Phospholipase D (PLD) activation in stimulated neutrophils results in the conversion of membrane phosphatidylcholine (PC) to phosphatidic acid (PA). This change in membrane phospholipid composition has two potentially positive effects on degranulation. It 1) replaces a nonfusogenic phospholipid with a fusogenic one and 2) increases the potential for interactions between membranes and the annexins. Modeling neutrophil degranulation, we examined the effect of PLD (Streptomyces chromofuscus) hydrolysis on the aggregation and fusion of liposomes in the presence and absence of annexin I. We found that PLD-mediated conversion of PC to PA lowered the [Ca2+] required for fusion. Annexin I increased the rate of fusion in the presence of PA, although it did not lower threshold [Ca2+], which remained above the physiological range. However, after hydrolysis by PLD, annexin I lowered the [Ca2+] required for aggregation by almost three orders of magnitude, to near physiological concentrations. These studies indicate that the activation of PLD and the production of PA may play a role in annexin-mediated membrane-membrane apposition.

During inflammation neutrophils receive multiple signals that are integrated, allowing a single modified response. One mechanism for this discrimination is receptor desensitization, a process whereby ligand-receptor binding is disassociated from cell activation. We examined the effect of heterologous receptor desensitization on neutrophil chemotaxis, calcium mobilization, and arachidonic acid production, using interleukin-8 (IL-8), C5a, and N-formyl-methionyl-leucyl-phenylalanine (fMLP). We observed reciprocal inhibition with respect to chemotaxis. We demonstrated that homologous desensitization, with respect to the mobilization of intracellular calcium stores, lasted approximately 15 min. Heterologous desensitization between the fMLP receptor and the C5a receptor was reciprocal; either stimulant would diminish the cells' response to stimulation by the other for approximately 3-5 min. However, we observed a unidirectional heterologous desensitization of the IL-8 receptor by both the fMLP and the C5a receptor. This unidirectional heterologous desensitization was observed with respect to both calcium mobilization and arachidonic acid production (i.e., prestimulation of the IL-8 receptor had no effect on subsequent stimulation by either fMLP or C5a).

Neutrophil stimulation results in the activation of a variety of phospholipases, including phospholipase A2 (PLA2), which releases arachidonic acid from the 2 position of membrane phospholipids, leaving a lysophospholipid. Because arachidonic acid is known to be a potent fusogen in vitro, we examined the effect of metabolism by PLA2 on the fusion of complex liposomes (liposomes prepared with a phospholipid composition similar to that found in neutrophil plasma membrane). We observed that PLA2 augmented the fusion of complex liposomes with each other as well as with specific granules isolated from human neutrophils, lowering the Ca2+ requirement for fusion by three orders of magnitude. Furthermore, although lysophospholipids inhibited fusion, the incorporation of arachidonic acid into liposome membranes overcame the inhibitory effects of the lysophospholipids. Thus with PLA2 and annexins we were able to obtain fusion of complex liposomes at concentrations of Ca2+ that are close to physiological. Our data suggest that the activation of PLA2 and the generation of arachidonic acid may be the major fusion-promoting event mediating neutrophil degranulation.

Several models have been developed to study neutrophil degranulation. At the most basic level, phospholipid vesicles have been used to investigate the lipid interactions occurring during membrane fusion. The two major forms of assays used to measure phospholipid vesicle fusion are based either on the dilution of tagged phospholipids within the membrane of the two fusing partners or the mixing of the aqueous contents of the vesicles. Although problems exist with both methods, the latter is considered to be more accurate and representative of true fusion. Using 8-aminonaphthalene-1,3,6-trisulphonic acid (ANTS) as a fluorescent marker, we have taken advantage of the quenching properties of p-xylenebispyridinium bromide ('DPX') to develop a simple aqueous-space mixing assay that can be used with any sealed vesicle. We compared our new assay with more conventional assays using liposomes composed of phosphatidic acid (PA) and phosphatidylethanolamine (PE), obtaining comparable results with respect to Ca2+-dependent fusion. We extended our studies to measure the fusion of neutrophil plasma-membrane vesicles as well as azurophil and specific granules with PA/PE (1:3) liposomes. Both specific granules and plasma-membrane vesicles fused with PA/PE liposomes at [Ca2+] as low as 500 microM, while azurophil granules showed no fusion at [Ca2+] as high as 12 mM. These differences in the ability of Ca2+ to induce fusion may be related to differences observed in whole cells with respect to secretion.