Distinct exocytotic responses of intact and permeabilised chromaffin cells after cleavage of the 25-kDa synaptosomal-associated protein (SNAP-25) or synaptobrevin by botulinum toxin A or B

Botulinum Neurotoxin Light Chains Expressed by Defective Herpes Simplex Virus Type-1 Vectors Cleave SNARE Proteins and Inhibit CGRP Release in Rat Sensory Neurons

A set of herpes simplex virus type 1 (HSV-1) amplicon vectors expressing the light chains (LC) of botulinum neurotoxins (BoNT) A, B, C, D, E and F was constructed. Their properties have been assessed in primary cultures of rat embryonic dorsal root ganglia (DRG) neurons, and in organotypic cultures of explanted DRG from adult rats. Following infection of primary cultures of rat embryonic DRG neurons, the different BoNT LC induced efficient cleavage of their corresponding target Soluble N-ethylmaleimide-sensitive-factor Attachment protein Receptor (SNARE) protein (VAMP, SNAP25, syntaxin). A similar effect was observed following infection by BoNT-A LC of organotypic cultures of adult rat DRG. To quantify and compare the functional activities of the different BoNT LC, the inhibition of calcitonin gene-related protein (CGRP) secretion was assessed in DRG neurons following infection by the different vectors. All BoNT-LC were able to inhibit CGRP secretion although to different levels. Vectors expressing BoNT-F LC displayed the highest inhibitory activity, while those expressing BoNT-D and -E LC induced a significantly lower CGRP release inhibition. Cleavage of SNARE proteins and inhibition of CGRP release could be detected in neuron cultures infected at less than one transducing unit (TU) per neuron, showing the extreme efficacy of these vectors. To our knowledge this is the first study investigating the impact of vector-expressed transgenic BoNT LC in sensory neurons.

Dynamin inhibition blocks botulinum neurotoxin type A endocytosis in neurons and delays botulism

The botulinum neurotoxins (BoNTs) are di-chain bacterial proteins responsible for the paralytic disease botulism. Following binding to the plasma membrane of cholinergic motor nerve terminals, BoNTs are internalized into an endocytic compartment. Although several endocytic pathways have been characterized in neurons, the molecular mechanism underpinning the uptake of BoNTs at the presynaptic nerve terminal is still unclear. Here, a recombinant BoNT/A heavy chain binding domain (Hc) was used to unravel the internalization pathway by fluorescence and electron microscopy. BoNT/A-Hc initially enters cultured hippocampal neurons in an activity-dependent manner into synaptic vesicles and clathrin-coated vesicles before also entering endosomal structures and multivesicular bodies. We found that inhibiting dynamin with the novel potent Dynasore analog, Dyngo-4a(TM), was sufficient to abolish BoNT/A-Hc internalization and BoNT/A-induced SNAP25 cleavage in hippocampal neurons. Dyngo-4a also interfered with BoNT/A-Hc internalization into motor nerve terminals. Furthermore, Dyngo-4a afforded protection against BoNT/A-induced paralysis at the rat hemidiaphragm. A significant delay of >30% in the onset of botulism was observed in mice injected with Dyngo-4a. Dynamin inhibition therefore provides a therapeutic avenue for the treatment of botulism and other diseases caused by pathogens sharing dynamin-dependent uptake mechanisms.

Accurate quantification of functional analogy among close homologs

Correctly evaluating functional similarities among homologous proteins is necessary for accurate transfer of experimental knowledge from one organism to another, and is of particular importance for the development of animal models of human disease. While the fact that sequence similarity implies functional similarity is a fundamental paradigm of molecular biology, sequence comparison does not directly assess the extent to which two proteins participate in the same biological processes, and has limited utility for analyzing families with several parologous members. Nevertheless, we show that it is possible to provide a cross-organism functional similarity measure in an unbiased way through the exclusive use of high-throughput gene-expression data. Our methodology is based on probabilistic cross-species mapping of functionally analogous proteins based on Bayesian integrative analysis of gene expression compendia. We demonstrate that even among closely related genes, our method is able to predict functionally analogous homolog pairs better than relying on sequence comparison alone. We also demonstrate that the landscape of functional similarity is often complex and that definitive “functional orthologs” do not always exist. Even in these cases, our method and the online interface we provide are designed to allow detailed exploration of sources of inferred functional similarity that can be evaluated by the user.

Common ancestry is a central tenet of modern biology, as genes from different species often show a high degree of sequence similarity, making it possible to study analogous processes across model organisms. However, many genes belong to large families with several duplicates and the relationship between genes from different species is often not one-to-one, complicating the transfer of experimental knowledge. We present a method that uses a large compendia of high-throughput expression data, that covers many genes that have not been analyzed in any other way, to systematically predict which genes are most likely to participate in the same biological process and thus have analogous function in different organisms. We show that our method agrees well with current experimental knowledge and we use it to investigate several families of genes that demonstrate the complexity of functional analogy.

Embryonic stem cell-derived neurons are a novel, highly sensitive tissue culture platform for botulinum research

There are no pharmacological treatments to rescue botulinum neurotoxin (BoNT)-mediated paralysis of neuromuscular signaling. In part, this failure can be attributed to the lack of a cell culture model system that is neuron-based, allowing detailed elucidation of the mechanisms underlying BoNT pathogenesis, yet still compatible with modern cellular and molecular approaches. We have developed a method to derive highly enriched, glutamatergic neurons from suspension-cultured murine embryonic stem (ES) cells. Hypothesizing that ES cell-derived neurons (ESNs) might comprise a novel platform to investigate the neurotoxicology of BoNTs, we evaluated the susceptibility of ESNs to BoNT/A and BoNT/E using molecular and functional assays. ESNs express neuron-specific proteins, develop synapses and release glutamate in a calcium-dependent manner under depolarizing conditions. They express the BoNT substrate SNARE proteins SNAP25, VAMP2 and syntaxin, and treatment with BoNT/A and BoNT/E holotoxin results in proteolysis of SNAP25 within 24 h with EC50s of 0.81 and 68.6 pM, respectively. Intoxication with BoNT/A results in the functional inhibition of potassium-induced, calcium-dependent glutamate release. ESNs remain viable and susceptible to intoxication for up to 90 days after plating, enabling longitudinal screens exploring toxin-specific mechanisms underlying persistence of synaptic blockade. The evidence suggests that derived neurons are a novel, biologically relevant model system that combines the verisimilitude of primary neurons with the genetic tractability and scalable expansion of a continuous cell line, and thus should significantly accelerate BoNT research and drug discovery while dramatically decreasing animal use.

Adeno-associated virus transfer of a gene encoding SNAP-25 resistant to botulinum toxin A attenuates neuromuscular paralysis associated with botulism

Advances in viral gene therapy have opened new possibilities for treating a range of motor neuron diseases, but these have not yet been translated into clinically applicable therapies because of difficulties in delivery to susceptible/damaged neurons, ambiguities in the identity of gene(s) implicated, and a paucity of means to quantify any physiological improvement. Most of these hurdles can be overcome by using the neuromuscular paralysis induced by botulinum neurotoxin type A (BoNT/A) as a prototype disease. Furthermore, because human botulism, occasionally fatal, causes prolonged muscle disablement as a result of the intraneuronal persistence of the toxin's SNAP-25 (S25)-cleaving protease, development of a genetic approach could lead to a potential treatment for this debilitating disease. Adeno-associated viral delivery of a cleavage-resistant S25 gene (S25-R198T) to chromaffin cells in vitro yielded exocytotically active S25-R198T that diminished subsequent blockade by BoNT/A of evoked catecholamine release. Evaluation in vivo, by administering this virus into rat spinal cord before injecting BoNT/A, showed a decreased inhibition of acetylcholine release as reflected in elevated retention of neuromuscular transmission. A similar, although smaller, protection of synaptic transmission from the toxin was seen after peripherally injecting the therapeutic virus. Such therapy also curtailed nerve sprouting normally induced by BoNT/A. This first demonstration of the utility of a DNA-based therapy for botulism paves the way for further advances in its treatment and for application to genetic disorders of motor neurons.

Stable silencing of SNAP-25 in PC12 cells by RNA interference

BACKGROUND

SNAP-25 is a synaptic protein known to be involved in exocytosis of synaptic vesicles in neurons and of large dense-core vesicles in neuroendocrine cells. Its role in exocytosis has been studied in SNAP-25 knockout mice, in lysed synaptosomes lacking functional SNAP-25 and in cells after treatment with botulinum toxins A or E that specifically cleave SNAP-25. These studies have shown that SNAP-25 appears to be required for most but not all evoked secretion. In order to further study the role of SNAP-25 in catecholamine secretion from PC12 cells we have used the recently developed technique of RNA interference to generate PC12 cell lines with virtually undetectable levels of SNAP-25. RNA interference is the sequence-specific silencing or knockdown of gene expression triggered by the introduction of double-stranded RNA into a cell. RNA interference can be elicited in mammalian cells in a number of ways, one of which is by the expression of small hairpin RNAs from a transfected plasmid. Selection of stably transfected cell lines expressing a small hairpin RNA allows one-time characterization of the degree and specificity of gene silencing and affords a continuing source of well-characterized knockdown cells for experimentation.

RESULTS

A PC12 cell line stably transfected with a plasmid expressing an shRNA targeting SNAP-25 has been established. This SNAP-25 knockdown cell line has barely detectable levels of SNAP-25, but normal levels of other synaptic proteins. Catecholamine secretion elicited by depolarization of the SNAP-25 knockdown cells was reduced to 37% of control.

CONCLUSION

Knockdown of SNAP-25 in PC12 cells reduces but does not eliminate evoked secretion of catecholamines. Transient expression of human SNAP-25 in the knockdown cells rescues the deficit in catecholamine secretion.

Revisiting the role of SNAREs in exocytosis and membrane fusion

For over a decade SNARE hypotheses have been proposed to explain the mechanism of membrane fusion, yet the field still lacks sufficient evidence to conclusively identify the minimal components of native fusion. Consequently, debate concerning the postulated role(s) of SNAREs in membrane fusion continues. The focus of this review is to revisit original literature with a current perspective. Our analysis begins with the earliest studies of clostridial toxins, leading to various cellular and molecular approaches that have been used to test for the roles of SNAREs in exocytosis. We place much emphasis on distinguishing between specific effects on membrane fusion and effects on other critical steps in exocytosis. Although many systems can be used to study exocytosis, few permit selective access to specific steps in the pathway, such as membrane fusion. Thus, while SNARE proteins are essential to the physiology of exocytosis, assay limitations often prevent definitive conclusions concerning the molecular mechanism of membrane fusion. In all, the SNAREs are more likely to function upstream as modulators or priming factors of fusion.

Regulated secretion: SNARE density, vesicle fusion and calcium dependence

SNAREs such as VAMP, SNAP-25 and syntaxin are essential for intracellular trafficking, but what are their exact molecular roles and how are their interactions with other proteins manifest? Capitalizing on the differential sensitivity of SNAREs to exogenous proteases, we quantified the selective removal of identified SNAREs from native secretory vesicles without loss of fusion competence. Using previously established fusion assays and a high sensitivity immunoblotting protocol, we analyzed the relationship between these SNARE proteins and Ca2+-triggered membrane fusion. Neither the extent of fusion nor the number of intermembrane fusion complexes per vesicle were correlated with the measured density of identified egg cortical vesicle (CV) SNAREs. Without syntaxin, CVs remained fusion competent. Surprisingly, for one (but not another) protease the Ca2+ dependence of fusion was correlated with CV SNARE density, suggesting a native protein complex that associates with SNAREs, the architecture of which ensures high Ca2+ sensitivity. As SNAREs may function during CV docking in vivo, and as further proteolysis after SNARE removal eventually ablates fusion, we hypothesize that the triggered steps of regulated fusion (Ca2+ sensitivity and the catalysis and execution of fusion) require additional proteins that function downstream of SNAREs.

Evaluation of the therapeutic usefulness of botulinum neurotoxin B, C1, E, and F compared with the long lasting type A. Basis for distinct durations of inhibition of exocytosis in central neurons

Seven types (A-G) of botulinum neurotoxin (BoNT) target peripheral cholinergic neurons where they selectively proteolyze SNAP-25 (BoNT/A, BoNT/C1, and BoNT/E), syntaxin1 (BoNT/C1), and synaptobrevin (BoNT/B, BoNT/D, BoNT/F, and BoNT/G), SNARE proteins responsible for transmitter release, to cause neuromuscular paralysis but of different durations. BoNT/A paralysis lasts longest (4-6 months) in humans, hence its widespread clinical use for the treatment of dystonias. Molecular mechanisms underlying these distinct inhibitory patterns were deciphered in rat cerebellar neurons by quantifying the half-life of the effect of each toxin, the speed of replenishment of their substrates, and the degradation of the cleaved products, experiments not readily feasible at motor nerve endings. Correlation of target cleavage with blockade of transmitter release yielded half-lives of inhibition for BoNT/A, BoNT/C1, BoNT/B, BoNT/F, and BoNT/E (31, 25, approximately 10, approximately 2, and approximately 0.8 days, respectively), equivalent to the neuromuscular paralysis times found in mice, with recovery of release coinciding with reappearance of the intact SNAREs. A limiting factor for the short neuroparalytic durations of BoNT/F and BoNT/E is the replenishment of synaptobrevin or SNAP-25, whereas pulse labeling revealed that extended inhibition by BoNT/A, BoNT/B, or BoNT/C1 results from longevity of each protease. These novel findings could aid development of new toxin therapies for patients resistant to BoNT/A and effective treatments for human botulism.

Ca2+-induced changes in SNAREs and synaptotagmin I correlate with triggered exocytosis from chromaffin cells: insights gleaned into the signal transduction using trypsin and botulinum toxins

Ca2+-triggered catecholamine exocytosis from chromaffin cells involves SNAP-25, synaptobrevin and syntaxin (known as SNAREs). Synaptotagmin I has been implicated as the Ca2+-sensor because it binds Ca2+, and this enhances its binding to syntaxin, SNAP-25 and phospholipids in vitro. However, most of these interactions are only mediated by [Ca2+]i two orders of magnitude higher than that needed to elicit secretion. Thus, the Ca2+ sensitivities of synaptotagmin I and the other SNAREs were quantified in situ. Secretion elicited from permeabilised cells by μM Ca2+ was accompanied,with almost identical Ca2+ dependencies, by changes in synaptotagmin I, SNAP-25, syntaxin and synaptobrevin that rendered them less susceptible to trypsin. The majority of the trypsin-resistant SNAREs were not associated with SDS-resistant complexes. None of these proteins acquired trypsin resistance in cells rendered incompetent for exocytosis by run-down. Removal of nine C-terminal residues from SNAP-25 by botulinum toxin A reduced both exocytosis and the SNAREs' acquisition of trypsin resistance but did not alter the Ca2+ sensitivity, except for synaptotagmin I. Even after synaptobrevin had been cleaved by botulinum toxin B, all the other proteins still responded to Ca2+. These data support a model whereby Ca2+ is sensed, probably by synaptotagmin I, and the signal passed to syntaxin and SNAP-25 before they interact with synaptobrevin.

Multiple forms of SNARE complexes in exocytosis from chromaffin cells: effects of Ca2+, MgATP and botulinum toxin type A

The changes that SNAREs undergo during exocytosis were studied in permeabilised chromaffin cells treated with Ca2+, MgATP or botulinum neurotoxin A. High-resolution 2D SDS-PAGE revealed multiple SDS-resistant SNARE complexes having a wide range of sizes and in which SNAP-25 and syntaxin predominate over synaptobrevin. Their formation increased upon Ca2+-stimulated exocytosis; notably, the 2D protocol proved much superior to 1D SDS-PAGE for the detection of large complexes and revealed that for forms with relative molecular mass greater than 100,000 stimulated induction was more significant than for smaller species. MgATP enhanced Ca2+-triggered catecholamine release but reduced the content of complexes. By contrast, botulinum neurotoxin type A inhibited exocytosis and altered the stoichiometry of the SNAP-25:syntaxin binary association, without lowering its abundance. The individual SNAREs were protected against trypsin proteolysis to varying extents in binary and ternary complexes of different sizes, suggestive of distinct folding intermediates. Our data suggest that Ca2+ triggers an early stage of SNARE complex formation causing an accumulation of partially folded intermediates, especially of binary forms, as well as their maturation into smaller, more protease resistant states. In addition, botulinum neurotoxin A inhibits exocytosis by perturbing the syntaxin:SNAP-25 ratio in binary intermediates.

Adrenal gland SNAP-25 expression is altered in thyroid hormone receptor knock-out mice

SNAP-25 is a protein in neurons and neuroendocrine cells, which is involved, together with syntaxin and VAMP, in neurotransmitter release and neurite outgrowth. Since the thyroid hormone receptors TR alpha and TR beta are essential for nervous system development, their possible role in regulating the expression of these vesicle trafficking proteins was examined by analysing SNAP-25 levels in TR alpha and TR beta knock-out mice. Immunoblotting and RT-PCR showed that SNAP-25 levels are increased in the adrenal gland, but not in cerebellum, in knock-out mice, while syntaxin-1 and VAMP-2 are unaffected in either tissue. Treatment of the pheochromocytoma-derived cell line PC12 with the thyroid hormone L-3,5,3'-triiodothyronine (T3) decreased SNAP-25 expression. Together, these data suggest that thyroid hormones exert a negative regulatory effect on SNAP-25 in adrenal medullary neuroendocrine cells.

SNAP-24, a Drosophila SNAP-25 homologue on granule membranes, is a putative mediator of secretion and granule-granule fusion in salivary glands

Fusion of vesicles with target membranes is dependent on the interaction of target (t) and vesicle (v) SNARE (soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor) proteins located on opposing membranes. For fusion at the plasma membrane, the t-SNARE SNAP-25 is essential. In Drosophila, the only known SNAP-25 isoform is specific to neuronal axons and synapses and additional t-SNAREs must exist that mediate both non-synaptic fusion in neurons and constitutive and regulated fusion in other cells. Here we report the identification and characterization of SNAP-24, a closely related Drosophila SNAP-25 homologue, that is expressed throughout development. The spatial distribution of SNAP-24 in the nervous system is punctate and, unlike SNAP-25, is not concentrated in synaptic regions. In vitro studies, however, show that SNAP-24 can form core complexes with syntaxin and both synaptic and non-synaptic v-SNAREs. High levels of SNAP-24 are found in larval salivary glands, where SNAP-24 localizes mainly to granule membranes rather than the plasma membrane. During glue secretion, the massive exocytotic event of these glands, SNAP-24 containing granules fuse with one another and the apical membrane, suggesting that glue secretion utilizes compound exocytosis and that SNAP-24 mediates secretion.

Trachynilysin mediates SNARE-dependent release of catecholamines from chromaffin cells via external and stored Ca2+

Trachynilysin, a 159 kDa dimeric protein purified from stonefish (Synanceia trachynis) venom, dramatically increases spontaneous quantal transmitter release at the frog neuromuscular junction, depleting small clear synaptic vesicles, whilst not affecting large dense core vesicles. The basis of this insensitivity of large dense core vesicles exocytosis was examined using a fluorimetric assay to determine whether the toxin could elicit catecholamine release from bovine chromaffin cells. Unlike the case of the motor nerve endings, nanomolar concentrations of trachynilysin evoked sustained Soluble N-ethylmaleimide-sensitive fusion protein Attachment Protein REceptor-dependent exocytosis of large dense core vesicles, but only in the presence of extracellular Ca2+. However, this response to trachynilysin does not rely on Ca2+ influx through voltage-activated Ca2+ channels because the secretion was only slightly affected by blockers of L, N and P/Q types. Instead, trachynilysin elicited a localized increase in intracellular fluorescence monitored with fluo-3/AM, that precisely co-localized with the increase of fluorescence resulting from caffeine-induced release of Ca2+ from intracellular stores. Moreover, depletion of the latter stores inhibited trachynilysin-induced exocytosis. Thus, the observed requirement of external Ca2+ for stimulation of large dense core vesicles exocytosis from chromaffin cells implicates plasma membrane channels that signal efflux of Ca2+ from intracellular stores. This study also suggests that the bases of exocytosis of large dense core vesicles from motor nerve terminals and neuroendocrine cells are distinct.

Sensitivity of embryonic rat dorsal root ganglia neurons to Clostridium botulinum neurotoxins

Clostridium botulinum neurotoxins (BoNT) are zinc dependent endopeptidases which, once internalised into the neuronal cytosol, block neurotransmission by proteolysis of membrane-associated proteins putatively involved in synaptic vesicle docking and fusion with the plasma membrane. Although many studies have used a variety of cellular systems to study the neurotoxins, most require relatively large amounts of toxin or permeabilisation to internalise the neurotoxin. We present here a primary culture of embryonic rat dorsal root ganglia (DRG) neurons that exhibits calcium-dependent substance P secretion when depolarised with elevated extracellular potassium and is naturally BoNT sensitive. The DRG neurons showed a different IC50 for each of the toxins tested with a 1000 fold difference between the most and least potent neurotoxins (0.05, 0.3, 30 and approximately 60 nM for A, C, F and B, respectively). BoNT/A cleavage of SNAP-25 was seen as early as 2 h, but substance P secretion was not significantly inhibited until 4 h intoxication and the effects of BoNT/A were observed for as long as 15 days. This primary neuronal culture system represents a new and sensitive cellular model for the in vitro study of the botulinum neurotoxins.

Rescue of exocytosis in botulinum toxin A-poisoned chromaffin cells by expression of cleavage-resistant SNAP-25. Identification of the minimal essential C-terminal residues

Botulinum neurotoxin (BoNT) types A and B selectively block exocytosis by cleavage of SNAP-25 and synaptobrevin, respectively; in humans, many months are required for full recovery from the resultant neuromuscular paralysis. To decipher the molecular basis for such prolonged poisoning, intoxication in adreno-chromaffin cells was monitored over 2 months. Exocytosis from BoNT/B-treated cells resumed after 56 days because of the appearance of intact synaptobrevin. However, inhibition continued in BoNT/A-treated cells, throughout the same interval, with a continued predominance of cleaved SNAP-25-(1-197) over the intact protein. When recovery from poisoning was attempted by transfection of the latter cells with the gene encoding full-length SNAP-25-(1-206), no restoration of exocytosis ensued even after 3 weeks. To ascertain if this failure was because of the persistence of the toxin's protease activity, the cells were transfected with BoNT/A-resistant SNAP-25 constructs; importantly, exocytosis was rescued. C-terminal truncation of the toxin-insensitive SNAP-25 revealed that residues 1-201, 1-202, 1-203 afforded a significant return of exocytosis, unlike shorter forms 1-197, -198, -199, or -200; accordingly, mutants M202A or L203A of full-length SNAP-25 rescued secretion. These findings give insights into the C-terminal functional domain of SNAP-25, demonstrate the longevity of BoNT/A protease, and provide the prospect of a therapy for botulism.

Protein kinase B stimulates the translocation of GLUT4 but not GLUT1 or transferrin receptors in 3T3-L1 adipocytes by a pathway involving SNAP-23, synaptobrevin-2, and/or cellubrevin

An interaction of SNAP-23 and syntaxin 4 on the plasma membrane with vesicle-associated synaptobrevin-2 and/or cellubrevin, known as SNAP (soluble N-ethyl-maleimide-sensitive factor attachment protein) receptors or SNAREs, has been proposed to provide the targeting and/or fusion apparatus for insulin-stimulated translocation of the GLUT4 isoform of glucose transporter to the plasma membrane. By microinjecting 3T3-L1 adipocytes with the Clostridium botulinum toxin B or E, which proteolyzed synaptobrevin-2/cellubrevin and SNAP-23, respectively, we investigated the role of these SNAREs in GLUT4, GLUT1, and transferrin receptor trafficking. As expected, insulin stimulated the translocation of GLUT4, GLUT1, and transferrin receptors to the plasma membrane. By contrast, a constitutively active protein kinase B (PKB-DD) only stimulated a translocation of GLUT4 and not GLUT1 or the transferrin receptor. The GLUT4 response to PKB-DD was abolished by toxins B or E, whereas the insulin-evoked translocation of GLUT4 was inhibited by approximately 65%. These toxins had no significant effect on insulin-stimulated transferrin receptor appearance at the cell surface. Thus, insulin appears to induce GLUT4 translocation via two distinct routes, only one of which involves SNAP-23 and synaptobrevin-2/cellubrevin, and can be mobilized by PKB-DD. The PKB-, SNAP-23-, and synaptobrevin-2/cellubrevin-independent GLUT4 translocation pathway may involve movement through recycling endosomes, together with GLUT1 and transferrin receptors.

The mechanism of histamine secretion from gastric enterochromaffin-like cells

Enterochromaffin-like (ECL) cells play a pivotal role in the peripheral regulation of gastric acid secretion as they respond to the functionally important gastrointestinal hormones gastrin and somatostatin and neural mediators such as pituitary adenylate cyclase-activating peptide and galanin. Gastrin is the key stimulus of histamine release from ECL cells in vivo and in vitro. Voltage-gated K(+) and Ca(2+) channels have been detected on isolated ECL cells. Exocytosis of histamine following gastrin stimulation and Ca(2+) entry across the plasma membrane is catalyzed by synaptobrevin and synaptosomal-associated protein of 25 kDa, both characterized as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein. Histamine release occurs from different cellular pools: preexisting vacuolar histamine immediately released by Ca(2+) entry or newly synthesized histamine following induction of histidine decarboxylase (HDC) by gastrin stimulation. Histamine is synthesized by cytoplasmic HDC and accumulated in secretory vesicles by proton-histamine countertransport via the vesicular monoamine transporter subtype 2 (VMAT-2). The promoter region of HDC contains Ca(2+)-, cAMP-, and protein kinase C-responsive elements. The gene promoter for VMAT-2, however, lacks TATA boxes but contains regulatory elements for the hormones glucagon and somatostatin. Histamine secretion from ECL cells is thereby under a complex regulation of hormonal signals and can be targeted at several steps during the process of exocytosis.

Constitutive calcium-independent release of Toxoplasma gondii dense granules occurs through the NSF/SNAP/SNARE/Rab machinery

The signals and the molecular machinery mediating release of dense matrix granules from pathogenic protozoan parasites are unknown. We compared the secretion of the endogenous dense granule marker GRA3 in Toxoplasma gondii with the release of a stably transfected foreign reporter, beta-lactamase, that localizes to parasite dense granules. Both proteins were released constitutively in a calcium-independent fashion, as shown using both intact and streptolysin O-permeabilized parasites. N-Ethylmaleimide and recombinant bovine Rab-guanine dissociation inhibitor inhibited beta-lactamase secretion in permeabilized parasites, whereas recombinant hamster N-ethylmaleimide-sensitive fusion protein and bovine alpha-SNAP augmented release. Guanosine 5'-3-O-(thio)triphosphate, but not cAMP, augmented secretion in the presence but not in the absence of ATP. The T. gondii NSF/SNAP/SNARE/Rab machinery participates in dense granule release using parasite protein components that can interact functionally with their mammalian homologues.

Biochemical and functional studies of cortical vesicle fusion: the SNARE complex and Ca2+ sensitivity

Cortical vesicles (CV) possess components critical to the mechanism of exocytosis. The homotypic fusion of CV centrifuged or settled into contact has a sigmoidal Ca2+ activity curve comparable to exocytosis (CV-PM fusion). Here we show that Sr2+ and Ba2+ also trigger CV-CV fusion, and agents affecting different steps of exocytotic fusion block Ca2+, Sr2+, and Ba2+-triggered CV-CV fusion. The maximal number of active fusion complexes per vesicle, <n\>Max, was quantified by NEM inhibition of fusion, showing that CV-CV fusion satisfies many criteria of a mathematical analysis developed for exocytosis. Both <n\>Max and the Ca2+ sensitivity of fusion complex activation were comparable to that determined for CV-PM fusion. Using Ca2+-induced SNARE complex disruption, we have analyzed the relationship between membrane fusion (CV-CV and CV-PM) and the SNARE complex. Fusion and complex disruption have different sensitivities to Ca2+, Sr2+, and Ba2+, the complex remains Ca2+- sensitive on fusion-incompetent CV, and disruption does not correlate with the quantified activation of fusion complexes. Under conditions which disrupt the SNARE complex, CV on the PM remain docked and fusion competent, and isolated CV still dock and fuse, but with a markedly reduced Ca2+ sensitivity. Thus, in this system, neither the formation, presence, nor disruption of the SNARE complex is essential to the Ca2+-triggered fusion of exocytotic membranes. Therefore the SNARE complex alone cannot be the universal minimal fusion machine for intracellular fusion. We suggest that this complex modulates the Ca2+ sensitivity of fusion.

Dual effects of botulinum neurotoxin A on the secretory stages of chromaffin cells

Truncation of the C-terminal domain of the synaptosomal associated protein of 25 kDa (SNAP-25) by botulinum neurotoxin A (BoNT A) has been shown to block neuroendocrine cell secretion. It is unclear, however, if toxin mechanism involved the affection of a single or more events of the exocytotic cascade. BoNT A induced changes in both the degree of inhibition and the kinetics of catecholamine secretion from populations of cultured bovine chromaffin cells. Ca2+-dependent secretion from digitonin-permeabilized cells showed partial inhibition associated with the alteration of a slow secretory phase at different toxin concentrations. In contrast, in intact cells stimulated by depolarization, cell treatment with low concentrations (1 nM) of the toxin affected the late phase of secretion, whereas 100 nM BoNT A-poisoned cells showed an alteration even of fast components. The high degree of inhibition associated with fast secretory component alteration was dependent on Ca2+ entry through the Ca2+ channels, as it was absent from cells permeated with the A23187 Ca2+ ionophore. Vesicle pools implicated in the effect of BoNT A on the secretory response from single cells were identified using amperometry. These studies supported the macroscopic view by showing that secretion from BoNT A-treated permeabilized cells presented specific inhibition of late vesicle fusions. Intact cells showed alterations in the late vesicle pool (t1/2 = 39 s) recruited during prolonged or repetitive KCI depolarizations using 1 nM BoNT A-treated cells as well as in an intermediate kinetic pool (t1/2 = 18 s) at higher toxin concentrations (100 nM). The faster resolved component (t1/2 = 8 s) or the membrane fusion event itself were not affected. Our results demonstrate that removal of the last nine C-terminal amino acids of SNAP-25 by BoNT A has a specific effect on two different and distal secretory stages in chromaffin cells.

Binary interactions of the SNARE proteins syntaxin-4, SNAP23, and VAMP-2 and their regulation by phosphorylation

The SNARE hypothesis proposes that synaptic vesicles dock at presynaptic membranes via interactions among the vesicular, integral membrane proteins VAMP (vesicle-associated membrane protein) and synaptotagmin and the target membrane proteins SNAP25 (synaptosome-associated protein with an Mr of 25 kDa) and syntaxin-1. Non-neuronal cells express isoforms of these proteins, believed to mediate secretory vesicle docking and/or fusion. Secretion in neuronal and non-neuronal systems differs in time course, Ca2+ dependence, and regulatory input. It is not known whether the non-neuronal protein isoforms form complexes akin to those of their neuronal counterparts. In this study, we defined the binding characteristics of three SNARE proteins: SNAP23, VAMP-2, and syntaxin-4. Binary, saturable interactions among all three partners (VAMP-2-syntaxin-4, VAMP-2-SNAP23, and SNAP23-syntaxin-4) were measured in vitro. Unlike its neuronal counterpart, SNAP23 did not potentiate VAMP-2 binding to its putative t-SNARE partner, syntaxin-4. The susceptibility of SNARE proteins to phosphorylation by exogenous kinases and their impact on binary interactions were explored. Syntaxin-4 was efficiently phosphorylated by casein kinase II (CKII) and cAMP-dependent protein kinase (PKA) (incorporating 0.8 and 3.9 mol of phosphate/mol of syntaxin-4, respectively), while syntaxin-1 was only strongly phosphorylated by CKII. Each of the syntaxin isoforms was weakly phosphorylated by protein kinase C (PKC) (<0.05 mol of phosphate/mol of syntaxin-4). Importantly, PKA but not casein kinase II phosphorylation of syntaxin-4 disrupted its binding to SNAP23. We hypothesize that PKA may modulate syntaxin-4-dependent SNARE complex formation to regulate exocytosis in non-neuronal cells.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Negative chronotropic effect of botulinum toxin on neonatal rat cardiac myocytes

We demonstrated that botulinum neurotoxin attenuated the spontaneous beating rate of cultured cardiac myocytes. Primary cultured cardiac myocytes were prepared from the ventricles of neonatal Wistar rats (1-3 days old). On 7 days after cell seeding, botulinum toxin type A incorporated into liposomes was added to the culture medium. At a final concentration of 5.0 micrograms/ml, botulinum toxin markedly attenuated the beating rate of cardiac myocytes within 2-4 hours. These results demonstrated the effect of SNARE-complex proteins on the spontaneous beating of cardiac myocytes.

Ca2+ or Sr2+ partially rescues synaptic transmission in hippocampal cultures treated with botulinum toxin A and C, but not tetanus toxin

Botulinum (BoNT/A-G) and tetanus toxins (TeNT) are zinc endopeptidases that cleave proteins associated with presynaptic terminals (SNAP-25, syntaxin, or VAMP/synaptobrevin) and block neurotransmitter release. Treatment of hippocampal slice cultures with BoNT/A, BoNT/C, BoNT/E, or TeNT prevented the occurrence of spontaneous or miniature EPSCs (sEPSCs or mEPSCs) as well as the [Ca2+]o-independent increase in their frequency induced by phorbol ester, 0.5 nM alpha-latrotoxin, or sucrose. [Ca2+]o-independent and -dependent release thus requires that the target proteins of clostridial neurotoxins be uncleaved. In contrast, significant increases in mEPSC frequency were produced in BoNT-treated, but not TeNT-treated, cultures by application of the Ca2+ ionophore ionomycin in the presence of 10 mM [Ca2+]o. The frequency of sEPSCs was increased in BoNT-treated, but not TeNT-treated, cultures by increasing [Ca2+]o from 2.8 to 5-10 mM or by applying 5 mM Sr2+. Large Ca2+ and Sr2+ influxes thus can rescue release after BoNT treatment, albeit less than in control cultures. The nature of the toxin-induced modification of Ca2+-dependent release was assessed by recordings from monosynaptically coupled CA3 cell pairs. The paired-pulse ratio of unitary EPSCs evoked by two presynaptic action potentials in close succession was 0.5 in control cultures, but it was 1.4 and 1.2 in BoNT/A- or BoNT/C-treated cultures when recorded in 10 mM [Ca2+]o. Log-log plots of unitary EPSC amplitude versus [Ca2+]o were shifted toward higher [Ca2+]o in BoNT/A- or BoNT/C-treated cultures, but their slope was unchanged and the maximal EPSC amplitudes were reduced. We conclude that BoNTs reduce the Ca2+ sensitivity of the exocytotic machinery and the number of quanta released.

Botulinum neurotoxin B inhibits insulin-stimulated glucose uptake into 3T3-L1 adipocytes and cleaves cellubrevin unlike type A toxin which failed to proteolyze the SNAP-23 present

Types A, B, and C1 botulinum neurotoxin (BoNT), a group of selective Zn2+-dependent endoproteases, have been instrumental in demonstrating that their respective substrates [synaptosomal-associated protein with Mr = 25 kDa (SNAP-25), synaptobrevin (Sbr), and syntaxin] are essential for regulated exocytosis from nerve terminals and neuroendocrine cells. The colocalization of Sbr, or its homologue cellubrevin (Cbr), in the majority of the glucose transporter-isotype 4 (GLUT4)-containing vesicles from adipocytes implicates their involvement in insulin-stimulated glucose uptake, which results in part from enhanced fusion of these vesicles with the plasmalemma. In this study, exposure of cultured 3T3-L1 adipocytes to BoNT/B in a low-ionic strength medium was found to block insulin-evoked glucose uptake by up to 64%. BoNT/B was shown by immunoblotting to cause extensive proteolysis of Cbr and Sbr resulting in a significant blockade of the insulin-stimulated translocation of GLUT4 to the plasmalemma. This establishes that these two toxin substrates contribute to the insulin-regulated fusion of GLUT4-containing vesicles with the plasmalemma, at least in this differentiated 3T3-L1 clone. Although SNAP-25 was not detectable in the differentiated adipocytes, its functional homologue SNAP-23 is abundant and largely confined to the plasmalemma. SNAP-23 proved to be resistant to cleavage by BoNT/A. Consistent with these results, type A did not block insulin-induced glucose uptake, precluding a demonstration of its likely importance in this process.

Importance of two adjacent C-terminal sequences of SNAP-25 in exocytosis from intact and permeabilized chromaffin cells revealed by inhibition with botulinum neurotoxins A and E

Types A and E botulinum neurotoxin (BoNT) are Zn2+-requiring endoproteases which cleave nine and twenty-six residues, respectively, from the C-terminus of synaptosomal-associated protein of Mr = 25 kDa (SNAP-25). Involvement of SNAP-25 in the exocytosis of large dense-core vesicles in bovine adrenochromaffin cells was examined by measuring cleavage of SNAP-25 in relation to the levels of Ca2+-evoked catecholamine release from cells exposed to BoNT/A or /E, either before or after permeabilization. The dose-dependency of inhibition of exocytosis correlated closely with the extents of SNAP-25 cleavage in cells permeabilized and then treated with BoNT/E. In intact cells exposed to 66 nM BoNT/A, virtually all of the SNAP-25 was truncated, accompanied by a near-complete inhibition of exocytosis; however, after their permeabilization a significant level of secretion was recorded upon Ca2+-stimulation. Importantly, this BoNT/A-resistant release from the permeabilized cells was dramatically lowered by subsequently adding BoNT/E, which further truncated the SNAP-25 fragment (lacking the C-terminal nine residues) that had been produced earlier by BoNT/A. Moreover, anti-SNAP-25 IgG decreased the BoNT/A-insensitive exocytosis. When permeabilized cells were exposed to either neurotoxin, both blocked MgATP-dependent secretion but only BoNT/E attenuated the energy-independent phase. These distinct inhibitory effects of the two neurotoxins demonstrate that residues 197-205 at the C-terminus of SNAP-25 are absolutely essential for exocytosis from intact cells whereas even after their removal a significant proportion of the exocytotic response can be elicited from permeabilized cells, but this is reliant on amino acids 180-196. Moreover, the latter but not residues 197-205 are implicated in a late, MgATP-independent step of exocytosis, which is blocked by BoNT/E but nonsusceptible to BoNT/A.

25 years since the discovery of presynaptic receptors: present knowledge and future perspectives

The release of neurotransmitters is modulated through presynaptic autoreceptors that are acted upon by the neurone's own transmitter. The presynaptic inhibitory terminal autoreceptors were first described for noradrenergic neurones (alpha 2-adrenoceptors) and subsequently for other neurotransmitters: dopamine (D2/D3), acetylcholine (M2), GABA (GABAB), histamine (H3) and serotonin (5-HT10). This negative feedback loop introduces terminal regulation into the neural secretory event. Saloman Langer describes how presynaptic terminal facilitatory receptors exist for the modulation of acetylcholine release and these autoreceptors are of the nicotinic subtype. Presynaptic release-modulating receptors represent targets for pharmacological intervention by exogenous compounds acting as agonists, partial agonists or antagonists. Compounds with such properties as the antidepressant mirtazepine (alpha 2-adrenoceptor antagonist) and the neuroleptic amisulpride (preferential presynaptic D2/D3 receptor antagonist) may represent the beginning of a new generation of innovative drugs with useful therapeutic properties.

Dissociation of SNAP-25 and VAMP-2 by MgATP in permeabilized adrenal chromaffin cells

In digitonin-permeabilized adrenal chromaffin cells, Ca(2+)-induced catecholamine release can be resolved into at least two sequential steps: a MgATP-dependent priming step and a MgATP-independent Ca(2+)-triggered step. Botulinum neurotoxins types A and E cleaved SNAP-25, and blocked MgATP-independent Ca(2+)-induced catecholamine release from the permeabilized chromaffin cells. When the permeabilized cells were primed by pretreatment with MgATP, the amount of SNAP-25 associated with VAMP-2 decreased, and the fraction of SNAP-25 proteolyzed by the neurotoxins increased. These results suggest that dissociation of SNAP-25 and VAMP-2 occurs during the MgATP-dependent priming step, and SNAP-25 plays some important roles in the subsequent MgATP-independent step.