Synaptotagmin-like protein 2a regulates lumen formation via Weibel-Palade body apical secretion of angiopoietin-2 during angiogenesis
ObjectiveVascular lumen formation requires the redistribution of intracellular proteins to instruct apico-basal polarity, thereby enforcing maturation of both luminal and basal domains. In the absence of proper apical signaling, lumen formation can be distorted leading to lumen collapse and cessation of blood flow. Synaptotagmin-like protein-2a (Slp2a) has been implicated in apical membrane signaling; however, the role of Slp2a in vascular lumen formation has never been assessed. Approach and ResultsOur results demonstrate that Slp2a is required for vascular lumen formation. Using a 3- dimensional sprouting assay, sub-cellular imaging, and zebrafish blood vessel development we establish that Slp2a resides at the apical membrane acting as a tether for Rab27a that decorates Weibel-Palade bodies (WPBs). Unique to endothelial tissue, we show that Slp2a regulates exocytic activity of WPBs, thus regulating release of WPB contents into the luminal space during angiogenesis. Angiopoietin-2 is a Tie-2 receptor ligand that is selectively released from WPB secretory granules. We identify a critical role for angiopoietin-2 in regulating endothelial lumenization and show that in the absence of Slp2a, WPB contents cannot fuse with the apical membrane. This disrupts the release of angiopoietin-2 and blocks Tie-2 signaling necessary for proper lumen formation. ConclusionsOur results demonstrate a novel requirement of Slp2a for vascular lumen formation. Moreover, we show that Slp2a is required for the exocytic release of WPB secretory granule cargo during vascular lumen development, and thus is a core upstream component of the WPB secretory pathway. Furthermore, we provide evidence that WPB-housed angiopoietin-2 is required for vascular lumen formation. HIGHLIGHTSO_LISynaptotagmin-like protein-2a (Slp2a) is required for vascular lumen formation via its interaction with Rab27a and Weibel Palade Body secretory granules. C_LIO_LISlp2a is recruited to the apical membrane where it regulates secretion of Weibel Palade Body components into the luminal space. C_LIO_LIIn the absence of Slp2a, Weibel Palade Body-housed angiopoietin-2 ligand cannot be exocytosed, this impedes activation of Tie-2 signaling required for lumen biogenesis. C_LIO_LIKnockout of Slp2a or Tie-2 in zebrafish blunts the formation of vascular lumens during angiogenic development. C_LI