Basement membrane and fibronectin matrix are distinct entities in the developing mouse blastocyst

The role of mesenchymal-epithelial transition in endometrial function

BACKGROUND

The human uterine endometrium undergoes significant remodeling and regeneration on a rapid and repeated basis, after parturition, menstruation, and in some cases, injury. The ability of the adult endometrium to undergo cyclic regeneration and differentiation/decidualization is essential for successful human reproduction. Multiple key physiologic functions of the endometrium require the cells of this tissue to transition between mesenchymal and epithelial phenotypes, processes known as mesenchymal-epithelial transition (MET) and epithelial-mesenchymal transition (EMT). Although MET/EMT processes have been widely characterized in embryonic development and in the context of malignancy, mounting evidence demonstrates the importance of MET/EMT in allowing the endometrium the phenotypic and functional flexibility necessary for successful decidualization, regeneration/re-epithelialization and embryo implantation.

OBJECTIVE AND RATIONALE

The objective of this review is to provide a comprehensive summary of the observations concerning MET and EMT and their regulation in physiologic uterine functions, specifically in the context of endometrial regeneration, decidualization and embryo implantation.

SEARCH METHODS

Using variations of the search terms 'mesenchymal-epithelial transition', 'mesenchymal-epithelial transformation', 'epithelial-mesenchymal transition', 'epithelial-mesenchymal transformation', 'uterus', 'endometrial regeneration', 'endometrial decidualization', 'embryo implantation', a search of the published literature between 1970 and 2018 was conducted using the PubMed database. In addition, we searched the reference lists of all publications included in this review for additional relevant original studies.

OUTCOMES

Multiple studies demonstrate that endometrial stromal cells contribute to the regeneration of both the stromal and epithelial cell compartments of the uterus, implicating a role for MET in mechanisms responsible for endometrial regeneration and re-epithelialization. During decidualization, endometrial stromal cells undergo morphologic and functional changes consistent with MET in order to accommodate embryo implantation. Under the influence of estradiol, progesterone and multiple other factors, endometrial stromal fibroblasts acquire epithelioid characteristics, such as expanded cytoplasm and rough endoplasmic reticulum required for greater secretory capacity, rounded nuclei, increased expression of junctional proteins which allow for increased cell-cell communication, and a reorganized actin cytoskeleton. During embryo implantation, in response to both maternal and embryonic-derived signals, the maternal luminal epithelium as well as the decidualized stromal cells acquire the mesenchymal characteristics of increased migration/motility, thus undergoing EMT in order to accommodate the invading trophoblast.

WIDER IMPLICATIONS

Overall, the findings support important roles for MET/EMT in multiple endometrial functions required for successful reproduction. The endometrium may be considered a unique wound healing model, given its ability to repeatedly undergo repair without scarring or loss of function. Future studies to elucidate how MET/EMT mechanisms may contribute to scar-free endometrial repair will have considerable potential to advance studies of wound healing mechanisms in other tissues.

p53 inhibits the proliferation of male germline stem cells from dairy goat cultured on poly-L-lysine

Male germline stem cells (mGSCs) can transmit genetic materials to the next generation and dedifferentiate into pluripotent stem cells. However, in livestock, mGSC lines are difficult to establish, because of the factors that affect their isolation and culture. The extracellular matrix serves as a substrate for attachment and affects the fate of these stem cells. Poly-L-lysine (PL), an extracellular matrix of choice, inhibits and/or kills cancer cells, and promotes the attachment of stem cells in culture. However, how it affects the characteristics and potentials of these stem cells in culture needs to be elucidated. Here, we isolated, enriched and cultured dairy goat mGSCs on five types of extracellular matrices. To explore the best extracellular matrix to use for culturing them, the characteristics and proliferation ability of the cells were determined. Results showed that the cells shared several characteristics with previously reported mGSCs, including the poor effect of PL on their proliferative and colony-forming abilities. Further examination showed upregulation of p53 expression in these cells, which could be inhibiting their proliferation. When a p53 inhibitor was included in the culture medium, it was confirmed to be responsible for the inhibition of proliferation in mGSCs. Optimal concentration of the inhibitor in the culture of these cells was 5 µM. Furthermore, addition of the p53 inhibitor increased the expression of the markers of self-renewal and cell cycle in goat mGSCs. In summary, suppressing p53 is beneficial for the proliferation of dairy goat mGSCs, cultured on PL.

Tissue specific microenvironments: a key tool for tissue engineering and regenerative medicine

The accumulated evidence points to the microenvironment as the primary mediator of cellular fate determination. Comprised of parenchymal cells, stromal cells, structural extracellular matrix proteins, and signaling molecules, the microenvironment is a complex and synergistic edifice that varies tissue to tissue. Furthermore, it has become increasingly clear that the microenvironment plays crucial roles in the establishment and progression of diseases such as cardiovascular disease, neurodegeneration, cancer, and ageing. Here we review the historical perspectives on the microenvironment, and how it has directed current explorations in tissue engineering. By thoroughly understanding the role of the microenvironment, we can begin to correctly manipulate it to prevent and cure diseases through regenerative medicine techniques.

Collagen-alginate microspheres as a 3D culture system for mouse embryonic stem cells differentiation to primordial germ cells

Germ cells differentiation of stem cells will aid treatment of adults with infertility. Biopolymers utilization provided synthetic extracellular matrix (ECM) and desired attributes in in vitro to improve conditions for stem cells attachment, proliferation and differentiation. Mixture of alginate as a biocompatible hydrogel, with collagen IV, could establish an in vitro 3 dimensional (3D) culture model. The objective of this study was investigation of the mouse ESCs differentiation capacity to putative primordial germ cells (PGCs) in the alginate and alginate-collagen IV microspheres (CAM). ESCs aggregated together to form embryoid bodies (EB) in CAM under basal medium supplemented with bone morphogenetic protein-4 (BMP4) as a differentiation factor. Viability and PGC differentiation of the stem cells in microspheres was evaluated by apoptosis and PGC related gene markers. Flow cytometry analysis was also used to detect of Mvh endogenous protein as a specific PGC marker. PGC gene and protein expression revealed that differentiation potential of ESCs to putative PGCs in CAM is significantly higher than control groups. Taking together, it was concluded that CAM demonstrated a great potential to use in PGCs differentiation and treatment of adults with infertility and may be a reliable means of producing mature germ cells.

Cigarette Smoking Affects Uterine Receptivity Markers

OBJECTIVE

Smoking negatively affects fertility and the rate of other endometrial diseases. To determine the effect of smoking on endometrial physiology, we evaluated 2 endometrial regulatory cytokines and receptivity markers, C-X-C motif chemokine ligand 12 (CXCL12) and fibroblast growth factor 2 (FGF2), both in vitro and in vivo.

STUDY DESIGN

The human endometrial stromal cell line (HESC) and primary human endometrial stromal cells were treated with cigarette smoking extract (CSE) or with vehicle control. Twenty female mice were randomly assigned to either cigarette smoke (CS) exposure for 8 weeks or to a nonsmoke (NS) group that received room air. Immunohistochemical analysis of CXCL12 and FGF2 expression was performed in mouse uterine tissue. Human endometrial samples were obtained from both nonsmokers and smokers. Real-time reverse transcription-polymerase chain reaction was performed for all cell cultures and human samples.

RESULTS

Compared to controls, CXCL12 and FGF2 mRNA expression were significantly decreased in CSE-exposed HESC and primary cells. In mice, immunohistochemical analysis showed that both CXCL12 and FGF2 protein expression was lower in the CS group compared to controls. Similarly, both CXCL12 and FGF2 expression were decreased in women who smoke compared to nonsmokers.

CONCLUSION

Decreased endometrial CXCL12 and FGF2 expression contribute to the impaired endometrial receptivity in women who smoke. Smoking is also associated with decreased rates of endometrial cancer and endometriosis; increased CXCL12 and FGF2 are implicated in both conditions. The changes in the expression of cytokines described here may explain the impact of smoking on all of these diseases. Tobacco has direct effects on normal endometrium that impacts endometrial health and disease.

Placental Growth Factor Is Secreted by the Human Endometrium and Has Potential Important Functions during Embryo Development and Implantation

Embryo implantation requires synchronized dialogue between the receptive endometrium and activated blastocyst via locally produced soluble mediators. During the mid-secretory (MS) phase of the menstrual cycle, increased glandular secretion into the uterine lumen provides important mediators that modulate the endometrium and support the conceptus during implantation. Previously we demonstrated the importance of vascular endothelial growth factor (VEGF) in the human uterus, particularly with respect to embryo implantation. In the current study, proteomic analysis of human uterine lavage fluid identified the presence of placental growth factor (PlGF) a homolog of VEGF, that binds the VEGF receptor 1 (VEGFR1). Analysis of immunostaining for PlGF in human endometrial tissue across the menstrual cycle (from both fertile and infertile women) revealed PlGF was predominantly localised to glandular and luminal epithelial cells, with staining in the decidualising stromal cells surrounding the maternal spiral arteries in the secretory phase of the menstrual cycle. Immunoreactive PlGF was also detected in subpopulations of endometrial leukocytes. Functional studies demonstrated that culturing mouse embryos with recombinant human (rh)PlGF enhanced blastocyst cell number and outgrowth. Furthermore, treatment of human endometrial epithelial cells (EEC) with rhPlGF enhanced EEC adhesion. Taken together, these data demonstrate that PlGF is abundant in the human endometrium, and secreted into the uterine lumen where it mediates functional changes in cellular adhesion with important roles in implantation.

Hanging on for the ride: adhesion to the extracellular matrix mediates cellular responses in skeletal muscle morphogenesis and disease

Skeletal muscle specification and morphogenesis during early development are critical for normal physiology. In addition to mediating locomotion, skeletal muscle is a secretory organ that contributes to metabolic homeostasis. Muscle is a highly adaptable tissue, as evidenced by the ability to increase muscle cell size and/or number in response to weight bearing exercise. Conversely, muscle wasting can occur during aging (sarcopenia), cancer (cancer cachexia), extended hospital stays (disuse atrophy), and in many genetic diseases collectively known as the muscular dystrophies and myopathies. It is therefore of great interest to understand the cellular and molecular mechanisms that mediate skeletal muscle development and adaptation. Muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction. This process requires carefully orchestrated interactions between cells and their extracellular matrix microenvironment. These interactions are dynamic, allowing muscle cells to sense biophysical, structural, organizational, and/or signaling changes within their microenvironment and respond appropriately. In many musculoskeletal diseases, these cell adhesion interactions are disrupted to such a degree that normal cellular adaptive responses are not sufficient to compensate for accumulating damage. Thus, one major focus of current research is to identify the cell adhesion mechanisms that drive muscle morphogenesis, with the hope that understanding how muscle cell adhesion promotes the intrinsic adaptability of muscle tissue during development may provide insight into potential therapeutic approaches for muscle diseases. Our objectives in this review are to highlight recent studies suggesting conserved roles for cell-extracellular matrix adhesion in vertebrate muscle morphogenesis and cellular adaptive responses in animal models of muscle diseases.

Insulin-like growth factor 1 increases apical fibronectin in blastocysts to increase blastocyst attachment to endometrial epithelial cells in vitro

STUDY QUESTION

Does insulin-like growth factor 1 (IGF1) increase adhesion competency of blastocysts to increase attachment to uterine epithelial cells in vitro?

SUMMARY ANSWER

IGF1 increases apical fibronectin on blastocysts to increase attachment and invasion in an in vitro model of implantation.

WHAT IS KNOWN ALREADY

Fibronectin integrin interactions are important in attachment of blastocysts to uterine epithelial cells at implantation.

STUDY DESIGN, SIZE, DURATION

Mouse blastocysts (hatched or near completion of hatching) were cultured in serum starved (SS) medium with varying treatments for 24, 48 or 72 h. Treatments included 10 ng/ml IGF1 in the presence or absence of the PI3 kinase inhibitor LY294002, an IGF1 receptor (IGF1R) neutralizing antibody or fibronectin. Effects of treatments on blastocysts were measured by attachment of blastocysts to Ishikawa cells, blastocyst outgrowth and fibronectin and focal adhesion kinase (FAK) localization and expression. Blastocysts were randomly allocated into control and treatment groups and experiments were repeated a minimum of three times with varying numbers of blastocysts used in each experiment. FAK and integrin protein expression on Ishikawa cells was quantified in the presence or absence of IGF1.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Fibronectin expression and localization in blastocysts was studied using immunofluorescence and confocal microscopy. Global surface expression of integrin αvβ3, β3 and β1 was measured in Ishikawa cells using flow cytometry. Expression levels of phosphorylated FAK and total FAK were measured in Ishikawa cells and blastocysts by western blot and image J analysis. Blastocyst outgrowth was quantified using image J analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

The presence of IGF1 significantly increased mouse blastocyst attachment to Ishikawa cells compared with SS conditions (P < 0.01). IGF1 treatment resulted in distinct apical fibronectin staining on blastocysts, which was reduced by the PI3 kinase inhibitor LY294002. This coincided with a significant increase in blastocyst outgrowth in the presence of IGF1 (P < 0.01) or fibronectin (P < 0.001), which was abolished by LY294002 (P < 0.001). Apical expression of integrin αvβ3, β3 and β1 in Ishikawa cells was unaltered by IGF1. However, IGF1 increased phosphorylated FAK (P < 0.05) and total FAK expression in Ishikawa cells. FAK signalling is linked to integrin activation and can affect the integrins' ability to bind and recognize extracellular matrix proteins such as fibronectin. Treatment of blastocysts with IGF1 before co-culture with Ishikawa cells increased their attachment (P < 0.05). This effect was abolished in the presence of LY294002 (P < 0.001) or an IGF1R neutralizing antibody (P < 0.05).

LIMITATIONS, REASONS FOR CAUTION

This study uses an in vitro model of attachment that uses mouse blastocysts and human endometrial cells. This involves a species crossover and although this use has been well documented as a model for attachment (as human embryo numbers are limited) the results should be interpreted carefully.

WIDER IMPLICATIONS OF THE FINDINGS

This study presents mechanisms by which IGF1 improves attachment of blastocysts to Ishikawa cells and documents for the first time how IGF1 can increase adhesion competency in blastocysts. Failure of the blastocyst to implant is the major cause of human assisted reproductive technology (ART) failure. As growth factors are absent during embryo culture, their addition to embryo culture medium is a potential avenue to improve IVF success. In particular, IGF1 could prove to be a potential treatment for blastocysts before transfer to the uterus in an ART setting.

Mechanobiology and developmental control

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

Lessons from the embryonic neural stem cell niche for neural lineage differentiation of pluripotent stem cells

Pluripotent stem cells offer an abundant and malleable source for the generation of differentiated cells for transplantation as well as for in vitro screens. Patterning and differentiation protocols have been developed to generate neural progeny from human embryonic or induced pluripotent stem cells. However, continued refinement is required to enhance efficiency and to prevent the generation of unwanted cell types. We summarize and interpret insights gained from studies of embryonic neuroepithelium. A multitude of factors including soluble molecules, interactions with the extracellular matrix and neighboring cells cooperate to control neural stem cell self-renewal versus differentiation. Applying these findings and concepts to human stem cell systems in vitro may yield more appropriately patterned cell types for biomedical applications.

Distribution of tubulointerstitial nephritis antigen-like 1 and structural matrix proteins in mouse embryos during preimplantation development in vivo and in vitro

Tubulointerstitial nephritis antigen-like 1 (TINAGL1) is a novel matricellular protein that interacts with structural matrix proteins and promotes cell adhesion and spreading. We have previously reported unique localization of TINAGL1 to the trophectoderm (TE) of mouse blastocysts. TINAGL1 was found to be upregulated in implantation-competent blastocysts after estrogen treatment using progesterone-treated delayed-implantation models. Moreover, colocalization of TINAGL1 and extracellular matrix (ECM) protein laminin 1 was detected in the Reichert membrane on embryonic days 6.5 and 7.5. Although these data suggested a role for TINAGL1 in the embryo development at postimplantation, its relevance to other ECM proteins during preimplantation development is not clear. In this study, we examined the expression of TINAGL1 and its relevance to other ECM proteins fibronectin (FN) and collagen type IV (ColIV) during in vivo development of preimplantation embryos, particularly at blastocyst stage in detail. Localizations of TINAGL1, FN, and ColIV were similar. In 1-cell to 8-cell embryos, they were expressed in cytoplasm of blastomeres, and in morulae they were localized in the outer cells. FN and ColIV were expressed primarily on outer surface of the cells. In blastocysts, FN and ColIV were distributed in the cytoplasm of TE, but, just prior to implantation, they became localized uniquely to the blastocoelic surface of TE. In in vitro fertilized (IVF) blastocysts, expression levels of TINAGL1 and FN were lower than in in vivo blastocysts. These results suggest that, during preimplantation development, TINAGL1 may be involved in roles of structural matrix proteins, whose expression in blastocysts may be affected by in vitro culture.

Extracellular matrix proteins secreted from both the endometrium and the embryo are required for attachment: a study using a co-culture model of rat blastocysts and Ishikawa cells

Integrins are expressed in a highly regulated manner at the maternal-fetal interface during implantation. However, the significance of extracellular matrix (ECM) ligands during the integrin-mediated embryo attachment to the endometrium is not fully understood. Thus, the distribution of fibronectin in the rat uterus and blastocyst was studied at the time of implantation. Fibronectin was absent in the uterine luminal epithelial cells but was intensely expressed in the trophoblast cells and the inner cell mass suggesting that fibronectin secreted from the blastocyst may be a possible bridging ligand for the integrins expressed at the maternal-fetal interface. An Arg-Gly-Asp (RGD) peptide was used to block the RGD recognition sites on integrins, and the effect on rat blastocyst attachment to Ishikawa cells was examined. There was a significant reduction in blastocyst attachment when either the blastocysts or the Ishikawa cells were pre-incubated with the RGD-blocking peptide. Thus, successful attachment of the embryo to the endometrium requires the interaction of integrins on both the endometrium and the blastocyst with the RGD sequence of ECM ligands, such as fibronectin. Pre-treatment of both blastocysts and Ishikawa cells with the RGD peptide also inhibited blastocyst attachment, but not completely, suggesting that ECM bridging ligands that do not contain the RGD sequence are also involved in embryo attachment.

Dissecting the first transcriptional divergence during human embryonic development

The trophoblast cell lineage is specified early at the blastocyst stage, leading to the emergence of the trophectoderm and the pluripotent cells of the inner cell mass. Using a double mRNA amplification technique and a comparison with transcriptome data on pluripotent stem cells, placenta, germinal and adult tissues, we report here some essential molecular features of the human mural trophectoderm. In addition to genes known for their role in placenta (CGA, PGF, ALPPL2 and ABCG2), human trophectoderm also strongly expressed Laminins, such as LAMA1, and the GAGE Cancer/Testis genes. The very high level of ABCG2 expression in trophectoderm, 7.9-fold higher than in placenta, suggests a major role of this gene in shielding the very early embryo from xenobiotics. Several genes, including CCKBR and DNMT3L, were specifically up-regulated only in trophectoderm, indicating that the trophoblast cell lineage shares with the germinal lineage a transient burst of DNMT3L expression. A trophectoderm core transcriptional regulatory circuitry formed by 13 tightly interconnected transcription factors (CEBPA, GATA2, GATA3, GCM1, KLF5, MAFK, MSX2, MXD1, PPARD, PPARG, PPP1R13L, TFAP2C and TP63), was found to be induced in trophectoderm and maintained in placenta. The induction of this network could be recapitulated in an in vitro trophoblast differentiation model.

Expression of mesenchymal-related genes by the bovine trophectoderm following conceptus attachment to the endometrial epithelium

In the course of experiments to identify and characterize the factors that function in bovine conceptuses during peri-attachment periods, various transcripts related to the epithelial-mesenchymal transition (EMT) were found. In this study, RNA was extracted from different sets of days 17, 20, and 22 (day 0=day of estrous) bovine conceptuses and subjected to real-time PCR analysis as well as Western blotting, from which abundances of N-cadherin (CDH2), vimentin, matrix metalloproteinase 2 (gelatinase A, 72 kDa gelatinase, 72 kDa type IV collagenase) (MMP2), and matrix metallopeptidase 9 (gelatinase B, 92 kDa gelatinase, 92 kDa type IV collagenase) (MMP9) mRNAs were determined on day 22, concurrent with (CDH1) mRNA and protein downregulation. Transcription factors in EMT processes were then analyzed and changes in snail homolog 2 (Drosophila) (SNAI), zinc finger E-box binding homeobox 1 (ZEB1), zinc finger E-box binding homeobox 2 (ZEB2), twist homolog 1 (Drosophila) (TWIST1), twist homolog 2 (Drosophila) (TWIST2), and Kruppel-like factor 8 (KLF8) transcripts were found in day 22 conceptuses, while confirming SNAI2 expression by Western blotting. Immunohistochemical analysis revealed that the day 22 trophectoderm expressed the mesenchymal markers N-cadherin and vimentin as well as the epithelial marker cytokeratin. In attempts to identify the molecular mechanisms by which the trophectoderm expressed EMT-related genes, growth factor receptors associated with EMT were analyzed. Upregulation of the growth factor receptor transcripts, fibroblast growth factor receptor 1 (FGFR1), platelet-derived growth factor receptor, alpha polypeptide (PDGFRA), platelet-derived growth factor receptor, beta polypeptide (PDGFRB), and transforming growth factor, beta receptor II (70/80 kDa) (TGFBR2) mRNAs, was found on day 22. The analysis was extended to determine the integrin (ITG) transcripts and found high levels of integrin, alpha 4 (antigen CD49D, alpha 4 subunit of VLA-4 receptor) (ITGA4), integrin, alpha 8 (ITGA8), integrin, beta 3 (platelet glycoprotein IIIa, antigen CD61) (ITGB3), and integrin, beta 5 (ITGB5) mRNAs on day 22. These observations indicate that after the conceptus-endometrium attachment, EMT-related transcripts as well as the epithelial marker cytokeratin were present in the bovine trophectoderm and suggest that the implantation process for noninvasive trophoblasts requires not only extracellular matrix expression but also partial EMT.

Analysis of fertility-related soluble mediators in human uterine fluid identifies VEGF as a key regulator of embryo implantation

Embryo implantation requires synchronized dialogue between the receptive endometrium and activated blastocyst via locally produced soluble mediators. During the midsecretory (MS) phase of the menstrual cycle, increased glandular secretion into the uterine lumen contains important mediators that modulate the endometrium and support the conceptus during implantation. This study aimed first to identify the growth factor and cytokine profile of human uterine fluid from fertile women during the midproliferative (MP; nonreceptive) and MS (receptive) phases of the cycle, and from women with unexplained infertility during the MS phase. The second aim was to determine important functions of endometrial secretions for embryo implantation. Analysis of uterine fluid using quantitative Luminex assays revealed the presence of over 30 cytokines and growth factors, of which eight [platelet-derived growth factor-AA, TNF-B, soluble IL-2 receptor-A, Fms-like tyrosine kinase 3 ligand, soluble CD40 ligand, IL-7, interferon-A2, and chemokine (C-X-C motif) ligand 1-3] were previously unknown in human uterine fluid. Comparison of the fertile MP, MS, and infertile MS cohorts revealed vascular endothelial growth factor (VEGF) levels are significantly reduced in uterine fluid during the MS phase in women with unexplained infertility compared with fertile women. Functional studies demonstrated that culturing mouse embryos with either MS-phase uterine fluid from fertile women or recombinant human VEGF significantly enhanced blastocyst outgrowth. Furthermore, treatment of human endometrial epithelial cells with uterine fluid or recombinant human VEGF-A significantly increased endometrial epithelial cell adhesion. Taken together, our data support the concept that endometrial secretions, including VEGF, play important roles during implantation. Identifying the soluble mediators in human uterine fluid and their actions during implantation provides insight into interactions essential for establishing pregnancy, fertility markers, and infertility treatment options.

The extracellular matrix dimension of skeletal muscle development

Cells anchor to substrates by binding to extracellular matrix (ECM). In addition to this anchoring function however, cell-ECM binding is a mechanism for cells to sense their surroundings and to communicate and coordinate behaviour amongst themselves. Several ECM molecules and their receptors play essential roles in muscle development and maintenance. Defects in these proteins are responsible for some of the most severe muscle dystrophies at every stage of life from neonates to adults. However, recent studies have also revealed a role of cell-ECM interactions at much earlier stages of development as skeletal muscle forms. Here we review which ECM molecules are present during the early phases of myogenesis, how myogenic cells interact with the ECM that surrounds them and the potential consequences of those interactions. We conclude that cell-ECM interactions play significant roles during all stages of skeletal muscle development in the embryo and suggest that this "extracellular matrix dimension" should be added to our conceptual network of factors contributing to skeletal myogenesis.

Collagen IV significantly enhances migration and transplantation of embryonic stem cells: involvement of α2β1 integrin-mediated actin remodeling

Embryonic stem (ES) cell transplantation represents a potential means for the treatment of degenerative diseases and injuries. As appropriate distribution of transplanted ES cells in the host tissue is critical for successful transplantation, the exploration of efficient strategies to enhance ES cell migration is warranted. In this study we investigated ES cell migration under the influence of various extracellular matrix (ECM) proteins, which have been shown to stimulate cell migration in various cell models with unclear effects on ES cells. Using two mouse ES (mES) cell lines, ESC 26GJ9012-8-2 and ES-D3 GL, to generate embryoid bodies (EBs), we examined the migration of differentiating cells from EBs that were delivered onto culture surfaces coated with or without collagen I, collagen IV, Matrigel, fibronectin, and laminin. Among these ECM proteins, collagen IV exhibited maximal migration enhancing effect. mES cells expressed α2 and β1 integrin subunits and the migration enhancing effect of collagen IV was prevented by RGD peptides as well as antibodies against α2 and β1 integrins, indicating that the enhancing effect of collagen IV on cell migration was mediated by α2β1 integrin. Furthermore, staining of actin cytoskeleton that links to integrins revealed well-developed stress fibers and long filopodia in mES cells cultured on collagen IV, and the actin-disrupting cytochalasin D abolished the collagen IV-enhanced cell migration. In addition, pretreatment of undifferentiated or differentiated mES cells with collagen IV resulted in improved engraftment and growth after transplantation into the subcutaneous tissue of nude mice. Finally, collagen IV pretreatment of osteogenically differentiated mES cells increased osteogenic differentiation-like tissue and decreased undifferentiation-like tissue in the grafts grown after transplantation. Our results demonstrated that collagen IV significantly enhanced the migration of differentiating ES cells through α2β1 integrin-mediated actin remodeling and could promote ES cell transplantation efficiency, which may be imperative to stem cell therapy.

Dynamic 3D cell rearrangements guided by a fibronectin matrix underlie somitogenesis

Somites are transient segments formed in a rostro-caudal progression during vertebrate development. In chick embryos, segmentation of a new pair of somites occurs every 90 minutes and involves a mesenchyme-to-epithelium transition of cells from the presomitic mesoderm. Little is known about the cellular rearrangements involved, and, although it is known that the fibronectin extracellular matrix is required, its actual role remains elusive. Using 3D and 4D imaging of somite formation we discovered that somitogenesis consists of a complex choreography of individual cell movements. Epithelialization starts medially with the formation of a transient epithelium of cuboidal cells, followed by cell elongation and reorganization into a pseudostratified epithelium of spindle-shaped epitheloid cells. Mesenchymal cells are then recruited to this medial epithelium through accretion, a phenomenon that spreads to all sides, except the lateral side of the forming somite, which epithelializes by cell elongation and intercalation. Surprisingly, an important contribution to the somite epithelium also comes from the continuous egression of mesenchymal cells from the core into the epithelium via its apical side. Inhibition of fibronectin matrix assembly first slows down the rate, and then halts somite formation, without affecting pseudopodial activity or cell body movements. Rather, cell elongation, centripetal alignment, N-cadherin polarization and egression are impaired, showing that the fibronectin matrix plays a role in polarizing and guiding the exploratory behavior of somitic cells. To our knowledge, this is the first 4D in vivo recording of a full mesenchyme-to-epithelium transition. This approach brought new insights into this event and highlighted the importance of the extracellular matrix as a guiding cue during morphogenesis.

Computer simulation of emerging asymmetry in the mouse blastocyst

The mechanism of embryonic polarity establishment in mammals has long been controversial. Whereas some claim prepatterning in the egg, we recently presented evidence that mouse embryonic polarity is not established until blastocyst and proposed the mechanical constraint model. Here we apply computer simulation to clarify the minimal cellular properties required for this morphology. The simulation is based on three assumptions: (1) behavior of cell aggregates is simulated by a 3D vertex dynamics model; (2) all cells have equivalent mechanical properties; (3) an inner cavity with equivalent surface properties is gradually enlarged. However, an initial attempt reveals a requirement for an additional assumption: (4) the surface of the cavity is firmer than intercellular surfaces, suggesting the presence of a basement membrane lining the blastocyst cavity, which is indeed confirmed by published data. The simulation thus successfully produces a structure recapitulating the mouse blastocyst. The axis of the blastocyst, however, remains variable,leading us to an additional assumption: (5) the aggregate is enclosed by a capsule, equivalent to the zona pellucida in vivo. Whereas a spherical capsule does not stabilize the blastocyst axis, an ellipsoidal capsule eventually orients the axis in accordance with its longest diameter. These predictions are experimentally verified by time-lapse recordings of mouse embryos. During simulation, equivalent cells form two distinct populations composed of smaller inner cells and larger outer cells. These results reveal a unique feature of early mammalian development: an asymmetry may emerge autonomously in an equivalent population with no need for a priori intrinsic differences.

Pluripotent stem cells and their niches

The ability of stem cells to self-renew and to replace mature cells is fundamental to ontogeny and tissue regeneration. Stem cells of the adult organism can be categorized as mono-, bi-, or multipotent, based on the number of mature cell types to which they can give rise. In contrast, pluripotent stem cells of the early embryo have the ability to form every cell type of the adult body. Permanent lines of pluripotent stem cells have been derived from preimplantation embryos (embryonic stem cells), fetal primordial germ cells (embryonic germ cells), and malignant teratocarcinomas (embryonal carcinoma cells). Cultured pluripotent stem cells can easily be manipulated genetically, and they can be matured into adult-type stem cells and terminally differentiated cell types in vitro, thereby, providing powerful model systems for the study of mammalian embryogenesis and disease processes. In addition, human embryonic stem cell lines hold great promise for the development of novel regenerative therapies. To fully utilize the potential of these cells, we must first understand the mechanisms that control pluripotent stem cell fate and function. In recent decades, the microenvironment or niche has emerged as particularly critical for stem cell regulation. In this article, we review how pluripotent stem cell signal transduction mechanisms and transcription factor circuitries integrate information provided by the microenvironment. In addition, we consider the potential existence and location of adult pluripotent stem cell niches, based on the notion that a revealing feature indicating the presence of stem cells in a given tissue is the occurrence of tumors whose characteristics reflect the normal developmental potential of the cognate stem cells.

Expression of nidogens in rat uterus and embryo during decidualization and implantation

The purpose of this study was to demonstrate the expression of nidogen-1 and nidogen-2 and their possible role in decidualization and implantation events during early pregnancy in rats. The tissue samples were examined from pregnant animals between gestational days 1-8 using immunocytochemistry. The uterine luminal epithelium, the glandular epithelium, and the myometrial smooth muscle cells stained strongly from gestational days 1-8 with both nidogen antibodies. At day 4 the decidual reaction areas began to appear in the stromal matrix and immunostaining of both nidogens revealed that the basement membrane of the surface epithelium was discontinuous. The differentiation of stromal cells into decidual cells was seen at gestational day 5 and both nidogens were weakly expressed in the decidualizing cells. At day 6, nidogen-2 immunoreactivity was higher in the primary decidual cells close to the embryo than nidogen-1, and during development of the decidual tissue both nidogens appeared in the endometrial stromal cells. At day 7, while expression of both nidogens declined in the primary decidual cells, their expression was markedly observed in the secondary decidual cells close to the myometrium. At day 8, expression of both nidogens was also observed to increase in the primary decidual cells. While nidogen-2 expression was seen in the parietal endoderm and primary ectoderm of the rat embryos at this developmental stage, nidogen-1 expression was only detected in the parietal endoderm. These results indicate that nidogen-1 and nidogen-2 could play important roles during embryogenesis, decidualization, and implantation in the endometrium of rat uterus.

Identification of an Enhancer That Controls Up-regulation of Fibronectin during Differentiation of Embryonic Stem Cells into Extraembryonic Endoderm

The extraembryonic endoderm is derived from inner cell mass cells of the blastocyst during early mouse embryogenesis. Formation of the extraembryonic endoderm, which later contributes to the yolk sac, appears to be a prerequisite for subsequent differentiation of the inner cell mass. While embryonic stem cells can be induced to differentiate into extraembryonic endoderm cells in vitro, the molecular mechanisms underlying this process are poorly understood. We used a promoter trap approach to search for genes that are expressed in embryonic stem cells and are highly up-regulated during differentiation to the extraembryonic endoderm fate. We showed that fibronectin fits this expression profile. Moreover we identified an enhancer in the 12th intron of the fibronectin locus that recapitulated the endogenous pattern of fibronectin expression. This enhancer carries Sox protein-binding sequences, and our analysis demonstrated that Sox7 and Sox17, which are highly expressed in the extraembryonic endoderm, were involved in enhancer activity.

EFFECTS OF INHIBITORS OF INTEGRIN BINDING ON CELLULAR OUTGROWTH FROM BOVINE INNER CELL MASSES IN VITRO

Bovine inner cell masses (ICM) cultured on fibronectin give rise to extensive cellular outgrowths containing endoderm. Peptides with the Glu-Ile-Leu-Asp-Val (EILDV) and Arg-Gly-Asp (RGD) sequences inhibit cell migration on fibronectin by binding to the fibronectin-recognition site in several integrins. To identify integrins involved in endodermal cell outgrowth on fibronectin and vitronectin, the effects of the EILDV and RGD peptides were evaluated in vitro. In experiment 1, ICM were cultured on fibronectin in medium containing 0.5 or 1.0 mg/ml EILDV or RGD (or both). Compared with 0 mg/ml, 0.5 mg/ml EILDV suppressed (P<0.10) outgrowth area overall, and 1.0 mg/ml EILDV reduced (P<0.05) outgrowth area after 72 h of culture. Compared with 0 mg/ml, 0.5 and 1.0 mg/ml RGD reduced (P<0.05) outgrowth area after 72 h of culture. Plasminogen activator activity in conditioned medium increased (P<0.05) in 0.5 mg/ml RGD but decreased (P<0.10) in 1.0 mg/ml RGD compared with 0 mg/ml RGD. In experiment 2, bovine ICM were cultured on vitronectin in medium containing 0.5 or 1.0 mg/ml RGD. Neither concentration of RGD (P>0.10) affected the extent of cellular outgrowth on vitronectin. Bovine endodermal cell migration on fibronectin can be modulated by the RGD and EILDV peptides. Despite inhibition, neither peptide completely prevented outgrowth on fibronectin. In contrast, cellular outgrowth on vitronectin was unaffected by RGD. The persistence of cellular outgrowth on fibronectin and the absence of inhibition by RGD for ICM cultured on vitronectin suggests that bovine endodermal cells can use alternative cellular adhesion systems, such as nonintegrin receptors, during outgrowth.

Trophoblast-specific expression and function of the integrin alpha 7 subunit in the peri-implantation mouse embryo

For implantation and placentation to occur, mouse embryo trophoblast cells must penetrate the uterine stroma to make contact with maternal blood vessels. A major component of the uterine epithelial basement membrane and underlying stromal matrix with which they interact is the extracellular matrix protein laminin. We have identified integrin alpha 7 beta 1 as a major receptor for trophoblast-laminin interactions during implantation and yolk sac placenta formation. It is first expressed by trophectoderm cells of the late blastocyst and by all trophectoderm descendants in the early postimplantation embryo through E8.5, then disappears except in cells at the interface between the allantois and the ectoplacental plate. Integrin alpha 7 expression is a general characteristic of the early differentiation stages of rodent trophoblast, given that two different cultured trophoblast cell lines also express this integrin. Trophoblast cells interact with at least three different laminin isoforms (laminins 1, 2/4, and 10/11) in the blastocyst and in the uterus at the time of implantation. Outgrowth assays using function-blocking antibodies show that alpha 7 beta 1 is the major trophoblast receptor for laminin 1 and a functional receptor for laminins 2/4 and 10/11. When trophoblast cells are cultured on substrates of these three laminins, they attach and spread on all three, but show decreased proliferation on laminin 1. These results show that the alpha 7 beta 1 integrin is expressed by trophoblast cells and acts as receptor for several isoforms of laminin during implantation. These interactions are not only important for trophoblast adhesion and spreading but may also play a role in regulating trophectoderm proliferation and differentiation.

Inner cell mass-specific expression of a cell adhesion molecule (PECAM-1/CD31) in the mouse blastocyst

Platelet/Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) is thought to be a vascular-specific protein, but its function has not been clearly defined. Here, we demonstrate by using confocal immunofluorescence microscopy that PECAM-1 is first detected in the mouse blastocyst, which contains no vascular cells, and its expression is restricted to the pluripotent inner cell mass (ICM) cells. Expression is localized to cell-cell borders of the ICM and is detected at the very first signs of blastocoel formation. Consistent with these observations is that embryonic transcripts of PECAM-1 mRNA, as detected by RT-PCR, greatly increase during the morula-to-blastocyst transition and seven of the eight known alternatively spliced isoforms of PECAM-1 are expressed in the blastocyst. The synthesis of PECAM-1 is independent of compaction, cytokinesis, and DNA replication, as it is detected in embryos that are chronologically at the blastocyst stage following culture of 8-cell embryos in Ca2+-free medium, or medium containing cytochalasin D or aphidicolin, respectively. By the late blastocyst stage, PECAM-1 expression is restricted to the pluripotent epiblast, at which point it has a mutually exclusive expression pattern to that of type IV collagen, a basement membrane marker. The reduction in PECAM-1 transcripts in retinoic acid-induced differentiation of F9 teratocarcinoma cells, a model of epiblast-to-primitive endoderm differentiation, confirmed the epiblast-specific expression of PECAM-1. By the egg cylinder stage of development, at which point the epiblast is no longer pluripotent, PECAM-1 is not detected. This ICM-specific pattern of expression suggests a novel developmental role of PECAM-1 that is independent of its function in vascular ontogeny.

Distinct roles for dystroglycan, (β)1 integrin and perlecan in cell surface laminin organization

Dystroglycan (DG) is a cell surface receptor for several extracellular matrix (ECM) molecules including laminins, agrin and perlecan. Recent data indicate that DG function is required for the formation of basement membranes in early development and the organization of laminin on the cell surface. Here we show that DG-mediated laminin clustering on mouse embryonic stem (ES) cells is a dynamic process in which clusters are consolidated over time into increasingly more complex structures. Utilizing various null-mutant ES cell lines, we define roles for other molecules in this process. In (β)1 integrin-deficient ES cells, laminin-1 binds to the cell surface, but fails to organize into more morphologically complex structures. This result indicates that (β)1 integrin function is required after DG function in the cell surface-mediated laminin assembly process. In perlecan-deficient ES cells, the formation of complex laminin-1 structures is defective, implicating perlecan in the laminin matrix assembly process. Moreover, laminin and perlecan reciprocally modulate the organization of the other on the cell surface. Taken together, the data support a model whereby DG serves as a receptor essential for the initial binding of laminin on the cell surface, whereas (β)1 integrins and perlecan are required for laminin matrix assembly processes after it binds to the cell.

Amino acids and preimplantation development of the mouse in protein-free potassium simplex optimized medium

Development of outbred CF1 mouse zygotes in vitro was studied in a chemically defined, protein-free medium both with and without amino acids. The addition of amino acids to protein-free potassium simplex optimized medium (KSOM) had little effect on the proportion of embryos that developed at least to the zona-enclosed blastocyst stage. In contrast, amino acids stimulated very significantly, in a dilution-dependent way, the proportion of blastocysts that at least partially or completely hatched. Amino acids also stimulated cell proliferation in both the trophectoderm and inner cell mass (ICM) cells, at rates that favored proliferation of cells in the ICM; had no effect on the incidence of cell death (oncosis or apoptosis); and improved development of the basement membranes, which form on the blastocoelic surface of the trophectoderm and between the primitive endoderm and the primitive ectoderm. Thus, KSOM, supplemented with amino acids but containing no protein supplements, supports development of a newly fertilized ovum to the late blastocyst stage, in which its normal, three-dimensional structure is preserved and in which the ICM has been partitioned into the primitive ectoderm and primitive endoderm.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.

Blastocyst implantation: the adhesion cascade

This review covers the sequence of cell adhesion events occurring during implantation of the mammalian embryo, concentrating on data from mouse and human. The analogy is explored between initial attachment of trophoblast to the uterine lining epithelium and that of neutrophils to the endothelial lining of blood vessels at sites of inflammation. The possible role of various carbohydrate ligands in initial attachment of the blastocyst is reviewed. The evidence for subsequent stabilization of cell adhesion via integrins or the trophinin-tastin complex is discussed.

Ectopic expression of the transforming growth factor beta type II receptor disrupts mesoderm organisation during mouse gastrulation

Transforming growth factor beta (TGFbeta) regulates the cell cycle and extracellular matrix (ECM) deposition of many cells in vitro. We have analysed chimaeric mouse embryos generated from embryonic stem cells with abnormal receptor expression to study the effect of TGFbeta on these processes in vivo and the consequences for normal development. The binding receptor for TGFbeta, TbetaRII, is first detected in the embryo proper around day 8.5 in the heart. Ectopic expression of TbetaRII from the blastocyst stage onward resulted in an embryonic lethal around 9.5 dpc. Analysis of earlier stages revealed that the primitive streak of TbetaRII chimaeras failed to elongate. Furthermore, although cells passed through the streak and initially formed mesoderm, they tended to accumulate within the streak. These defects temporally and spatially paralleled the expression of the TGFbeta type I receptor, which is first expressed in the node and primitive streak. We present evidence that classical TGFbeta-induced growth inhibition was probably the cause of insufficient mesoderm being available for paraxial and axial structures. The results demonstrate that (1) TGFbeta mRNA and protein detected previously in early postimplantation embryos is present as a biologically active ligand; and (2) assuming that ectopic expression of TbetaRII results in no other changes in ES cells, the absence of TbetaRII is the principle reason why the embryo proper is unresponsive to TGFbeta ligand until after gastrulation.

Absence of basement membranes after targeting the LAMC1 gene results in embryonic lethality due to failure of endoderm differentiation

The LAMC1 gene coding for the laminin gamma1 subunit was targeted by homologous recombination in mouse embryonic stem cells. Mice heterozygous for the mutation had a normal phenotype and were fertile, whereas homozygous mutant embryos did not survive beyond day 5.5 post coitum. These embryos lacked basement membranes and although the blastocysts had expanded, primitive endoderm cells remained in the inner cell mass, and the parietal yolk sac did not develop. Cultured embryonic stem cells appeared normal after targeting both LAMC1 genes, but the embryoid bodies derived from them also lacked basement membranes, having disorganized extracellular deposits of the basement membrane proteins collagen IV and perlecan, and the cells failed to differentiate into stable myotubes. Secretion of the linking protein nidogen and a truncated laminin alpha1 subunit did occur, but these were not deposited in the extracellular matrix. These results show that the laminin gamma1 subunit is necessary for laminin assembly and that laminin is in turn essential for the organization of other basement membrane components in vivo and in vitro. Surprisingly, basement membranes are not necessary for the formation of the first epithelium to develop during embryogenesis, but first become required for extra embryonic endoderm differentiation.

Identification of two distinct functions for TGF-beta in early mouse development

In this study the function of transforming growth factor-beta (TGF-beta) in preimplantation mouse embryos was examined. By RT-PCR, mRNA for the signalling type I (T beta R-I) and type II (T beta R-II) receptors for TGF-beta was shown to be present in two distinct time windows: in fertilized oocytes and at the blastocyst stage. The function of TGF-beta at these times was analysed in two ways. Firstly, the TGF-beta signalling pathway was blocked by injecting a DNA construct encoding a truncated T beta R-II, that acts as a dominant-negative receptor, in fertilized oocytes, and the effect on development was determined. Secondly, inner cell masses isolated at the blastocyst stage were cultured in vitro with and without TGF-beta under conditions that favour the outgrowth of parietal endoderm. The results show that TGF-beta signalling mediated by maternally expressed receptors is important for development of preimplantation embryos beyond the two-cell stage, and suggest a regulatory role for TGF-beta in the outgrowth of parietal endoderm.

Integrins and reproductive physiology: expression and modulation in fertilization, embryogenesis, and implantation

OBJECTIVE

To review the available information regarding the role of integrins in reproductive physiology and to discuss their potential clinical implications.

DESIGN

Studies that specifically relate to the expression and modulation of integrins in fertilization, embryogenesis, and implantation were identified through the literature and Medline searches.

RESULT(S)

Integrins are a class of adhesion molecules that participate in cell-to-cell and cell-to-substratum interactions and are present on essentially all human cells. All mammalian eggs express integrins at their surface, and the integrin alpha 6 beta 1 serves as a sperm receptor that mediates sperm-egg binding. In addition, certain integrin moieties appear to be regulated within the cycling endometrium. Specifically, the expression of beta 1 integrins in the early proliferative phase is restricted to the glandular epithelium, whereas stromal cells also express beta 1 integrins in the midsecretory phase. The expression of beta 1 integrins increases at the time of implantation and remains elevated in the decidua during early pregnancy. A disruption of integrin expression is associated with certain types of infertility in women. The apical surface of the mural trophectoderm does indeed possess functional integrins, and trophoblast interactions with extracellular matrix proteins largely depend on the integrin family of adhesion receptors.

CONCLUSION(S)

Integrins play particularly important roles in both fertilization and embryogenesis, including the process of implantation.

Cooperative interactions between extracellular matrix, integrins and parathyroid hormone-related peptide regulate parietal endoderm differentiation in mouse embryos

The outgrowth of parietal endoderm (PE) cells from precursor endodermal cells is one of the first differentiation events that occur in mouse embryos. We have analyzed the molecular determinants of this process by placing isolated inner cell masses (ICMs) on defined extracellular matrix substrata in microdrop cultures. Differentiation and outgrowth of PE required a fibronectin substratum. Laminin supported the adhesion and outgrowth of visceral endoderm (VE) and actively suppressed the differentiation of PE in mixtures of fibronectin and laminin. Collagen type IV, gelatin, vitronectin or entactin supported little or no endodermal outgrowth. Trophectoderm (TE) cells have been implied to be important in PE induction in vivo. We found that recombination of ICMs in culture with TE cells, or with medium conditioned by TE cells, greatly increased the differentiation of PE. TE cells stimulated PE outgrowth on substrata other than fibronectin. One cytokine secreted by trophoblast and endodermal cells, parathyroid hormone-related peptide (PTHrP), was critical for outgrowth on any substratum. A function-perturbing antibody to PTHrP reduced the number of PE cells, whereas the addition of PTHrP increased that number. Furthermore, addition of PTHrP changed the substratum requirements for outgrowth, making laminin, vitronectin and low concentrations of fibronectin permissive for PE outgrowth. Immunostaining with anti-integrin antibodies showed that fully differentiated PE cells outgrowing on fibronectin expressed alpha 5, alpha 6 and alpha v beta 3 integrins. However, analysis of outgrowths in the presence of function-perturbing antibodies to alpha 5, alpha 6 and alpha v beta 3 integrins showed that these integrins directed PE outgrowth only on fibronectin, laminin and vitronectin substrata, respectively. We have shown that there is a cooperative interplay of extracellular matrix, integrins and PTHrP that modulates PE outgrowth.

Deletion of beta 1 integrins in mice results in inner cell mass failure and peri-implantation lethality

Integrin receptors for extracellular matrix receptors are important effectors of cell adhesion, differentiation, and migration in cultured cells and are believed to be critical effectors of these processes during development. To determine when beta 1 integrins become critical during embryonic development, we generated mutant mice with a targeted disruption of the beta 1 integrin subunit gene. Heterozygous mutant mice were normal. Homozygous loss of beta 1 integrin expression was lethal during early postimplantation development. Homozygous embryos lacking beta 1 integrins formed normal-looking blastocysts and initiated implantation at E4.5. However, the E4.5 beta 1-null embryos in situ had collapsed blastocoeles, and whereas the trophoblast penetrated the uterine epithelium, extensive invasion of the decidua was not observed. Laminin-positive endoderm cells were detected in the inner cell mass area, but endoderm morphogenesis and migration were defective. By E5.5 beta 1-null embryos had degenerated extensively. In vitro analysis showed that trophoblast function in beta 1-null peri-implantation embryos was largely normal, including expression of tissue-specific markers, and outgrowth on fibronectin- and vitronectin-coated, although not on laminin-coated substrates. In contrast, the inner cell mass region of beta 1-null blastocyst outgrowths, and inner cell masses isolated from beta 1-null blastocysts, showed highly retarded growth and defective extraembryonic endoderm morphogenesis and migration. These data suggest that beta 1 integrins are required for normal morphogenesis of the inner cell mass and are essential mediators of growth and survival of cells of the inner cell mass. Failure of continued trophoblast development in beta 1-null embryos after inner cell mass failure could be attributable to either an intrinsic requirement for beta 1 integrins for later stages of trophoblast development, or to the lack of trophic signals from the beta 1-null inner cell mass.

Beta 1- and beta 3-class integrins mediate fibronectin binding activity at the surface of developing mouse peri-implantation blastocysts. Regulation by ligand-induced mobilization of stored receptor

Implanting mouse blastocysts adhere through their abembryonic surfaces to the endometrial extracellular matrix. Because blastocysts cultured on fibronectin in vitro dissociate to form trophoblast outgrowths, it is unclear whether this adhesion is initially mediated by fibronectin receptors on the apical or basolateral surface of the trophectoderm. Intact blastocysts were examined in a ligand binding assay utilizing the fibronectin cell binding domain attached to fluorescent microspheres. Fibronectin binding activity on the apical surface of the trophectoderm was confined to the abembryonic pole of the blastocyst, where trophoblast differentiation initiates, and was regulated temporally in accordance with blastocyst outgrowth. Soluble fibronectin (IC50 = 0.2 microM) or Gly-Arg-Gly-Asp-Ser-Pro, but not laminin, competitively inhibited fibronectin binding activity. Addition of antibodies against the alpha v, alpha 5, beta 1, or beta 3 integrin subunits also inhibited binding activity. Blastocysts cultured in the absence of an adhesive substratum exhibited fibronectin binding activity only after exposure to immobilized or soluble ligand. Potentiation of binding activity by ligand was unaffected by cycloheximide but was sensitive to brefeldin A inhibition of protein trafficking. These findings suggest that the interaction of fibronectin with the trophectoderm induces a translocation event that up-regulates fibronectin binding beta 1- and beta 3-class integrins on the apical surface.

Integrins and reproduction

The integrins are a large family of cell adhesion receptors, involved in cell-cell and cell-matrix interactions. At present, 20 different integrin heterodimers are known. They not only anchor cells to their proper locations, but also activately mediate the passage of information into the cell. They are involved in such diverse processes as immune response, lymphocyte homing, platelet aggregation, metastatic spread of certain malignancies, healing process of tissue injuries and, embryologic development. The role of integrins in reproduction had been only recently suggested. Several reasons make these molecules very attractive, due to their constant involvement from egg to birth. A normal expression of integrins can disrupt every reproductive stage. Most likely diagnostic tools and therapeutic propositions will emerge from the knowledge of these receptors. Integrins are a family of membrane glycoproteines that mediate cell-substratum or cell-cell adhesion. In respect of one fundamental principle of cellular biology consisting of 'what a cell touches has a major role in determining what a cell does', adhesion has a main part in many cell functions. Adhesion not only anchors cells to their proper locations, but also activately mediates the passage of information into the cell. Cellular adhesion is implicated in the immune response, lymphocyte homing, platelet aggregation, metastatic spread of certain malignancies, embryologic development and wound healing. The role of integrins in reproduction appears interesting. The aim of this review is to introduce these molecules, to outline their roles in cellular function and to consider their involvement in reproduction before foreseeing their potential implications for therapy.

Developmental regulation of integrin expression at the time of implantation in the mouse embryo

The trophectoderm layer of the mouse blastocyst differentiates at the late blastocyst stage to form the invasive trophoblast that mediates implantation of the embryo into the uterine wall. The first sign that trophoblast cells have developed an invasion-specific cell behavior appears about 10–15 hours after the embryo hatches from the zona pellucida, when the quiescent, non-adherent trophectoderm cells initiate protrusive activity and become adhesive to extracellular matrix. Our previous findings that trophoblast outgrowth on extracellular-matrix-coated substrata involves the integrin family of adhesion receptors (Sutherland, A. E., Calarco, P. G. and Damsky, C. H., 1988, J. Cell Biol. 106, 1331–1348), suggested that the onset of trophoblast adhesive and migratory behavior at the time of implantation may be due to changes in expression or distribution of integrin receptors. We have thus examined the mRNA and protein expression of individual integrin subunits during pre- and periimplantation development (E0-E7.5). A basic repertoire of integrins, including receptors for fibronectin (alpha 5 beta 1), laminin (alpha 6B beta 1) and vitronectin (alpha v beta 3), was expressed continuously throughout this period, whereas the expression of five other integrin subunits was developmentally regulated. The mRNA for three of these (alpha 2, alpha 6A and alpha 7) was first detected in the late blastocyst, coincident with endoderm differentiation and development of attachment competence. The mRNA for another (alpha 1) was not detected until after trophoblast outgrowth had begun, suggesting that its expression may be induced by contact with matrix. At E7.5, three of the temporally regulated integrins (alpha 1, apha 6A, alpha 7), all of which can form receptors for laminin, were detected only in the ectoplacental cone (differentiating trophoblast), and may thus play specific roles in trophoblast adhesion and/or differentiation. Because laminin expression is upregulated in decidualized uterine stroma in response to the implanting embryo, we examined trophoblast-laminin interactions, using laminin fragments and integrin antibodies to determine which integrin receptors were involved. Trophoblast cells attached and spread on both the E8 and P1′ fragments of laminin; however, the P1′ binding site was cryptic in intact laminin. Interaction with P1′ was RGD- and alpha v beta 3-dependent, whereas outgrowth on E8 was RGD-independent and not inhibited by antibodies to the laminin receptor alpha 6 beta 1, suggesting that alpha 7 beta 1 is the major trophoblast integrin E8 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin

To examine the role of fibronectin in vivo, we have generated mice in which the fibronectin gene is inactivated. Heterozygotes have one half normal levels of plasma fibronectin, yet appear normal. When homozygous, the mutant allele causes early embryonic lethality, proving that fibronectin is required for embryogenesis. However, homozygous mutant embryos implant and initiate gastrulation normally including extensive mesodermal movement. Neural folds also form but the mutant embryos subsequently display shortened anterior-posterior axes, deformed neural tubes and severe defects in mesodermally derived tissues. Notochord and somites are absent; the heart and embryonic vessels are variable and deformed, and the yolk sac, extraembryonic vasculature and amnion are also defective. These abnormalities can be interpreted as arising from fundamental deficits in mesodermal migration, adhesion, proliferation or differentiation as a result of the absence of fibronectin. The nature of these embryonic defects leads to reevaluation of suggested roles for fibronectin during early development based on results obtained in vitro and in embryos of other species.

Variants of the alpha 6 beta 1 laminin receptor in early murine development: distribution, molecular cloning and chromosomal localization of the mouse integrin alpha 6 subunit

Laminin (A:B1:B2) is a major component of the first basement membrane to appear in the developing mouse embryo. Its effects on morphogenesis and differentiation are mediated by interaction with cell surface receptors that are members of the integrin family. We have studied the expression of the alpha 6 subunit of murine alpha 6 beta 1 and its ligand, laminin, in preimplantation mouse embryos, embryo outgrowths and in embryonic stem (ES) cells and embryonal carcinoma (EC) cells. The alpha 6 subunit is present in the oocyte and throughout preimplantation development. Laminin A chain appears later than alpha 6 and has a more restricted distribution until the late blastocyst stage. alpha 6 beta 1 is strongly expressed in ES and EC cells; the levels of mRNA expression are not altered by differentiation. Molecular cloning of cDNA for the murine integrin alpha 6 subunit from a mammary gland lambda gt11 library showed, as in man, an open reading frame encoding two variants of alpha 6, alpha 6A and alpha 6B. The identity of the alpha 6 amino acid sequence to that in man and chicken is 93% and 73%, respectively. The gene for murine alpha 6 was mapped to chromosome 2. While undifferentiated ES and EC cells express only alpha 6B, alpha 6A is co-expressed in ES cells after differentiation is induced by retinoic acid. alpha 6B is also the only variant expressed in blastocyst stage embryos, but when blastocysts have grown out in culture both alpha 6A and alpha 6B are expressed reflecting the results in the cell lines. We suggest that the deposition of laminin in the embryo is a receptor-mediated process and that the shift in the expression of the variants, as the inner cell mass forms its first differentiated progeny, reflects a change in functional properties.