Discordant regulation of hepatic IGF-I mRNA and circulating IGF-I during compensatory growth in a teleost, the hybrid striped bass (Morone chrysopsxMorone saxatilis)

Nutritional programming in Nile tilapia (Oreochromis niloticus): Effect of low dietary protein on growth and the intestinal microbiome and transcriptome

Nutritional programming is the idea that early nutrient contributions can influence organismal structure or function and is documented in a variety of vertebrates, yet studies in fish are largely lacking. Tilapia are an important foodfish, with global production having increased rapidly since the 1990s. They exhibit high disease-resistance and grow well on formulated feeds which makes them an ideal aquaculture species, however incorporating high quality proteins into feeds can be costly. As feed constitutes 50-70% of total production costs in aquaculture, reducing protein content could curb these costs and increase revenue. Thus, we examined the effects of feeding Nile tilapia (O. niloticus) fry a restricted protein diet for the first 7-21 days on growth, gut microbial flora, and the intestinal transcriptome. Fish were fed either a 25% restricted or 48% control crude protein starter (ST) diet for up to 21 days and then switched to a 25% or 38% control crude protein growout (GO) diet. Fish fed a 25% ST diet for 14 days followed by a 38% GO diet had significantly higher lengths and weights and better feed efficiency than fish fed the control 48% ST and 38% GO diet after 56 days of culture. Growth of fry on the 25% ST, 7-day/38% GO and the 25% ST,7-day/25% GO diets did not differ from the those fed the control protein diets, while fish fed the 25% ST diet for 21 days had significantly lower growth and survival rates. We observed no significant differences in either alpha or beta diversity of the gut microbial flora between diets, however species richness (Shannon Index) was higher in fry fed the 25% protein ST diet regardless of the GO diet. Similarly, fish fed the 25% ST diet for 14 days followed by the 38% GO diet had minimal changes to the intestinal transcriptome relative to fish fed the control 48% ST and 38% GO diet. However, those fed 25% ST and GO diets for the entire 56 days exhibited substantial differences in the gut transcriptome from other groups showing gene expression profiles characteristic of detrimental changes to gut physiology, protein metabolism and immune function. Results suggest protein restriction for up to 14 days early in development leads to enhanced growth and feed efficiency with minimal effects on gut microbes or intestinal function. Protein restriction beyond this period appears detrimental to fish growth and health as underscored by expression of disease related genes and higher mortality rates.

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Advances in understanding the mitogenic, metabolic, and cell death signaling in teleost development: the case of greater amberjack (Seriola dumerili, Risso 1810)

Cell growth and differentiation signals of insulin-like growth factor-1 (IGF-1), a key regulator in embryonic and postnatal development, are mediated through the IGF-1 receptor (IGF-1R), which activates several downstream pathways. The present study aims to address crucial organogenesis and development pathways including Akt, MAPKs, heat shock response, apoptotic and autophagic machinery, and energy metabolism in relation to IGF-1R activation during five developmental stages of reared Seriola dumerili: 1 day prior to hatching fertilized eggs (D-1), hatching day (D0), 3 days post-hatching larvae (D3), 33 (D33) and 46 (D46) days post-hatching juveniles. During both the fertilized eggs stage and larval-to-juvenile transition, IGF-1R/Akt pathway activation may mediate the hypertrophic signaling, while p44/42 MAPK phosphorylation was apparent at S. dumerili post-hatching processes and juvenile organs completion. On the contrary, apoptosis was induced during embryogenesis and autophagy at hatching day indicating a potential involvement in morphogenetic rearrangements and yolk-sac reserves depletion. Larvae morphogenesis was accompanied by a metabolic turnover with increased substantial energy consumption. The findings of the present study demonstrate the developmental stages-specific shift in critical signaling pathways during the ontogeny of reared S. dumerili.

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.

Ecology and Prediction of Compensatory Growth: From Theory to Application in Forestry

Compensatory growth has been observed in forests, and it also appears as a common phenomenon in biology. Though it sometimes takes different names, the essential meanings are the same, describing the accelerated growth of organisms when recovering from a period of unfavorable conditions such as tissue damage at the individual level and partial mortality at the population level. Diverse patterns of compensatory growth have been reported in the literature, ranging from under-, to compensation-induced-equality, and to over-compensation. In this review and synthesis, we provide examples of analogous compensatory growth from different fields, clarify different meanings of it, summarize its current understanding and modeling efforts, and argue that it is possible to develop a state-dependent model under the conceptual framework of compensatory growth, aimed at explaining and predicting diverse observations according to different disturbances and environmental conditions. When properly applied, compensatory growth can benefit different industries and human society in various forms.

Testosterone Reduces Growth and Hepatic IGF-1 mRNA in a Female-Larger Lizard, Sceloporus undulatus: Evidence of an Evolutionary Reversal in Growth Regulation

Previous research has demonstrated that testosterone (T) can inhibit growth in female-larger species and stimulate growth in male-larger species, but the underlying mechanisms of this regulatory bipotentiality have not been investigated. In this study, we investigated the effects of T on the expression of hepatic insulin-like growth factor-1 (IGF-1) mRNA and circulating IGF-1 hormone in Sceloporus undulatus, a species of lizard in which females grow faster to become larger than males and in which T inhibits growth. Experiments were performed in captivity on mature female and male adults in the asymptotic phase of their growth curve and on actively growing, pre-reproductive juveniles. In adult males, the expression of hepatic IGF-1 mRNA increased following surgical castration and returned to control levels with T replacement; in intact adult females, exogenous T had no effect on IGF-1 mRNA expression. In juveniles, T significantly reduced both growth and the expression of hepatic IGF-1 mRNA to similar extents in intact females and in castrated males. The relative inhibitory effects of T on mRNA expression were greater in juveniles than in adults. Plasma IGF-1 hormone was about four times higher in juveniles than in adults, but T had no significant effect on IGF-1 hormone in either sex or in either age group. Our finding of inhibition of the expression of hepatic IGF-1 mRNA stands in contrast to the stimulatory effects of T in the published body of literature. We attribute our novel finding to our use of a species in which T inhibits rather than stimulates growth. Our findings begin to explain how T has the regulatory bipotentiality to be stimulatory in some species and inhibitory in others, requiring only an evolutionary reversal in the molecular regulation of growth-regulatory genes including IGF-1. Further comparative transcriptomic studies will be required to fully resolve the molecular mechanism of growth inhibition.

Enhanced biodiversity of gut flora and feed efficiency in pond cultured tilapia under reduced frequency feeding strategies

Feed constitutes 50-70% of total production costs of tilapia, one of the most widely cultured finfishes in the world. We evaluated reduced-feeding strategies for improving production efficiency of Nile tilapia (Oreochromis niloticus). In a 12-week pond trial, fish were fed daily, every other day, every third day, or not at all. Ponds were fertilized to enhance natural foods. In a fifth group fish were fed daily without pond fertilization. Fish fed daily with or without pond fertilization and fish fed every other day had higher specific growth rates, survivability, and net production than the other two treatments. Fish feed efficiency and benefit to cost ratio was highest for treatments fed in a pulsatile manner (i.e. fed every other day or every third day) with fish fed on alternate days providing the best net return among all groups. Fish fed on alternate days had more moderate gene expression levels of intestinal nutrient transporters which may allow for a more balanced and efficient nutrient uptake. Fecal microbe analyses identified 145 families of prokaryotic and 132 genera of eukaryotic organisms in tilapia. The highest diversity of prokaryotes was found in fish fed either every other day or daily in fertilized ponds and the highest diversity of eukaryotes was found in fish fed every other day. These studies indicate feeding Nile tilapia on alternate days along with weekly pond fertilization has no deleterious effects on growth, survivability, or production versus daily feeding regimes, but enhances feed efficiency by 76% and provides the greatest net return on investments. Our studies also suggest for the first time that combining alternate-day feeding with pond fertilization produces the greatest microbial biodiversity in the intestine that could contribute to enhanced feed efficiency and overall health of tilapia.

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.

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.

Microarray and metabolome analysis of hepatic response to fasting and subsequent refeeding in zebrafish (Danio rerio)

Background

Compensatory growth refers to the phenomenon in which organisms grow faster after the improvement of an adverse environment and is thought to be an adaptive evolution to cope with the alleviation of the hostile environment. Many fish have the capacity for compensatory growth, but the underlying cellular mechanisms remain unclear. In the present study, microarray and nontargeted metabolomics were performed to characterize the transcriptome and metabolome of zebrafish liver during compensatory growth.

Results

Zebrafish could regain the weight they lost during 3 weeks of fasting and reach a final weight similar to that of fish fed ad libitum when refed for 15 days. When refeeding for 3 days, the liver displayed hyperplasia accompanied with decreased triglyceride contents and increased glycogen contents. The microarray results showed that when food was resupplied for 3 days, the liver TCA cycle (Tricarboxylic acid cycle) and oxidative phosphorylation processes were upregulated, while DNA replication and repair, as well as proteasome assembly were also activated. Integration of transcriptome and metabolome data highlighted transcriptionally driven alterations in metabolism during compensatory growth, such as altered glycolysis and lipid metabolism activities. The metabolome data also implied the participation of amino acid metabolism during compensatory growth in zebrafish liver.

Conclusion

Our study provides a global resource for metabolic adaptations and their transcriptional regulation during refeeding in zebrafish liver. This study represents a first step towards understanding of the impact of metabolism on compensatory growth and will potentially aid in understanding the molecular mechanism associated with compensatory growth.

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.

Effects of fasting and refeeding on protein and glucose metabolism in Arctic charr

Refeeding, following a period of food deprivation will often lead to compensatory growth. Although many studies have focused on molecular mechanisms behind this accelerated growth response in fish, little is known on the roles of protein and metabolism. We also assessed, for the first time, the potential roles of miRNAs in regulating compensatory growth. Artcic charr, Salvelinus alpinus, a northern freshwater species, was deprived of food for 101 days and then fed to satiety for 126 days. The refeeding period resulted in compensatory growth, with a partial compensation of body mass. The feed deprivation period lead to a decrease in hepatosomatic index (HSI) and intestinal somatic index (ISI). HSI and ISI were then gradually replenished during early refeeding, following a lag phase prior to the compensatory growth response. mRNA transcripts regulating protein degradation via the autophagy pathway (Cathepsin D and Cathepsin L) in muscle were upregulated during feed restriction and downregulated after refeeding, which could allow for greater protein accretion in muscle, facilitating compensatory growth. Transcript levels from the ubiquitin proteasome pathway (Mafbx and Murf1) and the calpain system (Calpain 7 and Calpastatin) suggested that these pathways were not involved in regulating compensatory growth. Furthermore, we've shown that miRNAs (miR-29a and miR-223) could be involved in fish glycogen homeostasis during the early stages of refeeding. These findings provide a deeper understanding of the molecular mechanisms regulating growth in fish.

Compensatory Growth in Juveniles of Freshwater Redclaw Crayfish Cherax quadricarinatus Reared at Three Different Temperatures: Hyperphagia and Food Efficiency as Primary Mechanisms

Feeding restriction, as a trigger for compensatory growth, might be considered an alternative viable strategy for minimizing waste as well as production costs. The study assessed whether juvenile redclaw crayfish Cherax quadricarinatus (initial weight 0.99 ± 0.03 g) was able to compensate for feeding restriction at different temperatures (23 ± 1, 27 ± 1 and 31 ± 1 ° C). Hyperphagia, food utilization efficiency, energetic reserves, and hepatopancreas structure were analyzed. Three temperatures and two feeding regimes (DF-daily fed throughout the experiment and CF- 4 days food deprivation followed by 4 days of feeding, intermittently) were tested. The restriction period was from day 1 to 45, and the recovery period was from day 45 to 90. The previously restricted crayfish held at 23, 27, and 31 ± 1 ° C displayed complete body weight catch-up through compensatory growth following the restriction period with depressed growth. The mechanisms that might explain this response were higher feed intake (hyperphagia), and increased food utilization efficiency. Hepatopancreatic lipids were used as a metabolic fuel and hepatosomatic index was reduced in the previously restricted crayfish, but recovery at the same level of unrestricted crayfish occurred after the shift to daily feeding. The highest temperature affected adversely growth, feed intake, food efficiency, and metabolism of crayfish, whereas the lowest temperature and feeding restriction induced a more efficient growth of the crayfish.

Lrp13 is a novel vertebrate lipoprotein receptor that binds vitellogenins in teleost fishes

Transcripts encoding a novel member of the lipoprotein receptor superfamily, termed LDL receptor-related protein (Lrp)13, were sequenced from striped bass (Morone saxatilis) and white perch (Morone americana) ovaries. Receptor proteins were purified from perch ovary membranes by protein-affinity chromatography employing an immobilized mixture of vitellogenins Aa and Ab. RT-PCR revealed lrp13 to be predominantly expressed in striped bass ovary, and in situ hybridization detected lrp13 transcripts in the ooplasm of early secondary growth oocytes. Quantitative RT-PCR confirmed peak lrp13 expression in the ovary during early secondary growth. Quantitative mass spectrometry revealed peak Lrp13 protein levels in striped bass ovary during late-vitellogenesis, and immunohistochemistry localized Lrp13 to the oolemma and zona radiata of vitellogenic oocytes. Previously unreported orthologs of lrp13 were identified in genome sequences of fishes, chicken (Gallus gallus), mouse (Mus musculus), and dog (Canis lupus familiaris). Zebrafish (Danio rerio) and Nile tilapia (Oreochromis niloticus) lrp13 loci are discrete and share genomic synteny. The Lrp13 appears to function as a vitellogenin receptor and may be an important mediator of yolk formation in fishes and other oviparous vertebrates. The presence of lrp13 orthologs in mammals suggests that this lipoprotein receptor is widely distributed among vertebrates, where it may generally play a role in lipoprotein metabolism.

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.

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.

Circulating levels of plasma IGF-I during recovery from size-selective harvesting in Menidia menidia

Selection for growth-related traits in domesticated fishes often results in predictable changes within the growth hormone-insulin-like growth factor (GH-IGF-1) axis. Little is known about the mechanisms controlling changes in growth capacity resulting from fishery-induced evolution. We took advantage of a long-term study where Menidia menidia were selected for size at age over multiple generations to mimic fisheries-induced selection. This selection regime produced three populations with significant differences in intrinsic growth rate. These growth differences partially rebounded, but persisted even after selection was relaxed, resulting in fast, intermediate, and slow-growing lines. Plasma IGF-1 was measured in these populations as a potential target of selection on growth. IGF-1 was significantly correlated with current length and mass, and was positively correlated with growth rate (g d(-1)) in two lines, indicating it may be an appropriate indicator of growth capacity. The slow-growing line exhibited higher overall IGF-1 levels relative to the depressed IGF-1 seen in the fast-growing line, contrary to our prediction. We offer possible explanations for this unusual pattern and argue that somatic growth is likely to be under control of mechanism(s) downstream to IGF-1. IGF-1 provides an interesting basis for understanding endocrine control of growth in response to artificial selection and recovery.

Relationships between gill Na⁺,K⁺-ATPase activity and endocrine and local insulin-like growth factor-I levels during smoltification of masu salmon (Oncorhynchus masou)

We established profiles of insulin-like growth factor (IGF)-I mRNA in the liver, gill and white muscle and circulating IGF-I during smoltification of hatchery-reared masu salmon, and compared with that of gill Na(+),K(+)-ATPase (NKA) activity. Gill NKA activity peaked in May and dropped in June. Liver igf1 mRNA was high in March and decreased to low levels thereafter. Gill igf1 increased from March, maintained its high levels during April and May and decreased in June. Muscle igf1 mRNA levels were relatively high during January and April when water temperature was low. Serum IGF-I continuously increased from March through June. Serum IGF-I during March and May showed a positive correlation with NKA activity, although both were also related to fish size. These parameters were standardized with fork length and re-analyzed. As a result, serum IGF-I and gill igf1 were correlated with NKA activity. On the other hand, samples from desmoltification period (June) that had high serum IGF-I levels and low NKA activity disrupted the relationship. Expression of two IGF-I receptor (igf1r) subtypes in the gill decreased in June, which could account for the disruption by preventing circulating IGF-I from acting on the gill and retaining it in the blood. The present study suggests that the increase in gill NKA activity in the course of smoltification of masu salmon was supported by both endocrine and local IGF-I, and the decrease during desmoltification in freshwater was due at least in part to the down-regulation of gill IGF-I receptors.

Nutrition-regulated lipolysis in rainbow trout (Oncorhynchus mykiss) is associated with alterations in the ERK, PI3K-Akt, JAK-STAT, and PKC signaling pathways

Previous studies have shown that food deprivation, which occurs naturally in the life cycle of many species of fish, results in cessation of growth and catabolism of stored energy reserves, including lipids. In this study, we used rainbow trout (Oncorhynchus mykiss) to identify the cellular mechanisms involved with this metabolic shift. Fish were placed on one of five dietary regimes--fed continuously for 2 or 4 weeks, fasted continuously for 2 or 4 weeks, or fasted 2 weeks then refed 2 weeks--and the effects on organismal growth and lipid catabolism and on the activation state of signaling elements (e.g., Akt, ERK, JAK-STAT, PKC) in selected tissues were measured. Fasting for either 2 or 4 weeks significantly retarded growth in terms of body weight, body length, and body condition; refeeding restored growth such that body length and body condition were similar to measures seen in continuously fed fish. Fasting activated lipid catabolism by stimulating the mRNA expression and catalytic activity of hormone-sensitive lipase (HSL). Two HSL-encoding mRNAs have been characterized, and the expression of both forms of mRNA in 2- and 4-week fasted fish were significantly elevated over levels in fed fish in all tissues. In adipose tissue, liver, and white muscle, HSL activity was significantly elevated in 2- and 4-week fasted fish compared to fed animals; whereas in red muscle, HSL activity was significantly elevated compared to fed fish after 4 weeks of fasting. Refeeding reversed both fasting-associated HSL mRNA expression and HSL activity. Fasting resulted in the deactivation of Akt, JAK2, and STAT5 in adipose tissue, liver, and red and white muscle. By contrast, fasting activated ERK and PKC in all tissues measured. Refeeding reversed fasting-associated alterations in the activation state of all signal elements. These findings suggest that deactivation of Akt and JAK-STAT in conjunction with activation of ERK and PKC underlie fasting-associated growth retardation and lipolysis.

Yellowtail insulin-like growth factor 1: molecular cloning and response to various nutritional conditions

Insulin-like growth factor 1 (IGF1) plays an important role in fish growth. This study investigated the IGF1 response to various nutritional conditions in yellowtail. First, we cloned 1,075 bp of yellowtail IGF1 cDNA, which codes for a protein of 185 amino acids (aa). This is composed of 44 aa for the signal peptide; 68 aa for the mature peptide comprising the B, C, A, and D domains; and 73 aa for the E domain. The mature yellowtail IGF1 showed high identity to IGF1 of other teleosts. Insulin-like growth factor 1 mRNA expression in the liver and white muscle was measured to observe the IGF1 response to various nutritional conditions, because the liver has the highest IGF1 expression and white muscle comprises the largest fraction of the fish body. Only white muscle IGF1 mRNA expression decreased significantly by 3 wk of fasting and recovered by refeeding. In subsequent feeding ratio (1%, 2%, and 3%/BW/d) experiments, significant correlations to growth were observed in white muscle IGF1 mRNA expression at 2- and 6-wk points and in hepatic IGF1 mRNA expression at 4 wk point. These data suggest that IGF1 expression both in hepatic and white muscle is important for somatic growth in yellowtail. Furthermore, white muscle IGF1 mRNA expression showed better responses to somatic growth and nutrition status in our two experiments than hepatic IGF1 mRNA expression.

Normalizing for biology: accounting for technical and biological variation in levels of reference gene and insulin-like growth factor 1 (igf1) transcripts in fish livers

Feeding, fasting and re-feeding is a common experimental paradigm for studying growth endocrinology. Herein we demonstrate dynamic changes in the livers of coho salmon under these conditions and how changes in liver composition can influence quantification and interpretation of liver gene expression data. A three-week fast resulted in decreases in hepatosomatic index (liver size), liver glycogen content, and liver DNA concentration. In addition, significant differences were found in liver transcript levels from fed and fasted fish for the reference genes, arp and ef1a, when these were normalized to total RNA. We took the additional step of normalizing reference gene transcript levels to the liver homogenate RNA/DNA ratio to account for differences in RNA yield/cell and the number of cells sampled, normalizing to transcript number per cell rather than transcript number per unit RNA. After this additional step no significant differences in liver transcript levels of reference genes were found. The significance of these results was illustrated by normalizing liver transcript levels of insulin-like growth factor 1 (igf1) to ef1a transcript levels or ef1a transcript levels by RNA/DNA. The different normalization strategies resulted in differing patterns of change in igf1 transcript levels between fed and fasted fish. The novelty of this work rests in a two-step normalization process, attempting to account for both 1) technical errors in reverse transcription and qPCR reactions, and 2) biological variance in liver samples.

Early and sustained increase in the expression of hippocampal IGF-1, but not EPO, in a developmental rodent model of traumatic brain injury

Pediatric traumatic brain injury (pTBI) is the leading cause of traumatic death and disability in children in the United States. Impaired learning and memory in these young survivors imposes a heavy toll on society. In adult TBI (aTBI) models, cognitive outcome improved after administration of erythropoietin (EPO) or insulin-like growth factor-1 (IGF-1). Little is known about the production of these agents in the hippocampus, a brain region critical for learning and memory, after pTBI. Our objective was to describe hippocampal expression of EPO and IGF-1, together with their receptors (EPOR and IGF-1R, respectively), over time after pTBI in 17-day-old rats. We used the controlled cortical impact (CCI) model and measured hippocampal mRNA levels of EPO, IGF-1, EPOR, IGF-1R, and markers of caspase-dependent apoptosis (bcl2, bax, and p53) at post-injury days (PID) 1, 2, 3, 7, and 14. CCI rats performed poorly on Morris water maze testing of spatial working memory, a hippocampally-based cognitive function. Apoptotic markers were present early and persisted for the duration of the study. EPO in our pTBI model increased much later (PID7) than in aTBI models (12 h), while EPOR and IGF-1 increased at PID1 and PID2, respectively, similar to data from aTBI models. Our data indicate that EPO expression showed a delayed upregulation post-pTBI, while EPOR increased early. We speculate that administration of EPO in the first 1-2 days after pTBI would increase hippocampal neuronal survival and function.

Perspectives on concordant and discordant relations between insulin-like growth factor 1 (IGF1) and growth in fishes

Many physiological processes are modulated by the endocrine system, including growth. Insulin-like growth factor 1 is one of the primary hormones involved in growth regulation in vertebrates, including fishes. Current work on IGF1 in fishes is driven both by a desire to better understand mechanisms of growth as well as to develop a reliable index of growth rate. A review of studies relating IGF1 to growth broadly reveals positive and significant relations between IGF1 and growth; however, relations found in individual studies range from no correlation to highly significant correlations. Potential sources for this variation include both biological and methodological issues and range from differences in how growth is defined (changes in length or weight), the duration of growth assessed (weeks to months) and how growth is calculated (total change, rate, percent change); yet, these methodological concerns cannot account for all the variation found. A further review of the literature reveals a number of physiological conditions and environmental factors that might influence IGF1 level and the subsequent relation of that IGF1 level to growth rate. The term concordance is introduced to categorize factors that influence IGF1 and growth in a similar fashion, such that positive and significant relations between IGF1 and growth are maintained even though the factor stimulates changes in IGF1 level. Conversely, the term discordance is introduced to categorize factors that stimulate changes in the relations between IGF1 and growth, such that IGF1 is not an efficacious index of growth for both pre and post-stimulus fish combined. IGF1 and growth relations generally remain concordant after changes in nutrition (consumption rate or diet). Differences in IGF1 level of juvenile, maturing male and maturing female fish are common and IGF1-growth relations appear discordant between these groups. Acute changes in temperature and salinity induce discordant relations between IGF1 and growth but acclimation to persistent differences in environmental condition generally result in concordant relations. Overall, by discriminating between fish of differing physiological status and discerning and categorizing differences among environments one may effectively use IGF1 as a growth index for fishes.

Influences of the environment on the endocrine and paracrine fish growth hormone-insulin-like growth factor-I system

Insulin-like growth factor-I (IGF-I) is a key component of the complex system that regulates differentiation, development, growth and reproduction of fishes. The IGF-I gene is mainly expressed in the liver that represents the principal source of endocrine IGF-I but also in numerous other organs where the hormone most probably acts in an autocrine-paracrine manner. The primary stimulus for synthesis and release of IGF-I is growth hormone (GH) from the anterior pituitary. Thus, in analogy to mammals, it is usual to speak of a fish 'GH-IGF-I axis'. The GH-IGF-I system is affected by changes in the environment and probably represents a target of endocrine disrupting compounds (EDC) that impair many physiological processes in fishes. Thus, the review deals with the influences of changes in different environmental factors, such as food availability, temperature, photoperiod, season, salinity and EDCs, on GH gene expression in pituitary, IGF-I gene expression in liver and extrahepatic sites and the physiological effects resulting from the evoked alterations in endocrine and local IGF-I. Environmental influences certainly interact with each other but for convenience of the reader they will be dealt with in separate sections. Current trends in GH-IGF-I research are analysed and future focuses are suggested at the end of the sections.

Effects of stocking density and feed ration on growth and gene expression in the Senegalese sole (Solea senegalensis): potential effects on the immune response

Stocking density and ration size are two major factors influencing aquaculture production. To evaluate their effects on growth and immune system in Senegalese sole (Solea senegalensis) juveniles, a 2 x 2 experimental design using two rations (1.0% and 0.25% of the total fish biomass) and two different initial stocking densities (7 and 30 kg m(-2)) was performed throughout a 60 days culture period. Soles fed 1.0% showed a higher specific growth rate (SGR) than those fed 0.25% (3.3-fold). No differences in SGR at 60 days were found between densities in spite of reduced values were detected at high density after 20 days (soles fed 0.25%) and 40 days (soles fed 1%) suggesting a compensatory growth. Physiologically, plasma cortisol levels were elevated in soles at high density (45-fold higher than at 7 kg m(-2)) whereas no differences associated to the feeding ration were observed. To assess the effects at a molecular level, the mRNA levels of genes involved in cellular stress (heat shock proteins HSP70 and HSP90), growth (insulin-like growth factors IGF-I, the spliced variants IGF-Ia and IGFI-b, and IGF-II) and innate immune system (g-type lysozyme and hepcidin (HAMP1)) were quantified. No differences in HSP90 expression were detected between densities or rations. In contrast, IGF-I, IGF-Ia and IGF-II showed reduced transcript levels in liver and HSP70 in liver and kidney at high density. Finally, g-type lysozyme and HAMP1 expression was greatly affected by both factors exhibiting an important reduction in the transcript levels at high density and low ration. Overall, our results show that S. senegalensis juveniles might exhibit satisfactory SGR at high density although the high plasma cortisol levels indicate a crowding stress that could negatively affect the expression levels of some of the genes studied.

Plasma ghrelin and growth hormone regulation in response to metabolic state in hybrid striped bass: effects of feeding, ghrelin and insulin-like growth factor-I on in vivo and in vitro GH secretion

The regulation of growth hormone (GH) secretion by ghrelin during variable metabolic states is poorly understood. We examined plasma GH and ghrelin in hybrid striped bass (HSB) undergoing seasonally-based feeding and temperature manipulations. Fasting for 21 days (d) at 24 degrees C resulted in catabolism and up-regulation of plasma GH and ghrelin relative to fed controls. Continued fasting during cold-banking (14 degrees C, 90 d) resulted in a further 43-fold increase in ghrelin while GH remained elevated. A subsequent 19 day refeeding period at 24 degrees C elicited hyperphagic and compensatory growth responses, accompanied by declines in ghrelin and GH. We then tested the role of ghrelin in stimulating GH release in vivo and in vitro. Intraperitoneal injections of ghrelin resulted in dose-dependent increases in plasma GH after 6 hours (h). Ghrelin also increased GH release from HSB pituitaries during 6h incubations. Lastly, we assessed how metabolic state, ghrelin and insulin-like growth factor-I (IGF-I) affect in vitro pituitary GH release. Spontaneous GH release was 5.2-fold higher from pituitaries of fasted compared with fed animals. Ghrelin was equally effective in stimulating GH release from pituitaries of fed and starved animals, while it was ineffective in enhancing GH release from pituitaries of starved (21 d) then refed (4d) HSB. Incubation with IGF-I inhibited GH release regardless of metabolic state. These studies are the first to show that seasonally-based periods of feed deprivation and low temperature yield sustained increases in GH secretion that are likely mediated, at least partially, through elevated ghrelin, reduced IGF-I negative feedback and fasting-induced spontaneous GH release.

Cloning, tissue distribution and effects of food deprivation on pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) in Atlantic cod (Gadus morhua)

Full-length complementary deoxyribonucleic acid sequences encoding pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) were cloned from Atlantic cod (Gadus morhua) hypothalamus using reverse transcription and rapid amplification of complementary deoxyribonucleic acid ends. Semi-quantitative reverse transcriptase polymerase chain reaction shows that PRP/PACAP mRNA and PPSS 1 mRNA are widely distributed throughout cod brain. During development, PRP/PACAP and PPSS 1 were detected at the 30-somite stage and pre-hatching stage, respectively, and expression levels gradually increased up to the hatched larvae. PPSS 1, but not PRP/PACAP, appeared to be affected by food availability during early development. In juvenile cod, PPSS 1 expression levels increased and remained significantly higher than that of control fed fish throughout 30 days of starvation and during a subsequent 10 days refeeding period. In contrast, PRP/PACAP expression levels were not affected by 30 days of food deprivation, but a significant increase in expression levels was observed during the 10 days refeeding period in the experimental food-deprived group as compared to the control fed group. Our results suggest that PRP/PACAP and PPSS 1 may be involved in the complex regulation of growth, feeding and metabolism during food deprivation and refeeding in Atlantic cod.

Insulin-like growth factor-I of pejerrey, Odontesthes bonariensis: cDNA characterization, tissue distribution and expression profiles after growth hormone administration

The liver production of the insulin-like growth factor-I (IGF-I) is a key factor in the endocrine control of body growth by a growth hormone. As pejerrey Odontesthes bonariensis has been reported as a fish with low growth rates in captivity, basic research on this respect is needed in order to understand it. In this context, the pejerrey IGF-I cDNA was cloned and its hepatic expression was examined in fish after recombinant pejerrey growth hormone (pjGHr) administration. The full length of IGF-I transcript showed a high sequence similarity to other teleost sequences. The tissue distribution analysis by reverse transcriptase polymerase chain reaction in adult fish revealed that IGF-I expressed ubiquitously with the highest mRNA levels in the liver, posterior intestine and brain. No alternative IGF-I mRNA was found in the liver, as it was reported for other teleosts. IGF-I transcript was measured in the liver after pjGHr in vivo stimulation by means of quantitative real-time polymerase chain reaction assays. A dose-dependent response of IGF-I mRNA was observed after pjGHr administration, and reached a six-fold IGF-I maximum increase over control group when 2.5 microg pjGH/g-body weight (bw) was injected. Temporal analysis of hepatic IGF-I mRNA level showed that administration of a single dose of pjGHr into juvenile pejerrey resulted in a significant increase (P <0.02) 9 hours post-injection (hpi). These results add novel information on the nucleotide sequence of IGF-I in Atheriniformes and demonstrate that pjGHr could promote a dramatic response in liver, increasing the IGF-I mRNA level.

PAX3/7 expression coincides with MyoD during chronic skeletal muscle overload

Paired box (Pax) proteins 3 and 7 are key determinants for embryonic skeletal muscle development by initiating myogenic regulatory factor (MRF) gene expression. We show that Pax3 and 7 participate in adult skeletal muscle plasticity during the initial responses to chronic overload (< or =7 days) and appear to coordinate MyoD expression, a member of the MRF family of genes. Pax3 and 7 mRNA were higher than control within 12 h after initiation of overload, preceded the increase in MyoD mRNA on day 1, and peaked on day 2. On days 3 and 7, Pax7 mRNA remained higher than control, suggesting that satellite cell self-renewal was occurring. Pax3 and 7 and MyoD protein levels were higher than control on days 2 and 3. These data indicate that Pax3 and 7 coordinate the recapitulation of developmental-like regulatory mechanisms in response to growth-inducing stimuli in adult skeletal muscle, presumably through activation of satellite cells.

Salinity regulates claudin mRNA and protein expression in the teleost gill

The teleost gill carries out NaCl uptake in freshwater (FW) and NaCl excretion in seawater (SW). This transformation with salinity requires close regulation of ion transporter capacity and epithelial permeability. This study investigates the regulation of tight-junctional claudins during salinity acclimation in fish. We identified claudin 3- and claudin 4-like immunoreactive proteins and examined their expression and that of select ion transporters by performing Western blot in tilapia (Oreochromis mossambicus) gill during FW and SW acclimation. Transfer of FW tilapia to SW increased plasma osmolality, which was corrected after 4 days, coinciding with increased gill Na+-K+-ATPase and Na+-K+-2Cl(-) cotransporter expression. Gill claudin 3- and claudin 4-like proteins were reduced with exposure to SW. Transfer to FW increased both claudin-like proteins. Immunohistochemistry shows that claudin 3-like protein was localized deep in the FW gill filament, whereas staining was found apically in SW gill. Claudin 4-like proteins are localized predominantly in the filament outer epithelial layer, and staining appears more intense in the gill of FW versus SW fish. In addition, tilapia claudin 28a and 30 genes were characterized, and mRNA expression was found to increase during FW acclimation. These studies are the first to detect putative claudin proteins in teleosts and show their localization and regulation with salinity in gill epithelium. The data indicate that claudins may be important in permeability changes associated with salinity acclimation and possibly the formation of deeper tight junctions in FW gill. This may reduce ion permeability, which is a critical facet of FW osmoregulation.

Prolactin receptor, growth hormone receptor, and putative somatolactin receptor in Mozambique tilapia: tissue specific expression and differential regulation by salinity and fasting

In fish, pituitary growth hormone family peptide hormones (growth hormone, GH; prolactin, PRL; somatolactin, SL) regulate essential physiological functions including osmoregulation, growth, and metabolism. Teleost GH family hormones have both differential and overlapping effects, which are mediated by plasma membrane receptors. A PRL receptor (PRLR) and two putative GH receptors (GHR1 and GHR2) have been identified in several teleost species. Recent phylogenetic analyses and binding studies suggest that GHR1 is a receptor for SL. However, no studies have compared the tissue distribution and physiological regulation of all three receptors. We sequenced GHR2 from the liver of the Mozambique tilapia (Oreochromis mossambicus), developed quantitative real-time PCR assays for the three receptors, and assessed their tissue distribution and regulation by salinity and fasting. PRLR was highly expressed in the gill, kidney, and intestine, consistent with the osmoregulatory functions of PRL. PRLR expression was very low in the liver. GHR2 was most highly expressed in the muscle, followed by heart, testis, and liver, consistent with this being a GH receptor with functions in growth and metabolism. GHR1 was most highly expressed in fat, liver, and muscle, suggesting a metabolic function. GHR1 expression was also high in skin, consistent with a function of SL in chromatophore regulation. These findings support the hypothesis that GHR1 is a receptor for SL. In a comparison of freshwater (FW)- and seawater (SW)-adapted tilapia, plasma PRL was strongly elevated in FW, whereas plasma GH was slightly elevated in SW. PRLR expression was reduced in the gill in SW, consistent with PRL's function in freshwater adaptation. GHR2 was elevated in the kidney in FW, and correlated negatively with plasma GH, whereas GHR1 was elevated in the gill in SW. Plasma IGF-I, but not GH, was reduced by 4 weeks of fasting. Transcript levels of GHR1 and GHR2 were elevated by fasting in the muscle. However, liver levels of GHR1 and GHR2 transcripts, and liver and muscle levels of IGF-I transcripts were unaffected by fasting. These results clearly indicate tissue specific expression and differential physiological regulation of GH family receptors in the tilapia.

Resolving the growth-promoting and metabolic effects of growth hormone: Differential regulation of GH-IGF-I system components

Growth hormone regulates numerous processes in vertebrates including growth promotion and lipid mobilization. During periods of food deprivation, growth is arrested yet lipid depletion is promoted. In this study, we used rainbow trout on different nutritional regimens to examine the regulation of growth hormone (GH)-insulin-like growth factor-I (IGF-I) system elements in order to resolve the growth-promoting and lipid catabolic actions of GH. Fish fasted for 2 or 6 weeks displayed significantly reduced growth compared to their fed counterparts despite elevated plasma GH, while refeeding for 2 weeks following 4 weeks of fasting partially restored growth and lowered plasma GH. Fish fasted for 6 weeks also exhausted their mesenteric adipose tissue reserves. Sensitivity to GH in the liver was reduced in fasting fish as evidenced by reduced expression of GH receptor type 1 (GHR 1) and GHR 2 mRNAs and by reduced (125)I-GH binding capacity. Expression of GHR 1 and GHR 2 mRNAs also was reduced in the gill of fasted fish. In adipose tissue, however, sensitivity to GH, as indicated by GHR 1 expression and by (125)I-GH binding capacity, increased after 6 weeks of fasting in concert with the observed lipid depletion. Fasting-associated growth retardation was accompanied by reduced expression of total IGF-I mRNA in the liver, adipose and gill, and by reduced plasma levels of IGF-I. Sensitivity to IGF-I was reduced in the gill of fasted fish as indicated by reduced expression of type 1 IGF-I receptor (IGFR 1A and IGFR 1B) mRNAs. By contrast, fasting did not affect expression of IGFR 1 mRNAs or (125)I-IGF-I binding in skeletal muscle and increased expression of IGFR 1 mRNAs and (125)I-IGF-I binding in cardiac muscle. These results indicate that nutritional state differentially regulates GH-IGF-I system components in a tissue-specific manner and that such alterations disable the growth-promoting actions of GH and promote the lipid-mobilizing actions of the hormone.

IGF-I and branchial IGF receptor expression and localization during salinity acclimation in striped bass

The initial response of the IGF-I system and the expression and cellular localization of IGF type-I receptor (IGF-IR) were studied in the gill of a euryhaline teleost during salinity acclimation. Exposure of striped bass ( Morone saxatilis) to hyperosmotic and hypoosmotic challenges induced small, transitory (<24 h) deflections in hydromineral balance. Transfer from freshwater (FW) to seawater (SW) induced an initial decrease in plasma IGF-I levels after 24 h in both fed and fasted fish. There was an overall decrease in liver IGF-I mRNA levels after SW transfer, suggesting that decreased plasma levels may be due to a decline in hepatic IGF-I synthesis. No changes were observed in gill IGF-I mRNA, but SW transfer induced an increase in gill IGF-IR mRNA after 24 h. Transfer from SW to FW induced an increase in plasma IGF-I levels in fasted fish. In fed fish, no significant changes were observed in either plasma IGF-I, liver, or gill IGF-I mRNA, or gill IGF-IR mRNA levels. In a separate experiment, FW-acclimated fish were injected with saline or IGF-I prior to a 24-h SW challenge. Rapid regain of osmotic balance following SW transfer was hindered by IGF-I. Immunohistochemistry revealed for the first time in teleosts that IGF-IR and Na+-K+-ATPase are localized in putative chloride cells at the base of the lamellae, identifying these cells in the gill as a target for IGF-I and IGF-II. Overall the data suggest a hyperosmoregulatory role of IGF-I in this species.

Temperature affects insulin-like growth factor I and growth of juvenile southern flounder, Paralichthys lethostigma

Temperature profoundly influences growth of heterothermic vertebrates. However, few studies have investigated the effects of temperature on growth and insulin-like growth factor I (IGF-I) in fishes. The aim of this study was to examine effects of temperature on growth and establish whether IGF-I may mediate growth at different temperatures in southern flounder, Paralichthys lethostigma. In two experiments, juvenile flounder were reared at 23 and 28 degrees C and growth was monitored for either 117 or 197 days. Growth was similar across treatments in both experiments until fish reached approximately 100 mm total length. Body size then diverged with fish at 23 degrees C ultimately growing 65-83% larger than those at 28 degrees C. Muscle IGF-I mRNA, plasma IGF-I, and hepatosomatic index (HSI) were significantly higher in flounder at 23 degrees C, whereas hepatic IGF-I mRNA abundance did not differ with treatment. Muscle IGF-I mRNA was correlated with HSI, while plasma IGF-I was correlated with body size, hepatic IGF-I mRNA, and HSI. These results demonstrate a strong effect of temperature on flounder growth and show that temperature-induced variation in growth is associated with differences in systemic IGF-I and local (i.e., muscle) IGF-I mRNA levels. The results also support the use of plasma IGF-I and HSI as indicators of flounder growth status.