Von Willebrand factor remodeling during exocytosis from vascular endothelial cells

Abstract

In vascular endothelial cells, high molecular weight multimers of von Willebrand factor (VWF) are folded into tubular structures for storage in Weibel-Palade bodies. On stimulation, VWF is secreted and forms strings to induce primary hemostasis. The structural changes composing the transition of stored tubular VWF into secreted unfurled VWF strings are still unresolved even though they are vital for normal hemostasis. The secretory pod is a novel structure that we previously described in endothelial cells. It is formed on stimulation and has been postulated to function as a VWF release site. In this study, we investigated the actual formation of secretory pods and the subsequent remodeling of VWF into strings. Human umbilical vein endothelial cells were stimulated and studied using various imaging techniques such as live-cell imaging and correlative light and electron microscopy. We found by using live-cell imaging that secretory pods are formed through the coalescence of multiple Weibel-Palade bodies without involvement of other large structures. Secreted VWF expelled from secretory pods was found to adopt a globular conformation. We visualized that VWF strings derive from those globular masses of VWF. Flow experiments showed that, on secretion, the globular masses of VWF move to the edge of the cell, where they anchor and generate VWF strings. On secretion, VWF adopts a globular conformation that remodels into strings after translocation and anchoring at the edge of the cell. This finding reveals new pathophysiological mechanisms that could be affected in patients with von Willebrand disease.