Hugues Schweitz

MIT1, a black mamba toxin with a new and highly potent activity on intestinal contraction

Mamba intestinal toxin (MIT1) isolated from Dendroaspis polylepis venom is a 81 amino acid polypeptide cross‐linked by five disulphide bridges. MIT1 has a very potent action on guinea‐pig intestinal contractility. MIT1 (1 nM) potently contracts longitudinal ileal muscle and distal colon, and this contraction is equivalent to that of 40 mM K+. Conversely MIT1 relaxes proximal colon again as potently as 40 mM K+. The MIT1‐induced effects are antagonised by tetrodotoxin (1 μM) in proximal and distal colon but not in longitudinal ileum. The MIT1‐induced relaxation of the proximal colon is reversibly inhibited by the NO synthase inhibitor L‐NAME (200 μM). 125I‐labelled MIT1 binds with a very high affinity to both ileum and brain membranes (K d=1.3 pM and 0.9 pM, and B max=30 fmol/mg and 26 fmol/mg, respectively). MIT1 is a very highly selective toxin for a receptor present both in the CNS and in the smooth muscle and which might be an as yet unidentified K+ channel.

Purification and pharmacological characterization of peptide toxins from the black mamba (Dendroaspis polylepis) venom

This paper reports the purification of 28 different peptides from the venom of the snake Dendroaspis polylepis. These peptides represent 99% of the total peptide fraction in the venom. The 14 most cationic peptides form a structurally and functionally homogeneous group of analogs of the most abundant dendrotoxin toxin I (DTXI). They recognize antibodies raised against DTXI as well as brain membrane binding sites corresponding to K+ channels that are sensitive to DTXI and the bee venom peptide MCD. Similarly to DTXI these 14 peptides induce convulsions after intracerebroventricular injections in mice and induce GABA release from synaptosomes. However, members in this iso-DTXI family differ widely in their affinity for the DTXI/MCD receptors and in their contractility promoting action on intestinal smooth muscle. The 14 other less cationic peptides do not interact with the DTXI receptor or with DTXI antibodies and they do not evoke GABA release. Their targets seem to be essentially of a peripheral nature. Half of them contract guinea pig ileum. In this group of toxins there might be new tools to study membrane excitability.

Charybdotoxin blocks dendrotoxin-sensitive voltage-activated K+ channels.

Charybdotoxin, a short scorpion venom neurotoxin, which was thought to be specific for the blockade of Ca2+‐activated K+ channels also blocks a class of voltage‐sensitive K+ channels that are known to be the target of other peptide neurotoxins from snake and bee venoms such as dendrotoxin and MCD peptide. Charybdotoxin also inhibits 125‐dendrotoxin and 125I‐MCD peptide binding to their receptors. All these effects are observed with an IC50 of about 30 nM.

Analogies and differences in the mode of action and properties of binding sites (localization and mutual interactions) of two K+ channel toxins, MCD peptide and dendrotoxin I

Both the bee venom toxin, mast cell degranulating (MCD) peptide, and the snake toxin, dendrotoxin 1 (DTX1) induce epileptiform activity and paroxystic seizures after intracerebroventricular (i.c.v.) injection to rats. Although many of the properties of the two toxins, which are blockers of the same K+ channel, appear to be very similar, a number of differences have been found. (1) Induced seizures have an hippocampal origin for MCD and two different origins, situated in the cortex and in the limbic system, for DTX1. (2) A first i.c.v. administration of DTXI desensitizes against a second ipsilateral injection of the same peptide as we had previously observed for MCD. However no cross-desensitization was observed between the two different toxins. (3) The number of high affinity (Kd = 41 pM) binding sites for 125I-DTXI in synaptic membranes is about 5 times higher than the number of high affinity (Kd = 158 pM) binding sites for 125I-MCD. (4) Autoradiographic analysis of the distribution of high affinity 125I-DTX1 binding sites has been compared to our previous analysis of high affinity 125I-MCD binding sites. High levels of high affinity binding sites for both toxins seem to be localized in synapse-rich areas. However high affinity binding sites for the two toxins are not always co-localized. Analysis of the mutual interactions between DTXI and MCD binding sites has revealed the presence of classes of low affinity binding sites for MCD. In most areas of the brain, a large proportion of high affinity binding sites for DTXI is allosterically related to low affinity binding for MCD.

Lethal potency in mice of toxins from scorpion, sea anemone, snake and bee venoms following intraperitoneal and intracisternal injection

The lethal potency of 29 toxins from scorpion, sea anemone, snake and bee venoms was studied. Lethality following intracisternal injection of these toxins is considerably higher than following i.p. injection (except for the snake neurotoxins). This is of practical interest in the determination of the concentration of active toxins in a solution when only small amounts are available, as in the case of radiolabelled toxins.

Sleep cycle disturbances induced by apamin, a selective blocker of Ca2+-activated K+ channels

Intracerebroventricular injections of low doses of apamin, a specific blocker of a class of Ca(2+)-activated K+ channels, induced insomnia and a long-lasting suppression of deep slow sleep and paradoxical sleep. Injected animals showed a late but important rebound of paradoxical sleep. Even after the recovery of a normal sleep amount, the circadian cycle remained disturbed during all the recording duration (96 h).

A polypeptide toxin from the coral Goniopora Purification and action on Ca2+ channels

A polypeptide toxin has been isolated from Goniopora coral with an Mr of 19000. Goniopora toxin has the following properties: (i) it induces contraction of guinea pig ileum and this contraction is prevented by Ca2+-channel blockers; (ii) it stimulates 45Ca2+ influx in cardiac cells in culture and this stimulation is abolished by Ca2+-channel blockers; (iii) it prevents binding of (+)-[3H]PN 200-110 to the Ca2+-channel protein of skeletal muscle T-tubule membranes. All these results taken together suggest that Goniopora toxin is a Ca2+-channel activator.A polypeptide toxin has been isolated from Goniopora coral with an M r of 19000. Goniopora toxin has the following properties: (i) it induces contraction of guinea pig ileum and this contraction is prevented by Ca2+‐channel blockers; (ii) it stimulates 45Ca2+ influx in cardiac cells in culture and this stimulation is abolished by Ca2+‐channel blockers; (iii) it prevents binding of (+)‐[3H]PN 200‐110 to the Ca2+‐channel protein of skeletal muscle T‐tubule membranes. All these results taken together suggest that Goniopora toxin is a Ca2+‐channel activator.

A micro-radioimmunoassay for apamin

A radioimmunoassay is described which allows detection of quantities of apamin between 5 and 1200 fmoles (10-2400 pg) in 50 microliter aliquots. The assay requires a few milliliters of apamin antiserum diluted 10,000-20,000 times and 125I apamin at a specific radioactivity of 2000 Ci/mmole. This assay is specific for apamin.

Kalicludines and kaliseptine: Two different classes of sea anemone toxins for voltage-sensitive K+ channels

New peptides have been isolated from the sea anemone Anemonia sulcata which inhibit competitively the binding of 125I-dendrotoxin I (a classical ligand for K+ channel) to rat brain membranes and behave as blockers of voltage-sensitive K+ channels. Sea anemone kalicludines are 58-59-amino acid peptides cross-linked with three disulfide bridges. They are structurally homologous both to dendrotoxins which are snake venom toxins and to the basic pancreatic trypsin inhibitor (Kunitz inhibitor) and have the unique property of expressing both the function of dendrotoxins in blocking voltage-sensitive K+ channels and the function of the Kunitz inhibitor in inhibiting trypsin. Kaliseptine is another structural class of peptide comprising 36 amino acids with no sequence homology with kalicludines or with dendrotoxins. In spite of this structural difference, it binds to the same receptor site as dendrotoxin and kalicludines and is as efficient as a K+ channel inhibitor as the most potent kalicludine.