Thrombin spatial distribution determines protein C activation during hemostasis and thrombosis

Dual antithrombotic therapy dose-dependently alters hemostatic plug structure and function

BACKGROUND

Antithrombotic medications carry an inherent risk of bleeding, which may be exacerbated when anticoagulant and antiplatelet therapeutics are combined. Prior studies have shown different effects of antiplatelet vs anticoagulant drugs on the structure and function of hemostatic plugs in vivo.

OBJECTIVES

We examined whether dual antithrombotic treatment consisting of combined antiplatelet and anticoagulant therapeutics alters hemostatic plug structure and function differently from treatment with either therapeutic alone.

METHODS

Mice were treated with the P2Y12 antagonist clopidogrel and the factor Xa inhibitor rivaroxaban across a range of doses, either alone or in combination. The hemostatic response was assessed using a mouse jugular vein puncture injury model. Platelet accumulation and fibrin deposition were evaluated using quantitative multiphoton fluorescence microscopy, and bleeding times were recorded.

RESULTS

Mice treated with clopidogrel alone exhibited a decrease in platelet accumulation at the site of injury, with prolonged bleeding times only at the highest doses of clopidogrel used. Mice treated with rivaroxaban alone instead showed a reduction in fibrin deposition with no impact on bleeding. Mice treated with both clopidogrel and rivaroxaban exhibited platelet and fibrin accumulation that was similar to that with either drug given alone; however, dual antithrombotic therapy resulted in impaired hemostasis at doses that had no impact on bleeding when given in isolation.

CONCLUSION

Combined administration of antiplatelet and anticoagulant therapeutics exacerbates bleeding as compared to that with either drug alone, potentially via combined loss of both adenosine 5'-diphosphate- and thrombin-mediated platelet activation. These findings enhance our understanding of the bleeding risk associated with dual antithrombotic therapy.

Highly efficient photoelectrochemical aptasensor based on CdS/CdTe QDs co-sensitized TiO2 nanoparticles designed for thrombin detection

A photoelectrochemical (PEC) sensor for the sensitive detection of thrombin (TB) was established. Co-sensitized combination of TiO2 nanoparticles combined with modified cadmium sulfide and cadmium telluride quantum dots (CdS/CdTe QDs) was utilized as a photoactive material. Successful growth of CdS/CdTe quantum dots on mesoporous TiO2 films occured by successive ion-layer adsorption and reaction. This interesting formation of co-sensitive structure is conducive to enhancing the photocurrent response by improving the use rate of light energy. Additionally, the step-level structure of CdS/CdTe QDs and TiO2 NPs shows a wide range of visible light absorption, facilitating the dissociation of excitons into free electrons and holes. Consequently, the photoelectric response of the PEC analysis platform is significantly enhanced. This constructed PEC aptasensor shows good detection of thrombin with a low detection limit (0.033 pM) and a wide linear range (0.0001-100 nM) in diluted actual human serum samples. In addition, this PEC aptasensor also has the characteristics of good stability and good reproducibility, which provides a novel insight for the quantitative measurement of other similar analytes.

Visualization of Domain- and Concentration-Dependent Impact of Thrombomodulin on Differential Regulation of Coagulation and Fibrinolysis

BACKGROUND

 Thrombomodulin (TM) functions as a dual modulator-anticoagulant and antifibrinolytic potential-by the thrombin-dependent activation of protein C and thrombin-activatable fibrinolysis inhibitor (TAFI). Activated TAFI cleaves the C-terminal lysine of partially degraded fibrin and inhibits both plasminogen binding and its activation on the fibrin surface. We have reported previously that activated platelets initiate fibrin network formation and trigger fibrinolysis after the accumulation of tissue-type plasminogen activator and plasminogen.

OBJECTIVE

 To analyze the effects of domain-deletion variants of TM on coagulation and fibrinolysis at different concentrations.

METHODS

 Domain-deletion variants of TM, such as D123 (all extracellular regions), E3456 (minimum domains for thrombin-dependent activation of protein C and TAFI), and E456 (minimum domains for that of protein C but not TAFI), were used at 0.25 to 125 nM for turbidimetric assay to determine the clotting time and clot lysis time and to visualize fibrin network formation and lysis in platelet-containing plasma.

RESULTS AND CONCLUSIONS

 A low concentration of either D123 or E3456, but not of E456, prolonged clot lysis time, and delayed the accumulation of fluorescence-labeled plasminogen at the activated platelets/dense fibrin area due to effective TAFI activation. Conversely, only the highest concentrations of all three TM variants delayed the clotting time, though fibrin network formation in the vicinity of activated platelets was almost intact. TAFI activation might be affected by attenuation in thrombin activity after the clot formation phase. These findings suggest that the spatiotemporal balance between the anticoagulant and antifibrinolytic potential of TM is controlled in domain- and concentration-dependent manners.

The protein C pathways

PURPOSE OF REVIEW

To provide an overview of the state-of-the-art in protein C (PC) pathway research.

RECENT FINDINGS

The PC pathway is crucial for maintaining hemostasis to prevent venous thromboembolism. This is evident from genetic mutations that result in impaired PC pathway activity and contribute to increased venous thromboembolism risk in affected individuals. In addition to its anticoagulant role, activated PC (APC) also mediates a complex, pleiotropic role in the maintenance of vascular cell health, which it achieves via anti-inflammatory and antiapoptotic cell signaling on endothelial cells. Emerging data have demonstrated that cell signaling by APC, mediated by multiple receptor interactions on different cell types, also confers cytoprotective and anti-inflammatory benefits. Defects in both arms of the PC pathway are associated with increased susceptibility to thrombo-inflammatory disease in various preclinical thrombotic, proinflammatory and neurological disease models. Moreover, recent studies have identified attenuation of anticoagulant PC pathway activity as an exciting therapeutic opportunity to promote hemostasis in patients with inherited or acquired bleeding disorders.

SUMMARY

In this review, we provide an overview of some recent developments in our understanding of the PC pathways.

Antithrombotic potential of a single-domain antibody enhancing the activated protein C-cofactor activity of protein S

BACKGROUND

Protein S (PS) is a natural anticoagulant acting as a cofactor for activated protein C (APC) in the proteolytic inactivation of activated factors V (FVa) and VIII (FVIIIa), but also for tissue factor pathway inhibitor α (TFPIα) in the inhibition of activated factor X (FXa).

OBJECTIVE

For therapeutic purposes, we aimed at generating single-domain antibodies (sdAbs) that could specifically modulate the APC-cofactor activity of PS in vivo.

METHODS

A llama-derived immune library of sdAbs was generated and screened on recombinant human PS by phage display. PS binders were tested in a global activated partial thromboplastin time (APTT)-based APC-cofactor activity assay.

RESULTS

A PS-specific sdAb (PS003) was found to enhance the APC-cofactor activity of PS in our APTT-based assay, and this enhancing effect was greater for a bivalent form of PS003 (PS003biv). Further characterization of PS003biv demonstrated that PS003biv also enhanced the APC-cofactor activity of PS in a tissue factor (TF)-induced thrombin generation assay and stimulated APC in the inactivation of FVa, but not FVIIIa, in plasma-based assays. Furthermore, PS003biv was directed against the sex hormone-binding globulin (SHBG)-like domain but did not inhibit the binding of PS to C4b-binding protein (C4BP) and did not interfere with the TFPIα-cofactor activity of PS. In mice, PS003biv exerted an antithrombotic effect in a FeCl3 -induced thrombosis model, while not affecting physiological hemostasis in a tail-clip bleeding model.

DISCUSSION

Altogether, these results showed that pharmacological enhancement of the APC-cofactor activity of PS through an original anti-PS sdAb might constitute a promising and safe antithrombotic strategy.