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.)

Administration of EPA improves lipopolysaccharide-induced intestinal dysfunction in turbot (Scophthalmus maximus L.) through modulation of TORC1 signaling

Intestinal health is crucial for digestive and absorptive functions, which are associated with fish growth performance. Searching for nutraceuticals and bioactive ingredients to improve intestinal health has become urgent for the aquaculture industry. In the present study, the effects of eicosapentaenoic acid (EPA) on intestinal function were investigated in turbot with induced intestinal damage caused by lipopolysaccharide (LPS). Juvenile turbot (initial weight 100 ± 5 g) were subjected to a 10-day enforced feeding regimen. Each fish was fed twice daily with either a 100 mg amino acid mixture (CON), an additional 1 mg of LPS (LPS), or an additional combination of 1 mg LPS and 1.7 mg EPA (LE). The results indicated that EPA supplementation restored intestinal villi integrity and the mucosal barrier. The number of goblet cells and the gene expression of MUC-2 were increased by EPA. Additionally, EPA suppressed the expression of inflammatory factors (MyD88 and NF-κB p65) and pro-inflammatory cytokines (TNFα, IL-1β, and IFN-γ). Meanwhile, post-feeding plasma free amino acid levels as well as intestinal protein synthesis were improved by EPA supplementation. The target of rapamycin (TOR) signaling pathway was significantly activated by EPA. Furthermore, EPA supplementation positively influenced intestinal microbiota, reducing the abundance of prevalent pathogens while enhancing the abundance of probiotics. Collectively, the administration of EPA effectively mitigates LPS-induced damage to the intestinal barrier, inflammatory response, and dysbiosis of microbiota through modulation of the TOR signaling pathway.

Dietary fishmeal replacement by Clostridium autoethanogenum protein meal influences the nutritional and sensory quality of turbot (Scophthalmus maximus) via the TOR/AAR/AMPK pathways

Clostridium autoethanogenum protein (CAP) is a promising protein source for aquaculture; however, how CAP influences fish quality is worth extensive research. We randomly allocated 630 turbot with initial body weights of about 180 g into 6 groups, with fishmeal-based control diet or diet with CAP replacing 15% (CAP15), 30% (CAP30), 45% (CAP45), 60% (CAP60), or 75% (CAP75) of fishmeal protein. After a 70-d feeding trial, the fillet yield (P = 0.015) and content of protein (P = 0.017), collagen (P < 0.001), hydroxyproline (P < 0.001), C20:5n-3 (P = 0.007), and ∑n-3/∑n-6 polyunsaturated fatty acids ratio (P < 0.001) in turbot muscle was found to decrease linearly with increasing CAP. However, turbot fed CAP15 diet maintained these parameters (P > 0.05). By contrast, the muscle hardness increased linearly with increasing CAP (P = 0.004), accompanied by linear reduction of muscle fiber area (P = 0.003) and expression of myogenesis-related genes, including cathepsin D (ctsd P < 0.001) and muscle ring finger protein 1 (murf 1, P < 0.001). Phosphorylation of protein kinase B (Akt, P < 0.001), target of rapamycin (TOR, P = 0.001), eukaryotic initiation factor 4E-binding protein 1 (4E-BP1, P < 0.001), and ribosomal protein S6 (S6, P < 0.001) decreased linearly; however, phosphorylation of AMP-activated protein kinase (AMPK, P < 0.001), eukaryotic initiation factor 2α (eIF2α, P < 0.001), and the abundance of activating transcription factor 4 (ATF4, P < 0.001) increased with increasing CAP, suggesting that the TOR signaling pathway was inhibited, and the amino acid response (AAR) and AMPK pathways were activated. Additionally, expression of genes related to protein degradation, including myogenic factor 5 (myf 5, P < 0.001), myogenic differentiation (myod, P < 0.001), paired box 7 (pax 7, P < 0.001), and ctsd (P < 0.001), decreased linearly with increasing CAP. In conclusion, CAP could be used to replace up to 15% of fishmeal without negatively impacting turbot quality. However, higher levels of CAP decreased fillet yield, muscle protein content, and muscle fiber diameter while increasing muscle hardness, which could be attributed to the inhibition of the TOR pathway and activation of the AAR and AMPK pathways.

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.

The preferential utilization of hepatic glycogen as energy substrates in largemouth bass (Micropterus salmoides) under short-term starvation

To elucidate the underlying mechanism of the energy metabolism in largemouth bass (Micropterus salmoides), cultured fish (initial body weight: 77.57 ± 0.75 g) in the present study were starved for 0 h, 12 h, 24 h, 48 h, 96 h and 192 h, respectively. The proximate composition analysis showed that short-term starvation induced a significant up-regulation in crude protein proportion in hepatic of cultured fish (P < 0.05). However, short-term starvation significantly decreased the hepatosomatic index and the viscerosomatic index of cultured fish (P < 0.05). The exact hepatic glycogen content in the group starved for 92 h presented remarkable decrease (P < 0.05). Meanwhile, compared with the weight change of lipid and protein (mg) in hepatic (y = 0.0007x2 - 0.2827x + 49.402; y = 0.0013x2 - 0.5666x + 165.31), the decreasing trend of weight in glycogen (mg) was more pronounced (y = 0.0032x2 - 1.817x + 326.52), which suggested the preferential utilization of hepatic glycogen as energy substrates under short-term starvation. Gene expression analysis revealed that the starvation down-regulated the expression of insulin-like growth factor 1 and genes of TOR pathway, such as target of rapamycin (tor) and ribosomal protein S6 (s6) (P < 0.05). In addition, the starvation significantly enhanced expression of lipolysis-related genes, including hormone-sensitive lipase (hsl) and carnitine palmitoyl transferase I (cpt1), but down-regulated lipogenesis as indicated by the inhibited expression of fatty acids synthase (fas), acetyl-CoA carboxylase 1 (acc1) and acetyl-CoA carboxylase 2 (acc2) (P < 0.05). Starvation of 24 h up-regulated the expression of glycolysis genes, glucokinase (gk), phosphofructokinase liver type (pfkl) and pyruvate kinase (pk), and then their expression returned to the normal level. Meanwhile, the expression of gluconeogenesis genes, such as glucose-6-phosphatase catalytic subunit (g6pc), fructose-1,6-bisphosphatase-1 (fbp1) and phosphoenolpyruvate carboxy kinase (pepck), was significantly inhibited with the short-term starvation (P < 0.05). In conclusion, short-term starvation induced an overall decline in growth performance, but it could deplete the hepatic glycogen accumulation and mobilize glycogen for energy effectively.

Effects of Dietary Protein Levels on Growth, Digestive Enzyme Activity, Antioxidant Capacity, and Gene Expression Related to Muscle Growth and Protein Synthesis of Juvenile Greasyback Shrimp (Metapenaeus ensis)

An 8-week feeding trial was conducted to assess the effects on growth, antioxidant capacity, digestive enzyme activity, and gene expression related to muscle growth and protein synthesis of juvenile greasyback shrimp (Metapenaeus ensis) using five experimental diets containing 29.37%, 34.30%, 39.11%, 44.05%, and 49.32% of protein. The results demonstrated that juvenile greasyback shrimp consuming 39.11%, 44.05%, and 49.32% dietary protein had a significantly higher final body weight (FBW), weight gain (WG), feed conversion ratio (FCR), and specific growth rate (SGR) than other groups (p < 0.05). The protein efficiency ratio (PER) showed a significantly quadratic pattern with increasing dietary protein levels (p < 0.05). The highest trypsin and pepsin activities were observed in the group with a protein level of 44.05% (p < 0.05). Relatively higher superoxide dismutase (SOD) activity was found in groups with protein levels of 39.11% (p < 0.05). Alkaline phosphatase (AKP) and catalase (CAT) activity showed a significantly linear increasing pattern with increasing protein intake up to 44.05%, and then decreased gradually (p < 0.05). Compared to the dietary 29.37% protein level, the expression levels of myogenic regulatory factors (mef2α, mlc, and myf5) and mTOR pathway (mtor, s6k, akt, and pi3k)-related genes were significantly up-regulated in muscle with 39.11%, 44.05%, and 49.32% dietary protein levels (p < 0.05). The AAR pathway (gcn2, eif2α, and atf4)-related gene expression levels were significantly lower in muscles with 39.11%, 44.05%, and 49.32% protein levels than in other groups (p < 0.05). Based on the broken-line regression analysis of SGR, the estimated appropriate dietary protein requirement for juvenile greasyback shrimp is 38.59%.

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.

Squid industry by-product hydrolysate supplementation enhances growth performance of Penaeus monodon fed plant protein-based diets without fish meal

The poor growth of aquatic animals fed with diets containing high plant proteins has been attributed to low diet acceptability and feed value. Supplementation of protein hydrolysate, with high contents of free amino acids and soluble low molecular weight peptides, may increase the acceptability and feed value of a plant protein-based diet. In the present work, squid processing by-products were enzymatically hydrolyzed and used as a supplement in a plant protein-based diet, without fish meal, of Penaeus monodon to fully maximize the utilization of this marine resource. The hydrolysate was incorporated at 0, 0.5, and 1% levels in P. monodon diets containing 0 and 10% fish meal levels. Growth, digestive enzyme activities, muscle growth-, gut pro-inflammatory and immune-related gene expressions, and muscle morphometric measurements were evaluated as biological indices in an 8-week feeding trial. The squid by-product hydrolysate produced in the present study contains 90.25% protein, 5.84% lipid, and 3.91% ash, and has a molecular weight of 3.76 kDa. Supplementation at 1% hydrolysate in the experimental shrimp diet without fish meal resulted in the highest growth performance associated with increased feed intake, efficient feed and nutrient conversion and retention, enhanced digestive enzyme activities, upregulation of muscle growth- and immune-related genes, and suppression of the gut pro-inflammatory gene. The growth promotion is also linked with a significant increase in muscle mean fiber area, which suggests hypertrophic growth in shrimp. Generally, the supplementation of 1% squid by-product hydrolysate supported the growth of P. monodon fed on a plant protein-based diet without fish meal.

Effects of fish meal replacement with Chlorella meal on growth performance, pigmentation, and liver health of largemouth bass (Micropterus salmoides)

Chlorella meal is a potential protein source for aquafeeds. However, the physiological response of carnivorous fish fed Chlorella meal remains elusive. This study evaluated the effects of replacing dietary fish meal with Chlorella meal on growth performance, pigmentation, and liver health in largemouth bass. Five diets were formulated to replace dietary fish meal of 0% (C0, control), 25% (C25), 50% (C50), 75% (C75), and 100% (C100) with Chlorella meal, respectively. Total 300 fish (17.6 ± 0.03 g) were randomly assigned to 15 tanks (3 tanks/group). Fish were fed the experimental diet twice daily for 8 weeks. The increased dietary Chlorella meal quadratically influenced the final body weight (FBW), weight gain rate (WGR), specific growth rate (SGR), and feed intake (FI), which were significantly lower in the C100 group than in the other groups (P < 0.05). The feed conversion ratio (FCR) increased linearly or quadratically with dietary Chlorella meal. Dietary Chlorella meal linearly or quadratically increased the lutein content of plasma, liver, and dorsal muscle of largemouth bass (P < 0.05). Compared to the C0 group, all supplemented Chlorella meal groups significantly improved the yellowness (b∗) of the dorsal body (1.5 to 2.0 fold), abdominal body (1.5 to 1.8 fold), and dorsal muscle (3.8 to 5.4 fold) of largemouth bass (P < 0.05). In addition, compared to the C0 group, the liver vacuolation area of fish was significantly increased in the C75 and C100 groups (P < 0.05). Transcriptional levels of apoptosis-related genes of β-cell lymphoma-2 (bcl2), caspase-9-like (casp9), and caspase-3a (casp3) were markedly upregulated (0.9 to 1.6 fold) in the C100 group compared to the C0 group (P < 0.05). Based on the quadratic regression analysis between FBW, WGR, or SGR and dietary Chlorella meal level, largemouth bass had the best growth when replacing 31.7% to 32.6% of fish meal with 15.03% to 15.43% dietary Chlorella meal. The present results indicated that dietary supplementation with Chlorella meal (11.85% to 47.45%) significantly enhanced the pigmentation; however, total replacement of fish meal (40%) with Chlorella meal (47.45%) caused growth retardation, apoptosis, and liver damage in largemouth bass.

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.

Effects of Replacement of Dietary Fishmeal by Cottonseed Protein Concentrate on Growth Performance, Liver Health, and Intestinal Histology of Largemouth Bass (Micropterus salmoides)

An 8-week feeding trial was conducted to explore the effects of replacement of dietary fishmeal by cottonseed protein concentrate (CPC) on growth performance, liver health, and intestine histology of largemouth bass. Four isoproteic and isolipidic diets were formulated to include 0, 111, 222, and 333 g/kg of CPC, corresponding to replace 0% (D1), 25% (D2), 50% (D3), and 75% (D4) of fishmeal. Two hundred and forty largemouth bass (15.11 ± 0.02 g) were randomly divided into four groups with three replicates per group. During the experiment, fish were fed to apparent satiation twice daily. Results indicated that CPC could replace up to 50% fishmeal in a diet for largemouth bass without significant adverse effects on growth performance. However, weight gain rate (WGR), specific growth rate (SGR), feed efficiency (FE), and condition factor (K) of the largemouth bass were significantly decreased when 75% of dietary fishmeal that was replaced by CPC. The whole body lipid content was increased with the increasing of dietary CPC levels. Oil red O staining results indicated that fish fed the D4 diet showed an aggravated fat deposition in the liver. Hepatocytes exhibited serious degeneration, volume shrinkage, and inflammatory cells infiltration in the D4 group. Intestinal villi appeared shorter and sparse with severe epithelial damage in the D4 group. The transcription levels of anti-inflammatory cytokines, such as transforming growth factor β (tgf-β), interleukin 10 (il-10), and interleukin 11 β (il-11β), were downregulated in the D4 group. The lipid metabolism-related genes carnitine palmitoyl transferase 1 (cpt1), peroxisome proliferator-activated receptor α (pparα), and target of rapamycin (TOR) pathway were also significantly downregulated in the D4 group. It was concluded that suitable replacement of fishmeal by less than 222 g CPC/kg diet had a positive effect on growth performance of fish, but an excessive substitution of 75% fishmeal by CPC would lead to the suppressed growth, liver inflammation, and intestinal damage of largemouth bass.

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.

Dietary Astragalus polysaccharides ameliorates the growth performance, antioxidant capacity and immune responses in turbot (Scophthalmus maximus L.)

Supplying immunostimulants to aquatic feed has been an effective way to enhance the health of aquatic animals and substitute for antibiotics. In the present study, the potential effects of Astragalus polysaccharides (APS) were evaluated in turbot, Scophthalmus maximus. Two levels of APS (50 and 150 mg/kg) were added to the basal diet (CON) and a 63-day growth trial (initial weight 10.13 ± 0.04 g) was conducted. As the results showed, significant improvement on growth performance in the APS groups were observed. In addition, dietary 150 mg/kg APS significantly increased the total antioxidant capacity (T-AOC), glutathione peroxidase (GSH-PX) and lysozyme activities in liver. Meanwhile, APS diets induced the mRNA expression of toll-like receptors (TLRs) such as tlr5α, tlr5β, tlr8 and tlr21, while reduced the expression of tlr3 and tlr22. The expression of inflammatory genes myeloid differentiation factor 88 and nuclear factor kappa b p65 and pro-inflammatory cytokines tumor necrosis factor-α and interleukin-1β were up-regulated in APS groups while the expression of anti-inflammatory cytokine transforming growth factor beta was inhibited. Taken together, the present study indicated that Astragalus polysaccharides could remarkably enhance the growth performance, antioxidant activity and maintain an active immune response in turbot.

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.

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.

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.