Endobrevin/VAMP-8 Is the Primary v-SNARE for the Platelet Release Reaction

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Abstract
Platelet secretion is critical to hemostasis. Release of granular cargo is mediated by soluble NSF attachment protein receptors (SNAREs), but despite consensus on t-SNAREs usage, it is unclear which Vesicle Associated Membrane Protein (VAMPs: synaptobrevin/VAMP-2, cellubrevin/VAMP-3, TI-VAMP/VAMP-7, and endobrevin/VAMP-8) is required. We demonstrate that VAMP-8 is required for release from dense core granules, alpha granules, and lysosomes. Platelets from VAMP-8−/−mice have a significant defect in agonist-induced secretion, though signaling, morphology, and cargo levels appear normal. In contrast, VAMP-2+/−, VAMP-3−/−, and VAMP-2+/−/VAMP-3−/−platelets showed no defect. Consistently, tetanus toxin had no effect on secretion from permeabilized mouse VAMP-3−/−platelets or human platelets, despite cleavage of VAMP-2 and/or -3. Tetanus toxin does block the residual release from permeabilized VAMP-8−/−platelets, suggesting a secondary role for VAMP-2 and/or -3. These data imply a ranked redundancy of v-SNARE usage in platelets and suggest that VAMP-8−/−mice will be a useful in vivo model to study platelet exocytosis in hemostasis and vascular inflammation.