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J. Biol. Chem., Vol. 279, Issue 40, 41263-41266, October 1, 2004
Defects in Cell Adhesion and the Visceral Endoderm following Ablation of Nonmuscle Myosin Heavy Chain II-A in Mice*
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| ABSTRACT |
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-catenin localization to cell adhesion sites in both cell culture and in the intact embryos. The defects can be reproduced by introducing siRNA directed against NMHC II-A into wild-type embryonic stem cells. Our results suggest an essential role for a single, specific nonmuscle myosin isoform in maintaining cell-cell adhesions in the early mammalian embryo. | INTRODUCTION |
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Until now, the effects of ablating NMHC II-A have not been reported, although humans with a single amino acid mutation have been shown to manifest a number of abnormalities affecting the kidneys, platelets, lens, and inner ear (6). In this paper, we address the role of NMHC II-A during early mouse development and show that it plays an important role in cell-cell adhesion and the formation of a polarized visceral endoderm.
| EXPERIMENTAL PROCEDURES |
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RT-PCRcDNA was synthesized from RNA made from wild-type, A+/A, and A/A embryoid bodies using the Superscript I kit (Invitrogen) with 5 µg of RNA and random hexamer primers. Dilutions of cDNA from 1:20 to 1:80 were then amplified with specific primers (7).
Preparation and Plating of ES ColoniesES cells (5 x 104) were plated on sterile glass coverslips in 15% ES cell-qualified fetal bovine serum (FBS; Invitrogen) DMEM supplemented with LIF (103 units/ml, Chemicon, Temecula, CA) and cultured for 2 days in a 10% CO2 incubator to form colonies. Colonies were fixed and permeabilized with 4% paraformaldehyde and 0.5% Triton X-100 in 5% sucrose in PBS, rinsed with PBS, and treated with M.O.M. blocking solution (Vector Laboratories, Burlingame, CA).
Preparation of Embryoid Bodies and Time-lapse MicroscopyEmbryoid bodies were formed from either A/A, A+/A, or wild-type ES cells and cultured according to Robertson (8). Cells (5 x 104 cells) were cultured in DMEM + 10% ES cell-qualified FBS (Invitrogen) without LIF for 48 h on bacteriological nonadhesive plates. The embryoid bodies were collected, washed, and transferred to a regular tissue culture dish for 2 h prior to filming, and phase contrast (Zeiss Axiovert 25, Jena, Germany) time-lapse images were captured every 10 min for 2024 h using MetaMorph 6.12 software (Universal Imaging Corp., Downington, PA).
siRNA Electroporation into ES CellsA pool of siRNAs (25 pmol) specific for mouse NMHC II-A (SMARTpool, Dharmacon Research, Inc., Lafayette, CO) was electroporated into ES cells using the Amaxa Nucleofector instrument and program A23 (Amaxa Biosystems, Gaithersburg, MD). Cells were plated in 12-well plates for immunoblot analysis and in 6-well plates on 20-mm coverslips for immunofluorescence analysis. The vector pCX-EGFP (a gift of Andras Nagy, Toronto, Canada) was used as a control for electroporation efficiency (90%). Cells were cultured for 2472 h prior to analysis.
| RESULTS AND DISCUSSION |
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Heterozygous mice were indistinguishable from wild-type littermates. Because no A/A mice were born, embryos were dissected at various stages of development to determine at what stage the null mutation became lethal. No viable A/A embryos were found later than E7.5. In an effort to gain insight into the cause of the early lethality, we analyzed sections of mouse embryos between E5.5 and E7.5 for cell proliferation using BrdUrd staining and for apoptosis using TUNEL assays. There was no significant difference between normal and null embryos in either assay (see supplemental Fig. S2).
Fig. 2A shows the results of immunostaining normal and mutant embryos at E6.5 with antibodies raised to NMHC II-A and II-B, and Fig. 2B shows staining by antibodies to the transcription factor GATA-4. In normal embryos (Fig. 2A, panels df), antibody staining for NMHC II-A is found in all cell layers (Fig. 2A, panel e). A/A embryos were identified by a lack of staining with the NMHC II-A antibody and marked cellular disorganization (Fig. 2A, panels a and b). Staining with an antibody to NMHC II-C confirmed its absence at E6.5 (data not shown). It is of particular note that, although both the null and normal embryos stain with an antibody for NMHC II-B (Fig. 2A, panels c and f), this staining is absent or very low in the outer cell layer of the normal embryo, the visceral endoderm (Fig. 2A, panel f, bracket). Moreover, the shape of the cells forming the visceral endoderm differs markedly between normal and A/A mice, with development of a polarized columnar morphology in the former by E6.5 (Fig. 2A, panels d and e, bracket, enlarged in inset), while a cuboidal shape is retained in the case of A/A mice (Fig. 2A, panel a, and Fig. 2B, panel a, arrows).
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To further characterize the expression of markers of visceral endoderm development, we used ES cells cultured under conditions in which they aggregate to form embryoid bodies. As such, they maintain temporal and spatial relationships in vitro of certain marker proteins expressed in embryos in vivo. Fig. 3A shows RT-PCR analyses of 14-day-old wild-type, A+/A and A/A embryoid bodies for the indicated markers. GATA-4 and apo-E, markers for specification of the visceral endoderm, were positive in all three genotypes of embryoid bodies. However, some markers for proteins that are secreted from the visceral endoderm and that indicate maturation and function of the visceral endoderm were missing, apo-AI, or markedly decreased, AFP (
-fetoprotein), TTR (transthyretin), apo-B, and RBP (retinal-binding protein) in the A/A embryoid bodies (Fig. 3A). These results were also confirmed for five of the markers (GATA-4, apo-E, AFP, RBP, and TTR) by real-time PCR analysis (see supplemental Fig. S3 for GATA-4 and RBP).
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The disrupted organization of cell layers in A/A embryos and the shedding of cells by A/A embryoid bodies in suspension and their rapid dispersal on adhesive substrates suggested that absence of NMHC II-A might cause defects in the cell complexes needed for effective cell-cell adhesion. We therefore compared the localization of NMHC II-A and the cell adhesion molecule E-cadherin as well as its intracellular binding partner,
-catenin, in frozen sections of A/A and wild-type E6.5 embryos using immunofluorescence confocal microcopy. As shown in Fig. 4a, NMHC II-A, in addition to being present in the cytoplasm, localized to the contact areas between the cells in wild-type embryos, close to E-cadherin (Fig. 4a, NMHC II-A; Fig. 4b, E-cadherin). In contrast, in A/A mouse embryos, E-cadherin was diminished at the cell-cell contacts (Fig. 4, compare d and b). The same decreased localization was found for
-catenin in A/A embryos (Fig. 4, compare h and f).
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-catenin present in the areas of cell-cell contact in the A/A cells compared with the wild-type ES cells (Fig. 4, compare i with j for cadherin and k with l for
-catenin). Fig. 4m is an immunoblot showing that the content of E-cadherin and
-catenin was essentially the same in A+/A+, A+/A and A/A cells and supports the idea that those proteins are mislocalized and not decreased in the A/A cells.
To verify whether NMHC II-A is critical for the formation of other types of cell adhesions, we compared the distribution of NMHC II-A and II-B, actin, ZO-2 (a marker for tight junctions), and connexin-43 (a marker for gap junctions) in wild-type and A/A cultured ES cells. In wild-type colonies, similar to E6.5 embryos, NMHC II-A localized near plasma membranes and particularly to areas of cell-cell contacts (Fig. 5A, panel a), while NMHC II-B showed more pronounced circumferential staining at the external surface of colonies, with a less pronounced cell-cell border localization (Fig. 5A, panel c), NMHC II-B staining remained prominent at the external surface of the colonies, and its overall distribution was essentially unchanged in A/A cells, although there was a change in the rounded shape of the colonies (Fig. 5A, panel d). F-actin was localized relatively uniformly to the cell cortex near the plasma membrane in A+/A+ colonies (Fig. 5A, panel e). In A/A colonies, total cortical F-actin staining remained relatively high, but a reduction in F-actin staining at the external borders of the colonies was observed (Fig. 5A, panel f). As was found for E-cadherin and
-catenin, ZO-2 levels, but not connexin 43 levels, were decreased at cell-cell boundaries in A/A cultured ES cells (see supplemental Fig. S4).
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-catenin (Fig. 5B, panels g and h) at cell-cell interfaces compared with mock-transfected cells. Immunoblot analysis confirms that the decrease in NMHC II-A in these cells is not accompanied by a decrease in E-cadherin and
-catenin content, consistent with the displacement of these proteins from the cell boundaries (Fig. 5C). In addition, the blots show that there is no change in NMHC II-B and actin.
Above, we demonstrate the striking effects of nonmuscle myosin II-A ablation on early embryonic development. A/A embryos fail to develop past E7.5. The visceral endoderm does not mature molecularly and morphologically. The overall disarray of tissue and cellular organization could be reproduced effectively in vitro using embryoid bodies. A/A ES cells cannot maintain the typical compact morphology, and cells readily disperse from the embryoid bodies. We also show here that two critical cell-cell adherens junction proteins, E-cadherin and
-catenin, do not localize normally to cell-cell interfaces both in vivo and in vitro, and tight junction formation is also impaired. It is noteworthy that uvomorulin (cadherin)-deficient ES cells, similar to nonmuscle myosin II-A-deficient cells, cannot aggregate tightly (9) and that
-catenin-null embryos at E7 showed detachment of cells from the ectodermal cell layer and dispersal into the proamniotic cavity (10).
Myosins I and II are the motors that generate contractility in cells. As described by Krendel and Bonder (11), contractility driven rearrangement of actin bundles can modulate the spatial organization of cell-cell contacts. However, even though nonmuscle myosin II-B is present in both A/A embryos and A/A ES cells, this isoform cannot rescue the mislocalization and the subsequent tissue disarray or cell shedding. Indeed, recent work from a number of laboratories (1214) confirms that these two isoforms have different functions in the same cell and supports a role for myosin II in E-cadherin-mediated adhesions (15). Our data suggest a unique role for myosin II-A during early embryonic development. By exerting tension on actin, which is bound to the cadherin-catenin complex, this myosin plays an essential role in maintaining normal adhesion junctions and cellular organization of the early mouse embryo.
| FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains supplemental Experimental Procedures and supplemental Figs. S1S4. ![]()
These investigators made major contributions to this work. ![]()
¶ To whom correspondence should be addressed: NIH, Bldg. 10, Rm. 8N202, 10 Center Dr., MSC 1762, Bethesda, MD 20892-1762. Tel.: 301-496-1912; Fax: 301-402-1542; E-mail: contim{at}nhlbi.nih.gov.
1 The abbreviations used are: NMHC, nonmuscle myosin heavy chain; E, embryonic day; apo-E, apolipoprotein E; apo-A1, apolipoprotein A1; AFP,
-fetoprotein; TTR, transthyretin; RBP, retinal-binding protein; TUNEL, terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling; RT, reverse transcriptase; ES, embryonic stem; FBS, fetal bovine serum; DMEM, Dulbecco's modified Eagle's medium; LIF, leukemia inhibitory factor; PBS, phosphate-buffered saline; siRNA, small interfering RNA; BrdUrd, bromodeoxyuridine. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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