Novel Mouse Hemostasis Model for Real-Time Determination of Bleeding Time and Hemostatic Plug Composition

Introduction: Hemostasis is a rapid response by the body to stop bleeding at sites of vessel injury. Both platelets and fibrin are important for the formation of a hemostatic plug. Mice have been used to uncover the molecular mechanisms that regulate the activation of platelets and coagulation under physiological conditions. However, measuring hemostasis in mice is quite variable and current methods do not quantify platelet adhesion or fibrin formation at the site of injury.

Methods: We describe a novel hemostasis model that uses intravital fluorescence microscopy to quantify platelet adhesion, fibrin formation, and time to hemostatic plug formation in real-time. Repeated vessel injuries of ~50-100µm in diameter were induced using laser ablation technology in the saphenous vein of mice.

Results: Hemostasis in this model was strongly impaired in mice deficient in glycoprotein (GP)Ibα (IL4R/GPIb-tg) or talin-1 (talin1^f/f PF4-Cre), important regulators of platelet adhesiveness. In contrast, the time to hemostatic plug formation was only minimally affected in mice defective in the extrinsic (tissue factor (TF)^low) or the intrinsic (FIX^-/-) coagulation pathways, even though platelet adhesion was significantly reduced. Interestingly, fibrin accumulation was markedly increased in lesions of talin1^f/f PF4-Cre, IL4R/GPIb-tg and FIX^-/- mice, while it was decreased in TF^low mice. These findings suggest that prolonged plasma exposure to TF leads to increased thrombin and fibrin generation in the surrounding tissue. A partial reduction of platelet adhesiveness using clopidogrel led to instability within the hemostatic plug, especially when combined with impaired coagulation in TF^low mice.

Conclusions: In summary, we present a novel, highly sensitive method to quantify hemostatic plug formation in mice. This new model has several defining characteristics, including the use of intravital fluorescence microscopy to monitor the hemostatic process, the novel use of laser ablation technology to generate vascular lesions with a defined diameter, the ability to repeatedly disrupt the hemostatic process at the same site of injury, and the possibility to perform multiple injuries along the exposed saphenous vein. Based on its sensitivity towards platelet adhesion defects and its real-time imaging capability, we propose this model as an ideal tool to study the efficacy and safety of antiplatelet agents.