A presynaptically toxic secreted phospholipase A2 is internalized into motoneuron-like cells where it is rapidly translocated into the cytosol

The molecular mechanism of the presynaptic toxicity of secreted phospholipase A2 (sPLA2) neurotoxins, including that of ammodytoxin A (AtxA), has not been resolved. Here we report the action of AtxA on mouse motoneuron-like cells, on which it induced characteristic neurotoxic effects on synaptic vesicles and on the reorganization of F-actin. AtxA also released fatty acids from the plasmalemma. Its significantly less neurotoxic V31W mutant showed similar effects on cells but with a much higher rate of hydrolysis than the wild-type, indicating that high enzymatic activity alone is not sufficient for the observed effects. The neurotoxic action was observed by confocal microscopy of a fluorescently labelled AtxA and by electron microscopy of a nanogold-labelled toxin. The Atx-binding proteins were tagged by a photo-cross-linking reagent conjugated to the toxin. AtxA was taken up rapidly by the cells, where it interacted within minutes with calmodulin and 14-3-3 proteins in the cytosol. These data demonstrate, for the first time, the translocation of an sPLA2 from the extracellular space into the cytosol of a cell. Such an event may thus be important in explaining the action of a range of homologous endogenous sPLA2 enzymes in mammals whose roles in various cellular processes are not yet completely understood.