Intracellular location of syntaxin 7 in human neutrophils

Neutrophils are the first line of defense in the innate immune system. Neutrophils neutralize invading microorganisms mainly by phagocytosis, but the mechanism and molecules involved in this process are not well characterized. Because the endosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein syntaxin 7 regulates vesicle trafficking events in phagocytosis, we investigated the expression and subcellular localization of syntaxin 7 in human neutrophils. Here we have found that human peripheral blood neutrophils and neutrophil-differentiated HL-60 cells express syntaxin 7 at both mRNA and protein levels. Using biochemical and ultrastructural approaches, we found that syntaxin 7 was broadly located in the membranes of the three major cytoplasmic granules of human neutrophils, with a major location in azurophilic granules, which are mainly involved in phagocytosis. A secondary, but extensive, location of syntaxin 7 was in specific and tertiary granules, which resulted translocated to the plasma membrane upon cell activation that promoted mobilization of these organelles. These data reveal the presence of syntaxin 7 in the membranes of exocytosis-prone granules (specific and tertiary granules) and phagocytosis-related granules (azurophilic granules) in human neutrophils, and therefore it might play a role in both exocytosis and phagocytosis in human neutrophils.