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

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

State-dependent accessibility and electrostatic potential in the channel of the acetylcholine receptor. Inferences from rates of reaction of thiosulfonates with substituted cysteines in the M2 segment of the alpha subunit

Ion channel function depends on the chemical and physical properties and spatial arrangement of the residues that line the channel lumen and on the electrostatic potential within the lumen. We have used small, sulfhydryl-specific thiosulfonate reagents, both positively charged and neutral, to probe the environment within the acetylcholine (ACh) receptor channel. Rate constants were determined for their reactions with cysteines substituted for nine exposed residues in the second membrane-spanning segment (M2) of the alpha subunit. The largest rate constants, both in the presence and absence of ACh, were for the reactions with the cysteine substituted for alpha Thr244, near the intracellular end of the channel. In the open state of the channel, but not in the closed state, the rate constants for the reactions of the charged reagents with several substituted cysteines depended on the transmembrane electrostatic potential, and the electrical distance of these cysteines increased from the extracellular to the intracellular end of M2. Even at zero transmembrane potential, the ratios of the rate constants for the reactions of three positively charged reagents with alpha T244C, alpha L251C, and alpha L258C to the rate constant for the reaction of an uncharged reagent were much greater in the open than in the closed state. This dependence of the rate constants on reagent charge is consistent with an intrinsic electrostatic potential in the channel that is considerably more negative in the open state than in the closed state. The effects of ACh on the rate constants for the reactions of substituted Cys along the length of alpha M2, on the dependence of the rate constants on the transmembrane potential, and on the intrinsic potential support a location of a gate more intracellular than alpha Thr244.

The location of the gate in the acetylcholine receptor channel

The cation-conducting channel of the nicotinic acetylcholine (ACh) receptor is lined by the first (M1) and second (M2) membrane-spanning segments of each of its five subunits. Six consecutive residues, alphaS239 to alphaT244, in the alpha subunit M1-M2 loop and at the intracellular end of M2 were mutated to cysteine. The accessibility of the substituted cysteines were probed with small, cationic, sulfhydryl-specific reagents added extracellularly and intracellularly. In the closed state of the channel, there is a barrier to these reagents added from either side between alphaG240 and alphaT244. ACh induces the removal of this barrier, which acts as an activation gate. The residues alphaG240, alphaE241, alphaK242, and alphaT244 line a narrow part of the channel, in which this gate is located.

Kinked-helices model of the nicotinic acetylcholine receptor ion channel and its complexes with blockers: simulation by the Monte Carlo minimization method

A model of the nicotinic acetylcholine receptor ion channel was elaborated based on the data from electron microscopy, affinity labeling, cysteine scanning, mutagenesis studies, and channel blockade. A restrained Monte Carlo minimization method was used for the calculations. Five identical M2 segments (the sequence EKMTLSISVL10LALTVFLLVI20V) were arranged in five-helix bundles with various geometrical profiles of the pore. For each bundle, energy profiles for chlorpromazine, QX-222, pentamethonium, and other blocking drugs pulled through the pore were calculated. An optimal model obtained allows all of the blockers free access to the pore, but retards them at the rings of residues known to contribute to the corresponding binding sites. In this model, M2 helices are necessarily kinked. They come into contact with each other at the cytoplasmic end but diverge at the synaptic end, where N-termini of M1 segments may contribute to the pore. The kinks disengage alpha-helical H-bonds between Ala12 and Ser8. The uncoupled lone electron pairs of Ser8 carbonyl oxygens protrude into the pore, forming a hydrophilic ring that may be important for the permeation of cations. A split network of H-bonds provides a flexibility to the chains Val9-Ala12, the numerous conformations of which form only two or three intrasegment H-bonds. The cross-ectional dimensions of the interface between the flexible chains vary essentially at the level of Leu11. We suggest that conformational transitions in the chains Val9-Ala12 are responsible for the channel gating, whereas rotations of more stable alpha-helical parts of M2 segments may be necessary to transfer the channel in the desensitized state.

Caenorhabditis elegans levamisole resistance genes lev-1, unc-29, and unc-38 encode functional nicotinic acetylcholine receptor subunits

We show that three of the eleven genes of the nematode Caenorhabditis elegans that mediate resistance to the nematocide levamisole and to other cholinergic agonists encode nicotinic acetylcholine receptor (nAChR) subunits. unc-38 encodes an alpha subunit while lev-1 and unc-29 encode non-alpha subunits. The nematode nAChR subunits show conservation of many mammalian nAChR sequence features, implying an ancient evolutionary origin of nAChR proteins. Expression in Xenopus oocytes of combinations of these subunits that include the unc-38 alpha subunit results in levamisole-induced currents that are suppressed by the nAChR antagonists mecamylamine, neosurugatoxin, and d-tubocurarine but not alpha-bungarotoxin. The mutant phenotypes reveal that unc-38 and unc-29 subunits are necessary for nAChR function, whereas the lev-1 subunit is not. An UNC-29-GFP fusion shows that UNC-29 is expressed in body and head muscles. Two dominant mutations of lev-1 result in a single amino acid substitution or addition in or near transmembrane domain 2, a region important to ion channel conductance and desensitization. The identification of viable nAChR mutants in C. elegans provides an advantageous system in which receptor expression and synaptic targeting can be manipulated and studied in vivo.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Assigning functions to residues in the acetylcholine receptor channel region (review)

This review is concerned with the functional domains of the nicotinic acetylcholine receptor (AChR) involved in ion permeation. These comprise the ion pore and its gate. The latter allows the channel to be almost exclusively closed in the absence of agonist and favours ion flux in its presence. Early photoaffinity labelling experiments using open-channel blockers and site-directed mutagenesis studies identified M2 of each AChR subunit as the transmembrane domain lining the walls of the ion pore. Several biochemical, electrophysiological, and mutagenesis studies as well as molecular modelling and in vitro studies of ion channel formation with synthetic peptides corroborate these findings. Point mutations combined with electrophysiological techniques have contributed to dissecting the AChR channel region assigning functions to individual amino acid residues, thus revealing structural and functional stratification of the M2 channel domain. Specific residues have been found to be structural determinants of conductance, ion selectivity, gating, and desensitization. The three-dimensional structure of the AChR protein at 9A resolution suggests a possible arrangement of the M2 alpha-helices in the open and closed states, respectively. In spite of the current wealth of knowledge on the AChR ion channel stemming from the combination of experimental approaches discussed in this review, the mechanistic structure by which the interaction with the agonist favours the opening of the cationic channel remains unknown.

Point mutations in alpha bENaC regulate channel gating, ion selectivity, and sensitivity to amiloride

We have generated two site-directed mutants, K504E and K515E, in the alpha subunit of an amiloride-sensitive bovine epithelial Na+ channel, alpha bENaC. The region in which these mutations lie is in the large extracellular loop immediately before the second membrane-spanning domain (M2) of the protein. We have found that when membrane vesicles prepared from Xenopus oocytes expressing either K504E or K515E alpha bENaC are incorporated into planar lipid bilayers, the gating pattern, cation selectivity, and amiloride sensitivity of the resultant channel are all altered as compared to the wild-type protein. The mutated channels exhibit either a reduction or a complete lack of its characteristic burst-type behavior, significantly reduced Na+:K+ selectivity, and an approximately 10-fold decrease in the apparent inhibitory equilibrium dissociation constant (Ki) for amiloride. Single-channel conductance for Na+ was not affected by either mutation. On the other hand, both K504E and K515E alpha bENaC mutants were significantly more permeable to K+, as compared to wild type. These observations identify a lysine-rich region between amino acid residues 495 and 516 of alpha bENaC as being important to the regulation of fundamental channel properties.

Electrostatics of a simple membrane model using Green's functions formalism

The electrostatics of a simple membrane model picturing a lipid bilayer as a low dielectric constant slab immersed in a homogeneous medium of high dielectric constant (water) can be accurately computed using the exact Green's functions obtainable for this geometry. We present an extensive discussion of the analysis and numerical aspects of the problem and apply the formalism and algorithms developed to the computation of the energy profiles of a test charge (e.g., ion) across the bilayer and a molecular model of the acetylcholine receptor channel embedded in it. The Green's function approach is a very convenient tool for the computer simulation of ionic transport across membrane channels and other membrane problems where a good and computationally efficient first-order treatment of dielectric polarization effects is crucial.

A putative selectivity filter in the G-protein-coupled receptors for parathyroid hormone and secretion

The seven transmembrane segments (TMs) of many G-protein-coupled receptors (GPCRs) are thought to form a cavity into which cognate ligands insert, leading to receptor activation. Residues lining the cavity are often essential for optimal ligand binding and/or signal transduction. The present studies evaluated whether residues lining the cavity also contribute to specificity, using GPCRs for the polypeptides parathyroid hormone (PTH) and secretin as models. These ligands display no sequence homology with one another, and neither ligand cross-reacts with the other's receptor. However, mutation of a single amino acid in the second TM of the secretin receptor to the corresponding residue in the PTH receptor (N192I) resulted in a receptor that binds and signals in response to PTH. The reciprocal mutation in the PTH receptor (I234N) likewise unmasked responsiveness to secretin. Neither mutation significantly altered the response of the receptors to their own ligands. The results suggest a model of specificity wherein TM residues near the extracellular surface of the receptor function as a selectivity filter that restricts access of inappropriate ligands to an activation site in the transmembrane cavity.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Muscle-type nicotinic acetylcholine receptor delta subunit determines sensitivity to noncompetitive inhibitors, while gamma subunit regulates divalent permeability

Heterologous expression of nicotinic acetylcholine receptor (nAChR) RNAs in Xenopus oocytes was used to examine the structural basis for pharmacological and physiological differences between muscle-type and neuronal nAChRs. Neuronal nAChRs have a higher permeability to calcium than muscle-type nAChRs and display inward rectification. while muscle-type nAChRs have a linear current-voltage relation. In addition, neuronal nAChRs are more sensitive to inhibition by a class of compounds known as "ganglionic blockers". It has been shown previously that neuronal-muscle hybrid receptors show increased sensitivity to the use-dependent inhibitor of neuronal nAChRs, BTMPS, based on the presence of a neuronal beta subunit. In this study, we report that omission of gamma subunit RNA has a similar effect. alpha beta delta receptors exhibit prolonged inhibition by BTMPS; show a significant permeability to divalent ions, display inward rectification and are more sensitive to mecamylamine. However, while pharmacological effects are associated with the presence of an additional delta subunit, the physiological changes described seem to be associated with the presence or absence of a gamma subunit. These results suggest that, for nAChRs, as is also the case for non-NMDA ionotropic glutamate receptors, the crucial functional property of limiting calcium permeability can be served by a single subunit.

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh-evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5-6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.

Structure and function of glutamate and nicotinic acetylcholine receptors

The past year has seen remarkable progress in defining the structure of various ligand-gated ion channels. Images of opened and closed nicotinic acetylcholine receptors at 9 A resolution have now made it easier to identify the conformational changes underlying gating. In addition, recent studies on glutamate receptors have led to a radical revision of their postulated transmembrane topology: models for agonist-binding and allosteric domains now use sites previously thought to lie in cytoplasmic loops. Other areas that are being actively pursued include identification of the amino acids lining the ion channels, accurate measurements of Ca2+ fluxes, and tests of transmembrane topology in kainate receptor subunits.

Charge selectivity of the designed uncharged peptide ion channel Ac-(LSSLLSL)3-CONH2

Charge selectivity in ion channel proteins is not fully understood. We have studied charge selectivity in a simple model system without charged groups, in which an amphiphilic helical peptide, Ac-(Leu-Ser-Ser-Leu-Leu-Ser-Leu)3-CONH2, forms ion channels across an uncharged phospholipid membrane. We find these channels to conduct both K+ and Cl-, with a permeability ratio (based on reversal potentials) that depends on the direction of the KCl concentration gradient across the membrane. The channel shows high selectivity for K+ when [KCl] is lowered on the side of the membrane that is held at a positive potential (the putative C-terminal side), but only modest K+ selectivity when [KCl] is lowered on the opposite side (the putative N-terminal side). Neither a simple Nernst-Planck electrodiffusion model including screening of the helix dipole potential, nor a multi-ion, state transition model allowing simultaneous cation and anion occupancy of the channel can satisfactorily fit the current-voltage curves over the full range of experimental conditions. However, the C-side/N-side dilution asymmetry in reversal potentials can be simulated with either type of model.

Conductance mutations of the nicotinic acetylcholine receptor do not act by a simple electrostatic mechanism

Fixed negative charges in many cation channels raise the single-channel conductance, apparently by an electrostatic mechanism: their effects are accentuated in solutions of low ionic strength and attenuated at high ionic strength. The charges of specific amino acids near the ends of the proposed pore-lining M2 segment of the nicotinic acetylcholine receptor, termed the extracellular and cytoplasmic rings, have recently been shown to influence the single-channel K+ conductance (Imoto, K., C. Busch, B. Sakmann, M. Mishina, T. Konno, J. Nakai, H. Bujo, Y. Mori, K. Fukuda and S. Numa. 1988. Nature 335:645-648). We examined whether these charges might act by a direct electrostatic effect on the energy of ions in the pore, rather than indirectly by inducing a structural change. To this end, we measured the conductances of charge mutants over a range of K+ concentrations (ionic strengths). As expected, we found that negative charge mutations raise the conductance, and positive charge mutations lower it. The effects of cytoplasmic-ring mutations are accentuated at low ionic strength, but they are not completely attenuated at high ionic strength. The effects of extracellular-ring mutations are independent of ionic strength. These results are inconsistent with the simplest electrostatic model. We suggest a modified model that qualitatively accounts for the data.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Cholinergic responses in cloned human TE671/RD tumour cells

The cholinergic responses of the human tumour cell line TE671/RD were examined using digital Ca2+ imaging fluorescence microscopy and patch-clamp measurements. In response to stimulation of the muscarinic acetylcholine (ACh) receptor (mAChR), the intracellular concentration of Ca2+ ([Ca2+]i) rose about two-fold, in parallel with inositol 1,4,5-trisphosphate accumulation, measured by chromatographic techniques. By contrast, there was no increment of [Ca2+]i upon stimulation of the nicotinic ACh receptor (nAChR), nor after caffeine application. Electrophysiological experiments showed that TE671/RD cells lack functional voltage-activated Ca2+ channels. The stimulation of the nAChR induced transient whole-cell currents (IACh). Little or no current was detected in isotonic extracellular Ca2+, with Cs+ in the patch pipette. Cell pretreatment with muscarine reduced IACh by about 20%, without consistent modifications of current kinetics. Muscarine applied to the extra-patch membrane under the cell-attached configuration had no obvious effect on ACh-evoked unitary events. In conclusion, in human TE671/RD cells, muscarinic stimulation increases [Ca2+]i, while nicotinic stimulation does not. In addition, the nAChR exhibits peculiar ion permeability properties and is not functionally regulated by the breakdown of phosphoinositides.

Mutations at two distinct sites within the channel domain M2 alter calcium permeability of neuronal alpha 7 nicotinic receptor

The relative permeability for sodium, potassium, and calcium of chicken alpha 7 neuronal nicotinic receptor was investigated by mutagenesis of the channel domain M2. Mutations in the "intermediate ring" of negatively charged residues, located at the cytoplasmic end of M2 (site 1), reduce calcium permeability without significantly modifying other functional properties (activation and desensitization) of the receptor; a similar change of ion selectivity is also noticed when mutations at site 1 are done in the context of a receptor mutant that conducts ions in a desensitized state. Moreover, mutations of two adjacent rings of leucines at the synaptic end of M2 (site 2) have multiple effects. They abolish calcium permeability, increase the apparent affinity for acetylcholine by 10- to 100-fold, augment Hill numbers (up to 4.6-5.0) of acetylcholine dose-response relationships, slow rates of ionic response onset, and lower the extent of desensitization. Mutations at these two topographically distinct sites within M2 selectively alter calcium transport without affecting the relative permeabilities for sodium and potassium.

Ion channels: molecular basis of ion selectivity

Recent mutagenesis studies of the ion channel proteins have allowed us to identify amino acid residues critical in determining ion selectivity. Ion selectivity of a channel can be altered even by single amino acid substitutions. Functional analyses of mutants largely support views in classical biophysics that the pore size and the fixed charges are major determinants of ion selectivity. For full understanding of the molecular mechanism of ion selectivity, elucidation of the tertiary structure of channel proteins remains essential.

Divalent cation competition with [3H]saxitoxin binding to tetrodotoxin-resistant and -sensitive sodium channels. A two-site structural model of ion/toxin interaction

Monovalent and divalent cations competitively displace tetrodotoxin and saxitoxin (STX) from their binding sites on nerve and skeletal muscle Na channels. Recent studies of cloned cardiac (toxin-resistant) and brain (toxin-sensitive) Na channels suggest important structural differences in their toxin and divalent cation binding sites. We used a partially purified preparation of sheep cardiac Na channels to compare monovalent and divalent cation competition and pH dependence of binding of [3H]STX between these toxin-resistant channels and toxin-sensitive channels in membranes prepared from rat brain. The effects of several chemical modifiers of amino acid groups were also compared. Toxin competition curves for Na+ in heart and Cd2+ in brain yielded similar KD values to measurements of equilibrium binding curves. The monovalent cation sequence for effectiveness of [3H]STX competition is the same for cardiac and brain Na channels, with similar KI values for each ion and slopes of -1. The effectiveness sequence corresponds to unhydrated ion radii. For seven divalent cations tested (Ca2+, Mg2+, Mn2+, Co2+, Ni2+, Cd2+, and Zn2+) the sequence for [3H]STX competition was also similar. However, whereas all ions displaced [3H]STX from cardiac Na channels at lower concentrations, Cd2+ and Zn2+ did so at much lower concentrations. In addition, and by way of explication, the divalent ion competition curves for both brain and cardiac channels (except for Cd2+ and Zn2+ in heart and Zn2+ in brain) had slopes of less than -1, consistent with more than one interaction site. Two-site curves had statistically better fits than one-site curves. The derived values of KI for the higher affinity sites were similar between the channel types, but the lower affinity KI's were larger for heart. On the other hand, the slopes of competition curves for Cd2+ and Zn2+ were close to -1, as if the cardiac Na channel had one dominant site of interaction or more than one site with similar values for KI. pH titration of [3H]STX binding to cardiac channels showed a pKa of 5.5 and a slope of 0.6-0.9, compared with a pKa of 5.1 and slope of 1 for brain channels. Tetramethyloxonium (TMO) treatment abolished [3H]STX binding to cardiac and brain channels and STX protected channels, but the TMO effect was less dramatic for cardiac channels. Trinitrobenzene sulfonate preferentially abolished [3H]STX binding to brain channels by action at an STX protected site.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure, diversity, and ionic permeability of neuronal and muscle acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) form a family of ligand-gated, cation-selective channels that are concentrated at cholinergic synapses on vertebrate neurons and muscle cells. At the neuromuscular endplate, muscle nAChRs bind acetylcholine released by the presynaptic motor neuron. The receptors then undergo a conformational change that opens their ion channels. Cations move passively through the water-filled pores down their electrochemical gradients, completing synaptic transmission by depolarizing the postsynaptic muscle. The channel only weakly discriminates among permeant cations, which include all monovalent and divalent cations that are small enough to fit through the narrowest cross section. The membrane-spanning region of the pore is lined by uncharged domains that are bracketed by residues with net negative charge. The pore has large entrance vestibules, especially facing extracellularly. The narrowest cross-section is located near the cytoplasmic end of the membrane-spanning region, and this short narrow region probably provides the main cation binding site that is directly in the permeation pathway. Neuronal nAChRs share many of the properties of muscle nAChRs, but the neuronal receptor subtypes are more heterogenous genetically, pharmacologically, and functionally. There are especially important functional differences between muscle and neuronal nAChRs. For example, neuronal nAChRs are more highly permeable to Ca2+ and physiological levels of Ca2+ very potently modulate neuronal nicotinic currents. This variety of nAChRs suggests that these receptor/channels serve many roles in the excitable tissues of vertebrates.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

Pore size and negative charge as structural determinants of permeability in the Torpedo nicotinic acetylcholine receptor channel

To gain an insight into the molecular basis of ion permeation mechanism through the nicotinic acetylcholine receptor (AChR) channel, we have determined permeability ratios of organic cations relative to Na+ of specifically mutated Torpedo californica AChR channels expressed in Xenopus oocytes. The mutations involved mainly the side chains of the amino acid residues in the intermediate ring, where mutations have been found to exert strong effects on single-channel conductance and ion selectivity among alkali metal cations. The results obtained reveal that both the size and the net charge of the side chains of the intermediate ring are involved in determining the permeability, and provide experimental evidence that the pore size at the intermediate ring is a critical determinant of permeability. Our findings further suggest that changes in net charge exert effects on permeability by affecting the pore size of the channel.

Acetylcholine receptor channel structure probed in cysteine-substitution mutants

In order to understand the structural bases of ion conduction, ion selectivity, and gating in the nicotinic acetylcholine receptor, mutagenesis and covalent modification were combined to identify the amino acid residues that line the channel. The side chains of alternate residues--Ser248, Leu250, Ser252, and Thr254--in M2, a membrane-spanning segment of the alpha subunit, are exposed in the closed channel. Thus alpha 248-254 probably forms a beta strand, and the gate is closer to the cytoplasmic end of the channel than any of these residues. On channel opening, Leu251 is also exposed. These results lead to a revised view of the closed and open channel structures.

Mutations in the channel domain of a neuronal nicotinic receptor convert ion selectivity from cationic to anionic

Introduction by site-directed mutagenesis of three amino acids from the MII segment of glycine or gamma-aminobutyric acid (GABAA) receptors into the MII segment of alpha 7 nicotinic receptor was sufficient to convert a cation-selective channel into an anion-selective channel gated by acetylcholine. A critical mutation was the insertion of an uncharged residue at the amino-terminal end of MII, stressing the importance of protein geometrical constraints on ion selectivity.

The roles of serine and threonine sidechains in ion channels: a modelling study

The ion channel of the nicotinic acetylcholine receptor (nAChR) is believed to be lined by transmembrane M2 helices. A "4-8-12" sequence motif, comprising serine (S) or threonine (T) residues at positions 4, 8 and 12 of M2, is conserved between different members, anion and cation selective, of the nAChR superfamily. Parallel bundles of 4-8-12 motif-containing helices are considered as simplified models of ion channels. The relationship between S and T sidechain conformations and channel-ion interactions is explored via evaluation of interaction energies of K+ and of Cl- ions with channel models. Energy calculations are used to determine optimal chi 2 (C alpha-C beta-O gamma-H gamma) values in the presence of K+ or Cl- ions. 4-8-12 motif-containing bundles may form favourable interactions with either cations or anions, dependent upon the chi 2 values adopted. Parallel-helix and tilted-helix bundles are considered, as are heteromeric models designed to mimic the Torpedo nAChR. The main conclusion of the study is that conformational flexibility at chi 2 enables both S and T residues to form favourable interactions with anions or cations. Consequently, there is apparently no difference between S and T residues in their interactions with permeant ions, which suggests that the presence of T vs. S residues within the 4-8-12 motif is not a major mechanism whereby anion/cation selectivity may be generated. The implications of these studies with respect to more elaborate models of nAChR and related receptors are considered.

Elementary steps in synaptic transmission revealed by currents through single ion channels

An account is presented of how the molecular basis of synaptic transmission at peripheral and central synapses is elucidated by combining patch clamp and recombinant DNA techniques.

Immunolocalization and expression of functional and nonfunctional cell-to-cell channels from wild-type and mutant rat heart connexin43 cDNA

The carboxyl terminal cytoplasmic domain of distinct gap junction proteins may play an important role in assembly of functional channels as well as differential responsiveness to pH, voltage, and intracellular second messengers. Oligonucleotide-directed site-specific mutagenesis in a paired Xenopus laevis oocyte expression system was used to examine the expression of mRNAs encoding wild-type and carboxyl terminal mutant connexin43 (Cx43) proteins. Oocytes were stripped, injected with mRNA or distilled water (dH2O), preincubated for 16-20 hours, and then paired for 5-10 hours; this process was followed by electrophysiological recording using the dual voltage-clamp technique. Initial experiments compared the relative junctional conductances (Gjs) in oocyte pairs expressing Cx43 (382 amino acid residues) and two truncated mutants lacking most or a portion of the cytoplasmic carboxyl terminal. The shortest mutant (M241) contained 240 amino acid residues and was devoid of all phosphorylatable serine residues in the cytoplasmic tail; its length approximated the length of liver connexin26. The longest mutant (M257) tested contained 256 amino acid residues, including two serine residues. Oocyte pairs expressing M241 yielded a Gj similar to that of oocytes injected with dH2O, whereas M257 yielded a Gj similar to that of oocytes injected with Cx43. Immunoprecipitation studies showed that Cx43, M257, and M241 proteins were readily detectable in oocytes injected with their respective mRNAs, indicating that the lack of Gj observed with the M241 mRNA was not due to reduced translation. Immunocytochemical studies revealed that wild-type and both truncated mutants were localized to the area of cell-to-cell contact between the paired oocytes, indicating that protein targeting to the membrane was not inhibited in oocytes injected with M241 mRNA. Oocyte pairs expressing mutants in which serine residues were replaced with nonphosphorylatable amino acids (serine codon No. 255 AGC was converted to GCC, alanine, designated as M255S----A, and serine codon No. 244 AGC was converted to GGC, glycine, designated as M244S----G) showed Gjs similar to M257, indicating that these serine residues and, by inference, their phosphorylation state are not critical for expression of functional channels. The importance of the length of the carboxyl terminus was assessed by comparing the Gjs in a series of mutants that were intermediate in length between M257 and M241. Gradual shortening of the carboxyl terminus produced a gradual reduction of Gj relative to M257. However, simple deletion of amino acid residues 241-257 from the wild-type Cx43 did not affect Gj relative to M257.(ABSTRACT TRUNCATED AT 400 WORDS)

Threonine in the selectivity filter of the acetylcholine receptor channel

The acetylcholine receptor (AChR) is a cation selective channel whose biophysical properties as well as its molecular composition are fairly well characterized. Previous studies on the rat muscle alpha-subunit indicate that a threonine residue located near the cytoplasmic side of the M2 segment is a determinant of ion flow. We have studied the role of this threonine in ionic selectivity by measuring conductance sequences for monovalent alkali cations and bionic reversal potentials of the wild type (alpha beta gamma delta channel) and two mutant channels in which this threonine was replaced by either valine (alpha T264V) or glycine (alpha T264G). For the wild type channel we found the selectivity sequence Rb greater than Cs greater than K greater than Na. The alpha T264V mutant channel had the sequence Rb greater than K greater than Cs greater than Na. The alpha T264G mutant channel on the other hand had the same selectivity sequence as the wild type, but larger permeability ratios Px/PNa for the larger cations. Conductance concentration curves indicate that the effect of both mutations is to change both the maximum conductance as well as the apparent binding constant of the ions to the channel. A difference in Mg2+ sensitivity between wild-type and mutant channels, which is a consequence of the differences in ion binding, was also found. The present results suggest that alpha T264 form part of the selectivity filter of the AChR channel were large ions are selected according to their dehydrated size.

Unconventional pharmacology of a neuronal nicotinic receptor mutated in the channel domain

The putative channel-forming MII domains of the nicotinic, gamma-aminobutyric acid type A, and glycine receptors contain a highly conserved leucine residue. Mutation of this hydrophobic amino acid in the neuronal nicotinic receptor alpha 7 (Leu-247), reconstituted in Xenopus oocytes, modifies the ionic response to acetylcholine and alters desensitization. Furthermore, the Leu----Thr (L247T) mutant has two conducting states (46 pS and 80 pS), in contrast with the wild-type (WT) receptor, which has only one (45 pS). We now show that this mutant possesses a rather paradoxical pharmacology: antagonists of the WT receptor such as dihydro-beta-erythroidin, hexamethonium, or (+)-tubocurarine elicit ionic currents when applied to the L247T alpha 7 mutant and these responses are blocked by alpha-bungarotoxin. Furthermore, prolonged application of acetylcholine causes desensitization in the WT but leads to a potentiation of the responses to acetylcholine or dihydro-beta-erythroidin in the mutant. These data are consistent with a scheme in which mutation of Leu-247 renders a desensitized state in the WT channel a conducting state. They also strengthen the proposal that, in the WT, some competitive antagonists may stabilize desensitized states. Finally, these observations may shed light on properties of other ion channels, in particular the glutamate receptors, which display multiple conductance levels associated with various pharmacological agents.

A ring of uncharged polar amino acids as a component of channel constriction in the nicotinic acetylcholine receptor

The channel pore of the nicotinic acetylcholine receptor (AChR) has been investigated by analysing single-channel conductances of systematically mutated Torpedo receptors expressed in Xenopus oocytes. The mutations mainly alter the size and polarity of uncharged polar amino acid residues of the acetylcholine receptor subunits positioned between the cytoplasmic ring and the extracellular ring. From the results obtained, we conclude that a ring of uncharged polar residues comprising threonine 244 of the alpha-subunit (alpha T244), beta S250, gamma T253 and delta S258 (referred to as the central ring) and the anionic intermediate ring, which are adjacent to each other in the assumed alpha-helical configuration of the M2-containing transmembrane segment, together form a narrow channel constriction of short length, located close to the cytoplasmic side of the membrane. Our results also suggest that individual subunits, particularly the gamma-subunit, are asymmetrically positioned at the channel constriction.