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

Direct actions of growth hormone in rainbow trout, Oncorhynchus mykiss, skeletal muscle cells in vitro

The growth hormone (GH)-insulin-like growth factor-1 (IGF-1) system regulates skeletal muscle growth and function. GH has a major function of targeting the liver to regulate IGF-1 production and release, and IGF-1 mediates the primary anabolic action of GH on growth. However, skeletal muscle is a target tissue of GH as evidenced by dynamic GH receptor expression, but it is unclear if GH elicits any direct actions on extrahepatic tissues as it is difficult to distinguish the effects of IGF-1 from GH. Fish growth regulation is complex compared to mammals, as genome duplication events have resulted in multiple isoforms of GHs, GHRs, IGFs, and IGFRs expressed in most fish tissues. This study investigated the potential for GH direct actions on fish skeletal muscle using an in vitro system, where rainbow trout myogenic precursor cells (MPCs) were cultured in normal and serum-deprived media, to mimic in vivo fasting conditions. Fasting reduces IGF-1 signaling in the muscle, which is critical for disentangling the roles of GH from IGF-1. The direct effects of GH were analyzed by measuring changes in myogenic proliferation and differentiation genes, as well as genes regulating muscle growth and proteolysis. This study provides the first in-depth analysis of the direct actions of GH on serum-deprived fish muscle cells in vitro. Data suggest that GH induces the expression of markers for proliferation and muscle growth in the presence of serum, but all observed GH action was blocked in serum-deprived conditions. Additionally, serum deprivation alone reduced the expression of several proliferation and differentiation markers, while increasing growth and proteolysis markers. Results also demonstrate dynamic gene expression response in the presence of GH and a JAK inhibitor in serum-provided but not serum-deprived conditions. These data provide a better understanding of GH signaling in relation to serum in trout muscle cells in vitro.

Insulin-like growth factor-1 (Igf1) signaling responses to food consumption after fasting in the Pacific rockfish Sebastes carnatus

Fish adjust rates of somatic growth in the face of changing food consumption. As in other vertebrates, growth in fish is regulated by the growth hormone (Gh)/insulin-like growth factor-1 (Igf1) endocrine axis, and changes in food intake impact growth via alterations to Gh/Igf1 signaling. Understanding the time course by which the Gh/Igf1 axis responds to food consumption is crucial to predict how rapidly changes in food abundance might lead to altered growth dynamics. Here, we looked at the response times of plasma Igf1 and liver Igf1 signaling-associated gene expression to refeeding after food deprivation in juvenile gopher rockfish (Sebastes carnatus), one of several species of northern Pacific Ocean Sebastes rockfishes targeted by fisheries or utilized for aquaculture. Gopher rockfish were fasted for 30 d, after which a subset was fed to satiation for 2 h, while other rockfish continued to be fasted. Refed fish exhibited higher hepatosomatic index (HSI) values and increased Igf1 after food consumption. Gene transcripts for Gh receptor 1 (ghr1), but not ghr2, increased in the liver after eating. Transcripts encoding igf1 also increased in the liver of refed fish 2-4 d after feeding, only to return to levels similar as continually fasted rockfish by 9 d after feeding. Liver mRNA abundances for Igf binding protein (Igfbp) genes igfbp1a, igfbp1b, and igfbp3a declined within 2 d of feeding. These findings provide evidence that circulating Igf1 in rockfish reflects a fish's feeding experience within the previous few days, and suggest that feeding-induced increases in Igf1 are being mediated in part by altered liver sensitivity to Gh due to upregulated Gh receptor 1 expression.

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.

Recent advances in the crosstalk between adipose, muscle and bone tissues in fish

Control of tissue metabolism and growth involves interactions between organs, tissues, and cell types, mediated by cytokines or direct communication through cellular exchanges. Indeed, over the past decades, many peptides produced by adipose tissue, skeletal muscle and bone named adipokines, myokines and osteokines respectively, have been identified in mammals playing key roles in organ/tissue development and function. Some of them are released into the circulation acting as classical hormones, but they can also act locally showing autocrine/paracrine effects. In recent years, some of these cytokines have been identified in fish models of biomedical or agronomic interest. In this review, we will present their state of the art focusing on local actions and inter-tissue effects. Adipokines reported in fish adipocytes include adiponectin and leptin among others. We will focus on their structure characteristics, gene expression, receptors, and effects, in the adipose tissue itself, mainly regulating cell differentiation and metabolism, but in muscle and bone as target tissues too. Moreover, lipid metabolites, named lipokines, can also act as signaling molecules regulating metabolic homeostasis. Regarding myokines, the best documented in fish are myostatin and the insulin-like growth factors. This review summarizes their characteristics at a molecular level, and describes both, autocrine effects and interactions with adipose tissue and bone. Nonetheless, our understanding of the functions and mechanisms of action of many of these cytokines is still largely incomplete in fish, especially concerning osteokines (i.e., osteocalcin), whose potential cross talking roles remain to be elucidated. Furthermore, by using selective breeding or genetic tools, the formation of a specific tissue can be altered, highlighting the consequences on other tissues, and allowing the identification of communication signals. The specific effects of identified cytokines validated through in vitro models or in vivo trials will be described. Moreover, future scientific fronts (i.e., exosomes) and tools (i.e., co-cultures, organoids) for a better understanding of inter-organ crosstalk in fish will also be presented. As a final consideration, further identification of molecules involved in inter-tissue communication will open new avenues of knowledge in the control of fish homeostasis, as well as possible strategies to be applied in aquaculture or biomedicine.

Exploring the impacts of different fasting and refeeding regimes on Nile tilapia (Oreochromis niloticus L.): growth performance, histopathological study, and expression levels of some muscle growth-related genes

The current study investigated how different fasting and refeeding regimes would impact Nile tilapia growth performance, histopathological examination, and gene expression of myostatin, myogenin, GH, IGF-1, and NPYa. Nile tilapia fish (n = 120) were randomly allocated into four groups, including the control group fed on a basal diet for 6 weeks (F6), group A starved for 1 week and then refed for 5 weeks (S1F5), group B starved for 2 weeks and then refed for 4 weeks (S2F4), while group C starved for 4 weeks and then refed for 2 weeks (S4F2). Fasting provoked a decrease in body weight coincided with more extended starvation periods. Also, it induced muscle and liver histological alterations; the severity was correlated with the length of fasting periods. Gene expression levels of GH, MSTN, MYOG, and NPYa were significantly increased, while IGF1 was markedly depressed in fasted fish compared to the control group. Interestingly, refeeding after well-planned short fasting period (S1F5) modulated the histopathological alterations. To some extent, these changes were restored after refeeding. Restored IGF-I and opposing fasting expression profiles of the genes mentioned above thus recovered weights almost like the control group and achieved satisfactory growth compensation. Conversely, refeeding following more extended fasting periods failed to restore body weight. In conclusion, refeeding after fasting can induce a compensatory response. Still, the restoration capacity is dependent on the length of fasting and refeeding periods through exhibiting differential morphological structure and expressions pattern for muscle and growth-related genes.

Gene Expression Profile Provides Novel Insights of Fasting-Refeeding Response in Zebrafish Skeletal Muscle

Recently, fasting has been spotlighted from a healthcare perspective. However, the de-tailed biological mechanisms and significance by which the effects of fasting confer health benefits are not yet clear. Due to certain advantages of the zebrafish as a vertebrate model, it is widely utilized in biological studies. However, the biological responses to nutrient metabolism within zebrafish skeletal muscles have not yet been amply reported. Therefore, we aimed to reveal a gene expression profile in zebrafish skeletal muscles in response to fasting-refeeding. Accordingly, mRNA-sequencing and bioinformatics analysis were performed to examine comprehensive gene expression changes in skeletal muscle tissues during fasting-refeeding. Our results produced a novel set of nutrition-related genes under a fasting-refeeding protocol. Moreover, we found that five genes were dramatically upregulated in each fasting (for 24 h) and refeeding (after 3 h), exhibiting a rapid response to the provided conditional changes. The assessment of the gene length revealed that the gene set whose expression was elevated only after 3 h of refeeding had a shorter length, suggesting that nutrition-related gene function is associated with gene length. Taken together, our results from the bioinformatics analyses provide new insights into biological mechanisms induced by fasting-refeeding conditions within zebrafish skeletal muscle.

Dysregulation of Intestinal Physiology by Aflatoxicosis in the Gilthead Seabream (Sparus aurata)

Aflatoxin B1 (AFB1) is a mycotoxin often present in food. This study aimed to understand the physiological effects of AFB1 on the seabream (Sparus aurata) gastrointestinal system. In a first in vitro approach, we investigated ion transport using the short-circuit current (Isc) technique in Ussing chambers in the anterior intestine (AI). Application of apical/luminal AFB1 concentrations of 8 and 16 μM to healthy tissues was without effect on tissue transepithelial electrical resistance (TER), and apparent tissue permeability (Papp) was measured using fluorescein FITC (4 kD). However, it resulted in dose-related effects on Isc. In a second approach, seabream juveniles fed with different AFB1 concentrations (1 and 2 mg AFB1 kg⁻¹ fish feed) for 85 days showed significantly reduced gill Na⁺/K⁺-ATPase (NKA) and H⁺-ATPase (HA) activities in the posterior intestine (PI). Moreover, dietary AFB1 modified Isc in the AI and PI, significantly affecting TER in the AI. To understand this effect on TER, we analyzed the expression of nine claudins and three occludins as markers of intestinal architecture and permeability using qPCR. Around 80% of the genes presented significantly different relative mRNA expression between AI and PI and had concomitant sensitivity to dietary AFB1. Based on the results of our in vitro, in vivo, and molecular approaches, we conclude that the effects of dietary AFB1 in the gastrointestinal system are at the base of the previously reported growth impairment caused by AFB1 in fish.

The cellular basis of compensatory muscle growth in the teleost Odontesthes bonariensis

This study evaluates white muscle growth and in vivo cell proliferation during a fasting and refeeding trial, using pejerrey Odontesthes bonariensis as animal model, in order to better understand the cellular basis governing catch-up growth. Experiments consisted in two groups of fish, a control one continuously fed ad libitum, and a group fasted for 2 weeks and then fed for another 2 weeks. We examined how the formation of new muscle fibers and their increase in size were related to muscle precursor cell (MPC) proliferation under both experimental conditions. During fasting, the number of 5-ethynyl-2'-deoxyuridinepositive (EdU+) cells decreased along with myogenic regulatory factors (MRF) mRNA levels related to myoblast proliferation and differentiation, and the muscle stem cell-markerPax7 mRNA level increased. Analysis of myomere cross-sectional area, distribution of muscle fiber sizes and number of fibers per myomere showed that muscle hypertrophy but not hyperplasia was inhibited during fasting. Both higher igf2 mRNA level and the persistence of cell proliferation could be supporting new myofibre formation. On the other hand, an exacerbated MPC proliferation occurred during catch-up growth, and this increase in cell number could be contributing to the growth of both pre-existing and newly form small fibers. The finding that some MPCs proliferate during fasting and that muscle growth mechanisms, hyperplasia and hypertrophy, are differentially regulated could help to explain why re-fed fish could growth at higher rates, and why they return to the lost growth trajectory.

Ascorbic Acid Supplementation Improves Skeletal Muscle Growth in Pacu (Piaractus mesopotamicus) Juveniles: In Vivo and In Vitro Studies

In fish, fasting leads to loss of muscle mass. This condition triggers oxidative stress, and therefore, antioxidants can be an alternative to muscle recovery. We investigated the effects of antioxidant ascorbic acid (AA) on the morphology, antioxidant enzyme activity, and gene expression in the skeletal muscle of pacu (Piaractus mesopotamicus) following fasting, using in vitro and in vivo strategies. Isolated muscle cells of the pacu were subjected to 72 h of nutrient restriction, followed by 24 h of incubation with nutrients or nutrients and AA (200 µM). Fish were fasted for 15 days, followed by 6 h and 15 and 30 days of refeeding with 100, 200, and 400 mg/kg of AA supplementation. AA addition increased cell diameter and the expression of anabolic and cell proliferation genes in vitro. In vivo, 400 mg/kg of AA increased anabolic and proliferative genes expression at 6 h of refeeding, the fiber diameter and the expression of genes related to cell proliferation at 15 days, and the expression of catabolic and oxidative metabolism genes at 30 days. Catalase activity remained low in the higher supplementation group. In conclusion, AA directly affected the isolated muscle cells, and the higher AA supplementation positively influenced muscle growth after fasting.

Identifying the Related Genes of Muscle Growth and Exploring the Functions by Compensatory Growth in Mandarin Fish (Siniperca chuatsi)

How organisms display many different biochemical, physiological processes through genes expression and regulatory mechanisms affecting muscle growth is a central issue in growth and development. In Siniperca chuatsi, the growth-related genes and underlying relevant mechanisms are poorly understood, especially for difference of body sizes and compensatory growth performance. Muscle from 3-month old individuals of different sizes was used for transcriptome analysis. Results showed that 8,942 different expression genes (DEGs) were identified after calculating the RPKM. The DEGs involved in GH-IGF pathways, protein synthesis, ribosome synthesis and energy metabolisms, which were expressed significantly higher in small individuals (S) than large fish (L). In repletion feeding and compensatory growth experiments, eight more significant DEGs were used for further research (GHR2, IGFR1, 4ebp, Mhc, Mlc, Myf6, MyoD, troponin). When food was plentiful, eight genes participated in and promoted growth and muscle synthesis, respectively. Starvation can be shown to inhibit the expression of Mhc, Mlc and troponin, and high expression of GHR2, IGFR1, and 4ebp inhibited growth. Fasting promoted the metabolic actions of GHR2, IGFR1, and 4ebp rather than the growth-promoting actions. MyoD can sense and regulate the hunger, which also worked with Mhc and Mlc to accelerate the compensatory growth of S. chuatsi. This study is helpful to understand the regulation mechanisms of muscle growth-related genes. The elected genes will contribute to the selective breeding in future as candidate genes.

Effects of feed restriction on growth performance, lipid mobilization, and gene expression in rose spotted snapper (Lutjanus guttatus)

The effects of feed deprivation were evaluated for 1 week and 2 weeks in Lutjanus guttatus juveniles. A significant reduction in body mass was observed in both feed deprivation schemes, as well as in hepatosomatic, viscerosomatic and mesenteric fat indexes. The composition of fasted fish was characterized by a decrease in lipid content; the liver displayed an intense reduction of lipid reserves in both fasted groups, and increased expression of the lysosomal acid lipase. 1 week after re-feeding, both experimental groups showed an increase in specific growth rate, feed intake and feed conversion ratio. A recovery in hepatic lipid reserves was observed, and the expression of the lysosomal acid lipase decreased, although lipid content in both groups was still significantly lower than in control groups. Hepatic expression of the growth hormone receptor decreased after fasting, and remained low even after the fish were fed, whereas the expression of insulin-like growth factor 1 and 2 increased after fasting and decreased in both groups when fish were fed again. Altogether, these results showed a partial compensatory growth response in L. guttatus juveniles after fasting, with enhanced growth rates and improved feed efficiency. Fish used stored lipid reserves as the main energy source, and the expression of growth-related and lipid mobilization marker genes in the liver showed similar patterns in both fasting schemes. Based on the results, we suggest as much as 1-week fasting intervals during grow-out programmes to reduce visceral fat and increase growth rate in this species.

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.

Effects of food deprivation on plasma insulin-like growth factor-1 (Igf1) and Igf binding protein (Igfbp) gene transcription in juvenile cabezon (Scorpaenichthys marmoratus)

The growth hormone (GH)/insulin-like growth factor (Igf) endocrine axis regulates somatic growth in the face of changing environmental conditions. In actinopterygian fishes, food availability is a key modulator of the somatotropic axis, with lower food intake generally depressing liver Igf1 release to diminish growth. Igf1 signaling, however, also involves several distinct IGF binding proteins (Igfbps), and the functional roles of many of these Igfbps in affecting growth during shifting food availability remain uncertain. Here, we tested how complete food deprivation (fasting) affected gene transcription for paralogs of all six types of Igfbps in the liver and fast-twitch skeletal muscle of cabezon (Scorpaenichthys marmoratus), a nearshore marine fish important for recreational fisheries in the eastern North Pacific Ocean. Juvenile cabezon were maintained as either fed (6% mass food⋅g fish wet mass^-1⋅d^-1) or fasted for 14 d. Fasted fish exhibited a lower body condition (K), a depressed mass-specific growth rate (SGR), and reduced plasma concentrations of Igf1. In the liver, fasting reduced the relative abundance of gene transcripts encoding Igfbps igfbp2a and igfbp2b, while significantly elevating mRNA levels for igfbp1a, igfbp1b, igfbp3b, and igfbp4. Fasting also reduced hepatic mRNA levels of GH receptor-1 (ghr1) - but not GH receptor-2 (ghr2) - supporting the idea that changes in liver sensitivity to GH may underlie the decline in plasma Igf1 during food deprivation. In skeletal muscle, fasting downregulated gene transcripts encoding igf1, igfbp2b, igfbp5b, and igfbp6b, while also upregulating mRNAs for igf2 and ghr2. These data demonstrate isoform-specific regulation of Igfbps in liver and skeletal muscle in cabezon experiencing food deprivation and reinforce the idea that the repertoire of duplicated Igfbp genes that evolved in actinopterygian fishes supports a diverse scope of endocrine and paracrine functions.

Regulatory mechanisms involved in muscle and bone remodeling during refeeding in gilthead sea bream

The tolerance of fish to fasting offers a model to study the regulatory mechanisms and changes produced when feeding is restored. Gilthead sea bream juveniles were exposed to a 21-days fasting period followed by 2 h to 7-days refeeding. Fasting provoked a decrease in body weight, somatic indexes, and muscle gene expression of members of the Gh/Igf system, signaling molecules (akt, tor and downstream effectors), proliferation marker pcna, myogenic regulatory factors, myostatin, and proteolytic molecules such as cathepsins or calpains, while most ubiquitin-proteasome system members increased or remained stable. In bone, downregulated expression of Gh/Igf members and osteogenic factors was observed, whereas expression of the osteoclastic marker ctsk was increased. Refeeding recovered the expression of Gh/Igf system, myogenic and osteogenic factors in a sequence similar to that of development. Akt and Tor phosphorylation raised at 2 and 5 h post-refeeding, much faster than its gene expression increased, which occurred at day 7. The expression in bone and muscle of the inhibitor myostatin (mstn2) showed an inverse profile suggesting an inter-organ coordination that needs to be further explored in fish. Overall, this study provides new information on the molecules involved in the musculoskeletal system remodeling during the early stages of refeeding in fish.

Early-life exposure to 17β-estradiol and 4-nonylphenol impacts the growth hormone/insulin-like growth-factor system and estrogen receptors in Mozambique tilapia, Oreochromis mossambicus

It is widely recognized that endocrine disrupting chemicals (EDCs) released into the environment through anthropogenic activities can have short-term impacts on physiological and behavioral processes and/or sustained or delayed long-term developmental effects on aquatic organisms. While numerous studies have characterized the effects of EDCs on temperate fishes, less is known on the effects of EDCs on the growth and reproductive physiology of tropical species. To determine the long-term effects of early-life exposure to common estrogenic chemicals, we exposed Mozambique tilapia (Oreochromis mossambicus) yolk-sac fry to 17β-estradiol (E2) and nonylphenol (NP) and subsequently characterized the expression of genes involved in growth and reproduction in adults. Fry were exposed to waterborne E2 (0.1 and 1 μg/L) and NP (10 and 100 μg/L) for 21 days. After the exposure period, juveniles were reared for an additional 112 days until males were sampled. Gonadosomatic index was elevated in fish exposed to E2 (0.1 μg/L) while hepatosomatic index was decreased by exposure to NP (100 μg/L). Exposure to E2 (0.1 μg/L) induced hepatic growth hormone receptor (ghr) mRNA expression. The high concentration of E2 (1 μg/L), and both concentrations of NP, increased hepatic insulin-like growth-factor 1 (igf1) expression; E2 and NP did not affect hepatic igf2 and pituitary growth hormone (gh) levels. Both E2 (1 μg/L) and NP (10 μg/L) induced hepatic igf binding protein 1b (igfbp1b) levels while only NP (100 μg/L) induced hepatic igfbp2b levels. By contrast, hepatic igfbp6b was reduced in fish exposed to E2 (1 μg/L). There were no effects of E2 or NP on hepatic igfbp4 and igfbp5a expression. Although the expression of three vitellogenin transcripts was not affected, E2 and NP stimulated hepatic estrogen receptor (erα and erβ) mRNA expression. We conclude that tilapia exposed to E2 and NP as yolk-sac fry exhibit subsequent changes in the endocrine systems that control growth and reproduction during later life stages.

Interactions of long-term food ration variation and short-term fasting on insulin-like growth factor-1 (IGF-1) pathways in copper rockfish (Sebastes caurinus)

Variation in food intake affects somatic growth by altering the expression of hormones in the somatotropic endocrine axis including insulin-like growth factor-1 (IGF-1). Here, we examined IGF-1 pathway responses to long- and short-term variation in food availability in copper rockfish (Sebastes caurinus), a nearshore Pacific rockfish important for commercial and recreational fisheries. Juvenile copper rockfish were raised under differing ration amounts (3% or 9% mass feed·g^-1 fish wet mass·day^-1) for 140 d to simulate 'long-term' feeding variation, after which some fish from both rations were fasted for 12 d to generate 'short-term' conditions of food deprivation. Rockfish on the 9% ration treatment grew more quickly than those on the 3% ration and were larger in mass, length, and body condition (k) after 152 d. Fish on the 9% ration had higher blood glucose than those on the 3% ration, with fasting decreasing blood glucose in both ration treatments, indicating that both long-term and short-term feed treatments altered energy status. Plasma IGF-1 was higher in rockfish from the 9% ration than those in the 3% ration and was also higher in fed fish than fasted fish. Additionally, plasma IGF-1 related positively to individual variation in specific growth rate (SGR). The positive association between IGF-1 and SGR showed discordance in fish that had experienced different levels of food and growth over the long-term but not short-term, suggesting that long-term nutritional experience can influence the relationship between IGF-1 and growth in this species. Rockfish on the 3% ration showed a lower relative abundance of gene transcripts encoding igf1 in the liver, but higher hepatic mRNAs for IGF binding proteins igfbp1a and igfbp1b. Fasting similarly decreased the abundance of igf1 mRNAs in the liver of fish reared under both the 9% and 3% rations, while concurrently increasing mRNAs encoding the IGF binding proteins igfbp1a, -1b, and -3a. Hepatic mRNAs for igfbp2b, -5a, and -5b were lower with long-term ration variation (3% ration) and fasting. Fish that experienced long-term reduced rations also had higher mRNA levels for igfbp3a, -3b, and IGF receptors isoforms A (igf1rA) and B (igf1rB) in skeletal muscle, but lower mRNA levels for igf1. Fasting increased muscle mRNA abundance for igfbp3a, igf1rA, and igf1rB, and decreased levels for igfbp2a and igf1. These data show that a positive relationship between circulating IGF-1 and individual growth rate is maintained in copper rockfish even when that growth variation relates to differences in food consumption across varying time scales, but that long- and short-term variation in food quantity can shift basal concentrations of circulating IGF-1 in this species.

Histological, transcriptomic and in vitro analysis reveal an intrinsic activated state of myogenic precursors in hyperplasic muscle of trout

BACKGROUND

The dramatic increase in myotomal muscle mass in post-hatching fish is related to their ability to lastingly produce new muscle fibres, a process termed hyperplasia. The molecular and cellular mechanisms underlying fish muscle hyperplasia largely remain unknown. In this study, we aimed to characterize intrinsic properties of myogenic cells originating from hyperplasic fish muscle. For this purpose, we compared in situ proliferation, in vitro cell behavior and transcriptomic profile of myogenic precursors originating from hyperplasic muscle of juvenile trout (JT) and from non-hyperplasic muscle of fasted juvenile trout (FJT) and adult trout (AT).

RESULTS

For the first time, we showed that myogenic precursors proliferate in hyperplasic muscle from JT as shown by in vivo BrdU labeling. This proliferative rate was very low in AT and FJT muscle. Transcriptiomic analysis revealed that myogenic cells from FJT and AT displayed close expression profiles with only 64 differentially expressed genes (BH corrected p-val < 0.001). In contrast, 2623 differentially expressed genes were found between myogenic cells from JT and from both FJT and AT. Functional categories related to translation, mitochondrial activity, cell cycle, and myogenic differentiation were inferred from genes up regulated in JT compared to AT and FJT myogenic cells. Conversely, Notch signaling pathway, that signs cell quiescence, was inferred from genes down regulated in JT compared to FJT and AT. In line with our transcriptomic data, in vitro JT myogenic precursors displayed higher proliferation and differentiation capacities than FJT and AT myogenic precursors.

CONCLUSIONS

The transcriptomic analysis and examination of cell behavior converge to support the view that myogenic cells extracted from hyperplastic muscle of juvenile trout are intrinsically more potent to form myofibres than myogenic cells extracted from non-hyperplasic muscle. The generation of gene expression profiles in myogenic cell extracted from muscle of juvenile trout may yield insights into the molecular and cellular mechanisms controlling hyperplasia and provides a useful list of potential molecular markers of hyperplasia.

Oceanographic upwelling conditions influence signaling pathways involved in muscle growth of intertidal fish

Few studies have addressed the impact of the upwelling oceanographical conditions on biological processes, such as growth, using a molecular and physiological approach. Upwelling conditions are characterized by low temperature seawater and high nutrient availability, which represents an ideal opportunity to understand how habitat modulates animal performance at different levels of biological complexity. We aimed to assess intraspecific variations in weight, oxygen consumption, protein content, and key signaling pathways involved in muscle-growth (protein kinase B (AKT) and extracellular signal-regulated kinase (ERK)) under experimental trials considering high/low seawater temperatures with full/restricted food rations. For this purpose, we studied Girella laevifrons, one of the most abundant fish species inhabiting rocky intertidal zones along the Eastern South Pacific coasts. Using fish obtained from upwelling (U) and non-upwelling (NU) zones, we reported that U animals displayed higher growth performance during both contrasting trials, with a weight gain (~3 g), lower oxygen consumption (~12%), and higher protein contents (~20%). Only ERK showed significant differences during the trials (~2-fold downregulation between NU and U fish). We also found that U fish increased protein ubiquitination in high water temperature and restricted food ration in contrast to NU fish. Our results help to elucidate how upwelling conditions may influence fish growth at physiological and molecular levels. Still, future analyses are necessary to improve the information regarding the impact of U and NU condition on animals, as well as the possible applications of this data in the aquaculture industry.

Compensatory response of fathead minnow larvae following a pulsed in-situ exposure to a seasonal agricultural runoff event

Agriculturally-dominated waterways such as those found throughout the Midwestern United States often experience seasonal pulses of agrichemical contaminants which pose a potential hazard to aquatic organisms at varying life stages. The objective of this study was to characterize the developmental plasticity of fathead minnow larvae in a natural environment subject to a seasonal episodic perturbation in the form of a complex mixture of agricultural stressors. Fathead minnow larvae were maintained at the Elkhorn River Research Station for a 28-d in situ exposure to an agrichemical pulse event. Minnow larvae were sampled after 14 and 28days to characterize developmental plasticity through growth measures and relative gene expression. Concentrations of agrichemical contaminants measured in water using polar organic chemical integrative samplers and composite sediment samples throughout the 28-d exposure were quantified using gas chromatography-mass spectrometry. Elevated concentrations of acetochlor, atrazine, and metolachlor were indicative of inputs from agricultural sources and were associated with reductions in body mass, condition factor, and androgenic gene expression in river exposed fathead minnow larvae. However, following a 14-d in situ depuration during the post-pulse period, river exposed larvae overcompensated in previously suppressed biological endpoints. These results indicate that fathead minnow larvae are capable of compensatory responses following episodic exposure to agrichemical stressors.

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.

Di- and tripeptide transport in vertebrates: the contribution of teleost fish models

Solute Carrier 15 (SLC15) family, alias H⁺-coupled oligopeptide cotransporter family, is a group of membrane transporters known for their role in the cellular uptake of di- and tripeptides (di/tripeptides) and peptide-like molecules. Of its members, SLC15A1 (PEPT1) chiefly mediates intestinal absorption of luminal di/tripeptides from dietary protein digestion, while SLC15A2 (PEPT2) mainly allows renal tubular reabsorption of di/tripeptides from ultrafiltration, SLC15A3 (PHT2) and SLC15A4 (PHT1) possibly interact with di/tripeptides and histidine in certain immune cells, and SLC15A5 has unknown function. Our understanding of this family in vertebrates has steadily increased, also due to the surge of genomic-to-functional information from 'non-conventional' animal models, livestock, poultry, and aquaculture fish species. Here, we review the literature on the SLC15 transporters in teleost fish with emphasis on SLC15A1 (PEPT1), one of the solute carriers better studied amongst teleost fish because of its relevance in animal nutrition. We report on the operativity of the transporter, the molecular diversity, and multiplicity of structural-functional solutions of the teleost fish orthologs with respect to higher vertebrates, its relevance at the intersection of the alimentary and osmoregulative functions of the gut, its response under various physiological states and dietary solicitations, and its possible involvement in examples of total body plasticity, such as growth and compensatory growth. By a comparative approach, we also review the few studies in teleost fish on SLC15A2 (PEPT2), SLC15A4 (PHT1), and SLC15A3 (PHT2). By representing the contribution of teleost fish to the knowledge of the physiology of di/tripeptide transport and transporters, we aim to fill the gap between higher and lower vertebrates.

Transcriptome Analysis and Postprandial Expression of Amino Acid Transporter Genes in the Fast Muscles and Gut of Chinese Perch (Siniperca chuatsi)

The characterization of the expression and regulation of growth-related genes in the muscles of Chinese perch is of great interest to aquaculturists because of the commercial value of the species. The transcriptome annotation of the skeletal muscles is a crucial step in muscle growth-related gene analysis. In this study, we generated 52 504 230 reads of mRNA sequence data from the fast muscles of the Chinese perch by using Solexa/Illumina RNA-seq. Twenty-one amino acid transporter genes were annotated by searching protein and gene ontology databases, and postprandial changes in their transcript abundance were assayed after administering a single satiating meal to Chinese perch juveniles (body mass, approximately 100 g), following fasting for 1 week. The gut content of the Chinese perch increased significantly after 1 h and remained high for 6 h following the meal and emptied within 48-96 h. Expression of eight amino acid transporter genes was assayed in the fast muscles through quantitative real-time polymerase chain reaction at 0, 1, 3, 6, 12, 24, 48, and 96 h. Among the genes, five transporter transcripts were markedly up-regulated within 1 h of refeeding, indicating that they may be potential candidate genes involved in the rapid-response signaling system regulating fish myotomal muscle growth. These genes display coordinated regulation favoring the resumption of myogenesis responding to feeding.

Hepatic insulin-like growth-factor binding protein (igfbp) responses to food restriction in Atlantic salmon smolts

The growth hormone (Gh)/insulin-like growth-factor (Igf) system plays a central role in the regulation of growth in fishes. However, the roles of Igf binding proteins (Igfbps) in coordinating responses to food availability are unresolved, especially in anadromous fishes preparing for seaward migration. We assayed plasma Gh, Igf1, thyroid hormones and cortisol along with igfbp mRNA levels in fasted and fed Atlantic salmon (Salmo salar). Fish were fasted for 3 or 10days near the peak of smoltification (late April to early May). Fasting reduced plasma glucose by 3days and condition factor by 10days. Plasma Gh, cortisol, and thyroxine (T4) were not altered in response to fasting, whereas Igf1 and 3,5,3'-triiodo-l-thyronine (T3) were slightly higher and lower than controls, respectively. Hepatic igfbp1b1, -1b2, -2a, -2b1 and -2b2 mRNA levels were not responsive to fasting, but there were marked increases in igfbp1a1 following 3 and 10days of fasting. Fasting did not alter hepatic igf1 or igf2; however, muscle igf1 was diminished by 10days of fasting. There were no signs that fasting compromised branchial ionoregulatory functions, as indicated by unchanged Na(+)/K(+)-ATPase activity and ion pump/transporter mRNA levels. We conclude that dynamic hepatic igfbp1a1 and muscle igf1 expression participate in the modulation of Gh/Igf signaling in smolts undergoing catabolism.

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

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

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.

Effects of sustained exercise on GH-IGFs axis in gilthead sea bream (Sparus aurata)

The endocrine system regulates growth mainly through the growth hormone (GH)/insulin-like growth factors (IGFs) axis and, although exercise promotes growth, little is known about its modulation of these factors. The aim of this work was to characterize the effects of 5 wk of moderate sustained swimming on the GH-IGFs axis in gilthead sea bream fingerlings. Plasma IGF-I/GH ratio and tissue gene expression of total IGF-I and three splice variants, IGF-II, three IGF binding proteins, two GH receptors, two IGF-I receptors, and the downstream molecules were analyzed. Fish under exercise (EX) grew more than control fish (CT), had a higher plasma IGF-I/GH ratio, and showed increased hepatic IGF-I expression (mainly IGF-Ia). Total IGF-I expression levels were similar in the anterior and caudal muscles; however, IGF-Ic expression increased with exercise, suggesting that this splice variant may be the most sensitive to mechanical action. Moreover, IGFBP-5b and IGF-II increased in the anterior and caudal muscles, respectively, supporting enhanced muscle growth. Furthermore, in EX fish, hepatic IGF-IRb was reduced together with both GHRs; GHR-II was also reduced in anterior muscle, while GHR-I showed higher expression in the two muscle regions, indicating tissue-dependent differences and responses to exercise. Exercise also increased gene and protein expression of target of rapamycin (TOR), suggesting enhanced muscle protein synthesis. Altogether, these data demonstrate that moderate sustained activity may be used to increase the plasma IGF-I/GH ratio and to potentiate growth in farmed gilthead sea bream, modulating the gene expression of different members of the GH-IGFs axis (i.e., IGF-Ic, IGF-II, IGFBP-5b, GHR-I, and TOR).

Molecular characterization of Myf5 and comparative expression patterns of myogenic regulatory factors in Siniperca chuatsi

Myogenic regulatory factors (MRFs) are muscle-specific basic helix-loop-helix (bHLH) transcription factor that plays an essential role in regulating skeletal muscle development and growth. To investigate molecular characterization of Myf5 and compare the expressional patterns of the four MRFs, we cloned the Myf5 cDNA sequence and analyzed the MRFs expressional patterns using quantitative real-time polymerase chain reaction in Chinese perch (Siniperca chuatsi). Sequence analysis indicated that Chinese perch Myf5 and other MRFs shared a highly conserved bHLH domain with those of other vertebrates. Sequence alignment and phylogenetic tree showed that Chinese perch MRFs had the highest identity with the MRFs of Epinephelus coioides. Spatio-temporal expressional patterns revealed that the MRFs were primarily expressed in muscle, especially in white muscle. During embryonic development period, Myf5, MyoD and MyoG mRNAs had a steep increase at neurula stage, and their highest expressional level was predominantly observed at hatching period. Whereas the highest expressional level of the MRF4 was observed at the muscular effect stage. The expressional patterns of post-embryonic development showed that the Myf5, MyoD and MyoG mRNAs were highest at 90 days post-hatching (dph). Furthermore, starvation and refeeding results showed that the transcription of the MRFs in the fast skeletal muscle of Chinese perch responded quickly to a single meal after 7 days of fasting. It indicated that the MRFs might contribute to muscle recovery after refeeding in Chinese perch.

Hybridization between Yellowstone Cutthroat Trout and Rainbow Trout Alters the Expression of Muscle Growth-Related Genes and Their Relationships with Growth Patterns

Hybridization creates novel gene combinations that may generate important evolutionary novelty, but may also reduce existing adaptation by interrupting inherent biological processes, such as genotype-environment interactions. Hybridization often causes substantial change in patterns of gene expression, which, in turn, may cause phenotypic change. Rainbow trout (Oncorhynchus mykiss) and cutthroat trout (O. clarkii) produce viable hybrids in the wild, and introgressive hybridization with introduced rainbow trout is a major conservation concern for native cutthroat trout. The two species differ in body shape, which is likely an evolutionary adaptation to their native environments, and their hybrids tend to show intermediate morphology. The characterization of gene expression patterns may provide insights on the genetic basis of hybrid and parental morphologies, as well as on the ecological performance of hybrids in the wild. Here, we evaluated the expression of eight growth-related genes (MSTN-1a, MSTN-1b, MyoD1a, MyoD1b, MRF-4, IGF-1, IGF-2, and CAST-L) and the relationship of these genes with growth traits (length, weight, and condition factor) in six line crosses: both parental species, both reciprocal F1 hybrids, and both first-generation backcrosses (F1 x rainbow trout and F1 x cutthroat trout). Four of these genes were differentially expressed among rainbow, cutthroat, and their hybrids. Transcript abundance was significantly correlated with growth traits across the parent species, but not across hybrids. Our findings suggest that rainbow and cutthroat trout exhibit differences in muscle growth regulation, that transcriptional networks may be modified by hybridization, and that hybridization disrupts intrinsic relationships between gene expression and growth patterns that may be functionally important for phenotypic adaptations.

Effects of sex steroids on expression of genes regulating growth-related mechanisms in rainbow trout (Oncorhynchus mykiss)

Effects of a single injection of 17β-estradiol (E2), testosterone (T), or 5β-dihydrotestosterone (DHT) on expression of genes central to the growth hormone (GH)/insulin-like growth factor (IGF) axis, muscle-regulatory factors, transforming growth factor-beta (TGFβ) superfamily signaling cascade, and estrogen receptors were determined in rainbow trout (Oncorhynchus mykiss) liver and white muscle tissue. In liver in addition to regulating GH sensitivity and IGF production, sex steroids also affected expression of IGF binding proteins, as E2, T, and DHT increased expression of igfbp2b and E2 also increased expression of igfbp2 and igfbp4. Regulation of this system also occurred in white muscle in which E2 increased expression of igf1, igf2, and igfbp5b1, suggesting anabolic capacity may be maintained in white muscle in the presence of E2. In contrast, DHT decreased expression of igfbp5b1. DHT and T decreased expression of myogenin, while other muscle regulatory factors were either not affected or responded similarly for all steroid treatments. Genes within the TGFβ superfamily signaling cascade responded to steroid treatment in both liver and muscle, suggesting a regulatory role for sex steroids in the ability to transmit signals initiated by TGFβ superfamily ligands, with a greater number of genes responding in liver than in muscle. Estrogen receptors were also regulated by sex steroids, with era1 expression increasing for all treatments in muscle, but only E2- and T-treatment in liver. E2 reduced expression of erb2 in liver. Collectively, these data identify how physiological mechanisms are regulated by sex steroids in a manner that promotes the disparate effects of androgens and estrogens on growth in salmonids.

Global gene expression patterns of grass carp following compensatory growth

BACKGROUND

Compensatory growth is accelerated compared with normal growth and occurs when growth-limiting conditions are overcome. Most animals, especially fish, are capable of compensatory growth, but the mechanisms remain unclear. Further investigation of the mechanism of compensatory growth in fish is needed to improve feeding efficiency, reduce cost, and explore growth-related genes.

RESULTS

In the study, grass carp, an important farmed fish in China, were subjected to a compensatory growth experiment followed by transcriptome analysis by RNA-sequencing. Samples of fish from starved and re-feeding conditions were compared with the control. Under starved conditions, 4061 and 1988 differentially expressed genes (DEGs) were detected in muscle and liver tissue when compared the experimental group with control group, respectively. After re-feeding, 349 and 247 DEGs were identified in muscle and liver when the two groups were compared. Moreover, when samples from experimental group in starved and re-feeding conditions were compared, 4903 and 2444 DEGs were found in muscle and liver. Most of these DEGs were involved in metabolic processes, or encoded enzymes or proteins with catalytic activity or binding functions, or involved in metabolic and biosynthetic pathways. A number of the more significant DEGs were subjected to further analysis. Under fasting conditions, many up-regulated genes were associated with protein ubiquitination or degradation, whereas many down-regulated genes were involved in the metabolism of glucose and fatty acids. Under re-feeding conditions, genes participating in muscle synthesis and fatty acid metabolism were up-regulated significantly, and genes related to protein ubiquitination or degradation were down-regulated. Moreover, Several DEGs were random selected for confirmation by real-time quantitative PCR.

CONCLUSIONS

Global gene expression patterns of grass carp during compensatory growth were determined. To our knowledge, this is a first reported for a teleost fish. The results will enhance our understanding of the mechanism of compensatory growth in teleost fish.

MicroRNA signature in response to nutrient restriction and re-feeding in fast skeletal muscle of grass carp (Ctenopharyngodon idella)

The grass carp (Ctenopharyngodon idella) is one of the most important cultivated fish species in China. Mounting evidences suggests that microRNAs (miRNAs) may be key regulators of skeletal muscle among the grass carp, but the knowledge of the identity of myogenic miRNAs and role of miRNAs during skeletal muscle anabolic state remains limited. In the present study, we choose 8 miRNAs previously reported to act as muscle growth-related miRNAs for fasting-refeeding research. We investigated postprandial changes in the expression of 8 miRNAs following a single satiating meal in grass carp juveniles who had been fasting for one week and found that 7 miRNAs were sharply up-regulated within 1 or 3 h after refeeding, suggesting that they may be promising candidate miRNAs involved in a fast-response signaling system that regulates fish skeletal muscle growth.

Dusky damselfish Stegastes fuscus relational learning: evidences from associative and spatial tasks

This study investigated the ability of the dusky damselfish Stegastes fuscus to associate conditioned and unconditioned stimuli (single CS-US) and to find a specific place in a clueless ambiece (spatial learning). After tested for colour preference and showing no specific colour attractively, the fish were trained to associate a colour cue with a stimulus fish (conspecific). Fish were then challenged to locate the exact place where the stimulus fish was presented. Stegastes fuscus spent most time close to the zone where stimulus was presented, even without obvious marks for orientation. The results confirm that S. fuscus show single CS-US learning and suggest the fish ability for spatial orientation. Stegastes fuscus appears to use multiple senses (sight and lateral line) for cues association and recall, and appear to perform relational learning similar to mammals. These data suggest the importance of cognitive skill for reef fishes that may have contributed to their establishment and evolutionary success in such complex environment.

IGF-I and amino acids effects through TOR signaling on proliferation and differentiation of gilthead sea bream cultured myocytes

Skeletal muscle growth and development is controlled by nutritional (amino acids, AA) as well as hormonal factors (insulin-like growth factor, IGF-I); however, how its interaction modulates muscle mass in fish is not clearly elucidated. The purpose of this study was to analyze the development of gilthead sea bream cultured myocytes to describe the effects of AA and IGF-I on proliferating cell nuclear antigen (PCNA) and myogenic regulatory factors (MRFs) expression, as well as on the transduction pathways involved in its signaling (TOR/AKT). Our results showed that AA and IGF-I separately increased the number of PCNA-positive cells and, together produced a synergistic effect. Furthermore, AA and IGF-I, combined or separately, increased significantly Myogenin protein expression, whereas MyoD was not affected. These results indicate a role for these factors in myocyte proliferation and differentiation. At the mRNA level, AA significantly enhanced PCNA expression, but no effects were observed on the expression of the MRFs or AKT2 and FOXO3 upon treatment. Nonetheless, we demonstrated for the first time in gilthead sea bream that AA significantly increased the gene expression of TOR and its downstream effectors 4EBP1 and 70S6K, with IGF-I having a supporting role on 4EBP1 up-regulation. Moreover, AA and IGF-I also activated TOR and AKT by phosphorylation, respectively, being this activation decreased by specific inhibitors. In summary, the present study demonstrates the importance of TOR signaling on the stimulatory role of AA and IGF-I in gilthead sea bream myogenesis and contributes to better understand the potential regulation of muscle growth and development in fish.

Salinity and temperature variations reflecting on cellular PCNA, IGF-I and II expressions, body growth and muscle cellularity of a freshwater fish larvae

The present study assessed the influence of salinity and temperature on body growth and on muscle cellularity of Lophiosilurus alexaxdri vitelinic larvae. Slightly salted environments negatively influenced body growth of freshwater fish larvae and we observed that those conditions notably act as an environmental influencer on muscle growth and on local expression of hypertrophia and hypeplasia markers (IGFs and PCNA). Furthermore, we could see that salinity tolerance for NaCl 4gl(-)(1) diminishes with increasing temperature, evidenced by variation in body and muscle growth, and by irregular morphology of the lateral skeletal muscle of larvae. We saw that an increase of both PCNA and autocrine IGF-II are correlated to an increase in fibre numbers and fibre diameter as the temperature increases and salinity diminishes. On the other hand, autocrine IGF-I follows the opposite way to the other biological parameters assessed, increasing as salinity increases and temperature diminishes, showing that this protein did not participate in muscle cellularity, but participating in molecular/cellular repair. Therefore, slightly salted environments may provide adverse conditions that cause some obstacles to somatic growth of this species, suggesting some osmotic expenditure with a salinity increment.

Ploidy effects on genes regulating growth mechanisms during fasting and refeeding in juvenile rainbow trout (Oncorhynchus mykiss)

Diploid and triploid rainbow trout weighing approximately 3g were either fed for five weeks, or feed deprived for one week, followed by refeeding. During feed deprivation gastrointestinal somatic index decreased in diploids, but not triploids, and during refeeding, carcass growth rate recovered more quickly in triploids. Although not affected by ploidy, liver ghr2 and igfbp2b expression increased and igfbp1b decreased in fasted fish. Effects of ploidy on gene expression indicate potential mechanisms associated with improved recovery growth in triploids, which include decreased hepatic igfbp expression, which could influence IGF-I bioavailability, differences in tissue sensitivity to TGFbeta ligands due to altered tgfbr and smad expression, and differences in expression of muscle regulatory genes (myf5, mstn1a, and mstn1b). These data suggest that polyploidy influences the expression of genes critical to muscle development and general growth regulation, which may explain why triploid fish recover from nutritional insult better than diploid fish.

Molecular characterization and expression patterns of myogenin in compensatory growth of Megalobrama amblycephala

Myogenin (myog) is a muscle-specific basic helix-loop-helix (bHLH) transcription factor that plays an essential role in regulating skeletal muscle development and growth. To investigate molecular characterization of myog and the effect of starvation/refeeding on the gene expression, we isolated the myog cDNA sequence and analyzed the expression patterns using quantitative real-time polymerase chain reaction in Megalobrama amblycephala. Sequence analysis indicated that M. amblycephala myog shared an analogous structure with the highly conserved His/Cys-rich, bHLH and C-terminal helix III domains with other vertebrates. Sequence alignment and phylogenetic tree showed that M. amblycephala myog had the highest identity with the homologues of Ctenopharyngodon idella and Cyprinus carpio. Spatio-temporal expression patterns revealed that myog mRNA levels at the segmentation period and 12 h post-hatching (hph) were significantly higher than at other development stages (P<0.05). Furthermore, the highest myog expression level was predominantly observed in white muscle compared with the other types of muscle. Fish body weight continuously decreased during 21-day starvation and then significantly increased after 7days of refeeding and reached the similar level to the control at 21days of refeeding, indicating that the pattern of complete compensatory growth possibly occurred in M. amblycephala; meanwhile, the relative somatic growth rate after refeeding was also dramatically higher than the control group. In addition, the myog expression decreased during 21days of starvation and then exhibited a strong rebound effect after 7days of refeeding and subsequently declined gradually to the control level by 21days of refeeding.

Persian sturgeon insulin-like growth factor I: molecular cloning and expression during various nutritional conditions

The effects of different periods of starvation (1, 2, 3, and 4 weeks) and subsequent re-feeding (over a 4 week) on the compensatory growth performance and insulin-like growth factor I (IGF-I) mRNA expression in liver and white muscle were investigated in juvenile Persian sturgeon (Acipenser persicus). First, a fragment of 617 nucleotides coding for IGF-I was cloned from liver, which included an open reading frame of 486 nucleotides, encoding a 162 amino acid preproIGF-I. This is composed of a 45 aa for signal peptide, a 117 aa for the mature peptide comprising the B, C, A, and D domains, and a 47 aa for E domain. The mature Persian sturgeon IGF-I exhibits high sequence identities with other sturgeon species and teleost, ranging between 68 and 95 %. The pattern of IGF-I mRNA expression in the liver and white muscle was measured in response to different periods of starvation and subsequent re-feeding. Nutritional status influenced IGF-I mRNA expression pattern in both liver and muscle. IGF-I mRNA expression in the liver increased during starvation, before decreasing after re-feeding. Furthermore, white muscle IGF-I mRNA expression showed better responses to nutritional status and decreased following starvation and increased by re-feeding. However, changes in the expression of IGF-I mRNA were not significantly different between any of the treatments in both tissues. These data suggest that muscle and liver IGF-I mRNA expression do not have a regulatory role for somatic growth induced by compensatory growth in Persain sturgeon.

Metabolic endocrine factors involved in spawning recovery and rematuration of iteroparous female rainbow trout (Oncorhynchus mykiss)

To determine how energy balance affects metabolic hormones hypothesized to play a role in the onset of a new reproductive cycle in iteroparous salmonids, food availability after spawning was restricted in female rainbow trout. These fish were compared with a control group that was fed a standard brood stock ration. Bodyweight, length, and muscle lipid content were determined, and blood was collected from fish at regular intervals; a subset of fish from each group was sacrificed at each sampling time for the collection of liver and ovary tissue, and to calculate hepatosomatic index (HSI) and gonadosomatic index (GSI). Plasma hormone levels were quantified by radioimmunoassay, and tissue gene expression levels were analyzed using q-RT-PCR. The experiment was conducted twice, using two-year-old and three-year-old post-spawned fish. Food-restriction arrested ovarian growth and development within 15-20 weeks, as evidenced by lower GSI in restricted-ration fish. Food restriction also reduced Fulton's condition factor, muscle lipid content, and specific growth rate from one month onward, and reduced HSI after 3 months. In the liver, insulin-like growth factor (igf1 and igf2) gene expression was reduced in three-year-old food-restricted fish within 2 months; however, no effect of ration on igf1 or igf2 expression was detected in two-year-old fish. In both years, IGF binding protein-1 (igfbp1) gene expression decreased over time in both treatment groups. Liver leptin (slepA1) gene expression was lower in two-year-old food-restricted fish at 4 months. These results show that this feed restriction regime arrested reproductive development and affected factors associated with energy balance purported to play a role in initiating reproductive development within 2-4months after spawning.

Transient inactivation of myostatin induces muscle hypertrophy and overcompensatory growth in zebrafish via inactivation of the SMAD signaling pathway

Myostatin (MSTN) is the main negative regulator of muscle growth and development in vertebrates. In fish, little is known about the molecular mechanisms behind how MSTN inactivation triggers skeletal muscle enhancement, particularly regarding the signaling pathways involved in this process. Moreover, there have not been reports on the biotechnological applications of MSTN and its signal transduction. In this context, zebrafish underwent compensatory growth using fasting and refeeding trials, and MSTN activity was inactivated with dominant negative LAPD76A recombinant proteins during the refeeding period, when a rapid, compensatory muscle growth was observed. Treated fish displayed an overcompensation of growth characterized by higher muscle hypertrophy and growth performance than constantly fed, control fish. Treatment with LAPD76A recombinant proteins triggered inactivation of the SMAD signaling pathway in skeletal muscle, the main signal transduction used by MSTN to achieve its biological actions. Therefore, transient inactivation of MSTN during the compensatory growth of zebrafish led to a decrease in the SMAD signaling pathway in muscle, triggering muscle hypertrophy and finally improving growth performance, thus, zebrafish achieved an overcompensation of growth. The present study shows an attractive strategy for improving muscle growth in a fish species by mixing a classical strategy, such as compensatory growth, and a biotechnological approach, such as the use of recombinant proteins for inhibiting the biological actions of MSTN. The mix of both strategies may represent a method that could be applied in order to improve growth in commercial fish of interest for aquaculture.

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

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

Isolation and selection of suitable reference genes for real-time PCR analyses in the skeletal muscle of the fine flounder in response to nutritional status: assessment and normalization of gene expression of growth-related genes

In the present study, different reference genes were isolated, and their stability in the skeletal muscle of fine flounder subjected to different nutritional states was assessed using geNorm and NormFinder. The combinations between 18S and ActB; Fau and 18S; and Fau and Tubb were chosen as the most stable gene combinations in feeding, long-term fasting and refeeding, and short-term refeeding conditions, respectively. In all periods, ActB was identified as the single least stable gene. Subsequently, the expression of the myosin heavy chain (MYH) and the insulin-like growth factor-I receptor (IGF-IR) was assessed. A large variation in MYH and IGF-IR expression was found depending on the reference gene that was chosen for normalizing the expression of both genes. Using the most stable reference genes, mRNA levels of MYH decreased and IGF-IR increased during fasting, with both returning to basal levels during refeeding. However, the drop in mRNA levels for IGF-IR occurred during short-term refeeding, in contrast with the observed events in the expression of MYH, which occurred during long-term refeeding. The present study highlights the vast differences incurred when using unsuitable versus suitable reference genes for normalizing gene expression, pointing out that normalization without proper validation could result in a bias of gene expression.

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.

Endocrine regulation of compensatory growth in fish

Compensatory growth (CG) is a period of accelerated growth that occurs following the alleviation of growth-stunting conditions during which an organism can make up for lost growth opportunity and potentially catch up in size with non-stunted cohorts. Fish show a particularly robust capacity for the response and have been the focus of numerous studies that demonstrate their ability to compensate for periods of fasting once food is made available again. CG is characterized by an elevated growth rate resulting from enhanced feed intake, mitogen production, and feed conversion efficiency. Because little is known about the underlying mechanisms that drive the response, this review describes the sequential endocrine adaptations that lead to CG; namely during the precedent catabolic phase (fasting) that taps endogenous energy reserves, and the following hyperanabolic phase (refeeding) when accelerated growth occurs. In order to elicit a CG response, endogenous energy reserves must first be moderately depleted, which alters endocrine profiles that enhance appetite and growth potential. During this catabolic phase, elevated ghrelin and growth hormone (GH) production increase appetite and protein-sparing lipolysis, while insulin-like growth factors (IGFs) are suppressed, primarily due to hepatic GH resistance. During refeeding, temporal hyperphagia provides an influx of energy and metabolic substrates that are then allocated to somatic growth by resumed IGF signaling. Under the right conditions, refeeding results in hyperanabolism and a steepened growth trajectory relative to constantly fed controls. The response wanes as energy reserves are re-accumulated and homeostasis is restored. We ascribe possible roles for select appetite and growth-regulatory hormones in the context of the prerequisite of these catabolic and hyperanabolic phases of the CG response in teleosts, with emphasis on GH, IGFs, cortisol, somatostatin, neuropeptide Y, ghrelin, and leptin.

The effect of insulin, TNFα and DHA on the proliferation, differentiation and lipolysis of preadipocytes isolated from large yellow croaker (Pseudosciaena Crocea R.)

Fish final product can be affected by excessive lipid accumulation. Therefore, it is important to develop strategies to control obesity in cultivated fish to strengthen the sustainability of the aquaculture industry. As in mammals, the development of adiposity in fish depends on hormonal, cytokine and dietary factors. In this study, we investigated the proliferation and differentiation of preadipocytes isolated from the large yellow croaker and examined the effects of critical factors such as insulin, TNFα and DHA on the proliferation, differentiation and lipolysis of adipocytes. Preadipocytes were isolated by collagenase digestion, after which their proliferation was evaluated. The differentiation process was optimized by assaying glycerol-3-phosphate dehydrogenase (GPDH) activity. Oil red O staining and electron microscopy were performed to visualize the accumulated triacylglycerol. Gene transcript levels were measured using SYBR green quantitative real-time PCR. Insulin promoted preadipocytes proliferation, stimulated cell differentiation and decreased lipolysis of mature adipocytes. TNFα and DHA inhibited cell proliferation and differentiation. While TNFα stimulated mature adipocyte lipolysis, DHA showed no lipolytic effect on adipocytes. The expressions of adipose triglyceride lipase (ATGL), fatty acid synthase (FAS), lipoprotein lipase (LPL) and peroxisome proliferator-activated receptor α, γ (PPARα, PPARγ) were quantified during preadipocytes differentiation and adipocytes lipolysis to partly explain the regulation mechanisms. In summary, the results of this study indicated that although preadipocytes proliferation and the differentiation process in large yellow croaker are similar to these processes in mammals, the effects of critical factors such as insulin, TNFα and DHA on fish adipocytes development are not exactly the same. Our findings fill in the gaps in the basic data regarding the effects of critical factors on adiposity development in fish and will facilitate the further study of molecular mechanism by which these factors act in fish and the application of this knowledge to eventually control obesity in cultured species.

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.