Alpha-latrotoxin channels in neuroblastoma cells

Molecular architecture of black widow spider neurotoxins

Latrotoxins (LaTXs) are presynaptic pore-forming neurotoxins found in the venom of Latrodectus spiders. The venom contains a toxic cocktail of seven LaTXs, with one of them targeting vertebrates (α-latrotoxin (α-LTX)), five specialized on insects (α, β, γ, δ, ε- latroinsectotoxins (LITs), and one on crustaceans (α-latrocrustatoxin (α-LCT)). LaTXs bind to specific receptors on the surface of neuronal cells, inducing the release of neurotransmitters either by directly stimulating exocytosis or by forming Ca²⁺-conductive tetrameric pores in the membrane. Despite extensive studies in the past decades, a high-resolution structure of a LaTX is not yet available and the precise mechanism of LaTX action remains unclear. Here, we report cryoEM structures of the α-LCT monomer and the δ-LIT dimer. The structures reveal that LaTXs are organized in four domains. A C-terminal domain of ankyrin-like repeats shields a central membrane insertion domain of six parallel α-helices. Both domains are flexibly linked via an N-terminal α-helical domain and a small β-sheet domain. A comparison between the structures suggests that oligomerization involves major conformational changes in LaTXs with longer C-terminal domains. Based on our data we propose a cyclic mechanism of oligomerization, taking place prior membrane insertion. Both recombinant α-LCT and δ-LIT form channels in artificial membrane bilayers, that are stabilized by Ca²⁺ ions and allow calcium flux at negative membrane potentials. Our comparative analysis between α-LCT and δ-LIT provides first crucial insights towards understanding the molecular mechanism of the LaTX family.

The venom of Latrodectus spiders contains seven Latrotoxins (LaTXs), among them α-latrocrustatoxin (LCT) and δ- latroinsectotoxins δ-LIT. LaTXs bind to specific receptors on the surface of neuronal cells and target the molecular exocytosis machinery. Here, the authors present the cryo-EM structure of the α-LCT monomer and the δ-LIT dimer, which reveal that LaTXs are organized in four domains and they discuss the potential oligomerisation mechanism that takes place before LaTXs membrane insertion. Both recombinant α-LCT and δ-LIT form channels in artificial membrane bilayers, that are stabilized by Ca²⁺ ions.

Penelope's web: using alpha-latrotoxin to untangle the mysteries of exocytosis

For more than three decades, the venom of the black widow spider and its principal active components, latrotoxins, have been used to induce release of neurotransmitters and hormones and to study the mechanisms of exocytosis. Given the complex nature of alpha--latrotoxin (alpha-LTX) actions, this research has been continuously overshadowed by many enigmas, misconceptions and perpetual changes of the underlying hypotheses. Some of the toxin's mechanisms of action are still not completely understood. Despite all these difficulties, the extensive work of several generations of neurobiologists has brought about a great deal of fascinating insights into pre-synaptic processes and has led to the discovery of several novel proteins and synaptic systems. For example, alpha-LTX studies have contributed to the widespread acceptance of the vesicular theory of transmitter release. Pre-synaptic receptors for alpha-LTX--neurexins, latrophilins and protein tyrosine phosphatase sigma--and their endogenous ligands have now become centrepieces of their own areas of research, with a potential of uncovering new mechanisms of synapse formation and regulation that may have medical implications. However, any future success of alpha-LTX research will require a better understanding of this unusual natural tool and a more precise dissection of its multiple mechanisms.

Alpha-latrotoxin stimulates a novel pathway of Ca2+-dependent synaptic exocytosis independent of the classical synaptic fusion machinery

Alpha-latrotoxin induces neurotransmitter release by stimulating synaptic vesicle exocytosis via two mechanisms: (1) A Ca(2+)-dependent mechanism with neurexins as receptors, in which alpha-latrotoxin acts like a Ca(2+) ionophore, and (2) a Ca(2+)-independent mechanism with CIRL/latrophilins as receptors, in which alpha-latrotoxin directly stimulates the transmitter release machinery. Here, we show that the Ca(2+)-independent release mechanism by alpha-latrotoxin requires the synaptic SNARE-proteins synaptobrevin/VAMP and SNAP-25, and, at least partly, the synaptic active-zone protein Munc13-1. In contrast, the Ca(2+)-dependent release mechanism induced by alpha-latrotoxin does not require any of these components of the classical synaptic release machinery. Nevertheless, this type of exocytotic neurotransmitter release appears to fully operate at synapses, and to stimulate exocytosis of the same synaptic vesicles that participate in physiological action potential-triggered release. Thus, synapses contain two parallel and independent pathways of Ca(2+)-triggered exocytosis, a classical, physiological pathway that operates at the active zone, and a novel reserve pathway that is recruited only when Ca(2+) floods the synaptic terminal.

alpha-Latrotoxin and its receptors

alpha-Latrotoxin (alpha-LTX) from black widow spider venom induces exhaustive release of neurotransmitters from vertebrate nerve terminals and endocrine cells. This 130-kDa protein has been employed for many years as a molecular tool to study exocytosis. However, its action is complex: in neurons, alpha-LTX induces massive secretion both in the presence of extracellular Ca(2+) (Ca(2+) (e)) and in its absence; in endocrine cells, it usually requires Ca(2+) (e). To use this toxin for further dissection of secretory mechanisms, one needs an in-depth understanding of its functions. One such function that explains some alpha-LTX effects is its ability to form cation-permeable channels in artificial lipid bilayers. The mechanism of alpha-LTX pore formation, revealed by cryo-electron microscopy, involves toxin assembly into homotetrameric complexes which harbor a central channel and can insert into lipid membranes. However, in biological membranes, alpha-LTX cannot exert its actions without binding to specific receptors of the plasma membrane. Three proteins with distinct structures have been found to bind alpha-LTX: neurexin Ialpha, latrophilin 1, and receptor-like protein tyrosine phosphatase sigma. Upon binding a receptor, alpha-LTX forms channels permeable to cations and small molecules; the toxin may also activate the receptor. To distinguish between the pore- and receptor-mediated effects, and to study structure-function relationships in the toxin, alpha-LTX mutants have been used.

The origin of the skewed amplitude distribution of spontaneous excitatory junction potentials in poorly coupled smooth muscle cells

The skewed amplitude distribution of spontaneous excitatory junction potentials (sEJPs) in the mouse vas deferens and other electrically-coupled smooth muscle syncytia has been attributed to electrically-attenuated depolarizations resulting from the spontaneous release of quantized packets of ATP acting on remote smooth muscle cells (SMCs). However, in the present investigation surface SMCs of the mouse isolated vas deferens were poorly electrically coupled, with input resistances (176±18 MΩ, range: 141–221 MΩ, n=4) similar to those of dissociated cells. Furthermore, the amplitude of evoked EJPs was more variable in surface compared with deeper SMCs (F test, F=17.4, P<0.0001). Using simultaneous electrophysiology and confocal microscopy to investigate these poorly-coupled cells, it is shown that α-latrotoxin-stimulated sEJPs correlate, in timing (median delay ranged from −30 to −57 ms, P<0.05 in all experiments, n=5) and amplitude (Pearson product moment correlation, ρ>0.55 and P<0.001), with purinergic neuroeffector Ca²⁺ transients (NCTs) in SMCs. The temporal correlation between sEJPs of widely ranging amplitude with NCTs in the impaled SMC demonstrates that all sEJPs could arise from neurotransmitter action on the impaled cell and that the skewed distribution of sEJPs can be explained by the variable effect of packets of ATP on a single SMC. The amplitude correlation of sEJPs and NCTs argues against the attenuation of electrical signal amplitude along the length of a single SMC. The skewed sEJP amplitude distribution arising from neurotransmitter release on single SMCs is consistent with a broad neurotransmitter packet size distribution at sympathetic neuroeffector junctions.

Insecticidal toxins from black widow spider venom

The biological effects of Latrodectus spider venom are similar in animals from different phyla, but these symptoms are caused by distinct phylum-specific neurotoxins (collectively called latrotoxins) with molecular masses ranging from 110 to 140 kDa. To date, the venom has been found to contain five insecticidal toxins, termed α, β, γ, δ and ε-latroinsectotoxins (LITs). There is also a vertebrate-specific neurotoxin, α-latrotoxin (α-LTX), and one toxin affecting crustaceans, α-latrocrustatoxin (α-LCT). These toxins stimulate massive release of neurotransmitters from nerve terminals and act (1) by binding to specific receptors, some of which mediate an exocytotic signal, and (2) by inserting themselves into the membrane and forming ion-permeable pores. Specific receptors for LITs have yet to be identified, but all three classes of vertebrate receptors known to bind α-LTX are also present in insects. All LTXs whose structures have been elucidated (α-LIT, δ-LIT, α-LTX and α-LCT) are highly homologous and have a similar domain architecture, which consists of a unique N-terminal sequence and a large domain composed of 13–22 ankyrin repeats. Three-dimensional (3D) structure analysis, so far done for α-LTX only, has revealed its dimeric nature and an ability to form symmetrical tetramers, a feature probably common to all LTXs. Only tetramers have been observed to insert into membranes and form pores. A preliminary 3D reconstruction of a δ-LIT monomer demonstrates the spatial similarity of this toxin to the monomer of α-LTX.

α-Latrotoxin Induces Exocytosis by Inhibition of Voltage-dependent K+ Channels and by Stimulation of L-type Ca2+ Channels via Latrophilin in β-Cells

The spider venom alpha-latrotoxin (alpha-LTX) induces massive exocytosis after binding to surface receptors, and its mechanism is not fully understood. We have investigated its action using toxin-sensitive MIN6 beta-cells, which express endogenously the alpha-LTX receptor latrophilin (LPH), and toxin-insensitive HIT-T15 beta-cells, which lack endogenous LPH. alpha-LTX evoked insulin exocytosis in HIT-T15 cells only upon expression of full-length LPH but not of LPH truncated after the first transmembrane domain (LPH-TD1). In HIT-T15 cells expressing full-length LPH and in native MIN6 cells, alpha-LTX first induced membrane depolarization by inhibition of repolarizing K(+) channels followed by the appearance of Ca(2+) transients. In a second phase, the toxin induced a large inward current and a prominent increase in intracellular calcium ([Ca(2+)](i)) reflecting pore formation. Upon expression of LPH-TD1 in HIT-T15 cells just this second phase was observed. Moreover, the mutated toxin LTX(N4C), which is devoid of pore formation, only evoked oscillations of membrane potential by reversible inhibition of iberiotoxin-sensitive K(+) channels via phospholipase C, activated L-type Ca(2+) channels independently from its effect on membrane potential, and induced an inositol 1,4,5-trisphosphate receptor-dependent release of intracellular calcium in MIN6 cells. The combined effects evoked transient increases in [Ca(2+)](i) in these cells, which were sensitive to inhibitors of phospholipase C, protein kinase C, or L-type Ca(2+) channels. The latter agents also reduced toxin-induced insulin exocytosis. In conclusion, alpha-LTX induces signaling distinct from pore formation via full-length LPH and phospholipase C to regulate physiologically important K(+) and Ca(2+) channels as novel targets of its secretory activity.

Alpha-latrotoxin triggers extracellular Ca(2+)-dependent exocytosis and sensitizes fusion machinery in endocrine cells

alpha-Latrotoxin from the venom of black widow spider induces and augments neurotransmitter and hormone release by way of extracellular Ca(2+) influx and cellular signal transduction pathways. By using whole cell current and capacitance recording, the photolysis of caged Ca(2+), and Ca(2+) microfluorometry and amperometry, we investigated the stimulating effect and mechanism of alpha-latrotoxin on exocytosis in rat pancreatic beta cells, LbetaT2 cells and latrophilin plasmid-transfected INS-1 cells. Our data indicated that: (1) alpha-latrotoxin increased cytosolic Ca(2+) concentration through the formation of cation-permitting pores and subsequent Ca(2+) influx with the presence of extracellular Ca(2+); (2) alpha-latrotoxin stimulated exocytosis in normal bath solution and its stimulating effect on secretion was eradicated in Ca(2+)-free bath solution; and (3) alpha-latrotoxin sensitized the molecular machinery of fusion through activation of protein kinase C and increased the response of cells to Ca(2+) photolyzed by a flash of ultraviolet light. In summary, alpha-latrotoxin induced exocytosis by way of Ca(2+) influx and accelerated vesicle fusion by the sensitization of fusion machinery.

alpha-Latrotoxin increases spontaneous and depolarization-evoked exocytosis from pancreatic islet beta-cells

alpha-Latrotoxin (alpha-LT), a potent excitatory neurotoxin, increases spontaneous, as well as action potential-evoked, quantal release at nerve terminals and increases hormone release from excitable endocrine cells. We have investigated the effects of alpha-LT on single human, mouse and canine beta-cells. In isolated and combined measurements, alpha-LT, at nanomolar concentrations, induces: (i) rises in cytosolic Ca(2+), into the micromolar range, that are dependent on extracellular Ca(2+); (ii) large conductance non-selective cation channels; and (iii) Ca(2+)-dependent insulin granule exocytosis, measured as increases in membrane capacitance and quantal release of preloaded serotonin. Furthermore, at picomolar concentrations, alpha-LT potentiates depolarization-induced exocytosis often without evidence of inducing channel activity or increasing cytosolic Ca(2+). These results strongly support the hypothesis that alpha-LT, after binding to specific receptors, has at least two complementary modes of action on excitable cells. (i) alpha-LT inserts into the plasma membrane to form Ca(2+) permeable channels and promote Ca(2+) entry thereby triggering Ca(2+)-dependent exocytosis in unstimulated cells. (ii) At lower concentrations, where its channel forming activity is hardly evident, alpha-LT augments depolarization-evoked exocytosis probably by second messenger-induced enhancement of the efficiency of the vesicle recruitment or vesicle fusion machinery. We suggest that both modes of action enhance exocytosis from a newly described highly Ca(2+)-sensitive pool of insulin granules activated by global cytosolic Ca(2+) concentrations in the range of approximately 1 microm.

The multiple actions of black widow spider toxins and their selective use in neurosecretion studies

The black widow spider venom contains several large protein toxins--latrotoxins--that are selectively targeted against different classes of animals: vertebrates, insects, and crustaceans. These toxins are synthesised as large precursors that undergo proteolytic processing and activation in the lumen of the venom gland. The mature latrotoxins demonstrate strong functional structure conservation and contain multiple ankyrin repeats, which mediate toxin oligomerisation. The three-dimensional structure has been determined for alpha-latrotoxin (alphaLTX), a representative venom component toxic to vertebrates. This reconstruction explains the mechanism of alphaLTX pore formation by showing that it forms tetrameric complexes, harbouring a central channel, and that it is able to insert into lipid membranes. All latrotoxins cause massive release of neurotransmitters from nerve terminals of respective animals after binding to specific neuronal receptors. A G protein-coupled receptor latrophilin and a single-transmembrane receptor neurexin have been identified as major high-affinity receptors for alphaLTX. Latrotoxins act by several Ca(2+)-dependent and -independent mechanisms based on pore formation and activation of receptors. Mutant recombinant alphaLTX that does not form pores has been used to dissect the multiple actions of this toxin. As a result, important insights have been gained into the receptor signalling and the role of intracellular Ca(2+) stores in the effect of alphaLTX.

Spider and wasp neurotoxins: pharmacological and biochemical aspects

Venoms from several arthropods are recognized as useful sources of bioactive substances, such as peptides, acylpolyamines, and alkaloids, which show a wide range of pharmacological effects on synaptic transmission. In this work, we summarize and compile several biochemical and pharmacological aspects related to spider and wasp neurotoxins. Their inhibitory and stimulatory actions on ion channels, receptors, and transporters involved in mammalian and insect neurotransmission are considered.

Mechanism of alpha-latrotoxin action at nerve endings of neurohypophysis

The neurotoxin alpha-latrotoxin elicits spontaneous exocytosis of neurotransmitter from neurons and peptide hormones from endocrine cells. While the mechanism of action is not fully understood, both Ca(2+)-dependent and Ca(2+)-independent pathways participate in the facilitation of release, with the relative contribution of the pathways differing among neuronal and endocrine cell types. Here, we investigate the actions of alpha-latrotoxin on neuroendocrine nerve endings that emanate from central nervous system neurons and, therefore, are unique in that they possess properties of central nerve endings and endocrine cells. Using intracellular [Ca(2+)] measurements both calcium-independent receptors for latrotoxin (CIRL or latrophilin) and neurexin 1 alpha receptors were found to be functionally present. Interaction of alpha-latrotoxin with these receptors stimulated secretion of vasopressin and oxytocin neuropeptide. The secretory response was entirely dependent upon toxin-mediated extracellular Ca(2+) influx, although alpha-latrotoxin also consistently triggered mobilization of Ca(2+) from an intracellular store. The mobilization of intracellular Ca(2+) relied on alpha-latrotoxin-mediated Na(+) influx and was blocked by the protonophore FCCP, thereby implicating mitochondria as the Ca(2+) store being mobilized. Using the whole cell recording configuration of the patch clamp, we report that alpha-latrotoxin interaction with the CIRL receptor on these nerve endings resulted in ionic pore formation, generating unitary inward current steps of 20 pA and a channel conductance of approximately 220 pS in Ca(2+)-free saline. Thus, alpha-latrotoxin stimulates Ca(2+)-dependent exocytosis in neurohypophysial nerve endings through receptor interaction and insertion of Ca(2+) permeable membrane pores. While alpha-latrotoxin mobilizes intracellular Ca(2+) stores the elevation in [Ca(2+)] reached is insufficient to trigger measurable exocytosis.

Mutant alpha-latrotoxin (LTXN4C) does not form pores and causes secretion by receptor stimulation: this action does not require neurexins

Alpha-latrotoxin (LTX) causes massive release of neurotransmitters via a complex mechanism involving (i) activation of receptor(s) and (ii) toxin insertion into the plasma membrane with (iii) subsequent pore formation. Using cryo-electron microscopy, electrophysiological and biochemical methods, we demonstrate here that the recently described toxin mutant (LTXN4C) is unable to insert into membranes and form pores due to its inability to assemble into tetramers. However, this mutant still binds to major LTX receptors (latrophilin and neurexin) and causes strong transmitter exocytosis in synaptosomes, hippocampal slice cultures, neuromuscular junctions, and chromaffin cells. In the absence of mutant incorporation into the membrane, receptor activation must be the only mechanism by which LTXN4C triggers exocytosis. An interesting feature of this receptor-mediated transmitter release is its dependence on extracellular Ca2+. Because Ca2+ is also strictly required for LTX interaction with neurexin, the latter might be the only receptor mediating the LTXN4C action. To test this hypothesis, we used conditions (substitution of Ca2+ in the medium with Sr2+) under which LTXN4C does not bind to any member of the neurexin family but still interacts with latrophilin. We show that, in all the systems tested, Sr2+ fully replaces Ca2+ in supporting the stimulatory effect of LTXN4C. These results indicate that LTXN4C can cause neurotransmitter release just by stimulating a receptor and that neurexins are not critical for this receptor-mediated action.

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

Alpha-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 LTX(N4C) to selectively study the receptor-mediated actions of LTX. LTX(N4C) 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 LTX(N4C) is reversible and is not attenuated by La3+ that is known to block LTX pores. On the other hand, LTX(N4C) 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 LTX(N4C) 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 LTX(N4C) involves mobilization of Ca2+ from intracellular stores. Finally, in contrast to wild-type LTX, which inhibits evoked synaptic transmission probably attributable to pore formation, LTX(N4C) actually potentiates synaptic currents elicited by electrical stimulation of afferent fibers. We suggest that the mutant LTX(N4C), 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.

Trachynilysin, a neurosecretory protein isolated from stonefish (Synanceia trachynis) venom, forms nonselective pores in the membrane of NG108-15 cells

Trachynilysin, a protein toxin isolated from the venom of the stonefish Synanceia trachynis, has been reported to elicit massive acetylcholine release from motor nerve endings of isolated neuromuscular preparations and to increase both cytosolic Ca2+ and catecholamine release from chromaffin cells. In the present study, we used the patch clamp technique to investigate the effect of trachynilysin on the cytoplasmic membrane of differentiated NG108-15 cells in culture. Trachynilysin increased membrane conductance the most when the negativity of the cell holding membrane potential was reduced. The trachynilysin-induced current was carried by cations and reversed at about -3 mV in standard physiological solutions, which led to strong membrane depolarization and Ca2+ influx. La3+ blocked the trachynilysin current in a dose-, voltage-, and time-dependent manner, and antibodies raised against the toxin antagonized its effect on the cell membrane. The inside-out configuration of the patch clamp technique allowed the recording of single channel activity from which various multiples of 22 pS elementary conductance were resolved. These results indicate that trachynilysin forms pores in the NG108-15 cell membrane, and they advance our understanding of the toxin's mode of action on motor nerve endings and neurosecretory 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, acting via two Ca2+-dependent pathways, triggers exocytosis of two pools of synaptic vesicles

alpha-Latrotoxin stimulates three types of [(3)H]gamma-aminobutyric acid and [(14)C]glutamate release from synaptosomes. The Ca(2+)-independent component (i) is insensitive to SNAP-25 cleavage or depletion of vesicle contents by bafilomycin A1 and represents transmitter efflux mediated by alpha-latrotoxin pores. Two other components of release are Ca(2+)-dependent and vesicular but rely on distinct mechanisms. The fast receptor-mediated pathway (ii) involves intracellular Ca(2+) 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 Ca(2+) entering through alpha-latrotoxin pores; it requires PI 4-kinase and occurs mainly from depot vesicles. Lanthanum perturbs alpha-latrotoxin pores and blocks the two pore-mediated components (i, iii) but not the receptor-mediated release (ii). alpha-Latrotoxin mutant (LTX(N4C)) cannot form pores and stimulates only the Ca(2+)-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.

alpha-Latrotoxin and its receptors: neurexins and CIRL/latrophilins

alpha-Latrotoxin, a potent neurotoxin from black widow spider venom, triggers synaptic vesicle exocytosis from presynaptic nerve terminals. alpha-Latrotoxin is a large protein toxin (120 kDa) that contains 22 ankyrin repeats. In stimulating exocytosis, alpha-latrotoxin binds to two distinct families of neuronal cell-surface receptors, neurexins and CLs (Cirl/latrophilins), which probably have a physiological function in synaptic cell adhesion. Binding of alpha-latrotoxin to these receptors does not in itself trigger exocytosis but serves to recruit the toxin to the synapse. Receptor-bound alpha-latrotoxin then inserts into the presynaptic plasma membrane to stimulate exocytosis by two distinct transmitter-specific mechanisms. Exocytosis of classical neurotransmitters (glutamate, GABA, acetylcholine) is induced in a calcium-independent manner by a direct intracellular action of alpha-latrotoxin, while exocytosis of catecholamines requires extracellular calcium. Elucidation of precisely how alpha-latrotoxin works is likely to provide major insight into how synaptic vesicle exocytosis is regulated, and how the release machineries of classical and catecholaminergic neurotransmitters differ.

Role of calcium in neurotransmitter release evoked by alpha-latrotoxin or hypertonic sucrose

At the synapse, neurotransmitter release is triggered physiologically by Ca(2+) influx through voltage-gated Ca(2+) channels. Non-physiologically, release can be evoked by a potent neurotoxin, alpha-latrotoxin, and by hypertonic sucrose. Controversy has arisen on whether release evoked by alpha-latrotoxin and hypertonic sucrose requires extracellular Ca(2+) or Ca(2+) from intracellular stores. Using synaptosomes, we have studied the Ca(2+) dependence of alpha-latrotoxin and sucrose action in different neurotransmitter systems. In agreement with previous data, no requirement for extracellular Ca(2+) in sucrose-induced secretion of norepinephrine, dopamine, glutamate or GABA was detected. Unexpectedly, we observed large differences between these neurotransmitters in the Ca(2+) dependence of alpha-latrotoxin-stimulated release: norepinephrine release required Ca(2+), dopamine release was only partially Ca(2+) dependent, and glutamate and GABA release did not require Ca(2+). To test if Ca(2+) derived from intracellular Ca(2+) stores participates in neurotransmitter release triggered by alpha-latrotoxin or hypertonic sucrose, we employed thapsigargin, a Ca(2+)-ATPase inhibitor that empties Ca(2+) stores. Thapsigargin did not induce neurotransmitter release, nor did it inhibit subsequent release stimulated by KCl depolarization, hypertonic sucrose or alpha-latrotoxin. However, intracellular Ca(2+) performs an important regulatory function, since thapsigargin increased the size of the readily releasable pool as measured by stimulation with hypertonic sucrose. This effect required extracellular Ca(2+) and protein kinase C, suggesting that depletion of internal Ca(2+) stores leads to store-operated Ca(2+) entry. The resulting Ca(2+) influx does not trigger release by itself, but activates protein kinase C that increases the readily releasable pool of neurotransmitters. Our data show that internal and external Ca(2+) is not acutely involved in hypertonic sucrose-evoked neurotransmitter release, while alpha-latrotoxin-triggered release requires external Ca(2+) for a subset of neurotransmitters. Although internal Ca(2+) is not essential for release, it modulates its extent, implying that the emptying of intracellular stores by activation of presynaptic receptors plays an important regulatory role in neurotransmitter release.

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-latrotoxin forms calcium-permeable membrane pores via interactions with latrophilin or neurexin

In order to explore the mechanisms by which alpha-latrotoxin activates neurotransmitter release, we have characterized its effects by patch-clamp methods on cells heterologously expressing its receptors, latrophilin-1 or neurexin-Ialpha. Application of alpha-latrotoxin (1 nM) to cells expressing rat latrophilin or neurexin, but not mock-transfected cells, induced a cationic conductance. In cells expressing latrophilin, current development was slow in the absence of divalent cations, but was accelerated by Ca2+ or Mg2+. In cells expressing neurexin, alpha-latrotoxin did not elicit currents in the absence of Ca2+. The toxin-induced conductance was rectifying, persistent, permeable to monovalent and divalent cations, but blocked by La3+. Single-channel recording revealed a permanently open state, with the same unitary conductance irrespective of whether cells expressed latrophilin or neurexin. Therefore, while pore formation displayed differences consistent with the reported properties of alpha-latrotoxin binding to latrophilin and neurexin, the pores induced by alpha-latrotoxin had identical properties. These results suggest that after anchoring to either of its nerve terminal receptors, alpha-latrotoxin inserts into the membrane and constitutes a single type of transmembrane ion pore.

Characterization of the epitope for 4C4.1 mAb on alpha-latrotoxin using phage display-peptide libraries: prevention of toxin-dependent 45Ca(2+) uptake in non-neuronal human embryonic cells transiently expressing latrophilin

alpha-Latrotoxin, a protein toxin present in the venom of black widow spider, interacts with membrane receptors of neurons and other secretory cells to stimulate exocytosis. Two types of receptors have been identified and cloned. Our attention has been focused on the calcium independent receptor, a G-protein coupled receptor, named latrophilin to see whether alpha-latrotoxin interaction was capable to produce an ionotropic effect, in alternative to the metabotropic hypothesis. Expression of latrophilin receptor is sufficient for the alpha-latrotoxin effect to become manifest. By inducing the transient expression of latrophilin receptor in non-neuronal human embryonic cells, we made them susceptible to toxin action as demonstrated by the increase in 45Ca(2+) accumulation detected after toxin treatment. Since the presence of a monoclonal antibody against alpha-latrotoxin (4C4.1 mAb) was able to obliterate toxin-dependent effects, we further investigated the nature of toxin-antibody interaction by characterization of the binding epitope using phage display-peptide libraries. A conformational epitope was recognized and partially localized on a region of the peptide toxin whereby a tetrameric structure is formed and inserted into the membrane of target cells where it functions as a pore.

alpha-latrotoxin triggers transmitter release via direct insertion into the presynaptic plasma membrane

alpha-latrotoxin, a component of black widow spider venom, binds to presynaptic nerve terminals and stimulates massive neurotransmitter release. Previous studies have demonstrated that alpha-latrotoxin first binds to two high-affinity receptors on nerve terminals, neurexins and CLs (CIRLs and latrophilins), and then executes a critical, second step of unknown nature that stimulates neurotransmitter release. We now demonstrate that incubation of alpha-latrotoxin with synaptosomes at 0 degrees C results in its peripheral membrane association. Incubation at 37 degrees C, however, converts the toxin into an operationally integral membrane protein, and induces generation of a protease-resistant fragment that consists of the entire N-terminal domain of alpha-latrotoxin and becomes protease sensitive after lysis of synaptosomes. Our data suggest that alpha-latrotoxin inserts into the presynaptic plasma membrane after receptor binding, resulting in an intracellular location of the N-terminal sequences. Membrane insertion of the N-terminal domain of alpha-latrotoxin occurs spontaneously, independently of membrane recycling or transmembrane ion gradients. We postulate that alpha-latrotoxin acts intracellularly in triggering release, and propose that non-selective cation channels induced by alpha-latrotoxin may be a by-product of membrane insertion.

Alpha-latrotoxin and its receptors CIRL (latrophilin) and neurexin 1 alpha mediate effects on secretion through multiple mechanisms

Alpha-Latrotoxin and its plasma membrane receptors cause a number of distinct effects in secretory cells. First, by tethering alpha-latrotoxin to the plasma membrane, CIRL/latrophilin and neurexin 1 alpha facilitate alpha-latrotoxin-induced channel formation. The stimulation of secretion by alpha-latrotoxin in neuroendocrine cells is a consequence of Ca(2+) influx through these alpha-latrotoxin-induced channels. In addition to channel formation, alpha-latrotoxin enhances secretion in permeabilized cells through interaction with the plasma membrane receptor CIRL/latrophilin. Finally, overexpression of CIRL/latrophilin inhibits Ca(2+)-dependent secretion in permeabilized chromaffin cells in the absence of alpha-latrotoxin. This effect represents a 'constitutive' action of the G-protein coupled receptor to specifically inhibit an ATP-dependent priming step in the secretory pathway. The effect suggests that the receptor may have an important modulatory role in synaptic transmission.

Tetramerisation of alpha-latrotoxin by divalent cations is responsible for toxin-induced non-vesicular release and contributes to the Ca(2+)-dependent vesicular exocytosis from synaptosomes

A novel procedure of alpha-latrotoxin (alpha LTX) purification has been developed. Pure alpha LTX has been demonstrated to exist as a very stable homodimer. Such dimers further assemble into tetramers, and Ca(2+), Mg(2+) or higher toxin concentrations facilitate this process. However, when the venom is treated with EDTA, purified alpha LTX loses the ability to tetramerise spontaneously; the addition of Mg(2+) or Ca(2+) restores this ability. This suggests that alphaLTX has some intrinsically bound divalent cation(s) that normally support its tetramerisation. Single-particle cryoelectron microscopy and statistical image analysis have shown that: 1) the toxin has a non-compact, branching structure; 2) the alpha LTX dimers are asymmetric; and 3) the tetramers are symmetric and have a 25 A-diameter channel in the centre. Both alpha LTX oligomers bind to the same receptors in synaptosomes and rat brain sections. To study the effects of the dimers and tetramers on norepinephrine release from rat cerebrocortical synaptosomes, we used the EDTA-treated and untreated toxin preparations. The number of tetramers present in a preparation correlates with alpha LTX pore formation, suggesting that the tetramers are the pore-forming species of alpha LTX. The toxin actions mediated by the pore include: 1) Ca(2+) entry from the extracellular milieu; and 2) passive efflux of neurotransmitters via the pore that occurs independently of Ca(2+). The Ca(2+)-dependent alpha LTX-stimulated secretion conforms to all criteria of vesicular exocytosis but also depends upon intact intracellular Ca(2+) stores and functional phospholipase C (PLC). The Ca(2+)-dependent effect of the toxin is stronger when dimeric alpha LTX is used, indicating that higher receptor occupancy leads to its stronger activation, which contributes to stimulation of neuroexocytosis. In contrast, the Ca(2+)-independent release measured biochemically represents leakage of neurotransmitters through the toxin pore. These results are discussed in relation to the previously published observations.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

Calcium-independent receptor for alpha-latrotoxin and neurexin 1alpha [corrected] facilitate toxin-induced channel formation: evidence that channel formation results from tethering of toxin to membrane

alpha-Latrotoxin binding to the calcium-independent receptor for alpha-latrotoxin (CIRL-1), a putative G-protein-coupled receptor, stimulates secretion from chromaffin and PC12 cells. Using patch clamp techniques and microspectrofluorimetry, we demonstrate that the interaction of alpha-latrotoxin with CIRL-1 produces a high conductance channel that permits increases in cytosolic Ca(2+). alpha-Latrotoxin interaction with CIRL-1 transiently expressed in bovine chromaffin cells produced a 400-pS channel, which rarely closed under Ca(2+)-free conditions. The major effect of overexpressing CIRL-1 was to greatly increase the sensitivity of chromaffin cells to channel formation by alpha-latrotoxin. alpha-Latrotoxin interaction with CIRL-1 transiently overexpressed in non-neuronal human embryonic kidney 293 (HEK293) cells produced channels that were nearly identical with those observed in chromaffin cells. Channel currents were reduced by millimolar Ca(2+). At alpha-latrotoxin concentrations below 500 pM, channel formation occurred many seconds after binding of toxin to CIRL-1 indicating distinct steps in channel formation. In all cases there was a rapid, sequential addition of channels once the first channel appeared. An analysis of CIRL-1 mutants indicated that channel formation in HEK293 cells is unlikely to be transduced by a G-protein-dependent mechanism. alpha-Latrotoxin interaction with a fusion construct composed of the extracellular domain of CIRL-1 anchored to the membrane by the transmembrane domain of vesicular stomatitis virus glycoprotein, and with neurexin 1alpha, an alpha-latrotoxin receptor structurally unrelated to CIRL-1, produced channels virtually identical with those observed with wild-type CIRL-1. We propose that alpha-latrotoxin receptors recruit toxin to facilitate its insertion across the membrane and that alpha-latrotoxin itself controls the conductance properties of the channels it produces.

Alpha-latrotoxin stimulates inward current, rise in cytosolic calcium concentration, and exocytosis in at pituitary gonadotropes

Alpha-latrotoxin (LTX) from the black widow spider venom, stimulates neurotransmitter release from neuronal cells via Ca2+ -dependent as well as Ca2+ -independent mechanisms. In some peptide-secreting endocrine cells, however, LTX stimulates hormone release mainly via a Ca2+ -independent mechanism. Here we investigated the action of LTX in rat pituitary gonadotropes that secrete the peptide, LH. Using the patch-clamp technique in conjunction with the fluorescent Ca2+ indicator (indo-1) to simultaneously measure the cytosolic Ca2+ concentration ([Ca2+]i) and ionic current, we showed that LTX elicited bursts of inward current that were accompanied by [Ca2+]i elevations. In the presence of a physiological concentration of extracellular Ca2+, the unitary conductance of the LTX-induced current was about 300 pS, and only about 6.4% of the current was carried by Ca2+. The LTX-induced current was occasionally followed by intracellular Ca2+ release. At [Ca2+]i of 1 microM or more, exocytosis (detected by membrane capacitance measurement) was consistently triggered, and it was frequently followed by endocytosis. Thus, LTX triggers Ca2+ -dependent exocytosis in gonadotropes via extracellular Ca2+ entry as well as intracellular Ca2+ release. In approximately 25% of the cells, LTX could also trigger a slow exocytosis in the absence of [Ca2+]i elevation. Therefore, LTX has both Ca2+ -dependent and Ca2+ -independent actions in gonadotropes.

Norepinephrine exocytosis stimulated by alpha-latrotoxin requires both external and stored Ca2+ and is mediated by latrophilin, G proteins and phospholipase C

alpha-latrotoxin (LTX) stimulates massive release of neurotransmitters by binding to a heptahelical transmembrane protein, latrophilin. Our experiments demonstrate that latrophilin is a G-protein-coupled receptor that specifically associates with heterotrimeric G proteins. The latrophilin-G protein complex is very stable in the presence of GDP but dissociates when incubated with GTP, suggesting a functional interaction. As revealed by immunostaining, latrophilin interacts with G alpha q/11 and G alpha o but not with G alpha s, G alpha i or G alpha z, indicating that this receptor may couple to several G proteins but it is not promiscuous. The mechanisms underlying LTX-evoked norepinephrine secretion from rat brain nerve terminals were also studied. In the presence of extracellular Ca2+, LTX triggers vesicular exocytosis because botulinum neurotoxins E, Cl or tetanus toxin inhibit the Ca(2+)-dependent component of the toxin-evoked release. Based on (i) the known involvement of G alpha q in the regulation of inositol-1,4,5-triphosphate generation and (ii) the requirement for Ca2+ in LTX action, we tested the effect of inhibitors of Ca2+ mobilization on the toxin-evoked norepinephrine release. It was found that aminosteroid U73122, which inhibits the coupling of G proteins to phospholipase C, blocks the Ca(2+)-dependent toxin's action. Thapsigargin, which depletes intracellular Ca2+ stores, also potently decreases the effect of LTX in the presence of extracellular Ca2+. On the other hand, clostridial neurotoxins or drugs interfering with Ca2+ metabolism do not inhibit the Ca2(+)-independent component of LTX-stimulated release. In the absence of Ca2+, the toxin induces in the presynaptic membrane non-selective pores permeable to small fluorescent dyes; these pores may allow efflux of neurotransmitters from the cytoplasm. Our results suggest that LTX stimulates norepinephrine exocytosis only in the presence of external Ca2+ provided intracellular Ca2+ stores are unperturbed and that latrophilin, G proteins and phospholipase C may mediate the mobilization of stored Ca2+, which then triggers secretion.

alpha-latrotoxin action probed with recombinant toxin: receptors recruit alpha-latrotoxin but do not transduce an exocytotic signal

alpha-Latrotoxin stimulates neurotransmitter release probably by binding to two receptors, CIRL/latrophilin 1 (CL1) and neurexin Ialpha. We have now produced recombinant alpha-latrotoxin (LtxWT) that is as active as native alpha-latrotoxin in triggering synaptic release of glutamate, GABA and norepinephrine. We have also generated three alpha-latrotoxin mutants with substitutions in conserved cysteine residues, and a fourth mutant with a four-residue insertion. All four alpha-latrotoxin mutants were found to be unable to trigger release. Interestingly, the insertion mutant LtxN4C exhibited receptor-binding affinities identical to wild-type LtxWT, bound to CL1 and neurexin Ialpha as well as LtxWT, and similarly stimulated synaptic hydrolysis of phosphatidylinositolphosphates. Therefore, receptor binding by alpha-latrotoxin and stimulation of phospholipase C are insufficient to trigger exocytosis. This conclusion was confirmed in experiments with La3+ and Cd2+. La3+ blocked release triggered by LtxWT, whereas Cd2+ enhanced it. Both cations, however, had no effect on the stimulation by LtxWT of phosphatidylinositolphosphate hydrolysis. Our data show that receptor binding by alpha-latrotoxin and activation of phospholipase C do not by themselves trigger exocytosis. Thus receptors recruit alpha-latrotoxin to its point of action without activating exocytosis. Exocytosis probably requires an additional receptor-independent activity of alpha-latrotoxin that is selectively inhibited by the LtxN4C mutation and by La3+.

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.

Dissociation between Ca2+-triggered synaptic vesicle exocytosis and clathrin-mediated endocytosis at a central synapse

We have tested whether action potential-evoked Ca2+ influx is required to initiate clathrin-mediated synaptic vesicle endocytosis in the lamprey reticulospinal synapse. Exo- and endocytosis were temporally separated by a procedure involving tonic action potential stimulation and subsequent removal of extracellular Ca2+ (Ca2+e). A low concentration of Ca2+ ([Ca2+]e of 11 microM) was found to be required for the induction of early stages of endocytosis. However, the entire endocytic process, from the formation of clathrin-coated membrane invaginations to the generation of synaptic vesicles, proceeded in the absence of action potential-mediated Ca2+ entry. Our results indicate that the membrane of synaptic vesicles newly incorporated in the plasma membrane is a sufficient trigger of clathrin-mediated synaptic vesicle endocytosis.

Vesicle exocytosis stimulated by alpha-latrotoxin is mediated by latrophilin and requires both external and stored Ca2+

alpha-Latrotoxin (LTX) stimulates massive neurotransmitter release by two mechanisms: Ca2+-dependent and -independent. Our studies on norepinephrine secretion from nerve terminals now reveal the different molecular basis of these two actions. The Ca2+-dependent LTX-evoked vesicle exocytosis (abolished by botulinum neurotoxins) is 10-fold more sensitive to external Ca2+ than secretion triggered by depolarization or A23187; it does not, however, depend on the cation entry into terminals but requires intracellular Ca2+ and is blocked by drugs depleting Ca2+ stores and by inhibitors of phospholipase C (PLC). These data, together with binding studies, prove that latrophilin, which is linked to G proteins and inositol polyphosphate production, is the major functional LTX receptor. The Ca2+-independent LTX-stimulated release is not inhibited by botulinum neurotoxins or drugs interfering with Ca2+ metabolism and occurs via pores in the presynaptic membrane, large enough to allow efflux of neurotransmitters and other small molecules from the cytoplasm. Our results unite previously contradictory data about the toxin's effects and suggest that LTX-stimulated exocytosis depends upon the co-operative action of external and intracellular Ca2+ involving G proteins and PLC, whereas the Ca2+-independent release is largely non-vesicular.

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

alpha-Latrotoxin (alpha-LT) potently enhances quantal release of neurotransmitter from nerve terminals. To develop the adrenal chromaffin cell as a 'model' system for the study of mechanisms of toxin action, we used amperometry to examine secretion of catecholamines and spectrofluorometry to measure cytosolic Ca. Several key features of toxin action emerged. (1) Release occurs at concentrations of toxin >35 pM and the pattern of release changes from repeated brief bursts to more continuous discharges of varying duration as the toxin concentration increases. (2) Release requires extracellular calcium in the micromolar range, but not the activity of native voltage-dependent calcium channels. (3) Release is associated with a rise in cytosolic calcium to near micromolar range. (4) Provided calcium is later restored, release can be induced even though the toxin is applied and washed away in calcium-free saline. These features largely resemble those of alpha-LT action on nerve terminals. They suggest that in chromaffin cells, as in neurons, the Ca-dependence of toxin-enhanced quantal release is based on Ca entry through toxin-induced channels rather than a requirement of extracellular Ca for toxin binding.

Ca2+-independent insulin exocytosis induced by alpha-latrotoxin requires latrophilin, a G protein-coupled receptor

alpha-Latrotoxin (alpha-LTX) induces exocytosis of small synaptic vesicles (SSVs) in neuronal cells both by a calcium-independent mechanism and by opening cation-permeable pores. Since the basic molecular events regulating exocytosis in neurons and endocrine cells may be similar, we have used the exocytosis of insulin-containing large dense core vesicles (LDCVs) as a model system. In primary pancreatic beta-cells and in the derived cell lines INS-1 and MIN6, alpha-LTX increased insulin release in the absence of extracellular calcium, but the insulin-secreting cell lines HIT-T15 and RINm5F were unresponsive. alpha-LTX did not alter membrane potential or cytosolic calcium, and its stimulatory effect on exocytosis was still observed in pre-permeabilized INS-1 cells kept at 0.1 microM Ca2+. Consequently, pore formation or ion fluxes induced by alpha-LTX could be excluded. The Ca2+-independent alpha-LTX-binding protein, latrophilin, is a novel member of the secretin family of G protein-coupled receptors (GPCR). Sensitivity to alpha-LTX correlated with expression of latrophilin, but not with synaptotagmin I or neurexin Ialpha expression. Moreover, transient expression of latrophilin in HIT-T15 cells conferred alpha-LTX-induced exocytosis. Our results indicate that direct stimulation of exocytosis by a GPCR mediates the Ca2+-independent effects of alpha-LTX in the absence of altered ion fluxes. Therefore, direct regulation by receptor-activated heterotrimeric G proteins constitutes an important feature of the endocrine exocytosis of insulin-containing LDCVs and may also apply to SSV exocytosis in neurons.

Alpha-latrotoxin receptor, latrophilin, is a novel member of the secretin family of G protein-coupled receptors

alpha-Latrotoxin (LTX) stimulates massive exocytosis of synaptic vesicles and may help to elucidate the mechanism of regulation of neurosecretion. We have recently isolated latrophilin, the synaptic Ca2+-independent LTX receptor. Now we demonstrate that latrophilin is a novel member of the secretin family of G protein-coupled receptors that are involved in secretion. Northern blot analysis shows that latrophilin message is present only in neuronal tissue. Upon expression in COS cells, the cloned protein is indistinguishable from brain latrophilin and binds LTX with high affinity. Latrophilin physically interacts with a Galphao subunit of heterotrimeric G proteins, because the two proteins co-purify in a two-step affinity chromatography. Interestingly, extracellular domain of latrophilin is homologous to olfactomedin, a soluble neuronal protein thought to participate in odorant binding. Our findings suggest that latrophilin may bind unidentified endogenous ligands and transduce signals into nerve terminals, thus implicating G proteins in the control of synaptic vesicle exocytosis.

Drastic facilitation by alpha-latrotoxin of bovine chromaffin cell exocytosis without measurable enhancement of Ca2+ entry or [Ca2+]i

1. Latrotoxin (LTX, 1-3 nM) caused a gradual increase of the spontaneous catecholamine release rate in bovine adrenal chromaffin cells superfused with normal Krebs-Hepes solution containing 2.5 mM Ca2+. Ca2+ removal abolished this effect. LTX enhanced also the secretory responses to high K+ (35 or 70 mM) and to acetylcholine (ACh, 30 microM). 2. The application of Ca2+ pulses to cells previously superfused with a 0 Ca2+ solution (Krebs-Hepes deprived of CaCl2) induced secretory responses that gradually reached 400-800 nA of catecholamines, provided that LTX was present. The responses to ACh or 35 mM K+ pulses (in the presence of Ca2+) were also enhanced by LTX, from around 100-200 nA to over 1000 nA. Though such enhancement remained in the presence of Ca2+ channel blockers, it disappeared upon the lowering of [Na+]o or in electroporated cells. 3. Using protocols similar to those of secretion, LTX did not enhance basal 45Ca2+ uptake, whole-cell Ca2+ currents or basal [Ca2+]i. In fact, LTX attenuated the K(+)- or ACh-evoked increases in 45Ca2+ uptake and [Ca2+]i. 4. It is proposed that the secretory response to brief periods of Ca2+ reintroductions is triggered by local subplasmalemmal Ca2+i transients, produced by the Na(+)-Ca2+ exchanger of the plasma membrane working in the reverse mode. This situation might be physiologically reproduced during ACh stimulation of chromaffin cells, which is followed by the firing of Na(+)-dependent action potentials.

Heterogeneity of functional synaptic parameters among single release sites

Following alpha-latrotoxin application to cerebellar slices, bursts of miniature IPSCs (mIPSCs) were observed in interneurons of the molecular layer. Within bursts, mIPSCs had homogeneous amplitudes with a narrow Gaussian distribution. Analysis of successive event amplitudes revealed an interaction between consecutive IPSCs, indicating that bursts originate at single release sites. A mean receptor occupancy of 76% was calculated. IPSCs within a burst were analyzed using nonstationary noise analysis. The results indicate that individual release sites differ in the number, unitary conductance, and peak opening probability of their postsynaptic channels. In addition, the IPSC decay kinetics were very different among release sites. Finally, a significant correlation was found between several pairs of single site synaptic parameters.

alpha-Latrotoxin stimulates exocytosis by the interaction with a neuronal G-protein-coupled receptor

alpha-Latrotoxin is a potent stimulator of neurosecretion. Its action requires extracellular binding to high affinity presynaptic receptors. Neurexin I alpha was previously described as a high affinity alpha-latrotoxin receptor that binds the toxin only in the presence of calcium ions. Therefore, the interaction of alpha-latrotoxin with neurexin I alpha cannot explain how alpha-latrotoxin stimulates neurotransmitter release in the absence of calcium. We describe molecular cloning and functional expression of the calcium-independent receptor of alpha-latrotoxin (CIRL), which is a second high affinity alpha-latrotoxin receptor that may be the major mediator of alpha-latrotoxin's effects. CIRL appears to be a novel orphan G-protein-coupled receptor, a member of the secretin receptor family. In contrast with other known serpentine receptors, CIRL has two subunits of the 120 and 85 kDa that are the result of endogenous proteolytic cleavage of a precursor polypeptide. CIRL is found in brain where it is enriched in the striatum and cortex. Expression of CIRL in chromaffin cells increases the sensitivity of the cells to the effects of alpha-latrotoxin, demonstrating that this protein is functional in coupling to secretion. Syntaxin, a component of the fusion complex, copurifies with CIRL on an alpha-latrotoxin affinity column and forms stable complexes with this receptor in vitro. Interaction of CIRL with a specific presynaptic neurotoxin and with a component of the docking-fusion machinery suggests its role in regulation of neurosecretion.

Single-cell measurements of quantal secretion induced by alpha-latrotoxin from rat adrenal chromaffin cells: dependence on extracellular Ca2+

alpha-Latrotoxin (alpha-LT), from black widow spider venom, is a potent enhancer of the spontaneous quantal release of neurotransmitter from a variety of nerve terminals and clonal neurosecretory cells. Using electrochemical amperometry and estimation of membrane impedance by phase detection, we present evidence that alpha-LT induces exocytosis of catecholamines from rat adrenal chromaffin cells beginning as rapidly as 30 s after close application of the toxin. This release is largely dependent on adequate levels of extracellular Ca2+ ([Ca2+]o). Lowering [Ca2+]o from 2 mM to </= 10-20 microM reduces the alpha-LT-induced rise in membrane capacitance by at least sixfold, on average, and nearly abolishes alpha-LT-induced quantal amperometric events, while still permitting insertion of non-selective cation channels. Based on these experiments, we argue that the rapid onset of alpha-LT action in promoting massive quantal release from chromaffin cells is primarily due to an increase in the Ca2+ permeability of the plasma membrane through non-selective cation channels.

Isolation and biochemical characterization of a Ca2+-independent alpha-latrotoxin-binding protein

alpha-Latrotoxin, a black widow spider neurotoxin, can bind to high affinity receptors on the presynaptic plasma membrane and stimulate massive neurotransmitter release in the absence of Ca2+. Neurexins, previously isolated as alpha-latrotoxin receptors, require Ca2+ for their interaction with the toxin and, thus, may not participate in the Ca2+-independent alpha-latrotoxin activity. We now report the isolation of a novel protein that binds alpha-latrotoxin with high affinity in the presence of various divalent cations (Ca2+, Mg2+, Ba2+, and Sr2+) as well as in EDTA. This protein, termed here latrophilin, has been purified from detergent-solubilized bovine brain membranes by affinity chromatography on immobilized alpha-latrotoxin and concentrated on a wheat germ agglutinin affinity column. The single polypeptide chain of latrophilin is N-glycosylated and has an apparent molecular weight of 120,000. Sucrose gradient centrifugations demonstrated that latrophilin and alpha-latrotoxin form a stable equimolar complex. In the presence of the toxin, anti-alpha-latrotoxin antibodies precipitated iodinated latrophilin, whose binding to immobilized toxin was characterized by a dissociation constant of 0.5-0.7 nM. This presumably membrane-bound protein is localized to and differentially distributed among neuronal tissues, with about four times more latrophilin expressed in the cerebral cortex than in the cerebellum; subcellular fractionation showed that the protein is highly enriched in synaptosomal plasma membranes. Our data suggest that latrophilin may represent the Ca2+-independent receptor and/or molecular target for alpha-latrotoxin.

The timing of synaptic vesicle endocytosis

Alternative models to describe the endocytosis phase of synaptic vesicle recycling are associated with time scales of vesicle recovery ranging from milliseconds to tens of seconds. There have been suggestions that one of the major models, envisioned as a slow process that occurs only after complete fusion of the vesicle membrane with the neurolemma, might be applicable only under conditions of heavy, nonphysiological stimulation. Using FM 1-43 and similar fluorescent probes to label recycling synaptic vesicles in rat hippocampal neurons, we have measured the kinetics of endocytosis with a wide range of action-potential-driven exocytotic loads. Our results indicate that when either 5% or 25% of the vesicle pool is used, vesicles are recovered with a half-time on the order of 20 s (24 degrees C). This endocytosis rate was not influenced by operations designed to alter intracellular Ca2+ during membrane retrieval, suggesting that residual Ca2+ after strong stimuli probably does not greatly retard endocytosis. Finally, we have shown that vesicle-destaining kinetics are not strongly influenced by the substantially differing rates at which two marker dyes tested dissociate from membranes. This observation suggests that vesicles remain open long enough for essentially complete dissociation of even the slower dye (a few seconds) or, alternatively, that both dyes readily escape vesicle membrane by lateral diffusion through any exocytotic opening. These data seem most consistent with applicability of the slow-endocytosis, complete-fusion model at low as well as high levels of exocytosis.

Amperometric detection of stimulus-induced quantal release of catecholamines from cultured superior cervical ganglion neurons

Amperometry has been used for real-time electrochemical detection of the quantal release of catecholamines and indolamines from secretory granules in chromaffin and mast cells. Using improved-sensitivity carbon fiber electrodes, we now report the detection of quantal catecholamine release at the surface of somas of neonatal superior cervical ganglion neurons that are studded with axon varicosities containing synaptic vesicles. Local application of a bath solution containing high K+ or black widow spider venom, each of which greatly enhances spontaneous quantal release of transmitter at synapses, evoked barrages of small-amplitude (2-20 pA), short-duration (0.5-2 ms) amperometric quantal "spikes". The median spike charge was calculated as 11.3 fC. This figure corresponds to 3.5 x 10(4) catecholamine molecules per quantum of release, or approximately 1% that evoked by the discharge of the contents of a chromaffin granule.

Inhibition of acetylcholine release from mouse motor nerve by a P-type calcium channel blocker, omega-agatoxin IVA

1. The effects were studied of the central neurone P-type Ca2+ channel blockers, omega-agatoxin IVA, omega-conotoxin MVIIC (polypeptide toxins) and synthetic funnel-web spider polyamine toxin on acetylcholine release from mouse motor nerve. 2. omega-Agatoxin IVA decreased the quantal content of endplate potentials and blocked synaptic transmission in the nanomolar range in a reversible manner, whereas the other toxins depressed transmission in the hundred micromolar range. 3. The polyamine toxin, but not the polypeptide toxins, decreased the amplitude of the miniature endplate potential. The increase in the frequency of miniature endplate potentials evoked by high [K+], but not that evoked by alpha-latrotoxin, was effectively antagonized by omega-agatoxin IVA. 4. In the presence of omega-agatoxin IVA, high frequency nerve stimulation produced facilitation of endplate currents and tetanic contractions. 5. The results suggest that, under physiological conditions, the Ca2+ necessary for nerve action potential-evoked acetylcholine release is translocated via a subtype of the P-type Ca2+ channel sensitive to omega-agatoxin IVA.

Inhibition of endocytosis by elevated internal calcium in a synaptic terminal

During synaptic transmission in the nervous system, synaptic vesicles fuse with the plasma membrane of presynaptic terminals, releasing neurotransmitter by exocytosis. The vesicle membrane is then retrieved by endocytosis and recycled into new transmitter-containing vesicles. Exocytosis in synaptic terminals is calcium-dependent, and we now report that endocytosis also is regulated by the intracellular calcium concentration ([Ca2+]i). Capacitance measurements in synaptic terminals of retinal bipolar neurons revealed that endocytosis was strongly inhibited by elevated [Ca2+]i in the range achieved by Ca(2+)-current activation. The rate of membrane retrieval was steeply dependent on [Ca2+]i, with a Hill coefficient of 4 and half-inhibition at approximately 500 nM. At [Ca2+]i > or = 900 nM, endocytosis was entirely absent. The action of internal calcium on endocytosis represents a novel negative-feedback mechanism controlling the rate of membrane recovery in synaptic terminals after neurotransmitter secretion. As membrane retrieval is the first step in vesicle recycling, this mechanism may contribute to activity-dependent synaptic depression.