Tissue factor is required for uterine hemostasis and maintenance of the placental labyrinth during gestation

Expression of human tissue factor under the control of the mouse tissue factor promoter mediates normal hemostasis in knock-in mice

BACKGROUND

Tissue factor (TF) is expressed widely at the subluminal surface of blood vessels and serves as the primary cellular initiator of the extrinsic pathway of blood coagulation. Lack of TF in mice resulted in lethality in utero, but human TF (huTF) expressed at low levels from a human minigene rescued null mice from prenatal death. Although these low-TF expressing transgenic mice developed to term, they had a significantly shorter life span and exhibited hemorrhage and fibrosis in the heart.

METHODS

Human TF knock-in (TFKI) mice were generated by replacing the first two exons of the mouse (murine) TF (muTF) gene with the huTF complete coding sequence, thus placing it under the control of the endogenous muTF promoter.

RESULTS

Expression of huTF in the TFKI mice was similar to muTF in wild-type (wt) mice. The TFKI mice showed no microscopic evidence of spontaneous hemorrhage in the heart, nor cardiac fibrosis at up to 18 months of age. Immunohistochemistry showed that huTF was expressed in cells surrounding blood vessels in TFKI mice. Coagulation activity of brain homogenates from TFKI mice was comparable with that from wt brain. Cardiac hemorrhage similar to that of the low-TF transgenic mice occurred in the TFKI mice when huTF was blocked by a neutralizing anti-huTF monoclonal antibody.

CONCLUSION

We generated a transgenic mouse line that expresses huTF under the control of the endogenous muTF promoter at physiological levels. Our results suggest that huTF can fully reconstitute the murine coagulation system and mediate normal hemostasis.

Tissue factor-dependent blood coagulation is enhanced following delivery irrespective of the mode of delivery

BACKGROUND

The risk of thrombosis is clearly increased in the postpartum period. Mice with a targeted deletion of the transmembrane domain of tissue factor (TF) develop serious activation of blood coagulation and widespread thrombosis after delivery.

OBJECTIVE AND METHODS

We hypothesized that TF, abundantly present in placental tissue, is released during delivery, resulting in the activation of blood coagulation. We measured sensitive markers for TF-dependent activation of coagulation before and after induction of labor in two groups: a vaginal delivery (VAG) group and a cesarean section (CS) group.

RESULTS

One hour after delivery, soluble TF (sTF) significantly increased in both groups [VAG group (mean +/- SD) 226 +/- 42 to 380 +/- 42 pg mL(-1) and CS group 193 +/- 17 to 355 +/- 44 pg mL(-1)]. The day after delivery, sTF was somewhat less increased. Both groups also showed an increase in factor VIIa, indicating activation of the TF pathway of coagulation. Indeed, after delivery, TF-dependent coagulation, as measured by the TF clotting time assay, was significantly enhanced. Increased plasma levels of prothrombin fragment 1 + 2 and thrombin-antithrombin complexes demonstrated thrombin generation following delivery. TF pathway-dependent activation of coagulation upon delivery was not blocked by TF pathway inhibitor and was not dependent on the mode of delivery.

CONCLUSION

The postdelivery increase in TF-dependent activation of coagulation is likely to be a natural mechanism to prevent excessive blood loss during and after delivery, and may also indicate a novel mechanism by which puerperal women have an increased risk of venous thromboembolism.

Pregnancy-associated changes in the hemostatic system

Pregnancy, from implantation to parturition, presents unique and profound challenges to a women's hemostatic system. During pregnancy, potentially catastrophic bleeding can occur during implantation and endovascular trophoblast invasion of the maternal spiral arteries. The risk of hemorrhage reaches a peak during the third stage of labor when the placenta is shorn from the decidua basalis exposing 120 spiral arteries largely denuded of their smooth muscle, and thus, their ability to constrict in response to injury. These challenges are met by dramatic changes in the local uterine, and systemic hemostatic systems. The net effect of these changes is to increase the efficiency of clotting and to impair fibrinolysis. Unfortunately, they also lead to an increase in the prevalence of venous thromboembolism, which is otherwise uncommon in reproductive age women.

Role of the extrinsic pathway of blood coagulation in hemostasis and thrombosis

Hemostasis requires both platelets and the coagulation system. At sites of vessel injury, bleeding is minimized by the formation of a hemostatic plug consisting of platelets and fibrin. The traditional view of the regulation of blood coagulation is that the initiation phase is triggered by the extrinsic pathway, whereas amplification requires the intrinsic pathway. The extrinsic pathway consists of the transmembrane receptor tissue factor (TF) and plasma factor VII/VIIa (FVII/FVIIa), and the intrinsic pathway consists of plasma FXI, FIX, and FVIII. Under physiological conditions, TF is constitutively expressed by adventitial cells surrounding blood vessels and initiates clotting. In addition so-called blood-borne TF in the form of cell-derived microparticles (MPs) and TF expression within platelets suggests that TF may play a role in the amplification phase of the coagulation cascade. Under pathologic conditions, TF is expressed by monocytes, neutrophils, endothelial cells, and platelets, which results in an elevation of the levels of circulating TF-positive MPs. TF expression within the vasculature likely contributes to thrombosis in a variety of diseases. Understanding how the extrinsic pathway of blood coagulation contributes to hemostasis and thrombosis may lead to the development of safe and effective hemostatic agents and antithrombotic drugs.

Expression of adhesion molecules during normal pregnancy

We review the available information regarding the role of adhesive molecules as potential participants in the complex events of fertilization, embryogenesis, implantation and placentation. Studies that specifically relate to the expression and modulation of adhesive molecules in fertilization, embryogenesis, and implantation have been identified in the literature and by Medline searches. Cell-cell and cell-extracellular matrix interactions play a critical role in various developmental processes and in the cascade of events that lead to implantation and to the normal development of the fetus during pregnancy. Adhesion molecules influence, directly or indirectly, numerous aspects of cell behaviour, cell migration, cell growth, cell survival, cell proliferation, angiogenesis, invasion and metastasis.

A coagulation factor VII deficiency protects against acute inflammatory responses in mice

Upregulation of the activated Factor VII (FVIIa)/Tissue Factor complex, downregulation of natural anticoagulation pathways, and inhibition of fibrinolysis, are major contributors to coagulopathies associated with acute inflammation. Provision of FVIIa, and consequent downstream coagulation-related proteases, also stimulates further inflammatory changes, which can result in disseminated intravascular coagulation. Thus, the potential protective effects in vivo of a genetic-based reduction in FVII levels have been investigated in a murine model of acute inflammation, namely lipopolysaccharide (LPS)-induced lethal endotoxaemia. Mice with a total FVII deficiency do not survive the neonatal period. Therefore mice expressing low levels of FVII (FVII(tTA/tTA)), producing sufficient amounts of FVII for survival (approximately 5% of wild-type (WT) FVII), were employed to investigate in vivo pathways involved in the crosstalk between coagulation, inflammation, and survival, consequent to administration of a lethal dose of LPS. The FVII(tTA/tTA) mice presented with reduced mortality, coagulation, and inflammatory responses in comparison with similarly treated WT mice after administration of LPS. The attenuated inflammatory responses in FVII(tTA/tTA) mice were associated with downregulation of Egr-1 signalling. Administration, in vivo, of specific inhibitors of FXa and thrombin demonstrated that the inflammatory responses were unaltered in WT mice, but further reduced in FVII(tTA/tTA) mice. Therefore, a FVII deficiency enhances survival from lethal endotoxaemia both through attenuation of inflammatory responses that result directly from reduced FVIIa levels, and, indirectly, from downregulation of coagulation proteases downstream of the FVII-dependent cascade.

Role of tissue factor in hemostasis and thrombosis

Tissue factor (TF) is a transmembrane glycoprotein that functions as the primary cellular initiator of blood coagulation. Perivascular cells express TF and provide a hemostatic barrier to limit hemorrhage after vessel injury. In addition, TF is expressed in a tissue-specific manner with high levels in vital organs, such as the heart and lung. TF expression in these tissues may provide additional hemostatic protection from mechanical injury to blood vessels. Recent studies have also detected TF in the blood. This circulating TF is present in the form of microparticles (MPs), which are membrane vesicles shed from cells, and possibly platelets. At present, the cell types that contribute to this pool of TF-positive MPs have not been fully defined. Monocytes, endothelial cells and platelets are the most likely sources of this circulating TF. However, TF-positive MPs represent only a minor subset of circulating MPs. Importantly, TF-negative MPs also possess procoagulant activity. In various diseases, such as sepsis and cancer, TF is expressed by vascular cells and this leads to thrombosis. Levels of circulating TF are also elevated in these diseases and may contribute to thrombosis. Recent studies have analyzed the role of TF-positive MPs in thrombus propagation using different in vivo models. Circulating TF was found to contribute to thrombosis in some models but not others. Inhibition of TF activity in patients with TF expression in vascular cells and with elevated levels of circulating TF may decrease thrombosis associated with a variety of diseases.

Fetomaternal cross talk in the placental vascular bed: control of coagulation by trophoblast cells

Humans and rodents exhibit a peculiar type of placentation in which zygote-derived trophoblast cells, rather than endothelial cells, line the terminal maternal vascular space. This peculiar aspect of the placental vasculature raises important questions about the relative contribution of fetal and maternal factors in the local control of hemostasis in the placenta and how these might determine the phenotypic expression of thrombophilia-associated complications of pregnancy. Using genomewide expression analysis, we identify a panel of genes that determine the ability of fetal trophoblast cells to regulate hemostasis at the fetomaternal interface. We show that spontaneous differentiation of trophoblast stem cells is associated with the acquisition of an endothelial cell-like thromboregulatory gene expression program. This program is developmentally regulated and conserved between mice and humans. We further show that trophoblast cells sense, via the expression of protease activated receptors, the presence of activated coagulation factors. Engagement of these receptors results in cell-type specific changes in gene expression. Our observations define candidate fetal genes that are potential risk modifiers of maternal thrombophilia-associated pregnancy complications and provide evidence that coagulation activation at the fetomaternal interface can affect trophoblast physiology altering placental function in the absence of frank thrombosis.

c-Yes response to growth factor activation

Transmembrane receptors link the extracellular environment to the internal control elements of the cell. This signaling influences cell division, differentiation, survival, motility, adhesion, spreading and vesicular transport. Central to this signaling is the Src family of nonreceptor tyrosine kinases. The most studied kinase of this nine member family, c-Src, shares a similar structure, as well as a similar expression pattern to that of another Src family protein, c-Yes. Despite high conservation in sequence, molecular studies demonstrate that the functional domains of these kinases can contribute to specificity in signaling. At the cellular level, analysis of tight junction formation also serves as a model to differentiate c-Yes and c-Src signaling. Results suggest that c-Yes promotes formation of the tight junction by phosphorylating occludin, while c-Src signaling downregulates occludin formation in a Raf-1 dependent manner. In addition, pp62c-Yes knockout mice exhibit a specific physiological function phenotype that is distinct from c-src-/- mice. In these studies, c-yes-/- mice exhibit decreased transcytosis of pIgA from the blood to the bile, while c-src-/- mice exhibit deficits in osteoclasts function and bone resorption. Of particular interest in this review are receptor signals that specifically influence the actions of c-Yes. Growth factors that influence many Src family proteins include the PDGF-R, CSF-1 receptor and others. Since these receptors interact with various Src-family kinases, it is predicted that specific signaling is generated by differential recruitment to the cell membrane and/or differentiated interactions with substrates and binding partners. This review provides an overview of c-Yes interactions with specific receptor signaling pathways and how this interaction potentially influences the known physiological roles of c-Yes.

Tissue-specific hemostasis in mice

Blood coagulation is essential to maintain hemostasis in organisms with a vascular network. Formation of a fibrin-rich clot at a site of vessel injury is a highly complex process that is orchestrated by the coagulation protease cascade. This cascade is regulated by 3 major anticoagulant pathways. Removal of a clot is mediated by the fibrinolytic system. Defects in the regulation of clot formation lead to either hemorrhage or thrombosis. Tissue factor, the primary cellular initiator of blood coagulation, is a transmembrane receptor that is expressed in a tissue-specific manner. The 3 major anticoagulants are tissue factor pathway inhibitor, antithrombin, and protein C, the latter requiring a transmembrane receptor called thrombomodulin for its activation. Tissue factor pathway inhibitor and thrombomodulin are expressed by endothelial cells in a tissue-specific manner, whereas antithrombin and protein C circulate in the plasma. Fibrinolysis requires the activation of plasminogen to plasmin, which is mediated by tissue-type plasminogen activator and urokinase-type plasminogen activator. Interestingly, tissue-type plasminogen activator is expressed by a subset of endothelial cells of discrete size and location. These observations, together with the phenotypes of mice that have defects in the procoagulant, anticoagulant, and fibrinolytic pathways, indicate that hemostasis is regulated in a tissue-specific manner.

Tissue factor as a link between wounding and tissue repair

The initial phase of wound repair involves inflammation, induction of tissue factor (TF), formation of a fibrin matrix, and growth of new smooth muscle actin (alpha-SMA)-positive vessels. In diabetes, TF induction in response to cutaneous wounding, which ordinarily precedes increased expression of vascular endothelial growth factor (VEGF) and alpha-SMA transcription, is diminished, though not to a degree causing excessive local bleeding. Enhanced TF expression in wounds of diabetic mice caused by somatic TF gene transfer increased VEGF transcription and translation and, subsequently, enhanced formation of new blood vessels and elevated blood flow. Furthermore, increased levels of TF in wounds of diabetic mice enhanced wound healing; the time to achieve 50% wound closure was reduced from 5.5 days in untreated diabetic mice to 4.1 days in animals undergoing TF gene transfer (this was not statistically different from wound closure in nondiabetic mice). Thus, cutaneous wounds in diabetic mice display a relative deficiency of TF compared with nondiabetic controls, and this contributes to delayed wound repair. These data establish TF expression as an important link between the early inflammatory response to cutaneous wounding and reparative processes.

A balance between tissue factor and tissue factor pathway inhibitor is required for embryonic development and hemostasis in adult mice

Inactivation of the murine tissue factor (TF) gene or tissue factor pathway inhibitor 1 (TFPI) gene results in embryonic lethality, indicating that both are required for embryonic development. We have shown that expression of low levels of TF from a transgene (hTF) rescues TF-null embryos. However, low-TF mice (mTF(-/-)/hTF+) have hemostatic defects in the uterus, placenta, heart, and lung. In this study, we hypothesized that the death of TFPI-/- embryos was due to unregulated TF/FVIIa activity and that the hemostatic defects in low-TF mice were due to insufficient TF expression. Therefore, we attempted to rescue TFPI-/- embryos by reducing TF expression, and to restore hemostasis in low-TF mice by abolishing TFPI expression. Intercrossing TFPI(+/-)/mTF(+/-)/hTF+/- mice generated close to the expected number of TFPI(-/-)/low-TF mice at weaning age from 128 offspring, indicating rescue of TFPI-/- embryos from embryonic lethality. Conversely, a decrease in TFPI levels dose-dependently prolonged the survival of low-TF mice and rescued the hemorrhagic defects in the lung and placenta but not in the heart or uterus. These results indicate that the correct balance between TF and TFPI in different organs is required to maintain hemostasis during embryonic development and in adult mice.

Hematopoietic cell-derived microparticle tissue factor contributes to fibrin formation during thrombus propagation

Tissue factor (TF) is expressed on nonvascular cells and cells within the vessel wall and circulates in blood associated with microparticles. Although blood-borne TF accumulates into the developing thrombus during thrombus formation, the contribution of blood-borne TF and vessel wall TF to thrombin generation in vivo following vessel injury is unknown. To determine the source and role of blood-borne microparticle TF, we studied arterial thrombus formation in a living mouse using intravital microscopy. Platelet, TF, and fibrin accumulation in the developing thrombus was compared in wild-type and low TF mice. Compared to wild-type mice, low TF mice formed very small platelet thrombi lacking TF or fibrin. Wild-type and low TF mice received transplants of bone marrow from wild-type and low TF mice. Arterial thrombi in low TF bone marrow/wild-type chimeric mice had decreased size and decreased TF and fibrin levels. Arterial thrombi in wild-type bone marrow/low TF chimeric mice showed decreased platelet thrombus size but normal TF and fibrin levels. This demonstrates that blood-borne TF associated with hematopoietic cell-derived microparticles contributes to thrombus propagation.

Role of tissue factor in hemostasis, thrombosis, and vascular development

Tissue factor (TF) is best known as the primary cellular initiator of blood coagulation. After vessel injury, the TF:FVIIa complex activates the coagulation protease cascade, which leads to fibrin deposition and activation of platelets. TF deficiency causes embryonic lethality in the mouse and there have been no reports of TF deficiency in humans. These results indicate that TF is essential for life, most likely because of its central role in hemostasis. In addition, aberrant TF expression within the vasculature initiates life-threatening thrombosis in various diseases, such as sepsis, atherosclerosis, and cancer. Finally, recent studies have revealed a nonhemostatic role of TF in the generation of coagulation proteases and subsequent activation of protease activated receptors (PARs) on vascular cells. This TF-dependent signaling contributes to a variety of biological processes, including inflammation, angiogenesis, metastasis, and cell migration. This review focuses on the roles of TF in hemostasis, thrombosis, and vascular development.

Progestin-regulated expression of tissue factor in decidual cells: implications in endometrial hemostasis, menstruation and angiogenesis

Expression of tissue factor (TF), the primary initiator of hemostasis via thrombin formation, is induced during progesterone (P4)-stimulated decidualization of human endometrial stromal cells (HESCs), and remains elevated in decidualized HESCs of luteal and gestational endometrium. In HESC monolayers, progestins elevate TF mRNA and protein levels and estradiol (E2) plus progestin further enhance TF levels for weeks despite no response to E2 alone. This in vitro model mimics the chronic differential ovarian steroid upregulation of TF levels associated with in vivo decidualization. After incubation of HESCs with E2 plus progestin to elevate TF expression, the antiprogestin RU486 completely reversed this upregulation. Thus, progesterone withdrawal transformed decidualization-associated hemostasis of the luteal phase endometrium to the hemorrhagic milieu of menstruation. Transient transfections with TF promoter constructs containing SP and EGR-1 binding sites before and after inactivation by site-directed mutagenesis revealed that Sp1 mediates basal and progestin-enhanced TF transcriptional activity. Progesterone receptor involvement in TF expression was further confirmed since RU486 was a pure antagonist of progestin-enhanced TF mRNA and protein expression, and progestin-enhanced, but not basal, Sp1-mediated transcriptional activity. Enhanced TF mRNA and protein levels in HESCs require co-incubation with progestin and epidermal growth factor receptor (EGFR) agonist indicating that the EGFR mediates progestin-enhanced TF expression. A peak in the primary angiogenic agent, vascular endothelial growth factor (VEGF) in luteal phase endometrium may be indirectly regulated by P4. Neither E2, nor progestin, nor E2 plus progestin affected VEGF expression in glandular epithelial and stromal cells, whereas thrombin enhanced VEGF mRNA and protein levels in decidualized HESCs, but not in the epithelial cells. Transudation of clotting factors to perivascular decidual cell TF in the luteal phase would generate thrombin, enabling it to act as an autocrine enhancer of VEGF in decidualized HESCs. Abnormal uterine bleeding complicates long-term progestin only contraceptive use. After Norplant administration, endometrial VEGF levels are elevated and TF levels are selectively enhanced in decidualized HESCs at bleeding sites. Over-expressed VEGF causes blood vessels to become leaky, increasing clotting factor access to decidualized HESC-expressed TF to promote feed-forward thrombin and VEGF formation. Since thrombin and VEGF induce angiogenesis via separate endothelial cell receptors, they may synergize to elicit aberrant angiogenesis, and ultimately lead to focal bleeding.

The Fgl2/fibroleukin prothrombinase contributes to immunologically mediated thrombosis in experimental and human viral hepatitis

Fibrin deposition and thrombosis within the microvasculature is now appreciated to play a pivotal role in the hepatocellular injury observed in experimental and human viral hepatitis. Importantly, the pathways by which fibrin generation is elicited in viral hepatitis may be mechanistically distinct from the classical pathways of coagulation induced by mechanical trauma or bacterial lipopolysaccharide (LPS). In the setting of murine hepatitis virus strain-3 (MHV-3) infection, a member of the Coronaviridae, activated endothelial cells and macrophages express distinct cell-surface procoagulants, including a novel prothrombinase, Fgl2/fibroleukin, which are important for both the initiation and localization of fibrin deposition. To assess the role of Fgl2/fibroleukin in murine viral hepatitis we generated a Fgl2/fibroleukin-deficient mouse. Peritoneal macrophages isolated from Fgl2/fibroleukin-/- mice did not generate a procoagulant response when infected with MHV-3. Fibrin deposition and liver necrosis were markedly reduced, and survival was increased in mice infected with MHV-3. To address the relevance of Fgl2/fibroleukin in human chronic viral hepatitis we studied patients with minimal and marked chronic hepatitis B. We detected robust expression of Fgl2/fibroleukin mRNA transcripts and protein in liver tissue isolated from patients with marked chronic hepatitis B. Fibrin deposition was strongly associated with Fgl2/fibroleukin expression. Collectively, these data indicate a critical role for Fgl2/fibroleukin in the pathophysiology of experimental and human viral hepatitis.

The Fgl2/fibroleukin prothrombinase contributes to immunologically mediated thrombosis in experimental and human viral hepatitis

Fibrin deposition and thrombosis within the microvasculature is now appreciated to play a pivotal role in the hepatocellular injury observed in experimental and human viral hepatitis. Importantly, the pathways by which fibrin generation is elicited in viral hepatitis may be mechanistically distinct from the classical pathways of coagulation induced by mechanical trauma or bacterial lipopolysaccharide (LPS). In the setting of murine hepatitis virus strain-3 (MHV-3) infection, a member of the Coronaviridae, activated endothelial cells and macrophages express distinct cell-surface procoagulants, including a novel prothrombinase, Fgl2/fibroleukin, which are important for both the initiation and localization of fibrin deposition. To assess the role of Fgl2/fibroleukin in murine viral hepatitis we generated a Fgl2/fibroleukin-deficient mouse. Peritoneal macrophages isolated from Fgl2/fibroleukin-/- mice did not generate a procoagulant response when infected with MHV-3. Fibrin deposition and liver necrosis were markedly reduced, and survival was increased in mice infected with MHV-3. To address the relevance of Fgl2/fibroleukin in human chronic viral hepatitis we studied patients with minimal and marked chronic hepatitis B. We detected robust expression of Fgl2/fibroleukin mRNA transcripts and protein in liver tissue isolated from patients with marked chronic hepatitis B. Fibrin deposition was strongly associated with Fgl2/fibroleukin expression. Collectively, these data indicate a critical role for Fgl2/fibroleukin in the pathophysiology of experimental and human viral hepatitis.

Haemostatic mechanisms in human placenta

The placenta is a unique organ with dual blood circulation functioning throughout fetal development. The architecture and functions of the placenta, where maternal blood flows into the intervillous space, present haemostatic problems, mainly the risk of haemorrhage. Placental trophoblasts express and produce coagulation components, participating not only in haemostasis but also in placental vascular development and differentiation. The expression of tissue factor, membrane phosphatidylserine and fibrin render the trophoblasts pro-coagulant, thus compromising the risk of bleeding while exposing the placenta to pro-thrombotic risks. Local inhibitory mechanisms-TFPI-1 and TFPI-2, thrombomodulin, annexin V and the fibrinolytic system-limit coagulation activation and fibrin deposition. Pregnancy complications have been associated with abnormalities in the functions of these inhibitors. Haemostatic processes in placental cells change throughout gestation and are affected by the changing requirements of the organ.

The thrombomodulin-protein C system is essential for the maintenance of pregnancy

Disruption of the mouse gene encoding the blood coagulation inhibitor thrombomodulin (Thbd) leads to embryonic lethality caused by an unknown defect in the placenta. We show that the abortion of thrombomodulin-deficient embryos is caused by tissue factor-initiated activation of the blood coagulation cascade at the feto-maternal interface. Activated coagulation factors induce cell death and growth inhibition of placental trophoblast cells by two distinct mechanisms. The death of giant trophoblast cells is caused by conversion of the thrombin substrate fibrinogen to fibrin and subsequent formation of fibrin degradation products. In contrast, the growth arrest of trophoblast cells is not mediated by fibrin, but is a likely result of engagement of protease-activated receptors (PAR)-2 and PAR-4 by coagulation factors. These findings show a new function for the thrombomodulin-protein C system in controlling the growth and survival of trophoblast cells in the placenta. This function is essential for the maintenance of pregnancy.

Tissue factor deficiency causes cardiac fibrosis and left ventricular dysfunction

Exposure of blood to tissue factor (TF) activates the extrinsic (TF:FVIIa) and intrinsic (FVIIIa:FIXa) pathways of coagulation. In this study, we found that mice expressing low levels of human TF ( approximately 1% of wild-type levels) in an mTF(-/-) background had significantly shorter lifespans than wild-type mice, in part, because of spontaneous fatal hemorrhages. All low-TF mice exhibited a selective heart defect that consisted of hemosiderin deposition and fibrosis. Direct intracardiac measurement demonstrated a 30% reduction (P < 0.001) in left ventricular function in 8-month-old low-TF mice compared with age-matched wild-type mice. Mice expressing low levels of murine FVII ( approximately 1% of wild-type levels) exhibited a similar pattern of hemosiderin deposition and fibrosis in their hearts. In contrast, FIX(-/-) mice, a model of hemophilia B, had normal hearts. Cardiac fibrosis in low-TF and low-FVII mice appears to be caused by hemorrhage from cardiac vessels due to impaired hemostasis. We propose that TF expression by cardiac myocytes provides a secondary hemostatic barrier to protect the heart from hemorrhage.

Disruption of the endothelial cell protein C receptor gene in mice causes placental thrombosis and early embryonic lethality

The endothelial cell protein C receptor (EPCR) is a type 1 transmembrane protein found primarily on endothelium that binds both protein C and activated protein C with similar affinity. EPCR augments the activation of protein C by the thrombin-thrombomodulin complex. To determine the physiological importance of EPCR, we generated EPCR-deficient mice by homologous targeting in embryonic stem cells. Genotyping of progeny obtained from EPCR(+/-) interbreeding indicated that EPCR(-/-) embryos died on or before embryonic day 10.5 (E10.5). Reverse transcriptase-PCR confirmed the absence of EPCR mRNA in EPCR(-/-) embryos. EPCR(-/-) embryos removed from extra-embryonic membranes and tissues at day E7.5 and cultured in vitro developed beyond E10.5, suggesting a role for EPCR in the normal function of the placenta and/or at the materno-embryonic interface. Immunohistochemistry revealed the lack of EPCR in trophoblast giant cells of EPCR(-/-) embryos. These cells, which normally express EPCR, are in direct contact with the maternal circulation and its clotting factors. In EPCR(-/-) embryos, greatly increased fibrin deposition was detected around these cells. To prevent this fibrin deposition, EPCR(+/-)-crossed female mice received a daily subcutaneous injection of enoxaparin through pregnancy. Although some EPCR(-/-) embryos were rescued from midgestational lethality, this regimen yielded no EPCR(-/-) pups. We conclude that EPCR is essential for normal embryonic development. Moreover, EPCR plays a key role in preventing thrombosis at the maternal-embryonic interface.

Decidual cell-expressed tissue factor maintains hemostasis in human endometrium

We showed that decidualized stromal cells of luteal phase and pregnant human endometrium express tissue factor (TF), the primary initiator of hemostasis, thereby suggesting a mechanism by which perivascular decidual cells can mitigate the risk of hemorrhage during endovascular trophoblast invasion. Progestins enhanced TF mRNA and protein levels in monolayers of human endometrial stromal cells (HESCs), with estradiol (E2) + progestin, further enhancing TF levels despite a lack of response to E2 alone. This differential ovarian steroid response has been found for several decidualization markers. Further studies with cultured HESCs established that elevated TF levels are mediated by the progesterone receptor and are maintained for weeks in response to E2 plus progestin, thus simulating the chronic upregulation of TF levels observed in decidualized HESCs in vivo. Recent studies revealed that elevated TF expression during in vitro decidualization of HESCs involved both the EGFR and progesterone receptor. Thus, enhancement of TF mRNA and protein levels in the HESCs required co-incubation with a progestin (MPA) and an EGFR agonist such as EGF or TGF-alpha. In correspondence with co-elevation of EGFR and TF in decidualized HESCs in sections of luteal phase and pregnant endometrium, EGFR levels proved to be progestin-enhanced in the cultured HESCs. We established that progestin-enhanced TF expression in HESCs was trancriptionally regulated, then evaluated the relative roles of SP and EGR-1 sites on the TF promoter in regulating this expression. Transient transfections with a series of promoter constructs containing overlapping SP and EGR-1 sites and with constructs in which the EGR-1 and SP sites were systematically inactivated by site-directed mutagenesis established the dominance of SP sites in both basal and progestin-enhanced TF transcriptional activity. Additional experiments involving transient transfections with SPloverexpressing vectors and with a specific blocker of if Sp1 binding to its corresponding GC box specified the importance of the Sp1 transcription factor. These results were further validated by immunostaining, which revealed that the ratio of Sp1 to Sp3 increased during progestin-regulated decidualization of HESCs in vitro and in vivo. The absence of canonical estrogen and progesterone response elements from either the TF or Sp1 gene promoters suggests that the EGFR may help to mediate progestin-enhanced TF expression during decidualization of HESCs.

Genetic manipulation of fibrinogen and fibrinolysis in mice

Vascular integrity is maintained by a sophisticated system of circulating and cell associated hemostatic factors that control local platelet deposition, the conversion of soluble fibrinogen to an insoluble fibrin polymer, and the dissolution of fibrin matrices. However, hemostatic factors are likely to be biologically more important than merely maintaining vascular patency and controlling blood loss. Specific hemostatic factors have been associated with a wide spectrum of physiological processes, including development, reproduction, tissue remodeling, wound repair, angiogenesis, and the inflammatory response. Similarly, it has been proposed that hemostatic factors are important determinants of a variety of pathological processes, including vessel wall disease, tumor dissemination, infectious disease, and inflammatory diseases of the joint, lung, and kidney. The development of gene targeted mice either lacking or expressing modified forms of selected hemostatic factors has provided a valuable opportunity to test prevailing hypotheses regarding the biological roles of key coagulation and fibrinolytic system components in vivo. Genetic analyses of fibrin(ogen) and its interacting factors in transgenic mice have proven to be particularly illuminating, often challenging long standing concepts. This review summarizes the key findings made in recent studies of gene targeted mice with single and combined deficits in fibrinogen and fibrinolytic factors. Studies illustrating the role and interplay of these factors in disease progression are highlighted.

Phenotype of the endothelium in the human term placenta

The placental endothelium contributes to regulating transplacental exchange and maintaining the immunological maternofetal barrier. We characterized the endothelial phenotype in human normal term placentae with a panel of antibodies to endothelial antigens using a standardized immunofluorescence method. Placental endothelium strongly expressed vWF, PAL-E, H-antigen, thrombomodulin, PECAM-1, CD34, CD36, ICAM-1, CD44, thy-1, A10/33-1, VE-cadherin, caveolin-1 and HLA-G, whereas occludin, claudin-1, eNOS, angiotensin converting enzyme (ACE), ICAM-2, endoglin and integrin-alphathetabeta(3)were weakly expressed. PGI(2)synthase, tissue factor, E-selectin and VCAM-1 were not detected. Some antigens were heterogenously expressed along the vascular tree or within individual villi. Expression of ACE, eNOS, vWF, P-selectin, E-selectin, integrin alpha(v)beta(3)and endoglin was stronger in the maternal decidual vessels, while PECAM-1, CD44, thy-1 and caveolin-1 expression was stronger in fetal vessels. Some endothelial markers were present in trophoblasts and stroma. Endothelial proliferation was apparent in mature intermediate and terminal villi. There was limited inflammatory response to TNFalpha in explants, characterized by upregulation of vWF, P-selectin, PECAM-1 and CD44, downregulation of thrombomodulin, but no increase in ICAM-1 expression, nor induction of E-selectin, VCAM-1 or tissue factor. These patterns of heterogeneity, proliferative activity and inflammatory activation may underlie the specific physiological roles of the placental endothelium.

Binding of factor VIIa to tissue factor on human fibroblasts leads to activation of phospholipase C and enhanced PDGF-BB–stimulated chemotaxis

Tissue factor (TF) is the cellular receptor for factor FVIIa (FVIIa), and the complex is the principal initiator of blood coagulation. The effects of FVIIa binding to TF on cell migration and signal transduction of human fibroblasts, which express high amounts of TF, were studied. Fibroblasts incubated with FVIIa migrated toward a concentration gradient of PDGF-BB at approximately 100 times lower concentration than do fibroblasts not ligated with FVIIa. Anti-TF antibodies inhibited the increase in chemotaxis induced by FVIIa/TF. Moreover, a pronounced suppression of chemotaxis induced by PDGF-BB was observed with active site-inhibited FVIIa (FFR-FVIIa). The possibility that hyperchemotaxis was induced by a putative generation of FXa and thrombin activity was excluded. FVIIa/TF did not induce increased levels of PDGF β-receptors on the cell surface. Thus, the hyperchemotaxis was not a result of this mechanism. FVIIa induced the production of inositol-1,4,5-trisphosphate to the same extent as PDGF-BB; the effects of FVIIa and PDGF-BB were additive. FFR-FVIIa did not induce any release of inositol-1,4,5,-trisphosphate. Thus, binding of catalytically active FVIIa to TF can, independent of coagulation, modulate cellular responses, such as chemotaxis.

Binding of factor VIIa to tissue factor on human fibroblasts leads to activation of phospholipase C and enhanced PDGF-BB–stimulated chemotaxis

Tissue factor (TF) is the cellular receptor for factor FVIIa (FVIIa), and the complex is the principal initiator of blood coagulation. The effects of FVIIa binding to TF on cell migration and signal transduction of human fibroblasts, which express high amounts of TF, were studied. Fibroblasts incubated with FVIIa migrated toward a concentration gradient of PDGF-BB at approximately 100 times lower concentration than do fibroblasts not ligated with FVIIa. Anti-TF antibodies inhibited the increase in chemotaxis induced by FVIIa/TF. Moreover, a pronounced suppression of chemotaxis induced by PDGF-BB was observed with active site-inhibited FVIIa (FFR-FVIIa). The possibility that hyperchemotaxis was induced by a putative generation of FXa and thrombin activity was excluded. FVIIa/TF did not induce increased levels of PDGF β-receptors on the cell surface. Thus, the hyperchemotaxis was not a result of this mechanism. FVIIa induced the production of inositol-1,4,5-trisphosphate to the same extent as PDGF-BB; the effects of FVIIa and PDGF-BB were additive. FFR-FVIIa did not induce any release of inositol-1,4,5,-trisphosphate. Thus, binding of catalytically active FVIIa to TF can, independent of coagulation, modulate cellular responses, such as chemotaxis.

The retinoblastoma gene family members pRB and p107 coactivate the AP-1-dependent mouse tissue factor promoter in fibroblasts

Serum-stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of blood coagulation. This requires the rapid entry of c-Fos into specific AP-1 DNA-binding complexes and can be strongly inhibited by the adenovirus EIA 12S gene product. In this study, we utilized a panel of E1A mutants deficient in cellular protein binding to analyse the molecular basis for EIA inhibition of a minimal, c-Fos-dependent TF promoter/ reporter construct in mouse AKR-2B fibroblasts. Mutations which impaired binding of the retinoblastoma tumor suppressor protein family members pRB, p107, and p130 relieved E1A-mediated inhibition of transcription in response to serum-stimulation or c-Fos overexpression. Inhibition was restricted to the G0 to G1 transition, consistent with the specificity of E1A for hypophosphorylated forms of RB proteins. Although E1A mutants deficient in CBP/p300 binding retained the ability to inhibit TF transcription, deletion of the amino-terminal portion of the CBP/p300 interaction domain was required to permit rescue of TF promoter activity by coexpression of pRB. Moreover, ectopic p107 could effectively substitute for pRB in relieving E1A-mediated repression. In primary mouse embryo fibroblasts, activity of the minimal AP-1-dependent TF promoter was suppressed in Rb(-/-) cells compared to parallel Rb(+/-) and Rb(+/+) transfectants. Ectopic expression of either pRB or p107 markedly enhanced TF promoter activity in Rb(-/-) fibroblasts. Collectively, these data imply that pRB and p107 can cooperate with c-Fos to activate TF gene transcription in fibroblasts and suggest a requirement for another, as yet unidentified, E1A-binding protein.

Vascular protease receptors: integrating haemostasis and endothelial cell functions

The endothelium plays a crucial dynamic role as a protective interface between blood and the underlying tissues during the haemostatic process, which maintains blood flow in the circulation and prevents life-threatening blood loss. Following vessel wall injury with initial platelet adhesion and aggregation to exposed subendothelial extracellular matrix, the initiation, amplification, and control of haemostasis depend on structurally unrelated membrane-associated receptors for blood coagulation proteases including tissue factor, G-protein-coupled protease-activatable receptors, thrombomodulin, and protein C receptor, respectively. In addition to their regulatory role in haemostasis, the respective (pro-)enzyme ligands such as Factors VIIa and Xa, thrombin or protein C mediate specific signalling pathways in vascular cells related to migration, proliferation or adhesion. The functional importance of these receptors beyond haemostasis has been manifested by various lethal and pathological phenotypes in knock-out mice. These protease receptors thereby provide important molecular links in the vascular system and serve to integrate haemostasis with endothelial cell functions which are relevant for the (patho-)physiological responses to injury or inflammatory challenges.