Mechanisms of integrin signaling and a new anti-platelet/anti-inflammatory approach

Thrombotic cardiovascular diseases remain the leading cause of death in US and world. Blood platelets play key roles in both hemostasis and thrombosis. Not only do platelets adhere and aggregate to form thrombi at the site of vascular injury, platelets also facilitate coagulation (formation of a fibrin clot). Both platelet thrombus formation and coagulation are important contributors to the morbidity and mortality of thrombotic diseases such as heart attack and stroke. Thus, anti-platelet drugs and anti-coagulants were developed and are clinically used to prevent and treat thrombosis. However, current anti-platelet drugs and anti-coagulants have deficiencies. First, anti-platelet drugs and anti-coagulants all have the major adverse effect of bleeding, which can be life-threatening. Furthermore, anti-platelet drugs are not as effective in treating coagulation, and vice versa, but combined use of anti-platelets and anti-coagulants greatly exacerbates bleeding risk. Thus, it would be highly significant to develop dual anti-platelet and anti-coagulant drugs, which do not cause bleeding. Furthermore, thrombosis can be induced by chronic and acute inflammatory conditions such as atherosclerosis and sepsis. Conversely, thrombosis induces and exacerbates inflammation. Thus, it is also highly desirable to develop a drug that is more potent than aspirin in anti-thrombotic and anti-inflammatory efficacy. The Integrin family of adhesion receptors plays key roles in both in platelets and leukocytes. Platelet integrin aIIbb3 (GPIIb- IIIa), upon activation by inside-out signaling stimulated by agonists, not only mediates platelet adhesion and thrombus formation, but also transmits outside-in signals leading to thrombus expansion and occlusive thrombosis, but is dispensable for primary hemostasis. Leukocyte b2 integrins, aLb2 and amb2, mediate leukocyte adhesion and outside-in signaling, leading to cell migration, cytokine release, phagocytosis, etc., and thus play critical roles in inflammation. We recently showed that outside-in signaling of integrins requires the binding of G protein subunit Ga13 to an ExE motif conserved in the cytoplasmic domains of both b2 and b3. Project 1 of this proposal is to investigate the mechanisms of Ga13-dependent integrin outside-in signaling. Project 2 is to investigate the important role of b3 outside-in signaling in shear- and agonist-induced platelet procoagulant activity, and the conceptual basis for targeting Ga13-b3 interaction to develop a dual anti- platelet/anti-coagulant drug with minimal bleeding risk. Project 3 is to investigate the role of Ga13-dependent b2 integrin outside-in signaling in proinflammatory functions of leukocytes and in severe sepsis, and the conceptual basis for developing dual anti-thrombotic and anti-inflammatory drugs for treating severe sepsis and other thrombo-inflammatory conditions.