Fine-tuning the standard setting of objective structured practical examinations in clinical anatomy

The objective structured practical examination (OSPE) is a reliable assessment of practical skills in anatomy teaching. It is often administered as low-stake assessments to track progress at multiple time points in anatomy curricula. Standard-setting OSPEs to derive a pass mark and to ensure assessment quality and rigor is a complex task. This study compared standard-setting outcomes of clinical anatomy OSPEs determined by traditional criterion-referenced (Ebel) and norm-referenced ("mean minus standard deviation") methods in comparison to hybrid methods which apply both criterion-referenced and norm-referenced approaches in setting examination standards. The hybrid approaches utilized included the "Cohen method" and an adaptation of the "Taylor's method," which is an improvement on the Cohen method. These diverse standard-setting methods were applied retrospectively to 16 anatomy OSPEs conducted over 4 years for first- and second-year medical students in a graduate Doctor of Medicine Program at Griffith Medical School, Australia; and the pass marks, failure rates, and variances of failure rates were compared. The application of the adaptation of Taylor's method to standard set OSPEs produced pass marks and failure rates comparable to the Ebel method, whereas the variability of failure rates was higher with the Ebel method than with the Cohen and Taylor's methods. This underscores this study's adaptation of Taylor's method as a suitable alternative to the widely accepted but resource intensive, panel-based criterion-referenced standard-setting methods such as the Ebel method, where panelists with relevant expertise are unavailable, particularly for the multiple low-stakes OSPEs in an anatomy curriculum.

Artificial intelligence software in pulmonary nodule assessment

Background:

This study tests the impact of the addition of autonomous computed tomography (CT) interpreting software to radiologist assessment of pulmonary nodules.

Methods:

Computed tomography scans for nodule assessment were identified retrospectively. Lung cancer risk factors, initial radiologist (RAD) report, Philips Lung Nodule software report (computer-aided nodule (CAD)) and radiologist report following the review of CT images and CAD (RAD + CAD) were collected. Follow-up recommendations based on current guidelines were derived from each report.

Results:

In all, 100 patients were studied. Median maximal diameter of the largest nodule reported by RAD and RAD + CAD were similar at 10.0 and 9.0 mm, respectively ( p = 0.06) but were reported as larger by CAD at 11.8 mm ( p < 0.001). Follow-up recommendations derived from RAD + CAD were less intensive in 23 (23%) and more intensive in 34 (34%) than that of RAD.

Discussion:

This study suggests that autonomous software use can alter radiologist assessment of pulmonary nodules such that suggested follow-up is altered.

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.

Fulditoxin, representing a new class of dimeric snake toxins, defines novel pharmacology at nicotinic ACh receptors

BACKGROUND AND PURPOSE

Animal toxins have contributed significantly to our understanding of the neurobiology of receptors and ion channels. We studied the venom of the coral snake Micrurus fulvius fulvius and identified and characterized the structure and pharmacology of a new homodimeric neurotoxin, fulditoxin, that exhibited novel pharmacology at nicotinic ACh receptors (nAChRs).

EXPERIMENTAL APPROACH

Fulditoxin was isolated by chromatography, chemically synthesized, its structure determined by X-ray crystallography, and its pharmacological actions on nAChRs characterized by organ bath assays and two-electrode voltage clamp electrophysiology.

KEY RESULTS

Fulditoxin's distinct 1.95-Å quaternary structure revealed two short-chain three-finger α-neurotoxins (α-3FNTxs) non-covalently bound by hydrophobic interactions and an ability to bind metal and form tetrameric complexes, not reported previously for three-finger proteins. Although fulditoxin lacked all conserved amino acids canonically important for inhibiting nAChRs, it produced postsynaptic neuromuscular blockade of chick muscle at nanomolar concentrations, comparable to the prototypical α-bungarotoxin. This neuromuscular blockade was completely reversible, which is unusual for snake α-3FNTxs. Fulditoxin, therefore, interacts with nAChRs by utilizing a different pharmacophore. Unlike short-chain α-3FNTxs that bind only to muscle nAChRs, fulditoxin utilizes dimerization to expand its pharmacological targets to include human neuronal α4β2, α7, and α3β2 nAChRs which it blocked with IC₅₀ values of 1.8, 7, and 12 μM respectively.

CONCLUSIONS AND IMPLICATIONS

Based on its distinct quaternary structure and unusual pharmacology, we named this new class of dimeric Micrurus neurotoxins represented by fulditoxin as Σ-neurotoxins, which offers greater insight into understanding the interactions between nAChRs and peptide antagonists.

Role of individual disulfide bridges in the conformation and activity of spinoxin (α-KTx6.13), a potassium channel toxin from Heterometrus spinifer scorpion venom

Spinoxin (SPX; α-KTx6.13), isolated from venom of the scorpion Heterometrus spinifer, is a K⁺ channel-specific peptide toxin (KTx), which adopts a cysteine-stabilized α/β scaffold that is cross-linked by four disulfide bridges (Cys1-Cys5, Cys2-Cys6, Cys3-Cys7, and Cys4-Cys8). To investigate the role of the individual disulfide bonds in the structure-activity relationship of SPX, we synthesized four SPX analogs in which each pair of cysteine residues was replaced by alanine residues. The analysis of circular dichroism spectra and inhibitory activity against Kv1.3 channels showed that the SPX analogs lacking any of three specific disulfide bonds (Cys1-Cys5, Cys2-Cys6, and Cys3-Cys7) were unable to form the native secondary structure and completely lost inhibitory activities. Thus, we conclude that Cys1-Cys5, Cys2-Cys6, and Cys3-Cys7 are required for the inhibition of the Kv1.3 channel by SPX. In contrast, the analog lacking Cys4-Cys8 retained both native secondary structure and inhibitory activity. Interestingly, one of the isomers of the analog lacking Cys1-Cys5 also showed inhibitory activities, although its inhibition was ∼18-fold weaker than native SPX. This isomer had an atypical disulfide bond pairing (Cys3-Cys4 and Cys7-Cys8) that corresponds to that of maurotoxin (MTX), another α-KTx6 family member. These results indicate that the Cys1-Cys5 and Cys2-Cys6 bonds are important for restricting the toxin from forming an atypical (MTX-type) disulfide bond pairing among the remaining four cysteine residues (Cys3, Cys4, Cys7, and Cys8) in native SPX.

Active Sites of Spinoxin, a Potassium Channel Scorpion Toxin, Elucidated by Systematic Alanine Scanning

Peptide toxins from scorpion venoms constitute the largest group of toxins that target the voltage-gated potassium channel (Kv). Spinoxin (SPX) isolated from the venom of scorpion Heterometrus spinifer is a 34-residue peptide neurotoxin cross-linked by four disulfide bridges. SPX is a potent inhibitor of Kv1.3 potassium channels (IC50 = 63 nM), which are considered to be valid molecular targets in the diagnostics and therapy of various autoimmune disorders and cancers. Here we synthesized 25 analogues of SPX and analyzed the role of each amino acid in SPX using alanine scanning to study its structure-function relationships. All synthetic analogues showed similar disulfide bond pairings and secondary structures as native SPX. Alanine replacements at Lys(23), Asn(26), and Lys(30) resulted in loss of activity against Kv1.3 potassium channels, whereas replacements at Arg(7), Met(14), Lys(27), and Tyr(32) also largely reduced inhibitory activity. These results suggest that the side chains of these amino acids in SPX play an important role in its interaction with Kv1.3 channels. In particular, Lys(23) appears to be a key residue that underpins Kv1.3 channel inhibition. Of these seven amino acid residues, four are basic amino acids, suggesting that the positive electrostatic potential on the surface of SPX is likely required for high affinity interaction with Kv1.3 channels. This study provides insight into the structure-function relationships of SPX with implications for the rational design of new lead compounds targeting potassium channels with high potency.

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.

Irditoxin, a novel covalently linked heterodimeric three-finger toxin with high taxon-specific neurotoxicity

A novel heterodimeric three-finger neurotoxin, irditoxin, was isolated from venom of the brown treesnake Boiga irregularis (Colubridae). Irditoxin subunit amino acid sequences were determined by Edman degradation and cDNA sequencing. The crystal structure revealed two subunits with a three-finger protein fold, typical for "nonconventional" toxins such as denmotoxin, bucandin, and candoxin. This is the first colubrid three-finger toxin dimer, covalently connected via an interchain disulfide bond. Irditoxin showed taxon-specific lethality toward birds and lizards and was nontoxic toward mice. It produced a potent neuromuscular blockade at the avian neuromuscular junction (IC(50)=10 nM), comparable to alpha-bungarotoxin, but was three orders of magnitude less effective at the mammalian neuromuscular junction. Covalently linked heterodimeric three-finger toxins found in colubrid venoms constitute a new class of venom peptides, which may be a useful source of new neurobiology probes and therapeutic leads.

Identification of binding sites in the nicotinic acetylcholine receptor for TDBzl-etomidate, a photoreactive positive allosteric effector

Etomidate, one of the most potent general anesthetics used clinically, acts at micromolar concentrations as an anesthetic and positive allosteric modulator of gamma-aminobutyric acid responses, whereas it inhibits muscle-type nicotinic acetylcholine receptors (nAChRs) at concentrations above 10 microm. We report here that TDBzl-etomidate, a photoreactive etomidate analog, acts as a positive allosteric nAChR modulator rather than an inhibitor, and we identify its binding sites by photoaffinity labeling. TDBzl-etomidate (>10 microm) increased the submaximal response to acetylcholine (10 microm) with a 2.5-fold increase at 60 microm. At higher concentrations, it inhibited the binding of the noncompetitive antagonists [(3)H]tetracaine and [(3)H]phencyclidine to Torpedo nAChR-rich membranes (IC(50) values of 0. 8 mm). nAChR-rich membranes were photolabeled with [(3)H]TDBzl-etomidate, and labeled amino acids were identified by Edman degradation. For nAChRs photolabeled in the absence of agonist (resting state), there was tetracaine-inhibitable photolabeling of amino acids in the ion channel at positions M2-9 (deltaLeu-265) and M2-13 (alphaVal-255 and deltaVal-269), whereas labeling of alphaM2-10 (alphaSer-252) was not inhibited by tetracaine but was enhanced 10-fold by proadifen or phencyclidine. In addition, there was labeling in gammaM3 (gammaMet-299), a residue that contributes to the same pocket in the nAChR structure as alphaM2-10. The pharmacological specificity of labeling of residues, together with their locations in the nAChR structure, indicate that TDBzl-etomidate binds at two distinct sites: one within the lumen of the ion channel (labeling of M2-9 and -13), an inhibitory site, and another at the interface between the alpha and gamma subunits (labeling of alphaM2-10 and gammaMet-299) likely to be a site for positive allosteric modulation.

[3H]Benzophenone photolabeling identifies state-dependent changes in nicotinic acetylcholine receptor structure

Interactions of benzophenone (BP) with the Torpedo nicotinic acetylcholine receptor (nAChR) were characterized by electrophysiological analyses, radioligand binding assays, and photolabeling of nAChR-rich membranes with [3H]BP to identify the amino acids contributing to its binding sites. BP acted as a low potency noncompetitive antagonist, reversibly inhibiting the ACh responses of nAChRs expressed in Xenopus oocytes (IC50 = 600 microM) and the binding of the noncompetitive antagonist [3H]tetracaine to nAChR-rich membranes (IC50 = 150 microM). UV irradiation at 365 nm resulted in covalent incorporation of [3H]BP into the nAChR subunits (delta > alpha approximately beta > gamma), with photoincorporation limited to the nAChR transmembrane domain. Comparison of nAChR photolabeling in the closed state (absence of agonist) and desensitized state (equilibrated with agonist) revealed selective desensitized state labeling in the delta subunit of deltaPhe-232 in deltaM1 and deltaPro-286/deltaIle-288 near the beginning of deltaM3 that are within a pocket at the interface between the transmembrane and extracellular domains. There was labeling in the closed state within the ion channel at position M2-13 (alphaVal-255, betaVal-261, and deltaVal-269) that was reduced by 90% upon desensitization and labeling in the transmembrane M3 helices of the beta and gamma subunits (betaMet-285, betaMet-288, and gammaMet-291) that was reduced by 50-80% in the desensitized state. Labeling at the lipid interface (alphaMet-415 in alphaM4) was unaffected by agonist. These results provide a further definition of the regions in the nAChR transmembrane domain that differ in structure between the closed and desensitized states.

Synthesis of trifluoromethylaryl diazirine and benzophenone derivatives of etomidate that are potent general anesthetics and effective photolabels for probing sites on ligand-gated ion channels

To locate the binding sites of general anesthetics on ligand-gated ion channels, two derivatives of the intravenous general anesthetic etomidate (2-ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate), in which the 2-ethyl group has been replaced by photoactivable groups based on either aryl diazirine or benzophenone chemistry, have been synthesized and characterized pharmacologically. TDBzl-etomidate (4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate) and BzBzl-etomidate (4-benzoylbenzyl-1-(1-phenylethyl)-1H-imidazole-5-carboxylate are both potent general anesthetics with half-effective anesthetic concentrations of 700 and 220 nM, respectively. Both agents resembled etomidate in enhancing currents elicited by low concentrations of GABA on heterologously expressed GABAA receptors and in shifting the GABA concentration-response curve to lower concentrations. They also allosterically enhanced the binding of flunitrazepam to mammalian brain GABAA receptors. Both agents were also effective and selective photolabels, photoincorporating into some, but not all, subunits of the Torpedo nicotinic acetylcholine receptor to a degree that was allosterically regulated by an agonist or a noncompetitive inhibitor. Thus, they have the necessary pharmacological and photochemical properties to be useful in identifying the site of etomidate-induced anesthesia.

Site specificity of agonist-induced opening and desensitization of the Torpedo californica nicotinic acetylcholine receptor

Agonist-binding kinetics to the nicotinic acetylcholine receptor (AChR) from Torpedo californica were measured using sequential-mixing stopped-flow fluorescence methods to determine the contribution of each individual site to agonist-induced opening and desensitization. Timed dansyl-C6-choline (DC6C) binding followed by its dissociation upon mixing with high, competing agonist concentrations revealed four kinetic components: an initial, fast fluorescence decay, followed by a transient increase, and then two characteristic decays that reflect dissociation from the desensitized agonist sites. The transient increase resulted from DC6C binding to the open-channel based on its prevention by proadifen, a noncompetitive antagonist. Further characterization of DC6C channel binding by the inhibition of [3H]phencyclidine binding and by equilibrium measurements of DC6C fluorescence yielded KD values of 2-4 microM for the desensitized AChR and approximately 600 microM for the closed state. At this site, DC6C displayed a strongly blue-shifted emission spectrum, higher intrinsic fluorescence, and weaker energy transfer from tryptophans than when bound to either agonist site. The initial, fast fluorescence decay was assigned to DC6C dissociation from the alphadelta site of the AChR in its closed conformation, on the basis of inhibition with the site-selective antagonists d-tubocurarine and alpha-conotoxin MI. Fast decay amplitude data indicated an apparent affinity of 0.9 microM for the closed-state alphadelta site; the closed-state alphagamma-site affinity is inferred to be near 100 microM. These values and the known affinities for the desensitized conformation show that the alphagamma site drives AChR desensitization to a approximately 40-fold greater extent than the alphadelta site, undergoes energetically larger conformational changes, and is the primary determinant of agonist potency.

Photolabeling the Torpedo nicotinic acetylcholine receptor with 4-azido-2,3,5,6-tetrafluorobenzoylcholine, a partial agonist

The interactions of a photoreactive analogue of benzoylcholine, 4-azido-2,3,5,6-tetrafluorobenzoylcholine (APFBzcholine), with nicotinic acetylcholine receptors (nAChRs) were studied using electrophysiology and photolabeling. APFBzcholine acted as a low-efficacy partial agonist, eliciting maximal responses that were 0.3 and 0.1% of that of acetylcholine for embryonic mouse and Torpedo nAChRs expressed in Xenopus oocytes, respectively. Equilibrium binding studies of [3H]APFBzcholine with nAChR-rich membranes from Torpedo electric organ revealed equal affinities (K(eq) = 12 microM) for the two agonist binding sites. Upon UV irradiation at 254 nm, [3H]APFBzcholine was photoincorporated into the nAChR alpha, gamma, and delta subunits in an agonist-inhibitable manner. Photolabeled amino acids in the agonist binding sites were identified by Edman degradation of isolated, labeled subunit fragments. [3H]APFBzcholine photolabeled gammaLeu-109/deltaLeu-111, gammaTyr-111, and gammaTyr-117 in binding site segment E as well as alphaTyr-198 in alpha subunit binding site segment C. The observed pattern of photolabeling is examined in relation to the predicted orientation of the azide when APFBzcholine is docked in the agonist binding site of a homology model of the nAChR extracellular domain based upon the structure of the snail acetylcholine binding protein.

Assignment of voltage-gated potassium channel blocking activity to kappa-KTx1.3, a non-toxic homologue of kappa-hefutoxin-1, from Heterometrus spinifer venom

A new family of weak K(+) channel toxins (designated kappa-KTx) with a novel "bi-helical" scaffold has recently been characterized from Heterometrus fulvipes (Scorpionidae) venom. Based on the presence of the minimum functional dyad (Y5 and K19), kappa-hefutoxin-1 (kappa-KTx1.1) was investigated and found to block Kv 1.2 (IC(50) approximately 40 microM) and Kv 1.3 (IC(50) approximately 150 microM) channels. In the present study, kappa-KTx1.3, that shares approximately 60% identity with kappa-hefutoxin 1, has been isolated from Heterometrus spinifer venom. Interestingly, despite the presence of the functional dyad (Y5 and K19), kappa-KTx1.3 failed to reproduce the K(+) channel blocking activity of kappa-hefutoxin-1. Since the dyad lysine in kappa-KTx1.3 was flanked by another lysine (K20), it was hypothesized that this additional positive charge could hinder the critical electrostatic interactions known to occur between the dyad lysine and the Kv 1 channel selectivity filter. Hence, mutants of kappa-KTx1.3, substituting K20 with a neutral (K20A) or a negatively (K20E) or another positively (K20R) charged amino acid were synthesized. kappa-KTx1.3 K20E, in congruence with kappa-hefutoxin 1 with respect to subtype selectivity and affinity, produced blockade of Kv 1.2 (IC(50) = 36.8+/-4.9 microM) and Kv 1.3 (IC(50)=53.7+/-6.7 microM) but not Kv 1.1 channels. kappa-KTx1.3 K20A produced blockade of both Kv 1.2 (IC(50) = 36.9+/-4.9 microM) and Kv 1.3 (IC(50)=115.7+/-7.3 microM) and in addition, acquired affinity for Kv 1.1 channels (IC(50) =1 10.7+/-7.7 microM). kappa-KTx1.3 K20R failed to produce any blockade on the channel subtypes tested. These data suggest that the presence of an additional charged residue in a position adjacent to the dyad lysine impedes the functional block of Kv 1 channels produced by kappa-KTx1.3.

Three-finger alpha-neurotoxins and the nicotinic acetylcholine receptor, forty years on

The discovery, about forty years ago, of alpha-bungarotoxin, a three-finger alpha-neurotoxin from Bungarus multicinctus venom, enabled the isolation of the nicotinic acetylcholine receptor (nAChR), making it one of the most thoroughly characterized receptors today. Since then, the sites of interaction between alpha-neurotoxins and nAChRs have largely been delineated, revealing the remarkable plasticity of the three-finger toxin fold that has optimally evolved to utilize different combinations of functional groups to generate a panoply of target specificities to discern subtle differences between nAChR subtypes. New facets in toxinology have now broadened the scope for the use of alpha-neurotoxins in scientific discovery. For instance, the development of short, combinatorial library-derived, synthetic peptides that bind with sub-nanomolar affinity to alpha-bungarotoxin and prevent its interaction with muscle nAChRs has led to the in vivo neutralization of experimental alpha-bungarotoxin envenomation, while the successful introduction of pharmatopes bearing "alpha-bungarotoxin-sensitive sites" into toxin-insensitive nAChRs has permitted the use of various alpha-neurotoxin tags to localize and characterize new receptor subtypes. More ambitious strategies can now be envisaged for engineering rationally designed novel activities on three-finger toxin scaffolds to generate lead peptides of therapeutic value that target the nicotinic pharmacopoeia. This review details the progress made towards achieving this goal.

Identification of binding sites in the nicotinic acetylcholine receptor for [3H]azietomidate, a photoactivatable general anesthetic

To identify binding domains in a ligand-gated ion channel for etomidate, an intravenous general anesthetic, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with a photoactivatable analog, [(3)H]azietomidate. Based upon the inhibition of binding of the noncompetitive antagonist [(3)H]phencyclidine, azietomidate and etomidate bind with 10-fold higher affinity to nAChRs in the desensitized state (IC(50) = 70 microm) than in the closed channel state. In addition, both drugs between 0.1 and 1 mm produced a concentration-dependent enhancement of [(3)H]ACh equilibrium binding affinity, but they inhibited binding at higher concentrations. UV irradiation resulted in preferential [(3)H]azietomidate photoincorporation into the nAChR alpha and delta subunits. Photolabeled amino acids in both subunits were identified in the ion channel domain and in the ACh binding sites by Edman degradation. Within the nAChR ion channel in the desensitized state, there was labeling of alphaGlu-262 and deltaGln-276 at the extracellular end and deltaSer-258 and deltaSer-262 toward the cytoplasmic end. Within the acetylcholine binding sites, [(3)H]azietomidate photolabeled alphaTyr-93, alphaTyr-190, and alphaTyr-198 in the site at the alpha-gamma interface and deltaAsp-59 (but not the homologous position, gammaGlu-57). Increasing [(3)H]azietomidate concentration from 1.8 to 150 microm increased the efficiency of incorporation into amino acids within the ion channel by 10-fold and in the ACh sites by 100-fold, consistent with higher affinity binding within the ion channel. The state dependence and subunit selectivity of [(3)H]azietomidate photolabeling are discussed in terms of the structures of the nAChR transmembrane and extracellular domains.

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Non-conventional toxins from Elapid venoms

Non-conventional toxins constitute a poorly characterized class of three-finger toxins isolated exclusively from Elapidae venoms. These toxins are monomers of 62-68 amino acid residues and contain five disulfide bridges. However, unlike alpha/kappa-neurotoxins and kappa-neurotoxins which have the fifth disulfide bridge in their middle loop (loop II), the fifth disulfide bridge in non-conventional toxins is located in loop I (N-terminus loop). Overall, non-conventional toxins share approximately 28-42% identity with other three-finger toxins including alpha-neurotoxins, alpha/kappa-neurotoxins and kappa-neurotoxins. Recent structural studies have revealed that non-conventional toxins also display the typical three-finger motif. Non-conventional toxins are typically characterized by a lower order of toxicity (LD(50) approximately 5-80 mg/kg) in contrast to prototype alpha-neurotoxins (LD(50) approximately 0.04-0.3 mg/kg) and hence they are also referred to as 'weak toxins'. Further, it is generally assumed that non-conventional toxins target muscle (alpha(2)beta gamma delta) receptors with low affinities several orders of magnitude lower than alpha-neurotoxins and alpha/kappa-neurotoxins. However, it is now known that some non-conventional toxins also antagonize neuronal alpha 7 nicotinic acetylcholine receptors. Hence, non-conventional toxins are not a functionally homogeneous group and other, yet unknown, molecular targets for this class of snake venom toxins may exist. Non-conventional toxins may therefore be a useful source of ligands with novel biological activity targeting the plethora of neuronal nicotinic receptors as well as other physiological processes.

Crystallization and preliminary X-ray analysis of candoxin, a novel reversible neurotoxin from the Malayan krait Bungarus candidus

Candoxin, a novel three-finger toxin from Bungarus candidus, is a reversible antagonist of muscle (alphabetagammadelta) but a poorly reversible antagonist of neuronal alpha7 nicotinic acetylcholine receptors. It has a molecular weight of 7344 Da, with 66 amino-acid residues including ten half-cystines. The fifth disulfide bridge is located at the tip of loop I (Cys6-Cys11) instead of in loop II as found in other alpha-neurotoxins. Interestingly, candoxin lacks the segment cyclized by the fifth disulfide bridge at the tip of the middle loop of long-chain neurotoxins, which was reported to be critical for binding to alpha7 receptors. As a first step to determining its three-dimensional structure, candoxin was crystallized by the hanging-drop vapour-diffusion technique in conditions around 1.5 M sodium chloride, 10%(v/v) ethanol. The crystals formed belonged to the hexagonal system, space group P6(2)22, with unit-cell parameters a = 54.88, b = 54.88, c = 75.54 A, alpha = beta = 90, gamma = 120 degrees, and diffract to a resolution of 1.80 A. The crystallographic asymmetric unit contains one molecule of candoxin, with an estimated solvent content of 44.6%. Attempts to solve these structures by molecular-replacement methods have not been successful and a heavy-atom derivative search has been initiated.

Crystallization and preliminary X-ray analysis of bucain, a novel toxin from the Malayan krait Bungarus candidus

Bucain is a three-finger toxin, structurally homologous to snake-venom muscarinic toxins, from the venom of the Malayan krait Bungarus candidus. These proteins have molecular masses of approximately 6000-8000 Da and encompass the potent curaremimetic neurotoxins which confer lethality to Elapidae and Hydrophidae venoms. Bucain was crystallized in two crystal forms by the hanging-drop vapour-diffusion technique in 0.1 M sodium citrate pH 5.6, 15% PEG 4000 and 0.15 M ammonium acetate. Form I crystals belong to the monoclinic system space group C2, with unit-cell parameters a = 93.73, b = 49.02, c = 74.09 A, beta = 111.32 degrees, and diffract to a nominal resolution of 1.61 A. Form II crystals also belong to the space group C2, with unit-cell parameters a = 165.04, b = 49.44, c = 127.60 A, beta = 125.55 degrees, and diffract to a nominal resolution of 2.78 A. The self-rotation function indicates the presence of four and eight molecules in the crystallographic asymmetric unit of the form I and form II crystals, respectively. Attempts to solve these structures by molecular-replacement methods have not been successful and a heavy-atom derivative search has been initiated.

Autonomic effects of some scorpion venoms and toxins

1. The autonomic effects of venoms and toxins from several species of scorpions, including the Indian red scorpion Mesobuthus tamulus, the Chinese scorpion Buthus martensi Karsch and the Israeli scorpion Leiurus quinquestriatus quinquestriatus, all belonging to Buthidae, and the Asian black scorpions Heterometrus longimanus and Heterometrus spinifer, belonging to Scorpionidae, are reviewed. 2. The effects of the venoms of M. tamulus and L. q. quinquestriatus on noradrenergic and nitrergic transmission in the rat isolated anococcygeus muscle revealed that both venoms mediated their pharmacological effects via a prejunctional mechanism involving the activation of voltage-sensitive sodium channels with consequent release of neurotransmitters that mediate target organ responses, similar to the effects mediated by other alpha-scorpion toxins. 3. Two new toxins, Makatoxin I and Bukatoxin, were purified to homogeneity from the venom of B. martensi Karsch. Determination of their complete amino acid sequences confirmed that both toxins belonged to the class of alpha-scorpion toxins. The effects of both toxins on noradrenergic and nitrergic transmission in the rat anococcygeus muscle provided firm evidence that their pharmacological actions also closely resembled those mediated by other alpha-scorpion toxins on neuronal voltage-sensitive sodium channels. 4. The venoms of H. longimanus and H. spinifer were found to have high concentrations of noradrenaline (1.8 +/- 0.3 mmol/L) and relatively high concentrations of acetylcholine (79.8 +/- 1.7 micromol/L) together with noradrenaline (146.7 +/- 19.8 micromol/L), respectively, which can account for their potent direct cholinergic and noradrenergic agonist actions in the rat anococcygeus muscle. 5. Our studies confirmed that the rat anococcygeus muscle is an excellent nerve-smooth muscle preparation for investigating the effects of bioactive agents on noradrenergic and nitrergic transmission, as well as the direct agonist actions of these agents on post-synaptic alpha-adrenoceptors and M3 muscarinic cholinoceptors. Although many studies, including our own, have documented that scorpion venoms and toxins mediate their primary effects via a prejunctional mechanism that leads to the marked release of various autonomic neurotransmitters, our studies have shown that there are exceptions to this generally accepted phenomenon. In particular, we have provided firm evidence to show that the venoms from H. longimanus and H. spinifer do not have such a prejunctional site of action but, instead, the venoms mediate their autonomic effects through direct agonist actions on post-junctional muscarinic M3 cholinoceptors and alpha-adrenoceptors.

Pharmacological characterization of mikatoxin, an alpha-neurotoxin isolated from the venom of the New-Guinean small-eyed snake Micropechis ikaheka

Symptoms of envenomation by the New-Guinean small-eyed snake Micropechis ikaheka (Elapidae) include peripheral neurotoxicity and myotoxicity. We have now purified to homogeneity a long-chain neurotoxin, mikatoxin, from M. ikaheka venom by successive gel filtration and reverse-phase chromatography. Electrospray ionization mass spectrometry showed mikatoxin to be a homogenous peptide of MW 7775.6. Mikatoxin was devoid of any phospholipase A(2) activity associated with the crude venom and did not exhibit any intrinsic anticholinesterase activity. In the chick biventer cervicis muscle, it produced an irreversible, concentration-dependent block of responses to exogenously applied acetylcholine and carbachol as well as twitches evoked by nerve, but not by direct muscle stimulation. Moreover, mikatoxin, like alpha-bungarotoxin and erabutoxin-b, did not show significant fade response to train-of-four stimulation of the mouse phrenic nerve-hemi diaphragm muscle. It also failed to block ganglionic transmission in the guinea pig ileum and muscarinic responses in the rat anococcygeus muscle. Our study provides strong evidence for the presence of a neurotoxin (mikatoxin) in M. ikaheka venom that produces neuromuscular blockade in skeletal muscle attributable to selective and irreversible antagonism of postsynaptic nicotinic acetylcholine receptors of the neuromuscular junction and likely contributes to the peripheral neurotoxicity observed in M. ikaheka envenomation.

Candoxin, a novel toxin from Bungarus candidus, is a reversible antagonist of muscle (alphabetagammadelta ) but a poorly reversible antagonist of neuronal alpha 7 nicotinic acetylcholine receptors

In contrast to most short and long chain curaremimetic neurotoxins that produce virtually irreversible neuromuscular blockade in isolated nerve-muscle preparations, candoxin, a novel three-finger toxin from the Malayan krait Bungarus candidus, produced postjunctional neuromuscular blockade that was readily and completely reversible. Nanomolar concentrations of candoxin (IC(50) = approximately 10 nm) also blocked acetylcholine-evoked currents in oocyte-expressed rat muscle (alphabetagammadelta) nicotinic acetylcholine receptors in a reversible manner. In contrast, it produced a poorly reversible block (IC(50) = approximately 50 nm) of rat neuronal alpha7 receptors, clearly showing diverse functional profiles for the two nicotinic receptor subsets. Interestingly, candoxin lacks the helix-like segment cyclized by the fifth disulfide bridge at the tip of the middle loop of long chain neurotoxins, reported to be critical for binding to alpha7 receptors. However, its solution NMR structure showed the presence of some functionally invariant residues involved in the interaction of both short and long chain neurotoxins to muscle (alphabetagammadelta) and long chain neurotoxins to alpha7 receptors. Candoxin is therefore a novel toxin that shares a common scaffold with long chain alpha-neurotoxins but possibly utilizes additional functional determinants that assist in recognizing neuronal alpha7 receptors.

Characterization of the rat isolated retractor penis muscle as a model for the study of nitrergic transmission

INTRODUCTION

The anococcygeus and retractor penis muscles are part of the erectile machinery in male rodents. The rat anococcygeus muscle is a widely used smooth muscle preparation for the study of the effects of test substances on adrenergic, nitrergic, and cholinergic transmission. There is, however, little information available on the process of autonomic transmission in the rat retractor penis muscle, although its autonomic innervation has generally been assumed to be similar to that of the anococcygeus muscle because of the contiguous nature of the two muscles. The present study investigated the involvement of nitrergic transmission in mediating relaxant responses of the rat retractor penis muscle to electrical field stimulation.

METHODS

The retractor penis muscle was isolated from Sprague-Dawley rats and mounted in Krebs solution. Phentolamine (5 microM) was added to the bath to block the adrenergic responses of the muscle, which was then precontracted with carbachol (10 microM).

RESULTS

Electrical field stimulation (20-30 V, 1 ms pulse width, at 0.5-20 Hz for 10 s) of the carbachol precontracted muscle elicited frequency-dependent relaxant responses (0.9-68%). Tetrodotoxin (1 microM), N(G)-nitro-L-arginine (L-NOARG) (50 microM), N(G)-nitro-L-arginine methylester (L-NAME) (100 microM), and haemoglobin (100 microM) inhibited these relaxant responses by 99.3%, 93.9%, 86.9%, and 77.5%, respectively. L-Arginine (250 microM) (but not its D-isomer) reversed the blockade produced by L-NOARG (72.7%) and L-NAME (81.5%).

DISCUSSION

Our results provide clear evidence that the inhibitory (relaxant) responses of the rat retractor penis muscle to electrical field stimulation are mediated by nitric oxide involving the L-arginine-nitric oxide synthase-nitric oxide pathway. The rat retractor penis muscle is a versatile preparation that can replace the cumbersome preparations from the pig, ox, and horse, hitherto used as pharmacological models for the study of the retractor penis muscle.

Biochemical and pharmacological characterization of the venom of the black scorpion Heterometrus spinifer

The sting of the black scorpion Heterometrus spinifer, which can cause intense localized pain, has not been reported to produce lethal cardiovascular complications, which are well known to result from scorpion envenomation as a consequence of a massive release of catecholamines. Therefore, we have undertaken a biochemical and pharmacological characterization of the venom of H. spinifer. Pharmacologically, the venom (0.125 microL/mL) produced a marked, reversible contracture in the chick biventer cervicis muscle that was blocked by d-tubocurarine (2 microM) but not by tetrodotoxin (5 microM) and omega-conotoxin GVIA (3 microM). The anticholinesterase neostigmine (1 microM) potentiated the contracture by 5.3-fold. An ultra-filtrate fraction of MW < 3000 (F3K) of the venom produced a similar contracture in the biventer muscle, whereas the retentate of MW > 3000 did not. In the rat anococcygeus muscle, the venom produced a contractile response that was partially (37.4 +/- 1.6%) blocked by atropine (5 microM); phentolamine (5 microM) blocked the remaining response. Tetrodotoxin (5 microM) did not block the contractile response of the venom on the anococcygeus muscle. Electrospray ionization-mass spectrometry/mass spectrometry confirmed the presence of high concentrations of acetylcholine (79.8 +/- 1.7 microM) and norepinephrine (146.7 +/- 19.8 microM) in H. spinifer venom, which can fully account for the observed cholinergic and adrenergic effects. In contrast to scorpion venoms that selectively target neuronal ion channels in mediating transmitter release, our data show that H. spinifer venom does not possess such activity, which likely explains the apparent lack of lethality of black scorpion envenomation.

Functional site of bukatoxin, an alpha-type sodium channel neurotoxin from the Chinese scorpion (Buthus martensi Karsch) venom: probable role of the (52)PDKVP(56) loop

Alpha-toxins from scorpion venoms prolong the action potential of excitable cells by blocking sodium channel inactivation. We have purified bukatoxin, an alpha-toxin from scorpion (Buthus martensi Karsch) venom, to homogeneity. Bukatoxin produced marked relaxant responses in the carbachol-precontracted rat anococcygeus muscle (ACM), which were mediated through the L-arginine-nitric oxide synthase-nitric oxide pathway, consequent to a neuronal release of nitric oxide. Based on the presence of proline residues in the flanking segments of protein-protein interaction sites, we predicted the site between (52)PP(56) to be the potential interaction site of bukatoxin. A homology model of bukatoxin indicated the presence of this site on the surface. Buka11, a synthetic peptide designed based on this predicted site, produced a concentration-dependent nitric oxide-mediated relaxant response in ACM. Using alanine-substituted peptides, we have shown the importance (53)DKV(55) flanked by proline residues in the functional site of bukatoxin.