Occurrence of members of the insulin superfamily in central nervous system and digestive tract of protochordates

Insulin-like genes in ascidians: findings in Ciona and hypotheses on the evolutionary origins of the pancreas

Insulin plays an extensively characterized role in the control of sugar metabolism, growth and homeostasis in a wide range of organisms. In vertebrate chordates, insulin is mainly produced by the beta cells of the endocrine pancreas, while in non-chordate animals insulin-producing cells are mainly found in the nervous system and/or scattered along the digestive tract. However, recent studies have indicated the notochord, the defining feature of the chordate phylum, as an additional site of expression of insulin-like peptides. Here we show that two of the three insulin-like genes identified in Ciona intestinalis, an invertebrate chordate with a dual life cycle, are first expressed in the developing notochord during embryogenesis and transition to distinct areas of the adult digestive tract after metamorphosis. In addition, we present data suggesting that the transcription factor Ciona Brachyury is involved in the control of notochord expression of at least one of these genes, Ciona insulin-like 2. Finally, we review the information currently available on insulin-producing cells in ascidians and on pancreas-related transcription factors that might control their expression.

Three insulin-relaxin-like genes in Ciona intestinalis

The Ciona intestinalis genome harbors three insulin-like genes: INS-L1, -L2 and -L3. Conserved synteny between the Ciona-human genomes predicts that Ciona INS-Ls are orthologous to the vertebrate insulin-relaxin family, but this relation cannot be inferred from molecular phylogeny. A conserved protein core with six cysteines; typical arrangement of B-, C- and A-protein domains; pro-protein maturation mode; and putative insulin receptor-binding sites were identified in Ciona INS-L proteins. ESTs used to assemble exonic sequences of INS-Ls combined with qRT-PCR analysis provided evidence that the predicted genes are expressed in the developing and adult Ciona. Our results support that Ciona INS-L1 is orthologous to the vertebrate insulin-like/relaxin genes, INS-L2 to insulin genes and INS-L3 to IGF genes. Our analysis also implies that the insulin-like/relaxin ancestor switched receptor type from tyrosine kinase- to GPCR-type, whereas insulin-IGF subfamily retained the tyrosine kinase-type of receptor. We propose that this receptor-switch occurred after the time when urochordates branched from the common chordate lineage, but before the two genome-duplications at the root of the vertebrates.

Endocrinology of protochordates

Large-scale gene duplications occurred early in the vertebrate lineage after the split with protochordates. Thus, protochordate hormones and their receptors, transcription factors, and signaling pathways may be the foundation for the endocrine system in vertebrates. A number of hormones have been identified including cionin, a likely ancestor of cholecytokinin (CCK) and gastrin. Both insulin and insulin-like growth hormone (IGF) have been identified in separate cDNAs in a tunicate, whereas only a single insulin-like peptide was found in amphioxus. In tunicates, nine distinct forms of gonadotropin-releasing hormone (GnRH) are shown to induce gamete release, even though a pituitary gland and sex steroids are lacking. In both tunicates and amphioxus, there is evidence of some components of a thyroid system, but the lack of a sequenced genome for amphioxus has slowed progress in the structural identification of its hormones. Immunocytochemistry has been used to tentatively identify a number of hormones in protochordates, but structural and functional studies are needed. For receptors, protochordates have many vertebrate homologs of nuclear receptors, such as the thyroid, retinoic acid, and retinoid X receptors. Also, tunicates have cell surface receptors including the G-protein-coupled type, such as β-adrenergic, putative endocannabinoid, cionin (CCK-like), and two GnRH receptors. Several tyrosine kinase receptors include two epidermal growth factor (EGF) receptors (tunicates) and an insulin/IGF receptor (amphioxus). Interestingly, neither steroid receptors nor a full complement of enzymes for synthesis of sex steroids are encoded in the Ciona genome. Tunicates appear to have some but not all of the necessary molecules to develop a vertebrate-like pituitary or complete thyroid system.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

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.

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.

An immunohistochemical and morphometric analysis of insulin, insulin-like growth factor I, glucagon, somatostatin, and PP in the development of the gastro-entero-pancreatic system of Xenopus laevis

The ontogeny of the classical islet hormones insulin (INS), glucagon (GLUC), somatostatin (SOM), and pancreatic polypeptide (PP) as well as insulin-like growth factor I (IGF-I) in the gastro-entero-pancreatic (GEP) system of Xenopus laevis (stages 41-66) was studied using double immunofluorescence and morphometric analysis. As early as stage 41, clustered INS-immunoreactive (-IR) and isolated GLUC-IR cells occurred in the pancreas. The first SOM-IR cells appeared at stage 43, followed by PP-IR cells at stage 46. About 79% of the PP immunoreactivity was confined to a subpopulation of the GLUC-IR cells. Both the GLUC/PP-IR cells and the PP-IR cells were located in a distinct area of the pancreas. The first islets occurred in premetamorphosis (around stage 50) and comprised mainly INS-IR and GLUC-IR cells. The majority of SOM-IR, PP-IR, and GLUC/PP-IR cells was dispersed. The numbers of hormone cells remained quite constant until the end of prometamorphosis (stage 58). Around stages 60-62, the islets were partly disintegrated and the numbers of islet cells slightly decreased. At stage 63, the cell number began to increase and reached the levels typical for the adult around stage 66. After metamorphic climax, the islets were reformed. In the gastrointestinal tract, transient INS-IR cells occurred prior to the adaptation of the gastrointestinal tract to feeding (stages 41-44) and during metamorphosis when there is remodeling of the gastrointestinal tract (stages 60-63). Therefore, INS released from the transient mucosal INS-IR cells may be involved in the temporary proliferation of mucosal epithelial cells. The first GLUC-IR and SOM-IR cells were seen at stage 41. PP-IR cells followed at stage 46. In contrast to the islets, GLUC-IR and PP-IR cells constituted different cell populations. Around stage 46, the first IGF-I immunoreactions appeared in the GEP-system. In pancreas, IGF-I immunoreactivity was found in the GLUC/PP-IR, cells (85-99%) but was absent from INS-IR, GLUC-IR, and SOM-IR cells. The IGF-I-IR gastro-entero-endocrine cells, however, seemed to contain none of the classical islet hormones.

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.

Localization and expression of insulin-like growth factor in the teleost retina

Teleost fish retinas continue to add neurons throughout life, and evidence from in vitro experiments have implicated insulin-like growth factors (IGFs) in this process. To discover whether these factors are expressed in vivo, we have examined their expression in the cichlid fish, Haplochromis burtoni. Three lines of evidence show that IGFs are present in the fish retina. An IGF-I specific antibody, sm 1.2, binds preferentially to the retinal outer plexiform layer, in areas of cone photoreceptor synaptic endings. Northern blots of mRNA hybridized with riboprobes from trout IGF-I and IGF-II genes revealed transcripts of approximately 6.5 and 4.9 kb, respectively. The IGF-I probe detected an additional transcript of 1.2 kb in liver but not in retinal mRNA. In situ hybridization with digoxigenin-labeled riboprobes revealed that the IGF gene product is localized in the cone photoreceptors. These results show that cone photoreceptors are the source of IGFs in the fish retina, consistent with the hypothesis that IGFs play a role in regulating production of new neurons in the teleost retina.

Ontogeny of IGF-1 and the classical islet hormones in the turbot, Scophthalmus maximus

The ontogeny of the endocrine pancreas of a teleost, the turbot (Scophthalmus maximus), was investigated by the use of double immunofluorescence. Clustered insulin (INS)-immunoreactive (IR) cells were observed on the first day after hatching. During the following days, the islet largely increased in size and some smaller islets appeared. All islets consisted only of INS-IR cells. Between day 5 (onset of exogenous feeding) and 7, somatostatin (SOM) and glucagon (GLUC) cells appeared. In the large (principal) islet, the SOM-IR cells intermingled with the INS-IR cells. In the secondary islets, they occurred at the periphery. The GLUC-IR cells were located at the periphery in all islets. Subsequently, two-four additional small principal islets appeared. At day 11, pancreatic polypeptide (PP)-IR cells were present in principal islets and secondary islets. Starting with day 11, in all islets, insulin-like growth factor 1 (IGF-1) immunoreactivity was localized in numerous PP-IR cells and GLUC-IR and some SOM-IR cells. It also occurred in enteroendocrine cells that seemed to contain none of the classical islet hormones. The early appearance of INS correlates with its key role in the regulation of fish protein and lipid metabolism. Islet-derived IGF-1 might inhibit the regulation of INS secretion in a paracrine manner and may be highly involved in growth-promoting processes.

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.

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.

A high-molecular IGF-2 immunoreactive peptide (pro-IGF-2?) in the insulin cells of the islets of Langerhans in pancreas of man and rat

Histopathologically normal pancreatic parenchyma from 12 adult men and women, as well as that from 14 adult rats (Sprague-Dawley and Wistar strains), were investigated immunohistochemically with a mouse monoclonal antibody, raised against recombinant human pro-IGF-2. The antiserum showed no crossreactivity with insulin; IGF-1 had 0.1% of the reactivity of IGF-2. The immunohistochemical observations were checked by means of a radioimmunoassay (RIA), based on the same antibody, of an extract of a sample of one of the human pancreatic glands. Analogous investigations for insulin were made in parallel, using polyclonal insulin antisera. A high-molecular (12 kDa) IGF-2-like peptide was found in the islets of Langerhans, being localized to the insulin cells. These cells were identified as beta-cells by immunohistochemistry with insulin antisera on adjacent paraffin sections. From observations made by means of acid-gel-chromatography, the peptide was tentatively supposed to represent either pro-IGF-2, or a partially processed form of it.