A toxin purified from the venom of black widow spider affects the uptake and release of radioactive gamma-amino butyrate and N-epinephrine from rat brain synaptosomes

Latrophilins updated

Latrophilins (LPHN) are part of a yet unexplored family of receptors comprising three isoforms, LPHN1-3, and belonging to a unique branch of G protein-coupled receptors (GPCR) named adhesion GPCR (aGPCR). LPHN are considered to be prototypical models for the study of aGPCR as they are one of the most evolutionary conserved members. Previously described as the target for a potent neurotoxin from the black widow spider venom, LPHN are now being studied under a whole new perspective. Indeed, recent advances have provided a better understanding of different aspects of this prototypical family of receptors: 1) elucidation of LPHN ectodomain organization by crystallography has unveiled a new functional domain with great repercussion on all the other members of the aGPCR family, 2) proteomic approaches have opened the gate to unsuspected functional characteristics of LPHN cellular role, and 3) genetic approaches have provided hints into the physiological functions of LPHN in specific systems and organisms. Moreover, genomic linkage studies screening human patients from diverse genetic backgrounds have involved LPHN gene defects in human disorders such as attention-deficit hyperactivity disorder and cancer. In this review, we will provide a historical perspective addressing experimental research on these receptors while highlighting the new advances and discoveries concerning LPHN functions. As GPCR still represent the most studied targets for the development of pharmacological approaches aiming at alleviating human disorders, the relevance of studying LPHN retains a high pertinence to better understand these receptors for the treatment of human diseases.

Partial purification of ?-glycerotoxin, a presynaptic neurotoxin from the venom glands of the polychaete annelid glycera convoluta

The venom secreted from glands appended to the jaws of Glycera convoluta, a Polychaete Annelid, increases the spontaneous quantal release of transmitter from nerve terminals. The component that is biologically active on vertebrate cholinergic nerve terminals has recently been shown to be a high molecular weight protein. In the present work, the crude extract from the venom apparatus was shown to be toxic for mammals and crustaceans. It was fractionated by gel filtrations and ion exchange chromatographies. The biologically active component at frog neuromuscular junctions, ?-glycerotoxin, was purified more than 1,000-fold. It is distinct from the components that are toxic for crustaceans. Purified ?-glycerotoxin is a globular protein of 300,000 +/- 20,000 mol wt. It has a Stokes radius of 65 A and a sedimentation coefficient of 11 S. By its molecular properties, ?-glycerotoxin appears distinct from other neurotoxins such as ?-latrotoxin, which also trigger transmitter release.

Proteomic analysis of Latrodectus tredecimguttatus venom for uncovering potential latrodectism-related proteins

Black widow spider is one of the most poisonous spiders in the world. Up to now, there have been few systematic analyses of the spider venom components, and the mechanism of action of the venom has not been completely understood. In this work, we employed combinative proteomic strategy to analyze the venom collected from living adult spider Latrodectus tredecimguttatus by electrical stimulation. The experiments demonstrated that the venom is primarily composed of high molecular weight proteins and has high abundance proteins around 100 kDa. The content of peptides and proteins with low molecular weight is low. A total of 75 nonredundant venom proteins with distinct function were unambiguously identified. Besides the known black widow spider venom proteins including latrotoxins, a variety of hydrolases and other proteins with special activity were found in the venom, such as proteinase, phospholipase, phosphatase, nuclease, fucolectin, venom allergen antigen 5-like protein and trypsin inhibitor, and so on. Their possible biological actions and relationship with latrodectism were discussed. The results help to understand the complexity and action mechanism of L. tredecimguttatus venom.

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.

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.

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.

Black widow spider toxins: the present and the future

The venom of the black widow spider Latrodectus mactans tredisimguttatus was found to contain a family of high molecular weight toxic proteins inducing a sharp increase in transmitter secretion from the affected nerve endings, which are highly specific for vertebrates, or for insects, or for crustaceans. Along with the known alpha-latrotoxin, five latroinsectotoxins affecting the neurotransmitter release from presynaptic endings of insects and one latrocrustatoxin active only for crustaceans were isolated and studied in detail. Alpha-latrotoxin provokes a massive transmitter release from different nerve endings of vertebrates, whereas other toxins increase the secretion process either in insects or crustaceans. The cDNAs encoding the putative alpha-latrotoxin and two latroinsectotoxins (alpha-latroinsectotoxin and delta-latroinsectotoxin) precursors were cloned and sequenced. These toxins are polypeptides of about 1000 amino acids and share a high level of amino acid identity. Analysis of amino acid sequences of the three toxins reveals the central regions being almost entirely composed of series of ankyrin-like repeats. Taking into account the size and multifunctional properties of latrotoxin its molecule can be divided into several functional domains. Immunochemical experiments indicated the presence in the alpha-latrotoxin molecule of distinguishable functional domains responsible for ionophoric and secretogenic actions. The highly purified preparation of alpha-latrotoxin was shown to contain an additional component, a low molecular weight protein structurally related to crustacean hyperglycemic hormones. Several attempts were made to characterize and isolate alpha-latrotoxin receptor components. The existence of Ca-dependent and Ca-independent binding proteins was found in the presynaptic membrane preparations.

trans -Bilayer orientation and voltage-dependence of α-latrotoxin-induced channels

α-Latrotoxin, a polypeptide and potent presynaptic neurotoxin, interacts with artificial lipid bilayers inducing a large increase of conductance, when added to one ( cis side) of the two bathing solutions. These conductance changes are due to the presence of channels which, in 0.1 m monovalent cation solution, have conductances between 100 and 400 pS. Current-voltage relations of macroscopic and single channel conductances show marked non-ohmic behaviour being reduced at positive potentials, referred to cis side as virtual ground. The decrease in conductance at high positive voltage is modulated by lipid composition and abolished by digestion with pronase of a trans -bilayer protruding domain of the protein. The results are consistent with the notion that α-latrotoxin forms channels and provide evidence that the molecule is endowed with a specific mode of insertion and orientation in lipid bilayers.

Detection of intracellular free Na+ concentration of synaptosomes by a fluorescent indicator, Na(+)-binding benzofuran isophthalate: the effect of veratridine, ouabain, and alpha-latrotoxin

A novel fluorescent Na+ indicator, Na(+)-binding benzofuran isophthalate (SBFI), was used to follow changes in the intracellular free Na+ concentration ([Na+]i) of synaptosomes. The dye, when loaded into synaptosomes in the form of its acetoxymethyl ester, was responsive to changes of [Na+]. Calibration was made using the 340/380 nm excitation ratio when the cytoplasmic Na+ concentration was equilibrated with different concentrations of extracellular Na+ in the presence of 2 microM gramicidin D. The basal value of [Na+]i in synaptosomes in the presence of 140 mM extracellular Na+ was found to be 10.9 +/- 1.8 mM. Veratridine, which opens potential-dependent Na+ channels, caused a sudden increase in [Na+]i in a concentration-dependent manner (1-20 microM), whereas the effect of ouabain (20 and 50 microM), the inhibitor of the plasma membrane Na+,K(+)-ATPase, was more gradual. The rise in the fluorescence intensity upon addition of veratridine was prevented completely by 2 microM tetrodotoxin. alpha-Latrotoxin, the black widow spider toxin, caused an increase in the fluorescence intensity, which became evident 1 min after the addition of the toxin. The rate of increase was proportional to the concentration of the toxin (0.19-1.5 nM). This report confirms our earlier finding demonstrating a Na(+)-dependent component in the action of alpha-latrotoxin, and shows that changes in [Na+]i in synaptosomes can be followed by SBFI.

Effect of alpha-latrotoxin on acetylcholine release and intracellular Ca2+ concentration in synaptosomes: Na(+)-dependent and Na(+)-independent components

We studied the effect of alpha-latrotoxin (alpha LTX) on [14C]acetylcholine ([14C]ACh) release, intracellular Ca2+ concentration ([Ca2+]i). plasma membrane potential, and high-affinity choline uptake of synaptosomes isolated from guinea pig cortex. alpha LTX (10(-10)-10(-8) M) caused an elevation of the [Ca2+]i as detected by Fura 2 fluorescence and evoked [14C]ACh efflux. Two components in the action of the toxin were distinguished: one that required the presence of Na+ in the external medium and another that did not Displacement of Na+ by sucrose or N-methylglucamine in the medium considerably decreased the elevation of [Ca2+]i and [14C]ACh release by alpha LTX. The Na(+)-dependent component of the alpha LTX action was obvious in the inhibition of the high-affinity choline uptake of synaptosomes. Some of the toxin action on both [Ca2+]i and [14C]ACh release remained in the absence of Na+. Both the Na(+)-dependent and the Na(+)-independent components of the alpha LTX-evoked [14C]ACh release partly required the presence of either Mg2+ or Ca2+. The nonneurotransmitter [14C]choline was released along with [14C]ACh, but this release did not depend on the presence of either Na+ or Ca2+, indicating nonspecific leakage through the plasma membrane. We conclude that there are two factors in the release of ACh from synaptosomes caused by the toxin: (1) cation-dependent ACh release, which is related to (a) Na(+)-dependent divalent cation entry and (b) Na(+)-independent divalent cation entry, and (2) non-specific Na(+)- and divalent cation-independent leakage.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.

Alpha-latrotoxin: preparation and effects on calcium fluxes

A toxin that causes a massive presynaptic activation of transmitter release from nerve terminals is alpha-latrotoxin, isolated from Latrodectus tredecimguttatus spider venom. This toxin has been highly purified, utilizing as a biological assay a toxin-dependent increase in 45Ca(2+)-accumulation by PC12 cells. The purification protocol includes an ion-exchange step and a gel-filtration column, by fast-flow liquid chromatography. The resulting toxin is a polypeptide of about 125 kDa in molecular mass. At nmol concentrations it specifically activates calcium influx and transmitter secretion after interacting with neuronal acceptors of the presynaptic membrane. The inhibitory effect of trivalent ions (which may develop as degradation product of 45Ca2+) on toxin-dependent calcium accumulation by PC12 cells is described. The results obtained suggest that calcium fluxes directly involved in the neurosecretory event, may occur through newly formed toxin-dependent channels.

Purification and characterization of iotrochotin, a novel toxin from the Caribbean sponge Iotrochota birotulata, which selectively permeabilizes synaptosomes

A protein termed iotrochotin (IOT) was isolated from the exudate of the Caribbean sponge Iotrochota birotulata using as an assay its stimulation of the release of radioactivity from synaptosomes preloaded with [3H]choline. Sephadex G-50 chromatography of the exudate produced one peak, with a mol. wt of approximately 18,000, which was further resolved into two active fractions by anion exchange chromatography. The more tightly bound of the two fractions was characterized and referred to as IOT. The action of IOT was essentially complete by 0.5-1.0 min and was independent of the Ca2+ or Na+ content of the incubation mixture. Released radioactivity included about 50% each of [3H]acetylcholine and [3H]choline. Release of radioactivity increased as a function of IOT concentration and then reached a maximum. Extrapolated asymptotic release was nearly equal to that obtained by lysing the synaptosomes. IOT also released radioactivity from synaptosomes which had been preincubated with other tritiated neurotransmitters or with 2-[3H]deoxy-D-glucose. Lactate dehydrogenase and choline acetyltransferase activities were not released from synaptosomes by treatment with IOT, but were released by digitonin. IOT therefore releases some of the smaller molecular weight components of synaptosomes, but does not permeabilize the synaptosomal membrane in the same way as digitonin treatment.

Selective presynaptic insectotoxin (alpha-latroinsectotoxin) isolated from black widow spider venom

A homogenous protein of 120,000 mol. wt isolated from black widow spider (Lactrodectus mactans tredecimguttatus) venom and referred to as alpha-latroinsectotoxin was highly potent (4 nM) in the induction of an increase of the frequency of miniature excitatory postsynaptic potentials in blowfly (Calliphora vicina) larvae neuromuscular preparations. In the frog nerve ending, however, even 50 nM alpha-latroinsectotoxin failed to affect transmitter release. Pretreatment of insect preparations with alpha-latrotoxin or frog preparations with alpha-latroinsectotoxin did not prevent the specific effect of consequent applications of alpha-latroinsectotoxin (insect) and alpha-latrotoxin (frog), respectively. The binding of labelled [125I]alpha-latroinsectotoxin to insect and [125I]alpha-latrotoxin to bovine membrane preparations was saturable and highly specific. The presynaptic effect, but not the binding of alpha-latroinsectotoxin, was dependent on the presence of divalent cations in the external medium. Mg2+ could readily substitute for Ca2+ and increase of transmitter release induced by alpha-latroinsectotoxin also occurred in Ca(2+)-free solutions. Pretreatment of preparations with 300 micrograms/ml concanavalin A completely abolished both the presynaptic effect of alpha-latroinsectotoxin and its binding to insect membrane preparations. Thus, the phenomenology of alpha-latroinsectotoxin action on insects resembles in general that described for the action of alpha-latrotoxin on vertebrates. The selectivity of alpha-latrotoxin and alpha-latroinsectotoxin seems to be due to differences in the structure of neurotoxin receptors in nerve endings of vertebrates and insects, although the mode of presynaptic action has a great deal in common.

Alpha-latrotoxin releases both vesicular and cytoplasmic glutamate from isolated nerve terminals

alpha-Latrotoxin causes a massive release of endogenous glutamate from guinea-pig cerebrocortical synaptosomes. There appear to be two components to the release. In the first 2 min following addition of 1.3 nM alpha-latrotoxin, glutamate release is largely energy dependent. Superimposed upon this release is a more slowly developing but ultimately much more extensive release of cytoplasmic glutamate together with gamma-aminobutyric acid and nonvesicular amino acids such as aspartate and alpha-aminoisobutyrate. In parallel with this cytoplasmic release there is an extensive depletion of ATP, a massive rise in cytoplasmic free Ca2+ concentration, and a severe restriction of synaptosomal respiratory capacity. The cytoplasmic release is only partially Na+ dependent, eliminating a simple reversal of the plasma membrane acidic amino acid carrier. It is concluded that alpha-latrotoxin releases both transmitter and cytoplasmic pools of amino acids in synaptosomes and causes a major disruption of terminal integrity.

Behavioural observations and neurophysiological responses of cockroaches envenomated with Latrodectus katipo venom

1. The behaviour of American cockroaches envenomated with Latrodectus katipo venom was correlated with neurophysiological events recorded from an isolated cockroach CNS preparation to which venom was applied. 2. The injection of a venom extract into cockroaches caused convulsions (3 behavioural categories: quivering, jerking and hyperextension) which eventually led to paralysis and death. 3. Changes in the spontaneous activity of nerve 5 (from the metathoracic ganglion) corresponded to the time course of envenomation behaviours. An initial increase in discharge peaked 5-10 min after application and subsequently decreased to become irreversibly blocked.

Latrotoxin-induced fusion of liposomes with bilayer phospholipid membranes

Liposomes containing amphotericin B as ionophoric marker were used to investigate the fusion of bilayer phospholipid membranes with liposomes. It was found that latrotoxin isolated from black widow spider venom induced the fusion of liposomes with planar bilayer when liposomes and latrotoxin were administered at opposite sides of the membrane.

Characterization and some properties of the venom gland extract of a theridiid spider (Steatoda paykulliana) frequently mistaken for black widow spider (Latrodectus tredecimguttatus)

The simplified purification protocol established for the isolation of alpha-latrotoxin from the venom of the spider Latrodectus tredecimguttatus, has been employed for the purification of toxic components present in the venom of the spider Steatoda paykulliana. The venom of this spider, frequently mistaken for L. tredecimguttatus, is by tradition considered to cause an envenomation potentially dangerous to man. The venom of S. paykulliana has little toxic effect on guinea-pigs but is extremely toxic to houseflies (Musca domestica). No proteolytic activity was detectable. Interaction of microgram/ml amounts of the venom extract with artificial lipid membranes produces an increase of membrane conductance through the formation of stable ion-permeable channels modulated by the direction and size of the electric potential differences across the membrane. Higher concentrations of this venom are able to stimulate the release of transmitters from neurosecretory cells in a fashion reminiscent of black widow spider venom. Antibodies against the whole L. tredecimguttatus venom gave a few positive cross-reactions in the immunodiffusion test with S. paykulliana venom gland extract indicating the presence of common molecular sequences in the two venoms. Polyclonal antibodies against alpha-latrotoxin did not cross-react in the immunodiffusion test with S. paykulliana venom extracts, nor in the immunofluorescence assay with its cephalothorax sections, thus suggesting that the venom glands do not contain alpha-latrotoxin. A partial characterization of S. paykulliana venom has been performed and a high molecular weight protein toxic to houseflies has been partially purified.

A functional domain on the alpha-latrotoxin molecule, distinct from the binding site, involved in catecholamine secretion from PC12 cells: identification with monoclonal antibodies

Seven monoclonal antibodies (mAbs) have been produced against alpha-latrotoxin (alpha-Latx), the toxin component of black widow spider venom that stimulates release of neurotransmitters from PC12 cells. These mAbs were characterized by an enzyme-linked immunosorbent assay and by neutralization analysis of the secretagogue properties of the toxin. The production of a panel of mAbs, possibly directed against different epitopes of alpha-Latx, provides a useful set of reagents to dissect the molecular regions of the toxin having different functions and to describe steps of its mode of action in responsive cells. Attention was focused on one of these mAbs (4C4.1), which inhibits in a dose-dependent fashion both toxin-stimulated and crude venom stimulated dopamine release from PC12 cells, prevents toxin-induced 45Ca2+ accumulation in PC12, alters toxin-dependent phosphoinositide breakdown, and prevents toxin-induced channel formation in artificial lipid bilayers. Since, within certain experimental conditions, mAb 4C4.1 is able to recognize the toxin bound to cells, we conclude that its effects were not a consequence of a direct interference with binding. On the basis of kinetic analysis of mAb interference on toxin action, expressed as accumulation of inositol phosphates and transmitter secretion, we suggest that the described effects result primarily from the blockade of an event immediately successive to binding and central for the full expression of toxin action. The availability of mAb 4C4.1 now makes possible the molecular characterization of the toxin moiety responsible for such an event.

Alpha-latrotoxin and glycerotoxin differ in target specificity and in the mechanism of their neurotransmitter releasing action

alpha-Latrotoxin, a high molecular weight protein (130,000) purified from the venom of the black widow spider, and a partially purified neurotoxin, glycerotoxin, prepared from extracts of the jaw glands of the polichaete annelid Glycera convoluta, were previously found to induce similar effects (stimulation of quantal acetylcholine release) at the frog neuromuscular junction. In the present study parallel experiments performed with these two toxins revealed that only glycerotoxin was able to release acetylcholine from Torpedo electric organ synaptosomes, while alpha-latrotoxin did not affect release in this system. In contrast, alpha-latrotoxin stimulated release of dopamine from PC12 cells (a cloned neurosecretory cell line), whereas glycerotoxin was almost inactive. In rat brain synaptosomes both toxins were active. Preincubation of synaptosomal membranes with glycerotoxin was without effect on the subsequent binding of alpha-latrotoxin. Glycerotoxin application induced depolarization of synaptosomal plasma membrane, massive Ca2+ influx, marked increase of the cytosolic Ca2+ concentration, and stimulation of catecholamine release. The latter effect occurred to the same extent when glycerotoxin was applied either in complete medium (containing both Ca2+ and Mg2+), Ca2+-free medium or divalent cation-free medium. Some of these effects of glycerotoxin in rat brain synaptosomes (depolarization, increased Ca2+ influx and increased cytosolic Ca2+ concentration) resemble effects previously reported for alpha-latrotoxin. However, the secretory response induced by the latter was reduced in Ca2+-free, and abolished in divalent cation-free media. The different target specificity and the lack of binding competition of the two toxins could be due to their ability to recognize different receptors whose distribution overlap only in part in the cellular systems we have studied. The differences in action, on the other hand, could depend on postreceptor events, possibly related to the transmembrane insertion of toxin molecules demonstrated by others in artificial lipid membranes.

Specific localization of the alpha-latrotoxin receptor in the nerve terminal plasma membrane

The receptor for alpha-latrotoxin, the major protein component of the black widow spider venom, was investigated by the use of the purified toxin and of polyclonal, monospecific anti-alpha-latrotoxin antibodies. Experiments on rat brain synaptosomes (where the existence of alpha-latrotoxin receptors was known from previous studies) demonstrated that the toxin-receptor complex is made stable by glutaraldehyde fixation. At saturation, each such complex was found to bind on the average five antitoxin antibody molecules. In frog cutaneous pectoris muscles, the existence of a finite number of high-affinity receptors was revealed by binding experiments with 125I-alpha-latrotoxin (Kd = 5 X 10(-10) M; bmax = 1.36 +/- 0.16 [SE] X 10(9) sites/mg tissue, dry weight). Nonpermeabilized muscles were first treated with alpha-latrotoxin, and then washed, fixed, dissociated into individual fibers, and treated with anti-alpha-latrotoxin antibodies and finally with rhodamine-conjugated sheep anti-rabbit antibodies. In these preparations, muscle fibers and unmyelinated preterminal nerve branches were consistently negative, whereas bright specific fluorescent images, indicative of concentrated alpha-latrotoxin binding sites, appeared in the junctional region. These images closely correspond in size, shape, and localization to endplates decorated by the acetylcholinesterase reaction. The presynaptic localization of the specific fluorescence found at frog neuromuscular junctions is supported by two sets of findings: (a) fluorescent endplate images were not seen in muscles that had been denervated; and (b) the distribution of fluorescence in many fibers treated with alpha-latrotoxin at room temperature was the one expected from swollen terminal branches. Swelling of terminals is a known morphological change induced by alpha-latrotoxin in this preparation. When muscles were treated with either proteolytic enzymes (trypsin, collagenase) or detergents (Triton X-100) before exposure to alpha-latrotoxin, the specific fluorescent endplate images failed to appear. Taken together these findings indicate that the alpha-latrotoxin receptor is an externally exposed protein highly concentrated in the nerve terminal plasma membrane. Its density (number per unit area) at the frog neuromuscular junction can be calculated to be approximately 2,400/micron2.

The effect of alpha-latrotoxin on the neurosecretory PC12 cell line: studies on toxin binding and stimulation of transmitter release

alpha-Latrotoxin of black widow spider venom was found to bind with high affinity (KA = 1.8 X 10(9)M-1) to specific sites present in discrete number (approximately 6300/cell, approximately 12/micron2) at the surface membrane of PC12 cells. This binding correlated with (and therefore, probably caused) the secretory response produced by the toxin. Binding was enhanced (approximately 2-fold) in the presence of mM concentrations of various divalent cations (Ca2+, Mn2+ and Co2+) while Ba2+ and Sr2+ had a smaller effect and Mg2+ was inactive. Hypertonicity, concanavalin A and trypsin pretreatment of the cells blocked the binding interaction. The alpha-latrotoxin-induced stimulation of 3H-dopamine release was massive and occurred very rapidly when cells were exposed to the toxin in a Ca2+-containing Krebs-Ringer medium, whereas it occurred at a much slower rate in a Ca2+-free, Mg2+-containing Ringer. Introduction of Ca2+ into the latter medium resulted in a shift of the release rate from slow to fast. In contrast, in divalent cation-free medium the response was abolished. The toxin-induced secretory response was unaffected by Na+ and Ca2+ channel blockers (tetrodotoxin and D600) as well as by calmodulin inhibitors (calmidazolium and trifluoperazine). The effects of Ca2+ and Mg2+ were found to be concentration-dependent, with half maximal responses occurring at approximately 0.3 and 1.5 mM for the two divalent cations, respectively. Other divalent cations could substitute for Ca2+ and Mg2+, the relative efficacy being Sr2+ greater than Ca2+ greater than Ba2+ much greater than Mn2+ greater than Mg2+ greater than Co2+. Moreover, the response occurring at suboptimal concentration of Ca2+ (0.4 mM) was potentiated by the concomitant addition of either Mg2+, Mn2+ or Co2+. The effect(s) of divalent cations in supporting the alpha-latrotoxin-induced release response seem(s) to occur primarily at step(s) beyond toxin binding because (a) the stimulatory effects of the various cations on release were not matched by parallel effects on binding, and (b) Ca2+ maintained its ability to stimulate fast release even when toxin binding had occurred in a Ca2+-free medium. Delays in the release responses were observed when cells were exposed to alpha LTx in Na+-free, glucosamine or methylamine-based media, or depolarized with high K+ (in the presence of D600) before toxin treatment. Moreover, in these two conditions the ability of Mg2+ to support the alpha LTx response was considerably decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of alpha-latrotoxin of black widow spider venom on synaptosome ultrastructure. A morphometric analysis correlating its effects on transmitter release

The morphological effects of alpha-latrotoxin, the major component of black widow spider venom, were studied quantitatively in a crude synaptosome fraction (prepared from rat brain cortices) which was incubated at 37 degrees C for 10 min in Ringer solutions. Two toxin concentrations were employed, one causing a very large stimulation of transmitter release (approximately 65% and approximately 43% release of [3H]noradrenaline from preloaded synaptosomes, with and without Ca2+ in the incubation buffer), the other 50-60% as active. Incubated synaptosomes, fixed in suspension with aldehydes, were evenly dispersed in agarose before embedding, to assure randomized sampling in the subsequent morphometric analysis. In all the experimental conditions investigated, alpha-latrotoxin treatment caused a significant decrease in the density (number/unit area) of synaptic vesicles in synaptosome profiles. Such an effect was dose-dependent and partially Ca2+-dependent, in good agreement with the data on transmitter release. At high toxin concentration a moderate increase of synaptosome volume and surface area was observed, both with and without Ca2+. Mitochondrial swelling appeared only in synaptosomes treated in Ca2+ containing medium. These effects of alpha-latrotoxin are similar to those described previously at the neuromuscular junction. Thus, the toxin might be a tool of general use for studying vertebrate synapses.

alpha-latrotoxin of black widow spider venom depolarizes the plasma membrane, induces massive calcium influx, and stimulates transmitter release in guinea pig brain synaptosomes

The effect of alpha-latrotoxin from black widow spider venom upon guinea pig cerebral cortical synaptosomes is described. Plasma membrane potential (delta psi p), in situ mitochondrial membrane potential (delta psi m), Ca2+ transport, gamma-amino[3H]butyrate release, [3H]noradrenaline release, and synaptosomal ATP were monitored under parallel conditions. Potentials were determined both isotopically and with a tetraphenylphosphonium-selective electrode. alpha-Latrotoxin depolarizes delta psi p selectively, both in the presence and absence of Ca2+. A slight toxin-induced depolarization of delta psi m is a consequence of a massive Ca2+ uptake across the plasma membrane. Depolarization of delta psi p is insensitive to tetrodotoxin, and Ca2+ entry is only partially inhibited by verapamil. Release of [3H]noradrenaline and gamma-amino[3H]butyrate is markedly stimulated by the toxin in the presence of Ca2+, and this effect is only slightly reduced in Ca2+-free conditions.

Peripheral catecholamine release by alpha-latrotoxin in the rat

Intraarterial injection of alpha-latrotoxin (alpha LTx), the major toxin of the black widow spider (Latrodectus mactans tredecimguttatus) venom, into carotid catheterized rats, induced prompt and marked rises in plasma adrenaline and noradrenaline concentrations, indicating that the toxin stimulates catecholamine release from both the adrenal medulla and sympathetic nerve terminals. Pretreatment with the ganglionic blocker chlorisondamine greatly reduced the plasma adrenaline response to alpha LTx but had almost no effect on the noradrenaline response, indicating that alpha LTx-stimulation of sympathetic nerve terminals is direct, whereas catecholamine release from the adrenal medulla is probably mediated by preganglionic release of acetylcholine. In vitro, alpha LTx induced a dose-dependent release of 3H-noradrenaline from rat irides preincubated with this radioactive amine and this effect was not changed by chlorisondamine plus atropine. By contrast, the toxin had no effect on 3H-noradrenaline release from suspensions of cultured rat chromaffin cells. Specific, high affinity binding of 125I-alpha LTx in iris and adrenal medulla homogenates was found to be exceedingly low, suggesting that it might be restricted to nerve terminals. No 125I-alpha LTx binding was seen nor could any effect of the toxin on 14C-5-hydroxytryptamine release be found in rat blood platelet preparations. alpha LTx binding and its amine releasing effect seem, therefore, to be specific for neurons and absent from other cells, even those, like adrenomedullary cells and blood platelets, which share with neurons their origin and/or other important characteristics.

Studies on alpha-latrotoxin receptors in rat brain synaptosomes: correlation between toxin binding and stimulation of transmitter release

alpha-Latrotoxin (alpha-LT), the major component of black widow spider venom, is a high-molecular-weight protein that acts presynaptically by stimulating the release of stored neurotransmitters. The purified toxin was iodinated to high specific radioactivity by the Bolton-Hunter procedure, without appreciable loss of biological activity. By the use of the 125I-toxin, specific receptors were revealed in synaptosome fractions isolated from various regions of the rat brain, but not in nonneural tissues. The density of alpha-LT receptors [which are probably composed of, or include, membrane protein(s)] varies between 0.6 and 0.88 pmol/mg of synaptosome protein, their affinity is very high (KA of the order of 10(10) M-1), their association rate is fast, and their dissociation rate slow. They might belong to a single, homogeneous class. This last conclusion, however, is still uncertain, because results suggesting a possible heterogeneity were obtained by studying the dissociation of the toxin from synaptosomes incubated in high-salt buffer. Experiments in which the binding of alpha-LT and its dopamine release activity in striatal synaptosomes were investigated in parallel in a variety of experimental conditions support the hypothesis that occupation of the high-affinity receptors is the initial step in the alpha-LT activation of the presynaptic response.

Characterization of alpha-latrotoxin interaction with rat brain synaptosomes and PC12 cells

alpha-latrotoxin, a polypeptide neurotoxin purified from the venom of the spider Latrodectus mactans tredecimguttatus, induces a massive release of a variety of neurotransmitters from rat brain synaptosomes and a clonal pheochromocytoma cell line (PC12 cells). In both systems secretion of catecholamines is dose- and calcium-dependent. Efflux of catecholamines is coupled with a substantial release of intracellular ATP. Independent of alpha-latrotoxin with PC12 cells is followed by a rapid influx of calcium and sodium ions, the rate being dependent on toxin and calcium concentrations. By reductive methylation it is possible to radioactively label alpha-latrotoxin without appreciable loss of neurotoxicity. A sensitive binding assay in vitro allows the identification of a limited number of specific binding sites in central nervous system synaptic membranes and PC12 cells, for which tritiated alpha-latrotoxin displays nanomolar affinity.