Human endothelial cells size‐select their secretory granules for exocytosis to modulate their functional output

Background The secretory granules of endothelial cells, Weibel‐Palade bodies, are released in response to numerous extracellular signals. Their cargo is critical to many vascular functions including hemostasis and inflammation. This presents a fundamental problem: how can these cells initiate tailor‐made responses from the release of a single type of organelle, each with similar cargo? Each cell contains Weibel‐Palade bodies in a wide range of sizes, and we have shown that experimentally shortening these organelles disproportionately reduces their ability to initiate hemostasis in vitro, leaving leukocyte recruitment unaffected. Could the production of this range of sizes underpin differential responses? Objectives To determine whether different agonists drive the exocytosis of different sizes of Weibel‐Palade bodies. Methods We used a high‐throughput automated unbiased imaging workflow to analyze the sizes of Weibel‐Palade bodies within human umbilical vein endothelial cells (HUVECs) before and after agonist activation to determine changes in organelle size distributions. Results We found that a subset of agonists differentially evoke the release of the longest, most pro‐hemostatic organelles. Inhibiting the release of these longest organelles by just 15% gives a fall of 60% in an assay of secreted von Willebrand factor (vWF) function. Conclusions The size‐selection of granules for exocytosis represents a novel layer of control, allowing endothelial cells to provide diverse responses to different signals via the release of a single type of organelle.