Evolution of the insulin gene superfamily. Sequence of a preproinsulin-like growth factor cDNA from the Atlantic hagfish

Reassessment of an Innovative Insulin Analogue Excludes Protracted Action yet Highlights the Distinction between External and Internal Diselenide Bridges

Long-acting insulin analogues represent the most prescribed class of therapeutic proteins. An innovative design strategy was recently proposed: diselenide substitution of an external disulfide bridge. This approach exploited the distinctive physicochemical properties of selenocysteine (U). Relative to wild type (WT), Se-insulin[C7U^A , C7U^B ] was reported to be protected from proteolysis by insulin-degrading enzyme (IDE), predicting prolonged activity. Because of this strategy's novelty and potential clinical importance, we sought to validate these findings and test their therapeutic utility in an animal model of diabetes mellitus. Surprisingly, the analogue did not exhibit enhanced stability, and its susceptibility to cleavage by either IDE or a canonical serine protease (glutamyl endopeptidase Glu-C) was similar to WT. Moreover, the analogue's pharmacodynamic profile in rats was not prolonged relative to a rapid-acting clinical analogue (insulin lispro). Although [C7U^A , C7U^B ] does not confer protracted action, nonetheless its comparison to internal diselenide bridges promises to provide broad biophysical insight.

Sequential and Asynchronous Strengthening of the Influence of Temperature on the Endo- and Exocytosis of Insulin in the Isolated Vertebrata Hepatocytes: Summing up Previous Studies

Insulin internalization and processing of the Insulin Receptor Complex (IRC) inside the cell are important components of the intracellular Mechanism of Insulin Action (MIA). They define the continuation of intracellular signaling of IRC and allow utilization of the parts of the complex after ligand dissociation. Traditionally, changes in the insulin regulatory system associated with the vertebrate phylogenesis have been evaluated by changes of its two elements: the hormone and its receptor. A hormone-competent cell was considered as an evolutionarily completed element of insulin regulatory system. However, previous studies of the isolated hepatocytes of four classes of vertebrates (lamprey, frog, chicken, and rat) revealed significant differences in the state of internalization of 125I-insulin and intracellular IRC processing. Radical differences were noted in the regulation of 125I-insulin internalization and the intracellular fate of the IRC. Here, cytosolic efficient insulin degradation and a complete lack of 125I-insulin exocytosis were observed in the cyclostome cells, whereas in amphibians the hormone underwent lysosomal degradation and showed low levels of exocytosis, while birds and mammals were characterized by high volumes of the excreted 125Iinsulin containing proteolytic 125I-insulin fragments. Despite the established recognition of the importance of the temperature factor, a complete understanding of the molecular mechanisms underlying the temperature effects on MIA is still missing. This poorly studied problem of the MIA temperature dependence can be behind the differences in the effect of temperature on the intracellular action of insulin and IGF-I. In fact, at different phylogenetic stages, successive changes were reported for the temperature dependence of the 125Iinsulin internalization and exocytosis. The following regularities were reported for the effect of temperature on the 125I-insulin internalization in isolated hepatocytes of different origin: complete lack of receptibility of the process to temperature in lampreys, receptibility of the process in a narrow range of low temperatures (0-5°C) in amphibians, and flexible regulation of 125I-insulin internalization in a wide temperature range (6- 37°C) in the cells from endothermic organisms. Reported data make it possible to observe three stages in the alteration of temperature regulation of 125I-insulin internalization (in cells of cyclostomes, amphibians, and endothermic organisms) and two stages of temperature regulation of 125I-insulin exocytosis in cells of amphibians, birds, and mammals. The data presented in this study reflect the specificity of the developmental reorganization of the intracellular MIA regulation and hormone utilization, and emphasize the central role of temperature in selective MIA formation during vertebrate phylogenesis.

Lamprey IGF-Binding Protein-3 Has IGF-Dependent and -Independent Actions

Insulin-like growth factor-binding proteins (IGFBPs) are multifunctional proteins that possess IGF-dependent and -independent actions. Recent studies suggest that its IGF-independent action appeared early and that the IGF-binding function may have been acquired later in evolution. The timing of the emergence of IGF-dependent actions is unclear. Here, we identified and characterized an igfbp gene from sea lamprey, an agnathan, which was separated from the jawed vertebrates 450 million years ago. Phylogenetic and structural analyses suggested that the encoded protein belongs to the IGFBP-3 clade in the IGFBP family. Lamprey IGFBP-3 contains an IGF-binding domain (IBD), nuclear localization signal, and transactivation (TA) domain. Biochemical and functional analyses showed that these domains are all functional. Lamprey IGFBP-3 can bind IGFs and modulate IGF signaling when tested in mammalian cells. Lamprey IGFBP-3 also has the capacity to enter the nucleus and has strong TA activity. Forced expression of lamprey IGFBP-3, but not its IBD mutant, in zebrafish embryos decreased body growth and developmental speed. Lamprey IGFBP-3 inhibited BMP2 signaling in cultured cells and in zebrafish embryos, and this action is independent of its IGF-binding function. These results suggest that lamprey IGFBP-3 has both IGF-dependent and -independent actions and provide new insights into the functional evolution of the IGFBP family.

Hepatoselectivity and the evolution of insulin

In spite of major developments in insulin production, purification, pharmaceutical formulation and methods of delivery, problems remain both in the day to day management of insulin-treated diabetes and with regard to its long-term complications. The risks of hypoglycaemia and weight gain are major concerns particularly for the patient, and the persistence of microvascular and premature macrovascular complications as the main causes of morbidity and mortality in both type 1 and type 2 diabetes is a constant reminder that our therapeutic and management strategies are inadequate. One clear and striking difference between currently available insulin treatments and normal physiology is the relative difference in exposure to insulin of the liver versus peripheral tissues. Hepatoselective insulin analogues have the potential to restore the normal hepatic to peripheral gradient in insulin action. Here, we discuss the possible therapeutic potential that such analogues may have over currently available insulin preparations. These benefits could include a lower risk of hypoglycaemia, less weight gain and a potential reduction in microvascular and macrovascular complications. We explore the evolution of insulin with hepatoselectivity in mind and possible strategies to create hepatoselective insulins.

The skeleton as an endocrine organ

Surprising new discoveries in the field of skeletal biology show that bone cells produce endocrine hormones that regulate phosphate and glucose homeostasis. In this Review, we examine the features of these new endocrine pathways and discuss their physiological importance in the context of our current understanding of energy metabolism and mineral homeostasis. Consideration of evolutionary and comparative biology provides clues that a key driving force for the emergence of these hormonal pathways was the development of a large, energy-expensive musculoskeletal system. Specialized bone cells also evolved and produced endocrine hormones to integrate the skeleton in global mineral and nutrient homeostasis. The recognition of bone as a true endocrine organ represents a fertile area for further research and should improve the diagnosis and treatment of metabolic diseases such as osteoporosis and diabetes mellitus.

Amphioxus IGF-like peptide induces mouse muscle cell development via binding to IGF receptors and activating MAPK and PI3K/Akt signaling pathways

Insulin-like growth factors (IGFs) are identified in all vertebrates. An insulin/IGF hybrid polypeptide has also been identified in protochordate amphioxus. However, whether this hybrid polypeptide functions as vertebrate IGFs remains unknown. Here we established a primary culture system of mouse muscle satellite cells as an in vitro model to investigate the effects of amphioxus IGF-like molecule on muscle cell development. Like human IGF, recombinant IGF-like molecule was able to stimulate the proliferation of mouse muscle cells. Besides, it was able to bind to the cells and the partially purified IGF receptors from mouse muscle cells. Moreover, recombinant IGF-like molecule was capable of activating MAPK and PI3K/Akt pathways by stimulating phosphorylation of MAPK and Akt. This interaction of amphioxus IGF-like molecule with mammalian (mouse) IGF receptors and its induction of similar downstream signaling pathways add substantially to the hypothesis of the presence of IGF signaling in the common ancestor of protochordate/vertebrates.

Molecular cloning of IGF-1 and IGF-1 receptor and their expression pattern in the Chilean flounder (Paralichthys adspersus)

Insulin-like growth factor-1 and insulin-like growth factor-1 receptor (IGF-1 and IGF-1R) play main roles in vertebrate growth and development. In fish, besides contributing to somatic growth, both molecules exhibit pleiotropic functions. We isolated complete cDNAs sequences encoding for both IGF-1 and IGF-1R in the Chilean flounder by using RT-PCR and rapid amplification of cDNAs ends (RACE) techniques. We analyzed gene expression in pre-metamorphic larvae and different organs of juvenile fish through whole mount in situ hybridization and RT-PCR, respectively. In addition, we studied the presence of calcified skeletal structures in pre-metamorphic larvae through the fluorescent chromophore calcein. The IGF-1 cDNA sequence displays an open reading frame of 558 nucleotides, encoding a 185 amino acid preproIGF-1. Moreover, IGF-1R contains an open reading frame spanning 4239 nucleotides, rendering a 702 amino acid subunit alpha and a 676 amino acid subunit beta. The deduced mature IGF-1 and IGF-1R exhibited high sequence identities with their corresponding orthologs in fishes, especially those domains involved in biological activity. RT-PCR showed expression of IGF-1 and IGF-1R transcripts in all studied tissues, consistent with their pleiotropic functions. Furthermore, we observed IGF-1 expression in notochord and IGF-1R expression in notochord, somites and head in larvae of 8 and 9 days post fertilization. Complementarily, we detected in larvae of 8 days post fertilization the presence of calcified skeletal structures in notochord and head. Interestingly, both mRNAs and calcified structures were found in territories such as notochord, an embryonic midline structure essential for the pattern of surrounding tissues as nervous system and mesoderm. Our results suggest that IGF-1 and its receptor play an important role in the development of the nervous system, muscle and bone-related structures during larval stages.

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.

Zebrafish IGF genes: gene duplication, conservation and divergence, and novel roles in midline and notochord development

Insulin-like growth factors (IGFs) are key regulators of development, growth, and longevity. In most vertebrate species including humans, there is one IGF-1 gene and one IGF-2 gene. Here we report the identification and functional characterization of 4 distinct IGF genes (termed as igf-1a, -1b, -2a, and -2b) in zebrafish. These genes encode 4 structurally distinct and functional IGF peptides. IGF-1a and IGF-2a mRNAs were detected in multiple tissues in adult fish. IGF-1b mRNA was detected only in the gonad and IGF-2b mRNA only in the liver. Functional analysis showed that all 4 IGFs caused similar developmental defects but with different potencies. Many of these embryos had fully or partially duplicated notochords, suggesting that an excess of IGF signaling causes defects in the midline formation and an expansion of the notochord. IGF-2a, the most potent IGF, was analyzed in depth. IGF-2a expression caused defects in the midline formation and expansion of the notochord but it did not alter the anterior neural patterning. These results not only provide new insights into the functional conservation and divergence of the multiple igf genes but also reveal a novel role of IGF signaling in midline formation and notochord development in a vertebrate model.

Gene structure and functional characterization of growth hormone in dogfish, Squalus acanthias

Dogfish (Squalus acanthias) growth hormone (GH) was identified by cDNA cloning and protein purification from the pituitary gland. Dogfish GH cDNA encoded a prehormone of 210 amino acids (aa). Sequence analysis of purified GH revealed that the prehormone is composed of a signal peptide of 27 aa and a mature protein of 183 aa. Dogfish GH showed 94% sequence identity with blue shark GH, and also showed 37-66%, 26%, and 48-67% sequence identity with GH from osteichtyes, an agnathan, and tetrapods. The site of production was identified through immunocytochemistry to be cells of the proximal pars distalis of the pituitary gland. Dogfish GH stimulates both insulin-like growth factor-I and II mRNA levels in dogfish liver in vitro. The dogfish GH gene consisted of five exons and four introns, the same as in lamprey, teleosts such as cypriniforms and siluriforms, and tetrapods. The 5'-flanking region within 1082 bp of the transcription start site contained consensus sequences for the TATA box, Pit-1/GHF-1, CRE, TRE, and ERE. These results show that the endocrine mechanism for growth stimulation by the GH-IGF axis was established at an early stage of vertebrate evolution, and that the 5-exon-type gene organization might reflect the structure of the ancestral gene for the GH gene family.

An analysis of IGFBP evolution

OBJECTIVES

To perform a synonymous, non-synonymous codon mutational analysis of the IGFBP gene family and identify mechanisms by which the IGFBP subfamilies diverged.

METHODS

We identified 78 intact nucleotide sequences from 6 IGFBP subfamilies and 12 different species and used them for phylogenetic and synonymous, non-synonymous codon mutational analysis. Deletion and insertion comparisons were performed across subfamilies to determine if this might play a unique role in subfamily genesis.

RESULTS

IGFBP-2 was identified by phylogenetic analysis to be the most related subfamily of the IGFBP progenitor, followed by IGFBP-4. Insertions and deletions within the variable domains were associated with divergence of each subfamily from its progenitor, suggesting a common motif for IGFBP evolution. Insertions unique to mammals were also found within the amino terminus of IGFBP-2.

CONCLUSION

IGFBP subfamily divergence is associated with variable domain insertion or deletion and vigorous non-synonymous codon mutation. Our findings suggest strong selective pressure for IGFBP divergence in terrestrial vertebrates.

Cloning of Russian sturgeon (Acipenser gueldenstaedtii) growth hormone and insulin-like growth factor I and their expression in male and female fish during the first period of growth

In this study, the GH and IGF-I of the Russian sturgeon (rs), Acipenser gueldenstaedtii, were cloned and sequenced, and their mRNA gene expression determined. In addition, to improve our understanding of the GH function, the expression of this hormone was assessed in young males and females. Moreover, IGF-I expression was quantified in young males and compared to that in older ones. The nucleotide sequence of the rsGH cDNA was 980 bp long and had an open reading frame of 642 bp, beginning with the first ATG codon at position 39 and ending with the stop codon at position 683. A putative polyadenylation signal, AATAAA, was recognized 42 bp upstream of the poly (A) tail. The position of the signal- peptide cleavage site was predicted to be at position 111, yielding a signal peptide of 24 amino-acids (aa) and a mature peptide of 190 aa. When the rsGH aa sequence was compared with other species, the highest degree of identity was found to be with mammalians (66-70% identity), followed by anguilliformes and amphibia (61%) and other fish (39-47%). The level of rsGH mRNA was discovered to be similar in pituitaries of females and males of 5 age groups (1, 2, 3, 4, and 5- yr-old). In females and males, the levels did not change dramatically during the first 5 yr of growth. The partial nucleotide sequence of the rsIGF-I was 445 bp long and had an open reading frame of 396 bp, beginning with the ATG codon at position 50. The position of the signal-peptide cleavage site was predicted to be at position 187, yielding a signal peptide of 44 aa. The highest level of IGF-I mRNA expression was recorded in the kidney of adult sturgeons. The IGF-I mRNA expression levels in the intestine, pituitary gland, and liver were not significantly different. Low levels of expression were found in the brain, heart, and muscle. In most tissues, there was no significant difference between mRNA levels of one and 5-yr-old fish. In conclusion, based on the GH-sequence analysis, A. gueldenstaedtii is genetically distant from other teleosts. The expression of the GH mRNA was similar in males and females, and its level remained constant during the first 5 yr of growth. While the IGF-I mRNA expression differed amongst various tissues, the level in each tissue was similar in 1 and 5-yr-old fish.

Identification of two insulin-like growth factor IIs in the Japanese eel, Anguilla japonica: Cloning, tissue distribution, and expression after growth hormone treatment and seawater acclimation

To better understand the role of IGFs in Japanese eel, Anguilla japonica, we cloned insulin-like growth factor-II (IGF-II) cDNAs and examined their mRNA expression in several tissues. Two eel IGF-II cDNAs, eIGF-II-1 and eIGF-II-2, were cloned from the liver. A signal peptide and a mature peptide of both preproIGF-IIs were composed of 47 amino acids (aa) and 69 aa, but they differed at 17 aa and 13 aa, respectively. The E domain of eIGF-II-1 was 49 aa longer than that of eIGF-II-2, and differed at 22 aa within 52 aa. The highest eIGF-II-1 and II-2 mRNA levels were observed in the liver, with detectable levels also found in all tissues examined. The eIGF-II-1 mRNA levels in the liver, heart, and muscle were higher in females than in males, whereas those in the stomach and intestine were lower in the females. The eIGF-II-2 mRNA levels in the liver and swim-bladder were also higher in females than in males whereas those in the stomach, spleen, and intestine were lower in the females. The eIGF-II-1 mRNA levels in the liver were higher in large compared to small glass eels, while the eIGF-II-2 mRNA levels did not correlate with body weight. Both eIGF-II mRNA levels in the liver increased after eel GH treatments in vivo and in vitro. No differences in both eIGF-II mRNA levels were observed in the gills, liver, stomach and whole kidney between seawater- and freshwater-reared eels.

PACAP-related peptide (PRP)--molecular evolution and potential functions

PACAP-related peptide (PRP) and PACAP are structurally related peptides that are encoded in the same transcripts. In the past, it was believed that the mammalian PRPs are evolved from GHRHs in non-mammals. With the recent discovery of authentic GHRH and receptor genes in frog and fish, this review aims to (1) coin the name of all GHRH-like peptides in previous literature as PRPs and (2) provide the background for new research direction for PRP in vertebrates. As a goldfish receptor highly specific for PRP with distinct tissue distribution has previously been characterized, it is highly possible that PRP plays a physiological role in non-mammalian vertebrates and the function of PRP has somehow been lost in mammals as a consequence of the loss of its receptor in the genome. This information may provide clues to elucidate functions of PRP in the future.

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.

Neuroendocrinology of protochordates: Insights from Ciona genomics

The genome for two species of Ciona is available making these tunicates excellent models for studies on the evolution of the chordates. In this review most of the data is from Ciona intestinalis, as the annotation of the C. savignyi genome is not yet available. The phylogenetic position of tunicates at the origin of the chordates and the nature of the genome before expansion in vertebrates allows tunicates to be used as a touchstone for understanding genes that either preceded or arose in vertebrates. A comparison of Ciona, a sea squirt, to other model organisms such as a nematode, fruit fly, zebrafish, frog, chicken and mouse shows that Ciona has many useful traits including accessibility for embryological, lineage tracing, forward genetics, and loss- or gain-of-function experiments. For neuroendocrine studies, these traits are important for determining gene function, whereas the availability of the genome is critical for identification of ligands, receptors, transcription factors and signaling pathways. Four major neurohormones and their receptors have been identified by cloning and to some extent by function in Ciona: gonadotropin-releasing hormone, insulin, insulin-like growth factor, and cionin, a member of the CCK/gastrin family. The simplicity of tunicates should be an advantage in searching for novel functions for these hormones. Other neuroendocrine components that have been annotated in the genome are a multitude of receptors, which are available for cloning, expression and functional studies.

Proinsulin in development: New roles for an ancient prohormone

In postnatal organisms, insulin is well known as an essential anabolic hormone responsible for maintaining glucose homeostasis. Its biosynthesis by the pancreatic beta cell has been considered a model of tissue-specific gene expression. However, proinsulin mRNA and protein have been found in embryonic stages before the formation of the pancreatic primordium, and later, in extrapancreatic tissues including the nervous system. Phylogenetic studies have also confirmed that production of insulin-like peptides antecedes the morphogenesis of a pancreas, and that these peptides contribute to normal development. In recent years, other roles for insulin distinct from its metabolic function have emerged also in vertebrates. During embryonic development, insulin acts as a survival factor and is involved in early morphogenesis. These findings are consistent with the observation that, at these stages, the proinsulin gene product remains as the precursor form, proinsulin. Independent of its low metabolic activity, proinsulin stimulates proliferation in developing neuroretina, as well as cell survival and cardiogenesis in early embryos. Insulin/proinsulin levels are finely regulated during development, since an excess of the protein interferes with correct morphogenesis and is deleterious for the embryo. This fine-tuned regulation is achieved by the expression of alternative embryonic proinsulin transcripts that have diminished translational activity.

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.

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.

The MHC big bang

The human Major Histocompatibility Complex (MHC) shares similarities with three other chromosome regions in human. This could be the vestige of ancestral large scale duplications. We discuss here the possibility i) that these duplications occurred during two rounds of tetraploidization supposed to have taken place during chordate evolution before the jawed vertebrate radiation, and ii) that one of the quadruplicate regions, relaxed of functional constraints, gave rise to the vertebrate MHC by a quick round of gene cis-duplication and cis-exon shuffling. These different rounds of cis-duplications and exon shufflings allowed the emergence of new genes participating in novel biological functions i.e. adaptive immune responses. Cis-duplications and cis-exon shufflings are ongoing processes in the evolution of some of these genes in this region as they have occurred and were fixed at different times and in different lineages during vertebrate evolution. In contrast, other genes within the MHC have remained stable since the emergence of jawed vertebrates.

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.

Insulin-family peptide-receptor interaction at the early stage of vertebrate evolution

This is an overview of our studies on insulin and insulin-like growth factor-I (IGF-I) interactions with their own and each other's receptors in the lamprey (Lampetra fluviatilis L.), an extant representative of the ancient vertebrate group of Agnathans as compared to mammal (rat). Lamprey insulin receptor shows species specificity, namely, it binds its own insulin with higher affinity than mammalian hormone. Nevertheless, and unlike mammalian insulin receptor, lamprey receptor discriminates relatively poorly between insulin and IGF-I. Autophosphorylation patterns are identical for both receptors. In contrast, IGF-I receptors in lamprey tissues are very similar to mammalian IGF-I receptors confirming known evolutionary conservatism of IGF receptor system. Presumed common evolutionary origin of insulin and IGF-I receptors and poor ability of lamprey insulin receptor to discriminate between two ligands, implies that lamprey insulin receptor is closer to putative ancestral protein that IGF-I receptor. Contrary to the common belief, ambient temperatures for lampreys (4-15 degrees C) put no constraints on either downregulation of receptors or the endocytosis of hormone-receptor complexes.

Some of my not so favorite things about insulin and insulin-like growth factors in fish

Several topics concerning insulin and IGFs rarely discussed among comparative physiologists/endocrinologists are reviewed. The topics addressed include interpretation of radioimmunoassays for insulin and IGFs, the major sites of synthesis of these peptides, signal transduction via peptide-specific, and possibly via related peptide receptors. Finally, some thoughts are included on the development of the insulin superfamily of peptides in two phyletic lines of evolution, proto- and deuterostomian animals. The author considers all the above topics open to further exploration. If this review initiates more discussion leading to unorthodox input to the field, its goal is achieved.

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.

Cloning of a full-length insulin-like growth factor-I complementary DNA in the goldfish liver and ovary and development of a quantitative PCR method for its measurement

Five forms of insulin-like growth factor-I (IGF-I) complementary DNA (cDNA) were isolated by PCR from goldfish liver and ovary, using primers based on common carp IGF-I sequence. In the goldfish liver, we cloned and sequenced three IGF-I forms (1,2, and 3), and elucidated the full-length cDNA sequence using the 5'-and 3'-RACE. Two IGF-I forms (1 and 2) were cloned from the goldfish ovary and were found to have differences with respect to both size and nucleotide sequence compared to liver IGF-I. The entire liver IGF-I form 1 sequence was found to be 833 nucleotides long, containing a 483-nucleotide open reading from encoding 161 amino acids. The deduced amino acid sequence of the mature peptide was compared to IGF-I sequences of other vertebrates, and found to have 97 and 93% similarity to carp and salmon IGF-I, respectively, In this study we also developed a competitive quantitative PCR method and demonstrated an increase in IGF-I expression following treatments with growth hormone or gonadotropin-releasing hormone in the goldfish liver.

Isolation and characterization of tilapia (Oreochromis mossambicus) insulin-like growth factors gene and proximal promoter region

To understand the molecular mechanism which controls the transcription of the insulin-like growth factors (IGFs) gene, we have cloned and sequenced the cDNA for the proximal promoter region of the tilapia IGFs gene and have characterized its activity by chloramphenicol acetyltransferase (CAT) transient transfected expression assays. Tilapia (Oreochromis mossambicus) IGF-I cDNA (549 bp) was amplified by PCR from single-stranded cDNA of growth hormone (GH)-induced liver RNA using a pair of oligonucleotides specific for fish IGF-I as amplification primers. Tilapia IGF-I and IGF-II 5' termini were analyzed by rapid amplification of cDNA 5' ends (5'RACE). Analysis of the 5'RACE results revealed two transcription start sites in IGF-I and one transcription start site in IGF-II. Different fragments of the 5' flanking region were transfected into human lung adenocarcinoma cells. In the cell line, maximum promoter activity was located in the distal 657 basepairs of the IGF-I 5' flanking region and in the distal 450 basepairs of the IGF-II 5' flanking region. The in vivo actions of the IGFs promoter on developmental stage expression were investigated further in transgenic zebrafish in which an IGFs promoter-driven green fluorescent protein (GFP) encoding the cDNA transgene was microinjected into embryos. Morphologic and RT-PCR studies of the transgenic zebrafish indicated that IGF-I promoter-driven GFP transcripts appeared for the first time in the 1-K-cell stage and the IGF-II promoter-driven GFP transcripts appeared for the first time in the 32-cell stage. Fluorescent (GFP) distribution was apparent within 48 h in IGF-II-transgenic zebrafish embryos, especially in eye, muscle, corpuscle, floor plate, horizontal myoseptum, yolk sac extension, and yolk sac. These results indicate that the IGF-I and IGF-II promoters are active in tissue and in a development-specific manner. Our findings also indicate that the IGF-II promoter influences the growth of fish embryos earlier than does IGF-I, and IGF-II has higher levels of expression than does IGF-I. These results suggest that the IGF-II promoter plays a growth factor role in teleost embryo development.

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.

Ancient divergence of insulin and insulin-like growth factor

Studies on the evolutionary pathway of the insulin gene family suggest that insulin and insulin-like growth factor (IGF) became distinct molecules only after the vertebrates arose. A single molecule with identity to both insulin and IGF was reported in amphioxus. To study the origin of insulin, we selected tunicates because their ancestors are thought to be a nodal point in the evolution of vertebrates. This is the first report of separate insulin and IGF molecules from invertebrates. Two cDNAs were isolated from the tunicate Chelyosoma productum: One cDNA encodes a distinct preproinsulin with B, C, and A domains, whereas the other encodes tunicate preproIGF, including all five domains in their proper sequence. Both mRNAs are expressed in the nervous system, digestive tract, heart, and possibly the gonad but not in branchial basket or tunic. Hence, insulin and igf genes have similar expression patterns. In situ methods confirm the polymerase chain reaction evidence that tunicate insulin and igf mRNAs are expressed in cortical cells of the neural ganglion. We conclude that insulin and IGF have maintained separate gene lineages in both vertebrate and protochordate evolution and, thus, a distinct evolutionary history of more than 600 million years.

Insulin-like growth factors in poultry

A large amount of research, primarily in mammals, has defined to a great extent the pleiotropic effects of the IGF system on growth, development, and intermediary metabolism. Similar elucidations in poultry were hindered to some extent by the absence of native peptides (IGF-I and IGF-II) until their purification, followed by the production of recombinant chicken IGFs. In many ways IGF physiology in birds is similar to that in other species, including but not limited to the fact that IGF-I synthesis is both GH- and GH-independent, and that autocrine-paracrine IGF action is evident. However, it is clear that several unique differences in IGF physiology exist between birds and mammals. For example, more IGF is present in the free form in chickens, and the biological responses to the IGFs is different in several metabolic pathways in birds compared to mammals. To date, no unique IGF-II receptor has been identified in birds. Despite an increasing understanding of the IGFs in aves, several important questions remain to be answered. What is the role of IGF-II in embryo development and posthatch growth? Does an IGF-II receptor entity exist in nonmammalian species? How does nutrition affect IGF-I and IGF-II gene expression, and can this information be used to enhance poultry production? What is the biochemical composition of the IGFBPs, and what are their roles in birds? Can the genetic variation present in poultry be used to positively modify IGF gene expression and physiology? How do the IGFs regulate intermediary metabolism? What is the role of the IGFs in the etiology of several disease states associated with rapid growth in poultry, including tibial dyschondroplasia, obesity, ascites, and spiking mortality syndrome? Answers to these questions are relevant to our understanding of the basic mechanisms of IGF physiology as well as possibly assisting in the amelioration of problems found in modern poultry production.

Chicken insulin-like growth factor binding protein (IGFBP)-5: conservation of IGFBP-5 structure and expression during evolution

A complementary DNA (cDNA) clone encoding chicken insulin-like growth factor binding protein-5 (cIGFBP-5) was isolated. The nucleotide sequence of the 1236-bp clone encodes a mature polypeptide of 251 amino acids with a calculated molecular weight of approximately 28.2 kDa. The deduced amino acid sequence is 83% identical to human IGFBP-5. Labeled cIGFBP-5 cDNA detected a single mRNA transcript of approximately 6 kb by Northern blot analysis of various tissues obtained from embryonic and 6 weeks post-hatch chickens; interestingly, adult heart showed an approximately 10-fold increase in cIGFBP-5 mRNA relative to embryonic heart. The pattern of IGFBP-5 mRNA expression in chicken tissues was similar to that found in mammals during fetal and extrauterine life. In addition, IGFBP-5 mRNA was abundant in primary cultures of chicken myoblasts throughout in vitro differentiation and fusion. The conservation of IGFBP-5 primary structure and expression pattern across vertebrate species suggests conservation of important functions during evolution, particularly in muscle differentiation.

Divergence of insulin-like growth factors I and II in the elasmobranch, Squalus acanthias

Recent studies have shown that vertebrates, including teleostean fishes, amphibians, birds and mammals, contain two distinct insulin-like growth factor (IGF) genes. In contrast agnathans, represented by hagfish, apparently have only one IGF that has features characteristic of both IGF-I and IGF-II. Between these groups the elasmobranchs occupy a critical position in terms of the phylogeny of IGFs. We sought to determine if gene duplication and divergence of IGF-I and IGF-II occurred before or after divergence of elasmobranchs from other vertebrates by cloning IGF-like molecules from Squalus acanthias. Our analysis shows that Squalus liver produces two distinct IGF-like molecules. One has greater sequence identity to, and conserved features characteristic of, known IGF-I molecules, while the other is more IGF-II like. These results suggest that the prototypical IGF molecule duplicated and diverged in an ancestor of the extant gnathostomes.

Sequence and expression of cDNA for pituitary adenylate cyclase activating polypeptide (PACAP) and growth hormone-releasing hormone (GHRH)-like peptide in catfish

Growth hormone-releasing hormone (GHRH) and pituitary adenylate cyclase activating polypeptide (PACAP) are two neuropeptides that are associated with the release of pituitary growth hormone. Here a cDNA of 2501 base pairs encoding both a PACAP and a GHRH-like peptide was isolated from a brain cDNA library made from Thai catfish (Clarias macrocephalus). The organization is unlike that of the mammalian gene where PACAP and PACAP-related peptide (PRP) are encoded in one gene, and the GHRH peptide is on a separate gene. Northern analysis of catfish brain mRNA indicated that PACAP/GHRH-like mRNA has three sizes; bands of 6000, 2500, and 1000 bases suggest alternative splicing of the gene. Reverse transcriptase/PCR assay detected PACAP/GHRH-like mRNA in tissues from the brain, testis, ovary, and stomach, but not from the pancreas, pituitary, muscle, and liver. Our hypothesis that the two mammalian genes encoding GHRH or PACAP originated from a gene duplication between fish and tetrapods is supported by the present findings of similar mRNA organization and pattern of expression for the one fish gene and two mammalian genes.

Conservation of IGFBP structure during evolution: cloning of chicken insulin-like growth factor binding protein-5

The insulin-like growth factor binding proteins (IGFBPs) have conserved characteristics of their genomic organization, including similar locations of exon borders relative to nucleotides encoding conserved cysteine residues. Furthermore, the human IGFBP genes, as well as the human homeobox (HOX) genes, are localized to chromosomes 2, 7, 12, and 17. Although little is known about the evolution of the IGFBP genes, the association of human IGFBP and homeobox (HOX) genes at four chromosomal loci may indicate that their ancestral genes were linked prior to the first duplication of chromosomal DNA containing the ancestral HOX cluster. The hypothesis that IGFBPs are ancient proteins is supported by the reported detection of IGFBP activity in serum from the Agnathan species, Geotria australis, a primitive vertebrate. Further studies of IGFBPs in different species are needed to understand the evolution of this protein/gene family. Chicken provides a good intermediate model, since birds diverged from mammals approximately 300 million years ago. A complementary DNA (cDNA) clone encoding chicken insulin-like growth factor binding protein-5 (cIGFBP-5) was isolated. The deduced amino acid sequence is 83% identical to human IGFBP-5 and encodes a mature polypeptide of 251 amino acids. The conservation of IGFBP-5 primary structure across vertebrate species suggests maintenance of important functions during evolution.

IGF-2-like peptides are present in insulin cells of the elasmobranchian endocrine pancreas: an immunohistochemical and chromatographic study

Evidence for the presence of peptides, related to insulin-like growth factor 2 (IGF-2), has been obtained in the endocrine pancreas of the elasmobranchian species Raja clavata, the sting ray. By radioimmunoassay, IGF-2-like immunoreactivity was detected in Raja pancreas extract. Further characterization of this activity by acid gel chromatography revealed two distinct peaks of IGF-2-like immunoreactivity with apparent molecular weights of approximately 8.2 kDa and 4.5 kDa. Using the same IGF-2 antibody as well as antisera specific for mammalian IGF-1, insulin, glucagon, somatostatin and pancreatic polypeptide in double immunofluorescence studies, IGF-2-like immunoreactivity was located exclusively in insulin-immunoreactive cells. In contrast, IGF-1-like immunoreactivity was mainly observed in somatostatin- and glucagon-immunoreactive cells. A varying proportion (0-70%) of insulin-immunoreactive cells, however, displayed both IGF-1- and IGF-2-like immunoreactivity. Absorption studies indicated that the IGF-2-like peptides in Raja are different from mammalian and submammalian insulin and mammalian IGF-1, but similar to mammalian IGF-2. Thus, IGF-2-like peptides seem to occur during evolution as early as the phylogenetic development of the elasmobranchians. Furthermore, the results indicate a particularly conservative evolution of the islet IGF-2 system.

Expression of facilitative glucose transporter isoforms in human brain tumors

The expression of facilitative glucose transporter (GLUT) isoforms in human astrocytic tumors was examined. Reverse transcriptase-polymerase chain reaction of a surgically biopsied glioblastoma was carried out using the degenerative oligonucleotide primers corresponding to the sequences of the human facilitative glucose transporter family, and polymerase chain reaction products were hybridized with human GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 cDNA probes. The results showed that a biopsied glioblastoma expressed GLUT1, GLUT3, and GLUT4 glucose transporter genes. Northern blot analysis of total RNA (10 micrograms) from a biopsied glioblastoma showed the transcripts of only GLUT1 and GLUT3, suggesting that the expression of insulin-responsive glucose transporter GLUT4 mRNA is relatively low. Immunoblot analysis of biopsied glioblastoma tissues by polyclonal antibodies against the C-terminal synthetic peptides of GLUT1, GLUT3, and GLUT4 showed a single band of each polypeptide. However, elevated expression of GLUT1 and GLUT3 glucose transporters was not observed in the glioblastoma. Astrocytic tumor tissues (n = 14) were also examined immunohistochemically. Reactive products for GLUT1 were observed in the luminal surface of capillaries in all cases, whereas tumor cells were positive for GLUT1 in only two of 14 cases. GLUT3 was positive in astrocytic tumor cells in all cases. Three of 14 cases expressed the GLUT4 protein, which was localized in the cytoplasm of tumor cells. These results suggest that the facilitative glucose transport may be altered in astrocytic tumor cells and thus display a significant change in glucose metabolism.

Recombinant coho salmon insulin-like growth factor I. Expression in Escherichia coli, purification and characterization

Recombinant coho salmon insulin-like growth factor I (rsIGF-I) was produced in Escherichia coli, purified and characterized. The rsIGF-I expression vector was constructed by polymerase chain reaction and cloning into a plasmid containing a phage T7 RNA polymerase promoter. The rsIGF-I was recovered from bacterial inclusion bodies, solubilized under reducing conditions, immediately refolded, then fractionated by a two-step ion-exchange chromatography on DEAE-52 and Mono-S columns. It was further purified by HPLC on a reverse-phase Asahi-Pak C4P-50 C4 column. Purification of rsIGF-I was monitored by SDS/PAGE and immunoblot with anti-[human somatomedin C (SM C)/IGF-I] serum. The rsIGF-I appeared as a single band with molecular mass of 7 kDa, the same size as recombinant human IGF-I (rhIGF-I) and cross-reacted with anti-(human SM C/IGF-I) serum. The amino acid sequence of rsIGF-I contained an NH2-terminal methionine residue followed by the sequence predicted for mature sIGF-I. At concentrations in the range 3.9-250 ng/ml, rsIGF-I significantly stimulated sulfate uptake by the cultured branchial cartilage of coho salmon. The stimulatory effect of rsIGF-I was concentration dependent and slightly more potent than that of rhIGF-I at the highest concentration tested.

The branching of insulin-like growth factor 1 and insulin: an immunohistochemical analysis during phylogeny

The co-existence of insulin-like growth factor 1 (IGF-1) with the classical islet hormones insulin (INS), glucagon (GLUC), somatostatin (SOM) and pancreatic polypeptide (PP) in the endocrine pancreas of representative species of cyclostomes (Myxine glutinosa), cartilaginous fish (Raja clavata, Squalus acanthias) and bony fish (Cottus scorpius, Carassius auratus, Cyprinus carpio, Anguilla anguilla) was studied by the use of monoclonal and polyclonal antisera and the double immunofluorescence technique. In all species investigated, IGF-1-like-immunoreactive cells were found in the endocrine pancreas, however, in varying localization. In Myxine glutinosa, all INS-immunoreactive cells and some of the SOM-immunoreactive cells contained IGF-1-like-immunoreactivity. In Raja and Squalus, only a minority of the INS-immunoreactive cells also displayed IGF-1-like-immunoreactivity. The majority of the IGF-1-like-immunoreactivity was observed in SOM- and in GLUC-immunoreactive cells. Different results were obtained in bony fish. In Cottus, in the Brockmann bodies and the small islets IGF-1-like- and INS-immunoreactivities co-existed to 100%. In contrast, in the other bony fish studied IGF-1-like-immunoreactivity was not observed in INS-immunoreactive cells: in Cyprinus, IGF-1-like-immunoreactivity was found in GLUC-, PP- and SOM-immunoreactive cells and in Carassius and Anguilla, in SOM-immunoreactive cells only. Thus, in all bony fish species with the exception of Cottus, IGF-1 and insulin display a distinct cellular distribution, similar to that of mammals. The present results, thus, may indicate that the branching of IGF-1 and insulin has occurred at the phylogenetic level of bony fish.

Phylogeny of the insulin-like growth factors (IGFs) and receptors: a molecular approach

The IGFs (IGF-I and IGF-II) are essential for normal mammalian growth and development. Their actions are mediated primarily by their interactions with the type I IGF receptor (IGF-I receptor), a transmembrane tyrosine kinase. The ligands and the IGF-I receptor are structurally related to insulin and to the insulin receptor, respectively. Analysis of evolutionary conservation has often provided insights into essential regions of molecules such as hormones and their receptors. The genes for insulin and IGFs have been partially characterized in a number of vertebrate species extending evolutionarily from humans as far back as fish. The sequences of the exons encoding the mature insulin and IGF peptides are highly conserved among vertebrate species, and IGF-I-like molecules are found in species whose origins extend back as much as 550 million years. The insulin receptor is also highly conserved in vertebrate species, and an insulin-receptor-like molecule has been characterized in Drosophila. In contrast, IGF-I receptors have only been characterized in mammalian species and partially studied in Xenopus, in which the tyrosine kinase domain is highly conserved. Studies are presently being undertaken to analyze in more detail the regulation of the genes encoding this important family of growth factors and the structure/function relationships in the gene products themselves.