Gamma- and delta-subunits regulate the affinity and the cooperativity of ligand binding to the acetylcholine receptor

Receptor subunit compositions underly distinct potencies of a muscle relaxant in fast and slow muscle fibers

A line of studies in the 1960s–1980s suggested that muscle relaxants do not work uniformly on all skeletal muscles, though its mechanism has not been clarified. We showed here that a classical non-depolarizing muscle relaxant pancuronium inhibits fast muscle fibers at lower concentration compared to slow muscle fibers in zebrafish. The difference of effective concentration was observed in locomotion caused by tactile stimulation as well as in synaptic currents of the neuromuscular junction induced by motor neuron excitation. We further showed that this difference arises from the different composition of acetylcholine receptors between slow and fast muscle fibers in the neuromuscular junction of zebrafish. It will be interesting to examine the difference of subunit composition and sensitivity to muscle relaxants in other species.

Structural mechanism of muscle nicotinic receptor desensitization and block by curare

Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor.

Structural snapshots of TRPV1 reveal mechanism of polymodal functionality

Many transient receptor potential (TRP) channels respond to diverse stimuli and conditionally conduct small and large cations. Such functional plasticity is presumably enabled by a uniquely dynamic ion selectivity filter that is regulated by physiological agents. What is currently missing is a "photo series" of intermediate structural states that directly address this hypothesis and reveal specific mechanisms behind such dynamic channel regulation. Here, we exploit cryoelectron microscopy (cryo-EM) to visualize conformational transitions of the capsaicin receptor, TRPV1, as a model to understand how dynamic transitions of the selectivity filter in response to algogenic agents, including protons, vanilloid agonists, and peptide toxins, permit permeation by small and large organic cations. These structures also reveal mechanisms governing ligand binding substates, as well as allosteric coupling between key sites that are proximal to the selectivity filter and cytoplasmic gate. These insights suggest a general framework for understanding how TRP channels function as polymodal signal integrators.

A single historical substitution drives an increase in acetylcholine receptor complexity

Human adult muscle-type acetylcholine receptors are heteropentameric ion channels formed from four different, but evolutionarily related, subunits. These subunits assemble with a precise stoichiometry and arrangement such that two chemically distinct agonist-binding sites are formed between specific subunit pairs. How this subunit complexity evolved and became entrenched is unclear. Here we show that a single historical amino acid substitution is able to constrain the subunit stoichiometry of functional acetylcholine receptors. Using a combination of ancestral sequence reconstruction, single-channel electrophysiology, and concatenated subunits, we reveal that an ancestral β-subunit can not only replace the extant β-subunit but can also supplant the neighboring δ-subunit. By forward evolving the ancestral β-subunit with a single amino acid substitution, we restore the requirement for a δ-subunit for functional channels. These findings reveal that a single historical substitution necessitates an increase in acetylcholine receptor complexity and, more generally, that simple stepwise mutations can drive subunit entrenchment in this model heteromeric protein.

Back to the future: Rational maps for exploring acetylcholine receptor space and time

Global functions of nicotinic acetylcholine receptors, such as subunit cooperativity and compatibility, likely emerge from a network of amino acid residues distributed across the entire pentameric complex. Identification of such networks has stymied traditional approaches to acetylcholine receptor structure and function, likely due to the cryptic interdependency of their underlying amino acid residues. An emerging evolutionary biochemistry approach, which traces the evolutionary history of acetylcholine receptor subunits, allows for rational mapping of acetylcholine receptor sequence space, and offers new hope for uncovering the amino acid origins of these enigmatic properties.

Ancestral Reconstruction Approach to Acetylcholine Receptor Structure and Function

Acetylcholine receptors (AChRs) are members of a superfamily of proteins called pentameric ligand-gated ion channels, which are found in almost all forms of life and thus have a rich evolutionary history. Muscle-type AChRs are heteropentameric complexes assembled from four related subunits (α, β, δ, and ɛ). Here we reconstruct the amino acid sequence of a β subunit ancestor shared by humans and cartilaginous fishes (i.e., Torpedo). Then, by resurrecting this ancestral β subunit and co-expressing it with human α, δ, and ɛ subunits, we show that despite 132 substitutions, the ancestral subunit is capable of forming human/ancestral hybrid AChRs. Whole-cell currents demonstrate that the agonist acetylcholine has reduced potency for hybrid receptors, while single-channel recordings reveal that hybrid receptors display reduced conductance and open probability. Our results outline a promising strategy for studies of AChR evolution aimed at identifying the amino acid origins of AChR structure and function.

Differential Contribution of Subunit Interfaces to α9α10 Nicotinic Acetylcholine Receptor Function

Nicotinic acetylcholine receptors can be assembled from either homomeric or heteromeric pentameric subunit combinations. At the interface of the extracellular domains of adjacent subunits lies the acetylcholine binding site, composed of a principal component provided by one subunit and a complementary component of the adjacent subunit. Compared with neuronal nicotinic acetylcholine cholinergic receptors (nAChRs) assembled from α and β subunits, the α9α10 receptor is an atypical member of the family. It is a heteromeric receptor composed only of α subunits. Whereas mammalian α9 subunits can form functional homomeric α9 receptors, α10 subunits do not generate functional channels when expressed heterologously. Hence, it has been proposed that α10 might serve as a structural subunit, much like a β subunit of heteromeric nAChRs, providing only complementary components to the agonist binding site. Here, we have made use of site-directed mutagenesis to examine the contribution of subunit interface domains to α9α10 receptors by a combination of electrophysiological and radioligand binding studies. Characterization of receptors containing Y190T mutations revealed unexpectedly that both α9 and α10 subunits equally contribute to the principal components of the α9α10 nAChR. In addition, we have shown that the introduction of a W55T mutation impairs receptor binding and function in the rat α9 subunit but not in the α10 subunit, indicating that the contribution of α9 and α10 subunits to complementary components of the ligand-binding site is nonequivalent. We conclude that this asymmetry, which is supported by molecular docking studies, results from adaptive amino acid changes acquired only during the evolution of mammalian α10 subunits.

Improved resolution of single channel dwell times reveals mechanisms of binding, priming, and gating in muscle AChR

The acetylcholine receptor (AChR) from vertebrate skeletal muscle initiates voluntary movement, and its kinetics of activation are crucial for maintaining the safety margin for neuromuscular transmission. Furthermore, the kinetic mechanism of the muscle AChR serves as an archetype for understanding activation mechanisms of related receptors from the Cys-loop superfamily. Here we record currents through single muscle AChR channels with improved temporal resolution approaching half an order of magnitude over our previous best. A range of concentrations of full and partial agonists are used to elicit currents from human wild-type and gain-of-function mutant AChRs. For each agonist-receptor combination, rate constants are estimated from maximum likelihood analysis using a kinetic scheme comprised of agonist binding, priming, and channel gating steps. The kinetic scheme and rate constants are tested by stochastic simulation, followed by incorporation of the experimental step response, sampling rate, background noise, and filter bandwidth. Analyses of the simulated data confirm all rate constants except those for channel gating, which are overestimated because of the established effect of noise on the briefest dwell times. Estimates of the gating rate constants were obtained through iterative simulation followed by kinetic fitting. The results reveal that the agonist association rate constants are independent of agonist occupancy but depend on receptor state, whereas those for agonist dissociation depend on occupancy but not on state. The priming rate and equilibrium constants increase with successive agonist occupancy, and for a full agonist, the forward rate constant increases more than the equilibrium constant; for a partial agonist, the forward rate and equilibrium constants increase equally. The gating rate and equilibrium constants also increase with successive agonist occupancy, but unlike priming, the equilibrium constants increase more than the forward rate constants. As observed for a full and a partial agonist, the gain-of-function mutation affects the relationship between rate and equilibrium constants for priming but not for channel gating. Thus, resolving brief single channel currents distinguishes priming from gating steps and reveals how the corresponding rate and equilibrium constants depend on agonist occupancy.

Conotoxins targeting nicotinic acetylcholine receptors: an overview

Marine snails of the genus Conus are a large family of predatory gastropods with an unparalleled molecular diversity of pharmacologically active compounds in their venom. Cone snail venom comprises of a rich and diverse cocktail of peptide toxins which act on a wide variety of ion channels such as voltage-gated sodium- (Na_V), potassium- (K_V), and calcium- (Ca_V) channels as well as nicotinic acetylcholine receptors (nAChRs) which are classified as ligand-gated ion channels. The mode of action of several conotoxins has been the subject of investigation, while for many others this remains unknown. This review aims to give an overview of the knowledge we have today on the molecular pharmacology of conotoxins specifically interacting with nAChRs along with the structure–function relationship data.

Structure-function elucidation of a new α-conotoxin, Lo1a, from Conus longurionis

α-Conotoxins are peptide toxins found in the venom of marine cone snails and potent antagonists of various subtypes of nicotinic acetylcholine receptors (nAChRs). nAChRs are cholinergic receptors forming ligand-gated ion channels in the plasma membranes of certain neurons and the neuromuscular junction. Because nAChRs have an important role in regulating transmitter release, cell excitability, and neuronal integration, nAChR dysfunctions have been implicated in a variety of severe pathologies such as epilepsy, myasthenic syndromes, schizophrenia, Parkinson disease, and Alzheimer disease. To expand the knowledge concerning cone snail toxins, we examined the venom of Conus longurionis. We isolated an 18-amino acid peptide named α-conotoxin Lo1a, which is active on nAChRs. To the best of our knowledge, this is the first characterization of a conotoxin from this species. The peptide was characterized by electrophysiological screening against several types of cloned nAChRs expressed in Xenopus laevis oocytes. The three-dimensional solution structure of the α-conotoxin Lo1a was determined by NMR spectroscopy. Lo1a, a member of the α4/7 family, blocks the response to acetylcholine in oocytes expressing α7 nAChRs with an IC50 of 3.24 ± 0.7 μM. Furthermore, Lo1a shows a high selectivity for neuronal versus muscle subtype nAChRs. Because Lo1a has an unusual C terminus, we designed two mutants, Lo1a-ΔD and Lo1a-RRR, to investigate the influence of the C-terminal residue. Lo1a-ΔD has a C-terminal Asp deletion, whereas in Lo1a-RRR, a triple-Arg tail replaces the Asp. They blocked the neuronal nAChR α7 with a lower IC50 value, but remarkably, both adopted affinity for the muscle subtype α1β1δε.

Synergistic effect of sevoflurane and isoflurane on inhibition of the adult-type muscle nicotinic acetylcholine receptor by rocuronium

PURPOSE

Inhaled anesthetics increase the incidence of postoperative residual neuromuscular blockade, and the mechanism is still unclear. We have investigated the synergistic effect of low-concentration inhaled anesthetics and rocuronium on inhibition of the inward current of the adult-type muscle nicotinic acetylcholine receptor (ε-nAChR).

METHODS

Adult-type mouse muscle ε-nAChR was expressed in HEK293 cells by liposome transfection. The inward current of the ε-nAChR was activated by use of 10 μmol/L acetylcholine alone or in combination with different concentrations of sevoflurane, isoflurane, or rocuronium. The concentration-response curves of five cells were constructed, and the data yielded the 5, 25, and 50 % inhibitory concentrations (IC5, IC25, and IC50, respectively) for single-drug application. Subsequently, the functional channels were perfused by adding 0.5 IC5 of either sevoflurane or isoflurane (aqueous concentrations 140 and 100 μmol/L, respectively) to the solution, followed by addition of IC5, IC25, or IC50 rocuronium. The amount of inhibition was calculated to quantify their synergistic effect.

RESULTS

The inhibitory effect of rocuronium was enhanced by sevoflurane or isoflurane in a concentration-dependent manner. Sevoflurane or isoflurane (0.5 IC5) with rocuronium at IC5, IC25, and IC50 synergistically inhibited the current amplitude of adult-type muscle ε-nAChR. When the IC5 of rocuronium was used, isoflurane had a stronger synergistic effect than sevoflurane (p < 0.05). When rocuronium was applied at higher concentrations (IC25 and IC50), sevoflurane had an effect similar to that of isoflurane. For both inhaled anesthetics, the synergistic effect was more intense for rocuronium at IC5 than for rocuronium at IC25 or IC50.

CONCLUSION

Residual-concentration sevoflurane or isoflurane has a strong synergistic effect with rocuronium at clinically relevant residual concentrations. A lower rocuronium concentration resulted in a stronger synergistic effect.

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

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

Distinct properties of glycine receptor β+/α- interface: unambiguously characterizing heteromeric interface reconstituted in homomeric protein

The glycine receptor (GlyR) exists either in homomeric α or heteromeric αβ forms. Its agonists bind at extracellular subunit interfaces. Unlike subunit interfaces from the homomeric α GlyR, subunit interfaces from the heteromeric αβ GlyR have not been characterized unambiguously because of the existence of multiple types of interface within single receptors. Here, we report that, by reconstituting β+/α- interfaces in a homomeric GlyR (αChb+a- GlyR), we were able to functionally characterize the αβ GlyR β+/α- interfaces. We found that the β+/α- interface had a higher agonist sensitivity than that of the α+/α- interface. This high sensitivity was contributed primarily by loop A. We also found that the β+/α- interface differentially modulates the agonist properties of glycine and taurine. Using voltage clamp fluorometry, we found that the conformational changes induced by glycine binding to the β+/α- interface were different from those induced by glycine binding to the α+/α- interface in the α GlyR. Moreover, the distinct conformational changes found at the β+/α- interface in the αChb+a- GlyR were also found in the heteromeric αβ GlyR, which suggests that the αChb+a- GlyR reconstitutes structural components and recapitulates functional properties, of the β+/α- interface in the heteromeric αβ GlyR. Our investigation not only provides structural and functional information about the GlyR β+/α- interface, which could direct GlyR β+/α- interface-specific drug design, but also provides a general methodology for unambiguously characterizing properties of specific protein interfaces from heteromeric proteins.

A series of α7 nicotinic acetylcholine receptor allosteric modulators with close chemical similarity but diverse pharmacological properties

Acetylcholine activates nicotinic acetylcholine receptors (nAChRs) by binding to an extracellular site located at the interface of two adjacent subunits. In contrast, recent studies have provided evidence that positive allosteric modulators (PAMs) such as TQS (4-(naphthalen-2-yl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and allosteric agonists such as 4BP-TQS (4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) interact at an intrasubunit transmembrane site. Here, we describe the synthesis and pharmacological characterization of a series of chemically related allosteric modulators of the α7 nAChR. Minimal changes in the chemical structure of these compounds have been found to exert profound effects on their pharmacological properties. For example, compounds containing a bromine atom at either the ortho or meta position on the phenyl ring, such as 2BP-TQS (4-(2-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and 3BP-TQS (4-(3-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide), rather than at the para position (4BP-TQS), display no allosteric agonist activity but retain PAM activity on α7 nAChRs, demonstrating the importance of the location of the halogen atom on pharmacological properties. Replacement of the bromine atom in 4BP-TQS with either a chlorine [4CP-TQS (4-(4-chloroophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] or an iodine atom [4IP-TQS (4-(4-iodoophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] results in compounds that have pharmacological properties characteristic of allosteric agonists but display differences in activation rates, in inactivation rates, and in levels of desensitization. In contrast, replacement of the bromine atom in 4BP-TQS with a fluorine atom [4FP-TQS (4-(4-fluorophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] generated a compound that lacks allosteric agonist activity but acts a potentiator of responses to acetylcholine. In addition, 4FP-TQS was found to act as an antagonist of responses evoked by allosteric agonists such as 4BP-TQS. These findings provide evidence of the pharmacological diversity of compounds interacting with the allosteric transmembrane site on α7 nAChRs.

Molecular dissection of Cl--selective Cys-loop receptor points to components that are dispensable or essential for channel activity

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that bind neurotransmitters to open an intrinsic transmembrane ion channel pore. The recent crystal structure of a prokaryotic pLGIC from the cyanobacterium Gloeobacter violaceus (GLIC) revealed that it naturally lacks an N-terminal extracellular α helix and an intracellular domain that are typical of eukaryotic pLGICs. GLIC does not respond to neurotransmitters acting at eukaryotic pLGICs but is activated by protons. To determine whether the structural differences account for functional differences, we used a eukaryotic chimeric acetylcholine-glutamate pLGIC that was modified to carry deletions corresponding to the sequences missing in the prokaryotic homolog GLIC. Deletions made in the N-terminal extracellular α helix did not prevent the expression of receptor subunits and the appearance of receptor assemblies on the cell surface but abolished the capability of the receptor to bind α-bungarotoxin (a competitive antagonist) and to respond to the neurotransmitter. Other truncated chimeric receptors that lacked the intracellular domain did bind ligands; displayed robust acetylcholine-elicited responses; and shared with the full-length chimeric receptor similar anionic selectivity, effective open pore diameter, and unitary conductance. We suggest that the integrity of the N-terminal α helix is crucial for ligand accommodation because it stabilizes the intersubunit interfaces adjacent to the neurotransmitter-binding pocket(s). We also conclude that the intracellular domain of the chimeric acetylcholine-glutamate receptor does not modulate the ion channel conductance and is not involved in positioning of the pore-lining helices in the conformation necessary for coordinating a Cl- ion within the intracellular vestibule of the ion channel pore.

An acetylcholine receptor lacking both γ and ε subunits mediates transmission in zebrafish slow muscle synapses

Fast and slow skeletal muscle types in larval zebrafish can be distinguished by a fivefold difference in the time course of their synaptic decay. Single-channel recordings indicate that this difference is conferred through kinetically distinct nicotinic acetylcholine receptor (AChR) isoforms. The underlying basis for this distinction was explored by cloning zebrafish muscle AChR subunit cDNAs and expressing them in Xenopus laevis oocytes. Measurements of single-channel conductance and mean open burst duration assigned α₂βδε to fast muscle synaptic current. Contrary to expectations, receptors composed of only αβδ subunits (presumed to be α₂βδ₂ receptors) recapitulated the kinetics and conductance of slow muscle single-channel currents. Additional evidence in support of γ/ε-less receptors as mediators of slow muscle synapses was reflected in the inward current rectification of heterologously expressed α₂βδ₂ receptors, a property normally associated with neuronal-type nicotinic receptors. Similar rectification was reflected in both single-channel and synaptic currents in slow muscle, distinguishing them from fast muscle. The final evidence for α₂βδ₂ receptors in slow muscle was provided by our ability to convert fast muscle synaptic currents to those of slow muscle by knocking down ε subunit expression in vivo. Thus, for the first time, muscle synaptic function can be ascribed to a receptor isoform that is composed of only three different subunits. The unique functional features offered by the α₂βδ₂ receptor likely play a central role in mediating the persistent contractions characteristic to this muscle type.

Effects of skeletal muscle denervation on potency of rocuronium

Background: Rocuronium is an alternative to succinylcholine for rapid tracheal intubation after major thermal injury and other forms of critical illness that cause denervation changes in skeletal muscle. Rocuronium may decrease the potencies of non-depolarizing muscle relaxants.

Objectives: Examine whether potency of rocuronium changed during the first month after denervation, and investigate the effects of skeletal muscle denervation on potency of rocuronium.

Methods: The denervation mouse model was developed to create denervated individual cells from the flexor digitorum brevis of the hindfoot. The skeletal muscle cells were examined at day 0 in the innervated control and days 1, 4, 7, 14, 21, and 28 in the denervation group. Nicotinic acetylcholine receptors in the cells were activated with 30 M acetylcholine, alone or in combination with various concentrations of rocuronium. Currents were recorded with a whole-cell patch-clamp technique.

Results: Rocuronium reversibly inhibited acetylcholine-activated currents in a dose-dependent fashion at different times after denervation. The inhibition concentration for the half-maximal responses of rocuronium increased 1.2- (p >0.05), 1.8-, 2.8-, 2.3-, 2.1-, and 1.9-fold (p <0.01) at day 1, 4, 7, 14, 21, and 28 after denervation, respectively, compared to that at day 0 after denervation.

Conclusion: Rocuronium dose required to achieve satisfactory clinical effects changed at different durations after skeletal muscle denervation.

The α1 subunit of nicotinic acetylcholine receptors in the inner ear: transcriptional regulation by ATOH1 and co-expression with the γ subunit in hair cells

Acetylcholine is a key neurotransmitter of the inner ear efferent system. In this study, we identify two novel nAChR subunits in the inner ear: α1 and γ, encoded by Chrna1 and Chrng, respectively. In situ hybridization shows that the messages of these two subunits are present in vestibular and cochlear hair cells during early development. Chrna1 and Chrng expression begin at embryonic stage E13.5 in the vestibular system and E17.5 in the organ of Corti. Chrna1 message continues through P7, whereas Chrng is undetectable at post-natal stage P6. The α1 and γ subunits are known as muscle-type nAChR subunits and are surprisingly expressed in hair cells which are sensory-neural cells. We also show that ATOH1/MATH1, a transcription factor essential for hair cell development, directly activates CHRNA1 transcription. Electrophoretic mobility-shift assays and supershift assays showed that ATOH1/E47 heterodimers selectively bind on two E boxes located in the proximal promoter of CHRNA1. Thus, Chrna1 could be the first transcriptional target of ATOH1 in the inner ear. Co-expression in Xenopus oocytes of the α1 subunit does not change the electrophysiological properties of the α9α10 receptor. We suggest that hair cells transiently express α1γ-containing nAChRs in addition to α9α10, and that these may have a role during development of the inner ear innervation.

Postsynaptic development of the neuromuscular junction in mice lacking the gamma-subunit of muscle nicotinic acetylcholine receptor

The mammalian muscle nicotinic acetylcholine receptor (AChR) is composed of five membrane-spanning subunits and its composition differs between embryonic and adult muscles. In embryonic muscles, it is composed of two alpha-, one beta-, one delta-, and one gamma-subunit; the gamma-subunit is later replaced by the epsilon-subunit during postnatal development. This unique temporal expression pattern of the gamma-subunit suggests it may play specific roles in embryonic muscles. To address this issue, we examined the formation and function of the neuromuscular junction in mouse embryos deficient in the gamma-subunit. At embryonic day 15.5, AChR clusters were absent and the spontaneous miniature endplate potentials were undetectable in the mutant muscles. However, electrical stimulation of the nerves triggered muscle contraction and elicited postsynaptic endplate potential (EPP) in the mutant muscles, although the magnitude of the muscle contraction and the amplitudes of the EPPs were smaller in the mutant compared to the wild-type muscles. Reintroducing a wild-type gamma-subunit into the mutant myotubes restored the formation of AChR clusters in vitro. Together, these results have demonstrated that functional AChRs were present in the mutant muscle membrane, but at reduced levels. Thus, in the absence of the gamma-subunit, a combination of alpha, beta, and delta subunits may assemble into functional receptors in vivo. These results also suggest that the gamma-subunit maybe involved in interacting with rapsyn, a cytoplasmic protein required for AChR clustering.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

Mutant forms of the extracellular domain of the human acetylcholine receptor gamma-subunit with improved solubility and enhanced antigenicity. The importance of the Cys-loop

The muscle nicotinic acetylcholine receptor (AChR) is the prototype of the ligand-gated ion channels (or Cys-loop receptors), formed by 5 homologous subunits (alpha2betagammadelta or alpha2betagammaepsilon), and is the major autoantigen in the autoimmune disease, myasthenia gravis. Previously, we expressed the wild-type extracellular domain (ECD) of the gamma-subunit (gammaECD) of the AChR in yeast Pichia pastoris at 0.3-0.8 mg/L, in soluble but microaggregate form, to use as starting material for structural and antigenicity studies. To optimize these characteristics, we constructed and characterized four gammaECD variants: (a) mutants-1 (gammaC61S) and -2 (gammaC106S-C115S), where the non-conserved Cys of gammaECD were replaced by serines, (b) mutant-3 (gammaCysLoop), where the gamma Cys-loop region was substituted by the cognate region of the acetylcholine binding protein (AChBP) and (c) mutant-4 (gammaCysLoop-C106S-C115S), where both the C106S-C115S and Cys-loop mutations were combined. None of mutants-1 and -2 displayed any improvement, while mutant-3 and -4 were mostly in dimeric form and expressed at much higher levels (2.5 mg/L and 3.5 mg/L respectively). All four mutants and wild-type gammaECD were recognized by sera from myasthenic patients, but mutants-3 and -4 exhibited higher efficiency, compared to wild-type or mutants-1 and -2. These results suggest that the substitution of the Cys-loop region of any AChR ECD with the AChBP counterpart leads to AChR ECD of improved conformation, more suitable for structural and therapeutic studies.

Essential roles of the acetylcholine receptor gamma-subunit in neuromuscular synaptic patterning

Formation of the vertebrate neuromuscular junction (NMJ) takes place in a stereotypic pattern in which nerves terminate at select sarcolemmal sites often localized to the central region of the muscle fibers. Several lines of evidence indicate that the muscle fibers may initiate postsynaptic differentiation independent of the ingrowing nerves. For example, nascent acetylcholine receptors (AChRs) are pre-patterned at select regions of the muscle during the initial stage of neuromuscular synaptogenesis. It is not clear how these pre-patterned AChR clusters are assembled, and to what extent they contribute to pre- and post-synaptic differentiation during development. Here, we show that genetic deletion of the AChR gamma-subunit gene in mice leads to an absence of pre-patterned AChR clusters during initial stages of neuromuscular synaptogenesis. The absence of pre-patterned AChR clusters was associated with excessive nerve branching, increased motoneuron survival, as well as aberrant distribution of acetylcholinesterase (AChE) and rapsyn. However, clustering of muscle specific kinase (MuSK) proceeded normally in the gamma-null muscles. AChR clusters emerged at later stages owing to the expression of the AChR epsilon-subunit, but these delayed AChR clusters were broadly distributed and appeared at lower level compared with the wild-type muscles. Interestingly, despite the abnormal pattern, synaptic vesicle proteins were progressively accumulated at individual nerve terminals, and neuromuscular synapses were ultimately established in gamma-null muscles. These results demonstrate that the gamma-subunit is required for the formation of pre-patterned AChR clusters, which in turn play an essential role in determining the subsequent pattern of neuromuscular synaptogenesis.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Expression and characterization of soluble forms of the extracellular domains of the beta, gamma and epsilon subunits of the human muscle acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is a ligand-gated ion channel found in muscles and neurons. Muscle AChR, formed by five homologous subunits (alpha2 beta gamma delta or alpha2 beta gamma epsilon), is the major antigen in the autoimmune disease, myasthenia gravis (MG), in which pathogenic autoantibodies bind to, and inactivate, the AChR. The extracellular domain (ECD) of the human muscle alpha subunit has been heterologously expressed and extensively studied. Our aim was to obtain satisfactory amounts of the ECDs of the non-alpha subunits of human muscle AChR for use as starting material for the determination of the 3D structure of the receptor ECDs and for the characterization of the specificities of antibodies in sera from patients with MG. We expressed the N-terminal ECDs of the beta (amino acids 1-221; beta1-221), gamma (amino acids 1-218; gamma1-218), and epsilon (amino acids 1-219; epsilon1-219) subunits of human muscle AChR in the yeast, Pichia pastoris. beta1-221 was expressed at approximately 2 mg.L(-1) culture, whereas gamma1-218 and epsilon1-219 were expressed at 0.3-0.8 mg.L(-1) culture. All three recombinant polypeptides were glycosylated and soluble; beta1-221 was mainly in an apparently dimeric form, whereas gamma1-218 and epsilon1-219 formed soluble oligomers. CD studies of beta1-221 suggested that it has considerable beta-sheet secondary structure with a proportion of alpha-helix. Conformation-dependent mAbs against the ECDs of the beta or gamma subunits specifically recognized beta1-221 or gamma1-218, respectively, and polyclonal rabbit antiserum raised against purified beta1-221 bound to (125)I-labeled alpha-bungarotoxin-labeled human AChR. Moreover, immobilization of each ECD on Sepharose beads and incubation of the ECD-Sepharose matrices with MG sera caused a significant reduction in the concentrations of autoantibodies in the sera, showing specific binding to the recombinant ECDs. These results suggest that the expressed proteins present some near-native conformational features and are thus suitable for our purposes.

Structural dynamics of the M4 transmembrane segment during acetylcholine receptor gating

The transition state structures that link the stable end states of allosteric proteins are largely unresolved. We used single-molecule kinetic analysis to probe the dynamics of the M4 transmembrane segments during the closed<==>open isomerization of the neuromuscular acetylcholine receptor ion channel (AChR). We measured the slopes (phi) of the free energy relationships for 87 mutants, which reveal the open- versus closed-like characters of the mutated residues at the transition state and hence the sequence and organization of gating molecular motions. phi was constant throughout the length of the alpha subunit M4 segment with an average value of 0.54, suggesting that this domain moves as a unit, approximately midway through the reaction. Analysis of a hybrid construct indicates that the two alpha subunits move synchronously. Between subunits, the sequence of M4 motions is alpha-epsilon-beta. The AChR ion channel emerges as a dynamic nanomachine with many moving parts.

Effects of isoflurane on the actions of neuromuscular blockers on the muscle nicotine acetylcholine receptors

In this study, we tested the hypothesis that volatile anesthetic enhancement of muscle relaxation is the result of combined drug effects on the nicotinic acetylcholine receptors. The poly A m RNA from muscle by isolation were microinjected into Xenopus oocytes for receptor expression. Concentration-effect curves for the inhibition of Ach-induced currents were established for vecuronium, rocuranium, and isoflurane. Subsequently, inhibitory effects of NDMRs were studied in the presence of the isoflurane at a concentration equivalent to half the concentration producing a 50% inhibition alone. All tested drugs produced rapid and readily reversible concentration-dependent inhibition. The 50% inhibitory concentration values were 889 micromol/L (95% CI: 711-1214 micromol). 33.4 micromol (95% CI: 27.1-41.7 nmol) and 9.2 nmol (95% CI: 7.9-12.3 nmol) for isoflurane. rocuranium and vecuronium, respectively. Coapplication of isoflurane significantly enhanced the inhibitory effects of rocuranium and vecuronium, and it was especially so at low concentration of NMDRs. Isoflurane increases the potency of NDMRs, possibly by enhancing antagonist affinity at the receptor site.

Structural basis for epibatidine selectivity at desensitized nicotinic receptors

The agonist binding sites of the fetal muscle nicotinic acetylcholine receptor are formed at the interfaces of alpha-subunits and neighboring gamma- and delta-subunits. When the receptor is in the nonconducting desensitized state, the alpha-gamma site binds the agonist epibatidine 200-fold more tightly than does the alpha-delta site. To determine the structural basis for this selectivity, we constructed gamma/delta-subunit chimeras, coexpressed them with complementary wild-type subunits in HEK 293 cells, and determined epibatidine affinity of the resulting complexes. The results reveal three determinants of epibatidine selectivity: gamma104-117/delta106-delta119, gamma164-171/delta166-177, and gammaPro190/deltaAla196. Point mutations reveal that three sequence differences within the gamma104-117/delta106-delta119 region are determinants of epibatidine selectivity: gammaLys104/deltaTyr106, gammaSer111/deltaTyr113, and gammaTyr117/deltaTyr119. In the delta-subunit, simultaneous mutation of these residues to their gamma equivalent produces high affinity, gamma-like epibatidine binding. However, converting gamma to delta affinity requires replacement of the gamma104-117 segment with delta sequence, suggesting interplay of residues in this region. The structural basis for epibatidine selectivity is explained by computational docking of epibatidine to a homology model of the alpha-gamma binding site.

A fluorophore attached to nicotinic acetylcholine receptor beta M2 detects productive binding of agonist to the alpha delta site

To study conformational transitions at the muscle nicotinic acetylcholine (ACh) receptor (nAChR), a rhodamine fluorophore was tethered to a Cys side chain introduced at the beta 19' position in the M2 region of the nAChR expressed in Xenopus oocytes. This procedure led to only minor changes in receptor function. During agonist application, fluorescence increased by (Delta F/F) approximately 10%, and the emission peak shifted to lower wavelengths, indicating a more hydrophobic environment for the fluorophore. The dose-response relations for Delta F agreed well with those for epibatidine-induced currents, but were shifted approximately 100-fold to the left of those for ACh-induced currents. Because (i) epibatidine binds more tightly to the alpha gamma-binding site than to the alpha delta site and (ii) ACh binds with reverse-site selectivity, these data suggest that Delta F monitors an event linked to binding specifically at the alpha delta-subunit interface. In experiments with flash-applied agonists, the earliest detectable Delta F occurs within milliseconds, i.e., during activation. At low [ACh] (< or = 10 microM), a phase of Delta F occurs with the same time constant as desensitization, presumably monitoring an increased population of agonist-bound receptors. However, recovery from Delta F is complete before the slowest phase of recovery from desensitization (time constant approximately 250 s), showing that one or more desensitized states have fluorescence like that of the resting channel. That conformational transitions at the alpha delta-binding site are not tightly coupled to channel activation suggests that sequential rather than fully concerted transitions occur during receptor gating. Thus, time-resolved fluorescence changes provide a powerful probe of nAChR conformational changes.

Acetylcholine binding protein (AChBP): a secreted glial protein that provides a high-resolution model for the extracellular domain of pentameric ligand-gated ion channels

Acetylcholine binding protein (AChBP) has recently been identified from molluskan glial cells. Glial cells secrete it into cholinergic synapses, where it plays a role in modulating synaptic transmission. This novel mechanism resembles glia-dependent modulation of glutamate synapses, with several key differences. AChBP is a homolog of the ligand binding domain of the pentameric ligand-gated ion-channels. The crystal structure of AChBP provides the first high-resolution structure for this family of Cys-loop receptors. Nicotinic acetylcholine receptors and related ion-channels such as GABAA, serotonin 5HT3, and glycine can be interpreted in the light of the 2.7 A AChBP structure. The structural template provides critical details of the binding site and helps create models for toxin binding, mutational effects, and molecular gating.

Curariform antagonists bind in different orientations to acetylcholine-binding protein

Acetylcholine-binding protein (AChBP) recently emerged as a prototype for relating structure to function of the ligand binding domain of nicotinic acetylcholine receptors (AChRs). To understand interactions of competitive antagonists at the atomic structural level, we studied binding of the curare derivatives d-tubocurarine (d-TC) and metocurine to AChBP using computational methods, mutagenesis, and ligand binding measurements. To account for protein flexibility, we used a 2-ns molecular dynamics simulation of AChBP to generate multiple snapshots of the equilibrated dynamic structure to which optimal docking orientations were determined. Our results predict a predominant docking orientation for both d-TC and metocurine, but unexpectedly, the bound orientations differ fundamentally for each ligand. At one subunit interface of AChBP, the side chain of Tyr-89 closely approaches a positively charged nitrogen in d-TC but is farther away from the equivalent nitrogen in metocurine, whereas, at the opposing interface, side chains of Trp-53 and Gln-55 closely approach the metocurine scaffold but not that of d-TC. The different orientations correspond to approximately 170 degrees rotation and approximately 30 degrees degree tilt of the curare scaffold within the binding pocket. Mutagenesis of binding site residues in AChBP, combined with measurements of ligand binding, confirms the different docking orientations. Thus structurally similar ligands can adopt distinct orientations at receptor binding sites, posing challenges for interpreting structure-activity relationships for many drugs.

Conformation, action, and mechanism of action of neuromuscular blocking muscle relaxants

Since curare was introduced into clinical anaesthesia in 1942, efforts to create better neuromuscular blocking (NMB) muscle relaxants have continued. Today, muscle relaxation remains a mainstay of modern anaesthesia and intensive care. Through manipulation of the traditional structure-action relationships, many new and improved muscle relaxants have been created, and several have been brought to clinical use. However, structure-action relationship is inconsistent and has its limits. Using computer-aided molecular conformational analyses, the conformation-action relationships of NMB agents of various chemical classes have been explored. Conformation, no less than structure, of the NMB agents has shed new light on their mechanisms of action. By reflection, the conformations also suggest new details of the topology of the receptive sites of the nicotinic acetylcholine receptor modeled for the motor endplate of the skeletal muscle.

The nicotinic receptor ligand binding domain

The ligand binding domain (LBD) of the nicotinic acetylcholine receptor has served as a prototype for understanding molecular recognition in the family of neurotransmitter-gated ion channels. During the past fifty years, studies progressed from fundamental electrophysiological analyses of ACh-evoked ion flow, to biochemical purification of the receptor protein, pharmacological measurements of ligand binding, molecular cloning of receptor subunits, site-directed mutagenesis combined with functional analysis and recently, atomic structural determination. The emerging picture of the nicotinic receptor LBD is a specialized pocket of aromatic and hydrophobic residues formed at interfaces between protein subunits that changes conformation to convert agonist binding into gating of an intrinsic ion channel.

Lysine scanning mutagenesis delineates structural model of the nicotinic receptor ligand binding domain

Nicotinic acetylcholine receptors (AChR) and their relatives mediate rapid chemical transmission throughout the nervous system, yet their atomic structures remain elusive. Here we use lysine scanning mutagenesis to determine the orientation of residue side chains toward core hydrophobic or surface hydrophilic environments and use this information to build a structural model of the ligand binding region of the AChR from adult human muscle. The resulting side-chain orientations allow assignment of residue equivalence between AChR subunits and an acetylcholine binding protein solved by x-ray crystallography, providing the foundation for homology modeling. The resulting structural model of the AChR provides a picture of the ACh binding site and predicts novel pairs of residues that stabilize subunit interfaces. The overall results suggest that lysine scanning can provide the basis for structural modeling of other members of the AChR superfamily as well as of other proteins with repeating structures delimiting a hydrophobic core.

Characterization of the interactions between volatile anesthetics and neuromuscular blockers at the muscle nicotinic acetylcholine receptor

Volatile anesthetics enhance the neuromuscular blockade produced by nondepolarizing muscle relaxants (NDMRs). The neuromuscular junction is a postulated site of this interaction. We tested the hypothesis that volatile anesthetic enhancement of muscle relaxation is the result of combined drug effects on the nicotinic acetylcholine receptor. The adult mouse muscle nicotinic acetylcholine receptor (alpha(2), beta, delta, epsilon) was heterologously expressed in Xenopus laevis oocytes. Concentration-effect curves for the inhibition of acetylcholine-induced currents were established for vecuronium, d-tubocurarine, isoflurane, and sevoflurane. Subsequently, inhibitory effects of NDMRs were studied in the presence of the volatile anesthetics at a concentration equivalent to half the concentration producing a 50% inhibition alone. All individually tested compounds produced rapid and readily reversible concentration-dependent inhibition. The calculated 50% inhibitory concentration values were 9.9 nM (95% confidence interval [CI], 8.4-11.4 nM), 43.4 nM (95% CI, 33.6-53.3 nM), 897 microM (95% CI, 699-1150 microM), and 818 microM (95% CI, 685-1001 microM) for vecuronium, d-tubocurarine, isoflurane, and sevoflurane, respectively. Coapplication of either isoflurane or sevoflurane significantly enhanced the inhibitory effects of vecuronium and d-tubocurarine, especially so at small concentrations of NDMRs. Volatile anesthetics increase the potency of NDMRs, possibly by enhancing antagonist affinity at the receptor site. This effect may contribute to the clinically observable enhancement of neuromuscular blockade by volatile anesthetics.

IMPLICATIONS

Isoflurane and sevoflurane enhance the receptor blocking effects of nondepolarizing muscle relaxants on nicotinic acetylcholine receptors.

Conotoxin MI inhibits the alpha-delta acetylcholine binding site of the Torpedo marmorata receptor

The muscle-type nicotinic receptor has two pharmacologically distinguishable acetylcholine binding sites at the alpha-gamma and alpha-delta subunit interfaces; alpha-conotoxins can bind them selectively. As reported, alpha-conotoxin MI has greater affinity for the site near the alpha-delta interface of the BC(3)H1 cell receptor but, in the case of the Torpedo californica receptor, displays greater affinity for that near the alpha-gamma interface. To further investigate ligand selectivity, we study the conotoxin MI-Torpedo marmorata receptor interaction. In this work, we show the binding of alpha-conotoxin MI to the T. marmorata receptor and the influence of the antagonist alpha-Bungarotoxin and the agonist carbamylcholine on such binding; in addition, and contrasting with the results for the Torpedo californica receptor, we identify the alpha-delta subunit interface as the high affinity binding site. This is the first work describing different characteristics of the interaction between alpha-conotoxin MI and receptors from different species of the same genus.

The effects of general anaesthetics on ligand-gated ion channels

The experimental effort that has been expended in investigating the effects of general anaesthetics on LGICs has been enormous over the past decade. Members of all three LGIC superfamilies have been examined using electrophysiological techniques. Anaesthetics that have been examined include volatile anaesthetics, gaseous anaesthetics, alcohols, i.v. anaesthetics and non-immobilizers. Obsolete anaesthetics (ether, cyclopropane, butane) have been used in order to increase the variability of the structure and polarity of experimental compounds. The tools of molecular biology have been used to make chimeric receptors and to make single-site mutations. Interestingly, this work has been taking place in parallel with efforts to understand the structure of these proteins. Anaesthetic research often stimulates structural research as well as vice versa. There are some common themes in the interactions between anaesthetics and the three superfamilies of LGICs. In many cases, anaesthetics have both inhibitory and potentiating effects on the channels. It is likely that the number of examples of this will increase when experiments are designed to look specifically for one or the other type of effect. So we must conclude that there are multiple binding sites for anaesthetics on LGICs. The degree of inhibition or potentiation is not easily predictable. In retrospect, this is not surprising when we consider that the sensitivity of a channel to anaesthetics can be altered by a single amino-acid mutation. The large structural differences between the cys-loop, glutamate-activated and P2X superfamilies do not lead to large differences in anaesthetic sensitivity. It is the smaller, almost insignificant, changes that do this. This observation that small changes may lead to large effects reinforces the idea that at least some of the interactions between anaesthetics and LGICs are direct drug-protein interactions that are not mediated by the lipids. This review has not addressed the question of whether the effects of anaesthetics seen on LGICs are relevant to anaesthesia. This question cannot really be answered at present. Although potent effects can be observed on the channels themselves, we have only begun to try to understand whether these effects are important for a synapse, a neuronal circuit or the function of an animal's nervous system. We have studied the trees; now we must go on to study the forest and the ecosystem.

Yeast expression and NMR analysis of the extracellular domain of muscle nicotinic acetylcholine receptor alpha subunit

The alpha subunit of the nicotinic acetylcholine receptor (AChR) from Torpedo electric organ and mammalian muscle contains high affinity binding sites for alpha-bungarotoxin and for autoimmune antibodies in sera of patients with myasthenia gravis. To obtain sufficient materials for structural studies of the receptor-ligand complexes, we have expressed part of the mouse muscle alpha subunit as a soluble, secretory protein using the yeast Pichia pastoris. By testing a series of truncated fragments of the receptor protein, we show that alpha211, the entire amino-terminal extracellular domain of AChR alpha subunit (amino acids 1-211), is the minimal segment that could fold properly in yeast. The alpha211 protein was secreted into the culture medium at a concentration of >3 mg/liter. It migrated as a 31-kDa polypeptide with N-linked glycosylation on SDS-polyacrylamide gel. The protein was purified to homogeneity by isoelectric focusing electrophoresis (pI 5.8), and it appeared as a 4.5 S monomer on sucrose gradient at concentrations up to 1 mm ( approximately 30 mg/ml). The receptor domain bound monoclonal antibody mAb35, a conformation-specific antibody against the main immunogenic region of the AChR. In addition, it formed a high affinity complex with alpha-bungarotoxin (k(D) 0.2 nm) but showed relatively low affinity to the small cholinergic ligand acetylcholine. Circular dichroism spectroscopy of alpha211 revealed a composition of secondary structure corresponding to a folded protein. Furthermore, the receptor fragment was efficiently (15)N-labeled in P. pastoris, and proton cross-peaks were well dispersed in nuclear Overhauser effect and heteronuclear single quantum coherence spectra as measured by NMR spectroscopy. We conclude that the soluble AChR protein is useful for high resolution structural studies.

Isobolographic analysis of non-depolarising muscle relaxant interactions at their receptor site

Administration of certain combinations of non-depolarising muscle relaxants produces greater than expected neuromuscular blockade. Synergistic effects may be explained by drug interactions with the postsynaptic muscle nicotinic acetylcholine receptor. To investigate this hypothesis, the adult mouse muscle nicotinic acetylcholine receptor (alpha(2)beta delta epsilon) was heterologously expressed in Xenopus laevis oocytes and activated by the application of acetylcholine (10 microM). The effects of five individually applied muscle relaxants and six combinations of structurally similar and dissimilar compounds were studied. Drug combinations containing equipotent concentrations of two agents were tested and dose-response curves were determined. All compounds tested alone and in combination produced rapid and readily reversible, concentration-dependent inhibition. Isobolographic and fractional analyses indicated additive interactions for all six tested combinations. These findings suggest that synergistic neuromuscular blocking effects, observed for the administration of certain combinations of muscle relaxants, do not result from purely postsynaptic binding events at the muscle nicotinic acetylcholine receptor, but rather from differential actions on pre- and postsynaptic sites.

The potency of new muscle relaxants on recombinant muscle-type acetylcholine receptors

We studied the inhibition of fetal (gamma-nAChR) and adult (epsilon-nAChR) muscle-type nicotinic acetylcholine receptors by the two new nondepolarizing muscle relaxants (NDMRs) rocuronium and rapacuronium, the metabolite 3-desacetyl rapacuronium (Org 9488), and five other, longer-used NDMRs (pancuronium, vecuronium, mivacurium, d-tubocurarine, and gallamine). Receptors were expressed in Xenopus laevis oocytes by cytoplasmic injection of subunit complementary RNAs. Functional channels were activated with 10 microM acetylcholine, alone or in combination with various concentrations of the NDMRs. Currents were recorded with a whole-cell two-electrode voltage clamp technique. All NDMRs reversibly inhibited acetylcholine-activated currents in a dose-dependent fashion. Potencies of rapacuronium and Org 9488 were not statistically different at either gamma-nAChR (half-maximal response = 58.2 and 36.5 nM, respectively) or epsilon-nAChR (half-maximal response = 80.3 and 97.7 nM, respectively). The rank order of potencies at the epsilon-nAChR (pancuronium > vecuronium similar mivacurium > rocuronium similar d-tubocurarine > rapacuronium similar Org 9488 > gallamine) correlated highly with the clinical doses needed to produce 50% twitch depression at the adductor pollicis muscle in adults. Neuromuscular blockade by rapacuronium may be enhanced by its metabolite Org 9488. Different drug-receptor affinities of the tested NDMRs contribute to the differences in clinical dose requirements of these drugs needed to achieve appropriate muscle relaxation.

IMPLICATIONS

Potencies of nondepolarizing muscle relaxants, studied at muscle nicotinic acetylcholine receptors expressed in a recombinant expression system, correlate highly with the clinical doses needed in adults to produce 50% twitch depression at the adductor pollicis muscle.

Emerging structure of the nicotinic acetylcholine receptors

The conversion of acetylcholine binding into ion conduction across the membrane is becoming more clearly understood in terms of the structure of the receptor and its transitions. A high-resolution structure of a protein that is homologous to the extracellular domain of the receptor has revealed the binding sites and subunit interfaces in great detail. Although the structures of the membrane and cytoplasmic domains are less well determined, the channel lining and the determinants of selectivity have been mapped. The location and structure of the gates, and the coupling between binding sites and gates, remain to be established.

Snake alpha-neurotoxin binding site on the Egyptian cobra (Naja haje) nicotinic acetylcholine receptor Is conserved

Evolutionary success requires that animal venoms are targeted against phylogenetically conserved molecular structures of fundamental physiological processes. Species producing venoms must be resistant to their action. Venoms of Elapidae snakes (e.g., cobras, kraits) contain alpha-neurotoxins, represented by alpha-bungarotoxin (alpha-BTX) targeted against the nicotinic acetylcholine receptor (nAChR) of the neuromuscular junction. The model which presumes that cobras (Naja spp., Elapidae) have lost their binding site for conspecific alpha-neurotoxins because of the unique amino acid substitutions in their nAChR polypeptide backbone per se is incompatible with the evolutionary theory that (1) the molecular motifs forming the alpha-neurotoxin target site on the nAChR are fundamental for receptor structure and/or function, and (2) the alpha-neurotoxin target site is conserved among Chordata lineages. To test the hypothesis that the alpha-neurotoxin binding site is conserved in Elapidae snakes and to identify the mechanism of resistance against conspecific alpha-neurotoxins, we cloned the ligand binding domain of the Egyptian cobra (Naja haje) nAChR alpha subunit. When expressed as part of a functional Naja/mouse chimeric nAChR in Xenopus oocytes, this domain confers resistance against alpha-BTX but does not alter responses induced by the natural ligand acetylcholine. Further mutational analysis of the Naja/mouse nAChR demonstrated that an N-glycosylation signal in the ligand binding domain that is unique to N. haje is responsible for alpha-BTX resistance. However, when the N-glycosylation signal is eliminated, the nAChR containing the N. haje sequence is inhibited by alpha-BTX with a potency that is comparable to that in mammals. We conclude that the binding site for conspecific alpha-neurotoxin in Elapidae snakes is conserved in the nAChR ligand binding domain polypeptide backbone per se. This conclusion supports the hypothesis that animal toxins are targeted against evolutionarily conserved molecular motifs. Such conservation also calls for a revision of the present model of the alpha-BTX binding site. The approach described here can be used to identify the mechanism of resistance against conspecific venoms in other species and to characterize toxin-receptor coevolution.

The noncompetitive inhibitor quinacrine modifies the desensitization kinetics of muscle acetylcholine receptors

Quinacrine has been shown to act as a noncompetitive inhibitor of the nicotinic acetylcholine receptor (nAChR). However, its mechanism of action is still a matter of controversy. We analyzed in detail the action of quinacrine at both the single-channel and macroscopic current levels. The main effect of quinacrine is a profound concentration-dependent decrease in both the frequency of opening events and the duration of clusters elicited by high acetylcholine concentrations. Quinacrine also significantly increases (40-fold at 30 microM) the decay rate of macroscopic currents elicited by rapid perfusion of acetylcholine to outside-out patches. This decay is still well-described by a single exponential. Quinacrine has very little effect on the peak amplitude of the response, suggesting that it acts mainly on open channels. The recovery from desensitization after removal of acetylcholine is delayed in the presence of quinacrine. Results from both single-channel and macroscopic current recordings indicate that quinacrine increases the rate of nAChR desensitization and stabilizes the desensitized state. Interestingly, in equilibrium agonist-binding assays, quinacrine does not promote the typical high-affinity desensitized state. Thus, quinacrine seems to induce an intermediate state exhibiting the permeability but not the agonist binding properties of desensitization.

Human thymuses express incomplete sets of muscle acetylcholine receptor subunit transcripts that seldom include the delta subunit

In myasthenia gravis (MG) the muscle acetylcholine receptor (AChR) is the target of an immune response that might begin in the thymus. The thymus expresses binding sites for specific ligands of muscle AChR, a complex protein composed of alpha, beta, gamma (or epsilon) and delta subunits. The thymus expresses the AChR alpha subunit, but there is controversy regarding the expression in the thymus of the gamma, epsilon and delta subunits. We investigated the presence of messenger RNA (mRNA) for the different muscle AChR subunits in thymus tissue from 20 healthy subjects and 13 myasthenic patients. We detected mRNA for the alpha and epsilon subunits in all samples, for the beta subunit in all but one sample and for the gamma subunit in most samples although at lower levels than the epsilon subunit. Myasthenic thymuses expressed levels of gamma subunit mRNA similar to control thymuses but more abundant epsilon subunit mRNA. None of the myasthenic thymuses and only two control thymuses expressed detectable delta subunit mRNA. This supports the hypothesis that human thymus may express AChR proteins that do not include the delta subunit. Such receptors, which would have different antigenic structure than the muscle AChRs, might have a role in triggering the autoimmune response that causes MG.

Contributions of Torpedo nicotinic acetylcholine receptor gamma Trp-55 and delta Trp-57 to agonist and competitive antagonist function

Results of affinity-labeling studies and mutational analyses provide evidence that the agonist binding sites of the nicotinic acetylcholine receptor (nAChR) are located at the alpha-gamma and alpha-delta subunit interfaces. For Torpedo nAChR, photoaffinity-labeling studies with the competitive antagonist d-[(3)H]tubocurarine (dTC) identified two tryptophans, gammaTrp-55 and deltaTrp-57, as the primary sites of photolabeling in the non-alpha subunits. To characterize the importance of gammaTrp-55 and deltaTrp-57 to the interactions of agonists and antagonists, Torpedo nAChRs were expressed in Xenopus oocytes, and equilibrium binding assays and electrophysiological recordings were used to examine the functional consequences when either or both tryptophans were mutated to leucine. Neither substitution altered the equilibrium binding of dTC. However, the deltaW57L and gammaW55L mutations decreased acetylcholine (ACh) binding affinity by 20- and 7,000-fold respectively. For the wild-type, gammaW55L, and deltaW57L nAChRs, the concentration dependence of channel activation was characterized by Hill coefficients of 1.8, 1.1, and 1.7. For the gammaW55L mutant, dTC binding at the alpha-gamma site acts not as a competitive antagonist but as a coactivator or partial agonist. These results establish that interactions with gamma Trp-55 of the Torpedo nAChR play a crucial role in agonist binding and in the agonist-induced conformational changes that lead to channel opening.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i