IGF-I binding in primary culture of muscle cells of rainbow trout: changes during in vitro development

Nutritional status affects Igf1 regulation of skeletal muscle myogenesis, myostatin, and myofibrillar protein degradation pathways in gopher rockfish (Sebastes carnatus)

Insulin-like growth factor-1 (Igf1) regulates skeletal muscle growth in fishes by increasing protein synthesis and promoting muscle hypertrophy. When fish experience periods of insufficient food intake, they undergo slower muscle growth or even muscle wasting, and those changes emerge in part from nutritional modulation of Igf1 signaling. Here, we examined how food deprivation (fasting) modulates Igf1 regulation of liver and skeletal muscle gene expression in gopher rockfish (Sebastes carnatus), a nearshore rockfish of importance for commercial and recreational fisheries in the northeastern Pacific Ocean, to understand how food limitation impacts Igf regulation of muscle growth pathways. Rockfish were either fed or fasted for 14 d, after which a subset of fish from each group was treated with recombinant Igf1 from sea bream (Sparus aurata). Fish that were fasted lost body mass and had lower body condition, reduced hepatosomatic index, and lower plasma Igf1 concentrations, as well as a decreased abundance of igf1 gene transcripts in the liver, increased hepatic mRNAs for Igf binding proteins igfbp1a, igfbp1b, and igfbp3a, and decreased mRNA abundances for igfbp2b and a putative Igf acid labile subunit (igfals) gene. In skeletal muscle, fasted fish showed a reduced abundance of intramuscular igf1 mRNAs but elevated gene transcripts encoding Igf1 receptors A (igf1ra) and B (igf1rb), which also showed downregulation by Igf1. Fasting increased skeletal muscle mRNAs for myogenin and myostatin1, as well as ubiquitin ligase F-box only protein 32 (fbxo32) and muscle RING-finger protein-1 (murf1) genes involved in muscle atrophy, while concurrently downregulating mRNAs for myoblast determination protein 2 (myod2), myostatin2, and myogenic factors 5 (myf5) and 6 (myf6 encoding Mrf4). Treatment with Igf1 downregulated muscle myostatin1 and fbxo32 under both feeding conditions, but showed feeding-dependent effects on murf1, myf5, and myf6/Mrf4 gene expression indicating that Igf1 effects on muscle growth and atrophy pathways is contingent on recent food consumption experience.

Differentiation and Maturation of Muscle and Fat Cells in Cultivated Seafood: Lessons from Developmental Biology

Cultivated meat, also known as cultured or cell-based meat, is meat produced directly from cultured animal cells rather than from a whole animal. Cultivated meat and seafood have been proposed as a means of mitigating the substantial harms associated with current production methods, including damage to the environment, antibiotic resistance, food security challenges, poor animal welfare, and-in the case of seafood-overfishing and ecological damage associated with fishing and aquaculture. Because biomedical tissue engineering research, from which cultivated meat draws a great deal of inspiration, has thus far been conducted almost exclusively in mammals, cultivated seafood suffers from a lack of established protocols for producing complex tissues in vitro. At the same time, fish such as the zebrafish Danio rerio have been widely used as model organisms in developmental biology. Therefore, many of the mechanisms and signaling pathways involved in the formation of muscle, fat, and other relevant tissue are relatively well understood for this species. The same processes are understood to a lesser degree in aquatic invertebrates. This review discusses the differentiation and maturation of meat-relevant cell types in aquatic species and makes recommendations for future research aimed at recapitulating these processes to produce cultivated fish and shellfish.

Myomaker and Myomixer Characterization in Gilthead Sea Bream under Different Myogenesis Conditions

Skeletal muscle is formed by multinucleated myofibers originated by waves of hyperplasia and hypertrophy during myogenesis. Tissue damage triggers a regeneration process including new myogenesis and muscular remodeling. During myogenesis, the fusion of myoblasts is a key step that requires different genes' expression, including the fusogens myomaker and myomixer. The present work aimed to characterize these proteins in gilthead sea bream and their possible role in in vitro myogenesis, at different fish ages and during muscle regeneration after induced tissue injury. Myomaker is a transmembrane protein highly conserved among vertebrates, whereas Myomixer is a micropeptide that is moderately conserved. myomaker expression is restricted to skeletal muscle, while the expression of myomixer is more ubiquitous. In primary myocytes culture, myomaker and myomixer expression peaked at day 6 and day 8, respectively. During regeneration, the expression of both fusogens and all the myogenic regulatory factors showed a peak after 16 days post-injury. Moreover, myomaker and myomixer were present at different ages, but in fingerlings there were significantly higher transcript levels than in juveniles or adult fish. Overall, Myomaker and Myomixer are valuable markers of muscle growth that together with other regulatory molecules can provide a deeper understanding of myogenesis regulation in fish.

Rainbow trout slow myoblast cell culture as a model to study slow skeletal muscle, and the characterization of mir-133 and mir-499 families as a case study

Muscle fibres are classified as fast, intermediate and slow. In vitro myoblast cell culture model from fast muscle is a very useful tool to study muscle growth and development; however, similar models for slow muscle do not exist. Owing to the compartmentalization of fish muscle fibres, we have developed a slow myoblast cell culture for rainbow trout (Oncorhynchus mykiss). Slow and fast muscle-derived myoblasts have similar morphology, but with differential expression of slow muscle markers such as slow myhc, sox6 and pgc-1α We also characterized the mir-133 and mir-499 microRNA families in trout slow and fast myoblasts as a case study during myogenesis and in response to electrostimulation. Three mir-133 (a-1a, a-1b and a-2) and four mir-499 (aa, ab, ba and bb) paralogues were identified for rainbow trout and named base on their phylogenetic relationship to zebrafish and Atlantic salmon orthologues. Omy-mir-499ab and omy-mir-499bb had 0.6 and 0.5-fold higher expression in slow myoblasts compared with fast myoblasts, whereas mir-133 duplicates had similar levels in both phenotypes and little variation during development. Slow myoblasts also showed increased expression for omy-mir-499b paralogues in response to chronic electrostimulation (7-fold increase for omy-mir-499ba and 2.5-fold increase for omy-mir-499bb). The higher expression of mir-499 paralogues in slow myoblasts suggests a role in phenotype determination, while the lack of significant differences of mir-133 copies during culture development might indicate a different role in fish compared with mammals. We have also found signs of sub-functionalization of mir-499 paralogues after electrostimulation, with omy-mir-499b copies more responsive to electrical signals.

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.

Impact of Short-Term Fasting on The Rhythmic Expression of the Core Circadian Clock and Clock-Controlled Genes in Skeletal Muscle of Crucian Carp (Carassius auratus)

The peripheral tissue pacemaker is responsive to light and other zeitgebers, especially food availability. Generally, the pacemaker can be reset and entrained independently of the central circadian structures. Studies involving clock-gene expressional patterns in fish peripheral tissues have attracted considerable attention. However, the rhythmic expression of clock genes in skeletal muscle has only scarcely been investigated. The present study was designed to investigate the core clock and functional gene expression rhythms in crucian carp. Meanwhile, the synchronized effect of food restrictions (short-term fasting) on these rhythms in skeletal muscle was carefully examined. In fed crucian carp, three core clock genes (Clock, Bmal1a, and Per1) and five functional genes (Epo, Fas, IGF1R2, Jnk1, and MyoG) showed circadian rhythms. By comparison, four core clock genes (Clock, Bmal1a, Cry3, and Per2) and six functional genes (Epo, GH, IGF2, Mstn, Pnp5a, and Ucp1) showed circadian rhythms in crucian carp muscle after 7-day fasting. In addition, three core clock genes (Clock, Per1, and Per3) and six functional genes (Ampk1a, Lpl, MyoG, Pnp5a, PPARα, and Ucp1) showed circadian rhythms in crucian carp muscle after 15-day fasting. However, all gene rhythmic expression patterns differed from each other. Furthermore, it was found that the circadian genes could be altered by feed deprivation in crucian carp muscle through the rhythms correlation analysis of the circadian genes and functional genes. Hence, food-anticipatory activity of fish could be adjusted through the food delivery restriction under a light⁻dark cycle. These results provide a potential application in promoting fish growth by adjusting feeding conditions and nutritional state.

Regulation of compensatory growth by molecular mechanism in Labeo rohita juveniles under different feeding regimes

A study was carried out to assess the regulation of compensatory growth under different restriction feeding regimes in Labeo rohita juveniles by the interaction of various feed intake and growth regulating genes. A 60 day feeding trial was conducted with five treatment groups, Control (3% body weight, bw), T1 (alternate days), T2 (0.5% bw), T3 (1% bw) and T4 (2% bw) and feeding was done for first 30 days of the trial. For next 30 days, all the treatment groups were fed at a rate of 3% bw as in the control group. There was significant (p < 0.05) difference in the weight gain among the treatment groups with lowest FCR and highest PER was found in T2 group. Ghrelin gene mRNA levels were upregulated during first 30th days of the trial with highest expression levels in the T2 group. The expression levels of leptin gene mRNA were found significantly different (p < 0.05) among the treatments, which was down-regulated during initial 30 days and upregulated as the experiment progress towards 60th day. The IGF-1 mRNA expression levels were upregulated more in liver compared to the muscle tissue. The results of the study suggest that increased ghrelin levels and decreased leptin levels lead to hyperphagia during the onset of refeeding, which further triggers the compensatory growth in L. rohita. The present study describes the molecular mechanism behind the compensatory growth following a different feed restriction regime in L. rohita which is regulated due to the interaction of different energy homeostasis and growth regulating genes.

The DNA methylation status of MyoD and IGF-I genes are correlated with muscle growth during different developmental stages of Japanese flounder (Paralichthys olivaceus)

Many genes related to muscle growth modulate myoblast proliferation and differentiation and promote muscle hypertrophy. MyoD is a myogenic determinant that contributes to myoblast determination, and insulin-like growth factor 1 (IGF-I) interacts with MyoD to regulate muscle hypertrophy and muscle mass. In this study, we aimed to assess DNA methylation and mRNA expression patterns of MyoD and IGF-I during different developmental stages of Japanese flounder, and to examine the relationship between MyoD and IGF-I gene. DNA and RNA were extracted from muscles, and DNA methylation of MyoD and IGF-I promoter and exons was detected by bisulfite sequencing. The relative expression of MyoD and IGF-I was measured by quantitative polymerase chain reaction. IGF-I was measured by radioimmunoassay. Interestingly, the lowest expression of MyoD and IGF-I emerged at larva stage, and the mRNA expression was negatively associated with methylation. We hypothesized that many skeletal muscle were required to complete metamorphosis; thus, the expression levels of MyoD and IGF-I genes increased from larva stage and then decreased. The relative expression levels of MyoD and IGF-I exhibited similar patterns, suggesting that MyoD and IGF-I regulated muscle growth through combined effects. Changes in the concentrations of IGF-I hormone were similar to those of IGF-I gene expression. Our results the mechanism through which MyoD and IGF-I regulate muscle development and demonstrated that MyoD interacted with IGF-I to regulate muscle growth during different developmental stages.

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.

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.

mRNA-seq reveals skeletal muscle atrophy in response to handling stress in a marine teleost, the red cusk-eel (Genypterus chilensis)

Background

Fish reared under intensive conditions are repeatedly exposed to stress, which negatively impacts growth. Although most fish follow a conserved pattern of stress response, with increased concentrations of cortisol, each species presents specificities in the cell response and stress tolerance. Therefore, culturing new species requires a detailed knowledge of these specific responses. The red cusk-eel (Genypterus chilensis) is a new economically important marine species for the Chilean aquaculture industry. However, there is no information on the stress- and cortisol-induced mechanisms that decrease skeletal muscle growth in this teleost.

Results

Using Illumina RNA-seq technology, skeletal muscle sequence reads for G. chilensis were generated under control and handling stress conditions. Reads were mapped onto a reference transcriptome, resulting in the in silico identification of 785 up-regulated and 167 down-regulated transcripts. Gene ontology enrichment analysis revealed a significant up-regulation of catabolic genes associated with skeletal muscle atrophy. These results were validated by RT-qPCR analysis for ten candidates genes involved in ubiquitin-mediated proteolysis, autophagy and skeletal muscle growth. Additionally, using a primary culture of fish skeletal muscle cells, the effect of cortisol was evaluated in relation to red cusk-eel skeletal muscle atrophy.

Conclusions

The present data demonstrated that handling stress promotes skeletal muscle atrophy in the marine teleost G. chilensis through the expression of components of the ubiquitin-proteasome and autophagy-lysosome systems. Furthermore, cortisol was a powerful inductor of skeletal muscle atrophy in fish myotubes. This study is an important step towards understanding the atrophy system in non-model teleost species and provides novel insights on the cellular and molecular mechanisms that control skeletal muscle growth in early vertebrates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2232-7) contains supplementary material, which is available to authorized users.

Insulin-like growth factors effects on the expression of myogenic regulatory factors in gilthead sea bream muscle cells

Gilthead sea bream (Sparus aurata) is a widely cultured fish; however, muscle development regulation is poorly known. Myogenesis can be activated by the myogenic regulatory factors (MRFs: MyoD, Myf5, myogenin and MRF4) and by endocrine signals from the growth hormone (GH)/insulin-like growth factors (IGFs) axis. We cultured gilthead sea bream myocytes to better understand the role of IGFs in muscle growth and differentiation through the regulation of MRFs expression. First, we studied the expression pattern during culture development of IGFs and IGF-I splice variants. The expression of igf-II was highest at the beginning of the culture and decreased when the cells started to differentiate, similarly to that observed for total igf-I. Igf-Ib showed a paralleled expression pattern as that of total igf-I, whereas igf-Ic was more stable during culture progression. Next, we analyzed the expression of IGFs and MRFs after incubation of cells at day 4 with GH, IGF-I, IGF-II and combinations of them at 3, 6 and 18 h. IGF-II increased myod2 and myf5 expression, genes involved in early muscle cell proliferation. Moreover, IGF-I caused an increase on mrf4 and myogenin expression, both involved in the later stages of development corresponding to differentiation. Regarding the regulation of IGFs expression, igf-I was stimulated by GH and IGF-II alone and combined, whereas igf-II expression was increased in response to IGF-I, suggesting a nice model of crossed regulation. Overall, the present model could be very useful to understand the different regulatory roles of these endocrine and transcription factors on fish myogenesis.

Myostatin inhibits myosatellite cell proliferation and consequently activates differentiation: evidence for endocrine-regulated transcript processing

Myostatin is a potent negative regulator of muscle growth in mammals. Despite high structural conservation, functional conservation in nonmammalian species is only assumed. This is particularly true for fish due to the presence of several myostatin paralogs: two in most species and four in salmonids (MSTN-1a, -1b, -2a, and -2b). Rainbow trout are a rich source of primary myosatellite cells as hyperplastic muscle growth occurs even in adult fish. These cells were therefore used to determine myostatin's effects on proliferation whereas our earlier studies reported its effects on quiescent cells. As in mammals, recombinant myostatin suppressed proliferation with no changes in cell morphology. Expression of MSTN-1a was several fold higher than the other paralogs and was autoregulated by myostatin, which also upregulated the expression of key differentiation markers: Myf5, MyoD1, myogenin, and myosin light chain. Thus, myostatin-stimulated cellular growth inhibition activates rather than represses differentiation. IGF-1 stimulated proliferation but had minimal and delayed effects on differentiation and its actions were suppressed by myostatin. However, IGF-1 upregulated MSTN-2a expression and the processing of its transcript, which is normally unprocessed. Myostatin therefore appears to partly mediate IGF-stimulated myosatellite differentiation in rainbow trout. This also occurs in mammals, although the IGF-stimulated processing of MSTN-2a transcripts is highly unique and is indicative of subfunctionalization within the gene family. These studies also suggest that the myokine's actions, including its antagonistic relationship with IGF-1, are conserved and that the salmonid gene family is functionally diverging.

Adiponectin effects and gene expression in rainbow trout: an in vivo and in vitro approach

Here we present the presence of adiponectin and adiponectin receptors [type 1 (adipoR1) and type 2 (adipoR2)] in rainbow trout (Oncorhynchus mykiss) tissues and cell cultures together with the response to different scenarios. In response to fasting, adiponectin expression was up-regulated in adipose tissue, while the expression of its receptors increased in white and red muscle. Insulin injection decreased adipoR1 expression in white and red muscles. We deduce that the adipoRs in trout muscle show opposite responses to increasing insulin plasma levels, which may maintain sensitivity to insulin in this tissue. Adiponectin expression was inhibited by the inflammatory effect of lipopolysaccharide (LPS) in adipose tissue and red muscle. Moreover, results indicate that LPS may lead to mobilization of fat reserves, increasing adipoR1 expression in adipose tissue. The effects of LPS could be mediated through tumour necrosis factor α (TNFα), at least in red muscle. Insulin, growth hormone and TNFα all diminished expression of adipoR2 in adipocytes and adipoR1 in myotubes, while insulin increased the expression of adipoR2 in the muscle cells. Adiponectin activates Akt in rainbow trout myotubes, which may lead to an increase in fatty acid uptake and oxidation. Overall, our results show that the adiponectin system responds differently to various physiological challenges and that it is hormonally controlled in vivo and in vitro. To the best of our knowledge, this is the first time this has been demonstrated in teleosts, and it may be a valuable contribution to our understanding of adipokines in fish.

Metabolic Effects of Insulin and IGFs on Gilthead Sea Bream (Sparus aurata) Muscle Cells

Primary cultures of gilthead sea bream myocytes were performed in order to examine the relative metabolic function of insulin compared with IGF-I and IGF-II (insulin-like growth factors, IGFs) at different stages in the cell culture. In these cells, the in vitro effects of insulin and IGFs on 2-deoxyglucose (2-DG) and l-alanine uptake were studied in both myocytes (day 4) and small myotubes (day 9). 2-DG uptake in gilthead sea bream muscle cells was increased in the presence of insulin and IGFs in a time dependent manner and along with muscle cell differentiation. On the contrary, l-alanine uptake was also stimulated by insulin and IGFs but showed an inverse pattern, being the uptake higher in small myocytes than in large myotubes. The results of preincubation with inhibitors (PD-98059, wortmannin, and cytochalasin B) on 2-DG uptake indicated that insulin and IGFs stimulate glucose uptake through the same mechanisms, and evidenced that mitogenesis activator protein kinase (MAPK) and PI3K-Akt transduction pathways mediate the metabolic function of these peptides. In the same way, we observed that GLUT4 protein synthesis was stimulated in the presence of insulin and IGFs in gilthead sea bream muscle cells in a different manner at days 4 or 9 of the culture. In summary we describe here, for the first time, the effects of insulin and IGFs on 2-DG and l-alanine uptake in primary culture of gilthead sea bream muscle cells. We show that both MAPK and PI3K-Akt transduction pathways are needed in order to control insulin and IGFs actions in these cells. Moreover, changes in glucose uptake can be explained by the action of the GLUT4 transporter, which is stimulated in the presence of insulin and IGFs throughout the cell culture.

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

In this study, the effect of acute and chronic insulin treatments on major factors involved in the control of muscle protein turnover were investigated in rainbow trout (Oncorhynchus mykiss). We found that acute but not chronic insulin administration leads to the induction of the phosphorylation of several key factors (IRS1, TOR and 4E-BP1) involved in the control of the protein synthesis and to the concomitant down-regulation of the expression of ubiquitin-proteasome-related genes (atrogin1, C2, C9) and the calpains inhibitor calpastatin. In contrast, no modification of autophagy-related gene (LC3B, gabarpl1, atg4b) expressions was observed suggesting that the mechanisms controlling this proteolytic route have diverged throughout the evolution. Overall, these results provide a possible explanation of the growth-promoting properties of insulin previously described in fish and indicate that this hormone acutely administrated is able to exert a regulatory influence on various factors associated with growth in skeletal muscle.

Differential effects on proliferation of GH and IGFs in sea bream (Sparus aurata) cultured myocytes

Primary culture of gilthead sea bream skeletal muscle cells was used to examine the effects of growth hormone (GH) and insulin-like growth factors (IGFs) in fish muscle proliferation and growth. Proliferation was measured as the percentage of positive cells expressing the proliferating cell nuclear antigen (PCNA) analyzed by immunocytochemistry. First, the effects of GH from two different origins (mammals and fish) were tested. GH from human (hGH) did not stimulate proliferation except at 3h at the dose of 1 nM. On the other hand, sea bream GH (sbGH) significantly stimulated proliferation, without differences between the three incubation times studied (3, 6, and 18 h), at the dose of 10nM, demonstrating that the homologous hormone has a more potent effect. In addition, the results with the IGFs indicated that both peptides, IGF-I and IGF-II significantly stimulated proliferation of sea bream myocytes, but IGF-II showed higher effects than IGF-I, and even than those of sbGH. Finally, the combinations of peptide treatments (GHs with IGFs) indicated that IGF-I has higher effects on proliferation when it is combined with GHs compared with IGF-I alone, while IGF-II has similar effects alone or combined with either GH. These results indicate that IGF-II may have an important role on muscle proliferation that appears to be independent of GH. On the contrary, IGF-I seems to play a synergistic action with GH stimulating myocyte proliferation.

Live prey enrichment, with particular emphasis on HUFAs, as limiting factor in false percula clownfish (Amphiprion ocellaris, Pomacentridae) larval development and metamorphosis: molecular and biochemical implications

In fast growing organisms, like fish larvae, fatty acids provided through live prey are essential to satisfy high energy demand and are required to promote growth. Therefore, in recent decades, a great amount of research has been directed towards the development of lipid enrichment in order to improve larval fish survival and growth. However, in fish, the biochemical and molecular processes related to highly unsaturated fatty acid (HUFA) administration are still poorly understood. In the current study, the false percula clownfish, a short larval phase marine species, was used as an experimental model and the effects of a standard and a HUFAs-enriched diet were tested through a molecular, biochemical, ultrastructural and morphometric approach. Our results support the hypothesis that HUFA administration may improve larval development through the presence of better structured mitochondria, a higher synthesis of energy compounds and coenzymes with a central position in the metabolism, with respect to controls. This higher energy status was confirmed by better growth performance and a shorter larval phase in larvae fed with an enriched diet with respect to the control. This strategy of rapid growth and early energy storage may be considered positively adaptive and beneficial to the survival of this species.

Direct involvement of tumor necrosis factor-&alpha; in the regulation of glucose uptake in rainbow trout muscle cells

The proinflammatory cytokine TNF-α is known to have a direct action on skeletal muscle in mammals. However, little is known regarding the potential effects of cytokines on nonimmune tissues, particularly in skeletal muscle, in fish. The aim of this study was to investigate the effects of recombinant trout TNF-α (rtTNF-α) on skeletal muscle carbohydrate metabolism in rainbow trout (Oncorhynchus mykiss). We used a primary cell culture of muscle cells from rainbow trout to show that rtTNF-α stimulates glucose uptake in myoblasts and myotubes at concentrations that do not affect the viability of the cells, requiring de novo protein synthesis as shown by the impairment of rtTNF-α-stimulated glucose uptake by cycloheximide. With the use of specific inhibitors, we show that rtTNF-α-stimulated glucose uptake is mediated by the p38MAPK, NF-κB, and JNK pathways. Additionally, we provide evidence that the stimulatory effects of rtTNF-α on glucose uptake in trout skeletal muscle cells may be caused, at least in part, by an increase in the amount of GLUT4 at the plasma membrane. Incubation of trout muscle cells with conditioned medium from LPS-stimulated trout macrophages, enriched in TNF-α, increased glucose uptake. Our results indicate that recombinant, as well as native trout TNF-α, directly stimulates glucose uptake in trout muscle cells and provide evidence, for the first time in nonmammalian vertebrates, for a potential regulatory role of TNF-α in skeletal muscle metabolism.

In vitro characterization of proliferation and differentiation of trout satellite cells

Fish satellite cells have been extracted from various species, but the myogenic characteristics of these cells in culture remain largely unknown. We show here that 60%-70% of the adherent cells are myogenic based on their immunoreactivity for the myogenic regulatory factor MyoD. In DMEM containing 10% fetal calf serum (FCS), trout myoblasts display rapid expression of myogenin (18% of myogenin-positive cells at day 2) combined with rapid fusion into myotubes (50% of myogenin-positive nuclei and 30% nuclei in myosin heavy chain [MyHC]-positive cells at day 7). These kinetics of differentiation are reminiscent of the behavior of fetal myoblasts in mammals. However, not all the myogenic cells differentiate; this subpopulation of cells might correspond to the previously named "reserve" cells. More than 90% of the BrdU-positive cells are also positive for MyoD, indicating that myogenic cells proliferate in vitro. By contrast, less than 1% of myogenin-positive cells are positive for BrdU suggesting that myogenin expression occurs only in post-mitotic cells. In order to maximize either the proliferation or the differentiation of cells, we have defined new culture conditions based on the use of a proliferation medium (F10+10%FCS) and a differentiation medium (DMEM+2%FCS). Three days after switching the medium, the differentiation index (% MyHC-positive nuclei) is 40-fold higher than that in proliferation medium, whereas the proliferation index (% BrdU-positive nuclei) is three-fold lower. Stimulation of cell proliferation by insulin-like growth factor 1 (IGF1), IGF2, and FGF2 is greater in F10 medium. The characterization of these extracted muscle cells thus validates the use of this in vitro system of myogenesis in further studies of the myogenic activity of growth factors in trout.

Insulin and insulin-like growth factor I signaling pathways in rainbow trout (Oncorhynchus mykiss) during adipogenesis and their implication in glucose uptake

Primary cultures of rainbow trout ( Oncorhynchus mykiss ) adipocytes were used to examine the main signaling pathways of insulin and insulin-like growth factor I (IGF-I) during adipogenesis. We first determined the presence of IGF-I receptors (IGF-IR) and insulin receptors (IR) in trout preadipocytes ( day 5) and adipocytes ( day 14). IGF-IRs were more abundant and appeared to be in higher levels in differentiated cells than in preadipocytes, whereas IRs were detected in lower but constant levels throughout the culture. The cells were immunoreactive against ERK1/2 MAPK, and AKT/PI3K, components of the two main signal transduction pathways for insulin and IGF-I receptors. Stimulation of MAPK phosphorylation by IGF-I was higher in preadipocytes than in adipocytes, while no effects were observed in MAPK phosphorylation after incubation of cells with insulin. AKT phosphorylation increased in the presence of both insulin and IGF-I, with higher levels of stimulation in adipocytes than in preadipocytes. Activation of both pathways was blocked by the use of specific inhibitors of MAPK (PD98059) and AKT (wortmannin). We describe here, for the first time, the effects of IGF-I and insulin on 2-deoxyglucose uptake in primary culture of trout adipocytes. IGF-I was more potent in stimulating glucose uptake than insulin, and PD98059 and wortmannin inhibited the stimulation of glucose uptake by this growth factor, suggesting that IGF-I plays an important metabolic role in trout adipocytes. Our results suggest that differential activation of the MAPK and AKT pathways are involved in the IGF-I- and insulin-induced effects of trout adipocytes during the various stages of adipogenesis.

Endocrine control of oleic acid and glucose metabolism in rainbow trout (Oncorhynchus mykiss) muscle cells in culture

The effects of insulin and IGF-I on fatty acid (FA) and glucose metabolism were examined using oleic acid or glucose as tracers in differentiated rainbow trout (Oncorhynchus mykiss) myotubes. Insulin and IGF-I significantly reduced the production of CO(2) from oleic acid with respect to the control values. IGF-I also significantly reduced the production of acid-soluble products (ASP) and the concentration of FA in the medium, while cellular triacylglycerols (TAG) tended to increase. Only insulin produced a significant accumulation of glycogen inside the cells in glucose distribution experiments. Incubation with catecholamines did not affect oleic acid metabolism. Cells treated with rapamycin [a target of rapamycin (TOR) inhibitor] significantly increased the oxidation of oleic acid to CO(2) and ASP, while the accumulation of TAG diminished. Rosiglitazone (a peroxisome proliferator-activated receptor gamma agonist) and etomoxir (a CPT-1 inhibitor) produced a severe and significant reduction in the production of CO(2) and ASP. Rosiglitazone and etomoxir also produced a significant accumulation of FA outside and inside the cells, respectively. No significant effects of these drugs on glucose distribution were observed. These data indicate that insulin and IGF-I act as anabolic hormones in trout myotubes in both oleic acid and glucose metabolism, although glucose oxidation appears to be less sensitive than FA oxidation to insulin and IGF-I. The use of rapamycin, etomoxir, and rosiglitazone may help us to understand the mechanisms of regulation of lipid metabolism in fish.

Effects of insulin-like growth factor-I, insulin, and leucine on protein turnover and ubiquitin ligase expression in rainbow trout primary myocytes

The effects of insulin-like growth factor-I (IGF-I), insulin, and leucine on protein turnover and pathways that regulate proteolytic gene expression and protein polyubiquitination were investigated in primary cultures of 4-day-old rainbow trout myocytes. Supplementing media with 100 nM IGF-I increased protein synthesis by 13% (P < 0.05) and decreased protein degradation by 14% (P < 0.05). Treatment with 1 microM insulin increased protein synthesis by 13% (P < 0.05) and decreased protein degradation by 17% (P < 0.05). Supplementing media containing 0.6 mM leucine with an additional 2.5 mM leucine did not increase protein synthesis rates but reduced rates of protein degradation by 8% (P < 0.05). IGF-I (1 nM-100 nM) and insulin (1 nM-1 microM) independently reduced the abundance of ubiquitin ligase mRNA in a dose-dependent manner, with maximal reductions of approximately 70% for muscle atrophy F-box (Fbx) 32, 40% for Fbx25, and 25% for muscle RING finger-1 (MuRF1, P < 0.05). IGF-I and insulin stimulated phosphorylation of FOXO1 and FOXO4 (P < 0.05), which was inhibited by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, and decreased the abundance of polyubiquitinated proteins by 10-20% (P < 0.05). Supplementing media with leucine reduced Fbx32 expression by 25% (P < 0.05) but did not affect Fbx25 nor MuRF1 transcript abundance. Serum deprivation decreased rates of protein synthesis by 60% (P < 0.05), increased protein degradation by 40% (P < 0.05), and increased expression of all ubiquitin ligases. These data suggest that, similar to mammals, the inhibitory effects of IGF-I and insulin on proteolysis occur via P I3-kinase/protein kinase B signaling and are partially responsible for the ability of these compounds to promote protein accretion.

Insulin-like growth factor-I and genetic effects on indexes of protein degradation in response to feed deprivation in rainbow trout (Oncorhynchus mykiss)

This study determined the effect of genetic variation, feed deprivation, and insulin-like growth factor-I (IGF-I) on weight loss, plasma IGF-I and growth hormone, and indexes of protein degradation in eight full-sibling families of rainbow trout. After 2 wk of feed deprivation, fish treated with IGF-I lost 16% less (P < 0.05) wet weight than untreated fish. Feed deprivation increased growth hormone (P < 0.05) and decreased IGF-I (P < 0.05), but hormone levels were not altered by IGF-I. Plasma 3-methylhistidine concentrations were not affected by IGF-I but were decreased after 2 wk (P < 0.05) and increased after 4 wk (P < 0.05) of feed deprivation. In white muscle, transcript abundance of genes in the ubiquitin-proteasome, lysosomal, and calpain- and caspase-dependent pathways were affected by feed deprivation (P < 0.05). IGF-I prevented the feed deprivation-induced upregulation of MAFbx (F-box) and cathepsin transcripts and reduced abundance of proteasomal mRNAs (P < 0.05), suggesting that reduction of protein degradation via these pathways may be partially responsible for the IGF-I-induced reduction of weight loss. Family variations in gene expression, IGF-I concentrations, and weight loss during fasting suggest genetic variation in the fasting response, with considerable impact on regulation of proteolytic pathways. These data indicate that nutrient availability, IGF-I, and genetic variation affect weight loss, in part through alterations of proteolytic pathways in rainbow trout, and that regulation of genes within these pathways is coordinated in a way that supports a similar physiological response.

Expression of rainbow trout glucose transporters GLUT1 and GLUT4 during in vitro muscle cell differentiation and regulation by insulin and IGF-I

Insulin is an important factor for the maintenance of glucose homeostasis, enhancing glucose uptake in its target tissues in a process that has been conserved between fish and mammals. In fish skeletal muscle cells, like in mammals, insulin promotes GLUT4 translocation to the plasma membrane and, consequently, glucose uptake, but its role regulating the expression of glucose transporters in vitro has not been demonstrated to date. Thus, we investigated the expression of GLUT4 and GLUT1 throughout skeletal muscle cell differentiation and their regulation by insulin and IGF-I using a primary culture of trout muscle cells. GLUT4 expression gradually increased during the muscle cell differentiation process, whereas GLUT1 expression remained fairly constant. Insulin and IGF-I similarly increased the mRNA levels of GLUT4 in myoblasts and myotubes. On the other hand, IGF-I appeared to be more potent than insulin in stimulating GLUT1 expression, particularly at the myoblast stage. Therefore, this work provides the first demonstration in nonmammalian vertebrates that insulin and IGF-I may act directly on trout muscle cells to regulate the expression of GLUT4 and GLUT1.

Clinical, agricultural, and evolutionary biology of myostatin: a comparative review

The discovery of myostatin and our introduction to the "Mighty Mouse" over a decade ago spurred both basic and applied research and impacted popular culture as well. The myostatin-null genotype produces "double muscling" in mice and livestock and was recently described in a child. The field's rapid growth is by no means surprising considering the potential benefits of enhancing muscle growth in clinical and agricultural settings. Indeed, several recent studies suggest that blocking myostatin's inhibitory effects could improve the clinical treatment of several muscle growth disorders, whereas comparative studies suggest that these actions are at least partly conserved. Thus, neutralizing myostatin's effects could also have agricultural significance. Extrapolating between studies that use different vertebrate models, particularly fish and mammals, is somewhat confusing because whole genome duplication events have resulted in the production and retention of up to four unique myostatin genes in some fish species. Such comparisons, however, suggest that myostatin's actions may not be limited to skeletal muscle per se, but may additionally influence other tissues including cardiac muscle, adipocytes, and the brain. Thus, therapeutic intervention in the clinic or on the farm must consider the potential of alternative side effects that could impact these or other tissues. In addition, the presence of multiple and actively diversifying myostatin genes in most fish species provides a unique opportunity to study adaptive molecular evolution. It may also provide insight into myostatin's nonmuscle actions as results from these and other comparative studies gain visibility in biomedical fields.

Metabolic and mitogenic effects of IGF-II in rainbow trout (Oncorhynchus mykiss) myocytes in culture and the role of IGF-II in the PI3K/Akt and MAPK signalling pathways

Primary cultures of rainbow trout skeletal muscle cells were used to examine the role of insulin-like growth factor II (IGF-II) in fish muscle metabolism and growth, and to compare its main signal transduction pathways with those of IGF-I. IGF-II stimulated 2-deoxy-d-glucose (2-DG) uptake in trout myocytes at concentrations of between 5 and 100 nM, with similar maximal effects and temporal pattern to IGF-I (100 nM). The results of incubation with inhibitors (Wortmannin and CKB) indicated that IGF-II stimulates glucose uptake through the same mechanisms as IGF-I. In addition, IGF-II stimulated myoblast DNA synthesis (measured by thymidine incorporation) at relatively low concentrations (0.1-10 nM), with the maximum increase at 1 nM (167+/-17% with respect to control values). The cells were immunoreactive against ERK 1/2 MAPK and Akt/PKB, components of the two main signal transduction pathways for the IGF-I receptor. IGF-II stimulated the phosphorylation of the protein MAPK, especially at the proliferation stage (increases of up to 125.7+/-16.9% and 125.3+/-3.3% with respect to control in IGF-II- and IGF-I-treated cells, respectively). In contrast, the effects of both IGFs on the activation of the PI3K/Akt pathway were stronger in fully differentiated myocytes and in early-formed fibres (up to 359+/-18.5% in IGF-II-treated cells with respect to control). These results indicate that IGF-II has both mitogenic and metabolic effects in trout muscle cells, which are equivalent to those found in response to IGF-I. Both IGFs exert these effects though the same signalling pathways (MAPK and PI3K/Akt).

Fish glucose transporter (GLUT)-4 differs from rat GLUT4 in its traffic characteristics but can translocate to the cell surface in response to insulin in skeletal muscle cells

In mammals, glucose transporter (GLUT)-4 plays an important role in glucose homeostasis mediating insulin action to increase glucose uptake in insulin-responsive tissues. In the basal state, GLUT4 is located in intracellular compartments and upon insulin stimulation is recruited to the plasma membrane, allowing glucose entry into the cell. Compared with mammals, fish are less efficient restoring plasma glucose after dietary or exogenous glucose administration. Recently our group cloned a GLUT4-homolog in skeletal muscle from brown trout (btGLUT4) that differs in protein motifs believed to be important for endocytosis and sorting of mammalian GLUT4. To study the traffic of btGLUT4, we generated a stable L6 muscle cell line overexpressing myc-tagged btGLUT4 (btGLUT4myc). Insulin stimulated btGLUT4myc recruitment to the cell surface, although to a lesser extent than rat-GLUT4myc, and enhanced glucose uptake. Interestingly, btGLUT4myc showed a higher steady-state level at the cell surface under basal conditions than rat-GLUT4myc due to a higher rate of recycling of btGLUT4myc and not to a slower endocytic rate, compared with rat-GLUT4myc. Furthermore, unlike rat-GLUT4myc, btGLUT4myc had a diffuse distribution throughout the cytoplasm of L6 myoblasts. In primary brown trout skeletal muscle cells, insulin also promoted the translocation of endogenous btGLUT4 to the plasma membrane and enhanced glucose transport. Moreover, btGLUT4 exhibited a diffuse intracellular localization in unstimulated trout myocytes. Our data suggest that btGLUT4 is subjected to a different intracellular traffic from rat-GLUT4 and may explain the relative glucose intolerance observed in fish.

Role of insulin, insulin-like growth factors, and muscle regulatory factors in the compensatory growth of the trout (Oncorhynchus mykiss)

To examine the various mechanisms involved in compensatory growth in Oncorhynchus mykiss, an experimental protocol involving 1, 2 or 4 weeks of fasting followed by a single ad libitum re-feeding period of 4 weeks was designed for alevins. Morphological parameters including body weight, specific growth rates (SGR), and coefficient factor decreased significantly during fasting. Re-feeding accelerated growth and restored final body weight in groups previously fasted. Plasma insulin and glucose decreased in fasting, while normal levels were restored in all re-fed groups. The expression profile of insulin-like growth factors (IGFs) in liver and of the main muscle growth regulators in white muscle was examined using real-time quantitative RT-PCR. Fasting decreased the expression of IGF-I mRNA in both tissues, while re-feeding restored expression to control values. In contrast, IGF-II expression was not affected by any treatment in either tissue. Insulin- and IGF-I-binding assays in partial semi-purifications (of soluble proteins) in white skeletal muscle showed that insulin binding was not affected by either fasting or re-feeding, whereas fasting up-regulated IGF-I binding. The expression of IGFRIb mRNA in white skeletal muscle also increased with fasting, while IGFRIa increased with re-feeding, indicating that the two receptor isoforms are differentially regulated. The mRNA expression of myogenic regulator factors and fibroblast growth factors (FGFs) was not affected throughout the experiment, except for myogenin, which first decreased and then showed a rebound effect after 4 weeks of fasting. Myostatin mRNA expression did not change during fasting, although re-feeding caused a significant decrease. In conclusion, re-feeding of previously fasted trout induced compensatory growth. The differential regulation in muscle expression of IGF-I, IGF-I receptors, and myostatin indicates their contribution to this compensatory mechanism.

Growth factor messenger ribonucleic acid expression during differentiation of porcine embryonic myogenic cells

The growth factors, IGF-I and II, their binding proteins, IGFBP, and members of the transforming growth factor (TGF) superfamily (myostatin and TGFbeta1) are known to regulate proliferation and differentiation of myogenic cells. We hypothesized that changes in the relative expression of members of the IGF and TGFbeta systems play a significant role in regulating myogenesis in porcine embryonic myogenic cell (PEMC) cultures. Therefore, determining the expression patterns of these factors during PEMC myogenesis is important. Consequently, we used real-time PCR to explore the pattern of IGF-I; IGF-II; IGFBP-2, -3, and -5; IGF-type-I receptor; myogenin; myostatin; and TGFbeta1 mRNA expression during PEMC myogenesis. The progression of differentiation was assessed using creatine kinase activity and myogenin mRNA expression. As anticipated, creatine kinase activity was low in PEMC cultures at 48 h and increased 20-fold (P < 0.0001) between 48 h and its peak at 144 h. Similarly, myogenin mRNA was low at 48 h and increased approximately 5-fold (P < 0.0001) as differentiation progressed, peaking at 120 h and decreasing at 144 h. The patterns of IGF-I and IGFBP-2 mRNA expression were similar and were relatively lower in 48-h PEMC cultures, increasing approximately 5-fold (P < 0.0001) to their greatest levels at 120 h. In contrast, IGF-II and IGFBP-5 mRNA levels were relatively high at 48 h, peaking at 72 h, and steadily decreasing by 60 and 80%, respectively (P < 0.001), at 144 h. The level of IGF-type-I receptor mRNA was relatively high until 96 h of culture, after which it decreased 40% (P < 0.01), reaching a low at 144 h. Levels of IGFBP-3 mRNA were relatively high at 48 h, dropped approximately 40% to their lowest level at 72 h (P < 0.001), and then increased approximately 60% (P < 0.001) to their greatest levels at 144 h. Levels of TGFbeta1 mRNA decreased approximately 30% (P < 0.0001) between 48 and 96 h, then quickly rebounded to a peak at 120 h, and by 144 h had dropped to the levels seen at 72 h. Myostatin mRNA was at its greatest level at 48 h and declined rapidly between 72 and 96 h, finally decreasing by approximately 80% at 144 h (P < 0.0001). Our data demonstrate that these factors are differentially regulated during PEMC myogenesis and provide new information about their pattern of mRNA expression in cultured porcine muscle cells.

Long-term culture of muscle explants from Sparus aurata

Although there are mammalian myoblast cell lines, no fish myoblast cell line has been developed so far. The aim of this study was to develop a culture system of muscle explants for fish, as explants provide an approximation of the in vivo conditions for cell proliferation and differentiation, and enable a close comparison with events in muscle regenerating in vivo. Here we describe the main features of a long-term in vitro culture system for muscle explants from Sparus aurata fry. At the time of sampling, the original fibres were damaged and subsequently degenerated as shown by the loss of parvalbumin (PV) and presence of apoptotic nuclei. This mechanical damage provoked a myogenic response by activation of myogenic precursor cells. After a few days, new mononucleate cells aligned with the original fibres were seen in the explants, some with proliferating cell nuclear antigen (PCNA-) and Myf-5-positive nuclei, indicating proliferation and their myogenic fate. By 1 week, multinucleate cells with desmin immunoreactivity but PCNA- and Myf5-negative nuclei were present, equivalent to differentiated, postmitotic myotubes. Some of these myotubes were also immunoreactive for PV and insulin-like growth factors (IGFs). By 11 days, many of the myotubes were also immunoreactive for myostatin (MSTN). By 23 days, many of the myotubes had increased in diameter, were packed with myofibrils, and were strongly PV-positive and immunoreactive for MSTN, IGF-I and IGF-I receptor. This study shows that a proliferative process occurs in the explants despite the death of the original muscle fibres, and new muscle fibres expressing growth regulators are formed by regeneration from myogenic precursors present in the explants at the time of sampling.

Molecular cloning, recombinant expression, and growth-promoting effect of mud carp (Cirrhinus molitorella) insulin-like growth factor-I

A full-length cDNA encoding insulin-like growth factor-I (IGF-I) was cloned from mud carp (Cirrhinus molitorella) liver tissue using reverse transcription polymerase-chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) techniques. The IGF-I precursor cDNA consists of 822 bp in size with a 218 bp 5'-untranslated region and 118 bp 3'-untranslated region. The 486 bp open reading frame encodes a 161 amino acid peptide with a molecular weight of 17.9 kDa. The deduced IGF-I amino acid sequence shared 82.5-97% and 82.5-84% sequence identity with fish and mammalian counterparts, respectively. The mature IGF-I was overexpressed in Escherichia coli, and the expression level of recombinant mcIGF-I reached to 34.1% of the cell total protein. After purification and refolding of recombinant mcIGF-I, growth-promoting effect of recombinant mcIGF-I was investigated, the results showed that the recombinant mcIGF-I significantly enhanced the growth rate of juvenile tilapia. After 6-week treatment, the growth rates of group 1 and 2 were 53 and 67.3% higher than the saline-treated control group. The recombinant mcIGF-I was more effective than recombinant mcGH to enhance the growth rate of juvenile tilapia. The recombinant mcIGF-I-treated fish revealed no significant changes of content of protein, lipid, ash and moisture in muscle.

Environment and plasticity of myogenesis in teleost fish

Embryonic development in teleosts is profoundly affected by environmental conditions, particularly temperature and dissolved oxygen concentrations. The environment determines the rate of myogenesis, the composition of sub-cellular organelles, patterns of gene expression, and the number and size distribution of muscle fibres. During the embryonic and larval stages, muscle plasticity to the environment is usually irreversible due to the rapid pace of ontogenetic change. In the early life stages, muscle can affect locomotory performance and behaviour, with potential consequences for larval survival. Postembryonic growth involves myogenic progenitor cells (MPCs) that originate in the embryo. The embryonic temperature regime can have long-term consequences for the growth of skeletal muscle in some species, including the duration and intensity of myotube formation in adult stages. In juvenile and adult fish, abiotic (temperature, day-length, water flow characteristics, hypoxia) and biotic factors (food availability, parasitic infection) have complex effects on the signalling pathways regulating the proliferation and differentiation of MPCs, protein synthesis and degradation, and patterns of gene expression. The phenotypic responses observed to the environment frequently vary during ontogeny and are integrated with endogenous physiological rhythms, particularly sexual maturation. Studies with model teleosts provide opportunities for investigating the underlying genetic mechanisms of muscle plasticity that can subsequently be applied to non-model species of more ecological or commercial interest.

IGF-I and insulin receptor signal transduction in trout muscle cells

In this study, primary cultures of trout skeletal muscle cells were used to investigate the main signal transduction pathways of insulin and IGF-I receptors in rainbow trout muscle. At different stages of in vitro development (myoblasts on day 1, myocytes on day 4, and fully developed myotubes on day 11), we detected in these cells the presence of immunoreactivity against ERK 1/2 MAPK and Akt/PKB proteins, components of the MAPK and the phosphatidylinositol 3-kinase-Akt pathways, respectively, two of the main intracellular transduction pathways for insulin and IGF-I receptors. Both insulin and IGF-I activated both pathways, although the latter provoked higher immunoreactivity of phosphorylated MAPKs and Akt proteins. At every stage, increases in total MAPK immunoreactivity levels were observed when cells were stimulated with IGF-I or insulin, while total Akt immunoreactivity levels changed little under stimulation of peptides. Total Akt and total MAPK levels increased as skeletal muscle cells differentiated in culture. Moreover, when cells were incubated with IGF-I or insulin, MAPK-P immunoreactivity levels showed greater increases over the basal levels on days 1 and 4, with no effect observed on day 11. Although Akt-P immunoreactivity displayed improved responses on days 1 and 4 as well, a stimulatory effect was still observed on day 11. In addition, the present study demonstrates that purified trout insulin receptors possess higher phosphorylative activity per unit of receptor than IGF-I receptors. In conclusion, these results indicate that trout skeletal muscle culture is a suitable model to study the insulin and IGF-I signal transduction molecules and that there is a different regulation of MAPK and Akt pathways depending on the developmental stage of the muscle cells.

Growth hormone and insulin-like growth factors in fish: where we are and where to go

This communication summarizes viewpoints, discussion, perspectives, and questions, put forward at a workshop on "Growth hormone and insulin-like growth factors in fish" held on September 7th, 2004, at the 5th International Symposium on Fish Endocrinology in Castellon, Spain.

Metabolic and mitogenic effects of IGF-I and insulin on muscle cells of rainbow trout

The relative function of IGF-I and insulin on fish muscle metabolism and growth has been investigated by the isolation and culture at different stages (myoblasts at day 1, myocytes at day 4, and myotubes at day 10) of rainbow trout muscle cells. This in vitro model avoids interactions with endogenous peptides, which could interfere with the muscle response. In these cells, the effects of IGF-I and insulin on cell proliferation, 2-deoxyglucose (2-DG), and l-alanine uptake at different development stages, and the use of inhibitors were studied and quantified. Insulin (10-1,000 nM) and IGF-I (10-100 nM) stimulated 2-DG uptake in trout myocytes at day 4 in a similar manner (maximum of 124% for insulin and of 142% for IGF-I), and this stimulation increased when cells differentiated to myotubes (maximum for IGF-I of 193%). When incubating the cells with PD-98059 and especially cytochalasin B, a reduction in 2-DG uptake was observed, suggesting that glucose transport takes place through specific facilitative transporters. IGF-I (1-100 nM) stimulated the l-alanine uptake in myocytes at day 4 (maximum of 239%), reaching higher values of stimulation than insulin (100-1,000 nM) (maximum of 160%). This stimulation decreased when cells developed to myotubes at day 10 (118% for IGF-I and 114% for insulin). IGF-I (0.125-25 nM) had a significant effect on myoblast proliferation, measured by thymidine incorporation (maximum of 170%), and required the presence of 2-5% fetal serum (FBS) to promote thymidine uptake. On the other hand, insulin was totally ineffective in stimulating thymidine uptake. We conclude that IGF-I is more effective than insulin in stimulating glucose and alanine uptake in rainbow trout myosatellite cells and that the degree of stimulation changes when cells differentiate to myotubes. IGF-I stimulates cell proliferation in this model of muscle in vitro and insulin does not. These results indicate the important role of IGF-I on growth and metabolism of fish muscle.

Plasticity of muscle fibre number in seawater stages of Atlantic salmon in response to photoperiod manipulation

Atlantic salmon (Salmo salar L.) were fed to satiety and reared from approximately 60 g to 5000 g at ambient seawater temperatures. The effect of photoperiod manipulation on muscle growth was investigated from the start of the first sea winter. Continuous light treatment in winter/spring (1 November to 18 June) improved growth performance in fish, resulting in a 30% increase in mean body mass relative to the ambient photoperiod fish by 12 August, but had no effect on sexual maturation. Significant increases in body mass in the continuous light groups were observed after 126 days (P<0.01). The number of fast muscle fibres per trunk cross-section was determined in a subset of the fish and was 28.5% higher in the continuous light (799 x 10(3)) than the natural day length (644 x 10(3)) groups after only 40 days, corresponding to the period of decreasing natural day length. Subsequent rates of fibre recruitment were similar between treatments. At the end of the fibre recruitment phase of growth (combined June and August samples), the maximum number of fast muscle fibres was 23% higher in fish from the cages receiving continuous light (881 x 10(3)+/-32 x 10(3); N=19) than in the ambient photoperiod cages (717 x 10(3)+/-15 x 10(3); N=20) (P<0.001). Continuous light treatment was associated with a shift in the distribution of fibre diameters, reflecting the altered patterns of fibre recruitment. However, the mean rate of fibre hypertrophy showed no consistent difference between treatments. There was a linear relationship between the myonuclear content of isolated single fibres and fibre diameter. On average, there were 27% more myonuclei in 150 microm-diameter fibres in the continuous light (3118 myonuclei cm(-1)) than the ambient photoperiod (2448 myonuclei cm(-1)) fish. After 40 days, continuous light treatment resulted in a transient increase in the density of myogenic progenitor cells, identified using a c-met antibody, to a level 70% above that of fish exposed to natural light. It is suggested that short days inhibited the proliferation of myogenic progenitor cells and that this was overcome by transferring fish to continuous light, causing an increase in the number of times the myogenic precursor cells divided and/or a decrease in cell cycle time. The net increase in myogenic progenitor cells resulted in proportional increases in the number and myonuclear content of fibres. The subsequent hypertrophy of these additional fibres can explain the delayed increase in body mass observed with continuous light treatment.

Effect of refeeding on IGFI, IGFII, IGF receptors, FGF2, FGF6, and myostatin mRNA expression in rainbow trout myotomal muscle

Fish endure long periods of fasting and demonstrate an extensive capacity for rapid and complete recovery after refeeding. The underlying mechanisms through which nutrient intake activates an increase in somatic growth and especially in muscle growth is poorly understood. In this study we examined the expression profile of major muscle growth regulators in trout white muscle 4, 12, and 34 days after refeeding, using real-time quantitative RT-PCR. Mean insulin-like growth factor I (IGFI) mRNA level in muscle increased dramatically 8- and 15-fold, 4 and 12 days, respectively, after refeeding compared to fasted trout. This declined thereafter. Conversely, only a weak but gradual increase in mean insulin-like growth factor II (IGFII) mRNA level was observed during refeeding. Inversely to IGFI, mean IGF receptor Ia (IGFRIa) mRNA level declined after ingestion of food. In contrast, IGF receptor Ib (IGFRIb) mRNA level was not affected by refeeding. Mean fibroblast growth factor 2 (FGF2) mRNA level increased by 2.5-fold both 4 and 12 days after refeeding, whereas fibroblast growth factor 6 (FGF6) and myostatin mRNA levels were unchanged. Subsequent to IGFI and FGF2 gene activation, an increase in myogenin mRNA accumulation was observed at 12 days post-refeeding suggesting that an active differentiation of myogenic cells succeeds their proliferation. In conclusion, among the potential growth factors we examined in this study, IGFI and FGF2 were identified as candidate genes whose expression may contribute to muscle compensatory growth induced by refeeding.