Spirolide and gymnodimine macrocyclic imine phycotoxins belong to an emerging class of chemical agents associated with marine algal blooms and shellfish toxicity. Analysis of 13-desmethyl spirolide C and gymnodimine A by binding and voltage-clamp recordings on muscle-type α1 2 βγδ and neuronal α3β2 and α4β2 nicotinic acetylcholine receptors reveals subnanomolar affinities, potent antagonism, and limited subtype selectivity. Their binding to acetylcholine-binding proteins (AChBP), as soluble receptor surrogates, exhibits picomolar affinities governed by diffusion-limited association and slow dissociation, accounting for apparent irreversibility. Crystal structures of the phycotoxins bound to Aplysia-AChBP (≈2.4Å) show toxins neatly imbedded within the nest of aromatic side chains contributed by loops C and F on opposing faces of the subunit interface, and which in physiological conditions accommodates acetylcholine. The structures also point to three major features: (i) the sequence-conserved loop C envelops the bound toxins to maximize surface complementarity; (ii) hydrogen bonding of the protonated imine nitrogen in the toxins with the carbonyl oxygen of loop C Trp147 tethers the toxin core centered within the pocket; and (iii) the spirolide bisspiroacetal or gymnodimine tetrahydrofuran and their common cyclohexene-butyrolactone further anchor the toxins in apical and membrane directions, along the subunit interface. In contrast, the sequence-variable loop F only sparingly contributes contact points to preserve the broad receptor subtype recognition unique to phycotoxins compared with other nicotinic antagonists. These data offer unique means for detecting spiroimine toxins in shellfish and identify distinctive ligands, functional determinants and binding regions for the design of new drugs able to target several receptor subtypes with high affinity.acetylcholine binding protein | marine phycotoxins | nicotinic acetylcholine receptor | pharmacological and structural analyses | seafood poisoning

To understand how snake neurotoxins interact with nicotinic acetylcholine receptors, we have elaborated an experimentally based model of the ␣-cobratoxin-␣7 receptor complex. This model was achieved by using (i) a three-dimensional model of the ␣7 extracellular domain derived from the crystallographic structure of the homologous acetylcholine-binding protein, (ii) the previously solved x-ray structure of the toxin, and (iii) nine pairs of residues identified by cycle-mutant experiments to make contacts between the ␣-cobratoxin and ␣7 receptor. Because the receptor loop F occludes entrance of the toxin binding pocket, we submitted this loop to a dynamics simulation and selected a conformation that allowed the toxin to reach its binding site. T he ␣-neurotoxins from snake venom are potent antagonists that block nicotinic acetylcholine receptors (AChRs) and hence affect synaptic transmission (1-3). Despite many studies (reviewed in ref. 4), the molecular process associated with this efficient blockage remains unclear. To approach this question, we previously studied ␣-cobratoxin (␣-Cbtx), an ␣͞K neurotoxin that binds to both muscular and homopentameric neuronal receptors (␣7 and ␣8) with high affinities (4). This toxin, similar to other snake neurotoxins, is folded into three adjacent loops rich in ␤-sheet that emerge from a small globular core in which four disulfide bonds are located (5). By mutational analyses, the residues by which ␣-Cbtx interacts with the muscular-type or neuronal ␣7 receptors were identified previously (6, 7). The present study shows how functional residues account for the antagonistic properties of the toxin toward the ␣7 neuronal receptor. The ␣7 AChR possesses five identical ␣7 subunits (8) that offer five ligand-binding sites located at the interface of two subunits (9). These sites include residues located on the different functional loops described previously on the principal ␣7 (ϩ) face, loops A, B, and C and on the complementary ␣7 (Ϫ) face, loops D, E, and F (refs. 10-13; see Fig. 1). Until now, the residues of the ␣7 receptor involved in snake toxin binding have remained unknown.The aim of the present paper is fourfold. First, by an extensive mutational study we have identified ␣7 receptor residues involved in the interaction with ␣-Cbtx. Second, by using a double-mutant cycle approach we have disclosed several pairs of interacting residues in the toxin-receptor complex. Third, by using the three-dimensional (3D) structure of an AChBP that is similar functionally and structurally to the N-terminal domain of an AChR ␣-subunit (14), we used a 3D model for the ␣7 subunit extracellular region obtained by comparative modeling [see accompanying paper on page 3210 (15)]. Fourth, by using this model, a molecular dynamics simulation of the loop F region, and the constraints derived from our pairwise analysis, we propose an experimentally based 3D model of the complex between the ␣-Cbtx and ␣7 receptor, which explains the antagonistic properties of the snake toxin toward the neuronal recepto...

Long chain and short chain curaremimetic toxins from snakes possess 66 -74 residues with five disulfide bonds and 60 -62 residues with four disulfide bonds, respectively. Despite their structural differences all of these toxins bind with high affinity to the peripheral nicotinic acetylcholine receptors (AChR). Binding experiments have now revealed that long chain toxins only, like the neuronal -bungarotoxin, have a high affinity for a chimeric form of the neuronal ␣7 receptor, with K d values ranging from about 1 to 12 nM. In contrast, all other toxins bind to the chimeric ␣7 receptor with a low affinity, with K d values ranging between 3 and 22 M. These results are supported by electrophysiological recordings on both the wild-type and chimeric ␣7 receptors. Amino acid sequence analyses have suggested that high affinities for the neuronal receptor are associated with the presence of the fifth disulfide at the tip of the toxin second loop. In agreement with this conclusion, we show that a long chain toxin whose fifth disulfide is reduced and then dithiopyridylated has a low affinity (K d ‫؍‬ 12 M) for the neuronal ␣7 receptor, whereas it retains a high affinity (K d ‫؍‬ 0.35 nM) for the peripheral AChR. Thus, a long chain curaremimetic toxin having a reduced fifth disulfide bond behaves like a short chain toxin toward both the peripheral and neuronal AChR. Therefore, functional classification of toxins that bind to AChRs should probably be done by considering their activities on both peripheral and neuronal receptors.

Gymnodimines (GYMs) are phycotoxins exhibiting unusual structural features including a spirocyclic imine ring system and a trisubstituted tetrahydrofuran embedded within a 16‐membered macrocycle. The toxic potential and the mechanism of action of GYM‐A, highly purified from contaminated clams, have been assessed. GYM‐A in isolated mouse phrenic hemidiaphragm preparations produced a concentration‐ and time‐dependent block of twitch responses evoked by nerve stimulation, without affecting directly elicited muscle twitches, suggesting that it may block the muscle nicotinic acetylcholine (ACh) receptor (nAChR). This was confirmed by the blockade of miniature endplate potentials and the recording of subthreshold endplate potentials in GYM‐A paralyzed frog and mouse isolated neuromuscular preparations. Patch‐clamp recordings in Xenopus skeletal myocytes revealed that nicotinic currents evoked by constant iontophoretical ACh pulses were blocked by GYM‐A in a reversible manner. GYM‐A also blocked, in a voltage‐independent manner, homomeric human α7 nAChR expressed in Xenopus oocytes. Competition‐binding assays confirmed that GYM‐A is a powerful ligand interacting with muscle‐type nAChR, heteropentameric α3β2, α4β2, and chimeric α7‐5HT3 neuronal nAChRs. Our data show for the first time that GYM‐A broadly targets nAChRs with high affinity explaining the basis of its neurotoxicity, and also pave the way for designing specific tests for accurate GYM‐A detection in shellfish samples.

Venoms of elapid and hydrophid snakes contain a family of small toxic proteins called curarimimetic toxins or ␣-neurotoxins that bind with high affinity to muscular nicotinic acetylcholine receptors (AChRs) 1 and hence affect synaptic transmission (1, 2). All these toxins adopt a leaf-like shape with three adjacent loops rich in ␤-sheet that emerge from a small globular core where four disulfide bonds are invariably located (3-7). Notwithstanding their common fold and their similar biological function, ␣-neurotoxins are currently classified as short chain toxins with 60 -62 residues and four disulfide bonds and long chain toxins with 66 -74 residues and five disulfide bonds. In agreement with this old chemically based classification, we recently showed that the long chain toxins are also and uniquely capable of binding with high affinity to the neuronal ␣7 receptor (8). These preliminary data also indicated that the neuron-specific binding capacity may be associated with the unique presence in the long chain toxins of a small cyclic loop at the tip of their central loop. The goal of this work was therefore to identify as precisely as possible the determinants by which long chain toxins bind to the neuronal ␣7-AChR and to compare them with those involved when toxins bind to the muscular AChR.The toxin used in this study is ␣-cobratoxin (␣-Cbtx) (9) from Naja naja siamensis (probably Naja kaouthia (10)). It is a prototype of long chain curarimimetic toxins with a single polypeptide chain of 71 amino acids and five disulfide bonds. ␣-Cbtx binds with high affinity to the muscular type AChR from Torpedo marmorata (K d ϭ 58 pM) and the neuronal ␣7-AChR (K d ϭ 9 nM). Its three-dimensional structure is known from both NMR (11) and x-ray crystallographic studies (12). We recently submitted this toxin to an extensive site-directed mutagenesis and found that the residues by which it binds to the Torpedo AChR include a number of amino acids that are highly conserved throughout the family of curarimimetic toxins (13). These are Lys-23, Trp-25, Asp-27, Phe-29, and Arg-33, which belong to the concave face of the toxin loop II, and Lys-49, which belongs to the same face of loop III. The same residues of a short chain curarimimetic toxin are involved in binding to the same AChR (14,15). In addition, however, long and short chain curarimimetic toxins use specific residues for binding to the Torpedo AChR. These specific residues are located in the Cterminal tail and in loop I of long and short chain toxins, respectively (13).The goal of this study was 3-fold. First, using a set of 36 toxin mutants, we identified the residues by which ␣-Cbtx most likely binds to the neuronal ␣7 receptor. Second, we compared these data with those that previously indicated the residues by which the same toxin binds to the Torpedo AChR (13). Third, to identify the regions of the ␣7 receptor that are recognized by the toxin, we mutated different residues in various functional loops of the ␣7 receptor and studied the effect of these muta-* The costs of ...

Pinnatoxins belong to an emerging class of potent marine toxins of the cyclic imine group. Detailed studies of their biological effects have been impeded by unavailability of the complex natural product from natural sources. This work describes the development of a robust, scalable synthetic sequence relying on a convergent strategy that delivered a sufficient amount of the toxin for detailed biological studies and its commercialization for use by other research groups and regulatory agencies. A central transformation in the synthesis is the highly diastereoselective Ireland–Claisen rearrangement of a complex α,α-disubstituted allylic ester based on a unique mode for stereoselective enolization through a chirality match between the substrate and the lithium amide base. With synthetic pinnatoxin A, a detailed study has been performed that provides conclusive evidence for its mode of action as a potent inhibitor of nicotinic acetylcholine receptors selective for the human neuronal α7 subtype. The comprehensive electrophysiological, biochemical, and computational studies support the view that the spiroimine subunit of pinnatoxins is critical for blocking nicotinic acetylcholine receptor subtypes, as evidenced by analyzing the effect of a synthetic analogue of pinnatoxin A containing an open form of the imine ring. Our studies have paved the way for the production of certified standards to be used for mass-spectrometric determination of these toxins in marine matrices and for the development of tests to detect these toxins in contaminated shellfish.

Boiga dendrophila (mangrove catsnake) is a colubrid snake that lives in Southeast Asian lowland rainforests and mangrove swamps and that preys primarily on birds. We have isolated, purified, and sequenced a novel toxin from its venom, which we named denmotoxin. It is a monomeric polypeptide of 77 amino acid residues with five disulfide bridges. In organ bath experiments, it displayed potent postsynaptic neuromuscular activity and irreversibly inhibited indirectly stimulated twitches in chick biventer cervicis nerve-muscle preparations. In contrast, it induced much smaller and readily reversible inhibition of electrically induced twitches in mouse hemidiaphragm nerve-muscle preparations. More precisely, the chick muscle ␣ 1 ␤␥␦-nicotinic acetylcholine receptor was 100-fold more susceptible compared with the mouse receptor. These data indicate that denmotoxin has a bird-specific postsynaptic activity. We chemically synthesized denmotoxin, crystallized it, and solved its crystal structure at 1.9 Å by the molecular replacement method. The toxin structure adopts a non-conventional three-finger fold with an additional (fifth) disulfide bond in the first loop and seven additional residues at its N terminus, which is blocked by a pyroglutamic acid residue. This is the first crystal structure of a three-finger toxin from colubrid snake venom and the first fully characterized bird-specific toxin. Denmotoxin illustrates the relationship between toxin specificity and the primary prey type that constitutes the snake's diet. Three-finger toxins (3FTXs)3 form one of the most abundant, well recognized families of snake venom proteins. They share a similar structure and are characterized by three ␤-stranded finger-like loops, emerging from a globular core and stabilized by four conserved disulfide bridges. An additional disulfide linkage may sometimes be present in the first (non-conventional toxins) or second (long-chain ␣-neurotoxins and -toxins) loop (1-5). All 3FTXs are monomers except for -toxins, which are noncovalent homodimers isolated from Bungarus venoms. Minor structural differences in the three-finger fold, viz. the number of ␤-strands, overall morphology of the loops, and differential lengths of turns or C-terminal tails (6), lead to the recognition of varied targets and modulate the toxicity and specificity (7). Hence, 3FTXs affect a broad range of molecular targets, including ␣ 1 -nicotinic acetylcholine receptors (nAChRs; short-and longchain ␣-neurotoxins), ␣ 7 -nAChRs (long-chain ␣-neurotoxins), and ␣ 3 -and ␣ 4 -nAChRs (-toxins) (4, 5); muscarinic acetylcholine receptors (muscarinic toxins) (8); L-type calcium channels (calciseptine and FS2 toxin) (9, 10); integrin ␣ IIb ␤ 3 (dendroaspin) (11, 12); integrin ␣ v ␤ 3 (cardiotoxin A5) (13); acetylcholinesterase (fasciculins) (14); phospholipids and glycosphingolipids (cardiotoxins) (15); and blood coagulation protein factor VIIa (16). As the interaction with such a broad spectrum of target proteins results in a variety of pharmacological effects, the understanding...

␣-Cobratoxin, a long chain curaremimetic toxin from Naja kaouthia venom, was produced recombinantly (r␣-Cbtx) from Escherichia coli. It was indistinguishable from the snake toxin. Mutations at 8 of the 29 explored toxin positions resulted in affinity decreases for Torpedo receptor with ⌬⌬G higher than 1.1 kcal/mol. These are R33E > K49E > D27R > K23E > F29A > W25A > R36A > F65A. These positions cover a homogeneous surface of approximately 880 Å 2 and mostly belong to the second toxin loop, except Lys-49 and Phe-65 which are, respectively, on the third loop and C-terminal tail. The mutations K23E and K49E, and perhaps R33E, induced discriminative interactions at the two toxin-binding sites. When compared with the short toxin erabutoxin a (Ea), a number of structurally equivalent residues are commonly implicated in binding to muscular-type nicotinic acetylcholine receptor. These are Lys-23/Lys-27, Asp-27/Asp-31, Arg-33/Arg-33, Lys-49/Lys-47, and to a lesser and variable extent Trp-25/Trp-29 and Phe-29/ Phe-32. In addition, however, the short and long toxins display three major differences. First, Asp-38 is important in Ea in contrast to the homologous Glu-38 in ␣-Cbtx. Second, all of the first loop is insensitive to mutation in ␣-Cbtx, whereas its tip is functionally critical in Ea. Third, the C-terminal tail may be specifically critical in ␣-Cbtx. Therefore, the functional sites of long and short curaremimetic toxins are not identical, but they share common features and marked differences that might reflect an evolutionary pressure associated with a great diversity of prey receptors.