Project Summary/Abstract Endothelium plays a vital role in hemostasis and thrombosis. It is essential to maintain blood fluidity, but it also expresses and releases numerous “factors” that regulate blood cell activation and coagulation. Among these, von Willebrand factor (vWF) is an essential plasma hemostatic factor released by the endothelium. Deficiency of mature vWF is the most common bleeding disorder in humans. On the other hand, elevated plasma levels of vWF, or abnormal concentrations of its high-molecular weight multimers, is associated with increased risk of cardiovascular morbidity. As such, understanding of the molecular mechanisms that underlie regulated release of vWF have broad implications in hemostasis and thrombosis. One factor that plays a crucial role in normal vWF exocytosis from endothelial cells is unimpaired biogenesis of its storage granules, the Weibel-Palade bodies (WPBs), which is a highly complex process that remains poorly characterized. We have previously shown that deficiency of biogenesis of lysosome-related organelle complex 2 (BLOC-2) results in impaired vWF exocytosis. Since, BLOC-2, and other related trafficking proteins, are essential for biogenesis of lysosomal-related organelles, a group of specialized granules that includes platelet dense granules and melanosomes, it seems plausible that this protein complex is also required for biogenesis of WPB. We hypothesize that BLOC-2-mediated endosomal trafficking is critical for biogenesis of WPB. We further postulate that the exocyst complex plays an essential role in WPB trafficking by 1) interacting with BLOC-2 in its role in endosomal trafficking 2) and regulating soluble NSF adaptor protein receptor (SNARE)-mediated fusion of WPBs at the plasma membrane. In Aim 1, we will characterize cargo trafficking from the endosomes to the maturing WPBs and the dependence of this pathway on BLOC-2. In Aim 2, we will evaluate the role of the exocyst complex in BLOC-2-dependent endosomal trafficking and characterize the core trafficking machinery involved in this pathway. And, finally, in Aim 3, we will evaluate the regulatory function of the exocyst complex in SNARE-mediated fusion of WPBs at the plasma membrane. The applicant, Dr. Anish Sharda, is currently an Assistant Professor at Yale University with Dr. John Hwa as his primary mentor. He previously completed clinical as well as a research fellowship in hematology under Dr. Robert Flaumenhaft at BIDMC, Boston, who will continue to serve as his co-mentor. The Yale cardiovascular research center, with its distinguished record in innovation and mentorship, will provide an excellent environment for the applicant in pursuit of his career goals. The advisory committee of exceptional scientists that Dr. Sharda has established will bring diverse intellectual expertise to his training and scientific growth to meet his long-term goal of becoming an independent researcher.