Abstract

Weibel-Palade bodies (WPBs) are secretory granules that contain von Willebrand factor and P-selectin, molecules that regulate hemostasis and inflammation respectively. The presence of CD63/LAMP3 in the limiting membrane of WPBs has led to their classification as lysosome-related organelles. Many lysosome-related organelles contain intraluminal vesicles (ILVs) enriched in CD63 that are secreted into the extracellular environment during cell activation to mediate intercellular communication. To date there are no reports that WPBs contain or release ILVs. By light microscopy and live-cell imaging we show that CD63 is enriched in micro-domains within WPBs. Extracellular antibody recycling studies showed that CD63 in WPB micro-domains can originate from the plasma membrane. By cryo-electron tomography of frozen-hydrated endothelial cells we identify internal vesicles as a novel structural feature of the WPB lumen. By live-cell fluorescence microscopy we observe directly the exocytotic release of EGFP-CD63 ILVs as discrete particles from individual WPBs. WPB exocytosis provides a novel route for release of ILVs during endothelial cell stimulation.