E-BP. In conclusion, the TOR signaling pathway of abalone can be regulated by arginine, and SLC38A9 plays an essential role in this regulation.

Effects of mixed antimicrobial peptide on the growth performance, antioxidant and immune responses and disease resistance of Pengze crucian carp (Carassius auratus var. Pengze)

Antimicrobial peptides have broad-spectrum antibacterial properties and low drug resistance, and they demonstrate great potential as antibiotic substitutes. In this study, five dietary mixed antimicrobial peptide supplement groups were set and fed to Pengze crucian carp for 10 weeks. The 6 groups were G0 (control group) and 5 additional groups: G1 (100 mg/kg), G2 (200 mg/kg), G3 (400 mg/kg), G4 (800 mg/kg) and G5 (1600 mg/kg). The results showed that the final body weight (FBW), weight gain rate (WGR) and specific growth rate (SGR) of fish in G1 and G2 were higher than those of fish in the control group, and G1 was significantly higher than G0 (P < 0.05). In addition, the FBW, WGR, and SGR of the G3 group were significantly lower than those of the G0 group. The chymotrypsin, lipase and amylase activities of G1 and G2 were significantly upregulated compared with G0 and reached peak values in G1. The activity of T-AOC and SOD in the addition group was higher (except G2 and G4) than that in the control groups, and significantly increased in G3 compared to the control group. The activity of MDA in the addition group was lower than that in the control group (p > 0.05). The expression levels of TLR-4, MYD88 and TNF-α in the three organs of the addition group were higher than those in G0 and reached the peak value in G3 (p < 0.05). Furthermore, the expression levels of TLR-4, MYD88 and TNF-α in the three organs of G3 were significantly lower than those in G0 and lower than those in the other supplemented groups. The expression levels of IL-10 and IL-11 tended to be upregulated after A. hydrophila challenge, and G3 in different organs was significantly higher than that in other supplemented groups and G0. The results of this study show that an appropriate amount of mixed antimicrobial peptides can improve the growth performance and antioxidant and immune capabilities of Pengze crucian carp and can also play a positive role in the treatment of A. hydrophila infection.

Comprehensive physiological and transcriptomic analysis revealing the responses of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) to the replacement of fish meal with soy protein concentrate

Plant proteins are suitable and alternative to fish meals (FMs), with less cost compared with that of all other types of fish feeds. In recent years, soy protein concentrate (SPC) has emerged as a cost-effective alternative to FM; however, little is known regarding the effects of dietary SPC on general fish physiology and well-being. This study aimed to perform comprehensive physiological and transcriptomic analysis for testing the applicability of SPC as fish feeds in hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) [SPC replaced 0% (CK), 30% (SPC30), and 75% (SPC75) of FM protein]. Generally, SPC30 promoted fish survival and had less effects on the phenotype, while SPC75 reduced fish survival, promoted inflammation, and regulated multiple physiological responses. Thousands of differentially expressed genes (DEGs) by SPC were identified in the intestine, liver, and muscle, which were enriched in biological regulation, cellular process, metabolic process, single-organism process, cell, cell part, membrane, binding, and catalytic activity based on RNA-seq. Notably, some DEGs involved in amino acid and lipid metabolism in the digestive system highlighted the modulatory effect of SPC on these metabolic processes, consistent with the physiological responses including enzyme activities. The enriched aspects of these predominant DEGs might be directly related to the different effects of SPC30 and SPC75 on fish growth, digestibility, and underlying enzyme activities and histology. In conclusion, the comprehensive physiological and transcriptomic comparative analysis of CK, SPC30, and SPC75 was also effective in testing the applicability of SPC as fish feeds and in designing a proper diet with the best impact on the growth performance and health of fish in hybrid grouper.

Preliminary study of mechanisms of intestinal inflammation induced by plant proteins in juvenile hybrid groupers (♀Epinephelus fuscoguttatus×♂E. lanceolatu)

Fish fed a high plant protein diet exhibit intestinal inflammation, the mechanism of which needs to be clarified. We preliminarily elucidate the mechanism of the TLRs/MyD88-PI₃K/Akt signalling pathway in intestinal inflammation induced by plant proteins. The diets contained 60% fish meal (FM, controls), or had 45% of the fish meal protein replaced by soybean meal (SBM), peanut meal (PM), cottonseed meal (CSM) or cottonseed protein concentrate (CPC). After an 8-week feeding trial, fish were challenged by injection of Vibrio parahaemolyticus bacteria for 7 days until the fish stabilized. The results showed that the specific growth rate (SGR) of the FM group was higher than other groups. The SGR of the CPC group was higher than those of the SBM, PM and CSM groups. The catalase (CAT) contents in the serum of fish fed a plant protein diet were higher than in FM fish. The abundances of Rhodobacteraceae and Microbacteriaceae in the MI (mid intestine) were higher in the CPC group. The TLR-2 expressions in the MI and DI of plant protein-fed fish were up-regulated. The expressions of IL-6 in the PI and MI, of hepcidin and TLR-3 in the MI, and of TLR-3 in the DI, were all lower than those of fish fed FM. In the PI, MI and DI, the protein expressions of P-PI₃K/T-PI₃K in the SBM and PM groups were higher than in the FM group. After the challenge, the cumulative mortalities in the FM and CPC groups were lower than those of the SBM, PM and CSM groups. These results suggested that plant protein diets reduced antioxidant capacity and glycolipid metabolism, hindered the development of the intestine and reduced intestinal flora diversity. TLR-3 is involved in the immune regulation of the PI in CPC group, MI and DI in SBM, PM, CSM and CPC groups, while might be involved in the immune regulation of the PI in SBM, PM and CSM groups. Furthermore, PI₃K/Akt signaling does not participate in the regulation of PI and MI in the CSM group, MI and DI in the CPC group.

Integrative Transcriptomic and Small RNA Sequencing Reveals Immune-Related miRNA-mRNA Regulation Network for Soybean Meal-Induced Enteritis in Hybrid Grouper, Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂

A 10-week feeding experiment was conducted to reveal the immune mechanism for soybean meal-induced enteritis (SBMIE) in hybrid grouper, Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂. Four isonitrogenous and isolipidic diets were formulated by replacing 0, 10, 30, and 50% fish meal protein with soybean meal (namely FM, SBM10, SBM30, and SBM50, respectively). The weight gain rate of the SBM50 group was significantly lower than those of the other groups. Plica height, muscular layer thickness, and goblet cells of the distal intestine in the SBM50 group were much lower than those in the FM group. The intestinal transcriptomic data, including the transcriptome and miRNAome, showed that a total of 6,390 differentially expressed genes (DEGs) and 92 DEmiRNAs were identified in the SBM50 and FM groups. DEmiRNAs (10 known and 1 novel miRNAs) and their DE target genes were involved in immune-related phagosome, natural killer cell-mediated cytotoxicity, Fc gamma R-mediated phagocytosis, and the intestinal immune network for IgA production pathways. Our study is the first to offer transcriptomic and small RNA profiling for SBMIE in hybrid grouper. Our findings offer important insights for the understanding of the RNA profile and further elucidation of the underlying molecular immune mechanism for SBMIE in carnivorous fish.

Dietary valine levels affect growth, protein utilisation, immunity and antioxidant status in juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

A 6-week growth trial was conducted to evaluate the influences of dietary valine (Val) levels on growth, protein utilisation, immunity, antioxidant status and gut micromorphology of juvenile hybrid groupers. Seven isoenergetic, isoproteic and isolipidic diets were formulated to contain graded Val levels (1·21, 1·32, 1·45, 1·58, 1·69, 1·82 and 1·94 %, DM basis). Each experimental diet was hand-fed to triplicate groups of twelve hybrid grouper juveniles. Results showed that weight gain percentage (WG%), protein productive value, protein efficiency ratio, and feed efficiency were increased as dietary Val level increased, reaching a peak value at 1·58 % dietary Val. The quadratic regression analysis of WG% against dietary Val levels indicated that the optimum dietary Val requirement for hybrid groupers was estimated to be 1·56 %. Gut micromorphology and expression of growth hormone in pituitary, insulin-like growth factor 1, target of rapamycin and S6 kinase 1 in liver were significantly affected by dietary Val levels. In serum, fish fed 1·58 % dietary Val had higher superoxide dismutase, catalase, lysozyme activities and IgM concentrations than fish fed other dietary Val levels. Fish fed 1·58 % dietary Val had higher expression of NF-E2-related factor 2 in head kidney than fish fed other dietary Val levels. Generally, the optimum dietary Val requirement for maximal growth of hybrid groupers was estimated to be 1·56 % of DM, corresponding to 3·16 % of dietary protein, and dietary Val levels affected growth, protein utilisation, immunity and antioxidant status in hybrid groupers.

dl-Methionine supplementation in a low-fishmeal diet affects the TOR/S6K pathway by stimulating ASCT2 amino acid transporter and insulin-like growth factor-I in the dorsal muscle of juvenile cobia (Rachycentron canadum)

An 8-week feeding experiment was conducted to investigate the effects of dl-methionine (Met) supplementation in a low-fishmeal diet on growth, key gene expressions of amino acid transporters and target of rapamycin (TOR) pathway in juvenile cobia, Rachycentron canadum. Seven isonitrogenous and isolipidic diets were formulated, containing 0·72, 0·90, 1·00, 1·24, 1·41, 1·63 and 1·86 % Met. Weight gain and specific growth rates increased gradually with Met levels of up to 1·24 % and then decreased gradually. In dorsal muscle, mRNA levels of ASCT2 in the 1·00 % Met group were significantly up-regulated compared with 0·72, 1·63, and 1·86 %. The insulin-like growth factor-I (IGF-I) mRNA levels in the dorsal muscle of fish fed 1·00 and 1·24 % Met were higher than those in fish fed other Met levels. In addition, fish fed 1·24 % Met showed the highest mRNA levels of TOR and phosphorylation of TOR on Ser2448. The phosphorylation of ribosomal p70-S6 kinase (S6K) on Ser371 in the dorsal muscle of fish fed 1·86 % Met was higher than those in the 0·72 % group. In conclusion, straight broken-line analysis of weight gain rate against dietary Met level indicates that the optimal Met requirement for juvenile cobia is 1·24 % (of DM, or 2·71 % dietary protein). Met supplementation in a low-fishmeal diet increased cobia growth via a mechanism that can partly be attributed to Met's ability to affect the TOR/S6K signalling pathway by enhancing ASCT2 and IGF-I transcription in cobia dorsal muscle.

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.