51
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Kim S, Chamberlain AK, Bowie JU. A model of the closed form of the nicotinic acetylcholine receptor m2 channel pore. Biophys J 2005; 87:792-9. [PMID: 15298888 PMCID: PMC1304489 DOI: 10.1529/biophysj.103.039396] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
The nicotinic acetylcholine receptor is a neurotransmitter-gated ion channel in the postsynaptic membrane. It is composed of five homologous subunits, each of which contributes one transmembrane helix--the M2 helix--to create the channel pore. The M2 helix from the delta subunit is capable of forming a channel by itself. Although a model of the receptor was recently proposed based on a low-resolution, cryo-electron microscopy density map, we found that the model does not explain much of the other available experimental data. Here we propose a new model of the M2 channel derived solely from helix packing and symmetry constraints. This model agrees well with experimental results from solid-state NMR, chemical reactivity, and mutagenesis experiments. The model depicts the channel pore, the channel gate, and the residues responsible for cation specificity.
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Affiliation(s)
- Sanguk Kim
- Department of Chemistry and Biochemistry and UCLA-Department of Energy Center for Genomics and Proteomics, University of California, Los Angeles, Los Angeles, California 90095, USA
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52
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Jensen ML, Pedersen LN, Timmermann DB, Schousboe A, Ahring PK. Mutational studies using a cation-conducting GABAA receptor reveal the selectivity determinants of the Cys-loop family of ligand-gated ion channels. J Neurochem 2005; 92:962-72. [PMID: 15686498 DOI: 10.1111/j.1471-4159.2004.02970.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The present study evaluates how four key amino acid residue positions (- 4' to - 1') within the M1-M2 linker of the GABA(A) receptor beta subunit influences ion selectivity of a cation-conducting GABA receptor. Cation selectivity was found to be highly dependent on the side-chains of the amino acid residues present. The critical factor for cation selectivity was the presence of a negatively charged Glu or Asp residue in the -1' position. Receptors containing the neutral amino acids Gln or Asn or a positively charged Arg residue were anion selective. In the presence of a -1' Glu residue, the amino acids in adjacent positions were also found to be important determinants of cation selectivity. Moreover, the length of the M1-M2 linker as well as the presence of a Pro residue within this segment also affected ion selectivity, suggesting that the local environment and three-dimensional position of the -1' Glu are essential determinants of cation permeation. Conversely, no specific amino acid residues were found to be essential for anion selectivity, suggesting that the basic architecture of the selectivity segment of this class of receptor channels is optimally suited for anion conduction.
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53
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Jensen ML, Schousboe A, Ahring PK. Charge selectivity of the Cys-loop family of ligand-gated ion channels. J Neurochem 2005; 92:217-25. [PMID: 15663470 DOI: 10.1111/j.1471-4159.2004.02883.x] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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.
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54
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Erkkila BE, Weiss DS, Wotring VE. Picrotoxin-mediated antagonism of alpha3beta4 and alpha7 acetylcholine receptors. Neuroreport 2004; 15:1969-73. [PMID: 15305147 DOI: 10.1097/00001756-200408260-00027] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Picrotoxin (PTX) is a convulsant that antagonizes many inhibitory ligand-gated receptors. The mechanism of PTX block is believed to involve residues which line the pore in the second transmembrane domain (M2). The alpha(3)beta(4) and alpha(7) nicotinic acetylcholine receptors (nAChRs) have high homology to inhibitory LGICs in this M2 region and therefore could also be susceptible to block by PTX. Here, we report that PTX is an effective inhibitor at these nicotinic receptors (rat), with IC50 values of 96.1 +/- 5.5 and 194.9 +/- 19.2 microM for the alpha(3)beta(4) and alpha(7), respectively. These results provide insights into the structure-function relation of PTX-mediated antagonism in this family of ligand-activated receptors. Furthermore they should also be considered when employing PTX to selectively eliminate GABA- or glycine-mediated events.
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Affiliation(s)
- Brian E Erkkila
- Department of Neurobiology, University of Alabama at Birmingham School of Medicine, 1719 6th Ave South CIRC 410, Birmingham, AL 35294, USA
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55
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Abstract
The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.
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Affiliation(s)
- Joseph W Lynch
- School of Biomedical Sciences, Univ. of Queensland, Brisbane QLD 4072, Australia.
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56
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Premkumar A, Ewart GD, Cox GB, Gage PW. An amino-acid substitution in the influenza-B NB protein affects ion-channel gating. J Membr Biol 2004; 197:135-43. [PMID: 15042345 DOI: 10.1007/s00232-004-0648-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2003] [Indexed: 11/25/2022]
Abstract
The effects of site-directed mutations in NB, a protein encoded by the influenza B virus that has been shown to form cation-selective ion channels at pH 6.0, were studied on ion channel characteristics in artificial lipid bilayers. It was thought that the residues in the hydrophobic region of NB we selected for mutation might be involved in the transport of cations across the channel and that changes in these residues might affect channel properties such as gating and ion-selectivity. Serine residues at positions 20 and 28, threonine at position 24 and cysteine at position 26 were replaced by alanine. We found that the mutation S20A gave channels that did not gate and that remained open most of the time. Proton permeability of NB channels, as detected by fluorescence quenching, was also altered by the mutation S20A: channels were no longer proton-permeable. The other mutations, S28A, T24A and C26A, did not have any detectable effect on the activity or proton permeability of channels formed by NB. The results indicate that serine 20 may have an important role in normal function of NB channels.
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Affiliation(s)
- A Premkumar
- Division of Molecular Bioscience, John Curtin School of Medical Research, Australian National University, P.O. Box 334, Canberra City, ACT 2601, Australia
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57
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Lefebvre JL, Ono F, Puglielli C, Seidner G, Franzini-Armstrong C, Brehm P, Granato M. Increased neuromuscular activity causes axonal defects and muscular degeneration. Development 2004; 131:2605-18. [PMID: 15128655 DOI: 10.1242/dev.01123] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Before establishing terminal synapses with their final muscle targets,migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twistermutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor α-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.
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Affiliation(s)
- Julie L Lefebvre
- Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6058, USA
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58
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Filippova N, Wotring VE, Weiss DS. Evidence that the TM1-TM2 Loop Contributes to the ρ1 GABA Receptor Pore. J Biol Chem 2004; 279:20906-14. [PMID: 15007065 DOI: 10.1074/jbc.m401012200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Considerable evidence indicates the second transmembrane domain (TM2) of the gamma-aminobutyric acid (GABA) receptor lines the integral ion pore. To further delineate the structures that constitute the ion pore and selectivity filter of the rho1 GABA receptor, we used the substituted cysteine accessibility method with charged reagents to identify anion- and cation-accessible surfaces. Twenty-one consecutive residues were mutated to cysteine, one at a time, in the presumed intracellular end of the first transmembrane domain (TM1; Ala(271)-Met(276)), the entire linker connecting TM1 to TM2 (Leu(277)-Arg(287)), and the presumed intracellular end of TM2 (Ala(288)-Ala(291)). Positively (MTSEA(+)) and negatively (pCMBS(-)) charged sulfhydryl reagents, as well as Cd(2+), were added extracellularly to test accessibility of the engineered cysteines. Four of the mutants, all at the intracellular end of TM2 (R287C, V289C, P290C, A291C), were accessible to positively charged reagents, whereas seven mutants (A271C, T272C, L277C, W279C, V280C, P290C, A291C) were functionally modified by negatively charged pCMBS(-). These seven modified residues were at the intracellular end of TM2, in the TM1-TM2 linker, and at the intracellular end of TM1. In nearly all cases (excluding P290C), the rate and the degree of modification were state-dependent, with greater accessibility in the presence of agonist. Select cysteine mutants were combined with a point mutation (A291E) that converted the pore from chloride- to non-selective. In this case, positively charged reagents could modify residues in the TM1-TM2 linker (Leu(277) and Val(280)), supporting the notion that the modifying reagents were reaching their target through the pore. Taken together, our results suggest that, up to its intracellular end, the TM2 domain is not charge selective. In addition, we propose that the TM1-TM2 linker and the intracellular end of TM1 are along the pathway of the permeating ion. These findings may lend new insights into the structure of the GABA receptor pore.
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Affiliation(s)
- Natalia Filippova
- Department of Neurobiology, University of Alabama at Birmingham School of Medicine, 1719 Sixth Avenue South, CIRC410, Birmingham, AL 35294, USA
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59
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Cascio M. Structure and Function of the Glycine Receptor and Related Nicotinicoid Receptors. J Biol Chem 2004; 279:19383-6. [PMID: 15023997 DOI: 10.1074/jbc.r300035200] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Affiliation(s)
- Michael Cascio
- Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA.
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60
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Lemaitre V, Ali R, Kim CG, Watts A, Fischer WB. Interaction of amiloride and one of its derivatives with Vpu from HIV-1: a molecular dynamics simulation. FEBS Lett 2004; 563:75-81. [PMID: 15063726 DOI: 10.1016/s0014-5793(04)00251-0] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2004] [Revised: 02/26/2004] [Accepted: 03/02/2004] [Indexed: 11/24/2022]
Abstract
Vpu is an 81-residue membrane protein, with a single transmembrane segment that is encoded by HIV-1 and is involved in the enhancement of virion release via formation of an ion channel. Cyclohexamethylene amiloride (Hma) has been shown to inhibit ion channel activity. In the present 12-ns simulation study a putative binding site of Hma blockers in a pentameric model bundle built of parallel aligned helices of the first 32 residues of Vpu was found near Ser-23. Hma orientates along the channel axis with its alkyl ring pointing inside the pore, which leads to a blockage of the pore.
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Affiliation(s)
- V Lemaitre
- Biomembrane Structure Unit, Department of Biochemistry, Oxford University, South Parks Road, Oxford OX1 3QU, UK
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61
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Beg AA, Jorgensen EM. EXP-1 is an excitatory GABA-gated cation channel. Nat Neurosci 2003; 6:1145-52. [PMID: 14555952 DOI: 10.1038/nn1136] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2003] [Accepted: 09/15/2003] [Indexed: 11/09/2022]
Abstract
Gamma-aminobutyric acid (GABA) mediates fast inhibitory neurotransmission by activating anion-selective ligand-gated ion channels. Although electrophysiological studies indicate that GABA may activate cation-selective ligand-gated ion channels in some cell types, such a channel has never been characterized at the molecular level. Here we show that GABA mediates enteric muscle contraction in the nematode Caenorhabditis elegans via the EXP-1 receptor, a cation-selective ligand-gated ion channel. The EXP-1 protein resembles ionotropic GABA receptor subunits in almost all domains. In the pore-forming domain of EXP-1, however, the residues that confer anion selectivity are exchanged for those that specify cation selectivity. When expressed in Xenopus laevis oocytes, EXP-1 forms a GABA receptor that is permeable to cations and not anions. We conclude that some of the excitatory functions assigned to GABA are mediated by cation channels rather than by anion channels.
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Affiliation(s)
- Asim A Beg
- Neuroscience Program and Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, Utah 84112-0840, USA
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62
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Rodrigues-Pinguet N, Jia L, Li M, Figl A, Klaassen A, Truong A, Lester HA, Cohen BN. Five ADNFLE mutations reduce the Ca2+ dependence of the mammalian alpha4beta2 acetylcholine response. J Physiol 2003; 550:11-26. [PMID: 12754307 PMCID: PMC2343021 DOI: 10.1113/jphysiol.2003.036681] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Five nicotinic acetylcholine receptor (nAChR) mutations are currently linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). The similarity of their clinical symptoms suggests that a common functional anomaly of the mutations underlies ADNFLE seizures. To identify this anomaly, we constructed rat orthologues (S252F, +L264, S256L, V262L, V262M) of the human ADNFLE mutations, expressed them in Xenopus oocytes with the appropriate wild-type (WT) subunit (alpha4 or beta2), and studied the Ca2+ dependence of their ACh responses. All the mutations significantly reduced 2 mM Ca2+-induced increases in the 30 microM ACh response (P < 0.05). Consistent with a dominant mode of inheritance, this reduction persisted in oocytes injected with a 1:1 mixture of mutant and WT cRNA. BAPTA injections showed that the reduction was not due to a decrease in the secondary activation of Ca2+-activated Cl- currents. The S256L mutation also abolished 2 mM Ba2+ potentiation of the ACh response. The S256L, V262L and V262M mutations had complex effects on the ACh concentration-response relationship but all three mutations shifted the concentration-response relationship to the left at [ACh] >= 30 microM. Co-expression of the V262M mutation with a mutation (E180Q) that abolished Ca2+ potentiation resulted in 2 mM Ca2+ block, rather than potentiation, of the 30 microM ACh response, suggesting that the ADNFLE mutations reduce Ca2+ potentiation by enhancing Ca2+ block of the alpha4beta2 nAChR. Ca2+ modulation may prevent presynaptic alpha4beta2 nAChRs from overstimulating glutamate release at central excitatory synapses during bouts of synchronous, repetitive activity. Reducing the Ca2+ dependence of the ACh response could trigger seizures by increasing alpha4beta2-mediated glutamate release during such bouts.
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63
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Law RJ, Tieleman DP, Sansom MSP. Pores formed by the nicotinic receptor m2delta Peptide: a molecular dynamics simulation study. Biophys J 2003; 84:14-27. [PMID: 12524262 PMCID: PMC1302590 DOI: 10.1016/s0006-3495(03)74829-2] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
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.
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Affiliation(s)
- R J Law
- Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, United Kingdom
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64
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Greenfield LJ, Zaman SH, Sutherland ML, Lummis SCR, Niemeyer MI, Barnard EA, Macdonald RL. Mutation of the GABAA receptor M1 transmembrane proline increases GABA affinity and reduces barbiturate enhancement. Neuropharmacology 2002; 42:502-21. [PMID: 11955521 DOI: 10.1016/s0028-3908(01)00196-4] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
All GABA(A) receptor (GABAR) subunits include an invariant proline in a consensus motif in the first transmembrane segment (M1). In receptors containing bovine alpha1, beta1 and gamma2 subunits, we analyzed the effect of mutating this M1 proline to alanine in the alpha1 or beta1 subunit using 3 different expression systems. The beta1 subunit mutant, beta1(P228A), reduced the EC(50) for GABA about 10-fold in whole cell recordings in HEK293 cells and L929 fibroblasts. The corresponding alpha1 subunit mutant (alpha1(P233A)) also reduced the GABA EC(50) when expressed in Xenopus oocytes; alpha1(P233A)beta1gamma2S receptors failed to assemble in HEK293 cells. Binding of [(3)H]flumazenil and [(3)H]muscimol to transfected HEK293 cell membranes showed similar levels of receptor expression with GABARs containing beta1 or beta1(P228A) subunits and no change in the affinity for [(3)H]flumazenil; however, the affinity for [(3)H]muscimol was increased 6-fold in GABARs containing beta1(P228A) subunits. In L929 cells, presence of the beta1(P228A) subunit reduced enhancement by barbiturates without affecting enhancement by diazepam or alfaxalone. Single channel recordings from alpha1beta1gamma2S and alpha1beta1(P228A)gamma2L GABARs showed similar channel kinetics, but beta-mutant containing receptors opened at lower GABA concentrations. We conclude that the beta1 subunit M1 segment proline affects the linkage between GABA binding and channel gating and is critical for barbiturate enhancement. Mutation of the M1 proline in the alpha1 subunit also inhibited receptor assembly.
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Affiliation(s)
- L John Greenfield
- Departments of Neurology and Pharmacology, Medical College of Ohio, Toledo, OH, USA.
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65
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Li M, Farley RA, Lester HA. Voltage-dependent transient currents of human and rat 5-HT transporters (SERT) are blocked by HEPES and ion channel ligands. FEBS Lett 2002; 513:247-52. [PMID: 11904159 DOI: 10.1016/s0014-5793(02)02322-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The hyperpolarization-activated transient current of mammalian 5-hydroxytryptamine transporters (SERT) expressed in Xenopus oocytes was studied. Human (h) and rat (r) SERT transient currents are blocked by HEPES with changes in the waveform kinetics, and the blockade of hSERT has use-dependent properties. HEPES also changes the time course of the prepriming step, especially for hSERT. Transient currents at hSERT and rSERT are also blocked by spermine and spermidine in the mM range, and by fluoxetine, cocaine, QX-314, and QX-222 in the microM range. These pharmacological and kinetic properties of transient current blockade emphasize the similarities between the transient current and phenomena at ion channels.
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Affiliation(s)
- Ming Li
- Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, USA
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66
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The endogenous calcium-activated Cl channel in Xenopus oocytes: A physiologically and biophysically rich model system. CURRENT TOPICS IN MEMBRANES 2002. [DOI: 10.1016/s1063-5823(02)53026-x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
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67
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Picciotto MR, Caldarone BJ, Brunzell DH, Zachariou V, Stevens TR, King SL. Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications. Pharmacol Ther 2001; 92:89-108. [PMID: 11916531 DOI: 10.1016/s0163-7258(01)00161-9] [Citation(s) in RCA: 164] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) in the muscle, autonomic ganglia, and brain are targets for pharmacologically administered nicotine. Several of the subunits that combine to form neuronal nicotinic receptors have been deleted by knockout or mutated by knockin in mice using homologous recombination. We will review the biochemical, pharmacological, anatomical, physiological, and behavioral phenotypes of mice with genetically altered neuronal nAChR subunits. Clinically relevant mutations in nAChR genes will also be discussed. In addition, some of the signal transduction pathways activated through nAChRs will be described in order to delineate the longer-term changes that might result from persistent activation or inactivation of nAChRs. Genetically manipulated mice have greatly increased our understanding of the subunit composition and physiological properties of nAChRs in vivo. In addition, these mice have provided a model system to determine the molecular basis for many of the pharmacological actions of nicotine on neurotransmitter release and behavior. Genetic manipulations in mice have also elucidated the role of nAChR subunits in various disease states, and suggest several avenues for drug development.
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Affiliation(s)
- M R Picciotto
- Department of Psychiatry, Yale University School of Medicine, 34 Park Street, 3rd Floor Research, New Haven, CT 06508, USA.
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68
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69
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70
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Cordes FS, Kukol A, Forrest LR, Arkin IT, Sansom MS, Fischer WB. The structure of the HIV-1 Vpu ion channel: modelling and simulation studies. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1512:291-8. [PMID: 11406106 DOI: 10.1016/s0005-2736(01)00332-7] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Vpu is an 81 amino acid auxiliary protein in HIV-1 which exhibits channel activity. We used two homo-pentameric bundles with the helical transmembrane segments derived from FTIR spectroscopy in combination with a global molecular dynamics search protocol: (i) tryptophans (W) pointing into the pore, and (ii) W facing the lipids. Two equivalent bundles have been generated using a simulated annealing via a restrained molecular dynamics simulations (SA/MD) protocol. A fifth model was generated via SA/MD with all serines facing the pore. The latter model adopts a very stable structure during the 2 ns of simulation. The stability of the models with W facing the pore depends on the starting structure. A possible gating mechanism is outlined.
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Affiliation(s)
- F S Cordes
- Laboratory of Molecular Biophysics, Department of Biochemistry, Oxford University, UK
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71
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The lonotropic Nicotinic Acetylcholine Receptors. Cell Mol Neurobiol 2001. [DOI: 10.1016/b978-0-12-311624-6.50013-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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72
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Unwin N. The Croonian Lecture 2000. Nicotinic acetylcholine receptor and the structural basis of fast synaptic transmission. Philos Trans R Soc Lond B Biol Sci 2000; 355:1813-29. [PMID: 11205343 PMCID: PMC1692909 DOI: 10.1098/rstb.2000.0737] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Communication in the nervous system takes place at chemical and electrical synapses, where neurotransmitter-gated ion channels, such as the nicotinic acetylcholine (ACh) receptor, and gap junction channels control propagation of electrical signals from one cell to the next. Newly developed electron crystallographic methods have revealed the structures of these channels trapped in open as well as closed states, suggesting how they work. The ACh receptor has large vestibules extending from the membrane which shape the ACh-binding pockets and facilitate selective transport of cations across a narrow membrane-spanning pore. When ACh enters the pockets it triggers a concerted conformational change that opens the pore by destabilizing a gate in the middle of the membrane made by a ring of pore-lining alpha-helical segmets. The alternative 'open' configuration of pore-lining segments reshapes the lumen and creates new surfaces, allowing the ions to pass through. The gap junction channel uses a similar structural mechanism, involving coordinated rearrangements of alpha-helical segments in the plane of the membrane, to open its pore.
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Affiliation(s)
- N Unwin
- Medical Research Council Laboratory of Molecular Biology, Cambridge, UK
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73
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Chen Z, White MM. Forskolin modulates acetylcholine receptor gating by interacting with the small extracellular loop between the M2 and M3 transmembrane domains. Cell Mol Neurobiol 2000; 20:569-77. [PMID: 10930133 DOI: 10.1023/a:1007011911611] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
1. Forskolin acts as an allosteric modulator of muscle-type nicotinic acetylcholine receptors. Receptors from mouse muscle and Torpedo electroplax demonstrate differential sensitivity to inhibition by forskolin. Previous work from this laboratory suggested that the gamma subunit is responsible for this differential sensitivity. 2. We have used a series of mouse/Torpedo species-chimeric gamma subunits to further define the site of forskolin interaction with the gamma subunit. Analysis of the patterns of forskolin inhibition of receptors containing mouse/Torpedo chimeric gamma subunits along with the mouse alpha, beta, and delta subunits suggests that forskolin interacts with the small extracellular domain that links the M2 and M3 transmembrane domains (the M2-M3 linker). 3. We suggest that the M2-M3 linker domain plays an important role in the transduction of ligand binding to the conformational changes that result in channel opening.
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Affiliation(s)
- Z Chen
- Department of Pharmacology and Physiology, MCP Hahnemann University, Philadelphia, Pennsylvania 19129, USA
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74
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Abstract
Many ion channels and receptors display striking phenotypes for gain-of-function mutations but milder phenotypes for null mutations. Gain of molecular function can have several mechanistic bases: selectivity changes, gating changes including constitutive activation and slowed inactivation, elimination of a subunit that enhances inactivation, decreased drug sensitivity, changes in regulation or trafficking of the channel, or induction of apoptosis. Decreased firing frequency can occur via increased function of K+ or Cl- channels. Channel mutants also cause gain-of-function syndromes at the cellular and circuit level; of these syndromes, the cardiac long-QT syndromes are explained in a more straightforward way than are the epilepsies. G protein-coupled receptors are also affected by activating mutations.
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Affiliation(s)
- H A Lester
- Division of Biology, California Institute of Technology, Pasadena 91125, USA.
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75
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Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) protein forms a Cl(−) channel found in the plasma membranes of many epithelial cells, including those of the kidney, gut and conducting airways. Mutation of the gene encoding CFTR is the primary defect in cystic fibrosis, a disease that affects approximately 30 000 individuals in the United States alone. Alteration of CFTR function also plays an important role in the pathophysiology of secretory diarrhea and polycystic kidney disease. The basic mechanisms of permeation in this channel are not well understood. It is not known which portions of the protein contribute to forming the pore or which amino acid residues in those domains are involved in the biophysical processes of ion permeation. In this review, I will discuss (i) the present understanding of ion transport processes in the wild-type CFTR channel, (ii) the experimental approaches currently being applied to investigate the pore, and (iii) a proposed structure that takes into account the present data on mechanisms of ion selectivity in the CFTR channel and on blockade of the pore by open-channel blockers.
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Affiliation(s)
- N A McCarty
- Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322-3110, USA.
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76
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Eaton MJ, Labarca C, Eterovíc VA. M2 mutations of the nicotinic acetylcholine receptor increase the potency of the non-competitive inhibitor phencyclidine. J Neurosci Res 2000; 61:44-51. [PMID: 10861798 DOI: 10.1002/1097-4547(20000701)61:1<44::aid-jnr5>3.0.co;2-r] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Phencyclidine (PCP) is a non-competitive inhibitor of the nicotinic acetylcholine receptor (nAChR) with biphasic characteristics. At low and high micromolar concentrations, PCP inhibits nAChR from fetal mouse muscle, whereas at intermediate concentrations PCP does not inhibit the receptor. The present study was performed to determine whether the high and low concentration effects of PCP on mouse nAChR were due to interactions of this blocker with channel lining amino acids. In order to test this hypothesis, we examined the ability of PCP to inhibit acetylcholine-induced currents from wild-type nAChR and nAChR in which amino acid substitutions were made in the 6', 8' and 10' positions of the M2 transmembrane segments of the receptor. Fetal mouse nAChR from BC(3)H-1 cells were expressed in Xenopus laevis oocytes and studied using the two-electrode voltage clamp technique. The results of this study reveal that in native fetal muscle receptor, PCP potency is not affected by membrane potential between -80 mV and -30 mV. The potency of PCP is increased by mutations in M2 6', 8', and 10' positions. This increase in potency cannot be explained merely by either changes in hydrophobicity/hydrophilicity of amino acids at these positions or by side-chain size. A model proposing extra-luminal inhibitory and regulatory sites for PCP explains the lack of voltage-dependency, the biphasic effect of PCP, and the fact that all M2 mutations increased PCP potency (by disrupting the link with the regulatory sites).
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Affiliation(s)
- M J Eaton
- Department of Biochemistry and Nutrition, Center for Molecular and Behavioral Neuroscience, Universidad Central del Caribe, Bayamon, Puerto Rico.
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77
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Steinlein OK, Stoodt J, Mulley J, Berkovic S, Scheffer IE, Brodtkorb E. Independent occurrence of the CHRNA4 Ser248Phe mutation in a Norwegian family with nocturnal frontal lobe epilepsy. Epilepsia 2000; 41:529-35. [PMID: 10802757 DOI: 10.1111/j.1528-1157.2000.tb00205.x] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
PURPOSE To describe the clinical features of a family from Northern Norway in which autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is associated with a Ser248Phe amino acid exchange in the second transmembrane domain of the neuronal nicotinic acetylcholine receptor alpha4 subunit (CHRNA4). We also tested for evidence of a de novo mutation or founder effect by comparing haplotypes with the original Australian family where the Ser248Phe mutation was first described. METHODS Clinical details were obtained from 19 family members. Personal interviews and genetic analysis were carried out in 17. Parts of the coding region of CHRNA4 were sequenced, and two known polymorphisms (bp555/FokI, bp594/CfoI) were typed by restriction analysis. RESULTS Eleven individuals had ADNFLE. The haplotypes of the mutation-carrying alleles of affected individuals from the Northern Norwegian and the Australian ADNFLE family are different. The phenotypic expressions are remarkably similar. CONCLUSIONS The Ser248Phe mutation occurred independently in both families. Given the rarity of the disease, this suggests that not only the position of a mutation in the coding sequence but also the type of an amino acid exchange is important for the etiology of ADNFLE. The phenotypic similarity of these two families with different genetic backgrounds suggests that the Ser248Phe mutation largely determines the phenotype, with relatively little influence of other background genes.
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Affiliation(s)
- O K Steinlein
- Institute for Human Genetics, Friedrich-Wilhelms-University of Bonn, Bonn, Germany.
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78
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Olivier B, van Wijngaarden I, Soudijn W. 5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review. Eur Neuropsychopharmacol 2000; 10:77-95. [PMID: 10706989 DOI: 10.1016/s0924-977x(99)00065-6] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.
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Affiliation(s)
- B Olivier
- Department of Psychopharmacology, Faculty of Pharmacy, Rudolf Magnus Institute for Neurosciences, University of Utrecht, Utrecht, The Netherlands
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79
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Wilson GG, Pascual JM, Brooijmans N, Murray D, Karlin A. The intrinsic electrostatic potential and the intermediate ring of charge in the acetylcholine receptor channel. J Gen Physiol 2000; 115:93-106. [PMID: 10653890 PMCID: PMC2217203 DOI: 10.1085/jgp.115.2.93] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/1999] [Accepted: 12/09/1999] [Indexed: 11/28/2022] Open
Abstract
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.
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Affiliation(s)
- Gary G. Wilson
- From the Center for Molecular Recognition, College of Physicians and Surgeons, Columbia University, New York, New York 10032
| | - Juan M. Pascual
- From the Center for Molecular Recognition, College of Physicians and Surgeons, Columbia University, New York, New York 10032
- From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, New York 10032
| | - Natasja Brooijmans
- From the Center for Molecular Recognition, College of Physicians and Surgeons, Columbia University, New York, New York 10032
| | - Diana Murray
- From the Department of Biochemistry and Molecular Biophysics, College of Physicians and Surgeons, Columbia University, New York, New York 10032
| | - Arthur Karlin
- From the Center for Molecular Recognition, College of Physicians and Surgeons, Columbia University, New York, New York 10032
- From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, New York 10032
- From the Department of Biochemistry and Molecular Biophysics, College of Physicians and Surgeons, Columbia University, New York, New York 10032
- From the Department of Physiology and Cellular Biophysics, College of Physicians and Surgeons, Columbia University, New York, New York 10032
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80
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Ffrench-Constant RH, Anthony N, Aronstein K, Rocheleau T, Stilwell G. Cyclodiene insecticide resistance: from molecular to population genetics. ANNUAL REVIEW OF ENTOMOLOGY 2000; 45:449-466. [PMID: 10761585 DOI: 10.1146/annurev.ento.45.1.449] [Citation(s) in RCA: 121] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
This review follows progress in the analysis of cyclodiene insecticide resistance from the initial isolation of the mutant, through cloning of the resistance gene, to an examination of the distribution of resistance alleles in natural populations. Emphasis is given to the use of a resistant Drosophila mutant as an entry point to cloning the associated gamma-aminobutyric acid (GABA) receptor subunit gene, Resistance to dieldrin. Resistance is associated with replacements of a single amino acid (alanine302) in the chloride ion channel pore of the protein. Replacements of alanine302 not only directly affect the drug binding site but also allosterically destabilize the drug preferred conformation of the receptor. Resistance is thus conferred by a unique dual mechanism associated with alanine302, which is the only residue replaced in a wide range of different resistant insects. The underlying mutations appear either to have arisen once, or multiply, depending on the population biology of the pest insect. Although resistance frequencies decline in the absence of selection, resistance alleles can persist at relatively high frequency and may cause problems for compounds to which cross-resistance is observed, such as the novel fipronils.
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81
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Arias HR. Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors. Neurosci Biobehav Rev 1999; 23:817-43. [PMID: 10541058 DOI: 10.1016/s0149-7634(99)00020-2] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A great body of experimental evidence indicates that the main target for the pharmacological action of local anesthetics (LAs) is the voltage-gated Na+ channel. However, the epidural and spinal anesthesia as well as the behavioral effects of LAs cannot be explained exclusively by its inhibitory effect on the voltage-gated Na+ channel. Thus, the involvement of other ion channel receptors has been suggested. Particularly, two members of the neurotransmitter-gated ion channel receptor superfamily, the nicotinic acetylcholine receptor (AChR) and the 5-hydroxytryptamine receptor (5-HT3R type). In this regard, the aim of this review is to explain and delineate the mechanism by which LAs inhibit both ionotropic receptors from peripheral and central nervous systems. Local anesthetics inhibit the ion channel activity of both muscle- and neuronal-type AChRs in a noncompetitive fashion. Additionally, LAs inhibit the 5-HT3R by competing with the serotonergic agonist binding sites. The noncompetitive inhibitory action of LAs on the AChR is ascribed to two possible blocking mechanisms. An open-channel-blocking mechanism where the drug binds to the open channel and/or an allosteric mechanism where LAs bind to closed channels. The open-channel-blocking mechanism is in accord with the existence of high-affinity LA binding sites located in the ion channel. The allosteric mechanism seems to be physiologically more relevant than the open-channel-blocking mechanism. The inhibitory property of LAs is also elicited by binding to several low-affinity sites positioned at the lipid-AChR interface. However, there is no clearcut evidence indicating whether these sites are located at either the annular or the nonannular lipid domain. Both tertiary (protonated) and quaternary LAs gain the interior of the channel through the hydrophilic pathway formed by the extracellular ion channel's mouth with the concomitant ion flux blockade. Nevertheless, an alternative mode of action is proposed for both deprotonated tertiary and permanently-uncharged LAs: they may pass from the lipid membrane core to the lumen of the ion channel through a hydrophobic pathway. Perhaps this hydrophobic pathway is structurally related to the nonannular lipid domain. Regarding the LA binding site location on the 5-HT3R, at least two amino acids have been involved. Glutamic acid at position 106 which is located in a residue sequence homologous to loop A from the principal component of the binding site for cholinergic agonists and competitive antagonists, and Trp67 which is positioned in a stretch of amino acids homologous to loop F from the complementary component of the cholinergic ligand binding site.
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Affiliation(s)
- H R Arias
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad Nacional del Sur, Argentina.
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82
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Chang Y, Weiss DS. Allosteric activation mechanism of the alpha 1 beta 2 gamma 2 gamma-aminobutyric acid type A receptor revealed by mutation of the conserved M2 leucine. Biophys J 1999; 77:2542-51. [PMID: 10545355 PMCID: PMC1300529 DOI: 10.1016/s0006-3495(99)77089-x] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
A conserved leucine residue in the midpoint of the second transmembrane domain (M2) of the ligand-activated ion channel family has been proposed to play an important role in receptor activation. In this study, we assessed the importance of this leucine in the activation of rat alpha 1 beta 2 gamma 2 GABA receptors expressed in Xenopus laevis oocytes by site-directed mutagenesis and two-electrode voltage clamp. The hydrophobic conserved M2 leucines in alpha1(L263), beta2(L259), and gamma 2(L274) subunits were mutated to the hydrophilic amino acid residue serine and coexpressed in all possible combinations with their wild-type and/or mutant counterparts. The mutation in any one subunit decreased the EC(50) and created spontaneous openings that were blocked by picrotoxin and, surprisingly, by the competitive antagonist bicuculline. The magnitudes of the shifts in GABA EC(50) and picrotoxin IC(50) as well as the degree of spontaneous openings were all correlated with the number of subunits carrying the leucine mutation. Simultaneous mutation of the GABA binding site (beta 2Y157S; increased the EC(50)) and the conserved M2 leucine (beta 2L259S; decreased the EC(50)) produced receptors with the predicted intermediate agonist sensitivity, indicating the two mutations affect binding and gating independently. The results are discussed in light of a proposed allosteric activation mechanism.
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Affiliation(s)
- Y Chang
- Department of Neurobiology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0021, USA
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83
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Picard F, Bertrand S, Steinlein OK, Bertrand D. Mutated nicotinic receptors responsible for autosomal dominant nocturnal frontal lobe epilepsy are more sensitive to carbamazepine. Epilepsia 1999; 40:1198-209. [PMID: 10487182 DOI: 10.1111/j.1528-1157.1999.tb00848.x] [Citation(s) in RCA: 103] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
PURPOSE The recent linkage between a genetically transmissible form of epilepsy (ADNFLE) and mutations within the alpha4 subunit, one component of the major brain neuronal nicotinic acetylcholine receptor (nAChR), raises the question of the role of this receptor in epileptogenesis. Although acting by different mechanisms, the two genetic alterations so far identified both render the nAChR less efficient. In view of the high sensitivity of ADNFLE to carbamazepine (CBZ), we studied the effects of this drug and of valproate (VPA) on the human alpha4beta2 nAChR and its mutations. METHODS The alpha4beta2 nAChRs from control and mutant alpha4 subunits were reconstituted in Xenopus oocytes and investigated by using a dual-electrode voltage clamp technique. Acetylcholine (ACh)-evoked currents recorded in the absence or presence of antiepileptic drugs (AEDs) were studied to analyze the mode of action of these compounds. RESULTS ACh-evoked currents at the human alpha4beta2 nAChR were readily and reversibly inhibited by approximately 100 microM CBZ. This compound was found to be a noncompetitive inhibitor of the nAChR, which probably acts by entering the channel and causing a blockade by steric hindrance. Dose-response inhibition curves determined on the control receptor and on ADNFLE-mutant receptors showed a greater sensitivity of the mutants to CBZ, with median inhibitory concentrations (IC50s) in the range of the antiepileptic plasma levels of CBZ. In contrast, VPA had nearly no effect on control and mutant nAChRs. CONCLUSIONS CBZ inhibits the neuronal alpha4beta2 nAChRs at pharmacologic concentrations, with ADNFLE mutants displaying about threefold higher sensitivity to this compound. The increased sensitivity of these mutant receptors supports the hypothesis that the antiepileptic activity of CBZ can, at least to some extent, be attributed to the nAChR inhibition.
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Affiliation(s)
- F Picard
- Department of Physiology, Faculty of Medicine, University of Geneva, Switzerland
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84
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Webster JC, Francis MM, Porter JK, Robinson G, Stokes C, Horenstein B, Papke RL. Antagonist activities of mecamylamine and nicotine show reciprocal dependence on beta subunit sequence in the second transmembrane domain. Br J Pharmacol 1999; 127:1337-48. [PMID: 10455283 PMCID: PMC1760656 DOI: 10.1038/sj.bjp.0702686] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
We show that a portion of the TM2 domain regulates the sensitivity of beta subunit-containing rat neuronal nicotinic AChR to the ganglionic blocker mecamylamine, such that the substitution of 4 amino acids of the muscle beta subunit sequence into the neuronal beta4 sequence decreases the potency of mecamylamine by a factor of 200 and eliminates any long-term effects of this drug on receptor function. The same exchange of sequence that decreases inhibition by mecamylamine produces a comparable potentiation of long-term inhibition by nicotine. Inhibition by mecamylamine is voltage-dependent, suggesting a direct interaction of mecamylamine with sequence elements within the membrane field. We have previously shown that sensitivity to TMP (tetramethylpiperidine) inhibitors is controlled by the same sequence elements that determine mecamylamine sensitivity. However, inhibition by bis-TMP compounds is independent of voltage. Our experiments did not show any influence of voltage on the inhibition of chimeric receptors by nicotine, suggesting that the inhibitory effects of nicotine are mediated by binding to a site outside the membrane's electric field. An analysis of point mutations indicates that the residues at the 6' position within the beta subunit TM2 domain may be important for determining the effects of both mecamylamine and nicotine in a reciprocal manner. Single mutations at the 10' position are not sufficient to produce effects, but 6' 10' double mutants show more effect than do the 6' single mutants.
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Affiliation(s)
- J C Webster
- Department of Pharmacology and Therapeutics, University of Florida, Gainesville 32610-0267, USA
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85
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Corringer PJ, Bertrand S, Galzi JL, Devillers-Thiéry A, Changeux JP, Bertrand D. Mutational analysis of the charge selectivity filter of the alpha7 nicotinic acetylcholine receptor. Neuron 1999; 22:831-43. [PMID: 10230802 DOI: 10.1016/s0896-6273(00)80741-2] [Citation(s) in RCA: 135] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
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.
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Affiliation(s)
- P J Corringer
- Neurobiologie Moléculaire, Unité de Recherche Associée au Centre National de la Recherche Scientifique D1284, Institut Pasteur, Paris, France
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86
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Abstract
Ion channel defects produce a clinically diverse set of disorders that range from cystic fibrosis and some forms of migraine to renal tubular defects and episodic ataxias. This review discusses diseases related to impaired function of the skeletal muscle acetylcholine receptor and calcium channels of the motor nerve terminal. Myasthenia gravis is an autoimmune disease caused by antibodies directed toward the skeletal muscle acetylcholine receptor that compromise neuromuscular transmission. Congenital myasthenias are genetic disorders, a subset of which are caused by mutations of the acetylcholine receptor. Lambert-Eaton myasthenic syndrome is an immune disorder characterized by impaired synaptic vesicle release likely related to a defect of calcium influx. The disorders will illustrate new insights into synaptic transmission and ion channel structure that are relevant for all ion channel disorders.
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Affiliation(s)
- K Boonyapisit
- Department of Neurology, Case Western Reserve University School of Medicine, Department of Veterans Affairs Medical Center in Cleveland, University Hospitals of Cleveland, Ohio 44106, USA
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87
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Forman SA. Direct interactions of anesthetics and nonanesthetics with the nicotinic acetylcholine receptor pore. Toxicol Lett 1998; 100-101:169-78. [PMID: 10049138 DOI: 10.1016/s0378-4274(98)00182-9] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
(1) We review evidence that anesthetics inhibit peripheral nAChR cation translocation by binding directly to a protein site in the transmembrane pore. (2) This site is near the middle of the pore-forming M2 domains on alpha and beta subunits, but further from the homologous portions of gamma and delta subunits. (3) Interactions between both anesthetics and nonanesthetics with the nAChR pore site are determined primarily by hydrophobic forces rather than steric factors. (4) Anesthetics and nonanesthetics display different state-dependent accessibility to this site, suggesting a mechanism for the different in vivo actions of these two classes of drugs.
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Affiliation(s)
- S A Forman
- Department of Anesthesia and Critical Care, CLN-3, Massachusetts General Hospital, Boston 02114, USA.
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88
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Pascual JM, Karlin A. Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel. J Gen Physiol 1998; 112:611-21. [PMID: 9806969 PMCID: PMC2229442 DOI: 10.1085/jgp.112.5.611] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/1998] [Accepted: 09/03/1998] [Indexed: 11/21/2022] Open
Abstract
The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.
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Affiliation(s)
- J M Pascual
- Center for Molecular Recognition, Columbia University, New York 10032, USA
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89
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Affiliation(s)
- J P Changeux
- Neurobiologie Moléculaire Institut Pasteur, Paris, France.
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90
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ffrench-Constant RH, Pittendrigh B, Vaughan A, Anthony N. Why are there so few resistance-associated mutations in insecticide target genes? Philos Trans R Soc Lond B Biol Sci 1998; 353:1685-93. [PMID: 10021768 PMCID: PMC1692388 DOI: 10.1098/rstb.1998.0319] [Citation(s) in RCA: 100] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The genes encoding the three major targets of conventional insecticides are: Rdl, which encodes a gamma-aminobutyric acid receptor subunit (RDL); para, which encodes a voltage-gated sodium channel (PARA); and Ace, which encodes insect acetylcholinesterase (AChE). Interestingly, despite the complexity of the encoded receptors or enzymes, very few amino acid residues are replaced in different resistant insects: one within RDL, two within PARA and three or more within AChE. Here we examine the possible reasons underlying this extreme conservation by looking at the aspects of receptor and/or enzyme function that may constrain replacements to such a limited number of residues.
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91
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Abstract
Although the neuronal nicotinic acetylcholine receptor is found in most parts of the brain, not much is known about its functional significance. At least ten different subunits are expressed in the central nervous system, theoretically able to give rise to more than a thousand different receptor subtypes. Despite, or perhaps because of, this astonishing diversity, the biological role of this receptor type remains to be investigated. It has recently been found that a mutated alpha4-subunit is associated with an inherited epilepsy syndrome. A missense mutation replacing a serine in position 248 of the second transmembrane domain by phenylalanine leads to hypoactivity of the receptor due to accelerated desensitization and delayed resensitization. Thus, for the first time a link between a human disease and a mutated neuronal nicotinic acetylcholine receptor has been found, pointing to a possible involvement of this ligand-gated receptor family in the modulation of brain excitability levels.
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Affiliation(s)
- O Steinlein
- Institute of Human Genetics, University of Bonn, Germany
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92
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Affiliation(s)
- A Karlin
- Department of Biochemistry, Columbia University College of Physicians, New York, New York 10032-3702, USA
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93
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Zhang JT. Use of cell-free systems to determine P-glycoprotein transmembrane topology. Methods Enzymol 1998; 292:279-89. [PMID: 9711561 DOI: 10.1016/s0076-6879(98)92022-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
MESH Headings
- ATP Binding Cassette Transporter, Subfamily B, Member 1/biosynthesis
- ATP Binding Cassette Transporter, Subfamily B, Member 1/chemistry
- ATP Binding Cassette Transporter, Subfamily B, Member 1/isolation & purification
- Amino Acid Sequence
- Animals
- Cell Membrane/metabolism
- Cell Membrane/ultrastructure
- Cell-Free System
- Cloning, Molecular/methods
- DNA, Complementary
- DNA-Directed RNA Polymerases/metabolism
- Dogs
- Electrophoresis, Polyacrylamide Gel/methods
- Glycosylation
- Indicators and Reagents
- Microsomes/metabolism
- Models, Molecular
- Molecular Sequence Data
- Pancreas/metabolism
- Protein Biosynthesis
- Protein Conformation
- Recombinant Proteins/biosynthesis
- Recombinant Proteins/chemistry
- Recombinant Proteins/isolation & purification
- Transcription, Genetic
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Affiliation(s)
- J T Zhang
- Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555, USA
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94
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Arias HR. Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1376:173-220. [PMID: 9748559 DOI: 10.1016/s0304-4157(98)00004-5] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
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
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Affiliation(s)
- H R Arias
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, Consejo Nacional de Investigaciones Científicas y Técnicas, and Universidad Nacional del Sur, Blanca, Argentina.
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95
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Wick MJ, Mihic SJ, Ueno S, Mascia MP, Trudell JR, Brozowski SJ, Ye Q, Harrison NL, Harris RA. Mutations of gamma-aminobutyric acid and glycine receptors change alcohol cutoff: evidence for an alcohol receptor? Proc Natl Acad Sci U S A 1998; 95:6504-9. [PMID: 9600996 PMCID: PMC27833 DOI: 10.1073/pnas.95.11.6504] [Citation(s) in RCA: 179] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/1998] [Accepted: 03/24/1998] [Indexed: 02/07/2023] Open
Abstract
Alcohols in the homologous series of n-alcohols increase in central nervous system depressant potency with increasing chain length until a "cutoff" is reached, after which further increases in molecular size no longer increase alcohol potency. A similar phenomenon has been observed in the regulation of ligand-gated ion channels by alcohols. Different ligand-gated ion channels exhibit radically different cutoff points, suggesting the existence of discrete alcohol binding pockets of variable size on these membrane proteins. The identification of amino acid residues that determine the alcohol cutoff may, therefore, provide information about the location of alcohol binding sites. Alcohol regulation of the glycine receptor is critically dependent on specific amino acid residues in transmembrane domains 2 and 3 of the alpha subunit. We now demonstrate that these residues in the glycine alpha1 and the gamma-aminobutyric acid rho1 receptors also control alcohol cutoff. By mutation of Ser-267 to Gln, it was possible to decrease the cutoff in the glycine alpha1 receptor, whereas mutation of Ile-307 and/or Trp-328 in the gamma-aminobutyric acid rho1 receptor to smaller residues increased the cutoff. These results support the existence of alcohol binding pockets in these membrane proteins and suggest that the amino acid residues present at these positions can control the size of the alcohol binding cavity.
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MESH Headings
- Alcohols/metabolism
- Animals
- DNA, Complementary/analysis
- DNA, Complementary/genetics
- Humans
- Models, Molecular
- Mutation
- Receptors, Cell Surface/chemistry
- Receptors, Cell Surface/metabolism
- Receptors, GABA/chemistry
- Receptors, GABA/genetics
- Receptors, GABA/metabolism
- Receptors, Glycine/chemistry
- Receptors, Glycine/genetics
- Receptors, Glycine/metabolism
- Xenopus
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Affiliation(s)
- M J Wick
- Department of Pharmacology, University of Colorado Health Sciences Center, Denver, CO 80262, USA
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96
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Francis MM, Choi KI, Horenstein BA, Papke RL. Sensitivity to voltage-independent inhibition determined by pore-lining region of the acetylcholine receptor. Biophys J 1998; 74:2306-17. [PMID: 9591658 PMCID: PMC1299574 DOI: 10.1016/s0006-3495(98)77940-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Some noncompetitive inhibitors (e.g., ganglionic blockers) exhibit selectivity for the inhibition of neuronal nicotinic acetylcholine receptors (nAChRs). This study characterizes the mechanism of selective long-term inhibition of neuronal and muscle-neuronal chimeric nAChRs by bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (bis-TMP-10 or BTMPS), a bifunctional form of the potent ganglionic blocker tetramethylpiperidine. Long-term inhibition of neuronal nAChRs by bis-TMP-10 has been previously demonstrated to arise, at least in part, from the binding of the bis compound to neuronal beta-subunits. In this study, long-term inhibition is demonstrated to be dependent upon the presence of sequence element(s) within the pore-lining second transmembrane domain (tm2) of neuronal beta-subunits; however, the inhibitor binding site itself does not appear to be contained within the segment of the channel pore influenced by the membrane electric field. Specifically, our results imply that bis-TMP-10 interacts with an activation-sensitive element, the availability of which may be regulated by a sequence in the tm2 domain. Furthermore, we demonstrate a compound length requirement for long-term inhibition that would be consistent with binding to multiple sites located on the extracellular portion of the receptor.
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Affiliation(s)
- M M Francis
- Department of Neuroscience, University of Florida, Gainesville 32610, USA
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97
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Changeux JP, Bertrand D, Corringer PJ, Dehaene S, Edelstein S, Léna C, Le Novère N, Marubio L, Picciotto M, Zoli M. Brain nicotinic receptors: structure and regulation, role in learning and reinforcement. BRAIN RESEARCH. BRAIN RESEARCH REVIEWS 1998; 26:198-216. [PMID: 9651527 DOI: 10.1016/s0165-0173(97)00040-4] [Citation(s) in RCA: 219] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).
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Affiliation(s)
- J P Changeux
- Neurobiologie Moléculaire, CNRS URA 1284, Institut Pasteur, 25-28 rue du Dr. Roux, 75724, Paris Cedex 15, France
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98
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Léna C, Changeux JP. Allosteric nicotinic receptors, human pathologies. JOURNAL OF PHYSIOLOGY, PARIS 1998; 92:63-74. [PMID: 9782446 DOI: 10.1016/s0928-4257(98)80140-x] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Nicotinic acetylcholine receptors are ligand-gated ion channels present in muscle and brain. These allosteric oligomers may exist in several conformational states which include a resting state, an open-channel state, and a desensitized refractory state. Recent work has shown that point mutations in the nicotinic receptor may, altogether, abolish desensitization, increase apparent affinity for agonists and convert the effect of a competitive antagonist into an agonist response. These pleiotropic effects are interpreted in terms of the allosteric model. This paper reviews recent evidence that such mutations occur spontaneously in humans and may cause diseases such as congenital myasthenia or familial frontal lobe epilepsy. In addition, nicotinic receptors are involved in tobacco smoking. Accumulating evidence, including experiments with knock-out animals, indicates that addiction to nicotine is linked to the activation of beta 2-subunit containing nicotinic receptors in the dopaminergic mesolimbic neurons which are part of the reward systems in the brain. Current research also indicates that nicotinic agonists might serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia. This paper extends and updates a recently published review.
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Affiliation(s)
- C Léna
- CNRS UA 1284, Institut Pasteur, Paris, France
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99
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Chang Y, Weiss DS. Substitutions of the highly conserved M2 leucine create spontaneously opening rho1 gamma-aminobutyric acid receptors. Mol Pharmacol 1998; 53:511-23. [PMID: 9495819 DOI: 10.1124/mol.53.3.511] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
All members of the receptor-operated ion channel family that includes gamma-aminobutyric acid (GABA), glycine, nicotinic acetylcholine, and serotonin type 3 receptors have a conserved leucine near the center of the presumed second membrane-spanning domain. This leucine has been postulated to play a role in the gating of the pore. In this study, we examined the effects of mutating this leucine (L301) on the function of human homomeric rho1 GABA receptors. Oocytes expressing rho1 GABA receptors in which this leucine was substituted with alanine (A), glycine (G), serine (S), threonine (T), valine, or tyrosine, but not isoleucine or phenylalanine, demonstrated larger-than-normal resting conductances in the absence of GABA. This resting conductance had a reversal potential (and shifted reversal potential with chloride substitution) indistinguishable from that of the wild-type rho1 GABA-activated current. This resting conductance was antagonized by picrotoxin and, in the case of the A, G, S, and T substitutions, by GABA itself. Although the rho1 competitive antagonist 3-aminopropyl(methyl)-phosphinic acid did not block the resting conductance, this compound did competitively inhibit the GABA-mediated antagonism of the resting conductance. At higher concentrations, both 3-aminopropyl(methyl)-phosphinic acid and GABA directly activated the A, G, S, and T mutant receptors. Taken together, these data suggest that substitution of this highly conserved leucine with either small or polar residues produced rho1 GABA receptors that can open in the absence of GABA and support the hypothesis that this leucine may play a key role in the gating of the pore.
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Affiliation(s)
- Y Chang
- Department of Neurobiology, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA
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100
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Vafa B, Schofield PR. Heritable mutations in the glycine, GABAA, and nicotinic acetylcholine receptors provide new insights into the ligand-gated ion channel receptor superfamily. INTERNATIONAL REVIEW OF NEUROBIOLOGY 1998; 42:285-332. [PMID: 9476176 DOI: 10.1016/s0074-7742(08)60613-3] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Affiliation(s)
- B Vafa
- Garvan Institute of Medical Research, Darlinghurst, Sydney, Australia
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