Kappa-flavitoxin: isolation of a new neuronal nicotinic receptor antagonist that is structurally related to kappa-bungarotoxin

On characterizing the Red-headed Krait (Bungarus flaviceps) venom: Decomplexation proteomics, immunoreactivity and toxicity cross-neutralization by hetero-specific antivenoms

The Red-headed Krait (Bungarus flaviceps) is a medically important venomous snake species in Southeast Asia, while there is no specific antivenom available for its envenoming. This study investigated the venom composition through a decomplexation proteomic approach, and examined the immunoreactivity as well as cross-neutralization efficacy of two hetero-specific krait antivenoms, Bungarus candidus Monovalent Antivenom (BcMAV) and Bungarus fasciatus Monovalent Antivenom (BfMAV), against the venom of B. flaviceps from Peninsular Malaysia. A total of 43 non-redundant proteoforms belonging to 10 toxin families were identified in the venom proteome, which is dominated by phospholipases A₂ including beta-bungarotoxin lethal subunit (56.20 % of total venom proteins), Kunitz-type serine protease inhibitors (19.40 %), metalloproteinases (12.85 %) and three-finger toxins (7.73 %). The proteome varied in quantitative aspect from the earlier reported Indonesian (Sumatran) sample, suggesting geographical venom variation. BcMAV and BfMAV were immunoreactive toward the B. flaviceps venom, with BcMAV being more efficacious in immunological binding. Both antivenoms cross-neutralized the venom lethality with varying efficacy, where BcMAV was more potent than BfMAV by ~13 times (normalized potency: 38.04 mg/g vs. 2.73 mg/g, defined as the venom amount completely neutralized by one-gram antivenom protein), supporting the potential utility of BcMAV for para-specific neutralization against B. flaviceps venom.

Transcriptomic analysis of the venom gland of the red-headed krait (Bungarus flaviceps) using expressed sequence tags

Background

The Red-headed krait (Bungarus flaviceps, Squamata: Serpentes: Elapidae) is a medically important venomous snake that inhabits South-East Asia. Although the venoms of most species of the snake genus Bungarus have been well characterized, a detailed compositional analysis of B. flaviceps is currently lacking.

Results

Here, we have sequenced 845 expressed sequence tags (ESTs) from the venom gland of a B. flaviceps. Of the transcripts, 74.8% were putative toxins; 20.6% were cellular; and 4.6% were unknown. The main venom protein families identified were three-finger toxins (3FTxs), Kunitz-type serine protease inhibitors (including chain B of β-bungarotoxin), phospholipase A₂ (including chain A of β-bungarotoxin), natriuretic peptide (NP), CRISPs, and C-type lectin.

Conclusion

The 3FTxs were found to be the major component of the venom (39%). We found eight groups of unique 3FTxs and most of them were different from the well-characterized 3FTxs. We found three groups of Kunitz-type serine protease inhibitors (SPIs); one group was comparable to the classical SPIs and the other two groups to chain B of β-bungarotoxins (with or without the extra cysteine) based on sequence identity. The latter group may be functional equivalents of dendrotoxins in Bungarus venoms. The natriuretic peptide (NP) found is the first NP for any Asian elapid, and distantly related to Australian elapid NPs. Our study identifies several unique toxins in B. flaviceps venom, which may help in understanding the evolution of venom toxins and the pathophysiological symptoms induced after envenomation.

Neurotoxins acting at nicotinic receptors

Neurotoxins include, in the most general sense, all molecules that destroy or inhibit the proper functioning of the nervous system. Neurotoxins from animals and plants include alkaloids and peptides, many of which interact with physiological processes in a selective manner. The majority of neurotoxins disrupt the transmission of signals in the nervous system by interfering with synaptic transmission. Neurotoxins can act presynaptically to inhibit the release, uptake and recycling of neurotransmitters or postsynaptically, binding to receptors on the postsynaptic membrane and preventing their activation by neurotransmitters. A class of neurotoxins from plants and animals interact with nicotinic acetylcholine receptors, either at the neuromuscular junction, peripherally at neuronal ganglia or centrally, to produce neurotoxic effects. In this article we review current knowledge of some of these neurotoxins, their structure, pharmacology, importance as pharmaceutical tools as well as future prospects for the development of therapeutic molecules.

Isolation of the major lethal toxin in the venom of Bungarus flaviceps

The major lethal toxin in the venom of Bungarus flaviceps has been isolated by ion-exchange chromatography, absorption chromatography and RP-HPLC with a 14-fold purification and an overall yield of 16.5% of the lethal toxicity contained in crude venom. Its sublethal dose (LD(50)) determined in mice weighing 18-20 g was 0.25 (0.19-0.32) microg per mouse. The lethal toxin was pure according to disc- and SDS-PAGE as well as gel HPLC. Its apparent molecular weight determined by SDS-PAGE was 29 kDa. It is a basic protein consisting of two polypeptide chains having apparent molecular weights of 17 and 8 kDa, respectively. The toxin has PLA activity but is free of ACE activity.

Primary structure of gamma-bungarotoxin, a new postsynaptic neurotoxin from venom of Bungarus multicinctus

The primary structure of gamma-bungarotoxin, a new toxin from Bungarus multicinctus venom, was determined using mass spectrometry and Edman degradation. The toxin has a mass of 7524.7 D and consists of 68 residues having the following sequence: MQCKTCSFYT CPNSETCPDG KNICVKRSWT AVRGDGPKRE IRRECAATCP PSKLGLTVFC CTTDNCNH. Gamma-bungarotoxin is structurally similar to both kappa-bungarotoxin and elapid long postsynaptic neurotoxins. Its C-terminal nine residues are identical to those of the kappa-toxins. Its disulfide bond locations appear identical to those of several elapid toxins of unknown pharmacology and its hydrophobicity profile is also strikingly similar. However, with an LD50 of 0.15 microg/g i.v. in mice, gamma-bungarotoxin is 30-150-fold more toxic than other members of this latter class. Its toxicity is comparable to those of alpha-nicotinic acetylcholine receptor antagonists.

Binding of native kappa-neurotoxins and site-directed mutants to nicotinic acetylcholine receptors

The kappa-neurotoxins are useful ligands for the pharmacological characterization of nicotinic acetylcholine receptors because they are potent antagonists at only a subgroup of these receptors containing either alpha 3- or alpha 4-subunits (IC50 < or = 100 nM). Four of these highly homologous, 66 amino acid peptides have been purified from the venom of Bungarus multicinctus (kappa-bungarotoxin (kappa-Bgt), kappa 2-Bgt, kappa 3-Bgt] and Bungarus flaviceps [kappa-Fvt)]. Two approaches were taken to examine the binding of these toxins to nicotinic receptors. First, venom-derived kappa-Fvt and kappa-Bgt were radioiodinated and the specific binding was measured of these toxins to overlapping synthetic peptides (16-20 amino acids in length) prepared based on the known sequence of the nicotinic receptor alpha 3-subunit. At least two main regions of interaction between the toxins and the receptor subunit were identified, both lying in the N-terminal region of the subunit that is exposed to the extracellular space. The second approach examined the importance of several sequence position in kappa-Bgt for binding to alpha 3-containing receptors in autonomic ganglia and alpha 1-containing muscle receptors. This was done using site-directed mutants of kappa-Bgt produced by an Escherichia coli expression system. Arg-34 and position 36 were important for binding to both receptor subtypes, while replacing Gln-26 with Trp-26 (an invariant in alpha-neurotoxins) increased affinity for the muscle receptor by 8-fold. The results confirm that kappa-neurotoxins bind potently to the alpha 3-subunit and bind with considerably reduced affinity (Kd approximately 10 microM) to muscle receptors. Site-directed mutagenesis of recombinant kappa-Bgt is thus an important approach for the study of structure-function relationships between kappa-Bgt and nicotinic receptors.

Effects of a high molecular weight toxin from the sea anemone Condylactis gigantea on cholinergic responses

1. The effects of a high molecular weight toxin isolated from the sea anemone Condylactis gigantea (Condytoxina 2) on the cholinergic responses were studied in two different preparations: identified cells of a land snail and enzymatically dissociated mice sensory neurons. These neurons were studied using intracellular recording and concentration clamp techniques respectively. 2. The toxin produces a concentration-dependent dual effect on the cholinergic responses in both preparations. Thus the application of the toxin at concentrations up to 25 nmol/l produces a reversible block of the response whereas higher doses potentiates it. 3. These results suggest that Condytoxina 2 contains an active compound(s) with the capacity to bind to the nicotinic acetylcholine receptor of excitable cells in both snail and mice neurons. During this action complex allosteric interactions among the binding sites could occur.

Nicotinic acetylcholine receptor subtypes in human frontal cortex: changes in Alzheimer's disease

Molecular genetic and pharmacological studies have suggested that several subtypes of nicotinic acetylcholine receptors exist in the mammalian and avian brain. Combining 3H-(-)-nicotine, 125I-alpha-bungarotoxin, and 125I-kappa-bungarotoxin as ligands, we report here the first evidence for the existence in human frontal cortex of at least three different subtypes of nicotinic receptors. Autoradiographic analysis shows that specific 125I-kappa-bungarotoxin binding sites are concentrated mainly in several cortical layers. We also show that kappa-bungarotoxin, but not alpha-bungarotoxin decreases the evoked release of 3H-acetylcholine in rat cortical slices, indicating a likely presynaptic localization for some of the alpha-bungarotoxin-insensitive kappa-bungarotoxin sites in mammalian brain. The brains of patients with Alzheimer's disease show marked decreases in Bmax values for low-affinity 125I-kappa-bungarotoxin sites and both high- and low-affinity 3H-nicotine sites, whereas 125I-alpha-bungarotoxin sites are not significantly different in number from age-matched control brains. We conclude that Alzheimer's disease does not affect all subtypes of nicotinic receptors in the frontal cortex to the same extent.

Intracellular recording in avian brain of a nicotinic response that is insensitive to K-bungarotoxin

We examined nicotinic acetylcholine receptors in the avian brain using a combination of autoradiographic and intracellular electrophysiological techniques. We found that the lateral spiriform nucleus (SPL) in the mesencephalon has a very high density of 3H-nicotine binding sites but no detectable 125I-K-bungarotoxin (125I-K-BuTx) or 125I-alpha-bungarotoxin (125I-alpha-BuTx) bindings sites. Intracellular recordings in brain slices revealed that SPL neurons depolarize in response to nicotine and carbachol (in the presence of atropine). These depolarizations were blocked by the classic nicotinic antagonists d-tubocurarine and dihydro-beta-erythroidine. As predicted for nicotinic receptors with a high affinity for nicotine, neither K-BuTx nor alpha-BuTx blocked these nicotinic responses. Thus, although the existence of high-affinity 3H-nicotine binding sites has been known for some time, we now report the in situ detection of a functional nicotinic receptor that has a high affinity for nicotine and is K-BuTx-insensitive.

Kappa 2-bungarotoxin and kappa 3-bungarotoxin: two new neuronal nicotinic receptor antagonists isolated from the venom of Bungarus multicinctus

Neuronal nicotinic acetylcholine receptors are recognized with high affinity by two snake venom kappa-neurotoxins, kappa-bungarotoxin and kappa-flavitoxin. Native and radiolabeled kappa-neurotoxins have been used to localize and quantitate neuronal nicotinic receptors in a variety of species. We now report the identification of two new kappa-neurotoxins. kappa 2-Bungarotoxin and kappa 3-bungarotoxin were purified from the venom of Bungarus multicinctus collected in the province of Guangdong, China. kappa-Bungarotoxin has as yet not been found in this venom, although it is the only kappa-neurotoxin to be isolated thus far from Taiwanese Bungarus multicinctus. The geographical separation of Guangdong and Taiwan might account for this evolutionary divergence within the species. Both of the new kappa-neurotoxins are potent antagonists of nicotinic transmission in the chick ciliary ganglion. kappa 3-Bungarotoxin, the least potent of the kappa-neurotoxins, produces a complete blockage of nicotinic transmission in 60 min at 250 nM. Protection experiments using the short-acting nicotinic antagonists dihydro-beta-erythroidine and (+)-tubocurarine demonstrate that kappa 2-bungarotoxin blocks transmission by binding to the acetylcholine recognition sites of neuronal nicotinic receptors. The isoelectric point of kappa 2-bungarotoxin (pI = 8.9) is similar to that of kappa-bungarotoxin and kappa-flavitoxin, but kappa 3-bungarotoxin is considerably more basic, with pI greater than 11. Partial amino acid sequences are reported for both kappa 2-bungarotoxin and kappa 3-bungarotoxin. These sequences show a high degree of homology (approximately 80%) with other kappa-neurotoxins, and allow the determination of the critical differences between the kappa-neurotoxins and the structurally related alpha-neurotoxins. For example, all 4 kappa-neurotoxins lack a tryptophanyl residue which is invariant and important for function in the alpha-neurotoxins. The kappa-neurotoxins also differ from the alpha-neurotoxins by having an invariant prolinyl residue at a critical sequence position. Heterodimers were detected consisting of one subunit each of kappa 2-bungarotoxin and kappa 3-bungarotoxin. These heterodimers, which form between any combination of two kappa-neurotoxins, appear to be physiologically active and confirm that a further distinction between kappa-neurotoxins and alpha-neurotoxins is the strong tendency of the former to self-associate in solution. The present results help to establish the definition of 'kappa-neurotoxin'. These snake toxins are now being used by a number of laboratories in physiological and biochemical experiments on neuronal nicotinic receptors from a variety of species.(ABSTRACT TRUNCATED AT 400 WORDS)

Kappa-neurotoxins: heterodimer formation between different neuronal nicotinic receptor antagonists

The kappa-neurotoxins are a family of snake venom polypeptides that are competitive antagonists of acetylcholine at a variety of neuronal nicotinic receptors. We have previously determined that kappa-bungarotoxin, purified from the venom of Bungarus multicinctus, exists in solution entirely as a dimer of identical subunits. We now report that the three other known kappa-neurotoxins, namely, kappa 2-bungarotoxin and kappa 3-bungarotoxin from Bungarus multicinctus and kappa-flavitoxin from Bungarus flaviceps, also self-aggregate in solution. Furthermore, when two different kappa-neurotoxins are mixed, a heterodimer species spontaneously forms and reaches an equilibrium with the two homodimers after which 40-50% of the protein exists as the heterodimer. A cation-exchange high-pressure liquid chromatography procedure is described which readily separates kappa-neurotoxin heterodimers from the homodimers. Sedimentation equilibria experiments give an Mr = 15,500 +/- 1000 for kappa-flavitoxin and an Mr = 14,500 +/- 700 for a mixture of kappa-bungarotoxin and kappa-flavitoxin. Since the subunit molecular weights of kappa-bungarotoxin and kappa-flavitoxin are respectively 7313 and 7242, self-aggregation of these toxins in solution results in a preponderance of kappa-neurotoxin dimers. The stoichiometry of the heterodimer formed by kappa-bungarotoxin and kappa-flavitoxin is 1:1, as determined by amino acid sequence analysis. After isolation, the kappa-neurotoxin heterodimer partially dissociates and again reaches equilibrium with the homodimers, a process which requires 2-4 h at 23 degrees C. The ability to self-aggregate to form heterodimers and homodimers thus appears to be a common property of the kappa-neurotoxins.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa-bungarotoxin blocks an alpha-bungarotoxin-sensitive nicotinic receptor in the insect central nervous system

Snake venom kappa-neurotoxins are selective antagonists of nicotinic acetylcholine responses in avian, murine and bovine neurons, and have been used as probes for functionally defined vertebrate neuronal nicotinic receptors. The actions of kappa-bungarotoxin, a kappa-neurotoxin, have now been examined at a central invertebrate nicotinic receptor. kappa-Bungarotoxin is a potent antagonist (IC50 = 100 nM) of nicotinic responses, producing a long-lasting blockade of insect nicotinic acetylcholine receptors. The blockade appears to be competitive, and voltage-clamp experiments on an identified cockroach motorneuron indicate that the actions of kappa-bungarotoxin are not dependent on membrane potential. alpha-Bungarotoxin is also a potent antagonist at the cockroach central nicotinic receptor, and binds (Kd = 4.3 nM) to a nicotinic site in cockroach nervous tissue. kappa-Bungarotoxin recognizes this invertebrate nicotinic site with high affinity (Ki = 27 nM). A comparison of the pharmacological properties of insect nicotinic receptors with those of functionally defined receptors identified by kappa-neurotoxins in avian autonomic ganglia reveals several similarities. However, a striking exception is alpha-bungarotoxin, which is the most potent antagonist examined at cockroach nicotinic receptors, but fails to recognize functional autonomic ganglia nicotinic receptors even at very high concentrations. It is concluded that kappa-neurotoxins can be used as selective probes for neuronal nicotinic receptors in both vertebrates and invertebrates. Although invertebrates diverged from vertebrates over 600 million years ago, the results indicate that the neuronal nicotinic receptors found in species as diverse as cockroach and chick retain considerable structural similarity, and thus neuronal nicotinic receptors appear to be highly conserved membrane proteins.

Neural regulation of acetylcholine sensitivity in embryonic sympathetic neurons

The development of transmitter sensitivity is an important component of synaptic differentiation. Despite a wealth of information about the appearance of acetylcholine (AcCho) sensitivity at the neuromuscular junction, the onset and regulation of this critical aspect of synaptogenesis has not previously been examined for synapse formation between neurons. To determine whether there is a role of presynaptic input in the induction of AcCho sensitivity at interneuronal synapses, AcCho-induced currents were measured in embryonic sympathetic neurons before and after synapse formation in vitro. The total AcCho sensitivity of postsynaptic neurons was increased nearly 10-fold after innervation. The effects of innervation are mimicked by medium conditioned by preganglionic neurons, suggesting that presynaptic neurons regulate postsynaptic AcCho sensitivity by release of a soluble factor. These observations provide evidence that presynaptic input regulates neuronal sensitivity to an identified synaptic transmitter.

kappa-Bungarotoxin: binding of a neuronal nicotinic receptor antagonist to chick optic lobe and skeletal muscle

kappa-Bungarotoxin, a snake venom kappa-neurotoxin, is a potent neuronal nicotinic receptor antagonist. kappa-Neurotoxins are structurally related to the long-type alpha-neurotoxins (including alpha-bungarotoxin), which often fail to block neuronal nicotinic transmission, but which are potent antagonists of nicotinic receptors found on vertebrate skeletal muscle. The binding of kappa-bungarotoxin has now been examined in homogenates of chick skeletal muscle and optic lobe. In muscle, kappa-bungarotoxin binds to nicotinic receptors with 200-fold lower affinity than does alpha-bungarotoxin. The weakest known alpha-neurotoxin, L.s. III, is found to be 6.5-fold more potent than kappa-bungarotoxin. These findings support the conclusion that kappa-neurotoxins are selective for neuronal nicotinic receptors. In the optic lobe, 125I-alpha-bungarotoxin and 125I-L.s. III. A second nicotinic site, detected with high affinity by both alpha-neurotoxins, is only weakly bound by kappa-bungarotoxin. No evidence for a unique 125I-kappa-neurotoxin site is observed. Furthermore, kappa-bungarotoxin does not recognize the high affinity L-[3H]nicotine binding site in chick optic lobe which is distinct from the alpha-neurotoxin binding sites. Three subtypes of nicotinic sites can thus be defined in chick optic lobe, although which of these subtypes is involved in nicotinic transmission in the lobe remains to be conclusively determined.

Lophotoxin: selective blockade of nicotinic transmission in autonomic ganglia by a coral neurotoxin

Lophotoxin is a diterpene lactone isolated from gorgonian corals. The toxin has previously been shown to bind with high affinity to an acetylcholine recognition site located on skeletal muscle nicotinic receptors, producing an essentially irreversible blockade of neuromuscular transmission. Lophotoxin has also been shown to block nicotinic transmission in autonomic ganglia of the frog and in ileal strips of guinea pig and rabbit. The effects of lophotoxin have now been examined on neuronal nicotinic receptors in autonomic ganglia of the chick and rat. Low concentrations of lophotoxin (1 microM) produce a blockade of neuronal nicotinic transmission which is partially reversed by 3-5 h of washing out the toxin. The blockade produced by higher concentrations of lophotoxin (up to 32 microM) is not reversed during a similar washout period. Prior exposure to d-tubocurarine, a competitive nicotinic antagonist, can partially protect ganglia against exposure to lophotoxin. In contrast the local anesthetic QX-314, a noncompetitive nicotinic antagonist, does not protect ganglia against lophotoxin exposure. Lophotoxin binds to a site in ganglia identified by [125I]kappa-bungarotoxin which appears to be on the neuronal nicotinic receptor. Intracellular recordings reveal that lophotoxin has no effect on either muscarinic responses or on responses to gamma-aminobutyrate in autonomic ganglia. Passive and active membrane properties of the neurons are unaffected by lophotoxin except for the blockade of nicotinic responses. It is concluded that lophotoxin is a selective, high-affinity antagonist at the neuronal nicotinic receptor. The long-term nature of the blockade with lophotoxin suggests that the toxin will be of considerable value as a probe for characterizing the ganglionic nicotinic receptor.