Reconstitution of carbamylcholine-dependent sodium ion flux and desensitization of the acetylcholine receptor from Torpedo californica

Modulation of a rapid neurotransmitter receptor-ion channel by membrane lipids

Membrane lipids modulate the proteins embedded in the bilayer matrix by two non-exclusive mechanisms: direct or indirect. The latter comprise those effects mediated by the physicochemical state of the membrane bilayer, whereas direct modulation entails the more specific regulatory effects transduced via recognition sites on the target membrane protein. The nicotinic acetylcholine receptor (nAChR), the paradigm member of the pentameric ligand-gated ion channel (pLGIC) superfamily of rapid neurotransmitter receptors, is modulated by both mechanisms. Reciprocally, the nAChR protein exerts influence on its surrounding interstitial lipids. Folding, conformational equilibria, ligand binding, ion permeation, topography, and diffusion of the nAChR are modulated by membrane lipids. The knowledge gained from biophysical studies of this prototypic membrane protein can be applied to other neurotransmitter receptors and most other integral membrane proteins.

Fluorescence Studies of Nicotinic Acetylcholine Receptor and Its Associated Lipid Milieu: The Influence of Erwin London's Methodological Approaches

Erwin London dedicated considerable effort to understanding lipid interactions with membrane-resident proteins and how these interactions shaped the formation and maintenance of lipid phases and domains. In this endeavor, he developed ad hoc techniques that greatly contributed to advancements in the field. We have employed and/or modified/extended some of his methodological approaches and applied them to investigate lipid interaction with the nicotinic acetylcholine receptor (nAChR) protein, the paradigm member of the superfamily of rapid pentameric ligand-gated ion channels (pLGIC). Our experimental systems ranged from purified receptor protein reconstituted into synthetic lipid membranes having known effects on receptor function, to cellular systems subjected to modification of their lipid content, e.g., varying cholesterol levels. We have often employed fluorescence techniques, including fluorescence quenching of diphenylhexatriene (DPH) extrinsic fluorescence and of nAChR intrinsic fluorescence by nitroxide spin-labeled phospholipids, DPH anisotropy, excimer formation of pyrene-phosphatidylcholine, and Förster resonance energy transfer (FRET) from the protein moiety to the extrinsic probes Laurdan, DPH, or pyrene-phospholipid to characterize various biophysical properties of lipid-receptor interactions. Some of these strategies are revisited in this review. Special attention is devoted to the anionic phospholipid phosphatidic acid (PA), which stabilizes the functional resting form of the nAChR. The receptor protein was shown to organize its PA-containing immediate microenvironment into microdomains with high lateral packing density and rigidity. PA and cholesterol appear to compete for the same binding sites on the nAChR protein.

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.

Progress in nicotinic receptor structural biology

Here we begin by briefly reviewing landmark structural studies on the nicotinic acetylcholine receptor. We highlight challenges that had to be overcome to push through resolution barriers, then focus on what has been gleaned in the past few years from crystallographic and single particle cryo-EM studies of different nicotinic receptor subunit assemblies and ligand complexes. We discuss insights into ligand recognition, ion permeation, and allosteric gating. We then highlight some foundational aspects of nicotinic receptor structural biology that remain unresolved and are areas ripe for future exploration. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

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.

An allosteric link connecting the lipid-protein interface to the gating of the nicotinic acetylcholine receptor

The mechanisms underlying lipid-sensing by membrane proteins is of considerable biological importance. A unifying mechanistic question is how a change in structure at the lipid-protein interface is translated through the transmembrane domain to influence structures critical to protein function. Gating of the nicotinic acetylcholine receptor (nAChR) is sensitive to its lipid environment. To understand how changes at the lipid-protein interface influence gating, we examined how a mutation at position 418 on the lipid-facing surface of the outer most M4 transmembrane α-helix alters the energetic couplings between M4 and the remainder of the transmembrane domain. Human muscle nAChR is sensitive to mutations at position 418, with the Cys-to-Trp mutation resulting in a 16-fold potentiation in function that leads to a congenital myasthenic syndrome. Energetic coupling between M4 and the Cys-loop, a key structure implicated in gating, do not change with C418W. Instead, Trp418 and an adjacent residue couple energetically with residues on the M1 transmembrane α-helix, leading to a reorientation of M1 that stabilizes the open state. We thus identify an allosteric link connecting the lipid-protein interface of the nAChR to altered channel function.

Phylogenetic conservation of protein-lipid motifs in pentameric ligand-gated ion channels

Using the crosstalk between the nicotinic acetylcholine receptor (nAChR) and its lipid microenvironment as a paradigm, this short overview analyzes the occurrence of structural motifs which appear not only to be conserved within the nAChR family and contemporary eukaryotic members of the pentameric ligand-gated ion channel (pLGIC) superfamily, but also extend to prokaryotic homologues found in bacteria. The evolutionarily conserved design is manifested in: 1) the concentric three-ring architecture of the transmembrane region, 2) the occurrence in this region of distinct lipid consensus motifs in prokaryotic and eukaryotic pLGIC and 3) the key participation of the outer TM4 ring in conveying the influence of the lipid membrane environment to the middle TM1-TM3 ring and this, in turn, to the inner TM2 channel-lining ring, which determines the ion selectivity of the channel. The preservation of these constant structural-functional features throughout such a long phylogenetic span likely points to the successful gain-of-function conferred by their early acquisition. This article is part of a Special Issue entitled: Lipid-protein interactions.

Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function

Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

Membrane protein reconstitution for functional and structural studies

Membrane proteins are involved in various critical biological processes, and studying membrane proteins represents a major challenge in protein biochemistry. As shown by both structural and functional studies, the membrane environment plays an essential role for membrane proteins. In vitro studies are reliant on the successful reconstitution of membrane proteins. This review describes the interaction between detergents and lipids that aids the understanding of the reconstitution processes. Then the techniques of detergent removal and a few useful techniques to refine the formed proteoliposomes are reviewed. Finally the applications of reconstitution techniques to study membrane proteins involved in Ca(2+) signaling are summarized.

The role of the M4 lipid-sensor in the folding, trafficking, and allosteric modulation of nicotinic acetylcholine receptors

With the availability of high resolution structural data, increasing attention has focused on the mechanisms by which drugs and endogenous compounds allosterically modulate nicotinic acetylcholine receptor (nAChR) function. Lipids are potent modulators of the nAChR from Torpedo. Membrane lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations, as well as influencing the transitions between these conformational states. Structural and functional data highlight a role for the lipid-exposed M4 transmembrane α-helix of each subunit in lipid sensing, and suggest that lipids influence gating by altering the binding of M4 to the adjacent transmembrane α-helices, M1 and M3. M4 has also been implicated in both the folding and trafficking of nAChRs to the cell surface, as well as in the potentiation of nAChR gating by neurosteroids. Here, we discuss the roles of M4 in the folding, trafficking, and allosteric modulation of nAChRs. We also consider the hypothesis that variable chemistry at the M4-M1/M3 transmembrane α-helical interface in different nAChR subunits governs the capacity for potentiation by activating lipids. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors

General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

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.

Molecular mechanisms of acetylcholine receptor-lipid interactions: from model membranes to human biology

Lipids are potent modulators of the Torpedo nicotinic acetylcholine receptor. Lipids influence nicotinic receptor function by allosteric mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations. Recent structures reveal that lipids could alter function by modulating transmembrane α-helix/α-helix packing, which in turn could alter the conformation of the allosteric interface that links the agonist-binding and transmembrane pore domains-this interface is essential in the coupling of agonist binding to channel gating. We discuss potential mechanisms by which lipids stabilize different conformational states in the context of the hypothesis that lipid-nicotinic receptor interactions modulate receptor function at biological synapses.

Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function

Like all other monomeric or multimeric transmembrane proteins, receptors for neurotransmitters are surrounded by a shell of lipids which form an interfacial boundary between the protein and the bulk membrane. Among these lipids, cholesterol and sphingolipids have attracted much attention because of their well-known propensity to segregate into ordered platform domains commonly referred to as lipid rafts. In this review we present a critical analysis of the molecular mechanisms involved in the interaction of cholesterol/sphingolipids with neurotransmitter receptors, in particular acetylcholine and serotonin receptors, chosen as representative members of ligand-gated ion channels and G protein-coupled receptors. Cholesterol and sphingolipids interact with these receptors through typical binding sites located in both the transmembrane helices and the extracellular loops. By altering the conformation of the receptors ("chaperone-like" effect), these lipids can regulate neurotransmitter binding, signal transducing functions, and, in the case of multimeric receptors, subunit assembly and subsequent receptor trafficking to the cell surface. Several sphingolipids (especially gangliosides) also exhibit low/moderate affinity for neurotransmitters. We suggest that such lipids could facilitate (i) the attachment of neurotransmitters to the post-synaptic membrane and in some cases (ii) their subsequent delivery to specific protein receptors. Overall, various experimental approaches provide converging evidence that the biological functions of neurotransmitters and their receptors are highly dependent upon sphingolipids and cholesterol, which are active partners of synaptic transmission. Several decades of research have been necessary to untangle the skein of a complex network of molecular interactions between neurotransmitters, their receptors, cholesterol and sphingolipids. This sophisticated crosstalk between all four distinctive partners may allow a fine biochemical tuning of synaptic transmission.

A lipid-dependent uncoupled conformation of the acetylcholine receptor

Lipids influence the ability of Cys-loop receptors to gate open in response to neurotransmitter binding, but the underlying mechanisms are poorly understood. With the nicotinic acetylcholine receptor (nAChR) from Torpedo, current models suggest that lipids modulate the natural equilibrium between resting and desensitized conformations. We show that the lipid-inactivated nAChR is not desensitized, instead it adopts a novel conformation where the allosteric coupling between its neurotransmitter-binding sites and transmembrane pore is lost. The uncoupling is accompanied by an unmasking of previously buried residues, suggesting weakened association between structurally intact agonist-binding and transmembrane domains. These data combined with the extensive literature on Cys-loop receptor-lipid interactions suggest that the M4 transmembrane helix plays a key role as a lipid-sensor, translating bilayer properties into altered nAChR function.

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

A radioactive uptake assay to measure ion transport across ion channel–containing liposomes

Here we describe a procedure for incorporating ion channels into lipid vesicles (liposomes) and functional characterization of the channel population by assaying radioactive isotope uptake into these proteoliposomes. The technique as described will work only for potassium channels but can be easily modified, as suggested in the text, for other ion channels and transporters. Purified ion channel proteins in detergent micelles are combined with solubilized lipids. Detergent is subsequently removed from protein-lipid complexes by gel filtration or dialysis into high potassium (high [K+]) buffer. After freezing-thawing and sonication, the resultant larger liposomes are passed over another gel-filtration column to exchange an extraliposomal high [K+] to a low [K+] buffer, thus establishing a large K+ gradient across the liposomal membrane. Trace 86Rb is then added to the extraliposomal space and the reaction begins. If the ion channel is permeable to K+, the K+ inside exits the liposomes down its concentration gradient and the 86Rb outside accumulates in the intraliposomal space until equilibrium is reached. The reaction time course is monitored by measurement of accumulated 86Rb after removal of external 86Rb over an ion-exchange column. The 86Rb flux assay takes 2-5 hours depending on the reaction rate and the number of desired time points.

Structural basis for lipid modulation of nicotinic acetylcholine receptor function

The nicotinic acetylcholine receptor (AChR) is the archetype molecule in the superfamily of ligand-gated ion channels (LGIC). Members of this superfamily mediate fast intercellular communication in response to endogenous neurotransmitters. This review is focused on the structural and functional crosstalk between the AChR and lipids in the membrane microenvironment, and the modulation exerted by the latter on ligand binding and ion translocation. Experimental approaches using Laurdan extrinsic fluorescence and Förster-type resonance energy transfer (FRET) that led to the characterization of the polarity and molecular dynamics of the liquid-ordered phase AChR-vicinal lipids and the bulk membrane lipids, and the asymmetry of the AChR-rich membrane are reviewed first. The topological relationship between protein and lipid moieties and the changes in physical properties induced by exogenous lipids are discussed next. This background information lays the basis for understanding the occurrence of lipid sites in the AChR transmembrane region, and the selectivity of the protein-lipid interactions. Changes in FRET efficiency induced by fatty acids, phospholipid and cholesterol (Chol), led to the identification of discrete sites for these lipids on the AChR protein, and electron-spin resonance (ESR) spectroscopy has recently facilitated determination of the stoichiometry and selectivity for the AChR of the shell lipid. The influence of lipids on AChR function is discussed next. Combined single-channel and site-directed mutagenesis data fostered the recognition of lipid-sensitive residues in the transmembrane region, dissecting their contribution to ligand binding and channel gating, opening and closing. Experimental evidence supports the notion that the interface between the protein moiety and the adjacent lipid shell is the locus of a variety of pharmacologically relevant processes, including the action of steroids and other lipids.

Cooperativity and specificity of association of a designed transmembrane peptide

Thermodynamics studies aimed at quantitatively characterizing free energy effects of amino acid substitutions are not restricted to two state systems, but do require knowing the number of states involved in the equilibrium under consideration. Using analytical ultracentrifugation and NMR methods, we show here that a membrane-soluble peptide, MS1, designed by modifying the sequence of the water-soluble coiled-coil GCN4-P1, exhibits a reversible monomer-dimer-trimer association in detergent micelles with a greater degree of cooperativity in C14-betaine than in dodecyl phosphocholine detergents.

Thermal stability of Torpedo californica acetylcholine receptor in a cholesterol lipid environment

Controlled heating of acetylcholine receptor (AChR) vesicles inactivates the alpha-bungarotoxin (alpha-Bgtx) binding sites with a T50 (temperature at which 50% of the initial capacity to bind alpha-Bgtx remains) of 60 +/- 0.2 degrees C. The same value was obtained for receptor reconstituted in lipid vesicles from Torpedo electroplax where the % mol composition of cholesterol to phospholipid was 30. However, when the reconstitution was carried out in dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidic acid (DOPA) vesicles (3:1 molar ratio), T50 of the curves decreased to 56 +/- 0.2 degrees C and no carbamylcholine stimulated 22Na+ flux was detected. Inclusion of cholesterol in the DOPC-DOPA vesicles increased the toxin binding site stability. The maximal T50 of the toxin binding curves was 63 +/- 0.1 degrees C when the % mol cholesterol/mol DOPC:DOPA in the vesicles was 33. Under these conditions AChR was able to translocate ions, a property that was lost upon heating at 46 degrees C. Preincubation of AChR in the presence of d-tubocurarine, tetracaine or procaine did not affect T50 values of toxin binding. However, a slight increment in thermal stability was found when the receptor was preincubated in the presence of carbamylcholine. The results show that cholesterol requirements for protecting against thermal inactivation of toxin binding and ion gating properties are different and the carbamylcholine-bound receptor may have a different conformation.

Oligosaccharide composition of the neurotoxin-responsive sodium channel of mouse neuroblastoma and requirement of sialic acid for biological activity

A glycoprotein, M(r) 200,000, which has the biological activity of the neurotoxin-responsive Na+ channel, was isolated from a clonal line of mouse neuroblastoma cells, N-18. The glycoprotein was purified to homogeneity in 18% yield by methods used to purify glycoproteins, which included metabolic labeling of the cells with L-[3H]fucose and binding of the radioactive glycoproteins to WGA- and lentil-Sepharose, and DEAE-cellulose. The glycoprotein has biological activity of neurotoxin-responsive ion flux when reconstituted into artificial phospholipid vesicles. This activity was shown to depend on the presence of sialic acid since treatment of the purified, reconstituted glycoprotein with Vibrio cholerae neuraminidase abolished the response to neurotoxins of 86Rb flux. The [3H]fucose-containing glycopeptides derived by Pronase digestion of the glycoprotein were characterized by affinity to immobilized lectins and contained di-, tri-, and tetra-antennary oligosaccharides in a ratio of 2:4:3. Most of the glycopeptides were sialylated as shown by binding characteristics to immobilized serotonin-Sepharose with and without neuraminidase. The structure of the diantennary oligosaccharides was elucidated by 500-MHz 1H NMR spectroscopy. The Con A-bound fraction contains alpha-NeuNAc-(2-->6)-bound group on the GlcNAc5' antenna and an alpha-NeuNAc-(2-->3)-bound groups on the GlcNAc5 antenna. An alpha-L-fucosyl group is (1-->6)-bound to the Asn core GlcNAc1 residue.

Functional reconstitution of the nicotinic acetylcholine receptor by CHAPS dialysis depends on the concentrations of salt, lipid, and protein

The detergent CHAPS was found to be the preferable surfactant for the efficient purification and reconstitution of the Torpedo californica nicotinic acetylcholine receptor (AChR). The main result is that the incorporation of the AChR proteins into lipid vesicles by CHAPS dialysis was strongly dependent on the salt and protein concentrations. As monitored by sucrose gradients, by electron microscopy, and by agonist-induced lithium ion flux, the best reconstitution yields were obtained in 0.5 M NaCl at a protein concentration of 0.5 g/L and in 0.84 M NaCl at 0.15 g/L protein. Electron micrographs of receptor molecules, which were incorporated into vesicles, showed single, nonaggregated dimer (M(r) = 580,000) and monomer (M(r) = 290,000) species. CHAPS dialysis at NaCl concentrations less than 0.5 M largely reduced the receptor incorporation concomitant with protein aggregation. Electron micrographs of these preparations revealed large protein sheets or ribbons not incorporated into vesicles. The analysis of static and dynamic light scattering demonstrated that the detergent-solubilized AChR molecules aggregate at low lipid contents (less than or equal to 500 phospholipids/AChR dimer), independent of the salt concentration. AChR proteins eluted from an affinity column with a solution containing 8 mM CHAPS (but no added lipid) still contained 130 +/- 34 tightly bound phospholipids per dimer. The aggregates (about 10 dimers on the average) could be dissociated by readdition of lipid and, interestingly, also by increasing the CHAPS concentration up to 15 mM. This value is much higher than the CMC of CHAPS = 4.0 +/- 0.4 mM, which was determined by surface tension measurements. The data clearly suggest protein-micelle interactions in addition to the association of monomeric detergents with proteins. Furthermore, the concentration of the (free) monomeric CHAPS at the vesicle-micelle transformation in 0.5 M NaCl ([Dw]c = 3.65 mM) was higher than in 50 mM NaCl ([Dw]c = 2.8 mM). However, it is suggested that the main effect of high salt concentrations during the reconstitution process is an increase of the fusion (rate) of the ternary protein/lipid/CHAPS complexes with mixed micelles or with vesicular structures, similar to the salt-dependent fusion of vesicles.

Solubilization of dopamine D-1 receptors with a zwitterionic detergent DCHAPS and their reconstitution

1. Dopamine D-1 receptors of the bovine caudate nucleus were solubilized with different detergents. They were labelled with [3H]SCH 23390 and assayed by filtration through PEI-coated glass fibre filters and Sephadex G-50 columns. 2. DCHAPS was the best solubilizer among all detergents used and at 0.075% DCHAPS, 10 mg/ml protein, 30 min, 4 degrees C, gave the yield of 48.7%. 3. Reconstitution of solubilized receptors was performed using SM-2 Bio-Beads. Phosphatidylcholine did not improve reconstitution suggesting that DCHAPS solubilized sufficient amounts of the membrane phospholipids. 4. Loss of affinity of solubilized receptors to [3H]SCH 23390 binding was reversible. Apparent Kd values of 0.36 +/- 0.02, 21.3 +/- 3.2 and 0.77 +/- 0.05 nM were obtained for membrane-bound, solubilized and reconstituted receptors, respectively.

Filter assay technique and quench-flow experiments: examples of receptor-mediated transmembrane ion-exchange measured with membrane vesicles

Modifications to a quench-flow apparatus are described which allow a rapid, in-line filter assay with immediate washing, in conditions to give minimum background. A design for an effluent spout is presented, which decelerates the liquid by a large factor, prevents splashes, limits the area of the filter exposed to the sample and allows an immediate wash over a larger area. A design for a filter assay funnel for general use is also presented. These devices feature minimal contact of the funnels with the filter disc. Examples are given in which in-line filtration was used to follow transmembrane ion flux in membrane vesicle preparations. In measurements of transmembrane flux with membrane vesicles and radioisotope the filter assay background can be resolved into three components. These are, (1) the uptake of radioactivity by the filter, (2) the radioactivity inside the vesicles not taking part in the specific measurement and (3) the occlusion of radioactivity in aggregated membrane particles on the filter. These different components depend on the conditions in different ways. Techniques for minimizing the background in filter assays are discussed. The importance of rapid filtration and immediate washing is demonstrated. The examples given illustrate that the function of the acetylcholine receptor from E. electricus is not affected by diisopropylfluorophosphate in the conditions used, and that added GABA is not removed from solution in a brain membrane preparation by the GABA uptake mechanisms in the short times of the experiments.

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Reconstitution of acetylcholine receptors into planar lipid bilayers

Obviously, bilayer reconstitution experiments have largely contributed to the understanding of the AChR-channel function. Nevertheless, at present there are many unanswered questions concerning the minimum structural requirements for AChR-channel function, agonist cooperativity, and different types of AChR. Another complex of parameters important for receptor function which must be explored in much more detail, is the dependence of AChR-channel function on membrane composition and its physical state. This important but rather neglected field is predestined to be explored by reconstitution techniques. All the results on AChRs reconstituted in planar lipid bilayers cannot adequately be discussed without the data obtained by other techniques, thus coming back to the statements already mentioned in the introductory section about strategies for investigating ion channels in general. Only such a strategy can lead to a molecular understanding of channel function.

Comparison of acetylcholine and alpha-bungarotoxin binding sites in insects and vertebrates

1. The nervous tissue of locusts contains high affinity as well as low affinity binding sites for acetylcholine which display a similar nicotinic pharmacology. 2. Hill plot analysis indicated a non-cooperative binding of acetylcholine. 3. In membrane preparations from locust ganglia and mouse brain the number of binding sites for ACh was about ten fold lower than for BGTX, whereas in membranes from electric tissue both sites occurred in similar concentrations. 4. Drug binding studies suggest that the high affinity binding sites for ACh and BGTX in preparations from insect and mouse are different; whereas in electric tissue both sites are very similar. 5. Precipitation experiments using immobilized BGTX and specific antibodies indicated that in insect nervous tissue as in electric tissue the ACh and BGTX binding sites are located on the same receptor molecule and occupy distinct partially overlapping binding sites, whereas in the vertebrate brain both sites are located on distinct binding proteins.

Reconstitution of the nicotinic acetylcholine receptor using a lipid substitution technique

The nicotinic acetylcholine receptor was purified by affinity chromatography in the presence of dioleoylphosphatidylcholine (DOPC). A method for replacing the DOPC with other lipids was developed by using detergent solubilization with a large excess of the new lipid followed by sucrose density gradient centrifugation in detergent-free buffers to separate receptor-lipid complexes from excess lipid and detergent. Homogenous complexes of defined lipid composition could be easily prepared and the efficiency of substitution was independent of lipid type. However, the functional properties of the resulting lipid complexes depended on the lipid composition.

Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles

The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation.

Neurotoxin-modulated uptake of sodium by highly purified preparations of the electroplax tetrodotoxin-binding glycopeptide reconstituted into lipid vesicles

Using the dialysable detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate), the tetrodotoxin-binding protein from the electroplax of the electric eel has been purified to a high degree of both chemical homogeneity and toxin-binding activity. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the best preparations showed only a single microheterogeneous band at Mr approximately 260,000, despite attempts to visualize smaller bands by sample overloading. Upon dialysis, this material became incorporated into the membranes of small unilamellar vesicles, and in this form the purified protein exhibited tetrodotoxin-binding properties similar to the component in the original electroplax membrane. Furthermore, in the presence of activator neurotoxins the vesicles were able to accumulate isotopic sodium in a manner similar to that previously described for less active or less pure preparations of vesicles containing either mammalian or eel electroplax toxin-binding proteins. Quantitative consideration of the isotopic transport activity of this pure material, along with the high degree of purity of the protein, strongly suggests that the 260-kDa glycopeptide from electroplax is necessary and sufficient to account for the sodium channel function seen in these studies, and eliminates the possible involvement of smaller peptides in the channel phenomena observed.

Biochemical interactions of carbamates and ecothiophate with the activated conformation of nicotinic acetylcholine receptor

Purified Torpedo nobiliana electric organ acetylcholine receptor (AChR) was reconstituted into membranes containing natural phospholipids supplemented with cholesterol (25% w/w). The reconstituted system facilitates the study of the effects of drugs on the regulation of the AChR channel complex under both resting and carbachol (carb)-stimulated conditions. Neostigmine (Neo) was the only carbamate to induce activation of [3-H]-phencyclidine ([3-H]-PCP) binding to the channel sites, acting as a weak agonist. The activation of [3-H]-PCP binding is dependent upon the nature of the reconstituted systems, with carb/Neo activation ratios of 8, 3, and 1 for the intact purified AChR vesicles fraction (PVF), the PVF reconstituted in phospholipid/cholesterol (CRPVF), and the PVF reconstituted in phospholipid (RPVF), respectively. The carbamates Neo, physostigmine (Physo), and pyridostigmine (Pyrido) inhibited carb-activated [3-H]-PCP binding with Ki values of 10, 20, and 1,600 microM, respectively. The inhibition was mixed competitive-noncompetitive in nature. The characteristic response of CRPVF to carb-stimulated [22-Na] influx was inhibited by the three carbamates, with IC-50 values of 6, 50, and 1,000 microM for Neo, Physo, and Pyrido, respectively. The quaternary ammonium organophosphate ecothiophate (Eco) inhibited carb-stimulated [22-Na] influx with potency similar to that of Neo. Preincubation of AChR preparation with the carbamates and ecothiophate caused a reduction in the binding of [125-I]-alpha-bungarotoxin ([125-I]-alpha-BGT) with the following decreasing order of potency: Neo less than Physo less than Eco less than Pyrido. Calcium has a direct modulatory role on the time-course inhibition of [125-I]-alpha-BGT binding by these drugs. While we observed a high potency of Neo and Physo in inhibiting [125-I]-alpha-BGT binding, it was undetectable for the carbamate insecticide 2-methyl-2-(methylthio)propionaldehyde-O-(methylcarbamoyl)oxime (aldicarb). These data suggest that the potent anticholinesterase carbamate agents interact differently with the AChR and its ionic channel. Their interactions with the nicotinic AChR channel system can be described as (a) weakly agonist, (b) directly acting on the open conformation of the channel, and (c) blocking the AChR-binding sites.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.