Analysis of intracellular storage and regulated secretion of three von Willebrands disease-causing variants of von Willebrand factor

Authors:
Michaux G, Hewlett LJ, Messenger SL, Goodeve AC, Peake IR, Daly ME and Cutler DF
In:
Blood (2003) 102(7): 2452-2458
Experiment
Mutations in the VWF (von Willebrand factor) gene are responsible for von Willebrand disease, the most common inherited bleeding disorder in humans. In endothelial cells VWF is stored in Weibel-Palade bodies (WPB). Transient nucleofection of HEK293 cells with wild-type VWF cDNA lead to the formation of ?pseudo-WPBs which could be used to examine 3 VWF mutations leading to von Willebrand disease. All 3 mutations could induce formation of ?pseudo-WPBs? but with significant differences between WT- and mutant VWF-containing organelles. The most striking defects were delay in formation and a reduction in the length and number of pseudo-WPBs in proportion to the clinical severity of the mutation.
Abstract
The rapid exocytosis of von Willebrand factor (VWF) in response to vascular injury can be attributed to the fact that VWF is stored in the Weibel-Palade bodies (WPBs) of endothelial cells. We describe a system for examining the ability of VWF to drive both the formation of a storage compartment and the function of that compartment with respect to regulated secretion. Transient transfection of HEK293 cells with wild-type human VWF cDNA leads to the formation of numerous elongated organelles that resemble WPBs. These "pseudo-WPBs" exhibit the internal structure, as well as the ability to recruit membrane proteins including P-selectin, of bona fide WPBs. Finally, VWF was efficiently secreted upon stimulation by phorbol ester. We used this system to examine 3 VWF mutations leading to von Willebrand disease that affect VWF multimerization and constitutive secretion. Surprisingly we find that all 3 mutants can, to some extent, make pseudo-WPBs that recruit appropriate membrane proteins and that are responsive to secretagogues. The most striking defects are a delay in formation and a reduction in the length and number of pseudo-WPBs in proportion to the clinical severity of the mutation. Studies of pseudo-WPB formation in this system thus yield insights into the structure-function relationships underpinning the ability of VWF to form functional WPBs.