Intracellular storage and regulated secretion of von Willebrand factor in quantitative von Willebrand disease

Several missense mutations in the von Willebrand Factor (VWF) gene of von Willebrand disease (VWD) patients have been shown to cause impaired constitutive secretion and intracellular retention of VWF. However, the effects of those mutations on the intracellular storage in Weibel-Palade bodies (WPBs) of endothelial cells and regulated secretion of VWF remain unknown. We demonstrate, by expression of quantitative VWF mutants in HEK293 cells, that four missense mutations in the D3 and CK-domain of VWF diminished the storage in pseudo-WPBs, and led to retention of VWF within the endoplasmic reticulum (ER). Immunofluorescence and electron microscopy data showed that the pseudo-WPBs formed by missense mutant C1060Y are indistinguishable from those formed by normal VWF. C1149R, C2739Y, and C2754W formed relatively few pseudo-WPBs, which were often short and sometimes round rather than cigar-shaped. The regulated secretion of VWF was impaired slightly for C1060Y but severely for C1149R, C2739Y, and C2754W. Upon co-transfection with wild-type VWF, both intracellular storage and regulated secretion of all mutants were (partly) corrected. In conclusion, defects in the intracellular storage and regulated secretion of VWF following ER retention may be a common mechanism underlying VWD with a quantitative deficiency of VWF.