Alpha-Bungarotoxin binding to human muscle acetylcholine receptor: measurement of affinity, delineation of AChR subunit residues crucial to binding, and protection of AChR function by synthetic peptides

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Pre-innervated tissue-engineered muscle promotes a pro-regenerative microenvironment following volumetric muscle loss

Volumetric muscle loss (VML) is the traumatic or surgical loss of skeletal muscle beyond the inherent regenerative capacity of the body, generally leading to severe functional deficit. Formation of appropriate somato-motor innervations remains one of the biggest challenges for both autologous grafts as well as tissue-engineered muscle constructs. We aim to address this challenge by developing pre-innervated tissue-engineered muscle comprised of long aligned networks of spinal motor neurons and skeletal myocytes on aligned nanofibrous scaffolds. Motor neurons led to enhanced differentiation and maturation of skeletal myocytes in vitro. These pre-innervated tissue-engineered muscle constructs when implanted in a rat VML model significantly increased satellite cell density, neuromuscular junction maintenance, graft revascularization, and muscle volume over three weeks as compared to myocyte-only constructs and nanofiber scaffolds alone. These pro-regenerative effects may enhance functional neuromuscular regeneration following VML, thereby improving the levels of functional recovery following these devastating injuries.

Structure of the Native Muscle-type Nicotinic Receptor and Inhibition by Snake Venom Toxins

The nicotinic acetylcholine receptor, a pentameric ligand-gated ion channel, converts the free energy of binding of the neurotransmitter acetylcholine into opening of its central pore. Here we present the first high-resolution structure of the receptor type found in muscle-endplate membrane and in the muscle-derived electric tissues of fish. The native receptor was purified from Torpedo electric tissue and functionally reconstituted in lipids optimal for cryo-electron microscopy. The receptor was stabilized in a closed state by the binding of α-bungarotoxin. The structure reveals the binding of a toxin molecule at each of two subunit interfaces in a manner that would block the binding of acetylcholine. It also reveals a closed gate in the ion-conducting pore, formed by hydrophobic amino acid side chains, located ∼60 Å from the toxin binding sites. The structure provides a framework for understanding gating in ligand-gated channels and how mutations in the acetylcholine receptor cause congenital myasthenic syndromes.

Isolation and Pharmacological Characterization of α-Elapitoxin-Ot1a, a Short-Chain Postsynaptic Neurotoxin from the Venom of the Western Desert Taipan, Oxyuranus temporalis

Taipans (Oxyuranus spp.) are elapids with highly potent venoms containing presynaptic (β) and postsynaptic (α) neurotoxins. O. temporalis (Western Desert taipan), a newly discovered member of this genus, has been shown to possess venom which displays marked in vitro neurotoxicity. No components have been isolated from this venom. We describe the characterization of α-elapitoxin-Ot1a (α-EPTX-Ot1a; 6712 Da), a short-chain postsynaptic neurotoxin, which accounts for approximately 30% of O. temporalis venom. α-Elapitoxin-Ot1a (0.1–1 µM) produced concentration-dependent inhibition of indirect-twitches, and abolished contractile responses to exogenous acetylcholine and carbachol, in the chick biventer cervicis nerve-muscle preparation. The inhibition of indirect twitches by α-elapitoxin-Ot1a (1 µM) was not reversed by washing the tissue. Prior addition of taipan antivenom (10 U/mL) delayed the neurotoxic effects of α-elapitoxin-Ot1a (1 µM) and markedly attenuated the neurotoxic effects of α-elapitoxin-Ot1a (0.1 µM). α-Elapitoxin-Ot1a displayed pseudo-irreversible antagonism of concentration-response curves to carbachol with a pA₂ value of 8.02 ± 0.05. De novo sequencing revealed the main sequence of the short-chain postsynaptic neurotoxin (i.e., α-elapitoxin-Ot1a) as well as three other isoforms found in O. temporalis venom. α-Elapitoxin-Ot1a shows high sequence similarity (i.e., >87%) with other taipan short-chain postsynaptic neurotoxins.

Alpha neurotoxins

α-Neurotoxins have been isolated from hydrophid, elapid and, more recently, colubrid snake venoms. Also referred to as postsynaptic neurotoxins or 'curare mimetic' neurotoxins, they play an important role in the capture and/or killing of prey by binding to the nicotinic acetylcholine receptor on the skeletal muscle disrupting neurotransmission. They are also thought to cause respiratory paralysis in envenomed humans. This review will discuss the historical background into the discovery, isolation, structure and mechanism of action of the α-neurotoxins, including targets and cellular outcomes, and then will examine the potential uses of α-neurotoxins as pharmacological tools and/or as drug leads.

Actions of snake neurotoxins on an insect nicotinic cholinergic synapse

Here we examine the actions of six snake neurotoxins (alpha-cobratoxin from Naja naja siamensis, erabutoxin-a and b from Laticauda semifasciata; CM12 from N. haje annulifera, toxin III 4 from Notechis scutatus and a long toxin from N. haje) on nicotinic acetylcholine receptors in the cercal afferent, giant interneuron 2 synapse of the cockroach, Periplaneta americana. All toxins tested reduced responses to directly-applied ACh as well as EPSPs evoked by electrical stimulation of nerve XI with similar time courses, suggesting that their action is postsynaptic. Thus, these nicotinic receptors in a well-characterized insect synapse are sensitive to both long and short chain neurotoxins. This considerably expands the range of snake toxins that block insect nicotinic acetylcholine receptors and may enable further pharmacological distinctions between nAChR subtypes.

Expression of soluble ligand- and antibody-binding extracellular domain of human muscle acetylcholine receptor alpha subunit in yeast Pichia pastoris. Role of glycosylation in alpha-bungarotoxin binding

The N-terminal extracellular domain (amino acids 1-210; halpha-(1-210)) of the alpha subunit of the human muscle nicotinic acetylcholine receptor (AChR), bearing the binding sites for cholinergic ligands and the main immunogenic region, the major target for anti-AChR antibodies in patients with myasthenia gravis, was expressed in the yeast, Pichia pastoris. The recombinant protein was water-soluble and glycosylated, and fast protein liquid chromatography analysis showed it to be a monomer. halpha-(1-210) bound (125)I-alpha-bungarotoxin with a high affinity (K(d) = 5.1 +/- 2.4 nm), and this binding was blocked by unlabeled d-tubocurarine and gallamine (K(i) approximately 7.5 mm). Interestingly, (125)I-alpha-bungarotoxin binding was markedly impaired by in vitro deglycosylation of halpha-(1-210). Several monoclonal antibodies that show partial or strict conformation-dependent binding to the AChR were able to bind to halpha-(1-210), as did antibodies from a large proportion of myasthenic patients. These results suggest that the extracellular domain of the human AChR alpha subunit expressed in P. pastoris has an apparently near native conformation. The correct folding of the recombinant protein, together with its relatively high expression yield, makes it suitable for structural studies on the nicotinic acetylcholine receptor and for use as an autoantigen in myasthenia gravis studies.

A cyclic peptide with high affinity to alpha-bungarotoxin protects mice from the lethal effect of the toxin

Employing a combinatorial phage-peptide library, we previously identified the peptide MRYYESSLKSYPD (designated, library-peptide) that binds the snake toxin alpha-bungarotoxin (alpha-BTX) with a moderate binding constant of 10(-6)M (Balass et al., 1997. Proc. Natl. Acad. Sci. USA 94, 6054-6058). Under the experimental conditions employed, we found that the library-peptide did not protect mice from alpha-BTX lethality when injected concomitantly with the toxin. In order to improve the affinity of the peptide to alpha-BTX, we designed and synthesized the peptide CRYYESSLKSYCD (Met1 and Pro12 were replaced by cysteines), which following oxidation creates a single disulfide bond and forms a cyclic structure. The design of the cyclic peptide was based on our previous NMR analysis of the library-peptide/alpha-BTX complex (Scherf et al., 1997. Proc. Natl. Acad. Sci. USA 94, 6059-6064). The cyclic peptide binds alpha-BTX with affinity two orders of magnitude higher than that of the linear library selected peptide. Whereas the library peptide was ineffective, the cyclic peptide conferred protection from alpha-BTX lethality in mice, even when given 1h after the toxin injection. The cyclic peptide conferred complete protection from alpha-BTX lethality in mice when administered 40min prior to toxin injection. However, experiments with the whole venom of the snake Bungarus multicinctus showed that protection could be achieved only when the cyclic peptide was administered concomitantly with the venom.

Molecular targets for autoimmune and genetic disorders of neuromuscular transmission

The neuromuscular junction is the target of a variety of autoimmune, neurotoxic and genetic disorders, most of which result in muscle weakness. Most of the diseases, and many neurotoxins, target the ion channels that are essential for neuromuscular transmission. Myasthenia gravis is an acquired autoimmune disease caused in the majority of patients by antibodies to the acetylcholine receptor, a ligand-gated ion channel. The antibodies lead to loss of acetylcholine receptor, reduced efficiency of neuromuscular transmission and muscle weakness and fatigue. Placental transfer of these antibodies in women with myasthenia can cause fetal or neonatal weakness and occasionally severe deformities. Lambert Eaton myasthenic syndrome and acquired neuromyotonia are caused by antibodies to voltage-gated calcium or potassium channels, respectively. In the rare acquired neuromyotonia, reduced repolarization of the nerve terminal leads to spontaneous and repetitive muscle activity. In each of these disorders, the antibodies are detected by immunoprecipitation of the relevant ion channel labelled with radioactive neurotoxins. Genetic disorders of neuromuscular transmission are due mainly to mutations in the genes for the acetylcholine receptor. These conditions show recessive or dominant inheritance and result in either loss of receptors or altered kinetics of acetylcholine receptor channel properties. Study of these conditions has greatly increased our understanding of synaptic function and of disease aetiology.