Purification from Torpedo marmorata electric tissue of membrane fragments particularly rich in cholinergic receptor protein

Pursuing High-Resolution Structures of Nicotinic Acetylcholine Receptors: Lessons Learned from Five Decades

Since their discovery, nicotinic acetylcholine receptors (nAChRs) have been extensively studied to understand their function, as well as the consequence of alterations leading to disease states. Importantly, these receptors represent pharmacological targets to treat a number of neurological and neurodegenerative disorders. Nevertheless, their therapeutic value has been limited by the absence of high-resolution structures that allow for the design of more specific and effective drugs. This article offers a comprehensive review of five decades of research pursuing high-resolution structures of nAChRs. We provide a historical perspective, from initial structural studies to the most recent X-ray and cryogenic electron microscopy (Cryo-EM) nAChR structures. We also discuss the most relevant structural features that emerged from these studies, as well as perspectives in the field.

Sequential purification and characterization of Torpedo californica nAChR-DC supplemented with CHS for high-resolution crystallization studies

Over the past 10 years we have been developing a multi-attribute analytical platform that allows for the preparation of milligram amounts of functional, high-pure, and stable Torpedo (muscle-type) nAChR detergent complexes for crystallization purpose. In the present work, we have been able to significantly improve and optimize the purity and yield of nicotinic acetylcholine receptors in detergent complexes (nAChR-DC) without compromising stability and functionality. We implemented new methods in the process, such as analysis and rapid production of samples for future crystallization preparations. Native nAChR was extracted from the electric organ of Torpedo californica using the lipid-like detergent LysoFos Choline 16 (LFC-16), followed by three consecutive steps of chromatography purification. We evaluated the effect of cholesteryl hemisuccinate (CHS) supplementation during the affinity purification steps of nAChR-LFC-16 in terms of receptor secondary structure, stability and functionality. CHS produced significant changes in the degree of β-secondary structure, these changes compromise the diffusion of the nAChR-LFC-16 in lipid cubic phase. The behavior was reversed by Methyl-β-Cyclodextrin treatment. Also, CHS decreased acetylcholine evoked currents of Xenopus leavis oocyte injected with nAChR-LFC-16 in a concentration-dependent manner. Methyl-β-Cyclodextrin treatment do not reverse functionality, however column delipidation produced a functional protein similar to nAChR-LFC-16 without CHS treatment.

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.

Discovery of the First Neurotransmitter Receptor: The Acetylcholine Nicotinic Receptor

The concept of pharmacological receptor was proposed at the turn of the 20th century but it took almost 70 years before the first receptor for a neurotransmitter was isolated and identified as a protein. This review retraces the history of the difficulties and successes in the identification of the nicotinic acetylcholine receptor, the first neurotransmitter receptor to be identified.

A mechanism in agrin signaling revealed by a prevalent Rapsyn mutation in congenital myasthenic syndrome

Neuromuscular junction is a synapse between motoneurons and skeletal muscles, where acetylcholine receptors (AChRs) are concentrated to control muscle contraction. Studies of this synapse have contributed to our understanding of synapse assembly and pathological mechanisms of neuromuscular disorders. Nevertheless, underlying mechanisms of NMJ formation was not well understood. To this end, we took a novel approach - studying mutant genes implicated in congenital myasthenic syndrome (CMS). We showed that knock-in mice carrying N88K, a prevalent CMS mutation of Rapsyn (Rapsn), died soon after birth with profound NMJ deficits. Rapsn is an adapter protein that bridges AChRs to the cytoskeleton and possesses E3 ligase activity. In investigating how N88K impairs the NMJ, we uncovered a novel signaling pathway by which Agrin-LRP4-MuSK induces tyrosine phosphorylation of Rapsn, which is required for its self-association and E3 ligase activity. Our results also provide insight into pathological mechanisms of CMS.

Pinnatoxins' Deleterious Effects on Cholinergic Networks: From Experimental Models to Human Health

Pinnatoxins (PnTXs) are emerging neurotoxins that were discovered about 30 years ago. They are solely produced by the marine dinoflagellate Vulcanodinium rugosum, and may be transferred into the food chain, as they have been found in various marine invertebrates, including bivalves. No human intoxication has been reported to date although acute toxicity was induced by PnTxs in rodents. LD₅₀ values have been estimated for the different PnTXs through the oral route. At sublethal doses, all symptoms are reversible, and no neurological sequelae are visible. These symptoms are consistent with impairment of central and peripheral cholinergic network functions. In fact, PnTXs are high-affinity competitive antagonists of nicotinic acetylcholine receptors (nAChRs). Moreover, their lethal effects are consistent with the inhibition of muscle nAChRs, inducing respiratory distress and paralysis. Human intoxication by ingestion of PnTXs could result in various symptoms observed in episodes of poisoning with natural nAChR antagonists. This review updates the available data on PnTX toxicity with a focus on their mode of action on cholinergic networks and suggests the effects that could be extrapolated on human physiology.

The nicotinic acetylcholine receptor: a typical 'allosteric machine'

The concept of allosteric interaction was initially proposed to account for the inhibitory feedback mechanism mediated by bacterial regulatory enzymes. In contrast with the classical mechanism of competitive, steric, interaction between ligands for a common site, allosteric interactions take place between topographically distinct sites and are mediated by a discrete and reversible conformational change of the protein. The concept was soon extended to membrane receptors for neurotransmitters and shown to apply to the signal transduction process which, in the case of the acetylcholine nicotinic receptor (nAChR), links the ACh binding site to the ion channel. Pharmacological effectors, referred to as allosteric modulators, such as Ca²⁺ ions and ivermectin, were discovered that enhance the transduction process when they bind to sites distinct from the orthosteric ACh site and the ion channel. The recent X-ray and electron microscopy structures, at atomic resolution, of the resting and active conformations of several homologues of the nAChR, in combination with atomistic molecular dynamics simulations reveal a stepwise quaternary transition in the transduction process with tertiary changes modifying the boundaries between subunits. These interfaces host orthosteric and allosteric modulatory sites which structural organization changes in the course of the transition. The nAChR appears as a typical allosteric machine. The model emerging from these studies has led to the conception and development of several new pharmacological agents.This article is part of a discussion meeting issue 'Allostery and molecular machines'.

Enzymatic Activity of the Scaffold Protein Rapsyn for Synapse Formation

Neurotransmission is ensured by a high concentration of neurotransmitter receptors at the postsynaptic membrane. This is mediated by scaffold proteins that bridge the receptors with cytoskeleton. One such protein is rapsyn (receptor-associated protein at synapse), which is essential for acetylcholine receptor (AChR) clustering and NMJ (neuromuscular junction) formation. We show that the RING domain of rapsyn contains E3 ligase activity. Mutation of the RING domain that abolishes the enzyme activity inhibits rapsyn- as well as agrin-induced AChR clustering in heterologous and muscle cells. Further biological and genetic studies support a working model where rapsyn, a classic scaffold protein, serves as an E3 ligase to induce AChR clustering and NMJ formation, possibly by regulation of AChR neddylation. This study identifies a previously unappreciated enzymatic function of rapsyn and a role of neddylation in synapse formation, and reveals a potential target of therapeutic intervention for relevant neurological disorders.

Protein dynamics and the allosteric transitions of pentameric receptor channels

The recent application of molecular dynamics (MD) methodology to investigate the allosteric transitions of the acetylcholine receptor and its prokaryotic and eukaryotic pentameric homologs has yielded new insights into the mechanisms of signal transduction by these receptors. Combined with available data on X-ray structures, MD techniques enable description of the dynamics of the conformational change at the atomic level, intra-molecular propagation of this signal transduction mechanism as a concerted stepwise process at physiological timescales and the control of this process by allosteric modulators, thereby offering new perspectives for drug design.

Seeking structural specificity: direct modulation of pentameric ligand-gated ion channels by alcohols and general anesthetics

Alcohols and other anesthetic agents dramatically alter neurologic function in a wide range of organisms, yet their molecular sites of action remain poorly characterized. Pentameric ligand-gated ion channels, long implicated in important direct effects of alcohol and anesthetic binding, have recently been illuminated in renewed detail thanks to the determination of atomic-resolution structures of several family members from lower organisms. These structures provide valuable models for understanding and developing anesthetic agents and for allosteric modulation in general. This review surveys progress in this field from function to structure and back again, outlining early evidence for relevant modulation of pentameric ligand-gated ion channels and the development of early structural models for ion channel function and modulation. We highlight insights and challenges provided by recent crystal structures and resulting simulations, as well as opportunities for translation of these newly detailed models back to behavior and therapy.

The nicotinic acetylcholine receptor: the founding father of the pentameric ligand-gated ion channel superfamily

A critical event in the history of biological chemistry was the chemical identification of the first neurotransmitter receptor, the nicotinic acetylcholine receptor. Disciplines as diverse as electrophysiology, pharmacology, and biochemistry joined together in a unified and rational manner with the common goal of successfully identifying the molecular device that converts a chemical signal into an electrical one in the nervous system. The nicotinic receptor has become the founding father of a broad family of pentameric membrane receptors, paving the way for their identification, including that of the GABA(A) receptors.

The effects of ethanol on the structural stability of acetylcholine receptor and the activity of various molecular forms of acetylcholinesterase

The actions of ethanol on the structural stability of acetylcholine receptor (AchR)-enriched membrane vesicles and the activity of various molecular forms of acetylcholinesterase (AchE) were investigated, using the receptor and the enzyme isolated from the electric organ of Torpedo californica. In the presence of ethanol up to 200 mM, the thermogram of AchR-enriched membranes exhibited no significant decrease in the temperature (td) of receptor transition at 57 degrees C, but a decrease in the enthalpy change (delta Hd) indicated a slight ethanol-induced structural perturbation. The presence of 12.5 nmol alpha-bungarotoxin also caused a decrease in delta Hd. A complete loss of the receptor transition was observed at a higher concentration 500 nmol of alpha-bungarotoxin and no recovery of the transition was found with the addition of 200 mM ethanol. The results suggested a noncompetitive interaction of ethanol with the receptor. In the presence of 200-1000 mM ethanol, the activity of two soluble forms of AchE, a higher (117 S) aggregate and a lower (10 S) aggregate was not significantly affected. Comparing the activity of these two aggregates over a wide concentration range of ethanol (200-2000 mM) revealed no obvious difference in the level of ethanol effect between them. However, after removal of ethanol, the higher aggregate form of AchE exhibited a greater recoverability of the activity, suggesting a possible slightly greater structure-functional stability for it. Studies of soluble AchE and membrane-bound AchE showed that the presence of 200 or 600 mM ethanol caused a greater level of inhibition in membrane-bound enzyme than in soluble enzyme, possible due to a disruption of protein-lipid interaction needed to maintain the conformation of membrane-bound AchE. Interestingly, at a much higher concentration of ethanol (2.0 M), membrane-bound AchE became more resistant to ethanol than did the soluble forms of AchE. In this case, the effective concentration of ethanol felt by the enzyme was expected to be less for membrane-bound AchE, owing to ethanol's solubility in lipids.

Immunocytochemical localization of acetylcholine receptors in locust brain using auto-anti-idiotypic acetylcholine antibodies

Auto-anti-idiotypic antibodies have been detected in antisera of rabbits immunized with an acetylcholine (ACh) conjugate. These antibodies were found to bind to ACh receptor (ACh-R) purified from different species membranes. They competed with the ACh-R antagonist alpha-bungarotoxin and some agonists such as ACh conjugate and ACh itself. They did not recognize acetylcholinesterase. Their characterization 'in vitro' suggested their employment as an immunohistological marker for ACh-R. In the locust brain, specific immunoreactivity was found in neuropils of the protocerebrum, the optic lobes, the deutocerebrum and the tritocerebrum.

Two pools of cholesterol in acetylcholine receptor-rich membranes from Torpedo

The acetylcholine receptor (AChR)-containing electroplax membranes from Torpedo californica have a relatively high cholesterol content. Reconstitution studies suggest that this cholesterol may be important in preserving or modulating the function of the acetylcholine receptor-channel complex. We have manipulated cholesterol levels in intact Torpedo AChR-rich membrane fragments using small, unilamellar phosphatidylcholine liposomes. Conditions have been established that allow further subfractionation of sucrose gradient purified Torpedo electroplax membranes into AChR-rich and ATPase-rich populations and that, at the same time, achieve cholesterol depletion without phospholipid back exchange or fusion. The incubation of membranes with excess liposomes could only achieve about a 50% reduction in the molar ratio of cholesterol to phospholipid. In no case was the number of cholesterol molecules per AChR oligomer reduced below 36. The remaining cholesterol could not be depleted either by longer incubations or by multiple, sequential depletions. Cholesterol depletion was accompanied by a significant increase in bulk membrane fluidity as measured by electron spin resonance spectroscopy, but the equilibrium binding parameters of acetylcholine to its receptor were unaltered. This suggests strongly that there exist two pools of cholesterol in the AChR-rich Torpedo electroplax membrane: an easily depleted fraction that influences bulk fluidity, and a tightly-bound fraction perhaps surrounding the AChR oligomer.

Large-scale purification of alpha 2-adrenergic receptor-enriched membranes from human platelets. Persistent association of guanine nucleotides with nonpurified membranes

A simple large-scale purification of alpha 2-adrenergic receptor-enriched membranes from human platelets is described. Binding of the antagonist [3H]yohimbine is enriched 3-5-fold compared to a crude membrane fraction. Binding of low concentrations of the partial agonist 3-H-rho-aminoclonidine is increased 15-20-fold due to a higher binding affinity for the purified membranes. A soluble inhibitor of 3H-rho-aminoclonidine binding to purified membranes is found even in thrice-washed crude platelet membranes. The guanine nucleotides GDP and GTP are found to account for this inhibitory activity. Forskolin-stimulated adenylate cyclase activity is also enriched in the purified membrane fraction. Adenylate cyclase activity is inhibited by alpha 2-agonist to a comparable extent in all membrane fractions. This membrane preparation should prove useful in studies of alpha 2-adrenergic receptor mechanisms.

A new model of experimental auto-immune myasthenia gravis

Experimental auto-immune myasthenia gravis (EAMG) was observed in rabbits during the time course of immunization with an acetylcholine (ACh) conjugate: choline-glutaryl-protein. This synthesized antigenic determinant mimics the molecular structure of ACh. The presence of both anti-ACh and auto-anti-idiotypic antibodies was demonstrated. These latter antibodies recognized the ACh receptor, and could have been the triggering agents in this auto-immune condition. Clinical and electromyographic investigations confirmed the myasthenic symptomatology observed after immunization with the ACh conjugate.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.

Pressure reversal of the action of octanol on postsynaptic membranes from Torpedo

Octanol increases the binding of [3H]-acetylcholine to the desensitized state of the nicotinic receptor in postsynaptic membranes prepared from Torpedo californica. This increase in binding results from an increase in the affinity of [3H]-acetylcholine for its receptor without any change in the number of sites or the shape of the acetylcholine binding curve. High pressures of helium (300 atm) decrease [3H]-acetylcholine binding by a mechanism that changes only the affinity of acetylcholine binding. Helium pressure reverses the effect of octanol on the affinity of [3H]-acetylcholine for its receptor. This pressure reversal of the action of octanol at a postsynaptic membrane is consistent either with pressure counteracting an octanol-induced membrane expansion or with independent mechanisms for the actions of octanol and pressure. The data do not conform with a mechanism in which pressure displaces octanol from a binding site on the receptor protein.

Anti-acetylcholine receptor response achieved by immunization with a synthetic peptide from the receptor sequence

A synthetic peptide corresponding to the first twenty amino acids of the N-terminal region from the alpha-subunit of the Torpedo acetylcholine receptor cross reacts with antibodies to the receptor. A conjugate of this peptide to bovine serum albumin elicits in rabbits an immune response towards the synthetic peptide as well as towards the acetylcholine receptor. Blotting experiments demonstrate that the antipeptide antibodies react exclusively with the alpha-subunit of the acetylcholine receptor. Antibodies against synthetic peptides from various regions of the receptor sequence may provide useful reagents for structural and developmental analysis of the acetylcholine receptor as well as for the regulation of experimental autoimmune myasthenia gravis.

Characterization of a snake venom neurotoxin which blocks nicotinic transmission in the avian ciliary ganglion

Bungarus multicinctus venom was fractionated by ion exchange chromatography and the various fractions were assayed for their ability to block synaptic transmission through the chick ciliary ganglion. alpha-Bungarotoxin purified from this venom failed to block transmission at 50 micrograms/ml. A second neurotoxin, which we designate Toxin F, blocked transmission at 1-3 micrograms/ml and also blocked ganglionic depolarizations induced by carbachol. Toxin F was clearly distinguishable from alpha-bungarotoxin on the basis of molecular weight (estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and isoelectric point. Binding assays revealed that 125I-labeled toxin F bound to two sites in the ciliary ganglion: one site that was shared by alpha-bungarotoxin and toxin F and another site that was recognized solely by toxin F. Carbachol and d-tubocurarine displaced only that [125I]toxin F bound to the shared site and had no effect on [125I]toxin F bound to the site recognized by toxin F alone. The results suggest that toxin F blocks synaptic transmission in the chick ciliary ganglion by a postsynaptic mechanism. Further study is required to determine whether this effect of toxin F is mediated through a direct interaction with ganglionic nicotinic receptors.

Kinetic properties of soluble and membrane-bound acetylcholinesterase from electric eel

Using electric eel acetylcholinesterase (AChE) which was either membrane-bound (AChEm) or solubilized (AChEs), similar kinetics were seen in the absence of inhibitor or in the presence of edrophonium, trimethylammonium ion or paraoxon. Thus, both forms of the enzyme appear to behave similarly toward various inhibitors. However, in the presence of a probe sensitive to allosteric effects or changes in membrane fluidity, the two forms exhibit altered behavior. In the presence of F-, the relative rate of substrate hydrolysis by AChEm was reduced more rapidly than with AChEs, whether or not paraoxon was present. When inhibition by paraoxon (10(-7)-10(-4) M) was studied in the presence of F-, AChEs had a Hill coefficient of 1.0, whereas with AChEm the Hill coefficient changed from 0.8 to 1.5.

Crosslinking of proteins in acetylcholine receptor-rich membranes: association between the beta-subunit and the 43 kd subsynaptic protein

Acetylcholine receptor-rich membranes from the electric organ of Torpedo californica are enriched in the four subunits (alpha, beta, gamma, delta) of the acetylcholine receptor (AChR) and for polypeptides at 43 kd and 270 kd. Reaction of these membranes with 3H-N-ethylmaleimide (3H-NEM) demonstrates that most of the available free sulfhydryls reside on the 43 kd protein. Cross-linking reagents that contain NEM as one reactive group, and N-hydroxysuccinimide as the other, were used to study the topography of the 43 kd protein in AChR-rich membranes. Proteins from cross-linked membranes were resolved by SDS-PAGE and the composition of crosslinked products was determined by Western blots and monoclonal antibodies. A crosslinked product at 110 kd was labeled by a monoclonal antibody to the beta-subunit and by a monoclonal antibody to the 43 kd protein, but not by monoclonal antibodies to the alpha, gamma, or delta subunits. The 110 kd crosslink was not produced in the presence of 10 mM lithium diiodosalicylate, which dissociates the 43 kd protein from the membrane. Thus the 43 kd protein is intimately associated with the AChR and in close proximity to the beta-subunit.

Kappa-bungarotoxin: a probe for the neuronal nicotinic receptor in the avian ciliary ganglion

The interaction of snake alpha-neurotoxins with neuronal membranes has been examined in the chick ciliary ganglion. Some, but not all, alpha-neurotoxins block nicotinic transmission in this ganglion. alpha-Bungarotoxin (ABgT), the major alpha-neurotoxin in the venom of Bungarus multicinctus, does not block transmission at high concentrations (1.2 microM) although it binds (Kd = 1 nM) to a pharmacologically nicotinic site in the ganglion. A toxin (kappa-bungarotoxin, KBgT) has been purified from the venom of Bungarus multicinctus. KBgT has a molecular weight of 6500 daltons and a pI of 9.1. KBgT is a potent inhibitor of nicotinic transmission in the ciliary ganglion, producing a reversible (overal several hours) blockade at 75 nM. Pre-exposure of ganglia to 1.2 microM ABgT does not prevent the effects of KBgT, indicating that the blockade occurs at a site distinct from that recognized by ABgT. Binding of [125I]KBgT to ciliary ganglia reveals two binding sites: one which has previously been characterized by [125I]ABgT and one which is not identified by [125I]ABgT. Both of these [125I]KBgT binding sites are blocked following pre-treatment of ganglia with the irreversible nicotinic affinity agent bromoacetylcholine. A two-site model is proposed to account for these observations. One site (the ABgT binding site) is seen by both ABgT and KBgT, and has as yet no physiological function associated with it. The second site is recognized only by the physiologically active KBgT, and may represent binding of the toxin to the physiologically detected nicotinic receptor.

Interaction of monoclonal antibodies to Torpedo acetylcholine receptor with the receptor of skeletal muscle

Several monoclonal antibodies (mcAbs) elicited against the nicotinic acetylcholine receptor (AChR) from Torpedo react also with skeletal muscle AChR. Such mcAbs were used to define antigenic determinants on muscle AChR and to elucidate their effect on muscle AChR functions. Primary chick muscle cultures were used as a model for skeletal muscle. Of the four mcAbs studies only mcAb 5.5, which is directed against the cholinergic site in Torpedo AChR, blocks the binding of alpha-bungarotoxin (alpha-Bgt) to AChR in chick muscle cultures and inhibits carbamylcholine-induced sodium transport in these cells. The interaction of mcAb 5.5 with the cholinergic site on muscle AChR demonstrates the conservation of this site. Two mcAbs, 5.5 and 5.34, each of a different antigenic specificity but both directed against conformation-dependent antigenic determinants, accelerate the degradation of AChR in muscle cultures. From the reactivity of the various mcAbs with Triton-solubilized and membranous AChR it appears that there are some antigenic differences between the detergent solubilized and membranous forms of the receptor.

Isolation of a presynaptic plasma membrane fraction from Torpedo cholinergic synaptosomes: evidence for a specific protein

Synaptosomal plasma membranes were isolated from Torpedo cholinergic synaptosomes which had been purified as previously described or repurified by equilibrium centrifugation. The synaptosomal plasma membrane could be distinguished from postsynaptic membranes by the absence of postsynaptic specific markers (nicotinic AChR) and by its low intramembrane particle complement after freeze fracture. In addition, the presynaptic membrane fraction contained acetylcholinesterase. Gel electrophoresis permitted the identification of a major protein component of the presynaptic membrane fraction which had a molecular weight of 67,000. This protein was not found in postsynaptic membrane or synaptic vesicle fractions. Thus it appeared to be specific to the nerve terminal plasma membrane.

Monoclonal anti-acetylcholine-receptor antibodies directed against the cholinergic binding site

We have isolated 32 hybridoma cell lines producing monoclonal antibodies against the acetylcholine receptor from Torpedo californica. One of these lines, designated 5.5.G.12, secretes antibodies which are directed against the cholinergic binding site of the acetylcholine receptor. This specific antibody blocked the binding of alpha-bungarotoxin to the acetylcholine receptor. The binding of monoclonal antibody 5.5.G.12 to acetylcholine receptor was inhibited by alpha-neurotoxins and by other cholinergic ligands in accordance with their affinities to the nicotinic acetylcholine receptor. None of the other monoclonal antibodies obtained inhibited the binding of alpha-bungarotoxin to acetylcholine receptor, nor was their binding to the acetylcholine receptor inhibited by cholinergic ligands. The monoclonal antibody elicited against the binding site of Torpedo acetylcholine receptor bound also to acetylcholine receptors of various species and organs, demonstrating the wide structural homology between the cholinergic sites of various acetylcholine receptors.

Regulation of acetylcholine receptor phosphorylation by calcium and calmodulin

Acetylcholine receptor-enriched membranes prepared from frozen electric organ of Torpedo californica by differential centrifugation and density step gradient centrifugation were assayed for endogenous phosphorylation in the absence and presence of calmodulin and calcium. Each of the membrane fractions exhibited a 3- to 6-fold stimulation of endogenous phosphorylation by calcium and calmodulin. Both calcium and calmodulin were needed for maximal stimulation although calcium alone afforded a small, reproducible stimulation of endogenous phosphorylation. In the presence of fluoride, a phosphatase inhibitor, the calmodulin plus calcium stimulation was increased an additional 3-fold. The phosphorylation reaction was rapid, and maximal phosphorylation was achieved in 2 min. Stimulation of phosphorylation by calcium and calmodulin was completely inhibited by 25 microM trifluoperazine; at 50 microM it inhibited basal phosphorylation by 60%, suggesting that most of the basal phosphorylation may be due to the endogenous calmodulin present in our membrane preparation. NaDodSO4/polyacrylamide gel electrophoresis revealed that at least three of the phosphorylated species (both in the presence and in the absence of calcium and calmodulin) correspond to subunits of the purified acetylcholine receptor from T. californica (i.e., 65,000, 58,000, and 50,000 daltons) which are the beta, gamma, and delta subunits of the receptor.

Xenon NMR: chemical shifts of a general anesthetic in common solvents, proteins, and membranes

The rare gas xenon contains two NMR-sensitive isotopes in high natural abundance. The nuclide 129Xe has a spin of 1/2: 131Xe is quadrupolar with a spin of 3/2. The complementary NMR characteristics of these nuclei provide a unique opportunity for probing their environment. The method is widely applicable because xenon interacts with a useful range of condensed phases including pure liquids, protein solutions, and suspensions of lipid and biological membranes. Although xenon is chemically inert, it does interact with living systems; it is an effective general anesthetic. We have found that the range of chemical shifts of 129Xe dissolved in common solvents is ca. 200 ppm, which is 30 times larger than that found for 13C in methane dissolved in various solvents. Resonances were also observed for 131Xe in some systems; they were broader and exhibited much greater relaxation rates than did 129Xe. The use of 129Xe NMR as a probe of biological systems was investigated. Spectra were obtained from solutions of myoglobin, from suspensions of various lipid bilayers, and from suspensions of the membranes of erythrocytes and of the acetylcholine receptor-rich membranes of Torpedo californica. These systems exhibited a smaller range of chemical shifts. In most cases there was evidence of a fast exchange of xenon between the aqueous and organic environments, but the exchange was slow in suspensions of dimyristoyl lecithin vesicles.

Phospholipid chain immobilization and steroid rotational immobilization in acetylcholine receptor-rich membranes from Torpedo marmorata

1. The ESR spectra of both phosphatidylcholine and phosphatidylethanolamine spin labels reveal an immobilized lipid component (tau R greater than or equal to 50 ns), in addition to a fluid component (tau R approximately 1 ns), in acetylcholine receptor-rich membranes prepared from Torpedo marmorata electroplax according to the method of Cohen et al. (Cohen, J.B., Weber, M., Huchet, M. and Changeux, J.-P. (1972) FEBS Lett. 26, 43--27). 2. The ESR spectra of the androstanol spin label display a component corresponding to molecules which are immobilized with respect to rotation about the long molecular axis (tau R greater than or equal to 50 ns), in addition to the fluid lipid bilayer component in which the molecules are rotating rapidly about their long axes (tau R approximately 1 ns). This immobilized component is observed throughout the temperature range 2--22 degrees C, at an approximately constant relative intensity of approx. 45% of the total, which is quantitatively the same as previously observed with fatty acid spin labels.

The effects of alpha- and beta-neurotoxins from the venoms of various snakes on transmission in autonomic ganglia

We have previously shown that certain commercially available lots of alpha-bungarotoxin block transmission in ciliary and choroid neurons of both pigeon and chicken ciliary ganglia at a concentration of 10 microgram/ml (1.2 microM). The blockade is antagonized by pre-incubation with 100 microM tubocurarine. Further evidence that this blockade is produced by a postsynaptic action, as one would expect of an alpha-neurotoxin, are our findings that: (a) exposure to the toxin prevents the depolarization of ganglion cells normally seen in response to the cholinergic agonist, carbachol; and (b) the blocking activity of the toxin is removed by treatment with membranes purified from Torpedo electric organ containing an excess of alpha-neurotoxin binding sites. A high affinity binding site for [125I]alpha-bungarotoxin was characterized in the chicken ciliary ganglion. However, since it is labelled equally well by lots of alpha-bungarotoxin which block transmission and those that do not, this site does not appear to be involved in the blockade of transmission. alpha-Cobratoxin (from Naja naja siamensis), the alpha-neurotoxin L.s. III (from Laticauda semifasciata) and certain lots of alpha-bungarotoxin produce a partial blockade of transmission in ciliary neurons of the pigeon ciliary ganglion at a concentration of 10 microgram/ml (1.2 microM), but have no effect on transmission in choroid neurons. Two other alpha-neurotoxins from Laticauda semifasciata, erabutoxin a and erabutoxin b, have no effect on transmission in either cell population at this concentration. None of the alpha-neurotoxins tested had any effect on transmission in either the rat superior cervical ganglion or the rat pelvic ganglion at concentrations up to 100 microgram/ml (12 microM). Collagenase treatment of these ganglia, in an attempt to increase access of the toxins to ganglion cells, did not alter these negative results. beta-Bungarotoxin (0.5 microgram/ml, 0.02 microM) produces a complex blockade of transmission in both avian ciliary ganglia and rat superior cervical ganglia. Unlike the action of alpha-bungarotoxin, the blockade of ciliary ganglion transmission by beta-bungarotoxin is irreversible and is not prevented by pretreatment with tubocurarine.

Quantitation of cation transport by reconstituted membrane vesicles containing purified acetylcholine receptor

A stopped-flow spectroscopic technique was used to study the kinetics of ion transport by reconstituted membrane preparations containing purified acetylcholine receptor. Influx of thallium (I) into membrane vesicles was monitored as a decrease, due to quenching by the thallous ion, in the fluorescence of an entrapped fluorophore. In a reproducible manner, the reconstituted receptor responded to cholinergic agonists by mediating rapid ion transport in the millisecond time range. The effect of agonists was blocked by receptor desensitization and by histrionicotoxin and was absent in membrane vesicles lacking receptor. Analysis of the fast kinetics of cation transport produced by saturating concentrations of agonists yielded an estimated rate of transport through a single reconstituted receptor channel. Comparison of this rate with those reported for in vivo preparations and for purified membranes shows that the reconstituted protein closely resembles the physiologically active receptor.

Major component of acetylcholinesterase in Torpedo electroplax is not basal lamina associated

Electroplax tissue from Torpedo californica contains two major structural forms of the enzyme acetylcholinesterase. One form, composed of tetrameric protomers which are further aggregated by interactions among associated collagenous "tail fibers", has been well characterized previously. This form is associated in situ with the basal lamina. The other form is described and characterized herein. This latter form accounts for at least 50% of the acetylcholinesterase activity of the tissue. This enzyme associated with the tissue phospholipids. It aggregates in aqueous solution but readily dissociates to dimers in 1% sodium cholate solution, a solvent in which it is both soluble and catalytically fully active. The same dimer is obtained in sodium dodecyl sulfate solution where the enzyme is denatured. Denaturation in the presence of the reductant dithiothreitol results in the formation of a single 80000-dalton subunit. The purified enzyme contains no collagenous component. It is not derivable from the collagenous "tailed-enzyme" form in the tissue homogenate. However, the two enzymes have similar molecular weight catalytic subunits and the same substrate-dependent turnover numbers (per active site) for a variety of choline esters which are generally utilized to distinguish specific esterase function. In the tissue homogenate each form of the enzyme is associated with a characteristic structural component (phospholipid or collagen). By implication, acetylcholinesterase function is localized in situ in the phospholipid membrane as well as at the basal lamina.

Topographic studies of Torpedo acetylcholine receptor subunits as a transmembrane complex

The exposure of the four subunits of the acetylcholine receptor from Torpedo californica on both the extracellular and cytoplasmic faces of the postsynaptic membranes of the electroplaque cells has been investigated. Sealed membrane vesicles containing no protein components other than the receptor were isolated and were shown to have 95% of their synaptic surfaces facing the medium. The susceptibility of the four receptor subunits in these preparations to hydrolysis by trypsin both from the external and from the internal medium was used to investigate the exposure of the subunits on the synaptic and cytoplasmic surfaces of the membrane. It was shown by sodium dodecyl sulfate gel electrophoresis of the tryptic products that all four subunits are exposed on the extracellular surface to a similar degree. All four subunits are also exposed on the internal surface of the membrane, but the apparent degree of exposure varies with the subunit size, the larger subunits being more exposed. The results are discussed in terms of a possible topographic model of the receptor as a transmembrane protein complex.

Reconstitution of a functional acetylcholine receptor. Polypeptide chains, ultrastructure, and binding sites for acetylcholine and local anesthetics

The 'reconstitution cycle' is composed of the following sequence of operations. Highly purified receptor-rich membranes prepared from Torpedo marmorata electric organ are exposed to pH 11 to remove the 43,000-Mr protein and dispersed into solution by sodium cholate under conditions where more than 85% of the receptor protein is in its 9-S form. Elimination of the detergent by filtration on a Sephadex column (or dialysis) yields a 'reconstituted receptor' fraction, under conditions which conserve part of the endogenous lipids, or 'reconstituted vesicles' in the presence of an excess of exogenous lipids. The polypeptide composition of these fractions was analysed by sodium dodecylsulfate gel electrophoresis. Conditions are defined for quantitative measurements of the various polypeptide chains. The 40,000-Mr chain, which is labelled by the affinity reagent 4-(N-maleimido)phenyl [3H]trimethylammonium and therefore carries the acetylcholine receptor site, is the dominant polypeptide in the alkaline-treated membranes and the reconstituted acetylcholine receptor. Electron microscopy discloses that many of the alkaline-treated membranes no longer form closed vesicles and do not show the transverse asymmetry of the native membranes observed after tannic acid fixation. In the reconstituted receptor fractions, the receptor molecules reaggregate into discs and may be exposed on both faces of the discs. In the reconstituted vesicles, receptor rosettes are integrated to the lipid vesicles. With native membranes, the radioactive local anesthetic [3H]trimethisoquin binds to three classes of sites: non-specific, low-affinity and high-affinity. Carbamylcholine causes an increase in the number of high-affinity sites up to approximately 0.7 times the number of alpha-125I-bungarotoxin sites. This ratio, the three classes of binding sites, and their regulation by carbamylcholine are conserved through the reconstitution cycle.

Collagen-tailed and hydrophobic components of acetylcholinesterase in Torpedo marmorata electric organ

We have distinguished three fractions of acetylcholinesterase (AcChoE; acetylcholine acetylhydrolase, EC 3.1.1.7) from Torpedo marmorata electric organs, according to their solubilization characteristics. The low-salt-aggregating collagen-tailed forms are soluble in high-salt buffers; their hydrodynamic properties ae not modified in the presence of detergents. They constitute the A fraction, which amounts to about a third of the tissue's AcChoE activity. The low-salt-soluble (LSS) and detergent-soluble (DS) fractions are not sensitive to ionic strength and collagenase. In the presence of nonionic detergents or bile salts, both fractions behave as a monodisperse "6.3S" form, the properties of which have been investigated mostly in the case of Triton X-100. Disulfide bond reduction dissociates the detergent form into a smaller "5S" form. These two forms are thought to be, respectively, detergent-associated dimers and monomers. In the absence of detergent, the LSS fraction is polydisperse: it contains a major 8S component, 11S and 14S components, and faster-sedimenting aggregates, which appear to represent dimers, tetramers, and higher polymers. The heterogeneity of the 8S component in gel filtration suggests that it also contains variable noncatalytic elements. Upon removal of the detergent the DS fraction forms ill-defined aggregates. Trypsin induces quaternary rearrangements of part of the 8S component into 11S and 14S components, which are still convertible into the detergent form; therefore trypsin probably digests noncatalytic elements. Pronase and proteinase K, on the other hand, convert the enzyme into a dimeric form, G2, that does not interact with detergents, probably by cleaving a minor fragment of the subunit that is involved in hydrophobic interactions.