Platelet ?IIb?3 is required for normal hemostasis, but is also a validated drug target because of its essential role in pathologic thrombosis. ?IIb?3 is normally kept in an inactive bent conformation on circulating platelets, but rapidly switches to an active (ligand-binding) conformation in response to inside-out signaling. The ligand- occupied receptor then transmits outside-in signals via the ?? transmembrane (TM) and cytoplasmic domains that initiate platelet adhesion, a response inadvertently produced by current orthosteric inhibitory drugs, which has limited their clinical efficacy. The structural basis of bidirectional integrin signaling remains to be clarified. One model suggests that the ligand-binding site directly faces the plasma membrane in the bent conformation, with integrin genuextension required for access to ligand. However, the integrin ectodomain/TM tilt has not been defined experimentally. Further, regulated ?? TM domain association in integrins is driven by the ectodomain, which may explain some of the differences found in the reported NMR structures of the isolated ?IIb?3 TM domains. In preliminary studies, we produce a low-resolution cryo-EM structure of full-length ?IIb?3 in an unexpected orientation relative to the membrane, the result of its complex in cis with a tetraspanner, providing the first experimental definition of an integrin ectodomain/TM tilt, and novel insights into the structural basis of ?IIb?3 activation. We also develop a water soluble, stable and high affinity orthosteric inhibitor of ?IIb?3 that is not a partial agonist, and use it in a structure-guided approach to generate like compounds. We propose to expand on these preliminary data in three specific aims, utilizing multidisciplinary approaches that include electron cryomicroscopy, tomography, protein crystallography, structure-based drug design, new peptide synthesis technology and novel murine models of thrombosis.
Integrin ?IIb?3 is critical for platelet aggregation, which underlies hemostasis but also pathological arterial thrombosis in heart attacks that claim the life of one American every minute. Drugs targeting platelet ?IIb?3 have improved outcomes in heart attack victims, but serious adverse outcomes linked to inadvertent receptor activation by the drug have limited their clinical efficacy, and underscored the need for better understanding of how activation of ?IIb?3 is regulated and for development of safer anti-?IIb?3 therapeutics. In preliminary studies, we determined the first structure of ?IIb?3 in complex with an integral membrane protein, providing new insights into the regulation of ?IIb?3 activation, and a structure-guided approach for the design of novel and safer ligand-mimetic inhibitors of ?IIb?3, bases for the current proposal.
