Endothelial cells promote thrombosis and haemostasis 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 remodelling 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 utilise actomyosin mediated exocytosis. Here we characterise the role of β-PAK-interacting exchange factor (β-PIX) in WPB exocytosis. Inhibition of β-PIX function prevents dynamic cytoskeletal remodelling 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 exocytic actomyosin ring. Use of full length and truncated β-PIX demonstrated that the PAK interacting and GEF domain mediate cytoskeletal remodelling whereas only the full-length construct could rescue VWF secretion. β-PIX regulates both septin ring formation and cofilin mediated actin remodelling during actomyosin ring function. These data identify β-PIX as a regulator of endothelial exocytosis through supporting actomyosin mediated expulsion of VWF.
El-Mansi et al. (Thu,) studied this question.