Type: Oral
Session: 301. Vasculature, Endothelium, Thrombosis and Platelets: Basic and Translational: Insights Into the Role of Endothelium in Cardiovascular Disease and Thrombosis
Hematology Disease Topics & Pathways:
Research, Fundamental Science
Methods: All experiments were performed on human umbilical vein endothelial cells (HUVECs). RalA/B and exocyst depletion were performed via targeted siRNAs. vWF antigen was estimated using ELISA. RalB activation was determined by a commercial pulldown assay specific for the activated form of RalB. Expression of FLAG-tagged RalB was performed by lentiviral transduction. RalB mutants were generated using site-directed mutagenesis of FLAG-RalB (WT). Spinning disc confocal microscope was used for immunofluorescence (IF). Immuno-electron microscopy was performed using immune-gold labeling. FLAG-pulldown and immunoprecipitation (IP) was performed on cell lysates using antibody-labeled magnetic beads.
Results: RalB but not RalA depletion, or degradation with the RalB-selective inhibitor dihydroartemisinin, significantly impaired thrombin-induced vWF exocytosis (55±7% decrease). RalB was activated (GTP-bound) and phosphorylated upon thrombin stimulation of HUVECs. Correspondingly, expression of constitutively active RalB-G23V mutant, which stays in a GTP-bound state, significantly increased vWF exocytosis compared to RalB-WT (69±4.6% increase). IF and EM confirmed co-localization of RalB and exocyst on WPBs. Unexpectedly, FLAG pulldown and protein identification by mass spectrometry demonstrated that RalB-WT (GDP-bound;“off”) but not RalB-G23V (GTP-bound;“on”) interacts with exocyst, confirmed by immunoblotting. IP of resting and thrombin-stimulated native HUVEC lysates with RalB antibodies, as well as reverse-IP with exocyst antibodies confirmed this differential binding. Exocyst binding site mutation (RalB-WTD49E) and transduction of these mutants in HUVECs resulted in significantly augmented vWF exocytosis (40±3% increase), like constitutively active RalB-G23V mutant. Moreover, IF showed uniform cytoplasmic distribution of WPB cigars in RalB-WT HUVECs, whereas in RalB-WTD49E mutants WPBs were found at the plasma membrane, like RalB-G23V. This confirms that RalB-free exocyst tethers WPB to the plasma membrane (Figure 1). Serine phosphorylation site mutation (RalB-WTS198A), previously shown to regulate GTPase activity of RalB, hence GTP-GDP cycling, also significantly decreased RalB exocyst binding, in turn increasing vWF release. This shows that post-translational phosphorylation of RalB at serine 192, which is also accelerated by thrombin activation of endothelial cells, regulates RalB-exocyst interaction, hence vWF release.
Conclusions: We have identified an important regulatory role of RalB in exocyst-dependent WPB secretion and vWF release. WPB localized exocyst is associated with RalB in resting endothelium. Endothelial cell activation switches RalB ‘on’, and exocyst, now uncoupled from GTP-bound RalB, is free to tether WPBs to the plasma membrane and facilitate vWF release.
Disclosures: Mendez: Inventiva: Consultancy.