Identification of amino acids contributing to high and low affinity d-tubocurarine sites in the Torpedo nicotinic acetylcholine receptor

Allosteric inhibition of muscle-type nicotinic acetylcholine receptors by a neuromuscular blocking agent pancuronium

Muscle relaxants are indispensable for surgical anesthesia. Early studies suggested that a classical non-depolarizing muscle relaxant pancuronium competitively binds to the ligand binding site to block nicotinic acetylcholine receptors (nAChR). Our group recently showed that nAChR which has two distinct subunit combinations are expressed in zebrafish muscles, αβδε and αβδ, for which potencies of pancuronium are different. Taking advantage of the distinct potencies, we generated chimeras between two types of nAChRs and found that the extracellular ACh binding site is not associated with the pancuronium sensitivity. Furthermore, application of either 2 μM or 100 μM ACh in native αβδε or αβδ subunits yielded similar IC50 of pancuronium. These data suggest that pancuronium allosterically inhibits the activity of zebrafish nAChRs.

Structural mechanism of muscle nicotinic receptor desensitization and block by curare

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

Photoaffinity Labeling of Pentameric Ligand-Gated Ion Channels: A Proteomic Approach to Identify Allosteric Modulator Binding Sites

Photoaffinity labeling techniques have been used for decades to identify drug binding sites and to study the structural biology of allosteric transitions in transmembrane proteins including pentameric ligand-gated ion channels (pLGIC). In a typical photoaffinity labeling experiment, to identify drug binding sites, UV light is used to introduce a covalent bond between a photoreactive ligand (which upon irradiation at the appropriate wavelength converts to a reactive intermediate) and amino acid residues that lie within its binding site. Then protein chemistry and peptide microsequencing techniques are used to identify these amino acids within the protein primary sequence. These amino acid residues are located within homology models of the receptor to identify the binding site of the photoreactive probe. Molecular modeling techniques are then used to model the binding of the photoreactive probe within the binding site using docking protocols. Photoaffinity labeling directly identifies amino acids that contribute to drug binding sites regardless of their location within the protein structure and distinguishes them from amino acids that are only involved in the transduction of the conformational changes mediated by the drug, but may not be part of its binding site (such as those identified by mutational studies). Major limitations of photoaffinity labeling include the availability of photoreactive ligands that faithfully mimic the properties of the parent molecule and protein preparations that supply large enough quantities suitable for photoaffinity labeling experiments. When the ligand of interest is not intrinsically photoreactive, chemical modifications to add a photoreactive group to the parent drug, and pharmacological evaluation of these chemical modifications become necessary. With few exceptions, expression and affinity-purification of proteins are required prior to photolabeling. Methods to isolate milligram quantities of highly enriched pLGIC suitable for photoaffinity labeling experiments have been developed. In this chapter, we discuss practical aspects of experimental strategies to identify allosteric modulator binding sites in pLGIC using photoaffinity labeling.

Photolabeling a Nicotinic Acetylcholine Receptor (nAChR) with an (α4)3(β2)2 nAChR-Selective Positive Allosteric Modulator

Positive allosteric modulators (PAMs) of nicotinic acetylcholine (ACh) receptors (nAChRs) have potential clinical applications in the treatment of nicotine dependence and many neuropsychiatric conditions associated with decreased brain cholinergic activity, and 3-(2-chlorophenyl)-5-(5-methyl-1-(piperidin-4-yl)-1H-pyrrazol-4-yl)isoxazole (CMPI) has been identified as a PAM selective for neuronal nAChRs containing theα4 subunit. In this report, we compare CMPI interactions with low-sensitivity (α4)3(β2)2 and high-sensitivity (α4)2(β2)3 nAChRs, and with muscle-type nAChRs. In addition, we use the intrinsic reactivity of [(3)H]CMPI upon photolysis at 312 nm to identify its binding sites inTorpedonAChRs. Recording fromXenopusoocytes, we found that CMPI potentiated maximally the responses of (α4)3(β2)2nAChR to 10μM ACh (EC10) by 400% and with anEC50of ∼1µM. CMPI produced a left shift of the ACh concentration-response curve without altering ACh efficacy. In contrast, CMPI inhibited (∼35% at 10µM) ACh responses of (α4)2(β2)3nAChRs and fully inhibited human muscle andTorpedonAChRs with IC50values of ∼0.5µM. Upon irradiation at 312 nm, [(3)H]CMPI photoincorporated into eachTorpedo[(α1)2β1γδ] nAChR subunit. Sequencing of peptide fragments isolated from [(3)H]CMPI-photolabeled nAChR subunits established photolabeling of amino acids contributing to the ACh binding sites (αTyr(190),αTyr(198),γTrp(55),γTyr(111),γTyr(117),δTrp(57)) that was fully inhibitable by agonist and lower-efficiency, state-dependent [(3)H]CMPI photolabeling within the ion channel. Our results establish that CMPI is a potent potentiator of nAChRs containing anα4:α4 subunit interface, and that its intrinsic photoreactivy makes it of potential use to identify its binding sites in the (α4)3(β2)2nAChR.

Human Secreted Ly-6/uPAR Related Protein-1 (SLURP-1) Is a Selective Allosteric Antagonist of α7 Nicotinic Acetylcholine Receptor

SLURP-1 is a secreted toxin-like Ly-6/uPAR protein found in epithelium, sensory neurons and immune cells. Point mutations in the slurp-1 gene cause the autosomal inflammation skin disease Mal de Meleda. SLURP-1 is considered an autocrine/paracrine hormone that regulates growth and differentiation of keratinocytes and controls inflammation and malignant cell transformation. The majority of previous studies of SLURP-1 have been made using fusion constructs containing, in addition to the native protein, extra polypeptide sequences. Here we describe the activity and pharmacological profile of a recombinant analogue of human SLURP-1 (rSLURP-1) differing from the native protein only by one additional N-terminal Met residue. rSLURP-1 significantly inhibited proliferation (up to ~ 40%, EC₅₀ ~ 4 nM) of human oral keratinocytes (Het-1A cells). Application of mecamylamine and atropine,—non-selective inhibitors of nicotinic acetylcholine receptors (nAChRs) and muscarinic acetylcholine receptors, respectively, and anti-α7-nAChRs antibodies revealed α7 type nAChRs as an rSLURP-1 target in keratinocytes. Using affinity purification from human cortical extracts, we confirmed that rSLURP-1 binds selectively to the α7-nAChRs. Exposure of Xenopus oocytes expressing α7-nAChRs to rSLURP-1 caused a significant non-competitive inhibition of the response to acetylcholine (up to ~ 70%, IC₅₀ ~ 1 μM). It was shown that rSLURP-1 binds to α7-nAChRs overexpressed in GH₄C_l cells, but does not compete with ¹²⁵I-α-bungarotoxin for binding to the receptor. These findings imply an allosteric antagonist-like mode of SLURP-1 interaction with α7-nAChRs outside the classical ligand-binding site. Contrary to rSLURP-1, other inhibitors of α7-nAChRs (mecamylamine, α-bungarotoxin and Lynx1) did not suppress the proliferation of keratinocytes. Moreover, the co-application of α-bungarotoxin with rSLURP-1 did not influence antiproliferative activity of the latter. This supports the hypothesis that the antiproliferative activity of SLURP-1 is related to ‘metabotropic’ signaling pathway through α7-nAChR, that activates intracellular signaling cascades without opening the receptor channel.

Agonist activation of a nicotinic acetylcholine receptor

How does an agonist activate a receptor? In this article I consider the activation process in muscle nicotinic acetylcholine receptors (AChRs), a prototype for understanding the energetics of binding and gating in other ligand-gated ion channels. Just as movements that generate gating currents activate voltage-gated ion channels, movements at binding sites that generate an increase in affinity for the agonist activate ligand-gated ion channels. The main topics are: i) the schemes and intermediate states of AChR activation, ii) the energy changes of each of the steps, iii) the sources of the energies, iv) the three kinds of AChR agonist binding site and v) the correlations between binding and gating energies. The binding process is summarized as sketches of different conformations of an agonist site. The results suggest that agonists lower the free energy of the active conformation of the protein in stages by establishing favorable, local interactions at each binding site, independently. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Functional differences between neurotransmitter binding sites of muscle acetylcholine receptors

A muscle acetylcholine receptor (AChR) has two neurotransmitter binding sites located in the extracellular domain, at αδ and either αε (adult) or αγ (fetal) subunit interfaces. We used single-channel electrophysiology to measure the effects of mutations of five conserved aromatic residues at each site with regard to their contribution to the difference in free energy of agonist binding to active versus resting receptors (ΔGB1). The two binding sites behave independently in both adult and fetal AChRs. For four different agonists, including ACh and choline, ΔGB1 is ∼-2 kcal/mol more favorable at αγ compared with at αε and αδ. Only three of the aromatics contribute significantly to ΔGB1 at the adult sites (αY190, αY198, and αW149), but all five do so at αγ (as well as αY93 and γW55). γW55 makes a particularly large contribution only at αγ that is coupled energetically to those contributions of some of the α-subunit aromatics. The hydroxyl and benzene groups of loop C residues αY190 and αY198 behave similarly with regard to ΔGB1 at all three kinds of site. ACh binding energies estimated from molecular dynamics simulations are consistent with experimental values from electrophysiology and suggest that the αγ site is more compact, better organized, and less dynamic than αε and αδ. We speculate that the different sensitivities of the fetal αγ site versus the adult αε and αδ sites to choline and ACh are important for the proper maturation and function of the neuromuscular synapse.

Rescue of amyloid-Beta-induced inhibition of nicotinic acetylcholine receptors by a peptide homologous to the nicotine binding domain of the alpha 7 subtype

Alzheimer's disease (AD) is characterized by brain accumulation of the neurotoxic amyloid-β peptide (Aβ) and by loss of cholinergic neurons and nicotinic acetylcholine receptors (nAChRs). Recent evidence indicates that memory loss and cognitive decline in AD correlate better with the amount of soluble Aβ than with the extent of amyloid plaque deposits in affected brains. Inhibition of nAChRs by soluble Aβ40 is suggested to contribute to early cholinergic dysfunction in AD. Using phage display screening, we have previously identified a heptapeptide, termed IQ, homologous to most nAChR subtypes, binding with nanomolar affinity to soluble Aβ40 and blocking Aβ-induced inhibition of carbamylcholine-induced currents in PC12 cells expressing α7 nAChRs. Using alanine scanning mutagenesis and whole-cell current recording, we have now defined the amino acids in IQ essential for reversal of Aβ40 inhibition of carbamylcholine-induced responses in PC12 cells, mediated by α7 subtypes and other endogenously expressed nAChRs. We further investigated the effects of soluble Aβ, IQ and analogues of IQ on α3β4 nAChRs recombinantly expressed in HEK293 cells. Results show that nanomolar concentrations of soluble Aβ40 potently inhibit the function of α3β4 nAChRs, and that subsequent addition of IQ or its analogues does not reverse this effect. However, co-application of IQ makes the inhibition of α3β4 nAChRs by Aβ40 reversible. These findings indicate that Aβ40 inhibits different subtypes of nAChRs by interacting with specific receptor domains homologous to the IQ peptide, suggesting that IQ may be a lead for novel drugs to block the inhibition of cholinergic function in AD.

Physostigmine and galanthamine bind in the presence of agonist at the canonical and noncanonical subunit interfaces of a nicotinic acetylcholine receptor

Galanthamine and physostigmine are clinically used cholinomimetics that both inhibit acetylcholinesterase and also interact directly with and potentiate nAChRs. As with most nAChR-positive allosteric modulators, the location and number of their binding site(s) within nAChRs are unknown. In this study, we use the intrinsic photoreactivities of [(3)H]physostigmine and [(3)H]galanthamine upon irradiation at 312 nm to directly identify amino acids contributing to their binding sites in the Torpedo californica nAChR. Protein sequencing of fragments isolated from proteolytic digests of [(3)H]physostigmine- or [(3)H]galanthamine-photolabeled nAChR establish that, in the presence of agonist (carbamylcholine), both drugs photolabeled amino acids on the complementary (non-α) surface of the transmitter binding sites (γTyr-111/γTyr-117/δTyr172). They also photolabeled δTyr-212 at the δ-β subunit interface and γTyr-105 in the vestibule of the ion channel, with photolabeling of both residues enhanced in the presence of agonist. Furthermore, [(3)H]physostigmine photolabeling of γTyr-111, γTyr-117, δTyr-212, and γTyr-105 was inhibited in the presence of nonradioactive galanthamine. The locations of the photolabeled amino acids in the nAChR structure and the results of computational docking studies provide evidence that, in the presence of agonist, physostigmine and galanthamine bind to at least three distinct sites in the nAChR extracellular domain: at the α-γ interface (1) in the entry to the transmitter binding site and (2) in the vestibule of the ion channel near the level of the transmitter binding site, and at the δ-β interface (3) in a location equivalent to the benzodiazepine binding site in GABA(A) receptors.

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

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

The Cys-loop pentameric ligand-gated ion channel receptors: 50 years on

This year, 2011, the Department of Pharmacology at the University of Alberta celebrated its 50th anniversary. This timeframe covers nearly the entire history of Cys-loop pentameric ligand-gated ion channel (pLGIC) research. In this review we consider how major technological advancements affected our current understanding of pLGICs, and highlight the contributions made by members of our department. The individual at the center of our story is Susan Dunn; her passing earlier this year has robbed the Department of Pharmacology and the research community of a most insightful colleague. Her dissection of ligand interactions with the nAChR, together with their interpretation, was the hallmark of her extensive collaborations with Michael Raftery. Here, we highlight some electrophysiological studies from her laboratory over the last few years, using the technique that she introduced to the department in Edmonton, the 2-electrode voltage-clamp of Xenopus oocytes. Finally, we discuss some single-channel studies of the anionic GlyR and GABA(A)R that prefaced the introduction of this technique to her laboratory.

An ER-resident membrane protein complex regulates nicotinic acetylcholine receptor subunit composition at the synapse

Nicotinic acetylcholine receptors (nAChRs) are homo- or heteropentameric ligand-gated ion channels mediating excitatory neurotransmission and muscle activation. Regulation of nAChR subunit assembly and transfer of correctly assembled pentamers to the cell surface is only partially understood. Here, we characterize an ER transmembrane (TM) protein complex that influences nAChR cell-surface expression and functional properties in Caenorhabditis elegans muscle. Loss of either type I TM protein, NRA-2 or NRA-4 (nicotinic receptor associated), affects two different types of muscle nAChRs and causes in vivo resistance to cholinergic agonists. Sensitivity to subtype-specific agonists of these nAChRs is altered differently, as demonstrated by whole-cell voltage-clamp of dissected adult muscle, when applying exogenous agonists or after photo-evoked, channelrhodopsin-2 (ChR2) mediated acetylcholine (ACh) release, as well as in single-channel recordings in cultured embryonic muscle. These data suggest that nAChRs desensitize faster in nra-2 mutants. Cell-surface expression of different subunits of the 'levamisole-sensitive' nAChR (L-AChR) is differentially affected in the absence of NRA-2 or NRA-4, suggesting that they control nAChR subunit composition or allow only certain receptor assemblies to leave the ER.

[(3)H]Epibatidine photolabels non-equivalent amino acids in the agonist binding site of Torpedo and alpha4beta2 nicotinic acetylcholine receptors

Nicotinic acetylcholine receptor (nAChR) agonists, such as epibatidine and its molecular derivatives, are potential therapeutic agents for a variety of neurological disorders. In order to identify determinants for subtype-selective agonist binding, it is important to determine whether an agonist binds in a common orientation in different nAChR subtypes. To compare the mode of binding of epibatidine in a muscle and a neuronal nAChR, we photolabeled Torpedo alpha(2)betagammadelta and expressed human alpha4beta2 nAChRs with [(3)H]epibatidine and identified by Edman degradation the photolabeled amino acids. Irradiation at 254 nm resulted in photolabeling of alphaTyr(198) in agonist binding site Segment C of the principal (+) face in both alpha subunits and of gammaLeu(109) and gammaTyr(117) in Segment E of the complementary (-) face, with no labeling detected in the delta subunit. For affinity-purified alpha4beta2 nAChRs, [(3)H]epibatidine photolabeled alpha4Tyr(195) (equivalent to Torpedo alphaTyr(190)) in Segment C as well as beta2Val(111) and beta2Ser(113) in Segment E (equivalent to Torpedo gammaLeu(109) and gammaTyr(111), respectively). Consideration of the location of the photolabeled amino acids in homology models of the nAChRs based upon the acetylcholine-binding protein structure and the results of ligand docking simulations suggests that epibatidine binds in a single preferred orientation within the alpha-gamma transmitter binding site, whereas it binds in two distinct orientations in the alpha4beta2 nAChR.

Differential regulation of receptor activation and agonist selectivity by highly conserved tryptophans in the nicotinic acetylcholine receptor binding site

We have shown previously that a highly conserved Tyr in the nicotinic acetylcholine receptor (nAChR) ligand-binding domain (LBD) (alpha7 Tyr188 or alpha4 Tyr195) differentially regulates the activity of acetylcholine (ACh) and the alpha7-selective agonist 3-(4-hydroxy,2-methoxybenzylidene)anabaseine (4OH-GTS-21) in alpha4beta2 and alpha7 nAChR. In this study, we mutated two highly conserved LBD Trp residues in human alpha7 and alpha4beta2 and expressed the receptors in Xenopus laevis oocytes. Alpha7 receptors with Trp55 mutated to Gly or Tyr became less responsive to 4OH-GTS-21, whereas mutation of the homologous Trp57 in beta2 to Gly, Tyr, Phe, or Ala resulted in alpha4beta2 receptors that showed increased responses to 4OH-GTS-21. Mutation of alpha7 Trp55 to Val resulted in receptors for which the partial agonist 4OH-GTS-21 became equally efficacious as ACh, whereas alpha4beta2 receptors with the homologous mutation remained nonresponsive to 4OH-GTS-21. In contrast to the striking alterations in agonist activity profiles that were observed with mutations of alpha7 Trp55 and beta2 Trp57, mutations of alpha7 Trp149 or alpha4 Trp154 universally resulted in receptors with reduced function. Our data support the hypothesis that some conserved residues in the nAChR LBD differentially regulate receptor activation by subtype-selective agonists, whereas other equally well conserved residues play fundamental roles in receptor activation by any agonist. Residues like alpha7 Trp149 (alpha4 Trp154) may be considered pillars upon which basic receptor function depends, whereas alpha7 Trp55 (beta2 Trp57) and alpha7 Tyr188 (alpha4 Tyr195) may be fulcra upon which agonists may operate differentially in specific receptor subtypes, consistent with the hypothesis that ACh and 4OH-GTS-21 are able to activate nAChR in distinct ways.

Aromatic Residues {epsilon}Trp-55 and {delta}Trp-57 and the Activation of Acetylcholine Receptor Channels

The two transmitter binding sites of the neuromuscular acetylcholine (ACh) receptor channel contain several aromatic residues, including a tryptophan located on the complementary, negative face of each binding pocket. These two residues, Trp-55 in the epsilon subunit and Trp-57 in the delta subunit, were mutated (AEFHILRVY), and for most constructs the rate constants for acetylcholine binding and channel gating were estimated by using single channel kinetic analyses. The rate constants for unliganded channel opening and closing were also estimated for some mutants. From these measurements we calculated all of the equilibrium constants of the "allosteric" cycle as follows: diliganded gating, unliganded gating, dissociation from the C(losed) conformation, and dissociation from the O(pen) conformation. The results indicate the following. (i) These aromatic side chains play a relatively minor role in ACh receptor channel activation. (ii) The main consequence of mutations is to reduce the affinity of the O conformation of the binding site for ACh, with the effect being greater at the epsilon subunit. (iii) In epsilon (but not delta) the aromatic nature of the side chain is important in determining affinity, to a slightly greater degree in the O conformation. Phi value analyses (of both tryptophan residues) show Phi approximately 1 for both the ACh binding and diliganded gating reactions. (iv) This suggests that the structural boundaries of the dynamic elements of the gating conformational change may not be subunit-delimited, and (v) the mutated tryptophan residues experience energy changes that occur relatively early in both the ligand-binding and channel-gating reactions.

The kinetics of competitive antagonism of nicotinic acetylcholine receptors at physiological temperature

Detailed information about the ligand-binding site of nicotinic acetylcholine receptors has emerged from structural and mutagenesis experiments. However, these approaches provide only static images of ligand-receptor interactions. Kinetic measurements of changes in protein function are needed to develop a more dynamic picture. Previously, we measured association and dissociation rate constants for competitive inhibition of current through embryonic muscle acetylcholine receptor channels at 25 degrees C. Little is known about competitive antagonism at physiological temperatures. Here, we performed measurements at 37 degrees C and used thermodynamics to estimate the energetics of antagonism. We used rapid solution exchange protocols to determine equilibrium and kinetics of inhibition of acetylcholine-activated currents in outside-out patches by (+)-tubocurarine, pancuronium and cisatracurium. Kinetic rates as high as 600 s(-1) were resolved by this technique. Binding was primarily enthalpy driven. The 12 degrees C increase in temperature decreased equilibrium antagonist binding by 1.7- to 1.9-fold. In contrast, association and dissociation rate constants increased 1.9- to 6.0-fold. Activation energies for dissociation were 90 +/- 6, 106 +/- 8 and 116 +/- 10 kJ mol(-1) for cisatracurium, (+)-tubocurarine and pancuronium, respectively. The corresponding apparent activation energies for association were 38 +/- 6, 85 +/- 6 and 107 +/- 13 kJ mol(-1). The higher activation energy for association of (+)-tubocurarine and pancuronium compared with cisatracurium is notable. This may arise from either a more superficial binding site for the large antagonist cisatracurium compared to the other ligands, or from a change in receptor conformation upon binding of (+)-tubocurarine and pancuronium but not cisatracurium. Differences in ligand desolvation and ligand conformation are not likely to be important.

The conformation of acetylcholine at its target site in the membrane-embedded nicotinic acetylcholine receptor

The conformation of the neurotransmitter acetylcholine bound to the fully functional nicotinic acetylcholine receptor embedded in its native membrane environment has been characterized by using frequency-selective recoupling solid-state NMR. Six dipolar couplings among five resolved (13)C-labeled atoms of acetylcholine were measured. Bound acetylcholine adopts a bent conformation characterized with a quaternary ammonium-to-carbonyl distance of 5.1 A. In this conformation, and with its orientation constrained to that previously determined by us, the acetylcholine could be docked satisfactorily in the agonist pocket of the agonist-bound, but not the agonist-free, crystal structure of a soluble acetylcholine-binding protein from Lymnaea stagnali. The quaternary ammonium group of the acetylcholine was determined to be within 3.9 A of five aromatic residues and its acetyl group close to residues C187/188 of the principle and residue L112 of the complementary subunit. The observed >C O chemical shift is consistent with H bonding to the nicotinic acetylcholine receptor residues gammaY116 and deltaT119 that are homologous to L112 in the soluble acetylcholine-binding protein.

The role of loop F residues in determining differential d-tubocurarine potencies in mouse and human 5-hydroxytryptamine 3A receptors

The competitive antagonist d-tubocurarine (curare) has greater potency at mouse than at human 5-hydroxytryptamine 3A (5-HT3A) receptors, despite 84% amino acid sequence identity between the receptors. Within the ligand binding domain of this receptor are six loops (A-F). A previous report demonstrated that loop C of the 5-HT3A receptor contributed to differential potency between the receptors [Hope, A. G. et al. (1999) Mol. Pharmacol. 55, 1037-1043]. The present study tested the hypothesis that loop F plays a significant role in conferring interspecies curare potency differences. Wild-type, chimeric, and point mutant 5-HT3A receptors were expressed in Xenopus oocytes, and two-electrode voltage clamp electrophysiological recordings were performed. Our data suggest that loops C and F contribute to curare potency, given that the curare IC50's (concentration of drug that produces 50% inhibition of the response) for chimeric human receptors with substitutions of mouse residues in loop C (40.07 +/- 2.52 nM) or loop F (131.8 +/- 5.95 nM) were intermediate between those for the mouse (12.99 +/- 0.77 nM) and human (1817 +/- 92.36 nM) wild-type receptors. Two human point mutant receptors containing mouse receptor substitutions in loop F (H-K195E or H-V202I) had significantly lower curare IC50's than that of the human receptor. The human double mutant receptor, H-K195E,V202I, had the same curare IC50 (133.8 +/- 6.38 nM) as that of the human receptor containing all six loop F mouse substitutions. These results demonstrate that two loop F residues make a significant contribution in determining curare potency at the 5-HT3A receptor.

alpha-Conotoxin GI benzoylphenylalanine derivatives. 1H-NMR structures and photoaffinity labeling of the Torpedo californica nicotinic acetylcholine receptor

alpha-Conotoxins are small peptides from cone snail venoms that function as nicotinic acetylcholine receptor (nAChR)-competitive antagonists differentiating between nAChR subtypes. Current understanding about the mechanism of these selective interactions is based largely on mutational analyses, which identify amino acids in the toxin and nAChR that determine the energetics of ligand binding. To identify regions of the nAChR involved in alpha-conotoxin binding by use of photoactivated cross-linking, two benzoylphenylalanine (Bpa) analogs of alpha-conotoxin GI, GI(Bpa12) and GI(Bpa4), were synthesized by replacing the respective residues with Bpa, and their (1)H-NMR structures were determined. Both analogs preserved the GI conformation, but only GI(Bpa12) displaced (125)I-labeled GI from the Torpedo californica nAChR. (125)I-labeled GI(Bpa12) bound to two sites on the receptor (K(d) 13 and 1800 nM), and on UV irradiation specifically photolabeled the alpha, gamma and delta subunits. Photolabeling sites were mapped by selective proteolysis and enzymatic deglycosylation, combined with SDS/PAGE, HPLC and Edman degradation. In the alpha subunit, cobratoxin-inhibited incorporation was limited to the 22-kDa fragment beginning at alphaSer173 and containing the agonist-binding site segment C. In the gamma subunit, radioactivity was localized to two distinct peptides containing agonist-binding site segments F and D: nonglycosylated 24-kDa and glycosylated 13-kDa fragments starting at gammaAla167 and gammaAla49, respectively. The labeling of these fragments is discussed in terms of a model of GI(Bpa12) bound to the extracellular domain of the Torpedo nAChR homology model derived from the cryo-electron microscopy structure of Torpedo marmorata nAChR and X-ray crystal structures of snail acetylcholine-binding protein complexes with agonists and antagonists.

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling

An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.

Nicotine enhances intracellular nicotinic receptor maturation: A novel mechanism of neural plasticity?

Nicotine addiction, the primary cause of tobacco consumption, is mediated through nicotine binding to brain nicotinic acetylcholine receptor (nAChRs). Upon chronic exposure, nicotine elicits a cascade of events, starting with nAChR activation and desensitization, followed by a long term up-regulation that corresponds to an increase in the number of the high affinity nAChRs, a paradoxical process that occurs in the brain of smokers. Recent investigation of the maturation and trafficking of the major brain alpha4beta2 nAChR demonstrates that up-regulation is initiated in the endoplasmic reticulum soon after protein translation. The data thus far accumulated provide evidence that nicotine elicits up-regulation by promoting maturation of nAChR precursors that would otherwise be degraded. This "maturational enhancer" action of nicotine probably contributes to the long term effect of chronic nicotine, and suggests a novel mechanism of neuronal plasticity through an yet unknown endogenous substance which would modulate the receptor expression under physiological conditions.

Activation of the Torpedo nicotinic acetylcholine receptor. The contribution of residues alphaArg55 and gammaGlu93

The Torpedo nicotinic acetylcholine receptor is a heteropentamer (alpha2betagammadelta) in which structurally homologous subunits assemble to form a central ion pore. Viewed from the synaptic cleft, the likely arrangement of these subunits is alpha-gamma-alpha-delta-beta lying in an anticlockwise orientation. High affinity binding sites for agonists and competitive antagonists have been localized to the alpha-gamma and alpha-delta subunit interfaces. We investigated the involvement of amino acids lying at an adjacent interface (gamma-alpha) in receptor properties. Recombinant Torpedo receptors, expressed in Xenopus oocytes, were used to investigate the consequences of mutating alphaArg55 and gammaGlu93, residues that are conserved in most species of the peripheral nicotinic receptors. Based on homology modeling, these residues are predicted to lie in close proximity to one another and it has been suggested that they may form a salt bridge in the receptor's three-dimensional structure (Sine et al. 2002 J Biol Chem277, 29 210-29 223). Although substitution of alphaR55 by phenylalanine or tryptophan resulted in approximately a six-fold increase in the EC50 value for acetylcholine activation, the charge reversal mutation (alphaR55E) had no significant effect. In contrast, the replacement of gammaE93 by an arginine conferred an eight-fold increase in the potency for acetylcholine-induced receptor activation. In the receptor carrying the double mutations, alphaR55E-gammaE93R or alphaR55F-gammaE93R, the potency for acetylcholine activation was partially restored to that of the wild-type. The results suggest that, although individually these residues influence receptor activation, direct interactions between them are unlikely to play a major role in the stabilization of different conformational states of the receptor.

Endoplasmic reticulum retention and associated degradation of a GABAA receptor epilepsy mutation that inserts an aspartate in the M3 transmembrane segment of the alpha1 subunit

A GABA(A) receptor alpha1 subunit epilepsy mutation (alpha1(A322D)) introduces a negatively charged aspartate residue into the hydrophobic M3 transmembrane domain of the alpha1 subunit. We reported previously that heterologous expression of alpha1(A322D)beta2gamma2 receptors in mammalian cells resulted in reduced total and surface alpha1 subunit protein. Here we demonstrate the mechanism of this reduction. Total alpha1(A322D) subunit protein was reduced relative to wild type protein by a similar amount when expressed alone (86 +/- 6%) or when coexpressed with beta2 and gamma2S subunits (78 +/- 6%), indicating an expression reduction prior to subunit oligomerization. In alpha1beta2gamma2S receptors, endoglycosidase H deglycosylated only 26 +/- 5% of alpha1 subunits, consistent with substantial protein maturation, but in alpha1(A322D)beta2gamma2S receptors, endoglycosidase H deglycosylated 91 +/- 4% of alpha1(A322D) subunits, consistent with failure of protein maturation. To determine the cellular localization of wild type and mutant subunits, the alpha1 subunit was tagged with yellow (alpha1-YFP) or cyan (alpha1-CFP) fluorescent protein. Confocal microscopic imaging demonstrated that 36 +/- 4% of alpha1-YFPbeta2gamma2 but only 5 +/- 1% alpha1(A322D)-YFPbeta2gamma2 colocalized with the plasma membrane, whereas the majority of the remaining receptors colocalized with the endoplasmic reticulum (55 +/- 4% alpha1-YFPbeta2gamma2S, 86 +/- 3% alpha1(A322D)-YFP). Heterozygous expression of alpha1-CFPbeta2gamma2S and alpha1(A322D)-YFPbeta2gamma2S or alpha1-YFPbeta2gamma2S and alpha1(A322D)-CFPbeta2gamma2S receptors showed that membrane GABA(A) receptors contained primarily wild type alpha1 subunits. These data demonstrate that the A322D mutation reduces alpha1 subunit expression after translation, but before assembly, resulting in endoplasmic reticulum-associated degradation and membrane alpha1 subunits that are almost exclusively wild type subunits.

Structural dynamics of the M4 transmembrane segment during acetylcholine receptor gating

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

Imaging of receptor trafficking by using alpha-bungarotoxin-binding-site-tagged receptors

alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors mediate excitatory synaptic transmission and are dynamically regulated during synaptic plasticity in the CNS. The membrane trafficking of AMPA receptors to synapses is critical for the regulation of the efficacy of excitatory synaptic transmission. Direct imaging of AMPA receptors in various cell compartments is important to dissecting the regulation of distinct steps in receptor membrane trafficking. In this study, we have developed an approach for the imaging of receptor trafficking with subunits tagged with a 13-aa alpha-bungarotoxin (BTX)-binding site (BBS). The small polypeptide neurotoxin BTX has been used for decades to study the nicotinic acetylcholine receptor. Similar high-affinity ligands are rarely available for most receptors. Engineering the BBS tag into receptor subunits allowed the high-affinity binding of fluorescent, radioactive, and biotinylated BTX to the tagged receptor subunits. By using this approach, the total receptor expression, surface expression, internalization, and insertion of receptors into the plasma membrane could be visualized and quantified in fixed or live cells including cultured neurons. The BBS tag is a flexible approach for labeling membrane proteins and studying their dynamic trafficking.

Structural basis for epibatidine selectivity at desensitized nicotinic receptors

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

Solution conformation of alpha-conotoxin GIC, a novel potent antagonist of alpha3beta2 nicotinic acetylcholine receptors

Alpha-conotoxin GIC is a 16-residue peptide isolated from the venom of the cone snail Conus geographus. Alpha-conotoxin GIC potently blocks the alpha3beta2 subtype of human nicotinic acetylcholine receptor, showing a high selectivity for neuronal versus muscle subtype [McIntosh, Dowell, Watkins, Garrett, Yoshikami, and Olivera (2002) J. Biol. Chem. 277, 33610-33615]. We have now determined the three-dimensional solution structure of alpha-conotoxin GIC by NMR spectroscopy. The structure of alpha-conotoxin GIC is well defined with backbone and heavy atom root mean square deviations (residues 2-16) of 0.53 A and 0.96 A respectively. Structure and surface comparison of alpha-conotoxin GIC with the other alpha4/7 subfamily conotoxins reveals unique structural aspects of alpha-conotoxin GIC. In particular, the structural comparison between alpha-conotoxins GIC and MII indicates molecular features that may confer their similar receptor specificity profile, as well as those that provide the unique binding characteristics of alpha-conotoxin GIC.

An Extracellular Protein Microdomain Controls Up-regulation of Neuronal Nicotinic Acetylcholine Receptors by Nicotine

In smoker's brain, rodent brain, and in cultured cells expressing nicotinic receptors, chronic nicotine treatment induces an increase in the total number of high affinity receptors for acetylcholine and nicotine, a process referred to as up-regulation. Up-regulation induced by 1 mm nicotine reaches 6-fold for alpha3beta2 nicotinic receptors transiently expressed in HEK 293 cells, whereas it is much smaller for alpha3beta4 receptors, offering a rationale to investigate the molecular mechanism underlying up-regulation. In this expression system binding sites are mainly intracellular, as shown by [(3)H]epibatidine binding experiments and competition with the impermeant ligand carbamylcholine. Systematic analysis of beta2/beta4 chimeras demonstrates the following. (i) The extracellular domain critically contributes to up-regulation. (ii) Only residues belonging to two beta2 segments, 74-89 and 106-115, confer up-regulation to beta4, mainly by decreasing the amount of binding sites in the absence of nicotine; on an atomic three-dimensional model of the alpha3beta2 receptor these amino acids form a compact microdomain that mainly contributes to the subunit interface and also faces the acetylcholine binding site. (iii) The beta4 microdomain is sufficient to confer to beta2 a beta4-like up-regulation. (iv) This microdomain makes an equivalent contribution to the up-regulation differences between alpha4beta2 and alpha4beta4. We propose that nicotine, by binding to immature oligomers, elicits a conformational reorganization of the microdomain, strengthening the interaction between adjacent subunits and, thus, facilitating maturation processes toward high affinity receptors. This mechanism may be central to nicotine addiction, since alpha4beta2 is the subtype exhibiting the highest degree of up-regulation in the brain.

Identification of binding sites in the nicotinic acetylcholine receptor for [3H]azietomidate, a photoactivatable general anesthetic

To identify binding domains in a ligand-gated ion channel for etomidate, an intravenous general anesthetic, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with a photoactivatable analog, [(3)H]azietomidate. Based upon the inhibition of binding of the noncompetitive antagonist [(3)H]phencyclidine, azietomidate and etomidate bind with 10-fold higher affinity to nAChRs in the desensitized state (IC(50) = 70 microm) than in the closed channel state. In addition, both drugs between 0.1 and 1 mm produced a concentration-dependent enhancement of [(3)H]ACh equilibrium binding affinity, but they inhibited binding at higher concentrations. UV irradiation resulted in preferential [(3)H]azietomidate photoincorporation into the nAChR alpha and delta subunits. Photolabeled amino acids in both subunits were identified in the ion channel domain and in the ACh binding sites by Edman degradation. Within the nAChR ion channel in the desensitized state, there was labeling of alphaGlu-262 and deltaGln-276 at the extracellular end and deltaSer-258 and deltaSer-262 toward the cytoplasmic end. Within the acetylcholine binding sites, [(3)H]azietomidate photolabeled alphaTyr-93, alphaTyr-190, and alphaTyr-198 in the site at the alpha-gamma interface and deltaAsp-59 (but not the homologous position, gammaGlu-57). Increasing [(3)H]azietomidate concentration from 1.8 to 150 microm increased the efficiency of incorporation into amino acids within the ion channel by 10-fold and in the ACh sites by 100-fold, consistent with higher affinity binding within the ion channel. The state dependence and subunit selectivity of [(3)H]azietomidate photolabeling are discussed in terms of the structures of the nAChR transmembrane and extracellular domains.

Identification of nicotinic acetylcholine receptor amino acids photolabeled by the volatile anesthetic halothane

To identify inhalational anesthetic binding domains in a ligand-gated ion channel, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with [(14)C]halothane and determined by Edman degradation some of the photolabeled amino acids in nAChR subunit fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography. Irradiation at 254 nm for 60 s in the presence of 1 mM [(14)C]halothane resulted in incorporation of approximately 0.5 mol of (14)C/mol of subunit, with photolabeling distributed within the nAChR extracellular and transmembrane domains, primarily at tyrosines. GammaTyr-111 in ACh binding site segment E was labeled, while alphaTyr-93 in segment A was not. Within the transmembrane domain, alphaTyr-213 within alphaM1 and deltaTyr-228 within deltaM1 were photolabeled, while no labeled amino acids were identified within the deltaM2 ion channel domain. Although the efficiency of photolabeling at the subunit level was unaffected by agonist, competitive antagonist, or isoflurane, state-dependent photolabeling was seen in a delta subunit fragment beginning at deltaPhe-206. Labeling of deltaTyr-212 in the extracellular domain was inhibited >90% by d-tubocurarine, whereas addition of either carbamylcholine or isoflurane had no effect. Within M1, the level of photolabeling of deltaTyr-228 with [(14)C]halothane was increased by carbamylcholine (90%) or d-tubocurarine (50%), but it was inhibited by isoflurane (40%). Within the structure of the nAChR transmembrane domain, deltaTyr-228 projects into an extracellular, water accessible pocket formed by amino acids from the deltaM1-deltaM3 alpha-helices. Halothane photolabeling of deltaTyr-228 provides initial evidence that halothane and isoflurane bind within this pocket with occupancy or access increased in the nAChR desensitized state compared to the closed channel state. Halothane binding at this site may contribute to the functional inhibition of nAChRs.

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

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

A single M1 residue in the beta2 subunit alters channel gating of GABAA receptor in anesthetic modulation and direct activation

General anesthetics allosterically modulate the activity of neuronal gamma-aminobutyric acid, type A (GABAA), receptors. Previous mutational studies from our laboratory and others have shown that the regions in transmembrane domain 1 (M1) and pre-M1 of alpha and beta subunits in GABA receptors are essential for positive modulation of GABA binding and function by the intravenous (IV) general anesthetics. Mutation of beta2Gly-219 to Phe corresponded in rho nearly eliminated the modulatory effect of IV anesthetics in alpha1/beta2/gamma2S combination. However, the general anesthetics retained the ability to directly open the channel of mutant G219F, and the apparent affinity for GABA was increased, and desensitization rate was reduced. In this study, we made additional single mutations such as 219 Ser, Cys, Ile, Asp, Arg, Tyr, and Trp. The larger side chains of the replacement residues produced the greatest reduction in enhancement of GABA currents by IV anesthetics at clinical concentrations (Trp > Tyr = Phe > Arg > Asp > Ile > Cys > Ser = wild type). Compared with a 2-3-fold response in wild type, pentobarbital and propofol enhanced less than 0.5-fold; etomidate and alphaxalone modulation was reduced from more than 4- to 1-fold in G219F, G219Y, and G219W. A linear correlation was observed between the volume of the residue at position 219 and the loss of modulation. An identical correlation was found for the effect of modulation on left-shift in the GABA EC50 value; furthermore, the same rank order of residues, related to size, was found for reduction in the maximal direct channel-gating by pentobarbital (1 mm) and etomidate (100 mum) and for increased apparent affinity for direct gating by the IV anesthetics.

Solution conformation of alphaA-conotoxin EIVA, a potent neuromuscular nicotinic acetylcholine receptor antagonist from Conus ermineus

We report the solution three-dimensional structure of an alphaA-conotoxin EIVA determined by nuclear magnetic resonance spectroscopy and restrained molecular dynamics. The alphaA-conotoxin EIVA consists of 30 amino acids representing the largest peptide among the alpha/alphaA-family conotoxins discovered so far and targets the neuromuscular nicotinic acetylcholine receptor with high affinity. alphaA-Conotoxin EIVA consists of three distinct structural domains. The first domain is mainly composed of the Cys3-Cys11-disulfide loop and is structurally ill-defined with a large backbone root mean square deviation of 1.91 A. The second domain formed by residues His12-Hyp21 is extremely well defined with a backbone root mean square deviation of 0.52 A, thus forming a sturdy stem for the entire molecule. The third C-terminal domain formed by residues Hyp22-Gly29 shows an intermediate structural order having a backbone root mean square deviation of 1.04 A. A structurally ill-defined N-terminal first loop domain connected to a rigid central molecular stem seems to be the general structural feature of the alphaA-conotoxin subfamily. A detailed structural comparison between alphaA-conotoxin EIVA and alphaA-conotoxin PIVA suggests that the higher receptor affinity of alphaA-conotoxin EIVA than alphaA-conotoxin PIVA might originate from different steric disposition and charge distribution in the second loop "handle" motif.

Structure and Function of AChBP, Homologue of the Ligand-Binding Domain of the Nicotinic Acetylcholine Receptor

Acetylcholine-binding protein (AChBP) is a novel protein with high similarity to the extracellular domain of the nicotinic acetylcholine receptor. AChBP lacks the transmembrane domains and intracellular loops typical for the nAChRs. AChBP is secreted from glia cells in the central nervous system of the freshwater snail, Lymnaea stagnalis, where it modulates synaptic transmission. AChBP forms homopentamers with pharmacology that resembles the alpha(7)-type of nicotinic receptors. As such, AChBP is a good model for the ligand-binding domain of the nAChRs. In the crystal structure of AChBP at 2.7 A, each protomer has a modified immunoglobulin fold. Almost all residues previously shown to be involved in ligand binding in the nicotinic receptor are found in a pocket at the subunit interface, which is lined with aromatic residues. The AChBP crystal structure explains many of the biochemical studies on the nicotinic acetylcholine receptors. Surprisingly, the interface between protomers is relatively weakly conserved between families in the superfamily of pentameric ligand-gated ion channels. The lack of conservation has implications for the mechanism of gating of the ion channels.

A cysteine-linkable, short cleavable photoprobe with dual functionality to explore protein-protein interfaces

We developed a bifunctional photoprobe with dual functionality, that can be specifically tethered to cysteinyl residues of peptides and proteins through a short cleavable disulfide bond. Thus, an aryldiazonium moiety is positioned at approximately 8.5 A from the modified cysteinyl alpha-carbon, leading to one of the shortest cleavable linkages. In a sodium azide-containing buffer, the aryldiazonium moiety is transformed into an aryl azide. Therefore, with one bifunctional photoprobe two types of photogenerated species can be obtained: a hydrophilic and positively charged arylcation or a hydrophobic nitrene. We coupled the aryldiazonium probe, in a site-directed manner, to a nicotinic acetylcholine receptor competitive antagonist, obtained by chemical engineering of an analogue of a snake alpha-neurotoxin. In this molecule, Arg33, which is known to interact with the receptor, was replaced by a cysteine residue, where the photoprobe could be attached. Under inactinic light, this novel photosensitive snake toxin behaved as a reversible ligand on the Torpedo acetylcholine receptor. However, when irradiated at 391 nm, it generated a highly reactive arylcation which labeled mostly the receptor alpha-subunit, confirming the location of the tip of the second toxic loop near this receptor subunit. Finally, we showed that reduction of the disulfide bond, linking the ligand to the photocoupled receptor, allowed introduction of radioactivity on the labeled residue(s), opening the way to further characterization and avoiding the synthesis of a radioactive bifunctional photoprobe.

Identification of amino acids in the nicotinic acetylcholine receptor agonist binding site and ion channel photolabeled by 4-[(3-trifluoromethyl)-3H-diazirin-3-yl]benzoylcholine, a novel photoaffinity antagonist

[(3)H]4-[(3-trifluoromethyl)-3H-diazirin-3-yl]benzoylcholine (TDBzcholine) was synthesized and used as a photoaffinity probe to map the orientation of an aromatic choline ester within the agonist binding sites of the Torpedo nicotinic acetylcholine receptor (nAChR). TDBzcholine acts as a nAChR competitive antagonist that binds at equilibrium with equal affinity to both agonist sites (K(D) approximately 10 microM). Upon UV irradiation (350 nm), nAChR-rich membranes equilibrated with [(3)H]TDBzcholine incorporate (3)H into the alpha, gamma, and delta subunits in an agonist-inhibitable manner. The specific residues labeled by [(3)H]TDBzcholine were determined by N-terminal sequence analysis of subunit fragments produced by enzymatic cleavage and purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and/or reversed-phase high-performance liquid chromatography. For the alpha subunit, [(3)H]TDBzcholine photoincorporated into alphaCys-192, alphaCys-193, and alphaPro-194. For the gamma and delta subunits, [(3)H]TDBzcholine incorporated into homologous leucine residues, gammaLeu-109 and deltaLeu-111. The photolabeling of these amino acids suggests that when the antagonist TDBzcholine occupies the agonist binding sites, the Cys-192-193 disulfide and Pro-194 from the alpha subunit Segment C are oriented toward the agonist site and are in proximity to gammaLeu-109/deltaLeu-111 in Segment E, a conclusion consistent with the structure of the binding site in the molluscan acetylcholine binding protein, a soluble protein that is homologous to the nAChR extracellular domain.

Interaction of d-tubocurarine analogs with mutant 5-HT(3) receptors

d-Tubocurarine is a potent competitive antagonist of both the muscle-type nicotinic acetylcholine receptor (AChR) and the serotonin type-3 receptor (5HT(3)R). We have previously used a series of structural analogs of d-tubocurarine to demonstrate that the ligand-binding domains of both receptors share common structural features. We have now extended these studies to examine the interaction of a series of d-tubocurarine analogs with 5HT(3)Rs containing mutations at either of two residues within the ligand-binding domain of the receptor (W90F and R92A). The W90F mutation results in an approximately 2-4-fold decrease in the affinity of the analogs relative to wild-type receptors, while the R92A results in an approximately 8-10-fold increase in affinity. However, since the effect of a given mutation is more or less equivalent for all analogs, neither residue W90 nor R92 is likely to make a specific interaction with d-tubocurarine itself. Rather, these two residues are likely to play a role in determining both the geometry of the binding site, as well as the overall environment that a ligand encounters in the binding site.

Contributions of the non-alpha subunit residues (loop D) to agonist binding and channel gating in the muscle nicotinic acetylcholine receptor

The agonist binding site of the nicotinic acetylcholine receptor has a loop-based structure, and is formed by residues located remotely to each other in terms of primary structure. Amino acid residues in sites delta57 and delta59, and the equivalent residues in the epsilon; subunit, have been identified as part of the agonist binding site and designated as loop D. The effects of point mutations in sites delta57, delta59, epsilon;55 and epsilon;57 on agonist binding and channel gating were studied. The mutated receptors were expressed transiently in HEK 293 cells and the currents were recorded using the cell-attached single-channel patch clamp technique. The results demonstrate that the mutations mainly affect channel gating with the major portion of the effect due to a reduction in the channel opening rate constant. For both the delta57/epsilon;55 and the delta59/epsilon;57 site, a mutation in the epsilon; subunit had a greater effect on channel gating than a mutation in the delta subunit. In all instances, agonist binding was affected to a lesser degree than channel gating. Previous data have placed the delta57 and delta59 residues in or near the agonist binding pocket. The data presented here suggest that these two residues (and the homologous sites in the epsilon; subunit) are not involved in specific interactions with the nicotinic agonist and that they affect the activation of the nicotinic receptor by shaping the overall structure of the agonist binding site.

Orientation of d-tubocurarine in the muscle nicotinic acetylcholine receptor-binding site

Ligand modification and receptor site-directed mutagenesis were used to examine binding of the competitive antagonist, d-tubocurarine (dTC), to the muscle-type nicotinic acetylcholine receptor (AChR). By using various dTC analogs, we measured the interactions of specific dTC functional groups with amino acid positions in the AChR gamma-subunit. Because data for mutations at residue gammaTyr(117) were the most consistent with direct interaction with dTC, we focused on that residue. Double mutant thermodynamic cycle analysis showed apparent interactions of gammaTyr(117) with both the 2-N and the 13'-positions of dTC. Examination of a dTC analog with a negative charge at the 13'-position failed to reveal electrostatic interaction with charged side-chain substitutions at gamma117, but the effects of side-chain substitutions remained consistent with proximity of Tyr(117) to the cationic 2-N of dTC. The apparent interaction of gammaTyr(117) with the 13'-position of dTC was likely mediated by allosteric changes in either dTC or the receptor. The data also show that cation-pi electron stabilization of the 2-N position is not required for high affinity binding. Molecular modeling of dTC within the binding pocket of the acetylcholine-binding protein places the 2-N in proximity to the residue homologous to gammaTyr(117). This model provides a plausible structural basis for binding of dTC within the acetylcholine-binding site of the AChR family that appears consistent with findings from photoaffinity labeling studies and with site-directed mutagenesis studies of the AChR.

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

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

Identification of residues at the alpha and epsilon subunit interfaces mediating species selectivity of Waglerin-1 for nicotinic acetylcholine receptors

Waglerin-1 (Wtx-1) is a 22-amino acid peptide that is a competitive antagonist of the muscle nicotinic receptor (nAChR). We find that Wtx-1 binds 2100-fold more tightly to the alpha-epsilon than to the alpha-delta binding site interface of the mouse nAChR. Moreover, Wtx-1 binds 100-fold more tightly to the alpha-epsilon interface from mouse nAChR than that from rat or human sources. Site-directed mutagenesis of residues differing in the extracellular domains of rat and mouse epsilon subunits indicates that residues 59 and 115 mediate the species difference in Wtx-1 affinity. Mutation of residues 59 (Asp in mouse, Glu in rat epsilon) and 115 (Tyr in mouse, Ser in rat epsilon) converts Wtx-1 affinity for the alpha-epsilon interface of one species to that of the other species. Studies of different mutations at position 59 indicate both steric and electrostatic contributions to Wtx-1 affinity, whereas at position 115, both aromatic and polar groups contribute to affinity. The human nAChR also has lower affinity for Wtx-1 than mouse nAChR, but unlike rat nAChR, residues in both alpha and epsilon subunits mediate the affinity difference. In human nAChR, polar residues (Ser-187 and Thr-189) confer low affinity, whereas in mouse nAChR aromatic residues (Trp-187 and Phe-189) confer high affinity. The overall results show that non-conserved residues at the nAChR binding site, although not crucial for activation by ACh, govern the potency of neuromuscular toxins.

Mapping the agonist binding site of the nicotinic acetylcholine receptor by cysteine scanning mutagenesis: antagonist footprint and secondary structure prediction

To further define the surface of the Torpedo californica nicotinic acetylcholine receptor (nAChR) contributing to the agonist binding site structure, we used the substituted Cys accessibility method to identify novel residues and determined the "footprint" of residues protected from modification by the reversible competitive antagonist d-tubocurarine (dTC). nAChRs containing single Cys substitutions within regions of the alpha- or gamma-subunit primary structure known to contribute to the agonist binding site were expressed in Xenopus laevis oocytes. Cys substitutions in binding site segments A (alphaTyr-93 and alphaAsn-94), C (alphaTyr-198), and D (gammaGlu-57) had been shown previously to be accessible for modification. We now introduced cysteines from alphaAsp-195 to alphaIle-201 and from gammaAla-106 to gammaAsp-113 and identified positions accessible for modification in segments C (alphaAsp-195, alphaThr-196, alphaPro-197, alphaAsp-200, and alphaIle-201) and E (gammaAsn-107 and gammaLeu-109). dTC protected against alkylation in segments D (gammaGlu-57) and E (gammaLeu-109) but not in segment A (alphaTyr-93 and alphaAsn-94). In segment C, dTC protection experiments revealed a pattern in which every other residue (alpha196, alpha198, and alpha200, but not alpha197 or alpha201) was protected from alkylation. This pattern of protection provides evidence that bound dTC is near amino acids in segments C, D, and E but not in segment A, and identifies a beta-strand surface within segment C contributing to the binding site. These results are discussed in terms of a homology model, based on the molluscan acetylcholine binding protein crystal structure, of the T. californica nAChR agonist binding site.

Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications

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.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Acetylcholine receptor delta subunit mutations underlie a fast-channel myasthenic syndrome and arthrogryposis multiplex congenita

Limitation of movement during fetal development may lead to multiple joint contractures in the neonate, termed arthrogryposis multiplex congenita. Neuromuscular disorders are among the many different causes of reduced fetal movement. Many congenital myasthenic syndromes (CMSs) are due to mutations of the adult-specific epsilon subunit of the acetylcholine receptor (AChR), and, thus, functional deficits do not arise until late in gestation. However, an earlier effect on the fetus might be predicted with some defects of other AChR subunits. We studied a child who presented at birth with joint contractures and was subsequently found to have a CMS. Mutational screening revealed heteroallelic mutation within the AChR delta subunit gene, delta 756ins2 and delta E59K. Expression studies demonstrate that delta 756ins2 is a null mutation. By contrast, both fetal and adult AChR containing delta E59K have shorter than normal channel activations that predict fast decay of endplate currents. Thus, delta E59K causes dysfunction of fetal as well as the adult AChR and would explain the presence of joint contractures on the basis of reduced fetal movement. This is the first report of the association of AChR gene mutations with arthrogryposis multiplex congenita. It is probable that mutations that severely disrupt function of fetal AChR will underlie additional cases.