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

Optimal dietary lipid levels alleviated adverse effects of high temperature on growth, lipid metabolism, antioxidant and immune responses in juvenile turbot (Scophthalmus maximus L.)

Fish physiological health is often negatively impacted by high-temperature environments and there are few studies on how dietary lipids affect fish growth and physiology when exposed to heat stress. The main objective of this research was to examine the impact of dietary lipid levels on growth and physiological status of juvenile turbot (Scophthalmus maximus L.) and determine if dietary lipid concentration could alleviate the possible adverse effects of heat stress. Five diets containing 6.81%, 9.35%, 12.03%, 14.74%, and 17.08% lipid, respectively, were formulated and fed to turbot (initial weight 5.13 ± 0.02 g) under high-temperature conditions (24.0-25.0 °C). Meanwhile, the diet with 12.03% lipid (considered by prior work to be an optimal dietary lipid level) was fed to turbot of the same size at normal temperature. Results suggested that, among the different dietary lipid levels under high-temperature conditions, fish fed the optimal lipid (12.03%) exhibited better growth compared to non-optimal lipid groups, as evidenced by higher weight gain and specific growth rate. Simultaneously, the optimal lipid diet may better maintain lipid homeostasis, as attested by lower liver and serum lipid, along with higher liver mRNA levels of lipolysis-related genes (pgc1α, lipin1, pparα, lpl and hl) and lower levels of synthesis-related genes (lxr, fas, scd1, pparγ, dgat1 and dgat2). Also, the optimal lipid diet might mitigate oxidative damage by improving antioxidant enzyme activity, decreasing malondialdehyde levels, and up-regulating oxidation-related genes (sod1, sod2, cat, gpx and ho-1). Furthermore, the optimal lipid may enhance fish immunity, as suggested by the decrease in serum glutamic-oxalacetic/pyruvic transaminase activities, down-regulation of pro-inflammatory genes and up-regulation of anti-inflammation genes. Correspondingly, the optimal lipid level suppressed MAPK signaling pathway via decreased phosphorylation levels of p38, JNK and ERK proteins in liver. In summary, the optimal dietary lipid level facilitated better growth and physiological status in turbot under thermal stress.

EPA and DHA promote cell proliferation and enhance activity of the Akt-TOR-S6K anabolic signaling pathway in primary muscle cells of turbot (Scophthalmus maximus L.)

Fish growth and health are predominantly governed by dietary nutrient supply. Although the beneficial effects of omega-3 polyunsaturated fatty acids supplementation have been shown in a number of fish species, the underlying mechanisms are still mostly unknown. In this study, we conducted an investigation into the effects of EPA and DHA on cell proliferation, nutrient sensing signaling, and branched-chain amino acids (BCAA) transporting in primary turbot muscle cells. The findings revealed that EPA and DHA could stimulate cell proliferation, promote protein synthesis and inhibit protein degradation through activation of target of rapamycin (TOR) signaling pathway, a pivotal nutrient-sensing signaling cascade. While downregulating the expression of myogenin and myostatin, EPA and DHA increased the level of myogenic regulatory factors, such as myoD and follistatin. Furthermore, we observed a significant increase in the concentrations of intracellular BCAAs following treatment with EPA or DHA, accompanied by an upregulation of the associated amino acid transporters. Our study providing valuable insights into the mechanisms underlying the growth-promoting effects of omega-3 fatty acids in fish.

Recycling of animal protein wastes in the formulation of feed for Labeo rohita and Mystus vittatus-a comparative evaluation

Lactic acid bacteria (LAB) are key players in the fermentation of organic wastes and their recycling as feedstuff for fish. Whey, a common dairy byproduct in India, is a cheap source of LAB and can be used to ferment animal byproducts. An experimental study was designed to explore whether the whey fermented animal protein blend (WFAPB) could be used as a fishmeal replacer in the formulation of feed for both stomach-less carp fish Labeo rohita and stomach-bearing catfish Mystus vittatus. Experiments were performed with five isoproteinous, isolipidous, and isoenergetic feeds with WFAPB replacing fishmeal (FM) by 0% (T1), 25% (T2), 50% (T3), 75% (T4), and 100% (T5). Fifteen days of laboratory experiments with these experimental feeds revealed that more than 50% FM replacement level could result in excess postprandial absorption (6 h) of some essential and non-essential amino acids in the plasma of both fish. The postprandial absorption was more in M. vittatus than L. rohita. Ninety-day experiments were conducted in outdoor cement vats to measure growths and deposition of amino acids (AA) in muscle. Regression analysis was performed to find the optimal FM replacement based on four growth parameters and fifteen AA deposition in muscle. A two-phase fuzzy methodology was used to obtain Pareto-optimal replacement levels for each fish. The results demonstrated that FM replacement levels were 7.63% and 36.79% respectively for L. rohita and M. vittatus when only four growth parameters were considered. However, based on the FM replacement level that maximized deposition of 15 amino acids and growth parameters, it was found that 12.23% and 40.02% replacement of FM by the WFAPB was ideal respectively for L. rohita and M. vittatus. The results revealed that only a fraction of both essential and non-essential amino acids absorbed in plasma could be converted into protein and deposited as bound amino acids in the muscle. It is concluded that fermentation by whey is an inexpensive, easily available, and environmentally sustainable technique to recycle animal protein in the formulation of feed for fish, and the stomach-bearing carnivorous fish are more efficient in utilizing fermented animal protein blend than the stomach-less carps.

Growth performance, plasma and hepatic biochemistry of jundiá Rhamdia quelen fed dephytinized rice bran protein concentrate

A 45-day feeding assay was carried out to evaluate the effects of crescent levels of dephytinized rice bran protein concentrate (DRBPC) on growth performance, nutrient deposition, plasma and liver parameters of jundiá Rhamdia quelen. Five experimental diets were formulated with inclusion of 0 (control), 10, 15, 20, and 30% of DRBPC. In total 500 jundiás (initial body weight 6.28 ± 0.12 g) were allocated in 20 tanks (230 L) to give four groups for each treatment. Fish were fed to apparent satiation for 45 days. Weight gain and specific growth rate were evaluated by cubic regression analysis (P < 0.05) and displayed maximal growth on the inclusion level of 25% of DRBPC. The results indicated that fish fed DRBPC15 and DRBPC30 had lower body protein deposition and hepatosomatic index compared to CONTROL diet, respectively. No significant differences (P > 0.05) were assessed in plasma parameters. The alanine aminotransferase activity was higher in fish fed DRBPC30 compared to CONTROL group. The present study has demonstrated that DRBPC displayed significant nutritional quality for the jundiá. Thus, this new ingredient could be included as a protein source in fish for minimizing the use of fish meal.

Selenium deficiency-induced high concentration of reactive oxygen species restricts hypertrophic growth of skeletal muscle in juvenile zebrafish by suppressing TORC1-mediated protein synthesis

Se deficiency causes impaired growth of fish skeletal muscle due to the retarded hypertrophy of muscle fibres. However, the inner mechanisms remain unclear. According to our previous researches, we infer this phenomenon is associated with Se deficiency-induced high concentration of reactive oxygen species (ROS), which could suppress the target of rapamycin complex 1 (TORC1) pathway-mediated protein synthesis by inhibiting protein kinase B (Akt), an upstream protein of TORC1. To test this hypothesis, juvenile zebrafish (45 d post-fertilisation) were fed a basal Se-adequate diet or a basal Se-deficient diet or them supplemented with an antioxidant (DL-α-tocopherol acetate, designed as VE) or a TOR activator (MHY1485) for 30 d. Zebrafish fed Se-deficient diets exhibited a clear Se-deficient status in skeletal muscle, which was not influenced by dietary VE and MHY1485. Se deficiency significantly elevated ROS concentrations, inhibited Akt activity and TORC1 pathway, suppressed protein synthesis in skeletal muscle, and impaired hypertrophy of skeletal muscle fibres. However, these negative effects of Se deficiency were partly (except that on ROS concentration) alleviated by dietary MHY1485 and completely alleviated by dietary VE. These data strongly support our speculation that Se deficiency-induced high concentration of ROS exerts a clear inhibiting effect on TORC1 pathway-mediated protein synthesis by regulating Akt activity, thereby restricting the hypertrophy of skeletal muscle fibres in fish. Our findings provide a mechanistic explanation for Se deficiency-caused retardation of fish skeletal muscle growth, contributing to a better understanding of the nutritional necessity and regulatory mechanisms of Se in fish muscle physiology.

Screening and Stability Analysis of Reference Genes for Gene Expression Normalization in Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × Pelteobagrus vachelli ♂) Fed Diets Containing Different Soybean Meal Levels

In this study, we screened the expression stability of six reference genes (18S rRNA, β-actin, GAPDH, EF1a, B2M, and HPRT1) in hybrid yellow catfish (n = 6), considering the SBM levels, sampling time points, and different tissues. Four different statistical programs, BestKeeper, NormFinder, Genorm, and Delta Ct, combined with a method that comprehensively considered all results, were used to evaluate the expression stability of these reference genes systematically. The results showed that SBM levels significantly impacted the expression stability of most of the reference genes studied and that this impact was time-, dose-, and tissue-dependent. The expression stability of these six reference genes varied depending on tissue, sampling time point, and SBM dosage. Additionally, more variations were found among different tissues than among different SBM levels or sampling time points. Due to its high expression, 18S rRNA was excluded from the list of candidate reference genes. β-actin and GAPDH in the liver and β-actin, HPRT1 and EF1a in the intestine were the most stable reference genes when SBM levels were considered. HPRT1, and EF1a in tissues sampled at 2 W and EF1a and β-actin in tissues sampled at 4 and 6 W were proposed as two stable reference genes when different tissues were considered. When the sampling time points were considered, β-actin, EF1a, and HPRT1 were the top three stable reference genes in the intestine. In contrast, β-actin and B2M are the most stable reference genes in the liver. In summary, β-actin, EF1a, and HPRT1 were the more stable reference genes in this study. The stability of reference genes depends on the tissues, sampling time points, and SBM diet levels in hybrid yellow catfish. Therefore, attention should be paid to these factors before selecting suitable reference genes for normalizing the target genes.

Partial Replacement of Fishmeal with Poultry By-Product Meal in Diets for Coho Salmon (Oncorhynchus kisutch) Post-Smolts

The present study evaluated the effects of partially substituting fish meal (FM) with poultry by-product meal (PBPM) on the growth, muscle composition, and tissue biochemical parameters of coho salmon (Oncorhynchus kisutch) post-smolts. Five isonitrogenous (7.45% nitrogen) and isoenergetic (18.61 MJ/kg gross energy) experimental diets were made by substituting 0%, 10%, 20%, 40%, and 60% FM protein with PBPM protein, which were designated accordingly as PBPM0 (the control), PBPM10, PBPM20, PBPM40, and PBPM60, respectively. Each diet was fed to triplicates of ten post-smolts (initial individual body weight, 180.13 ± 1.32 g) in three floating cages three times daily (6:50, 11:50, and 16:50) to apparent satiation for 84 days. Both specific growth rate (SGR) and protein efficiency ratio did not differ significantly (p > 0.05) among the control, PBPM10, and PBPM20 groups, which were remarkably (p < 0.05) higher than those of the PBPM40 and PBPM60 groups. Feed conversion ratio varied inversely with SGR. The PBPM replacement had no remarkable effects on the morphological indices and proximal muscle components. The control and PBPM10 groups led to significantly higher muscle contents of leucine, lysine, and methionine than groups of higher PBPM inclusion. The groups of PBPM40 and PBPM60 obtained significantly (p < 0.05) higher serum alanine aminotransferase and aspartate aminotransferase activities than the control and low PBPM inclusion groups. The control group had significantly higher albumin and total cholesterol contents than the groups with PBPM inclusion. The control group had significantly higher triglycerides content than the PBPM60 group. The PBPM60 group had significantly lower contents of high-density lipoprotein, low-density lipoprotein, and total protein than the control and PBPM10 groups. The high PBPM replacement level up to 40% and 60% had adverse effects on hepatic malondialdehyde levels. The catalase and superoxide dismutase activities were not affected by low PBPM inclusion, but significantly decreased in high-PBPM-inclusion groups. Based on broken-line regression analysis of SGR and PER, the optimum dietary PBPM replacing level was evaluated to be 16.63-17.50% of FM protein for coho salmon post-smolts.

Early Molecular Immune Responses of Turbot (Scophthalmus maximus L.) Following Infection with Aeromonas salmonicida subsp. salmonicida

Turbot aquaculture production is an important economic activity in several countries around the world; nonetheless, the incidence of diseases, such furunculosis, caused by the etiological agent A. salmonicida subsp. salmonicida, is responsible for important losses to this industry worldwide. Given this perspective, this study aimed to evaluate early immune responses in turbot (S. maximus L.) following infection with A. salmonicida subsp. salmonicida. For this, 72 fish were individually weighed and randomly distributed into 6 tanks in a circulating seawater system. For the bacterial challenge, half of the individuals (3 tanks with 36 individuals) were infected using a peritoneal injection with the bacterial suspension, while the other half of individuals were injected with PBS and kept as a control group. Several factors linked to the innate immune response were studied, including not only haematological (white blood cells, red blood cells, haematocrit, haemoglobin, mean corpuscular volume, mean cell haemoglobin, mean corpuscular haemoglobin concentration, neutrophils, monocytes, lymphocytes, thrombocytes) and oxidative stress parameters, but also the analyses of the expression of 13 key immune-related genes (tnf-α, il-1β, il-8, pparα-1, acox1, tgf-β1, nf-kB p65, srebp-1, il-10, c3, cpt1a, pcna, il-22). No significant differences were recorded in blood or innate humoral parameters (lysozyme, anti-protease, peroxidase) at the selected sampling points. There was neither any evidence of significant changes in the activity levels of the oxidative stress indicators (catalase, glutathione S-transferase, lipid peroxidation, superoxide dismutase). In contrast, pro-inflammatory (tnf-α, il-1β), anti-inflammatory (il-10), and innate immune-related genes (c3) were up-regulated, while another gene linked with the lipid metabolism (acox1) was down-regulated. The results showed new insights about early responses of turbot following infection with A. salmonicida subsp. salmonicida.

Exercise training combined with a high-fat diet improves the flesh flavour, texture and nutrition of gibel carp (Carassius auratus gibelio)

To meet the consumer demand for high-quality flesh sources, this study investigated the impacts of exercise training (ET) combined with a high-fat diet (HFD) on flesh quality. The results showed that HFD increased muscular fat content but reduced hardness, flexibility and adhesiveness. ET decreased fat content but increased flesh water holding capacity, hardness and stickiness. In terms of flavour, ET decreased the umami and sweet amino acid contents, which were restored when concomitantly feeding the HFD. Metabolomics further revealed that ET and HFD mainly affect the alanine, aspartate and glutamate metabolism, the citrate cycle and purine metabolism. The E-nose and volatile metabolomics analysis demonstrated that the combination of ET and HFD improved the aroma of flesh by enhancing the content of key flavour compounds within flesh such as hexadecenoic acid, ethyl ester and methyl stearate. This research provides a new strategy for improving the flesh quality of cultured fish.

Dietary β-Hydroxy-β-Methylbutyrate Supplementation Affects Growth Performance, Digestion, TOR Pathway, and Muscle Quality in Kuruma Shrimp (Marsupenaeus japonicas) Fed a Low Protein Diet

An 8-week feeding trial was performed to evaluate the effects of dietary β-hydroxy-β-methylbutyrate (HMB) supplementation on growth performance and muscle quality of kuruma shrimp (Marsupenaeus japonicas) (initial weight: 2.00 ± 0.01 g) fed a low protein diet. The positive control diet (HP) with 490 g/kg protein and negative control diet (LP) with 440 g/kg protein were formulated. Based on the LP, 0.25, 0.5, 1, 2 and 4 g/kg β-hydroxy-β-methylbutyrate calcium were supplemented to design the other five diets named as HMB0.25, HMB0.5, HMB1, HMB2 and HMB4, respectively. Results showed that compared with the shrimp fed LP, the HP, HMB1 and HMB2 groups had significantly higher weight gain and specific growth rate, while significantly lower feed conversion ratio (p < 0.05). Meanwhile, intestinal trypsin activity was significantly elevated in the above three groups than that of the LP group. Higher dietary protein level and HMB inclusion upregulated the expressions of target of rapamycin, ribosomal protein S6 kinase, phosphatidylinositol 3-kinase, and serine/threonine-protein kinase in shrimp muscle, accompanied by the increases in most muscle free amino acids contents. Supplementation of 2 g/kg HMB in a low protein diet improved muscle hardness and water holding capacity of shrimp. Total collagen content in shrimp muscle increased with increasing dietary HMB inclusion. Additionally, dietary inclusion of 2 g/kg HMB significantly elevated myofiber density and sarcomere length, while reduced myofiber diameter. In conclusion, supplementation of 1-2 g/kg HMB in a low protein diet improved the growth performance and muscle quality of kuruma shrimp, which may be ascribed to the increased trypsin activity and activated TOR pathway, as well as elevated muscle collagen content and changed myofiber morphology caused by dietary HMB.

Effects of a Phytogenic Supplement Containing Olive By-Product and Green Tea Extracts on Growth Performance, Lipid Metabolism, and Hepatic Antioxidant Capacity in Largemouth Bass (Micropterus salmoides) Fed a High Soybean Meal Diet

A 10-week growth trial was conducted to investigate the effects of a phytogenic feed additive (PFA) containing olive by-products and green tea extracts supplemented to a reduced fishmeal/high soybean meal diet on the growth performance, hepatic antioxidant capacity, lipid metabolism, and liver health of largemouth bass (Micropterus salmoides). Three experimental diets were tested: (1) a control high fishmeal (40%) and low soybean meal (15.57%) diet (named HFM), (2) a reduced fishmeal (30%) and high soybean meal (30.97%) diet (named HSB), and (3) a HSB diet supplemented with the PFA at 500 mg/kg (named HSB+P). Each diet was assigned to four replicate tanks, each containing 30 largemouth bass (initial body weight, IBW = 48.33 ± 0.01 g). The results showed that increasing the soybean meal content in the diet did not negatively affect growth performance, whereas supplementation with PFA significantly increased weight gain and specific growth rate of largemouth bass compared to both HFM and HSB groups. Reducing fishmeal and increasing soybean meal in the diet caused oxidative stress with a higher content of ROS in the liver. However, the hepatic antioxidant capacity was enhanced, with reduced ROS and increased GSH-Px levels in the HSB+P group. Moreover, the decrease of plasma TG, LDL-C, and LDL-C/TC, and downregulation of lipogenesis and cholesterol synthesis gene expression in liver, indicated that supplementation with the PFA improved fish lipid metabolism. Protein retention efficiency was also significantly increased in largemouth bass fed the diet with PFA supplementation, which regulated (enhanced) AKT-mTOR phosphorylation. These results clearly indicated that a PFA containing olive by-product and green tea extracts can positively improve growth performance, protein retention efficiency, antioxidant capacity, and lipid metabolism of largemouth bass fed a reduced fishmeal/high soybean meal diet.

Dietary ribose supplementation improves flesh quality through purine metabolism in gibel carp (Carassius auratus gibelio)

Since the aquaculture industry is currently observing a deterioration in the flesh quality of farmed fish, the use of nutrients as additives to improve the flesh quality of farmed fish species is a viable strategy. The aim of this study was to investigate the effect of dietary D-ribose (RI) on the nutritional value, texture and flavour of gibel carp (Carassius auratus gibelio). Four diets were formulated containing exogenous RI at 4 gradient levels: 0 (Control), 0.15% (0.15RI), 0.30% (0.30RI) and 0.45% (0.45RI). A total of 240 fish (150 ± 0.31 g) were randomly distributed into 12 fibreglass tanks (150 L per tank). Triplicate tanks were randomly assigned to each diet. The feeding trial was carried out in an indoor recirculating aquaculture system for 60 d. After the feeding trial, the muscle and liver of gibel carp were analysed. The results showed that RI supplementation did not result in any negative impact on the growth performance and 0.30RI supplementation significantly increased the whole-body protein content compared to the control group. The contents of collagen and glycogen in muscle were enhanced by RI supplementation. The alterations in the flesh indicated that RI supplementation improved the texture of the flesh in terms of its water-holding capacity and hardness, therefore improving the taste. Dietary RI facilitated the deposition of amino acids and fatty acids in the muscle that contributed to the meaty taste and nutritional value. Furthermore, a combination of metabolomics and expression of key genes in liver and muscle revealed that 0.30RI activated the purine metabolism pathways by supplementing the substrate for nucleotide synthesis and thereby promoting the deposition of flavour substance in flesh. This study offers a new approach for providing healthy, nutritious and flavourful aquatic products.

Evaluation of Spray-Dried Bovine Hemoglobin Powder as a Dietary Animal Protein Source in Nile Tilapia, Oreochromis niloticus

The present study evaluated the potential effects of dietary inclusion of spray-dried bovine hemoglobin powder (SDBH) on the growth, gene expression of peptide and amino acid transporters, insulin growth factor-1 (IGF-1) and myostatin, digestive enzymes activity, intestinal histomorphology and immune status, immune-related gene expression, and economic efficiency in Nile tilapia, Oreochromis niloticus. Two hundred twenty-five fingerlings (32.38 ± 0.05 g/fish) were distributed into five treatments with five dietary inclusion levels of SDBH: 0, 2.5, 5, 7.5, and 10% for a ten-week feeding period. Dietary inclusion of SDBH linearly increased the final body weight (FBW), total weight gain (TWG), specific growth rate (SGR), and protein efficiency ratio (PER). Additionally, a linear decrease in feed conversion ratio (FCR) and daily feed intake relative to the daily BW was reported in the highest inclusion levels (7.5 and 10%). Dietary inclusion of SDBH was associated with a significant increase in the intestinal villous height (VH), villous width (VW), villous height: crypt depth ratio (VH: CD), and muscle coat thickness (MCT), with the highest values reported in SDBH7.5 group. Increased serum growth hormone levels and decreased serum leptin hormone levels were also reported by increasing the SDBH level. The serum glucose level was decreased in the SDBH7.5 and SDBH10 groups. The digestive enzymes' activity (amylase and protease) was increased by increasing the SDBH inclusion level. An up-regulation in the expression of peptide and amino acid transporters, IGF-1, and down-regulation of myostatin was reported in the SDBH2.5 to SDBH7.5 groups. Spleen sections showed more lymphoid elements, especially in the SDBH2.5 and SDBH7.5 groups. The SDBH inclusion increased the serum lysozyme activity, nitric oxide (NO), and complement 3 (C3) levels, with the highest values recorded in the SDBH5 group. The phagocytic % and the phagocytic index were increased by increasing the SDBH inclusion %. The expressions of immune-related genes (transforming growth factor-beta (TGF-β), Toll-like receptor 2 (TLR2), and interleukin 10 (IL10)) were up-regulated by SDBH inclusion with the highest expression in the SDBH5 group. Economically, the feed costs and feed costs/kg gain were linearly decreased in the SDBH7.5 and SDBH10 diets. In conclusion, spray-dried bovine hemoglobin powder could be used as a protein source for up to 10% of the diets of Nile tilapia for better growth and immune status of fish.

Nrf2 pathway in vegetable oil-induced inflammation of large yellow croaker (Larimichthys crocea)

This study was conducted to investigate the effects and regulation of dietary vegetable oil (VO, enriched with α-linolenic acid [ALA] and linoleic acid [LNA]) on the nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor-κB (NF-κB) pathways in large yellow croaker. In vivo study showed that the VO diet significantly decreased the activity of antioxidant enzymes and antioxidant enzyme-related mRNA expression in the liver tissue, in comparison with the fish oil (FO) diet (P < 0.05). The suppression of antioxidant capacity might be due to the decrease of nuclear Nrf2 protein translocation, Nrf2 binding to antioxidant response element (ARE) sequences, and subsequently, antioxidant genes transcription as electrophoretic mobility shift assay (EMSA) and luciferase assay showed. VO-derivated ALA and LNA exerted a lower antioxidant capacity than FO-derivated DHA and EPA, characterized by significantly lower nucleus Nfr2 protein expression but significantly higher ROS production values in primary hepatocytes (P < 0.05). The pro-inflammatory genes (tumor necrosis factor α [TNFα] and interleukin 1β [IL1β]) expression was significantly higher in the liver tissue of fish fed the VO diet which might be due to the activation of the NF-κB pathway (P < 0.05). Knockdown of the Nrf2 gene negatively affected the anti-inflammatory effect of fatty acids by increasing the expression of TNFα and the IL1β gene and nuclear p65 protein (P < 0.05). In general, the results indicated that dietary vegetable oil decreased antioxidant capacity but induced inflammatory responses through the Nrf2/NF-κB pathway.

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.

Functional Properties of Protein Hydrolysates on Growth, Digestive Enzyme Activities, Protein Metabolism, and Intestinal Health of Larval Largemouth Bass (Micropterus salmoides)

The present study was conducted to investigate the effects of dietary inclusion of protein hydrolysates on growth performance, digestive enzyme activities, protein metabolism, and intestinal health in larval largemouth bass (Micropterus salmoides). The experimental feeding trial presented in this study was based on five isonitrogenous and isolipidic diets formulated with graded inclusion levels of protein hydrolysates, and it showed that protein hydrolysates improved growth performance, reduced larval deformity rate, and increased the activity of digestive enzymes, including pepsin and trypsin. Gene expression results revealed that the supplementation of protein hydrolysates upregulated the expression of intestinal amino acid transporters LAT2 and peptide transporter 2 (PepT2), as well as the amino acid transporters LAT1 in muscle. Dietary provision of protein hydrolysates activated the target of rapamycin (TOR) pathway including the up-regulation of TOR and AKT1, and down-regulation of 4EBP1. Additionally, the expression of genes involved in the amino acids response (AAR) pathway, ATF4 and REDD1, were inhibited. Protein hydrolysates inhibited the transcription of some pro-inflammatory cytokines, including IL-8 and 5-LOX, but promoted the expression of anti-inflammatory cytokines TGF-β and IL-10. The 16S rRNA analysis, using V3-V4 region, indicated that dietary protein hydrolysates supplementation reduced the diversity of the intestine microbial community, increased the enrichment of Plesiomonas and reduced the enrichment of Staphylococcus at the genus level. In summary, protein hydrolysates have been shown to be an active and useful supplement to positively complement other protein sources in the diets for largemouth bass larvae, and this study provided novel insights on the beneficial roles and possible mechanisms of action of dietary protein hydrolysates in improving the overall performance of fish larvae.

Muscle Nutritive Metabolism Changes after Dietary Fishmeal Replaced by Cottonseed Meal in Golden Pompano (Trachinotus ovatus)

Our previous study demonstrated that based on growth performance and feed utilization, cottonseed meal (CSM) could substitute 20% fishmeal (FM) without adverse effect on golden pompano (Trachinotus ovatus). Muscle deposition was also an important indicator to evaluate the efficiency of alternative protein sources. Therefore, the present study was conducted to explore the changes of physiobiochemical and nutrient metabolism in muscle after FM replaced by CSM. Four isonitrogenous and isolipidic experimental diets (42.5% crude protein, 14.0% crude lipid) were formulated to replace 0% (CSM0 diet), 20% (CSM20 diet), 40% (CSM40 diet), and 60% (CSM60 diet) of FM with CSM. Juvenile fish (24.8 ± 0.02 g) were fed each diet for 6 weeks. The results presented, which, compared with the CSM0 diet, CSM20 and CSM40 diets, had no effect on changing the muscle proximate composition and free essential amino acid (EAA) concentration. For glycolipid metabolism, the CSM20 diet did not change the mRNA expression of hexokinase (hk), glucose transport protein 4 (glut4), glucagon-like peptide 1 receptor (glp-1r), while over 20% replacement impaired glucose metabolism. However, CSM20 and CSM40 diets had no effect on altering lipid metabolism. Mechanistically, compared with the CSM0 diet, the CSM20 diet did not change muscle nutritive metabolism through keeping the activities of the nutrient sensing signaling pathways stable. Higher replacement would break this balance and lead to muscle nutritive metabolism disorders. Based on the results, CSM could substitute 20–40% FM without affecting the muscle nutritive deposition. All data supplemented the powerful support for our previous conclusion that CSM could successfully replace 20% FM based on growth performance.

The Gut Microbiota (Microbiome) in Cardiovascular Disease and Its Therapeutic Regulation

In the last two decades, considerable interest has been shown in understanding the development of the gut microbiota and its internal and external effects on the intestine, as well as the risk factors for cardiovascular diseases (CVDs) such as metabolic syndrome. The intestinal microbiota plays a pivotal role in human health and disease. Recent studies revealed that the gut microbiota can affect the host body. CVDs are a leading cause of morbidity and mortality, and patients favor death over chronic kidney disease. For the function of gut microbiota in the host, molecules have to penetrate the intestinal epithelium or the surface cells of the host. Gut microbiota can utilize trimethylamine, N-oxide, short-chain fatty acids, and primary and secondary bile acid pathways. By affecting these living cells, the gut microbiota can cause heart failure, atherosclerosis, hypertension, myocardial fibrosis, myocardial infarction, and coronary artery disease. Previous studies of the gut microbiota and its relation to stroke pathogenesis and its consequences can provide new therapeutic prospects. This review highlights the interplay between the microbiota and its metabolites and addresses related interventions for the treatment of CVDs.

Agouti-related protein as the glucose signaling sensor in the central melanocortin circuits in regulating fish food intake

Agouti-related protein (AgRP) is a neuropeptide synthesized by AgRP/NPY neurons and transcribed as 132 amino acids in humans and 142 amino acids (AgRP1) in Japanese seabass (Lateolabrax maculatus) fish. AgRP neurons are activated by hormonal signals of energy deficits and inhibited by signals of energy surpluses and have been demonstrated to have the ability to sense the dynamics of blood glucose concentrations as the "glucose sensor" in mammals. It is widely recognized that AgRP is an endogenous antagonist of the melanocortin-3 and -4 receptors (MC3R and MC4R) in the hypothalamus, exhibiting potent orexigenic activity and control of energy homeostasis. Most fish, especially carnivorous fish, cannot make efficient use of carbohydrates. When carbohydrates like corn or wheat bran are added as energy sources, they often cause feeding inhibition and metabolic diseases. When fishmeal is replaced by plant protein, this does not completely eliminate carbs, limiting the utilization of carbohydrates and plant proteins in aquaculture. Our previous study showed that AgRP, and not neuropeptide Y (NPY) is the principal protein molecule that correlates well with feeding behavior in Japanese seabass from anorexia to adaptation. The Ghrelin/Leptin-mTOR-S6K1-NPY/AgRP/POMC feed intake regulatory pathway responds to the plant-oriented protein which contains glucose. However, its regulatory function and mechanism are still not clear. This review offers an integrative overview of how glucose signals converge on a molecular level in AgRP neurons of the arcuate nucleus of the hypothalamus. This is in order to control fish food intake and energy homeostasis.

Growth, serum biochemical parameters, salinity tolerance and antioxidant enzyme activity of rainbow trout (Oncorhynchus mykiss) in response to dietary taurine levels

This study evaluated the effect of dietary taurine levels on growth, serum biochemical parameters, salinity adaptability, and antioxidant activity of rainbow trout (Oncorhynchus mykiss). Four diets were formulated with taurine supplements at 0, 0.5, 1, and 2% w/v (abbreviated as T₀, T_0.5, T₁, and T₂, respectively). Rainbow trouts (initial weight of 80.09 ± 4.72 g) were stocked in tanks (180 L capacity), and were fed these diets for six weeks and subsequently underwent salinity acclimation. Physiological indicators were determined before salinity acclimation at 1, 4, 7, and 14 days afterwards. Results showed that there were no significant differences in growth performance (final mean weight ranged from 182.35 g to 198.48 g; percent weight gain was between 127.68% and 147.92%) of rainbow trout in freshwater stage, but dietary taurine supplement significantly increased serum-free taurine content. After entering seawater, the Na⁺-K⁺-ATPase activity of T₂ group returned to its freshwater levels, and the serum cortisol content was significantly higher than T₀ and T_0.5 groups. At the end of this experiment, the liver superoxide dismutase activity in the T₀ and T_0.5 groups was significantly lower than in the T₁ and T₂ groups, and the liver catalase in the T₀ group was the lowest whereas that in the T₂ group was the highest. Muscle malondialdehyde content was the highest in the T₀ group, and the lowest in the T₂ group. Based on the results of this study, supplement of dietary taurine (0.5-2%) enhanced the salinity tolerance in rainbow trout, which increased with the higher taurine concentration.

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.

Post-Prandial Amino Acid Changes in Gilthead Sea Bream

Following a meal, a series of physiological changes occurs in fish as they digest, absorb and assimilate ingested nutrients. This study aims to assess post-prandial free amino acid (FAA) activity in gilthead sea bream consuming a partial marine protein (fishmeal) replacement. Sea bream were fed diets where 16 and 27% of the fishmeal protein was replaced by plant protein. The essential amino acid (EAA) composition of the white muscle, liver and gut of sea bream was strongly correlated with the EAA composition of the 16% protein replacement diet compared to the 27% protein replacement diet. The mean FAA concentration in the white muscle and liver changed at 4 to 8 h after a meal and was not different to pre-feeding (0 h) and at 24 h after feeding. It was confirmed in this study that 16% replacement of marine protein with plant protein meets the amino acid needs of sea bream. Overall, the present study contributes towards understanding post-prandial amino acid profiles during uptake, tissue assimilation and immediate metabolic processing of amino acids in sea bream consuming a partial marine protein replacement. This study suggests the need to further investigate the magnitude of the post-prandial tissue-specific amino acid activity in relation to species-specific abilities to regulate metabolism due to dietary nutrient utilization.

Bacillus amyloliquefaciens ameliorates high-carbohydrate diet-induced metabolic phenotypes by restoration of intestinal acetate-producing bacteria in Nile Tilapia

Poor utilisation efficiency of carbohydrate always leads to metabolic phenotypes in fish. The intestinal microbiota plays an important role in carbohydrate degradation. Whether the intestinal bacteria could alleviate high-carbohydrate diet (HCD)-induced metabolic phenotypes in fish remains unknown. Here, a strain affiliated to Bacillus amyloliquefaciens was isolated from the intestine of Nile tilapia. A basal diet (CON), HCD or HCD supplemented with B. amy SS1 (HCB) was used to feed fish for 10 weeks. The beneficial effects of B. amy SS1 on weight gain and protein accumulation were observed. Fasting glucose and lipid deposition were decreased in the HCB group compared with the HCD group. High-throughput sequencing showed that the abundance of acetate-producing bacteria was increased in the HCB group relative to the HCD group. Gas chromatographic analysis indicated that the concentration of intestinal acetate was increased dramatically in the HCB group compared with that in the HCD group. Glucagon-like peptide-1 was also increased in the intestine and serum of the HCB group. Thus, fish were fed with HCD, HCD supplemented with sodium acetate at 900 mg/kg (HLA), 1800 mg/kg (HMA) or 3600 mg/kg (HHA) diet for 8 weeks, and the HMA and HHA groups mirrored the effects of B. amy SS1. This study revealed that B. amy SS1 could alleviate the metabolic phenotypes caused by HCD by enriching acetate-producing bacteria in fish intestines. Regulating the intestinal microbiota and their metabolites might represent a powerful strategy for fish nutrition modulation and health maintenance in future.

Effects of High Levels of Dietary Linseed Oil on the Growth Performance, Antioxidant Capacity, Hepatic Lipid Metabolism, and Expression of Inflammatory Genes in Large Yellow Croaker (Larimichthys crocea)

A growth experiment was conducted to evaluate the effects of dietary fish oil (FO) replaced by linseed oil (LO) on the growth performance, antioxidant capacity, hepatic lipid metabolism, and expression of inflammatory genes in large yellow croaker (Larimichthys crocea). Fish (initial weight: 15.88 ± 0.14 g) were fed four experimental diets with 0% (the control), 33.3%, 66.7%, and 100% of FO replaced by LO. Each diet was randomly attributed to triplicate seawater floating cages (1.0 × 1.0 × 2.0 m) with 60 fish in each cage. Results showed that the growth performance of fish fed the diet with 100% LO was markedly decreased compared with the control group (P < 0.05), while no remarkable difference was observed in the growth performance of fish fed diets within 66.7% LO (P > 0.05). The percentage of 18:3n-3 was the highest in the liver and muscle of fish fed the diet with 100% LO among the four treatments. When dietary FO was entirely replaced by LO, fish had a markedly higher total cholesterol, total triglyceride, low-density lipoprotein cholesterol content, and alanine transaminase activity in the serum than the control group (P < 0.05). The concentration of malondialdehyde was markedly higher, while the activity of catalase was markedly lower in fish fed the diet with 100% LO than the control group (P < 0.05). When dietary FO was entirely replaced by LO, hepatic lipid content, transcriptional levels of fatp1 and cd36, and CD36 protein expression were significantly higher, while transcriptional level of cpt-1 and CPT-1 protein expression were significantly lower than the control group (P < 0.05). As for the gene expression of cytokines, fish fed the diet with 100% LO had markedly higher transcriptional levels of il-1β, tnfα, and il-6 than the control group (P < 0.05). In conclusion, the substitution of 66.7% FO with LO had no significant effects on the growth performance of fish, while 100% LO decreased the growth performance and increased the inflammation and hepatic lipid content of fish. The increase of hepatic lipid content was probably due to the increased fatty acid uptake and decreased fatty acid oxidation in fish.

Physical and nutrient stimuli differentially modulate gut motility patterns, gut transit rate, and transcriptome in an agastric fish, the ballan wrasse

The effects of nutrient and mechanical sensing on gut motility and intestinal metabolism in lower vertebrates remains largely unknown. Here we present the transcriptome response to luminal stimulation by nutrients and an inert bolus on nutrient response pathways and also the response on gut motility in a stomachless fish with a short digestive tract; the ballan wrasse (Labrus berggylta). Using an in vitro model, we differentiate how signals initiated by physical stretch (cellulose and plastic beads) and nutrients (lipid and protein) modulate the gut evacuation rate, motility patterns and the transcriptome. Intestinal stretch generated by inert cellulose initiated a faster evacuation of digesta out of the anterior intestine compared to digestible protein and lipid. Stretch on the intestine upregulated genes associated with increased muscle activity, whereas nutrients stimulated increased expression of several neuropeptides and receptors which are directly involved in gut motility regulation. Although administration of protein and lipid resulted in similar bulbous evacuation times, differences in intestinal motility, transit between the segments and gene expression between the two were observed. Lipid induced increased frequency of ripples and standing contraction in the middle section of the intestine compared to the protein group. We suggest that this difference in motility was modulated by factors [prepronociceptin (pnoca), prodynorphin (pdyn) and neuromedin U (nmu), opioid neurotransmitters and peptides] that are known to inhibit gastrointestinal motility and were upregulated by protein and not lipid. Our findings show that physical pressure in the intestine initiate contractions propelling the bolus distally, directly towards the exit, whereas the stimuli from nutrients modulates the motility to prolong the residence time of digesta in the digestive tract for optimal digestion.

Effects of lysine and leucine in free and different dipeptide forms on the growth, amino acid profile and transcription of intestinal peptide, and amino acid transporters in turbot (Scophthalmus maximus)

This study was conducted to evaluate the effects of different dipeptides (lysine-leucine, lysine-glycine, and leucine-glycine) and free amino acids (lysine and leucine) on the growth, gene expression of intestinal peptide and amino acid transporters, and serum free amino acid concentrations in turbot. Fish (11.98 ± 0.03 g) were fed four experimental diets supplementing with crystalline amino acids (CAA), lysine-leucine (Lys-Leu), lysine-glycine (Lys-Gly), and leucine-glycine (Gly-Leu). Fish protein hydrolysate (FPH) containing a mixture of free amino acids and small peptides was designed as a positive control diet. There was no significant difference in the growth and feed utilization among three dipeptide diets (Lys-Leu, Lys-Gly, and Gly-Leu). Compared with the CAA group, feed efficiency ratio was significantly higher in the Lys-Leu and Lys-Gly groups, and protein efficiency ratio was significantly higher in the Lys-Leu group. For peptide transporter, oligopeptide transporter 1 (PepT1) mRNA level was not affected by dietary treatments. For amino acid transporters, lower expression of B0 neutral amino acid transporter 1 (B0AT1) and proton-coupled amino acid transporter 1 (PAT1) were observed in fish fed the dipeptide and FPH diets compared with the CAA diet. In conclusion, juvenile turbot fed Lys-Leu, Gly-Leu, and Lys-Gly had a similar growth performance, whereas lysine and leucine in the Lys-Leu form can be utilized more efficiently for feed utilization than those in free amino acid from. In addition, compared to free amino acids, dipeptides and fish protein hydrolysate in diets may down-regulate the expression of amino acid transporters but did not affect the expression of PepT1.

Low Dietary Fish Meal Induced Endoplasmic Reticulum Stress and Impaired Phospholipids Metabolism in Juvenile Pacific White Shrimp, Litopenaeus vannamei

This study mainly evaluated the low dietary fish-meal (FM) on growth performance, immune competence and metabolomics response of juvenile Pacific white shrimp, Litopenaeus vannamei reared at low salinity (7‰). Five experimental diets with graded levels (25, 20, 15, 10, and 5%) of FM were formulated. Weight gain, feed utilization and survival were decreased with the decreasing FM levels. When dietary FM decreased, glucose, cholesterol, total bile acids, and triglyceride in hemolymph decreased. Fatty acid synthesis was promoted and fatty acid lipolysis was reduced in hepatopancreas of shrimp fed low dietary FM. Endoplasmic reticulum (ER) stress related genes expression in hepatopancreas were down-regulated and in intestine were upregulated by low dietary FM. Inhibitor kappa B kinaseβ expression in intestine increased with the dietary FM levels, while mRNA levels of dorsal in hepatopancreas showed the opposite tendency. Hematoxylin and eosin (H&E) stain and transmission electron microscope analysis of intestinal samples indicated that low FM diets induced intestinal morphological damage, ER swollen and chromatin condensation. UPLC-Q/TOF-MS analysis indicated that degree of unsaturation of the fatty acid chains of phospholipids in hemolymph decreased with the decreasing dietary FM levels. Lysophospholipids and bile acids metabolism were disturbed by high levels of FM sparing in diet. These results indicated when dietary FM contents decreased, ER stress of shrimp was induced. The decreased unsaturated degree of phospholipids, decreased contents of lysophospholipids, altered lipid metabolism and ER stress may responsible for the impaired growth performance and health of shrimp fed a low FM diet.

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.

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.

Molecular cloning and the involvement of IRE1α-XBP1s signaling pathway in palmitic acid induced - Inflammation in primary hepatocytes from large yellow croaker (Larimichthys crocea)

Apart from mitigating endoplasmic reticulum (ER) stress, vast studies have demonstrated the crucial role of inositol-requiring transmembrane kinase and endonuclease 1α (IRE1α) - spliced X-box binding protein 1 (XBP1s) signaling pathway in inflammatory response in mammals. In addition, palmitic acid (PA)-induced inflammation has been verified in large yellow croaker (Larimichthys crocea). However, whether the IRE1α-XBP1s signaling pathway is involved in inflammatory response caused by PA remains poorly studied in fish. The present study was aimed at elucidating the role of the IRE1α-XBP1s signaling pathway in inflammatory response induced by PA in primary hepatocytes from large yellow croaker. In the present study, the full-length cDNA of ire1α and xbp1s were cloned and comprised 3793 bp and 1789 bp with an open reading frame of 3279 bp and 1170 bp, encoding 1093 and 390 amino acids, respectively. IRE1α protein possessed a protein kinase and endoribonuclease domain and XBP1s protein possessed a basic-leucine zipper domain. The IRE1α protein and XBP1s protein located to the ER membrane and nucleus respectively. The ire1α and xbp1s were widely transcribed in various tissues with the higher level in intestine, liver, adipose and head kidney. The ER stress-inducing agent tunicamycin (Tm) and PA treatment significantly activated the IRE1α-XBP1s signaling pathway and increased the pro-inflammatory genes expression including tumor necrosis factor α (tnfα), interleukin 6 (il-6) and interleukin 1β (il-1β) (P < 0.05). When KIRA6, the IRE1α kinase inhibitor, was used to block the IRE1α-XBP1s signaling pathway, the Tm and PA-induced pro-inflammatory genes expression was significantly suppressed (P < 0.05). These data indicated that the IRE1α-XBP1s signaling pathway was involved in the PA-induced inflammatory response in large yellow croaker.

Effects of dietary multi-strain probiotics supplementation in a low fishmeal diet on growth performance, nutrient utilization, proximate composition, immune parameters, and gut microbiota of juvenile olive flounder (Paralichthys olivaceus)

A 12-week feeding trial was conducted to evaluate the effects of multi-strain probiotics (MSP) in a low fish meal (FM) diet on overall performance, gut microbiota, selected non-specific immune responses and antioxidant enzyme activities of olive flounder (Paralichthys olivaceus) juveniles. A total of 225 healthy olive flounders (initial mean body weight, 13.5 ± 0.01 g) were randomly separated into 3 groups of 75 fish, each group having three replicates of 25 fish; first group was fed with a FM-based control diet (Con), 2nd group was fed with a low-FM diet containing a blend of plant and animal protein meals replacing 30% of the FM protein (FM30), and 3rd group was fed with the FM30 diet supplemented with 10⁸-10⁹ CFU kg^-1 of the MSP (Pro). With the exception of lipid retention, which was significantly lower in fish fed the FM30 diet compared to the other two treatments, no other statistically significant differences were recorded with respect to any of the other growth and nutrient utilization parameters. Myeloperoxidase and lysozyme activities of fish fed the Pro diet were much higher and significantly different than those of fish fed the FM30 diet. Glutathione peroxidase activity was significantly higher in Pro- than in Con-fed fish, which, in turn, was significantly higher than FM30-fed fish. Expression of immune-related genes including IL-1β, IL-6, and TNF-α was markedly upregulated in livers of the fish fed Pro diet compared to those fed the Con and FM30 diets. Furthermore, supplementation of MSP in FM30 diet enriched the Lactobacillus abundance in the fish gut as well as predictive gene functions in relation to lipid and carbohydrate metabolisms. These data suggested that the MSP could reduce the potential adverse effects of the low-FM diet and might be used as a healthy immunostimulant for olive flounder.

Resveratrol attenuates oxidative stress and inflammatory response in turbot fed with soybean meal based diet

Adding immunopotentiators to plant protein based diets has been a feasible way to improve fish growth performance and healthy status. In this study, an 8-week trial was carried out to explore the effects of resveratrol, a natural polyphenolic compound, on growth performance, anti-oxidative capacity and immune responses in turbot fed soybean meal based diet. As the results showed, replacement 45% fish meal with soybean meal (SBM) significantly depressed the fish growth, feed utilization and the heights of villi and microvilli in distal intestine. The mRNA levels of hepatic antioxidant enzymes, including superoxide dismutase (sod), glutathione peroxidase (gsh-px) and peroxiredoxin 6 (prx 6), were highly inhibited in SBM group. The inflammation related genes in intestine were also responsive to soybean meal. Supplying resveratrol showed no effects on fish growth performance but significantly restored the intestinal morphology and improved the mRNA levels of hepatic antioxidant enzymes as well as the activity of SOD. Meanwhile, resveratrol significantly improved the mRNA levels of anti-inflammatory cytokine transforming growth factor-β and inhibited the expression of pro-inflammatory cytokines tumor necrosis factor-α (tnf-ɑ), interleukin-1β (il-1β) and interleukin-8 (il-8). The results indicate that resveratrol could attenuate the oxidative stress and inflammatory response induced by soybean meal in turbot. This study shows resveratrol is an effective immunopotentiator to carnivorous fishes fed plant protein sources.

Establishment and characterization of a fibroblast-like cell line from the muscle of turbot (Scophthalmus maximus L.)

A continuous fibroblast-like cell line, TMF (turbot muscle fibroblasts), was established from juvenile turbot Scophthalmus maximus muscle with the method of trypsin digestion. It has been subcultured more than 60 passages for over 150 days. The TMF cells were cultured in L-15 medium supplemented with HEPES, fetal bovine serum (FBS), GlutaMAX, and basic fibroblast growth factor (bFGF). The optimal temperature for TMF culture was 24 °C. TMF cells were predominantly composed of fibroblastic-like cells, and the transcription factor 4 (TCF-4) was highly expressed in TMF cells. Chromosome analysis revealed that it had a diploid chromosome number of 2n = 44. The transfection efficiency achieved 54.95 ± 6.59%, and the cell mortality rate was about 8.70% when transfected with the nucleofection method. Meanwhile, the TMF cells showed a sensitive response to amino acid levels and activation target of rapamycin (TOR) signaling pathway. These results indicate that TMF was a potential tool to explore the signal transduction of teleost in vitro.

Replacing Fish Meal with Defatted Insect Meal (Yellow Mealworm Tenebrio molitor) Improves the Growth and Immunity of Pacific White Shrimp (Litopenaeus vannamei)

Simple Summary

Globally, Pacific white shrimp is one of the most commonly commercialized marine species in aquaculture. In recent years, the commercial production of this shrimp has experienced major challenges, including the availability and cost of fish meal—the main protein source of commercial shrimp—as well as frequent outbreaks of early mortality syndrome (EMS) among shrimp stock. In the present study, we investigated whether substituting fish meal with a defatted yellow mealworm meal could overcome these challenges. Our results from a series of feeding trials show that it is possible to partially or completely replace fish meal with this insect meal in isoproteic and isoenergetic diets. The insect meal improved growth and feed conversion performances of shrimp; optimum performances were achieved at 50% of fish meal replacement. Furthermore, shrimp that were fed the insect meal and then challenged with the pathogenic bacteria that cause EMS (Vibrio parahaemolyticus) had significantly improved survival rates and reduced immunosuppression. We conclude that an insect meal comprised of Tenebrio molitor mealworm, is a suitable alternative to fish meal in the commercial production of shrimp because of the meal’s high protein value and the presence of chitin/other bioactive substances that counter pathogen infection.

Abstract

Recently, ecological and economic issues have affected fish meal (FM) supply, the main source of protein for shrimp. This triggered a search for alternative dietary protein sources for shrimp production. We studied the consequences of replacing FM with a defatted insect meal, ŸnMeal^TM (YM), comprised of yellow mealworm (Tenebrio molitor). Growth and immune parameters of juvenile Pacific white shrimp (Litopenaeus vannanmei) were compared after an eight-week feeding trial. Shrimp were kept in aquaria with densities of 60 and 40 shrimp/m² and fed one of five diets in which a proportion of FM was replaced by YM. All diets were isoproteic, isoenergetic, and balanced in lysine and methionine. After the feeding trial, shrimp were challenged with pathogenic bacteria (Vibrio parahaemolyticus). Growth and feed conversion parameters improved when YM was included in shrimp diets; with the highest weight gain and best food conversion ratio (FCR) achieved when 50% of FM was replaced by YM versus the control diet that contained no YM (initial weight: 1.60 g/shrimp; growth: 5.27 vs. 3.94 g/shrimp; FCR 1.20 vs. 1.59). In challenged shrimp, mortality rates were significantly less among groups that received YM, with a 76.9% lower mortality rate in the 50% FM replacement group versus the control.

High percentage of dietary palm oil suppressed growth and antioxidant capacity and induced the inflammation by activation of TLR-NF-κB signaling pathway in large yellow croaker (Larimichthys crocea)

A 70-day feeding trial was conducted to investigate the effects of dietary fish oil (FO) replaced by palm oil (PO) on growth, biochemical and antioxidant response as well as inflammatory response in the liver of large yellow croaker (initial weight 15.87 ± 0.14 g). Four iso-proteic and iso-lipidic experimental diets were formulated with 0% (the control group), 33.3%, 66.7% and 100% FO replaced by PO. Fish fed the diet with 100% PO showed significantly lower growth performance than the control group. As expected, the contents of C16:0, C18:1n-9 and C18:2n-6 were increased with increasing dietary PO levels. There were remarkable increases in total cholesterol (TC) and low-density lipoprotein-cholesterol (LDL-C) levels in fish fed the diet with 100% PO compared to the control group. Moreover, dietary PO significantly increased activities of plasma alanine transaminase (ALT) and aspartate aminotransferase (AST) in fish fed the diet with 100% PO compared to the control group. The total antioxidant capacity (T-AOC) and the activity of catalase (CAT) in plasma were significantly decreased in fish fed the diet with 100% PO compared to the control group, and meanwhile no significant differences were found in T-AOC and CAT activity in fish fed diets with no more than 66.7% PO. Fish fed the diet with 100% PO exerted significantly higher toll like receptors (TLRs) and myeloid differentiation factor (MyD88) mRNA expression levels than the control group. The IFNγ, IL-1β and TNFα mRNA expressions were increased with increasing dietary PO levels. The increase of pro-inflammatory gene expression may be due to the activation of NF-κB signaling as the ratio of nucleus p65 to total p65 protein was elevated with the increase of dietary PO levels. These results showed that relatively higher PO levels in diets suppressed the growth and antioxidant capacity as well as induced the inflammatory response by activating TLR-NF-κB signaling pathway in juvenile large yellow croaker.

Dietary replacement of fish-meal impaired protein synthesis and immune response of juvenile Pacific white shrimp, Litopenaeus vannamei at low salinity

An 8-week feeding trial was conducted to evaluate the effect of fish-meal replacement on growth performance, protein synthesis and immune response of juvenile Pacific white shrimp, Litopenaeus vannamei reared at low salinity (7‰). Five isonitrogenous and isolipidic diets were formulated to contain graded levels (25, 20, 15, 10 and 5%) of fish-meal. High quality alternative solutions were performed, crystalline amino acids, phytase, mannan oligosaccharides and some micro-nutrients were supplemented in the low fish-meal diets. Each diet was randomly assigned to triplicate tanks, each tank with 30 shrimp (mean weight 0.3 g), the shrimp were fed 3 times a day. Weight gain and survival were decreased with the decreasing dietary fish meal levels. When dietary fish-meal decreased, the gene expression of TOR, Raptor and eIF4E2 in hepatopancreas were decreased with the decreasing fish meal levels, eIF4E2 in intestine was decreased while 4E-BP was increased with the decreasing fish meal levels. The mRNA level of SOD in hepatopancreas decreased, and the expression of GPx and CAT increased with the decreasing FM levels. The Toll pathway was affected by dietary FM levels, the expression of Toll2, TNFSF, MyD88, Rho and p38 in intestine were increased with the decreasing FM levels. The results indicated that at low salinity condition, fish meal level lower than 15% would inhibit the protein synthesis and harm to the health of shrimp.

Cottonseed protein concentrate (CPC) suppresses immune function in different intestinal segments of hybrid grouper ♀Epinephelus fuscoguttatus×♂Epinephelus lanceolatu via TLR-2/MyD88 signaling pathways

Cottonseed protein concentrate (CPC) has similar amino acid composition compared with fish meal, and has the characteristics of low gossypol and low toxicity. The present study was conducted to investigate the growth performance, antioxidant capacity and different intestinal segments immune responses of hybrid grouper to replacement dietary fish meal ofCPC. Six iso-nitrogenous (50% crude protein) and iso-lipidic (10% crude lipid) diets were formulated: a reference diet (FM) containing 60% fishmeal and five experimental diets (12%, 24%, 36%, 48 and 60%) in which fishmeal protein was substituted at different levels by CPC to feed fish (initial body weight: 11 ± 0.23 g) for 8 weeks. Thena challenge test with injection of Vibrio parahaemolyticus was conducted for 7 days until the fish stabilized. The results showed that specific growth rate (SGR) was the highest with 24% replacement level and feed conversion ratio (FCR)was significantly increased when the replacement level reached 48% (P < 0.05). The content of malonaldehyde (MDA) in the serum was significantly increased when the replacement level reached 36% (P < 0.05). The plica height in the proximal, mid and distal intestine were significantly decreased with the replacement level up to 48% (P < 0.05). Hepatic fat deposition wasaggravatedwhen the replacement level reached 36% (P < 0.05). The expression of IL-6, TNF-α, and IL-1β mRNAs were significantly up-regulated (P < 0.05). The hepcidin mRNA expression was significantly down-regulated (P < 0.05). In proximal intestine (PI) and mid intestine (MI), IFN-γ mRNA expression was significantly up-regulated (P < 0.05). These results suggested that the CPC decreased hybrid grouper growth performance and inflammation function, and different inflammation function responses in PI,MI, and distal intestine (DI) were mediated partly by the TLR-2/MyD88 signaling pathway. According to the analysis of specific growth rate, the dietary optimum replacement level and maximum replacement level were estimated to be 17% and 34%, respectively.

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.

Soybean saponin modulates nutrient sensing pathways and metabolism in zebrafish

Soybean saponin (SA) is known as a major anti-nutritional factor that causes metabolic disturbances and growth reduction in fish. However, the mechanisms underlying these effects were far from fully understood. In particular, the influences of SA on nutrient sensing and downstream metabolic pathways remain largely unexplored. Using zebrafish as an animal model, this study was conducted to examine the phenotypic and molecular responses after dietary SA treatment for 2 weeks. SA at both 5 and 10 g/kg diet levels significantly reduced growth performance and feed efficiency, and damaged the morphology of the intestinal mucosa. SA stimulated AMP-activated protein kinase but reduced target of rapamycin (TOR) activities in both feeding trial and cellular studies. Furthermore, SA increased the mRNA expressions of growth axis genes including growth hormone, insulin-like growth factor 1, growth hormone receptor A, and growth hormone receptor B, but decreased insulin-like growth factor-binding protein 2 at both mRNA and protein levels. SA also increased the expressions of key metabolic enzymes involved in glutamine synthetase, glutamate dehydrogenase and lipolysis, hormone-sensitive lipase and lipoprotein lipase. Our results demonstrated that SA modulated nutrient sensing pathways and metabolism, thus provide new aspects on the explanation of the physiological effects of SA.

Nutrient sensing and metabolic changes after methionine deprivation in primary muscle cells of turbot (Scophthalmus maximus L.)

The low methionine content in plant-based diets is a major limiting factor for feed utilization by animals. However, the molecular consequences triggered by methionine deficiency have not been well characterized, especially in fish species, whose metabolism is unique in many aspects and important for aquaculture industry. In the present study, the primary muscle cells of turbot (Scophthalmus maximus L.) were isolated and treated with or without methionine for 12 h in culture. The responses of nutrient sensing pathways, the proteomic profiling of metabolic processes, and the expressions of key metabolic molecules were systematically examined. Methionine deprivation (MD) suppressed target of rapamycin (TOR) signaling, activated AMP-activated protein kinase (AMPK) and amino acid response (AAR) pathways. Reduced cellular protein synthesis and increased protein degradation by MD led to increased intracellular free amino acid levels and degradations. MD also reduced glycolysis and lipogenesis while stimulated lipolysis, thus resulted in decreased intracellular lipid pool. MD significantly enhanced energy expenditure through stimulated tricarboxylic acid (TCA) cycle and oxidative phosphorylation. Collectively, our results identified a comprehensive set of transcriptional, proteomic, and signaling responses generated by MD and provided the molecular insight into the integration of cell homeostasis and metabolic controls in fish species.

Dietary vegetable oil suppressed non-specific immunity and liver antioxidant capacity but induced inflammatory response in Japanese sea bass (Lateolabrax japonicus)

High percentage of dietary vegetable oil (VO) induced negative effects on immunity in numerous fish species. The present study was conducted to investigate whether VO could exert anti-immunological effects by regulating non-specific immunity, liver antioxidant capacity and nuclear factor kappa beta (NF-κB) signaling in Japanese sea bass (Lateolabrax japonicus). Three iso-nitrogenous and iso-lipid diets were formulated by replacing 0% (FO, the control), 50% (FV) and 100% (VO) of fish oil with vegetable oil. Each diet was randomly fed to triplicate groups of fish for 10 weeks. Results showed that the alternative complement pathway (ACP) activity and the disease resistance were significantly lower in fish fed VO diets compared with the control group (P < 0.05). Liver superoxide dismutase (SOD), catalase (CAT) and glutathion peroxidase (GPx) enzyme activities, as well as total antioxidant capacity (T-AOC) significantly decreased in fish fed VO diets (P < 0.05). Meanwhile, significantly low level of liver SOD1 and CAT mRNA, nuclear factor erythroid 2-related factor 2 (Nrf2) of both mRNA and protein were observed in fish fed VO diets when compared with fish fed FO diets (P < 0.05). However, the transcription level of TNFα and IL1β was significantly higher in the liver of fish fed VO diets, which might be attributed to the activation of NF-κB signaling pathway since the protein expression of p65, one of the key members of NF-κB family, was significantly increased (P < 0.05). These results suggested that dietary VO could lower the ACP activity, disease resistance and liver antioxidant capacity, but it could also exacerbate inflammatory response by activating NF-κB signaling pathway in Japanese sea bass.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

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.

Vegetable oil induced inflammatory response by altering TLR-NF-κB signalling, macrophages infiltration and polarization in adipose tissue of large yellow croaker (Larimichthys crocea)

High level of vegetable oil (VO) in diets could induce strong inflammatory response, and thus decrease nonspecific immunity and disease resistance in most marine fish species. The present study was conducted to investigate whether dietary VO could exert these anti-immunological effects by altering TLR-NF-κB signalling, macrophages infiltration and polarization in adipose tissue of large yellow croaker (Larimichthys crocea). Three iso-nitrogenous and iso-lipid diets with 0% (FO, fish oil, the control), 50% (FV, fish oil and vegetable oil mixed) and 100% (VO, vegetable oil) vegetable oil were fed to fish with three replicates for ten weeks. The results showed that activities of respiratory burst (RB) and alternative complement pathway (ACP), as well as disease resistance after immune challenge were significantly decreased in large yellow croaker fed VO diets compared to FO diets. Inflammatory response of experimental fish was markedly elevated by VO reflected by increase of pro-inflammatory cytokines (IL1β and TNFα) and decrease of anti-inflammatory cytokine (arginase I and IL10) genes expression. TLR-related genes expression, nucleus p65 protein, IKKα/β and IκBα phosphorylation were all significantly increased in the AT of large yellow croaker fed VO diets. Moreover, the expression of macrophage infiltration marker proteins (cluster of differentiation 68 [CD68] and colony-stimulating factor 1 receptor [CSF1R]) was significantly increased while the expression of anti-inflammatory M2 macrophage polarization marker proteins (macrophage mannose receptor 1 [MRC1] and cluster of differentiation 209 [CD209]) was significantly decreased in the AT of large yellow croaker fed VO diets. In conclusion, VO could induce inflammatory responses by activating TLR-NF-κB signalling, increasing macrophage infiltration into adipose tissue and polarization of macrophage in large yellow croaker.

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.

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.