Evoked Weibel-Palade Body Exocytosis Modifies the Endothelial Cell Surface by Releasing a Substrate-Selective Phosphodiesterase.

Weibel Palade bodies (WPB) are lysosome-related secretory organelles of endothelial cells. Commonly known for their main cargo, the platelet and leukocyte receptors von-Willebrand factor (VWF) and P-selectin, WPB play a crucial role in hemostasis and inflammation. Here, the authors identify the glycerophosphodiester phosphodiesterase domain-containing protein 5 (GDPD5) as a WPB cargo protein and show that GDPD5 is transported to WPB following uptake from the plasma membrane via an unique endocytic transport route. GDPD5 cleaves GPI-anchored, plasma membrane-resident proteins within their GPI-motif, thereby regulating their local activity. The authors identify a novel target of GDPD5 , the complement regulator CD59, and show that it is released from the endothelial surface by GDPD5 following WPB exocytosis. This results in increased deposition of complement components and can enhance local inflammatory and thrombogenic responses. Thus, stimulus-induced WPB exocytosis can modify the endothelial cell surface by GDPD5-mediated selective release of a subset of GPI-anchored proteins.

Acidification of endothelial Weibel-Palade bodies is mediated by the vacuolar-type H+-ATPase.

Weibel-Palade bodies (WPB) are unique secretory granules of endothelial cells that store the procoagulant von-Willebrand factor (VWF) in a highly compacted form. Upon exocytosis the densely packed VWF unfurls into long strands that expose binding sites for circulating platelets and thereby initiate the formation of a platelet plug at sites of blood vessel injury. Dense packing of VWF requires the establishment of an acidic pH in the lumen of maturing WPB but the mechanism responsible for this acidification has not yet been fully established. We show here that subunits of the vacuolar-type H+-ATPase are present on mature WPB and that interference with the proton pump activity of the ATPase employing inhibitors of different chemical nature blocks a reduction in the relative internal pH of WPB. Furthermore, depletion of the V-ATPase subunit V0d1 from primary endothelial cells prevents WPB pH reduction and the establishment of an elongated morphology of WPB that is dictated by the densely packed VWF tubules. Thus, the vacuolar-type H+-ATPase present on WPB is required for proper acidification and maturation of the organelle.

Spire1 and Myosin Vc promote Ca2+-evoked externalization of von Willebrand factor in endothelial cells.

Weibel-Palade bodies (WPB) are endothelial cell-specific storage granules that regulate vascular hemostasis by releasing the platelet adhesion receptor von Willebrand factor (VWF) following stimulation. Fusion of WPB with the plasma membrane is accompanied by the formation of actin rings or coats that support the expulsion of large multimeric VWF fibers. However, factor(s) organizing these actin ring structures have remained elusive. We now identify the actin-binding proteins Spire1 and Myosin Vc (MyoVc) as cytosolic factors that associate with WPB and are involved in actin ring formation at WPB-plasma membrane fusion sites. We show that both, Spire1 and MyoVc localize only to mature WPB and that upon Ca2+ evoked exocytosis of WPB, Spire1 and MyoVc together with F-actin concentrate in ring-like structures at the fusion sites. Depletion of Spire1 or MyoVc reduces the number of these actin rings and decreases the amount of VWF externalized to the cell surface after histamine stimulation.

Weibel Palade Bodies: Unique Secretory Organelles of Endothelial Cells that Control Blood Vessel Homeostasis.

Vascular endothelial cells produce and release compounds regulating vascular tone, blood vessel growth and differentiation, plasma composition, coagulation and fibrinolysis, and also engage in interactions with blood cells thereby controlling hemostasis and acute inflammatory reactions. These interactions have to be tightly regulated to guarantee smooth blood flow in normal physiology, but also allow specific and often local responses to blood vessel injury and infectious or inflammatory insults. To cope with these challenges, endothelial cells have the remarkable capability of rapidly changing their surface properties from non-adhesive (supporting unrestricted blood flow) to adhesive (capturing circulating blood cells). This is brought about by the evoked secretion of major adhesion receptors for platelets (von-Willebrand factor, VWF) and leukocytes (P-selectin) which are stored in a ready-to-be-used form in specialized secretory granules, the Weibel-Palade bodies (WPB). WPB are unique, lysosome related organelles that form at the trans-Golgi network and further mature by receiving material from the endolysosomal system. Failure to produce correctly matured VWF and release it through regulated WPB exocytosis results in pathologies, most importantly von-Willebrand disease, the most common inherited blood clotting disorder. The biogenesis of WPB, their intracellular motility and their fusion with the plasma membrane are regulated by a complex interplay of proteins and lipids, involving Rab proteins and their effectors, cytoskeletal components as well as membrane tethering and fusion machineries. This review will discuss aspects of WPB biogenesis, trafficking and exocytosis focussing on recent findings describing factors contributing to WPB maturation, WPB-actin interactions and WPB-plasma membrane tethering and fusion.