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. Author manuscript; available in PMC: 2011 Apr 1.
Published in final edited form as: Arterioscler Thromb Vasc Biol. 2011 Jan 13;31(4):821–826. doi: 10.1161/ATVBAHA.110.220293

Murine hematopoietic cell tissue factor pathway inhibitor limits thrombus growth

Susan A Maroney 1, Brian C Cooley 2, Josephine P Ferrel 1, Catherine E Bonesho 1, Alan E Mast 1,3
PMCID: PMC3060281  NIHMSID: NIHMS268370  PMID: 21233452

Abstract

Objective

Tissue factor-factor VIIa (TF-fVIIa) initiates blood coagulation and is found on microparticles that accumulate within intravascular thrombi. Tissue factor pathway inhibitor (TFPI), a fXa-dependent inhibitor of TF-fVIIa, is produced by megakaryocytes and is present in platelets. We sought to determine the role of platelet TFPI in regulation of thrombus growth.

Methods and Results

Western blot analyses demonstrated that murine platelets produce TFPIα, the most evolutionarily conserved alternatively spliced isoform of TFPI. A mouse model of hematopoietic cell TFPI deficiency was developed by transplanting irradiated TFPI+/− mice with TFPI−/− fetal liver cells. Platelets from transplanted mice totally lack TFPI inhibitory activity. An electrolytic vascular injury model was used to assess thrombus growth in the femoral vein and carotid artery. Mice lacking hematopoietic TFPI developed larger femoral vein and carotid artery thrombi than TFPI+/− mice transplanted with TFPI+/+ hematopoietic cells, as evidenced by increased platelet accumulation.

Conclusions

Hematopoietic TFPI limits thrombus growth following vascular injury. Since platelets are the primary hematopoietic cell accumulating within a growing thrombus, these findings suggest that TFPI present within platelets functions to limit intravascular thrombus growth, likely through inhibition of the procoagulant activity of blood borne TF.

Keywords: Tissue factor pathway inhibitor, tissue factor, platelet, thrombosis, vascular injury

Tissue factor (TF) is the primary protein that initiates blood coagulation in vivo. It binds to factor VIIa (fVIIa) forming a catalytic complex that activates factors IX and X (fIX and fX) that lead to thrombin generation and the formation of fibrin. Under physiological conditions, TF is localized to extravascular cells and hemostasis occurs when it is exposed to flowing blood following traumatic vascular injury 1. Under pathophysiological conditions, such as inflammation, cells within the vasculature can express TF resulting in thrombotic disease 2–4. For example, TF released on microparticles from activated leukocytes can integrate with activated platelets and contribute to development of intravascular thrombosis 5–7. The potential detrimental effects of intravascular TF are dampened by tissue factor pathway inhibitor (TFPI), an anticoagulant protein primarily produced by microvascular endothelial cells and megakaryocytes 8–11.

Three alternatively spliced isoforms of TFPI--TFPIα, TFPIβ and TFPIγ--have been identified that differ in their C-terminal domain structure and mechanism for association with cell surfaces 12. Alternatively spliced isoforms of TFPI are produced in a tissue-specific manner in mice. All mouse tissues produce mRNA for all three isoforms, but in the vascular beds of adult tissues only the TFPIβ mRNA is found to be translated into protein, while placenta and embryonic tissues produce TFPIα protein 13. We demonstrate that adult mouse platelets produce exclusively TFPIα protein, as previously shown for human platelets 11. The conserved expression of TFPIα in adult mouse platelets that occurs when all other tissues switch to production of TFPIβ suggests that platelet TFPIα may have a physiologically relevant role in limiting intravascular TF activity that is not performed by endothelial TFPI.

Novotny and coworkers demonstrated that blood samples obtained from the site of a skin wound have progressively increasing concentration (two- to threefold) of TFPI.10 They hypothesized that this increase is due to TFPI released from platelets that acts to limit TF and fXa activity at the site of a tissue wound. Additionally, since platelets accumulate at the site of vascular injury, platelet TFPI is optimally localized to inhibit blood-borne forms of TF that incorporate within a growing intravascular thrombus. A mouse model of hematopoietic cell TFPI deficiency was developed to investigate the ability of platelet TFPI to locally modulate thrombus formation. Data obtained from this model demonstrate that platelet TFPI significantly moderates late-stage thrombus growth following vascular injury.

Methods

Western Blot Analysis

Mouse heart tissue and platelets were examined for TFPI isoform expression by western blot analysis following precipitation with fXa and deglycosylation as previously described 13.

Generation of a murine model of platelet TFPI deficiency

All animal experiments were approved by the Medical College of Wisconsin Institutional Animal Care and Use Committee. TFPI heterozygous C57Bl/6J knockout mice (3) were a gift from Dr. George Broze Jr. Fetal livers from TFPI+/+ and TFPI−/− embryos were harvested at E14.5, processed into single cell suspensions and cryopreserved in 90% FBS, 10% DMSO containing 67 nM trehalose. C57/BL6 TFPI+/− recipient male mice 8–12 weeks old were lethally irradiated with 1100 cGy using a Gamma Cell 40 Exactor (Best Theratronic Ltd., Ottawa, Canada), anesthetized using 120 mg/kg ketamine and 15 mg/kg xylazine intraperitoneally and transplanted with TFPI+/+ or TFPI−/− fetal liver cells via retro orbital injection. Genomic DNA from peripheral blood was used to genotype transplanted mice.

Platelet TFPI activity assays

Platelets were isolated from transplanted mice using blood collected from the inferior vena cava. Blood was diluted 2-fold with Buffered Saline Glucose Citrate (129 mM NaCl, 13.6 mM sodium citrate, 11.1 mM glucose, 1.6mM KH2PO4, 8.6 mM NaH2PO4, pH 6.5) and centrifuged at 130g for 10 minutes to obtain platelet rich plasma. Platelets were pelleted at 700g for 10 minutes, re-suspended in BSGC buffer, and counted using an Animal Blood Counter (SCIL Animal Care Co., Gurnee, IL). Following multiple freeze-thaw cycles, platelet lysates were prepared and used at a final concentration of 9 × 106 platelets per well to analyze for TFPI activity using factor Xa generation assays.11 In some experiments lysates were pre-incubated with polyclonal anti-mouse TFPI antibody (a gift from Dr. Robert Simari) for one hour at 23°C.

Mouse CBC analysis

Blood was collected from the inferior vena cava of mice 8 weeks post-transplant in 10% (v/v) 0.13M sodium citrate. CBC analyses were performed on whole blood using the Animal Blood Counter.

Mouse TFPI ELISA assay

MAXISORP CS80 96-well plates (Thermo Fisher Scientific Inc, Waltham, MA) were coated with bovine fXa (10ug/ml) in PBS (150 mM NaCl, 10 mM NaH2PO4, pH 7.5) overnight at 4°C. After washing three times with PBS containing 0.05% Tween-20 (also used in subsequent washes), plates were blocked with 0.05% casein at 37°C for 1 hour. Plasma samples were diluted 1:10 in PBS and incubated at 37°C for 1.5 hours. Plates were washed and polyclonal rabbit anti-mouse TFPI was incubated at room temperature for 1.5 hrs. The plates were again washed, then, blocked with casein containing 0.025% normal goat serum at room temperature for 30 min. The blocking buffer was discarded and goat anti-rabbit Alexa Fluor 488 (Invitrogen, Eugene, OR) incubated at room temperature for 30 min. The plate was washed and fluorescence measured using a VICTOR2™ V analyzer (Perkin Elmer, Waltham, MA).

TAT assay

Plasma TAT was determined using an ELISA assay according to the manufacturer’s instructions (Siemens, Deerfield, IL).

Venous and arterial injury models

In vivo platelet function was evaluated in fetal liver recipient mice at 8–12 weeks post-transplant. Anesthesia was induced using intraperitoneal sodium pentobarbital (50 mg/kg). The thrombus induction models and fluorescence imaging system are described elsewhere (Cooley, submitted for publication). Briefly, mice were pre-injected with platelets obtained from donor mice of the same genotype/transplant group and pre-labeled with Vybrant DiD (Invitrogen); monoclonal anti-fibrin antibody (does not bind fibrinogen -- hybridoma cells were a gift from Dr. Marshall Runge) labeled with Alexa Fluor-532 (Invitrogen) was coinjected. Electrolytic injuries were created in femoral veins and carotid arteries using a steel microsurgical needle applied to the outer surface of each vessel, with positive direct current (1.5 volts for veins, 3.0 volts for arteries) delivered for 30 seconds. Vessels were illuminated uniformly with beam-expanded green (532 nm) and red (650 nm) laser light. Fluorescent images were captured over a 60-minute interval with a low-light video camera attached to an operating microscope at 100X magnification; video images were taken every two minutes for analysis of relative fluorophore intensity (ImageJ software) within the thrombus zone and normalized for inter-animal comparisons (Cooley, submitted for publication).

Statistics

For plasma assays, Student’s t-test and ANOVA analyses were performed using GraphPad Prism software (San Diego, CA). For the vascular injury model, 10, 20, 30, and 60 minutes were selected as representative of the growth and stabilization of the clots for use in statistical analysis. Quantitative data were analyzed with ANOVA, using the posthoc Student LSD test for comparisons between platelet genotypes at each time point. A p-value <0.05 was used to assign statistical significance.

Results

Murine platelets make TFPIα

Platelet TFPI isoform production was examined using western blot analysis following protein deglycosylation since glycosylated TFPIα and TFPIβ migrate at the same molecular weight 13, 14. Deglycosylated mouse platelet TFPI migrates as a single band at the molecular weight of TFPIα (Figure 1). In a simultaneously run control reaction to assess the activity of the deglycosylation enzymes, mouse heart TFPI migrated at the molecular weight of TFPIβ demonstrating that the deglycosylation reactions were effective. Thus, the slower migration of deglycosylated platelet TFPI is due to its larger size (that of TFPIα) and not to incomplete deglycosylation of another TFPI isoform.

Figure 1. Murine platelets make TFPIα.

Figure 1

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Western blot analysis of murine heart and platelet TFPI was performed following tissue lysis, fXa precipitation and deglycosylation. Molecular size markers (kDa) are indicated on the left. The migration patterns of deglycosylated mouse TFPIα and TFPIβ have been previously reported and are as indicated 13. (A) Non-deglycosylated mouse heart TFPI; (B) Deglycosylated mouse heart TFPI; (C) Non-deglycosylated mouse platelet TFPI; (D) Deglycosylated mouse platelet TFPI. Mouse heart TFPI was deglycosylated simultaneously with platelet TFPI as a positive control for enzyme activity. The images are from two exposures of the same western blot. The image of platelet TFPI was exposed longer because of the smaller amount of TFPI in the sample.

Generation of a mouse model of hematopoietic TFPI deficiency

A model of hematopoietic TFPI deficiency was generated to investigate the contribution of circulating blood cell TFPI in the regulation of intravascular TF activity and thrombus formation. Since TFPI null mice die during embryonic development 15, E14.5 fetal livers were used to produce mice lacking functional hematopoietic TFPI. Genotyping of whole blood from the TFPI+/− recipient mice confirmed the presence of either TFPI+/+ or TFPI−/− blood cells and lack of detectable TFPI+/− blood cells (Figure 2A). There were no significant differences in hematocrit, WBC, or platelet count between mice transplanted with TFPI+/+ or TFPI−/− fetal liver cells (Figure 2B). Platelet TFPI activity assays confirmed that the platelets from mice transplanted with TFPI−/− fetal liver cells lacked functional TFPI. A control sample without platelet lysate generated 26 fmole fXa/min. This was reduced to 14 fmole fXa/min in the presence of platelet from wild type platelets. Pre-incubation of the wild type lysates with anti-TFPI antibody increased the rate of fXa generation to 20 fmole/min. In contrast, in the presence of TFPI null platelet lysates fXa generation was 22 fmole/min, a rate that was not altered by the presence of anti-TFPI antibody.

Figure 2.

Figure 2

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A: Analysis of peripheral blood cell DNA demonstrates appropriate TFPI genotype in mice transplanted with TFPI+/+ or TFPI−/− fetal liver cells. (A) Base pair marker, (B) TFPI+/+ control, (C) TFPI+/− control, (D) TFPI−/− control, (E) TFPI+/− mice transplanted with TFPI+/+ fetal liver cells, (F) TFPI+/− mice transplanted with TFPI−/− fetal liver cells.

B: Absence of platelet TFPI does not alter blood counts. Whole blood obtained from the vena cava was analyzed using an automated hematology analyzer. Mice transplanted with liver cells from TFPI+/+ (n = 6) or TFPI−/− (n = 5) fetuses have essentially identical hematocrit (Hct), white blood cell (WBC) count and platelet count.

Loss of hematopoietic TFPI does not significantly alter the plasma TFPI concentration

An ELISA assay that pulls down TFPI with fXa was used to detect functionally active plasma TFPI. We have previously demonstrated that fXa precipitated plasma from TFPI+/− mice contains only full-length TFPI 13 and not both the full-length and the K1 deleted forms that are detected when pulling down with anti-TFPI antibody 15. No significant differences in the plasma TFPI concentration were found between non-transplanted TFPI+/− mice and TFPI+/− mice transplanted with either TFPI+/+ or TFPI−/− fetal liver cells (Figure 3A). Although there is a trend for lower plasma TFPI in mice with TFPI−/− cells, there is not a trend for higher plasma TFPI in mice with TFPI+/+ cells, which is consistent with the hematopoietic cells not contributing to active plasma TFPI.

Figure 3.

Figure 3

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A: Platelet TFPI null mice have normal plasma TFPI concentration. Plasma TFPI concentration was measured using a mouse TFPI ELISA assay. There were no significant differences between non-transplanted TFPI+/− mice (▲) and TFPI+/−mice transplanted with TFPI+/+ (▼) or TFPI−/− (♦) fetal liver cells. The plasma TFPI concentration of wild type mice is shown for comparison (■)

B: Absence of platelet TFPI does not promote a generalized prothrombotic state. Plasma TAT concentration was measured using a commercially available ELISA assay. There are no significant differences in plasma TAT levels of TFPI+/− mice transplanted with TFPI+/+ (▲) or TFPI−/− (▼) fetal liver cells.

Loss of platelet TFPI does not produce a generalized procoagulant state

There were no differences in plasma TAT between mice transplanted with TFPI+/+ or TFPI−/− fetal liver cells indicating that the lack of hematopoietic TFPI does not produce a generalized procoagulant state (Figure 3B).

Hematopoietic TFPI limits thrombus size following vascular injury by reducing platelet accumulation within the growing thrombus

In the venous electrolytic injury model mice with TFPI+/+ or TFPI−/− platelets had essentially identical rates of platelet accumulation during the first 12 minutes following injury (Figures 4A and 4B). Thereafter, platelet accumulation continued in the mice with TFPI−/− platelets, while it slowed, peaked, and began to decrease at about 14 minutes in the mice with TFPI+/+ platelets. At 30 minutes, the mice transplanted with TFPI−/− platelets had significantly more platelets in the thrombus than the mice with TFPI+/+ platelets (p=0.039). An associated difference in the amount of fibrin was not observed.

Figure 4. Mice with TFPI−/− platelets form enlarged thrombi following venous or arterial electrolytic injury.

Figure 4

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The femoral vein (A and B) and carotid artery (C and D) were studied in an electrolytic vascular injury model. The relative normalized fluorescence intensity of platelets (A and C) and fibrin (B and D) within the thrombi were measured over time. TFPI+/− mice transplanted with TFPI−/− fetal liver cells (○) produced larger thrombi than those transplanted with TFPI+/+ fetal liver cells (□) in both the venous (n = 7 for mice with TFPI+/+ platelets; n = 5 for mice with TFPI−/− platelets) and arterial (n = 5 for mice with TFPI+/+ platelets; n = 7 for mice with TFPI−/− platelets) model systems primarily due to increased numbers of platelets accumulating within the growing blood clot (p=0.039 for the venous model, p=0.024 for the arterial model). There were no significant differences in the amount of fibrin present within the thrombi of the different groups of mice in either the venous or arterial model system.

The arterial injury model produced the same general pattern of thrombus growth observed in the venous model (Figures 4C and 4D). The rate of platelet accumulation was approximately the same for the first 6 minutes following injury before they began to diverge. Total platelet accumulation at 30 minutes was significantly higher in the mice with TFPI−/− platelets (p= 0.024). Again, an associated difference in the amount of fibrin was not observed.

Discussion

Endothelial cells are thought to be the major source of intravascular TFPI8. Yet other cells within the vasculature make TFPI, including platelets and monocytes10, 11, 16. We have demonstrated that mouse platelets produce TFPIα. Thus, in regards to the production of alternatively spliced isoforms of TFPI, platelets are similar to placenta and embryonic tissues that also produce TFPIα, rather than adult tissue vascular beds that produce TFPIβ 13. The third Kunitz domain and basic C-terminal region present in TFPIα, which are lacking in TFPIβ, have been evolutionarily conserved from fish to primates, over 430Myr 13, suggesting that platelet TFPI may have a physiological role in limiting thrombus formation following vascular injury.

A mouse model of TFPI deficiency was developed by transplanting fetal liver cells from TFPI+/+ or TFPI−/− embryos into lethally irradiated adult TFPI+/− recipients. The TFPI+/+ and TFPI−/− recipient mice had comparable hematocrit, WBC count and platelet count. TFPI activity assays performed using washed platelet lysates demonstrated the absence of functional TFPI in the mice transplanted with TFPI−/− fetal liver cells, definitively confirming successful generation of mice lacking platelet TFPI.

TFPI+/− mice were chosen as transplant recipients because they have one-half the plasma TFPI concentration of wild type mice and their use limits confounding effects of plasma TFPI on thrombus growth in the vascular injury models while maximizing the effects of platelet TFPI. There was no difference in the active plasma TFPI concentration in the mice transplanted with TFPI+/+ or TFPI−/− fetal liver cells demonstrating that hematopoietic cells do not significantly contribute to the active TFPI in mouse plasma. This finding is consistent with mouse plasma containing predominantly TFPIβ with little to no TFPIα13.

Plasma TAT levels were not elevated in mice transplanted with TFPI−/− fetal liver cells demonstrating that lack of hematopoietic TFPI does not produce a procoagulant state. This finding is consistent with the cell surface expression and release of TFPI from only highly activated platelets11 and suggests that hematopoietic TFPI acts locally at the site of vascular injury rather than as a systemic anticoagulant.

Platelets accumulate at the site of microvascular injury in three distinct phases 4. In the first phase there is a net accumulation of platelets within the thrombus. During the second phase there is a net loss of platelets. During the third phase the platelets stabilize at a plateau. Formation of a fibrin mesh network is necessary for stabilization of the platelet thrombus 17. TF present on microparticles released from activated leukocytes incorporates into the growing blood clot via interactions between P-selectin expressed on the surface of activated platelets and P-selectin glycoprotein ligand-1 present on the microparticles 7, 18 The importance of blood-borne TF in thrombus development has been demonstrated in a ferric chloride vascular injury model performed in mice with hemophilia A where microparticle-associated TF significantly improved the kinetics of clot formation 19.

The effect of hematopoietic TFPI on thrombus growth was assessed in models of large vessel arterial and venous injury in the platelet TFPI null mice, using models similar to previously published models20, 21. Both vessel types demonstrated similar patterns of thrombus growth. The initial phase of platelet accumulation within the thrombus was not affected by the presence of TFPI+/+ or TFPI−/− platelets. However, in mice with TFPI−/− platelets the platelet accumulation phase lasted longer, generating a larger thrombus when compared to mice with TFPI+/+ platelets. This pattern of thrombus growth is consistent with platelet TFPI altering the kinetics of clot formation by limiting platelet accumulation and the thrombus stabilization that occurs as a result of blood-borne TF accumulating within the developing thrombus 2, 6, 7. Inhibition of the later stages of thrombus formation by platelet TFPI suggests that it may act to limit thrombus propagation and prevent clinical diseases, such as myocardial infarction and deep venous thrombosis, which are a result of total vessel occlusion.

Fibrin deposition within thrombi produced in both the venous and arterial injury models was essentially identical in both groups of mice. This is a somewhat unexpected result because a rational hypothesis would be that reduced inhibition of the blood-borne TF as it incorporates into a growing clot would result in a measurable increase in the amount of fibrin within the clot. However, studies performed using a laser-induced arteriole injury model have demonstrated that in wild type mice most of the fibrin deposition in the thrombi occurs near the vessel wall/platelet thrombus interface, with lesser amounts further from the vessel wall 22. Furthermore, it has been demonstrated that thrombin cleavage of platelet PAR4 is necessary for propagation of the platelet thrombus at a distance from the vessel wall 22. Interpretation of the data presented here in the context of these previously published results suggests that blood-borne TF that incorporates within a growing thrombus at a distance from the vessel wall may function primarily to generate thrombin that promotes platelet activation through PAR4 cleavage rather than generating additional fibrin within the clot.

Since there are no differences in the active TFPI concentration in the two groups of mice, plasma TFPI is not responsible for the differences in thrombus size observed. TFPI has been identified on the surface of human monocytes 16, a leukocyte that would be expected to be altered by fetal liver transplantation. Monocytes lacking TFPI are unlikely to contribute to the pro-coagulant phenotype observed because they account for only 1.4% of the WBC in C57Bl/6J male mice (Mouse Phenome Database, http://Jax.org), and do not initially accumulate at sites of vascular injury. It is possible that increased procoagulant activity on microparticles derived from monocytes lacking TFPI may contribute to the increased thrombus size observed in the mice transplanted with TFPI null fetal liver cells. However, mice lacking monocyte TFPI do not produce increased clot volume following arterial ferric chloride injury23. Thus, platelets that are directly accumulating at the site of vascular injury are likely the predominant source of TFPI contributing to the difference in thrombus growth observed in the vascular injury models.

Mouse and human platelets both produce TFPIα, and similar to human platelets11, mouse platelets express 10- to 20-fold more TFPI on their surface following dual activation with thrombin and calcium ionophore than with either agonist alone (S.A Maroney and A.E. Mast, unpublished data). Nevertheless, interpretation of the data presented here in the context of human disease is somewhat limited because the predominant plasma isoforms of TFPI are different between mice (TFPIβ) and humans (TFPIα) 13. In addition, humans have a large pool of heparin-releasable TFPIα that is not present in mice 24, 25. Since TFPIα and TFPIβ may have different anticoagulant potency when circulating in plasma, it is not clear whether or not the human plasma pool of TFPI would compensate for a lack of platelet TFPI to a greater degree than is observed in the mouse model presented here. However, the localization of platelets at the site of vascular injury suggests that platelet TFPI is an important anticoagulant protein that acts to regulate thrombus growth independently of the presence of plasma or endothelium pools of TFPI.

Acknowledgements

This work was funded by grants HL068835 to AEM, HL091469 to SAM and EB007582 to BCC.

Footnotes

Disclosure of Conflict of Interests

A.E.M. is the recipient of a research grant from Novo Nordisk, Inc. The other authors have no conflicting financial interests.

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