| This study is conducted on turbot. Scophthalmus maximus L., which was wildly cultured as an important commercial carnivorous species in northern China. Feeding experiments were conducted to investigate the effects of different diets on fish growth performance, feed utilization, physiological and biochemical indices in juvenile turbot. Based on these studies, it was aimed to investigate the mechanisms of involved in the related signaling pathway. The results and conclusions are summarized as follows:1 Fishmeal replacement by mixed plant proteins and maggot meal on growth performance, nutrient sensing and metabolism in juvenile turbot(Scophthalmus maximus L.)Fishmeal replacement in aquafeeds remains a major challenge. In the present study, an experiment was designed to replace fishmeal by mixed plant proteins and maggot meal in turbot diet. Six diets were formulated:a reference diet (FM) containing 63%fish meal and four experimental diets (35(0%),35(3%).40(0%)). 40(3%)) in which fishmeal was substituted at different levels by plant proteins with 0-3% maggot meal. Turbot (4.90±0.03 g) was fed with these diets for 9 weeks. The whole body content of protein, lipid and ash in fish fed all five experimental diets were similar to those of FM group. Moreover, fishmeal replacement did not significantly affect its muscle essential fatty acid composition, especially those polyunsaturated fatty acids (HUFA). which reflects the quality of flesh. These suggested that mixed plant proteins with maggot meal can replace 40%(35%) fishmeal without affecting its growth performance and flesh quality in turbot diet. Further studies showed that in fish fed 40(0%)diet. the digestibility of dry diet, protein and energy were all significantly lower than those fed Con. Moreover, the digestive enzymes in intestine such as amylase and proteinase were also lower than those fed Con. These suggested that decreased digestive enzyme activities in digestive tract resulted in the declined digestibility of diet and finally caused the bad growth performance such as Specific Growth Rate. Weight Gain and Feed efficiency Rate of fish in 40(0%). Moreover, free amino acid analysis showed that muscle taurine content increased with dietary addition, while plasma hyd-pro content was significantly lower.with significantly higher plasma proline contentthan those fed Con diet. This suggested the metabolism between proline and hy-pro is not good in juvenile turbot. and fish cannot turnover proline into hyd-pro, which is also the reason for bad growth performance in fish fed 40(0%) diet. However, when maggot meal was added, the growth performance of fish fed 40(3%) diet was similar to those fed Con. with better diet digestibility and digestive enzyme activity than fish fed 40(0%)diet. These all suggested the good prospect and great economic value of maggot meal usage in juvenile turbot diet.More efforts were taken to illustrate the inner mechanism which may constrain the growth performance of turbot fed 40(0%). TOR signaling pathway is the main signaling factor controlling nutrient sensing and protein synthesis. In this experiment. TOR and 4EBP1 mRNA and protein relative abundance were not influenced by fishmeal treatment.However, p-TOR and p-S6 protein level is significantly decreased in 40(0%) compared to Con. These also suggested that in fish, TOR signaling pathway modulated protein synthesis and organism growth at the post-translational level other than transcriptional or translational level.2. Dietary sulfur amino acid modulations of taurine biosynthesis in juvenile turbot(Scophthalmus maximus L.)Taurine is essential for development, normal growth and homeostasis in various fish species. However, very limited information was available on the regulation of taurine biosynthesis in fish. In the present study, the effects of dietary sulfur amino acids supplementation on taurine biosynthesis were examined in juvenile turbots. Fish were fed with a casein-based semi-purified basal diet or diets supplemented with methionine at 0.5%(MM).1.5%(HM). or cysteine at 0.3%(MC).0.6%(HC). or taurine at 1.5%(MT).2.5%(HT). twice per day for 2 weeks. Growth performance, taurine concentration in multiple tissues, hepatic activities and mRNA relative abundance of cysteine dioxygenase (CDO) and hepatic cysteine sulfinic acid decarboxylase (CSD) activities were measured. Methionine. cysteine and taurine supplementations promoted growth performance in turbot. Methionine and cysteine supplementation stimulated CDO activities, while no significant mRNA relative abundance elevation was observed. Besides, no significant changes of hepatic CSD activities were observed. Dietary supplementation of methionine. cysteine and taurine increased taurine concentration in liver of turbot These results suggest that juvenile turbot have considerable taurine biosynthesis capability and can be modulated by dietary sulfur amino acids, but may not be enough for its endogenous requirement.3 Roles of target of rapamycin in nutrient metabolism and physiology of juvenile turbot (Scophthalmus maximus L.)High dietary protein inclusion is characteristic for fish nutrition and a major cost for aquaculture industry, which demands improved dietary conversion into tissue proteins in fish. However, the regulatory mechanism for fish protein synthesis was not clear. In particular, the roles of target of rapamycin (TOR), the major nutrition responsive molecule identified in other species, remained elusive in fish. In the present study, we examined the nutritional physiology of turbot after chronic rapamycin inhibition. Our results showed that long-term TOR inhibition by rapamycin inhibited growth performance, feed utilization and protein retention. Hepatic glycolysis was decreased after rapamycin treatment, with increasedhepatic gluconeogenesis. Hepatic lipogenic genes including FAS. Soat2. Dgat2 was significantly reduced after rapamycin treatment, in consistent with lower plasma cholesterol level, while fatty acid β-oxidation. indicated by Cptla was significantly increased after rapamycin treatment.Hepatic gap junction was affected by rapamycin treatment with decreased connexin 43-protein expression, while rapamycin treatment increased intestinal goblet cell. These all suggested that mTOR plays important roles in growth and nutrient metabolism.4 Modulation of hepatic lipid metabolism and immune system by different levels of linseed oil in juvenile turbot (Scophthalmus maximus L.)It has been an increasing trend to replace fish oil (FO) in aquaculture diets with vegetable oils (VO). driven by the short supply of FO derived from wild fish stocks. However, little is known of the fish health consequences and metabolic changes after such substitution. The effects on lipid metabolic gene expression, immune parameters and transferrin gene expression were determined in turbot(Psetta maxima) by increasing dietary LO levels as 0.1/3.2/3 and 100% of lipid sources. Dietary LO had no major effect on growth and feed efficiency of turbot when the substitution level was not over 2/3. LO replacement decreased the whole fish protein contents while 1/3 LO replacing level showed the largest body lipid content. HUFA biosynthesis, especially fatty acids desaturation reflected by FADS and cholesterol biosynthesis reflected by Soat2 gene expression was significantly increased when FO was completely replaced by LO. Fatty acid β-oxidation. shown by Cptla gene expression, was decreased at complete fish oil replacing level by LO. The regulating factors. PPARa did not show any significant change while SREBP1 showed a significant decrease at lower replacing level but same to FO at 100% replacing level, suggesting its role in controlling FA incorporation into cholesteryl esters. These are just paralyzed with serum metabolites as TCHO and HDL increased at lower replacing level but decreased at complete replacing level. Serum AST increased with increasing dietary LO level but ALT did not show any changes. For the immune parameters, serum LMZ activity was significantly inhibited at 2/3 and 100% replacing level while total SOD activity was not significantly changed. In addition. T-AOC activity was significantly decreased in 100% replacing level. Further studies are still needed to illustrate the inner mechanism of nutritional regulation.5 Potential of additive Schizochytrium sp. to linseed oil based turbot diet:Effect on growth, muscle fatty acid, serum metabolites and lipid metabolism of juvenile turbot, Scophthalmus maximus L.A laboratory experiment was conducted to evaluate the effects of supplementation of marine algae. Schizochytrium sp. to linseed oil based diet on the growth performance, fatty acid composition, lipid metabolism and serum immune parameters of juvenile turbot. Linseed oil was used to full)-replace fish oil in turbot diet without or with Schizochytrium sp. addition, and all these three iso-nitrogenous and iso-lipogenic diets were fed twice daily to satisfaction for 9 weeks to juvenile turbot (5.00 g ±0.10) iin a circulating system. WGR and FER were significantly decreased after linseed oil replacing fish oil while recovered to similar level like FO after Schizochytrium sp. supplementation. Schizochytrium sp. supplementation significantly decreased moisture content but increased protein and lipid level. Moreover, muscle n-3HUFA content was significantly decreased after linseed oil replacement, but showed moderate recover after Schizochytrium sp. supplementation, especially muscle DHA content. Lipid metabolic genes, such as Cptla and LPL was significantly decreased in LO group, while recovered after Schizochytrium sp. supplementation. In addition, serum ALP. CAT and T-AOC activity were also recovered after Schizochytrium sp. supplementation. These results suggested the potential of Schizochytrium sp. using as a feed additive to turbot diet to improve growth, lipid metabolism and immunity. |
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