kappa-Bungarotoxin. Self-association of a neuronal nicotinic receptor probe

Snake three-finger α-neurotoxins and nicotinic acetylcholine receptors: molecules, mechanisms and medicine

Snake venom three-finger α-neurotoxins (α-3FNTx) act on postsynaptic nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ) to produce skeletal muscle paralysis. The discovery of the archetypal α-bungarotoxin (α-BgTx), almost six decades ago, exponentially expanded our knowledge of membrane receptors and ion channels. This included the localisation, isolation and characterization of the first receptor (nAChR); and by extension, the pathophysiology and pharmacology of neuromuscular transmission and associated pathologies such as myasthenia gravis, as well as our understanding of the role of α-3FNTxs in snakebite envenomation leading to novel concepts of targeted treatment. Subsequent studies on a variety of animal venoms have yielded a plethora of novel toxins that have revolutionized molecular biomedicine and advanced drug discovery from bench to bedside. This review provides an overview of nAChRs and their subtypes, classification of α-3FNTxs and the challenges of typifying an increasing arsenal of structurally and functionally unique toxins, and the three-finger protein (3FP) fold in the context of the uPAR/Ly6/CD59/snake toxin superfamily. The pharmacology of snake α-3FNTxs including their mechanisms of neuromuscular blockade, variations in reversibility of nAChR interactions, specificity for nAChR subtypes or for distinct ligand-binding interfaces within a subtype and the role of α-3FNTxs in neurotoxic envenomation are also detailed. Lastly, a reconciliation of structure-function relationships between α-3FNTx and nAChRs, derived from historical mutational and biochemical studies and emerging atomic level structures of nAChR models in complex with α-3FNTxs is discussed.

Proteomic characterization and comparison of venoms from two elapid snakes (Bungarus multicinctus and Naja atra) from China

Bungarus multicinctus (many-banded krait) and Naja atra (Chinese cobra) are widely distributed and medically important venomous snakes in China; however, their venom proteomic profiles have not been fully compared. Here, we fractionated crude venoms and analyzed them using a combination of proteomic techniques. Three-finger toxins (3-FTx) and phospholipase A2 (PLA2) were most abundant in both species, respectively accounting for 32.6% and 66.4% of total B. multicinctus venom, and 84.3% and 12.2% of total N. atra venom. Venoms from these two species contained one common protein family and six less abundant species-specific protein families. The proteomic profiles of B. multicinctus and N. atra venoms and analysis of toxicological activity in mice suggested that 3-FTx and PLA2 are the major contributors to clinical symptoms caused by envenomation. The venoms differed in enzymatic activity, likely the result of inter-specific variation in the amount of related venom components. Antivenomics assessment revealed that a small number of venom components (3-FTxs and PLA2s in B. multicinctus, and 3-FTxs in N. atra) could not be immunocaptured completely, suggesting that we should pay attention to enhancing the immune response of these components in designing commercial antivenoms for B. multicinctus and N. atra.

BIOLOGICAL SIGNIFICANCE

The proteomic profiles of venoms from two medically important snake species - B. multicinctus and N. atra - have been explored. Quantitative and qualitative differences are evident in both venoms when proteomic profiles and transcriptomic results are compared; this is a reminder that combined approaches are needed to explore the precise composition of snake venom. Two protein families (3-FTx and PLA2) of high abundance in these snake venoms are major players in the biochemical and pharmacological effects of envenomation. Elucidation of the proteomic profiles of these snake venoms is helpful in understanding composition-function relationships and will facilitate the clinical application of antivenoms.

Isolation and structural and pharmacological characterization of α-elapitoxin-Dpp2d, an amidated three finger toxin from black mamba venom

We isolated a novel, atypical long-chain three-finger toxin (TFT), α-elapitoxin-Dpp2d (α-EPTX-Dpp2d), from black mamba (Dendroaspis polylepis polylepis) venom. Proteolytic digestion with trypsin and V8 protease, together with MS/MS de novo sequencing, indicated that the mature toxin has an amidated C-terminal arginine, a posttranslational modification rarely observed for snake TFTs. α-EPTX-Dpp2d was found to potently inhibit α7 neuronal nicotinic acetylcholine receptors (nAChR; IC₅₀, 58 ± 24 nM) and muscle-type nAChR (IC₅₀, 114 ± 37 nM) but did not affect α3β2 and α3β4 nAChR isoforms at 1 μM concentrations. Competitive radioligand binding assays demonstrated that α-EPTX-Dpp2d competes with epibatidine binding to the Lymnea stagnalis acetylcholine-binding protein (Ls-AChBP; IC₅₀, 4.9 ± 2.3 nM). The activity profile and binding data are reminiscent of classical long-chain TFTs with a free carboxyl termini, suggesting that amidation does not significantly affect toxin selectivity. The crystal structure of α-EPTX-Dpp2d was determined at 1.7 Å resolution and displayed a dimeric toxin assembly with each monomer positioned in an antiparallel orientation. The dimeric structure is stabilized by extensive intermolecular hydrogen bonds and electrostatic interactions, which raised the possibility that the toxin may exist as a noncovalent homodimer in solution. However, chemical cross-linking and size-exclusion chromatography coupled with multiangle laser light scattering (MALLS) data indicated that the toxin is predominantly monomeric under physiological conditions. Because of its high potency and selectivity, we expect this toxin to be a valuable pharmacological tool for studying the structure and function of nAChRs.

Directed evolution of three-finger toxin to produce serine protease inhibitors

Directed evolution is a very popular strategy for improving biophysical properties and even for generating proteins with novel functions. Recent advances in combinatorial protein engineering mean it is now possible to develop protein scaffolds that could substitute for whole antibody-associated properties as emerging therapeutic proteins. In particular, disulfide-rich proteins are attractive templates for directed evolution in the search for novel molecules because they can regulate the activities of receptors, enzymes, and other molecules. Previously, we demonstrated that functional regulatory molecules against interleukin-6 receptor (IL-6R) could be obtained by directed evolution of the three-finger toxin (3F) scaffold. In the present study, trypsin was selected as a target for directed evolution to further explore the potential use of the 3F cDNA display library. After seven rounds of selection, the DNA sequences converged. The recombinant proteins produced by the selected candidates had inhibitory activity against trypsin (Ki of 33-450 nM). Three of the six groups had Ki values that were comparable to bovine pancreatic trypsin inhibitor and soybean trypsin inhibitor. Two of the candidates also had inhibitory effects against chymotrypsin and kallikrein. This study suggests that 3F protein is suitable for the preparation of high-diversity libraries that can be utilized to obtain protease inhibitors. In addition to our previous successful targeting of IL-6R, the technique developed in our studies may have wide applications in the generation of regulatory molecules for targets of interest, such as receptors, enzymes for research, diagnostic applications, and therapeutic uses.

Inter-residue coupling contributes to high-affinity subtype-selective binding of α-bungarotoxin to nicotinic receptors

The crystal structure of a pentameric α7 ligand-binding domain chimaera with bound α-btx (α-bungarotoxin) showed that of the five conserved aromatic residues in α7, only Tyr¹⁸⁴ in loop C of the ligand-binding site was required for high-affinity binding. To determine whether the contribution of Tyr¹⁸⁴ depends on local residues, we generated mutations in an α7/5HT(3A) (5-hydroxytryptamine type 3A) receptor chimaera, individually and in pairs, and measured ¹²⁵I-labelled α-btx binding. The results show that mutations of individual residues near Tyr¹⁸⁴ do not affect α-btx affinity, but pairwise mutations decrease affinity in an energetically coupled manner. Kinetic measurements show that the affinity decreases arise through increases in the α-btx dissociation rate with little change in the association rate. Replacing loop C in α7 with loop C from the α-btx-insensitive α2 or α3 subunits abolishes high-affinity α-btx binding, but preserves acetylcholine-elicited single channel currents. However, in both the α2 and α3 construct, mutating either residue that flanks Tyr¹⁸⁴ to its α7 counterpart restores high-affinity α-btx binding. Analogously, in α7, mutating both residues that flank Tyr¹⁸⁴ to the α2 or α3 counterparts abolishes high-affinity α-btx binding. Thus interaction between Tyr¹⁸⁴ and local residues contributes to high-affinity subtype-selective α-btx binding.

Neurotoxicity in snakebite--the limits of our knowledge

Snakebite is classified by the WHO as a neglected tropical disease. Envenoming is a significant public health problem in tropical and subtropical regions. Neurotoxicity is a key feature of some envenomings, and there are many unanswered questions regarding this manifestation. Acute neuromuscular weakness with respiratory involvement is the most clinically important neurotoxic effect. Data is limited on the many other acute neurotoxic manifestations, and especially delayed neurotoxicity. Symptom evolution and recovery, patterns of weakness, respiratory involvement, and response to antivenom and acetyl cholinesterase inhibitors are variable, and seem to depend on the snake species, type of neurotoxicity, and geographical variations. Recent data have challenged the traditional concepts of neurotoxicity in snake envenoming, and highlight the rich diversity of snake neurotoxins. A uniform system of classification of the pattern of neuromuscular weakness and models for predicting type of toxicity and development of respiratory weakness are still lacking, and would greatly aid clinical decision making and future research. This review attempts to update the reader on the current state of knowledge regarding this important issue.

Structural and functional characterization of a novel homodimeric three-finger neurotoxin from the venom of Ophiophagus hannah (king cobra)

Snake venoms are a mixture of pharmacologically active proteins and polypeptides that have led to the development of molecular probes and therapeutic agents. Here, we describe the structural and functional characterization of a novel neurotoxin, haditoxin, from the venom of Ophiophagus hannah (King cobra). Haditoxin exhibited novel pharmacology with antagonism toward muscle (alphabetagammadelta) and neuronal (alpha(7), alpha(3)beta(2), and alpha(4)beta(2)) nicotinic acetylcholine receptors (nAChRs) with highest affinity for alpha(7)-nAChRs. The high resolution (1.5 A) crystal structure revealed haditoxin to be a homodimer, like kappa-neurotoxins, which target neuronal alpha(3)beta(2)- and alpha(4)beta(2)-nAChRs. Interestingly however, the monomeric subunits of haditoxin were composed of a three-finger protein fold typical of curaremimetic short-chain alpha-neurotoxins. Biochemical studies confirmed that it existed as a non-covalent dimer species in solution. Its structural similarity to short-chain alpha-neurotoxins and kappa-neurotoxins notwithstanding, haditoxin exhibited unique blockade of alpha(7)-nAChRs (IC(50) 180 nm), which is recognized by neither short-chain alpha-neurotoxins nor kappa-neurotoxins. This is the first report of a dimeric short-chain alpha-neurotoxin interacting with neuronal alpha(7)-nAChRs as well as the first homodimeric three-finger toxin to interact with muscle nAChRs.

Protein complexes in snake venom

Snake venom contains mixture of bioactive proteins and polypeptides. Most of these proteins and polypeptides exist as monomers, but some of them form complexes in the venom. These complexes exhibit much higher levels of pharmacological activity compared to individual components and play an important role in pathophysiological effects during envenomation. They are formed through covalent and/or non-covalent interactions. The subunits of the complexes are either identical (homodimers) or dissimilar (heterodimers; in some cases subunits belong to different families of proteins). The formation of complexes, at times, eliminates the non-specific binding and enhances the binding to the target molecule. On several occasions, it also leads to recognition of new targets as protein-protein interaction in complexes exposes the critical amino acid residues buried in the monomers. Here, we describe the structure and function of various protein complexes of snake venoms and their role in snake venom toxicity.

Naturally occurring disulfide-bound dimers of three-fingered toxins: a paradigm for biological activity diversification

Disulfide-bound dimers of three-fingered toxins have been discovered in the Naja kaouthia cobra venom; that is, the homodimer of alpha-cobratoxin (a long-chain alpha-neurotoxin) and heterodimers formed by alpha-cobratoxin with different cytotoxins. According to circular dichroism measurements, toxins in dimers retain in general their three-fingered folding. The functionally important disulfide 26-30 in polypeptide loop II of alpha-cobratoxin moiety remains intact in both types of dimers. Biological activity studies showed that cytotoxins within dimers completely lose their cytotoxicity. However, the dimers retain most of the alpha-cobratoxin capacity to compete with alpha-bungarotoxin for binding to Torpedo and alpha7 nicotinic acetylcholine receptors (nAChRs) as well as to Lymnea stagnalis acetylcholine-binding protein. Electrophysiological experiments on neuronal nAChRs expressed in Xenopus oocytes have shown that alpha-cobratoxin dimer not only interacts with alpha7 nAChR but, in contrast to alpha-cobratoxin monomer, also blocks alpha3beta2 nAChR. In the latter activity it resembles kappa-bungarotoxin, a dimer with no disulfides between monomers. These results demonstrate that dimerization is essential for the interaction of three-fingered neurotoxins with heteromeric alpha3beta2 nAChRs.

Urokinase regulates vitronectin binding by controlling urokinase receptor oligomerization

Adhesion of monocytes to the extracellular matrix is mediated by a direct high affinity interaction between cell-surface urokinase-type plasminogen activator (uPA) receptor (uPAR) and the extracellular matrix protein vitronectin. We demonstrate a tight connection between uPA-regulated uPAR oligomerization and high affinity binding to immobilized vitronectin. We find that binding of soluble uPAR (suPAR) to immobilized vitronectin is strictly ligand-dependent with a linear relationship between the observed binding and the concentration of ligand added. Nevertheless, a comparison of experimentally obtained binding curves to those generated using a simple equilibrium model suggests that the high affinity vitronectin-binding pro-uPA.suPAR complex contains two molecules of suPAR. In co-immunoprecipitation experiments, using different epitope-tagged suPAR molecules, suPAR/suPAR co-immunoprecipitation displayed a similar uPA dose dependence as that observed for vitronectin binding, demonstrating that the high affinity vitronectin-binding complex indeed contains oligomeric suPAR. Structurally, the kringle domain of uPA was found to be critical for the formation of the vitronectin-binding competent complex because the amino-terminal fragment, but not the growth factor-like domain, behaved as a full-length uPA. Our data represent the first demonstration of functional, ligand-induced uPAR oligomerization having extensive implications for glycosylphosphatidylinositol-anchored receptors in general, and for the biology of the uPA/uPAR system in particular.

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.

Critical interactions at the dimer interface of kappa-bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist

The alpha- and kappa-neurotoxins are polypeptide antagonists of nicotinic acetylcholine receptors derived from snake venom. They are structurally very similar but differ in their specificity for receptor subtype and in their native aggregation state. While the alpha-neurotoxins are monomeric, the kappa-neurotoxins occur as homodimers. The crystal structure shows that there is a correlation in the distance between essential arginine residues in the kappa-bungarotoxin dimer and the distance between the acetylcholine binding sites in the pentameric receptor. This has lead to an investigation of the critical interactions at the dimer interface of kappa-bungarotoxin. Mutations of residues that the crystal structure indicates participate in dimer interaction were found to fall into two general groups: those that do not affect the dimerization state or activity of kappa-bungarotoxin as single mutants, and those that interfere with it to such an extent that the protein is no longer able to fold properly. In general, those residues that fall into the latter group are found to be invariant in kappa-neurotoxins and not found in alpha-neurotoxins. The results suggest that the extent of both the main chain-main chain beta-sheet hydrogen bond interaction and van der Waals interactions between Phe 49 and Ile 20 are required for dimer formation. These studies provide a basis for understanding why the kappa-neurotoxins readily dimerize in solution and the alpha-neurotoxins do not and also suggest that there is a possible interrelationship between dimer formation and protein folding in kappa-bungarotoxin.

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.

Facile production of native-like kappa-bungarotoxin in yeast: an enhanced system for the production of a neuronal nicotinic acetylcholine receptor probe

Research on the mammalian central nervous system had been hindered by the limited number and meager supply of naturally occurring toxins that can be used as pharmacological reagents. The kappa-neurotoxins in particular are not found abundantly in nature and are difficult to obtain and isolate in quantities sufficient for research purposes. Here we report the expression and isolation of relatively large quantities of the kappa-neurotoxin, kappa-bungarotoxin, in an active form using a yeast, Pichia pastoris, expression system. The resultant product of the expression system has a short amino-terminal amino acid extension relative to venom-derived kappa-bungarotoxin, but is equivalent to the native toxin in physical and biological properties, as judged by the CD spectra, the ability to form dimers in solution, and the activity on chick ciliary ganglia. The yeast system produces approximately 0.2 mg from a 2 liter culture and the purification takes approximately 2 days. In contrast, E. coli, the only other available expression system for this toxin, produces one-fifth to one-half as much active material from a 5 liter high-density fermentation and the resulting protein takes over a week to purify. No high mol. wt disulfide-bonded aggregates were found in the yeast expression system product, indicating that the product is that of a biologically assisted folding process. This has significant implications not only for the efficient production of native toxin but also for the production of mutant proteins to study the structure-function relationship in these proteins.

alpha 4 beta 2 subunit combination specific pharmacology of neuronal nicotinic acetylcholine receptors in N1E-115 neuroblastoma cells

Pharmacological characteristics of native neuronal nicotinic acetylcholine receptor-mediated ion currents in mouse N1E-115 neuroblastoma cells have been investigated by superfusion of voltage clamped cells with known concentrations of the agonists acetylcholine, nicotine and cytisine, and the antagonists alpha-bungarotoxin and neuronal bungarotoxin. The sensitivity of the nicotinic acetylcholine receptor for agonists followed the agonist potency rank-order: nicotine approximately acetylcholine >> cytisine. The EC50 values of acetylcholine and nicotine are 78 microM and 76 microM, respectively. Equal concentrations of acetylcholine and nicotine induce inward currents with approximately the same peak amplitude whereas cytisine induces much smaller inward currents. Acetylcholine-induced currents are unaffected by high concentrations of alpha-bungarotoxin. Conversely, at 10 and 90 nM neuronal bungarotoxin reduces the amplitude of the 1 mM acetylcholine-induced inward current to 47% and 11% of control values, respectively. Both the agonist potency rank-order and the differential sensitivity to snake toxins of nicotinic receptors in N1E-115 cells are consistent with the known pharmacological profile of alpha 4 beta 2 nicotinic receptors expressed in Xenopus oocytes and distinct from those of all other nicotinic acetylcholine receptors of known functional subunit compositions. All data indicate that the native nicotinic acetylcholine receptor in N1E-115 cells is an assembly of alpha 4 and beta 2 subunits, the putative major subtype of nicotinic acetylcholine receptor in the brain.

Evidence for a fast-exchange conformational process in alpha-bungarotoxin

Anomalous behavior of the post-synaptic protein neurotoxin, alpha-bungarotoxin (alpha-bgt), has been observed during reverse-phase HPLC. Purified samples of this toxin from two distinct sources elute from reverse-phase columns as two separate peaks. The protein species represented by these two peaks are in rapid equilibrium, the relative ratio of which displays a pH dependency with a pKa of approximately 3. This equilibrium does not involve the dimerization or aggregation of the toxin and appears to be relatively unique to alpha-bungarotoxin in that similar behavior is not displayed by several other available alpha-neurotoxins. pH-dependent conformational changes have been documented for several alpha-neurotoxins whose crystal structures have been determined (alpha-bungarotoxin, alpha-cobratoxin, and erabutoxin b). One or more of these may account for the observed behavior of alpha-bungarotoxin on reverse-phase HPLC.

Crystallization of kappa-bungarotoxin: preliminary X-ray data obtained from the venom-derived protein

kappa-Bungarotoxin is a 66 residue polypeptide found in the venom of the Taiwanese banded krait, Bungarus multicinctus. It binds tightly to neuronal nicotinic acetylcholine receptors and inhibits nerve transmission mediated by these postsynaptic receptors. It is related, by similarity in amino acid sequence, to alpha-bungarotoxin and other alpha-neurotoxins, but differs sharply in physiologic action. The alpha-neurotoxins inhibit nerve transmission in nicotinic acetylcholine receptors associated with vertebrate skeletal muscle and fish electric organs. The kappa-neurotoxins inhibit nerve transmission in neuronal nicotinic acetylcholine receptors such as those found in chick ciliary ganglia. The kappa-neurotoxins display a low level of interaction with receptors that are strongly affected by alpha-neurotoxins, but alpha-neurotoxins are completely without effect on receptors that are affected by kappa-bungarotoxin. The structural basis for this physiologic differentiation is not known. Crystals of kappa-bungarotoxin have now been obtained that diffract to at least 2.3 A. These crystals are hexagonal, space group P6, and have dimensions of a = b = 80.2 A, c = 39.6 A, and angles of alpha = beta = 90 degrees and gamma = 120 degrees. Each unit cell contains 12 molecules of the 66 residue protein or two molecules per asymmetric unit. Comparison of the structure of kappa-bungarotoxin, which will result from further diffraction analysis of these crystals, with the structures of the alpha-neurotoxins that have been determined may provide information on the structural basis of physiologic action in these acetylcholine receptor antagonists.

The interaction of kappa-bungarotoxin with the nicotinic receptor of bovine chromaffin cells

Whole-cell recording techniques were used to examine acetylcholine-induced nicotinic currents in isolated bovine chromaffin cells. The effects on these currents of kappa-bungarotoxin, a snake venom kappa-neurotoxin, were tested. Exposure of cells to kappa-bungarotoxin (600 nM for 40 min) produced a prolonged blockade of nicotinic currents. The mechanism of this blockade was examined in several ways. Firstly, the pre-exposure of cells to trimetaphan, a competitive nicotinic antagonist, protected against the action of subsequent additions of kappa-bungarotoxin. Secondly, voltage-clamp measurements indicated that the degree of blockade produced by kappa-bungarotoxin was independent of cell membrane potential. Unlike (+)-tubocurarine, kappa-bungarotoxin had no direct agonist effects on nicotinic receptors. It is concluded from the present functional studies and from previously reported binding studies that kappa-bungarotoxin blocks nicotinic responses in bovine chromaffin cells by binding to regions overlying acetylcholine sites on nicotinic receptors.

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: 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.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Long-term blockade by toxin F of nicotinic synaptic potentials in cultured sympathetic neurons

The effects of a recently identified blocker of neuronal nicotinic transmission, toxin F, were studied in cultured sympathetic neurons. Single principal neurons, dissociated from superior cervical ganglia of newborn rats, were grown on cardiac myocytes in microculture. The toxin blocked nicotinic synaptic potentials in these cultures but had no effect on muscarinic interactions. When toxin F was applied by addition to the perfusion medium, the concentration required for blocking most of the nicotinic potential was 40 nM, and the recovery from blockade was slow (t1/2 = 95 +/- 64 min). When the toxin was briefly applied by pressure ejection from a pipette, the concentration in the pipette necessary for blockade was 21 microM, and 30-60% of the response recovered within a few minutes while the remainder recovered slowly (t1/2 of the remainder = 105 +/- 82 min). One possible explanation for the difference in recovery time is that toxin F binds initially with low affinity to the nicotinic receptor but with time the toxin receptor complex converts to a high affinity state. The presence of dihydro-beta-erythroidine during perfusion of toxin F prevented the long-lasting blockade by the toxin, suggesting that toxin F and dihydro-beta-erythroidine act through a common binding site. The specificity, potency, and slow reversibility of the effects of toxin F make it useful as a probe for studying neuronal nicotinic receptors of cultured sympathetic neurons.

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

A peptide, termed kappa-flavitoxin (kappa-flavitoxin), has been purified from the venom of the red-headed krait, Bungarus flaviceps, by low- and high-pressure liquid chromatography. kappa-Flavitoxin has a pI of 8.8 and an apparent molecular weight on sodium dodecyl-sulfate-polyacrylamide gel electrophoresis of 6500 Da. kappa-Flavitoxin is a potent inhibitor of nicotinic transmission in autonomic ganglia, producing a complete and long-lasting blockade at doses as low as 50 nM. Intracellular recordings reveal a selective blockade of neuronal nicotinic receptors by the toxin, with no effects on other active or passive properties of neuronal membranes. kappa-Flavitoxin shares a number of pharmacological and biochemical properties with kappa-bungarotoxin, purified from the venom of Bungarus multicinctus. The two peptides exhibit considerable homology in their amino acid sequences. Radiolabeled kappa-flavitoxin binds to a nicotinic site in ciliary ganglia previously identified by kappa-bungarotoxin, which appears to be associated with the neuronal nicotinic receptor. This site is not recognized by alpha-bungarotoxin, which also does not block nicotinic transmission in this ganglion.