Distribution of acceptors for beta-bungarotoxin in the central nervous system of the rat

Beta-agkistrodotoxin inhibits large-conductance calcium-activated potassium channels in rat hippocampal CA1 pyramidal neurons

Effect of beta-agkistrodotoxin (beta-AgTx), a presynaptic neurotoxin purified from snake venom, on large-conductance calcium-activated potassium channels (BK(Ca)) was studied in rat hippocampal CA1 pyramidal neurons using inside-out configuration of patch-clamp technique. The results showed that in equimolar K+ (150 mM) and 1 microM intracellular Ca2+ conditions, internal application of beta-AgTx inhibited the activity of BK(Ca) by reducing open probability (P(o)) of the channels in a concentration-dependent manner. High concentration (74 nM) of beta-AgTx completely eliminated opening of the channels. However, 37 nM beta-AgTx (at -40 mV) decreased P(o) from 0.49+/-0.07 to 0.03+/-0.03, switched two open time constants (0.51+/-0.32 and 8.77+/-1.63 ms) to be a single time constant of 0.46+/-0.40 ms. The results indicate that inhibition of BK(Ca) by beta-AgTx may account for the facilitatory phase of the toxin on acetylcholine release from nerve terminals.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

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.

Cloning and functional expression of B chains of beta-bungarotoxins from Bungarus multicinctus (Taiwan banded krait)

The cDNA species encoding the B chains (B1 and B2) of beta-bungarotoxins (beta-Bgt) were constructed from the cellular RNA isolated from the venom glands of Bungarus multicinctus (Taiwan banded krait). The deduced amino acid sequences of the B chains were different from those determined previously by a protein sequencing technique. One additional Arg residue is inserted between Val-19 and Arg-20 of the B1 chain. Similarly the insertion of one additional Val residue between Val-19 and Arg-20 of the B2 chain is noted. Thus the B chains should comprise 61 amino acid residues. Moreover, the residues at positions 44-46 are Gly-Asn-His, in contrast with a previous result showing the sequence His-Gly-Asn. Instead of Asp, the residues at positions 41 and 43 are Asn. The B chain was subcloned into the expression vector pET-32a(+) and transformed into Escherichia coli strain BL21(DE3). The recombinant B chain was expressed as a fusion protein and purified on a His-Bind resin column. The yield of affinity-purified fusion protein was increased markedly by replacing Cys-55 of the B chain with Ser. However, the isolated B(C55S) chain became insoluble in aqueous solution after removal of the fused protein from the affinity-purified product, suggesting that protein-protein interactions might be crucial for stabilizing the structure of the B chain. The B(C55S) chain fusion protein showed activity in blocking the voltage-dependent K+ channel, but did not inhibit the binding of beta-Bgt to synaptosomal membranes. These results, together with the finding that modification of His-48 of the A chain of beta-Bgt caused a marked decrease in the ability to bind toxin to its acceptor proteins, suggest that the B chain is involved in the K+ channel blocking action observed with beta-Bgt, and that the binding of beta-Bgt to neuronal receptors is not heavily dependent on the B chain.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

Ultrastructural localization of a voltage-gated K+ channel alpha subunit (KV 1.2) in the rat cerebellum

A highly specific monoclonal antibody and pre-embedding immunocytochemistry were employed to examine the distribution of the K+ channel, alpha subunit K(V)1.2 in the rat cerebellum. At the light microscopic level, the heaviest immunoreactivity was seen in the basket cell pinceau at the base of Purkinje cells, with lighter staining of basket and Golgi cell bodies and a punctate pattern in the granule cell and molecular layers. Electron microscopy was performed to identify the ultrastructural location of K(V)1.2 alpha subunit in these labelled structures. This revealed that the labelling of the pinceau was confined to the preterminal axonal plexus, the area immediately around the Purkinje axon initial segment being relatively devoid of staining. Basket cell parent axons were not immunostained, but gave rise to heavily stained fine processes. Immunoreactivity was also seen in myelinated axons in the granule cell layer and in the medial cerebellar nucleus, the staining being most concentrated at the juxtaparanodal regions of the axons. An unusual pattern of staining was seen in some mossy fibre terminals, with staining restricted to fine protuberances of mossy fibre glomeruli. Structures contacted by these protuberances included adjoining glial processes. Immunostaining was absent from Purkinje cell bodies, dendrites, their axon initial segments and their terminals in the medial cerebellar nucleus. In this study, the alpha subunit K(V)1.2 was localized to a number of different cell types in the cerebellum. Each neuronal type displays a distinct subcellular distribution of the subunit.

ATP sensitive potassium channels: potential drug targets in neuropsychopharmacology

1. K channels are a diverse and ubiquitous class of proteins that regulate a number of biological functions. 2. Ligands for the study of a variety of K channels are available. These include "openers" and antagonists for the ATP sensitive K channel and peptide toxins such as apamin and charybdotoxin that block other subtypes. 3. Antagonists of the ATP sensitive K channel are useful in the treatment of type II diabetes while "openers" of this channel are being tested in asthma and cardiovascular disease. 4. Intracerebroventricular administration of K channel "openers" block experimentally induced seizures in rodents through a hyperpolarization of neurons. K channel openers may also be useful in the treatment of neurodegenerative diseases, pain and cerebral ischemia. 5. A key to the development of psychopharmacological agents to modify brain K channel function is CNS selectivity. The promise of the ATP sensitive K channel openers suggests a bright future for this mechanism.

Antibodies specific for distinct Kv subunits unveil a heterooligomeric basis for subtypes of alpha-dendrotoxin-sensitive K+ channels in bovine brain

The authentic subunit compositions of neuronal K+ channels purified from bovine brain were analyzed using a monoclonal antibody (mAb 5), reactive exclusively with the Kv1.2 subunit of the latter and polyclonal antibodies specific for fusion proteins containing C-terminal regions of four mammalian Kv proteins. Western blotting of the K+ channels isolated from several brain regions, employing the selective blocker alpha-dendrotoxin (alpha-DTX), revealed the presence in each of four different Kvs. Variable amounts of Kv1.1 and 1.4 subunits were observed in the K+ channels purified from cerebellum, corpus striatum, hippocampus, cerebral cortex, and brain stem; on the other hand, contents of Kv1.6 and 1.2 subunits appeared uniform throughout. Each Kv-specific antibody precipitated a different proportion (anti-Kv1.2 > 1.1 >> 1.6 > 1.4) of the channels detectable with radioiodinated alpha-DTX in every brain region, consistent with a widespread distribution of these oligomeric subtypes. Such heterooligomeric combinations were further documented by the lack of additivity upon their precipitation with a mixture of antibodies to Kv1.1 and Kv1.2; moreover, cross-blotting of the multimers precipitated by mAb 5 showed that they contain all four Kv proteins. Collectively, these findings demonstrate that subtypes of alpha-DTX-susceptible K+ channels are prevalent throughout mammalian brain which are composed of different Kv proteins assembled in complexes, shown previously to also contain auxiliary beta-subunits [Parcej, D. N., Scott, V. E. S., & Dolly, J.O. (1992) Biochemistry 31, 11084-11088].

Comparison of the distribution of binding sites for the potassium channel ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in the rat brain

Potassium channels represent a diverse and promising target for drug development. Pharmacological subtypes of K channels have begun to emerge based on the development of both organic molecules and peptide toxins which possess subtype selectivity. In order to evaluate the neuroanatomical distribution of these subtypes we have utilized the ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in an autoradiographic study of rat brain. In the rat brain, these ligands have selectivity for the low conductance Ca(2+)-activated, voltage-gated K channels and ATP-sensitive K channels respectively. The distribution of binding sites for these three ligands were distinctly different. [125I]Apamin binding was highest in various thalamic and hippocampal structures, while only low to moderate levels of [125I]charybdotoxin binding were seen in these regions. In contrast, very high levels of [125I]charbydotoxin were seen in white matter regions such as the lateral olfactory tract and fasciculus retroflexus. High levels of [125I]charybdotoxin binding were also seen in gray matter-containing regions such as the zona incerta, medial geniculate and superior colliculus, where low to moderate [125I]apamin binding was found. [125I]Iodoglyburide presented a more uniform binding with the highest levels in the globus pallidus, islands of Calleja, anteroventral nucleus of the thalamus and zonas reticulata of the substantia nigra. These results indicate that subtypes of K channels have very different distributions in the brain. As such, the results imply differing CNS actions for potential modulators of K channel subtypes.

Autoradiographic localization of [125I]charybdotoxin binding sites in rat brain

Charybdotoxin, a 37 amino acid peptide isolated from scorpion venom, is a potent inhibitor of potassium channel function. [125I]charybdotoxin was originally believed to be a selective ligand for the Ca(2+)-sensitive channel in many tissues, but it appears to bind only to a voltage-sensitive potassium channel in brain. We found high densities of [125I]charybdotoxin binding in lateral olfactory tract, interpeduncular nucleus and a variety of mesencephalic nuclei. Moderate levels were found in the cerebral cortex, medial thalamus, hypothalamus and selected thalamic nuclei. These results indicate that [125I]charybdotoxin identifies a potassium channel or channels with a unique distribution in the brain.

K+ channel sub-types in rat brain: characteristic locations revealed using beta-bungarotoxin, alpha- and delta-dendrotoxins

Sub-types of fast-activating, voltage-dependent K+ channels were localized in rat brain using the specific probe, alpha-dendrotoxin, in conjunction with the putative K+ channel ligands, delta-dendrotoxin and beta-bungarotoxin. Sheet-film autoradiography of brain sections labelled with radioiodinated delta-dendrotoxin showed that its acceptors occur in most synapse-rich and gray matter regions, and nerve tracts; all of this labelling was abolished by alpha-dendrotoxin or its homologue, toxin I. Other structurally related peptides from mamba snake venom, beta- and gamma-dendrotoxin, were much less effective in preventing delta-dendrotoxin labelling. In common with the sites for alpha-dendrotoxin and beta-bungarotoxin, delta-dendrotoxin acceptors were enriched in cerebral cortex, thalamus and molecular layer of both the cerebellum and dentate gyrus of the hippocampus. However, delta-dendrotoxin failed to show significant binding to the Purkinje cell layer of the cerebellar cortex and stratum lacunosum moleculare of the hippocampal formation, areas labelled prominently by the other two probes. Evidence of this apparent heterogeneity in the toxin-binding proteins was consolidated by the observed inability of delta-dendrotoxin to inhibit 125I-labelled alpha-dendrotoxin or beta-bungarotoxin binding to these specified regions. Thus, delta-dendrotoxin, like beta-bungarotoxin, discriminates between sub-types of alpha-dendrotoxin acceptors but in different fashions. Whilst beta-bungarotoxin interacts preferentially with a sub-population in synaptic areas, delta-dendrotoxin distinguished sub-types in certain synaptic and gray matter regions and, in this, resembles mast cell degranulating peptide, a ligand known to block an alpha-dendrotoxin-sensitive K+ current.

Phospholipid hydrolysis and loss of membrane integrity following treatment of rat brain synaptosomes with beta-bungarotoxin, notexin, and Naja naja atra and Naja nigricollis phospholipase A2

The effects of the phospholipase A2 (PLA2) toxins, beta-bungarotoxin and notexin, and the PLA2 enzymes from Naja naja atra and Naja nigricollis snake venoms on the plasma membrane integrity of synaptosomes were examined. Synaptosomes were isolated from rat brain cerebral cortex, corpus striatum and hippocampus. Osmotic activity, lactate dehydrogenase leakage, and leakage of 2-deoxy-D-(1-3H)-glucose-6-phosphate were monitored (37 degrees C, 10-120 min) following incubation with 0.5, 5 and 50 nM concentrations of toxins and enzymes. Damage to the synaptosomal plasma membrane was time and concentration but not tissue dependent. The potencies of the treatments were as follows: N. n. atra PLA2 greater than or equal to N. nigricollis PLA2 greater than notexin greater than beta-bungarotoxin. Chelation of Ca2+ with 5 mM EDTA completely inhibited plasma membrane disruption caused by beta-bungarotoxin and N. n. atra PLA2. One mg/ml of bovine serum albumin also blocked the disruptive action of N. n. atra PLA2, while 8 mg/ml was required to antagonize beta-bungarotoxin. A correlation between phospholipid hydrolysis and loss of membrane integrity was also observed. The generation of phospholipid hydrolytic products may be critical in the permeabilization of synaptic plasma membranes by these toxins and enzymes, however, they do not explain the presynaptic specificity and potency of beta-bungarotoxin and notexin.

Potassium channel toxins

Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.

Interactions between discrete neuronal membrane binding sites for the putative K+-channel ligands beta-bungarotoxin, dendrotoxin and mast-cell-degranulating peptide

1. beta-Bungarotoxin, a presynaptically active neurotoxin from the venom of Bungarus multicinctus, was radiolabelled with 125I and its binding to synaptic membranes from rat brain was analyzed. The interaction of these binding sites with those for dendrotoxin (a convulsant polypeptide from mamba venom) and mast-cell-degranulating peptide (from bee venom) was examined in the light of the known effects of all three toxins on voltage-dependent K+ currents. 2. When measured in Krebs/phosphate buffer, the binding appeared monotonic at low concentrations of radioiodinated beta-bungarotoxin (Kd 0.4 nM; Bmax 0.42 pmol/mg protein); higher concentrations of labelled toxin revealed an additional binding component of lower affinity, but computer analysis of the data failed to provide well-defined estimates of its Kd and Bmax values. 3. Equilibrium binding experiments conducted in imidazole-based buffers yielded distinctly biphasic Scatchard plots; computer analysis of the data revealed two populations of sites [Kd 0.26 (+/- 0.30) nM and 6.14 (+/- 5.68) nM; Bmax 0.16 (+/- 0.20) and 2.65 (+/- 1.21) pmol/mg protein]. 4. In Krebs medium, beta-bungarotoxin was a very weak antagonist of the binding of 125I-labelled dendrotoxin. In imidazole medium, however, the efficacy of the inhibition was markedly increased; analysis of this inhibition showed it to be non-competitive. 5. Dendrotoxin inhibited the binding of radioiodinated beta-bungarotoxin in Krebs medium with high potency, although the interaction was by a complex, non-competitive mechanism. 6. Mast-cell-degranulating peptide inhibited non-competitively the binding of both radiolabelled dendrotoxin and beta-bungarotoxin but with relatively low potency. 7. A speculative schematic model of the dendrotoxin/beta-bungarotoxin/mast-cell-degranulating peptide binding component(s) is proposed. Findings are discussed in terms of the likely involvement of these sites with voltage-dependent K+-channel proteins.

Distribution in the rat central nervous system of acceptor sub-types for dendrotoxin, a K+ channel probe

Dendrotoxin, a snake polypeptide that facilitates the release of neurotransmitters, is a putative ligand for certain voltage-dependent, rapidly-activating K+ channels. Using a 125I-labelled derivative, the location of high-affinity acceptors for this toxin in the rat central nervous system was established by quantitative sheet film autoradiography. A widespread distribution of binding sites was observed, with high densities of acceptors being found in most gray matter regions and along nerve tracts. Heterogeneity in these acceptors was deduced from their differential interaction with beta-bungarotoxin, another probe that perturbs transmitter release. Whereas the latter blocked the majority of dendrotoxin sites in gray matter areas, it competed much less efficaciously for the acceptors in white matter. These collective findings demonstrate the occurrence of dendrotoxin acceptor sub-types which display characteristic distributions in the central nervous system. Notably, this heterogeneity can be related to electrophysiological evidence for the presence in neurons of multiple, dendrotoxin-sensitive, K+ conductances, though some of these remain to be shown directly in brain preparations.

Dendrotoxin, 4-aminopyridine, and beta-bungarotoxin act at common loci but by two distinct mechanisms to induce Ca2+-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes

The release of endogenous glutamate from guinea-pig cerebrocortical synaptosomes evoked by dendrotoxin, beta-bungarotoxin, and 4-aminopyridine is compared. Dendrotoxin and 4-aminopyridine cause Ca2+-dependent release, representing a partial depletion of the KCl-releasable transmitter pool. The decrease in the plasma membrane potential caused by 4-aminopyridine or dendrotoxin and the evoked release of glutamate from a transmitter pool accord with the inhibitory action of these agents on certain K+ conductances. In contrast, the massive release of glutamate evoked by beta-bungarotoxin is produced in the presence of Ca2+ but not of Sr2+, a result consistent with a generalised permeabilisation of synaptosomal plasma membranes. Although dendrotoxin inhibits the binding of beta-bungarotoxin and the resultant synaptosomal lysis, demonstration of a direct effect of beta-bungarotoxin binding per se on K+ permeability is impractical owing to its phospholipase A2 activity.

Existence of different populations of the dendrotoxin I binding protein associated with neuronal K+ channels

The binding sites of dendrotoxin I, mast cell degranulating peptide, and beta-bungarotoxin are thought to be associated with neuronal K+ channels. The different binding sites seem to reside on the same molecular assembly as each toxin can allosterically inhibit the binding of the others. Affinity chromatography on a beta-BTX Aca 22 affinity column has shown that there is an heterogeneous population of dendrotoxin I binding proteins. Two subtypes were separated: DTXI binding proteins with low affinity for beta-BTX (60-70% of total) and DTXI binding proteins with high affinity for beta-BTX (30-40% of total). Binding of 125I-DTXI and 125I-MCD to the former subtype is inhibited by beta-BTX with a low affinity (IC50 = 560 nM), while inhibition at the latter subtype occurs with a high affinity (IC50 = 10-16 nM). The DTXI binding subtype with low affinity for beta-BTX contains most (85-90%) of the binding sites for 125I-MCD.