Re‐evaluation of mouse tissue factor pathway inhibitor and comparison of mouse and human tissue factor pathway inhibitor physiology

Abstract

Essentials Mouse models are often used to define roles of tissue factor pathway inhibitor (TFPI) in man. TFPI isoform-specific KOs reveal unexpected differences between mouse and human TFPI physiology. Mouse plasma contains 20 times more TFPI than man, derived from TFPIγ, a form not found in man. TFPIγ null mice, expressing only TFPI isoforms α and β, may better reflect the human situation. SUMMARY: Background Mouse models can provide insight into the pathophysiology of human thrombosis and hemostasis. Tissue factor pathway inhibitor (TFPI) regulates coagulation through protein S (PS)-enhanced factor (F) Xa inhibition and FXa-dependent inhibition of FVIIa/tissue factor (TF) activity. TFPI is expressed as isoforms α and β in man, and α, β and γ in the mouse. Objective Assess the reliability of extending TFPI-related studies in mice to humans. Method Compare mouse and human TFPI physiology using a variety of methods. Results Mouse TFPI and human TFPI are similar in regard to: (i) the mechanisms for FVIIa/TF and FXa inhibition; (ii) TFPIα is a soluble form and TFPIβ is glycosyl phosphatidyl inositol (GPI) membrane anchored; (iii) the predominant circulating form of TFPI in plasma is lipoprotein-associated; (iv) low levels of TFPIα circulate in plasma and increase following heparin treatment; and (v) TFPIα is the isoform in platelets. They differ in that: (i) mouse TFPI circulates at a ~20-fold higher concentration; (ii) mouse lines with isolated isoform deletions show this circulating mouse TFPI is derived from TFPIγ; (iii) sequences homologous to the mouse TFPIγ exon are present in many species, including man, but in primates are unfavorable for splicing; and (iv) tandem mass spectrometry (MS/MS) detects sequences for TFPI isoforms α and β in human plasma and α and γ in mouse plasma. Conclusion To dissect the pathophysiological roles of human TFPIα and TFPIβ, studies in TFPIγ null mice, expressing only α and β, only α or only β should better reflect the human situation.

Defective fibrin deposition and thrombus stability in Bambi−/− mice are mediated by elevated anticoagulant function

“…15 In mice, a shorter soluble TFPI isoform (TFPIγ) consisting of the first two Kunitz domains also exists in circulation. [16][17][18] Thrombomodulin is a type-I transmembrane protein expressed by EC. 19 Thrombomodulin binds thrombin with high affinity and enhances the conversion of protein C into activated protein C (APC) by ~1000-fold.…”

“…Both isoforms inhibit TF‐FVIIa in a FXa‐dependent manner, with TFPIα likely being more important for regulating hemostasis on platelet surfaces, whereas TFPIβ probably functions primarily on the endothelial cell surface to which it is attached . In mice, a shorter soluble TFPI isoform (TFPIγ) consisting of the first two Kunitz domains also exists in circulation . Thrombomodulin is a type‐I transmembrane protein expressed by EC .…”

Hemostatic efficacy of marstacimab alone or in combination with bypassing agents in hemophilia plasmas and a mouse bleeding model

“…In the mouse, the levels of plasma TFPI are approximately 20 times higher compared with humans. 32 The properties of the human isoforms TFPIα and TFPIβ are similar in the mouse, including platelet TFPIα. However, the predominant form in mouse plasma, TFPIγ, is an isoform not found in humans.…”

Intrauterine lethality in Tfpi gene disrupted mice is differentially suppressed during mid‐ and late‐gestation by platelet TFPIα overexpression

“…11 In mice, TFPIα is the major isoform expressed by megakaryocytes and present in platelets, 10 TFPIβ is the major isoform on endothelium in tissue vascular beds, 12 and TFPIγ is the major isoform in plasma. 13 In addition to its location in platelets, TFPIα is a heparin-releasable protein and is detectable in mouse plasma (~20% of total plasma TFPI) following heparin infusion. 12 All isoforms are capable of inhibiting the tissue factor-factor VIIa (TF-FVIIa) catalytic complex and factor Xa (FXa) via the first (K1) and second (K2) Kunitz domains, respectively.…”