Synaptotagmin 5 regulates calcium-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca2+ concentration ([Ca2+]i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of Von Willebrand factor (VWF) from endothelial cells, however, the identity of WPB-associated Ca2+-sensors involved in transducing acute increases in [Ca2+]i into granule exocytosis remain unknown. Here we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVEC) and is recruited to WPBs to regulate Ca2+-driven WPB exocytosis. Western blot analysis of HUVEC identified SYT5 protein, and exogenously expressed SYT5-mEGFP localized almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca2+-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca2+ sensor regulating Ca2+-dependent secretion of stored mediators from vascular endothelial cells.