Tissue factor pathway inhibitor-γ is an active alternatively spliced form of tissue factor pathway inhibitor present in mice but not in humans

Summary.  Background:  Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of tissue factor procoagulant activity produced as two alternatively spliced isoforms, TFPIα and TFPIβ, which differ in domain structure and mechanism for cell surface association. 3' Rapid amplification of cDNA ends was used to search for new TFPI isoforms. TFPIγ, a new alternatively spliced form of TFPI, was identified and characterized. Methods:  The tissue expression, cell surface association and anticoagulant activity of TFPIγ were characterized and compared to those of TFPIα and TFPIβ through studies of mouse and human tissues and expression of recombinant proteins in Chinese hamster ovary (CHO) cells. Results:  TFPIγ is produced by alternative splicing using the same 5'-splice donor site as TFPIβ and a 3'-splice acceptor site 276 nucleotides beyond the stop codon of TFPIβ in exon 8. The resulting protein has the first two Kunitz domains connected to an 18 amino acid C-terminal region specific to TFPIγ. TFPIγ mRNA is differentially produced in mouse tissues but is not encoded within the human TFPI gene. When expressed in CHO cells, TFPIγ is secreted into conditioned media and effectively inhibits tissue factor procoagulant activity. Conclusions:  TFPIγ is a third alternatively spliced form of TFPI that is widely expressed in mouse tissues but not made by human tissues. It contains the first two Kunitz domains and is a secreted, rather than a cell surface-associated, protein. It is a functional anticoagulant and may partially explain the resistance of mice to coagulopathy in tissue factor-mediated models of disease.     

Temporal expression of alternatively spliced forms of tissue factor pathway inhibitor in mice

Summary.  Background: Mouse tissue factor pathway inhibitor (TFPI) is produced in three alternatively spliced isoforms that differ in domain structure and mechanism for cell surface binding. Tissue expression of TFPI isoforms in mice was characterized as an initial step for identification of their physiological functions. Methods and Results: Sequence homology demonstrates that TFPIα existed over 430 Ma while TFPIβ and TFPIγ evolved more recently. In situ hybridization studies of heart and lung did not reveal any cells exclusively expressing a single isoform. Although our previous studies have demonstrated that TFPIα mRNA is more prevalent than TFPIβ or TFPIγ mRNA in mouse tissues, western blot studies demonstrated that TFPIβ is the primary protein isoform produced in adult tissues, while TFPIα is expressed during embryonic development and in placenta. Consistent with TFPIβ as the primary isoform produced within adult vascular beds, the TFPI isoform in mouse plasma migrates like TFPIβ in SDS-PAGE and mice have a much smaller heparin-releasable pool of plasma TFPIα than humans. Conclusions: The data demonstrate that alternatively spliced isoforms of TFPI are temporally expressed in mouse tissues at the level of protein production. TFPIα and TFPIβ are produced in embryonic tissues and in placenta while adult tissues produce almost exclusively TFPIβ.     

A novel anticoagulant activity assay of tissue factor pathway inhibitor I (TFPI).

Tissue factor (TF) pathway inhibitor I (TFPI) is the physiological inhibitor of TF-induced blood coagulation. Circulating blood contains full-length TFPI and TFPI truncated at the C-terminal end. Previous studies have shown that full-length TFPI exerts a stronger anticoagulant effect on diluted prothrombin time (DPT) than truncated TFPI, and it has been suggested that full-length TFPI is biologically more important in vivo. The objective of this study was to develop and validate an assay of TFPI anticoagulant activity. TFPI anticoagulant activity was assayed using a modified DPT assay. Plasmas were incubated in the absence and the presence of TFPI-blocking antibodies. Results were expressed as a ratio with the clotting time in the presence of anti-TFPI as the denominator. The ratio was normalized against a ratio obtained with a reference plasma. The assay was compared with assays of TFPI free antigen, total antigen, and bound TFPI, and TFPI chromogenic substrate activity. We performed all tests in 436 healthy individuals. The normalized TFPI anticoagulant ratio was strongly associated with TFPI free antigen (r = 0.73) but was weakly associated with TFPI chromogenic substrate activity (r = 0.46), TFPI total antigen (r = 0.48), and bound TFPI (r = 0.30). TFPI chromogenic substrate activity was strongly associated with TFPI total antigen (r = 0.73). We have developed a novel assay of TFPI anticoagulant activity in plasma, which may be considered a functional assay of full-length TFPI. Further studies are needed to establish the role of TFPI anticoagulant activity for thrombotic disorders.     

Low tissue factor pathway inhibitor (TFPI) together with low antithrombin allows sufficient thrombin generation in neonates

Summary.  Neonates have an excellent hemostasis despite, in comparison to adults, markedly decreased and delayed ability to generate thrombin. Only 30–50% of peak adult thrombin activity can be produced in neonatal plasma by means of conventional in vitro assays. We show that in contrast to conventional activation, activation with small amounts of lipidated tissue factor (<10 pmol L⁻¹) results in shorter clotting times and faster activated factor X- and thrombin generation in neonates compared with adults due to the concomitant action of low tissue factor pathway inhibitor and antithrombin. The concentrations of both inhibitors in cord plasma are approximately 50% of the respective adult values. After addition of 2.5 pmol L⁻¹ lipidated tissue factor, cord plasma clotted ∼90 s earlier than adult plasma and the amount of free thrombin generated was ∼90% of adult value (291 ± 14 vs. 329 ± 16 nmol L⁻¹ min⁻¹, P  < 0.01). Our results might help to explain the clinically observed excellent hemostasis of neonates despite low levels of procoagulant factors.     

基于通用质粒标准品的实时荧光定量PCR检测组织因子剪接异构体F3tv1及F3tv2

目的建立基于通用质粒标准品的实时荧光定量PCR(qPCR)法检测人组织因子(TF)两种剪接异构体(F3tv1和F3tv2)。方法设计基因特异性上游引物与通用下游引物,通过引物末端延伸法将2种扩增产物进行序列简并,用于构建通用质粒标准品。用qPCR法检测人白血病细胞株THP-1、Jurkat及外周血单核细胞中F3tv1与F3tv2的表达,分析该法的线性范围与特异性。结果所建qPCR方法对F3tv1与F3tv2扩增特异,线性范围均为108～101copies/μL。THP-1与外周血单核细胞中F3tv1相对表达量分别为(5.70×10-4)±(2.62×10-5)与(1.79×10-4)±(4.37×10-5);F3tv2分别为(4.94×10-5)±(2.19×10-6)与(2.98±2.18)×10-6,Jurkat细胞株F3tv1与F3tv2均未检出。结论建立的qPCR方法可对TF两种剪接异构体同时进行精确、定量地检测,为深入研究TF的选择性剪接调控机制提供实验依据。     

Thrombin generation in factor VIII-depleted neonatal plasma: nearly normal because of physiologically low antithrombin and tissue factor pathway inhibitor

BACKGROUND: Bleeding in hemophilic neonates has a low incidence. A possible explanation for this could be the peculiarities of the neonatal hemostatic system, especially low levels of the inhibitors tissue factor pathway inhibitor (TFPI) and antithrombin (AT). OBJECTIVE: We investigated the influence of an elevation of these inhibitors to adult levels on the thrombin generation (TG) in normal neonatal plasma and factor (F) VIII-depleted neonatal plasma by means of incubation with anti-FVIII-antibodies. PATIENTS/METHODS: TG was measured after activation with low amounts of tissue factor (TF) by using Calibrated Automated Thrombography. RESULTS: TG in FVIII-depleted neonatal plasma was nearly as high as in normal neonatal plasma. TG decreased after elevation of AT in both neonatal plasmas. After elevation of TFPI TG decreased much more in FVIII-depleted neonatal plasma than in normal neonatal plasma. After elevation of both inhibitors their synergistic effect led to a stronger decrease of TG in FVIII-depleted neonatal plasma. TG measured in plasma of one hemophilic newborn showed the same pattern as in FVIII-depleted neonatal plasma. CONCLUSION: Our observation provides a biochemical basis for the rare bleeding in hemophilic neonates and shows the important role of the natural inhibitors in the hemostatic system of hemophilic patients.     

Identification and characterization of murine alternatively spliced tissue factor

Tissue factor (TF) is a transmembrane glycoprotein that initiates coagulation and plays a critical role in regulating hemostasis and thrombosis. We have recently reported a naturally occurring, soluble form of human tissue factor (asTF) generated by alternative splicing. This splice variant has a novel C-terminus with no homology to that of the full-length TF (flTF), lacks a transmembrane domain, and is active in the presence of phospholipids. Mouse models offer unique opportunities to examine the relative importance of flTF and asTF in mediating thrombosis, the response to arterial injury, and ischemic damage. To that end, we have identified and characterized murine asTF (masTF). Like the human splice variant, masTF lacks a transmembrane domain and has a unique C-terminus. We have generated antibodies specific to masTF and murine flTF (mflTF) to examine the expression of both forms of TF. masTF antigen is widely and abundantly expressed, with a pattern similar to that of mflTF, in adult tissues, in experimentally induced thrombi, and during development. These studies demonstrate that masTF contributes to the pool of total TF and may thus play an important role in mediating TF-dependent processes.     

子痫前期患者组织因子、组织因子途径抑制物与血清可溶性血管内皮生长因子受体1的相关性

目的探讨子痫前期患者外周血组织因子(TF)、组织因子途径抑制物(TFPI)与及可溶性血管内皮生长因子受体1(s Flt-1)水平的相关性。方法酶联免疫吸附试验(ELISA)检测健康孕妇、非重度子痫前期孕妇、重度子痫前期孕妇血浆TF、TFPI水平以及血清s Flt-1水平;免疫组化检测各组胎盘TF抗原的表达。结果 3组患者血浆及胎盘TF、血清s Flt-1水平均有显著差异,且随病变程度的增加逐渐升高。3组TF/TFPI比值有显著差异,随病变程度的增加亦逐渐升高。与非重度PE组相比,重度PE组血清s Flt-1水平显著升高(P0.05)。在子痫前期患者中,血浆TF、TF/TFPI比值与血清s Flt-1水平呈正相关。3组中胎盘TF表达与血清s Flt-1水平无相关性。在TF165.9 pg/m L组中,血浆、胎盘TF、血浆TFPI与血清s Flt-1水平呈正相关。结论子痫前期患者血浆TF、TF/TFPI比值与血清s Flt-1水平存在正相关关系。     

腔隙性脑梗死患者血浆组织因子和组织因子途径抑制物抗原水平的测定

目的探讨腔隙性脑梗死(lacunarinfarct,LI)患者血浆组织因子(tissue factor,TF)和组织因子途径抑制物(tissuefactor pathwayinhibitor,TFPI)测定的临床意义以及组织因子途径在LI发病中的作用.方法择确诊的LI患者63例,采用酶联免疫吸附的方法测定血浆TF和TFPI相关指标抗原水平,与正常对照组比较并对不同危险因素患者组之间的结果进行分析.结果①与正常对照组比较,LI患者组血浆TF抗原水平显著增高(217.4±101.3pg/ml对140.9±27.1pg/ml,P=0.0003)、游离TFPI抗原水平降低(41.4±16.7 ng/ml对30.0±18.6 ng/ml,P=0.005);②合并高血压、糖尿病和血脂异常LI患者血浆TFPI相关指标的改变不同;③LI患者血浆t-TFPI和tr-TFPI抗原水平与血浆TF抗原水平相关.结论LI患者血浆组织因子途径改变表现为凝血活性增高和抗凝活性减低.     

Inhibition of tissue factor pathway inhibitor by the aptamer BAX499 improves clotting of hemophilic blood and plasma

Summary. Background: Tissue factor pathway inhibitor (TFPI) is the major inhibitor of tissue factor‐initiated coagulation, making it an interesting and novel therapeutic target in hemophilia treatment. The aptamer BAX499 (formerly ARC19499) is designed to improve hemostasis by specifically inhibiting TFPI. Objectives: The aim of the study was to examine the concentration‐dependent augmentation of clotting by BAX499. Methods: Whole blood clot formation was quantified by rotational thromboelastometry and thromboelastography, and thrombin generation in platelet‐poor plasma was assessed with the calibrated automated thrombogram, in samples from patients with congenital hemophilia A (N = 55) and B (N = 11), patients with acquired hemophilia A (N = 1), and healthy controls (N = 37). Results: BAX499 significantly improved clotting of samples from hemophilic patients in a concentration‐dependent manner, resulting in clotting profiles in samples from patients with severe hemophilia that were similar to those of healthy controls. Conclusion: BAX499 improved ex vivo clotting parameters in blood and plasma from patients with hemophilia A and B with different severity of disease, and also in a patient with acquired hemophilia. These results further support the contention that anti TFPI strategies may be an effective treatment for hemophilic patients.     

组织因子途径抑制物活性测定及其临床应用

目的 探索组织因子途径抑制物（ＴＦＰＩ）活性测定及其临床应用,方法 采用简易稀释凝血活酶时间方法 对８２例肝脏病,２３例糖尿病,１０２例恶性肿瘤,４６例肾脏病患者进行ＴＦＰＩ活性检测,结果 除了急性肝炎,其他各类疾病ＴＦＰＩ活性均高于正常对照,急性白血病患者治疗前后ＴＦＰＩ活性也有显著差异,结论：ＴＦＰＩ的活性测定能反映各类疾病中的血管病变的严重性,并能对疾病转归作了判断。     

Improvement of spatial fibrin formation by the anti‐TFPI aptamer BAX499: changing clot size by targeting extrinsic pathway initiation

Summary. Background: Tissue factor pathway inhibitor (TFPI) is a major regulator of clotting initiation and a promising target for pro‐ and anticoagulation therapy. The aptamer BAX499 (formerly ARC19499) is a high‐affinity specific TFPI antagonist designed to improve hemostasis. However, it is not clear how stimulation of coagulation onset by inactivating TFPI will affect spatial and temporal clot propagation. Objective: To examine the BAX499 effect on clotting in a spatial, reaction‐diffusion experimental system in comparison with that of recombinant activated factor VII (rVIIa). Methods: Clotting in plasma activated by immobilized tissue factor (TF) was monitored by videomicroscopy. Results: BAX499 dose‐dependently improved coagulation in normal and hemophilia A plasma activated with TF at 2 pmole m^?2 by shortening lag time and increasing clot size by up to ～2‐fold. The effect was TFPI specific as confirmed by experiments in TFPI‐depleted plasma with or without TFPI supplementation. Clotting improvement was half‐maximal at 0.7 nm of BAX499 and reached a plateau at 10 nm, remaining there at concentrations up to 1000 nm . The BAX499 effect decreased with TF surface density increase. RVIIa improved clotting in hemophilia A plasma activated with TF at 2 or 20 pmole m^?2, both by shortening lag time and increasing spatial velocity of clot propagation; its effects were strongly concentration dependent. Conclusions: BAX499 significantly improves spatial coagulation by inhibiting TFPI in a spatially localized manner that is different to that observed with rVIIa.     

Disposition of tissue factor pathway inhibitor during cardiopulmonary bypass

BACKGROUND: The tissue factor (TF) factor (F) VIIa complex activates coagulation FIX and FX to initiate coagulation, and also cleaves protease activated receptors (PARs) to initiate inflammatory processes in vascular cells. Tissue factor pathway inhibitor (TFPI) is the only specific inhibitor of the TF-FVIIa complex, regulating both its procoagulant and pro-inflammatory properties. Upon heparin infusion during cardiopulmonary bypass (CPB), a heparin releasable pool of endothelial associated TFPI circulates in plasma. Following protamine neutralization of heparin, the plasma TFPI level decreases, but does not return completely to baseline, suggesting that during CPB a fraction of the plasma TFPI becomes heparin-independent. We have investigated the structural and functional properties of plasma TFPI during CPB to further characterize how TFPI is altered during this procedure. METHODS: We enrolled 17 patients undergoing first-time cardiac surgery involving CPB. Plasma samples were obtained at baseline, 5 min and 1 h after start of CPB (receiving heparin), 10 min after protamine administration (off CPB) and 24 h following surgery. Samples were analyzed for full-length and free (non-lipoprotein bound) TFPI antigen by enzyme-linked immunosorbent assay (ELISA) and for TFPI anticoagulant activity using an amidolytic assay. Western blot analysis was used to identify TFPI species of varying molecular weights in three additional patients. Dunnett's test for post hoc comparisons was used for statistical analysis. RESULTS: The ELISA and Western blot data indicated that an increase in full-length TFPI accounted for most of the heparin releasable TFPI. Following heparin neutralization with protamine, the full-length TFPI antigen returned to baseline levels while the free TFPI antigen and the total plasma TFPI activity remained elevated. This was associated with the appearance of a new 38 kDa form of plasma TFPI identified by Western blot analysis. The 38 kDa form of TFPI did not react with an antibody directed against the C-terminal region of TFPI indicating it has undergone proteolysis within this region. All TFPI measurements returned to baseline 24 h following CPB. CONCLUSIONS: During CPB the full-length form of TFPI is the predominant form in plasma because of its prompt release from the endothelial surface following heparin administration. Upon heparin neutralization with protamine, full-length TFPI redistributes back to the endothelial surface. However, a new 38 kDa TFPI fragment is generated during CPB and remains circulating in plasma, indicating that TFPI undergoes proteolytic degradation during CPB. This degradation may result in a decrease in endothelium-associated TFPI immediately post-CPB, and may contribute to the procoagulant and proinflammatory state that often complicates CPB.