Integration of insulin and amino acid signals that regulate hepatic metabolism-related gene expression in rainbow trout: role of TOR

Precision formulation, a new concept to improve dietary amino acid absorption based on the study of cationic amino acid transporters

Amino acid (AA) transporters (AAT) control AA cellular fluxes across membranes, contributing to maintain cellular homeostasis. In this study, we took advantage of rainbow trout metabolic feature, which highly relies on dietary AA, to explore the cellular and physiological consequences of unbalanced diets on AAT dysregulations with a particular focus on cationic AAs (CAA), frequently underrepresented in plant-based diets. Results evidenced that 24 different CAAT are expressed in various trout tissues, part of which being subjected to AA- and CAA-dependent regulations, with y+LAT2 exchanger being prone to the strongest dysregulations. Moreover, CAA were shown to control two major AA-dependent activation pathways (namely mTOR and GCN2) but at different strength according to the CAA considered. A new feed formulation strategy has been put forward to improve specifically the CAA supplemented absorption in fish together with their growth performance. Such "precision formulation" strategy reveals high potential for nutrition practices, especially in aquaculture.

Identification and Functional Analysis of foxo Genes in Chinese Tongue Sole (Cynoglossus semilaevis)

The Chinese tongue sole (Cynoglossus semilaevis) is a traditional, precious fish in China. Due to the large growth difference between males and females, the investigation of their sex determination and differentiation mechanisms receives a great deal of attention. Forkhead Box O (FoxO) plays versatile roles in the regulation of sex differentiation and reproduction. Our recent transcriptomic analysis has shown that foxo genes may participate in the male differentiation and spermatogenesis of Chinese tongue sole. In this study, six Csfoxo members (Csfoxo1a, Csfoxo3a, Csfoxo3b, Csfoxo4, Csfoxo6-like, and Csfoxo1a-like) were identified. Phylogenetic analysis indicated that these six members were clustered into four groups corresponding to their denomination. The expression patterns of the gonads at different developmental stages were further analyzed. All members showed high levels of expression in the early stages (before 6 months post-hatching), and this expression was male-biased. In addition, promoter analysis found that the addition of C/EBPα and c-Jun transcription factors enhanced the transcriptional activities of Csfoxo1a, Csfoxo3a, Csfoxo3b, and Csfoxo4. The siRNA-mediated knockdown of the Csfoxo1a, Csfoxo3a, and Csfoxo3b genes in the testicular cell line of Chinese tongue sole affected the expression of genes related to sex differentiation and spermatogenesis. These results have broadened the understanding of foxo's function and provide valuable data for studying the male differentiation of tongue sole.

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

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

Fish Meal Replacement by Mixed Plant Protein in the Diets for Juvenile Yellow Catfish Pelteobagrus fulvidraco: Effects on Growth Performance and Health Status

Increasing dietary replacement levels of fish meal by alternative plant proteins are of value for aquaculture. Here, a 10-week feeding experiment was undertaken to explore the effects of fish meal replacement by mixed plant protein (at a 2 : 3 ratio of cottonseed meal to rapeseed meal) on growth performance, oxidative and inflammatory responses, and mTOR pathway of yellow catfish Pelteobagrus fulvidraco. Yellow catfish (2.38 ± 0.1 g, mean ± SEM) were randomly divided into 15 indoors fiberglass tanks, 30 fish each tank, and fed five isonitrogenous (44% crude protein) and isolipidic (9% crude fat) diets with fish meal replaced by mixed plant protein at 0% (the control), 10% (RM10), 20% (RM20), 30% (RM30), and 40% (RM40), respectively. Among five groups, fish fed the control, and RM10 diets tended to have higher growth performance, higher protein content, and lower lipid content in livers. Dietary mixed plant protein substitute increased hepatic free gossypol content and damaged liver histology and reduced the serum total essential amino acids, total nonessential amino acids, and total amino acid contents. Yellow catfish fed the control, and RM10 diets tended to have higher antioxidant capacity. Dietary mixed plant protein replacement tended to promote proinflammatory responses and inhibited mTOR pathway. Based on the second regression analysis of SGR against mixed plant protein substitutes, the optimal replacement level of fish meal by mixed plant protein was 8.7%.

Involvement of Mechanistic Target of Rapamycin (mTOR) in Valine Orexigenic Effects in Rainbow Trout

This study was aimed at clarifying the importance of a mechanistic target of rapamycin (mTOR) in the central orexigenic effect of valine in fish. For this, rainbow trout (Oncorhynchus mykiss) were intracerebroventricularly (ICV) injected with valine alone or in the presence of rapamycin as the mTOR inhibitor, and two experiments were performed. In the first experiment, we evaluated feed intake levels. In the second experiment, we evaluated in the hypothalamus and telencephalon the following: (1) the phosphorylation status of mTOR and its downstream effectors ribosomal protein S6 and p70 S6 kinase 1 (S6K1), (2) the abundance and phosphorylation status of transcription factors involved in appetite regulation, and (3) the mRNA levels of key neuropeptides associated with homeostatic regulation of feed intake in fish. Rising central levels of valine clearly resulted in an orexigenic response in rainbow trout. This response occurred in parallel with mTOR activation in both the hypothalamus and telencephalon, as supported by depressant changes in proteins involved in mTOR signalling (S6 and S6K1). Also, these changes disappeared in the presence of rapamycin. However, it is not clear which precise mechanisms link the activation of mTOR and the alteration in feed intake levels since we did not observe changes in mRNA levels of appetite-regulatory neuropeptides as well as in the phosphorylation status and levels of integrative proteins.

Nitrate exposure reprograms hepatic amino acid and nutrient sensing pathways prior to exercise: A metabolomic and transcriptomic investigation in zebrafish (Danio rerio)

Scope: Nitrate supplementation is a popular ergogenic aid that improves exercise performance by reducing oxygen consumption during exercise. We investigated the effect of nitrate exposure and exercise on metabolic pathways in zebrafish liver.

Materials and methods: Fish were exposed to sodium nitrate (606.9 mg/L), or control water, for 21 days and analyzed at intervals during an exercise test. We utilized untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis and measured gene expression of 24 genes central to energy metabolism and redox signaling.

Results: We observed a greater abundance of metabolites involved in endogenous nitric oxide (NO) metabolism and amino acid metabolism in nitrate-treated liver at rest, compared to rested controls. In the absence of exercise, nitrate treatment upregulated expression of genes central to nutrient sensing (pgc1a), protein synthesis (mtor) and purine metabolism (pnp5a and ampd1) and downregulated expression of genes involved in mitochondrial fat oxidation (acaca and cpt2).

Conclusion: Our data support a role for sub-chronic nitrate treatment in the improvement of exercise performance, in part, by improving NO bioavailability, sparing arginine, and modulating hepatic gluconeogenesis and glycolytic capacity in the liver.

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.

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.

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

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

Alterations of amino acid metabolism and intestinal microbiota in Chinese mitten crab (Eriocheir sinensis) fed on formulated diet and iced trash fish

Formulated diet (FD) and iced trash fish (ITF) are common diets during E. sinensis farming. However, whether FD can completely replace ITF during long-term E. sinensis farming is still unclear. Thus this study was conducted to compare the differences in amino acid metabolism and intestinal microbiota of the E. sinensis fed on different diets. The crabs were randomly divided into three groups fed on FD, ITF and mixed diet (MD, FD: ITF = 1:1), respectively. The results showed that there were no significant differences in amino acid composition among FD, MD and ITF groups. The activities of AST and ALT, and the mRNA levels of amino acid metabolism-related genes were significantly up-regulated in FD or/and MD groups compared with ITF group. The diversity of intestinal bacterial community was similar between the FD and ITF groups, but the relative abundance of dominant taxa showed marked differences between the two groups. At the phylum level, the relative abundance of Firmicutes was significantly higher, but the relative abundance of Proteobacteria was significantly lower in the FD group than that in ITF group. Meanwhile, at genus level, the relative abundance of Candidatus_Hepatoplasma in FD was higher than that in ITF group. Data related to functional prediction demonstrated that the significantly differenced pathways between the two groups were observed in metabolism (Pyrimidine metabolism, Glycolysis/Gluconeogenesis and Citrate cycle) and environmental information processing (transporters). The overall results indicated that replacement of ITF by FD did not affect amino acid composition, but altered amino acid metabolism and the relative abundance of intestinal microbiota. Our data provided a valuable reference for FD application replacing ITF during long-term E. sinensis farming.

Appropriate leucine supplementation promotes glucose metabolism and enhances energy homeostasis in juvenile crucian carp (Carassius auratus gibelio var. CAS III)

In order to characterize the molecular mechanisms by which leucine regulates carbohydrate metabolism and energy homeostasis, juvenile crucian carps (Carassius auratus gibelio var. CAS III) fed with a high carbohydrate diet were supplemented with different levels of dietary leucine: 0% (Leu0), 0.4% (Leu4), 0.8% (Leu8), 1.2% (Leu12), 1.6% (Leu16), 2.0% (Leu20), and 5.0% (Leu50). After 8 weeks, RNA sequencing was performed on samples collected from the Leu0, Leu8, Leu12 and Leu50 groups. Differentially expressed genes were then detected and analyzed. The results showed a total of 91.6 Gb of clean bases were generated. Moreover, a total of 1131, 5254, and 1539 DEGs were detected in Leu8, Leu12, and Leu50 compared with Leu0, respectively, encompassing 161 common DEGs. STEM analysis elucidated four significant clusters of DEGs that were associated with "glycerophospholipid metabolism," "glycerolipid metabolism," "PPAR signaling pathway," and "adipocytokine signaling pathway." Moreover, the mRNA expression levels of acyl-CoA synthetase long chain family member 5 (ACSL5), choline kinase beta (CHKB), cryptochrome-1 (CRY1), lon protease homolog 2, peroxisomal isoform X2 (LONP2), lipin 1 (LPIN1), membrane bound O-acyltransferase domain containing 2 (MBOAT2), phosphoenolpyruvate carboxykinase 1 (PEPCK), and uridine-cytidine kinase 2b (UCK2b) were then further investigated in all leucine treatment groups at starvation times of 0 h, 24 h, and 48 h. The results revealed that the expression levels of UCK2b and MBOAT2 were negatively correlated with the addition of leucine, whereas CHKB, LONP2, CRY1, PEPCK, and LPIN1 were positively correlated. In conclusion, dietary leucine supplementation below 1.2% enhanced carbohydrate metabolism in juvenile crucian carp fed with a high-carbohydrate diet, whereas concentrations above 2.0% is a better choice for energy homeostasis under starvation.

Alanine alters the carbohydrate metabolism of rainbow trout: glucose flux and cell signaling

In rainbow trout, dietary carbohydrates are poorly metabolized compared with other macronutrients. One prevalent hypothesis suggests that high dietary amino acid levels could contribute to the poor utilization of carbohydrates in trout. In mammals, alanine is considered an important gluconeogenic precursor, but has recently been found to stimulate AMP-activated protein kinase (AMPK) to reduce glucose levels. In trout, the effect of alanine on glucose flux is unknown. The goal of this study was to determine the effects of 4 h exogenous alanine infusion on glucose metabolism in rainbow trout. Glucose flux, and the rate of glucose appearance (Ra) and disposal (Rd) were measured in vivo. Key glycolytic and gluconeogenic enzyme expression and activity, and cell signaling molecules relevant to glucose metabolism were assessed in the liver and muscle. The results show that alanine inhibits glucose Ra (from 13.2±2.5 to 7.3±1.6 μmol kg-1 min-1) and Rd (from 13.2±2.5 to 7.4±1.5 μmol kg-1 min-1) and the slight mismatch between Ra and Rd caused a reduction in glycemia, similar to the effects of insulin in trout. The reduction in glucose Rd can be partially explained by a reduction in glut4b expression in red muscle. In contrast to mammals, trout alanine-dependent glucose-lowering effects did not involve hepatic AMPK activation, suggesting a different mechanistic basis. Interestingly, protein kinase B (AKT) activation increased only in muscle, similar to effects observed in insulin-infused trout. We speculate that alanine-dependent effects were probably mediated through stimulation of insulin secretion, which could indirectly promote alanine oxidation to provide the needed energy.

New insights into methylmercury induced behavioral and energy-related gene transcriptional responses in European glass eel (Anguilla anguilla)

The effect of methylmercury (MeHg) was investigated in glass eel migration behavior and metabolism. To migrate up estuary, glass eels synchronize their swimming activity to the flood tide and remain on or in the substratum during ebb tide. Following seven days of exposure to MeHg (100 ng L^-1), glass eels migration behavior was expressed by their swimming synchronization to the water current reversal every 6.2 h (mimicking the alternation of flood and ebb tides) and their swimming activity level. In relation to their behavior, we then analyzed the energy-related gene expression levels in individual head, viscera and muscle. Results showed that MeHg decreased the number of glass eels synchronized to the change in water current direction and their swimming activity level. This last effect was more pronounced in non-synchronized fish than in synchronized ones, supporting the idea that non-synchronized glass eels could be more vulnerable to stress. As regard the expression of energy-related genes, no significant difference was observed between control and MeHg-exposed fish. In contrast, when the swimming activity levels were plotted against transcriptional responses, positive correlations were evidenced in viscera and especially in the head of exposed glass eels but not in control. Finally, it is noteworthy that non-synchronized glass eels displayed lower expression level of metabolism genes than their synchronized counterpart, but only in the head. Altogether, these results support the interest of focusing on the head to investigate the facultative migration behavior in glass eels and the effect of environmental stressors on this rhythmic behavior.

Early leucine programming on protein utilization and mTOR signaling by DNA methylation in zebrafish (Danio rerio)

Background

Early nutritional programming affects a series of metabolism, growth and development in mammals. Fish also exhibit the developmental plasticity by early nutritional programming. However, little is known about the effect of early amino acid programming on growth and metabolism.

Methods

In the present study, zebrafish (Danio rerio) was used as the experimental animal to study whether early leucine stimulation can programmatically affect the mechanistic target of rapamycin (mTOR) signaling pathway, growth and metabolism in the later life, and to undercover the mechanism of epigenetic regulation. Zebrafish larvas at 3 days post hatching (dph) were raised with 1.0% leucine from 3 to 13 dph during the critical developmental stage, then back to normal water for 70 days (83 dph).

Results

The growth performance and crude protein content of zebrafish in the early leucine programming group were increased, and consistent with the activation of the mTOR signaling pathway and the high expression of genes involved in the metabolism of amino acid and glycolipid. Furthermore, we compared the DNA methylation profiles between the control and leucine-stimulated zebrafish, and found that the methylation levels of CG-differentially methylated regions (DMGs) and CHH-DMGs of genes involved in mTOR signaling pathway were different between the two groups. With quantitative PCR analysis, the decreased methylation levels of CG type of Growth factor receptor-bound protein 10 (Grb10), eukaryotic translation initiation factor 4E (eIF4E) and mTOR genes of mTOR signaling pathway in the leucine programming group, might contribute to the enhanced gene expression.

Conclusions

The early leucine programming could improve the protein synthesis and growth, which might be attributed to the methylation of genes in mTOR pathway and the expression of genes involved in protein synthesis and glycolipid metabolism in zebrafish. These results could be beneficial for better understanding of the epigenetic regulatory mechanism of early nutritional programming.

RTH-149 Cell Line, a Useful Tool to Decipher Molecular Mechanisms Related to Fish Nutrition

Nowadays, aquaculture provides more than 50% of fish consumed worldwide but faces new issues that challenge its sustainability. One of them relies on the replacement of fish meal (FM) in aquaculture feeds by other protein sources without deeply affecting the whole organism's homeostasis. Multiple strategies have already been tested using in vivo approaches, but they hardly managed to cope with the multifactorial problems related to the complexities of fish biology together with new feed formulations. In this context, rainbow trout (RT) is particularly concerned by these problems, since, as a carnivorous fish, dietary proteins provide the amino acids required to supply most of its energetic metabolism. Surprisingly, we noticed that in vitro approaches considering RT cell lines as models to study RT amino acid metabolism were never previously used. Therefore, we decided to investigate if, and how, three major pathways described, in other species, to be regulated by amino acid and to control cellular homeostasis were functional in a RT cell line called RTH-149-namely, the mechanistic Target Of Rapamycin (mTOR), autophagy and the general control nonderepressible 2 (GCN2) pathways. Our results not only demonstrated that these three pathways were functional in RTH-149 cells, but they also highlighted some RT specificities with respect to the time response, amino acid dependencies and the activation levels of their downstream targets. Altogether, this article demonstrated, for the first time, that RT cell lines could represent an interesting alternative of in vivo experimentations for the study of fish nutrition-related questions.

Adequate supply of dietary taurine stimulates expression of molecular markers of growth and protein turnover in juvenile barramundi (Lates calcarifer)

A trial was conducted to investigate the effect of dietary taurine (Tau) supply on the plasma amino acid composition and hepatic expression of several genes in juvenile barramundi (Lates calcarifer) after feeding. Triplicate tanks of fish (average weight, 89.3 g) were fed diets containing either a deficient (1 g kg^-1), adequate (8 g kg^-1) or excessive (19 g kg^-1) level of dietary Tau. Liver tissues collected before feeding, and at 2- and 4-h post-feeding, were analysed for expression of genes involved in pathways of sulphur amino acid turnover, Tau biosynthesis and transport, target of rapamycin (TOR) signalling, the somatotropic axis and protein turnover. The treatment had no significant effect on the profiles of any amino acid in plasma collected over time after feeding, other than Tau and glycine. The expression profile of cystine and Tau synthetic genes suggested an effect of Tau excess on the metabolism of cystine. Markers of two pathways of Tau biosynthesis appear to be active in this species, providing proof that this species possesses the ability to synthesise Tau from SAA precursors. A marker for the regulation of Tau transport and homeostasis was shown to be directly regulated by Tau availability, whilst a link between adequate supply of Tau and TOR pathway-mediated growth stimulation was also apparent. An observed depression in expression of genes of the somatotropic axis, coupled with upregulation of the proteolytic and TOR-suppressing genes, in response to excessive Tau supply in the diet, signalled that excessive Tau may not be conducive to optimal growth in this species.

Short-Term Responses to Fatty Acids on Lipid Metabolism and Adipogenesis in Rainbow Trout (Oncorhynchus mykiss)

Fish are rich in n-3 long-chain polyunsaturated fatty acids (LC-PUFA) such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. Due to the increasing use of vegetable oils (VO), their proportion in diets has lowered, affecting lipid metabolism and fillet composition. Rainbow trout cultured preadipocytes were treated with representative FA found in fish oils (EPA and DHA) or VO (linoleic, LA and alpha-linolenic, ALA acids), while EPA and LA were also orally administered, to evaluate their effects on adipogenesis and lipid metabolism. In vitro, all FA increased lipid internalization, with ALA producing the highest effect, together with upregulating the FA transporter fatp1. In vivo, EPA or LA increased peroxisome proliferator-activated receptors ppara and pparb transcripts abundance in adipose tissue, suggesting elevated β-oxidation, contrary to the results obtained in liver. Furthermore, the increased expression of FA synthase (fas) and the FA translocase/cluster of differentiation (cd36) in adipose tissue indicated an enhanced uptake of lipids and lipogenesis de novo, whereas stable or low hepatic expression of genes involved in lipid transport and turnover was found. Thus, fish showed a similar tissue metabolic response to the short-term availability of EPA or LA in vivo, while in vitro VO-derived FA demonstrated greater potential inducing fat accumulation.

Leucine Improved Growth Performance, Muscle Growth, and Muscle Protein Deposition Through AKT/TOR and AKT/FOXO3a Signaling Pathways in Hybrid Catfish Pelteobagrus vachelli × Leiocassis longirostris

(1) Background: l-leucine (Leu) plays a positive role in regulating protein turnover in skeletal muscle in mammal. However, the molecular mechanism for the effects of Leu on muscle growth and protein deposition is not clearly demonstrated in fish. This study investigated the effects of dietary Leu on growth performance and muscle growth, protein synthesis, and degradation-related signaling pathways of hybrid catfish (Pelteobagrus vachelli♀ × Leiocassis longirostris♂). (2) Methods: A total of 630 hybrid catfish (23.19 ± 0.20 g) were fed 6 different experimental diets containing graded levels of Leu at 10.0 (control), 15.0, 20.0, 25.0, 30.0, 35.0, and 40.0 g Leu kg^-1 for 8 weeks. (3) Results: Results showed that dietary Leu increased percent weight gain (PWG), specific growth rate (SGR), FI (feed intake), feed efficiency (FE), protein efficiency ratio (PER), muscle fibers diameter, and muscle fibers density; up-regulated insulin-like growth factor I (IGF-I), insulin-like growth factor I receptor (IGF-IR), proliferating cell nuclear antigen (PCNA), myogenic regulation factors (MyoD, Myf5, MyoG, and Mrf4), and MyHC mRNA levels; increased muscle protein synthesis via regulating the AKT/TOR signaling pathway; and attenuated protein degradation via regulating the AKT/FOXO3a signaling pathway. (4) Conclusions: These results suggest that Leu has potential role to improve muscle growth and protein deposition in fish, which might be due to the regulation of IGF mRNA expression, muscle growth related gene, and protein synthesis and degradation-related signaling pathways. Based on the broken-line model, the Leu requirement of hybrid catfish (23.19-54.55 g) for PWG was estimated to be 28.10 g kg^-1 of the diet (73.04 g kg^-1 of dietary protein). These results will improve our understanding of the mechanisms responsible for muscle growth and protein deposition effects of Leu in fish.

Kinetic study of the expression of genes related to hepatic steatosis, glucose and lipid metabolism, and cellular stress during overfeeding in mule ducks

Induced by overfeeding, hepatic steatosis is a process exploited for the "foie gras" production in mule ducks. To better understand the mechanisms underlying its development, the physiological responses of mule ducks overfed with corn for a duration of 11 days were analyzed. A kinetic analysis of glucose and lipid metabolism and cell protection mechanisms was performed on 96 male mule ducks during overfeeding with three sampling times (after the 4th, the 12th, and the 22nd meal). Gene expression and protein analysis realized on the liver, muscle, and abdominal fat showed an activation of a cholesterol biosynthetic pathway during the complete overfeeding period mainly in livers with significant correlations between its weight and its cholesterolemia (r = 0.88; P < 0.0001) and between the liver weight and the hmgcr and soat1 expression (r = 0.4, P < 0.0001 and r = 0.67; P < 0.0001, respectively). Results also revealed an activation of insulin and amino acid cells signaling a pathway suggesting that ducks boost insulin sensitivity to raise glucose uptake and use via glycolysis and lipogenesis. Cellular stress analysis revealed an upregulation of key autophagy-related gene expression atg8 and sqstm1(P < 0.0001) during the complete overfeeding period, mainly in the liver, in contrast to an induction of cyp2e1(P < 0.0001), suggesting that autophagy could be suppressed during steatosis development. This study has highlighted different mechanisms enabling mule ducks to efficiently handle the starch overload by keeping its liver in a nonpathological state. Moreover, it has revealed potential biomarker candidates of hepatic steatosis as plasma cholesterol for the liver weight.

Effect of dietary threonine on growth performance and muscle growth, protein synthesis and antioxidant-related signalling pathways of hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂

The experiment was conducted to investigate the effects of dietary threonine (Thr) on growth performance and muscle growth, protein synthesis and antioxidant-related signalling pathways of hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. A total of 1200 fish (14·19 (se 0·13) g) were randomly distributed into six groups with four replicates each, fed six diets with graded level of Thr (9·5, 11·5, 13·5, 15·4, 17·4 and 19·3 g/kg diets) for 56 d. Results showed (P < 0·05) that dietary Thr (1) increased percentage weight gain, specific growth rate, feed efficiency and protein efficiency ratio; (2) up-regulated growth hormone (GH), insulin-like growth factor 1 (IGF-1), proliferating cell nuclear antigen, myogenic regulation factors (MyoD, Myf5, MyoG and Mrf4) and myosin heavy chain (MyHC) mRNA levels; (3) increased muscle protein content via regulating the protein kinase B/target of rapamycin signalling pathway and (4) decreased malondialdehyde and protein carbonyl contents, increased catalase, glutathione-S-transferase, glutathione reductase and GSH activities, up-regulated mRNA levels of antioxidant enzymes related to NFE2-related factor 2 and γ-glutamylcysteine ligase catalytic subunit. These results suggest that Thr has a potential role to improve muscle growth and protein synthesis, which might be due to the regulation of GH-IGF system, muscle growth-related gene, antioxidative capacity and protein synthesis-related signalling pathways. Based on the quadratic regression analysis of specific growth rate, the Thr requirement of hybrid catfish (14·19-25·77 g) was estimated to be 13·77 g/kg of the diet (33·40 g/kg of dietary protein).

Effects of dietary protein level on growth performance, digestive enzyme activity, and gene expressions of the TOR signaling pathway in fingerling Pelteobagrus fulvidraco

An 8-week feeding trial was conducted to investigate effects of dietary protein levels (37, 40, and 43%) on the growth performance, feed utilization, digestive enzyme activity, and gene expressions of target of rapamycin (TOR) signaling pathway in fingerling yellow catfish. One hundred and eighty fingerlings (average weight 0.77 ± 0.03 g) were equally distributed across four replicate tanks for each of the three treatments, with 15 fish per tank. No difference (P > 0.05) was observed in initial body weight, survival rate (SR), hepatosomatic index (HSI), viscera index (VSI), dressing percentage (DP), and condition factor (CF) among all the treatments. The diet containing 40% protein increased significantly (P < 0.05) final body weight, weight gain rate (WGR), specific growth rate (SGR), protein efficiency ratio (PER), nitrogen retention (NRE), and energy retention (ERE) in fish. The highest protease activity in the stomach and intestine was observed in the P40 group (P < 0.05), while amylase and lipase were not significantly different (P > 0.05). The transcriptional levels of IGF-1, IGF-1R, and Akt were significantly (P < 0.05) higher in fish fed P40 or P43 than those of fish fed P37. TOR and S6K1 mRNA expressions were significantly (P < 0.05) increased in the P40 groups. Hence, the diet containing 40% protein would be suitable for the optimum growth and effective protein utilization of fingerling Pelteobagrus fulvidraco. In vitro, the transcriptional levels of IGF-1, IGF-1R, Akt, TOR, and S6K1 in hepatocyte supplemented with a 40-μM mixed amino acids were significantly (P < 0.05) higher compared to other treatments. No difference (P > 0.05) was observed in eukaryotic translation initiation factor 4E-binding protein 1 in vivo and in vitro among all the treatments. Effects of dietary protein level on growth performance likely are involved in the activation of TOR signaling pathway in fingerling Pelteobagrus fulvidraco.

The Administration of Chitosan-Tripolyphosphate-DNA Nanoparticles to Express Exogenous SREBP1a Enhances Conversion of Dietary Carbohydrates into Lipids in the Liver of Sparus aurata

In addition to being essential for the transcription of genes involved in cellular lipogenesis, increasing evidence associates sterol regulatory element binding proteins (SREBPs) with the transcriptional control of carbohydrate metabolism. The aim of this study was to assess the effect of overexpression SREBP1a, a potent activator of all SREBP-responsive genes, on the intermediary metabolism of Sparus aurata, a glucose-intolerant carnivorous fish. Administration of chitosan-tripolyphosphate nanoparticles complexed with a plasmid driving expression of the N-terminal transactivation domain of SREBP1a significantly increased SREBP1a mRNA and protein in the liver of S. aurata. Overexpression of SREBP1a enhanced the hepatic expression of key genes in glycolysis-gluconeogenesis (glucokinase and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase), fatty acid synthesis (acetyl-CoA carboxylase 1 and acetyl-CoA carboxylase 2), elongation (elongation of very long chain fatty acids protein 5) and desaturation (fatty acid desaturase 2) as well as reduced nicotinamide adenine dinucleotide phosphate production (glucose-6-phosphate 1-dehydrogenase) and cholesterol synthesis (3-hydroxy-3-methylglutaryl-coenzyme A reductase), leading to increased blood triglycerides and cholesterol levels. Beyond reporting the first study addressing in vivo effects of exogenous SREBP1a in a glucose-intolerant model, our findings support that SREBP1a overexpression caused multigenic effects that favoured hepatic glycolysis and lipogenesis and thus enabled protein sparing by improving dietary carbohydrate conversion into fatty acids and cholesterol.

Profiling the rainbow trout hepatic miRNAome under diet-induced hyperglycemia

Carnivorous rainbow trout exhibit prolonged postprandial hyperglycemia when fed a diet exceeding 20% carbohydrate content. This poor capacity to utilize carbohydrates has led to rainbow trout being classified as "glucose-intolerant" (GI). The metabolic phenotype has spurred research to identify the underlying cellular and molecular mechanisms of glucose intolerance, largely because carbohydrate-rich diets provide economic and ecological advantages over traditionally used fish meal, considered unsustainable for rainbow trout aquaculture operations. Evidence points to a contribution of hepatic intermediary carbohydrate and lipid metabolism, as well as upstream insulin signaling. Recently, microRNAs (miRNAs), small noncoding RNAs acting as negative posttranscriptional regulators affecting target mRNA stability and translation, have emerged as critical regulators of hepatic control of glucose-homeostasis in mammals, revealing that dysregulated hepatic miRNAs might play a role in organismal hyperglycemia in metabolic disease. To determine whether hepatic regulatory miRNA networks may contribute to GI in rainbow trout, we induced prolonged postprandial hyperglycemia in rainbow trout by using a carbohydrate-rich diet and profiled genome-wide hepatic miRNAs in hyperglycemic rainbow trout compared with fasted trout and trout fed a diet devoid of carbohydrates. Using small RNA next-generation sequencing and real-time RT-PCR validation, we identified differentially regulated hepatic miRNAs between these groups and used an in silico approach to predict bona fide mRNA targets and enriched pathways. Diet-induced hyperglycemia resulted in differential regulation of hepatic miRNAs compared with fasted fish. Some of the identified miRNAs, such as miRNA-27b-3p and miRNA-200a-3p, are known to be responsive to hyperglycemia in the liver of hyperglycemic glucose-tolerant fish and mammals, suggesting an evolutionary conserved regulation. Using Gene Ontology term-based enrichment analysis, we identify intermediate carbohydrate and lipid metabolism and insulin signaling as potential targets of posttranscriptional regulation by hyperglycemia-regulated miRNAs and provide correlative expression analysis of specific predicted miRNA-target pairs. This study identifies hepatic miRNAs in rainbow trout that exhibit differential postprandial expression in response to diets with different carbohydrate content and predicts posttranscriptionally regulated target mRNAs enriched for pathways involved in glucoregulation. Together, these results provide a framework for testable hypotheses of functional involvement of specific hepatic miRNAs in GI in rainbow trout.

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

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

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.

Dietary leucine affects glucose metabolism and lipogenesis involved in TOR/PI3K/Akt signaling pathway for juvenile blunt snout bream Megalobrama amblycephala

The present study evaluated the mechanisms governing insulin signaling, glucose metabolism, and lipogenesis in juvenile fish fed with different dietary leucine levels. Fish were fed six practical diets with graded leucine levels ranging from 0.90 to 2.94% of dry basis for 8 weeks. The trial results showed that, compared to the control group (0.90%), optimal dietary leucine level (1.72%) resulted in the up-regulation of mRNA expression related to insulin signaling pathway, including target of rapamycin (TOR), insulin receptor substrate 1 (IRS-1), phosphoinositide 3-kinase (PI3K), and protein kinase B (Akt). However, an excessive leucine level (2.94%) led to protein S6 kinase 1 (S6K1) overexpression and inhibited TOR, IRS-1, PI3K, and Akt mRNA expressions. The protein level of TOR, S6K1, IRS-1, PI3K, and Akt showed a similar result with mRNA level of these genes. Optimal dietary leucine level (1.72%) significantly improved plasma insulin content, while high level of leucine showed an inhibiting phenomenon. Optimal dietary leucine level (1.72%) could reduce plasma glucose by enhancing the ability of glycometabolism including improving glucose transporter 2 (GLUT2), glucokinase (GK) expressions and down-regulating phosphoenolpyruvate carboxykinase (PEPCK) expression. While an excessive leucine level (2.94%) resulted in high plasma glucose by inhibiting the ability of glycometabolism including lowering GLUT2 and GK expressions, and improving glucose-6-phosphatase (G6Pase) and PEPCK expressions. The relative expressions of pyruvate kinase (PK) and glycogen synthase (GS) were not significantly affected by dietary leucine levels. Dietary leucine level of 1.33% could improve plasma triglyceride content (TG) by enhancing lipogenesis including improving sterol-response element-binding protein 1 (SREBP1), fatty acid synthase (FAS), acetyl CoA carboxylase (ACC), and glucose-6-phosphate dehydrogenase (G6PDH) expressions compared to the control group (0.90%). Total cholesterol (TC) was not significantly affected by dietary leucine levels. The present results indicate that optimal leucine level could improve glycolysis and fatty acid synthesis through improving insulin sensitivity in juvenile blunt snout bream. However, excessive dietary leucine level resulted in high plasma glucose, which led to insulin resistance by inhibiting the gene expressions of insulin signaling pathway and activating gluconeogenesis-related gene expression.

Forkhead box O1 in turbot Scophthalmus maximus: Molecular characterization, gene structure, tissue distribution and the role in glucose metabolism

Forkhead box O1 (foxo1) is a transcription factor and plays important roles in glucose metabolism. In the present study, foxo1 in turbot Scophthalmus maximus was cloned and characterized. The siRNA of foxo1 was used to investigate the functions of foxo1 in turbot hepatocytes glucose metabolism. After that, a 10-week feeding trial with two different dietary carbohydrate levels (15% and 21%, respectively) was conducted to analyze the function of foxo1 in glucose metabolism in vivo. Results showed that the foxo1 was identified as 2176 bp (base pair) with a 2025 bp open reading frame, which encoded 675 amino acids. Sequence analysis showed that foxo1 of turbot was highly homologous to most of fishes. Tissue distribution analysis revealed that the highest expression of foxo1 was in liver. After in vitro analysis, foxo1-specific small interfering RNA (sifoxo1) treatment significantly decreased the expressions of cytosolic phosphoenolpyruvate carboxykinase (cpepck) and glucose-6-phosphatase1(g6pase1) in primary hepatocytes. Expression of mitochondrial phosphoenolpyruvate carboxykinase (mpepck) was not significantly inhibited. In contrast, the expression of glucose-6-phosphatase2 (g6pase2) increased significantly. After the in vivo study (feeding trial), with the decreased expression of foxo1 in turbot due to high dietary carbohydrate level (21%), the expression of g6pase2 was significantly upregulated. However, the expression of glucokinase (gk) was not changed significantly. These increased the level of blood glucose and hepatic glycogen. In conclusion, data from both in vitro (primary hepatocytes) and in vivo (feeding trial) showed that downregulated foxo1 in turbot could not result in significant depression of gluconeogenesis and activation of glycolysis. This could be one of the reasons why intake of high level of carbohydrate resulted in prolonged hyperglycemia in turbot.

The Autophagic Flux Inhibitor Bafilomycine A1 Affects the Expression of Intermediary Metabolism-Related Genes in Trout Hepatocytes

Autophagy is an evolutionarily conserved process of cellular self-eating which emerged these last years as a major adaptive metabolic response to various stresses such as fasting, hypoxia, or environmental pollutants. However, surprisingly very few data is currently available on its role in fish species which are directly exposed to frequent environmental perturbations. Here, we report that the treatment of fasted trout hepatocytes with the autophagy inhibitor Bafilomycine A1 lowered the mRNA levels of many of the gluconeogenesis-related genes and increased those of genes involved in intracellular lipid stores. Concurrently, intracellular free amino acid levels dropped and the expression of the main genes involved in the endoplasmic reticulum (ER) stress exhibited a sharp increase in autophagy inhibited cells. Together these results highlight the strong complexity of the crosstalk between ER, autophagy and metabolism and support the importance of considering this function in future studies on metabolic adaptation of fish to environmental stresses.

Pck-ing up steam: Widening the salmonid gluconeogenic gene duplication trail

Rainbow trout have, as salmonid fish species, undergone sequential genome duplication events in their evolutionary history. In addition to a teleost-specific whole genome duplication approximately 320-350 million years ago, rainbow trout and salmonids in general underwent an additional salmonid lineage-specific genome duplication event approximately 80 million years ago. Through the recent sequencing of salmonid genome sequences, including the rainbow trout, the identification and study of duplicated genes has become available. A particular focus of interest has been the evolution and regulation of rainbow trout gluconeogenic genes, as recent molecular and gene expression evidence points to a possible contribution of previously uncharacterized gluconeogenic gene paralogues to the rainbow trout long-studied glucose intolerant phenotype. Since the publication of the initial rainbow trout genome draft, resequencing and annotation have further improved genome coverage. Taking advantage of these recent improvements, we here identify a salmonid-specific genome duplication of ancestral mitochondrial phosphoenolpyruvate carboxykinase 2 isoenzyme, we termed pck2a and pck2b. Cytosolic phosphoenolpyruvate carboxykinase (Pck1) and, more recently mitochondrial Pck2, are considered to be the rate-limiting enzymes in de novo gluconeogenesis. Following in silico confirmation of salmonid pck2a and pck2b evolutionary history, we simultaneously profiled cytosolic pck1 and mitochondrial pck2a and pck2b expression in rainbow trout liver under several experimental conditions known to regulate hepatic gluconeogenesis. Cytosolic pck1 abundance was increased by nutritional (diets with a high protein to carbohydrate ratio compared to diets with a low carbohydrate to protein ratio) and glucoregulatory endocrine factors (glucagon and cortisol), revealing that the well-described transcriptional regulation of pck1 in mammals is present in rainbow trout. Conversely, and in contrast to mammals, we here describe endocrine regulation of pck2a (decrease in abundance in response to glucagon infusion), and nutritional, social-status-dependent and hypoxia-dependent regulation of pck2b. Specifically, pck2b transcript abundance increased in trout fed a diet with a low protein to carbohydrate ratio compared to a diet with a high protein to carbohydrate ratio, in dominant fish compared to subordinate fish as well as hypoxia. This specific and differential expression of rainbow trout pck2 ohnologues is indicative of functional diversification, and possible functional consequences are discussed in light of the recently highlighted gluconeogenic roles of mitochondrial pck2 in mammalian models.

Unexpected effect of insulin on glucose disposal explains glucose intolerance of rainbow trout

The physiological reasons why salmonids show glucose intolerance are unclear. In mammals, rapid clearance of a glucose load is mainly achieved through insulin-mediated inhibition of hepatic glucose production ( R_a) and stimulation of glucose disposal ( R_d), but the effects of insulin on R_a and R_d glucose have never been measured in fish. The goal of this study was to characterize the impact of insulin on the glucose kinetics of rainbow trout in vivo. Glucose fluxes were measured by continuous infusion of [6-³H]glucose before and during 4 h of insulin administration. The phosphorylated form of the key signaling proteins Akt and S6 in the insulin cascade were also examined, confirming activation of this pathway in muscle but not liver. Results show that insulin inhibits trout R_d glucose from 8.6 ± 0.6 to 5.4 ± 0.5 µmol kg^-1 min^-1: the opposite effect than classically seen in mammals. Such a different response may be explained by the contrasting effects of insulin on gluco/hexokinases of trout versus mammals. Insulin also reduced trout R_a from 8.5 ± 0.7 to 4.8 ± 0.6 µmol·kg^-1·min^-1, whereas it can almost completely suppresses R_a in mammals. The partial inhibition of R_a glucose may be because insulin only affects gluconeogenesis but not glycogen breakdown in trout. The small mismatch between the responses to insulin for R_d (-37%) and R_a glucose (-43%) gives trout a very limited capacity to decrease glycemia. We conclude that the glucose intolerance of rainbow trout can be explained by the inhibiting effect of insulin on glucose disposal.

Social status affects lipid metabolism in rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout ( Oncorhynchus mykiss) confined in pairs form social hierarchies in which socially subordinate fish display characteristic traits, including reduced growth rates and altered glucose metabolism. These effects are, in part, mediated by chronically elevated cortisol levels and/or reduced feeding. To determine the effects of social status on lipid metabolism, trout were held in pairs for 4 days, following which organismal and liver-specific indexes of lipid metabolism were measured. At the organismal level, circulating triglycerides were elevated in dominant trout, whereas subordinate trout exhibited elevated concentrations of circulating free fatty acids (FFAs) and lowered plasma total cholesterol levels. At the molecular level, increased expression of lipogenic genes in dominant trout and cpt1a in subordinate trout was identified, suggesting a contribution of increased de novo lipogenesis to circulating triglycerides in dominant trout and reliance on circulating FFAs for β-oxidation in the liver of subordinates. Given the emerging importance of microRNAs (miRNA) in the regulation of hepatic lipid metabolism, candidate miRNAs were profiled, revealing increased expression of the lipogenic miRNA-33 in dominant fish. Because the Akt-TOR-S6-signaling pathway is an important upstream regulator of hepatic lipid metabolism, its signaling activity was quantified. However, the only difference detected among groups was a strong increase in S6 phosphorylation in subordinate trout. In general, the changes observed in lipid metabolism of subordinates were not mimicked by either cortisol treatment or fasting alone, indicating the existence of specific, emergent effects of subordinate social status itself on this fuel.

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.

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

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

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

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

Effect of different levels of protein concentrates supplementation on the growth performance, plasma amino acids profile and mTOR cascade genes expression in early-weaned yak calves

Objective

This study evaluated the effects of different levels of protein concentrate supplementation on the growth performance of yak calves, and correlated the growth rate to changes occurring in the plasma- amino acids, -insulin profile, and signaling activity of mammalian target of rapamycin (mTOR) cascade to characterize the mechanism through which the protein synthesis can be improved in early weaned yaks.

Methods

For this study, 48 early (3 months old) weaned yak calves were selected, and assigned into four dietary treatments according to randomized complete block design. The four blocks were balanced for body weight and sex. The yaks were either grazed on natural pasture (control diet) in a single herd or the grazing yaks was supplemented with one of the three protein rich supplements containing low (17%; LP), medium (19%; MP), or high (21%; HP) levels of crude proteins for a period of 30 days.

Results

Results showed that the average daily gain of calves increased (0.14 vs 0.23–0.26 kg; p<0.05) with protein concentrates supplementation. The concentration of plasma methionine increased (p<0.05; 8.6 vs 10.1–12.4 μmol/L), while those of serine and tyrosine did not change (p>0.05) when the grazing calves were supplemented with protein concentrates. Compared to control diet, the insulin level of calves increased (p<0.05; 1.86 vs 2.16–2.54 μIU/mL) with supplementation of protein concentrates. Addition of protein concentrates up-regulated (p<0.05) expression of mTOR-raptor, mammalian vacuolar protein sorting 34 homolog, the translational regulators eukaryotic translation initiation factor 4E binding protein 1, and S6 kinase 1 genes in both Longissimus dorsi and semitendinosus. In contrast, the expression of sequestosome 1 was down-regulated in the concentrate supplemented calves.

Conclusion

Our results show that protein supplementation improves the growth performance of early weaned yak calves, and that plasma methionine and insulin concentrations were the key mediator for gene expression and protein deposition in the muscles.

Characterization and mechanism of phosphoinositide 3-kinases (PI3Ks) members in insulin-induced changes of protein metabolism in yellow catfish Pelteobagrus fulvidraco

In the present study, seven phosphoinositide 3-kinase (PI3K) members (PI3KCa, PI3KCb, PI3KCd, PI3KCg, PI3KC2a, PI3KC2b and PI3KC3, respectively) were isolated and characterized from yellow catfish Pelteobagrus fulvidraco, and their roles in insulin-induced changes of protein metabolism were determined. These seven PI3Ks can be divided into three classes, class I (including PI3KCa, PI3KCb, PI3KCd and PI3KCg), class II (including PI3KC2a and PI3KC2b) and class III (only including PI3KC3). Compared with mammals, all of these members share similar domain structure. Their mRNAs were widely expressed across ten tested tissues (liver, white muscle, spleen, brain, gill, mesenteric fat, intestine, heart, kidney and ovary), but at variable levels. In the in vivo study, insulin treatment significantly increased hepatic protein content at 3h, accompanied with reduced plasma total amino acid contents and liver ALT activity, and with increased total RNA content and the mRNA levels of PI3KCb, PI3KC2a, AKT2, mTORC1 and S6K1 in liver. At 6h and 12h, insulin injection showed no significant effect on liver protein content and plasma total amino acid, but reduced liver ALT activity and increased liver total RNA and the mRNA levels of AKT2, mTORC1 and S6K1 in liver at 6h. In the in vitro study, insulin incubation also tended to increase protein content of hepatocytes, accompanied with reduced cell medium total amino acid contents and hepatocytes ALT activity, and increased total RNA content and the mRNA levels of PI3KCb, PI3KC2a, AKT2, mTORC1 and S6K1 in hepatocytes. However, insulin treatment showed no significant effect on GDH activity and mRNA expression of PI3KCa, PI3KCd, PI3KCg, PI3KC2b, PI3KC3 and eEF2 both in the in vivo and in vitro studies. Effects of insulin on the mRNA levels of eIF-4E and 4E-BP1 were different between the in vivo and in vitro studies, and also time-dependent. Compared to single insulin group, insulin+wortmannin group increased ALT activity at 6h but reduced T-RNA content at 6 and 12h. AKT2 and S6K1 mRNA levels at 6 and 12h, mRNA levels of mTORC1, 4E-BP1 and eEF2 at 3 and 6h, and EIF-4E mRNA levels at 3 and 12h, PI3KCb and PI3KC2a mRNA levels were significantly lower in insulin+wortmannin group than those in single insulin group. Thus, our study demonstrated that among seven PI3K members, PI3KCb and PI3KC2a were more sensitive to the insulin signaling pathway, and insulin stimulated hepatic protein synthesis in yellow catfish through PI3K signaling pathway.

An analysis of partial efficiencies of energy utilisation of different macronutrients by barramundi (Lates calcarifer) shows that starch restricts protein utilisation in carnivorous fish

This study examined the effect of including different dietary proportions of starch, protein and lipid, in diets balanced for digestible energy, on the utilisation efficiencies of dietary energy by barramundi (Lates calcarifer). Each diet was fed at one of three ration levels (satiety, 80 % of initial satiety and 60 % of initial satiety) for a 42-d period. Fish performance measures (weight gain, feed intake and feed conversion ratio) were all affected by dietary energy source. The efficiency of energy utilisation was significantly reduced in fish fed the starch diet relative to the other diets, but there were no significant effects between the other macronutrients. This reduction in efficiency of utilisation was derived from a multifactorial change in both protein and lipid utilisation. The rate of protein utilisation deteriorated as the amount of starch included in the diet increased. Lipid utilisation was most dramatically affected by inclusion levels of lipid in the diet, with diets low in lipid producing component lipid utilisation rates well above 1·3, which indicates substantial lipid synthesis from other energy sources. However, the energetic cost of lipid gain was as low as 0·65 kJ per kJ of lipid deposited, indicating that barramundi very efficiently store energy in the form of lipid, particularly from dietary starch energy. This study defines how the utilisation efficiency of dietary digestible energy by barramundi is influenced by the macronutrient source providing that energy, and that the inclusion of starch causes problems with protein utilisation in this species.

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.

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

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

Nutritive values, flavor amino acids, healthcare fatty acids and flesh quality improved by manganese referring to up-regulating the antioxidant capacity and signaling molecules TOR and Nrf2 in the muscle of fish

Flesh quality, amino acid and fatty acid composition, antioxidant status and related molecule expression in fish muscle were estimated by feeding grass carp with diets containing 3.65-27.86mg/kg diet of manganese (Mn) for 8weeks. Results demonstrated that optimal Mn increased toughness, collagen content, and pH, and decreased the cooking loss, and cathepsin B and L activities to enhance the flesh quality of fish. Meanwhile, optimal Mn increased the protein, lipid, the total essential amino acid (AA) (especially umami AA), and healthcare fatty acids, C18: 1c+t, C20: 3n-3, C20: 4 and DHA contents. These might be partially related to the decreased lipid peroxidation and protein oxidation, and the enhanced activities of Mn superoxide dismutase (MnSOD), catalase (CAT) and glutathione peroxidase (GPx) modulated by their gene expression, Nrf2 and TOR signaling. We firstly demonstrated that Mn improved flesh quality, flavor and healthcare function in fish muscle.

Transcriptome Analyses Reveal Lipid Metabolic Process in Liver Related to the Difference of Carcass Fat Content in Rainbow Trout (Oncorhynchus mykiss)

Excessive accumulation of carcass fat in farm animals, including fish, has a significant impact on meat quality and on the cost of feeding. Similar to farmed animals and humans, the liver can be considered one of the most important organs involved in lipid metabolism in rainbow trout (Oncorhynchus mykiss). RNA-seq based whole transcriptome sequencing was performed to liver tissue of rainbow trout with high and low carcass fat content in this study. In total 1,694 differentially expressed transcripts were identified, including many genes involved in lipid metabolism, such as L-FABP, adiponectin, PPAR-α, PPAR-β, and IGFBP1a. Evidence presented in this study indicated that lipid metabolic process in liver may be related to the difference of carcass fat content. The relevance of PPAR-α and PPAR-β as molecular markers for fat storage in liver should be worthy of further investigation.

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

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

Essential amino acid ratios and mTOR affect lipogenic gene networks and miRNA expression in bovine mammary epithelial cells

Background

The objective of this study was to study how changing the ratio of Lys to Thr, Lys to His, and Lys to Val affects the expression of lipogenic genes and microRNA (miRNA) in bovine mammary epithelial cells.

Results

Triplicate cultures with the respective “optimal” amino acid (AA) ratio (OPAA = Lys:Met 2.9:1; Thr:Phe 1.05:1; Lys:Thr 1.8:1; Lys:His 2.38:1; Lys:Val 1.23:1) plus rapamycin (OPAARMC; positive control), OPAA, Lys:Thr 2.1:1 (LT2.1), Lys:Thr 1.3:1 (LT1.3), Lys:His 3.05:1 (LH3.0), or Lys:Val 1.62:1 (LV1.6) were incubated in lactogenic medium for 12 h. The expression of 15 lipogenic genes and 7 miRNA were evaluated. Responses to LT2.1, LT1.3, LH3.0, and LV1.6 relative to the control (OPAARMC) included up-regulated expression of ACSS2, FABP3, ACACA, FASN, SCD, LPIN1, INSIG1, SREBF1, PPARD, and NR1H3 (commonly known as LXR-α). Furthermore, LV1.6 up-regulated expression of ACSL1, DGAT1, and RXRA and down-regulated PPARG expression. Although no effect of OPAA on expression of PPARG was observed, compared with the control, OPAA up-regulated expression of the PPAR targets ACSS2, FABP3, ACACA, FASN, SCD, LPIN1, INSIG1, and SREBF1. Compared with the control, the expression of the anti-lipogenic MIR27AB was down-regulated by OPAA, LT2.1, LT1.3 and LH3.0. In contrast, compared with the control, the expression of the pro-lipogenic MIR21 was up-regulated by LT2.1, LT1.3, LH3.0, and LV1.6.

Conclusions

The observed up-regulation of lipogenic gene networks and the changes in expression of key miRNA involved in the control of lipogenic balance are indicative of a potentially important role of EAA ratios and mTOR signaling in the regulation of milk fat synthesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-016-0104-x) contains supplementary material, which is available to authorized users.

IGF-I and IGF-II effects on local IGF system and signaling pathways in gilthead sea bream (Sparus aurata) cultured myocytes

The insulin-like growth factors (IGFs) have a fundamental role in a vast range of functions acting through a tyrosine-kinase receptor (IGF-IR). IGFs in muscle can affect the expression of components of the local IGF system, myogenic regulatory factors (MRFs), proliferating (proliferating cell nuclear antigen, PCNA) or differentiating molecules (myosin heavy chain, MHC) and, lead to the activation of different signaling pathways. The response of all these genes to IGFs incubation at two different times in day 4 cultured myocytes of gilthead sea bream was analyzed. Both IGFs increased the expression of IGF-I and IGFBP-5, but showed different effects on the receptors, with IGF-I suppressing the expression of both isoforms (IGF-IRa and IGF-IRb) and IGF-II up-regulating only IGF-IRb. Moreover, the protein levels of PCNA and target of rapamycin (TOR) increased after IGF-II incubation, although a decline in Myf5 and a rise in MHC gene expression was caused by IGF-I. Taken together, these results provide evidence for the importance of IGFs on controlling muscle development and growth in gilthead sea bream and suggest that each IGF may be preferentially acting through a specific IGF-IR. Moreover, the data support the hypothesis that IGF-II has a more important role during proliferation, whereas IGF-I seems to be relevant for the differentiation phase of myogenesis.