Background: Upon vessel injury, platelets adhere to exposed matrix constituents via specific membrane receptors, including the von Willebrand factor receptor glycoprotein (GP)Ib-IX-V complex and integrins beta 1 and beta 3. In platelets, the Fes/CIP4-homology Bin-Amphiphysin-Rvs protein PACSIN2 associates with the cytoskeletal and scaffolding protein filamin A (FlnA), linking GPIb alpha and integrins to the cytoskeleton.Objectives: Here we investigated the role of PACSIN2 in platelet function.Methods: Platelet parameters were evaluated in mice lacking PACSIN2 and platelet integrin beta 1.Results: Pacsin2(-/-) mice displayed mild thrombocytopenia, prolonged bleeding time, and delayed thrombus formation in a ferric chloride-mediated carotid artery injury model, which was normalized by injection of control platelets. Pacsin2(-/-) platelets formed unstable thrombi that embolized abruptly in a laser-induced cremaster muscle injury model. Pacsin2(-/-) platelets had hyperactive integrin beta 1, as evidenced by increased spreading onto surfaces coated with the collagen receptor alpha 2 beta 1-specific peptide GFOGER and increased binding of the antibody 9EG7 directed against active integrin beta 1. By contrast, Pacsin2(-/-) platelets had normal integrin alpha IIb beta 3 function and expressed P-selectin normally following stimulation through the collagen receptor GPVI or with thrombin. Deletion of platelet integrin beta 1 in Pacsin2(-/-) mice normalized platelet count, hemostasis, and thrombus formation. A PACSIN2 peptide mimicking the FlnA-binding site mediated the pull-down of a FlnA rod 2 construct by integrin beta 7, a model for integrin beta-subunits.Conclusions: Pacsin2(-/-) mice displayed severe thrombus formation defects due to hyperactive platelet integrin beta 1. The data suggest that PACSIN2 binding to FlnA negatively regulates platelet integrin beta 1 hemostatic function.
