Molecular and functional characterization of two distinct IGF binding protein-6 genes in zebrafish

Gene expression analyses of GH/IGF axis in triploid crucian carp with growth heterosis

Hybridization and polyploid breeding are the main approaches used to obtain new aquaculture varieties. Allotriploid crucian carp (3n) with rapid growth performance was generated by mating red crucian carp (RCC) with allotetraploids (4n). Fish growth is controlled by the growth hormone (GH)/insulin-like growth factor (IGF) axis. In the present study, we examined the expression characteristics of GH/IGF axis genes in hybrids F1, 4n, 3n, RCC and common carp (CC). The results showed that GHRa, GHRb, IGF1, IGF2, and IGF-1Ra were highly expressed in 3n compared with RCC and CC, whereas IGF3 was undetectable in the liver in RCC, CC and 3n. GHRa and GHRb had low expression in the 4n group. In hybrid F1, GHRa expression was low, whereas GHRb was highly expressed compared to the levels in RCC and CC. Moreover, in hybrid F1, the expression of IGF3 was higher, and the expression of IGF1 and IGF2 was lower than that in the RCC and CC, whereas the expression of IGF-1Ra was similar to that in RCC and CC. For the IGFBP genes, IGFBP1 had higher expression in 3n compared than that in RCC and CC, while other IGFBP genes were not high expressed in 3n. Among the genes detected in this study, 11 genes were nonadditively expressed in 3n, with 5 genes in the transgressive upregulation model. We proposed that the 11 nonadditive expression of GH/IGF axis genes is related to growth heterosis in 3n. This evidence provides new insights into hybridization and polyploid breeding from the perspective of hormone regulation.

Molecular cloning and expression profiling of insulin-like growth factor 2 and IGF-binding protein 6 in Clarias magur (Hamilton 1822)

Growth is an important trait in aquaculture and the major genes that regulate it are Insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs). In this study, the full-length coding sequences of IGF2 and IGFBP6 genes in the Indian catfish Clarias magur were cloned and characterized. The full-length cDNA sequences of IGF2 and IGFBP6 were 885 bp (ORF 642 bp) and 928 bp (ORF 600 bp), encoding 213 and 199 amino acids, respectively. Bioinformatics analyses revealed that the magur IGF2 and IGFBP6 proteins are hydrophilic and secretory in nature. Sequence alignment with other teleosts and mammalian orthologues shows conservation of the functional domains. Gene expression analysis in 6 individuals each of high (298 ± 5.0 g) and low (210 ± 6.0 g) growth performing families showed significantly (p < 0.05) higher expression (2.5-3 fold) of IGF2, and lower expression (∼2.5 fold) of IGFBP6 in liver and muscle of fast-growing fish. This study suggests that IGF2 could be playing a major role in the growth regulation of magur. These genes and their expression patterns could be developed into growth-associated markers for magur and other catfishes.

Stanniocalcin 1a regulates organismal calcium balance and survival by suppressing Trpv6 expression and inhibiting IGF signaling in zebrafish

Stanniocalcin 1 (Stc1) is well known for its role in regulating calcium uptake in fish by acting on ionocytes or NaR cells. A hallmark of NaR cells is the expression of Trpv6, a constitutively open calcium channel. Recent studies in zebrafish suggest that genetical deletion of Stc1a and Trpv6 individually both increases IGF signaling and NaR cell proliferation. While trpv6-/- fish suffered from calcium deficiency and died prematurely, stc1a-/- fish had elevated body calcium levels but also died prematurely. The relationship between Stc1a, Trpv6, and IGF signaling in regulating calcium homeostasis and organismal survival is unclear. Here we report that loss of Stc1a increases Trpv6 expression in NaR cells in an IGF signaling-dependent manner. Treatment with CdCl2, a Trpv6 inhibitor, reduced NaR cell number in stc1a -/- fish to the sibling levels. Genetic and biochemical analysis results suggest that Stc1a and Trpv6 regulate NaR cell proliferation via the same IGF pathway. Alizarin red staining detected abnormal calcium deposits in the yolk sac region and kidney stone-like structures in stc1a -/- fish. Double knockout or pharmacological inhibition of Trpv6 alleviated these phenotypes, suggesting that Stc1a inhibit epithelial Ca2+ uptake by regulating Trpv6 expression and activity. stc1a-/- mutant fish developed cardiac edema, body swelling, and died prematurely. Treatment of stc1a-/- fish with CdCl2 or double knockout of Trpv6 alleviated these phenotypes. These results provide evidence that Stc1a regulates calcium homeostasis and organismal survival by suppressing Trpv6 expression and inhibiting IGF signaling in ionocytes.

IPPD-induced growth inhibition and its mechanism in zebrafish

N-isopropyl-N-phenyl-1,4-phenylenediamine (IPPD) is used as a ubiquitous antioxidant worldwide, it is an additive in tire rubber easily discharged into the surrounding environment. At present, there is no study concerning the subacute toxicity of IPPD on fish. We used zebrafish embryos (2 h post-fertilization) exposed to IPPD for 5 days at concentrations of 0, 0.0012, 0.0120 and 0.1200 mg/L to investigate its toxic effects of embryonic development, disruption of growth hormone/insulin-like growth factor (GH/IGF) and hypothalamic-pituitary-thyroid (HPT) axis. The results showed that IPPD exposure decreased hatchability, weakened movement ability, reduced body length, and caused multiple types of deformities in zebrafish embryos. The expression of genes involved to GH/IGF and HPT axis were altered after exposure to IPPD in zebrafish larvae. Meanwhile, exposure to IPPD significantly decreased thyroxine (T4) and 3,5,3'-triiodothyronine (T3) contents in larvae, which indicated that HPT axis was in a disturbed state. Moreover, treatment of IPPD decreased the enzymatic activities of superoxide dismutase (SOD) and catalase (CAT) as well as levels of glutathione (GSH). While the contents of malondialdehyde (MDA) were elevated after exposure to IPPD. The present study thus demonstrated that IPPD induced oxidative stress, caused developmental toxicity and disrupted the GH/IGF and HPT axis of zebrafish, which could be responsible for developmental impairment and growth inhibition.

Transcriptomes of testis and pituitary from male Nile tilapia (O. niloticus L.) in the context of social status

African cichlids are well established models for studying social hierarchies in teleosts and elucidating the effects social dominance has on gene expression. Ascension in the social hierarchy has been found to increase plasma levels of steroid hormones, follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) as well as gonadosomatic index (GSI). Furthermore, the expression of genes related to gonadotropins and steroidogenesis and signaling along the brain-pituitary-gonad axis (BPG-axis) is affected by changes of an animal’s social status. In this study, we use RNA-sequencing to obtain an in-depth look at the transcriptomes of testes and pituitaries from dominant and subordinate male Nile tilapia living in long-term stable social hierarchies. This allows us to draw conclusions about factors along the brain-pituitary-gonad axis that are involved in maintaining dominance over weeks or even months. We identify a number of genes that are differentially regulated between dominant and subordinate males and show that in high-ranking fish this subset of genes is generally upregulated. Genes differentially expressed between the two social groups comprise growth factors, related binding proteins and receptors, components of Wnt-, Tgfβ- and retinoic acid-signaling pathway, gonadotropin signaling and steroidogenesis pathways. The latter is backed up by elevated levels of 11-ketotestosterone, testosterone and estradiol in dominant males. Luteinizing hormone (Lh) is found in higher concentration in the plasma of long-term dominant males than in subordinate animals. Our results both strengthen the existing models and propose new candidates for functional studies to expand our understanding of social phenomena in teleost fish.

Alteration of organ size and allometric scaling by organ-specific targeting of IGF signaling

The size of an organ is proportional to the other body parts or the whole body. This relationship is known as allometry. Understanding how allometry is determined is a fundamental question in biology. Here we tested the hypothesis that local insulin-like growth factor (Igf) signaling is critical in regulating organ size and its allometric scaling by organ-specific expression of Igf binding protein (Igfbp). Overexpression of Igfbp2a or 5b in the developing zebrafish eye, heart, and inner ear resulted in a disproportional reduction in their growth relative to the body. Stable transgenic zebrafish with lens-specific Igfbp5b expression selectively reduced adult eye size. The action is Igf-dependent because an Igf-binding deficient Igfbp5b mutant had no effect. Targeted expression of a dominant-negative Igf1 receptor (dnIgf1r) in the lens caused a similar reduction in relative eye growth. Furthermore, co-expression of IGF-1 with an Igfbp restored the eye size. Finally, co-expression of a constitutively active form of Akt with Igfbp or dnIgf1r restored the relative eye growth. These data suggest that local Igf availability and Igf signaling activity are critical determinants of organ size and allometric scaling in zebrafish.

Production of two recombinant insulin-like growth factor binding protein-1 subtypes specific to salmonids

Salmonids have four subtypes of insulin-like growth factor binding protein (IGFBP)-1, termed -1a1, -1a2, -1b1 and 1b2, owing to teleost- and a lineage-specific whole-genome duplications. We have previously produced recombinant proteins of masu salmon IGFBP-1a1 and -1b2 and conducted functional analysis. To further characterize salmonid-specific IGFBP-1s, we cloned cDNAs encoding mature proteins of IGFBP-1a2 and -1b1 from the liver of masu salmon (Oncorhynchus masou). IGFBP-1a2 and -1b1 shared a 56% amino acid sequence homology whereas their homologies with their counterparts (i.e. -1a1 and -1b2) were 77% and 82%, respectively. We next expressed recombinant masu salmon (rs) IGFBP-1a2 and -1b1 with fusion partners thioredoxin (Trx) and a His-tag using the pET-32a(+) vector system in Escherichia coli. Trx.His.rsIGFBP-1s were detected in the insoluble faction, solubilized in a buffer containing urea, and isolated by Ni-affinity chromatography. They were refolded by dialysis and cleaved from the fusion partners by enterokinase. rsIGFBP-1a2 and -1b1 were purified by reversed-phase high performance liquid chromatography. Purified rsIGFBP-1a2 and -1b1 had the ability to bind digoxigenin-labeled human IGF-I on ligand blotting. We then examined the effects of rsIGFBP-1a1, -1a2, -1b1 and -1b2 in combination with human IGF-I on growth hormone (GH) release from cultured pituitary cells of masu salmon. IGF-I alone reduced GH release while the addition of rsIGFBP-1a1, -1b1 or -1b2, but not rsIGFBP-1a2, diminished the suppressive effect of IGF-I. Addition of rsIGFBP-1s without IGF-I had no effect on GH release. These results show that rsIGFBP-1b1, along with rsIGFBP-1a1 and -1b2, inhibits IGF-I action on the pituitary in masu salmon. The lack of the effect by rsIGFBP-1a2 suggests that salmon IGFBP-1 subtypes underwent subfunction partitioning and have different degrees of IGF-inhibitory action.

Expression of acid-labile subunit (ALS) in developing and adult zebrafish and its role in dorso-ventral patterning during development

Mammalian acid-labile subunit (ALS) is a serum protein that binds binary complexes between Insulin-like growth factors (IGFs) and Insulin-like growth factor-binding proteins (IGFBPs) extending their half-life and keeping them in the vasculature. Human ALS deficiency (ACLSD), due to homozygous or compound heterozygous mutations in IGFALS, leads to moderate short stature with reduced levels of IGF-I and IGFBP-3. There is only one corresponding zebrafish ortholog gene and it has not yet been studied. In this study we elucidate the role of igfals during zebrafish development. In zebrafish embryos igfals mRNA is expressed throughout development, mainly in the brain and subsequently also in the gut and swimbladder. To determine its role during development, we knocked down igfals gene product using morpholinos (MOs). Igfals morphant embryos displayed dorsalization in different degrees of severity, including a shortened trunk and loss of tail. Furthermore, co-injection of human IGFALS (hIGFALS) mRNA was able to rescue the MO-induced phenotype. Finally, overexpression of either hIGFALS or zebrafish igfals (zigfals) mRNA leads to ventralization of embryos including a reduced head and enlarged tail. These findings suggest that als plays an important role in dorso-ventral patterning during zebrafish development.

Oxygen and embryonic growth: the role of insulin-like growth factor signaling

Oxygen is indispensable for the efficient release of chemical energy from nutrient molecules in cells. Therefore, the local oxygen tension is one of the most critical factors affecting physiological processes. In most viviparous species, many pathological conditions result in abnormal oxygen tension in the uterus, which modifies the growth and development of the fetus. Insulin-like growth factor (IGF/Igf) is one of the most important hormones for the regulation of somatic growth in animals. Changes in oxygen levels modulate the activity of the IGF/Igf signaling system, which in turn regulates the embryonic growth rate. In general, there are serious difficulties associated with monitoring and studying rodent embryos in utero. The zebrafish is a convenient experimental model to study the relationship between embryonic growth and environmental conditions. Most importantly, the fish model makes it possible to rapidly evaluate embryonic growth and development under entirely controlled environments without interfering with the mother organism. In this review, firstly an overview is given of the fluctuation of environmental oxygen, the IGF-system, and the advantages of the zebrafish model for studying embryonic growth. Then, the relationships of dynamic environmental oxygen and embryonic growth rate are outlined with a specific focus on the changes in the IGF/Igf-system in the zebrafish model. This review will shed light on the fine-tuning mechanisms of the embryonic IGF/Igf-system under different oxygen levels, including constant normoxia, hypoxia, and re-oxygenation.

The metalloproteinase Papp-aa controls epithelial cell quiescence-proliferation transition

Human patients carrying PAPP-A2 inactivating mutations have low bone mineral density. The underlying mechanisms for this reduced calcification are poorly understood. Using a zebrafish model, we report that Papp-aa regulates bone calcification by promoting Ca2+-transporting epithelial cell (ionocyte) quiescence-proliferation transition. Ionocytes, which are normally quiescent, re-enter the cell cycle under low [Ca2+] stress. Genetic deletion of Papp-aa, but not the closely related Papp-ab, abolished ionocyte proliferation and reduced calcified bone mass. Loss of Papp-aa expression or activity resulted in diminished IGF1 receptor-Akt-Tor signaling in ionocytes. Under low Ca2+ stress, Papp-aa cleaved Igfbp5a. Under normal conditions, however, Papp-aa proteinase activity was suppressed and IGFs were sequestered in the IGF/Igfbp complex. Pharmacological disruption of the IGF/Igfbp complex or adding free IGF1 activated IGF signaling and promoted ionocyte proliferation. These findings suggest that Papp-aa-mediated local Igfbp5a cleavage functions as a [Ca2+]-regulated molecular switch linking IGF signaling to bone calcification by stimulating epithelial cell quiescence-proliferation transition under low Ca2+ stress.

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.

In vivo effects of 17α-ethinylestradiol, 17β-estradiol and 4-nonylphenol on insulin-like growth-factor binding proteins (igfbps) in Atlantic salmon

Feminizing endocrine disrupting compounds (EDCs) affect the growth and development of teleost fishes. The major regulator of growth performance, the growth hormone (Gh)/insulin-like growth-factor (Igf) system, is sensitive to estrogenic compounds and mediates certain physiological and potentially behavioral consequences of EDC exposure. Igf binding proteins (Igfbps) are key modulators of Igf activity, but their alteration by EDCs has not been examined. We investigated two life-stages (fry and smolts) of Atlantic salmon (Salmo salar), and characterized how the Gh/Igf/Igfbp system responded to waterborne 17α-ethinylestradiol (EE₂), 17β-estradiol (E₂) and 4-nonylphenol (NP). Fry exposed to EE₂ and NP for 21 days had increased hepatic vitellogenin (vtg) mRNA levels while hepatic estrogen receptor α (erα), gh receptor (ghr), igf1 and igf2 mRNA levels were decreased. NP-exposed fry had reduced body mass and total length compared to controls. EE₂ and NP reduced hepatic igfbp1b1, -2a, -2b1, -4, -5b2 and -6b1, and stimulated igfbp5a. In smolts, hepatic vtg mRNA levels were induced following 4-day exposures to all three EDCs, while erα only responded to EE₂ and E₂. EDC exposures did not affect body mass or fork length; however, EE₂ diminished plasma Gh and Igf1 levels in parallel with reductions in hepatic ghr and igf1. In smolts, EE₂ and E₂ diminished hepatic igfbp1b1, -4 and -6b1, and stimulated igfbp5a. There were no signs of compromised ionoregulation in smolts, as indicated by unchanged branchial ion pump/transporter mRNA levels. We conclude that hepatic igfbps respond (directly and/or indirectly) to environmental estrogens during two key life-stages of Atlantic salmon, and thus may modulate the growth and development of exposed individuals.

Somatotropic Axis Regulation Unravels the Differential Effects of Nutritional and Environmental Factors in Growth Performance of Marine Farmed Fishes

The Gh/Prl/Sl family has evolved differentially through evolution, resulting in varying relationships between the somatotropic axis and growth rates within and across fish species. This is due to a wide range of endogenous and exogenous factors that make this association variable throughout season and life cycle, and the present minireview aims to better define the nutritional and environmental regulation of the endocrine growth cascade over precisely defined groups of fishes, focusing on Mediterranean farmed fishes. As a result, circulating Gh and Igf-i are revitalized as reliable growth markers, with a close association with growth rates of gilthead sea bream juveniles with deficiency signs in both macro- or micro-nutrients. This, together with other regulated responses, promotes the use of Gh and Igf-i as key performance indicators of growth, aerobic scope, and nutritional condition in gilthead sea bream. Moreover, the sirtuin-energy sensors might modulate the growth-promoting action of somatotropic axis. In this scenario, transcripts of igf-i and gh receptors mirror changes in plasma Gh and Igf-i levels, with the ghr-i/ghr-ii expression ratio mostly unaltered over season. However, this ratio is nutritionally regulated, and enriched plant-based diets or diets with specific nutrient deficiencies downregulate hepatic ghr-i, decreasing the ghr-i/ghr-ii ratio. The same trend, due to a ghr-ii increase, is found in skeletal muscle, whereas impaired growth during overwintering is related to increase in the ghr-i/ghr-ii and igf-ii/igf-i ratios in liver and skeletal muscle, respectively. Overall, expression of insulin receptors and igf receptors is less regulated, though the expression quotient is especially high in the liver and muscle of sea bream. Nutritional and environmental regulation of the full Igf binding protein 1-6 repertoire remains to be understood. However, tissue-specific expression profiling highlights an enhanced and nutritionally regulated expression of the igfbp-1/-2/-4 clade in liver, whereas the igfbp-3/-5/-6 clade is overexpressed and regulated in skeletal muscle. The somatotropic axis is, therefore, highly informative of a wide-range of growth-disturbing and stressful stimuli, and multivariate analysis supports its use as a reliable toolset for the assessment of growth potentiality and nutrient deficiencies and requirements, especially in combination with selected panels of other nutritionally regulated metabolic biomarkers.

Insulin-Like Growth Factor-Binding Proteins of Teleost Fishes

The insulin-like growth factor (Igf) binding protein (Igfbp) family has a broad range of physiological functions and a fascinating evolutionary history. This review focuses on the Igfbps of teleost fishes, where genome duplication events have diversified gene repertoire, function, and physiological regulation-with six core Igfbps expanded into a family of over twenty genes in some lineages. In addition to briefly summarizing the current state of knowledge on teleost Igfbp evolution, function, and expression-level regulation, we highlight gaps in our understanding and promising areas for future work.

IGF-Binding Proteins: Why Do They Exist and Why Are There So Many?

Insulin-like growth factors (IGFs) are key growth-promoting peptides that act as both endocrine hormones and autocrine/paracrine growth factors. In the bloodstream and in local tissues, most IGF molecules are bound by one of the members of the IGF-binding protein (IGFBP) family, of which six distinct types exist. These proteins bind to IGF with an equal or greater affinity than the IGF1 receptor and are thus in a key position to regulate IGF signaling globally and locally. Binding to an IGFBP increases the half-life of IGF in the circulation and blocks its potential binding to the insulin receptor. In addition to these classical roles, IGFBPs have been shown to modulate IGF signaling locally under various conditions. Although members of the IGFBP family share significant sequence homology, they each have unique structural features and play distinct roles. These IGFBP genes also have different modes of regulation and distinct expression patterns. Some IGFBPs have been found to bind to their own receptors or to translocate into the interior compartments of cells where they may execute IGF-independent actions. In spite of this functional and regulatory diversity, it has been puzzling that loss-of-function studies have yielded relatively little information about the physiological functions of IGFBPs. In this review, we suggest that evolution has tended to retain an array of IGFBPs in order to facilitate fine-tuning of IGF signaling. We explore the emerging explanation that many IGFBP functions have evolved to allow the targeted adjustment of IGF signaling under stressful or irregular conditions, which would likely not be revealed in a standard laboratory setting.

Circulating insulin-like growth factor binding proteins in fish: Their identities and physiological regulation

Insulin-like growth factor binding proteins (IGFBPs) play crucial roles in regulating the availability of IGFs to receptors and prolong the half-lives of IGFs. There are six IGFBPs present in the mammalian circulation with IGFBP-3 being most abundant. In mammals IGFBP-3 is the major carrier of circulating IGFs, facilitated by forming a ternary complex with IGF and an acid-labile subunit (ALS). IGFBP-1 is generally inhibitory to IGF action by preventing it from interacting with its receptors. In teleosts, the third-round of vertebrate whole genome duplication created paralogs of each IGFBP, except IGFBP-4. In the fish circulation, three major IGFBPs are typically detected at molecular ranges of 20-25, 28-32 and 40-50kDa. However, their identities are not well established. Three major circulating IGFBPs in Chinook salmon have been identified through protein purification and cDNA cloning. Salmon 28- and 22-kDa IGFBPs are co-orthologs of IGFBP-1, termed IGFBP-1a and -1b, respectively. They are induced under catabolic conditions such as stress and fasting but their responses are somewhat different, with IGFBP-1b being the most sensitive of the two. Cortisol stimulates production and secretion of these IGFBP-1 subtypes while, unlike in mammals, insulin may not be a primary suppressor. Salmon 41-kDa IGFBP, a major carrier of IGF-I, is not IGFBP-3, as might be expected extrapolating from mammals, but is in fact IGFBP-2b. Salmon IGFBP-2b levels in plasma are high when fish are fed, and GH treatment increases its circulating levels similar to mammalian IGFBP-3. These findings suggest that salmon IGFBP-2b acquired the role and regulation similar to mammalian IGFBP-3. Multiple replications of fish IGFBPs offer a unique opportunity to investigate molecular evolution of IGFBPs.

Variation in branchial expression among insulin-like growth-factor binding proteins (igfbps) during Atlantic salmon smoltification and seawater exposure

Background

In preparation for migration from freshwater to marine habitats, Atlantic salmon (Salmo salar L.) undergo smoltification, a transformation that includes the acquisition of hyposmoregulatory capacity. The growth hormone (Gh)/insulin-like growth-factor (Igf) axis promotes the development of branchial ionoregulatory functions that underlie ion secretion. Igfs interact with a suite of Igf binding proteins (Igfbps) that modulate hormone activity. In Atlantic salmon smolts, igfbp4,−5a,−5b1,−5b2,−6b1 and−6b2 transcripts are highly expressed in gill. We measured mRNA levels of branchial and hepatic igfbps during smoltification (March, April, and May), desmoltification (July) and following seawater (SW) exposure in March and May. We also characterized parallel changes in a broad suite of osmoregulatory (branchial Na⁺/K⁺-ATPase (Nka) activity, Na⁺/K⁺/2Cl⁻cotransporter 1 (nkcc1) and cystic fibrosis transmembrane regulator 1 (cftr1) transcription) and endocrine (plasma Gh and Igf1) parameters.

Results

Indicative of smoltification, we observed increased branchial Nka activity, nkcc1 and cftr1 transcription in May. Branchial igfbp6b1 and -6b2 expression increased coincidentally with smoltification. Following a SW challenge in March, igfbp6b1 showed increased expression while igfbp6b2 exhibited diminished expression. igfbp5a,−5b1 and−5b2 mRNA levels did not change during smolting, but each had lower levels following a SW exposure in March.

Conclusions

Salmonids express an especially large suite of igfbps. Our data suggest that dynamic expression of particular igfbps accompanies smoltification and SW challenges; thus, transcriptional control of igfbps may provide a mechanism for the local modulation of Igf activity in salmon gill.

Igf Binding Proteins Protect Undifferentiated Spermatogonia in the Zebrafish Testis Against Excessive Differentiation

IGF binding proteins (IGFBPs) modulate the availability of IGFs for their cognate receptors. In zebrafish testes, IGF3 promotes the proliferation and differentiation of type A undifferentiated (A_und) spermatogonia, and igf3 expression is strongly elevated by FSH but also responds to T₃. Here we report the effects of FSH and T₃ on igfbp transcript levels in adult zebrafish testis. We then examined T₃ and FSH effects on zebrafish spermatogenesis and explored the relevance of IGFBPs in modulating these T₃ or FSH effects, using a primary tissue culture system for adult zebrafish testis. T₃ up-regulated igfbp1a and igfbp3 expression, whereas FSH reduced igfbp1a transcript levels. To quantify effects on spermatogenesis, we determined the mitotic index and relative section areas occupied by A_und, type A differentiating, or type B spermatogonia. In general, T₃ and FSH stimulated spermatogonial proliferation and increased the areas occupied by spermatogonia, suggesting that both self-renewal and differentiating divisions were stimulated. Preventing IGF/IGFBP interaction by NBI-31772 further increased T₃- or FSH-induced spermatogonial proliferation. However, under these conditions the more differentiated type A differentiating and B spermatogonia occupied larger surface areas at the expense of the area held by A_und spermatogonia. Clearly decreased nanos2 transcript levels are in agreement with this finding, and reduced amh expression may have facilitated spermatogonial differentiation. We conclude that elevating IGF3 bioactivity by blocking IGFBPs shifted T₃- or FSH-induced signaling from stimulating spermatogonial self-renewal as well as differentiation toward predominantly stimulating spermatogonial differentiation, which leads to a depletion of type A_und spermatogonia.

Cross Talk Between Growth and Immunity: Coupling of the IGF Axis to Conserved Cytokine Pathways in Rainbow Trout

Although disease and infection is associated with attenuated growth, the molecular pathways involved are poorly characterized. We postulated that the IGF axis, a central governor of vertebrate growth, is repressed during infection to promote resource reallocation towards immunity. This hypothesis was tested in rainbow trout (Oncorhynchus mykiss) challenged by Aeromonas salmonicida (AS), a Gram-negative bacterial pathogen, or viral hemorrhagic septicemia virus (VHSv) at hatch, first feeding, and 3 weeks after first feeding. Quantitative transcriptional profiling was performed for genes encoding both IGF hormones, 19 salmonid IGF binding proteins (IGFBPs) and a panel of marker genes for growth and immune status. There were major differences in the developmental response of the IGF axis to AS and VHSv, with the VHSv challenge causing strong down-regulation of many genes. Despite this, IGFBP-1A1 and IGFBP-6A2 subtypes, each negative regulators of IGF signaling, were highly induced by AS and VHSv in striking correlation with host defense genes regulated by cytokine pathways. Follow-up experiments demonstrated a highly significant coregulation of IGFBP-1A1 and IGFBP-6A2 with proinflammatory cytokine genes in primary immune tissues (spleen and head kidney) when trout were challenged by a different Gram-negative bacterium, Yersinia ruckeri. Based on our findings, we propose a model where certain IGFBP subtypes are directly regulated by cytokine signaling pathways, allowing immediate modulation of growth and/or immune system phenotypes according to the level of activation of immunity. Our findings provide new and comprehensive insights into cross talk between conserved pathways regulating teleost growth, development, and immunity.

Recent insights into the actions of IGFBP-6

IGFBP-6 is an O-linked glycoprotein that preferentially binds IGF-II over IGF-I. It is a relatively selective inhibitor of IGF-II actions including proliferation, survival and differentiation of a wide range of cells. IGFBP-6 has recently been shown to have a number of IGF-independent actions, including promotion of apoptosis in some cells and inhibition of angiogenesis. IGFBP-6 also induces migration of tumour cells including rhabdomyosarcomas by an IGF-independent mechanism. This chemotactic effect is mediated by MAP kinases. IGFBP-6 binds to prohibitin-2 on the cell surface and the latter is required for IGFBP-6-induced migration by a mechanism that is independent of MAP kinases. IGFBP-6 may enter the nucleus and modulate cell survival and differentiation. IGFBP-6 expression is decreased in a number of cancer cells and it has been postulated to act as a tumour suppressor. IGFBP-6 expression is increased in a smaller number of cancers, which may reflect a compensatory mechanism to control IGF-II actions or IGF-independent actions. The relative balance of IGF-dependent and IGF-independent actions of IGFBP-6 in vivo together with the related question regarding the roles of IGFBP-6 binding to IGF and non-IGF ligands are keys to understanding the physiological role of this protein.

Cloning, molecular characterization and expression analysis of insulin-like growth factor binding protein-2 (IGFBP-2) cDNA in goldfish, Carassius auratus

In the present study, a full-length cDNA encoding the insulin-like growth factor binding protein-2 (IGFBP-2) was cloned from the liver of goldfish (Carassius auratus) by rapid amplification of cDNA ends technique. The goldfish IGFBP-2 cDNA sequence was 1,513 bp long and had an open reading frame of 825 bp encoding a predicted polypeptide of 274 amino acid residues. Semi-quantitative RT-PCR results revealed that goldfish IGFBP-2 mRNA was expressed in all detected tissues. In liver, central nervous system and pituitary gland, goldfish IGFBP-2 expressed at high levels, followed by anterior intestine, middle intestine and kidney. In posterior intestine, ovary, skin, fat, spleen, muscle and gill, the goldfish IGFBP-2 expression levels were very low. Fasting and refeeding experiment showed that the mRNA expression of goldfish IGFBP-2 was up-regulated significantly in liver compared to the fed group and restored rapidly to normal level after refed. However, the mRNA expressions of IGFBP-2 in hypothalamus and pituitary of goldfish were insensitive to fasting. Furthermore, the mRNA expressions of IGFBP-2 in hypothalamus, pituitary and liver were varied in periprandial changes and significantly down-regulated at 2 and 4 h after meal. These results imply that the IGFBP-2 mRNA expression may be associated with anabolic and catabolic metabolism and regulated by metabolic factors in goldfish.

Nonhuman genetics. Genomic basis for the convergent evolution of electric organs

Little is known about the genetic basis of convergent traits that originate repeatedly over broad taxonomic scales. The myogenic electric organ has evolved six times in fishes to produce electric fields used in communication, navigation, predation, or defense. We have examined the genomic basis of the convergent anatomical and physiological origins of these organs by assembling the genome of the electric eel (Electrophorus electricus) and sequencing electric organ and skeletal muscle transcriptomes from three lineages that have independently evolved electric organs. Our results indicate that, despite millions of years of evolution and large differences in the morphology of electric organ cells, independent lineages have leveraged similar transcription factors and developmental and cellular pathways in the evolution of electric organs.

Regulation of skeletal muscle growth in fish by the growth hormone--insulin-like growth factor system

The growth hormone (GH)-insulin-like growth factor (IGF) system is the key promoter of growth in vertebrates; however, how this system modulates muscle mass in fish is just recently becoming elucidated. In fish, the GH induces muscle growth by modulating the expression of several genes belonging to the myostatin (MSTN), atrophy, GH, and IGF systems as well as myogenic regulatory factors (MRFs). The GH controls the expression of igf1 via Janus kinase 2 (JAK2)/signal transducers and activators of the transcription 5 (STAT5) signaling pathway, but it seems that it is not the major regulator. These mild effects of the GH on igf1 expression in fish muscle seem to be related with the presence of higher contents of truncated GH receptor1 (tGHR1) than full length GHR (flGHR1). IGFs in fish stimulate myogenic cell proliferation, differentiation, and protein synthesis through the MAPK/ERK and PI3K/AKT/TOR signaling pathways, concomitant with abolishing protein degradation and atrophy via the PI3K/AKT/FOXO signaling pathway. Besides these signaling pathways control the expression of several genes belonging to the atrophy and IGF systems. Particularly, IGFs and amino acid control the expression of igf1, thus, suggesting other of alternative signaling pathways regulating the transcription of this growth factor. The possible role of IGF binding proteins (IGFBPs) and the contribution of muscle-derived versus hepatic-produced IGF1 on fish muscle growth is also addressed. Thus, a comprehensive overview on the GH-IGF system regulating fish skeletal muscle growth is presented, as well as perspectives for future research in this field.

Insulin-like growth factor-binding protein-6 and cancer

The IGF (insulin-like growth factor) system is essential for physiological growth and it is also implicated in a number of diseases including cancer. IGF activity is modulated by a family of high-affinity IGF-binding proteins, and IGFBP-6 is distinctive because of its marked binding preference for IGF-II over IGF-I. A principal role for IGFBP-6 is inhibition of IGF-II actions, but recent studies have indicated that IGFBP-6 also has IGF-independent effects, including inhibition of angiogenesis and promotion of cancer cell migration. The present review briefly summarizes the IGF system in physiology and disease before focusing on recent studies on the regulation and actions of IGFBP-6, and its potential roles in cancer cells. Given the widespread interest in IGF inhibition in cancer therapeutics, increasing our understanding of the mechanisms underlying the actions of the IGF ligands, receptors and binding proteins, including IGFBP-6, will enhance our ability to develop optimal treatments that can be targeted to the most appropriate patients.

The stress-response gene redd1 regulates dorsoventral patterning by antagonizing Wnt/β-catenin activity in zebrafish

REDD1/redd1 is a stress-response gene that is induced under various stressful conditions such as hypoxia, DNA damage, and energy stress. The increased REDD1 inhibits mTOR signaling and cell growth. Here we report an unexpected role of Redd1 in regulating dorsoventral patterning in zebrafish embryos and the underlying mechanisms. Zebrafish redd1 mRNA is maternally deposited. Although it is ubiquitously detected in many adult tissues, its expression is highly tissue-specific and dynamic during early development. Hypoxia and heat shock strongly induce redd1 expression in zebrafish embryos. Knockdown of Redd1 using two independent morpholinos results in dorsalized embryos and this effect can be rescued by injecting redd1 mRNA. Forced expression of Redd1 ventralizes embryos. Co-expression of Redd1 with Wnt3a or a constitutively active form of β-catenin suggests that Redd1 alters dorsoventral patterning by antagonizing the Wnt/β-catenin signaling pathway. These findings have unraveled a novel role of Redd1 in early development by antagonizing Wnt/β-catenin signaling.

Dynamic transcriptional regulation of autocrine/paracrine igfbp1, 2, 3, 4, 5, and 6 in the skeletal muscle of the fine flounder during different nutritional statuses

The IGF-binding proteins (IGFBPs) play a dual role in the regulation of the activity and bioavailability of IGFs in different tissues. Diverse evidence has shown that IGFBPs can inhibit and/or potentiate IGF actions. In this study, igfbp1, 2, 3, 4, 5, and 6 were isolated in the fine flounder, a flat fish species that shows slow growth and inherent Gh resistance in muscle. Subsequently, the expression of all igfbps was assessed in the skeletal muscle of flounder that underwent different nutritional statuses. igfbp1 was not expressed in muscle during any of the nutritional conditions, whereas igfbp3 and igfbp5 were the lowest and the highest igfbps expressed respectively. A dynamic expression pattern was found in all the igfbps expressed in skeletal muscle, which depended on the nutritional status and sampling period. During the fasting period, igfbp2, 4, and 5 were downregulated, whereas igfbp3 was upregulated during part of the fasting period. The restoration of food modulated the expression of the igfbps dynamically, showing significant changes during both the long- and short-term refeeding. igfbp3 and igfbp6 were downregulated during short-term refeeding, whereas igfbp5 was upregulated, and igfbp2 and igfbp4 remained stable. During long-term refeeding, the expression of igfbp2, 4, 5, and 6 increased, while igfbp3 remained unchanged. In conclusion, this study shows for the first time the isolation of all igfbps in a single fish species, in addition to describing a dynamic nutritional and time-dependent response in the expression of igfbps in the skeletal muscle of a nonmammalian species.

Identification and expression analysis of IGFBP-1 gene from Japanese flounder (Paralichthys olivaceus)

Insulin-like growth factor binding protein-1 (IGFBP-1) plays an important role in IGF regulating vertebrate growth and development. In this study, we cloned IGFBP-1 cDNA from Japanese flounder (Paralichthys olivaceus) liver. The full-length cDNA is 1070 bp, including a 5'-untranslated region (UTR) of 69 bp, a 3'-UTR of 272 bp, and an open reading frame (ORF) of 729 bp encoding a polypeptide of 242 amino acids. Real-time quantitative PCR revealed that IGFBP-1 mRNA is mainly expressed in the liver, and a small amount of mRNAs was also found in other adult tissues. There are maternal transcripts of IGFBP-1 gene, and relatively low mRNA levels were observed in different embryonic stages. A higher level of IGFBP-1 mRNA was detected at 3 days post hatching (dph), and it got to the highest level at 29 dph (metamorphic climax), and finally brought back to a lower level at the end of metamorphosis. The expression of IGFBP-1 mRNA was greatly up-regulated in thyroid hormone (TH)-treated larvae, and declined after thiourea (TU) treatment. These results provide basic information for further studies on the role of IGF system in the P. olivaceus development and metamorphosis.

Circulating salmon 28- and 22-kDa insulin-like growth factor binding proteins (IGFBPs) are co-orthologs of IGFBP-1

Circulating insulin-like growth factor binding proteins (IGFBPs) play pivotal roles in stabilizing IGFs and regulating their availability to target tissues. In the teleost circulation, three major IGFBPs are typically detected by ligand blotting with molecular masses around 20-25, 28-32 and 40-45kDa. However, their identity is poorly established and often confused. We previously identified salmon 22- and 41-kDa forms as IGFBP-1 and -2b, respectively. In the present study, we cloned the cDNA of 28-kDa IGFBP from Chinook salmon (Oncorhynchus tshawytscha) as well as rainbow trout (Oncorhynchus mykiss) based on the partial N-terminal amino acid sequence of purified protein and identified it as an ortholog of IGFBP-1. Structural and phylogenetic analyses revealed that the 28-kDa IGFBP is more closely related to human IGFBP-1 and zebrafish IGFBP-1a than the previously identified salmon IGFBP-1 (i.e. 22-kDa IGFBP). We thus named salmon 28- and 22-kDa forms as IGFBP-1a and -1b, respectively. Salmon IGFBP-1a contains a potential PEST region involved in rapid protein turnover and phosphorylation sites typically found in mammalian IGFBP-1, although the PEST and phosphorylation scores are not as high as those of human IGFBP-1. There was a striking difference in tissue distribution patterns between subtypes; Salmon igfbp-1a was expressed in a variety of tissues while igfbp-1b was almost exclusively expressed in the liver, suggesting that IGFBP-1a has more local actions. Direct seawater exposure (osmotic stress) of Chinook salmon parr caused increases in both IGFBP-1s in plasma, while IGFBP-1b appeared to be more sensitive. The presence of two co-orthologs of IGFBP-1 in the circulation in salmon, and most likely in other teleosts, provides a good opportunity to investigate subfunction partitioning of duplicated IGFBP-1 during postnatal growth.

Insulin-like growth factor (IGF) signalling and genome-wide transcriptional regulation in fast muscle of zebrafish following a single-satiating meal

Male zebrafish (Danio rerio) were fasted for 7 days and fed to satiation over 3 h to investigate the transcriptional responses to a single meal. The intestinal content at satiety (6.3% body mass) decreased by 50% at 3 h and 95% at 9 h following food withdrawal. Phosphorylation of the insulin-like growth factor (IGF) signalling protein Akt peaked within 3 h of feeding and was highly correlated with gut fullness. Retained paralogues of IGF hormones genes were regulated with feeding, with igf1a showing a pronounced peak in expression after 3 h and igf2b after 6 h. Igf-I receptor transcripts were markedly elevated with fasting, and decreased to their lowest levels 45 min after feeding. igf1rb transcripts increased more quickly than igf1ra transcripts as the gut emptied. Paralogues of the insulin-like growth factor binding proteins (IGFBPs) were constitutively expressed, except for igfbp1a and igfbp1b transcripts, which were significantly elevated with fasting. Genome-wide transcriptional responses were analysed using the Agilent 44K oligonucleotide microarray and selected genes validated by qPCR. Fasting was associated with the upregulation of genes for the ubiquitin-proteasome degradation pathway, anti-proliferative and pro-apoptotic genes. Protein chaperones (unc45b, hspd1, hspa5, hsp90a.1, hsp90a.2) and chaperone interacting proteins (ahsa1 and stip1) were upregulated 3 h after feeding along with genes for the initiation of protein synthesis and mRNA processing. Transcripts for the enzyme ornithine decarboxylase 1 showed the largest increase with feeding (11.5-fold) and were positively correlated with gut fullness. This study demonstrates the fast nature of the transcriptional responses to a meal and provides evidence for differential regulation of retained paralogues of IGF signalling pathway genes.

Characterization of three IGFBP mRNAs in Atlantic croaker and their regulation during hypoxic stress: potential mechanisms of their upregulation by hypoxia

Insulin-like growth factor-binding proteins (IGFBPs) play important roles in downregulating IGF activity and growth and development in vertebrates under hypoxic stress. However, the mechanisms of hypoxia regulation of IGFBPs in teleost fishes are unknown. The involvement of reactive oxygen species (ROS) and hypoxia-inducible factors (HIFs) in hypoxia upregulation of IGFBPs in Atlantic croaker were investigated. Three croaker IGFBPs, IGFBP-1, IGFBP-2, and IGFBP-5, were cloned and characterized. Chronic hypoxia exposure [dissolved oxygen (DO): 1.7 mg/l for 2-4 wk] caused significant increases in hepatic and neural IGFBP-1 mRNA expression compared with tissue mRNA levels in fish held under normoxic conditions (6.5 mg DO/l). Moreover, longer-term chronic hypoxia exposure (2-2.7 mg DO/l for 15-20 wk) caused significant increases in mRNA levels of all three IGFBPs in both liver and brain tissues. Hypoxia exposure also markedly increased superoxide radical (O(2)(·-), an index of ROS) production and HIF-1α mRNA and HIF-2α protein expression in croaker livers. Pharmacological treatment with an antioxidant attenuated the hypoxia-induced increases in O(2)(·-) production and HIFα mRNA and protein expression as well as the elevation of IGFBP-1 mRNA levels. These results suggest that the upregulation of IGFBP expression under hypoxia stress is due, in part, to alterations in the antioxidant status, which may involve ROS and HIFs.

IGF binding protein-6 expression in vascular endothelial cells is induced by hypoxia and plays a negative role in tumor angiogenesis

Hypoxia stimulates tumor angiogenesis by inducing the expression of angiogenic molecules. The negative regulators of this process, however, are not well understood. Here, we report that hypoxia induced the expression of insulin-like growth factor binding protein-6 (IGFBP-6), a tumor repressor, in human and rodent vascular endothelial cells (VECs) via a hypoxia-inducible factor (HIF)-mediated mechanism. Addition of human IGFBP-6 to cultured human VECs inhibited angiogenesis in vitro. An IGFBP-6 mutant with at least 10,000-fold lower binding affinity for IGFs was an equally potent inhibitor of angiogenesis, suggesting that this action of IGFBP-6 is IGF-independent. The functional relationship between IGFBP-6 and vascular endothelial growth factor (VEGF), a major hypoxia-inducible angiogenic molecule, was examined. While VEGF alone increased angiogenesis in vitro, co-incubation with IGFBP-6 abolished VEGF-stimulated angiogenesis. The in vivo role of IGFBP-6 in angiogenesis was tested in flk1:GFP zebrafish embryos, which exhibit green fluorescence protein in developing vascular endothelium, permitting visualization of developing blood vessels. Injection of human IGFBP-6 mRNA reduced the number of embryonic inter-segmental blood vessels by ∼40%. This anti-angiogenic activity is conserved in zebrafish because expression of zebrafish IGFBP-6b had similar effects. To determine the anti-angiogenic effect of IGFBP-6 in a tumor model, human Rh30 rhabdomyosarcoma cells stably transfected with IGFBP-6 were inoculated into athymic BALB/c nude mice. Vessel density was 52% lower in IGFBP-6-transfected xenografts than in vector control xenografts. These results suggest that the expression of IGFBP-6 in VECs is up-regulated by hypoxia and IGFBP-6 inhibits angiogenesis in vitro and in vivo.

Evolution of the insulin-like growth factor binding protein (IGFBP) family

The evolution of the IGF binding protein (IGFBP) gene family has been difficult to resolve. Both chromosomal and serial duplications have been suggested as mechanisms for the expansion of this gene family. We have identified and annotated IGFBP sequences from a wide selection of vertebrate species as well as Branchiostoma floridae and Ciona intestinalis. By combining detailed sequence analysis with sequence-based phylogenies and chromosome information, we arrive at the following scenario: the ancestral chordate IGFBP gene underwent a local gene duplication, resulting in a gene pair adjacent to a HOX cluster. Subsequently, the gene family expanded in the two basal vertebrate tetraploidization (2R) resulting in the six IGFBP types that are presently found in placental mammals. The teleost fish ancestor underwent a third tetraploidization (3R) that further expanded the IGFBP repertoire. The five sequenced teleost fish genomes retain 9-11 of IGFBP genes. This scenario is supported by the phylogenies of three adjacent gene families in the HOX gene regions, namely the epidermal growth factor receptors (EGFR) and the Ikaros and distal-less (DLX) transcription factors. Our sequence comparisons show that several important structural components in the IGFBPs are ancestral vertebrate features that have been maintained in all orthologs, for instance the integrin interaction motif Arg-Gly-Asp in IGFBP-2. In contrast, the Arg-Gly-Asp motif in IGFBP-1 has arisen independently in mammals. The large degree of retention of IGFBP genes after the ancient expansion of the gene family strongly suggests that each gene evolved distinct and important functions early in vertebrate evolution.

Circulating salmon 41-kDa insulin-like growth factor binding protein (IGFBP) is not IGFBP-3 but an IGFBP-2 subtype

In vertebrates, most circulating insulin-like growth factor (IGF) is bound to multiple forms of IGF-binding proteins (IGFBPs) that differ both structurally and functionally. In mammals, the largest reservoir of IGF in the circulation comes from a large (150kDa) ternary complex comprised of IGF bound to IGFBP-3, which is bound to an acid label subunit (ALS), and this variant of IGFBP is regulated by growth hormone (GH) and feed intake. Although multiple variants of IGFBPs ranging from 20 to 50kDa have been found in fishes, no ternary complex is present and it has been assumed that the majority of circulating IGF is bound to fish IGFBP-3. Consistent with this assumption is previous work in salmon showing the presence of a 41-kDa IGFBP that is stimulated by GH, decreases with fasting and increases with feeding. However, the hypothesis that the salmon 41-kDa IGFBP is structurally homologous to mammalian IGFBP-3 has not been directly tested. To address this issue, we cloned cDNAs for several Chinook salmon IGFBPs, and found that the cDNA sequence of the 41-kDa IGFBP is most similar to that of mammalian IGFBP-2 and dissimilar to IGFBP-3. We found an additional IGFBP (termed IGFBP-2a) with high homology to mammalian IGFBP-2. These results demonstrate that salmon 41-kDa IGFBP is not IGFBP-3, but a paralog of IGFBP-2 (termed IGFBP-2b). Salmon IGFBP-2s are also unique in terms of having potential N-glycosylation sites and splice variants. Additional research on non-mammalian IGFBPs is needed to fully understand the molecular/functional evolution of the IGFBP family and the significance of the ternary complex in vertebrates.

Acute handling disturbance modulates plasma insulin-like growth factor binding proteins in rainbow trout (Oncorhynchus mykiss)

The effects of acute stressor exposure on proximal (growth hormone [GH]) and distal (insulin-like growth factor-I [IFG-I] and insulin-like growth factor-binding proteins [IFGBPs]) components of the somatotropic axis are poorly understood in finfish. Rainbow trout (Oncorhynchus mykiss) were exposed to a 5-min handling disturbance to mimic an acute stressor episode, and levels of plasma GH, IGF-I, and IGFBPs at 0, 1, 4, and 24 h post-stressor exposure were measured. An unstressed group was also sampled at the same clock times (09:00, 10:00, 13:00, and 08:00 [the following day]) as acute stress sampling to determine temporal changes in the above somatotropic axis components. The acute stressor transiently elevated plasma cortisol and glucose levels at 1 and 4 h post-stressor exposure, whereas no changes were seen in the unstressed group. Plasma GH levels were not affected by handling stress or sampling time in the unstressed animals. Plasma IGF-I levels were significantly depressed at 1 and 4 h post-stressor exposure, but no discernible temporal pattern was seen in the unstressed animals. Using a western ligand blotting technique, we detected plasma IGFBPs of 21, 32, 42, and 50 kDa in size. The plasma levels of the lower-molecular-weight IGFBPs (21 and 32 kDa) were unaffected by handling stressor, nor were there any discernible temporal patterns in the unstressed animals. By contrast, the higher-molecular-weight IGFBPs (42 and 50 kDa) were affected by stress or time of sampling. Levels of the 42-kDa IGFBP levels significantly decreased over the sampling period in unstressed control animals, but this temporal drop was eliminated in stressed animals. Levels of the 50-kDa IGFBPs also decreased significantly over the sampling time in unstressed trout, whereas handling disturbance transiently increased levels of this IGFBP at 1 h but not at 4 and 24 h post-stressor exposure compared with the control group. Overall, our results suggest that acute stress adaptation involves modulation of plasma IGF-1 and high-molecular-mass IGFBP levels (42 and 50 kDa) in rainbow trout.

Transcriptional regulation of the IGF signaling pathway by amino acids and insulin-like growth factors during myogenesis in Atlantic salmon

The insulin-like growth factor signalling pathway is an important regulator of skeletal muscle growth. We examined the mRNA expression of components of the insulin-like growth factor (IGF) signalling pathway as well as Fibroblast Growth Factor 2 (FGF2) during maturation of myotubes in primary cell cultures isolated from fast myotomal muscle of Atlantic salmon (Salmo salar). The transcriptional regulation of IGFs and IGFBP expression by amino acids and insulin-like growth factors was also investigated. Proliferation of cells was 15% d⁻¹ at days 2 and 3 of the culture, increasing to 66% d⁻¹ at day 6. Three clusters of elevated gene expression were observed during the maturation of the culture associated with mono-nucleic cells (IGFBP5.1 and 5.2, IGFBP-6, IGFBP-rP1, IGFBP-2.2 and IGF-II), the initial proliferation phase (IGF-I, IGFBP-4, FGF2 and IGF-IRb) and terminal differentiation and myotube production (IGF2R, IGF-IRa). In cells starved of amino acids and serum for 72 h, IGF-I mRNA decreased 10-fold which was reversed by amino acid replacement. Addition of IGF-I and amino acids to starved cells resulted in an 18-fold increase in IGF-I mRNA indicating synergistic effects and the activation of additional pathway(s) leading to IGF-I production via a positive feedback mechanism. IGF-II, IGFBP-5.1 and IGFBP-5.2 expression was unchanged in starved cells, but increased with amino acid replacement. Synergistic increases in expression of IGFBP5.2 and IGFBP-4, but not IGFBP5.1 were observed with addition of IGF-I, IGF-II or insulin and amino acids to the medium. IGF-I and IGF-II directly stimulated IGFBP-6 expression, but not when amino acids were present. These findings indicate that amino acids alone are sufficient to stimulate myogenesis in myoblasts and that IGF-I production is controlled by both endocrine and paracrine pathways. A model depicting the transcriptional regulation of the IGF pathway in Atlantic salmon muscle following feeding is proposed.

Duplicated zebrafish insulin-like growth factor binding protein-5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity

Insulin-like growth factor binding protein (IGFBP)-5 is a secreted protein that binds to IGF and modulates IGF actions. IGFBP-5 is also found in the nucleus of mammalian cells and has transactivation activity. The structural basis of this transactivation activity and its role in mediating IGF-independent actions are not clear. Here we report that there are 2 igfbp-5 genes in zebrafish and other teleost fish. In zebrafish, igfbp-5a and -5b are expressed in spatially restricted, mostly nonoverlapping domains during early development. The IGF binding site is conserved in both zebrafish IGFBP-5s, and they are both secreted and capable of IGF binding. Both proteins contain a consensus bipartite nuclear localization signal and were found in the nucleus when introduced into cultured cells. Although zebrafish IGFBP-5b possesses transactivation activity, zebrafish IGFBP-5a lacks this activity. Mutational analysis demonstrated that 2 unique amino acids in positions 22 and 56 of IGFBP-5a are responsible for its lack of transactivation activity. These findings suggest that the duplicated zebrafish IGFBP-5s have evolved divergent regulatory mechanisms and distinct biological properties by partitioning of ancestral structural domains and provide new evidence for a conserved role of the IGF binding, nuclear localization, and transactivation domain of this multifunctional IGFBP.