Background: Granulocyte exocytosis is proposed to be critically dependent on the interaction of soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs) located on granules/vesicles (v-SNAREs) and plasma membrane (t-SNAREs). Previous studies indicated that the v-SNARE, vesicle-associated membrane protein (VAMP)-2, as well as t-SNAREs (SNAP-23, syntaxin-4 and -6) are implicated in exocytosis from human granulocytes. Vesicle-associated membrane proteins-7 and -8 have been implicated in endosome/lysosome trafficking, however, their role in granulocyte exocytosis remains obscure.
Objective: We sought to investigate the expression and functional role of SNARE isoforms in the secretion of different granule-derived mediators in human eosinophils and neutrophils.
Methods: The expression of SNAREs was determined by subcellular fractionation and flow cytometry. SNARE-specific antibodies were examined for their ability to impair mediator release from permeabilized eosinophils and neutrophils.
Results: Vesicle-associated membrane proteins-7 and -8 were localized to granule and membrane-enriched fractions in eosinophils and neutrophils, whereas syntaxin-6 was not detectable. In permeabilized cells, anti-VAMP-7, but not anti-VAMP-8, antibody impaired the secretion of all mediators examined (in eosinophils, eosinophil peroxidase and eosinophil-derived neurotoxin; in neutrophils, myeloperoxidase, lactoferrin and matrix metalloprotease-9) in a dose-dependent manner. In contrast, anti-VAMP-2 modestly and selectively impaired secretion from small granules and vesicles. Syntaxin-4, but not syntaxin-6, was found to interact with SNAP-23 and was partially involved in mediator secretion from multiple compartments.
Conclusion: Our observations indicate for the first time a critical role for VAMP-7 in both eosinophil and neutrophil mediator release.