Increasing dietary levels of the n-3 long-chain PUFA, EPA and DHA, improves the growth, welfare, robustness and fillet quality of Atlantic salmon in sea cages.

The present study evaluated the effects of increasing the dietary levels of EPA and DHA in Atlantic salmon (Salmo salar) reared in sea cages, in terms of growth performance, welfare, robustness and overall quality. Fish with an average starting weight of 275 g were fed one of four different diets containing 10, 13, 16 and 35 g/kg of EPA and DHA (designated as 1·0, 1·3, 1·6 and 3·5 % EPA and DHA) until they reached approximately 5 kg. The 3·5 % EPA and DHA diet showed a significantly beneficial effect on growth performance and fillet quality compared with all other diets, particularly the 1 % EPA and DHA diet. Fish fed the diet containing 3·5 % EPA and DHA showed 400-600 g higher final weights, improved internal organ health scores and external welfare indicators, better fillet quality in terms of higher visual colour score and lower occurrence of dark spots and higher EPA and DHA content in tissues at the end of the feeding trial. Moreover, fish fed the 3·5 % EPA and DHA diet showed lower mortality during a naturally occurring cardiomyopathy syndrome outbreak, although this did not reach statistical significance. Altogether, our findings emphasise the importance of dietary EPA and DHA to maintain good growth, robustness, welfare and fillet quality of Atlantic salmon reared in sea cages.

A Piece of the Puzzle-Possible Mechanisms for Why Low Dietary EPA and DHA Cause Hepatic Lipid Accumulation in Atlantic Salmon (Salmo salar).

The present study aimed at elucidating the effects of graded levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the hepatic metabolic health of Atlantic salmon reared in sea cages. Diets containing 10, 13, 16 and 35 g/kg EPA + DHA (designated diets 1.0, 1.3, 1.6 and 3.5, respectively) were fed in triplicate through a full production cycle from an average starting weight of 275 g to slaughter size (~5 kg). Feeding low dietary EPA + DHA altered the hepatic energy metabolism, evidenced by reductions in tricarboxylic acid cycle intermediates originating from ß-oxidation, which was compensated by elevated activity in alternative energy pathways (pentose phosphate pathway, branched chain amino acid catabolism and creatine metabolism). Increases in various acylcarnitines in the liver supported this and indicates issues with lipid metabolism (mitochondrial ß-oxidation). Problems using lipids for energy in the lower EPA + DHA groups line up well with observed increases in liver lipids in these fish. It also aligns with the growth data, where fish fed the highest EPA + DHA grew better than the other groups. The study showed that diets 1.0 and 1.3 were insufficient for maintaining good liver metabolic health. However, diet 3.5 was significantly better than diet 1.6, indicating that diet 1.6 might also be suboptimal.

Long-term feeding of Atlantic salmon with varying levels of dietary EPA + DHA alters the mineral status but does not affect the stress responses after mechanical delousing stress.

Atlantic salmon were fed diets containing graded levels of EPA + DHA (1·0, 1·3, 1·6 and 3·5 % in the diet) and one diet with 1·3 % of EPA + DHA with reduced total fat content. Fish were reared in sea cages from about 275 g until harvest size (about 5 kg) and were subjected to delousing procedure (about 2·5 kg), with sampling pre-, 1 h and 24 h post-stress. Delousing stress affected plasma cortisol and hepatic mRNA expression of genes involved in oxidative stress and immune response, but with no dietary effects. Increasing EPA + DHA levels in the diet increased the trace mineral levels in plasma and liver during mechanical delousing stress period and whole body at harvest size. The liver Se, Zn, Fe, Cu, and Mn and plasma Se levels were increased in fish fed a diet high in EPA + DHA (3·5 %) upon delousing stress. Furthermore, increased dietary EPA + DHA caused a significant increase in mRNA expression of hepcidin antimicrobial peptide (HAMP), which is concurrent with downregulated transferrin receptor (TFR) expression levels. High dietary EPA + DHA also significantly increased the whole-body Zn, Se, and Mn levels at harvest size fish. Additionally, the plasma and whole-body Zn status increased, respectively, during stress and at harvest size in fish fed reduced-fat diet with less EPA + DHA. As the dietary upper limits of Zn and Se are legally added to the feeds and play important roles in maintaining fish health, knowledge on how the dietary fatty acid composition and lipid level affect body stores of these minerals is crucial for the aquaculture industry.

The autophagy response during adipogenesis of primary cultured rainbow trout (Oncorhynchus mykiss) adipocytes.

Adipogenesis is a tightly regulated process, and the involvement of autophagy has been recently proposed in mammalian models. In rainbow trout, two well-defined phases describe the development of primary cultured adipocyte cells: proliferation and differentiation. Nevertheless, information on the transcriptional profile at the onset of differentiation and the potential role of autophagy in this process is scarce. In the present study, the cells showed an early and transient induction of several adipogenic transcription factors genes' expression (i.e., cebpa and cebpb) along with the morphological changes (round shape filled with small lipid droplets) typical of the onset of adipogenesis. Then, the expression of various lipid metabolism-related genes involving the synthesis (fas), uptake (fatp1 and cd36), accumulation (plin2) and mobilization (hsl) of lipids, characteristic of the mature adipocyte, increased. In parallel, several autophagy markers (i.e., atg4b, gabarapl1 and lc3b) mirrored the expression of those adipogenic-related genes, suggesting a role of autophagy during in vitro fish adipogenesis. In this regard, the incubation of preadipocytes with lysosomal inhibitors (Bafilomycin A1 or Chloroquine), described to prevent autophagy flux, delayed the process of adipogenesis (i.e., cell remodelling), thus suggesting a possible relationship between autophagy and adipocyte differentiation in trout. Moreover, the disruption of the autophagic flux altered the expression of some key adipogenic genes such as cebpa and pparg. Overall, this study contributes to improve our knowledge on the regulation of rainbow trout adipocyte differentiation, and highlights for the first time in fish the involvement of autophagy on adipogenesis, suggesting a close-fitting connection between both processes.

The probiotic Lactobacillus rhamnosus mimics the dark-driven regulation of appetite markers and melatonin receptors' expression in zebrafish (Danio rerio) larvae: Understanding the role of the gut microbiome.

The use of probiotics has been recently considered a novel therapeutic strategy to prevent pathologies such as obesity; however, the specific mechanisms of action by which probiotics exert their beneficial effects on metabolic health remain unclear. The aim of the present study was to investigate the short-term effects of a probiotic Lactobacillus rhamnosus supplementation (PROB) on appetite regulation, growth-related markers, and microbiota diversity in zebrafish (Danio rerio) larvae, compared to a group subjected to a constant darkness photoperiod (DARK), as well as to evaluate the effects of both treatments on melatonin receptors' expression. After a 24 h treatment, both PROB and DARK conditions caused a significant increase in leptin a expression. Moreover, mRNA abundances of leptin b and proopiomelanocortin a were elevated in the PROB group, and DARK showed a similar tendency, supporting a negative regulation of appetite markers by the treatments. Moreover, both PROB and DARK also enhanced the abundances of melatonin receptors transcript (melatonin receptor 1 ba and bb) and protein (melatonin receptor 1) suggesting a potential involvement of melatonin in mediating these effects. Nevertheless, treatments did not exhibit a significant effect on the expression of most of the growth hormone/insulin-like growth factor axis genes evaluated. Finally, only the DARK condition significantly modulated gut microbiota diversity at such short time, altogether highlighting the rapid effects of this probiotic on modulating appetite regulatory and melatonin receptors' expression, without a concomitant variation of gut microbiota.

Genistein Induces Adipogenic and Autophagic Effects in Rainbow Trout (Oncorhynchus mykiss) Adipose Tissue: In Vitro and In Vivo Models.

Soybeans are one of the most used alternative dietary ingredients in aquafeeds. However, they contain phytoestrogens like genistein (GE), which can have an impact on fish metabolism and health. This study aimed to investigate the in vitro and in vivo effects of GE on lipid metabolism, apoptosis, and autophagy in rainbow trout (Oncorhynchus mykiss). Primary cultured preadipocytes were incubated with GE at different concentrations, 10 or 100 µM, and 1 µM 17ß-estradiol (E2). Furthermore, juveniles received an intraperitoneal injection of GE at 5 or 50 µg/g body weight, or E2 at 5 µg/g. In vitro, GE 100 µM increased lipid accumulation and reduced cell viability, apparently involving an autophagic process, indicated by the higher LC3-II protein levels, and higher lc3b and cathepsin d transcript levels achieved after GE 10 µM. In vivo, GE 50 µg/g upregulated the gene expression of fatty acid synthase (fas) and glyceraldehyde-3-phosphate dehydrogenase in adipose tissue, suggesting enhanced lipogenesis, whereas it increased hormone-sensitive lipase in liver, indicating a lipolytic response. Besides, autophagy-related genes increased in the tissues analyzed mainly after GE 50 µg/g treatment. Overall, these findings suggest that an elevated GE administration could lead to impaired adipocyte viability and lipid metabolism dysregulation in rainbow trout.

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 ß-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.

Photoperiod Manipulation Affects Transcriptional Profile of Genes Related to Lipid Metabolism and Apoptosis in Zebrafish (Danio rerio) Larvae: Potential Roles of Gut Microbiota.

Gut microbiota plays a fundamental role in maintaining host's health by controlling a wide range of physiological processes. Administration of probiotics and manipulation of photoperiod have been suggested as modulators of microbial composition and are currently undergoing an extensive research in aquaculture as a way to improve health and quality of harvested fish. However, our understanding regarding their effects on physiological processes is still limited. In the present study we investigated whether manipulation of photoperiod and/or probiotic administration was able to alter microbial composition in zebrafish larvae at hatching stage. Our findings show that probiotic does not elicit effects while photoperiod manipulation has a significant impact on microbiota composition. Moreover, we successfully predicted lipid biosynthesis and apoptosis to be modulated by microbial communities undergoing continuous darkness. Interestingly, expression levels of caspase 3 gene (casp3) and lipid-related genes (hnf4a, npc1l1, pparγ, srebf1, agpat4 and fitm2) were found to be significantly overexpressed in dark-exposed larvae, suggesting an increase in the occurrence of apoptotic processes and a lipid metabolism impairment, respectively (p < 0.05). Our results provide the evidence that microbial communities in zebrafish at early life stages are not modulated by a short administration of probiotics and highlight the significant effect that dark photoperiod elicits on zebrafish microbiota and potentially on health.

n-3 Canola oil effectively replaces fish oil as a new safe dietary source of DHA in feed for juvenile Atlantic salmon.

Limited availability of fish oils (FO), rich in n-3 long-chain (≥C20) PUFA, is a major constraint for further growth of the aquaculture industry. Long-chain n-3 rich oils from crops GM with algal genes are promising new sources for the industry. This project studied the use of a newly developed n-3 canola oil (DHA-CA) in diets of Atlantic salmon fingerlings in freshwater. The DHA-CA oil has high proportions of the n-3 fatty acids (FA) 18 : 3n-3 and DHA and lower proportions of n-6 FA than conventional plant oils. Levels of phytosterols, vitamin E and minerals in the DHA-CA were within the natural variation of commercial canola oils. Pesticides, mycotoxins, polyaromatic hydrocarbons and heavy metals were below lowest qualifiable concentration. Two feeding trials were conducted to evaluate effects of two dietary levels of DHA-CA compared with two dietary levels of FO at two water temperatures. Fish increased their weight approximately 20-fold at 16°C and 12-fold at 12°C during the experimental periods, with equal growth in salmon fed the FO diets compared with DHA-CA diets. Salmon fed DHA-CA diets had approximately the same EPA+DHA content in whole body as salmon fed FO diets. Gene expression, lipid composition and oxidative stress-related enzyme activities showed only minor differences between the dietary groups, and the effects were mostly a result of dietary oil level, rather than the oil source. The results demonstrated that DHA-CA is a safe and effective replacement for FO in diets of Atlantic salmon during the sensitive fingerling life-stage.

Interaction between dietary fatty acids and genotype on immune response in Atlantic salmon (Salmo salar) after vaccination: A transcriptome study.

A pivotal matter to aquaculture is the sourcing of sustainable resources as ingredients to aquafeeds. Levels of plant delivered oils as source of fatty acids (FA) in aquafeeds have reached around 70% resulting in reduced levels of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in salmon fillet composition. EPA and DHA can modulate inflammation and immune response, so it is crucial to understand how fish immune response is affected by low LC n-3 PUFA diet and if this diet can have a detrimental effect on vaccine response. Atlantic salmon (Salmo salar) can produce EPA/DHA from α-linolenic acid (ALA) and this endogenous capacity can be explored to develop families with higher tolerance to low LC n-3 PUFA diets. Here we analyze innate and adaptive immune response in Atlantic salmon to a commercial vaccine after being fed low levels of EPA and DHA, and we also compare three strains of salmon selected by their endogenous capacity of synthesizing LC- n-3 PUFA. A total of 2,890 differentially expressed genes (DEGs) were identified (p-value adjusted < 0.1) when comparing vaccinated fish against control non-vaccinated. Gene ontology (GO) and KEGG analysis with 442 up/downregulated genes revealed that most DEGs were both related to immune response as well as part of important immune related pathways, as "Toll-like receptor" and "Cytokine-Cytokine interaction". Adaptive response was also addressed by measuring antigen specific IgM, and titers were significantly higher than in the pre-immune fish at 62 days post-immunization. However, diet and strain had no/little effect on vaccine-specific IgM or innate immune responses. Atlantic salmon therefore display robustness in its response to vaccination even when feed low levels of LC n-3 PUFA.

Fatty acids from fish or vegetable oils promote the adipogenic fate of mesenchymal stem cells derived from gilthead sea bream bone potentially through different pathways.

Fish are rich in n-3 long-chain polyunsaturated fatty acids (LC-PUFA), such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, thus they have a great nutritional value for human health. In this study, the adipogenic potential of fatty acids commonly found in fish oil (EPA and DHA) and vegetable oils (linoleic (LA) and alpha-linolenic (ALA) acids), was evaluated in bone-derived mesenchymal stem cells (MSCs) from gilthead sea bream. At a morphological level, cells adopted a round shape upon all treatments, losing their fibroblastic form and increasing lipid accumulation, especially in the presence of the n-6 PUFA, LA. The mRNA levels of the key transcription factor of osteogenesis, runx2 significantly diminished and those of relevant osteogenic genes remained stable after incubation with all fatty acids, suggesting that the osteogenic process might be compromised. On the other hand, transcript levels of the main adipogenesis-inducer factor, pparg increased in response to EPA. Nevertheless, the specific PPARγ antagonist T0070907 appeared to suppress the effects being caused by EPA over adipogenesis. Moreover, LA, ALA and their combinations, significantly up-regulated the fatty acid transporter and binding protein, fatp1 and fabp11, supporting the elevated lipid content found in the cells treated with those fatty acids. Overall, this study has demonstrated that fatty acids favor lipid storage in gilthead sea bream bone-derived MSCs inducing their fate into the adipogenic versus the osteogenic lineage. This process seems to be promoted via different pathways depending on the fatty acid source, being vegetable oils-derived fatty acids more prone to induce unhealthier metabolic phenotypes.

A long-term growth hormone treatment stimulates growth and lipolysis in gilthead sea bream juveniles.

The enhancement of the endocrine growth hormone (GH)/insulin-like growth factor I (IGF-I) system by the treatment with a sustained release formulation of a recombinant bovine GH (rBGH), is a good strategy to investigate growth optimization in aquaculture fish species. To further deepen into the knowledge of rBGH effects in fish and to estimate the growth potential of juveniles of gilthead sea bream, the present work evaluated rBGH injection on growth, GH/IGF-I axis and lipid metabolism modulation, and explored the conservation of GH effects provoked by the in vivo treatment using in vitro models of different tissues. The rBGH treatment increased body weight and specific growth rate (SGR) in juveniles and potentiated hyperplastic muscle growth while reducing circulating triglyceride levels. Moreover, the results demonstrated that the in vivo treatment enhanced also lipolysis in both isolated hepatocytes and adipocytes, as well as in day 4 cultured myocytes. Furthermore, these cultured myocytes extracted from rBGH-injected fish presented higher gene expression of GH/IGF-I axis-related molecules and myogenic regulatory factors, as well as stimulated myogenesis (i.e. increased protein expression of a proliferation and a differentiation marker) compared to Control fish-derived cells. These data, suggested that cells in vitro can retain some of the pathways activated by in vivo treatments in fish, what can be considered an interesting line of applied research. Overall, the results showed that rBGH stimulates somatic growth, including specifically muscle hyperplasia, as well as lipolytic activity in gilthead sea bream juveniles.

Recombinant bovine growth hormone (rBGH) enhances somatic growth by regulating the GH-IGF axis in fingerlings of gilthead sea bream (Sparus aurata).

The growth hormone (GH)/insulin-like growth factors (IGFs) endocrine axis is the main growth-regulator system in vertebrates. Some authors have demonstrated the positive effects on growth of a sustained-release formulation of a recombinant bovine GH (rBGH) in different fish species. The aim of this work was to characterize the effects of a single injection of rBGH in fingerlings of gilthead sea bream on growth, GH-IGF axis, and both myogenic and osteogenic processes. Thus, body weight and specific growth rate were significantly increased in rBGH-treated fish respect to control fish at 6weeks post-injection, whereas the hepatosomatic index was decreased and the condition factor and mesenteric fat index were unchanged, altogether indicating enhanced somatic growth. Moreover, rBGH injection increased the plasma IGF-I levels in parallel with a rise of hepatic mRNA from total IGF-I, IGF-Ic and IGF-II, the binding proteins IGFBP-1a and IGFBP-2b, and also the receptors IGF-IRb, GHR-I and GHR-II. In skeletal muscle, the expression of IGF-Ib and GHR-I was significantly increased but that of IGF-IRb was reduced; the mRNA levels of myogenic regulatory factors, proliferation and differentiation markers (PCNA and MHC, respectively), or that of different molecules of the signaling pathway (TOR/AKT) were unaltered. Besides, the growth inhibitor myostatin (MSTN1 and MSTN2) and the hypertrophic marker (MLC2B) expression resulted significantly enhanced, suggesting altogether that the muscle is in a non-proliferative stage of development. Contrarily in bone, although the expression of most molecules of the GH/IGF axis was decreased, the mRNA levels of several osteogenic genes were increased. The histology analysis showed a GH induced lipolytic effect with a clear decrease in the subcutaneous fat layer. Overall, these results reveal that a better growth potential can be achieved on this species and supports the possibility to improve growth and quality through the optimization of its culture conditions.

Proteolytic systems' expression during myogenesis and transcriptional regulation by amino acids in gilthead sea bream cultured muscle cells.

Proteolytic systems exert an important role in vertebrate muscle controlling protein turnover, recycling of amino acids (AA) or its use for energy production, as well as other functions like myogenesis. In fish, proteolytic systems are crucial for the relatively high muscle somatic index they possess, and because protein is the most important dietary component. Thus in this study, the molecular profile of proteolytic markers (calpains, cathepsins and ubiquitin-proteasome system (UbP) members) were analyzed during gilthead sea bream (Sparus aurata) myogenesis in vitro and under different AA treatments. The gene expression of calpains (capn1, capn3 and capns1b) decreased progressively during myogenesis together with the proteasome member n3; whereas capn2, capns1a, capns1b and ubiquitin (ub) remained stable. Contrarily, the cathepsin D (ctsd) paralogs and E3 ubiquitin ligases mafbx and murf1, showed a significant peak in gene expression at day 8 of culture that slightly decreased afterwards. Moreover, the protein expression analyzed for selected molecules presented in general the same profile of the mRNA levels, which was confirmed by correlation analysis. These data suggest that calpains seem to be more important during proliferation, while cathepsins and the UbP system appear to be required for myogenic differentiation. Concerning the transcriptional regulation by AA, the recovery of their levels after a short starvation period did not show effects on cathepsins expression, whereas it down-regulated the expression of capn3, capns1b, mafbx, murf1 and up-regulated n3. With regards to AA deficiencies, the major changes occurred at day 2, when leucine limitation suppressed ctsb and ctsl expression. Besides at the same time, both leucine and lysine deficiencies increased the expression of mafbx and murf1 and decreased that of n3. Overall, the opposite nutritional regulation observed, especially for the UbP members, points out an efficient and complementary role of these factors that could be useful in gilthead sea bream diets optimization.

Caffeic acid and hydroxytyrosol have anti-obesogenic properties in zebrafish and rainbow trout models.

Some natural products, known sources of bioactive compounds with a wide range of properties, may have therapeutic values in human health and diseases, as well as agronomic applications. The effect of three compounds of plant origin with well-known dietary antioxidant properties, astaxanthin (ATX), caffeic acid (CA) and hydroxytyrosol (HT), on zebrafish (Danio rerio) larval adiposity and rainbow trout (Onchorynchus mykiss) adipocytes was assessed. The zebrafish obesogenic test (ZOT) demonstrated the anti-obesogenic activity of CA and HT. These compounds were able to counteract the obesogenic effect produced by the peroxisome proliferator-activated receptor gamma (PPARγ) agonist, rosiglitazone (RGZ). CA and HT suppressed RGZ-increased PPARγ protein expression and lipid accumulation in primary-cultured rainbow trout adipocytes. HT also significantly reduced plasma triacylglycerol concentrations, as well as mRNA levels of the fasn adipogenic gene in the adipose tissue of HT-injected rainbow trout. In conclusion, in vitro and in vivo approaches demonstrated the anti-obesogenic potential of CA and HT on teleost fish models that may be relevant for studying their molecular mode of action. Further studies are required to evaluate the effect of these bioactive components as food supplements for modulating adiposity in farmed fish.

Tributyltin and triphenyltin exposure promotes in vitro adipogenic differentiation but alters the adipocyte phenotype in rainbow trout.

Numerous environmental pollutants have been identified as potential obesogenic compounds affecting endocrine signaling and lipid homeostasis. Among them, well-known organotins such as tributyltin (TBT) and triphenyltin (TPT), can be found in significant concentrations in aquatic environments. The aim of the present study was to investigate in vitro the effects of TBT and TPT on the development and lipid metabolism of rainbow trout (Onchorynchus mykiss) primary cultured adipocytes. Results showed that TBT and TPT induced lipid accumulation and slightly enhanced peroxisome proliferator-activated receptor gamma (PPARγ) and CCAAT enhancer binding protein alpha (C/EBPα) protein expression when compared to a control, both in the presence or absence of lipid mixture. However, the effects were higher when combined with lipid, and in the absence of it, the organotins did not cause complete mature adipocyte morphology. Regarding gene expression analyses, exposure to TBT and TPT caused an increase in fatty acid synthase (fasn) mRNA levels confirming the pro-adipogenic properties of these compounds. In addition, when added together with lipid, TBT and TPT significantly increased cebpa, tumor necrosis factor alpha (tnfa) and ATP-binding cassette transporter 1 (abca1) mRNA levels suggesting a synergistic effect. Overall, our data highlighted that TBT and TPT activate adipocyte differentiation in rainbow trout supporting an obesogenic role for these compounds, although by themselves they are not able to induce complete adipocyte development and maturation suggesting that these adipocytes might not be properly functional.

Moderate and sustained exercise modulates muscle proteolytic and myogenic markers in gilthead sea bream (Sparus aurata).

Swimming activity primarily accelerates growth in fish by increasing protein synthesis and energy efficiency. The role of muscle in this process is remarkable and especially important in teleosts, where muscle represents a high percentage of body weight and because many fish species present continuous growth. The aim of this work was to characterize the effects of 5 wk of moderate and sustained swimming in gene and protein expression of myogenic regulatory factors, proliferation markers, and proteolytic molecules in two muscle regions (anterior and caudal) of gilthead sea bream fingerlings. Western blot results showed an increase in the proliferation marker proliferating cell nuclear antigen (PCNA), proteolytic system members calpain 1 and cathepsin D, as well as vascular endothelial growth factor protein expression. Moreover, quantitative real-time PCR data showed that exercise increased the gene expression of proteases (calpains, cathepsins, and members of the ubiquitin-proteasome system in the anterior muscle region) and the gene expression of the proliferation marker PCNA and the myogenic factor MyoD in the caudal area compared with control fish. Overall, these data suggest a differential response of the two muscle regions during swimming adaptation, with tissue remodeling and new vessel formation occurring in the anterior muscle and enhanced cell proliferation and differentiation occurring in the caudal area. In summary, the present study contributes to improving the knowledge of the role of proteolytic molecules and other myogenic factors in the adaptation of muscle to moderate sustained swimming in gilthead sea bream.

Lysine and Leucine Deficiencies Affect Myocytes Development and IGF Signaling in Gilthead Sea Bream (Sparus aurata).

Optimizing aquaculture production requires better knowledge of growth regulation and improvement in diet formulation. A great effort has been made to replace fish meal for plant protein sources in aquafeeds, making necessary the supplementation of such diets with crystalline amino acids (AA) to cover the nutritional requirements of each species. Lysine and Leucine are limiting essential AA in fish, and it has been demonstrated that supplementation with them improves growth in different species. However, the specific effects of AA deficiencies in myogenesis are completely unknown and have only been studied at the level of hepatic metabolism. It is well-known that the TOR pathway integrates the nutritional and hormonal signals to regulate protein synthesis and cell proliferation, to finally control muscle growth, a process also coordinated by the expression of myogenic regulatory factors (MRFs). This study aimed to provide new information on the impact of Lysine and Leucine deficiencies in gilthead sea bream cultured myocytes examining their development and the response of insulin-like growth factors (IGFs), MRFs, as well as key molecules involved in muscle growth regulation like TOR. Leucine deficiency did not cause significant differences in most of the molecules analyzed, whereas Lysine deficiency appeared crucial in IGFs regulation, decreasing significantly IGF-I, IGF-II and IGF-IRb mRNA levels. This treatment also down-regulated the gene expression of different MRFs, including Myf5, Myogenin and MyoD2. These changes were also corroborated by a significant decrease in proliferation and differentiation markers in the Lysine-deficient treatment. Moreover, both Lysine and Leucine limitation induced a significant down-regulation in FOXO3 gene expression, which deserves further investigation. We believe that these results will be relevant for the production of a species as appreciated for human consumption as it is gilthead sea bream and demonstrates the importance of an adequate level of Lysine in fishmeal diet formulation for optimum growth.

Contribution of in vitro myocytes studies to understanding fish muscle physiology.

Research on the regulation of fish muscle physiology and growth was addressed originally by classical in vivo approaches; however, systemic interactions resulted in many questions that could be better considered through in vitro myocyte studies. The first paper published by our group in this field was with Tom Moon on brown trout cardiomyocytes, where the insulin and IGF-I receptors were characterized and the down-regulatory effects of an excess of peptides demonstrated. We followed the research on cultured skeletal muscle cells through the collaboration with INRA focused on the characterization of IGF-I receptors and its signaling pathways through in vitro development. Later on, we showed the important metabolic role of IGFs, although these studies were only the first stage of a prolific area of work that has offered a useful tool to advance in our knowledge of the endocrine and nutritional regulation of fish growth and metabolism. Obviously, the findings obtained in vitro serve the purpose to propose the scenario that will need confirmation in vivo, but this technique has made possible many different, easy, fast and better controlled studies. In this review, we have summarized the main advances that the use of cultured muscle cells has permitted, focusing mainly in the role of IGFs regulating fish metabolism and growth. Although many articles have already appeared using this model system in salmonids, gilthead sea bream or zebrafish, it is reasonable to expect new studies with cultured cells using innovative approaches that will help to understand fish physiology and its regulation.