PLA(2) promotes fusion between PMN-specific granules and complex liposomes

Neutrophil stimulation results in the activation of a variety of phospholipases, including phospholipase A(2) (PLA(2)), 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 PLA(2) on the fusion of complex liposomes (liposomes prepared with a phospholipid composition similar to that found in neutrophil plasma membrane), We observed that PLA(2) 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 PLA(2) and annexins we were able to obtain fusion of complex liposomes at concentations of Ca2+ that are close to physiological, Our data suggest that the activation of PLA(2) and the generation of arachidonic acid may be the major fusion-promoting event mediating neutrophil degranulation.