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

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 haemostasis 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 haemostasis in vitro, leaving leukocyte recruitment unaffected. Could the production of this range of sizes underpin differential responses?To determine whether different agonists drive the exocytosis of different sizes of Weibel-Palade bodies.We used a high-throughput automated unbiased imaging workflow to analyse the sizes of Weibel-Palade bodies within HUVECs before and after agonist activation to determine changes in organelle size distributions.We found that a subset of agonists differentially evoke the release of the longest, most pro-haemostatic organelles. Inhibiting the release of these longest organelles by just 15% gives a fall of 60% in an assay of secreted VWF function.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.