Effect of acute and chronic insulin administrations on major factors involved in the control of muscle protein turnover in rainbow trout (Oncorhynchus mykiss)

Autophagy in farm animals: current knowledge and future challenges

Autophagy (a process of cellular self-eating) is a conserved cellular degradative process that plays important roles in maintaining homeostasis and preventing nutritional, metabolic, and infection-mediated stresses. Surprisingly, little attention has been paid to the role of this cellular function in species of agronomical interest, and the details of how autophagy functions in the development of phenotypes of agricultural interest remain largely unexplored. Here, we first provide a brief description of the main mechanisms involved in autophagy, then review our current knowledge regarding autophagy in species of agronomical interest, with particular attention to physiological functions supporting livestock animal production, and finally assess the potential of translating the acquired knowledge to improve animal development, growth and health in the context of growing social, economic and environmental challenges for agriculture.Abbreviations: AKT: AKT serine/threonine kinase; AMPK: AMP-activated protein kinase; ASC: adipose-derived stem cells; ATG: autophagy-related; BECN1: beclin 1; BNIP3: BCL2 interacting protein 3; BVDV: bovine viral diarrhea virus; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CMA: chaperone-mediated autophagy; CTSB: cathepsin B; CTSD: cathepsin D; DAP: Death-Associated Protein; ER: endoplasmic reticulum; GFP: green fluorescent protein; Gln: Glutamine; HSPA8/HSC70: heat shock protein family A (Hsp70) member 8; IF: immunofluorescence; IVP: in vitro produced; LAMP2A: lysosomal associated membrane protein 2A; LMS: lysosomal membrane stability; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MDBK: Madin-Darby bovine kidney; MSC: mesenchymal stem cells; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; NBR1: NBR1 autophagy cargo receptor; NDV: Newcastle disease virus; NECTIN4: nectin cell adhesion molecule 4; NOD1: nucleotide-binding oligomerization domain 1; OCD: osteochondritis dissecans; OEC: oviduct epithelial cells; OPTN: optineurin; PI3K: phosphoinositide-3-kinase; PPRV: peste des petits ruminants virus; RHDV: rabbit hemorrhagic disease virus; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy.

Revealing Alteration in the Hepatic Glucose Metabolism of Genetically Improved Carp, Jayanti Rohu Labeo rohita Fed a High Carbohydrate Diet Using Transcriptome Sequencing

Although feed cost is the greatest concern in aquaculture, the inclusion of carbohydrates in the fish diet, and their assimilation, are still not well understood in aquaculture species. We identified molecular events that occur due to the inclusion of high carbohydrate levels in the diets of genetically improved 'Jayanti rohu' Labeo rohita. To reveal transcriptional changes in the liver of rohu, a feeding experiment was conducted with three doses of gelatinized starch (20% (control), 40%, and 60%). Transcriptome sequencing revealed totals of 15,232 (4464 up- and 4343 down-regulated) and 15,360 (4478 up- and 4171 down-regulated) differentially expressed genes. Up-regulated transcripts associated with glucose metabolisms, such as hexokinase, PHK, glycogen synthase and PGK, were found in fish fed diets with high starch levels. Interestingly, a de novo lipogenesis mechanism was found to be enriched in the livers of treated fish due to up-regulated transcripts such as FAS, ACCα, and PPARγ. The insulin signaling pathways with enriched PPAR and mTOR were identified by Kyoto Encyclopedia of Genes and Genome (KEGG) as a result of high carbohydrates. This work revealed for the first time the atypical regulation transcripts associated with glucose metabolism and lipogenesis in the livers of Jayanti rohu due to the inclusion of high carbohydrate levels in the diet. This study also encourages the exploration of early nutritional programming for enhancing glucose efficiency in carp species, for sustainable and cost-effective aquaculture production.

Mitochondrial Fatty Acid β-Oxidation Inhibition Promotes Glucose Utilization and Protein Deposition through Energy Homeostasis Remodeling in Fish

BACKGROUND

Fish cannot use carbohydrate efficiently and instead utilize protein for energy supply, thus limiting dietary protein storage. Protein deposition is dependent on protein turnover balance, which correlates tightly with cellular energy homeostasis. Mitochondrial fatty acid β-oxidation (FAO) plays a crucial role in energy metabolism. However, the effect of remodeled energy homeostasis caused by inhibited mitochondrial FAO on protein deposition in fish has not been intensively studied.

OBJECTIVES

This study aimed to identify the regulatory role of mitochondrial FAO in energy homeostasis maintenance and protein deposition by studying lipid, glucose, and protein metabolism in fish.

METHODS

Carnitine-depleted male Nile tilapia (initial weight: 4.29 ± 0.12 g; 3 mo old) were established by feeding them with mildronate diets (1000 mg/kg/d) for 6 wk. Zebrafish deficient in the carnitine palmitoyltransferase 1b gene (cpt1b) were produced by using CRISPR/Cas9 gene-editing technology, and their males (154 ± 3.52 mg; 3 mo old) were used for experiments. Normal Nile tilapia and wildtype zebrafish were used as controls. We assessed nutrient metabolism and energy homeostasis-related biochemical and molecular parameters, and performed 14C-labeled nutrient tracking and transcriptomic analyses.

RESULTS

The mitochondrial FAO decreased by 33.1-88.9% (liver) and 55.6-68.8% (muscle) in carnitine-depleted Nile tilapia and cpt1b-deficient zebrafish compared with their controls (P < 0.05). Notably, glucose oxidation and muscle protein deposition increased by 20.5-24.4% and 6.40-8.54%, respectively, in the 2 fish models compared with their corresponding controls (P < 0.05). Accordingly, the adenosine 5'-monophosphate-activated protein kinase/protein kinase B-mechanistic target of rapamycin (AMPK/AKT-mTOR) signaling was significantly activated in the 2 fish models with inhibited mitochondrial FAO (P < 0.05).

CONCLUSIONS

These data show that inhibited mitochondrial FAO in fish induces energy homeostasis remodeling and enhances glucose utilization and protein deposition. Therefore, fish with inhibited mitochondrial FAO could have high potential to utilize carbohydrate. Our results demonstrate a potentially new approach for increasing protein deposition through energy homeostasis regulation in cultured animals.

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.

New Insights on Intermediary Metabolism for a Better Understanding of Nutrition in Teleosts

The rapid development of aquaculture production throughout the world over the past few decades has led to the emergence of new scientific challenges to improve fish nutrition. The diet formulations used for farmed fish have been largely modified in the past few years. However, bottlenecks still exist in being able to suppress totally marine resources (fish meal and fish oil) in diets without negatively affecting growth performance and flesh quality. A better understanding of fish metabolism and its regulation by nutrients is thus mandatory. In this review, we discuss four fields of research that are highly important for improving fish nutrition in the future: ( a) fish genome complexity and subsequent consequences for metabolism, ( b) microRNAs (miRNAs) as new actors in regulation of fish metabolism, ( c) the role of autophagy in regulation of fish metabolism, and ( d) the nutritional programming of metabolism linked to the early life of fish.

Essential amino acids exhibit variable effects on protein degradation in rainbow trout (Oncorhynchus mykiss) primary myocytes

The functional role of amino acids as regulators of protein degradation was investigated using primary myogenic precursor cell culture as in vitro model of rainbow trout white muscle. Seven-day old myocytes were starved of amino acids for two hours then exposed to media that contained amino acid treatments, during which protein degradation rates were analyzed over five hours by measuring cellular release of ³H-tyrosine. Increasing concentrations of essential amino acids (EAA) reduced protein degradation rates; this effect was dose-dependent within the physiological range found in plasma. Addition of leucine or phenylalanine at 5 mM and 2.5 mM, respectively, decreased rates of protein degradation compared to media without amino acid supplementation, suggesting that these amino acids directly regulate muscle proteolysis. Protein degradation rates were similar in cells exposed to media without EAA and media lacking only leucine, further supporting a role for leucine as a central regulator of protein turnover. Addition of 5 mM lysine or valine to media without amino acids increased protein degradation; this response was attenuated as EAA were added back into media, supporting that a lysine or valine imbalance is costly for muscle protein retention. In summary, there is evidence for amino acids as both positive and negative regulators of protein turnover in rainbow trout muscle. These findings suggest that there may be an optimal plasma amino acid profile that minimizes protein turnover and that this could be achieved through diet formulation.

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.

IRS1 and IRS2: molecular characterization, tissue expression and transcriptional regulation by insulin in yellow catfish Pelteobagrus fulvidraco

The insulin receptor substrate (IRS) proteins, in particular, IRS1 and IRS2, are the key downstream players of insulin signaling pathway and the regulation of lipid metabolism. In the present study, two genes of IRS (IRS1 and IRS2) were isolated and characterized from yellow catfish Pelteobagrus fulvidraco. Their molecular characterizations, tissue expressions, and transcriptional levels by insulin both in vivo and in vitro were determined. The validated complementary DNAs encoding for IRS1 and IRS2 were 3693 and 3177 bp in length, encoding proteins of 1230 and 1058 amino acid residues, respectively. Similarly to mammals, amino acid sequence alignment revealed that IRSs contained an N-terminal pleckstrin homology (PH) domain, a phosphotyrosine-binding (PTB) domain, and several C-terminal multiple sites of tyrosine phosphorylation. Both IRS1 and IRS2 were widely expressed across the ten tissues (liver, white muscle, spleen, brain, gill, mesenteric fat, anterior intestine, heart, mid-kidney, and ovary), but at the variable levels. Insulin injection at 1 μg/g in vivo significantly stimulated the messenger RNA (mRNA) expression of IRS2, but not IRS1 mRNA expression levels in the liver of yellow catfish after 48 h. In hepatocytes of yellow catfish, insulin incubation significantly stimulated the IRS1 (at a 1000 nM insulin group) and IRS2 (at both 100 and 1000 nM insulin groups) mRNA expressions, which indicated that IRS2 was more sensitive than IRS1 to insulin stimulation in the liver of yellow catfish, and IRS2 played a more important role in mediating insulin's effects on the liver metabolism. The present study serves to increase our understanding into the function of IRS in fish.

Molecular mechanisms of OLIG2 transcription factor in brain cancer

Oligodendrocyte lineage transcription factor 2 (OLIG2) plays a pivotal role in glioma development. Here we conducted a comprehensive study of the critical gene regulatory networks involving OLIG2. These include the networks responsible for OLIG2 expression, its translocation to nucleus, cell cycle, epigenetic regulation, and Rho-pathway interactions. We described positive feedback loops including OLIG2: loops of epigenetic regulation and loops involving receptor tyrosine kinases. These loops may be responsible for the prolonged oncogenic activity of OLIG2. The proposed schemes for epigenetic regulation of the gene networks involving OLIG2 are confirmed by patient survival (Kaplan-Meier) curves based on the cancer genome atlas (TCGA) datasets. Finally, we elucidate the Coherent-Gene Modules (CGMs) networks-framework of OLIG2 involvement in cancer. We showed that genes interacting with OLIG2 formed eight CGMs having a set of intermodular connections. We showed also that among the genes involved in these modules the most connected hub is EGFR, then, on lower level, HSP90 and CALM1, followed by three lower levels including epigenetic genes KDM1A and NCOR1. The genes on the six upper levels of the hierarchy are involved in interconnections of all eight CGMs and organize functionally defined gene-signaling subnetworks having specific functions. For example, CGM1 is involved in epigenetic control. CGM2 is significantly related to cell proliferation and differentiation. CGM3 includes a number of interconnected helix-loop-helix transcription factors (bHLH) including OLIG2. Many of these TFs are partially controlled by OLIG2. The CGM4 is involved in PDGF-related: angiogenesis, tumor cell proliferation and differentiation. These analyses provide testable hypotheses and approaches to inhibit OLIG2 pathway and relevant feed-forward and feedback loops to be interrogated. This broad approach can be applied to other TFs.

Flesh Shear Force, Cooking Loss, Muscle Antioxidant Status and Relative Expression of Signaling Molecules (Nrf2, Keap1, TOR, and CK2) and Their Target Genes in Young Grass Carp (Ctenopharyngodon idella) Muscle Fed with Graded Levels of Choline

Six groups of grass carp (average weight 266.9 ± 0.6 g) were fed diets containing 197, 385, 770, 1082, 1436 and 1795 mg choline/kg, for 8 weeks. Fish growth, and muscle nutrient (protein, fat and amino acid) content of young grass carp were significantly improved by appropriate dietary choline. Furthermore, muscle hydroxyproline concentration, lactate content and shear force were improved by optimum dietary choline supplementation. However, the muscle pH value, cooking loss and cathepsins activities showed an opposite trend. Additionally, optimum dietary choline supplementation attenuated muscle oxidative damage in grass carp. The muscle antioxidant enzyme (catalase and glutathione reductase did not change) activities and glutathione content were enhanced by optimum dietary choline supplementation. Muscle cooking loss was negatively correlated with antioxidant enzyme activities and glutathione content. At the gene level, these antioxidant enzymes, as well as the targets of rapamycin, casein kinase 2 and NF-E2-related factor 2 transcripts in fish muscle were always up-regulated by suitable choline. However, suitable choline significantly decreased Kelch-like ECH-associated protein 1 a (Keap1a) and Kelch-like ECH-associated protein 1 b (Keap1b) mRNA levels in muscle. In conclusion, suitable dietary choline enhanced fish flesh quality, and the decreased cooking loss was due to the elevated antioxidant status that may be regulated by Nrf2 signaling.

Contribution of the autophagy-lysosomal and ubiquitin-proteasomal proteolytic systems to total proteolysis in rainbow trout (Oncorhynchus mykiss) myotubes

The ubiquitin-proteasome system (UPS) is recognized as the major contributor to total proteolysis in mammalian skeletal muscle, responsible for 50% or more of total protein degradation in skeletal muscle, whereas the autophagic-lysosome system (ALS) plays a more minor role. While the relative contribution of these systems to muscle loss is well documented in mammals, little is known in fish species. The current study uses myotubes derived from rainbow trout myogenic precursor cells as an in vitro model of white muscle tissue. Cells were incubated in complete or serum-deprived media or media supplemented with insulin-like growth factor-1 (IGF-1) and exposed to selective proteolytic inhibitors to determine the relative contribution of the ALS and UPS to total protein degradation in myotubes in different culture conditions. Results indicate that the ALS is responsible for 30-34% and 50% of total protein degradation in myotubes in complete and serum-deprived media, respectively. The UPS appears to contribute much less to total protein degradation at almost 4% in cells in complete media to nearly 17% in serum-deprived cells. IGF-1 decreases activity of both systems, as it inhibited the upregulation of both proteolytic systems induced by serum deprivation. The combined inhibition of both the ALS and UPS reduced degradation by a maximum of 55% in serum-deprived cells, suggesting an important contribution of other proteolytic systems to total protein degradation. Collectively, these data identify the ALS as a potential target for strategies aimed at improving muscle protein retention and fillet yield through reductions in protein degradation.

High dietary lipid level is associated with persistent hyperglycaemia and downregulation of muscle Akt-mTOR pathway in Senegalese sole (Solea senegalensis)

High levels of dietary lipids are incorporated in feeds for most teleost fish to promote growth and reduce nitrogen waste. However, in Senegalese sole (Solea senegalensis) previous studies revealed that increasing the level of dietary lipids above 8% negatively affect growth and nutrient utilization regardless of dietary protein content. It has been shown that glucose regulation and metabolism can be impaired by high dietary fat intake in mammals, but information in teleost fish is scarce. The aim of this study was to assess the possible effect of dietary lipids on glucose metabolism in Senegalese sole with special emphasis on the regulation of proteins involved in the muscle insulin-signalling pathway. Senegalese sole juveniles (29 g) were fed two isonitrogenous diets (53% dry matter) for 88 days. These two diets were one with a high lipid level (∼17%, HL) and a moderate starch content (∼14%, LC), and the other being devoid of fish oil (4% lipid, LL) and with high starch content (∼23%, HC). Surprisingly, feeding Senegalese sole the HL/LC diet resulted in prolonged hyperglycaemia, while fish fed on LL/HC diet restored basal glycaemia 2 h after feeding. The hyperglycaemic phenotype was associated with greater glucose-6-phosphatase activity (a key enzyme of hepatic glucose production) and lower citrate synthase activity in the liver, with significantly higher liver glycogen content. Sole fed on HL/LC diet also had significantly lower hexokinase activity in muscle, although hexokinase activity was low with both dietary treatments. The HL/LC diet was associated with significant reductions in muscle AKT, p70 ribosomal S6-K1 Kinase (S6K-1) and ribosomal protein S6 (S6) 2 h after feeding, suggesting down regulation of the AKT-mTOR nutrient signalling pathway in these fish. The results of this study show for the first time that high level of dietary lipids strongly affects glucose metabolism in Senegalese sole.

Effect of sexual maturation on muscle gene expression of rainbow trout: RNA-Seq approach

Muscle degradation occurs as a response to various physiological states that are regulated by specific molecular mechanisms. Previously, we characterized the metabolic changes of muscle deterioration of the female rainbow trout at full sexual maturity and spawning (Salem et al., Physiol. Genomics 2006;28:33–45; J. Proteomics 2010;73:778–789). Muscle deterioration in this model represents nutrient mobilization as a response to the energetic overdemands of the egg/ovarian growth phase. Our recent studies showed that most of the changes in muscle growth and quality start 2–3 months before spawning. Gravid fish exhibited reduced intramuscular fat that is lower in saturated and monounsaturated fatty acids and higher in polyunsaturated fatty acids compared to sterile fish. In this study, RNA-Seq was used to explain the mechanisms underlying changes during this phase of sexual maturity. Furthermore, to minimize changes due to nutrient deficits, fish were fed on a high-plane of nutrition. The RNA-Seq technique identified a gene expression signature that is consistent with metabolic changes of gravid fish. Gravid fish exhibited increased abundance of transcripts in metabolic pathways of fatty acid degradation and up-regulated expression of genes involved in biosynthesis of unsaturated fatty acids. In addition, increased expression of genes involved in the citric acid cycle and oxidative phosphorylation was observed for gravid fish. This muscle transcriptomic signature of fish fed on a high nutritional plane is quite distinct from that previously described for fish at terminal stages of maturity and suggest that female rainbow trout approaching spawning, on high nutritional planes, likely mobilize intramuscular fat rather than protein to support gonadal maturation.

Characterisation and expression of myogenesis regulatory factors during in vitro myoblast development and in vivo fasting in the gilthead sea bream (Sparus aurata)

The aim of this study was to characterise a primary cell culture isolated from fast skeletal muscle of the gilthead sea bream. Gene expression profiles during culture maturation were compared with those obtained from a fasting-refeeding model which is widely used to modulate myogenesis in vivo. Myogenesis is controlled by numerous extracellular signals together with intracellular transcriptional factors whose coordinated expression is critical for the appropriate development of muscle fibres. Full-length cDNAs for the transcription factors Myf5, Mrf4, Pax7 and Sox8 were cloned and sequenced for gilthead sea bream. Pax7, sox8, myod2 and myf5 levels were up-regulated during the proliferating phase of the myogenic cultures coincident with the highest expression of proliferating cell nuclear antigen (PCNA). In contrast, myogenin and mrf4 transcript abundance was highest during the differentiation phase of the culture when myotubes were present, and was correlated with increased myosin heavy chain (mhc) and desmin expression. In vivo, 30days of fasting resulted in muscle fibre atrophy, a reduction in myod2, myf5 and igf1 expression, lower number of Myod-positive cells, and decreased PCNA protein expression, whereas myogenin expression was not significantly affected. Myostatin1 (mstn1) and pax7 expression were up-regulated in fasted relative to well-fed individuals, consistent with a role for Pax7 in the reduction of myogenic cell activity with fasting. The primary cell cultures and fasting-feeding experiments described provide a foundation for the future investigations on the regulation of muscle growth in gilthead sea bream.

Macronutrient composition of the diet affects the feeding-mediated down regulation of autophagy in muscle of rainbow trout (O. mykiss)

Autophagy functions as an important catabolic mechanism by mediating the turnover of intracellular organelles and protein complexes through a lysosome dependent degradative pathway. Although the induction of autophagy by starvation has been extensively studied, we still know very little about how autophagy is regulated under normal nutritional conditions. The purpose of the present study was to characterize both in vivo and in vitro the response of the autophagy-lysosomal degradative pathway to nutrient (amino acids and carbohydrates) availability in the muscle of the carnivorous rainbow trout. We report that meal feeding is accompanied by a rapid activation of Akt, FoxO1 and the Target of Rapamycin (TOR) signaling pathways and a concomitant decrease of autophagosome formation. We also show that this effect occurs only when the proportion of dietary proteins increases at the expense of carbohydrates. Concurrently, our in vitro study on primary culture of trout muscle cells demonstrates an opposite effect of amino acids and glucose on the regulation of autophagy-lysosomal pathways. More specifically, the addition of amino acids in cell culture medium inhibited the formation of autophagosomes, whereas the addition of glucose had an opposite effect. The effect of amino acids was accompanied by an activation of TOR, considered as an important regulator of autophagosomal formation. However, the mechanisms involved in the effect of glucose were independent of Akt, TOR and AMPK and remain to be determined. Together, these results demonstrated the specific role of macronutrients as well as that of their interactions in the regulation of autophagy and highlight the interest to consider the macronutrient composition of the diets in the control of this degradative pathway.

Myostatin induces atrophy of trout myotubes through inhibiting the TORC1 signaling and promoting Ubiquitin-Proteasome and Autophagy-Lysosome degradative pathways

Myostatin (MSTN) is well known as a potent inhibitor of muscle growth in mammals and has been shown to both inhibit the growth promoting TORC1 signaling pathway and promote Ubiquitin-Proteasomal and Autophagy-Lysosomal degradative routes. In contrast, in non-mammalian species, despite high structural conservation of MSTN sequence, functional conservation is only assumed. Here, we show that treatment of cultured trout myotubes with human recombinant MSTN (huMSTN) resulted in a significant decrease of their diameter by up to 20%, validating the use of heterologous huMSTN in our in vitro model to monitor the processes by which this growth factor promotes muscle wasting in fish. Accordingly, huMSTN stimulation prevented the full activation by IGF1 of the TORC1 signaling pathway, as revealed by the analysis of the phosphorylation status of 4E-BP1. Moreover, the levels of the proteasome-dependent protein Atrogin1 exhibited an increase in huMSTN treated cells. Likewise, we observed a stimulatory effect of huMSTN treatment on the levels of LC3-II, the more reliable marker of the Autophagy-Lysosomal degradative system. Overall, these results show for the first time in a piscine species the effect of MSTN on several atrophic and hypertrophic pathways and support a functional conservation of this growth factor between lower and higher vertebrates.

Postprandial regulation of growth- and metabolism-related factors in zebrafish

Zebrafish (Danio rerio) have been proposed as a possible model organism for nutritional physiology. However, this potential has not yet been realized and studies on the field remain scarce. In this work, we investigated in this species the effect of a single meal as well as that of an increase in the ratio of dietary carbohydrates/proteins on the postprandial expression of several hepatic and muscle metabolism-related genes and proteins. Fish were fed once either a commercial diet (experiment 1) or one of two experimental diets (experiment 2) containing different protein and carbohydrate levels after 72 h of starvation. Refeeding induced the postprandial expression of genes of glycolysis (GK, HK1) and lipogenesis (FAS, G6PDH, ACCa) and inhibited those of gluconeogenesis (cPEPCK) and beta-oxidation (CPT1b) in the viscera. In the muscle, refeeding increased transcript levels of myogenesis (Myf5, Myogenin), inhibited those of Ub-proteasomal proteolytic system (Atrogin1, Murf1a, Murf1b), and induced the activation of key signaling factors of protein synthesis (Akt, 4EBP1, S6K1, S6). However, diet composition had a low impact on the studied factors. Together, these results highlight some specificity of the zebrafish metabolism and demonstrate the interest and the limits of this species as a model organism for nutritional physiology studies.

Apparent low ability of liver and muscle to adapt to variation of dietary carbohydrate:protein ratio in rainbow trout (Oncorhynchus mykiss)

The rainbow trout (Oncorhynchus mykiss) exhibits high dietary amino acid requirements and an apparent inefficiency to use dietary carbohydrates. Using this species, we investigated the metabolic consequences of long-term high carbohydrates/low protein feeding. Fish were fed two experimental diets containing either 20 % carbohydrates/50 % proteins (C20P50), or high levels of carbohydrates at the expense of proteins (35 % carbohydrates/35 % proteins – C35P35). The expression of genes related to hepatic and muscle glycolysis (glucokinase (GK), pyruvate kinase and hexokinase) illustrates the poor utilisation of carbohydrates irrespective of their dietary levels. The increased postprandial GK activity and the absence of inhibition of the gluconeogenic enzyme glucose-6-phosphatase activity support the hypothesis of the existence of a futile cycle around glucose phosphorylation extending postprandial hyperglycaemia. After 9 weeks of feeding, the C35P35-fed trout displayed lower body weight and feed efficiency and reduced protein and fat gains than those fed C20P50. The reduced activation of eukaryotic translation initiation factor 4-E binding protein 1 (4E-BP1) in the muscle in this C35P35 group suggests a reduction in protein synthesis, possibly contributing to the reduction in N gain. An increase in the dietary carbohydrate:protein ratio decreased the expression of genes involved in amino acid catabolism (serine dehydratase and branched-chain α-keto acid dehydrogenase E1α and E1β), and increased that of carnitine palmitoyltransferase 1, suggesting a higher reliance on lipids as energy source in fish fed high-carbohydrate and low-protein diets. This probably also contributes to the lower fat gain. Together, these results show that different metabolic pathways are affected by a high-carbohydrate/low-protein diet in rainbow trout.

Postprandial regulation of hepatic microRNAs predicted to target the insulin pathway in rainbow trout

Rainbow trout are carnivorous fish and poor metabolizers of carbohydrates, which established this species as a model organism to study the comparative physiology of insulin. Following the recent characterisation of key roles of several miRNAs in the insulin action on hepatic intermediary metabolism in mammalian models, we investigated the hypothesis that hepatic miRNA expression is postprandially regulated in the rainbow trout and temporally coordinated in the context of insulin-mediated regulation of metabolic gene expression in the liver. To address this hypothesis, we used a time-course experiment in which rainbow trout were fed a commercial diet after short-term fasting. We investigated hepatic miRNA expression, activation of the insulin pathway, and insulin regulated metabolic target genes at several time points. Several miRNAs which negatively regulate hepatic insulin signaling in mammalian model organisms were transiently increased 4 h after the meal, consistent with a potential role in acute postprandial negative feed-back regulation of the insulin pathway and attenuation of gluconeogenic gene expression. We equally observed a transient increase in omy- miRNA-33 and omy-miRNA-122b 4 h after feeding, whose homologues have potent lipogenic roles in the liver of mammalian model systems. A concurrent increase in the activity of the hepatic insulin signaling pathway and the expression of lipogenic genes (srebp1c, fas, acly) was equally observed, while lipolytic gene expression (cpt1a and cpt1b) decreased significantly 4 h after the meal. This suggests lipogenic roles of omy-miRNA-33 and omy-miRNA-122b may be conserved between rainbow trout and mammals and that these miRNAs may furthermore contribute to acute postprandial regulation of de novo hepatic lipid synthesis in rainbow trout. These findings provide a framework for future research of miRNA regulation of hepatic metabolism in trout and will help to further elucidate the metabolic phenotype of rainbow trout.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

High levels of dietary fat impair glucose homeostasis in rainbow trout

This study was designed to assess the effects of dietary fat levels on glucose homeostasis in rainbow trout under prolonged hyperglycaemia induced by high carbohydrate intake. Trout were fed identical amounts of one of two iso-energetic diets containing either a low (LFD, 3%) or a high fat level (HFD, 20%) and similar amounts of digestible carbohydrates (26-30%) for 14 days. While a single high fat meal reduced glycaemia compared with a low fat meal, the consumption of a high fat diet for 14 days resulted in prolonged hypergylcaemia and reduced plasma glucose clearance in response to an exogenous glucose or insulin challenge. The hyperglycaemic phenotype in trout was characterised by a reduction of the activities of lipogenic and glucose phosphorylating enzymes with a concomitant stimulation of enzymes involved in glucose production in the liver and reduced glycogen levels in the white muscle. Impaired glucose tolerance (IGT) was further associated with a significant reduction of insulin receptor substrate 1 (IRS1) protein content in muscle, and with a poor response of HFD fed fish to an exogenous insulin load, suggestive of impaired insulin signalling in trout fed with a HFD. To our knowledge, this is the first study showing that a teleost can also develop a high fat-induced IGT, characterised by persistent hyperglycaemia and reduced insulin sensitivity, established symptoms of IGT and the prediabetic insulin-resistant state in mammals. Our results also provide evidence that persistent hyperglycaemia after a high carbohydrate meal stems from a metabolic interaction between dietary macronutrients rather than from high carbohydrate intake alone.