Sodium-dependent increase in quantal secretion induced by brevetoxin-3 in Ca2+-free medium is associated with depletion of synaptic vesicles and swelling of motor nerve terminals in situ

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.

Analysis of Caribbean ciguatoxin-1 effects on frog myelinated axons and the neuromuscular junction

Caribbean ciguatoxin-1 (C-CTX-1) induced, after about 1h exposure, muscle membrane depolarisation and repetitive post-synaptic action potentials (APs) in frog neuromuscular preparations. This depolarising effect was also observed in a Ca(2+)-free medium with a strong enhancement of spontaneous quantal transmitter release, compared with control conditions. The ciguatoxin-induced increase in release could be accelerated when Ca(2+) was present in the extracellular medium. C-CTX-1 also enhanced nerve-evoked quantal acetylcholine (ACh) release. At normal neuromuscular junctions loaded with the fluorescent dye FM1-43, C-CTX-1 induced swelling of nerve terminals, an effect that was reversed by hyperosmotic d-mannitol. In myelinated axons, C-CTX-1 increased nodal membrane excitability, inducing spontaneous and repetitive APs. Also, the toxin enlarged the repolarising phase of APs in control and tetraethylammonium-treated axons. Overall, our data suggest that C-CTX-1 affects nerve excitability and neurotransmitter release at nerve terminals. We conclude that C-CTX-1-induced up-regulation of Na(+) channels and the inhibition of K(+) channels, at low nanomolar concentrations, produce a variety of functional dysfunctions that are in part responsible for the human muscle skeletal symptoms observed in ciguatera. All these dysfunctions seem to result from the subtle balance between ionic currents, intracellular Na(+) and Ca(2+) concentrations, and engaged second messengers.

Brevenal inhibits pacific ciguatoxin-1B-induced neurosecretion from bovine chromaffin cells

Ciguatoxins and brevetoxins are neurotoxic cyclic polyether compounds produced by dinoflagellates, which are responsible for ciguatera and neurotoxic shellfish poisoning (NSP) respectively. Recently, brevenal, a natural compound was found to specifically inhibit brevetoxin action and to have a beneficial effect in NSP. Considering that brevetoxin and ciguatoxin specifically activate voltage-sensitive Na⁺ channels through the same binding site, brevenal has therefore a good potential for the treatment of ciguatera. Pacific ciguatoxin-1B (P-CTX-1B) activates voltage-sensitive Na⁺ channels and promotes an increase in neurotransmitter release believed to underpin the symptoms associated with ciguatera. However, the mechanism through which slow Na⁺ influx promotes neurosecretion is not fully understood. In the present study, we used chromaffin cells as a model to reconstitute the sequence of events culminating in ciguatoxin-evoked neurosecretion. We show that P-CTX-1B induces a tetrodotoxin-sensitive rise in intracellular Na⁺, closely followed by an increase in cytosolic Ca²⁺ responsible for promoting SNARE-dependent catecholamine secretion. Our results reveal that brevenal and β-naphtoyl-brevetoxin prevent P-CTX-1B secretagogue activity without affecting nicotine or barium-induced catecholamine secretion. Brevenal is therefore a potent inhibitor of ciguatoxin-induced neurotoxic effect and a potential treatment for ciguatera.

Matrix metalloproteinase-7 modulates synaptic vesicle recycling and induces atrophy of neuronal synapses

Matrix metalloproteinase-7 (MMP-7) belongs to a family of zinc dependent endopeptidases that are expressed in a variety of tissues including the brain. MMPs are known to be potent mediators of pericellular proteolysis and likely mediators of dynamic remodelling of neuronal connections. While an association between proteases and the neuronal synapse is emerging, a full understanding of this relationship is lacking. Here, we show that MMP-7 alters the structure and function of presynaptic terminals without affecting neuronal survival. Bath application of recombinant MMP-7 to cultured rat neurons induced long-lasting inhibition of vesicular recycling as measured by synaptotagmin 1 antibody uptake assays and FM4-64 optical imaging. MMP-7 application resulted in reduced abundance of vesicular and active zone proteins locally within synaptic terminals although their general levels remained unaltered. Finally, chronic application of the protease resulted in synaptic atrophy, including smaller terminals and fewer synaptic vesicles, as determined by electron microscopy. Together these results suggest that MMP-7 is a potent modulator of synaptic vesicle recycling and synaptic ultrastructure and that elevated levels of the enzyme, as may occur with brain inflammation, may adversely influence neurotransmission.

Internalization of a GFP-tetanus toxin C-terminal fragment fusion protein at mature mouse neuromuscular junctions

The distribution, dynamics, internalization, and retrograde axonal traffic of a fusion protein composed of green fluorescent protein (GFP) and the atoxic C-terminal fragment of tetanus toxin (TTC) were studied after its in vivo injection. Confocal microscopy and immunogold electron microscopy revealed that the fusion protein (GFP-TTC) rapidly clustered in motor nerve terminals of the neuromuscular junction. Clathrin-coated pits, and axolemma infoldings located between active zones appeared to be involved in the internalization of the fusion protein. Biochemical analysis of detergent-extracted neuromuscular preparations showed that the GFP-TTC fusion protein was associated with lipid microdomains. We suggest that GFP-TTC clustering in these lipid microdomains favors the recruitment of other proteins involved in its endocytosis and internalization in motor nerve terminals. During its retrograde trafficking, GFP-TTC accumulated in different axonal compartments than those used by cholera toxin B-subunit suggesting that these two proteins are transported by different pathways and cargos.

Na+ influx triggers bleb formation on inner hair cells

Large blebs form rapidly on apical membranes of sensory inner hair cells (IHCs) when the organ of Corti is freshly isolated from adult guinea pigs. Bleb formation had two distinguishable phases. Initially, we identified small particles labeled with fluorescent annexin V; these rapidly coalesced into larger aggregates. After particle aggregation, a single membrane bleb emerged from cuticular plate at the vestigial kinocilium location, eventually reaching approximately 10 microm maximum spherical diameter; blebs this size often detached from IHCs. Development of blebs was associated with elevated concentration of intracellular Na(+); blocking Na(+) influx through mechanotransduction and ATP channels in the apical pole of IHCs or by replacement of Na(+) with N-methyl-D-glucamine prevented Na(+) loading and bleb formation. Depletion of intracellular ATP, blocking cAMP synthesis, inhibition of vesicular transport with brefeldin A, or inhibition of phosphatidylinositol 3-kinase with 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one (LY-294002) significantly reduced bleb formation in the presence of a Na(+) load. Neither the mechanism of blebbing nor the size growth of the IHC blebs was associated with cellular apoptosis or necrosis. Bleb formation was not significantly reduced by disassembling microtubules or decreasing intracellular hydrostatic pressure. Moreover, no polymerized actin was observed in the lumen of blebs. We conclude that IHC bleb formation differs from classic blebbing mechanisms and that IHC blebs arise from imbalance of endocytosis and exocytosis in the apical plasma membrane, linked to Na(+) loading that occurs in vitro.

Altered brain activity in brevetoxin-exposed bluegill, Lepomis macrochirus, visualized using in vivo 14C 2-deoxyglucose labeling

This study investigated the neurological effects of sublethal brevetoxin (PbTx-2) exposure in bluegill (Lepomis macrochirus) by measuring alterations in 2-deoxyglucose (2-DG) uptake in the brains of exposed fish. Changes in regional brain activity were quantified using digitized autoradiographs from exposed and control fish. Brains of brevetoxin-exposed fish had significantly higher labeling of 2-DG than brains of control fish. Regional increases in labeling were observed in the optic lobes, telencephalon, and cerebrum of PbTx-2 exposed fish. From these observations, we conclude that sublethal brevetoxin exposure in vivo in bluegill increases neurological stimulation, measured through quantification of [14C]2-DG uptake in the brain. Increases in the uptake of [14C]2-DG from this study may be indicative of differences in neural activity in the PbTx-exposed fish and are likely associated with the action of PbTx-2 on voltage-gated sodium channels (VGSC), as well as neurological alterations in calcium and neurotransmitter release downstream resulting from VGSC activation. These techniques quantify physiological alterations in fish brain activity resulting from exposure to brevetoxin and possibly other harmful algal bloom toxins.

Sodium channel toxins and neurotransmitter release

Voltage-dependent sodium channels (VDSC) are an important class of ion channels in excitable cells, where they are responsible for the generation and conduction of action potential. In addition, the release of neurotransmitters from nerve terminals is influenced by sodium channel activity. The function of VDSC is subject to modulation by various neurotoxins, such as scorpion toxins, which have long been used as tools in the investigation of neurotransmitter release. This opens an interesting perspective concerning modulation of neurotransmission via pharmacological manipulation of sodium channel properties, which can lead to a better understanding of their physiological and pathological roles. Here we briefly review the studies of neurotoxins acting on sodium channels, focusing primarily on the view of the mechanisms of neurotransmitter release.

[Mode of action of botulinum neurotoxin: pathological, cellular and molecular aspect]

Several bacteria of the Clostridium genus (C. botulinum) produce 150 kDa di-chainal protein toxins referred as botulinum neurotoxins or BoNTs. They associate with non-toxic companion proteins and form a complex termed botulinum toxin or BoTx. The latter is used in clinic for therapeutic purpose. BoNTs affect cholinergic nerve terminals in periphery where they block acetylcholine release, thereby causing dysautonomia and motorparalysis (i.e. botulism). The cellular action of BoNTs can be depicted according to a three steps model: binding, internalisation and intraneuronal action. The toxins heavy chain mediates binding to specific receptors followed by endocytotic internalisation of BoNT/receptor complex. BoNT receptors may comprise gangliosides and synaptic vesicle-associated proteins as synaptotagmins. Vesicle recycling induces BoNT internalisation. Upon acidification of vesicles, the light chain of the neurotoxin is translocated into the cytosol. Here, this zinc-endopeptidase cleaves one or two among three synaptic proteins (VAMP-synaptobrevin, SNAP25, and syntaxin). As the three protein targets of BoNT play major role in fusion of synaptic vesicles at the release sites, their cleavage is followed by blockage of neurotransmitter exocytosis. The duration of the paralytic effect of the BoNTs is determined by 1) the turnover of their protein target; 2) the time-life of the toxin light chain in the cytosol, and 3) the sprouting of new nerve-endings that are retracted when the poisoned nerve terminal had recovered its full functionality.

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.

How botulinum and tetanus neurotoxins block neurotransmitter release

Botulinum neurotoxins (BoNT, serotypes A-G) and tetanus neurotoxin (TeNT) are bacterial proteins that comprise a light chain (M(r) approximately 50) disulfide linked to a heavy chain (M(r) approximately 100). By inhibiting neurotransmitter release at distinct synapses, these toxins cause two severe neuroparalytic diseases, tetanus and botulism. The cellular and molecular modes of action of these toxins have almost been deciphered. After binding to specific membrane acceptors, BoNTs and TeNT are internalized via endocytosis into nerve terminals. Subsequently, their light chain (a zinc-dependent endopeptidase) is translocated into the cytosolic compartment where it cleaves one of three essential proteins involved in the exocytotic machinery: vesicle associated membrane protein (also termed synaptobrevin), syntaxin, and synaptosomal associated protein of 25 kDa. The aim of this review is to explain how the proteolytic attack at specific sites of the targets for BoNTs and TeNT induces perturbations of the fusogenic SNARE complex dynamics and how these alterations can account for the inhibition of spontaneous and evoked quantal neurotransmitter release by the neurotoxins.

Direct interaction between synaptotagmin and the intracellular loop I-II of neuronal voltage-sensitive sodium channels

Synaptotagmin, a synaptic vesicle protein involved in Ca(2+)-regulated exocytosis, displayed direct high affinity interaction with neuronal sodium channels. Monoclonal antibodies directed against synaptotagmins I and II adsorbed in a concentration-dependent and -specific manner [(3)H]saxitoxin prelabeled sodium channels extracted with detergent from nerve endings. Conversely, co-immunoprecipitation of synaptotagmin was achieved by antibodies against sodium channel subunits. Consistent with the co-immunoprecipitation assays, solubilized [(3)H]saxitoxin-prelabeled sodium channels were trapped on immobilized maltose binding protein (MBP)-synaptotagmin I. In vitro recombinant protein assays were employed to identify the interaction site of synaptotagmin I, which was located on the cytoplasmic loop between domains I and II of the sodium channel alphaIIA subunit. The co-immunoprecipitated synaptotagmin-sodium channel complexes were found to be Ca(2+)-dependent; this effect was mimicked by Ba(2+) and Sr(2+) but not Mg(2+). Finally the complex was shown to be distinct from the synaptotagmin-SNARE protein complex that can selectively interact with presynaptic calcium channels (N and P/Q types). Thus, our findings demonstrate an unexpected and direct interaction between sodium channels and synaptotagmin. The Ca(2+)-regulated association between sodium channels and a protein implicated in vesicular fusion may have intriguing consequences for the establishment and regulation of neuronal excitability.

Electrospray ionization tandem mass spectrometry for structural elucidation of protonated brevetoxins in red tide algae

Brevetoxins, the toxic components of "red tide" algae, all share one of two robust polycyclic ether backbone structures, but they are distinguished by differing side-chain substituents. Electrospray ionization mass spectrometry analyses of brevetoxins have shown that the polyether structure invariably has a very high affinity for sodium cations that results in the production of abundant (M + Na)+ ions even when sodium cations are only present as impurities. Because the ionic charge tends to remain localized on the sodium atom and because at least two bonds must be broken in order to produce polycyclic backbone fragmentation, it is extremely difficult to obtain abundant product ions (other than Na+) from (M + Na)+ brevetoxin precursor ions in low-energy collision-induced dissociation (CID) MS/MS experiments. This report establishes that acid additives (oxalic acid, trifluoroacetic acid, and particularly hydrochloric acid) in aqueous methanol solutions can promote high yields of protonated brevetoxin molecules (MH+ ions) for Btx-1, -2, and -9 brevetoxins. Most importantly, unlike their (M + Na)+ counterparts, MH+ precursor ions offer readily detectable product ions in CID MS/MS experiments, even under low-energy collisions. This direct structural characterization approach has provided decomposition information from brevetoxins that was previously inaccessible, including the identification of diagnostic product ions for "type A" brevetoxins (m/z 611) and "type B" brevetoxins (m/z 779, 473, 179) and characteristic ions for Btx-1 (m/z 221, 139), Btx-2 (m/z 153), and Btx-9 (m/z 157, 85). Precursor ion scans and constant neutral loss scans are proposed to enable screening of individual type A or type B brevetoxins present in naturally occurring mixtures.

Ciguatoxin reduces larval survivability in finfish

Ciguatoxins are lipophilic polyether toxins which concentrate in the viscera and flesh of coral reef associated finfish (Hessel et al., 1960). In this study, we quantify the adverse effects of ciguatoxin on fish embryos by microinjection into the egg yolk of medaka (Oryzias latipis) embryos. Embryos microinjected with 0.1-0.9 pg/egg (ppb) of ciguatoxin exhibit cardiovascular, muscular, and skeletal abnormalities and those injected with higher levels (1.0-9.0 pg/egg) exhibit significantly reduced hatching success. The sensitivity of embryonic fish to direct oocyte exposure indicates that maternal transfer of low levels of ciguatoxin may represent an unrecognized threat to the reproductive success of reef fish and a previously undetected ecological consequence of proliferation of ciguatoxin-producing algae in reef systems increasingly impacted by human perturbations.

A new conotoxin isolated from Conus consors venom acting selectively on axons and motor nerve terminals through a Na+-dependent mechanism

A novel conotoxin was isolated and characterized from the venom of the fish-hunting marine snail Conus consors. The peptide was identified by screening chromatography fractions of the crude venom that produced a marked contraction and extension of the caudal and dorsal fins in fish, and noticeable spontaneous contractions of isolated frog neuromuscular preparations. The peptide, named CcTX, had 30 amino acids and the following scaffold: X11CCX7CX2CXCX3C. At the frog neuromuscular junction, CcTx at nanomolar concentrations selectively increased nerve terminal excitability so that a single nerve stimulation triggered trains of repetitive or spontaneous synaptic potentials and action potentials. In contrast, CcTx had no noticeable effect on muscle excitability even at concentrations 100 x higher than those that affected motor nerve terminals, as revealed by direct muscle stimulation. In addition, CcTx increased miniature endplate potential (MEPP) frequency in a Ca2+-free medium supplemented with ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid (EGTA). Blockade of voltage-dependent sodium channels with tetrodotoxin (TTX) either prevented or suppressed the increase of MEPP frequency induced by the toxin. CcTx also produced a TTX-sensitive depolarization of the nodal membrane in single myelinated axons giving rise, in some cases, to repetitive and/or spontaneous action potential discharges. In addition, CcTx increased the nodal volume of myelinated axons, as determined using confocal laser scanning microscopy. This increase was reversed by external hyperosmolar solutions and was prevented by pretreatment of axons with TTX. It is suggested that CcTx, by specifically activating neuronal voltage-gated sodium channels at the resting membrane potential, produced Na+ entry into nerve terminals and axons without directly affecting skeletal muscle fibres. CcTx belongs to a novel family of conotoxins that targets neuronal voltage-gated sodium channels.

Neurotoxins targetting receptor site 5 of voltage-dependent sodium channels increase the nodal volume of myelinated axons

The effects of a C57 type ciguatoxin (CTX-3C) and two types of brevetoxins (PbTx-1 and PbTx-3), known to bind to receptor site 5 of the neuronal voltage-dependent Na+ channel-protein, were studied on the morphology of living frog myelinated axons using confocal laser scanning microscopy. During the action of CTX-3C, PbTx-1, and PbTx-3 (10-50 nM), a marked swelling of nodes of Ranvier was observed without apparent modification of internodal parts of axons. In all cases, toxin-induced nodal swelling attained a steady-state within 75-100 min that was well maintained during an additional 90-115 min. The nodal swelling was reversed by an external hyperosmotic solution containing 100 mM D-mannitol and could be completely prevented by blocking voltage-dependent Na+ channels with 1 microM tetrodotoxin. It is suggested that CTX-3C, PbTx-1, and PbTx-3 by activating Na+ channels cause a continuous Na+ entry into axons, increasing internal Na+ concentration. Such an increase directly or indirectly disturbs the osmotic equilibrium between intra- and extra-axonal media, resulting in an influx of water, which is responsible for the long-lasting nodal swelling. Similar results were previously reported with two C60 type ciguatoxins (CTX-1B and CTX-4B). Thus, it is concluded that the four types of toxins targetting receptor site 5 of neuronal voltage-dependent Na+ channels, not only enhance nerve membrane excitability but also, on a long-term basis, cause a marked increase in the axonal volume.

Ciguatoxins and brevetoxins, neurotoxic polyether compounds active on sodium channels

Ciguatoxins (CTXs) and brevetoxins (PbTxs) modify the activation and inactivation processes of voltage-sensitive sodium channels (VSSC). In this study, the specific binding to rat brain synaptosomes of two commercial PbTxs, five purified CTXs and their derivatives was evaluated in competition with various concentrations of radiolabelled brevetoxin ([3H]PbTx-3). The results indicate that all CTXs bind specifically and with high affinity to sodium channels. Statistical analysis of the calculated inhibition constants identified two classes of toxins: the PbTxs and the less polar CTXs, and a group of CTXs of very high affinity. Relatively small chemical differences between the CTXs gave rise to significant differences in their affinity to the rat brain sodium channels. Cytotoxic effects associated with sodium channel activation were evaluated for the two classes of toxins on murine neuroblastoma cells, and their acute toxicity was determined in mice. CTXs have shown high affinities to VSSC of rat brain membranes and strong cytotoxic effects on neuroblastoma cells which correlate with their very low LD50 in mice. For PbTxs, it is different. Although binding with high affinity to VSSC and giving rise to significant cytotoxic effects, they are known to be poorly toxic intraperitoneally to mice. Furthermore, within the CTXs family, even though the most toxic compound (CTX-1B) has the highest affinity and the less toxic one (CTX-4B) the lowest affinity, a detailed analysis of the data pointed out a complex situation: (i) high affinity and toxicity seem to be related to the hydroxylation of the molecule on the A-ring rather than to the backbone type, (ii) acute toxicity in mice does not follow exactly the sodium-dependent cytotoxicity on neuroblastoma cells. These data suggest that the high toxicity of CTXs is related to sodium-dependent disturbances of the excitable membranes but might also involve other cellular mechanisms.