Thesis Maaike Schillemans

Summary

Endothelial cells (ECs), the cells that form the inner lining of the entire vascular system, are critically involved in the regulation of hemostasis and inflammation. To perform these specialized functions ECs store a cocktail of bioactive compounds in secretory granules called Weibel-Palade bodies (WPBs), including the hemostatic protein von Willebrand factor (VWF). Upon activation WPBs release their content into the circulation through exocytosis, after which secreted VWF promotes the arrest of bleeding by recruiting platelets to sites of vascular injury. In this thesis we aimed to get a better understanding of the molecular mechanisms that control WPB exocytosis.

Chapter 2 reviews recent advances in our understanding of WPB exocytosis and provides an integrated view on WPB release, including the secretory pathways, stimuli and signaling cascades, and machinery that directs and facilitates exocytosis. An important group of regulators of vesicle trafficking and release is the large family of soluble NSF attachment receptor (SNARE) proteins. In Chapters 3 to 5 we have studied the role of the endothelial SNARE protein network in WPB release, which led to the identification of two novel regulators of VWF secretion, syntaxin-3 and VAMP8. Blood outgrowth endothelial cells from patients with mutations in STX3 (Microvillous Inclusion Disease; chapter 3) or AP3B1 (Hermansky-Pudlak Syndrome type 2; chapter 4) were used as ex vivo model systems to study the endothelial context of secretory disease. Unbiased proteomic interaction screens  were used in Chapter 5 to further map the composition of the WPB exocytotic machinery. Chapter 6 describes a novel approach to generate VWF deficient cord blood-derived BOECs using CRISPR gene editing.