Osteopontin expression in uterine stroma indicates a decidualization-like differentiation during ovine pregnancy

Cytokine regulation during the formation of the fetal-maternal interface: focus on cell-cell adhesion and remodelling of the extra-cellular matrix

The establishment of human pregnancy requires the orchestration of substantial cell differentiation and tissue remodelling processes in the context of a complex dialogue between the receptive endometrium and the implanting blastocyst, and is therefore dependent upon a complex sequence of signalling events. Cytokines play an important role in each step of implantation, modulating expression of adhesion molecules on both the fetal and maternal surfaces, regulating expression of the proteases that remodel the extra-cellular matrix, and promoting invasion and differentiation of trophoblasts. Here we review the role of cytokines in regulating the establishment of the fetal-maternal interface, with a particular focus on regulation of the functional expression of CAMs, the ECM and of the proteinases that modulate their function.

Enhanced focal adhesion assembly reflects increased mechanosensation and mechanotransduction at maternal–conceptus interface and uterine wall during ovine pregnancy

The integrity of the fetal–maternal interface is critical for proper fetal nourishment during pregnancy. Integrins are important adhesion molecules present at the interface during implantation; however,in vivoevidence for integrin activation and focal adhesion formation at the maternal–conceptus interface is limited. We hypothesized that focal adhesion assembly in uterine luminal epithelium (LE) and conceptus trophectoderm (Tr) results from integrin binding of extracellular matrix (ECM) at this interface to provide increased tensile forces and signaling to coordinate utero-placental development. An ovine model of unilateral pregnancy was used to evaluate mechanotransduction events leading to focal adhesion assembly at the maternal–conceptus interface and within the uterine wall. Animals were hysterectomized on days 40, 80, or 120 of pregnancy, and uteri immunostained for integrins (ITGAV, ITGA4, ITGA5, ITGB1, ITGB3, and ITGB5), ECM proteins (SPP1, LGALS15, fibronectin (FN), and vitronectin (VTN)), cytoskeletal molecules (ACTN and TLN1), and a signal generator (PTK2). Focal adhesion assembly in myometrium and stroma was also studied to provide a frame of reference for mechanical stretch of the uterine wall. Large focal adhesions containing aggregates of ITGAV, ITGA4, ITGA5, ITGB1, ITGB5, ACTN, and PTK2 were detected in interplacentomal uterine LE and Tr of gravid but not non-gravid uterine horns and increased during pregnancy. SPP1 and LGALS15, but not FN or VTN, were present along LE and Tr interfaces in both uterine horns. These data support the idea that focal adhesion assembly at the maternal–conceptus interface reflects adaptation to increasing forces caused by the growing fetus. Cooperative binding of multiple integrins to SPP1 deposited at the maternal–conceptus interface forms an adhesive mosaic to maintain a tight connection between uterine and placental surfaces along regions of epitheliochorial placentation in sheep.

Stanniocalcin 1 is a luminal epithelial marker for implantation in pigs regulated by progesterone and estradiol

Stanniocalcin 1 (STC1) is a glycoprotein that decreases calcium and increases phosphate in cells/tissues. This investigation examined endocrine regulation of STC1 in endometria of pigs during the estrous cycle and pregnancy. STC1 mRNA was present exclusively in luminal epithelium (LE) between d 12 and 15 of the estrous cycle, increased between d 12 and d 20, and was not detectable by d 30 of pregnancy. STC1 protein was also detected in uterine flushings. To determine effects of estrogen and progesterone, pigs were ovariectomized and treated with these hormones alone or together. Progesterone, but not estrogen, induced STC1 in LE. Cotreatment with progesterone and estrogen further stimulated STC1 over progesterone alone. To determine effects of pseudopregnancy, nonpregnant gilts were given daily injections of estradiol benzoate from d 11 to d 14. STC1 was not expressed in LE on d 90 of pseudopregnancy, suggesting that the estradiol given to induce pseudopregnancy and/or long-term exposure to progesterone are required for down-regulation of STC1. To determine effects of long-term progesterone, without effects of estradiol, pigs were ovariectomized on d 12, given daily injections of progesterone through d 39, and hysterectomized on d 40 after estrus. STC1 was expressed in LE of progesterone-treated pigs, suggesting that estrogen is involved in down-regulation of STC1. We conclude that STC1 is induced in LE by progesterone and further stimulated by estrogen, and its down-regulation in LE by d 25 likely requires exposure of the progestinized uterus to estrogen. The temporal and cell type-specific expression of STC1 makes this gene a unique marker for implantation in pigs.

Molecular profile of endothelial invasion of three-dimensional collagen matrices: insights into angiogenic sprout induction in wound healing

Sprouting angiogenesis is a multistep process consisting of basement membrane degradation, endothelial cell (EC) activation, proliferation, invasion, lumen formation, and sprout stabilization. Such complexity is consistent with a requirement for orchestration of individual gene expression alongside multiple signaling pathways. To better understand the mechanisms that direct the transformation of adherent ECs on the surface of collagen matrices to develop multicellular invading sprouts, we analyzed differential gene expression with time using a defined in vitro model of EC invasion driven by the combination of sphingosine-1-phosphate, basic FGF, and VEGF. Gene expression changes were confirmed by real-time PCR and Western blot analyses. A cohort of cell adhesion molecule genes involved in adherens junction and cell-extracellular matrix (ECM) interactions were upregulated, whereas a set of genes associated with tight junctions were downregulated. Numerous genes encoding ECM proteins and proteases were induced, indicating that biosynthesis and remodeling of ECM is indispensable for sprouting angiogenesis. Knockdown of a highly upregulated gene, a disintegrin and metalloproteinase with thrombospondin-type repeats-1 (ADAMTS1), decreased invasion responses, confirming a role for ADAMTS1 in mediating EC invasion. Furthermore, differential expression of multiple members of the Wnt and Notch pathways was observed. Functional experiments indicated that inhibition and activation of the Notch signaling pathway stimulated and inhibited EC invasion responses, respectively. This study has enhanced the molecular road map of gene expression changes that occur during endothelial invasion and highlighted the utility of three-dimensional models to study EC morphogenesis.

Progesterone and placentation increase secreted phosphoprotein one (SPP1 or osteopontin) in uterine glands and stroma for histotrophic and hematotrophic support of ovine pregnancy

Secreted phosphoprotein one (SPP1, osteopontin) may regulate conceptus implantation and placentation. We investigated effects of progesterone (P(4)) and the conceptus on expression and localization of SPP1 in the ovine uterus. Steady-state levels of SPP1 mRNA in the endometrium of unilaterally pregnant ewes did not differ significantly between nongravid and gravid horns within their respective days of pregnancy; however, levels did increase as pregnancy progressed. SPP1 mRNA was detectable in the glandular epithelium (GE) of both nongravid and gravid horns via in situ hybridization. SPP1 protein was localized to the apical surface of the luminal epithelium of both nongravid and gravid uterine horns. Gravid horns exhibited extensive stromal SPP1 on Days 40 through 120, whereas SPP1 was markedly lower in the stroma of nongravid uterine horns through Day 80 of pregnancy. By Day 120, stromal expression of SPP1 between nongravid and gravid horns was similar. Long-term P(4) treatment of ovariectomized ewes induced SPP1 in the uterine stroma and GE. A bioactive 45-kDa SPP1 fragment was purified from uterine secretions and promoted ovine trophectoderm cell attachment in vitro. Interestingly, increased stromal cell expression of SPP1 was positively associated with vascularization as assessed by von Willebrand factor staining. Finally, ovine uterine artery endothelial cells produced SPP1 during outgrowth into three-dimensional collagen matrices in an in vitro model system that recapitulates angiogenesis. Collectively, P(4) induces and the conceptus further stimulates SPP1 in uterine GE and stroma, where SPP1 likely influences histotrophic and hematotrophic support of conceptus development.

Osteopontin promotes the development of natural killer cells from hematopoietic stem cells

The detailed mechanisms driving the development of natural killer (NK) cells from hematopoietic stem cells remain to be clearly elucidated. Here, we show that osteopontin (OPN) is a key factor for NK development. OPN-deficient mice evidenced severe impairments of NK development in bone marrow (BM) and spleen in which the NK populations that express CD122 and NK cell receptors were reduced. However, the absence of intrinsic OPN expression did not affect NK development, whereas the absence of OPN in the microenvironment caused a significant reduction in NK population. The expression of OPN was induced by interleukin (IL)-15 in BM stromal cells, and the defect in NK differentiation in IL-15(-/-) hematopoietic precursor cells (HPC) was recovered by addition of recombinant OPN, suggesting that the microenvironmental OPN may be a key factor in IL-15-mediated NK differentiation. In addition, OPN-driven NK maturation was reduced in T-bet-deficient HPC, suggesting that T-bet is required for OPN-mediated NK development. Collectively, these results show that paracrine OPN signaling drives NK-lineage commitment, thus ultimately promoting NK cell development. Disclosure of potential conflicts of interest is found at the end of this article.

Expression of alphaV and beta3 integrin subunits during implantation in pig

Integrin are adhesion molecules involved in uterine-conceptus interactions during the perimplantation period. In this study, the expression of alphaV and beta3 integrin subunits in endometrium during implantation in pigs was investigated. The immunohistochemical location was performed on paraformaldehyde-fixed, paraffin-embedded tissue sections, and the mRNA expression of alphaV was detected in endometrium. In addition, serum levels of estradiol, progesterone, follicle-stimulating hormone, and luteinizing hormone were measured on Days 0, 12, 18, and 25 of pregnancy. The results indicate that endometrium expressed integrin alphaV and beta3 in all stages examined. The most intensive staining for integrin alphaV and beta3 was observed in endometrial stroma in porcine pregnancy on Day 18. The mRNA of alphaV integrin strongly expressed on Day 18, and moderately expressed on Days 12 and 25. The correlation between serum hormone level and the mRNA expression of alphaV integrin was not significant. The expression patterns of integrin alphaV and beta3 during implantation provide insights into the important physiological function of alphaVbeta3 integrin in pig, and the strong expression of integrin alphaV and beta3 in mid-implantation may indicate its crucial role in successful implantation and embryo survival.

Gene expression profiling of the rat endometriosis model

PROBLEM

To investigate the molecular mechanism of endometriosis, gene expression profiling was analyzed in a rat endometriosis model.

METHOD OF STUDY

An endometriosis model was induced by uterine autotransplantation in the peritoneal cavity on a female-SD rat (8 weeks old). As control samples, the normal uterine tissues were used. The gene expression was compared between endometriotic lesions and normal uterine tissues by cDNA microarray analysis, quantitative real time RT-PCR and immunohistochemistry.

RESULTS

The expression of 71 genes was upregulated and that of 45 genes was downregulated in the endometriotic lesions compared to normal uterine tissues. The upregulated genes included genes encoding cytokines, chemokines, growth factors and cell adhesion molecules. The levels of transcripts of osteopontin, Lyn, Vav1, Runx1, and l-selectin in the endometriotic lesions were 130, 10, 10, 12 and 46-fold higher than the respective levels in the eutopic endometrial samples.

CONCLUSION

The results suggest that osteopontin, Lyn, Vav1, Runx1, and l-selectin play important roles in the pathogenesis of endometriosis.

Pig conceptuses secrete estrogen and interferons to differentially regulate uterine STAT1 in a temporal and cell type-specific manner

Conceptus trophectoderm and uterine luminal epithelial cells interact via endocrine, paracrine, and autocrine modulators to mediate pregnancy recognition and implantation. Pig conceptuses not only release estrogens for pregnancy recognition but also secrete interferons during implantation. Because interferon-stimulated genes are increased by interferons secreted for pregnancy recognition in ruminants, we asked whether the interferon-stimulated gene, STAT1, is up-regulated in pig endometrium by conceptus estrogens and/or interferons. STAT1 expression in response to day of pregnancy, estrogen injection, and intrauterine infusion of conceptus secretory proteins in pigs indicated 1) estrogen increases STAT1 in luminal epithelial cells, 2) conceptus secretory proteins that contain interferons increase STAT1 in stroma, 3) STAT1 increases in close proximity to the conceptus, and 4) early estrogen results in conceptus death and no STAT1 in stroma. The interactions of estrogen and interferons to regulate cell-type-specific expression of STAT1 highlight the complex interplay between endometrium and conceptus for pregnancy recognition and implantation.

Pig conceptuses increase uterine interferon-regulatory factor 1 (IRF1), but restrict expression to stroma through estrogen-induced IRF2 in luminal epithelium

Pig conceptuses secrete estrogen for pregnancy recognition, and they secrete interferons (IFNs) gamma and delta during the peri-implantation period. The uterine effects of pig IFNs are not known, although ruminant conceptuses secrete IFN tau for pregnancy recognition, and this increases the expression of IFN-stimulated genes (ISGs) in the endometrium. In sheep, the transcriptional repressor interferon-regulatory factor 2 (IRF2) is expressed in the endometrial luminal epithelium (LE) and appears to restrict IFN tau induction of most ISGs, including IRF1, to the stroma and glands. Interestingly, MX1, which is an ISG in sheep, is also expressed in the endometrial stroma of pregnant pigs. The objective of the present study was to determine if estrogen and/or conceptus secretory proteins (CSPs) that contain IFNs regulate IRF1 and IRF2 in pig endometria. The endometrial levels of IRF1 and IRF2 were low throughout the estrus cycle. After Day 12 of pregnancy, the levels of the classical ISGs, which include IRF1, STAT2, MIC, and B2M, increased in the overall endometrium, with expression of IRF1 and STAT2 being specifically localized to the stroma. IRF2 increased in the LE after Day 12. To determine the effects of estrogen, pigs were treated with 17 beta-estradiol benzoate (E2). To determine the CSP effects, pigs were treated with E2 and implanted with mini-osmotic pumps that delivered control serum proteins (CX) to one ligated uterine horn and CSP to the other horn. Estrogen increased the level of IRF2 in the endometrial LE. The administration of E2 and infusion of CSP increased the level of IRF1 in the stroma. These results suggest that conceptus estrogen induces IRF2 in the LE and limits the induction of IRF1 by conceptus IFNs to the stroma. The cell-specific expression of IRF1 and IRF2 in the pig endometrium highlights the complex and overlapping events that are associated with gene expression during the peri-implantation period, when pregnancy recognition signaling and uterine remodeling for implantation and placentation are necessary for successful pregnancy.

Glycosylation dependent cell adhesion molecule 1-like protein and L-selectin expression in sheep interplacentomal and placentomal endometrium

Glycosylation dependent cell adhesion molecule 1 (GlyCAM-1), a mucin component of sheep histotroph produced by glandular epithelium (GE) during early pregnancy, is hypothesized to function in implantation. However, GlyCAM-1 is present in uterine tissues subsequent to implantation suggesting additional functions of this l-selectin-binding ligand. This study focused on uterine GlyCAM-1 expression during placentome development in sheep. Western blot analysis of day 50 pregnant sheep identified 45, 40, and 25 kDa bands in interplacentomal endometrium, 40 and 25 kDa bands in placentomes, and 80 and 40 kDa bands in chorioallantois. The GlyCAM-1 proteins in interplacentomal regions were comparable to those detected in day 15-19 pregnant sheep, however, the 80 kDa form was unique to chorioallantois, and the absence of the 45 kDa GlyCAM-1 in placentomes indicated differences between interplacentomal and placentomal endometrium. Immunofluorescence identified GlyCAM-1 in lumenal epithelium (LE), stromal fibroblasts, and vascular smooth muscle cells. To better define its cellular distribution, GlyCAM-1 was co-localized with either epithelium-specific cytokeratin, smooth muscle-specific alpha-smooth muscle actin (alpha SMA), or stromal-specific vimentin. In interplacentomal endometrium, GlyCAM-1 co-localized with cytokeratin in LE but not in GE. GlyCAM-1 did not co-localize with alpha SMA, and was localized in the extracellular matrix of vimentin-positive stroma. In placentomes, GlyCAM-1 did not co-localize with cytokeratin, but did co-localize with alpha SMA and vimentin. Thus, in contrast to interplacentomal regions, GlyCAM-1 in placentomes was predominantly localized in vasculature rather than epithelial cells. Further, leukocytes expressing L-selectin were localized to the endothelial surface of GlyCAM-1-expressing vessels within placentomes. These data suggest that GlyCAM-1 assumes distinct functions in compartment-specific regions of the sheep uterus.

Steroid regulation of cell specific secreted phosphoprotein 1 (osteopontin) expression in the pregnant porcine uterus

Secreted phosphoprotein 1 (SPP1, commonly referred to as osteopontin and formerly known as bone sialoprotein 1, early T-lymphocyte activation 1) is an extracellular matrix/adhesion molecule that is upregulated in the pregnant uterus of all mammals examined to date. This study focused on the pig, which has true epitheliochorial placentation and exhibits induction of SPP1 mRNA in luminal epithelium (LE) just before conceptus attachment and in glandular epithelium (GE) after Day 30 of pregnancy. The objective of this study was to determine steroid regulation of SPP1 mRNA and protein in porcine uterine epithelium. To examine the effect of estrogen, cyclic gilts were treated daily (Days 11-14) with 5 mg estradiol benzoate (i.m.) and hysterectomized on Day 15. To evaluate the long-term effect of pseudopregnancy, cyclic gilts were given daily injections (Days 11-15) with steroid as above and hysterectomized on Day 90. In situ hybridization showed high expression of SPP1 mRNA only in LE contiguous with apposing conceptus tissue on Day 15 of pregnancy. In contrast, estrogen injection resulted in moderate but uniform SPP1 mRNA in all LE of Day 15 nonpregnant gilts, with expression maintained through Day 90 of pseudopregnancy. SPP1 mRNA also localized to the GE of Day 90 pseudopregnant gilts, similar to expression in late gestation. Consistent with in situ hybridization results, SPP1 protein localized to the apical surface of LE in all estrogen-treated gilts and in the GE on Day 90 of pseudopregnancy. We conclude that, in pregnant pigs, SPP1 is induced by conceptus estrogen in uterine LE and is regulated in GE in a manner coincident with CL/placental progesterone production.

Caprine uterine and placental osteopontin expression is distinct among epitheliochorial implanting species

Osteopontin (OPN) is the most highly up-regulated extracellular matrix/adhesion molecule in the uterus of humans and domestic animals as it becomes receptive to implantation. Studies in sheep and pigs have shown that OPN is a component of ovine and porcine histotroph characterized by a complex temporal and spatial pattern of uterine and conceptus expression involving immune, epithelial, and stromal cells. It is proposed that these expression events are orchestrated to contribute to conceptus attachment and placentation. However, differences in OPN expression between sheep and pigs have been detected that relate to differences in placentation. Therefore, this study examined OPN expression in the caprine uterus and conceptus to gain insight into mechanisms underlying OPN function(s) during pregnancy through comparative analysis of differences in placentation between pigs, sheep, and goats. Goats were hysterectomized (n = 5/day) on Days 5, 11, 13, 15, 17 or 19 of the estrous cycle, and Days 5, 11, 13, 15, 17, 19 or 25 of pregnancy. Slot-blot hybridization showed increases in endometrial OPN mRNA beginning on Day 17 of the estrous cycle and Day 19 of pregnancy. In situ hybridization localized OPN mRNA to endometrial glandular epithelium (GE), Day 25 myometrium, and cells scattered within the placenta hypothesized to be immune. Immunofluorescence microscopy detected OPN protein on the apical surface of endometrial lumenal epithelium (LE), in GE, and on conceptus (Tr). Western blot analysis detected primarily the native 70-kDa OPN protein in endometrial extracts from the estrous cycle and pregnancy, as well as in uterine flushings from pregnant goats. Co-induction of OPN and alpha-smooth muscle actin, but not desmin proteins, was observed in uterine stroma by Day 25 of pregnancy. OPN in cyclic GE, Day 25 myometrium, and desmin-negative endometrial stroma is unique and reflects subtle differences among superficial implanting species that correlate with the depth of Tr invasion.

Conceptus signals for establishment and maintenance of pregnancy

Establishment and maintenance of pregnancy results from signaling by the conceptus (embryo/fetus and associated extraembryonic membranes) and requires progesterone produced by the corpus luteum (CL). In most mammals, hormones produced by the trophoblast maintain progesterone production by acting directly or indirectly to maintain the CL. In domestic animals (ruminants and pigs), hormones from the trophoblast are antiluteolytic in that they act on the endometrium to prevent uterine release of luteolytic prostaglandin F2 alpha (PGF). In cyclic and pregnant sheep, progesterone negatively autoregulates expression of the progesterone receptor (PR) gene in the endometrial luminal (LE) and superficial glandular epithelium (GE). Available evidence in cyclic sheep indicates that loss of the PR is closely followed by increases in epithelial estrogen receptors (ER) and then oxytocin receptors (OTR), allowing oxytocin to induce uterine release of luteolytic PGF pulses. In pregnant sheep, the conceptus trophoblast produces interferon tau (IFN tau) that acts on the endometrium to inhibit transcription of the ER alpha gene directly and the OTR gene indirectly to abrogate development of the endometrial luteolytic mechanism. Subsequently, sequential, overlapping actions of progesterone, IFN tau, placental lactogen (PL) and growth hormone (GH) comprise a hormonal servomechanism that regulates endometrial gland morphogenesis and terminal differentiated function to maintain pregnancy in sheep. In pigs, the conceptus trophoblast produces estrogen that alters the direction of PGF secretion from an endocrine to exocrine direction, thereby sequestering luteolytic PGF within the uterine lumen. Conceptus estrogen also increases expression of fibroblast growth factor 7 (FGF-7) in the endometrial LE that, in turn, stimulates proliferation and differentiated functions of the trophectoderm, which expresses the FGF-7 receptor. Strategic manipulation of these physiological mechanisms can offer therapeutic schemes to improve uterine capacity, conceptus survival and reproductive health.

Conceptus signals for establishment and maintenance of pregnancy

Establishment and maintenance of pregnancy results from signaling by the conceptus (embryo/fetus and associated extraembryonic membranes) and requires progesterone produced by the corpus luteum. In most mammals, hormones produced by the trophoblast maintain progesterone production by acting directly or indirectly to maintain the corpus luteum. In domestic animals (ruminants and pigs), hormones from the trophoblast are antiluteolytic in that they act on the endometrium to prevent uterine release of luteolytic prostaglandin F2alpha. In cyclic and pregnant sheep, progesterone negatively autoregulates progesterone receptor gene expression in the endometrial luminal and superficial glandular epithelium. In cyclic sheep, loss of the progesterone receptor is closely followed by increases in epithelial estrogen receptors and then oxytocin receptors, allowing oxytocin to induce uterine release of luteolytic prostaglandin F2alpha pulses. In pregnant sheep, the conceptus trophoblast produces interferon tau that acts on the endometrium to inhibit transcription of the estrogen receptor alpha gene directly and the oxytocin receptor gene indirectly to abrogate development of the endometrial luteolytic mechanism. Subsequently, sequential, overlapping actions of progesterone, interferon tau, placental lactogen, and growth hormone comprise a hormonal servomechanism that regulates endometrial gland morphogenesis and terminal differentiated function to maintain pregnancy in sheep. In pigs, the conceptus trophoblast produces estrogen that alters the direction of prostaglandin F2alpha secretion from an endocrine to exocrine direction, thereby sequestering luteolytic prostaglandin F2alpha within the uterine lumen. Conceptus estrogen also increases expression of fibroblast growth factor 7 in the endometrial lumenal epithelium that, in turn, stimulates proliferation and differentiated functions of the trophectoderm, which expresses the fibroblast growth factor 7 receptor. Strategic manipulation of these physiological mechanisms may improve uterine capacity, conceptus survival, and reproductive health.

Osteopontin: roles in implantation and placentation

Osteopontin (OPN) is an acidic member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family of extracellular matrix proteins/cytokines that undergoes extensive posttranslational modification, including phosphorylation, glycosylation, and cleavage, yielding molecular mass variants ranging in size from 25 to 75 kDa. The result is a versatile protein(s) with multiple functions arising from its role as a mediator of cell-cell and cell-extracellular matrix (ECM) communication that encompass both normal and tumorigenic developmental processes, immunological responses during inflammation and wound healing, and biomineralization. Studies in primates, pigs, sheep, and rodents have revealed that OPN is a major constituent of the uterine-placental microenvironment with influence as 1) a component of histotroph required for adhesion and signal transduction at the uterine-placental interface throughout pregnancy, 2) a gene product expressed by uterine stroma contributing to a decidualization-like transformation that correlates with the degree of conceptus invasiveness, and 3) a product of resident uterine and placental immune cells that may regulate their behavior and cytokine production. This minireview summarizes information regarding uterine and placental expression of OPN that has accumulated over the past 15 yr, and we briefly describe structural/functional properties of this protein that are likely relevant to its role(s) during pregnancy. Comparative studies have offered insights into the potential hormonal/cytokine, cellular, and molecular mechanisms underlying OPN-mediated adhesion, remodeling, and cell-cell/cell-ECM communication within the uterus and placenta. OPN has the potential to profoundly impact pregnancy, and investigators are now challenged to focus on the mechanistic nature of the functions of this multifaceted and major component of the uterine-placental microenvironment.

Osteopontin is synthesized by uterine glands and a 45-kDa cleavage fragment is localized at the uterine-placental interface throughout ovine pregnancy

Osteopontin (OPN) is a phosphorylated and glycosylated, secreted protein that is present in various epithelial cells and biological fluids. On freezing and thawing or treatment with proteases, the native 70-kDa protein gives rise to 45- and 24-kDa fragments. Secreted OPN functions as an extracellular matrix (ECM) protein that binds cell surface receptors to mediate cell-cell adhesion, cell-ECM communication, and cell migration. In sheep and humans, OPN is proposed to be a secretory product of uterine glandular epithelium (GE) that binds to uterine luminal epithelium (LE) and conceptus trophectoderm to mediate conceptus attachment, which is essential to maintain pregnancy through the peri-implantation period. Cell-cell adhesion, communication, and migration likely are important at the interface between uterus and placenta throughout pregnancy, but to our knowledge, endometrial and/or placental expression of OPN beyond the peri-implantation period has not been documented in sheep. Therefore, the present study determined temporal and spatial alterations in OPN mRNA and protein expression in the ovine uterus between Days 25 and 120 of pregnancy. The OPN mRNA in total ovine endometrium increased 30-fold between Days 40 and 80 of gestation. In situ hybridization and immunofluorescence analyses revealed that the predominant source of OPN mRNA and protein throughout pregnancy was the uterine GE. Interestingly, the 45-kDa form of OPN was detected exclusively, continuously, and abundantly along the apical surface of LE, on conceptus trophectoderm, and along the uterine-placental interface of both interplacentomal and placentomal regions through Day 120 of pregnancy. The 45-kDa OPN is a proteolytic cleavage fragment of the native 70-kDa OPN, and it is the most abundant form in uterine flushes during early pregnancy. The 45-kDa OPN is more stimulatory to cell attachment and cell migration than the native 70-kDa protein. Collectively, the present results support the hypothesis that ovine OPN is a component of histotroph secreted by the uterine GE that accumulates at the uterine-placental interface to influence maternal-fetal interactions throughout gestation in sheep.