Uterine fibronectin mRNA content and localization are modulated during implantation

3D biomimetic platform reveals the first interactions of the embryo and the maternal blood vessels

The process of implantation and the cellular interactions at the embryo-maternal interface are intrinsically difficult to analyze, as the implanting embryo is concealed by the uterine tissues. Therefore, the mechanisms mediating the interconnection of the embryo and the mother are poorly understood. Here, we established a 3D biomimetic culture environment that harbors the key features of the murine implantation niche. This culture system enabled direct analysis of trophoblast invasion and revealed the first embryonic interactions with the maternal vasculature. We found that implantation is mediated by the collective migration of penetrating strands of trophoblast giant cells, which acquire the expression of vascular receptors, ligands, and adhesion molecules, assembling a network for communication with the maternal blood vessels. In particular, Pdgf signaling cues promote the establishment of the heterologous contacts. Together, the biomimetic platform and our findings thereof elucidate the hidden dynamics of the early interactions at the implantation site.

Transcriptomic changes in the pre-implantation uterus highlight histotrophic nutrition of the developing marsupial embryo

Early pregnancy is a critical time for successful reproduction; up to half of human pregnancies fail before the development of the definitive chorioallantoic placenta. Unlike the situation in eutherian mammals, marsupial pregnancy is characterised by a long pre-implantation period prior to the development of the short-lived placenta, making them ideal models for study of the uterine environment promoting embryonic survival pre-implantation. Here we present a transcriptomic study of pre-implantation marsupial pregnancy, and identify differentially expressed genes in the Sminthopsis crassicaudata uterus involved in metabolism and biosynthesis, transport, immunity, tissue remodelling, and uterine receptivity. Interestingly, almost one quarter of the top 50 genes that are differentially upregulated in early pregnancy are putatively involved in histotrophy, highlighting the importance of nutrient transport to the conceptus prior to the development of the placenta. This work furthers our understanding of the mechanisms underlying survival of pre-implantation embryos in the earliest live bearing ancestors of mammals.

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.

Extracellular matrix remodelling in rat endometrium during early pregnancy: the role of fibronectin and laminin

The endometrial extracellular matrix (ECM) remodelling has a crucial role in the establishment of a successful pregnancy. In addition to its basic function such as regulation of cell function, differentiation, migration, proliferation, the substantial alterations in the endometrial ECM may play a specific role in the trophoblast invasion, placentation, cell death and formation of the proper and functional implantation chamber around the embryo. In the present study, immunolocalizations of fibronectin and laminin were determined using avidin-biotin complex-peroxidase in rat implantation sites during 7-10 days of pregnancy. Both proteins were present in the basal membrane of blood vessels and in decidual matrix whereas they were absent or had very weak reactivity in the primary decidual zone on day 7. When placentation has begun, the immunoreactivity of both proteins was increased in the placental bed and in the basal membrane of blood vessels of the mesometrial region. The immunolocalization of both proteins seemed to be decreased in the antimesometrial decidua, however, it was increased in the mesometrial decidual matrix on days 9 and 10. Therefore, it could be suggested laminin and fibronectin demonstrating dynamic expressions in relation with the morphological differentiation of endometrial stroma may play crucial roles in the control of trophoblast adhesion and invasion, in placentation and angiogenesis, in the determination of cell shape and fate thus contributing the endometrial receptivity and a successful pregnancy.

Immunocytochemical studies of adhesion molecules on mouse UNK cells and their extracellular matrix ligands during mouse pregnancy

Uterine natural killer (uNK) cells are the dominant lymphocytes of pregnant mammals' uterus. Studies have identified four differentiation stage of mouse uNK cells based on Dolichos biflorus lectin cytochemistry, and their distribution showed preferential domain in the uterus through out the pregnancy. This work was done to investigate the expression of alpha5, alpha6, and beta7 integrins on uNK cells and their ligands distribution. Section of mouse uterus from sixth to seventeenth gestational days were submitted to immunocytochemistry and positive reactions for alpha5, alpha6, and beta7 integrins were found on uNK from eighth to tenth gestational days but not after twelfth gestational days. Fibronectin reactions were seemed from sixth to tenth gestational days around uNK from the myometrium and endometrium close to the myometrium. No reaction for fibronectin was seen in the decidualized and nondecidualized endometrium near the placenta. Laminin reaction was seen just in the antimesometrial side. beta7 integrin seems to be the active receptor to bind with VCAM-1 or MAdCAM-1 of endothelial cells, promoting the uNK cross through the vessels. The absence of laminin in an uNK domain suggests these cells are not dependent of laminin and alpha6 integrin for their establishment. However, fibronectin seems to support uNK migration, proliferation, differentiation, and survival in the uterus by binding with alpha5 integrin. The loss of alpha5 integrin ligation by the down regulation of fibronectin could inhibits these events and further studies are need to investigate whether unligated alpha5 can actively and initiate apoptosis, maybe in a caspase 8-dependent way that has been called integrin-mediated death.

Heparan sulfate proteoglycans and their binding proteins in embryo implantation and placentation

Complex interactions occur among embryonic, placental and maternal tissues during embryo implantation. Many of these interactions are controlled by growth factors, extracellular matrix and cell surface components that share the ability to bind heparan sulfate (HS) polysaccharides. HS is carried by several classes of cell surface and secreted proteins called HS proteoglycan that are expressed in restricted patterns during implantation and placentation. This review will discuss the expression of HS proteoglycans and various HS binding growth factors as well as extracellular matrix components and HS-modifying enzymes that can release HS-bound proteins in the context of implantation and placentation.

Blastocysts don't go it alone. Extrinsic signals fine-tune the intrinsic developmental program of trophoblast cells

The preimplantation embryo floats freely within the oviduct and is capable of developing into a blastocyst independently of the maternal reproductive tract. While establishment of the trophoblast lineage is dependent on expression of developmental regulatory genes, further differentiation leading to blastocyst implantation in the uterus requires external cues emanating from the microenvironment. Recent studies suggest that trophoblast differentiation requires intracellular signaling initiated by uterine-derived growth factors and integrin-binding components of the extracellular matrix. The progression of trophoblast development from the early blastocyst stage through the onset of implantation appears to be largely independent of new gene expression. Instead, extrinsic signals direct the sequential trafficking of cell surface receptors to orchestrate the developmental program that initiates blastocyst implantation. The dependence on external cues could coordinate embryonic activities with the developing uterine endometrium. Biochemical events that regulate trophoblast adhesion to fibronectin are presented to illustrate a developmental strategy employed by the peri-implantation blastocyst.

Decidualization induces the expression and activation of an extracellular protease neuropsin in mouse uterus

Uterine decidualization is accompanied by the remodeling of the cell-matrix and cell-cell interactions around the endometrial stromal cells to allow an appropriate invasion of trophoblasts. This remodeling is thought to require the proteolysis of extracellular matrix proteins or cell adhesion molecules; however, the molecular mechanism remains poorly understood. In this study, decidualization induced the expression and activation of an extracellular serine protease neuropsin in the mouse uterus. Although nonpregnant uteri contained little neuropsin, the protein content and enzymatic activity increased markedly and peaked at the midgestational period in pregnant uteri. Neuropsin expression and activity was also upregulated in artificially induced deciduomata but not in nondecidualized pseudopregnant uteri. Neuropsin is the first extracellular protease to show the evident induction of expression and activity by decidualization and might contribute to the remodeling of extracellular components after decidualization.

Fluorescence polarization assay and SDS-PAGE confirms matrilysin degrades fibronectin and collagen IV whereas gelatinase A degrades collagen IV but not fibronectin

Matrilysin and gelatinase A are hypothesized to have significant roles in uterine and ovarian function. However, proteolytic activity assays for these enzymes are limited. We describe the development of simple and rapid assays for the proteolysis of fluorescein-labeled full-length substrates, collagen IV (Col-IV) and fibronectin (FN), and demonstrate the selectivity of matrilysin (MMP-7) compared to gelatinase A (MMP-2) for fibronectin. Changes in fluorescence intensity (FIU) and fluorescence polarization (mP) resulting from the protease activity of matrilysin and gelatinase A were measured. These studies show that the fluorescently labeled substrates, Col-IV and FN, are as reliable and amenable to rapid in vitro assay as peptide substrates. In addition, they are easier to use than previously described, non-fluorescent methods. The results demonstrate that assays using full-length, biological matrix proteins are more sensitive indicators of MMP-specific substrate activity than peptide based assays.

Activation of nuclear factor kappaB and induction of apoptosis by tumor necrosis factor-alpha in the mouse uterine epithelial WEG-1 cell line

In order to better understand how tumor necrosis factor (TNF)-alpha may contribute to the local regulation of uterine cell death, cultures of mouse uterine epithelial WEG-1 cells were exposed to TNF-alpha and observed at different time intervals. Earliest decrease in cell viability was observed after 31 h of exposure to 50 ng/ml mouse TNF-alpha and was associated with the expression of several markers of apoptosis. Treatment with human TNF-alpha or addition of a neutralizing antibody against TNF-alpha receptor protein 80 to mouse TNF-alpha resulted in attenuated induction of apoptosis, suggesting that coengagement of the two TNF-alpha receptor types is required for maximal impact. Ceramide analogs failed to replicate the effect of TNF-alpha and the stress-activated protein kinase signaling pathway was not activated by the cytokine. Treatment with mouse TNF-alpha resulted in an increase in nuclear factor (NF)kappaB activity that receded after 24 h. The impact of human TNF-alpha on NFkappaB activation was more moderate. Addition of either one of three different inhibitors of NFkappaB (SN50, PDTC, and A771726) to mouse TNF-alpha sensitized WEG-1 cells to the toxicity of the cytokine. Our data suggest that WEG-1 cells initiate their response to TNF-alpha with an increase in NFkappaB activation that may have transiently biased these cells toward cell death resistance.

Integrin trafficking regulates adhesion to fibronectin during differentiation of mouse peri-implantation blastocysts

Trophoblast cells of the peri-implantation blastocyst differentiate from a polarized epithelium, the trophectoderm, into invasive cells having an apical surface occupied by integrins that mediate adhesion to the extracellular matrix. Blastocyst differentiation was assessed during serum-free culture using a fibronectin binding assay with intact mouse blastocysts. Fibronectin binding activity became elevated during a 24-h "window" after approximately 72 h of culture. Blastocyst differentiation was unaffected by transcriptional inhibition with alpha-amanitin, however, exposure of cavitating morulae to the drug significantly delayed the onset of maximal fibronectin-binding activity. Inhibition of de novo protein synthesis with cycloheximide delayed development only when added during the first 24 h of blastocyst culture, indicating that proteins required for adhesion to fibronectin were synthesized at least 24 h before blastocyst differentiation was completed. Since blastocyst differentiation did not appear to be regulated temporally by gene expression, the possible role of protein trafficking was investigated using the inhibitor, brefeldin A. Brefeldin A caused a reversible, dose-dependent decrease in fibronectin-binding activity when added to the culture medium between 48 and 72 h of culture. During the period of brefeldin A sensitivity, alpha 5 beta 1 integrin, a major fibronectin receptor, translocated to the apical surface of trophoblast cells, as determined by immunohistochemistry and confocal microscopy. Mouse blastocysts expressed other integrins that recognize the central cell-binding domain of fibronectin, including the alpha v integrins and alpha llb beta 3, but not alpha4 which recognizes the lllCS site. Trafficking of alpha 5 beta 1, and possibly other integrins, to the apical surface of trophoblast cells appears to be a critical step in the differentiation of the mouse blastocyst to an invasive phenotype.

Localization of connective tissue growth factor during the period of embryo implantation in the mouse

A role for connective tissue growth factor (CTGF) in reproductive function has been suggested from recent studies in the pig. To extend these findings, we have analyzed the immunohistochemical localization of CTGF during the estrous cycle and early pregnancy in mice. During the diestrous and early proestrous stages, CTGF was localized at high levels to both luminal and glandular uterine epithelial cells and at much lower levels in the stroma or myometrium. Epithelial expression of CTGF was considerably reduced at estrus. On Days 1.5-3.5 of pregnancy, CTGF was localized mainly to the uterine epithelial cells, which showed a substantially reduced level of CTGF on Day 4.5. On Days 5.5 and 6.5, CTGF was present at high levels in uterine decidual cells. CTGF was detected in the trophectoderm and inner cell mass of the preimplantation embryo on Day 4.5 and became preferentially localized to embryonic endoderm and mesoderm on Days 5.5-6.5. Multiple mass forms of CTGF (Mr 14 000-38 000) were present in endometrial extracts and uterine luminal flushings. Collectively, these data support a role for CTGF in uterine cell growth, migration, adhesion, and extracellular matrix production during the estrous cycle and early pregnancy, as well as in early development of the embryo.

Common pattern of CD44 isoforms is expressed in morphogenetically active epithelia

CD44 is a widely distributed cell surface glycoprotein that is expressed as many isoforms arising from a single gene by alternative splicing. An important ligand for the widespread CD44 isoform, CD44s (standard form of CD44), is hyaluronan but ligands for the numerous variant isoforms are not as well characterized. Although it has been documented that CD44 is present at critical sites and stages of morphogenesis of several organs, the nature of the isoforms expressed and their precise localization have not been described. In this study we have determined the identity and distribution of CD44 isoforms expressed during development of several embryonic mouse organs by a combination of immunohistochemistry, in situ hybridization, reverse transcription-polymerase chain reaction, and DNA sequencing. As expected from previous studies, numerous CD44 variants are expressed by actively proliferating and invaginating epithelia at sites of epithelial-mesenchymal interaction, e.g., in the developing tooth, nose and hair follicle. Our results show that most prominent amongst these variants at these sites are CD44(v3-v10), CD44(v4-v10), and CD44(v6-v10), as was also found previously in the apical ectodermal ridge of the developing limb (Yu et al. [1996]). The common pattern of expression of these three particular variants in active epithelia implies that they play an important role in morphogenesis. Also very prominent in morphogenetically active epithelia is CD44s; hyaluronan is uniformly absent from these epithelia and often also from associated mesenchyme with which they interact, supporting the previously documented role of CD44s in endocytic removal of hyaluronan.

Expression of multiple CD44 isoforms in the apical ectodermal ridge of the embryonic mouse limb

Previous immunohistochemical studies have shown that CD44 is highly enriched within the apical ectodermal ridge of the developing limb (Wheatley et al. [1993] Development 119: 295-306), but the particular isoforms of CD44 were not identified. We show here that CD44s (standard or "hemopoietic" isoform) and several CD44 variants, especially V3-V10, V4-V10, and V6-V10, are concentrated in the apical ectodermal ridge in the early mouse limb. Since CD44s is a major cell surface receptor for hyaluronan, we compared its localization with that of hyaluronan. In the early limb bud, hyaluronan is distributed throughout the mesoderm but is absent from all regions of the ectoderm. Hyaluronan is especially enriched in the basement membrane separating ectoderm and mesoderm, except beneath the apical ectodermal ridge where it is absent. Since CD44s is a known endocytic receptor for hyaluronan, its presence in ridge ectoderm could lead to degradation of hyaluronan destined for the neighboring region of basement membrane, thus facilitating interaction of the ridge with underlying mesoderm. The CD44 (V3-V10) isoform found in the ridge is expressed elsewhere as a proteoglycan with heparan sulfate chains that bind fibroblast growth factors. Since fibroblast growth factors are present in the ridge and are essential for limb morphogenesis, CD44 (V3-V10) is likely to act as a cofactor or modulator in the growth-promoting action or maintenance of the ridge.

Fibronectin isoform distribution in the mouse. I. The alternatively spliced EIIIB, EIIIA, and V segments show widespread codistribution in the developing mouse embryo

Fibronectins (FNs) are extracellular matrix glycoproteins that are essential for embryonic development. In order to gain clues to possible developmental roles played by the particular isoforms of FN, we used indirect immunofluorescence microscopy to examine and compare the distributions of the alternatively spliced EIIIB, EIIIA, and V segments, as well as the total pool of FNs, in serial sections from mouse embryos. Antibodies to each of these segments produced staining patterns that colocalized during gastrulation (E7.5) and during early morphogenesis of somites and notochord (E9.5). During the period of continuing organogenesis in the latter half of gestation (E10.5 to E16.5), the antibodies generally continued to produce similar staining patterns localized to epithelial basement membranes, stromal connective tissues, blood vessel walls, and muscles. However, as development proceeded, there was a gradual decline in the intensity of staining for the spliced segments relative to the total pool of FN, with a particularly noticeable decline in staining for EIIIB and EIIIA segments in certain glandular organs, including the liver. A specific reduction in expression of these latter two segments was also evident in the uterus and placenta at early timepoints in gestation. However, the most dramatic difference in the expression of the spliced segments occurred in developing hyaline cartilage, which showed a selective reduction in staining for the EIIIA segment that was evident in the axial skeletal precursors by E12.5 and complete throughout the embryo by E15.5. Our findings suggest that the alternatively spliced EIIIB, EIIIA, and V segments are included in the FN that is required for the morphogenesis of "FN dependent" structures, including somites, notochord, and the vasculature. Conversely, these segments would appear to play divergent, and sometimes exclusive, biological roles in specific tissues such as liver, cartilage, and placenta.

Localization and expression of fibronectin during mouse decidualization in vitro: mechanisms of cell:matrix interactions

During implantation, the embryonic trophoblast aggressively invades the uterine stroma. The resulting uterine reaction, decidualization, involves differentiation of new cell morphologies and remodeling of the extracellular matrix. This creates an environment that first permits invasion, then controls this invasion to allow the establishment of the placenta. The production, organization, and cellular interactions with the matrix are thought to underlie decidual functions. We have begun a reductional analysis of the components of the decidual matrix, focusing on extracellular fibronectin (FN). Using decidual cell cultures prepared from day 7 implantation sites, the synthesis, extracellular organization, and details of decidual cell:FN interaction were studied. Employing immunofluorescence, immunoprecipitation, and dot blot analysis, decidualizing cultures showed a constitutive level of FN synthesis and deposition. The differentiating cells organized extracellular FN in patterns similar to that seen in vivo. The predominant, flattened dendritic decidual cells organized FN in long, thin fibrils. Large, rounded decidual cells, limited to the primary decidual zone in vivo, showed FN limited to punctate membrane patches and short, thick fibrils. Using double labeling techniques, FN expression was co-localized with actin microfilament (MF) bundles during the cytoskeletal changes associated with the differentiation of both decidual cell types. The function of MFs in maintaining morphology was demonstrated by cytochalasin B perturbation. Attachment of decidual cells to FN was calcium dependent and gly-arg-gly-asp-ser-pro (GRGDSP) sensitive, with dendritic decidual cells expressing the alpha 5 and beta 1 integrin subunits. This suggests that an integrin system functions to attach decidual MF bundles to extracellular FN. This work shows that during decidual matrix remodeling, constitutive levels of FN are maintained to provide an extracellular framework to stabilize the decidual cytoskeleton and support morphological differentiation of decidual cells.

Molecular interactions between fibronectin and integrins during mouse blastocyst outgrowth

To investigate the mechanism of trophoblast adhesion to fibronectin, we cultured blastocysts in serum-free medium on proteolytic fibronectin fragments containing its major functional domains, and localized fibronectin-binding integrins in outgrowing trophoblast cells by immunofluorescent staining. Outgrowth comparable to that obtained with intact fibronectin was observed using a 120 kD chymotryptic fragment containing the central cell-binding domain (FN-120) and the Arg-Gly-Asp (RGD) recognition sequence. A 40 kD COOH-terminal chymotryptic fragment of fibronectin containing both a heparin-binding region and an alternate (non-RGD) cell-binding site was inactive in supporting trophoblast adhesion. Three synthetic peptides derived from the heparin-binding domain, including the CS1 alternate cell-binding site, were also unable to promote trophoblast cell adhesion. A 75 kD recombinant protein, ProNectin F, containing 13 copies of the cell recognition epitope of fibronectin, Val-Thr-Gly-Arg-Gly-Asp-Ser-Pro-Ala-Ser, vigorously supported blastocyst outgrowth. Blastocyst outgrowth was not significantly different when surfaces were precoated with cellular fibronectin, which contains an alternatively spliced type III repeat and is the form actually encountered in vivo. Several putative fibronectin receptors were localized in trophoblast outgrowths by immunofluorescent labeling. Antibodies reactive with integrin subunits alpha 3, alpha 5, alpha IIb, alpha v, beta 1 and beta 3, but not alpha 4, all bound to trophoblast cells. Antibodies raised against either the beta 1 or beta 3 integrin subunits significantly inhibited fibronectin-mediated outgrowth. These findings demonstrate the key role of the central cell-binding domain of fibronectin in trophoblast adhesion, and suggest four RGD-binding integrins, alpha 3 beta 1, alpha 5 beta 1, alpha IIb beta 3, and alpha v beta 3, that could mediate trophoblast adhesion in vitro and may play an important role during implantation.

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.

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.