Identification of IGF-1 receptors in primitive vertebrates

Cyclic Glycine-Proline (cGP) Normalises Insulin-Like Growth Factor-1 (IGF-1) Function: Clinical Significance in the Ageing Brain and in Age-Related Neurological Conditions

Insulin-like growth factor-1 (IGF-1) function declines with age and is associated with brain ageing and the progression of age-related neurological conditions. The reversible binding of IGF-1 to IGF binding protein (IGFBP)-3 regulates the amount of bioavailable, functional IGF-1 in circulation. Cyclic glycine-proline (cGP), a metabolite from the binding site of IGF-1, retains its affinity for IGFBP-3 and competes against IGF-1 for IGFBP-3 binding. Thus, cGP and IGFBP-3 collectively regulate the bioavailability of IGF-1. The molar ratio of cGP/IGF-1 represents the amount of bioavailable and functional IGF-1 in circulation. The cGP/IGF-1 molar ratio is low in patients with age-related conditions, including hypertension, stroke, and neurological disorders with cognitive impairment. Stroke patients with a higher cGP/IGF-1 molar ratio have more favourable clinical outcomes. The elderly with more cGP have better memory retention. An increase in the cGP/IGF-1 molar ratio with age is associated with normal cognition, whereas a decrease in this ratio with age is associated with dementia in Parkinson disease. In addition, cGP administration reduces systolic blood pressure, improves memory, and aids in stroke recovery. These clinical and experimental observations demonstrate the role of cGP in regulating IGF-1 function and its potential clinical applications in age-related brain diseases as a plasma biomarker for-and an intervention to improve-IGF-1 function.

Pleiotropic Effect of Hormone Insulin-Like Growth Factor-I in Immune Response and Pathogenesis in Leishmaniases

Leishmaniases are diseases caused by several Leishmania species, and many factors contribute to the development of the infection. Because the adaptive immune response does not fully explain the outcome of Leishmania infection and considering that the initial events are crucial in the establishment of the infection, we investigated one of the growth factors, the insulin-like growth factor-I (IGF-I), found in circulation and produced by different cells including macrophages and present in the skin where the parasite is inoculated. Here, we review the role of IGF-I in leishmaniasis experimental models and human patients. IGF-I induces the growth of different Leishmania species in vitro and alters the disease outcome increasing the parasite load and lesion size, especially in L. major- and L. amazonensis-infected mouse leishmaniasis. IGF-I affects the parasite interacting with the IGF-I receptor present on Leishmania. During Leishmania-macrophage interaction, IGF-I acts on the arginine metabolic pathway, resulting in polyamine production both in macrophages and Leishmania. IGF-I and cytokines interact with reciprocal influences on their expression. IL-4 is a hallmark of susceptibility to L. major in murine leishmaniasis, but we observed that IGF-I operates astoundingly as an effector element of the IL-4. Approaching human leishmaniasis, patients with mucosal, disseminated, and visceral diseases presented surprisingly low IGF-I serum levels, suggesting diverse effects than parasite growth. We observed that low IGF-I levels might contribute to the inflammatory response persistence and delayed lesion healing in human cutaneous leishmaniasis and the anemia development in visceral leishmaniasis. We must highlight the complexity of infection revealed depending on the Leishmania species and the parasite's developmental stages. Because IGF-I exerts pleiotropic effects on the biology of interaction and disease pathogenesis, IGF-I turns up as an attractive tool to explore biological and pathogenic processes underlying infection development. IGF-I pleiotropic effects open further the possibility of approaching IGF-I as a therapeutical target.

Identification and expression analysis of 26 oncogenes of the receptor tyrosine kinase family in channel catfish after bacterial infection and hypoxic stress

Receptor tyrosine kinases (RTKs) are high-affinity cell surface receptors for many polypeptide growth factors, cytokines and hormones. RTKs are not only key regulators of normal cellular processes, but are also involved in the progression of many types of tumors, and responses to various biotic and abiotic stresses. Catfish is a primary aquaculture species in the United States, while its industry is drastically hindered by several major diseases including enteric septicemia of catfish (ESC) that is caused by Edwardsiella ictaluri. Disease outbreaks are often accompanied by hypoxic stress, which affects the performance and survival of fish by reducing disease resistance. In this study, we identified 26 RTK oncogenes in the channel catfish genome, and determined their expression profiles after ESC infection and hypoxic stress. The 26 RTK genes were divided into four subfamilies according to phylogenetic analysis, including TIE (2 genes), ErbB (6 genes), EPH (14 genes), and INSR (4 genes). All identified RTKs possess a similar molecular architecture including ligand-binding domains, a single transmembrane helix and a cytoplasmic region, which suggests that these genes could play conserved biological roles. The expression analysis revealed that eight RTKs were significantly regulated after bacterial infection, with dramatic induction of insulin receptor genes including INSRb, IGF1Ra, and IGF1Rb. Upon hypoxic stress, EPHB3a, EGFR, ErbB4b, and IGF1Rb were expressed at higher levels in the tolerant catfish, while EPHA2a, EPHA2, TIE1 and INSRa were expressed at higher levels in the intolerant catfish. These results suggested the involvement of RTKs in immune responses and hypoxic tolerance.

The role for IGF-1-derived small neuropeptides as a therapeutic target for neurological disorders

INTRODUCTION

Exogenous IGF-1 protects the brain from ischemic injury and improves function. However, its clinical application to neurological disorders is limited by its large molecular size, poor central uptake and mitogenic potential.

AREAS COVERED

In this review, the authors have discussed the efficacy, pharmacokinetics and mechanisms of IGF-1 derivatives on protecting acute brain injury, preventing memory impairment and improving recovery from neurological degenerative conditions evaluated in various animal models. We have included natural metabolites of IGF-1, glycine-proline-glutamate (GPE), cleaved from N-terminal IGF-1 and cyclic glycine-proline (cGP) as well as the structural analogues of GPE and cGP, glycine-2-methyl-proline-glutamate and cyclo-l-glycyl-l-2-allylproline, respectively. In addition, the regulatory role for cGP in bioavailability of IGF-1 has also been discussed.

EXPERT OPINION

These small neuropeptides provide effective neuroprotection by offering an improved pharmacokinetic profile and more practical route of administration compared with IGF-1 administration. Developing modified neuropeptides to overcome the limitations of their endogenous counterparts represents a novel strategy of pharmaceutical discovery for neurological disorders. The mechanism of action may involve a regulation of IGF-1 bioavailability.

Cyclic glycine-proline regulates IGF-1 homeostasis by altering the binding of IGFBP-3 to IGF-1

The homeostasis of insulin-like growth factor-1 (IGF-1) is essential for metabolism, development and survival. Insufficient IGF-1 is associated with poor recovery from wounds whereas excessive IGF-1 contributes to growth of tumours. We have shown that cyclic glycine-proline (cGP), a metabolite of IGF-1, can normalise IGF-1 function by showing its efficacy in improving the recovery from ischemic brain injury in rats and inhibiting the growth of lymphomic tumours in mice. Further investigation in cell culture suggested that cGP promoted the activity of IGF-1 when it was insufficient, but inhibited the activity of IGF-1 when it was excessive. Mathematical modelling revealed that the efficacy of cGP was a modulated IGF-1 effect via changing the binding of IGF-1 to its binding proteins, which dynamically regulates the balance between bioavailable and non-bioavailable IGF-1. Our data reveal a novel mechanism of auto-regulation of IGF-1, which has physiological and pathophysiological consequences and potential pharmacological utility.

Structural basis of the aberrant receptor binding properties of hagfish and lamprey insulins

The insulin from the Atlantic hagfish (Myxine glutinosa) has been one of the most studied insulins from both a structural and a biological viewpoint; however, some aspects of its biology remain controversial, and there has been no satisfying structural explanation for its low biological potency. We have re-examined the receptor binding kinetics, as well as the metabolic and mitogenic properties, of this phylogenetically ancient insulin, as well as that from another extant representative of the ancient chordates, the river lamprey (Lampetra fluviatilis). Both insulins share unusual binding kinetics and biological properties with insulin analogues that have single mutations at residues that contribute to the hexamerization surface. We propose and demonstrate by reciprocal amino acid substitutions between hagfish and human insulins that the reduced biological activity of hagfish insulin results from unfavorable substitutions, namely, A10 (Ile to Arg), B4 (Glu to Gly), B13 (Glu to Asn), and B21 (Glu to Val). We likewise suggest that the altered biological activity of lamprey insulin may reflect substitutions at A10 (Ile to Lys), B4 (Glu to Thr), and B17 (Leu to Val). The substitution of Asp at residue B10 in hagfish insulin and of His at residue A8 in both hagfish and lamprey insulins may help compensate for unfavorable changes in other regions of the molecules. The data support the concept that the set of unusual properties of insulins bearing certain mutations in the hexamerization surface may reflect those of the insulins evolutionarily closer to the ancestral insulin gene product.

Growth hormone differentially regulates growth and growth-related gene expression in closely related fish species

Zebrafish (Danio rerio) have become an important model organism for developmental biology and human health studies. We recently demonstrated differential growth patterns between the zebrafish and a close relative the giant danio (Danio aequipinnatus), where the giant danio appears to exhibit indeterminate growth similar to most fish species important for commercial production, while zebrafish exhibit determinate growth more similar to mammalian growth. This study focused on evaluating muscle growth regulation differences in adult zebrafish and giant danio utilizing growth hormone treatment as a mode of growth manipulation. Growth hormone treatment resulted in increased overall growth in giant danio, but failed to increase growth in the zebrafish. Growth hormone treatment increased muscle IGF-I and GHrI gene expression in both species, but to a larger degree in the giant danio. In contrast, zebrafish exhibited a larger increase in IrA and IGF-IrB gene expression in muscle in response to GH treatment. In addition muscle myostatin levels were differentially regulated between the two species, with a down-regulation observed in rapidly growing, GH-treated giant danio and an up-regulation in zebrafish not actively growing in response to GH. This is the first report of differential expression of growth-regulating genes in closely related fish species exhibiting opposing growth paradigms. These results further support the role that the zebrafish and giant danio can play important model organisms for determinate and indeterminate growth.

Expression profiles of growth-related genes during the very early development of rainbow trout embryos reared at two incubation temperatures

Quantitative RT-PCR was used to determine the profiles of expression of 10 growth- or development-related genes in rainbow trout (Oncorhynchus mykiss) embryos prior to the formation of the somatotropic (ST) axis (pituitary somatotrops and liver); embryos were sampled immediately after fertilization and water-hardening (t(0)), 1-h post-fertilization, and 1-, 2-, 5-, 7-, 10- and 13-days post-fertilization (dpf); expression profiles were examined in embryos reared at two temperatures (6.0 and 8.5 degrees C), which had different developmental rates. Accumulation of mRNA encoding for GH1, GH2, IGF-1, IGF-2, two isoforms of GH receptors (GHR1, GHR2), two isoforms of IGF receptors (IGF-RIa, IGF-RIb) and two isoforms of thyroid receptor (TR), TRalpha and TRbeta, was measured. All of these genes were expressed in the t(0) samples, but the rates of expression of the different genes varied markedly. For most of the genes examined, the expression rates tended to fall within the first hour after fertilization, and remained at the lower level for between 2 and 7 days, after which there was a significant (P <0.05) and progressive increase in the number of accumulated copies of mRNA. This increase is probably associated with the commencement of embryonic genome transcription activity (EGTA), and it was generally, although not always, found later in embryos that were reared at 6.0 degrees C compared with the faster developing embryos reared at 8.5 degrees C. The study suggests that the EGTA begins between 2- and 5-dpf, with a staged increase in EGTA between 5- and 13-dpf.

Autoradiographic and immunohistochemical localization of insulin-like growth factor-I receptor binding sites in brain of the brown trout, Salmo trutta

Insulin-like growth factor-I (IGF-I), a peptide closely related to insulin, is known to play crucial roles in brain development. While the central sites of action of IGF-I in higher vertebrates are now well established, surprisingly little is known in the teleost model where the brain undergoes continual, indeterminate, growth. In this study, we have mapped the distribution of putative IGF-I receptor (IGF-IR) binding sites in the brain of the brown trout using both ligand binding in vitro autoradiography and immunohistochemistry. The presence of IGF binding proteins (IGFBPs) was further studied by competitive inhibition using unlabelled IGF-I and des-(1-3)-IGF-I. In both juvenile and adult trout brain, [125I]IGF-I binding was highest in cerebellum and optic tectum, both regions of the teleost brain known to grow the most actively throughout life. At the cellular level, IGF-IR immunoreactivity was confirmed on cell bodies and dendrites, particularly of larger presumptive neurons including purkinje cells and dendritic fibres throughout the molecular layer of the cerebellum. Abundant IGF-IR expression in hypothalamic regions may further be related to neuron growth while a possible hypophysiotropic role will require further investigation. Competitive inhibition studies employing des-(1-3)-IGF-I also suggest IGFBPs are present in all regions exhibiting high [125I]IGF-I ligand binding and confirms the presence of this important regulatory component of the IGF-I system in the teleost brain. The importance of the IGF-I system in brain development, particularly in regions such as the cerebellum, together with the continual lifetime growth of the fish central nervous system, suggest the teleost brain is an extremely useful site for studying the actions of IGF-I in relation to neuron proliferation, growth, and survival in an adult brain.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

The effects of recombinant bovine somatotropin (rbST) on tissue IGF-I, IGF-I receptor, and GH mRNA levels in rainbow trout, Oncorhynchus mykiss

Numerous studies demonstrated that rbST increased growth rates in several fish species, and several species exhibit GH production in tissues other than the pituitary. The role of tissue GH and IGF-I in regulating fish growth is poorly understood. Therefore an experiment was conducted to examine the effects of rbST treatment on tissue GH, IGF-I, and IGF-I receptor-A (rA) expression in rainbow trout. Rainbow trout (550 +/- 10 g) received either intra-peritoneal injections of rbST (120 microg/g body weight) or vehicle on days 0 and 21, and tissue samples were collected on days 0, 0.5, 1, 3, 7, and 28 (n = 6/day/trt). Total RNA was isolated and assayed for steady-state levels of IGF-I, IGF-IrA, and GH mRNA using quantitative RT-PCR. Insulin-like growth factor-I mRNA levels increased in liver, gill, gonad, muscle, brain, and intestine in response to rbST treatment (P < 0.10). Liver IGF-I mRNA increased (P < 0.01) 0.5 day after treatment and remained elevated throughout the trial. Intestine IGF-I mRNA increased (P < 0.05) in treated fish from day 1 to day 3, then decreased to day 7 and increased again at day 28, and remained elevated above control levels throughout the trial. Gill IGF-I mRNA levels increased (P < 0.05) 1 day after treatment and remained elevated throughout the trial. Heart IGF-IrA mRNA levels decreased (P < 0.05) while gonad GH mRNA levels increased (P < 0.10) following rbST treatment. These results demonstrate that rbST treatment increased IGF-I mRNA levels in extra-hepatic tissues, and decreased heart IGF-IrA and increased gonad GH mRNA levels. Because the primary source for endocrine IGF-I is liver, the increased IGF-I mRNA reported in extra-hepatic tissues may indicate local paracrine/autocrine actions for IGF-I for local physiological functions.

Disparate regulation of insulin-like growth factor-binding proteins in a primitive, ictalurid, teleost (Ictalurus punctatus)

Vertebrate growth is principally controlled by growth hormone (GH) and, its intermediary, insulin-like growth factor-I (IGF-I). The actions of IGF-I are modulated by high-affinity binding proteins called insulin-like growth factor binding-proteins (IGFBPs). Channel catfish exhibit atypical responses (increased percentage body fat and reduced percentage protein) to GH treatment, despite GH-dependent IGF-I production. Among possible explanations for this atypical response to GH treatment is an unusual regulation of blood IGFBPs. In this species, there has been one report of a single 33-kDa plasma binding protein. To examine the occurrence and regulation of plasma IGFBPs in this species, two strains of channel catfish (Norris and USDA-103) were treated with weekly injections of recombinant bovine GH at different temperatures (21 degrees C versus 26 degrees C). In a separate experiment involving catfish of a different strain, endogenous GH levels were altered via injection of the GH secretagogue, bGHRH(1-29)-amide, and held in fresh water or transferred to brackish water (12 ppt). Following these treatments, the type and regulation of plasma IGFBPs in these catfish strains were examined by Western ligand blotting. We have identified five IGFBPs (19, 35, 44, 47, and >80 kDa) in catfish plasma that are differentially altered by experimental treatment and genetic lineage. Levels of the 19-kDa IGFBP were elevated in catfish of Norris and USDA-103 strains that were exposed to a higher environmental temperature (26 degrees C versus 21 degrees C), but was not seen in those animals used for the GH secretagogue/salinity study. In most vertebrates, treatment with GH increases levels of plasma IGFBP-3 (approximately 40-50 kDa). In the USDA-103 and Norris catfish strains, bGH injection reduced plasma levels of the 44- and 47-kDa IGFBPs. Similarly, elevations in plasma GH levels in GH secretagogue-treated and brackish water-adapted catfish resulted in reductions of the 44- and 47-kDa IGFBPs as well as a reduction in presence of a 35-kDa IGFBP that was not detected in the Norris or USDA-103 strains. Reduced levels of the 35, 44, and 47 kDa IGFBPs, seen in the plasma of the GH secretagogue-treated and brackish water-adapted animals, suggests that the atypical response of channel catfish to GH treatment is not attributed to the use of heterologous (bovine) GH. This negative response of the 35-47 kDa IGFBPs to GH has not been reported in any teleost or vertebrate (healthy) and may be partly responsible for the atypical physiological responses of channel catfish to GH treatment.

Insulin-like growth factor II mRNA is expressed in neurones of the brain of the bony fish Oreochromis mossambicus, the tilapia

The physiological meaning of insulin-like growth factor II (IGF-II) is still enigmatic. IGF-II occurs in the adult mammalian brain where it is expressed in the mesodermal portion of the choroid plexus and the meninges, but results on its presence in cells of neuroepithelial origin are controversial. However, IGF-II mRNA is transiently expressed in neurones during mammalian early development. In bony fish, IGF-II mRNA is also present in the adult brain but nothing is known about its synthesis sites. Thus, the present study using in situ hybridization with digoxigenin-labelled RNA species-specific probes investigates the cellular distribution of IGF-II mRNA in the adult brain of a bony fish, the tilapia (Oreochromis mossambicus). As in mammals, IGF-II mRNA was strongly expressed in the choroid plexus and meninges. Thus, IGF-II synthesis by choroid plexus and meninges seems to have a long evolutionary history and may be common to all vertebrates. However, as shown by the detailed investigation of landmark nuclei and regions, IGF-II mRNA occurred also in numerous neurones at all levels of the tilapia brain. The distinct localization of IGF-II mRNA in neurones might indicate that neuronal IGF-II acts as transmitter or modulator. However, the widespread occurrence of the IGF-II-producing neurones argues against this assumption and most probably suggests that IGF-II plays a role in the differentiation, maintenance and regeneration of neurones. It is further assumed that the sustained neuronal IGF-II expression in the brain of the adult tilapia correlates with continued post-embryonic up to life-long brain growth as has been shown in many teleost fishes.

Localization of IGF-I, IGF-I receptor, and IGFBP-2 in developing Umbrina cirrosa (Pisces: Osteichthyes)

In this study, the distribution of IGF-I, IGF type I receptor (IGF-IR), and IGF-binding protein 2 (IGFBP-2) was investigated during larval and post-larval developmental stages of the shi drum (Umbrina cirrosa) by immunohistochemistry using antisera raised against Sparus aurata IGF-I and IGF-IR, and against mouse IGFBP-2. Immunoreactivity of the mitogenic marker PCNA (proliferating cell nuclear antigen) was used for assessment of cellular proliferation. Distribution of IGF-I mRNA was studied by in situ hybridization. IGF-I immunoreactivity was detected in liver and developing intestine already in 1-5 day post-hatching larvae. From day 11, immunostaining in the intestine was evident in the enterocytes of the anterior intestine and in the apical zone of the epithelium of developing posterior intestine. Positive reaction with IGF-I antibody was also detected in chondrocytes, in the epithelium of the skin, gills and in the central nervous system (CNS), and lateral muscle. At hatching IGF-IR immunoreactivity was already detectable in developing CNS, notochord, and skin. From day 6 immunostaining was evident in the olfactory epithelium, in eyes and from day 11 in the developing olfactory bulbs and CNS. Positive reaction with IGF-IR antibody was also detected in chondrocytes, in the epithelium of the skin, gills, heart, and in the lateral muscle. Immunoreactive IGFBP-2, as detected by anti-mouse IGFBP-2 antiserum, exhibited generally a similar distribution pattern to that of IGF-I and IGF-IR. In situ hybridization, which has been performed by using riboprobes from S. aurata cDNA, revealed IGF-I mRNA in skeletal musculature, liver, and CNS. These data strongly suggest a role for the IGF system during development and growth of U. cirrosa.

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

To characterize and study the variations of IGF-I binding during the development of trout muscle cells, in vitro experiments were conducted using myocyte cultures, and IGF-I binding assays were performed in three stages of cell development: mononuclear cells (day 1), small myotubes (day 4), and large myotubes (day 10). Binding experiments were done by incubating cells with IGF-I for 12 h at 4 degrees C. Specific IGF-I binding increased with the concentration of labeled IGF-I and reached a plateau at 32 pM. The displacement of cold human and trout IGF-I showed a very similar curve (EC(50) = 1.19 +/- 0.05 and 0.95 +/- 0.05 nM, respectively). IGF binding proteins did not interfere significantly because displacement of labeled IGF-I by either cold trout recombinant IGF-I or Des (1-3) IGF-I resulted in similar curves. Insulin did not displace labeled IGF-I even at very high concentrations (>1 microM), which indicates the specificity of IGF-I binding. The amount of receptor (R(0)) increased from 253 +/- 51 fmol/mg DNA on day 1 to 766 +/- 107 fmol/mg DNA on day 10. However, the affinity (K(d)) of IGF-I receptors did not change significantly during this development (from 1.29 +/- 0.19 to 0.79 +/- 0.13 nM). On the basis of our results, we conclude that rainbow trout muscle cells in culture express specific IGF-I receptors, which increase their number with development from mononuclear cells to large myotubes.

Characterization of pituitary IGF-I receptors: modulation of prolactin and growth hormone

There have been no studies in any vertebrate that have localized insulin-like growth factor (IGF)-I receptors in prolactin (PRL) cells or that have correlated pituitary binding to the potency of IGF-I in regulating both PRL and growth hormone (GH) secretion. We show that IGF-I binds with high affinity and specificity to the pituitary gland of hybrid striped bass (Morone saxatilis x M. chrysops). IGF-I and IGF-II were equipotent in inhibiting saturable (125)I-IGF-I binding, whereas insulin was ineffective. IGF-I binds with similar affinity to the rostral pars distalis (>95% PRL cells) as the whole pituitary gland and immunohistochemistry colocalizes IGF-I receptors and PRL in this same region. Des(1-3)IGF-I, a truncated analog of IGF-I that binds with high affinity to IGF-I receptors but weakly to IGF-I binding proteins (IGFBPs), showed a similar inhibition of saturable (125)I-IGF-I binding, but it was more potent than IGF-I in stimulating PRL and inhibiting GH release. These results are the first to localize IGF-I receptors to PRL cells, correlate IGF-I binding to its efficacy in regulating GH and PRL secretion, as well as demonstrate that IGFBPs may play a significant role in modulating the disparate actions of IGF-I on PRL and GH secretion.

PCR-cloning and gene expression studies in common carp (Cyprinus carpio) insulin-like growth factor-II

Insulin-like growth factor-II (IGF-II) is a member of a growth factor family related to fetal growth in mammals but its physiological role has not been clearly identified in fish. In teleosts, the basic mechanism of the growth hormone (GH)-IGF axis is known to be operative but in a different manner. For instance, IGF-I exhibits GH dependence whereas for IGF-II, its GH dependence varies in different fish species. In this study, we used polymerase chain reaction (PCR) to obtain a common carp IGF-II (ccIGF-II) cDNA fragment and methods of rapid amplification of cDNA ends (RACEs) to obtain a full-length ccIGF-II sequence. The ccIGF-II encodes for a predicted amino acid sequence showing identities of 70.6%, 68.7%, 63.4% and 35% in comparison with salmon, barramundi, tilapia and human IGF-II, respectively. The nucleotide identity between the open reading frame (ORF) of the ccIGF-II and ccIGF-I cDNA sequence is only 36.2%. Distribution of ccIGF-II mRNA levels in common carp tissues was also studied; ccIGF-II expressed in hepatopancreas, heart, and many other tissues in adult carps are similar to the levels of ccIGF-I except in gills and testis. ccIGF-II levels were significantly higher than that of ccIGF-I in most juvenile tissues except in hepatopancreas, where ccIGF-I was higher (threefold) than that of ccIGF-II. The levels of ccIGF-I were also higher than ccIGF-II in carp larvae, from pre-hatched stage to day 30 post-hatching. Injection of porcine GH (pGH) increased the IGF-I and IGF-II mRNA levels in the hepatopancreas and brain of juvenile carps. However, hepatic IGF-I mRNA levels were induced more than IGF-II by pGH, whereas ccIGF-II levels gave a higher response than IGF-I in the brain in response to GH induction.

Characterization of the receptor for insulin-like growth factor on Leishmania promastigotes

Insulin-like growth factor (IGF)-I constitutively present in the skin is one of the first growth factors that Leishmania parasites encounter after transmission to the vertebrate host. We have previously shown that IGF-I is a potent growth-promoting factor for Leishmania parasites. IGF-I binds specifically to a single-site putative receptor at the parasite membrane, triggering a cascade of phosphorylation reactions. In the present article we characterize the receptor for IGF-I on Leishmania (Leishmania) mexicana promastigotes. The receptor is a monomeric glycoprotein with a molecular mass of 65 kDa and is antigenically related to the alpha chain of human type 1 IGF-I receptor. Upon IGF-I stimulation the receptor undergoes autophosphorylation on tyrosine residues with activation of its signaling pathway. Activation of the IGF-I receptor also leads to phosphorylation of an 185-kDa molecule that is homologous to the substrate of the insulin receptor present in human cells, the insulin receptor substrate 1 (IRS-1).

Receptors for insulin-like growth factor-I (IGF-I) predominate over insulin receptors in skeletal muscle throughout the life cycle of brown trout, Salmo trutta

Insulin and IGF-I binding has been studied in brown trout (Salmo trutta) wheat germ agglutinin semipurified receptors from embryos (organogenesis), larvae (yolk sac), juveniles (2.98 +/- 0.21 g bw) and adults (111.6 +/- 6.92 and 522 +/- 53 g bw). Embryos and larvae were sampled at 5 and 12 weeks after fertilization (December 1999 and February 2000) and juvenile and adults were taken simultaneously (July 1999) and under the same feeding conditions to minimize potential nutritional and seasonal effects. Insulin receptor number was maximal at 12 weeks (144 fmol/mg glycoprotein) and progressively decreased in subsequent samplings. No alterations in affinity were detected (K(d) range, 0.21-0.32 nM) and changes in number of receptor paralleled changes in total specific binding. IGF-I receptor number was highest at 5 weeks (1044 fmol/mg) and was significantly higher than values for insulin in all samplings. The affinity of IGF-I receptor did not change (K(d) range, 0.11-0.18 nM) but was consistently higher than that for the insulin receptor. A more rapid decrease of IGF-I binding and receptor number was found with age. However, the ratio of insulin/IGF-I binding established in 12-week-old larvae (0.18 +/- 0.01) was thereafter maintained at very similar values in juveniles and adults (0.15-0.17). Tyrosine kinase activity (TKA) for insulin receptors ranged between 136 and 183% and there were no significant changes with age. For the IGF-I receptor, TKA ranged from 174 to 281% and was significantly higher in 5-week-old larvae coincident with the highest levels of receptor number and declined gradually in parallel with binding levels. In conclusion, the greater abundance of IGF-I receptors during embryonic and larval development is maintained throughout juvenile and adult stages. This would suggest a key role for IGF-I in the growth and metabolism of trout muscle.

IGFs stimulate zebrafish cell proliferation by activating MAP kinase and PI3-kinase-signaling pathways

Insulin-like growth factor (IGF)-I and -II have been cloned from a number of teleost species, but their cellular actions in fish are poorly defined. In this study, we show that both IGF-I and -II stimulated zebrafish embryonic cell proliferation and DNA synthesis in a concentration-dependent manner, whereas insulin had little mitogenic activity. Affinity cross-linking and immunoblotting studies revealed the presence of IGF receptors with the characteristics of the mammalian type I IGF receptor. Competitive binding assay results indicated that the binding affinities of the zebrafish IGF-I receptors to IGF-I, IGF-II, and insulin are 1.9, 2.6, and >190 nM, indicating that IGF-I and -II bind to the IGF-I receptor(s) with approximately equal high affinity. To further investigate the cellular mechanism of IGF actions, we have studied the effects of IGFs on two major signal transduction pathways: mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3 kinase). IGFs activated MAPK in zebrafish embryonic cells in a dose-dependent manner. This activation occurred within 5 min of IGF-I stimulation and disappeared after 1 h. IGF-I also caused a concentration-dependent activation of protein kinase B, a downstream target of PI3 kinase, this activation being sustained for several hours. Inhibition of MAPK activation by the MAPK kinase inhibitor PD-98059 inhibited the IGF-I-stimulated DNA synthesis. Similarly, use of the PI3 kinase inhibitor LY-294002 also inhibited IGF-I-stimulated DNA synthesis. When both the MAPK and PI3 kinase pathways were inhibited using a combination of these compounds, the IGF-I-stimulated DNA synthesis was completely negated. These results indicate that both IGF-I and -II are potent mitogens for zebrafish embryonic cells and that activation of both the MAPK and PI3 kinase-signaling pathways is required for the mitogenic action of IGFs in zebrafish embryonic cells.

Contribution of residues A54 and L55 of the human insulin-like growth factor-II (IGF-II) A domain to Type 2 IGF receptor binding specificity

The underlying specificity of the interaction between insulin-like growth factor-II (IGF-II) and mammalian Type 2 insulin-like growth factor/cation-independent mannose 6 phosphate receptor (IGF2R) is not understood. We have mutated residues A54 and L55 of IGF-II in the second A domain helix to arginine (found in the corresponding positions of IGF-I) and measured IGF2R binding. There is a 4- and 3.3-fold difference in dissociation constants for A54R IGF-II and L55R IGF-II, respectively, and a 6.6-fold difference for A54R L55R IGF-II compared with IGF-II as measured by BlAcore analysis using purified rat IGF2R. This is also confirmed using cross-linking and soluble rat placental membrane receptor binding assays. Binding to the type I IGF receptor (IGF1R) and IGF binding protein-2 (IGFBP-2) is not altered. We can, therefore, conclude that residues at positions 54 and 55 in IGF-II are important for and equally contribute to IGF2R binding.

Insulin-like growth factor receptors and their ligands in gonads of a hermaphroditic species, the gilthead seabream (Sparus aurata): expression and cellular localization

Expression of insulin-like growth factor (IGF)-I, IGF-II, and IGF type I receptor (IGF-1R) genes was studied in gonads at different developmental stages of the protandrous hermaphroditic species the gilthead seabream (Sparus aurata) by reverse transcription-polymerase chain reaction and Northern blot analysis. Both IGF-I and IGF-II mRNA levels were highest in bisexual gonads and decreased during gonadal development. Regardless of the stage of gametogenesis, IGF-II mRNA levels exceeded those of IGF-I. Transcripts for IGF-1R RNA were detected in gonads at all stages studied. A major transcript of 11 kb was found in gonads and in gill arch and brain, but it was not found in liver and muscle. Distribution of the two types of IGF-1R and IGF-I in gonads was studied by immunohistochemistry. Immunoreactive IGF-I was found in the granulosa and theca cells of follicles at different vitellogenic stages and in oocytes at the chromatin-nucleolus and perinucleolus stage. In the testis, immunoreactive IGF-I was found in somatic cells of the cyst wall, interstitial cells, and spermatogonia A. In addition, IGF-1R was detected in the membrane of previtellogenic oocytes and in the theca and granulosa cells of vitellogenic and late vitellogenic follicles. In the testis, a positive reaction was identified in spermatogonia A and spermatids for the germ cells and in somatic cells of the cyst walls and interstitial cells. Local expression and production of IGFs and their receptors in fish gonads support a role for the IGF system in fish gonadal physiology.

Comparison of recombinant barramundi and human insulin-like growth factor (IGF)-I in juvenile barramundi (Lates calcarifer): in vivo metabolic effects, association with circulating IGF-binding proteins, and tissue localisation

The in vivo actions of human and fish insulin-like growth factor (IGF)-I have been compared to extend the understanding of the metabolism of IGFs in fish and to identify potential differences in their actions. The effects of acute administration of these proteins on the incorporation of glucose into muscle glycogen and leucine into liver protein in juvenile barramundi were investigated. In these in vivo metabolic assays, both baramundi IGF-I (bIGF-I) and human IGF-I (hIGF-I) increase the incorporation of D-[14C]glucose into muscle glycogen and [14C]leucine into liver protein. The distribution of radio-labeled human and barramundi IGF-I in the circulation and their uptake by tissue was also compared in juvenile barramundi (Lates calcarifer). Analysis of trichloroacetic acid-precipitable radioactivity in sequential samples following bolus injection of radiolabeled IGFs revealed that hIGF-I was degraded faster than bIGF-I. Neutral gel chromatography of these samples suggested that this difference is due to reduced affinity of hIGF-I, compared to bIGF-I, for the IGF-binding proteins (IGFBPs) present in the barramundi. Tissue uptake of [125I]-labeled hIGF-I and bIGF-I was similar except that [ 125I]bIGF-I uptake by the kidney exceeded that of hIGF-I. It is suggested that while some of the in vivo actions of IGFs in fish are conserved, functional differences between mammalian and teleostean IGFs exist, particularly with respect to their interactions with fish IGFBPs.

Ontogeny of the insulin-like growth factor system (IGF-I, IGF-II, and IGF-1R) in gilthead seabream (Sparus aurata): expression and cellular localization

There is evidence for the presence of an insulin-like growth factor (IGF) system during fish development. The pattern of gene expression of IGF-I, IGF-II, and their cognate receptors during early development of gilthead seabream (Sparus aurata) was studied by reverse transcription-polymerase chain reaction (RT-PCR). Transcripts for IGF-I, IGF-II, and IGF-1R were detected throughout development in unfertilized eggs, embryos, and larvae, suggesting that these mRNAs are products of both the maternal and the embryonic genomes. Analysis of IGF-1R mRNA in various adult tissues using RT-PCR revealed expression in all tissues studied, with the highest levels in gill cartilage, skin, kidney, heart, pyloric caeca, and brain. The distribution of the two types of IGF-1R and IGF-I in gilthead seabream larvae was studied by immunohistochemistry and found to be tissue-specific and age-dependent. IGF-I and its receptors are widely distributed and appear in various tissues of seabream larvae. IGF-I immunoreactivity was highest in skeletal muscle and pancreas. The general distribution of the two types of IGF receptors in larval tissues appeared similar except for the muscle and the corpus cerebelli, in which IGF-1R was detected only by SpIR6 antisera. Both IGF-I and IGF-II may thus play a role during early development of teleosts, as in other vertebrates.

Insulin-like growth factor-I and -II in the ovary of a bony fish, Oreochromis mossambicus, the tilapia: in situ hybridisation, immunohistochemical localisation, Northern blot and cDNA sequences

There is accumulating evidence that insulin-like growth factor (IGF)-I and IGF-II are present in the mammalian ovary but comparable studies on bony fish remain scarce. Thus, the present study aims to analyse several parameters of the IGFs in the ovary of a bony fish, the tilapia, (Oreochromis mossambicus). Molecular biological and morphological techniques were applied. The IGF-I and IGF-II cDNA sequences established from the ovary indicate that the same molecules are present in ovary and liver. Northern blot analysis revealed four IGF-I mRNA transcripts (6.0, 3.9, 1.9, 0.5 kb) and three IGF-II mRNA transcripts (5.0, 4.0, 2.0 kb) in ovary and liver. The amounts of IGF-I and IGF-II mRNA in the ovary were considerably high when compared to those in liver (IGF-I: 80.7%; IGF-II: 63.7%). The expression of IGF-I mRNA and IGF-II mRNA in the ovary were studied by in situ hybridisation and the peptides located by immunohistochemistry. The expression of IGF-I varied between the different developmental stages. Both IGF-I mRNA and IGF-I immunoreactivity were present in small oocytes. Moderate IGF-I expression and immunoreactivity occurred in granulosa cells of follicles at the lipid stage. A high IGF-I expression was observed in the granulosa and theca cells surrounding oocytes at the yolk globule stages and mature oocytes but neither IGF-I mRNA nor IGF-I immunoreactivity occurred in oocytes of the later stages. Thus, the IGF-I production seems to change from the young oocyte to the surrounding follicle cells at the later stages. In contrast, IGF-II mRNA and IGF-II-immunoreactivity occurred only in granulosa cells of the late follicle stages. The results suggest that both IGF-I and IGF-II are involved in the maturation of bony fish oocytes and in follicle development in a paracrine/autocrine manner. IGF-I and IGF-II may exert their effects at different stages of development. Furthermore, the intraovarian IGF-I and IGF-II systems seem to have a long phylogenetic history indicating the importance of the IGFs in reproductive biology.

Characterization of teleost insulin receptor family members

Insulin from mammals and fish has been used to determine insulin-binding affinities and receptor numbers with remarkable similarities between these two vertebrates, suggesting functional conservation. Yet, the nature and structure of teleost insulin receptors are not known. Therefore, the cloning and mRNA characterization of rainbow trout insulin receptors were undertaken. Three insulin receptor cDNAs were isolated by screening a cDNA library, confirmed as separate genes by genomic Southern hybridization, and designated as rainbow trout insulin receptor a (rtIR a), rainbow trout insulin receptor b (rtIR b), and rainbow trout insulin receptor c (rtIR c). A high degree of amino acid identity was observed between rainbow trout insulin receptors (rtIRs) and their human homolog, confirming the structural similarities between mammalian and fish insulin receptors. Reverse transcription-polymerase chain reaction from total RNA using either oligo(dT) or random hexamer primers resulted in a diminished ability to detect rtIR a and rtIR b mRNA when oligo(dT) was used, suggesting developmental and tissue-specific polyadenylation. The highest steady-state levels of rtIR mRNAs were consistently detected in juvenile and adult pyloric caeca (which also contained adipose and pancreatic tissue), while the lowest levels were consistently found in muscle. A high level of rtIR b and rtIR c mRNA was also found in ovary, while a high level of rtIR a was found in adult brain. Significant differences were also found between steady-state rtIR mRNA levels in corresponding juvenile and adult tissues. These results suggest a complex expression pattern of insulin receptor mRNAs in partial tetraploid fish.

Characterization of teleost insulin receptor family members. II. Developmental expression of insulin-like growth factor type I receptor messenger RNAs in rainbow trout

The insulin-like growth factor (IGF) system in teleosts consists of two ligands, IGF-I and IGF-II, multiple binding proteins, and high-affinity transmembrane receptors. There exists a large gap in our knowledge of the structure and expression of receptors mediating the biological effects of the IGFs in teleosts. For example, nucleotide sequence data other than those from the kinase domain, evidence of multiple genes, mRNA expression pattern and polyadenylation status in multiple tissues at different developmental stages, and quantitation of mRNA levels in multiple tissues are not known for any teleost. In the study described here, two rainbow trout IGF type I receptor cDNAs (rtIGFR Ia and rtIGFR Ib) were isolated by a 5' rapid amplification of cDNA ends method and confirmed as separate genes by genomic Southern blot hybridization. The predicted amino acid sequences are 85% identical to each other in the tyrosine kinase domain. Both cDNAs are more homologous to mammalian IGF type I receptors than to insulin receptors. Reverse transcription-polymerase chain reaction from total RNA using either oligo(dT) or random hexamers as primers resulted in a diminished ability to detect IGF receptor mRNAs when oligo(dT) was used, suggesting developmental and tissue-specific polyadenylation. The highest steady-state mRNA levels of rtIGFR Ia were found in juvenile gill and adult heart, while the highest levels of rtIGFR Ib were found in adult pyloric caeca, which also contained diffuse pancreatic and adipose tissue. The lowest steady-state mRNA levels of both rtIGFR Ia and rtIGFR Ib were found in juvenile heart, liver, muscle, and spleen, and adult liver. Significant differences in steady-state mRNA levels were also found between juveniles and adults. These results suggest a complex expression pattern of IGF type I receptor mRNAs in partial tetraploid fish.

Differential effect of insulin-like growth factor I on in vitro gonadotropin (I and II) and growth hormone secretions in rainbow trout (Oncorhynchus mykiss) at different stages of the reproductive cycle

The short-term effect of insulin-like growth factor I (IGF-I) on GTH I (FSH-like), GTH II (LH-like), and GH production by cultured rainbow trout pituitary cells was studied in immature fish of both sexes, at early gametogenesis and in spermiating and periovulatory animals. IGF-I had no effect on basal GTH I and GTH II release, whereas it always inhibited basal GH, showing decreasing intensity with the gonad maturation. In absence of IGF-I, GTH I and GTH II cells were always responsive to GnRH, whereas no response was observed for GH cells whatever the sexual stage. The action of IGF-I on the sensitivity to GnRH differs between GTH and GH cells. The former requires a coincubation with IGF-I (10(-6) M)/GnRH to show an increase in sensitivity, independent of the sexual stage. To be responsive to GnRH, the GH cells require longer exposure to IGF-I, the efficiency of which decreases with gonad maturation. The action of IGF-I (10(-6) M) on GTH cell sensitivity to GnRH does not seem to be related to a mitogenic effect or to an improvement in cell survival. It seems to be IGF-I specific, not passing via the insulin receptor. Certain hypotheses on the putative role of IGF-I and GnRH as a link between growth and puberty are suggested.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).

Molecular cloning and tissue expression of the insulin-like growth factor II prohormone in the bony fish Cottus scorpius

The cDNA encoding pro-IGF-II of an advanced teleost fish, Cottus scorpius (Scorpaeniformes), the daddy sculpin, was isolated from liver by RT-PCR and molecular cloning. Like other IGFs, the deduced 168 amino acid peptide contains B-, C-, A-, D-, and E-domains and six cysteine residues (CysB9, CysB21, CysA6, CysA7, CysA11, and CysA20) necessary for the maintenance of tertiary structure. At the amino acid level, the sculpin IGF-II prohormone exhibits 85-92% homology to pro-IGF-II of other bony fish but only 51% homology to human. The mature sculpin IGF-II peptide comprises 70 amino acids. Its A-, B-, and D-domains exhibit homologies as high as 91, 91, and 100%, respectively, when compared with the other bony fish species studied. The high sequence homologies may indicate a particular physiological impact of IGF-II in bony fish. RT-PCR followed by Southern blotting revealed an IGF-II mRNA transcript of the expected size in liver, pyloric and splenic islets, stomach, small and large intestine, kidney, gill, testis, ovary, brain, and heart. The local production of IGF-II in many organs indicates that IGF-II is involved in organ-specific functions in a paracrine/autocrine manner. Furthermore, the results show that all bony fish organs which have been demonstrated to express IGF-I mRNA also express IGF-II mRNA.

Insulin-like growth factor I is a growth-promoting factor for Leishmania promastigotes and amastigotes

Leishmaniases are diseases caused by protozoa of the genus Leishmania that affect more than 20 million people in the world. The initial phase of the infection is fundamental for either the progression or control of the disease. The Leishmania parasites are injected in the skin as promastigotes and then, after been phagocytized by the host macrophages, rapidly transform into amastigotes. In this phase different nonspecific cellular and humoral elements participate. We have shown previously that insulin-like growth factor (IGF)-I that is constitutively present in the skin induces growth of Leishmania promastigotes. In the present paper we show further evidence for the importance of this factor: (i) IGF-I also can induce a growth response in Leishmania (Leishmania) mexicana amastigotes; (ii) IGF-I binds specifically to a putative single-site receptor on both promastigotes and amastigotes; (iii) IGF-I induces a rapid tyrosine phosphorylation of parasite proteins with different molecular mass in promastigotes and amastigotes of L. (L.) mexicana; and, finally, (iv) the cutaneous lesion in the mice when challenged by IGF-I-preactivated Leishmania (Viannia) panamensis is increased significantly because of inflammatory process and growth of parasites. We thus suggest that IGF-I is another important host factor participating in the Leishmania-host interplay in the early stage during the establishment of the infection and presumably also in the later stages.

Evolution of insulin-like growth factor-I (IGF-I) action: in vitro characterization of vertebrate IGF-I proteins

While there is considerable structural evidence that IGFs share a long evolutionary history, less is known about the conservation of IGF action. These studies have primarily been hampered by the small amounts of purified IGFs that have been available for testing. More recently, however, we have adopted recombinant strategies to produce milligram quantities of IGFs for biological studies. Thus we have been able to compare the properties of rat, kangaroo, chicken, salmon and barramundi IGF-I, proteins that differ from human IGF-I by 3, 6, 8, 14 and 16 amino acids respectively. While we have found that the IGF-I proteins exhibit similar biological activities and type-I IGF receptor binding affinities, regardless of whether mammalian, avian or piscine cell lines are used, there was a trend suggesting that the fish proteins at least, were most effective in studies using homologous systems. Thus, salmon IGF-I was not as potent as human IGF-I in bioassays in mammalian cells, but was as effective as human IGF-I in piscine cells. As expected, the IGF-I proteins competed poorly for binding to type-2 receptors present on ovine placental membranes. Interestingly however, the two fish IGF-I proteins exhibited greater affinity for this receptor than the other IGF-I proteins, hence reminiscent of the results previously found with recombinant hagfish IGF. Despite these small differences, these results taken together indicate that the IGF-I proteins appear to have been remarkably conserved in both structure and in vitro action during vertebrate radiation.

The phylogeny of the insulin-like growth factors

The insulin-like growth factors are major regulators of growth and development in mammals and their presence in lower vertebrates suggests that they played a similarly fundamental role throughout vertebrate evolution. While originally perceived simply as mediators of growth hormone, on-going research in mammals has revealed several hierarchical layers of complexity in the regulation of ligand bioavailability and signal transduction. Our understanding of the biological role and mechanisms of action of these important growth factors in mammals patently requires further elucidation of the IGF hormone system in the simple model systems that can be found in lower vertebrates and protochordates. This review contrasts our knowledge of the IGF hormone system in mammalian and nonmammalian models through comparison of tissue and developmental distributions and gene structures of IGF system components in different taxa. We also discuss the evolutionary origins of the system components and their possible evolutionary pathways.

IGF-I in the bony fish Cottus scorpius: cDNA, expression and differential localization in brain and islets

The cDNA encoding prepro-insulin-like growth factor (IGF)-I of a teleost, Cottus scorpius, (Scorpaeniformes) was established from liver by RT-PCR and molecular cloning. Typically, the deduced 184 amino acid protein contains a signal peptide, B-, C-, A-, D- and E-domains and all residues necessary for maintenance of tertiary structure. C. scorpius IGF-I shares only approximately 57% identity with C. scorpius insulin in the A-domain and 7% in the B-domain. RT-PCR followed by Southern blotting revealed a transcript in liver, pancreatic islets, stomach, small and large intestine, kidney, gill, testis, ovary, heart and brain indicating paracrine/autocrine actions of locally produced IGF-I. IGF-I- and insulin-immunoreactivities coexisted in the islets, but did not in other sites such as brain. Thus, in contrast to other bony fish, sculpin insulin cells most probably produce IGF-I. The results also challenge the current model of the IGF/insulin evolution.

Appearance of insulin and insulin-like growth factor-I (IGF-I) receptors throughout the ontogeny of brown trout (Salmo trutta fario)

Insulin and IGF-I receptors were characterized in glycoprotein fractions prepared by affinity chromatography from different developmental stages of brown trout. The specificity of insulin and IGF-I binding was demonstrated by crossed-competition assays: unlabelled insulin displaced bound radiolabelled insulin at concentrations 45-fold lower than unlabelled IGF-I, whilst unlabelled IGF-I displaced bound radiolabelled IGF-I at concentrations 2,000-fold lower than unlabelled insulin. The affinity of these receptors did not change significantly during trout development. Insulin-specific binding was detectable 3 weeks after spawning, after which it increased to a maximum in fry weighing 0.4 g, and decreased progressively to adult levels. IGF-I specific binding was detectable in newly laid eggs and increased to a maximum during organogenesis in eyed eggs. It then decreased progressively during subsequent stages of development to adult levels. The apparent molecular weight (Mr) of the alpha-subunit of brown trout insulin and IGF-I receptors was smaller than that of the alpha-subunit of the rat insulin receptor. Receptor tyrosine kinase activity was stimulated in a dose-dependent manner by insulin and IGF-I. Insulin and IGF-I stimulated tyrosine kinase activity and reached a maximum of 201 +/- 17.6 and 240 +/- 29.6% of basal phosphorylation, respectively.

Insulin-like growth factor-I binds in the inner plexiform layer and circumferential germinal zone in the retina of the goldfish

Results of the previous study suggest that insulin-related peptides regulate proliferation of retinal progenitors in the adult goldfish. Because of their known roles in retinal neurogenesis, we have chosen to focus future studies on insulin-like growth factor I (IGF-I) and the IGF-I receptor. In the study described here, we characterized the spatial distribution and specificity of IGF-I binding sites in the retina of the adult goldfish by performing receptor-binding autoradiography with [125I]-IGF-I alone and with unlabeled IGF-I-related molecules (IGF-I, IGF-II, insulin, and des-[1-3]-IGF-I) as competitive inhibitors of [125I]-IGF-I binding. The results of these experiments show that IGF-I binds in two locations in the retina of the adult goldfish, within the inner plexiform layer of the differentiated retina and the circumferential germinal zone. The competition experiments suggest that [125I]-IGF-I binds at sites specific for IGF-I, and that both IGF-I receptors and IGF-I binding proteins are present in the retina.

Nutritional and developmental regulation of insulin-like growth factors in fish

The insulin-like growth factors (IGF) are evolutionarily ancient growth factors present in all vertebrates. The central importance of IGF for normal development and growth has been illustrated by the severe growth-retarded phenotype exhibited by IGF-I, IGF-II or IGF-I receptor "knockout" mice. Although we know much about the gross effects of IGF on the overall size of the fetus and the clinical manifestations that result from fetal and neonatal deficiency of IGF (i.e., severe growth retardation leads to dwarfism), very little is known about the in vivo actions of IGF during embryogenesis at the cellular and molecular levels. Most research on the developmental role of IGF has relied on rodent models, and attempts to elucidate the molecular and cellular basis of IGF actions have been hampered by the inaccessibility of the mammalian fetus enclosed in the uterus. During the past decade, there has been growing support for the concept that the IGF have been highly conserved in all vertebrates. Both IGF-I and IGF-II are present in fish, and their structures are highly conserved. Human and fish IGF-I are equally potent in mammalian and fish bioassay systems. Insulin-like growth factor mRNA is found in all life stages of fish, ranging from unfertilized egg to adult. The temporal and spatial expression patterns of fish IGF-I seem to be similar to those in mammals. Nutritional status and growth hormone both have a profound effect on IGF-I expression in fish, as they do in mammals. These features suggest that the IGF system is highly conserved between teleost fish and mammals. Because fish embryos develop externally, they provide excellent animal models for understanding the regulatory roles of IGF, IGF receptor and IGF-binding proteins in vertebrate embryonic development. Current research on the developmental and nutritional roles of IGF in fish will undoubtedly contribute to knowledge of the basic physiology of vertebrates in general.

Localization of IGF-I and IGF-I receptor mRNA in Sparus aurata larvae

Studies of the ontogeny of IGF-I mRNA during embryonic and larval development of the gilthead sea bream Sparus aurata showed its expression during these early developmental stages. The present study applies in situ hybridization to localize IGF-I and IGF receptor mRNAs in 16-day larvae of S. aurata. Paraffin sections were hybridized with homologous RNA probes labeled by [35S]UTP. IGF-I mRNA expression was found mainly in chondrocytes, in both the gill arches and cranial cartilage, in skeletal muscle, in the brain, in the pancreas, in the retina, and in the epithelial cells surrounding the lens. A strong positive reaction for IGF receptor mRNA was found in skeletal muscle, in the pancreas, and in the lymphoid tissue found in the intertubular tissue of the kidney. Signals were less intense in brain and chondrocytes. It is suggested that in teleosts, as in higher vertebrates, IGF-I may be involved in the regulation of tissue growth and differentiation in an autocrine/paracrine manner.

Insulin-like growth factor I in the teleost Oreochromis mossambicus, the tilapia: gene sequence, tissue expression, and cellular localization

Using reverse transcription-PCR and molecular cloning, the complementary DNA sequence encoding preproinsulin-like growth factor I (IGF-I) of a teleost, the tilapia (Oreochromis mossambicus) was established from liver. At the amino acid level, tilapia IGF-I shows all residues necessary for the maintenance of tertiary structure and shares about 80% identity with IGF-I from other teleosts. The B and A domains of tilapia IGF-I show more than 90% homology to those of other teleosts and 86-93% to those of human. However, in contrast to salmonids, the C domain of tilapia is truncated. Reverse transcription-PCR analysis followed by Southern blotting with an internal probe specific for tilapia IGF-I indicated a transcript in liver, pancreas, gut, kidney, head kidney, gill, ovary, testis, eye, and brain. In correlation, parenchymal cells were identified as likely local production sites by the use of immunohistochemistry. IGF-I immunoreactivity was confined to D cells in pancreatic islets, gastroentero-endocrine cells, cells of renal proximal tubules, interrenal cells of the head kidney, gill chondrocytes, chloride cells of the gill epithelium, granulosa cells in the ovary, spermatocytes and Sertoli cells in testis, and neurons in retina and brain. The local production of IGF-I in multiple organs of the tilapia indicates paracrine/autocrine actions of IGF-I involved in organ-specific functions. The results further demonstrate that the primary structure of IGF-I, especially in the B and A domains, is highly conserved during phylogeny.

Characterization of an insulin-like growth factor (IGF) receptor and the insulin-like effects of IGF-1 in the bony fish, Lates calcarifer

The present work is part of a broad phylogenetic study of the insulin superfamily of peptides in lower vertebrates. In the bony fish barramundi (Lates calcarifer), the presence of IGF receptors were investigated in the liver by means of competitive binding studies. The results suggested the presence of a type 1-like but no type 2-like IGF receptor. We also demonstrated insulin-like effects of intraperitoneally injected recombinant human (rh)-IGF-1 in barramundi with rh-IGF-1 and rh-insulin showing similar effects with respect to induction of hypoglycemia and stimulation of incorporation of [14C]-glucose into muscle glycogen.

Insulin-like growth factor (IGF-I) mRNA and IGF-I receptor in trout testis and in isolated spermatogenic and Sertoli cells

Few data exist concerning the occurrence and potential role of an insulin-like growth factor (IGF) system in fish gonads. Using Northern and slot blot hybridization with a specific salmon IGF-I cDNA, we confirmed that IGF-I transcription occurs in trout testis. Testicular IGF-I mRNA abundance may be increased by long-term GH treatment in juvenile fish, while shorter treatment with growth hormone (GH) or a gonadotropin (GTH-II) in maturing males had no statistically significant effect. Radiolabelled recombinant human IGF-I binds with high affinity to crude trout testis preparation, to cultured isolated testicular cells, and to a membrane fraction of these cells (Ka = 0.2 to 0.7 x 10(10) M-1; Bmax = 10 to 20 fmol/10(7) cells, and 68 fmol/mg protein of membrane). The binding site was identified as type 1 IGF receptor by its binding specificity (IGF-I > IGF-II >>> insulin) and the molecular size of its alpha-subunit labelled with 125I-IGF-I (M(r)125-140 kDa). 125I-IGF-II also bound to the type 1 receptor whereas IGF-II/ mannose 6 phosphate receptors could not be detected. Separation of isolated testicular cells by Percoll gradient and centrifugal elutriation provided populations enriched in different types of intratubular cells. IGF-I mRNA (detected by reverse transcription + polymerase chain reaction [PCR]) and IGF-I receptors (measured by competitive binding) were observed to a greater extent in Sertoli cell-enriched populations and in spermatogonia with primary spermatocytes. Therefore, IGF-I is a potential paracrine/autocrine regulator inside the spermatogenic compartment and appears as a possible mediator of GH action at the gonadal level in fish.