The role of the small GTPase Rap1 and environmental cues in the regulation of adhesion and cytoskeletal dynamics in migrating platelets - PROJECT SUMMARY/ABSTRACT Platelets are small anucleate blood cells that coordinate the hemostatic response to vascular damage through the formation of a platelet plug. Our lab has previously shown the rapid activation of the primary platelet integrin, ⍺IIbβ3, by the small GTPase Rap1 and its effector Talin is essential for platelet plug formation. This response must be tightly controlled as hypoactive platelets can cause prolonged bleeding while hyperactive platelets are associated with thrombotic disease. Platelets also play a crucial role in inflammation and immunity. Platelets are among the first cells recruited to sites of inflammation to aid in the recruitment of neutrophils and scavenge bacteria to prevent dissemination to other organs. Additionally, neutrophils exit the blood vessel between endothelial cells and leave behind gaps in the endothelium. Recent evidence shows single platelets adhere and migrate on the inflamed endothelium to plug the holes to prevent bleeding into the surrounding tissue, in a process known as inflammatory hemostasis. Overactivation of platelets and immune cells in areas of inflammation can lead to immunothrombosis, or the formation of thrombi in response to pathogens or damaged cells. Thus, a better understanding of the molecular mechanisms underlying platelet migration at sites of inflammation may lead to the development of new treatments for immunothrombosis. Platelet migration also relies on integrin activation; however, the mechanisms appear to be different than platelet plug formation. Our lab and others have shown that Talin1, but not Rap1 or ⍺IIbβ3, is required for inflammatory hemostasis. Previous studies also demonstrated that 1 and 3 integrins are functionally redundant in inflammatory hemostasis. Based on these foundational studies, I hypothesize that the coordinated formation and turnover of multiple integrin-ligand connections is critical to platelet function in inflammation. My preliminary data show that Talin1 is required for platelet migration, while Rap1 is dispensable for migration but plays a role in the establishment of platelet polarity. Additionally, my studies are the first to show that platelets, like other migratory cells, can cycle integrin engagement and migrate on a diverse array of surfaces, including fibronectin, vitronectin, and low concentrations of fibrinogen. The seminal questions that my proposal seeks to address in two aims are: (1) how do key regulators of platelet integrin affinity facilitate platelet migration and polarity required for bacterial scavenging and inflammatory hemostasis?, and (2) which integrin receptors, ligands, and binding motifs enable platelet migration in vitro and in vivo? Successful completion of this work will provide novel mechanistic insights on how platelet adhesive signaling at sites of inflammation differs from that at sites of mechanical injury, information that is critical for the development of novel therapies that target platelet function in inflammatory disease conditions.
