Weibel-Palade bodies : an exciting way out!

Abstract

Weibel-Palade bodies are specialized secretory granules found in endothelial cells. These vesicles are able to store a variety of proteins with different biological functions such as the multimeric glycoprotein von Willebrand factor (VWF) that is required for normal haemostasis, the leukocyte adhesion receptor P-selectin, and the chemokine interleukin-8 (IL-8). Correct delivery of these proteins to the cell membrane or to the circulation is essential for the biological role of these proteins. Indeed, regulated exocytosis of Weibel-Palade bodies serves several physiological functions including inflammatory and haemostatic responses. However, the mechanisms such as sorting of proteins to Weibel-Palade bodies, Weibel-Palade body trafficking in the cell and secretion of their content into the circulation, are far of being understood. In this thesis, I tried to get more insight into the life cycle of Weibel-Palade bodies and the extracellular function of one of their constituents. The first step of the life cycle of Weibel-Palade bodies is the sorting process of proteins into these storage organelles. Chapter 2 indicates that VWF plays an active role in sequestering IL-8 into Weibel-Palade bodies. Chapter 3 describes the existence of different pools of Weibel-Palade bodies: some vesicles barely moved as if they were tethered, others seemed to travel in a stochastic manner. Upon arrival at the plasma membrane, tethered Weibel-Palade bodies are trapped in the actin cortex of endothelial cells awaiting a trigger for release. Upon stimulation of endothelial cells, either with Ca2+- or cAMP-raising agonists, we observed fusion of individual vesicles with the plasma membrane, gradual release and diffusion of Weibel-Palade body content into the extracellular compartment. As a result of slow exocytosis, endothelial cells may present at their surface focal sites with high concentrations VWF, IL-8 and P-selectin which, in turn, may play a role in focal adhesion of blood constituents to the endothelium upon vascular injury. In addition to exocytosis, cAMP-raising agonists induce clustering of a subset of Weibel-Palade bodies in the perinuclear region of the cell. This may be a mechanism to prevent excessive exocytosis of Weibel-Palade bodies. Chapter 4 indicates that RalA, a small GTP-binding protein shown to be associated with Weibel-Palade bodies, is involved in the regulated exocytosis of these vesicles. RalA may interact with a putative exocyst leading to docking of Weibel-Palade bodies at the plasma membrane. VWF-propeptide, a Weibel-Palade constituent, was used as a marker for endothelial cell perturbation in Chapter 6 to determine whether endothelial cell activation is the primary event in the pathogenesis of thrombotic thrombocytopenic purpura (TTP). We concluded that endothelial cell activation is not the primary event leading to TTP. Vascular perturbation seems a consequence rather than a cause of the disease. In Chapter 7 the potential role of the VWF-propeptide as an inflammatory mediator was investigated. Neutrophil adhesion induced by IL-8 increased significantly upon co-stimulation with VWF-propeptide suggesting that the propeptide acts synergistically with IL-8 upon appropriate stimulation of the endothelium.

Taken together, biogenesis and exocytosis of Weibel-Palade bodies are essential in the processes underlying hemostatic, thrombotic and inflammatory responses