α-Latrotoxin, Acting via Two Ca2+-dependent Pathways, Triggers Exocytosis of Two Pools of Synaptic Vesicles

Abstract

α-Latrotoxin stimulates three types of [3H] γ-aminobutyric acid and [14C] glutamate release from synaptosomes. The Ca2+-independent component (i) is insensitive to SNAP-25 cleavage or depletion of vesicle contents by bafilomycin A1 and represents transmitter efflux mediated by α-latrotoxin pores. Two other components of release are Ca2+-dependent and vesicular but rely on distinct mechanisms. The fast receptor-mediated pathway (ii) involves intracellular Ca2+ stores and acts upon sucrose-sensitive readily releasable vesicles; this mechanism is insensitive to inhibition of phosphatidylinositol 4-kinase (PI 4-kinase). The delayed pore-dependent exocytotic component (iii) is stimulated by Ca2+ entering through α-latrotoxin pores; it requires PI 4-kinase and occurs mainly from depot vesicles. Lanthanum perturbs α-latrotoxin pores and blocks the two pore-mediated components (i, iii) but not the receptor-mediated release (ii). α-Latrotoxin mutant (LTXN4C) cannot form pores and stimulates only the Ca2+-dependent receptor-mediated amino acid exocytosis (ii) (detectable biochemically and electrophysiologically). These findings explain experimental data obtained by different laboratories and implicate the toxin receptors in the regulation of the readily releasable pool of synaptic vesicles. Our results also suggest that, similar to noradrenergic vesicles, amino acid-containing vesicles at some point in their cycle require PI 4-kinase.