Insulin gene expression in chicken ontogeny: pancreatic, extrapancreatic, and prepancreatic

Characterization and Expression of Turkey Prolactin Regulatory Element Binding in the Anterior Pituitary Gland and Pancreas During Embryogenesis

The PRL regulatory element-binding (PREB) protein is a transcription factor that was originally cloned from the rat anterior pituitary gland and characterized as a regulator of the PRL promoter. It is also strongly expressed in several extrapituitary tissues; however, its functional role is not well understood to date. In this study, we aimed to clone and characterize the turkey PREB gene and investigate its mRNA expression in the anterior pituitary gland and pancreas during embryogenesis. Based on the conserved sequence of chicken and mammalian PREB cDNAs, a turkey PREB cDNA fragment was obtained, and after sequencing of the fragment, the 5′-and 3′-ends of mRNA were amplified and determined. To identify the PREB gene structure, polymerase chain reaction (PCR) amplification was performed. The turkey PREB gene consists of 9 exons and 8 introns, and it encodes a 411-amino-acid protein. The expression of PREB mRNA in the anterior pituitary gland was measured during embryogenesis. Levels of PREB mRNA significantly increased at embryonic day 22, with maximum levels being detected on day 25 of ontogeny, which correlated with similar changes in levels of PRL mRNA. The highest level of PREB mRNA was detected on day 19 in the pancreas. However, the highest level of insulin mRNA was detected at embryonic day 25. These results indicate that PREB may be involved in the expression of PRL mRNA in the anterior pituitary gland, whereas insulin mRNA may be expressed independently of the expression of PREB mRNA in the pancreas during embryogenesis.

Expression of Ins1 and Ins2 genes in mouse fetal liver

A possible cure for diabetes is explored by using non-pancreatic cells such as fetal hepatocytes. The expression of insulin and transcription factors for insulin is investigated in mouse fetal liver. We detected mRNAs for insulin I (Ins1) and insulin II (Ins2) and proinsulin- and mature insulin-positive cells in mouse fetal liver by reverse transcription plus the polymerase chain reaction and immunohistochemistry. Glucagon, somatostatin and pancreatic polypeptide were not expressed throughout development. Mouse Ins2 and Ins1 promoters were transiently activated in mouse fetal hepatocytes of embryonic days 13.5 and 16.5, respectively. Pancreatic and duodenal homeobox 1 (Pdx1) mRNA was not expressed during development of the liver. In contrast, mRNAs and proteins of neurogenic differentiation (NeuroD)/β cell E-box transactivator 2 (Beta2) and v-maf musculoaponeurotic fibrosarcoma oncogene homolog (MafA) were almost simultaneously expressed with insulin genes in the liver. Ins2 and Ins1 promoters were activated in hepatoma cells by the transfection of the expression vector for NeuroD/Beta2 alone and by the combination of NeuroD/Beta2 and MafA, respectively. These results indicate that the expression of NeuroD/Beta2 and MafA is linked temporally with the transcription of Ins2 and Ins1 genes in mouse fetal liver and suggest the potential usage of fetal hepatocytes to make insulin-producing β cells by introducing transcription factors.

Proinsulin: from hormonal precursor to neuroprotective factor

In the last decade, non-canonical functions have been described for several molecules with hormone-like activities in different stages of vertebrate development. Since its purification in the 1960s, proinsulin has been one of the best described hormonal precursors, though it has been overwhelmingly studied in the context of insulin, the mature protein secreted by the pancreas. Beginning with our discovery of the presence and precise regulation of proinsulin mRNA in early neurulation and neurogenesis, we uncovered a role for proinsulin in cell survival in the developing nervous system. We subsequently demonstrated the ability of proinsulin to prevent pathological cell death and delay photoreceptor degeneration in a mouse model of retinitis pigmentosa. In this review, we focus on the evolution of proinsulin/insulin, beginning with insulin-like peptides expressed in mainly the neurosecretory cells of some invertebrates. We summarize findings related to the regulation of proinsulin expression during development and discuss the possible effects of proinsulin in neural cells or tissue, and its potential as a neuroprotective molecule.

Insulin receptor isoforms and insulin receptor/insulin-like growth factor receptor hybrids in physiology and disease

In mammals, the insulin receptor (IR) gene has acquired an additional exon, exon 11. This exon may be skipped in a developmental and tissue-specific manner. The IR, therefore, occurs in two isoforms (exon 11 minus IR-A and exon 11 plus IR-B). The most relevant functional difference between these two isoforms is the high affinity of IR-A for IGF-II. IR-A is predominantly expressed during prenatal life. It enhances the effects of IGF-II during embryogenesis and fetal development. It is also significantly expressed in adult tissues, especially in the brain. Conversely, IR-B is predominantly expressed in adult, well-differentiated tissues, including the liver, where it enhances the metabolic effects of insulin. Dysregulation of IR splicing in insulin target tissues may occur in patients with insulin resistance; however, its role in type 2 diabetes is unclear. IR-A is often aberrantly expressed in cancer cells, thus increasing their responsiveness to IGF-II and to insulin and explaining the cancer-promoting effect of hyperinsulinemia observed in obese and type 2 diabetic patients. Aberrant IR-A expression may favor cancer resistance to both conventional and targeted therapies by a variety of mechanisms. Finally, IR isoforms form heterodimers, IR-A/IR-B, and hybrid IR/IGF-IR receptors (HR-A and HR-B). The functional characteristics of such hybrid receptors and their role in physiology, in diabetes, and in malignant cells are not yet fully understood. These receptors seem to enhance cell responsiveness to IGFs.

Insulin signaling in the central nervous system: Learning to survive

Insulin is best known for its role in peripheral glucose homeostasis. Less studied, but not less important, is its role in the central nervous system. Insulin and its receptor are located in the central nervous system and are both implicated in neuronal survival and synaptic plasticity. Interestingly, over the past few years it has become evident that the effects of insulin, on neuronal survival and synaptic plasticity, are mediated by a common signal transduction cascade, which has been identified as "the PI3K route". This route has turned out to be a major integrator of insulin signaling in the brain. A pronounced feature of this insulin-activated route is that it promotes survival by directly inactivating the pro-apoptotic machinery. Interestingly, it is this same route that is required for the induction of long-term potentiation and depression, basic processes underlying learning and memory. This leads to the hypothesis that the PI3K route forms a direct link between learning and memory and neuronal survival. The implications of this hypothesis are far reaching, since it provides an explanation why insulin has beneficial effects on learning and memory and how synaptic activity can prevent cellular degeneration. Applying this knowledge may provide novel therapeutic approaches in the treatment of neurodegenerative diseases such as Alzheimer's disease.

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.

Developmental regulation of a proinsulin messenger RNA generated by intron retention

Proinsulin gene expression regulation and function during early embryonic development differ remarkably from those found in postnatal organisms. The embryonic proinsulin protein content decreased from gastrulation to neurulation in contrast with the overall proinsulin messenger RNA increase. This is due to increasing levels of a proinsulin mRNA variant generated by intron 1 retention in the 5' untranslated region. Inclusion of intron 1 inhibited proinsulin translation almost completely without affecting nuclear export or cytoplasmic decay. The novel proinsulin mRNA isoform expression was developmentally regulated and tissue specific. The proportion of intron retention increased from gastrulation to organogenesis, was highest in the heart tube and presomitic region, and could not be detected in the pancreas. Notably, proinsulin addition induced cardiac marker gene expression in the early embryonic stages when the translationally active transcript was expressed. We propose that regulated unproductive splicing and translation is a mechanism that regulates proinsulin expression in accordance with specific requirements in developing vertebrates.

Upstream AUGs in embryonic proinsulin mRNA control its low translation level

Proinsulin is expressed prior to development of the pancreas and promotes cell survival. Here we study the mechanism affecting the translation efficiency of a specific embryonic proinsulin mRNA. This transcript shares the coding region with the pancreatic form, but presents a 32 nt extended leader region. Translation of proinsulin is markedly reduced by the presence of two upstream AUGs within the 5' extension of the embryonic mRNA. This attenuation is lost when the two upstream AUGs are mutated to AAG, leading to translational efficiency similar to that of the pancreatic mRNA. The upstream AUGs are recognized as initiator codons, because expression of upstream ORF is detectable from the embryonic transcript, but not from the mutated or the pancreatic mRNAs. Strict regulation of proinsulin biosynthesis appears to be necessary, since exogenous proinsulin added to embryos in ovo decreased apoptosis and generated abnormal developmental traits. A novel mechanism for low level proinsulin expression thus relies on upstream AUGs within a specific form of embryonic proinsulin mRNA, emphasizing its importance as a tightly regulated developmental signal.

Synthesis and differentially regulated processing of proinsulin in developing chick pancreas, liver and neuroretina

Regulated preproinsulin gene expression in nonpancreatic tissues during development has been demonstrated in rodents, Xenopus and chicken. Little is known, however, about the synthesis and processing of the primary protein product, proinsulin, in comparison with these events in pancreas. Using specific antisera and immunocytochemistry, immunoblot and HPLC criteria, we characterize the differential processing of proinsulin in developing neuroretina, liver and pancreas. The chick embryo pancreas expresses the convertase PC2, and largely processes proinsulin to insulin. In contrast, little or no mature PC2 is present in embryonic liver and neuroretina and the (pro)insulin immunoactivity identified is predominantly proinsulin.

An avian model for comparative studies of insulin teratogenicity

This study was designed to explore the effects of purified insulin during early stages of chick embryo development, and to search for variations between different molecular structures of the hormone. Chicken embryos were treated in ovo with a single dose of insulin (porcine or bovine), in only one stage of development between day 0 and day 9. Two susceptible periods were found. The earliest period (day 0 to day 3), characterized by abnormalities in the caudal vertebrae and a high mortality rate, was followed by a period with a different set of malformations, a syndrome classified as achondroplasia. The rate of achondroplastic embryos was significantly higher with porcine rather than with bovine insulin. Paradoxically, insulin at physiological doses has stimulatory effects in growth and development but, in contrast, has inhibitory effects at higher doses. The precise signalling cascade of events in the target cells is still unclear. The possible interpretations of our results are discussed. The similarity between the insulin-induced abnormalities in the chicken embryos and the caudal regression syndrome, the most common malformation found in infants of diabetic women, suggests a common mechanism. This circumstance offers the chicken embryos as an excellent in vivo model for research on the mechanism of action of insulin during normal and abnormal development.

Developmental regulation and the role of insulin and insulin receptor in metanephrogenesis

The insulin family of peptides and their receptors influence cellular growth in very early preimplantation embryos. In this study their expression and role in renal organogenesis was investigated. By immunofluorescence microscopy and in situ hybridization, insulin receptor (IR) expression was seen in the ureteric bud branches and early nephron precursors in mouse metanephroi harvested at day 13 of gestation. The expression gradually decreased in successive stages of gestation, and it was confined mainly to renal tubules in 1-week-old mice. Similar developmental regulation of the IR and insulin was observed by reverse transcriptase-polymerase chain reaction (RT-PCR) analyses. Addition of insulin into the culture medium at low concentrations, ranging from 40 to 400 ng/ml, induced trophic changes and increased [3H]thymidine incorporation in the embryonic renal explants, and inclusion of IR beta-subunit-specific antisense oligodeoxynucleotide caused marked dysmorphogenesis and growth retardation of the metanephroi. Specificity of the antisense effect was reflected by immunoprecipitation experiments in which translational blockade of the beta subunit of the IR was observed. RT-PCR analyses revealed that the alpha subunit of the IR was unaffected by the antisense treatment of metanephric explants. Concomitantly, de novo synthesis of morphogenetic regulatory extracellular matrix proteins, especially the proteoglycans, was decreased. Gel-shift analyses indicated a failure in the activation of c-fos promoter region binding protein(s) by insulin in the antisense oligodeoxynucleotide-treated explants. These studies suggest that insulin and its putative receptor are developmentally regulated in the murine embryonic metanephros, and they play a role in renal organogenesis, possibly by affecting other modulators of morphogenesis-i.e., extracellular matrix proteins and protooncogenes.

The developing CNS: a scenario for the action of proinsulin, insulin and insulin-like growth factors

The multifunctional cytokines of the family of insulin and insulin-like growth factors (IGFs) have not yet gained general recognition as essential cell signals for the development of the vertebrate nervous system. This is, in part, a consequence of previous constraints in our thinking, focused for many years on the endocrine roles of these factors in late mammalian development and postnatal stages. The cellular distribution of the components of the insulin and IGFs signalling system in the developing mammalian and avian CNS is remarkably conserved. While receptors are widespread, the much less abundant factors and modulatory proteins are highly regulated in time and space. Progression of neural development through the steps of cell proliferation, differentiation, maturation and survival is stimulated, at least in culture, by proinsulin and insulin and the IGFs. Thus, these factors might be important autocrine and paracrine signals during development of the CNS.

Insulin and insulin-like growth factor system components gene expression in the chicken retina from early neurogenesis until late development and their effect on neuroepithelial cells

To better understand the role of insulin-related growth factors in neural development, we have characterized by in situ hybridization in chicken embryonic retina the patterns of gene expression for insulin, insulin-like growth factor I (IGF-I), their respective receptors and the IGF binding protein 5 (IGFBP5) from early stages (E6) until late stages (E18)--an analysis not performed yet in any species. In addition, we studied the effect of insulin and IGF-I on cultured neuroepithelial cells. Insulin receptor mRNA and IGF-I receptor mRNA were both present and showed a similar, widespread pattern throughout retina development. Insulin mRNA could be detected only by reverse transcription coupled to polymerase chain reaction. IGF-I mRNA was concentrated in the ciliary processes and extraocular muscles early in development (embryonic day 6; E6) and in maturing retinal ganglion cells subsequently (E9-15). IGFBP5 mRNA was preferentially localized in the more differentiated central retinal zone and was maximally concentrated in the inner nuclear and ganglion cell layers at E9. These findings suggest a near constitutive expression of insulin receptor and IGF-I receptor genes, while IGF-I and IGFBP5 showed a highly focal spatiotemporal regulation of gene expression. Insulin and IGF-I, already at 10(-8) M, increased the proportion of PM1-positive neuroepithelial cells found in E5 retinal cultures without affecting significantly the total number of proliferating cells. Together, these data support the finding that, during early neurogenesis in chicken retina, insulin and IGF-I have a specific paracrine/autocrine action. This action, as well as possible effects elicited subsequently, may be dictated by restricted-local synthesis of the ligands and limited access to the factors contained in the vitreous humour. In the case of IGF's role, local IGFBPs expression can contribute to the fine modulation.

Analysis of pyruvate dehydrogenase expression in embryonic mouse brain: localization and developmental regulation

Brain malformations and neurological dysfunctions are often seen in pyruvate dehydrogenase (PDH) deficient patients. To understand these clinical presentations, we have analyzed the localization and developmental expression of PDH in the embryonic mouse nervous system. Immunostaining was performed to localize PDH E1 alpha protein. PDH activities were measured before and after activation. PDH E1 alpha mRNA levels were quantitated by reverse transcriptase-polymerase chain reaction. Abundant PDH E1 alpha protein was localized in the central nervous system and other neural tissues in embryos at embryonic day (E) 11 onwards. The PDH activity was very low in E9 brain and it increased continuously until the end of gestation. The proportion of active form of PDH increased significantly in E15 brain. Analysis of the PDH E1 alpha mRNA showed that only the X-linked form of the gene was transcribed. The overall mRNA level of E9 brain was approximately 93% of the adult value. It decreased gradually during embryogenesis. A large increase took place at the end of gestation. The mRNA level of PDH was approximately 100 times higher than that of the acetoacetyl-CoA thiolase gene. These results suggest that PDH E1 alpha transcripts of E9 brain are not translated at a high level. The appearance of PDH activity and its increase during E11 and E15 are mainly due to increased levels of translation and activation of PDH. Increased PDH activity at the end of gestation is attributed to an increase in transcription. Our data to a large extent explain pathological presentations in PDH E1 alpha deficient patients with congenital brain disorders.

Differential expression of the two nonallelic proinsulin genes in the developing mouse embryo

In the mouse, insulin is produced from two similar but nonallelic genes that encode proinsulins I and II. We have investigated expression of these two genes during mouse embryonic development, using a PCR to detect the two gene transcripts and immunocytochemistry to visualize the two corresponding proteins. At appearance of the dorsal pancreatic anlage at day 9.5 of gestation, both mRNAs could be detected in the embryos, and both proteins were present together in the same cells of the developing pancreas. At days 9.5 and 10.5, when the ventral anlage appears, there were fewer proinsulin II mRNAs than proinsulin I mRNAs. At day 12.5 this ratio was reversed. Proinsulin II mRNA, but not proinsulin I mRNA, could be detected at day 8.5 in the prepancreatic embryo. Proinsulin II mRNA, but not proinsulin I mRNA, was also found in the heads of embryos at day 9.5 and at all later stages studied. These results indicate that the two proinsulin genes are regulated independently, at least in part. They also suggest that insulin might play a role as a growth factor in the developing mouse brain.

Insulin improves survival but does not maintain function of cultured chick wing bud apical ectodermal ridge

Previously we demonstrated that high levels of insulin (5 micrograms/ml) permit the survival of isolated chick apical ectodermal ridge in culture (Boutin and Fallon, Dev. Biol., 104:111-116, 1984). Here we address whether lower levels of insulin or insulin-like growth factors (IGFs) can also improve the survival of cultured apical ectodermal ridge and whether ridge function is maintained along with ridge survival. Neither IGF I nor IGF II (100 ng/ml) decreased ridge cell death; however, cell death was significantly decreased with 50 ng/ml insulin. No further improvement was obtained in the presence of both IGF I and insulin. These data suggest that insulin improved the survival of the isolated apical ectodermal ridge by binding its own receptor. To test for the maintenance of function, stage 20 ridges were cultured for 0, 6, 12, 18, or 24 hr with or without insulin (5 micrograms/ml or 5 ng/ml) and used to make recombinant limbs. Isolated ridges cultured for 12 hr or more produced fewer outgrowths and these were rarely distally complete. The medium in which the ridges had been cultured did not influence ridge activity, despite the major differences in cell survival. Recombinants made with ridges cultured with limb mesoderm for 18 hr did not yield outgrowths as often as those with freshly isolated ridges, but most of the limbs that did form were distally complete. These results suggest that the decline in function of cultured, isolated apical ectodermal ridge was not due merely to ridge cell death but rather, at least in part, to its separation from limb mesoderm.

Demonstration of insulin and glucagon mRNA in routinely fixed and processed pancreatic tissue by in-situ hybridization

Human insulin and glucagon mRNA were identified in routinely processed pancreatic tissue by non-radioactive in-situ hybridization using digoxigenin-labelled oligonucleotide probes. Cocktails of synthetic oligonucleotides complementary to human insulin and glucagon mRNA were labelled with digoxigenin using terminal deoxynucleotidyl transferase (Tdt). Specific hybrids were detected with alkaline phosphatase-labelled anti-digoxigenin antibody and visualized by BCIP-nitroblue tetrazolium indicator substrate. The results showed highly sensitive and specific staining of islet cells on a range of routinely formalin-fixed and paraffin-embedded tissues. Post-mortem pancreatic tissue from adults and stillborn neonates yielded acceptable signals as long as tissue morphology was well preserved. Preliminary investigations using pancreatic endocrine cell tumours gave clear easily interpretable signals which were comparable to conventional immunostaining. The application of this technique promises to be of value in the investigation of pancreatic disease.

Two nonallelic insulin genes in Xenopus laevis are expressed differentially during neurulation in prepancreatic embryos

Insulin, traditionally regarded as a metabolic hormone, also can potently stimulate growth and differentiation in many cell types. To study further the potential role of insulin during early embryogenesis, we have used the amphibian Xenopus laevis, a versatile model of vertebrate development. Using (i) nucleotide sequences of two previously cloned cDNAs that correspond to two different nonallelic Xenopus insulin genes (both of which are expressed in the adult pancreas) and (ii) a modification of the highly sensitive reverse transcription-polymerase chain reaction (RT-PCR) method developed in our laboratory, designated RNA template-specific PCR (RS-PCR), we now find that mRNAs for both Xenopus insulins I and II are present in mature (stage VI) oocytes but not in less-mature oocytes (stages I and IV) or in unfertilized eggs. The Xenopus insulin II gene is differentially expressed during early neurulation (stage 13), while only the insulin I gene is expressed at stage 21, when the neural tube is closing and cephalization is beginning. During later stages (i.e., stage 26) there is a region in the head that appears to be transcribing only the insulin I gene, while mRNAs for both insulins I and II are present in the body region. These findings show that the two nonallelic insulin genes are expressed differentially in Xenopus embryos in a stage- and region-specific manner; because appropriate receptors are also present, we suggest a role for insulin during early nervous system development well before the emergence of pancreatic beta cells.

Insulin-like growth factor-I and insulin as growth and differentiation factors in chicken embryogenesis

The avian embryo has been a useful model system for studies on the role of insulin and its close relative insulin-like growth factor-I (IGF-I) in development. The unfertilized chicken egg contains both peptides from maternal origin, and the embryo expresses insulin and IGF-I before the major organs are formed. Insulin receptors and IGF-I receptors are found in the blastoderm and in all tissues examined during organogenesis. When exogenous insulin or IGF-I are added to the embryo, growth and differentiation events are stimulated. By contrast, insulin antibodies and insulin receptor antibodies retard embryo development. In embryos cultured ex ovo, in which growth is impaired, the levels of serum IGF-I are decreased.

Genes for the insulin receptor and the insulin-like growth factor I receptor are expressed in the chicken embryo blastoderm and throughout organogenesis

The early expression of insulin and insulin-like growth factor I (IGF-I) in the chicken embryo suggests that these peptides play an important role in early development. The receptors for insulin and IGF-I, however, had not been studied at the molecular level in this model. We report two chicken sequences that, by comparison with known tyrosine kinases, appear to correspond to the tyrosine kinase domain of the insulin receptor homologue (CTK-1) and the IGF-I receptor homologue (CTK-2). Using reverse-transcription of RNA, amplification with the polymerase chain reaction (RT-PCR), and gene-specific hybridization, we demonstrate that the two genes, CTK-1 and CTK-2, are expressed in embryos at least as early as the blastoderm (Day 0), during neurulation (Day 1), and in early (Days 2-3) and late (Day 9) organogenesis.

Initial cholinergic differentiation in embryonic chick retina is responsive to insulin and cell-cell interactions

Previous work [Kyriakis et al., Proc. Natl. Acad. Sci. U.S.A., 84 (1987) 7463-7467] had shown that insulin, when added during a window of binding from embryonic days 9-11, stimulates the normal developmental increase in choline acetyltransferase (ChAT) activity (a marker for cholinergic differentiation) in cultured embryonic chick retinal neurons. Here, we investigated the effect of insulin and IGF 1 on embryonic chick retinal neurons at the stage of development (embryonic day 6) when ChAT activity is first expressed. We investigated insulin peptide effects in retinal tissue developing in vitro as well as in cultures of retinal cells. We show that insulin also stimulated the initial embryonic increase in ChAT activity but had no stimulatory effect on glutamic acid decarboxylase activity (a marker for GABAergic differentiation), an enzyme whose activity also increases developmentally in the same retinal neurons. In fact, insulin inhibited the expression of GAD activity in the retina. The insulin-mediated increase in ChAT activity was independent of normal cell-cell interactions but could not replace them. Insulin also stimulated choline uptake but only after a two day delay, suggesting that the normal program for cholinergic differentiation in the chick retina was induced by insulin. IGF 1 did not have any effect on either cholinergic or GABAergic differentiation. We conclude that cholinergic differentiation in chick embryo retinal neurons is dependent on both insulin- and cell contact-mediated signals.

Insulin receptors and insulin-like growth factor I receptors are functional during organogenesis of the lens

Insulin and insulin-like growth factor I (IGF-I) stimulate overall growth and development of the chick embryo in early organogenesis. Turning to individual organs, to clarify the cellular effects of these peptides and the activity of the receptors involved, we had demonstrated with developing lens that insulin and IGF-I increase the accumulation of delta-crystallin mRNA, a marker for lens differentiation, in part by stimulation of transcription. In this study we expand our previous work on lens receptors to an earlier time in organogenesis, day 4, which marks the beginning of differentiation of the lens epithelial cells into elongated fibers. Insulin receptors are demonstrable by affinity cross-linking in epithelial cells at day 6, and specific binding of [125I]insulin and [125I]IGF-I is detectable in day 4 lenses. Insulin and IGF-I stimulation of substrate phosphorylation in the presence of solubilized receptors occurs only with high concentrations (10-100 nM) of either peptide in day 4 lenses, while a clear response with low concentrations (1 nM) is elicited by day 6 of development. Low concentrations of both insulin and IGF-I (0.1-1 nM) increase the incorporation of [3H]leucine and [3H]uridine in day 6 lens cells, suggesting that each peptide acts through its own receptor. These results confirm and extend the finding of insulin and IGF-I receptors in the developing chicken lens, and demonstrate their functional activity. This embryonic model should be valuable for further analysis of the action of insulin and IGF-I in growth and differentiation processes during early development.

Endocrinization of the early embryo: an emerging role for hormones and hormone-like factors

In the last decade, we witnessed the extension of endocrinologically based concepts and molecules to many other arenas of intercellular communication, e.g. immunology, hematology and cancer biology. At the start of the new decade we are witnessing the beginning of a similar transformation in our understanding of early embryogenesis, i.e. that hormones, growth factors and other hormone-like agents and their receptors, familiar to us in other contexts, may be the long-sought mediators of many key events in early embryogenesis. Why these agents were overlooked before and how they have started to emerge is the theme of this essay. The title 'Endocrinization of the Early Embryo' refers to both the biological and intellectual developments.

In situ autoradiography and ligand-dependent tyrosine kinase activity reveal insulin receptors and insulin-like growth factor I receptors in prepancreatic chicken embryos

We previously reported specific cross-linking of 125I-labeled insulin and 125I-labeled insulin-like growth factor I (IGF-I) to the alpha subunit of their respective receptors in chicken embryos of 20 somites and older. To achieve adequate sensitivity and localize spatially the receptors in younger embryos, we adapted an autoradiographic technique using whole-mounted chicken blastoderms. Insulin receptors and IGF-I receptors were expressed and could be localized as early as gastrulation, before the first somite is formed. Relative density was analyzed by a computer-assisted image system, revealing overall slightly higher binding of IGF-I than of insulin. Structures rich in both types of receptors were predominantly of ectodermal origin: Hensen's node in gastrulating embryos and neural folds, neural tube and optic vesicles during neurulation. The signal transduction capability of the receptors in early organogenesis was assessed by their ability to phosphorylate the exogenous substrate poly(Glu80Tyr20). Ligand-dependent tyrosine phosphorylation was demonstrable with both insulin and IGF-I in glycoprotein-enriched preparations from embryos at days 2 through 6 of embryogenesis. There was a developmentally regulated change in ligand-dependent tyrosine kinase activity, with a sharp increase from day 2 to day 4, in contrast with a small increase in the ligand binding. Binding of 125I-labeled IGF-I was, with the solubilized receptors, severalfold higher than binding of 125I-labeled insulin. However, the insulin-dependent phosphorylation was as high as the IGF-I-dependent phosphorylation at each developmental stage.