Functional architecture of the nicotinic acetylcholine receptor: from electric organ to brain

Topology of ligand binding sites on the nicotinic acetylcholine receptor

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

Expression of the nicotinic acetylcholine receptor subunit, alpha9, in the guinea pig cochlea

Acetylcholine is a major neurotransmitter of the cochlear efferent system. Based on its high level of expression in hair cells, the recently cloned nicotinic receptor subunit, alpha9 [Elgoyhen et al., Cell 79 (1994) 705-715], is likely to be the postsynaptic receptor for acetylcholine in hair cells either as a homomeric complex or with other subunits yet to be identified. To further study this receptor, we cloned and sequenced alpha9 cDNA from the guinea pig organ of Corti library [Wilcox and Fex, Hear. Res. 62 (1992) 124-126]. The sequence of the guinea pig alpha9 cDNA is similar to that of the rat, with identities of 85% and 89% at the nucleotide and amino acid levels, respectively. Most differences are in the cytoplasmic loop domain between the transmembrane segments 3 and 4. We also observed minor differences in the putative ligand binding regions. Pharmacological differences between acetylcholine receptors on outer hair cells of rat and guinea pig have been reported, and the minor structural changes we observe could account for these differences. Reverse transcription-polymerase chain reaction analysis showed a high expression of alpha9 in the organ of Corti while expression was low or not detected in the spiral ganglion. In situ hybridization histochemistry showed expression of alpha9 mRNA in both inner and outer hair cells, with much higher expression in outer hair cells than in inner hair cells. In the inner hair cell, silver grains were more abundant over the basal part of the cell than over the apical part. Immunocytochemistry showed a pattern of distribution of the alpha9 protein similar to that seen for mRNA with in situ hybridization. Immunolabeling was most intense at the bases of both inner and outer hair cells. To determine the effect of hair cell loss on alpha9 expression, hair cells were destroyed by either systemic or local application of kanamycin. This treatment led to a down regulation of alpha9 in hair cells; this down regulation appeared to precede hair cell degeneration. In the spiral ganglion, a transient up regulation of alpha9, as determined by RT-PCR, was seen 4-6 weeks after kanamycin treatment.

Differential effects of dimethyl sulfoxide on nicotinic acetylcholine receptors from mouse muscle and Torpedo electrocytes

The present study examined the effects of 0.1% dimethyl sulfoxide (DMSO) on nicotinic acetylcholine receptors (nAChR) from mouse muscle and Torpedo californica electrocytes. Receptors were expressed in Xenopus laevis oocytes and studied with voltage-clamp. When applied simultaneously with acetylcholine, DMSO did not inhibit current amplitude of either receptor. Preincubation with DMSO for 1 min reduced current amplitude by approximately 50% from oocytes expressing electrocyte receptor. Preincubation did not affect the muscle receptor. With electric organ membranes, 0.1% DMSO did not block either [alpha-(125)I]bungarotoxin binding to the nAChR agonist site or [3H]phencyclidine binding to its high affinity site on resting or desensitized receptor. These data suggest that DMSO might be affecting the electrocyte receptor through a second messenger system.

Tyrosine phosphorylation of nicotinic acetylcholine receptor mediates Grb2 binding

Tyrosine phosphorylation of the nicotinic acetylcholine receptor (AChR) is associated with an altered rate of receptor desensitization and also may play a role in agrin-induced receptor clustering. We have demonstrated a previously unsuspected interaction between Torpedo AChR and the adaptor protein Grb2. The binding is mediated by the Src homology 2 (SH2) domain of Grb2 and the tyrosine-phosphorylated delta subunit of the AChR. Dephosphorylation of the delta subunit abolishes Grb2 binding. A cytoplasmic domain of the delta subunit contains a binding motif (pYXNX) for the SH2 domain of Grb2. Indeed, a phosphopeptide corresponding to this region of the delta subunit binds to Grb2 SH2 fusion proteins with relatively high affinity, whereas a peptide lacking phosphorylation on tyrosine exhibits no binding. Grb2 is colocalized with the AChR on the innervated face of Torpedo electrocytes. Furthermore, Grb2 specifically copurifies with AChR solubilized from postsynaptic membranes. These data suggest a novel role for tyrosine phosphorylation of the AChR in the initiation of a Grb2-mediated signaling cascade at the postsynaptic membrane.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Evidence of alpha-helix slidings during bacteriorhodopsin photocycle-energetics coupling

The three-dimensional structure of bacteriorhodopsin indicates that the all-trans-cis retinal bending causes alpha-helix slidings during the bacteriorhodopsin photocycle. For the elucidation of alpha-helix slidings taking place during the bacteriorhodopsin photocycle, we calculated ASAs of hydrophobic and hydrophilic atoms translocated by alpha-helix slidings with thermodynamic criteria found previously. Thermodynamic parameters calculated from ASAs (calculated delta Gtransfer and T delta S) were consistent with those (observed delta Gtransfer and T delta S) obtained empirically. These findings indicate that alpha-helix slidings take place during bacteriorhodopsin photocycle-energetics coupling. These mechanisms not only explain various phenomena, including some mutants forming a long-lived intermediate, but also predict various yet-unobserved phenomena, including the generation of heat in early photocycle intermediates.

Mouse-Torpedo chimeric alpha-subunit used to probe channel-gating determinants on the nicotinic acetylcholine receptor primary sequence

1. To determine if structural domains are important for nicotinic acetylcholine receptor (nAChr) channel function, six mouse-Torpedo chimeric alpha-subunits were constructed (Fig. 2) and coexpressed with Torpedo californica beta-, gamma-, and delta-subunits in Xenopus laevis oocytes. 2. nAChRs containing a chimeric alpha-subunit were examined by voltage- and patch-clamp methods to determine their functional characteristics. Dose-response curves from voltage-clamped oocytes were used to estimate EC50's and Hill coefficients. Whole-cell currents were normalized against the alpha-bungarotoxin (alpha-BTX) binding sites to obtain normalized responses to acetylcholine (ACh). Open time constants at 4 microM ACh were used to examine single-channel behavior. 3. The EC50 for ACh was modulated by the N-terminal half of the alpha-subunit. When the Torpedo subunit sequence between position 1 and position 268 was replaced by mouse sequence, the EC50 shifted toward the value for the wild-type mouse subunit. Replacement of either the 1-159 or the 160-268 positions of the Torpedo sequence with the mouse sequence lowered the EC50. This suggests that at least two regions play a role in determining the EC50. 4. When the primary sequence (160-268) of the Torpedo alpha-subunit was introduced in the mouse alpha-subunit (T160-268), the expressed chimeric receptor was nonfunctional. The inverse chimera (M160-268) was functional and the open time constant and EC50 were similar to those of mouse but the normalized response was characteristic of Torpedo. 5. The normalized macroscopic response to ACh (300 microM) of the chimera containing the mouse alpha-subunit showed a ninefold increase relative to the Torpedo wild type. Receptors which contain the C terminal of the mouse alpha-subunit also show an increase in the maximum normalized current. Receptors with the alpha-subunit which contain the Torpedo C-terminal sequence have a lower normalized response. 6. The combined results suggest that AChR channel function is modulated by structural determinants within the primary sequence. These structural domains might modulate channel function through specific allosteric interactions. The lack of response of the T160-268 chimera suggests that a critical interaction essential for the coupling of agonist binding and channel gating was disrupted. This result suggests that the interaction of structural domains within the nAChR primary structure are essential for channel function and that these intractions could be very specific within different nAChR species.

Pharmacology of neuronal nicotinic acetylcholine receptor subtypes

The search for the physiological function of nicotinic receptors on neurons in the brain began with their discovery. It was initially assumed that, as in ganglia and at the neuromuscular junction, nicotinic receptors would gate fast synaptic transmission in the brain. The best functional evidence now, however, points to a role in modifying the release of other transmitters. This does not preclude a postsynaptic role in transmission for nicotinic receptors in the brain, but attempts to locate such a synapse have not been successful. If fast nicotinic synapses are present in the brain, they are probably low in number and may be masked by other more prevalent synapses (such as glutamatergic) so identification will not be easy. The extent of diversity of nicotinic receptors is substantial. At the molecular level this is reflected in the number of different genes that encode receptor subunits and the multiple possible combinations of subunits that function in expression systems. From the cellular level there is a broad diversity of properties of native receptors in neurons. Some useful pharmacological tools allow the limited identification of subunits in native receptors. For example, block by alpha-bungarotoxin identifies alpha 7, alpha 8, or alpha 9 subunits; activation of a receptor by cytisine indicates an alpha 7 or beta 4 subunit; and neuronal bungarotoxin block identifies a beta 2 subunit. Despite the clues to identity gained by careful use of these agents, we have not been able to identify all the components of any native receptor based on pharmacological properties assessed from expression studies. When both pharmacological and biophysical properties of a receptor are taken into consideration, none of the combinations tested in oocytes mimics native receptors exactly. The reason for this discrepancy has been debated at length; it is possible that oocytes do not faithfully manufacture neuronal nicotinic receptors. For example, they may not correctly modify the protein after translation or they may allow a combination of subunits that do not occur in vivo. Another possibility is that correct combinations of subunits have not yet been tested in oocytes. Data from immunoprecipitation experiments suggest that many receptors contain three or more different subunits. Results from further experiments injecting combinations of three or more subunits into oocytes may be enlightening. The diversity of receptors may allow targeting of subtypes to specific locations. Nicotinic receptors are located presynaptically, preterminally, and on the cell soma. The function of the nicotinic receptors located on innervating axons is presumably to modify the release of other neurotransmitters. It is an attractive hypothesis that nicotinic receptors might be involved in modifying the weight of central synapses; however, in none of the regions where this phenomenon has been described is there any evidence for axoaxonal contacts. The presynaptic receptors described so far are pharmacologically unique; therefore, if there are different subtypes of nicotinic receptors modifying the release of different transmitters, they may provide a means of exogenously modifying the release of a particular transmitter with drugs. There are still many basic unanswered questions about nicotinic receptors in the brain. What are the compositions of native nicotinic receptors? What is their purpose on neurons? Although there is clearly a role presynaptically, what is the function of those located on the soma? Neuronal nicotinic receptors are highly permeable to calcium, unlike muscle nicotinic receptors, and this may have important implications for roles in synaptic plasticity and development. Finally, why is there such diversity? (ABSTRACT TRANCATED)

Functional interaction of Src family kinases with the acetylcholine receptor in C2 myotubes

Tyrosine phosphorylation of the beta subunit of the acetylcholine receptor (AChR) has been postulated to play a role in AChR clustering during development of the neuromuscular junction. We have investigated the mechanism of this phosphorylation in mammalian C2 myotubes and report that the tyrosine kinase Src binds and phosphorylates glutathione S-transferase fusion proteins containing the N-terminal half of the cytoplasmic loop of the beta subunit. No binding occurs to the related kinases Fyn or Yes or to the corresponding regions from the gamma and delta subunits. Furthermore, AChRs affinity-isolated from C2 myotubes using alpha-bungarotoxin-Sepharose were specifically associated with Src and Fyn and had tyrosine-phosphorylated beta subunits. We suggest that AChRs are initially phosphorylated by Src and subsequently bind Fyn in a phosphotyrosine-dependent manner. These interactions are likely to play an important role in construction of the specialized postsynaptic membrane during synaptogenesis.

Assembly of the nicotinic acetylcholine receptor. The first transmembrane domains of truncated alpha and delta subunits are required for heterodimer formation in vivo

To investigate the mechanism of assembly of the mouse muscle acetylcholine receptor, we have expressed truncated N-terminal fragments of the alpha and delta subunits in COS cells and have examined their ability to fold, to associate into heterodimers, and to form a ligand-binding site. Truncated fragments of the alpha subunit that include all, part, or none of the first transmembrane domain (M1) folded to acquire alpha-bungarotoxin binding activity. Neither the full-length alpha subunit nor any of the fragments were expressed on the cell surface, although the shortest folded fragment lacking a transmembrane domain was secreted into the medium. When coexpressed with the delta subunit, the alpha subunit fragment possessing M1 formed a heterodimer containing a ligand-binding site, but shorter fragments, which lack transmembrane segments, did not associate with the delta subunit. N-terminal delta subunit fragments gave similar results. An N-terminal delta subunit fragment that contains M1 associated with the alpha subunit to form a heterodimer, while a fragment lacking M1 did not. These results show that a complete M1 domain is necessary for association of truncated N-terminal alpha and delta subunits into a heterodimer with high affinity ligand binding activity.

Membrane tethering enables an extracellular domain of the acetylcholine receptor alpha subunit to form a heterodimeric ligand-binding site

The first step of assembly of the nicotinic acetylcholine receptor (AChR) of adult skeletal muscle is the specific association of the alpha subunit with either delta or epsilon subunits to form a heterodimer with a ligand-binding site. Previous experiments have suggested that het erodimer formation in the ER arises from interaction between the luminal, NH2-terminal domains of the subunits. When expressed in COS cells with the delta subunit, however, the truncated NH2-terminal domain of the subunit folded correctly but did not form a heterodimer. Association with the delta subunit occurred only when the NH2-terminal domain was retained in the ER and was tethered to the membrane by its own M1 transmembrane domain, by the transmembrane domain of another protein, or by a glycolipid link. In each case, the ligand-binding sites of the resulting heterodimers were indistinguishable from that formed when the full-length alpha subunit was used. Attachment to the membrane may promote interaction by concentrating or orienting the subunit; alternatively, a membrane-bound factor may facilitate subunit association.

Binding of native kappa-neurotoxins and site-directed mutants to nicotinic acetylcholine receptors

The kappa-neurotoxins are useful ligands for the pharmacological characterization of nicotinic acetylcholine receptors because they are potent antagonists at only a subgroup of these receptors containing either alpha 3- or alpha 4-subunits (IC50 < or = 100 nM). Four of these highly homologous, 66 amino acid peptides have been purified from the venom of Bungarus multicinctus (kappa-bungarotoxin (kappa-Bgt), kappa 2-Bgt, kappa 3-Bgt] and Bungarus flaviceps [kappa-Fvt)]. Two approaches were taken to examine the binding of these toxins to nicotinic receptors. First, venom-derived kappa-Fvt and kappa-Bgt were radioiodinated and the specific binding was measured of these toxins to overlapping synthetic peptides (16-20 amino acids in length) prepared based on the known sequence of the nicotinic receptor alpha 3-subunit. At least two main regions of interaction between the toxins and the receptor subunit were identified, both lying in the N-terminal region of the subunit that is exposed to the extracellular space. The second approach examined the importance of several sequence position in kappa-Bgt for binding to alpha 3-containing receptors in autonomic ganglia and alpha 1-containing muscle receptors. This was done using site-directed mutants of kappa-Bgt produced by an Escherichia coli expression system. Arg-34 and position 36 were important for binding to both receptor subtypes, while replacing Gln-26 with Trp-26 (an invariant in alpha-neurotoxins) increased affinity for the muscle receptor by 8-fold. The results confirm that kappa-neurotoxins bind potently to the alpha 3-subunit and bind with considerably reduced affinity (Kd approximately 10 microM) to muscle receptors. Site-directed mutagenesis of recombinant kappa-Bgt is thus an important approach for the study of structure-function relationships between kappa-Bgt and nicotinic receptors.

Anionic residue in the alpha-subunit of the nicotinic acetylcholine receptor contributing to subunit assembly and ligand binding

To ascertain the anionic sites on the nicotinic receptor to which acetylcholine and other quaternary ammonium ligands bind, we have examined the role of an aspartyl residue (Asp-152) in the alpha-subunit. Prior photolytic labeling with agonist analogues of the neighboring residues Trp-149 and Tyr-151 suggests that their side chains reside on the binding face (also termed the (+)- or counterclockwise face) of the alpha-subunit. Asp-152 presents an anionic charge in the vicinity of these aromatic residues. Modification of the aspartate to asparagine (D152N) creates a glycosylation signal (Asn-152-Gly-Ser), and we find, on the basis of altered electrophoretic migration, that glycosylation occurs at this position upon cotransfection of the mutant alpha-subunit with beta-, gamma-, and delta-subunits. Glycosylation results in a reduction in the capacity of the receptor to assemble; this reduction is manifest in the initial step of dimer formation between the alphagamma- and alphadelta-subunits. The alpha-subunit mutant receptor reaching the assembled pentamer exhibits an altered selectivity for certain ligands. Little reduction in alpha-bungarotoxin binding is observed, whereas affinities for agonists and competitive alkaloid antagonists are reduced substantially. Separation of the contributions of charge removal and glycosylation addition shows that both factors affect agonist affinity, with the charge influence being far more predominant. These findings raise the possibility that a component of the coulombic attraction stabilizing the binding of agonists comes from the aspartyl residue at position 152 in the alpha-subunit.

Determination of the chemical mechanism of neurotransmitter receptor-mediated reactions by rapid chemical kinetic methods

When neurotransmitters bind to their specific receptors in the membrane of nerve and muscle cells they induce conformational transitions leading to the formation of open receptor-channels and desensitized receptor forms. A knowledge of the rate and equilibrium constants associated with these transitions is required to (i) relate the mechanism of the receptor-mediated reaction to the resulting changes in transmembrane voltage that trigger signal transmission between neurons, (ii) calculate changes in transmembrane voltage that result from the interaction of diverse excitatory and inhibitory receptors in the same cell, and (iii) understand the mechanism by which receptor function is affected by activators, inhibitors, including clinically important compounds, and diseases of the nervous system. The conformational transitions of interest occur in the millisecond and the sub-millisecond time region. Chemical kinetic techniques for studying reactions mediated by membrane-bound neurotransmitter receptors in cells or vesicles in this time domain were not available. Here we describe the development and use of a laser pulse photolysis technique suitable for chemical kinetic investigations of neurotransmitter receptors in the mu s and ms time region. The type of information that can be obtained is also discussed.

The energetics of ligand binding at catecholamine receptors

One of the key events in the actions of agonists and antagonists is their binding to receptors. Understanding this event is of interest in terms of understanding receptor function but it also has immense practical relevance for the design of drugs. If the ligand-binding process could be understood in detail, including the nature of the interactions made between ligand and receptor, then this could help in the design of more-selective drugs. The interaction of a ligand with its receptor is clearly of importance in determining the specificity of ligand action but ligand-receptor interaction also initiates the processes of signalling that are exhibited in the efficacy of ligand action. Here Philip Strange considers these events for catecholamine receptors, concentrating mostly on dopamine receptors; where necessary the discussion is widened to include other receptor systems.

Activation and cooperative multi-ion block of single nicotinic-acetylcholine channel currents of Ascaris muscle by the tetrahydropyrimidine anthelmintic, morantel

1. We have investigated activation and block, by the tetrahydropyrimidine anthelmintic, morantel, of nicotinic-acetylcholine receptor (AChR) currents in membrane vesicles isolated from somatic muscle cells of the nematode parasite Ascaris suum. Standard single-channel recording techniques were employed. Morantel in the pipette (6 nM to 600 microM), activated single nicotinic AChR currents. 2. Kinetic properties of the main-conductance state of morantel-activated currents were investigated in detail throughout the concentration range, 0.6 microM to 600 microM. Open-time distributions were best fitted by a single exponential. Mean open-times were slightly voltage-dependent, increasing from 0.9 ms at +75 mV to 1.74 ms at -75 mV in the presence of 0.6 microM morantel. At low concentrations, closed-time distributions were best fitted by the sum of two or three exponential components. 3. As the concentration of morantel was increased (100-600 microM), fast-flickering open channel-block was observed at positive potentials, even though morantel, a cation, was only present at the extracellular surface of the membrane. The block rate was dependent on morantel concentration and both block rate and duration of block increased as the potential became less positive. A simple channel-block mechanism did not explain properties of this block. 4. At negative potentials, as the morantel concentration increased, a complex block was observed. With increases in morantel concentration two additional gap components appeared in closed-time distributions: one was short with a duration (approximately 13 ms) independent of morantel concentration; the other was long with a duration that increased with morantel concentration (up to many minutes). In combination, these two components produced a marked reduction in probability of channel opening (Po) with increasing morantel concentration. The relationship between the degrees of block and morantel concentration had a Hill coefficient of 1.6, suggesting the involvement of at least two blocking molecules. The data were analysed by use of a simple sequential double block kinetic model.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

The differential antagonism by bicuculline and SR95531 of pentobarbitone-induced currents in cultured hippocampal neurons

In voltage clamped cultured hippocampal neurons, application of gamma-aminobutyric acid (GABA) or pentobarbitone induced chloride current in a dose-dependent manner. The dose dependence of these agonists were well described by ED50 and Hill coefficients of 14.7 +/- 7 microM and 1.2 +/- 0.5, and 299 +/- 17.3 microM and 1.6 +/- 0.1, for GABA and pentobarbitone, respectively. The effects of two GABAA receptor antagonists, bicuculline and 2-(3-carboxypropyl)-3-amino-6-methoxyphenyl-pyridazinium bromide (SR95531) were evaluated by co-application of increasing concentrations of the antagonists with a fixed equipotent (approximately ED30) dose of GABA or pentobarbitone. Both bicuculline and SR95531 blocked the GABA induced current with ID50 and Hill coefficients of 0.74 +/- 0.07 microM and 0.96 +/- 0.07, and 0.44 +/- 0.02 microM and 1.22 +/- 0.06, respectively. Bicuculline similarly blocked the pentobarbitone induced current with a ID50 and Hill coefficient of 0.69 +/- 0.04 microM and 1.2 +/- 0.1. However, pentobarbitone induced current was poorly blocked by SR95531 retaining 86.5% of current amplitude at a concentration of SR95531, 200 times the IC50 for GABA induced current. Current induced by etomidate, another intravenous general anesthetic with GABAA receptor agonistic property, is likewise resistant to SR95531 blockade. Three-dimensional modeling of bicuculline and SR95531 with alignment of the molecules along the suggested GABA-receptor binding moiety indicates that these two antagonist molecules have distinct steric properties. We suggest that GABA and pentobarbitone act at nearby but non-identical sites on the hippocampal GABAA receptor to open the chloride ionophore, and that these sites can be distinguished by bicuculline and SR95531. This is the first demonstration that the prototypic GABAA site antagonists bicuculline and SR95531 have different effects on currents induced by GABA and pentobarbitone.

Arrest of subunit folding and assembly of nicotinic acetylcholine receptors in cultured muscle cells by dithiothreitol

In this study we have used cultured muscle cells to investigate the role of disulfide bond formation in the sequence of molecular events leading to nicotinic acetylcholine receptor (AChR) assembly and surface expression. We have observed that disulfide bond formation in newly synthesized AChR alpha-subunits occurs 5-20 min after translation and that this modification can be blocked by dithiothreitol (DTT), a membrane-permeant thiol-reducing agent. DTT treatment was found to arrest AChR alpha-subunit conformational maturation, assembly, and appearance on the cell surface, showing that these events are dependent on prior formation of disulfide bonds. Subunits prevented from maturation by the reducing agent do not irreversibly misfold or aggregate, since upon removal of DTT, AChR alpha-subunits undergo formation of disulfide bonds and resume folding, oligomerization, and surface expression. We have previously found that nascent alpha-subunits form transient complexes with the molecular chaperone calnexin immediately after subunit synthesis (Gelman, M.S., Chang, W., Thomas, D. Y., Bergeron, J. J. M., and Prives, J. M. (1995) J. Biol. Chem. 270, 15085-15092) and have now observed that both the formation and the subsequent dissociation of these complexes are unaffected by DTT treatment. Thus, alpha-subunits appear to dissociate from calnexin independently of their undergoing disulfide bond formation and achieving conformational maturation. This finding together with the absence of irreversible misfolding of DTT-arrested alpha-subunits suggests that calnexin may act to prevent misfolding by aiding in the initial folding events and is not an essential participant in the late stages of alpha-subunit maturation.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Characterization of nicotinic receptors in immortalized hippocampal neurons

Nicotinic acetylcholine receptors, particularly nicotinic alpha-bungarotoxin (alpha-BGT) receptors, are present in relatively high concentrations in rat hippocampus. Because of the difficulties encountered in studying receptors using primary cells in culture, especially for biochemical work, we investigated the possibility of using an immortalized cell line from embryonic rat hippocampus (H19-7). RNase protection assays show that alpha 4, alpha 7 and beta 2 neuronal nicotinic receptor subunit mRNAs are present in differentiated but not undifferentiated H19-7 cells, while alpha 2, alpha 3, alpha 5 and beta 3 subunit mRNAs were not detectable under either condition. In line with these results, the present data demonstrate that the H19-7 cells express cell surface nicotinic alpha-BGT binding sites, which were maximal after seven days of differentiation in culture. The receptors were saturable, of high affinity (Kd = 1.30 nM and Bmax = 11.70 fmol/10(5) cells) and had a pharmacological profile similar to that observed for CNS alpha-BGT receptors. On the other hand, although alpha 4 and beta 2 neuronal nicotinic subunit mRNAs were present in differentiated H19-7 cells, no [3H]cytisine binding was observed. Because immortalized cell lines have the advantage that they provide a limitless supply of cells as compared to primary cell cultures, but yet are not malignant in origin, the present results may suggest that the H19-7 immortalized hippocampal cell line represent a useful CNS model system for examining alpha-BGT nicotinic receptors.

Insights into the activation mechanism of rho1 GABA receptors obtained by coexpression of wild type and activation-impaired subunits

To gain insight into the activation mechanism of homomeric ligand-gated receptor-channels, we examined human homomeric rho1 GABA receptors with fewer than the normal number of agonist binding sites. This was accomplished by coexpressing different ratios of wild type and activation-impaired rho1 subunits. Dose-response relations from oocytes coexpressing wild type and mutant subunits were comprised of two components in terms of GABA sensitivity; one 'wild type'-like and the other 'mutant'-like. Applying the binomial hypothesis to subunit coassembly enabled use to correlate these two components of the GABA dose-response relations to the underlying chimaeric receptor subtypes. We demonstrate that the receptors activate near normal provided that they are comprised of at least three wild type subunits. Our data are consistent with five equivalent and independent GABA binding sites of which only three need bind GABA to open the pore. The two additional binding sites may increase the GABA sensitivity of the rho1 receptor and, when bound by agonist, stabilize the open state.

Cation-pi interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp

Cations bind to the pi face of an aromatic structure through a surprisingly strong, non-covalent force termed the cation-pi interaction. The magnitude and generality of the effect have been established by gas-phase measurements and by studies of model receptors in aqueous media. To first order, the interaction can be considered an electrostatic attraction between a positive charge and the quadrupole moment of the aromatic. A great deal of direct and circumstantial evidence indicates that cation-pi interactions are important in a variety of proteins that bind cationic ligands or substrates. In this context, the amino acids phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) can be viewed as polar, yet hydrophobic, residues.

Nuclear magnetic resonance (NMR) analysis of ligand receptor interactions: the cholinergic system--a model

Elucidation of the molecular mechanisms that govern ligand-receptor recognition is essential to the rational design of specific pharmacological reagents. Whereas often the receptor and its binding site are the target of investigation, study of the ligand in its free and bound state can also reveal important information regarding this recognition process. Nuclear magnetic resonance (NMR) spectroscopy can be extremely useful for such studies. In this review, we discuss the attributes of NMR in the study of ligand receptor interactions. The cholinergic receptor and its binding to the neurotransmitter, acetylcholine, and cholinergic antagonists serve as a model system, illustrating the power of ligand analysis by NMR. The results discussed prove that the region of residues alpha 180-205 of the nicotinic acetylcholine receptor are an essential component of the cholinergic binding site and that ligand binding involves a positively charged hydrophobic motif.

Cocaine: mechanism of inhibition of a muscle acetylcholine receptor studied by a laser-pulse photolysis technique

Effects of cocaine on the muscle nicotinic acetylcholine receptor were investigated by using a chemical kinetic technique with a microsecond time resolution. This membrane-bound receptor regulates signal transmission between nerve and muscle cells, initiates muscle contraction, and is inhibited by cocaine, an abused drug. The inhibition mechanism is not well understood because of the lack of chemical kinetic techniques with the appropriate (microsecond) time resolution. Such a technique, utilizing laser-pulse photolysis, was recently developed; by using it the following results were obtained. (i) The apparent cocaine dissociation constant of the closed-channel receptor form is approximately 50 microM. High carbamoylcholine concentration and, therefore, increased concentrations of the open-channel receptor form, decrease receptor affinity for cocaine approximately 6-fold. (ii) The rate of the receptor reaction with cocaine is at least approximately 30-fold slower than the channel-opening rate, resulting in a cocaine-induced decrease in the concentration of open receptor channels without a concomitant decrease in the channel-opening or -closing rates. (iii) The channel-closing rate increases approximately 1.5-fold as the cocaine concentration is increased from 20 to 60 microM but then remains constant as the concentration is increased further. The results are consistent with a mechanism in which cocaine first binds rapidly to a regulatory site of the receptor, which can still form transmembrane channels. Subsequently, a slow step (t1/2 approximately 70 ms) leads to a receptor form that cannot form transmembrane channels, and acetylcholine receptor-mediated signal transmission is, therefore, blocked. Implications for the search for therapeutic agents that alleviate cocaine poisoning are mentioned.

Cloning of the amiloride-sensitive FMRFamide peptide-gated sodium channel

The peptide Phe-Met-Arg-Phe-NH2 (FMRFamide) and structurally related peptides are present both in invertebrate and vertebrate nervous systems. Although they constitute a major class of invertebrate peptide neurotransmitters, the molecular structure of their receptors has not yet been identified. In neurons of the snail Helix aspersa, as well as in Aplysia bursting and motor neurons, FMRFamide induces a fast excitatory depolarizing response due to direct activation of an amiloride-sensitive Na+ channel. We have now isolated a complementary DNA from Helix nervous tissue; when expressed in Xenopus oocytes, it encodes an FMRFamide-activated Na+ channel (FaNaCh) that can be blocked by amiloride. The corresponding protein shares a very low sequence identity with the previously cloned epithelial Na+ channel subunits and Caenorhabditis elegans degenerins, but it displays the same overall structural organization. To our knowledge, this is the first characterization of a peptide-gated ionotropic receptor.

Fourier transform infrared and hydrogen/deuterium exchange reveal an exchange-resistant core of alpha-helical peptide hydrogens in the nicotinic acetylcholine receptor

The structure of the nicotinic acetylcholine receptor (nAChR) has been studied using a novel combination of hydrogen/deuterium exchange and attenuated total reflectance Fourier transform infrared spectroscopy. Fourier transform infrared spectra show marked changes in both the amide I and amide II bands upon exposure of the nAChR to 2H2O. The substantial decrease in intensity of the amide II band reflects the exchange of roughly 30% of the peptide hydrogens within seconds of exposure to 2H2O, 50% after 30 min, 60% after 12 h, and 75% after prolonged exposure for several days at room temperature or lower temperatures. The 30% of peptide hydrogens that exchange within seconds is highly exposed to solvent and likely involved in random and turn conformations, whereas the 25% of exchange-resistant peptide hydrogens is relatively inaccessible to solvent and likely located in the transmembrane domains of the nAChR. Marked changes occur in the amide I contour within seconds of exposure of the nAChR to 2H2O as a result of relatively large downshifts in the frequencies of amide I component bands assigned to turns and random structures. In contrast, only subtle change occur in the amide I contour between 3 min and 12 h after exposure to 2H2O as a result of slight downshifts in the frequencies of alpha-helix and beta-sheet vibrations. It is demonstrated that the time courses and relative magnitudes of the amide I component band shifts can be used both as an aid in the assignment of component bands to specific secondary structures and as a probe of the exchange rates of different types of secondary structures in the nAChR. Significantly, the intensities of the band shifts reflecting the exchange of alpha-helical secondary structures are relatively weak indicating that a large proportion of the 25% exchange resistant peptides adopt an alpha-helical conformation. Conversely, no evidence is found for the existence of a large number of exchange-resistant beta-strands. The exchange kinetics suggest a predominantly alpha-helical secondary structure for the transmembrane domains of the nAChR.

Two subsites in the binding domain of the acetylcholine receptor: an aromatic subsite and a proline subsite

The ligand binding site of the nicotinic acetylcholine receptor (AcChoR) is localized in the alpha-subunit within a domain containing the tandem Cys-192 and -193. By analyzing the binding-site region of AcChoR from animal species that are resistant to alpha-neurotoxins, we have previously shown that four residues in this region, at positions 187, 189, 194, and 197, differ between animals sensitive (e.g., mouse) and resistant (e.g., mongoose and snake) to alpha-bungarotoxin (alpha-BTX). In the present study, we performed site-directed mutagenesis on a fragment of the mongoose AcChoR alpha-subunit (residues 122-205) and exchanged residues 187, 189, 194, and 197, either alone or in combination, with those present in the mouse alpha-subunit sequence. Only the mongoose fragment in which all four residues were mutated to the mouse ones exhibited alpha-BTX binding similar to that of the mouse fragment. The mongoose double mutation in which Leu-194 and His-197 were replaced with proline residues, which are present at these positions in the mouse AcChoR and in all other toxin binders, bound alpha-BTX to approximately 60% of the level of binding exhibited by the mouse fragment. In addition, replacement of either Pro-194 or -197 in the mouse fragment with serine and histidine, respectively, markedly decreased alpha-BTX binding. All other mutations resulted in no or just a small increase in alpha-BTX binding. These results have led us to propose two subsites in the binding domain for alpha-BTX: the proline subsite, which includes Pro-194 and -197 and is critical for alpha-BTX binding, and the aromatic subsite, which includes amino acid residues 187 and 189 and determines the extent of alpha-BTX binding.

Spontaneous opening of the acetylcholine receptor channel in developing muscle cells from normal and dystrophic mice

Single-channel activity was recorded from cell-attached patches on skeletal muscle cells isolated from wild-type mice and from mice carrying the dy or mdx mutations. Spontaneous openings of the nicotinic acetylcholine receptor channel (nAChR) were detected in virtually all recordings from either dy/dy or dy/+ myotubes, but only infrequently from wild-type or mdx myotubes. Spontaneous openings were also present in most recordings from undifferentiated myoblasts from all of the mouse strains studied. The biophysical properties of the spontaneous activity were similar to those of the embryonic form of the nAChR in the presence of acetylcholine (ACh). Examination of the single-channel currents evoked by low concentrations of ACh showed a reduced sensitivity to the agonist in the dystrophic dy and mdx myotubes, but not in wild-type myotubes. The results suggest that alterations in nAChR function are associated with the pathogenesis of muscular dystrophy in the dy mouse.

Electron spin resonance studies of acyl chain motion in reconstituted nicotinic acetylcholine receptor membranes

The electron spin resonance spectra of spin-label positional isomers of stearic acid (n-SASL) incorporated into nicotinic acetylcholine receptors (nAcChoR) reconstituted into dioleoylphosphatidylcholine (DOPC) were deconvoluted into bilayer- and protein-associated components by subtraction under conditions of slow exchange. The selectivity of n-SASL (n = 6, 9, 12, and 14) for the lipid-protein interface of the nAcChoR was threefold greater than that of DOPC and independent of the spin label position. The temperature at which exchange became apparent as judged from lineshape broadening of the mobile lipid component spectrum was dependent upon the position of the spin-label moiety; near the bilayer center, exchange broadening occurred at lower temperatures than it did closer to the lipid headgroup. This suggests that the lipid headgroup region of boundary lipids is relatively fixed, whereas its acyl chain whips on and off the protein with increasing frequency near the bilayer center. Motions on the microsecond time scale were examined by microwave power saturation. Each n-SASL saturated more readily when incorporated into vesicles containing the nAcChoR than when in pure DOPC liposomes. Therefore, lipid mobility is perturbed by the nAcChoR on the microsecond time scale with an apparent magnitude that is relatively modest, probably due to exchange on this time scale.

Role of the endoplasmic reticulum chaperone calnexin in subunit folding and assembly of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (AChR) is a pentameric complex assembled from four different gene products by mechanisms that are inadequately understood. In this study we investigated the role of the endoplasmic reticulum (ER)-resident molecular chaperone calnexin in AChR subunit folding and assembly. We have shown that calnexin interacts with nascent AChR alpha-subunits (AChR-alpha) in muscle cell cultures and in COS cells transfected with mouse AChR-alpha. In chick muscle cells maximal association of labeled alpha-subunits with calnexin was observed immediately after a 15-min pulse with [35S]methionine/cysteine and subsequently declined with a t1/2 of approximately 20 min. The decrease in association with calnexin was concomitant with the folding of the alpha-subunit to achieve conformational maturation shortly before assembly. Brefeldin A did not inhibit AChR subunit assembly or the dissociation of calnexin from the assembling subunits, confirming that the ER is the site of AChR assembly and that calnexin dissociation is not affected under conditions in which the exit of assembled AChR from the ER is blocked. These results indicate that calnexin participates directly in the molecular events that lead to AChR assembly.

Allosteric regulation of [3H]vinblastine binding to P-glycoprotein of MCF-7 ADR cells by dexniguldipine

Plasma membranes were prepared from the P-glycoprotein expressing human breast cancer cell line MCF-7 ADR. [3H]Vinblastine bound to these membranes saturably with a Bmax of 24 pmol/mg of protein and KD of 23 nM. In contrast, membranes from the parent cells MCF-7 WT, which do not express P-glycoprotein, did not bind [3H]vinblastine with high affinity. Cytotoxics known to be transported by P-glycoprotein inhibited the binding of [3H]vinblastine, as did multidrug reversing agents including the 1,4-dihydropyridine, dexniguldipine-HCl (Ki, 15 nM). In dissociation kinetic experiments, dexniguldipine-HCl accelerated the dissociation of [3H]vinblastine from P-glycoprotein, indicating a negative heterotropic allosteric mechanism of action through a drug binding site distinct from that of vinblastine. Other 1,4-dihydropyridines tested also accelerated [3H]vinblastine dissociation from P-glycoprotein, however, multidrug reversing drugs of different chemical classes, including quinidine, verapamil and cyclosporin A did not. These results suggest that P-glycoprotein of MCF-7 ADR cell membranes possesses at least two drug acceptor sites which are allosterically coupled: receptor site-1 which binds vinca alkaloids, and receptor site-2 which binds 1,4-dihydropyridines such as dexniguldipine-HCl, which had the highest affinity of the tested derivatives.

Additive effect of ADP and CGRP in modulation of the acetylcholine receptor channel in Xenopus embryonic myocytes

1. We have previously shown that the activation of either protein kinase A (PKA) or protein kinase C (PKC) enhanced the responses of muscle membrane to acetylcholine (ACh) by increasing the mean open time of embryonic-type ACh channels in Xenopus cultured myocytes. In the present study, we further investigated the interaction between these two kinases in the modulation of ACh channels by using the receptor ligands, adenosine diphosphate (ADP) and calcitonin gene-related peptide (CGRP) which selectively activate PKC and PKA, respectively. 2. ADP concentration-dependently increased the mean open time of embryonic-type ACh channels and 0.3 mM ADP is sufficient to achieve the maximal potentiating effect. alpha, beta-Methylene ATP and PMA (phorbol 12-myristate 13-acetate) but not adenosine, AMP, dibutyryl cyclic GMP have similar potentiating action. 3. Suramin (0.3 mM) pretreatment abolished the potentiating effect of ADP but left that of PMA unchanged. 4. CGRP increased the mean open time of embryonic-type ACh channels in a concentration-dependent manner and 1 microM CGRP produced the maximal effect. 5. The maximal effects of both ADP (0.3 mM) and CGRP (1 microM) in the prolongation of mean open time of ACh channels were additive. 6. These results suggest that the modulation of embryonic-type ACh channels by the endogenously released ligands via the activation of PKA and PKC is additive and possibly different sites of ACh channels may be involved in the potentiation effect of either PKC or PKA.

Binding of monoclonal antibodies against the carboxyl terminal segment of the nicotinic receptor delta subunit suggests an unusual transmembrane disposition of this sequence region

Monoclonal antibodies (mAbs) specific for the carboxyl terminal region of the delta subunit of Torpedo nicotinic acetylcholine receptor (AChR), derived from mice immunized with AChR or a synthetic carboxyl terminal sequence of the delta subunit (C delta-mAbs), were used to determine the transmembrane disposition of their epitope(s) by immunoelectron microscopy, using AChR-rich postsynaptic membrane fragments from Torpedo electroplax. Some C delta-mAbs recognized only the cytoplasmic side of the membranes, some both sides to a similar extent, and others bound mostly, but not exclusively, to the cytoplasmic side. Binding of C delta-mAbs to the membranes was specifically blocked by synthetic peptides containing the carboxyl terminal region of the delta subunit. Control anti-AChR mAbs specific for the alpha or the delta subunits, whose epitopes have known transmembrane topology, uniquely recognized the expected side of the postsynaptic membrane. Residues involved in C delta-mAb binding were identified using single residue substituted peptide analogues of the sequence delta 481-501. All C delta-mAbs recognized epitopes within the same sequence segment, delta 485-493, at the carboxyl terminal of the AChR delta subunit. These results suggest that the delta subunit of the AChR might have alternative conformations, leading to exposure of the same sequence region on the extracellular or the cytoplasmic surface. Several Pro residues are present in this region. The alternative cis or trans conformation of one or more of them might result in different folding patterns of the carboxyl terminal sequence of the delta subunit, as described for a viral protein [Liddington, R. C., Yan, Y., Moulai, J., Sahli, R., Benjamin, T. L., & Harrison, S. C. (1991) Nature 354, 278-284.

Diversity and patterns of regulation of nicotinic receptor subtypes

In our studies we explored the functional relevance of nAChR diversity, in part from the perspective of nAChR as ideal targets for regulatory influences, including those mediated via actions of ligands at other "interacting" receptors. We explored possible mechanisms for nAChR regulation and roles played by nAChR subtype and subunit diversity in those processes. We showed that regulatory factors can influence nAChR numbers at transcriptional and posttranscriptional levels and can affect nAChR function and subcellular distribution. We also demonstrated that nAChR expression can be influenced (1) by nicotinic ligands, (2) by second messengers, (3) by growth factors, (4) by agents targeting the nucleus, and (5) by agents targeting the cytoskeleton. We found common effects of some regulatory influences on more than one nAChR subtype, and we found instances where regulatory influences differ for different cell and nAChR types. Even from the very limited number of these initial studies, it is evident that nAChR subunit and subtype diversity, which alone can provide diversity in nAChR functions, localization, and ligand sensitivity, dovetails with diversity in cellular signaling mechanisms that can affect nAChR expression to amplify the potential functional plasticity of cholinoceptive cells. As examples, we discussed potential roles for nAChR diversity and regulatory plasticity in synapse remodeling and in changes in neuronal circuit conditions. These examples illustrate how nAChR diversity could play important roles in the regulation of nervous system function.

Phosphorylation of the nicotinic acetylcholine receptor by protein tyrosine kinases

Most neurotransmitter receptors examined to date are either regulated by phosphorylation or contain consensus sequences for phosphorylation by protein kinases. The nicotinic acetylcholine receptor (AChR), which mediates depolarization at the neuromuscular junction, has served as a model for the study of the structure, function, and regulation of ligand-gated ion channels. The AChR is phosphorylated by protein kinase A, protein kinase C, and an unidentified protein tyrosine kinase. Tyrosine phosphorylation of the AChR is correlated with a modulation of the rate of receptor desensitization and is associated with AChR clustering. We showed that agrin, a neuronally derived extracellular matrix protein, induces AChR clustering and tyrosine phosphorylation. In addition, we identified two protein tyrosine kinases, Fyn and Fyk, that appear to be involved in the regulation of synaptic transmission at the neuromuscular junction by phosphorylating the AChR. The two kinases are highly expressed in Torpedo electric organ, a tissue enriched in synaptic components including the AChR. As demonstrated by coimmunoprecipitation, Fyn and Fyk associate with the AChR. Furthermore, the AChR is phosphorylated in Fyn and Fyk immunoprecipitates. We investigated the molecular basis for the association of the AChR with Fyn and Fyk using fusion proteins derived from the kinases. The AChR bound specifically to the SH2 domain fusion proteins of Fyn and Fyk. The association of the AChR with the SH2 domains is dependent on the state of AChR tyrosine phosphorylation and is mediated by the delta subunit of the receptor. These data provide evidence that the protein tyrosine kinases Fyn and Fyk may act to phosphorylate the AChR in vivo.

Snake venom toxins, unlike smaller antagonists, appear to stabilize a resting state conformation of the nicotinic acetylcholine receptor

Previous studies have shown that the pattern and degree of 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID) photoincorporation into the nicotinic acetylcholine receptor (nAChR) can be used as a sensitive measure of nAChR conformation. Upon desensitization by prolonged exposure to agonists, certain drugs and detergents, or reconstitution into desensitizing lipids, the levels of [125I]TID incorporation into the subunits of the nAChR are dramatically reduced. In this study, we characterized the effects of the snake venom proteins alpha-bungarotoxin and alpha-cobrotoxin, as well as the smaller antagonists tubocurarine and gallamine, on [125I]TID incorporation into the subunits of both partially-purified nAChR in native lipids, or affinity-purified nAChR reconstituted into different combinations of lipids. Unlike all other compounds previously tested, alpha-bungarotoxin and alpha-cobrotoxin reproducibly increased the level of [125I]TID incorporation into all four subunits of nAChR reconstituted into dioleoylphosphatidylcholine, dioleoylphosphatidic acid and cholesterol. Gallamine had little or no effect on [125I]TID incorporation at any concentration tested (0.1 microM-5 mM). Tubocurarine had no effect on [125I]TID incorporation at low concentrations, but at higher concentrations reduced the level of [125I]TID labeling. The snake venom proteins may shift the population of nAChR, which exists as a mixture of resting state and desensitized conformations, entirely to the resting state. However, the binding of the snake venom toxins does not appear sufficient to induce the resting state conformation in nAChR which have been desensitized by other means, such as solubilization in desensitizing detergents or reconstitution in densitizing lipids.

Nicotinic receptor binding site probed with unnatural amino acid incorporation in intact cells

The nonsense codon suppression method for unnatural amino acid incorporation has been applied to intact cells and combined with electrophysiological analysis to probe structure-function relations in the nicotinic acetylcholine receptor. Functional receptors were expressed in Xenopus oocytes when tyrosine and phenylalanine derivatives were incorporated at positions 93, 190, and 198 in the binding site of the alpha subunit. Subtle changes in the structure of an individual side chain produced readily detectable changes in the function of this large channel protein. At each position, distinct features of side chain structure dominated the dose-response relation, probably by governing the agonist-receptor binding.

Transcriptional analysis of acetylcholine receptor alpha 3 gene promoter motifs that bind Sp1 and AP2

In this study, we performed an analysis of the neuronal nicotinic acetylcholine receptor alpha 3 subunit gene promoter region, -238/+47, to identify cis and trans elements that are important for basal activity in PC12 cells. Sequence analyses of the alpha 3 promoter and footprint assays revealed an Sp1 binding site between -79 and -57 (termed the alpha 3 GA motif) and an AP2 binding site between -30 and -7. Using mobility shift analysis, we found that PC12 cell extracts contain proteins that specifically bind to the alpha 3 GA motif and are immunologically related to Sp1. Mutation of the alpha 3 GA motif, which prevented binding of Sp1, resulted in a 75% decrease in promoter activity. Mutation of the AP2 site resulted in only a minor loss of promoter activity, which is consistent with the lack of AP2 binding activity in PC12 extracts. In Drosophila Schneider line 2 (S2) cell cotransfection assays, Sp1 activated the alpha 3 promoter in a GA motif-dependent manner. Furthermore, multimerization of the GA motif upstream of the beta-globin TATA box conferred Sp1 responsiveness. Our results indicate that Sp1 can activate transcription through direct interaction with the alpha 3 GA motif and that this motif plays a major role in alpha 3 promoter basal activity in PC12 cells.

A mutated acetylcholine receptor subunit causes neuronal degeneration in C. elegans

Neurotoxicity through abnormal activation of membrane channels is a potential cause of neurodegenerative disease. Here we show that a gain-of-function mutation, deg.3(u662), leads to the degeneration of a small set of neurons in the nematode C. elegans. The deg.3 gene encodes a nicotinic acetylcholine receptor alpha subunit, which in the region of transmembrane domain II is most similar to the neuronal alpha 7 subunits from rat and chicken. The u662 mutation changes a residue in the second transmembrane domain, the domain thought to form the channel pore. A similar change in the equivalent amino acid in the chick protein produces channels that desensitize slowly. Channel hyperactivity may underlie the degenerations seen in the C. elegans deg.3(u662) mutants, since antagonists of nicotinic acetylcholine receptors suppress the deg-3(u662) mutant phenotypes.

Abnormal avoidance learning in mice lacking functional high-affinity nicotine receptor in the brain

Nicotine affects many aspects of behaviour including learning and memory through its interaction with neuronal nicotinic acetylcholine receptors (nAChR). Functional nAChRs are pentameric proteins containing at least one type of alpha-subunit and one type of beta-subunit. The involvement of a particular neuronal nicotinic subunit in pharmacology and behaviour was examined using gene targeting to mutate beta 2, the most widely expressed nAChR subunit in the central nervous system. We report here that high-affinity binding sites for nicotine are absent from the brains of mice homozygous for the beta 2-subunit mutation. Further, electrophysiological recording from brain slices reveals that thalamic neurons from these mice do not respond to nicotine application. Finally, behavioural tests demonstrate that nicotine no longer augments the performance of beta 2-1- mice on passive avoidance, a test of associative memory. Paradoxically, mutant mice are able to perform better than their non-mutant siblings on this task.

Characterization of the nicotinic acetylcholine receptor beta 3 gene. Its regulation within the avian nervous system is effected by a promoter 143 base pairs in length

Genomic and cDNA clones encoding the chicken neuronal nicotinic acetylcholine receptor beta 3 subunit were isolated and sequenced. The beta 3 gene consists of six protein-encoding exons and the deduced protein has the structural features found in all other members of the neuronal nicotinic acetylcholine receptor subunit family. Although they are undetectable in most brain compartments, beta 3 mRNAs are relatively abundant in the developing retina and in the trigeminal ganglion. In situ hybridization and immunohistochemical analysis demonstrated that in retina, beta 3 transcripts and protein are confined to subpopulations of cells in the inner nuclear and ganglion cell layers. Beta 3 is expressed in the proximal and distal regions of the developing trigeminal ganglion, i.e. in both placode- and neural crest-derived neurons. Transient transfection assays in cells freshly dissociated from selected regions of the central nervous system at different developmental stages allowed the identification of genetic elements involved in the neuronal-selective expression of the beta 3 gene. A promoter fragment 143 base pairs in length and containing TATA, CAAT, and other consensus sequences is sufficient to restrict reporter gene expression to a subpopulation of retinal neurons. This promoter is totally inactive upon transfection into neuronal and non-neuronal cells from other regions of the central nervous system.

Labeling studies of photolabile philanthotoxins with nicotinic acetylcholine receptors: mode of interaction between toxin and receptor

BACKGROUND

The nicotinic acetylcholine receptors (nAChRs) and glutamate receptors are ligand-gated cation channels composed of five separate polypeptide chains. A 43 kDa protein of unknown function is noncovalently associated with the cytoplasmic side of nAChR in vivo. The venoms of many wasps and spiders contain toxins that block the activity of these channels. Philanthotoxin-433 (PhTX-433) is a non-competitive channel blocker found in the venom of the wasp Philanthus. We have used a photolabile derivative to investigate how PhTX-433 interacts with nAChRs.

RESULTS

A radiolabeled PhTX analog, containing a photolabile group substituted on one of its aromatic rings, photocrosslinked to all five subunits (alpha, alpha 1, beta, gamma, delta) of purified nAChR in the absence of the 43 kDa protein. In the presence of the 43 kDa protein, the alpha subunit was preferentially labeled. Proteolysis of the receptor after crosslinking indicated that the hydrophobic end (head) of the PhTx-433 analog bound to the cytoplasmic loop(s) of the alpha-subunit. Binding is inhibited by other non-competitive channel blockers such as the related polyamine-amide toxins from spiders and chlorpromazine.

CONCLUSIONS

These results, coupled with previous structure/activity studies, lead to a putative model of the binding of PhTx and related polyamine toxins to nAChRs in vitro. The 43 kDa protein appears to influence the orientation of toxin binding. Further binding studies are necessary to confirm the model and to define how toxins enter the receptor and how they are oriented within it. A precise understanding of ligand/receptor interaction is crucial for the design of drugs specific for a particular subtype of receptor.

Dexniguldipine-HCl is a potent allosteric inhibitor of [3H]vinblastine binding to P-glycoprotein of CCRF ADR 5000 cells

Cell membranes were prepared from the multidrug resistant, P-glycoprotein expressing human lymphoblastoid cell line CCRF-ADR 5000. The P-glycoprotein of these membranes possessed high affinity binding sites for [3H]vinblastine, with a Kd of 8 +/- 2 nM and Bmax of 17 +/- 8 pmol/mg of protein. The binding of [3H]vinblastine to P-glycoprotein was not ATP-dependent, and was inhibited by cytotoxic drugs with the following potency order; vincristine > doxorubicin > etoposide. The 1,4-dihydropyridine and multidrug resistance reversing agent, dexniguldipine-HCl, inhibited binding with a Ki value of 37 nM. The multidrug resistance reversing agent cyclosporin A, and the cytotoxics doxorubicin and etoposide did not alter the kinetics of [3H]vinblastine dissociation from P-glycoprotein; however, the 1,4-dihydropyridines dexniguldipine-HCl and nicardipine accelerated dissociation of [3H]vinblastine. These data suggest that P-glycoprotein possesses at least two allosterically coupled drug acceptor sites; receptor site 1 which binds vinblastine, doxorubucin, etoposide and cyclosporin A, and receptor site 2 which binds dexniguldipine-HCl and other 1,4-dihydropyridines.