Key Points

  • Exocyst interacts with BLOC-2 in the delivery of endosomal cargo to maturing Weibel Palade bodies essential for vWF multimerization.

  • Exocyst serves as a clamp, impeding vWF exocytosis, which can be reversibly inhibited to facilitate vWF release.

von Willebrand factor (vWF) is an essential hemostatic protein that is synthesized in endothelial cells and stored in Weibel-Palade bodies (WPBs). Understanding the mechanisms underlying WPB biogenesis and exocytosis could enable therapeutic modulation of endogenous vWF, yet optimal targets for modulating vWF release have not been established. Since biogenesis of lysosomal related organelle-2 (BLOC-2) functions in the biogenesis of platelet dense granules and melanosomes, which like WPBs are lysosome-related organelles (LROs), we hypothesized that BLOC-2-dependent endolysosomal trafficking is essential for WPB biogenesis and sought to identify BLOC-2 interacting proteins. Depletion of BLOC-2 caused misdirection of cargo-carrying transport tubules from endosomes, resulting in immature WPBs that lack endosomal input. Immunoprecipitation of BLOC-2 identified the exocyst complex as a binding partner. Depletion of the exocyst complex phenocopied BLOC-2 depletion, resulting in immature WPBs. Furthermore, releasates of immature WPBs from either BLOC-2 or exocyst-depleted endothelial cells lacked high-molecular weight forms of vWF, demonstrating the importance of BLOC-2/exocyst-mediated endosomal input during vWF maturation. Yet BLOC-2 and exocyst showed very different effects on vWF release. While BLOC-2 depletion impaired exocytosis, exocyst depletion augmented WPB exocytosis, indicating that it acts as a clamp. Exposure of endothelial cells to a small molecule inhibitor of exocyst, Endosidin2, reversibly augmented secretion of mature WPBs containing high molecular weight forms of vWF. These studies show that while BLOC-2 and exocyst cooperate in WPB formation, only exocyst serves to clamp WPB release. Exocyst function in vWF maturation and release are separable, a feature that can be exploited to enhance vWF release.

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