Roles for Rho/ROCK and Vinculin in Parietal Endoderm Migration

The first cell migration event in the mouse embryo is the movement of parietal endoderm cells from the surface of the inner cell mass facing the blastocoel cavity to line the inner surface of the trophectoderm. F9 embryoid bodies provide an in vitro model for this event. They have an inner core of undifferentiated stem cells surrounded by an outer visceral endoderm layer. When plated on a laminin coated substrate, visceral endoderm transitions to parietal endoderm and migrates onto the dish, away from the attached embryoid body. We now show that this outgrowth contains abundant focal complexes and focal adhesions, as well as lamellipodia and filopodia. Treatment with the ROCK inhibitor Y-27632 promotes a 2-fold increase in outgrowth, and a transition from focal adhesions and associated stress fibers, to focal complexes and a decrease in stress fibers. ROCK inhibition also leads to an increase in lamellipodia. Inhibition of RhoA by transfection of a vector encoding C3 transferase, direct administration of the C3 enzyme, or transfection of a vector encoding p190 Rho GTPase Activating Protein also promotes outgrowth and an apparent transition from focal adhesions to focal complexes. Parietal endoderm outgrowth generated using vinculin-deficient F9 stem cells migrates 2-fold further than wild type cultures, but this outgrowth retains the morphology of wild type parietal endoderm, including focal adhesions and stress fibers. Addition of Y-27632 to vinculin-null outgrowth cultures further stimulates migration an additional 2-fold, supporting the conclusion that Rho/ROCK and vinculin regulate parietal endoderm outgrowth by distinct pathways.     

Expression of fibroblast growth factor by F9 teratocarcinoma cells as a function of differentiation

F9 teratocarcinoma stem cells treated with retinoic acid (RA) and dibutyryl cAMP (but2 cAMP) differentiate into embryonic parietal endoderm. Using heparin-affinity chromatography, endothelial cell proliferation assays, immunoprecipitation, and Western analysis with antibodies specific for acidic and basic fibroblast growth factors (FGFs), we detected biologically active FGF in F9 cells only after differentiation. A bovine basic FGF cDNA probe hybridized to 2.2-kb mRNAs in both F9 stem and parietal endoderm cells and to a 3.8-kb mRNA in F9 stem cells. A genomic DNA probe for acidic FGF hybridized to a 5.8-6.0-kb mRNA in both F9 stem and parietal endoderm cells, and to a 6.0-6.3-kb mRNA only in parietal endoderm cells. Although these FGF mRNAs were present in the stem cells, we could find no evidence that F9 stem cells synthesized FGFs, whereas differentiated F9 cells synthesized both acidic and basic FGF-like proteins. We conclude that biologically active factors with properties characteristic of acidic and basic FGF are expressed by F9 parietal endoderm cells after differentiation. Differentiating embryonic parietal endoderm thus may serve as a source of FGF molecules in the developing blastocyst, where these factors appear to play a central role in subsequent embryogenesis.     

Retinoic acid modulation of transmembrane signaling. Analysis in F9 teratocarcinoma cells

F9 embryonal mouse teratocarcinoma cells were differentiated to a primitive endoderm-like phenotype by retinoic acid and to a parietal endoderm-like phenotype by retinoic acid in combination with dibutyryl cyclic AMP. The secretion of tissue plasminogen activator (tPA) is a characteristic of the cells displaying the differentiated phenotypes. The fundamental question of whether tPA secretion is regulated acutely by G-protein-mediated transmembrane signaling was explored. Cells differentiated to primitive and parietal endoderm demonstrated a rapid tPA response to stimulation by beta-adrenergic agonist (isoproterenol). Adenylyl cyclase activity in response to isoproterenol and GTP, but not forskolin, was greater in primitive and parietal endoderm than F9 stem cells. Both primitive and parietal endoderm cells, but not F9 stem cells, displayed beta-adrenergic stimulation of cyclic AMP accumulation. Retinoic acid induced F9 stem cells to the primitive endoderm phenotype and increased beta-adrenergic receptor levels 3-fold. Gi alpha 2 levels declined, G beta-subunits increased, and Gs alpha levels were unchanged following differentiation to primitive endoderm. In parietal endoderm cells beta-adrenergic receptors increased 2-fold over F9 stem cells, Gi alpha 2 levels declined even further than in primitive endoderm, G beta-subunits increased compared to F9 stem cells, and Gs alpha levels again were unchanged. The marked potentiation of short-term stimulation of tPA secretion in the differentiated state may be best explained by the retinoic acid-induced increase in expression of beta-adrenergic receptors coupled with a decline in Gi alpha 2 levels. Short-term regulation by G-protein-linked receptors represents a novel mode for the control of tPA secretion.     

Roles for Rho/ROCK and vinculin in parietal endoderm migration

The first cell migration event in the mouse embryo is the movement of parietal endoderm cells from the surface of the inner cell mass facing the blastocoel cavity to line the inner surface of the trophectoderm. F9 embryoid bodies provide an in vitro model for this event. They have an inner core of undifferentiated stem cells surrounded by an outer visceral endoderm layer. When plated on a laminin coated substrate, visceral endoderm transitions to parietal endoderm and migrates onto the dish, away from the attached embryoid body. We now show that this outgrowth contains abundant focal complexes and focal adhesions, as well as lamellipodia and filopodia. Treatment with the ROCK inhibitor Y-27632 promotes a 2-fold increase in outgrowth, and a transition from focal adhesions and associated stress fibers, to focal complexes and a decrease in stress fibers. ROCK inhibition also leads to an increase in lamellipodia. Inhibition of RhoA by transfection of a vector encoding C3 transferase, direct administration of the C3 enzyme, or transfection of a vector encoding p190 Rho GTPase Activating Protein also promotes outgrowth and an apparent transition from focal adhesions to focal complexes. Parietal endoderm outgrowth generated using vinculin-deficient F9 stem cells migrates 2-fold further than wild type cultures, but this outgrowth retains the morphology of wild type parietal endoderm, including focal adhesions and stress fibers. Addition of Y-27632 to vinculin-null outgrowth cultures further stimulates migration an additional 2-fold, supporting the conclusion that Rho/ROCK and vinculin regulate parietal endoderm outgrowth by distinct pathways.     

The location and expression of fibroblast growth factor (FGF) in F9 visceral and parietal embryonic cells after retinoic acid-induced differentiation

It is well-established that fibroblast growth factors (FGFs) participate in mesoderm formation and patterning in the developing embryo. To identify cells in mammalian embryos that produce and/or respond to FGFs, we utilized the F9 teratocarcinoma cell system. Undifferentiated F9 cells resemble inner cell mass (ICM) cells of the mouse blastocyst by several criteria including having a characteristic high nuclear to cytoplasmic ratio and by their expression of stage-specific embryonic antigens. F9 stem cells differ from ICM cells by their low spontaneous rate of differentiation and their differentiation potential. ICM cells are heterogeneous with a proportion of the cells maintaining totipotency. In contrast, F9 stem cells appear capable of forming only endodermal derivatives. Retinoic acid (RA) treatment of F9 stem cells is required for them to differentiate, and under different culturing conditions the F9 cells will form either extraembryonic parietal or visceral endoderm. We have previously shown that FGF is synthesized by F9 parietal endoderm, but not by F9 stem cells. Our present study demonstrates that F9 aggregate cultures that contain visceral endoderm cells produce cell-associated-heparin-binding mitogens for 3T3 and endothelial cells, factors with characteristics of FGFs. Furthermore, our studies detect endothelial cell-mitogens within the extracellular matrix (ECM) of F9 parietal endoderm cells, not detected within F9 stem cell 'matrices'. Parietal endoderm cell matrix mitogens could be removed by prior treatment of the ECM with buffers containing heparin or 2 M NaCl, and could be neutralized by basic FGF antibodies.     

Galpha13 activation rescues moesin-depletion induced apoptosis in F9 teratocarcinoma cells

Mouse F9 cells differentiate into primitive endoderm when treated with retinoic acid (RA) and into parietal endoderm in response to RA and dibutyryl (db-) cAMP. G protein signaling either blocks or mimics RA-induced differentiation, the latter signaling through the Wnt-beta-catenin pathway. In our study, we found that a constitutively active Galpha13 mutant induces F9 cells to differentiate into parietal endoderm in the absence of exogenous agents. Galpha13 expression and subsequent differentiation are accompanied by beta-catenin translocation to the nucleus. Differentiation and changes in cell morphology are supported by rearrangements to the F-actin cytoskeleton. ERM (ezrin-radixin-moesin) proteins, known to link F-actin to transmembrane receptors, are also redistributed during differentiation. Furthermore, morpholino antisense and shRNA approaches show that moesin expression is essential since its knockdown leads to altered F-actin distribution and subsequent apoptosis. Moesin-depleted cells, however, remain attached to the substrate when Galpha13 is constitutively expressed, but they do not differentiate into extraembryonic endoderm. Our study demonstrates a link between Galpha13 signaling that regulates differentiation of F9 cells through primitive to parietal endoderm and a moesin requirement for cell survival.     

Wnt6 induces the specification and epithelialization of F9 embryonal carcinoma cells to primitive endoderm

Epithelial-to-mesenchymal transitions (EMTs) play key roles in the normal development of an organism as well as its demise following the metastasis of a malignant tumour. An EMT during early mouse development results in the differentiation of primitive endoderm into the parietal endoderm that forms part of the parietal yolk sac. In the embryo, primitive endoderm develops from cells in the inner cell mass, but the signals that instruct these cells to become specified and adopt an epithelial fate are poorly understood. The mouse F9 teratocarcinoma cell line, a model that can recapitulate the in vivo primitive to parietal endoderm EMT, has been used extensively to elucidate the signalling cascades involved in extraembryonic endoderm differentiation. Here, we identified Wnt6 as a gene up-regulated in F9 cells in response to RA and show that Wnt6 expressing cells or cells exposed to Wnt6 conditioned media form primitive endoderm. Wnt6 induction of primitive endoderm is accompanied by beta-catenin and Snail1 translocation to the nucleus and the appearance of cytokeratin intermediate filaments. Attenuating glycogen synthase kinase 3 activity using LiCl gave similar results, but the fact that cells de-differentiate when LiCl is removed reveals that other signalling pathways are required to maintain cells as primitive endoderm. Finally, Wnt6-induced primitive endodermal cells were tested to determine their competency to complete the EMT and differentiate into parietal endoderm. Towards that end, results show that up-regulating protein kinase A activity is sufficient to induce markers of parietal endoderm. Together, these findings indicate that undifferentiated F9 cells are responsive to canonical Wnt signalling, which negatively regulates glycogen synthase kinase 3 activity leading to the epithelialization and specification of primitive endoderm competent to receive additional signals required for EMT. Considering the ability of F9 cells to mimic an in vivo EMT, the identification of this Wnt6-beta-catenin-Snail signalling cascade has broad implications for understanding EMT mechanisms in embryogenesis and metastasis.     

Extraembryonic Endoderm cells as a model of endoderm development

In recent years the multipotent extraembryonic endoderm (XEN) stem cells have been the center of much attention. In vivo, XEN cells contribute to the formation of the extraembryonic endoderm, visceral and parietal endoderm and later on, the yolk sac. Recent data have shown that the distinction between embryonic and extraembryonic endoderm is not as strict as previously thought due to the integration, and not the displacement, of the visceral endoderm into the definitive embryonic endoderm. Therefore, cells from the extraembryonic endoderm also contribute to definitive endoderm. Many research groups focused on unraveling the potential and ability of XEN cells to both support differentiation and/or differentiate into endoderm‐like tissues as an alternative to embryonic stem (ES) cells. Moreover, the conversion of ES to XEN cells, shown recently without genetic manipulations, uncovers significant and novel molecular mechanisms involved in extraembryonic endoderm and definitive endoderm development. XEN cell lines provide a unique model for an early mammalian lineage that complements the established ES and trophoblast stem cell lines. Through the study of essential genes and signaling requirements for XEN cells in vitro, insights will be gained about the developmental program of the extraembryonic and embryonic endodermal lineage in vivo. This review will provide an overview on the current literature focusing on XEN cells as a model for primitive endoderm and possibly definitive endoderm as well as the potential of using these cells for therapeutic applications.     

Ras/MAPK pathway confers basement membrane dependence upon endoderm differentiation of embryonic carcinoma cells

The formation of extraembryonic endoderm is one of the earliest steps in the differentiation of pluripotent cells of the inner cell mass during the early stages of embryonic development. The primitive endoderm cells and the derived parietal and visceral endoderm cells gain the capacity to produce collagen IV and laminin. The deposition of these components results in the formation of basement membrane and epithelium of the endoderm, with polarized cells covering the inner surface of the blastocoels. We used retinoic acid-induced endoderm differentiation of stem cell-like F9 embryonic carcinoma cells to study the role of the Ras pathway and its regulation in the formation of the visceral endoderm. Upon endoderm differentiation of F9 cells induced by retinoic acid, c-Fos expression, the downstream target of the Ras pathway, is suppressed by uncoupling Elk-1 phosphorylation/activation to MAPK activity. However, attachment to matrix gel greatly enhances the activation of MAPK in endoderm cells but not in undifferentiated F9 cells. Enhanced MAPK activation as a result of contact with basement membrane is able to compensate for reduced Elk-1 phosphorylation and c-Fos expression. We conclude that endoderm differentiation renders the activation of the Ras pathway basement membrane dependent, contributing to the epithelial organization of the visceral endoderm.     

The production of distinct forms of plasminogen activator by mouse embryonic cells

We have examined cultured parietal endoderm, visceral endoderm, and extraembryonic mesoderm cells from the mouse embryo for production of the protease plasminogen activator. All of these cell types synthesize and secrete the enzyme, but the molecular characteristics of the plasminogen activators differ as defined by the apparent molecular weight in sodium dodecyl sulfate-polyacrylamide gels, the antigenic properties as defined by two antisera to distinct plasminogen activators, and the interaction with an inhibitor present in fetal bovine serum. The parietal endoderm plasminogen activator has a predominant molecular weight of 79,000, is immunoprecipitated and inhibited by an antiserum raised against a human melanoma plasminogen activator, but not by an antiserum against mouse urokinase, and is only partially inhibited by the serum inhibitor. The visceral endoderm and extraembryonic mesoderm plasminogen activators, which are identical by all criteria, have molecular weights of 48,000, are inactivated only by the anti-urokinase antibodies, and are inhibited by an inhibitor in fetal bovine serum. These results establish the presence of at least two different forms of plasminogen activator in the early mouse embryo. The distinctive nature of the enzyme produced by parietal endoderm can be used as a diagnostic marker for this cell type at this stage of development. When F9 teratocarcinoma stem cells are induced to differentiate by retinoic acid and dibutyryl cAMP, they secrete a plasminogen activator of the parietal endoderm type.     

Retinoic acid induces discrete Wnt-signaling-dependent differentiation in F9 cells

Retinoic acid (RA) induces F9 cells, the mouse teratocarcinoma cells, to differentiate into primitive endoderm and further into visceral and parietal endoderm depending on the culture conditions. To elucidate the instructive mechanisms involved in the differentiation steps we investigated the effects of Wnt-signaling members, Wnt3a and β-catenin, on the differentiation of F9 cells and β-catenin-deficient F9 cells (βT cells). RA up-regulated the expression of differentiation markers for primitive, visceral and parietal endoderm in F9 cells but not for visceral endoderm in βT cells. Wnt3a or leukemia inhibitory factor (LIF) inhibited the RA-induced differentiation in F9 cells. LIF but not Wnt3a could inhibit differentiation in βT cells. RA evoked ZO-1α+ signals at cell-to-cell contacts in F9 cells in a Wnt3a sensitive manner. The results suggest that Wnt3a inhibits differentiation into endoderm through a pathway involving β-catenin, and β-catenin might be necessary in the process leading from primitive to visceral endoderm in F9 cells.     

CD9-alpha6beta1 interactions in migratory parietal endoderm cells

Tetraspanins modulate the function of a variety of membrane proteins, including integrin receptors. We show here that the tetraspanin CD9 preferentially coimmunoprecipitates with the alpha6beta1 integrin heterodimer in F9-derived parietal endoderm cells in comparison to F9 stem cells. We also show that CD9 function-blocking antibody inhibits parietal endoderm migration in an embryoid body outgrowth assay. In addition, both CD9 and alpha6beta1 colocalize with vinculin to apparent focal adhesion sites in parietal endoderm cells. The data presented here suggests a role for CD9 in localizing the integrin to the focal adhesion. In addition, the data suggest a role for CD9 in alpha6beta1 mediated migration of parietal endoderm.     

The role of extracellular matrix in the migration and differentiation of parietal endoderm from teratocarcinoma embryoid bodies

Embryoid bodies formed from teratocarcinoma stem cells differentiate an outer layer consisting of parietal and visceral endoderm or of visceral endoderm exclusively. We have previously shown that when these embryoid bodies are plated on collagen-coated substrates a parietal endoderm-like cell migrates onto the substrate, whereas all of the visceral endoderm remains associated with the stem cell mass, suggesting a role for substrate contact in parietal endoderm differentiation. We now identify fibronectin as the migration-promoting component in these cultures, and note that laminin and collagen type IV are 10-fold less effective at promoting both attachment and endoderm outgrowth. The RGDS tetrapeptide (arg-gly-asp-ser) from the cell attachment domain of fibronectin can specifically block attachment and outgrowth on both fibronectin- and laminin-coated substrates. In addition, the involvement of the 140-kD fibronectin receptor is demonstrated using an antibody directed against this molecule.     

Hormonal induction of differentiation in teratocarcinoma stem cells: generation of parietal endoderm by retinoic acid and dibutyryl cAMP

It has previously been shown that retinoic acid induces multiple phenotypic changes in cultures of F9 teratocarcinoma stem cells. In this paper we demonstrate that these retinoid-generated cells can be converted to yet another cell type by compounds that elevate cAMP concentrations. The phenotype of the new cell type is characterized by the synthesis of plasminogen activator, laminin and type IV collagen, and by very low levels of alkaline phosphatase and lactate dehydrogenase. The secretion of plasminogen activator and type IV collagen, and low levels of alkaline phosphatase and lactate dehydrogenase, have been previously shown to be properties of parietal endoderm, an extraembryonic cell which is generated early in mouse embryonesis. We show here that parietal endoderm also synthesizes laminin. The cell type generated by retinoic acid and dibutyryl cAMP treatment is therefore indistinguishable from definitive parietal endoderm. Analysis of the final phenotype indicates that it is not dependent upon the continued presence of either compound, and that cAMP agents are active only on cells that have been treated with retinoic acid.     

The expression and activity of D-type cyclins in F9 embryonal carcinoma cells: modulation of growth by RXR-selective retinoids

The growth rate of malignant F9 embryonal carcinoma cells slows considerably following all-trans-retinoic acid-induced differentiation into benign parietal endoderm. To determine the mechanism of this process, we examined the expression of cyclins D1, D2, and D3 and the activity of their associated kinases. Cyclin D1 and D3 mRNA levels decreased during complete differentiation induced by all-trans-retinoic acid and dibutyryl cAMP, while the levels of cyclin D2 and the cyclin-dependent kinase (Cdk) inhibitor p27 mRNAs increased. Ultimately, terminally differentiated cells possessed 50% of the Cdk4-associated kinase activity observed in undifferentiated cells. Since numerous genes are differentially regulated during parietal endoderm differentiation, it is difficult to determine whether retinoic acid affects cell cycle gene expression directly or if these changes are caused by differentiation. We found that the retinoid X receptor (RXR)-selective agonists LG100153 and LG100268 significantly inhibited F9 cell growth without causing overt terminal differentiation as assessed by anchorage-independent growth and differentiation-associated gene expression. As seen in cells induced to differentiate by the RAR agonist all-trans-retinoic acid, RXR activation led to an increase in the number of cells in G1 phase. RXR agonists also sharply induced the levels of the Cdk regulatory subunits, cyclin D2 and D3. However, Cdk4-dependent kinase activity was reduced by RXR-selective retinoid treatment. These observations suggest that some retinoids can directly inhibit proliferation and regulate Cdk4-dependent kinase activity without inducing terminal differentiation.     

Developmentally regulated expression of cell surface carbohydrates during mouse embryogenesis

Cell surface carbohydrates undergo marked alterations during mouse embryogenesis. In preimplantation embryos, many carbohydrate markers show stage-specific expression in diverse ways. In early postimplantation embryos, certain carbohydrate markers are localized in defined regions in the embryo. Important carriers of stage-specific carbohydrates are the lactoseries structure (Gal beta 1----4GlcNAc) and the globoseries structure (Gal alpha 1----4Gal). Notably, the glycoprotein-bound large carbohydrate of poly-N-acetyllactosamine-type ([Gal beta 1----4GlcNAc beta 1----3]n) carries a number of markers preferentially expressed in early embryonic cells. These markers are of practical value in analyzing embryogenesis and cell differentiation. For example, in order to monitor in vitro differentiation of multipotential embryonal carcinoma cells, stage-specific embryonic antigen-1 (SSEA-1) and the Lotus agglutinin receptor have been used as markers of the undifferentiated cells, and the Dolichos agglutinin receptor has been used as a marker of extraembryonic endoderm cells. Developmental control of cell surface carbohydrates is attained by controlled expression of activities of key glycosyltransferases; for example, the activity of N-acetylglucosaminide alpha 1----3 fucosyltransferase is lost during in vitro differentiation of embryonal carcinoma cells to parietal endoderm cells, in parallel to the disappearance of SSEA-1. Accumulating evidence suggests that poly-N-acetyllactosamine-type glycans that are abundant in early embryonic cells are involved in cell surface recognition of these cells.