Rings of anionic amino acids as structural determinants of ion selectivity in the acetylcholine receptor channel

Tentonin 3 is a pore-forming subunit of a slow inactivation mechanosensitive channel

Mechanically activating (MA) channels transduce numerous physiological functions. Tentonin 3/TMEM150C (TTN3) confers MA currents with slow inactivation kinetics in somato- and barosensory neurons. However, questions were raised about its role as a Piezo1 regulator and its potential as a channel pore. Here, we demonstrate that purified TTN3 proteins incorporated into the lipid bilayer displayed spontaneous and pressure-sensitive channel currents. These MA currents were conserved across vertebrates and differ from Piezo1 in activation threshold and pharmacological response. Deep neural network structure prediction programs coupled with mutagenetic analysis predicted a rectangular-shaped, tetrameric structure with six transmembrane helices and a pore at the inter-subunit center. The putative pore aligned with two helices of each subunit and had constriction sites whose mutations changed the MA currents. These findings suggest that TTN3 is a pore-forming subunit of a distinct slow inactivation MA channel, potentially possessing a tetrameric structure.

Thermodynamics and Kinetics of Ion Permeation in Wild-Type and Mutated Open Active Conformation of the Human α7 Nicotinic Receptor

Molecular studies of human pentameric ligand-gated ion channels (LGICs) expressed in neurons and at neuromuscular junctions are of utmost importance in the development of therapeutic strategies for neurological disorders. We focus here on the nicotinic acetylcholine receptor nAChR-α7, a homopentameric channel widely expressed in the human brain, with a proven role in a wide spectrum of disorders including schizophrenia and Alzheimer’s disease. By exploiting an all-atom structural model of the full (transmembrane and extracellular) protein in the open, agonist-bound conformation we recently developed, we evaluate the free energy and the mean first passage time of single-ion permeation using molecular dynamics simulations and the milestoning method with Voronoi tessellation. The results for the wild-type channel provide the first available mapping of the potential of mean force in the full-length α7 nAChR, reveal its expected cationic nature, and are in good agreement with simulation data for other channels of the LGIC family and with experimental data on nAChRs. We then investigate the role of a specific mutation directly related to ion selectivity in LGICs, the E-1′ → A-1′ substitution at the cytoplasmatic selectivity filter. We find that the mutation strongly affects sodium and chloride permeation in opposite directions, leading to a complete inversion of selectivity, at variance with the limited experimental results available that classify this mutant as cationic. We thus provide structural determinants for the observed cationic-to-anionic inversion, revealing a key role of the protonation state of residue rings far from the mutation, in the proximity of the hydrophobic channel gate.

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.

Lack of negatively charged residues at the external mouth of Kir2.2 channels enable the voltage-dependent block by external Mg2+

Kir channels display voltage-dependent block by cytosolic cations such as Mg2+ and polyamines that causes inward rectification. In fact, cations can regulate K channel activity from both the extracellular and intracellular sides. Previous studies have provided insight into the up-regulation of Kir channel activity by extracellular K+ concentration. In contrast, extracellular Mg2+ has been found to reduce the amplitude of the single-channel current at milimolar concentrations. However, little is known about the molecular mechanism of Kir channel blockade by external Mg2+ and the relationship between the Mg2+ blockade and activity potentiation by permeant K+ ions. In this study, we applied an interactive approach between theory and experiment. Electrophysiological recordings on Kir2.2 and its mutants were performed by heterologous expression in Xenopus laevis oocytes. Our results confirmed that extracellular Mg2+ could reduce heterologously expressed WT Kir2.2 currents in a voltage dependent manner. The kinetics of inhibition and recovery of Mg2+ exhibit a 3∼4s time constant. Molecular dynamics simulation results revealed a Mg2+ binding site located at the extracellular mouth of Kir2.2 that showed voltage-dependent Mg2+ binding. The mutants, G119D, Q126E and H128D, increased the number of permeant K+ ions and reduced the voltage-dependent blockade of Kir2.2 by extracellular Mg2+.

Tracking the molecular evolution of calcium permeability in a nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors are a family of ligand-gated nonselective cationic channels that participate in fundamental physiological processes at both the central and the peripheral nervous system. The extent of calcium entry through ligand-gated ion channels defines their distinct functions. The α9α10 nicotinic cholinergic receptor, expressed in cochlear hair cells, is a peculiar member of the family as it shows differences in the extent of calcium permeability across species. In particular, mammalian α9α10 receptors are among the ligand-gated ion channels which exhibit the highest calcium selectivity. This acquired differential property provides the unique opportunity of studying how protein function was shaped along evolutionary history, by tracking its evolutionary record and experimentally defining the amino acid changes involved. We have applied a molecular evolution approach of ancestral sequence reconstruction, together with molecular dynamics simulations and an evolutionary-based mutagenesis strategy, in order to trace the molecular events that yielded a high calcium permeable nicotinic α9α10 mammalian receptor. Only three specific amino acid substitutions in the α9 subunit were directly involved. These are located at the extracellular vestibule and at the exit of the channel pore and not at the transmembrane region 2 of the protein as previously thought. Moreover, we show that these three critical substitutions only increase calcium permeability in the context of the mammalian but not the avian receptor, stressing the relevance of overall protein structure on defining functional properties. These results highlight the importance of tracking evolutionarily acquired changes in protein sequence underlying fundamental functional properties of ligand-gated ion channels.

Crystallographic studies of pharmacological sites in pentameric ligand-gated ion channels

BACKGROUND

Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical transmission of nerve signals in the central and peripheral nervous system. On the functional side, these molecules respond to the binding of a neurotransmitter (glycine, GABA, acetylcholine or 5HT3) in the extracellular domain (ECD) by opening their ionotropic pore in the transmembrane domain (TMD). The response to the neurotransmitter binding can be modulated by several chemical compounds acting at topographically distinct sites, as documented by a large body of literature. Notably, these receptors are the target of several classes of world-wide prescribed drugs, including general anesthetics, smoking cessation aids, anxiolytics, anticonvulsants, muscle relaxants, hypnotics and anti-emetics. On the structural side recent progress has been made on the crystallization of pLGICs in its different allosteric states, especially pLGICs of bacterial origin. Therefore, structure-function relationships can now be discussed at the atomic level for pLGICs.

SCOPE OF REVIEW

This review focuses on the crystallographic structure of complexes of pLGICs with a number of ligands of pharmacological interest. First, we review structural data on two key functional aspects of these receptors: the agonist-induced activation and ion transport itself. The molecular understanding of both these functional aspects is important, as they are those that most pharmacological compounds target. Next, we describe modulation sites that have recently been documented by X-ray crystallography. Finally, we propose a simple geometric classification of all these pharmacological sites in pLGICs, based on icosahedrons.

MAJOR CONCLUSIONS

This review illustrates the wealth of structural insight gained by comparing all available structures of members of the pLGIC family to rationalize the pharmacology of structurally diverse drugs acting at topographically distinct sites. It will be highlighted how sites that had been described earlier using biochemical techniques can be rationalized using structural data. Surprisingly, the use of icosahedral symmetry allows to link together several modulation sites, in a way that was totally unanticipated.

GENERAL SIGNIFICANCE

Overall, understanding the interplay between the different modulation sites at the structural level should help the design of future drugs targeting pLGICs. This article is part of a Special Issue entitled structural biochemistry and biophysics of membrane proteins.

Structure and pharmacology of pentameric receptor channels: from bacteria to brain

Orthologs of the pentameric receptor channels that mediate fast synaptic transmission in the central and peripheral nervous systems have been found in several bacterial species and in a single archaea genus. Recent X-ray structures of bacterial and invertebrate pentameric receptors point to a striking conservation of the structural features within the whole family, even between distant prokaryotic and eukaryotic members. These structural data reveal general principles of molecular organization that allow allosteric membrane proteins to mediate chemoelectric transduction. Notably, several conformations have been solved, including open and closed channels with distinct global tertiary and quaternary structure. The data reveal features of the ion channel architecture and of diverse categories of binding sites, such as those that bind orthosteric ligands, including neurotransmitters, and those that bind allosteric modulators, such as general anesthetics, ivermectin, or lipids. In this review, we summarize the most recent data, discuss insights into the mechanism of action in these systems, and elaborate on newly opened avenues for drug design.

A novel congenital myasthenic syndrome due to decreased acetylcholine receptor ion-channel conductance

Muscle acetylcholine receptor ion channels mediate neurotransmission by depolarizing the postsynaptic membrane at the neuromuscular junction. Inherited disorders of neuromuscular transmission, termed congenital myasthenic syndromes, are commonly caused by mutations in genes encoding the five subunits of the acetylcholine receptor that severely reduce endplate acetylcholine receptor numbers and/or cause kinetic abnormalities of acetylcholine receptor function. We tracked the cause of the myasthenic disorder in a female with onset of first symptoms at birth, who displayed mildly progressive bulbar, respiratory and generalized limb weakness with ptosis and ophthalmoplegia. Direct DNA sequencing revealed heteroallelic mutations in exon 8 of the acetylcholine receptor ε-subunit gene. Two alleles were identified: one with the missense substitution p.εP282R, and the second with a deletion, c.798_800delCTT, which result in the loss of a single amino acid, residue F266, within the M2 transmembrane domain. When these acetylcholine receptor mutations were expressed in HEK 293 cells, the p.εP282R mutation caused severely reduced expression on the cell surface, whereas p.εΔF266 gave robust surface expression. Single-channel analysis for p.εΔF266 acetylcholine receptor channels showed the longest burst duration population was not different from wild-type acetylcholine receptor (4.39 ± 0.6 ms versus 4.68 ± 0.7 ms, n = 5 each) but that the amplitude of channel openings was reduced. Channel amplitudes at different holding potentials showed that single-channel conductance was significantly reduced in p.εΔF266 acetylcholine receptor channels (42.7 ± 1.4 pS, n = 8, compared with 70.9 ± 1.6 pS for wild-type, n = 6). Although a phenylalanine residue at this position within M2 is conserved throughout ligand-gated excitatory cys-loop channel subunits, deletion of equivalent residues in the other subunits of muscle acetylcholine receptor did not have equivalent effects. Modelling the impact of p.εΔF266 revealed only a minor alteration to channel structure. In this study we uncover the novel mechanism of reduced acetylcholine receptor channel conductance as an underlying cause of congenital myasthenic syndrome, with the 'low conductance' phenotype that results from the p.εΔF266 deletion mutation revealed by the coinheritance of the low-expressor mutation p.εP282R.

Structural basis for the selective permeability of channels made of communicating junction proteins

The open state(s) of gap junction channels is evident from their permeation by small ions in response to an applied intercellular (transjunctional/transchannel) voltage gradient. That an open channel allows variable amounts of current to transit from cell-to-cell in the face of a constant intercellular voltage difference indicates channel open/closing can be complete or partial. The physiological significance of such open state options is, arguably, the main concern of junctional regulation. Because gap junctions are permeable to many substances, it is sensible to inquire whether and how each open state influences the intercellular diffusion of molecules as valuable as, but less readily detected than current-carrying ions. Presumably, structural changes perceived as shifts in channel conductivity would significantly alter the transjunctional diffusion of molecules whose limiting diameter approximates the pore's limiting diameter. Moreover, changes in junctional permeability to some molecules might occur without evident changes in conductivity, either at macroscopic or single channel level. Open gap junction channels allow the exchange of cytoplasmic permeants between contacting cells by simple diffusion. The identity of such permeants, and the functional circumstances and consequences of their junctional exchange presently constitute the most urgent (and demanding) themes of the field. Here, we consider the necessity for regulating this exchange, the possible mechanism(s) and structural elements likely involved in such regulation, and how regulatory phenomena could be perceived as changes in chemical vs. electrical coupling; an overall reflection on our collective knowledge of junctional communication is then applied to suggest new avenues of research. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Ligand activation of the prokaryotic pentameric ligand-gated ion channel ELIC

While the pentameric ligand-gated ion channel ELIC has recently provided first insight into the architecture of the family at high resolution, its detailed investigation was so far prevented by the fact that activating ligands were unknown. Here we describe a study on the functional characterization of ELIC by electrophysiology and X-ray crystallography. ELIC is activated by a class of primary amines that include the neurotransmitter GABA at high micro- to millimolar concentrations. The ligands bind to a conserved site and evoke currents that slowly desensitize over time. The protein forms cation selective channels with properties that resemble the nicotinic acetylcholine receptor. The high single channel conductance and the comparably simple functional behavior make ELIC an attractive model system to study general mechanisms of ion conduction and gating in this important family of neurotransmitter receptors.

Modulation of the Ca(2+) permeability of human endplate acetylcholine receptor-channel

In slow-channel congenital myasthenic syndrome, point mutations of the endplate acetylcholine receptor (AChR) prolong channel openings, leading to excessive Ca(2+) entry with ensuing endplate degeneration and myasthenic symptoms. The Ca(2+) permeability of the human endplate AChR-channel is quite high, and is further increased by two slow-channel mutations in its ɛ subunit, worsening the pathological cascade. To gain further support to the hypothesis that the ɛ subunit plays a crucial role in controlling Ca(2+) permeability of endplate AChR-channel, in this work we measured the fractional Ca(2+) current (P(f), i.e., the percentage of the total current carried by Ca(2+) ions) of a panel of AChR carrying slow-channel mutations in the α, β and ɛ subunits detected in patients (α(N217K), α(S226Y), α(C418W), β(V266A), β(V266M), ɛ(I257F), ɛ(V265A) and ɛ(L269F)). We confirm that only mutations in the ɛ subunit altered Ca(2+) permeability of AChR-channels, with ɛ(L269F) increasing P(f) (10% vs. 7% of wild type AChR) and ɛ(I257F) decreasing it (to 4.6%). We also found that, for ɛ(L269F)-AChR, the Ca(2+) permeability and ACh-induced cell death can be normalized by clinically relevant concentrations of salbutamol or verapamil, providing the first evidence that the Ca(2+) permeability of AChR-channels can be modulated and this treatment may provide protection against excitotoxic insults.

In glycine and GABA(A) channels, different subunits contribute asymmetrically to channel conductance via residues in the extracellular domain

Single-channel conductance in Cys-loop channels is controlled by the nature of the amino acids in the narrowest parts of the ion conduction pathway, namely the second transmembrane domain (M2) and the intracellular helix. In cationic channels, such as Torpedo ACh nicotinic receptors, conductance is increased by negatively charged residues exposed to the extracellular vestibule. We now show that positively charged residues at the same loop 5 position boost also the conductance of anionic Cys-loop channels, such as glycine (α1 and α1β) and GABA(A) (α1β2γ2) receptors. Charge reversal mutations here produce a greater decrease on outward conductance, but their effect strongly depends on which subunit carries the mutation. In the glycine α1β receptor, replacing Lys with Glu in α1 reduces single-channel conductance by 41%, but has no effect in the β subunit. By expressing concatameric receptors with constrained stoichiometry, we show that this asymmetry is not explained by the subunit copy number. A similar pattern is observed in the α1β2γ2 GABA(A) receptor, where only mutations in α1 or β2 decreased conductance (to different extents). In both glycine and GABA receptors, the effect of mutations in different subunits does not sum linearly: mutations that had no detectable effect in isolation did enhance the effect of mutations carried by other subunits. As in the nicotinic receptor, charged residues in the extracellular vestibule of anionic Cys-loop channels influence elementary conductance. The size of this effect strongly depends on the direction of the ion flow and, unexpectedly, on the nature of the subunit that carries the residue.

Structural basis of open channel block in a prokaryotic pentameric ligand-gated ion channel

The flow of ions through cation-selective members of the pentameric ligand-gated ion channel family is inhibited by a structurally diverse class of molecules that bind to the transmembrane pore in the open state of the protein. To obtain insight into the mechanism of channel block, we have investigated the binding of positively charged inhibitors to the open channel of the bacterial homolog GLIC by using X-ray crystallography and electrophysiology. Our studies reveal the location of two regions for interactions, with larger blockers binding in the center of the membrane and divalent transition metal ions binding to the narrow intracellular pore entry. The results provide a structural foundation for understanding the interactions of the channel with inhibitors that is relevant for the entire family.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Role of acetylcholine receptor domains in ion selectivity

The nicotinic acetylcholine receptor (nAChR) is a ligand gated ion channel protein, composed of three domains: a transmembrane domain (TM-domain), extracellular domain (EC-domain), and intracellular domain (IC-domain). Due to its biological importance, much experimental and theoretical research has been carried out to explore its mechanisms of gating and selectivity, but there are still many unresolved issues, especially on the ion selectivity. Moreover, most of the previous theoretical work has concentrated on the TM-domain or EC-domain of nAChR, which may be insufficient to understand the entire structure-function relation. In this work, we perform molecular dynamics, Brownian dynamics simulations and continuum electrostatic calculations to investigate the role of different nAChR domains in ion conduction and selectivity. The results show that although both the EC and IC domains contain strong negative charges that create large cation concentrations at either end of the pore, this alone is not sufficient to create the observed cation selectivity and may play a greater role in determining the channel conductance. The presence of cations in the wide regions of the pore can screen out the protein charge allowing anions to enter, meaning that local regions of the TM-domain are most likely responsible for discriminating between ions. These new results complement our understanding about the ion conduction and selectivity mechanism of nAChR.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Structure of a potentially open state of a proton-activated pentameric ligand-gated ion channel

The X-ray structure of a pentameric ligand-gated ion channel from Erwinia chrysanthemi (ELIC) has recently provided structural insight into this family of ion channels at high resolution. The structure shows a homo-pentameric protein with a barrel-stave architecture that defines an ion-conduction pore located on the fivefold axis of symmetry. In this structure, the wide aqueous vestibule that is encircled by the extracellular ligand-binding domains of the five subunits narrows to a discontinuous pore that spans the lipid bilayer. The pore is constricted by bulky hydrophobic residues towards the extracellular side, which probably serve as barriers that prevent the diffusion of ions. This interrupted pore architecture in ELIC thus depicts a non-conducting conformation of a pentameric ligand-gated ion channel, the thermodynamically stable state in the absence of bound ligand. As ligand binding promotes pore opening in these ion channels and the specific ligand for ELIC has not yet been identified, we have turned our attention towards a homologous protein from the cyanobacterium Gloebacter violaceus (GLIC). GLIC was shown to form proton-gated channels that are activated by a pH decrease on the extracellular side and that do not desensitize after activation. Both prokaryotic proteins, ELIC and GLIC form ion channels that are selective for cations over anions with poor discrimination among monovalent cations, characteristics that resemble the conduction properties of the cation-selective branch of the family that includes acetylcholine and serotonin receptors. Here we present the X-ray structure of GLIC at 3.1 A resolution. The structure reveals a conformation of the channel that is distinct from ELIC and that probably resembles the open state. In combination, both structures suggest a novel gating mechanism for pentameric ligand-gated ion channels where channel opening proceeds by a change in the tilt of the pore-forming helices.

Gating at the mouth of the acetylcholine receptor channel: energetic consequences of mutations in the alphaM2-cap

Gating of nicotinic acetylcholine receptors from a C(losed) to an O(pen) conformation is the initial event in the postsynaptic signaling cascade at the vertebrate nerve-muscle junction. Studies of receptor structure and function show that many residues in this large, five-subunit membrane protein contribute to the energy difference between C and O. Of special interest are amino acids located at the two transmitter binding sites and in the narrow region of the channel, where C↔O gating motions generate a low↔high change in the affinity for agonists and in the ionic conductance, respectively. We have measured the energy changes and relative timing of gating movements for residues that lie between these two locations, in the C-terminus of the pore-lining M2 helix of the α subunit (‘αM2-cap’). This region contains a binding site for non-competitive inhibitors and a charged ring that influences the conductance of the open pore. αM2-cap mutations have large effects on gating but much smaller effects on agonist binding, channel conductance, channel block and desensitization. Three αM2-cap residues (αI260, αP265 and αS268) appear to move at the outset of channel-opening, about at the same time as those at the transmitter binding site. The results suggest that the αM2-cap changes its secondary structure to link gating motions in the extracellular domain with those in the channel that regulate ionic conductance.

Charge scan reveals an extended region at the intracellular end of the GABA receptor pore that can influence ion selectivity

Selective permeability is a fundamental property of ion channels. The Cys-loop receptor superfamily is composed of both excitatory (ACh, 5-HT) and inhibitory (GABA, glycine) neurotransmitter-operated ion channels. In the GABA receptor, it has been previously shown that the charge selectivity of the integral pore can be altered by a single mutation near the intracellular end of the second transmembrane-spanning domain (TM2). We have extended these findings and now show that charge selectivity of the anionic rho1 GABA receptor can be influenced by the introduction of glutamates, one at a time, over an 8-amino acid stretch (-2' to 5') in the proposed intracellular end of TM2 and the TM1-TM2 intracellular linker. Depending on the position, glutamate substitutions in this region produced sodium to chloride permeability ratios (P(Na)+(/Cl)-) varying from 0.64 to 3.4 (wild type P(Na)+(/Cl)- = 0). In addition to providing insight into the mechanism of ion selectivity, this functional evidence supports a model proposed for the homologous nicotinic acetylcholine receptor in which regions of the protein, in addition to TM2, form the ion pathway.

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.

Chimeric Mutations in the M2 Segment of the 5-Hydroxytryptamine-gated Chloride Channel MOD-1 Define a Minimal Determinant of Anion/Cation Permeability

The ionic selectivity of ligand-gated ion channels (LGICs) determines whether receptor activation produces an excitatory or inhibitory response. The determinants of anion/cation selectivity were investigated for a new member of the LGIC superfamily, MOD-1, a serotonin-gated chloride channel cloned from the nematode Caenorhabditis elegans. In common with other anionic LGICs (glycine receptors and GABA(A) receptors), the selectivity triple mutant in the pore-forming M2 segment (proline insertion, Ala --> Glu substitution at the central ring, and Thr --> Val at the hydrophobic ring) converted the selectivity of MOD-1 from anionic to cationic. Unlike other LGICs, however, this mutant in MOD-1 was highly selective for K+ over other cations. Subsets of this selectivity triple mutant were studied to define the minimal change required for conversion from anion-permeable to cation-permeable. The double mutant at the central ring of charge (deltaPro-269/A270E) produced a non-selective cation channel. Charge reversal at the central ring alone (A270E) was sufficient to convert MOD-1 to cation-permeable. These results refine the determinants of ion-charge selectivity in LGICs and demonstrate the critical role of the central ring of charge formed by the M2 segments.

Conduction through the inward rectifier potassium channel, Kir2.1, is increased by negatively charged extracellular residues

Ion channel conductance can be influenced by electrostatic effects originating from fixed “surface” charges that are remote from the selectivity filter. To explore whether surface charges contribute to the conductance properties of Kir2.1 channels, unitary conductance was measured in cell-attached recordings of Chinese hamster ovary (CHO) cells transfected with Kir2.1 channels over a range of K⁺ activities (4.6–293.5 mM) using single-channel measurements as well as nonstationary fluctuation analysis for low K⁺ activities. K⁺ ion concentrations were shown to equilibrate across the cell membrane in our studies using the voltage-sensitive dye DiBAC₄(5). The dependence of γ on the K⁺ activity (a_K) was fit well by a modified Langmuir binding isotherm, with a nonzero intercept as a_K approaches 0 mM, suggesting electrostatic surface charge effects. Following the addition of 100 mM N-methyl-d-glucamine (NMG⁺), a nonpermeant, nonblocking cation or following pretreatment with 50 mM trimethyloxonium (TMO), a carboxylic acid esterifying agent, the γ–a_K relationship did not show nonzero intercepts, suggesting the presence of surface charges formed by glutamate or aspartate residues. Consistent with surface charges in Kir2.1 channels, the rates of current decay induced by Ba²⁺ block were slowed with the addition of NMG or TMO. Using a molecular model of Kir2.1 channels, three candidate negatively charged residues were identified near the extracellular mouth of the pore and mutated to cysteine (E125C, D152C, and E153C). E153C channels, but not E125C or D152C channels, showed hyperbolic γ–a_K relationships going through the origin. Moreover, the addition of MTSES to restore the negative charges in E53C channels reestablished wild-type conductance properties. Our results demonstrate that E153 contributes to the conductance properties of Kir2.1 channels by acting as a surface charge.

Charge selectivity of the Cys-loop family of ligand-gated ion channels

The determinants of charge selectivity of the Cys-loop family of ligand-gated ion channels have been studied for more than a decade. The investigations have mainly covered homomeric receptors e.g. the nicotinic acetylcholine receptor alpha7, the glycine receptor alpha1 and the serotonin receptor 5-HT(3A). Only recently, the determinants of charge selectivity of heteromeric receptors have been addressed for the GABA(A) receptor alpha2beta3gamma2. For all receptor subtypes, the selectivity determinants have been located to an intracellular linker between transmembrane domains M1 and M2. Two features of the M1-M2 linker appear to control ion selectivity. A central role for charged amino acid residues in selectivity has been almost universally observed. Furthermore, recent studies point to an important role of the size of the narrowest constriction in the pore. In the present review, these determinants of charge selectivity of the Cys-loop family of ligand-gated ion channels will be discussed in detail.

Ligand-gated ion channels: mechanisms underlying ion selectivity

Anion/cation selectivity is a critical property of ion channels and underpins their physiological function. Recently, there have been numerous mutagenesis studies, which have mapped sites within the ion channel-forming segments of ligand-gated ion channels that are determinants of the ion selectivity. Site-directed mutations to specific amino acids within or flanking the M2 transmembrane segments of the anion-selective glycine, GABA(A) and GABA(C) receptors and the cation-selective nicotinic acetylcholine and serotonin (type 3) receptors have revealed discrete, equivalent regions within the ion channel that form the principal selectivity filter, leading to plausible molecular mechanisms and mathematical models to describe how ions preferentially permeate these channels. In particular, the dominant factor determining anion/cation selectivity seems to be the sign and exposure of charged amino acids lining the selectivity filter region of the open channel. In addition, the minimum pore diameter, which can be influenced by the presence of a local proline residue, also makes a contribution to such ion selectivity in LGICs with smaller diameters increasing anion/cation selectivity and larger ones decreasing it.

Ca2+ permeability of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in muscle cells and neurons, as well as in an increasing number of other cell types. The nAChR channels are permeable to cations, including Ca(2+). Ca(2+) entry through nAChR channels has been shown to modulate several Ca(2+)-dependent cellular processes, such as neurotransmitter release, synaptic plasticity, and cell motility. The value of Ca(2+) permeability associated to a particular nAChR subtype thus represents an important indication for its physiological role. This review summarizes the quantitative data on Ca(2+) permeability obtained from several nAChR subtypes in native and heterologous systems. Different experimental approaches are compared, and the structural determinants of Ca(2+) permeability are discussed.

A single ring of charged amino acids at one end of the pore can control ion selectivity in the 5-HT3 receptor

1. To determine the mechanisms by which cation- or anion-specific channels select between these ions, we have examined the role of electrostatic factors in a typical ligand-gated ion channel, the 5-hydroxytryptamine3 (5-HT3) receptor, by removal and/or insertion of rings of conserved charged residues at either end of the pore. 2. Neutralization of the negatively charged ring at the intracellular end of the channel (E-1'A) results in a nonselective channel (PNa/PCl=0.89). 3. Insertion of positively charged residues at the extracellular end of the pore either results in a nonfunctional receptor (A24'K) or one that remains cation-selective (PNa/PCl=110; S19'R). 4. Combining the removal of a negatively charged ring (E-1'A) with the insertion of a positively charged ring (S19'R), however, results in a channel that is predominantly anion-selective (PNa/PCl=0.37). 5. The data suggest that for the cation-selective 5-HT3 receptor, the control of selectivity exerted by charged rings at either end of the pore is dominated by the ring of negatively charged residues at the intracellular side of the channel. As changing the charge at this position has also been shown to change ionic selectivity in anion-selective receptors, these data suggest that electrostatic factors can control selectivity in the whole Cys-loop family.

Nervous system targets of RNA editing identified by comparative genomics

An unknown number of precursor messenger RNAs undergo genetic recoding by modification of adenosine to inosine, a reaction catalyzed by the adenosine deaminases acting on RNA (ADARs). Discovery of these edited transcripts has always been serendipitous. Using comparative genomics, we identified a phylogenetic signature of RNA editing. We report the identification and experimental verification of 16 previously unknown ADAR target genes in the fruit fly Drosophila and one in humans-more than the sum total previously reported. All of these genes are involved in rapid electrical and chemical neurotransmission, and many of the edited sites recode conserved and functionally important amino acids. These results point to a pivotal role for RNA editing in nervous system function.

Structural effects of quinacrine binding in the open channel of the acetylcholine receptor

Noncompetitive inhibitors of the nicotinic acetylcholine (ACh) receptors suppress cation flux directly by binding in and blocking the open channel or indirectly by stabilizing closed states of the receptor. The lidocaine derivative QX-314 and the acridine derivative quinacrine act directly as open channel blockers, but can act indirectly as well. The binding site for quinacrine in the open channel of mouse-muscle ACh receptor was mapped in cysteine-substituted mutants of the alpha subunit expressed with wild-type beta, gamma, and delta subunits. In the open state, substituted cysteines in the inner half of the second membrane-spanning segment (M2), but not in the outer half, were protected by quinacrine from reaction with 2-aminoethyl methanethiosulfonate. In addition, an alkylating derivative, quinacrine mustard, affinity labeled a subset of the substituted cysteines in M2, but only in the open state. These results, mapped onto a model of the open channel surrounded by five alpha-helical M2s, imply that quinacrine binds midway down M2 in the same site previously mapped for QX-314. A cysteine substituted for a residue in the outer third of alphaM1, which reacted with 2-aminoethyl methanethiosulfonate only in the presence of ACh, reacted faster in the additional presence of quinacrine or QX-314. It is proposed that channel opening involves both the opening of the resting gate at the inner end of M2 and the removal of an obstruction formed by the outer end of M1 that retards diffusion of blockers into the closed channel. Blocker binding in the open channel causes a further change in structure.

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1). structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2). which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a). muscarinic receptor subtypes, (b). G(i) proteins, and (c). muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M(2) and M(1) receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of G(i) proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M(2) receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease.

Pores formed by the nicotinic receptor m2delta Peptide: a molecular dynamics simulation study

The M2delta peptide self-assembles to form a pentameric bundle of transmembrane alpha-helices that is a model of the pore-lining region of the nicotinic acetylcholine receptor. Long (>15 ns) molecular dynamics simulations of a model of the M2delta(5) bundle in a POPC bilayer have been used to explore the conformational dynamics of the channel assembly. On the timescale of the simulation, the bundle remains relatively stable, with the polar pore-lining side chains remaining exposed to the lumen of the channel. Fluctuations at the helix termini, and in the helix curvature, result in closing/opening transitions at both mouths of the channel, on a timescale of approximately 10 ns. On average, water within the pore lumen diffuses approximately 4x more slowly than water outside the channel. Examination of pore water trajectories reveals both single-file and path-crossing regimes to occur at different times within the simulation.

Cation-selective mutations in the M2 domain of the inhibitory glycine receptor channel reveal determinants of ion-charge selectivity

Ligand-gated ion channel receptors mediate neuronal inhibition or excitation depending on their ion charge selectivity. An investigation into the determinants of ion charge selectivity of the anion-selective alpha1 homomeric glycine receptor (alpha1 glycine receptor [GlyR]) was undertaken using point mutations to residues lining the extra- and intracellular ends of the ion channel. Five mutant GlyRs were studied. A single substitution at the intracellular mouth of the channel (A-1'E GlyR) was sufficient to convert the channels to select cations over anions with P(Cl)/P(Na) = 0.34. This result delimits the selectivity filter and provides evidence that electrostatic interactions between permeating ions and pore residues are a critical factor in ion charge selectivity. The P-2'Delta mutant GlyR retained its anion selectivity (P(Cl)/P(Na) = 3.81), but it was much reduced compared with the wild-type (WT) GlyR (P(Cl)/P(Na) = 27.9). When the A-1'E and the P-2'Delta mutations were combined (selectivity double mutant [SDM] GlyR), the relative cation permeability was enhanced (P(Cl)/P(Na) = 0.13). The SDM GlyR was also Ca(2+) permeable (P(Ca)/P(Na) = 0.29). Neutralizing the extracellular mouth of the SDM GlyR ion channel (SDM+R19'A GlyR) produced a more Ca(2+)-permeable channel (P(Ca)/P(Na) = 0.73), without drastically altering monovalent charge selectivity (P(Cl)/P(Na) = 0.23). The SDM+R19'E GlyR, which introduces a negatively charged ring at the extracellular mouth of the channel, further enhanced Ca(2+) permeability (P(Ca)/P(Na) = 0.92), with little effect on monovalent selectivity (P(Cl)/P(Na) = 0.19). Estimates of the minimum pore diameter of the A-1'E, SDM, SDM+R19'A, and SDM+R19'E GlyRs revealed that these pores are larger than the alpha1 GlyR, with the SDM-based GlyRs being comparable in diameter to the cation-selective nicotinic acetylcholine receptors. This result provides evidence that the diameter of the ion channel is also an important factor in ion charge selectivity.

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.

Polar residues of the second transmembrane domain influence cation permeability of the ATP-gated P2X(2) receptor

P2X receptors are simple polypeptide channels that mediate fast purinergic depolarizations in both nerve and muscle. Although the depolarization results mainly from the influx of Na(+), these channels also conduct a significant Ca(2+) current that is large enough to evoke transmitter release from presynaptic neurons. We sought to determine the molecular basis of this Ca(2+) conductance by a mutational analysis of recombinant P2X(2) receptors. Wild type and 31 mutant P2X(2) receptors were expressed in HEK-293 cells and studied under voltage-clamp. We found that the relative Ca(2+) permeability measured from the reversal potentials of ATP-gated currents was unaffected by neutralizing fixed charge (Asp(315), Asp(349)) near the mouths of the channel pore. By contrast, mutations that changed the character or side chain volume of three polar residues (Thr(336), Thr(339), Ser(340)) within the pore led to significant changes in P(Ca)/P(Cs). The largest changes occurred when Thr(339) and Ser(340) were replaced with tyrosine; these mutations almost completely abolished Ca(2+) permeability, reduced P(Li)/P(Cs) by about one-half, and shifted the relative permeability sequence of Cs(+), Rb(+), K(+), and Na(+) to their relative mobility in water. Our results suggest that the permeability sequence of the P2X(2) receptor arises in part from interactions of permeating cations with the polar side chains of three amino acids located in a short stretch of the second transmembrane domain.

Location of the polyamine binding site in the vestibule of the nicotinic acetylcholine receptor ion channel

To map the structure of a ligand-gated ion channel, we used the photolabile polyamine-containing toxin MR44 as photoaffinity label. MR44 binds with high affinity to the nicotinic acetylcholine receptor in its closed channel conformation. The binding stoichiometry was two molecules of MR44 per receptor monomer. Upon UV irradiation of the receptor-ligand complex, (125)I-MR44 was incorporated into the receptor alpha-subunit. From proteolytic mapping studies, we conclude that the site of (125)I-MR44 cross-linking is contained in the sequence alpha His-186 to alpha Leu-199, which is part of the extracellular domain of the receptor. This sequence partially overlaps in its C-terminal region with one of the three loops that form the agonist-binding site. The agonist carbachol and the competitive antagonist alpha-bungarotoxin had only minor influence on the photocross-linking of (125)I-MR44. The site where the hydrophobic head group of (125)I-MR44 binds must therefore be located outside the zone that is sterically influenced by agonist bound at the nicotinic acetylcholine receptor. In binding and photocross-linking experiments, the luminal noncompetitive inhibitors ethidium and triphenylmethylphosphonium were found to compete with (125)I-MR44. We conclude that the polyamine moiety of (125)I-MR44 interacts with the high affinity noncompetitive inhibitor site deep in the channel of the nicotinic acetylcholine receptor, while the aromatic ring of this compound binds in the upper part of the ion channel (i.e. in the vestibule) to a hydrophobic region on the alpha-subunit that is located in close proximity to the agonist binding site. The region of the alpha-subunit labeled by (125)I-MR44 should therefore be accessible from the luminal side of the vestibule.

Binding of polyamine-containing toxins in the vestibule of the nicotinic acetylcholine receptor ion channel

Several wasp venoms contain philanthotoxins (PhTXs) that act as noncompetitive inhibitors (NCIs) on cation-selective ion channels including the nicotinic acetylcholine receptor (nAChR). In the search for a ligand with high affinity and specificity for the nAChR we tested a series of newly developed PhTX analogues. Modulation of the structural elements of PhTXs can significantly influence their binding affinities. This approach resulted in the development of the photolabile compound MR44. In photoaffinity labelling studies 125I-MR44 was used to map the ligand-binding site at the Torpedo californica nAChR. Upon UV irradiation of the receptor-ligand complex, 125I-MR44 was mainly incorporated into the receptor alpha-subunit. Proteolytic mapping and microsequencing identified the site of 125I-MR44 cross-linking within the sequence alphaHis-186 to alphaLeu-199 that in its C-terminal region partially overlaps with the agonist-binding site. Since bound agonists had only minor influence on 125I-MR44 photocrosslinking, the site where the hydrophobic head group of 125I-MR44 binds must be located outside the zone that is sterically influenced by agonists bound at the nAChR. A possible site of interaction of 125I-MR44 would be the N-terminal region of the labelled sequence, in which aromatic amino-acid residues are accumulated. We suggest that the polyamine moiety of 125I-MR44 interacts with the high affinity non-competitive inhibitor site deep in the ion channel, while the aromatic ring of this compound binds in the vestibule of the nAChR to a hydrophobic region on the alpha-subunit that is located close to the agonist binding site.

M2 pore mutations convert the glycine receptor channel from being anion- to cation-selective

Three mutations in the M2 transmembrane domains of the chloride-conducting alpha1 homomeric glycine receptor (P250Delta, A251E, and T265V), which normally mediate fast inhibitory neurotransmission, produced a cation-selective channel with P(Cl)/P(Na), = 0.27 (wild-type P(Cl)/P(Na) = 25), a permeability sequence P(Cs) > P(K) > P(Na) > P(Li), an impermeability to Ca(2+), and a reduced glycine sensitivity. Outside-out patch measurements indicated reversed and accentuated rectification with extremely low mean single channel conductances of 3 pS (inward current) and 11 pS (outward current). The three inverse mutations, to those analyzed in this study, have previously been shown to make the alpha7 acetylcholine receptor channel anion-selective, indicating a common location for determinants of charge selectivity of inhibitory and excitatory ligand-gated ion channels.

A molecular link between inward rectification and calcium permeability of neuronal nicotinic acetylcholine alpha3beta4 and alpha4beta2 receptors

Many nicotinic acetylcholine receptors (nAChRs) expressed by central neurons are located at presynaptic nerve terminals. These receptors have high calcium permeability and exhibit strong inward rectification, two important physiological features that enable them to facilitate transmitter release. Previously, we showed that intracellular polyamines act as gating molecules to block neuronal nAChRs in a voltage-dependent manner, leading to inward rectification. Our goal is to identify the structural determinants that underlie the block by intracellular polyamines and govern calcium permeability of neuronal nAChRs. We hypothesize that two ring-like collections of negatively charged amino acids (cytoplasmic and intermediate rings) near the intracellular mouth of the pore mediate the interaction with intracellular polyamines and also influence calcium permeability. Using site-directed mutagenesis and electrophysiology on alpha(4)beta(2) and alpha(3)beta(4) receptors expressed in Xenopus oocytes, we observed that removing the five negative charges of the cytoplasmic ring had little effect on either inward rectification or calcium permeability. However, partial removal of negative charges of the intermediate ring diminished the high-affinity, voltage-dependent interaction between intracellular polyamines and the receptor, abolishing inward rectification. In addition, these nonrectifying mutant receptors showed a drastic reduction in calcium permeability. Our results indicate that the negatively charged glutamic acid residues at the intermediate ring form both a high-affinity binding site for intracellular polyamines and a selectivity filter for inflowing calcium ions; that is, a common site links inward rectification and calcium permeability of neuronal nAChRs. Physiologically, this molecular mechanism provides insight into how presynaptic nAChRs act to influence transmitter release.

The intrinsic electrostatic potential and the intermediate ring of charge in the acetylcholine receptor channel

A ring of aligned glutamate residues named the intermediate ring of charge surrounds the intracellular end of the acetylcholine receptor channel and dominates cation conduction (Imoto et al. 1988). Four of the five subunits in mouse-muscle acetylcholine receptor contribute a glutamate to the ring. These glutamates were mutated to glutamine or lysine, and combinations of mutant and native subunits, yielding net ring charges of -1 to -4, were expressed in Xenopus laevis oocytes. In all complexes, the alpha subunit contained a Cys substituted for alphaThr244, three residues away from the ring glutamate alphaGlu241. The rate constants for the reactions of alphaThr244Cys with the neutral 2-hydroxyethyl-methanethiosulfonate, the positively charged 2-ammonioethyl-methanethiosulfonate, and the doubly positively charged 2-ammonioethyl-2'-ammonioethanethiosulfonate were determined from the rates of irreversible inhibition of the responses to acetylcholine. The reagents were added in the presence and absence of acetylcholine and at various transmembrane potentials, and the rate constants were extrapolated to zero transmembrane potential. The intrinsic electrostatic potential in the channel in the vicinity of the ring of charge was estimated from the ratios of the rate constants of differently charged reagents. In the acetylcholine-induced open state, this potential was -230 mV with four glutamates in the ring and increased linearly towards 0 mV by +57 mV for each negative charge removed from the ring. Thus, the intrinsic electrostatic potential in the narrow, intracellular end of the open channel is almost entirely due to the intermediate ring of charge and is strongly correlated with alkali-metal-ion conductance through the channel. The intrinsic electrostatic potential in the closed state of the channel was more positive than in the open state at all values of the ring charge. These electrostatic properties were simulated by theoretical calculations based on a simplified model of the channel.

The 4'lysine in the putative channel lining domain affects desensitization but not the single-channel conductance of recombinant homomeric 5-HT3A receptors

The 5-HT3 receptor is a transmitter-gated ion channel of the Cys-loop superfamily. Uniquely, 5-HT3 receptor subunits (5-HT3A and 5-HT3B) possess a positively charged lysine residue within the putative channel lining M2 domain (4' position). Using whole cell recording techniques, we examined the role of this residue in receptor function using wild-type (WT) and mutant 5-HT3A receptor subunits of murine origin transiently expressed in human embryonic kidney (HEK 293) cells. WT 5-HT3A receptors mediated rapidly activating currents in response to 5-HT (10-90 % rise time, 103 ms; EC50, 2.34 microM; Hill coefficient, nH, 2.87). The currents rectified inwardly, reversed in sign at a potential of -9 mV and desensitized in the continuous presence of agonist (half-time of desensitization, t(1/2), 2.13 s). 5-HT3A receptor subunits in which the 4'lysine was mutated to arginine, glutamine, serine or glycine formed functional receptors. 5-HT EC50 values were approximately 2-fold lower than for WT 5-HT3A receptors, but Hill coefficients, kinetics of current activation, rectification, and reversal potentials were unaltered. Each of the mutants desensitized more slowly than the WT 5-HT3A receptor, with the arginine and glycine mutations exhibiting the greatest effect (5-fold reduction). The rank order of effect was arginine > glycine > serine > glutamine. The single-channel conductance of the WT 5-HT3A receptor, as assessed by fluctuation analysis of macroscopic currents, was 390 fS. A similar value was obtained for the 4'lysine mutant receptors. Thus it appears unlikely that 4'lysine is exposed to the channel lumen. Mutation of residues immediately adjacent to 4'lysine to glutamate or lysine resulted in lack of receptor expression or function. We conclude that 4'lysine does not form part of the channel lining, but may play an important role in 5-HT3 receptor desensitization.

Structure-activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

Several wasp venoms contain philanthotoxins (PhTXs), natural polyamine amides, which act as noncompetitive inhibitors (NCIs) on the nicotinic acetylcholine receptor (nAChR). Effects of varying the structure of PhTXs and poly(methylene tetramine)s on the binding affinity have been investigated. Using the fluorescent NCI ethidium in a displacement assay Kapp values of these compounds have been determined. We found that an increase in size of the PhTX's hydrophobic head group significantly increased the binding affinity, while inserting positive charge almost completely destroyed it. Elongating the PhTX polyamine chain by introducing an additional aminomethylene group decreased the binding affinity, whereas a terminal lysine improved it. In general, poly(methylene tetramine)s showed higher binding affinities than PhTX analogues. The stoichiometry of PhTX binding was determined to be two PhTX molecules per receptor monomer. PhTXs appeared to bind to a single class of nonallosterically interacting binding sites and bound PhTX was found to be completely displaced by well-characterized luminal NCIs. To elucidate the site of PhTX binding, a photolabile, radioactive PhTX derivative was photocross-linked to the nAChR in its closed channel conformation resulting in labeling yields for the two alpha and the beta, gamma and delta subunits of 10.4, 11.1, 4.0 and 7.4%, respectively. Based on these findings we suggest that PhTXs and poly(methylene tetramine)s enter the receptor's ionic channel from the extracellular side. The hydrophobic head groups most likely bind to the high-affinity NCI site, while the positively charged polyamine chains presumably interact with the negatively charged selectivity filter located deep in the channel lumen.

Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha7 receptors expressed in Xenopus oocytes were investigated using electrophysiology and site-directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding responses obtained from a first extracellular domain mutant, phenylalanine(189)tyrosine (P0.05). The dissociation constants (K(D)) of the wild type (4.7 nM) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were estimated by an electrophysiological approach. The similarity of the results suggests that the mutation did not lead to a widespread disruption of structure-function relationships, although a slight change in nicotine sensitivity may have occurred. In contrast, the mutations (Tyr(190)Gln, first extracellular domain), (Glu(261)Ala, M2 region) severely compromised receptor function. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic receptors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha7 receptors have parallels with heteromeric nicotinic receptors, but there may also be some marked differences.

Mutational analysis of the charge selectivity filter of the alpha7 nicotinic acetylcholine receptor

In the alpha7 nicotinic acetylcholine receptors, we analyze the contribution of mutations E237A and V251T, together with the proline insertion P236', in the conversion of the charge selectivity from cationic to anionic. We show that the triple mutant exhibits spontaneous openings displaying anionic selectivity. Furthermore, at position 251, hydrophilic or even negatively charged residues are compatible with an anionic channel. In contrast, the additional proline yields an anionic channel only when inserted between positions 234 and 237; insertion before 234 yields a cationic channel and after 238 alters the receptor surface expression. The coiled 234-238 loop thus directly contributes to the charge selectivity filter of the alpha7 channel.

(In)validity of the constant field and constant currents assumptions in theories of ion transport

Constant electric fields and constant ion currents are often considered in theories of ion transport. Therefore, it is important to understand the validity of these helpful concepts. The constant field assumption requires that the charge density of permeant ions and flexible polar groups is virtually voltage independent. We present analytic relations that indicate the conditions under which the constant field approximation applies. Barrier models are frequently fitted to experimental current-voltage curves to describe ion transport. These models are based on three fundamental characteristics: a constant electric field, negligible concerted motions of ions inside the channel (an ion can enter only an empty site), and concentration-independent energy profiles. An analysis of those fundamental assumptions of barrier models shows that those approximations require large barriers because the electrostatic interaction is strong and has a long range. In the constant currents assumption, the current of each permeating ion species is considered to be constant throughout the channel; thus ion pairing is explicitly ignored. In inhomogeneous steady-state systems, the association rate constant determines the strength of ion pairing. Among permeable ions, however, the ion association rate constants are not small, according to modern diffusion-limited reaction rate theories. A mathematical formulation of a constant currents condition indicates that ion pairing very likely has an effect but does not dominate ion transport.