Mice expressing nonpolymerizable fibrinogen have reduced arterial and venous thrombosis with preserved hemostasis

ABSTRACT

Elevated circulating fibrinogen levels correlate with increased risk for both cardiovascular and venous thromboembolic diseases. In vitro studies show that formation of a highly dense fibrin matrix is a major determinant of clot structure and stability. Here, we analyzed the impact of nonpolymerizable fibrinogen on arterial and venous thrombosis as well as hemostasis in vivo using FgaEK mice that express normal levels of a fibrinogen that cannot be cleaved by thrombin. In a model of carotid artery thrombosis, FgaWT/EK and FgaEK/EK mice were protected from occlusion with 4% ferric chloride (FeCl3) challenges compared with wild-type (FgaWT/WT) mice, but this protection was lost, with injuries driven by higher concentrations of FeCl3. In contrast, fibrinogen-deficient (Fga-/-) mice showed no evidence of occlusion, even with high-concentration FeCl3 challenge. Fibrinogen-dependent platelet aggregation and intraplatelet fibrinogen content were similar in FgaWT/WT, FgaWT/EK, and FgaEK/EK mice, consistent with preserved fibrinogen-platelet interactions that support arterial thrombosis with severe challenge. In an inferior vena cava stasis model of venous thrombosis, FgaEK/EK mice had near complete protection from thrombus formation. FgaWT/EK mice also displayed reduced thrombus incidence and a significant reduction in thrombus mass relative to FgaWT/WT mice after inferior vena cava stasis, suggesting that partial expression of nonpolymerizable fibrinogen was sufficient for conferring protection. Notably, FgaWT/EK and FgaEK/EK mice had preserved hemostasis in multiple models as well as normal wound healing times after skin incision, unlike Fga-/- mice that displayed significant bleeding and delayed healing. These findings indicate that a nonpolymerizable fibrinogen variant can significantly suppress occlusive thrombosis while preserving hemostatic potential in vivo.

Robust coagulation activation and coagulopathy in mice with experimental acetaminophen-induced liver failure

BACKGROUND

Patients with acetaminophen (APAP)-induced acute liver failure (ALF) display both hyper- and hypocoagulable changes not necessarily recapitulated by standard hepatotoxic doses of APAP used in mice (eg, 300 mg/kg).

OBJECTIVES

We sought to examine coagulation activation in vivo and plasma coagulation potential ex vivo in experimental settings of APAP-induced hepatotoxicity and repair (300-450 mg/kg) and APAP-induced ALF (600 mg/kg) in mice.

RESULTS

APAP-induced ALF was associated with increased plasma thrombin-antithrombin complexes, decreased plasma prothrombin, and a dramatic reduction in plasma fibrinogen compared with lower APAP doses. Hepatic fibrin(ogen) deposits increased independent of APAP dose, whereas plasma fibrin(ogen) degradation products markedly increased in mice with experimental ALF. Early pharmacologic anticoagulation (+2 hours after 600 mg/kg APAP) limited coagulation activation and reduced hepatic necrosis. The marked coagulation activation evident in mice with APAP-induced ALF was associated with a coagulopathy detectable ex vivo in plasma. Specifically, prolongation of the prothrombin time and inhibition of tissue factor-initiated clot formation were evident even after restoration of physiological fibrinogen concentrations. Plasma endogenous thrombin potential was similarly reduced at all APAP doses. Interestingly, in the presence of ample fibrinogen, ∼10 times more thrombin was required to clot plasma from mice with APAP-induced ALF compared with plasma from mice with simple hepatotoxicity.

CONCLUSION

The results indicate that robust pathologic coagulation cascade activation in vivo and suppressed coagulation ex vivo are evident in mice with APAP-induced ALF. This unique experimental setting may fill an unmet need as a model to uncover mechanistic aspects of the complex coagulopathy of ALF.

Thromboembolic risk with gender-affirming hormone therapy: potential role of global coagulation and fibrinolysis assays

Gender-affirming hormonal therapies are a critical component of the care of transgender individuals. Transgender people are commonly prescribed estrogen or testosterone to promote male-to-female or female-to-male transitions and to preserve gender-specific characteristics long-term. However, some exogenous hormones, especially certain estrogen preparations, are an established risk factor of thrombosis. As the number of individuals seeking gender-based care is rising, there is an urgent need to identify and characterize the mechanisms underlying hormone-associated thrombosis and incorporate this information into clinical algorithms for diagnosis and management. Herein, we discuss historical evidence on the incidence of thrombosis and changes in plasma composition in transgender and cisgender cohorts. We present 3 case studies to demonstrate knowledge gaps in thrombosis risk stratification and prediction tools. We also present data from in vitro coagulation and fibrinolysis assays and discuss how information from these kinds of assays may be used to help guide the clinical management of transgender individuals.

Tissue factor pathway inhibitor is a potential modifier of bleeding risk in factor XI deficiency

BACKGROUND

Factor (F) XI deficiency is associated with increased bleeding risk in some individuals. Neither FXI levels nor clinical clotting assays predict the bleeding risk. Compared with controls, FXI-deficient bleeders have reduced clot formation, decreased fibrin network density, and increased susceptibility to fibrinolysis. Tissue factor pathway inhibitor (TFPI) was recently implicated as a modifying factor in individuals with bleeding of unknown cause.

OBJECTIVES

To determine the potential of TFPI in modifying the bleeding risk in FXI-deficient individuals.

METHODS

The effects of TFPI on thrombin generation and clot formation, structure, and fibrinolysis in FXI-deficient plasma were measured in vitro in the absence or presence of inhibitory anti-TFPI antibody or exogenous recombinant TFPIα. Total plasma TFPI concentration was measured in 2 independent cohorts of controls and FXI-deficient individuals classified as bleeders or nonbleeders (cohort 1: 10 controls and 16 FXI-deficient individuals; cohort 2: 48 controls and 57 FXI-deficient individuals) and correlated with ex vivo plasma clot formation and fibrinolysis parameters associated with bleeding risk.

RESULTS

In an in vitro FXI deficiency model, inhibition of TFPI enhanced thrombin generation and clot formation, increased the network density, and decreased fibrinolysis, whereas an increase in TFPI had the opposite effects. Compared with controls, plasma from FXI-deficient bleeders had higher TFPI concentration. Total plasma TFPI concentrations correlated with parameters from ex vivo clotting and fibrinolysis assays that differentiate FXI-deficient bleeders and nonbleeders.

CONCLUSION

Coagulation and fibrinolysis parameters that differentiate FXI-deficient nonbleeders and bleeders were altered by plasma TFPIα. Total plasma TFPI was increased in FXI-deficient bleeders. TFPI may modify the bleeding risk in FXI-deficient individuals.

High risk oral contraceptive hormones do not directly enhance endothelial cell procoagulant activity in vitro

BACKGROUND

Oral contraceptive (OC) use increases venous thromboembolism risk 2-5-fold. Procoagulant changes can be detected in plasma from OC users even without thrombosis, but cellular mechanisms that provoke thrombosis have not been identified. Endothelial cell (EC) dysfunction is thought to initiate venous thromboembolism. It is unknown whether OC hormones provoke aberrant procoagulant activity in ECs.

OBJECTIVE

Characterize the effect of high-risk OC hormones (ethinyl estradiol [EE] and drospirenone) on EC procoagulant activity and the potential interplay with nuclear estrogen receptors ERα and ERβ and inflammatory processes.

METHODS

Human umbilical vein and dermal microvascular ECs (HUVEC and HDMVEC, respectively) were treated with EE and/or drospirenone. Genes encoding the estrogen receptors ERα and ERβ (ESR1 and ESR2, respectively) were overexpressed in HUVEC and HDMVEC via lentiviral vectors. EC gene expression was assessed by RT-qPCR. The ability of ECs to support thrombin generation and fibrin formation was measured by calibrated automated thrombography and spectrophotometry, respectively.

RESULTS

Neither EE nor drospirenone, alone or together, changed expression of genes encoding anti- or procoagulant proteins (TFPI, THBD, F3), integrins (ITGAV, ITGB3), or fibrinolytic mediators (SERPINE1, PLAT). EE and/or drospirenone did not increase EC-supported thrombin generation or fibrin formation, either. Our analyses indicated a subset of individuals express ESR1 and ESR2 transcripts in human aortic ECs. However, overexpression of ESR1 and/or ESR2 in HUVEC and HDMVEC did not facilitate the ability of OC-treated ECs to support procoagulant activity, even in the presence of a pro-inflammatory stimulus.

CONCLUSIONS

The OC hormones EE and drospirenone do not directly enhance thrombin generation potential of primary ECs in vitro.

Novel venous thromboembolism mouse model to evaluate the role of complete and partial factor XIII deficiency in pulmonary embolism risk

BACKGROUND

Venous thrombosis (VT) and pulmonary embolism (PE), collectively venous thromboembolism (VTE), cause high mortality and morbidity. Factor XIII (FXIII) crosslinks fibrin to enhance thrombus stability and consequently may influence PE risk. Elucidating mechanisms contributing to PE is limited by a lack of models that recapitulate human PE characteristics.

OBJECTIVE

We aimed to develop a mouse model that permits embolization of red blood cell (RBC)- and fibrin-rich VT and determine the contribution of FXIII to PE risk.

METHODS AND RESULTS

In a thrombin-infusion PE model, F13a+/+ , F13a+/- , and F13a-/-  mice had similar incidence of microthrombi in the lungs; however, thrombi were small, with low RBC content (≤7%), unlike human PEs (~70%). To identify a model producing PE consistent with histological characteristics of human PE, we compared mouse femoral vein electrolytic injury, femoral vein FeCl3 injury, and infrarenal vena cava (IVC) stasis models of VT. Electrolytic and FeCl3  models produced small thrombi with few RBCs (5% and 4%, respectively), whereas IVC stasis produced large thrombi with higher RBC content (68%) that was similar to human PEs. After IVC stasis and ligature removal (de-ligation) to permit thrombus embolization, compared to F13a+/+ mice, F13a+/-  and F13a-/-  mice had similar and increased PE incidence, respectively.

CONCLUSIONS

Compared to thrombin infusion-, electrolytic injury-, and FeCl3 -based models, IVC stasis produces thrombi that are more histologically similar to human thrombi. IVC stasis followed by de-ligation permits embolization of existing RBC- and fibrin-rich thrombi. Complete FXIII deficiency increases PE incidence, but partial deficiency does not.

Comparison of the coagulopathies associated with COVID-19 and sepsis

BACKGROUND

Coronavirus disease 2019 (COVID-19) is associated with activation of coagulation that mainly presents as thrombosis. Sepsis is also associated with activation of coagulation that mainly presents as disseminated intravascular coagulation. Many studies have reported increased levels of plasma d-dimer in patients with COVID-19 that is associated with severity, thrombosis, and mortality.

OBJECTIVES

The aim of this study was to compare levels of circulating extracellular vesicle tissue factor (EVTF) activity and active plasminogen activator inhibitor 1 (PAI-1) in plasma from patients with COVID-19 or sepsis.

METHODS

We measured levels of d-dimer, EVTF activity, and active PAI-1 in plasma samples from patients with COVID-19 (intensive care unit [ICU], N = 15; and non-ICU, N = 20) and patients with sepsis (N = 35).

RESULTS

Patients with COVID-19 had significantly higher levels of d-dimer, EVTF activity, and active PAI-1 compared with healthy controls. Patients with sepsis had significantly higher levels of d-dimer and EVTF activity compared with healthy controls. Levels of d-dimer were significantly lower in patients with COVID-19 compared with patients with sepsis. Levels of EVTF activity were significantly higher in ICU patients with COVID-19 compared with patients with sepsis. Levels of active PAI-1 were significantly higher in patients with COVID-19 compared with patients with sepsis.

CONCLUSIONS

High levels of both EVTF activity and active PAI-1 may promote thrombosis in patients with COVID-19 due to simultaneous activation of coagulation and inhibition of fibrinolysis. The high levels of active PAI-1 in patients with COVID-19 may limit plasmin degradation of crosslinked fibrin and the release of d-dimer. This may explain the lower levels of D-dimer in patients with COVID-19 compared with patients with sepsis.

Murine cadherin-6 mediates thrombosis in vivo in a platelet-independent manner

BACKGROUND

Platelet adhesion is the critical process mediating stable thrombus formation. Previous reports of cadherin-6 on human platelets have demonstrated its role in platelet aggregation and thrombus formation.

OBJECTIVES

We aimed to further characterize the importance of cadherin-6 in thrombosis in vivo.

METHODS

Cadherin-6 platelet expression was evaluated by western blotting, flow cytometry, and immunoprecipitation. Thrombosis was evaluated using the FeCl3 and Rose Bengal carotid artery models in C57Bl6 mice treated with anti-cadherin-6 or IgG and wild-type or Cdh6-/- mice. Platelet function was compared in wild-type and Cdh6-/- mice using tail-clip assays, aggregometry, and flow cytometry.

RESULTS

Human platelet expression of cadherin-6 was confirmed at ~3000 copies per platelet. Cdh6-/- mice or those treated with anti-cadherin-6 antibody showed an increased time to occlusion in both thrombosis models. Cadherin-6 was not expressed on mouse platelets, and there were no differences in tail bleeding times, platelet aggregation, or platelet activation in wild-type versus Cdh6-/- mice.

CONCLUSIONS

Cadherin-6 plays an essential role in thrombosis in vivo. However, cadherin-6 is not expressed on murine platelets. These data are in contrast to human platelets, which express a functional cadherin-6/catenin complex. The essential, platelet-independent role for cadherin-6 in hemostasis may allow it to be an effective and safe therapeutic target.

COVID-19 and Sepsis Are Associated With Different Abnormalities in Plasma Procoagulant and Fibrinolytic Activity

OBJECTIVE

Coronavirus disease 2019 (COVID-19) is associated with derangement in biomarkers of coagulation and endothelial function and has been likened to the coagulopathy of sepsis. However, clinical laboratory metrics suggest key differences in these pathologies. We sought to determine whether plasma coagulation and fibrinolytic potential in patients with COVID-19 differ compared with healthy donors and critically ill patients with sepsis. Approach and Results: We performed comparative studies on plasmas from a single-center, cross-sectional observational study of 99 hospitalized patients (46 with COVID-19 and 53 with sepsis) and 18 healthy donors. We measured biomarkers of endogenous coagulation and fibrinolytic activity by immunoassays, thrombin, and plasmin generation potential by fluorescence and fibrin formation and lysis by turbidity. Compared with healthy donors, patients with COVID-19 or sepsis both had elevated fibrinogen, d-dimer, soluble TM (thrombomodulin), and plasmin-antiplasmin complexes. Patients with COVID-19 had increased thrombin generation potential despite prophylactic anticoagulation, whereas patients with sepsis did not. Plasma from patients with COVID-19 also had increased endogenous plasmin potential, whereas patients with sepsis showed delayed plasmin generation. The collective perturbations in plasma thrombin and plasmin generation permitted enhanced fibrin formation in both COVID-19 and sepsis. Unexpectedly, the lag times to thrombin, plasmin, and fibrin formation were prolonged with increased disease severity in COVID-19, suggesting a loss of coagulation-initiating mechanisms accompanies severe COVID-19.

CONCLUSIONS

Both COVID-19 and sepsis are associated with endogenous activation of coagulation and fibrinolysis, but these diseases differently impact plasma procoagulant and fibrinolytic potential. Dysregulation of procoagulant and fibrinolytic pathways may uniquely contribute to the pathophysiology of COVID-19 and sepsis.