Time of initiation and site of action of the mouse chromosome 11 imprinting effects

Generation of Bimaternal and Bipaternal Mice from Hypomethylated Haploid ESCs with Imprinting Region Deletions

Unisexual reproduction is widespread among lower vertebrates, but not in mammals. Deletion of the H19 imprinted region in immature oocytes produced bimaternal mice with defective growth; however, bipaternal reproduction has not been previously achieved in mammals. We found that cultured parthenogenetic and androgenetic haploid embryonic stem cells (haESCs) display DNA hypomethylation resembling that of primordial germ cells. Through MII oocyte injection or sperm coinjection with hypomethylated haploid ESCs carrying specific imprinted region deletions, we obtained live bimaternal and bipaternal mice. Deletion of 3 imprinted regions in parthenogenetic haploid ESCs restored normal growth of fertile bimaternal mice, whereas deletion of 7 imprinted regions in androgenetic haploid ESCs enabled production of live bipaternal mice that died shortly after birth. Phenotypic analyses of organ and body size of these mice support the genetic conflict theory of genomic imprinting. Taken together, our results highlight the factors necessary for crossing same-sex reproduction barriers in mammals.

The role of genomic imprinting in biology and disease: an expanding view

Genomic imprinting is an epigenetic phenomenon that results in monoallelic gene expression according to parental origin. It has long been established that imprinted genes have major effects on development and placental biology before birth. More recently, it has become evident that imprinted genes also have important roles after birth. In this Review, I bring together studies of the effects of imprinted genes from the prenatal period onwards. Recent work on postnatal stages shows that imprinted genes influence an extraordinarily wide-ranging array of biological processes, the effects of which extend into adulthood, and play important parts in common diseases that range from obesity to psychiatric disorders.

Imprinted genes in mouse placental development and the regulation of fetal energy stores

Imprinted genes, which are preferentially expressed from one or other parental chromosome as a consequence of epigenetic events in the germline, are known to functionally converge on biological processes that enable in utero development in mammals. Over 100 imprinted genes have been identified in the mouse, the majority of which are both expressed and imprinted in the placenta. The purpose of this review is to provide a summary of the current knowledge regarding imprinted gene function in the mouse placenta. Few imprinted genes have been assessed with respect to their dosage-related action in the placenta. Nonetheless, current data indicate that imprinted genes converge on two key functions of the placenta, nutrient transport and placental signalling. Murine studies may provide a greater understanding of certain human pathologies, including low birth weight and the programming of metabolic diseases in the adult, and complications of pregnancy, such as pre-eclampsia and gestational diabetes, resulting from fetuses carrying abnormal imprints.

Paternal deletion of Meg1/Grb10 DMR causes maternalization of the Meg1/Grb10 cluster in mouse proximal Chromosome 11 leading to severe pre- and postnatal growth retardation

Mice with maternal duplication of proximal Chromosome 11 (MatDp(prox11)), where Meg1/Grb10 is located, exhibit pre- and postnatal growth retardation. To elucidate the responsible imprinted gene for the growth abnormality, we examined the precise structure and regulatory mechanism of this imprinted region and generated novel model mice mimicking the pattern of imprinted gene expression observed in the MatDp(prox11) by deleting differentially methylated region of Meg1/Grb10 (Meg1-DMR). It was found that Cobl and Ddc, the neighboring genes of Meg1/Grb10, also comprise the imprinted region. We also found that the mouse-specific repeat sequence consisting of several CTCF-binding motifs in the Meg1-DMR functions as a silencer, suggesting that the Meg1/Grb10 imprinted region adopted a different regulatory mechanism from the H19/Igf2 region. Paternal deletion of the Meg1-DMR (+/DeltaDMR) caused both upregulation of the maternally expressed Meg1/Grb10 Type I in the whole body and Cobl in the yolk sac and loss of paternally expressed Meg1/Grb10 Type II and Ddc in the neonatal brain and heart, respectively, demonstrating maternalization of the entire Meg1/Grb10 imprinted region. We confirmed that the +/DeltaDMR mice exhibited the same growth abnormalities as the MatDp(prox11) mice. Fetal and neonatal growth was very sensitive to the expression level of Meg1/Grb10 Type I, indicating that the 2-fold increment of the Meg1/Grb10 Type I is one of the major causes of the growth retardation observed in the MatDp(prox11) and +/DeltaDMR mice. This suggests that the corresponding human GRB10 Type I plays an important role in the etiology of Silver-Russell syndrome caused by partial trisomy of 7p11-p13.

Nedd4 controls animal growth by regulating IGF-1 signaling

The ubiquitin ligase Nedd4 has been proposed to regulate a number of signaling pathways, but its physiological role in mammals has not been characterized. Here we present an analysis of Nedd4-null mice to show that loss of Nedd4 results in reduced insulin-like growth factor 1 (IGF-1) and insulin signaling, delayed embryonic development, reduced growth and body weight, and neonatal lethality. In mouse embryonic fibroblasts, mitogenic activity was reduced, the abundance of the adaptor protein Grb10 was increased, and the IGF-1 receptor, which is normally present on the plasma membrane, was mislocalized. However, surface expression of IGF-1 receptor was restored in homozygous mutant mouse embryonic fibroblasts after knockdown of Grb10, and Nedd4(-/-) lethality was rescued by maternal inheritance of a disrupted Grb10 allele. Thus, in vivo, Nedd4 appears to positively control IGF-1 and insulin signaling partly through the regulation of Grb10 function.

Type 2 diabetes mellitus in a non-obese mouse model induced by Meg1/Grb10 overexpression

We assessed the possibility of C57BL/6-Tg (Meg1/Grb10)isn(Meg1 Tg) mice as a non-obese type 2 diabetes (2DM) animal model. Meg1 Tg mice were born normal, but their weight did not increase as much as normal after weaning and showed about 85% of normal size at 20 weeks of age. Body mass index of Meg1 Tg mice was also smaller than that of control mice. The glucose tolerance test and insulin tolerance test showed that Meg1 Tg mice had reduced ability to normalize the blood glucose level. Blood urea nitrogen (BUN) in Meg1 Tg mice (19.6 +/- 1.2 mg/dl) was significantly lower than in controls (22.0 +/- 0.8 mg/dl), while plasma triglyceride, insulin, adiponectin, and resistin levels were significantly higher (202.0 +/- 23.4 mg/dl vs 146.3 +/- 23.4 mg/dl, 152.4 +/- 16.3 pg/ml vs 88.1 +/- 16.9 pg/ml, 74.4 +/- 10.9 microg/ml vs 48.3 +/- 7.0 microg/ml, and 4.0 +/- 0.2 ng/ml vs 3.6 +/- 0.2 ng/ml, respectively). Body, visceral fat weight and liver weights were significantly lower (19.6 +/- 0.4 g vs 24.3 +/- 0.3 g, 376.7 +/- 29.6 mg to 507.5 +/- 23.0 mg, and 906.0 +/- 41.8 mg to 1,001.0 +/- 15.1 mg, respectively). Thus, hyperinsulinemia observed in Meg1 Tg mice indicates that their insulin signaling pathway is somehow inhibited. With high fat diet, the diabetes onset rate of Meg1 Tg mice increased up to 60%. These results suggest that Meg1 Tg mice resemble human 2DM.

Regulation of growth and metabolism by imprinted genes

A small sub-set of mammalian genes are subject to regulation by genomic imprinting such that only one parental allele is active in at least some sites of expression. Imprinted genes have diverse functions, notably including the regulation of growth. Much attention has been devoted to the insulin-like growth factor signalling pathway that has a major influence on fetal size and contains two components encoded by the oppositely imprinted genes, Igf2 (a growth promoting factor expressed from the paternal allele) and Igf2r (a growth inhibitory factor expressed from the maternal allele). These genes fit the parent-offspring conflict hypothesis for the evolution of genomic imprinting. Accumulated evidence indicates that at least one other fetal growth pathway exists that has also fallen under the influence of imprinting. It is clear that not all components of growth regulatory pathways are encoded by imprinted genes and instead it may be that within a pathway the influence of a single gene by each of the parental genomes may be sufficient for parent-offspring conflict to be enacted. A number of imprinted genes have been found to influence energy homeostasis and some, including Igf2 and Grb10, may coordinate growth with glucose-regulated metabolism. Since perturbation of fetal growth can be correlated with metabolic disorders in adulthood these imprinted genes are considered as candidates for involvement in this phenomenon of fetal programming.

Interactions between imprinting effects: summary and review

Mice with uniparental disomies (uniparental duplications) for defined regions of certain chromosomes, or certain disomies, show a range of developmental abnormalities most of which affect growth. These defects can be attributed to incorrect dosages of maternal or paternal copies of imprinted genes lying within the regions involved. Combinations of certain partial disomies result in interactions between the imprinting effects that seemingly independently affect foetal and/or placental growth in different ways or modify neonatal and postnatal development. The findings are generally in accord with the 'conflict hypothesis' for the evolution of genomic imprinting but do not demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that favours embryonic/foetal development, at consequent cost to the mother, will have been subject to evolutionary selection for only paternal allele expression. Reciprocally, any gene that reduces embryonic/foetal growth to limit disadvantage to the mother will have been selected for only maternal allele expression. It is concluded that survival of the placenta is core to the evolution of imprinting.

Grb10 and Grb14: enigmatic regulators of insulin action--and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans.

Meg1/Grb10 overexpression causes postnatal growth retardation and insulin resistance via negative modulation of the IGF1R and IR cascades

The Meg1/Grb10 protein has been implicated as an adapter protein in the signaling pathways from insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) in vitro. To elucidate its in vivo function, four independent Meg1/Grb10 transgenic mouse lines were established, and the effects of excess Meg1/Grb10 on both postnatal growth and glucose metabolism were examined. All of the Meg1/Grb10 transgenic mice showed growth retardation after weaning (3-4 weeks), which indicates that ectopic overexpression of Meg1/Grb10 inhibits postnatal growth that is mediated by IGF1 via IGF1R. In addition, the mice became hyperinsulinemic owing to high levels of insulin resistance, which demonstrates that Meg1/Grb10 also modulates the insulin receptor cascade negatively in vivo. Type II diabetes arose frequently in the two transgenic lines, which also showed impaired glucose tolerance. In these mice, severe atrophy of the pancreatic acinus cells was associated with high-level production of Meg1/Grb10 in the pancreas. These results suggest that Meg1/Grb10 inhibits the function of both insulin and IGF1 receptors in these cells, since a similar phenotype has been reported for Ir and Igf1r double knockout mice. Taken together, these results indicate that Meg1/Grb10 interacts with both insulin and IGF1 receptors in vivo, and negatively regulates the IGF growth pathways via these receptors.

Interactions between imprinting effects in the mouse

Mice with uniparental partial or complete disomies for any one of 11 identified chromosomes show abnormal phenotypes. The abnormalities, or imprinting effects, can be attributable to an incorrect dosage of maternal or paternal copies of imprinted gene(s) located within the regions involved. Here we show that combinations of partial disomies may result in interactions between imprinting effects that seemingly independently affect fetal and/or placental growth in different ways or modify neonatal and postnatal imprinting effects. Candidate genes within the regions have been identified. The findings are generally in accord with the "conflict hypothesis" for the evolution of genomic imprinting but do not clearly demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that represses or limits embryonic/fetal growth to the advantage of the mother--by any developmental means--will have been subject to evolutionary selection for paternal allele repression. Likewise, any gene that favors embryonic/fetal development at consequent cost to the mother--by any developmental means--will have faced selection for maternal allele repression. The classical Igf2-Igf2r axis may therefore be unique. The findings involve reinterpretation of older imprinting data and consequently revision of the mouse imprinting map.

Grb10 adapter protein as regulator of insulin-like growth factor receptor signaling

Grb10 is a member of a superfamily of adapter proteins that includes Grb10, 7, 14, and a protein of Caenorhabditis elegans called Mig10. Grb10 proteins are binding partners for several trans-membrane tyrosine-kinase receptors, including the insulin-like growth factor receptor (IGF-IR) and the insulin receptor (IR). Many recent reports have suggested a very important role of Grb10 in regulating IGF-IR signaling. In this review, we will focus on the role of Grb10 in IGF-I-induced mitogenesis and we will discuss the recent findings that show the involvement of Grb10 in the regulation of ligand-induced ubiquitination, internalization, and stability of the IGF-IR.

Disruption of the imprinted Grb10 gene leads to disproportionate overgrowth by an Igf2-independent mechanism

To investigate the function of the Grb10 adapter protein, we have generated mice in which the Grb10 gene was disrupted by a gene-trap insertion. Our experiments confirm that Grb10 is subject to genomic imprinting with the majority of Grb10 expression arising from the maternally inherited allele. Consistent with this, disruption of the maternal allele results in overgrowth of both the embryo and placenta such that mutant mice are at birth approximately 30% larger than normal. This observation establishes that Grb10 is a potent growth inhibitor. In humans, GRB10 is located at chromosome 7p11.2-p12 and has been associated with Silver-Russell syndrome, in which approximately 10% of those affected inherit both copies of chromosome 7 from their mother. Our results indicate that changes in GRB10 dosage could, in at least some cases, account for the severe growth retardation that is characteristic of Silver-Russell syndrome. Because Grb10 is a signaling protein capable of interacting with tyrosine kinase receptors, we tested genetically whether Grb10 might act downstream of insulin-like growth factor 2, a paternally expressed growth-promoting gene. The result indicates that Grb10 action is essentially independent of insulin-like growth factor 2, providing evidence that imprinting acts on at least two major fetal growth axes in a manner consistent with parent-offspring conflict theory.

Inhibition of hGrb10 binding to the insulin receptor by functional domain-mediated oligomerization

hGrb10 is a newly identified Src homology 2 (SH2) and pleckstrin homology (PH) domain-containing protein that binds to autophosphorylated receptor tyrosine kinases, including the insulin and insulin-like growth factor receptors. To identify potential downstream proteins that interact with hGrb10, we screened a yeast two-hybrid cDNA library using the full-length hGrb10gamma as bait. A fragment of hGrb10, which included the IPS (insert between the PH and SH2 domain) and the SH2 domains, was found to bind with high affinity to the full-length protein. The interaction between the IPS/SH2 domain and the full-length hGrb10 was further confirmed by in vitro glutathione S-transferase fusion protein binding studies. Gel filtration assays showed that hGrb10 underwent tetramerization in mammalian cells. The interaction involved at least two functional domains, the IPS/SH2 region and the PH domain, both of which interacted with the NH2-terminal amino acid sequence of hGrb10gamma (hGrb10gamma DeltaC, residues 4-414). Competition studies showed that hGrb10gamma DeltaC inhibited the binding of hGrb10 to the tyrosine-phosphorylated insulin receptor, suggesting that this region may play a regulatory role in hGrb10/insulin receptor interaction. We present a model for hGrb10 tetramerization and its potential role in receptor tyrosine kinase signal transduction.

Identification of the Meg1/Grb10 imprinted gene on mouse proximal chromosome 11, a candidate for the Silver-Russell syndrome gene

In a systematic screen for maternally expressed imprinted genes using subtraction hybridization with androgenetic and normal fertilized mouse embryos, seven candidate maternally expressed genes (Megs) have been isolated, including the H19 and p57(Kip2) genes that are known to be maternally expressed. Herein, we demonstrate that an imprinted gene, Meg1, is apparently identical to Grb10 (growth factor receptor-bound protein 10), which is located on mouse proximal chromosome 11. Grb10 protein was reported to bind to the insulin receptor and/or the insulin-like growth factor (IGF) I receptor via its src homology 2 domain and to inhibit the associated tyrosine kinase activity that is involved in the growth promoting activities of insulin and IGFs (IGF-I and -II). Thus, it is probable that Meg1/Grb10 is responsible for the imprinted effects of prenatal growth retardation or growth promotion caused by maternal or paternal duplication of proximal chromosome 11 with reciprocal deficiencies (MatDp.prox11 or PatDp.prox11), respectively. In the human, it has been reported that the maternal uniparental disomy 7 is responsible for the Silver-Russell syndrome (SRS) whose effects include pre- and postnatal growth retardation and other dysmorphologies. The human homologue GRB10 on chromosome 7q11.2-12 is a candidate gene for Silver-Russell syndrome.

The mouse chromosome 7 distal imprinting domain maps to G-bands F4/F5

Mouse Chromosome (Chr) 7 distal to band F3 on the physical map is known to be subject to imprinting, maternal duplication (MatDp) of the region leading to a late embryonic lethality, while paternal duplication (PatDp) causes death in utero before 11.5 dpc. Using a new mouse reciprocal translocation T(7;11)65H to produce MatDp for distal Chr 7, we have mapped the region subject to imprinting more precisely to bands 7F4/F5 on the cytogenetic map. Fluorescence in situ hybridization (FISH) studies on mitotic and meiotic chromosomes of a T65H heterozygote show that the imprinted gene Igf2 is located in the same region. This was confirmed by the finding that embryos with MatDp of bands 7F4/F5 did not express Igf2. We suggest that other members of the imprinted domain containing Igf2, namely Mash2, H19, Ins2, and p57(K1P2), are also located in 7F4/F5 and that some or all of these genes may be responsible for the two imprinting lethalities seen with MatDp and PatDp for this region.