Biosynthesis of von Willebrand factor

Biosynthesis of VWF occurs in vascular endothelial cells and megakaryocytes. In endothelial cells VWF is stored in rod-shaped endothelial cell-specific storage organelles, the Weibel-Palade bodies. Besides VWF, these Weibel-Palade bodies contain a number of other proteins, including P-selectin, angiopoietin-2, osteoprotegerin and a number of other components. Upon stimulation of endothelial cells by agonist such as thrombin or epinephrine, Weibel-Palade bodies undergo exocytosis, resulting in release or surface expression of their contents. The elongated shape of Weibel-Palade bodies has been attributed to the packaging of VWF multimers into helical structures. Analysis by electron microscopy reveals tubular-like structures that most likely are composed of tightly packed helically organized VWF multimers. Following their release Weibel-Palade bodies, VWF tubules are rapidly converted into ultra-large VWF strings that are anchored to the surface of endothelial cells. These ultra-large VWF strings provide multiple attachment sites for blood platelets (see figure below).

We have recently investigated the effect of the shear stress-induced transcription factor KLF2 on clustering and composition of WPBs using peripheral blood derived endothelial cells. Lentiviral over-expression of KLF2 resulted in a 4.5 fold increase in number of WPBs per cell when compared to mock-transduced endothelial cells (see first figure below). Unexpectedly, the average length of WPBs was significantly reduced. In mock-transduced endothelial cells WPBs had an average length of 1.8 µm whereas in KLF2 overexpressing cells WPBs had a average length of 1.4 µm. Overexpression of KLF2 abolished the perinuclear clustering of WPBs observed following stimulation with cAMP-raising agonists such as epinephrine. We previously hypothesized that perinuclear clustering of WPBs provides a means to limit excessive release of bioactive components from these organelles. We subsequently explored the pro-inflammatory P-selectin and angiopoietin-2 (Ang-2). Ang-2 has been shown to promote vascular leakage and endothelial cell migration thereby contributing to vascular remodeling. P-selectin was readily visualized in both KLF2 and mock-transduced endothelial cells. In contrast, confocal microscopy revealed that WPBs in KLF2-transduced cells did not contain Ang-2. Together our findings suggest show that KLF2 not only regulates the dynamics of WPBs but also regulates the size and contents of this highly versatile storage pool in endothelial cells.

Increased number of Weibel-Palade bodies (green) in KLF2 transduced endothelial cells. The periphery of the cell is shown by staining for ß-catenin (red).

Ultra-large VWF strings on the surface of endothelial cells. Stings are visualized by adhering blood platelets (arrowheads).