Pattern of the insulin-like growth factor II gene expression during early mouse embryogenesis

The alternative stimulatory G protein alpha-subunit XLalphas is a critical regulator of energy and glucose metabolism and sympathetic nerve activity in adult mice

The complex imprinted Gnas locus encodes several gene products including G(s)alpha, the ubiquitously expressed G protein alpha-subunit required for receptor-stimulated cAMP generation, and the neuroendocrine-specific G(s)alpha isoform XLalphas. XLalphas is only expressed from the paternal allele, whereas G(s)alpha is biallelically expressed in most tissues. XLalphas knock-out mice (Gnasxl(m+/p-)) have poor suckling and perinatal lethality, implicating XLalphas as critical for postnatal feeding. We have now examined the metabolic phenotype of adult Gnasxl(m+/p-) mice. Gnasxl(m+/p-) mice had reduced fat mass and lipid accumulation in adipose tissue, with increased food intake and metabolic rates. Gene expression profiling was consistent with increased lipid metabolism in adipose tissue. These changes likely result from increased sympathetic nervous system activity rather than adipose cell-autonomous effects, as we found that XLalphas is not normally expressed in adult adipose tissue, and Gnasxl(m+/p-) mice had increased urinary norepinephrine levels but not increased metabolic responsiveness to a beta3-adrenergic agonist. Gnasxl(m+/p-) mice were hypolipidemic and had increased glucose tolerance and insulin sensitivity. The similar metabolic profile observed in some prior paternal Gnas knock-out models results from XLalphas deficiency (or deficiency of the related alternative truncated protein XLN1). XLalphas (or XLN1) is a negative regulator of sympathetic nervous system activity in mice.

The 3' portion of the mouse H19 Imprinting-Control Region is required for proper tissue-specific expression of the Igf2 gene

Genomic imprinting at the H19/Igf2 locus is governed by a cis-acting Imprinting-Control Region (ICR), located 2 kb upstream of the H19 gene. This region possesses an insulator function which is activated on the unmethylated maternal allele through the binding of the CTCF factor. It has been previously reported that paternal transmission of the H19(SilK) deletion, which removes the 3' portion of H19 ICR, leads to the loss of H19 imprinting. Here we show that, in the liver, this reactivation of the paternal H19 gene is concomitant to a dramatic decrease in Igf2 mRNA levels. This deletion alters higher-order chromatin architecture, Igf2 promoter usage and tissue-specific expression. Therefore, when methylated, the 3' portion of the H19 ICR is a bi-functional regulatory element involved not only in H19 imprinting but also in 'formatting' the higher-order chromatin structure for proper tissue-specific expression of both H19 and Igf2 genes.

The H19 gene: regulation and function of a non-coding RNA

The H19 gene encodes a 2.3-kb non-coding mRNA which is strongly expressed during embryogenesis. This gene belongs to an imprinted cluster, conserved on mouse chromosome 7 and human chromosome 11p15. H19 is maternally expressed and the neighbouring Igf2 gene is transcribed from the paternal allele. These two genes are co-expressed in endoderm- and mesoderm-derived tissues during embryonic development, which suggests a common mechanism of regulation. The regulatory elements (imprinted control region, CTCF insulation, different enhancer sequences, promoters of the two genes, matrix attachment regions) confer a differential chromatin architecture to the two parental alleles leading to reciprocal expression. The role of the H19 gene is unclear but different aspects have been proposed. H19 influences growth by way of a cis control on Igf2 expression. Although H19(-/-) mice are viable, a role for this gene during development has been suggested by viable H19(-/-) parthenogenetic mice. Finally it has been described as a putative tumour suppressor gene. H19 has been studied by numerous laboratories over the last fifteen years, nevertheless the function of this non-coding RNA remains to be elucidated.

Imprinted genes, placental development and fetal growth

In mammals, imprinted genes have an important role in feto-placental development. They affect the growth, morphology and nutrient transfer capacity of the placenta and, thereby, control the nutrient supply for fetal growth. In particular, the reciprocally imprinted Igf2-H19 gene complex has a central role in these processes and matches the placental nutrient supply to the fetal nutrient demands for growth. Comparison of Igf2P0 and complete Igf2 null mice has shown that interplay between placental and fetal Igf2 regulates both placental growth and nutrient transporter abundance. In turn, epigenetic modification of imprinted genes via changes in DNA methylation may provide a mechanism linking environmental cues to placental phenotype, with consequences for development both before and after birth. Changes in expression of imprinted genes, therefore, have major implications for developmental programming and may explain the poor prognosis of the infant born small for gestational age and the wide spectrum of adult-onset diseases that originate in utero.

The Expression of Insulin-like Growth Factor and Insulin-like Growth Factor Binding Protein mRNAs in Mouse Placenta

Insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) are paracrine regulators of tissue growth and development, and are expressed at the sites of biological action. To study the role of the IGFs and IGFBPs in mouse placental development, we determined the temporal and spatial expression patterns of the mRNAs at embryonic days 10.5 to 18.5 by in situ hybridization. IGF-II mRNA was expressed strongly in mesoderm and fetal blood vessels of early placenta and in labyrinthine trophoblast of later placenta. In the junctional zone, IGF-II mRNA was expressed first in spongiotrophoblasts, later strongly in glycogen cells and variably in giant cells. IGFBP-2 mRNA was expressed weakly in spongiotrophoblasts and glycogen cells. IGFBP-2, -5 and -6 mRNAs were detected in the stroma of the metrial gland. Myometrium expressed IGFBP-2 mRNA strongly, IGFBP-6 mRNA moderately and IGFBP-5 mRNA weakly. The endothelium of maternal blood vessels in decidua expressed IGFBP-3 and -5 mRNAs, and some deeper vessels expressed IGFBP-4 mRNA. In the yolk sac, IGF-II mRNA was expressed in endoderm and mesoderm, whereas IGFBP-1, -2 and -4 mRNAs were expressed only in endoderm, and IGFBP-4 mRNA in mesoderm. Strong expression of IGF-II mRNA in glycogen cells suggests a role in the autocrine/paracrine regulation of invasion. Similar to rat and guinea pig, but in contrast to man and primates, IGFBP mRNAs, except IGFBP-4, were not expressed in mouse decidua. However, IGFBP-3, -4 and -5 mRNAs were expressed in endothelium of maternal blood vessels, and IGFBP-2 and -6 mRNAs in myometrium, where IGFBPs may play a critical role in regulating trophoblast invasion. These findings suggest possible biological roles of the peptides at the feto-maternal interface.

Insulin-like growth factor 2 as a candidate gene influencing growth and carcass traits and its bialleleic expression in chicken

We have identified DNA polymorphisms in the gene of insulin-like growth factor 2 by PCR-SSCP in a resource population, which was generated by Silky reciprocally crossing to Broilers. A C --> G mutation was detected in the exon 2 (at position 71) by sequencing. This single nucleotide polymorphism (SNP) was found to be associated with production traits. Chicken with BB genotype showed more chest angle width but less 3 week body weight and glandular stomach weight than chicken with AA genotype (P < 0.05); while the heterozygote (AB genotype) chicken had more abdominal fat weight, eviscerated yield with giblet than AA homozygote chicken. Further analysis showed that there were different genetic effects on some traits between heterozygote AB (paternal allele given first) and heterozygote BA: chickens with genotype BA had more birth weight and breast weight but less abdominal fat weight than chickens with genotype AB (P < 0.05), which could be hypothetically contributed by genome imprinting. Therefore, Silky chickens were selected for production of heterozygotes to confirm whether IGF2 locus was imprinting. Progeny from heterozygote x homozygote reciprocal cross was assayed for expression after the genotype was determined. The transcription of IGF2 was detected by RT-PCR-SSCP. IGF2 gene was expressed bialleleically in 1-day-old neonatal liver and 90-day-old liver, kidney, heart, and muscle of both heterozygote AB and BA chickens. Therefore, IGF2 was not an imprinting gene in chicken. The different genetic effects between the heterozygote AB and BA remain to be elucidated.

Spatial differences in hematopoiesis but not in stem cells indicate a lack of regional patterning in definitive hematopoietic stem cells

Most tissues are patterned so that progenitors in different locations are programmed to have different properties. Stem cells from different regions of the nervous system acquire intrinsic differences in their properties as they migrate through distinct environments. Hematopoietic stem cells (HSCs) also migrate through diverse environments throughout life, raising the question of whether HSCs also acquire at least transient changes in their properties as they are exposed to diverse environments. Although we observed significant differences in hematopoiesis between the fetal liver and fetal spleen, we were not able to detect phenotypic, functional, or gene expression differences between the HSCs in these organs. Regional differences in definitive hematopoiesis are therefore not determined by regional differences between HSCs. We were also not able to detect phenotypic, functional, or gene expression differences between HSCs in different adult bone marrow compartments. Our failure to detect differences among stem cells from different regions of the hematopoietic system at the same time during development suggests that the hematopoietic system has evolved mechanisms to prevent the spatial reprogramming of HSC properties as they migrate between distinct environments.

Delayed and disturbed morphogenesis of the umbilical blood vessels in insulin-like growth factor-II deficient conceptuses (Igf2m+/p-)

Insulin-like growth factor-II (IGF-II) deficiency occurs when a conceptus inherits an inactive gene from the father (Igf2m+/p-): fetal wet weight is reduced to 60% of wild-type, with the decline starting at E11. The umbilical cord vessels of mutant and wild-type were compared. At E8.0-E8.5, the timing of somite formation and chorioallantoic fusion was not altered. At E14.5-E16.5, the left umbilical artery degenerated approximately 1 day later in Igf2m+/p- conceptuses when compared with the wild-type. In the common umbilical artery at E15.5, muscle volume was reduced by one third in IGF-II deficiency. Treating the umbilical arteries as ideal tubes, the values of radius(4)/length suggest that blood flow through the placenta may be reduced by more than half in the Igf2m+/p- conceptuses.

FoxA proteins regulate H19 endoderm enhancer E1 and exhibit developmental changes in enhancer binding in vivo

Multiple enhancers govern developmental and tissue-specific expression of the H19-Igf2 locus, but factors that bind these elements have not been identified. Using chromatin immunoprecipitation, we have found two FoxA binding sites in the H19 E1 enhancer. Mutating these sites diminishes E1 activity in hepatoma cells. Additional chromatin immunoprecipitations show that FoxA binds to E1 in fetal liver, where H19 is abundantly expressed, but that binding decreases in adult liver, where H19 is no longer transcribed, even though FoxA proteins are present at both times. FoxA proteins are induced when F9 embryonal carcinoma cells differentiate into visceral endoderm (VE) and parietal endoderm (PE). We show that FoxA binds E1 in VE cells, where H19 is expressed, but not in PE cells, where H19 is silent. This correlation between FoxA binding and H19 expression indicates a role for FoxA in regulating H19, including developmental activation in the yolk sac and liver and postnatal repression in the liver. This is the first demonstration of a tissue-specific factor involved in developmental control of H19 expression. These data also indicate that the presence of FoxA proteins is not sufficient for binding but that additional mechanisms must govern the accessibility of FoxA proteins to their cognate binding sites within the H19 E1 enhancer.

High molecular weight form insulin-like growth factor II-producing mesenteric sarcoma causing hypoglycemia

An 81-year-old woman presented with frequent episodes of hypoglycemia. Her serum level of insulin was normal, but her serum insulin-like growth factor (IGF)-II level was high. She was found to have a spindle cell sarcoma originated from the mesentery of the sigmoid colon, which was completely resected. Postoperatively, hypoglycemia ameliorated with concomitant reduction in serum IGF-II levels. Immunohistochemical study revealed positive immunostaining for IGF-II in tumor cells, and the abundant expression of IGF-II mRNA was demonstrated by RT-PCR. The presence of high molecular weight (HMW) form IGF-II in patient's serum was confirmed by immunoblotting. This is the first report of a patient with HMW form IGF-II-producing mesenteric sarcoma causing hypoglycemia.

Amino acid transport regulates blastocyst implantation

Mouse blastocyst outgrowth in vitro and probably implantation in vivo require amino acid signaling via the target of rapamycin (TOR) pathway. This signaling does not simply support protein synthesis and trophoblast differentiation. Rather, it regulates development of trophoblast protrusive activity and may act as a developmental checkpoint for implantation. Moreover, intracellular amino acids per se are insufficient to elicit TOR signaling. Instead, de novo transport of amino acids, and particularly of leucine, stimulate mTOR activity at the blastocyst stage. The activity of the broad-scope and yet leucine-selective amino acid transport system B0,+ could produce such increases in intracellular amino acid concentrations. For example, system B0,+ uses a Na+ gradient to drive amino acid uptake, and the Na+ concentration in uterine secretions increases by nearly two-fold about 18 h before implantation. The resultant mTOR signaling could trigger polyamine, insulin-like growth factor II, and nitric oxide production in blastocysts and the increased cell motility sometimes associated with synthesis of these bioactive molecules.

Comparative Biology of the IGF System in Endometrium, Decidua, and Placenta, and Clinical Implications for Foetal Growth and Implantation Disorders

The insulin like growth factors and their binding proteins appear to play a central role during implantation and establishment of pregnancy in all species studied. Although there are similarities among species in the cell types that express IGFs and IGFBPs and their regulation during implantation and pregnancy, there are also significant differences. Understanding of the role of the IGF system in placental function in the human is of immense clinical importance, because serious complications of pregnancy such as intrauterine growth restriction and pre-eclampsia are thought to be associated with alterations in IGF system during early pregnancy and later in gestation. Research in laboratory and domestic animals, including transgenic and gene targeting studies in mice, has significantly improved our understanding of the role of IGF system in placental and foetal development. This paper reviews the diversity in the expression and regulation of IGF system in the decidua and placenta at the foetal-maternal interface in the human and different animal species, which may benefit in directing future studies in understanding of various complications of human pregnancy.

Furin gene (fur) regulation in differentiating human megakaryoblastic Dami cells: involvement of the proximal GATA recognition motif in the P1 promoter and impact on the maturation of furin substrates

The convertase furin is involved in the maturation of key growth/aggregation mediators synthesized by the platelet producers, megakaryocytes, but the regulation of furin in these cells remains unknown. Computer-assisted search of the furin promoter sequence revealed multiple potential binding motifs for GATA-1, suggesting that furin is expressed and regulated in these cells. Using megakaryoblastic Dami cells, we observed that fur mRNA expression increased gradually on phorbol 12-myristate 13-acetate-induced differentiation, reaching maximum levels (8.3-fold increase) at 10 days. Transient transfections with P1, P1A, or P1B fur-LUC-promoter constructs revealed that in Dami cells, the P1 promoter is the strongest and the most sensitive to forced expression of GATA-1. Coexpression of GATA-1 and its comodulator, Friend of GATA-1 (FOG-1), resulted in a cooperative increase in P1 activity. Deletion analysis indicated that important GATA-1-regulated sequences are located in the most proximal region of the P1 promoter. Further analysis revealed 2 potential GATA-binding motifs at positions -66 and +62. Point mutation of each of the 2 motifs indicated that the intactness of the first GATA site is required for full basal and GATA-1-stimulated promoter activity. Finally, the inhibition of furin activity through gene transfer of the inhibitor alpha1-AT-PDX led to a block in maturation of the furin substrates transforming growth factor-beta1 and platelet-derived growth factor. Taken together, these results indicate that the most proximal GATA element in the P1 promoter is needed for fur gene expression in megakaryoblastic cells. They also suggest that proper regulation of the fur gene in megakaryocytes has an impact on the activation of furin substrates involved in megakaryocyte maturation and platelet functions.

Glucose and pyruvate metabolism of preimplantation goat blastocysts following in vitro fertilization and parthenogenetic activation

The energy metabolism of preimplantation embryos can be used to predict viability and postimplantation development. Although preimplantation development and mean blastocyst cell numbers of goat in vitro-fertilized (IVF) embryos and chemically activated parthenogenotes are comparable, mammalian parthenogenotes are not viable, with most dying shortly after implantation. The objective of this study was to compare glucose and pyruvate metabolism of IVF goat blastocysts with that of parthenogenetic blastocysts developing from chemically activated oocytes. Embryos derived from IVF and parthenogenotes produced by exposing oocytes to either ionomycin or ethanol followed by 6-dimethylaminopurine (6-DMAP) were cultured in G1.2/G2.2 sequential culture media. Metabolism was determined for individual blastocysts using [5-3H]glucose and [2-14C]pyruvate to determine glycolytic and Kreb's cycle activity, respectively. Data were analyzed by ANOVA. A significantly higher percentage of activated oocytes underwent cleavage and developed to the blastocyst stage compared to IVF oocytes (p < 0.05). There was no significant difference in glucose or pyruvate metabolism between IVF and parthenogenetically activated blastocysts. Mean glucose metabolism through glycolysis was 154.9 +/- 29.1, 130.3 +/- 17.1, and 129 +/- 16.5 pmol/embryo/3 h for IVF, ethanol-activated, and ionomycin-activated blastocysts, respectively. Mean pyruvate metabolism through the Kreb's cycle was 28.1 +/- 8.0, 15.8 +/- 4.2, and 24.4 +/- 4.4 in pmol/embryo/3 h for IVF, ethanol-activated, and ionomycin-activated blastocysts, respectively. Our results suggest that known differences in postimplantation development observed in IVF versus parthenogenetic embryos cannot be attributed to differences in pyruvate or glucose metabolism in the preimplantation blastocysts. Thus, these activation protocols result in embryos capable of appropriate regulation of key metabolic enzymes.

Exogenous amino acids regulate trophectoderm differentiation in the mouse blastocyst through an mTOR-dependent pathway

At the late blastocyst stage, the epithelial trophectoderm cells of the mammalian embryo undergo a phenotypic change that allows them to invade into the uterine stroma and make contact with the maternal circulation. This step can be regulated in vitro by the availability of amino acids. Embryos cultured in defined medium lacking amino acids cannot form trophoblast cell outgrowths on fibronectin, an in vitro model of implantation, but remain viable for up to 3 days in culture and will form outgrowths when transferred into complete medium. The amino acid requirement is a developmentally regulated permissive event that occurs during a 4- to 8-h period at the early blastocyst stage. Amino acids affect spreading competence specifically by regulating the onset of protrusive activity and not the onset of integrin activation. Rapamycin, a specific inhibitor of the kinase mTOR/FRAP/RAFT1, blocks amino acid stimulation of embryo outgrowth, demonstrating that mTOR is required for the initiation of trophectoderm protrusive activity. Inhibition of global protein translation with cycloheximide also inhibits amino acid-dependent signals, suggesting that mTOR regulates the translation of proteins required for trophoblast differentiation. Our data suggest that mTOR activity has a developmental regulatory function in trophectoderm differentiation that may serve to coordinate embryo and uterus at the time of implantation.

Ontogeny of the H19 gene in sheep and effect of maternal fasting on its expression in the fetus

Mapped on the same chromosome as the Insulin-like Growth Factor II (IGF-II), an important factor regulating fetal growth, the H19 gene, is believed to play a role during embryogenesis and to share similar regulatory elements with IGF-II possibly by an enhancer competition system. This study was designed to characterize the ontogeny of H19 in sheep and the effect of maternal fasting on the expression of fetal IGF-II and H19 mRNA. A partial cDNA clone for the ovine H19 gene was isolated and used as a probe for RNase protection analysis. The ontogeny of H19 in liver, skeletal muscle and heart of ovine fetuses at 62,100 and 130 days, lambs at 1 month and adult sheep revealed high tissue levels of H19 mRNA during fetal life that decreased significantly after birth. Maternal fasting significantly decreased fetal liver H19 mRNA expression but did not alter fetal IGF-II mRNA expression. These results suggest that H19, like IGF-II, may play an important role in the regulation of fetal growth and define an environmental condition whereby these two genes are regulated independently.

TGFbeta2 activation status during cardiac morphogenesis

Transforming growth factor beta (TGFbeta) is secreted as a biologically inactive complex by many cell types in vitro, but little is known of TGFbeta's activation status in vivo. This study examined the in vivo expression of active and total (active + acid-activatable) TGFbeta2 in embryonic chicken hearts during cardiac morphogenesis (Hamburger-Hamilton stage 10-24). The concentration of TGFbeta2 was measured by an enzyme-linked immunoassay that recognized active TGFbeta2. Whole heart homogenates were either left untreated to measure active TGFbeta2 or treated with acid before assay to measure total (active + acid-activatable) TGFbeta2. Total TGFbeta2 concentration increased more than 16-fold between stage 10/11 and stage 24. Active TGFbeta2 concentration was highest at stage 14/15, but overall remained relatively constant varying at most by 2.8-fold. When expressed relative to total TGFbeta2, the amount of active TGFbeta2 progressively declined from 70% in stage 10/11 hearts to 7% in stage 24 hearts. The distribution of active and total TGFbeta2 was examined by immunostaining with an antibody against active TGFbeta2. Before immunostaining, sections were either treated with acid or left untreated to determine the distribution of total and active TGFbeta2, respectively. Active TGFbeta2 immunostaining was first detected in the endothelium, myocardium, and cardiac jelly of stage 14 hearts. Acid treatment had no effect on the distribution or intensity of immunostaining at this stage. Faint, active TGFbeta2 immunostaining was restricted to the ventricular myocardium in stage 18 hearts. Acid treatment resulted in a marked increase in staining intensity in the ventricle, but no staining was observed in the atrium or outflow tract. In stage 24 hearts, faint active TGFbeta2 staining was detected in the ventricle before acid treatment. After acid treatment, patches of intense punctate stain were found in all regions of the embryonic heart. Increases in TGFbeta2 concentration and immunostaining intensity after acidification suggest that a significant amount of TGFbeta2 is in the latent form. Stage-dependent differences in activation status suggest that activation may be a developmentally regulated process in the chick heart and support the notion that activation is an important step in regulating TGFbeta actions in vivo.

A complex duplication created by gene targeting at the imprinted H19 locus results in two classes of methylation and correlated Igf2 expression phenotypes

Imprinting of the mouse H19 and Igf2 genes is dependent on the presence of an intervening imprinting control region (ICR) situated 2 kb upstream of H19 and approximately 70 kb downstream of Igf2. Several recent studies have provided substantial evidence that the unmethylated maternal ICR acts as an insulator that prevents activation of Igf2 by a suite of enhancers downstream of the H19 gene. The methylated paternal ICR and H19 promoter have no activity, allowing sole activation of Igf2 expression. We have produced mice in which a duplication of the H19/Igf2 ICR produces, in each generation, two classes of methylation levels that correlated with two Igf2 imprinting phenotypes. One hypermethylated class also shows activation of the normally silent Igf2 gene, whereas the other hypomethylated class shows only slight activation of Igf2, in agreement with methylation's role in ICR function. This study describes a rare, possibly unique type of mutation that induces two distinct phenotypes in each generation.

Extensive tissue-specific variation of allelic methylation in the Igf2 gene during mouse fetal development: relation to expression and imprinting

The imprinted Igf2 gene is active only on the paternal allele in most tissues. Its imprinting involves a cis-acting imprinting-control region (ICR) located upstream of the neighboring and maternally expressed H19 gene. It is thought that differential methylation of the parental alleles at the ICR is crucial for parental imprinting of both genes. Differentially methylated regions (DMRs) have also been identified within the Igf2 gene and their differential methylation is thought to be established during early development. To gain further insight into the function of these DMRs, we performed a quantitative analysis of their allelic methylation levels in different tissues during fetal development and the postnatal period in the mouse. Surprisingly, we found that the methylation levels of Igf2 DMRs vary extensively during fetal development, mostly on the expressed paternal allele. In particular, in skeletal muscle, differential allelic methylation in both DMR 1 and DMR 2 occurs only after birth, whereas correct paternal monoallelic expression is always observed, including in the embryonic stages. This suggests that differential methylation in the DMR 1 and DMR 2 of the Igf2 gene is dispensable for its imprinting in skeletal muscle. Furthermore, progressive methylation of the Igf2 paternal allele appears to be correlated with concomitant postnatal down-regulation and silencing of the gene. We discuss possible relations between Igf2 allelic methylation and expression during fetal development.

Coordinate control of muscle cell survival by distinct insulin-like growth factor activated signaling pathways

Peptide growth factors control diverse cellular functions by regulating distinct signal transduction pathways. In cultured myoblasts, insulin-like growth factors (IGFs) stimulate differentiation and promote hypertrophy. IGFs also maintain muscle cell viability. We previously described C2 skeletal muscle lines lacking expression of IGF-II. These cells did not differentiate, but underwent progressive apoptotic death when incubated in differentiation medium. Viability could be sustained and differentiation enabled by IGF analogues that activated the IGF-I receptor; survival was dependent on stimulation of phosphatidylinositol 3-kinase (PI3-kinase). We now find that IGF action promotes myoblast survival through two distinguishable PI3-kinase-regulated pathways that culminate in expression of the cyclin-dependent kinase inhibitor, p21. Incubation with IGF-I or transfection with active PI3-kinase led to rapid induction of MyoD and p21, and forced expression of either protein maintained viability in the absence of growth factors. Ectopic expression of MyoD induced p21, and inhibition of p21 blocked MyoD-mediated survival, thus defining one PI3-kinase-dependent pathway as leading first to MyoD, and then to p21 and survival. Unexpectedly, loss of MyoD expression did not impede IGF-mediated survival, revealing a second pathway involving activation by PI3-kinase of Akt, and subsequent induction of p21. Since inhibition of p21 caused death even in the presence of IGF-I, these results establish a central role for p21 as a survival factor for muscle cells. Our observations also define a MyoD-independent pathway for regulating p21 in muscle, and demonstrate that distinct mechanisms help ensure appropriate expression of this key protein during differentiation.

IGF-II promotes mesoderm formation

IGF-II is abundant in the nascent mesoderm of the gastrulating mouse embryo. Its function at this developmental stage is unknown. We investigated it by following the in vitro and in vivo differentiation of several androgenetic, biparental, parthenogenetic, and androgenetic Igf2 -/- murine ES cell lines; these cells differed in endogenous IGF-II levels because Igf2 is paternally expressed in the mouse embryo in most tissues. The expression of mesoderm markers and the subsequent formation of muscle structures were correlated with endogenous IGF-II level during teratoma formation and during in vitro differentiation. In addition, the absence of Igf2 in androgenetic Igf2 -/- ES cells led to a severe impairment of mesoderm development, demonstrating the dependence of the preferential mesoderm development of androgenetic ES cells upon Igf2 activity, among the numerous known imprinted genes. The addition of exogenous IGF-II to in vitro differentiation culture medium led to a specific increase in the expression of mesoderm markers. Thus, we propose a novel model in which the binding of IGF-II to its principal signaling receptor, IGF1R, at the surface of mesoderm precursor cells increases the formation of mesoderm cells.

H19 RNA binds four molecules of insulin-like growth factor II mRNA-binding protein

H19 RNA is a major oncofetal 2.5-kilobase untranslated RNA of unknown function. The maternally expressed H19 gene is located 90 kilobase pairs downstream from the paternally expressed insulin-like growth factor II (IGF-II) gene on human chromosome 11 and mouse chromosome 7; and due to their reciprocal imprinting and identical spatiotemporal expression, it is assumed that the two genes are functionally coupled. Here we show that human H19 RNA contains four attachment sites for the oncofetal IGF-II mRNA-binding protein (IMP) with apparent K(d) values in the 0.4-1.3 nm range. The multiple attachment sites are clustered within a 700-nucleotide segment encoded by exons 4 and 5. This 3'-terminal segment targets H19 RNA to lamellipodia and perinuclear regions in dispersed fibroblasts where IMP is also localized. The results suggest that IMP participates in H19 RNA localization and provides a link between the IGF-II and H19 genes at post-transcriptional events during mammalian development.

Patched target Igf2 is indispensable for the formation of medulloblastoma and rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children (Dagher, R., and Helman, L. (1999) Oncologist 4, 34-44), whereas medulloblastoma, a highly malignant tumor of the cerebellum, accounts for 20% of childhood brain tumors (Goodrich, L. V., and Scott, M. P. (1998) Neuron 21, 1243-1257). Both tumors are associated with a deficiency in the tumor suppressor Patched (PTCH) in Gorlin syndrome (Gorlin, R. J. (1987) Medicine (Baltimore) 66, 98-113), and they are present in the corresponding murine models. RMS in Ptch mutant mice consistently contain elevated levels of the tumor growth-promoting insulin-like growth factor 2 (Igf2). We have investigated the mechanism of Igf2 overexpression and its significance in medulloblastoma and RMS tumorigenesis. Here we report that Igf2 is indispensable for the formation of medulloblastoma and RMS in Ptch mutants. Overexpression of Igf2 in RMS in these mice does not involve loss of imprinting, uniparental disomy, amplification of the Igf2 locus, or polyploidy. Since Igf2 is also overexpressed in non-tumor tissue deficient in Ptch, these observations suggest that Ptch regulates Igf2 levels through a transcriptional mechanism. They also identify Igf2 as a potential target for medulloblastoma and RMS.

Comparative genomic sequencing identifies novel tissue-specific enhancers and sequence elements for methylation-sensitive factors implicated in Igf2/H19 imprinting

A differentially methylated region (DMR) and endoderm-specific enhancers, located upstream and downstream of the mouse H19 gene, respectively, are known to be essential for the reciprocal imprinting of Igf2 and H19. To explain the same imprinting patterns in non-endodermal tissues, additional enhancers have been hypothesized. We determined and compared the sequences of human and mouse H19 over 40 kb and identified 10 evolutionarily conserved downstream segments, 2 of which were coincident with the known enhancers. Reporter assays in transgenic mice showed that 5 of the other 8 segments functioned as enhancers in specific mesodermal and/or ectodermal tissues. We also identified a conserved 39-bp element that appeared repeatedly within the DMR and formed complexes with specific nuclear factors. Binding of one of the factors was inhibited when the target sequence contained methylated CpGs. These complexes may contribute to the presumed boundary function of the unmethylated DMR, which is proposed to insulate maternal Igf2 from the enhancers. Our results demonstrate that comparative genomic sequencing is highly efficient in identifying regulatory elements.

Spatial and temporal patterns of expression of messenger RNA for insulin-like growth factors and their binding proteins in the placenta of man and laboratory animals

To better understand the role of the insulin-like growth factors (IGF-I and -II) and their binding proteins (IGFBPs 1-6) in placental development and function, it is important to review similarities and differences between species in expression of the respective mRNAs. In human placenta, IGF-II mRNA is expressed in chorionic mesoderm and first trimester villous cytotrophoblast, but not in syncytiotrophoblast. In contrast, in rhesus monkey placenta, IGF-II mRNA is expressed in syncytiotrophoblast but not in chorionic mesoderm. IGFBP-3 mRNA is present in the chorionic mesoderm of placental villi from both these species and may modulate IGF-II action through a paracrine mechanism. In rodent placentae, IGF-II mRNA is expressed both in fetal mesoderm and in the trophoblast of the placental labyrinth. In guinea pig, where IGFBP-5 mRNA is expressed in the marginal and interlobular syncytium and IGF-II mRNA in the labyrinth, interaction between IGF-II and IGFBP-5 mRNA may be involved in vascularization of the placenta by fetal vessels. In sheep placenta, IGF-II mRNA is expressed, not in the trophoblast layer, but in the fetal mesoderm immediately adjacent to it. In the basal plate of human, rhesus monkey and baboon placentae, extravillous trophoblasts express IGF-II mRNA and uterine decidual cells IGFBP 1-6 mRNAs. The inference is that there is interaction between IGF-II and IGFBPs at the maternal-fetal interface of the primate placenta during trophoblast invasion and decidualization. IGFBP-1 expressed by the decidua may also interact with alpha(5)beta(1)integrin expressed by the extravillous trophoblast. The placentae of rodents are also of the invasive type. Glycogen cells of the mouse placenta are analogous with human extravillous trophoblast and express IGF-II mRNA. However, expression of IGFBP mRNAs in the mouse, as in the guinea pig, is confined to non-decidualized endometrium and myometrium. IGF-II mRNA is strongly expressed by trophoblasts invading uterine vessels in human and guinea pig placentae. Interactions probably occur between IGF-II expressed by these trophoblasts and IGFBPs expressed in the vessel walls. However, it is possible that IGFBPs expressed by maternal vessels are associated with processes that are independent of trophoblast invasion. Thus, IGFBP-3 mRNA is highly expressed in the maternal blood vessels of the non-deciduate sheep placenta. Findings to date highlight the diversity in the expression of the IGF system among placentae of man and different laboratory animals, and even between closely related species. Comparative studies will continue to be required to understand the functional role of IGFs and IGFBPs in each species.

Variable accumulation of insulin-like growth factor II in mouse tissues deficient in insulin-like growth factor II receptor

The insulin-like growth factor II receptor mediates endocytosis of insulin-like growth factor II, resulting in growth factor degradation in lysosomes. This degradation is an important regulator of growth factor activity in vivo, as shown by the phenotype of receptor deficient mice. Recent evidence suggests that the insulin-like growth factor II receptor functions as a tumour suppressor in humans, and that loss of receptor function leads to increased levels of the growth factor in tumours. It is difficult to establish such a causal relationship in human tumours however, since most tumours have undergone several genetic changes by the time they are examined. Using mouse embryos deficient in receptor expression, and an insulin-like growth factor II-specific radioimmunoassay, we tested the hypothesis that lack of receptor function leads to local accumulation of insulin-like growth factor II. We found that mutant blood and skeletal muscle had excess insulin-like growth factor II, but that mutant lungs and liver had no accumulation. Mutant hearts had less growth factor than wild-type hearts, an unexpected observation, since the normal embryonic heart expresses very high levels of insulin-like growth factor II receptor, and mutant mice apparently die of congestive heart failure. The placentas of mutant mice were larger than those of wild-type, but this did not correlate with an excess of placental insulin-like growth factor II. These results indicate that lack of insulin-like growth factor II receptor can lead to local excess of the growth factor but that such excess is not a necessary consequence of receptor-deficiency.

Biallelic transcription of Igf2 and H19 in individual cells suggests a post-transcriptional contribution to genomic imprinting

The H19 and insulin-like growth factor 2 (Igf2) genes in the mouse are models for genomic imprinting during development. The genes are located only 90 kb apart in the same transcriptional orientation [1], but are reciprocally imprinted: Igf2 is paternally expressed while H19 is maternally expressed. It has been suggested that expression of H19 and repression of Igf2 (or the converse) on a given chromosome are mechanistically linked and that the parental imprint operates at the level of transcription [2]. Although expression of Igf2 and H19 is thought to be monoallelic, the data have so far been obtained exclusively by looking at steady-state RNA levels using techniques that reflect the average activity of the genes in a cell population [3] [4]. Here, we have adapted a fluorescent in situ hybridisation (FISH) method to detect nascent RNA molecules of Igf2 and H19 at the initial transcription sites in the nuclei of wild-type mouse embryonic liver cells. Nine different transcription patterns were observed, reflecting a high heterogeneity of transcription at the single-cell level. Our observations suggest that regulation of Igf2 and H19 by parental imprinting is much more complex than previously proposed and acts at both transcriptional and post-transcriptional levels.

Enhancer competition between H19 and Igf2 does not mediate their imprinting

The linked H19 and Igf2 genes on mouse distal chromosome 7 are subject to genomic imprinting. Competition between the promoters of the genes for transcription from shared enhancers has been proposed as an explanation for the coordinate expression and reciprocal imprinting of these two genes. To test this model, we have used Cre-loxP technology to generate in mice a conditional deletion of the H19 promoter and structural gene that leaves no transcription unit in the locus. Contrary to the prediction of enhancer competition we find that transcriptional activity from the H19 promoter is not required for the imprinted silencing of the Igf2 gene.

Proparathyroid hormone processing by the proprotein convertase-7: comparison with furin and assessment of modulation of parathyroid convertase messenger ribonucleic acid levels by calcium and 1,25-dihydroxyvitamin D3

We previously showed that the processing of proparathyroid hormone (proPTH) to PTH was accomplished most efficiently by furin (17). Colocalization studies demonstrated that furin is expressed in the parathyroid, whereas proprotein convertase (PC)1 and PC2 are not. Since that time, another member of the PC family, called PC7, has been identified. Here we show, using coinfection studies, that PC7, as well as furin, can appropriately cleave PTH from proPTH. ProPTH and PTH were purified from cell extracts by reversed-phase HPLC and were identified by Western blot analysis and delayed extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Colocalization studies, using Northern blot and reverse transcriptase-PCR analyses, showed that PC7 messenger RNA (mRNA) is expressed in the parathyroid gland. Therefore, PC7, like furin, has the potential to be involved in the physiological processing of proPTH to PTH. The two major regulators of parathyroid cell synthetic and secretory activity are the extracellular fluid calcium and 1,25-dihydroxyvitamin D [1,25(OH)2D] levels. We investigated whether either of these agents might modulate processing of proPTH to PTH by altering parathyroid convertase gene expression. In both in vitro and in vivo systems in which regulation of PTH mRNA levels were clearly apparent, there was no effect of either calcium or 1,25(OH)2D3 on parathyroid furin or PC7 mRNA levels. This is in contrast to the processing of proinsulin to insulin in the pancreatic beta-cell, which is up-regulated by glucose stimulation of PC1 and PC2 synthesis.

Insulin-like growth factor-2 regulation of conceptus composition: effects of the trophectoderm and inner cell mass genotypes in the mouse

The purpose of this study was to measure the effect of insulin-like growth factor-2 deficiency on the growth of the mouse conceptus. Initial observations on normal development in the 129J/Sv strain established that wet and dry weights were reduced by 35% when the insulin-like growth factor-2 gene was inactive. The DNA contents were reduced by only 15%. We exchanged the inner cell mass and trophectoderm between mouse blastocysts that had or lacked an active insulin-like growth factor-2 gene. At embryonic Day 16.5, lack of this gene's activity in the derivatives of either tissue decreased the fluid volumes of the exocelomic and amniotic cavities. The wet weights of the "fetal placentas," the yolk sacs, and the fetuses were also decreased. However, the tissue wet weight decrease could not be accounted for by the change in DNA content, indicating that cell-associated biomass had changed. The conclusions are 1) that insulin-like growth factor-2 levels regulate the composition of the fetus and extra-embryonic tissues and 2) that trophectoderm and inner cell mass derivatives cooperate to control extra-cellular fluid volume in the conceptus.

Gpc3 expression correlates with the phenotype of the Simpson-Golabi-Behmel syndrome

Interest in glypican-3 (GPC3), a member of the glypican-related integral membrane heparan sulfate proteoglycans (GRIPS) family, has increased with the finding that it is mutated in the Simpson-Golabi-Behmel overgrowth syndrome (Pilia et al. [1996] Nat. Genet. 12:241-247). The working model suggested that the membrane-bound protein acts locally to limit tissue and organ growth and that it may function by interacting with insulin-like growth factor 2 (IGF2) to limit its local effective level. Here we have tested two predictions of the model. In situ hybridization with the mouse gene cDNA was used to study the expression pattern during embryonic and fetal development. In agreement with predictions, the gene is expressed in precisely the organs that overgrow in its absence; and the patterns of expression of Gpc3 and those reported for Igf2 are strictly correlated.

Insulin-like 4 (INSL4) gene expression in human embryonic and trophoblastic tissues

Polypeptide growth factors play an important role in the regulation of human embryonic development. Insulin-like 4 gene (INSL4) is a member of the insulin family, which includes insulin, IGF-I, IGF-II, relaxin, and INSL3. Using RT-PCR, we previously found abundant INSL4 mRNA in the human placenta. In this study, we examined the chronology and spatial expression of this gene in sections of human placenta and conceptus by means of in situ hybridization. Expression of the IGF-II gene was studied as a positive control. INSL4 distribution was tissue- and cell-specific. Indeed, INSL4 mRNA was most abundant in syncytiotrophoblast cells. In fetal tissues, INSL4 mRNA was identified in the perichondrium of all four limbs, vertebrae, and ribs. Moreover, INSL4 mRNA was abundant in interbone ligaments. These findings indicate that the INSL4 gene may play an important role in trophoblast development and regulation of bone formation. IGF-II mRNA, in agreement with the literature, are mainly located in the mesodermal core in the villous trophoblast and in most embryonic tissues.

Expression of the insulin-like growth factors and their receptors in term placentas: a comparison between normal and IUGR births

Intrauterine growth retardation (IUGR) is defined as growth retarded to be below the tenth centile. The insulin-like growth factors and their receptors are implicated in pre- and postnatal growth and development, and it is believed that alteration in their activity may contribute to IUGR. In this study nine normal and nine intrauterine growth retarded births were followed and term placentas examined for expression of the insulin-like growth factors and their receptors. It was found that the expression of insulin-like growth factor 1 (IGF1), insulin-like growth factor 2 (IGF2), and the insulin, IGF1 and IGF2 receptor transcripts (IGF1R and IGF2R, respectively) was present in all term placentas examined. Expression of insulin was not detected. Quantitative polymerase chain reaction (PCR) was used to compare transcription levels in term placentas from normal with IUGR births. There was no significant difference in the levels of transcripts for IGF1, insulin receptor, or IGF2R between normal and IUGR term placentas. However, the IUGR term placentas had significantly higher levels of IGF2 and IGF1R expression compared with the normal term placentas. The increase in the transcription of IGF2 and IGF1R in IUGR term placentas may represent a counter regulatory mechanism in response to the growth retardation.

Expression and imprinting of the insulin-like growth factor II gene in neonatal mouse cerebellum

Insulin-like growth factor II (IGF-II) plays significant roles in the growth and development of mammals through the regulation of mitogenesis and cell survival. Previously, IGF-II mRNA transcripts within the CNS were detected in the choroid plexus and leptomeninges (DeChiara et al., 1991). The objective of this study was to determine the expression pattern of IGF-II mRNA in different cell types of the cerebellum during development. We report here that the IGF-II gene is transcribed in granule and glial cells within the cerebellar parenchyma at various times during the early postnatal period in mice. IGF-II gene expression is further regulated by parent-specific imprinting such that only the paternal IGF-II allele is expressed in granule cells. In contrast, choroid plexus and leptomeninges express IGF-II mRNAs biallelically, indicating that cell type-specific regulation of genomic imprinting occurs within the mammalian CNS.

Biallelic expression of the H19 and IGF2 genes in hepatocellular carcinoma

The imprinted genes, H19 and insulin-like growth factor II (IGF2), have been demonstrated to be necessary for embryonal development in humans. Both genes are reciprocally imprinted, with expression of the maternal H19 and paternal IGF2 alleles, and are normally characterized by monoallelic expression. Recently, loss of imprinting of these genes producing biallelic expression has been observed in childhood tumors including Wilms' tumors (WT), embryonal rhabdomyosarcoma, and adulthood tumors such as lung cancer. To test the existence of loss of imprinting in hepatocellular carcinoma (HCC), we analyzed the status of imprinting of H19 and IGF2 genes in three independent tumors, three HCC and one hepatoblastoma cell lines using AluI and ApaI polymorphisms of these genes, respectively. In contrast to the previous report, all the cases except one tumor and one HCC cell line showed biallelic expression of both H19 and IGF2 genes. Unlike WT, loss of imprinting (LOI) of IGF2 in HCC was not linked to down-regulation of H19 expression, but rather associated with coexpression for H19 and IGF2. Thus, Hl9 and IGF2 expression can be uncoupled in tumors with LOI. The frequent biallelic expression of H19 and IGF2 in hepatocellular carcinoma might play a causal role in the epigenetic mechanism involved in tumor development and/or process.

Perinatal lethality in H19 enhancers-Igf2 transgenic mice

The insulin-like growth factor II (IGFII) is a mitogen for a number of cell types in vitro and is required for normal embryonic growth. It has been hypothesized that overexpression of IGF2 is responsible for the increased growth and tumor predisposition in patients with Beckwith-Wiedemann syndrome. Association of increased levels of IGFII with increased growth is also incorporated in a current model for the evolution of Igf2 imprinting. Different experimental approaches to increasing IGFII levels in the mouse have yielded different results with respect to its effects on growth, viability, and tumor development. To investigate the consequences of IGf2 overexpression in the embryonic period, without alterations in the activity of other genes, we produced transgenic mice that express the Igf2 gene under the control of the H19 enhancers. Transgene expression in the embryonic period had no significant effect on the overall size of the embryos, but was associated with perinatal lethality in homozygous, and some heterozygous, mice. A large fraction of homozygous mice also developed a cleft palate. These findings indicate that overexpression of Igf2 can have an adverse effect on viability in the absence of a pronounced effect on overall body growth. The results are consistent with the view that growth and perinatal viability are affected differently by Igf2 overexpression in endodermal and mesodermal tissues.

Imprinting of Igf2 and H19 from a 130 kb YAC transgene

A stringent test for imprint control elements is to examine their function at ectopic loci in transgenic experiments. Igf2 and H19 are part of a larger imprinting region and as a first step, we examined these reciprocally imprinted genes in transgenic experiments using a 130 kb YAC clone. After paternal inheritance, H19 was appropriately repressed and Igf2 was expressed, irrespective of copy number or genetic background. After maternal inheritance H19 was consistently expressed, albeit with some variability. The levels of H19 expression per copy of the transgene inversely correlated with Igf2 (-lacZ) expression in cis. The consistent imprinting of H19 from this YAC contrasts with the previously described imprinting of mini-H19 transgenes, which only occurs at multi-copy loci, is inconsistent, and is prone to genetic background effects. We propose a novel model in which silencing of the H19 gene is the default state and its activation after maternal inheritance is the key mechanistic event for imprinting in this region. In addition, in situ analysis of the Igf2-lacZ reporter indicates that additional mesoderm-specific enhancers are present within the YAC clone. No obvious phenotype was detected from the excess gene dosage of H19.

Production of biologically active recombinant tilapia insulin-like growth factor-II polypeptides in Escherichia coli cells and characterization of the genomic structure of the coding region

Insulin-like growth factor-II (IGF-II) is a fetal growth factor in humans, but has not been clearly identified in fish up to now. For a detailed understanding of the physiological response of fish IGF-II, the first step was to clone tilapia IGF-II cDNA from the brain cDNA library, coding the region of genomic DNA, and also expressing tilapia IGF-II polypeptides from Escherichia coli. Tilapia cDNA sequences total 1,977 bp, and predicted nucleotide sequences and amino acid sequences of tilapia share 77.9% and 90.7% homology identity with rainbow trout IGF-II, respectively. The genomic structure of the tilapia prepro-IGF-II coding region is very difficult to sequence in mammals and birds. The cloned tilapia IGF-II gene coding region appears much more complex than in other vertebrates. In tilapia IGF-II, the first coding exon I encoding part of the signal peptide sequence is 25 amino acids shorter than the first coding exon of mammals and birds. The other 23 amino acids of the signal peptide, and the first amino acids of the B domain and C domain are encoded by tilapia coding exon 2. The C, A, and D domains, and the first 20 amino acids of the E peptide are encoded by tilapia coding exon 3. The other E peptides and the 3' untranslated region (UTR) region are encoded by tilapia coding exon 4. These data show that the IGF-II genes have significantly differing structures in vertebrate evolution, and there are differences of interrupting introns in the IGF-I genomic structure compared with mammals. To obtain recombinant biologically active polypeptides, tilapia IGF-II B-C-A-D domains were amplified using the polymerase chain reaction (PCR), then ligated with glutathione S-transferase (GST, pGEX-2T vector). Tilapia recombinant IGF-II protein was purified and characterized in E. coli. The fusion protein was also digested with thrombin and appeared as a recombinant IGF-II polypeptide single band with a molecular mass of 7 kD. The recombinant tilapia IGF-II protein biological function was measured by stimulation of [3H]thymidine incorporation. The assay concentration was set up from 0 to 120 nM to stimulate tilapia ovary cell line (TO-2) significantly to uptake thymidine. The results suggest that the recombinant IGF-II protein was dose dependent.

Wortmannin inhibits IGF-dependent differentiation in the mouse myogenic cell line C2

Evidence has accumulated that suggests that insulin-like growth factors (IGFs) exert a positive influence on myoblast differentiation. We have undertaken to study the signalling events required for differentiation resulting from type-1 IGF receptor stimulation in C2 myoblasts, where autocrine production of IGF-II was abolished by means of antisense RNA. Exposure of the cells to IGFs leads to a rapid and sustained activation of phosphatidyl-inositol 3-kinase followed by the expression of Myod, myogenin and differentiation. The fungal metabolite, wortmannin, inhibits both PI 3-kinase and muscle differentiation with an IC 50 in the nanomolar range. IGFs are also known to cause a rapid activation of MAP kinase. However, the synthetic inhibitor of MEK, PD098059, which prevents MAP kinase activation, does not affect myoblast differentiation. These results provide evidence that PI 3-kinase, but not MAP kinase, is required for insulin-like growth factor receptor-dependent differentiation of muscle cells.

Loss of imprinting of human insulin-like growth factor II gene, IGF2, in acute myeloid leukemia

Genomic imprinting is a non-Mendelian form of gene regulation resulting in differential allelic expression of a gene and plays an important role in the development of certain types of cancer and genetic diseases. Imprinting of IGF2 was demonstrated in normal placenta and fetal kidney by showing monoallelic paternal expression. In contrast, several kinds of tumor showed biallelic expression of IGF2, suggesting that relaxation of the normal imprinting of this growth cytokine may be a feature of some tumors. In this study we wished to determine whether relaxation of IGF2 imprinting was also common in human leukemias. An intragenic Apa I polymorphism within the 3' untranslated region of the gene was used to examine allele-specific transcription in leukemic blasts derived from 24 primary patients with acute myeloid leukemia by RT-PCR. Leukemic samples from 12 of the 24 leukemia patients were identified as heterozygous for IGF2 and potentially informative for the RT-PCR assay. RNA-PCR from these informative leukemia samples studied showed biallelic expression of IGF2, indicating loss of normal imprinting of this gene to be very frequent. In conclusion, constitutional loss of monoallelic expression in 50% of the leukemia samples tested suggests that abnormal imprinting of IGF2 could play an important role in either leukemogenesis or cytokine dysregulation for leukemic cells.

The early intracellular signaling pathway for the insulin/insulin-like growth factor receptor family in the mammalian central nervous system

Several studies support the idea that the polypeptides belonging to the family of insulin and insulin-like growth factors (IGFs) play an important role in brain development and continue to be produced in discrete areas of the adult brain. In numerous neuronal populations within the olfactory bulb, the cerebral and cerebellar cortex, the hippocampus, some diencephalic and brainstem nuclei, the spinal cord and the retina, specific insulin and IGF receptors, as well as crucial components of the intracellular receptor signaling pathway have been demonstrated. Thus, mature neurons are endowed with the cellular machinery to respond to insulin and IGF stimulation. Studies in vitro and in vivo, using normal and transgenic animals, have led to the hypothesis that, in the adult brain, IGF-I not only acts as a trophic factor, but also as a neuromodulator of some higher brain functions, such as long-term potentiation and depression. Furthermore, a trophic effect on certain neuronal populations becomes clearly evident in the ischemic brain or neurodegenerative disorders. Thus, the analysis of the early intracellular signaling pathway for the insulin/IGF receptor family in the brain is providing us with new intriguing findings on the way the mammalian brain is sculpted and operates.

Genomic imprinting in disorders of growth

This review has briefly considered some of the vast amount of information that has been gathered on genomic imprinting and its role in PWS, AS, BWS and Russell-Silver syndrome. The pace of investigation into the phenomenon of imprinting will undoubtedly continue, because our understanding remains far from complete. Newer approaches to identifying imprinted genes based on their expression rather than their location are likely to uncover currently unknown genes. We can also look forward to more insight into the fascinating complexities of the imprinting process.

Regulation of avian precardiac mesoderm development by insulin and insulin-like growth factors

Endoderm within the heart forming regions of vertebrate embryos has pronounced effects on myocardial cell development. Previous studies have suggested that these effects are mediated by soluble growth factors, in particular fibroblast growth factor 2 (FGF-2) and activin-A. Since both insulin and insulin-like growth factors (IGFs) are present in developing avian embryos at the time of heart formation, we have investigated the potential role of these molecules in promoting development of premyocardial cells in quail. Culture of precardiac mesoderm explants from stage 5 quail embryos in medium containing insulin, IGF-I, or IGF-II increased proliferation of premyocardial cells, with maximal stimulation observed at approximately 25 nM for each ligand. A direct comparison of the proliferative response of precardiac mesoderm to endoderm, fetal calf serum, insulin, IGF-I, IGF-II, activin-A, and FGF-2 showed that FGF-2 and activin-A increased proliferation of premyocardial cells approximately 2-fold, while insulin, IGF-I, and IGF-II stimulated proliferation approximately 3-fold. Insulin and IGF-I enhanced the rate of myocyte differentiation, similar to previously reported effects of endoderm. In contrast, exposure of precardiac mesoderm explants to transforming growth factor beta (TGF beta) reduced proliferation of premyocardial cells and moderated the proliferative effects of IGF-I. TGF beta did not block the differentiation of stage 5 premyocardial cells. Reverse transcription-polymerase chain reaction (RT-PCR) analyses showed that mRNAs encoding insulin, IGF-II, insulin receptor, and IGF-I receptor were present in both precardiac mesoderm and endoderm, as well as in the forming heart at stage 8. Since premyocardial cells can survive and differentiate in a defined medium lacking these factors, precardiac mesoderm may produce IGF-II and insulin at levels that are sufficient to stimulate myocyte development. Taken together, these results suggest that insulin and/or IGF-II may promote cardiac development in vivo by both autocrine and paracrine mechanisms. Cardiogenesis may therefore be promoted by the combined action of several classes of growth factors.