Mouse models of platelet function in vivo

Much of our current knowledge about the molecular mechanisms responsible for platelet activation and thrombin generation comes from in vitro studies where system components were examined in isolation. However, a comprehensive understanding of the hemostatic system requires study of how the various cellular and molecular components are integrated within, and influenced by, the dynamic physical environment present at sites of vascular injury in vivo. In this regard, animal models have played an integral role in studies of platelet biology since Bizzozero first recognized “blutplattchen” (i.e., platelets) as the cellular element of blood that responds to vascular injury in 1881 [1,2]. Current approaches take advantage of genetically modified mice to study molecular mechanisms within complex physiologic milieus. A variety of mechanisms for eliciting a hemostatic or thrombotic response in mice have been developed, and most have been adapted for use in multiple vascular beds. This review series explores several of these vascular injury models, with a particular emphasis on the mechanisms of injury and their relationship to events as they might occur in humans. As this review series focuses on mouse hemostasis and thrombosis models, it is worth noting at the outset that several important differences in the hemostatic system of mice and humans have been identified. Two obvious examples are platelet size and count (mouse platelets are smaller and more numerous) [3]. At the molecular level, the expression of some platelet genes is different [4], including the thrombin receptors present on the platelet surface (PARs 3 and 4 in mice; 1 and 4 in humans) [5,6]. Multiple differences in the coagulation system have been noted between mice and humans, including differences in coagulation factor concentrations, splicing variants (e.g., TFPI [7]), and biodistribution (e.g., FXI [8]). Clinically important pathologic features, such as joint bleeding and hemarthroses in hemophilia, are not alwayswell replicated.Nonetheless, these differences do not preclude the use of mouse models for the study of hemostasis and thrombosis in general. Overall, the similarities between the human and mouse hemostatic systems greatly outnumber the differences, and mousemodels continue to provide insights into themolecularmechanisms regulating platelet activation and thrombin generation in vivo. It is important, however, that investigators recognize the differences that inevitably exist between humans and any experimental model, so that results may be interpreted appropriately. Among the conceptually simplest of hemostasis models, the bleeding time assaywas first developed to examine clinical bleeding defects in humans by Duke in 1910 [9]. The finding that prolonged bleeding time correlated with low platelet counts provided early evidence that platelets were indeed an integral component of the hemostatic response [9]. Variations of this model have been adapted to a number of animal species for experimental purposes. Mohammed et al. review twoof themost commonmouse bleeding timemodels, the tail bleeding time and saphenous vein bleeding models. Besides these, recent studies have also utilized intravital imaging approaches to visualize the hemostatic response following perforating injuries induced with a laser. The review by Stalker discusses the use of these and other variations of laser injury models coupled with intravital imaging to study platelet and coagulation function in vivo. In addition to models of hemostasis and more general platelet function in vivo, several mouse models intended to mimic aspects of pathologic thrombosis in humans have been developed. Such models are complicated by underlying differences in the anatomy and physiology of mice and humans. For example, deep venous thrombosis (DVT) in humans is characterized by thrombus formation behind the valves of large veins [10,11]. Replicating this feature of DVT in mice is difficult as they do not possess the same complement of venous valves due to their small size and quadruped nature. Instead, DVT is commonly modeled in mice by introducing flow perturbations such as those associated with the pathogenesis of DVT in humans. Campos and Brill review the variety of mouse venous thrombosis models used in the field, noting strengths and weaknesses of each. To mimic human arterial thrombosis, FeCl3-induced injury models result in an occlusive platelet-rich thrombus approximating the clinically observed “white thrombi” common in arterial thrombosis. Grover and Mackman provide a critical review of commonly used FeCl3 induced thrombosis models, highlighting recent advances in our understanding of the molecular mechanisms responsible for FeCl3-induced vascular damage. To mimic atherothrombosis in settings of dyslipidemia and vessel wall disease, several mouse models of induced atherosclerotic plaque rupture have been developed in recent years. Karel et al. review the multiple approaches used to induce plaque rupture in mice and offer insights into their relationship with atherothrombosis as it occurs in humans. Finally, the role of platelets outside of traditional hemostasis and thrombosis contexts is increasingly appreciated [12]. Among these is the contribution of platelets to the maintenance of vascular integrity in settings of inflammation [13]. Le Chapelain et al. review the use of a traditional immune complex-mediated vascular inflammation model, the reversed passive Arthus reaction, to study the contribution of platelets to vascular integrity. Taken together, the goal of this series of reviews is to provide the readership of Platelets a state-of-the-art overview of commonly used mouse models for the study of platelet function in vivo. The pros and cons of various models, as well as their relationship to human pathophysiology, are discussed. Correspondence: Timothy J. Stalker, Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. E-mail: tstalker@pennmedicine.upenn. edu http://www.tandfonline. com/iplt ISSN: 0953-7104 (print), 1369-1635 (electronic)