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

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

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

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.

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.

Differential expression of the two GH genes during embryonic development of rainbow trout Oncorhynchus mykiss in relation with the IGFs system

The Growth hormone (GH)/insulin-like growth factor (IGF) system promotes embryonic growth in higher vertebrates. Such a system exists in salmonids, but exhibits an additional level of complexity resulting from a recent whole genome tetraploidisation. Thus, two nonallelic GH genes are present in the trout genome. Although the two GH genes are similar, the possibility remains that the two genes have evolved separately, acquiring a distinct expression pattern. In this study, using whole mounted in situ hybridisation, we observed a one stage delay between the appearance of GH-2 (Stage 22) and GH-1 (Stage 23) soon after pituitary formation (Stage 21). In addition, by double in situ hybridisation, we clearly evidenced two types of somatotroph, one expressing only GH-2 and the other type both GH-1 and GH-2 at Stage 24. Consequently, at this stage more cells expressed GH-2 than GH-1 as confirmed by quantitative RT-PCR. However at hatching, as in adult, the difference between the expression of the two GH genes was no longer observed. In addition, our immunohistochemical studies did not show any delay between the expression of the mRNA and its translation as a protein at Stage 24. A comparison of the expression pattern of the IGF system components (IGF-1, IGF-2, and the receptor type I) determined by real time RT-PCR, have shown an IGF-1 mRNA increase concomitantly to the appearance of GH expression. On the whole, our results demonstrate a differential regulation of GH-1 and GH-2 genes in rainbow trout embryo. The relationship observed between the expression of different component of the GH/IGF system seems to indicate that this system could be functional early on during embryonic development.

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.

Identification of a type II insulin-like growth factor receptor in fish embryos

To determine whether fish have an insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6-P) receptor similar to that of mammals, we have performed binding, cross-linking, and immunoprecipitation experiments with wheat-germ-agglutinin- and mannose 6-phosphate (M6-P)-affinity-purified receptor preparations from fish embryos. In both receptor preparations, IGF-II binding was specific, because labeled IGF-II could only be completely displaced by cold IGF-II but not by IGF-I or insulin. Labeled IGF-II bound to a protein with a molecular mass of approximately 250 kDa, which could be immunoprecipitated with an antibody against the rat IGF-II receptor. IGF-II stimulated tyrosine kinase activity in wheat germ agglutinin preparations and was more potent than insulin or IGF-I, but neither peptide stimulated tyrosine kinase activity in M6-P preparations. Two fish cell lines (CHSE-214 and EPC) were used to confirm the IGF-II binding data obtained in the receptor preparations, revealing the presence of highly specific IGF-II binding and the absence of insulin binding. Furthermore, a decrease of the IGF-I receptors on the cell surface did not alter IGF-II binding in EPC cells. In conclusion, we have detected the presence of IGF-II/M6-P receptors in fish embryos that are similar in structure and specificity for their ligand to those found in mammals.

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.