Platelet Endocytosis in Hemostasis and Innate Immunity

To survey the vasculature, platelets constantly test the blood stream’s composition and the integrity of the blood vessel walls. The processes by which most cells “sample” their environment are collectively known as endocytosis. However, aside from a few specific examples, it is unclear how important endocytosis is to platelet function. We hypothesize that endocytosis is critical for at least two acute platelet functions: thrombus formation and innate immune responses. To support our hypothesis, we will focus on the roles that platelet endocytosis plays in modulating surface integrins during thrombosis and in facilitating TLR-based signaling during innate immune responses. Our preliminary data show that platelet endocytosis is important for spreading on fibrinogen, clot retraction, and hemostasis. Endocytosis is also important for platelet activation by specific TLR agonists. Using novel, genetically altered mouse strains (Arf6-/-, VAMP-3-/-, and Syntaxin-2/4-/-), which are defective at different endocytosis steps, we will address the roles, routes, and mechanisms of platelet endocytosis in three specific aims: Aim 1. Determine the effects of defective platelet endocytosis on hemostasis; Aim 2. Determine the effects of defective platelet endocytosis on innate immune responses; Aim 3. Determine the mechanisms and routes of the platelet endocytosis system. Clearly endocytosis contributes to granule cargo loading; however, its role in other acute platelet functions is poorly understood. We hypothesize that endocytosis contributes to how platelets sense their environment and how they respond to what they detect. We present a view of platelets as “active” monitors of the vasculature, sampling their environment while circulating and during thrombosis. Our work will yield insights into platelet membrane trafficking during thrombosis and platelet function during sepsis, viremia, and other systemic infections; perhaps reshaping our use of anti-thrombotic drugs in these disease states. Finally, understanding platelet endocytosis may uncover strategies to load platelets with therapeutics that can be used to ameliorate cardiovascular diseases.