Platelet alphaIIbbeta3 activation and its therapeutic targeting

Massachusetts General Hospital, Boston MA

Investigators

Linked publications, trials & patents

Abstract

Abstract Platelet integrin αIIbβ3 plays a central and nonredundant role in hemostasis and thrombosis. It is normally kept in a default inactive conformation in resting free-flowing platelets in the circulation. Despite extensive efforts, it remains unclear how αIIbβ3 is normally kept inactive in the native membrane environment of circulation platelets despite the presence of μM concentrations of its physiologic ligand fibrinogen. In response to agonists like ADP or thrombin, generated at sites of vascular injury, αIIbβ3 rapidly switches to a primed (ligand-competent) state and engages ligands to initiate the formation of the hemostatic plug, a process that includes platelet aggregation and clot retraction. How ligand-binding triggers conformational changes leading to platelet adhesion in the full- length integrin has not been determined. And although much is known about the process of platelet aggregation, studies evaluating the active state of the integrin during clot retraction are lacking. Supported by the previous funding cycle, we achieved major advances in the integrin field by determining the first structures of full-length αIIbβ3 in a new membrane-mimetic environment using cryo-electron microscopy (cryo-EM). We also elucidated the profound conformational changes occurring when the full-length integrin is ligand-occupied. These conformational changes are prevented in the full-length αIIbβ3 when bound by new synthetic ligand-mimetic compounds we developed termed pure antagonists. The pure antagonists were as effective as current agonistic αIIbβ3 drugs in preventing thrombosis in mice but without causing bleeding, a major drawback of the agonistic drugs. These advances revealed several unexpected and novel features of the full- length αIIbβ3 structures that we propose to pursue to establish how this integrin is kept inactive on circulating platelets, thus avoiding disease. We also propose to take advantage of the unique properties of the pure antagonists to define the activation state of the integrin in formation of the contracting platelet-fibrin meshwork during clot retraction. The results from these proposed studies are expected to provide a more complete understanding of how αIIbβ3 activity is regulated in health and disease and offer new insights into the process of clot retraction that is key to making the impermeable seal in hemostasis.