Signaling Mechanisms of Platelet Adhesion Receptors: Integrins and GPIb-IX

Platelets play important roles in both hemostasis and thrombosis. Antiplatelet therapy has become a key pharmacological method in prevention and treatment of cardiovascular, and cerebrovascular diseases, such as coronary syndromes and ischemic stroke. Two critical adhesion receptors on platelets are integrins and GPIb-IX. Many anti-platelet drugs target these receptors, but their development and usage are limited by the risk of severe side effects, including potentially fatal bleeding and thrombocytopenia. My study is focusing on understanding the signaling mechanisms of both receptors and developing new anti-platelet approaches for treating cardiovascular diseases more safely. My research includes two major projects: Integrin beta3 directly inhibits the Galpha13-p115RhoGEF interaction to regulate G protein signaling and platelet exocytosis. The integrins and G protein-coupled receptors (GPCRs) are both fundamental in cell biology. GPCRs are known to stimulate integrin activation. However, it is unclear whether integrins regulate GPCR signaling. We show that beta3 integrins negatively regulate GPCR signaling by directly inhibiting the Galpha13-p115RhoGEF interaction. Furthermore, whereas beta3 deficiency or integrin antagonists inhibited integrin-dependent platelet aggregation and exocytosis (granule secretion), they enhanced GPCR-stimulated RhoA activation and integrin-independent secretion. In contrast, a beta3-derived Galpha13-binding peptide or Galpha13 knockout inhibited GPCR-stimulated RhoA activation and both integrin-independent and dependent platelet secretion without affecting primary platelet aggregation. In a mouse model of myocardial ischemia/reperfusion injury (MI/RI) in vivo, the beta3-derived Galpha13-binding peptide inhibited platelet secretion of granule constituents, which exacerbated inflammation and MI/RI. These data establish crucial integrin-GPCR crosstalk, providing a rationale for therapeutic approaches that inhibit exocytosis in platelets and possibly other cells without adverse effects associated with loss of cell adhesion and fatal bleeding. Direct binding of Lyn to GPIbbeta transmits two-way GPIb-IX signaling to stimulate platelet activation and VWF binding. The platelet glycoprotein (GP) Ib-IX mediates platelet adhesion to von Willebrand factor (VWF) and interacts with thrombin and other ligands to mediate thrombosis and inflammation. Ligand binding to GPIb-IX also transmits platelet activation signals. GPIb-IX signaling requires Src family kinase (SFK) Lyn, and SFK isoforms (Src and Lyn) co-immunoprecipitated with the GPIb-IX complex in platelet lysates. However, it is unclear whether Lyn directly binds to GPIb-IX, and if so, where the Lyn binding site is. It is also unclear whether Lyn binding to GPIb-IX is required for Lyn activation and GPIb-IX signaling. Here we show that Lyn directly binds to the beta subunit of GPIb between amino acids 144-161 at the transmembrane domain and cytoplasmic domain interface. We synthesized a myristoylated inhibitor peptide, mPLbeta, with sequence derived from the GPIbbeta-Lyn binding site. This peptide inhibited Lyn binding to GPIb-IX, GPIb-IX-dependent Lyn/SFK activation, platelet activation, stable platelet adhesion to VWF and ristocetin-induced platelet aggregation. Importantly, mPLbeta also partially inhibited ristocetin-induced platelet agglutination even in the presence of integrin inhibitors, which is minimally affected by platelet activation inhibitor forskolin or PGE1. Biomembrane force probe study shows that mPLbeta reduces the VWF-A1 domain-GPIb adhesion frequency, and also inhibits force-induced A1-mediated intraplatelet calcium elevation. Furthermore, mPLbeta inhibited arterial thrombosis in vivo with a mild effect on tail bleeding. These data demonstrate that direct binding of Lyn to GPIbbeta mediates two-way GPIb-IX signaling to activate platelets and modulate VWF-GPIb interaction. Targeting the Lyn-GPIbbeta interaction thus has the potential for treating GPIb-IX-dependent thrombosis and inflammation.