OC 41.4 - PACSIN2 regulates platelet integrin β1 hemostatic function

Background: Upon vascular injury, platelets adhere to exposed matrix constituents through specific membrane receptors, including the von Willebrand factor receptor GPIbα and integrins β1 and β3, thereby initiating thrombus formation. The F-BAR protein PACSIN2 colocalizes with GPIbα in platelets, where it associates with the cytoskeletal and scaffolding protein filamin A (FlnA).

Aims: We investigated the role of platelet PACSIN2 in regulating thrombus formation in vivo.

Methods: We determined platelet parameters in mice lacking PACSIN2 and platelet integrin β1.

Results: Pacsin2-/- mice displayed delayed thrombus formation in a ferric chloride-mediated carotid artery injury model. The phenotype was intrinsic to platelets, as it was conserved in chimeric mice lacking PACSIN2 in blood cells only and normalized by injection of control platelets. Pacsin2-/- platelets repeatedly formed unstable thrombi that embolized abruptly in a laser-induced cremaster muscle injury model. Following stimulation with thrombin and the GPVI-specific collagen-related peptide, Pacsin2-/- platelets displayed increased activation of the integrin β1, as evidenced by 9EG7 antibody binding, and showed increased spreading to surfaces coated with the integrin β1-specific peptide GFOGER. By contrast, Pacsin2-/- platelets activated the integrin αIIbβ3 and expressed P-selectin similar to controls. Pacsin2-/- mice were crossed with Itgb1fl/fl Pf4-iCre (Itgb1Plt-/-) mice specifically lacking platelet integrin β1 to determine whether hyperactive platelet integrin β1 contributed to the thrombus formation defects. Integrin β1 deletion normalized thrombus formation in the ferric chloride-mediated carotid artery and laser-induced cremaster muscle injury models. Importantly, Itgb1Plt-/- Pacsin2-/- platelets formed stable thrombi that did not embolize.

Conclusion(s): We conclude that Pacsin2-/- mice displayed thrombus formation defects due to hyperactivation of platelet integrin β1. We hypothesize that PACSIN2 binding to FlnA negatively regulates platelet integrin β1 hemostatic function and are currently exploring how plasma fibronectin binding to platelet integrin α5β1 contributes to the thrombus formation defects of Pacsin2-/- mice.