Determination of three-dimensional solution structure of waglerin I, a toxin from Trimeresurus wagleri, using 2D-NMR and molecular dynamics simulation

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Azemiopsin from Azemiops feae viper venom, a novel polypeptide ligand of nicotinic acetylcholine receptor

Azemiopsin, a novel polypeptide, was isolated from the Azemiops feae viper venom by combination of gel filtration and reverse-phase HPLC. Its amino acid sequence (DNWWPKPPHQGPRPPRPRPKP) was determined by means of Edman degradation and mass spectrometry. It consists of 21 residues and, unlike similar venom isolates, does not contain cysteine residues. According to circular dichroism measurements, this peptide adopts a β-structure. Peptide synthesis was used to verify the determined sequence and to prepare peptide in sufficient amounts to study its biological activity. Azemiopsin efficiently competed with α-bungarotoxin for binding to Torpedo nicotinic acetylcholine receptor (nAChR) (IC₅₀ 0.18 ± 0.03 μm) and with lower efficiency to human α7 nAChR (IC₅₀ 22 ± 2 μm). It dose-dependently blocked acetylcholine-induced currents in Xenopus oocytes heterologously expressing human muscle-type nAChR and was more potent against the adult form (α1β1ϵδ) than the fetal form (α1β1γδ), EC₅₀ being 0.44 ± 0.1 μm and 1.56 ± 0.37 μm, respectively. The peptide had no effect on GABA_A (α1β3γ2 or α2β3γ2) receptors at a concentration up to 100 μm or on 5-HT₃ receptors at a concentration up to 10 μm. Ala scanning showed that amino acid residues at positions 3–6, 8–11, and 13–14 are essential for binding to Torpedo nAChR. In biological activity azemiopsin resembles waglerin, a disulfide-containing peptide from the Tropidechis wagleri venom, shares with it a homologous C-terminal hexapeptide, but is the first natural toxin that blocks nAChRs and does not possess disulfide bridges.

Neurotoxins acting at nicotinic receptors

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

Identification of residues at the alpha and epsilon subunit interfaces mediating species selectivity of Waglerin-1 for nicotinic acetylcholine receptors

Waglerin-1 (Wtx-1) is a 22-amino acid peptide that is a competitive antagonist of the muscle nicotinic receptor (nAChR). We find that Wtx-1 binds 2100-fold more tightly to the alpha-epsilon than to the alpha-delta binding site interface of the mouse nAChR. Moreover, Wtx-1 binds 100-fold more tightly to the alpha-epsilon interface from mouse nAChR than that from rat or human sources. Site-directed mutagenesis of residues differing in the extracellular domains of rat and mouse epsilon subunits indicates that residues 59 and 115 mediate the species difference in Wtx-1 affinity. Mutation of residues 59 (Asp in mouse, Glu in rat epsilon) and 115 (Tyr in mouse, Ser in rat epsilon) converts Wtx-1 affinity for the alpha-epsilon interface of one species to that of the other species. Studies of different mutations at position 59 indicate both steric and electrostatic contributions to Wtx-1 affinity, whereas at position 115, both aromatic and polar groups contribute to affinity. The human nAChR also has lower affinity for Wtx-1 than mouse nAChR, but unlike rat nAChR, residues in both alpha and epsilon subunits mediate the affinity difference. In human nAChR, polar residues (Ser-187 and Thr-189) confer low affinity, whereas in mouse nAChR aromatic residues (Trp-187 and Phe-189) confer high affinity. The overall results show that non-conserved residues at the nAChR binding site, although not crucial for activation by ACh, govern the potency of neuromuscular toxins.