Porphyrin derivatives as potent and selective blockers of neuronal Kv1 channels.

Selective inhibitors of voltage-activated K(+) channels are needed for the treatment of multiple sclerosis. In this work it was discovered that porphyrins bearing 2-4 carbon alkyl ammonium side chains predominantly blocked the Kv1.1 current whilst Kv1.2 was susceptible to a porphyrin bearing polyamine side chains.

Innocuous full-length botulinum neurotoxin targets and promotes the expression of lentiviral vectors in central and autonomic neurons.

Fragments of botulinum neurotoxin (BoNT) have been explored as potential targeting moieties and carriers of biomolecules into neurons, although with lower binding and translocation efficiency compared with intact proteins. This study exploits a detoxified recombinant form of full-length BoNT/B (BoTIM/B) fused with core streptavidin (CS-BoTIM/B) for lentiviral targeting to central and autonomic neurons. CS-BoTIM/B underwent an activity-dependent entry into cultured spinal cord neurons. Coupling CS-BoTIM/B to biotinylated lentivirus-encoding green fluorescent protein (GFP) endowed considerable neuron selectivity to the vector as evident from the preferential expression of the reporter in neurons co-cultured with skeletal muscle cells. CS-BoTIM/B-guided lentiviral transduction with the expression of a SNARE protein, SNAP-25 (S25), rendered non-susceptible to proteolysis by three BoNT serotypes, yielded a sizable decrease in cleaved S25 upon exposure of spinal cord neurons to these toxins. This was accompanied by synaptic transmission being spared from blockade by BoNT/A or BoNT/E, reflecting adequate translation and functional competence of recombinant multi-toxin-resistant S25. The augmented neurotropism conveyed on the lentivirus by CS-BoTIM/B was also demonstrated in vivo through enhanced expression of a reporter in intramural ganglionic neurons in the rat trachea, after injection of the targeted GFP-encoding lentivirus. Thus, a novel and realistic prospect for gene therapy of peripheral neuropathies is offered in this study through lentiviral targeting to neurons by CS-BoTIM/B.

The structure and mode of action of different botulinum toxins.

The seven serotypes (A-G) of botulinum neurotoxin (BoNT) are proteins produced by Clostridium botulinum and have multifunctional abilities: (i) they target cholinergic nerve endings via binding to ecto-acceptors (ii) they undergo endocytosis/translocation and (iii) their light chains act intraneuronally to block acetylcholine release. The fundamental process of quantal transmitter release occurs by Ca2+-regulated exocytosis involving sensitive factor attachment protein-25 (SNAP-25), syntaxin and synaptobrevin. Proteolytic cleavage by BoNT-A of nine amino acids from the C-terminal of SNAP-25 disables its function, causing prolonged muscle weakness. This unique combination of activities underlies the effectiveness of BoNT-A haemagglutinin complex in treating human conditions resulting from hyperactivity at peripheral cholinergic nerve endings. In vivo imaging and immunomicroscopy of murine muscles injected with type A toxin revealed that the extended duration of action results from the longevity of its protease, persistence of the cleaved SNAP-25 and a protracted time course for the remodelling of treated nerve-muscle synapses. In addition, an application in pain management has been indicated by the ability of BoNT to inhibit neuropeptide release from nociceptors, thereby blocking central and peripheral pain sensitization processes. The widespread cellular distribution of SNAP-25 and the diversity of the toxin's neuronal acceptors are being exploited for other therapeutic applications.

Predominant expression of Kv1.3 voltage-gated K+ channel subunit in rat prostate cancer cell lines: electrophysiological, pharmacological and molecular characterisation.

Voltage-gated K+ currents expressed in two rat prostate cancer ("Dunning") cell lines of markedly different metastatic ability were characterised using electrophysiological, pharmacological and molecular approaches. Whole-cell patch-clamp recordings showed that both strongly metastatic MAT-LyLu and weakly metastatic AT-2 cell lines possessed outward (delayed-rectifier type) K+ currents, which activated at around -40 mV. From the parameters measured, several characteristics of the two cell lines were similar. However, a number of statistically significant differences were noted for MAT-LyLu versus the AT-2 cells as follows: (1) current densities were smaller; (2) the slope factor for channel activation was smaller; (3) the voltage at which current was half-inactivated, and the slope factor for channel inactivation were greater; (4) the time constants for current decay at -20 and 0 mV were smaller; and (5) the residual peak current was larger following 60 s of repetitive voltage pulses for stimulation frequencies in the range 0.05-0.2 Hz. On the other hand, the K+ currents in both cell lines showed similar pharmacological profiles. Thus, the currents were blocked by 4-aminopyridine, tetraethylammonium, verapamil, margatoxin, and charybdotoxin, with highly similar IC(50)s for given blockers. The electrophysiological and pharmacological data taken together suggested expression of voltage-gated K+ channels of the Kv1 family, expression of the Kv1.3 subunit being predominant. Western blot and RT-PCR tests both confirmed that the cells indeed expressed Kv1.3 and to a lesser extent Kv1.4 and Kv1.6 channel alpha-subunits. In view of the similarity of channel expression in the two cell lines, voltage-gated K+ channel activity may not be a primary determinant of metastatic potential in the rat model of prostate cancer, but the possible contribution of K+ channel activity to the metastatic process is discussed.

Episodic ataxia type-1 mutations in the Kv1.1 potassium channel display distinct folding and intracellular trafficking properties.

Episodic ataxia type 1 (EA-1) is a neurological disorder arising from mutations in the Kv1.1 potassium channel alpha-subunit. EA-1 patients exhibit substantial phenotypic variability resulting from at least 14 distinct EA-1 point mutations. We found that EA-1 missense mutations generate mutant Kv1.1 subunits with folding and intracellular trafficking properties indistinguishable from wild-type Kv1.1. However, the single identified EA-1 nonsense mutation exhibits intracellular aggregation and detergent insolubility. This phenotype can be transferred to co-assembled Kv1 alpha- and Kv beta-subunits associated with Kv1.1 in neurons. These results suggest that as in many neurodegenerative disorders, intracellular aggregation of misfolded Kv1.1-containing channels may contribute to the pathophysiology of EA-1.

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.

Glial heterogeneity in expression of the inwardly rectifying K(+) channel, Kir4.1, in adult rat CNS.

Previous electrophysiological evidence has indicated that astrocytes and oligodendrocytes express inwardly rectifying K(+) channels both in vitro and in vivo. Here, for the first time, we have undertaken light microscopic immunohistochemical studies demonstrating the location of one such channel, Kir4.1, in both cell types in regions of the rat CNS. Some astrocytes such as those in the deep cerebellar nuclei, Bergmann glia, retinal Müller cells, and a subset in hippocampus express Kir4.1 immunoreactivity, but not others including those in white matter. Oligodendrocytes also express this protein, strongly in perikarya and to a lesser extent in their processes. Expression of Kir4.1 in astrocytes and oligodendrocytes would enable these cells to clear extracellular K(+) through this channel, whereas nonexpressors might use other mechanisms.

A late phase of exocytosis from synaptosomes induced by elevated [Ca2+]i is not blocked by Clostridial neurotoxins.

Treatment of rat cerebrocortical synaptosomes with botulinum toxin types E and C1 or tetanus toxin removed the majority of intact SNAP-25, syntaxin 1A/1B, and synaptobrevin and diminished Ca(2+)-dependent K+ depolarization-induced noradrenaline secretion. With botulinum toxin type E, <10% of intact SNAP-25 remained and K(+)-evoked release of glutamate and GABA was inhibited. The large component of noradrenaline release evoked within 120 s by inclusion of the Ca2+ ionophore A23187 with the K+ stimulus was also attenuated by these toxins; additionally, botulinium neurotoxin type E blocked the first 60 s of ionophore-induced GABA and glutamate exocytosis. However, exposure to A23187 for longer periods induced a phase of secretion nonsusceptible to any of these toxins (>120 s for noradrenaline; >60 s for glutamate or GABA). Most of this late phase of release represented exocytosis because of its Ca2+ dependency, ATP requirement, and sensitivity to a phosphatidylinositol 4-kinase inhibitor. Based on these collective findings, we suggest that the ionophore-induced elevation of [Ca2+]i culminates in the disassembly of complexes containing nonproteolyzed SNAP receptors protected from the toxins that can then contribute to neuroexocytosis.

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.

Recreation of neuronal Kv1 channel oligomers by expression in mammalian cells using Semliki Forest virus.

The multiple roles of voltage-sensitive K(+) channels (Kv1 subfamily) in brain are served by subtypes containing pore-forming alpha (1.1-1.6) and auxiliary beta subunits, usually in an (alpha)(4)(beta)(4) stoichiometry. To facilitate structure/activity analysis, combinations that are prevalent in neurones and susceptible to alpha-dendrotoxin (alphaDTX) were reproduced in mammalian cells, using Semliki Forest virus. Infected Chinese hamster ovary cells expressed N-glycosylated Kv1.1 and 1.2 alpha subunits (M(r) approximately 60 and 62 K) that assembled and bound [(125)I]-alphaDTX with high affinity; an appreciable proportion appeared on the cell surface, with Kv1.2 showing a 5-fold enrichment in a plasma membrane fraction. To obtain 'native-like' alpha/beta complexes, beta1.1 or 2.1 (M(r) approximately 42 and 39 K, respectively) was co-expressed with Kv1.1 or 1.2. This slightly enhanced N-glycosylation and toxin binding, most notable with beta2. 1 and Kv1.2. Solubilization of membranes from cells infected with Kv. 1.2 and beta2.1, followed by Ni(2+) chromatography, gave a purified alpha1.2/beta2.1 complex with a size of approximately 405 K and S(20, W) = 15.8 S. Importantly, these values indicate that four alpha and beta subunits co-assembled as in neurones, a conclusion supported by the size ( approximately 260 K) of the homo-tetramer formed by Kv1.2 alone. Thus, an authentic K(+) channel octomer has been reconstructed; oligomeric species were also found in plasma membranes. To create 'authentic-like' hetero-oligomeric channels, Kv1.1 and 1.2 were co-expressed and shown to have assembled by the precipitation of both with IgGs specific for either. Consistently, confocal microscopy of cells labeled with these antibodies showed that the relatively low surface content of Kv1.1 was increased by Kv1.2. [(125)I]-alphaDTX binding to these complexes was antagonized by DTX(k), a probe selective for Kv1.1, in a manner that mimicks the pattern observed for the Kv1.1/1.2-containing channels in neuronal membranes.

A functional spliced-variant of beta 2 subunit of Kv1 channels in C6 glioma cells and reactive astrocytes from rat lesioned cerebellum.

Voltage-gated K(+) channels (Kv1) are important in glia, being required for cell proliferation. Herein, reactive astrocytes from a rat cerebellar lesion were shown to contain Kv1.1, -1.2, -1.3, -1.4, and -1.6 alpha plus beta1.1 subunits, as well as an unusual beta2.1 constituent; the latter was also found in a glioblastoma C6 cell line, together with Kv1.1, -1.3, and -1.6 and beta1.1 subunits. Reverse transcriptase-polymerase chain reaction revealed a novel product of the beta2 gene in C6 cells and reactive astrocytes, but not in cultured astrocytes or rat normal brain. Its cloning identified a variant, Kvbeta2.1A, alternatively spliced between I24 and Y39. Despite this 14 residue deletion, Kvbeta2.1A assembled cotranslationally with Kv1.1 or -1.2 and, when coexpressed with Kv1. 4 in oocytes, increased the inactivation rate of this K(+) current. Whereas the full-length beta2.1 gave a large increase in the amplitude of the Kv1.1 current in oocytes, the effect of beta2.1A varied from a modest elevation of the current to a slight suppression in some cases. In summary, this is the first report of the existence of an alternatively spliced product of the Kvbeta2.1 gene in C6 cells and reactive astrocytes, and supports the involvement of its core region (residues 39 onward) in assembly with alpha subunits while excluding a contribution of the adjacent 14 residues to accelerating the inactivation of Kv1.4.

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.

Identification of residues in dendrotoxin K responsible for its discrimination between neuronal K+ channels containing Kv1.1 and 1.2 alpha subunits.

Dendrotoxin (DTX) homologues are powerful blockers of K+ channels that contain certain subfamily Kv1 (1.1-1.6) alpha- and beta-subunits, in (alpha)4(beta)4 stoichiometry. DTXk inhibits potently Kv1.1-containing channels only, whereas alphaDTX is less discriminating, but exhibits highest affinity for Kv1.2. Herein, the nature of interactions of DTXk with native K+ channels composed of Kv1.1 and 1.2 (plus other) subunits were examined, using 15 site-directed mutants in which amino acids were altered in the 310-helix, beta-turn, alpha-helix and random-coil regions. The mutants' antagonism of high-affinity [125I]DTXk binding to Kv1. 1-possessing channels in rat brain membranes and blockade of the Kv1. 1 current expressed in oocytes were quantified. Also, the levels of inhibition of [125I]alphaDTX binding to brain membranes by the DTXk mutants were used to measure their high- and low-affinity interactions, respectively, with neuronal Kv1.2-containing channels that possess Kv1.1 as a major or minor constituent. Displacement of toxin binding to either of these subtypes was not altered by single substitution with alanine of three basic residues in the random-coil region, or R52 or R53 in the alpha-helix; accordingly, representative mutants (K17A, R53A) blocked the Kv1.1 current with the same potency as the natural toxin. In contrast, competition of the binding of the radiolabelled alphaDTX or DTXk was dramatically reduced by alanine substitution of K26 or W25 in the beta-turn whereas changing nearby residues caused negligible alterations. Consistently, W25A and K26A exhibited diminished functional blockade of the Kv1.1 homo-oligomer. The 310-helical N-terminal region of DTXk was found to be responsible for recognition of Kv1.1 channels because mutation of K3A led to approximately 1246-fold reduction in the inhibitory potency for [125I]DTXk binding and a large decrease in its ability to block the Kv1.1 current; the effect of this substitution on the affinity of DTXk for Kv1.2-possessing oligomers was much less dramatic (approximately 16-fold).

alpha subunit compositions of Kv1.1-containing K+ channel subtypes fractionated from rat brain using dendrotoxins.

K+ channels from the Kv1 subfamily contain four alpha-subunits and the combinations (from Kv1.1-1.6) determine susceptibility to dendrotoxin (DTX) homologues. The subunit composition of certain subtypes in rat brain was investigated using DTXk which only interacts with Kv1.1-containing channels and alphaDTX (and its closely related homologue DTXi) that binds preferentially to Kv1. 2-possessing homo- or hetero-oligomers. Covalent attachment of [125I]DTXk bound to channels in synaptic membranes unveiled subunits of Mr = 78 000 and 96 000. Immunoprecipitation of these solubilized and dissociated cross-linked proteins with IgG specific for each of the alpha-subunits identified Kv1.1, 1.2 and 1.4; this led to assemblies of Kv1.1/1.2 and 1.1/1.4 being established. Kv1. 2-enriched channels, purified from rat brain by chromatography on immobilized DTXi, contained Kv1.1, 1.2 and 1.6 confirming one of the above-noted pairs and indicating an additional Kv1.1-containing oligomer (Kv1.1/1.2/1.6); the notable lack of Kv1.4 excludes a Kv1. 1/1.2/1.4 combination. On the other hand, channels with Kv1.1 as a constituent, isolated using DTXk, possessed Kv1.4 in addition to those found in the DTXi-purified oligomers; this provides convergent support for the occurrence of the three combinations established above but adds a possible fourth (Kv1.1/1.4/1.6), though this was not confirmed. Moreover, sequential purification on DTXi and DTXk resins yielded channels containing only Kv1.1/1.2 but with an apparent predominance of Kv1.1, reaffirming the latter multimer.

Subunit composition of Kv1 channels in human CNS.

The alpha subunits of Shaker-related K+ channels (Kv1.X) show characteristic distributions in mammalian brain and restricted coassembly. Despite the functional importance of these voltage-sensitive K+ channels and involvement in a number of diseases, little progress has been achieved in deciphering the subunit composition of the (alpha)4(beta)4 oligomers occurring in human CNS. Thus, the association of alpha and beta subunits was investigated in cerebral grey and white matter and spinal cord from autopsy samples. Immunoblotting established the presence of Kv1.1, 1.2, and 1.4 in all the tissues, with varying abundance. Sequential immunoprecipitations identified the subunits coassembled. A putative tetramer of Kv1.3/1.4/1.1/1.2 was found in grey matter. Both cerebral white matter and spinal cord contained the heterooligomers Kv1.1/1.4 and Kv1.1/1.2, similar to grey matter, but both lacked Kv1.3 and the Kv1.4/1.2 combination. An apparent Kv1.4 homooligomer was detected in all the samples, whereas only the brain tissue possessed a putative Kv1.2 homomer. In grey matter, Kvbeta2.1 was coassociated with the Kv1.1/1.2 combination and Kv1.2 homooligomer. In white matter, Kvbeta2.1 was associated with Kv1.2 only, whereas Kvbeta1.1 coprecipitated with all the alpha subunits present. This represents the first description of Kv1 subunit complexes in the human CNS and demonstrates regional variations, indicative of functional specialisation.

Expression and characterisation of the heavy chain of tetanus toxin: reconstitution of the fully-recombinant dichain protein in active form.

Tetanus toxin, composed of a disulphide-linked heavy (HC) and light (LC) chain, preferentially blocks the release of inhibitory neurotransmitters in the spinal cord by Zn2+-dependent proteolytic cleavage of synaptobrevin. This intoxication involves binding via HC to ecto-acceptors on peripheral nerve endings, followed by internalisation and retrograde transportation to its prime site of action in central neurons. To facilitate exploitation of the toxin's unique activities, HC was expressed at a high level in Escherichia coli as a fusion with maltose binding protein; after cleavage by thrombin, free HC was isolated and its identity confirmed by Western blotting and N-terminal microsequencing. The expressed and native HC gave very similar circular dichroism spectra, excluding any gross differences in their folded structures. Recombinant HC antagonised the neuromuscular paralysing activity of the native toxin, by competing for binding to neuronal ecto-acceptors. The HC was reconstituted with bacterially-expressed LC to create disulphide-bridged dichain toxin that blocked neuromuscular transmission. The fully-recombinant toxin produced spastic paralysis in mice characteristic of the blockade of central inhibitory synapses, revealing that it undergoes axonal transport to the spinal cord, like the native toxin but with a reduced efficacy. This first report of the large-scale production of recombinant tetanus toxin in active form should facilitate studies on the use of engineered innocuous forms of the toxin as neuronal transport vehicles.

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.

Functional repair of motor endplates after botulinum neurotoxin type A poisoning: biphasic switch of synaptic activity between nerve sprouts and their parent terminals.

Blockade of acetylcholine release by botulinum neurotoxin type A at the neuromuscular junction induces the formation of an extensive network of nerve-terminal sprouts. By repeated in vivo imaging of N-(3-triethyl ammonium propyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide uptake into identified nerve endings of the mouse sternomastoid muscle after a single intramuscular injection of the toxin, inhibition of stimulated uptake of the dye at the terminals was detected within a few days, together with an increase in staining of the newly formed sprouts. After 28 days, when nerve stimulation again elicited muscle contraction, regulated vesicle recycling occurred only in the sprouts [shown to contain certain soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAREs) and to abut acetylcholine receptors] and not at the parent terminals. Therefore, only these sprouts could be responsible for nerve-muscle transmission at this time. However, a second, distinct phase of the rehabilitation process followed with a return of vesicle turnover to the original terminals, accompanied by an elimination of the by then superfluous sprouts. This extension and later removal of "functional" sprouts indicate their fundamental importance in the repair of paralyzed endplates, a finding with ramifications for the vital process of nerve regeneration.

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.

Seizures and hippocampal damage produced by dendrotoxin-K in rats is prevented by the 21-aminosteroid U-74389G.

The epileptogenic and neurodegenerative effects of dendrotoxin K (DTx-K), from Dendroaspis polylepsis, a specific blocker of a noninactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats pretreated with the 21-aminosteriod U-74389G, a scavenger of free oxygen radicals. Administration of 35 pmol DTx-K elicited in all of the treated animals (n = 6) motor seizures and bilateral electrocortical (ECoG) discharges after a latent period of approximately 5 min. At 24 h, histological examination of brain (n = 6) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation. Quantitation of damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer and dentate gyrus granule cell layer relative to the corresponding brain regions of rats (n = 6) injected with bovine serum albumin (300 ng), which per se was ineffective in all respects. DTx-K (35 pmol) also caused a significant loss of CA3 pyramidal neurons ipsilateral to the site of toxin injection. Systemic (i.p.) administration of U-74389G (5 mg/kg given 30 min beforehand) delayed the onset of motor and ECoG seizures and reduced the number of epileptogenic discharges typically observed in rats receiving an injection of DTx-K (35 pmol) alone. Similarly, this treatment prevented the damage inflicted to the hippocampus by the toxin and in no instance was significant neuronal loss observed. At variance with these results, pretreatment with U-74389G (up to 10 mg/kg i.p.) failed to prevent seizures and CA1 hippocampal damage evoked by intra-hippocampal injection of alpha-DTx (35 pmol), a DTx-K homologue which preferentially inhibits a slowly inactivating, voltage-dependent K+ conductance in nerve cells. In conclusion, the present data support a role for free oxygen radicals in mediating hippocampal damage induced by DTx-K, but not alpha-DTx, and confirm the original deduction that these DTx homologues are complementary neurobiological tools to study mechanisms of seizures and neuronal death.