alpha-Latrotoxin-induced quantal release of catecholamines from rat adrenal chromaffin cells

Protein-tyrosine phosphatase-sigma is a novel member of the functional family of alpha-latrotoxin receptors

Receptor-like protein-tyrosine phosphatase sigma (PTPvarsigma) is essential for neuronal development and function. Here we report that PTPvarsigma is a target of alpha-latrotoxin, a strong stimulator of neuronal exocytosis. alpha-Latrotoxin binds to the cell adhesion-like extracellular region of PTPvarsigma. This binding results in the stimulation of exocytosis. The toxin-binding site is located in the C-terminal part of the PTPvarsigma ectodomain and includes two fibronectin type III repeats. The intracellular catalytic domains of PTPvarsigma are not required for the alpha-latrotoxin binding and secretory response triggered by the toxin in chromaffin cells. These features of PTPvarsigma resemble two other previously described alpha-latrotoxin receptors, neurexin and CIRL. Thus, alpha-latrotoxin represents an unusual example of the neurotoxin that has three independent, equally potent, and yet structurally distinct targets. The known structural and functional characteristics of PTPvarsigma, neurexin, and CIRL suggest that they define a functional family of neuronal membrane receptors with complementary or converging roles in presynaptic function via a mechanism that involves cell-to-cell and cell-to-matrix interaction.

alpha-Latrotoxin releases calcium in frog motor nerve terminals

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

Latrophilin, neurexin, and their signaling-deficient mutants facilitate alpha -latrotoxin insertion into membranes but are not involved in pore formation

Pure alpha-latrotoxin is very inefficient at forming channels/pores in artificial lipid bilayers or in the plasma membrane of non-secretory cells. However, the toxin induces pores efficiently in COS-7 cells transfected with the heptahelical receptor latrophilin or the monotopic receptor neurexin. Signaling-deficient (truncated) mutants of latrophilin and latrophilin-neurexin hybrids also facilitate pore induction, which correlates with toxin binding irrespective of receptor structure. This rules out the involvement of signaling in pore formation. With any receptor, the alpha-latrotoxin pores are permeable to Ca(2+) and small molecules including fluorescein isothiocyanate and norepinephrine. Bound alpha-latrotoxin remains on the cell surface without penetrating completely into the cytosol. Higher temperatures facilitate insertion of the toxin into the plasma membrane, where it co-localizes with latrophilin (under all conditions) and with neurexin (in the presence of Ca(2+)). Interestingly, on subsequent removal of Ca(2+), alpha-latrotoxin dissociates from neurexin but remains in the membrane and continues to form pores. These receptor-independent pores are inhibited by anti-alpha-latrotoxin antibodies. Our results indicate that (i) alpha-latrotoxin is a pore-forming toxin, (ii) receptors that bind alpha-latrotoxin facilitate its insertion into the membrane, (iii) the receptors are not physically involved in the pore structure, (iv) alpha-latrotoxin pores may be independent of the receptors, and (v) pore formation does not require alpha-latrotoxin interaction with other neuronal proteins.

The sea anemone toxin Bc2 induces continuous or transient exocytosis, in the presence of sustained levels of high cytosolic Ca2+ in chromaffin cells

We have isolated and characterized a new excitatory toxin from the venom of the sea anemone Bunodosoma caissarum, named Bc2. We investigated the mechanism of action of the toxin on Ca(2+)-regulated exocytosis in single bovine adrenal chromaffin cells, monitoring simultaneously fura-2 fluorescence measurements and electrochemical recordings using a carbon fiber microelectrode. Bc2 induced quantal release of catecholamines in a calcium-dependent manner. This release was associated with a sustained rise in cytosolic Ca(2+) and displayed two different patterns of response: a continuous discharge of prolonged duration that changed to a transient burst as the toxin concentration (or incubation time) increased. Continuous secretion was dependent on the activity of native voltage-dependent Ca(2+) channels and showed a pattern similar to that of alpha-latrotoxin; however, its kinetics adjusted better to that of continuous cell depolarization with high K(+) concentration. In contrast, transient secretion was independent of Ca(2+) entry through native voltage-dependent Ca(2+) channels and showed inhibition of late vesicle fusion that was accompanied by "freezing" of F-actin disassembly. These new features make Bc2 a promising new tool for studying the machinery of neurotransmitter release.

alpha-latrocrustatoxin increases neurotransmitter release by activating a calcium influx pathway at crayfish neuromuscular junction

alpha-latrocrustatoxin (alpha-LCTX), a component of black widow spider venom (BWSV), produced a 50-fold increase in the frequency of spontaneously occurring miniature excitatory postsynaptic potentials (mEPSPs) at crayfish neuromuscular junctions but did not alter their amplitude distribution. During toxin action, periods of high-frequency mEPSP discharge were punctuated by periods in which mEPSP frequency returned toward control levels. EPSPs were increased in amplitude during periods of enhanced mEPSP discharge. alpha-LCTX had no effect when applied in Ca(2+)-free saline, but subsequent addition of Ca(2+) caused an immediate enhancement of mEPSP frequency even when alpha-LCTX was previously washed out of the bath with Ca(2+)-free saline. Furthermore removal of Ca(2+) from the saline after alpha-LCTX had elicited an effect immediately blocked the action on mEPSP frequency. Thus alpha-LCTX binding is insensitive to Ca(2+), but toxin action requires extracellular Ca(2+) ions. Preincubation with wheat germ agglutinin prevented the effect of alpha-LCTX but not its binding. These binding characteristics suggest that the toxin may bind to a crustacean homologue of latrophilin/calcium-independent receptor for latrotoxin, a G-protein-coupled receptor for alpha-latrotoxin (alpha-LTX) found in vertebrates. alpha-LCTX caused "prefacilitation" of EPSP amplitudes, i.e., the first EPSP in a train was enhanced in amplitude to a greater degree than subsequent EPSPs. A similar alteration in the pattern of facilitation was observed after application of the Ca(2+) ionophore, A23187, indicating that influx of Ca(2+) may mediate the action of alpha-LCTX. In nerve terminals filled with the Ca(2+) indicator, calcium green 1, alpha-LCTX caused increases in the fluorescence of the indicator that lasted for several minutes before returning to rest. Neither fluorescence changes nor toxin action on mEPSP frequency were affected by the Ca(2+) channel blockers omega-agatoxin IVA or Cd(2+), demonstrating that Ca(2+) influx does not occur via Ca(2+) channels normally coupled to transmitter release in this preparation. The actions of alpha-LCTX could be reduced dramatically by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid. We conclude that induction of extracellular Ca(2+) influx into nerve terminals is sufficient to explain the action of alpha-LCTX on both spontaneous and evoked transmitter release at crayfish neuromuscular junctions.

The latrophilin family: multiply spliced G protein-coupled receptors with differential tissue distribution

Latrophilin is a brain-specific Ca2+-independent receptor of alpha-latrotoxin, a potent presynaptic neurotoxin. We now report the finding of two novel latrophilin homologues. All three latrophilins are unusual G protein-coupled receptors. They exhibit strong similarities within their lectin, olfactomedin and transmembrane domains but possess variable C-termini. Latrophilins have up to seven sites of alternative splicing; some splice variants contain an altered third cytoplasmic loop or a truncated cytoplasmic tail. Only latrophilin-1 binds alpha-latrotoxin; it is abundant in brain and is present in endocrine cells. Latrophilin-3 is also brain-specific, whereas latrophilin-2 is ubiquitous. Together, latrophilins form a novel family of heterogeneous G protein-coupled receptors with distinct tissue distribution and functions.

alpha-Latrotoxin alters spontaneous and depolarization-evoked quantal release from rat adrenal chromaffin cells: evidence for multiple modes of action

alpha-Latrotoxin (alpha-LT) potently enhances both "spontaneous" and "depolarization-evoked" quantal secretion from neurons. Here we have used the patch-clamped rat adrenal chromaffin cell to examine simultaneously the effects of alpha-LT on membrane current or voltage, cytosolic Ca, and membrane capacitance, the latter used as an assay for exocytosis. In chromaffin cells exposed to toxin concentrations of >100 pM, the development of large conductance, Ca-permeable ion channels, accompanied by a rise in cytosolic Ca to levels near 1 microM, precedes the initiation of spontaneous exocytosis. These channels appear to be induced de novo, because they occur concurrently with massive reduction or pharmacological block of voltage-dependent Na and Ca currents. However, enhancement of depolarization-evoked release, seen in many cells at <50 pM toxin, often occurs in the absence of a rise in background cytosolic Ca or de novo channel activity. These results favor Ca entry through toxin-induced channels underlying initiation of spontaneous release and direct modulation of the secretory machinery by the toxin-bound receptor contributing to enhancement of depolarization-evoked secretion as well as spontaneous release.