Endothelial β-PIX (ARHGEF7) supports actomyosin mediated expulsion of VWF through dynamic reorganization of the cytoskeleton.

Abstract

Endothelial cells promote thrombosis and hemostasis through the secretion of von Willebrand Factor (VWF) from their secretory granules-the Weibel-Palade bodies (WPBs). In response to specific stimuli, dynamic actin nucleation and remodeling of the cytoskeleton facilitates the expulsion of ultra-large VWF multimers to elevate plasma VWF and to form a platform for platelet capture and thrombus formation. P21 activated kinase 2 (PAK2) is a crucial regulator of the actin cytoskeleton and is essential for VWF secretion in response to secretagogues which utilize actomyosin mediated exocytosis. Here, we characterize the role of β-PAK-interacting exchange factor (β-PIX) in WPB exocytosis. Inhibition of β-PIX function prevents dynamic cytoskeletal remodeling resulting in reduced VWF secretion. Depletion of β-PIX using siRNA reduced the number of WPB fusion events, prolonged the time taken for GFP-VWF to be secreted post-fusion and delayed kinetics of the exocytic actomyosin ring. Use of full length and truncated β-PIX demonstrated that the PAK interacting and GEF domain mediate cytoskeletal remodeling whereas only the full-length construct could rescue VWF secretion. β-PIX regulates both septin ring formation and cofilin mediated actin remodeling during actomyosin ring function. These data identify β-PIX as a regulator of endothelial exocytosis through supporting actomyosin-mediated expulsion of VWF.