The α-latrotoxin mutant LTXN4C enhances spontaneous and evoked transmitter release in CA3 pyramidal neurons

Abstract

α-Latrotoxin (LTX) stimulates vesicular exocytosis by at least two mechanisms that include (1) receptor binding-stimulation and (2) membrane pore formation. Here, we use the toxin mutant LTXN4C to selectively study the receptor-mediated actions of LTX. LTXN4C binds to both LTX receptors (latrophilin and neurexin) and greatly enhances the frequency of spontaneous and miniature EPSCs recorded from CA3 pyramidal neurons in hippocampal slice cultures. The effect of LTXN4C is reversible and is not attenuated by La3+ that is known to block LTX pores. On the other hand, LTXN4C action, which requires extracellular Ca2+, is inhibited by thapsigargin, a drug depleting intracellular Ca2+ stores, by 2-aminoethoxydiphenyl borate, a blocker of inositol(1,4,5)-trisphosphate-induced Ca2+ release, and by U73122, a phospholipase C inhibitor. Furthermore, measurements using a fluorescent Ca2+ indicator directly demonstrate that LTXN4C increases presynaptic, but not dendritic, free Ca2+ concentration; this Ca2+ rise is blocked by thapsigargin, suggesting, together with electrophysiological data, that the receptor-mediated action of LTXN4C involves mobilization of Ca2+ from intracellular stores. Finally, in contrast to wild-type LTX, which inhibits evoked synaptic transmission probably attributable to pore formation, LTXN4C actually potentiates synaptic currents elicited by electrical stimulation of afferent fibers. We suggest that the mutant LTXN4C, lacking the ionophore-like activity of wild-type LTX, activates a presynaptic receptor and stimulates Ca2+ release from intracellular stores, leading to the enhancement of synaptic vesicle exocytosis.