Dietary carbohydrate-to-protein ratio affects TOR signaling and metabolism-related gene expression in the liver and muscle of rainbow trout after a single meal

Dietary Protein Optimization for Growth and Immune Enhancement in Juvenile Hybrid Sturgeon (Acipenser baerii × A. schrenckii): Balancing Growth Performance, Serum Biochemistry, and Expression of Immune-Related Genes

This study aimed to evaluate the effects of dietary protein levels on growth performance, serum indices, body amino acid composition, and intestinal gene expression in juvenile hybrid sturgeon (Acipenser baerii × A. schrenckii). Hybrid sturgeons (initial weight 29.21 ± 2.04 g) were fed isolipidic diets containing 30%, 33%, 36%, 39%, 42% or 45% crude protein for 12 weeks (n = 18 tanks, 30 fish/tank). Results showed significant differences between treatments, where weight gain and protein efficiency ratio peaked optimally between 35.9% and 38.3% dietary protein. Serum parameters such as glucose, alanine aminotransferase, aspartate aminotransferase, superoxide dismutase, and lipid peroxidation levels varied significantly with changes in dietary protein levels. Specifically, the highest enzymatic activities and growth parameters were observed in groups fed with 33% to 39% protein, enhancing whole-body concentrations of lysine, leucine, phenylalanine, proline, and glutamic acid. Immune parameters such as immunoglobulin M and lysozyme activity also showed peak levels at higher protein concentrations, particularly notable at 42% for lysozyme and 36% for both component 3 and immunoglobulin M. Gene expression related to immune and growth pathways, including MyD88, TLR1, IL-8, IL-6, NF-κB, and IL1β, was significantly upregulated at protein levels of 33% to 36%, with a noted peak in expression at 39% for TLR1, IL-10, and TOR signaling genes, before diminishing at higher protein levels. Overall, the dietary protein requirement for juvenile hybrid sturgeon ranges from 35.9% to 38.3% crude protein.

Eicosapentaenoic acid (EPA) improves grass carp (Ctenopharyngodon idellus) muscle development and nutritive value by activating the mTOR signaling pathway

Skeletal muscle is the mainly edible part of fish. Eicosapentaenoic acid (EPA) is a crucial nutrient for fish. This study investigated the effect of EPA on the muscle development of grass carp along with the potential molecular mechanisms in vivo and in vitro. Muscle cells treated with 50 μM EPA in vitro showed the elevated proliferation, and the expression of mammalian target of rapamycin (mTOR) signaling pathway-related genes was upregulated (P < 0.05). In vivo experiments, 270 grass carp (27.92 g) were fed with one of the three experimental diets for 56 days: control diet (CN), 0.3% EPA-supplement diet (EPA), and the diet supplemented with 0.3% EPA and 30 mg/kg rapamycin (EPA + Rap). Fish weight gain rate (WGR) was improved in EPA group (P < 0.05). There was no difference in the viscerosomatic index (VSI) and body height (BH) among all groups (P > 0.05), whereas the carcass ratio (CR) and body length in the EPA group were obviously higher than those of other groups (P < 0.05), indicating that the increase of WGR was due to muscle growth. In addition, both muscle fiber density and muscle crude protein also increased in EPA group (P < 0.05). The principal component analysis showed that total weight of muscle amino acid in EPA group ranked first. Dietary EPA also increased protein levels of the total mTOR, S6k1, Myhc, Myog, and Myod in muscle (P < 0.05). In conclusion, EPA promoted the muscle development and nutritive value via activating the mTOR signaling pathway.

Evaluation of Fermented Soybean Meal to Replace a Portion Fish Meal on Growth Performance, Antioxidant Capacity, Immunity, and mTOR Signaling Pathway of Coho Salmon (Oncorhynchus kisutch)

In this study, we evaluated the effects of fermented soybean meal (FSBM) or/and unfermented SBM replacing a portion of fish meal (FM) on the growth performance, antioxidant capacity, immunity, and mechanistic target of rapamycin (mTOR) signaling pathway of juvenile coho salmon (Oncorhynchus kisutch). Four groups of juvenile coho salmon (initial weight 152.23 ± 3.21 g) in triplicate were fed for 12 weeks on four different iso-nitrogen and iso-lipid experimental diets: G0 diet (28% FM protein, control group), G1 diet (18% FM protein and 10% SBM protein), G2 diet (18% FM protein, 5% SBM protein, and 5% FSBM protein), and G3 diet (18% FM protein and 10% FSBM protein). The main results were compared with the G0 diet; the weight gain rate, specific growth rate, and condition factor of juveniles in G3 were increased significantly (p < 0.05). The content of muscle crude protein, the total protein, glucose, albumin, total cholesterol in serum, and the total antioxidant capacity in the liver of juveniles in G3 was increased significantly (p < 0.05). The activities of pepsin, trypsin, α-amylase, and lipase in the intestine, the superoxide dismutase, catalase, and alkaline phosphatase in the liver of juveniles in G3 were increased significantly (p < 0.05). The expression levels of phosphatidylinositide 3-kinases, serine/threonine kinase, mTOR, and ribosomal protein S6 kinase 1 genes in the liver of juveniles in G3 were upregulated significantly (p < 0.05). The feed coefficient ratio, viscerosomatic index, the contents of muscle moisture, and malondialdehyde in the liver of juveniles in G3 were decreased significantly (p < 0.05). The expression levels of tumor necrosis factor α, interleukin 1β, and interleukin 6 genes in the liver of juveniles in G3 were downregulated significantly (p < 0.05). However, there was no significant effect (p > 0.05) on the survival rate, food intake, and muscle crude lipid and ash of juveniles among the experimental groups. In conclusion, FSBM to replace a portion FM had a positive effect on the growth performance, protein deposition, antioxidant enzyme activity, digestive enzyme activity, protein synthesis, and immune-related genes of juvenile coho salmon.

Sequential Activations of ChREBP and SREBP1 Signals Regulate the High-Carbohydrate Diet-Induced Hepatic Lipid Deposition in Gibel Carp (Carassius gibelio)

The present study investigated the sequential regulation signals of high-carbohydrate diet (HCD)-induced hepatic lipid deposition in gibel carp (Carassius gibelio). Two isonitrogenous and isolipidic diets, containing 25% (normal carbohydrate diet, NCD) and 45% (HCD) corn starch, were formulated to feed gibel carp (14.82 ± 0.04 g) for 8 weeks. The experimental fish were sampled at 2^nd, 4^th, 6^th, and 8^th week. In HCD group, the hyperlipidemia and significant hepatic lipid deposition (oil red O area and triglyceride content) was found at 4^th, 6^th, and 8^th week, while the significant hyperglycemia was found at 2^nd, 4^th, and 8^th week, compared to NCD group (P < 0.05). HCD induced hepatic lipid deposition via increased hepatic lipogenesis (acc, fasn, and acly) but not decreased hepatic lipolysis (hsl and cpt1a). When compared with NCD group, HCD significantly elevated the hepatic sterol regulatory element binding proteins 1 (SREBP1) signals (positive hepatocytes and fluorescence intensity) at 4^th, 6^th, and 8^th week (P < 0.05). The hepatic SREBP1 signals increased from 2^nd to 6^th week, but decreased at 8^th week due to substantiated insulin resistance (plasma insulin levels, plasma glucose levels, and P-AKT^Ser473 levels) in HCD group. Importantly, the hepatic carbohydrate response element binding protein (ChREBP) signals (positive hepatocytes, fluorescence intensity, and expression levels) were all significantly elevated by HCD-induced glucose-6-phosphate (G6P) accumulation at 2^nd, 4^th, 6^th, and 8^th week (P < 0.05). Compared to 2^nd and 4^th week, the hepatic ChREBP signals and G6P contents was significantly increased by HCD at 6^th and 8^th week (P < 0.05). The HCD-induced G6P accumulation was caused by the significantly increased expression of hepatic gck, pklr, and glut2 (P < 0.05) but not 6pfk at 4^th, 6^th, and 8^th week, compared to NCD group. These results suggested that the HCD-induced hepatic lipid deposition was mainly promoted by SREBP1 in earlier stage and by ChREBP in later stage for gibel carp. This study revealed the sequential regulation pathways of the conversion from feed carbohydrate to body lipid in fish.

The mammalian insulin antagonist S961 does not exhibit insulin receptor antagonism in rainbow trout in vivo

Due to their reported 'glucose-intolerant' phenotype, rainbow trout have been the focus of comparative studies probing underlying endocrine mechanisms at the organismal, tissue and molecular level. A particular focus has been placed on the investigation of the comparative role of insulin, an important glucoregulatory hormone, and its interaction with macronutrients. A limiting factor in the comparative investigation of insulin is the current lack of reliable assays to quantify circulating mature and thus bioactive insulin. To circumvent this limitation, tissue-specific responsiveness to postprandial or exogenous insulin has been quantified at the level of post-translational modifications of cell signalling proteins. These studies revealed that the insulin responsiveness of these proteins and their post-translational modifications are evolutionarily highly conserved and thus provide useful and quantifiable proxy indices to investigate insulin function in rainbow trout. While the involvement of specific branches of the intracellular insulin signalling pathway (e.g., mTor) in rainbow trout glucoregulation have been successfully probed through pharmacological approaches, it would be useful to have a functionally validated insulin receptor antagonist to characterize the glucoregulatory role of the insulin receptor pathway in its entirety for this species. Here, we report two separate in vivo experiments to test the ability of the mammalian insulin receptor antagonist, S961, to efficiently block insulin signalling in liver and muscle in response to endogenously released insulin and to exogenously infused bovine insulin. We found that, irrespective of the experimental treatment or dose, activation of the insulin pathway in liver and muscle was not inhibited by S961, showing that its antagonistic effect does not extend to rainbow trout.

Transcriptome analysis provides insights into the molecular mechanism of liver inflammation and apoptosis in juvenile largemouth bass Micropterus salmoides fed low protein high starch diets

The present study was conducted to investigate the regulatory mechanism of liver injury in largemouth bass Micropterus salmoides (LMB) fed low protein high starch diets. Two isolipidic and isoenergetic diets were formulated with different protein and starch ratios, being named as diets P49S9 (48.8 % protein and 9.06 % starch) and P42S18 (42.4 % protein and 18.2 % starch). Each diet was fed to triplicate replicates of LMB (initial body weight, 4.65 ± 0.01 g) juveniles. Fish were fed to visual satiation for 8 weeks. The results indicated that though the P42S18 fish up-regulated the feeding ratio to meet their protein requirements, feeding efficiency ratio and growth performance were impaired in treatment P42S18 as compared to treatment P49S9. Periodic acid-Schiff (PAS) staining showed glycogen accumulated in the liver of LMB fed low protein high starch diets, and the reason should be attributed to down-regulated expression of the glycogenolytic glycogen debranching enzyme. Lower liver lipid level was associated with feeding low protein high starch diets in LMB, which should be resulted from the changes in hepatic glycerolipid metabolism regulated by lipoprotein lipase (representative of triglyceride synthesis, up-regulated) and diacylglycerol acyltransferase (representative of triglyceride breakdown, down-regulated). Though fasting plasma glucose level was comparable, treatment P42S18 performed inferior glucose tolerance to treatment P49S9. Hematoxylin-eosin (HE) and TdT-mediated dUTP Nick-End Labeling (TUNEL) staining suggested that feeding low protein high starch diets induced disruption of structural integrity, inflammation and apoptosis in the hepatocytes of LMB. As expected, KEGG pathways analysis indicated that many of the up-regulated differentially expressed genes were enriched in AGE (advanced glycation end product)/RAGE (receptor for AGE), Toll-like receptor and apoptosis signaling pathways. Our transcriptome data revealed that feeding low protein high starch diets might promote the accumulation of AGEs in LMB, which bound to RAGE and subsequently induced PI3K/Akt signal pathway. The activation of Akt induced NF-κB translocation into the nucleus thus releasing proinflammatory factors including tumor necrosis factor-α (TNF-α) and interleukin-8. The release of these inflammatory factors concomitantly induced T cell stimulation and natural killer cells chemotactic effects through Toll-like receptor signaling pathway. Besides mediating inflammation and immune response, TNF-α signal transduction participated in mediating apoptosis through the receptor of TNF (TNF-R1) pathway by up-regulating the expression of caspase 8 and cytochrome c. In conclusion, our results demonstrated that feeding low protein and high starch diets induced hepatocytes inflammation and apoptosis in LMB through the PI3K/Akt/NF-κB signaling pathway.

Sodium oxamate reduces lactate production to improve the glucose homeostasis of Micropterus salmoides fed high-carbohydrate diets

The study was performed to evaluate the effects of the reduced lactate production by sodium oxamate (SO) on growth performance, lactate and glucose and lipid metabolism, and glucose tolerance of Micropterus salmoides fed high-carbohydrate (CHO) diets. In in vitro study, primary hepatocytes were incubated for 48 h in a control medium (5.5 mM glucose), a high-glucose medium (25 mM glucose, HG), or a SO-containing high-glucose medium (25 mM glucose + 50 mM SO, HG-SO). Results indicated lactate and triglyceride (TG) levels, and lactate dehydrogenase a (LDH-a) expression in the HG-SO group were remarkably lower than those of the HG group. In in vivo study, M. salmoides (5.23 ± 0.03 g) were fed four diets containing a control diet (10% CHO, C) and three SO contents [0 (HC), 100 (HC-SO1), and 200 (HC-SO2) mg·kg^-1, respectively] of high-CHO diets (20% CHO) for 11 wk. High-CHO diets significantly reduced weight gain rate (WGR), specific growth rate (SGR), p-AMPK-to-t-AMPK ratio, and expression of insulin receptor substrate 1 (IRS1), insulin-like growth factor I (IGF-I), insulin-like growth factor I receptor (IGF-IR), fructose-1,6-biphosphatase (FBPase), peroxisome proliferator-activated receptor α (PPARα), and carnitine palmitoyl transferase 1α (CPT1α) compared with the C group, whereas the opposite was true for plasma levels of glucose, TG, lactate, tissue glycogen, and lipid contents, and expression of LDH-a, monocarboxylate transporter 1 and 4 (MCT1 and MCT4), insulin, glucokinase (GK), pyruvate dehydrogenase E1 subunit (PDH), sterol-regulatory element-binding protein 1 (SREBP1), fatty acid synthase (FAS). The HC-SO2 diets remarkably increased WGR, SGR, p-AMPK-to-t-AMPK ratio, and expression of IRS1, IGF-I, IGF-IR, GK, PDHα, PDHβ, FAS, acetyl-CoA carboxylase 1 (ACC1), PPARα, and CPT1α compared with the HC group. Besides, HC-SO2 diets also enhanced glucose tolerance of fish after a glucose loading. Overall, the reduced lactate production by SO benefits growth performance and glucose homeostasis of high-CHO-fed M. salmoides through the enhancement of glycolysis, lipogenesis, and fatty acid β-oxidation coupled with the suppression of glycogenesis and gluconeogenesis.

Effects of Dietary Carbohydrate Levels on Growth Performance, Body Composition, Antioxidant Capacity, Immunity, and Liver Morphology in Oncorhynchus mykiss under Cage Culture with Flowing Freshwater

The purpose of this study is to investigate the effects of dietary carbohydrate levels on growth performance, body composition, antioxidant capacity, immunity, and liver morphology in Oncorhynchus mykiss under cage culture with flowing freshwater. Fish (initial body weight 25.70 ± 0.24 g) were fed five isonitrogenous (420 g/kg protein) and isolipidic (150 g/kg lipid) diets containing 50.6, 102.1, 151.3, 200.9 and 251.8 g/kg carbohydrate levels, respectively. The results indicated that fish fed diets containing 50.6-200.9 g/kg carbohydrate showed significantly higher growth performance, feed utilization, and feed intake than those fed 251.8 g/kg dietary carbohydrate levels. Based on the analysis of the quadratic regression equation for weight gain rate, the appropriate dietary carbohydrate requirement of O. mykiss was estimated to be 126.2 g/kg. 251.8 g/kg carbohydrate level activated Nrf2-ARE signaling pathway, suppressed superoxide dismutase activity and total antioxidant capacity, and increased MDA content in the liver. Besides, fish fed a diet containing 251.8 g/kg carbohydrate showed a certain degree of hepatic sinus congestion and dilatation in the liver. Dietary 251.8 g/kg carbohydrate upregulated the mRNA transcription level of proinflammatory cytokines and downregulated the mRNA transcription level of lysozyme and complement 3. Whole-body compositions were not affected by dietary carbohydrate levels. In conclusion, 251.8 g/kg carbohydrate level suppressed the growth performance, antioxidant capacity and innate immunity, resulting in liver injury and inflammatory response of O. mykiss. A diet containing more than 200.9 g/kg carbohydrate is not efficiently utilized by O. mykiss under cage culture with flowing freshwater.

Dietary carbohydrate-to-protein ratio influences growth performance, hepatic health and dynamic of gut microbiota in atlantic salmon (Salmo salar)

Atlantic salmon (Salmo salar) fed a carbohydrate-rich diet exhibit suboptimal growth performance, along with other metabolic disturbances. It is well known that gut microbes play a pivotal role in influencing metabolism of the host, and these microbes can be modified by the diet. The main goal of the present study was to determine the effect of feeding graded levels of digestible carbohydrates to Atlantic salmon on the distal intestine digesta microbiota at 3 sampling times (i.e., weeks 4, 8 and 12), during a 12-week trial. A low carbohydrate-to-high protein diet (LC/HP, 0% wheat starch), a medium carbohydrate-to-medium protein diet (MC/MP, 15% wheat starch) or a high carbohydrate-to-low protein diet (HC/LP, 30% wheat starch) was fed to triplicate fish tanks (27 to 28 fish per tank). We performed an in-depth characterization of the distal intestine digesta microbiota. Further, growth parameters, liver histology and the expression of genes involved in hepatic neolipogenesis in fish were measured. Fish fed a HC/LP diet showed greater hepatosomatic and viscerosomatic indexes (P = 0.026 and P = 0.018, respectively), lower final weight (P = 0.005), weight gain (P = 0.003), feed efficiency (P = 0.033) and growth rate (P = 0.003) compared with fish fed the LC/HP diet. Further, feeding salmon a high digestible carbohydrate diet caused greater lipid vacuolization, steatosis index (P = 0.007) and expression of fatty acid synthase (fas) and delta-6 fatty acyl desaturase (d6fad) (P = 0.001 and P = 0.001, respectively) in the liver compared with fish fed the LC/HP diet. Although, the major impact of feeding a carbohydrate-rich diet to Atlantic salmon in beta diversity of distal intestine digesta microbiota was observed at week 4 (HC/LP vs MC/MP and HC/LP vs LC/HP; P = 0.007 and P = 0.008, respectively) and week 8 (HC/LP vs MC/MP; P = 0.04), no differences between experimental groups were detected after 12 weeks of feeding. Finally, at the end of the trial, there was a negative correlation between lactic acid bacteria (LAB) members, including Leuconostoc and Lactobacillus, with hepatic steatosis level, the hepatosomatic and viscerosomatic indexes as well as the expression of fas and d6fad. Weissella showed negative correlation with hepatic steatosis level and the hepatosomatic index. Finally, further research to explore the potential use of LAB as probiotics to improve liver health in carnivorous fish fed fatty liver-induced diet is warranted.

Dietary lysine level affects digestive enzyme, amino acid transport and hepatic intermediary metabolism in turbot (Scophthalmus maximus)

Lysine is one of the most important essential amino acids in fish, especially in the feed formulated with high levels of plant ingredients. Lysine restriction always led to growth inhibition and poor feed utilization. However, little information was available on its effects on digestion, absorption, and metabolism response in fish. In the present study, three experimental diets were formulated with three lysine levels, 1.69% (LL group), 3.32% (ML group), and 4.90% (HL group). A 10-week feeding trial was carried out to explore the effects of dietary lysine levels on the digestive enzymes, amino acid transporters, and hepatic intermediary metabolism in turbot (Scophthalmus maximus). As the results showed, the activities of lipase and trypsin in ML group were higher than in other groups. Lysine restriction inhibited the expression levels of peptides and amino acid transporters such as PpeT1, y⁺LAT2, b^0,+AT, and rBAT but significantly induced the expression of CAT1. Meanwhile, lysine deficiency elevated the content of T-CHO and LDL-C in plasma, while a higher HDL-C/LDL-C ratio was observed in ML group. For hepatic intermediary metabolism, the increase of lysine level induced the mRNA expression of G6Pase1 and FBPase, but no differences were observed in the expression of the key regulators in glycolysis pathway, such as GK and PK. Furthermore, an appropriate increase in the level of lysine promoted the genes involved in lipolysis, including PPARα, ACOX1, CPT1A, and LPL. However, no differences were observed in the expression of PPARγ, FAS, SREBP1, and LXR, which were important genes related to lipid synthesis. These results provide clues on the metabolic responses on dietary lysine in teleost.

Taurine alleviates endoplasmic reticulum stress, inflammatory cytokine expression and mitochondrial oxidative stress induced by high glucose in the muscle cells of olive flounder (Paralichthysolivaceus)

The aim of the present study was to evaluate the effects of taurine on endoplasmic reticulum stress, inflammatory cytokine expression and mitochondrial oxidative stress induced by high glucose in primary cultured muscle cells of olive flounder (Paralichthys olivaceus). Three experimental groups were designed as follows: muscle cells of olive flounder incubated with three kinds of medium containing 5 mM glucose (control), 33 mM glucose (HG) or 33 mM glucose + 10 mM taurine (HG + T), respectively. Results showed that taurine addition significantly alleviated the decreased activity of superoxide dismutase (SOD) and the ratio of reduced to oxidized glutathione (GSH/GSSG) induced by high glucose. The increase of cellular reactive oxygen species (ROS), malondialdehyde content and cell apoptosis induced by high glucose were alleviated by taurine. Besides, gene expression of glucose-regulated protein 78, PKR-like ER kinase, tumor necrosis factor-α, interleukin-6, interleukin-1β, interleukin-8, muscle atrophy F-box protein and muscle RING-finger protein 1 were significantly up-regulated in the HG group, and taurine addition decreased the expression of these genes. High glucose led to the swelling of the endoplasmic reticulum (ER). Meanwhile, the nuclear translocation of nuclear factor κB (NF-κB) and the release of cytochrome C from mitochondria induced by high glucose were suppressed by taurine addition. These results demonstrated that taurine alleviated ERS, inflammation and mitochondrial oxidative stress induced by high glucose in olive flounder muscle cells. The ROS production, NF-κB signaling pathway and mitochondria function were the main targets of the biological effects of taurine under high glucose condition.

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.

Fine-Tuning of Postprandial Responses via Feeding Frequency and Leucine Supplementation Affects Dietary Performance in Turbot (Scophthalmus maximus L.)

BACKGROUND

Feeding-induced cell signaling and metabolic responses affect utilization of dietary nutrients but are rarely taken advantage of to improve animal nutrition.

OBJECTIVES

We hypothesized that by modulating postprandial kinetics and signaling, improved dietary utilization and growth performance could be achieved in animals.

METHODS

Juvenile turbot (Scophthalmus maximus L.) with an initial mean ± SD weight of 10.1 ± 0.01 g were used. Two feeding frequencies (FFs), either 1 or 3 meals/d at a fixed 2.4% daily body weight ration, and 2 diets that were or were not supplemented with 1% crystalline leucine (Leu), were used in the 10-wk feeding trial. At the end of the trial, a 1-d force-feeding experiment was conducted using the aforementioned FF and experimental diets. Samples were collected for the analysis of postprandial kinetics of aminoacidemia, mechanistic target of rapamycin (mTOR) signaling activities, protein deposition, as well as the mRNA expression levels of key metabolic checkpoints at consecutive time points after feeding.

RESULTS

Increased FF and leucine supplementation significantly enhanced fish growth by 7.68% ± 0.53% (means ±SD) and 7.89% ± 1.25%, respectively, and protein retention by 4.01% ± 0.59% and 4.44% ± 1.63%, respectively, in feeding trial experiments. The durations of postprandial aminoacidemia and mTOR activation were extended by increased FF, whereas leucine supplementation enhanced mTOR signaling without influencing the postprandial free amino acids kinetics. Increased FF and leucine supplementation enhanced muscle protein deposition 21.6% ± 6.85% and 22.3% ± 1.52%, respectively, in a 24-h postfeeding period.

CONCLUSIONS

We provided comprehensive characterization of the postprandial kinetics of nutrient sensing and metabolic responses under different feeding regimens and leucine supplementation in turbot. Fine-tuning of postprandial kinetics could provide a new direction for better dietary utilization and animal performances in aquaculture.

Metabolic responses of Chinese perch (Siniperca chuatsi) to different levels of dietary carbohydrate

There are great differences in metabolic responses to different levels of carbohydrate among different carnivorous fish species. To explore metabolic responses of Chinese perch to moderate and high level of dietary carbohydrates, three diets containing 7.3% (LC), 17.5% (MC), and 27.5% (HC) of carbohydrates were provided to Chinese perch for 56 days. The results showed that MC and HC groups exhibited an increase in weight gain (WG) and hepatic glycogen content, and a decrease in feed conversion efficiency, compared with the LC group. The MC and HC groups also showed the increase in mRNA levels of phosphofructokinase and citrate synthase related to the aerobic oxidation pathway, which might be responsible for the increase in WG. Moreover, compared with the LC group, the HC group exhibited high levels of plasma indices (glucose, pyruvic acid, lactic acid, total triglyceride, total cholesterol, and low-density lipoprotein) and liver lipid resulting from the increased mRNA levels of fatty acid synthesis-related genes (ATP citrate lyase, acetyl-CoA carboxylase α, and fatty acid synthase), low level of crude protein caused by inhibition of TOR pathway, and liver damage induced by low antioxidant capacity and infiltration of inflammatory cells, but the MC group did not. The above results indicated that 17.5% dietary carbohydrate might be utilized effectively in Chinese perch and part carbohydrates were converted into glycogen to maintain glucose homeostasis; 27.5% dietary carbohydrate could not be fully utilized. The 27.5% carbohydrate diet induced the up-regulation of aerobic oxidation, glycogen synthesis, and fat synthesis pathways which might not be sufficient to maintain glucose homeostasis.

Integration of Nutrient Sensing in Fish Hypothalamus

The knowledge regarding hypothalamic integration of metabolic and endocrine signaling resulting in regulation of food intake is scarce in fish. Available studies pointed to a network in which the activation of the nutrient-sensing (glucose, fatty acid, and amino acid) systems would result in AMP-activated protein kinase (AMPK) inhibition and activation of protein kinase B (Akt) and mechanistic target of rapamycin (mTOR). Changes in these signaling pathways would control phosphorylation of transcription factors cAMP response-element binding protein (CREB), forkhead box01 (FoxO1), and brain homeobox transcription factor (BSX) leading to food intake inhibition through changes in the expression of neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opio melanocortin (POMC), and cocaine and amphetamine-related transcript (CART). The present mini-review summarizes information on the topic and identifies gaps for future research.

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.

Effect of dietary selenium on postprandial protein deposition in the muscle of juvenile rainbow trout (Oncorhynchus mykiss)

Se, an essential biological trace element, is required for fish growth. However, the underlying mechanisms remain unclear. Protein deposition in muscle is an important determinant for fish growth. This study was conducted on juvenile rainbow trout (Oncorhynchus mykiss) to explore the nutritional effects of Se on protein deposition in fish muscle by analysing the postprandial dynamics of both protein synthesis and protein degradation. Trout were fed a basal diet supplemented with or without 4 mg/kg Se (as Se yeast), which has been previously demonstrated as the optimal supplemental level for rainbow trout growth. After 6 weeks of feeding, dietary Se supplementation exerted no influence on fish feed intake, whereas it increased fish growth rate, feed efficiency, protein retention rate and muscle protein content. Results of postprandial dynamics (within 24 h after feeding) of protein synthesis and degradation in trout muscle showed that dietary Se supplementation led to a persistently hyperactivated target of rapamycin complex 1 pathway and the suppressive expression of numerous genes related to the ubiquitin-proteasome system and the autophagy-lysosome system after the feeding. However, the ubiquitinated proteins and microtubule-associated light chain 3B (LC3)-II:LC3-I ratio, biomarkers for ubiquitination and autophagy activities, respectively, exhibited no significant differences among the fish fed different experimental diets throughout the whole postprandial period. Overall, this study demonstrated a promoting effect of nutritional level of dietary Se on protein deposition in fish muscle by accelerating postprandial protein synthesis. These results provide important insights about the regulatory role of dietary Se in fish growth.

Cholesterol metabolism regulation mediated by SREBP-2, LXRα and miR-33a in rainbow trout (Oncorhynchus mykiss) both in vivo and in vitro

Cholesterol metabolism is greatly affected in fish fed plant-based diet. The regulation of cholesterol metabolism is mediated by both transcriptional factors such as sterol regulatory element-binding proteins (SREBPs) and liver X receptors (LXRs), and posttranscriptional factors including miRNAs. In mammals, SREBP-2 and LXRα are involved in the transcriptional regulation of cholesterol synthesis and elimination, respectively. In mammals, miR-33a is reported to directly target genes involved in cholesterol catabolism. The present study aims to investigate the regulation of cholesterol metabolism by SREBP-2 and LXRα and miR-33a in rainbow trout using in vivo and in vitro approaches. In vivo, juvenile rainbow trout of ~72 g initial body weight were fed a total plant-based diet (V) or a marine diet (M) containing fishmeal and fish oil. In vitro, primary cell culture hepatocytes were stimulated by graded concentrations of 25-hydroxycholesterol (25-HC). The hepatic expression of cholesterol synthetic genes, srebp-2 and miR-33a as well as miR-33a level in plasma were increased in fish fed the plant-based diet, reversely, their expression in hepatocytes were inhibited with the increasing 25-HC in vitro. However, lxrα was not affected neither in vivo nor in vitro. Our results suggest that SREBP-2 and miR-33a synergistically enhance the expression of cholesterol synthetic genes but do not support the involvement of LXRα in the regulation of cholesterol elimination. As plasma level of miR-33a appears as potential indicator of cholesterol synthetic capacities, this study also highlights circulating miRNAs as promising noninvasive biomarker in aquaculture.

Nutritional regulation of glucose metabolism-related genes in the emerging teleost model Mexican tetra surface fish: a first exploration

Astyanax mexicanus has gained importance as a laboratory model organism for evolutionary biology. However, little is known about its intermediary metabolism, and feeding regimes remain variable between laboratories holding this species. We thus aimed to evaluate the intermediary metabolism response to nutritional status and to low (NC) or high (HC) carbohydrate diets in various organs of the surface-dwelling form of the species. As expected, glycaemia increased after feeding. Fish fed the HC diet had higher glycaemia than fish fed the NC diet, but without displaying hyperglycaemia, suggesting that carbohydrates are efficiently used as an energy source. At molecular level, only fasn (Fatty Acid Synthase) transcripts increased in tissues after refeeding, suggesting an activation of lipogenesis. On the other hand, we monitored only moderate changes in glucose-related transcripts. Most changes observed were related to the nutritional status, but not to the NC versus HC diet. Such a metabolic pattern is suggestive of an omnivorous-related metabolism, and this species, at least at adult stage, may adapt to a fish meal-substituted diet with high carbohydrate content and low protein supply. Investigation to identify molecular actors explaining the efficient use of such a diet should be pursued to deepen our knowledge on this species.

Glucose regulates protein turnover and growth-related mechanisms in rainbow trout myogenic precursor cells

Rainbow trout are considered glucose intolerant because they are poor utilizers of glucose, despite having functional insulin receptors and glucose transporters. Following high carbohydrate meals, rainbow trout are persistently hyperglycemic, which is likely due to low glucose utilization in peripheral tissues including the muscle. Also, rainbow trout myogenic precursor cells (MPCs) treated in vitro with insulin and IGF1 increase glucose uptake and protein synthesis, whereas protein degradation is decreased. Given our understanding of glucose regulation in trout, we sought to understand how glucose concentrations affect protein synthesis, protein degradation; and expression of genes associated with muscle growth and proteolysis in MPCs. We found that following 24 h and 48 h of treatment with low glucose media (5.6 mM), myoblasts had significant decreases in protein synthesis. Also, low glucose treatments affected the expression of both mstn2a and igfbp5. These findings support that glucose is a direct regulator of protein synthesis and growth-related mechanisms in rainbow trout muscle.

Effects of CCK-8 and GLP-1 on fatty acid sensing and food intake regulation in trout

We hypothesize that cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1) are involved in the modulation of metabolic regulation of food intake by fatty acids in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with 1 ng/g of CCK-8 and with 2 ng/g of GLP-1 on food intake, expression of neuropeptides involved in food intake control and the activity of fatty acid-sensing systems in hypothalamus and hindbrain. Food intake decreased up to 24 h post-treatment to 49.8-72.3% and 3.1-17.8% for CCK-8 and GLP-1, respectively. These anorectic responses are associated with changes in fatty acid metabolism and an activation of fatty acid-sensing mechanisms in the hypothalamus and hindbrain. These changes occurred in parallel with those in the expression of anorexigenic and orexigenic peptides. Moreover, we observed that the activation of fatty acid sensing and the enhanced anorectic potential elicited by CCK-8 and GLP-1 treatments occurred in parallel with the activation of mTOR and FoxO1 and the inhibition of AMPKα, BSX and CREB. The results are discussed in the context of metabolic regulation of food intake in fish.

Autophagy-related genes in rainbow trout Oncorhynchus mykiss (Walbaum) gill epithelial cells and their role in nutrient restriction

Autophagy is primarily an adaptive response to provide nutrients and energy following exposure to stress and starvation but can also regulate muscle mass and impact infectious disease susceptibility. Expression of 10 autophagy-related (Atg) genes in rainbow trout was monitored throughout the autophagosome formation cycle. The Atg gene sequences of rainbow trout were compared to other species to identify highly conserved regions and to generate primers. Phylogeny trees created with rainbow trout and 14 other species demonstrate that rainbow trout Atg gene sequences have greatest similarity to Atlantic salmon and other fish species. RTgill-W1 cells were subjected to nutrient restriction and compared to cells in normal nutrient conditions using quantitative reverse transcriptase polymerase chain reaction to assess changes in Atg gene expression. Nutrient restriction had a direct impact on Atg gene expression, with atg4, atg9, atg12, lc3, gabarap and becn1 undergoing the greatest differential expression (p < 0.05), most dramatically on Day 3. This was corroborated by Western blot detection of LC3, which also showed a peak of autophagy activity at Day 3 post-nutrient restriction. Atg gene expression revealed autophagy flux in RTgill-W1, as well as, those genes that were most significantly altered by nutrient restriction.

Metformin improves the glucose homeostasis of Wuchang bream fed high-carbohydrate diets: a dynamic study

After a 12-week feeding trial, the glucose tolerance test was performed in Megalobrama amblycephala to evaluate the effects of metformin on the metabolic responses of glycolipids. Plasma insulin peaked at 2 h, then decreased to the basal value at 8-12 h post-injection. Plasma triglyceride levels and liver glycogen contents of the control group was decreased significantly during the first 2 and 1 h, respectively. Then, they returned to basal values at 12 h. During the whole sampling period, the high-carbohydrate groups had significantly higher levels of plasma metabolites and liver glycogen than those of the control group, and metformin supplementation enhanced these changes (except insulin levels). Glucose administration lowered the transcriptions of ampk α1, ampk α2, pepck, g6pase, fbpase, cpt IA and aco, the phosphorylation of Ampk α and the activities of the gluconeogenic enzymes during the first 2-4 h, while the opposite was true of glut 2, gs, gk, pk, accα and fas. High-carbohydrate diets significantly increased the transcriptions of ampk α1, ampk α2, glut 2, gs, gk, pk, accα and fas, the phosphorylation of Ampk α and the activities of the glycolytic enzymes during the whole sampling period, while the opposite was true for the remaining indicators. Furthermore, metformin significantly upregulated the aforementioned indicators (except accα and fas) and the transcriptions of cpt IA and aco. Overall, metformin benefits the glucose homeostasis of Megalobrama amblycephala fed high-carbohydrate diets through the activation of Ampk and the stimulation of glycolysis, glycogenesis and fatty acid oxidation, while depressing gluconeogenesis and lipogenesis.

Growth Performance, Digestive Enzymes, and TOR Signaling Pathway of Litopenaeus vannamei Are Not Significantly Affected by Dietary Protein Hydrolysates in Practical Conditions

Protein hydrolysates have been widely reported as the protein source of aquatic feed. However, previous studies on protein hydrolysates focused on fish under experimental conditions. In this study, a 6-week feeding trial in a greenhouse was conducted to investigate the effects of partially replacing fishmeal by protein hydrolysates on growth performance, digestive enzymes, and TOR signaling pathway of Litopenaeus vannamei under practical conditions. This involved randomly selecting 72,000 shrimps (initial body weight 2.26 ± 0.02 g) and placing them in groups inside nine concrete tanks (L 5 m × W 5 m × H 1 m) (3 treatments × 3 replicates × 8000 individuals per concrete tank). Two isonitrogenous (430 g kg^-1) and isolipidic (80 g kg^-1) diets were prepared: fishmeal diet (FM) containing 400g kg^-1 fishmeal, and protein hydrolysates diet (PH) in which 15% of the fishmeal was replaced by protein hydrolysates. A commercial diet (CD) was used as reference. The final weight (FW), percent weight gain (PWG), specific growth ratio (SGR), and total weight for each tank (TW) of L. vannamei fed with FM and PH diets were not significantly different (P > 0.05). However, shrimp fed with PH diet had significantly higher FW, PWG, SGR, and TW values than those fed with CD diet (P < 0.05). Trypsin activity was significantly lower in shrimp fed with CD diet, than in shrimp fed with FM and PH diets (P < 0.05). However, trypsin activity of L. vannamei fed with FM and PH diets were not significantly different (P > 0.05). The mRNA expression of tor, s6k, and 4e-bp genes were not significantly affected between FM and PH diets, while tor and s6k expression levels of CD diet were significantly down-regulated. Based on the above data, 15% replacement of fishmeal with protein hydrolysates did not make any difference on shrimps compared with FM group. Therefore, protein hydrolysates can partially replace fishmeal as the protein source of shrimp formula feed in practical conditions.

Hepatic glucose metabolic responses to digestible dietary carbohydrates in two isogenic lines of rainbow trout

Rainbow trout (Oncorhynchus mykiss) was recognized as a typical ‘glucose-intolerant’ fish and poor dietary carbohydrate user. Our first objective was to test the effect of dietary carbohydrates themselves (without modification of dietary protein intake) on hepatic glucose gene expression (taking into account the paralogs). The second aim was to research if two isogenic trout lines had different responses to carbohydrate intake, showing one with a better use dietary carbohydrates. Thus, we used two isogenic lines of rainbow trout (named A32h and AB1h) fed with either a high carbohydrate diet or a low carbohydrate diet for 12 weeks. We analysed the zootechnical parameters, the plasma metabolites, the hepatic glucose metabolism at the molecular level and the hormonal-nutrient sensing pathway. Globally, dietary carbohydrate intake was associated with hyperglycaemia and down regulation of the energy sensor Ampk, but also with atypical regulation of glycolysis and gluconeogenesis in the liver. Indeed, the first steps of glycolysis and gluconeogenesis catalysed by the glucokinase and the phospenolpyruvate carboxykinase are regulated at the molecular level by dietary carbohydrates as expected (i.e. induction of the glycolytic gck and repression of the gluconeogenic pck); by contrast, and surprisingly, for two other key glycolytic enzymes (phosphofructokinase enzyme – pfkl and pyruvate kinase – pk) some of the paralogs (pfklb and pklr) are inhibited by carbohydrates whereas some of the genes coding gluconeogenic enzymes (the glucose-6-phosphatase enzyme g6pcb1b and g6pcb2a gene and the fructose1-6 biphosphatase paralog fbp1a) are induced. On the other hand, some differences for the zootechnical parameters and metabolic genes were also found between the two isogenic lines, confirming the existence of genetic polymorphisms for nutritional regulation of intermediary metabolism in rainbow trout. In conclusion, our study determines some new and unexpected molecular regulations of the glucose metabolism in rainbow trout which may partly lead to the poor utilization of dietary carbohydrates and it underlines the existence of differences in molecular regulation of glucose metabolism between two isogenic lines which provides arguments for future selection of rainbow trout.

Summary: Using isogenic lines, this study determines some new, unexpected molecular regulation of the glucose metabolism in rainbow trout, which may partly lead to the poor utilization of dietary carbohydrates.

Evidence for the presence in rainbow trout brain of amino acid-sensing systems involved in the control of food intake

To assess the hypothesis of central amino acid-sensing systems involved in the control of food intake in fish, we carried out two experiments in rainbow trout. In the first one, we injected intracerebroventricularly two different branched-chain amino acids (BCAAs), leucine and valine, and assessed food intake up to 48 h later. Leucine decreased and valine increased food intake. In a second experiment, 6 h after similar intracerebroventricular treatment we determined changes in parameters related to putative amino acid-sensing systems. Different areas of rainbow trout brain present amino acid-sensing systems responding to leucine (hypothalamus and telencephalon) and valine (telencephalon), while other areas (midbrain and hindbrain) do not respond to these treatments. The decreased food intake observed in fish treated intracerebroventricularly with leucine could relate to changes in mRNA abundance of hypothalamic neuropeptides [proopiomelanocortin (POMC), cocaine- and amphetamine-related transcript (CART), neuropeptide Y (NPY), and agouti-related peptide (AgRP)]. These in turn could relate to amino acid-sensing systems present in the same area, related to BCAA and glutamine metabolism, as well as mechanistic target of rapamycin (mTOR), taste receptors, and general control nonderepressible 2 (GCN2) kinase signaling. The treatment with valine did not affect amino acid-sensing parameters in the hypothalamus. These responses are comparable to those characterized in mammals. However, clear differences arise when comparing rainbow trout and mammals, in particular with respect to the clear orexigenic effect of valine, which could relate to the finding that valine partially stimulated two amino acid-sensing systems in the telencephalon. Another novel result is the clear effect of leucine on telencephalon, in which amino acid-sensing systems, but not neuropeptides, were activated as in the hypothalamus.

A reassessment of the carnivorous status of salmonids: Hepatic glucokinase is expressed in wild fish in Kerguelen Islands

Salmonids belong to a high trophic level and are thus considered as strictly carnivorous species, metabolically adapted for high catabolism of proteins and low utilisation of dietary carbohydrates. However they conserved a "mammalian-type" nutritional regulation of glucokinase encoding gene and its enzymatic activity by dietary carbohydrates which remains puzzling regarding their dietary regime. The present study investigates the hypothesis that this conservation could be linked to a real consumption by trout of this nutrient in their natural habitat. To do so, brown trout were sampled in the sub-Antarctic Kerguelen Islands, a site presenting oligotrophic hydrosystems and no local freshwater fish fauna prior the introduction of salmonids fifty years ago. Qualitative and quantitative analysis of carbohydrate content within Kerguelen trout stomachs demonstrate that these animals are fed on food resources containing digestible carbohydrates. Additionally, glycaemia and more particularly gck mRNA level and gck enzymatic activity prove that Kerguelen trout digest and metabolise dietary carbohydrates. Physiological and molecular analyses performed in the present study thus strongly evidence for consumption of dietary carbohydrates by wild trout in natural environments. Investigating differences between Kerguelen individuals, we found that smaller individuals presented higher glycaemia, as well as higher carbohydrates contents in stomach. However no relationship between scaled mass index and any physiological indicator was found. Thus it appears that Kerguelen trout do not turn to carbohydrate diet because of a different condition index, or that the consumption of carbohydrates does not lead to a generally degraded physiological status. As a conclusion, our findings may explain the evolutionary conservation of a "mammalian-type" nutritional regulation of gck by dietary carbohydrates in these carnivorous fish.

The Mitotic and Metabolic Effects of Phosphatidic Acid in the Primary Muscle Cells of Turbot (Scophthalmus maximus)

Searching for nutraceuticals and understanding the underlying mechanism that promote fish growth is at high demand for aquaculture industry. In this study, the modulatory effects of soy phosphatidic acids (PA) on cell proliferation, nutrient sensing, and metabolic pathways were systematically examined in primary muscle cells of turbot (Scophthalmus maximus). PA was found to stimulate cell proliferation and promote G1/S phase transition through activation of target of rapamycin signaling pathway. The expression of myogenic regulatory factors, including myoD and follistatin, was upregulated, while that of myogenin and myostatin was downregulated by PA. Furthermore, PA increased intracellular free amino acid levels and enhanced protein synthesis, lipogenesis, and glycolysis, while suppressed amino acid degradation and lipolysis. PA also was found to increased cellular energy production through stimulated tricarboxylic acid cycle and oxidative phosphorylation. Our results identified PA as a potential nutraceutical that stimulates muscle cell proliferation and anabolism in fish.

Hypothalamic mechanisms linking fatty acid sensing and food intake regulation in rainbow trout

We assessed in rainbow trout hypothalamus the effects of oleate and octanoate on levels and phosphorylation status of two transcription factors, FoxO1 and CREB, possibly involved in linking activation of fatty acid sensing with modulation of food intake through the expression of brain neuropeptides. Moreover, we assessed changes in the phosphorylation status of three proteins possibly involved in modulation of these transcription factors such as Akt, AMPK and mTOR. In a first experiment, we evaluated, in pools of hypothalamus incubated for 3 h and 6 h at 15°C in a modified Hanks' medium containing 100 or 500 µM oleate or octanoate, the response of fatty acid sensing, neuropeptide expression and phosphorylation status of proteins of interest. The activation of fatty acid sensing and enhanced anorectic potential occurred in parallel with the activation of Akt and mTOR, and the inhibition of AMPK. The changes in these proteins would relate to a neuropeptide expression through changes in the phosphorylation status of transcription factors under their control, such as CREB and FoxO1, which displayed inhibitory (CREB) or activatory (FoxO1) responses when tissues were incubated with oleate or octanoate. In a second experiment, we incubated hypothalamus for 6 h with 500 µM oleate or octanoate alone or in the presence of specific inhibitors of Akt, AMPK, mTOR, CREB or FoxO1. The presence of inhibitors counteracted the effects of oleate or octanoate on the phosphorylation status of the proteins of interest. The results support, for the first time in fish, the involvement of these proteins in the regulation of food intake by fatty acids.

Dietary arginine affects the insulin signaling pathway, glucose metabolism and lipogenesis in juvenile blunt snout bream Megalobrama amblycephala

This study evaluated the mechanisms governing insulin resistance, glucose metabolism and lipogenesis in juvenile fish fed with graded levels of dietary arginine. The results showed that, compared with the control group (0.87%), 2.31% dietary arginine level resulted in the upregulation of the relative gene expression of IRS-1, PI3K and Akt in the insulin signaling pathway, while 2.70% dietary arginine level led to inhibition of these genes. 1.62% dietary arginine level upregulated glycolysis by increasing GK mRNA level; 2.70% dietary arginine level upregulated gluconeogenesis and resulted in high plasma glucose content by increasing PEPCK and G6P mRNA level. Furthermore, 2.70% dietary arginine level significantly lowered GLUT2 and increased PK mRNA levels. 1.62% dietary arginine level significantly upregulated ACC, FAS and G6PDH mRNA levels in the fat synthesis pathway and resulted in high plasma TG content. These results indicate that 1.62% dietary arginine level improves glycolysis and fatty acid synthesis in juvenile blunt snout bream. However, 2.70% dietary arginine level results in high plasma glucose, which could lead to negative feedback of insulin resistance, including inhibition of IRS-1 mRNA levels and activation of gluconeogenesis-related gene expression. This mechanism seems to be different from mammals at the molecular level.

Short- and long-term metabolic responses to diets with different protein:carbohydrate ratios in Senegalese sole (Solea senegalensis, Kaup 1858)

Four isolipidic and isoenergetic diets with different protein:carbohydrate (CH) contents (48:38, 52:34, 56:30, 60:26) were fed to juvenile Senegalese sole (22·01 (sem 0·01) g) during 104 d. Oral glucose tolerance tests were performed at the beginning (4 d) and at the end (104 d) of the experiment to assess the effect of the dietary treatment on glucose tolerance. Samples of blood, liver and muscle of all dietary groups were also obtained at the initial and final phases of the trial at different postprandial times (0, 1, 5 and 10 h after feeding) in order to analyse glucose and NEFA in plasma, and metabolites and enzyme activities involved in glycogen metabolism, glycolysis, gluconeogenesis and lipogenesis pathways in liver and muscle. The results obtained in this study suggest a good glucose tolerance in Senegalese sole. This species tolerated important amounts of CH in the diet without showing any deleterious signs in terms of growth or any metabolic disorders. After 104 d of feeding diets with an important amount of CH (48:38 and 52:34), the control of glycaemia was maintained and even postprandial glucose levels in plasma were (in general) lower than at the beginning of the experiment. This reasonable tolerance to glucose is also reflected by an increased use of glucose through glycolysis in liver (indicated by glucokinase activity), and the absence of changes in lipogenic potential in the same tissue (indicated by ATP citrate lyase activity). No clear changes were induced in the muscle by the dietary treatments.

Exposure to an acute hypoxic stimulus during early life affects the expression of glucose metabolism-related genes at first-feeding in trout

Rainbow trout (Oncorhynchus mykiss) is considered a "glucose-intolerant" species. With the aim of programming trout to improve their metabolic use of dietary carbohydrates, we hypothesised that a hypoxic stimulus applied during embryogenesis could later affect glucose metabolism at the first-feeding stage. An acute hypoxic stimulus (2.5 or 5.0 mg·L-1 O2) was applied for 24 h to non-hatched embryos or early hatched alevins followed by a challenge test with a high carbohydrate diet at first-feeding. The effectiveness of the early hypoxic stimulus was confirmed by the induction of oxygen-sensitive markers such as egln3. At first-feeding, trout previously subjected to the 2.5 mg·L-1 O2 hypoxia displayed a strong induction of glycolytic and glucose transport genes, whereas these glucose metabolism-related genes were affected much less in trout subjected to the less severe (5.0 mg·L-1 O2) hypoxia. Our results demonstrate that an acute hypoxic stimulus during early development can affect glucose metabolism in trout at first-feeding.

Changes in the levels and phosphorylation status of Akt, AMPK, CREB, and FoxO1 in hypothalamus of rainbow trout under conditions of enhanced glucosensing activity

There is no available information in fish about mechanisms linking glucosensing activation and changes in the expression of brain neuropeptides controlling food intake. Therefore, we assessed in rainbow trout hypothalamus the effects of raised levels of glucose on the levels and phosphorylation status of two transcription factors, FoxO1 and CREB, possibly involved in linking those processes. Moreover, we also aimed to assess the changes in the levels and phosphorylation status of two proteins possibly involved in the modulation of these transcription factors such as Akt and AMPK. Therefore, we evaluated in pools of hypothalamus incubated for 3h and 6h at 15 °C in modified Hanks’ medium containing 2, 4, or 8 mM D-glucose the response of parameters related to glucosensing mechanisms, neuropeptide expression, and levels and phosphorylation status of proteins of interest. The activation of hypothalamic glucosensing systems and the concomitant enhanced anorectic potential occurred in parallel with activation of Akt and inhibition of AMPK. The changes in these proteins would relate to neuropeptide expression through changes in the levels and phosphorylation status of transcription factors under their control, such as CREB and FoxO1, which displayed inhibitory (CREB) or activatory (FoxO1) responses to increased glucose.

Dietary gossypol suppressed postprandial TOR signaling and elevated ER stress pathways in turbot (Scophthalmus maximus L.)

Gossypol is known to be a polyphenolic compound toxic to animals. However, its molecular targets are far from fully characterized. To evaluate the physiological and molecular effects of gossypol, we chose turbot (Scophthalmus maximus L.), a carnivorous fish, as our model species. Juvenile turbots (7.83 ± 0.02 g) were fed diets containing gradient levels of gossypol at 0 (G0), 600 (G1), and 1,200 (G2) mg/kg diets for 11 wk. After the feeding trial, fish growth, body protein, and fat contents were significantly reduced in the G2 group compared with those of the G0 group (P < 0.05). Gossypol had little impact on digestive enzyme activities and intestine morphology. However, gossypol caused liver fibrosis and stimulated chemokine and proinflammatory cytokine secretions. More importantly, gossypol suppressed target of rapamycin (TOR) signaling and induced endoplasmic reticulum (ER) stress pathway in both the feeding experiment and cell cultures. Our results demonstrated that gossypol inhibited TOR signaling and elevated ER stress pathways both in vivo and in vitro, thus providing new mechanism of action of gossypol in nutritional physiology.

Looking at the metabolic consequences of the colchicine-based in vivo autophagic flux assay

Monitoring autophagic flux in vivo or in organs remains limited and the ideal methods relative to the techniques possible with cell culture may not exist. Recently, a few papers have demonstrated the feasibility of measuring autophagic flux in vivo by intraperitoneal (IP) injection of pharmacological agents (chloroquine, leupeptin, vinblastine, and colchicine). However, the metabolic consequences of the administration of these drugs remain largely unknown. Here, we report that 0.8 mg/kg/day IP colchicine increased LC3-II protein levels in the liver of fasted trout, supporting the usefulness of this drug for studying autophagic flux in vivo in our model organism. This effect was accompanied by a decrease of plasma glucose concentration associated with a fall in the mRNA levels of gluconeogenesis-related genes. Concurrently, triglycerides and lipid droplets content in the liver increased. In contrast, transcript levels of β-oxidation-related gene Cpt1a dropped significantly. Together, these results match with the reported role of autophagy in the regulation of glucose homeostasis and intracellular lipid stores, and highlight the importance of considering these effects when using colchicine as an in vivo "autophagometer."

Postprandial hepatic protein expression in trout Oncorhynchus mykiss a proteomics examination

Following a meal, a series of physiological changes occurs in animals as they digest, absorb and assimilate ingested nutrients, the kinetics of these responses depends on metabolic rate and nutrient quality. Here we investigated the hepatic proteome in the ectothermic teleost, the rainbow trout, following a single meal to define the postprandial expression of hepatic proteins. The fish were fed a high marine fishmeal/fish oil single meal following a period of 24 h without feeding. Liver protein profiles were examined by 2D gel electrophoresis just before feeding (time 0 h) and at 6 and 12 h after feeding. Of a total of 588 protein spots analysed in a temporal fashion, 49 differed significantly in abundance between the three time groups (ANOVA, p<0.05), before and after feeding, 15 were increased and 34 were decreased in abundance after feeding. Amino acid metabolism-regulated proteins such as phenylalanine-4-hydroxylase and proliferating cell antigen were increased in abundance 12 and 6 h following the meal, suggesting by this time that the fish were increasing their protein turnover to utilize efficiently their dietary protein consumption. Overall, these results highlight some specificity of the trout metabolism and identify postprandial response of metabolism-related proteins 6–12 h after feeding a single meal.

•

The effect of a single meal on the postprandial expression of hepatic proteins in fish is shown.

•

Temporal changes occurred in the trout liver proteome following a single meal.

•

There is a postprandial response of metabolism-related proteins 6–12 h after feeding a single meal.

Comparative Study on the Cellular and Systemic Nutrient Sensing and Intermediary Metabolism after Partial Replacement of Fishmeal by Meat and Bone Meal in the Diet of Turbot (Scophthalmus maximus L.)

This study was designed to examine the cellular and systemic nutrient sensing mechanisms as well as the intermediary metabolism responses in turbot (Scophthalmus maximus L.) fed with fishmeal diet (FM diet), 45% of FM replaced by meat and bone meal diet (MBM diet) or MBM diet supplemented with essential amino acids to match the amino acid profile of FM diet (MBM+AA diet). During the one month feeding trial, feed intake was not affected by the different diets. However, MBM diet caused significant reduction of specific growth rate and nutrient retentions. Compared with the FM diet, MBM diet down-regulated target of rapamycin (TOR) and insulin-like growth factor (IGFs) signaling pathways, whereas up-regulated the amino acid response (AAR) signaling pathway. Moreover, MBM diet significantly decreased glucose and lipid anabolism, while increased muscle protein degradation and lipid catabolism in liver. MBM+AA diet had no effects on improvement of MBM diet deficiencies. Compared with fasted, re-feeding markedly activated the TOR signaling pathway, IGF signaling pathway and glucose, lipid metabolism, while significantly depressed the protein degradation signaling pathway. These results thus provided a comprehensive display of molecular responses and a better explanation of deficiencies generated after fishmeal replacement by other protein sources.

A broader look at ammonia production, excretion, and transport in fish: a review of impacts of feeding and the environment

For nearly a century, researchers have studied ammonia production and excretion in teleost fish. Stemming from past investigations a body of knowledge now exists on various aspects including biochemical mechanisms of ammonia formation and specific routes and tissues used for ammonia transport, culminating in a current detailed theoretical model of branchial transport, including the molecular identities of the moieties involved. However, typical studies examining ammonia balance use routine laboratory conditions and fasted fish. While avoiding additional variables that influence nitrogen balance, these studies are arguably idealistic and do not reflect the natural variety of conditions that fish encounter. Further studies have revealed the impacts of extrinsic factors (e.g. salinity, pH, temperature) on ammonia handling in fasted fish whereas others have explored intrinsic factors, such as life history and developmental impacts. One routine challenge for ammonia balance that fish encounter is feeding and digestion. Fortunately, many new studies have revealed the impact of feeding and digestion on several aspects of ammonia balance; from production to excretion and to transport, and several have done so incorporating supplemental extrinsic and/or intrinsic factors. Together, these complex studies reveal realistic responses to multifactorial challenges encountered by animals in the wild and begin to provide a holistic view of ammonia balance in freshwater teleost fish.

Ceramides are involved in the regulation of food intake in rainbow trout (Oncorhynchus mykiss)

We hypothesize that ceramides are involved in the regulation of food intake in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with C6:0 ceramide on food intake. In a second experiment, we assessed the effects in brain areas of ceramide treatment on neuropeptide expression, fatty acid-sensing systems, and cellular signaling pathways. Ceramide treatment induced a decrease in food intake, a response opposed to the orexigenic effect described in mammals, which can be related to enhanced mRNA abundance of cocaine and amphetamine-related transcript and proopiomelanocortin and decreased mRNA abundance of Agouti-related protein and neuropeptide Y. Fatty acid-sensing systems appear to be inactivated by ceramide treatment. The mRNA abundance of integrative sensors AMPK and sirtuin 1, and the phosphorylation status of cellular signaling pathways dependent on protein kinase B, AMPK, mammalian target of rapamycin (mTOR), and forkhead box protein O1 (FoxO1) are generally activated by ceramide treatment. However, there are differences between hypothalamus and hindbrain in the phosphorylation status of AMPK (decreased in hypothalamus and increased in hindbrain), mTOR (decreased in hypothalamus and increased in hindbrain), and FoxO1 (increased in hypothalamus and decreased in hindbrain) to ceramide treatment. The results suggest that ceramides are involved in the regulation of food intake in rainbow trout through mechanisms comparable to those characterized previously in mammals in some cases.

Transcriptome Analysis and Postprandial Expression of Amino Acid Transporter Genes in the Fast Muscles and Gut of Chinese Perch (Siniperca chuatsi)

The characterization of the expression and regulation of growth-related genes in the muscles of Chinese perch is of great interest to aquaculturists because of the commercial value of the species. The transcriptome annotation of the skeletal muscles is a crucial step in muscle growth-related gene analysis. In this study, we generated 52 504 230 reads of mRNA sequence data from the fast muscles of the Chinese perch by using Solexa/Illumina RNA-seq. Twenty-one amino acid transporter genes were annotated by searching protein and gene ontology databases, and postprandial changes in their transcript abundance were assayed after administering a single satiating meal to Chinese perch juveniles (body mass, approximately 100 g), following fasting for 1 week. The gut content of the Chinese perch increased significantly after 1 h and remained high for 6 h following the meal and emptied within 48-96 h. Expression of eight amino acid transporter genes was assayed in the fast muscles through quantitative real-time polymerase chain reaction at 0, 1, 3, 6, 12, 24, 48, and 96 h. Among the genes, five transporter transcripts were markedly up-regulated within 1 h of refeeding, indicating that they may be potential candidate genes involved in the rapid-response signaling system regulating fish myotomal muscle growth. These genes display coordinated regulation favoring the resumption of myogenesis responding to feeding.

Chronic rapamycin treatment on the nutrient utilization and metabolism of juvenile turbot (Psetta maxima)

High dietary protein inclusion is necessary in fish feeds and also represents a major cost in the aquaculture industry, which demands improved dietary conversion into body proteins in fish. In mammals, the target of rapamycin (TOR) is a key nutritionally responsive molecule governing postprandial anabolism. However, its physiological significance in teleosts has not been fully examined. In the present study, we examined the nutritional physiology of turbot after chronic rapamycin inhibition. Our results showed that a 6-week inhibition of TOR using dietary rapamycin inclusion (30 mg/kg diet) reduced growth performance and feed utilization. The rapamycin treatment inhibited TOR signaling and reduced expression of key enzymes in glycolysis, lipogenesis, cholesterol biosynthesis, while increasing the expression of enzymes involved in gluconeogenesis. Furthermore, rapamycin treatment increased intestinal goblet cell number in turbot, while the expressions of Notch and Hes1 were down regulated. It was possible that stimulated goblet cell differentiation by rapamycin was mediated through Notch-Hes1 pathway. Therefore, our results demonstrate the important role of TOR signaling in fish nutritional physiology.

Effects of dietary carbohydrate on hepatic de novo lipogenesis in European seabass (Dicentrarchus labrax L.)

Farmed seabass have higher adiposity than their wild counterparts and this is often attributed to carbohydrate (CHO) feeding. Whether this reflects a reduction in fat oxidation, increased de novo lipogenesis (DNL), or both, is not known. To study the effects of high CHO diets on hepatic TG biosynthesis, hepatic TG deuterium ((2)H) enrichment was determined following 6 days in (2)H-enriched tank water for fish fed with a no-CHO control diet (CTRL), and diets with digestible starch (DS) and raw starch (RS). Hepatic fractional synthetic rates (FSRs, percent per day(-1)) were calculated for hepatic TG-glyceryl and FA moieties through (2)H NMR analysis. Glyceryl FSRs exceeded FA FSRs in all cases, indicating active cycling. DS fish did not show increased lipogenic potential compared to CTRL. RS fish had lower glyceryl FSRs compared with the other diets and negligible levels of FA FSRs despite similar hepatic TG levels to CTRL. DS-fed fish showed higher activity for enzymes that can provide NADPH for lipogenesis, relative to CTRL in the case of glucose-6-phosphate dehydrogenase (G6PDH) and relative to RS for both G6PDH and 6-phosphogluconate dehydrogenase. This approach indicated that elevated hepatic adiposity from DS feeding was not attributable to increased DNL.

The five glucose-6-phosphatase paralogous genes are differentially regulated by insulin alone or combined with high level of amino acids and/or glucose in trout hepatocytes

A recent analysis of the newly sequenced rainbow trout (Oncorhynchus mykiss) genome suggested that duplicated gluconeogenic g6pc paralogues, fixed in this genome after the salmonid-specific 4th whole genome duplication, may have a role in the setting up of the glucose-intolerant phenotype in this carnivorous species. This should be due to the sub- or neo-functionalization of their regulation. In the present short communication we thus addressed the question of the regulation of these genes by insulin, hormone involved in the glucose homeostasis, and its interaction with glucose and amino acids in vitro. The stimulation of trout hepatocytes with insulin revealed an atypical up-regulation of g6pcb2 ohnologues and confirmed the sub- or neo-functionalization of the five g6pc genes at least at the regulatory level. Intriguingly, when hepatocytes were cultured with high levels of glucose and/or AAs in presence of insulin, most of the g6pc paralogues were up-regulated. It strongly suggested a cross-talk between insulin and nutrients for the regulation of these genes. Moreover these results strengthened the idea that g6pc duplicated genes may significantly contribute to the setting up of the glucose-intolerant phenotype in trout via their atypical regulation by insulin alone or in interaction with nutrients. These findings open new perspectives to better understand in vivo glucose-intolerant phenotype in trout fed a high carbohydrate diet.

Postprandial nutrient-sensing and metabolic responses after partial dietary fishmeal replacement by soyabean meal in turbot (Scophthalmus maximus L.)

In this study, we chose a carnivorous fish, turbot (Scophthalmus maximus L.), to examine its nutrient-sensing and metabolic responses after ingestion of diets with fishmeal (FM), or 45% of FM replaced by soyabean meal (34·6% dry diet) balanced with or without essential amino acids (EAA) to match the amino acid profile of FM diet for 30 d. After a 1-month feeding trial, fish growth, feed efficiency and nutrient retention were markedly reduced by soyabean meal-incorporated (SMI) diets. Compared with the FM diet, SMI led to a reduction of postprandial influx of free amino acids, hypoactivated target of rapamycin signalling and a hyperactivated amino acid response pathway after refeeding, a status associated with reduced protein synthesis, impaired postprandial glycolysis and lipogenesis. These differential effects were not ameliorated by matching an EAA profile of soyabean meal to that of the FM diet through dietary amino acid supplementation. Therefore, this study demonstrated that the FM diet and SMI diets led to distinct nutrient-sensing responses, which in turn modulated metabolism and determined the utilisation efficiency of diets. Our results provide a new molecular explanation for the role of nutrient sensing in the inferior performance of aquafeeds in which FM is replaced by soyabean meal.

Glucose metabolism ontogenesis in rainbow trout (Oncorhynchus mykiss) in the light of the recently sequenced genome: new tools for intermediary metabolism programming

The rainbow trout (Oncorhynchus mykiss), a carnivorous fish species, displays a 'glucose-intolerant' phenotype when fed a high-carbohydrate diet. The importance of carbohydrate metabolism during embryogenesis and the timing of establishing this later phenotype are currently unclear. In addition, the mechanisms underlying the poor ability of carnivorous fish to use dietary carbohydrates as a major energy substrate are not well understood. It has recently been shown in trout that duplicated genes involved in glucose metabolism may participate in establishing the glucose-intolerant phenotype. The aim of this study was therefore to provide new understanding of glucose metabolism during ontogenesis and nutritional transition, taking into consideration the complexity of the trout genome. Trout were sampled at several stages of development from fertilization to hatching, and alevins were then fed a non-carbohydrate or a high-carbohydrate diet during first feeding. mRNA levels of all glucose metabolism-related genes increased in embryos during the setting up of the primitive liver. After the first meal, genes rapidly displayed expression patterns equivalent to those observed in the livers of juveniles. g6pcb2.a (a glucose 6-phosphatase-encoding gene) was up-regulated in alevins fed a high-carbohydrate diet, mimicking the expression pattern of gck genes. The g6pcb2.a gene may contribute to the non-inhibition of the last step of gluconeogenesis and thus to establishing the glucose-intolerant phenotype in trout fed a high-carbohydrate diet as early as first feeding. This information is crucial for nutritional programming investigations as it suggests that first feeding would be too late to programme glucose metabolism in the long term.