Development of Neutrophil Granule Diversity

The neutrophil has intracellular stores of both membrane proteins and soluble proteins that may be incorporated into the plasma membrane and exocytosed, respectively, at different times to meet the demands for assisting the neutrophil in adhesion to endothelium (secretory vesicles), for migration through basement membranes (gelatinase granules), and for phagocytosis, killing, and digestion of microorganisms (specific granules and azurophil granules). One reason for segregating the proteins into different subpopulations of granules is that some proteins cannot exist in the same compartment (NGAL digested if present in azurophil granules). Another reason is that the content of the different granules is needed at different times and places. The background for the diversity of neutrophil granules is the timing of biosynthesis, which again most likely is the result of transcriptional control, although this is not yet proven. It is possible that the control of exocytosis is also determined by the same mechanism--a carefully controlled timing of biosynthesis of fusion proteins that ties the content of granules to the likelihood that a given stimulus will result in exocytosis of that individual granule subset.