Regulation of Stimulated and Basal Release of Weibel-Palade Bodies By Syntaxin-3 Containing SNARE-Complexes

Vascular endothelial cells contain unique rod-shaped secretory granules, called Weibel-Palade bodies (WPBs), which contain a number of haemostatic, angiogenic and inflammatory mediators. Several components that are critical for regulated WPB exocytosis have been identified, including the small GTPase Rab27A and its effector synaptotagmin-like protein 4-a (Slp4-a), but the mechanism remains unclear. We have previously identified syntaxin binding protein 1 (STXBP1) as an endogenous Slp4-a binding partner involved in WPB release, along with the SNARE proteins syntaxin-2 and -3. In this study we investigated the possible role of syntaxin-2 and -3 in WPB exocytosis.

We characterized the subcellular location of these syntaxins in endothelial cells using immunocytochemistry. Syntaxin-2 was primarily associated with the plasma membrane where it localized at VE-caderin-based adherens junctions and at integrin-based adhesions to the extracellular matrix. Interestingly, the t-SNARE syntaxin-3 was primarily associated with WPBs. To further explore its role in WPB biology we mapped the endothelial interaction partners of syntaxin-3 through an unbiased mass spectrometry approach using pull downs of lentivirally-expressed mEGFP-syntaxin-3 with anti-GFP nanobeads. Among its interaction partners are various SNAREs and associated proteins such as syntaxin binding proteins 2 and 5 (STXBP2/5), N-ethylmaleimide-sensitive factor (NSF), SNAP23 and α-SNAP, suggesting we successfully pulled down a SNARE complex and its regulatory machinery that are involved in exocytosis. We further addressed the role of syntaxin-3 in stimulated WPB exocytosis using siRNA-mediated knockdowns and found that depletion of syntaxin-3 led to a significant potentiation of Ca²⁺- as well as cAMP-mediated VWF secretion. In contrast, we observed a decrease in basal (unstimulated) VWF secretion after silencing of syntaxin-3, which amounts to a significantly increased intracellular VWF content. The potentiation of VWF secretion after depletion of syntaxin-3 was almost completely attributable to an increased WPB pool size; when corrected for increased WPB content the probability of stimulated release of a WPB was unaltered in the absence of syntaxin-3.

Our data position syntaxin-3 as a WPB-linked SNARE-protein that regulates basal secretion of VWF.