Location of a delta-subunit region determining ion transport through the acetylcholine receptor channel

Inhibition of the nicotinic acetylcholine receptor by barbiturates and by procaine: do they act at different sites?

1. The effects of three barbiturates and the local anesthetic procaine on the ion channel function of mouse nicotinic acetylcholine receptor (nAChR) muscle subtype expressed in Xenopus laevis oocytes were examined by whole-cell voltage-clamp technique. 2. A concentration-response curve for the specific nicotinic agonist dimethylphenylpiperazinium iodide (DMPP) was first determined. This agonist produced increasing whole-cell currents up to a concentration of 100 microM (EC50 = 13 microM), then decreased responses at higher concentrations. 3. The barbiturates (amobarbital, secobarbital, pentobarbital) and procaine produced reversible inhibition of DMPP-induced currents at clinically used concentrations. The two classes of drugs differed in the voltage dependence of the inhibition: procaine-induced inhibition was increased at more negative transmembrane holding potentials (-90 vs. -45 mV); whereas amobarbital-induced inhibition did not vary at different transmembrane potentials. 4. Mutant forms of the nAChR, containing single amino acid changes in the M2 regions of alpha and beta subunits, showed increased sensitivity to procaine but no change in sensitivity to amobarbital-induced inhibition. 5. These electrophysiologic studies provide further evidence that barbiturates and local anesthetics produce inhibition of the nAChR at different sites.

Block by calcium of ATP-activated channels in pheochromocytoma cells

We have investigated the effects of Ca2+ on Na+ influx through ATP-activated channels in pheochromocytoma PC12 cells using single channel current recordings. Under cell-attached patch-clamp conditions with 150 mM Na+ and 2 mM Ca2+ in the pipette, the unitary current activity showed an open level of about -4.3 pA at -150 mV. The channel opening was interrupted by flickery noise as well as occasional transition to a subconducting state of about -1.7 pA at -150 mV. The open level was decreased with increased external Ca2+, suggesting that external Ca2+ blocks Na+ permeation. We assessed the block by Ca2+ as the mean amplitude obtained with heavy filtration according to Pietrobon et al. (Pietrobon, D., B. Prod'hom, and P. Hess, 1989. J. Gen. Physiol. 94:1-21). The block was concentration dependent with a Hill coefficient of 1 and a half-maximal concentration of approximately 6 mM. A similar block was observed with other divalent cations, and the order of potency was Cd2+ > Mn2+ > Mg2+ not equal to Ca2+ > Ba2+. High Ca2+, Mg2+ and Ba2+ did not block completely, probably because they can carry current in the channel. The block by external Ca2+ did not exhibit voltage dependence between -100 and -210 mV. In the inside-out patch-clamp configuration, the amplitude of inward channel current obtained with 150 mM external Na+ was reduced by increased internal Ca2+. The reduction was observed at lower concentrations than that by external Ca2+. Internal Ba2+ and Cd2+ induced similar reduction in current amplitude. This inhibitory effect of internal Ca2+ was voltage dependent; the inhibition was relieved with hyperpolarization. The results suggest that both external and internal Ca2+ can block Na+ influx through the ATP-activated channel. A simple one-binding site model with symmetric energy barriers is not sufficient to explain the Ca2+ block from both sides.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Excess divalent cations activate Ca(2+)-mobilizing receptors in pancreatic acinar cells

In single, enzymatically dissociated, rat pancreatic acinar cells, external application of excess divalent cations (Ca2+, Sr2+, Ba2+, Ni2+ and Mg2+ over 50 mM) induced Ca(2+)-dependent current responses monitored with the whole-cell recording technique. Inclusion of either EGTA, heparin or GDP[beta S] in the internal solution or treatment of acinar cells with a phorbol ester abolished the divalent-cation-induced responses. In contrast, internal inositol trisphosphate (InsP3) or GTP[gamma S] potentiated the responses. The results indicate that excess divalent cations activate membrane surface receptors or receptor/effector complexes, thereby inducing InsP3-mediated Ca2+ mobilization. The mechanism may be due to modulation of the receptors by changes in electrical profile through indirect action of divalent cations on membrane surface charges, i.e. neutralization of anionic charges. This proposal was supported by the evidence that the trivalent cation, La3+, and the polyvalent cation, protamine, both at much lower concentrations, could induce Ca(2+)-dependent responses, which were abolished by internal application of heparin, GDP[ beta S] or a high concentration of EGTA or by protein kinase C activation with a phorbol ester.

Acetylcholine receptor/channel molecules of insects

Acetylcholine-gated ion channels of the nicotinic type are abundant in the nervous system of insects. The channels are permeable to Na+, K+ and probably Ca(2+), and unlike most vertebrate neuronal nicotinic acetylcholine receptors the receptor/channel molecule is blocked by alpha-bungarotoxin (alpha-Bgt). Such alpha-Bgt-sensitive receptors are present at synapses and on cell bodies of insect neurones. Single channel recordings have shown the existence of multiple conductances of nAChRs. Studies on several different insect preparations have provided evidence for more than one open state and several closed states of insect nAChRs. Functional insect nAChR channels have now been investigated in situ, following reconstitution of a purified protein in bilayers, and as a result of expressing in Xenopus oocytes messenger RNA encoding receptor subunits.

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

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

Chimeric rat Na,K-ATPase alpha 1/alpha 3* isoforms. Analysis of the structural basis for differences in Na+ requirements in the alpha 1 and alpha 3* isoforms

Na,K-ATPase molecules containing the alpha 1, alpha 2*, and alpha 3* isoforms expressed in HeLa cells exhibit a two- to threefold difference in their K0.5 for Na+ (alpha 1 = alpha 2* < alpha 3*). To investigate the structural basis for this difference, chimeric alpha 1/alpha 3* isoform cDNAs were constructed and expressed in HeLa cells. Na,K-ATPase containing each alpha isoform chimera was analyzed for its Na+ dependence properties. Results of these experiments do not reveal a region in the alpha 1 or alpha 3* isoform that is clearly responsible for the apparent affinity for Na+. It is possible that molecular interactions involving amino acids that span virtually the entire Na,K-ATPase molecule contribute to the determination of this parameter.

Structure and function of inhibitory glycine receptors

Ion transport by peptide channels has been the major theme in the work of the late P. Läuger. His theoretical and experimental approaches provided the basis for a deeper understanding of pore-mediated ion permeation through biological membranes. This review on a ligand–gated ion channel protein from the mammalian brain is dedicated to the memory of this outstanding scientist.

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

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

Acetylcholine receptor channel structure probed in cysteine-substitution mutants

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

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

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

Asymmetry of the rat acetylcholine receptor subunits in the narrow region of the pore

The acetylcholine receptor (AChR) channel is a pentameric protein in which every subunit contributes to the conducting parts of the pore. Recent studies of rat nicotinic AChR channels mutated in the alpha-subunit revealed that a threonine residue (alpha T264) in the transmembrane segment M2 forms part of the narrow region of the channel. We have mutated the residues at homologous positions in the beta-, gamma-, and delta-subunits and measured the resulting change in channel conductance. For all subunits the conductance is inversely related to the volume of the amino acid residue, suggesting that they form part of the channel narrow region. Exchanges of residues between subunits do not alter the conductance, suggesting a ring-like structure formed by homologous amino acids. To investigate the relative contribution of amino acid residues at these positions in determining the channel conductance, receptors carrying the same amino acid in each subunit in the narrow region were constructed. They form functional channels in which the conductance is inversely related to the volume of the amino acids in the narrow region. Channels in which the narrow region is formed by four serines and one valine have the same conductance if the valine is located in the alpha-, beta-, or gamma-subunits, but it is smaller if the valine is located in the delta-subunit. The results suggest a structural asymmetry of the AChR channel in its narrow region formed by the hydroxylated amino acids of alpha-, gamma- and delta-subunits, where the delta-subunit serine is a main determinant of the channel conductance.

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

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

Neuromuscular function and polymorphism of the acetylcholine receptor gamma gene

Examination of inbred and wild mice has shown that the gene coding for the acetylcholine receptor gamma subunit is polymorphic and the polymorphism correlates with differences in neuromuscular function tested by two different methods. Polymorphism of the AChR gamma gene was demonstrated after digestion of genomic DNA with Pstl and Xbal. These two restriction enzymes demonstrated RFLP patterns specific for C57Bl/6J (PSXS) and C3H/HeJ (PRXR) mice, respectively. Examination of F1 (C3H/HeJ x C57Bl/6J) and F2 hybrid populations, and other murine inbred strains, showed the inheritance and strain specificity of the RFLPs. Testing wild mice demonstrated that the PSXS form of the AChR gamma gene is the most common in the wild. The AChR gamma and AChR delta subunits are closely linked and carried on the same chromosome as several contractile proteins. Because these genes are essential for correct neuromuscular development and activity, we investigated the possibility that the observed AChR gamma polymorphisms mark a haplotype which correlates with variations in neuromuscular function. Analysis of exercise times showed a correlation between AChR gamma polymorphism and neuromuscular function. To our knowledge, this is the first description of AChR polymorphism cosegregating with variations in neuromuscular function.

Threonine in the selectivity filter of the acetylcholine receptor channel

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

Functional properties of acetylcholine receptors coexpressed with the 43K protein in heterologous cell systems

The nicotinic acetylcholine (ACh) receptor is an integral membrane protein which mediates synaptic transmission at the skeletal neuromuscular junction. A key event in the development of the neuromuscular junction is the formation of high density aggregates of ACh receptors in the postsynaptic membrane. Receptor clustering has been attributed, in part, to their association with a peripheral membrane protein of Mr 43,000 (43K protein). We have addressed whether the association of the 43K protein can alter the single channel properties of the ACh receptor, and thus influence neuromuscular transmission at developing synapses, by expressing ACh receptors with and without the 43K protein in heterologous expression systems. We found that coexpression of the 43K protein with the receptor did not significantly alter either its single channel conductance or its mean channel open time. This was true in oocytes and also in COS cells where it was possible to localize 43K-induced clusters by fluorescence microscopy and to record from those clustered receptors. These data are in agreement with previous single channel studies which have shown that the properties of diffusely distributed and clustered receptors in native muscle cells from both mice and Xenopus do not differ.

Expression of fusion proteins of the nicotinic acetylcholine receptor from mammalian muscle identifies the membrane-spanning regions in the alpha and delta subunits

We have investigated the topology of the alpha and delta subunits of the nicotinic acetylcholine receptor (AChR) from mammalian muscle synthesized in an in vitro translation system supplemented with dog pancreatic microsomes. Fusion proteins were expressed in which a carboxy-terminal fragment of bovine prolactin was attached downstream of each of the major putative transmembrane domains, M1-M4 and MA, in the AChR subunits. The orientation of the prolactin domain relative to the microsomal membrane was then determined for each protein by a proteolysis protection assay. Since the prolactin domain contains no information which either directs or prevents its translocation, its transmembrane orientation depends solely on sequences within the AChR subunit portion of the fusion protein. When subunit-prolactin fusion proteins with the prolactin domain fused after either M2 or M4 were tested, prolactin-immunoreactive peptides that were larger than the prolactin domain itself were recovered. No prolactin-immunoreactive peptides were recovered after proteolysis of fusion proteins containing prolactin fused after M1, M3, or MA. These results support a model of AChR subunit topology in which M1-M4, but not MA, are transmembrane domains and the carboxy terminus is extracellular.

Voltage-dependent block by magnesium of neuronal nicotinic acetylcholine receptor channels in rat phaeochromocytoma cells

1. The effects of Mg2+ on the single-channel conductance of neuronal nicotinic acetylcholine receptors were examined using receptors expressed by the rat phaeochromocytoma cell line, PC12. PC12 cells express at least three conductance classes of channels that are activated by acetylcholine, the largest conductance class being the most prevalent. This receptor channel is blocked by intracellular and extracellular Mg2+. 2. The effects of Mg2+ are asymmetrical; at a given concentration, internal Mg2+ is more effective at blocking outward currents than external Mg2+ is at blocking inward currents. Receptor channels are blocked at concentrations of Mg2+ that are low compared to the concentration of the main permeant cation, Na+, and the block is voltage dependent. 3. The block by Mg2+ is not complete as Mg2+ can permeate the channel. With 80 mM-extracellular Mg2+ (no extracellular Na+), the channel has an inward slope conductance of 2.9 pS. 4. The block by extracellular Mg2+ can be described by a one site, two barrier model for the channel which includes a negative surface charge on the external surface of the membrane. The parameters of the model place the binding site for Mg2+ at 52% of the membrane field from the outside with an apparent dissociation constant of 14 mM. However, the same parameters cannot describe the block by intracellular Mg2+. The deviations from the model suggest that the receptor channel may have more than one binding site for Mg2+.

Mutations in the channel domain alter desensitization of a neuronal nicotinic receptor

A variety of ligand-gated ion channels undergo a fast activation process after the rapid application of agonist and also a slower transition towards desensitized or inactivated closed channel states when exposure to agonist is prolonged. Desensitization involves at least two distinct closed states in the acetylcholine receptor, each with an affinity for agonists higher than those of the resting or active conformations. Here we investigate how structural elements could be involved in the desensitization of the acetylcholine-gated ion channel from the chick brain alpha-bungarotoxin sensitive homo-oligomeric alpha 7 receptor, using site-directed mutagenesis and expression in Xenopus oocytes. Mutations of the highly conserved leucine 247 residue from the uncharged MII segment of alpha 7 suppress inhibition by the open-channel blocker QX-222, indicating that this residue, like others from MII, faces the lumen of the channel. But, unexpectedly, the same mutations decrease the rate of desensitization of the response, increase the apparent affinity for acetylcholine and abolish current rectification. Moreover, unlike wild-type alpha 7, which has channels with a single conductance level, the leucine-to-threonine mutant has an additional conducting state active at low acetylcholine concentrations. It is possible that mutation of Leu 247 renders conductive one of the high-affinity desensitized states of the receptor.

Primary structure and functional expression of the 5HT3 receptor, a serotonin-gated ion channel

The neurotransmitter serotonin (5HT) activates a variety of second messenger signaling systems and through them indirectly regulates the function of ion channels. Serotonin also activates ion channels directly, suggesting that it may also mediate rapid, excitatory responses. A complementary DNA clone containing the coding sequence of one of these rapidly responding channels, a 5HT3 subtype of the serotonin receptor, has been isolated by screening a neuroblastoma expression library for functional expression of serotonin-gated currents in Xenopus oocytes. The predicted protein product has many of the features shared by other members of the ligand-gated ion channel family. The pharmacological and electrophysiological characteristics of the cloned receptor are largely consistent with the properties of native 5HT3 receptors. Messenger RNA encoding this receptor is found in the brain, spinal cord, and heart. This receptor defines a new class of excitatory ligand-gated channels.

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

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

Acetylcholine receptor channel gating and conductance involve extracellular disulfide bond(s)

Disulfide bonds are critical determinants of the function of the acetylcholine receptor at the vertebrate neuromuscular junction. In the present study, the role of these bonds in acetylcholine receptor channel gating and conductance was investigated at the single channel level. Disulfide bond reducing agents decreased the single channel conductance of both ligand-gated and spontaneously opening acetylcholine receptor channels, indicating that the observed decrease in conductance is not due to blockade of the channel lumen by agonist molecules. In addition, the reducing agents increased the opening frequency of both liganded and unliganded acetylcholine receptor channels. Use of inside-out patches and both membrane permeant and impermeant reducing agents demonstrated that the disulfide bonds involved are all extracellular. These findings indicate that both channel gating and conductance involve conformational changes in extracellular regions of the acetylcholine receptor.

Actions of n-alcohols on nicotinic acetylcholine receptor channels in cultured rat myotubes

1. The actions of the n-alcohols from pentanol to dodecanol on nicotinic acetylcholine receptor (nAChR) channels were investigated by recording single ACh-activated channel activity from inside-out membrane patches isolated from cultured rat myotubes. Alcohols were applied to the cytoplasmic side of the membrane; aqueous concentrations ranged from 11.7 mM-pentanol to 0.02 mM-dodecanol. 2. The intermediate-chain alcohols (pentanol to octanol) caused channel currents to fluctuate between the fully open and closed state level so that openings occurred in bursts interrupted by brief gaps. Closed time distributions were fitted well with two exponential components, the fast component representing the closures within a burst. The number of gaps within a burst was dependent on alcohol concentration whereas gap duration was independent of concentration but increased with increasing chain length of the alcohol up to octanol. 3. Nonanol and decanol reduced the mean duration of bursts of openings but did not cause an increase in the number of short closed intervals within a burst. Beyond decanol there was a decline in the ability of the n-alcohols to affect channel function. A saturated solution of undecanol (0.07 mM) reduced the mean open time by 33 +/- 17%, whereas a saturated solution of dodecanol had no significant effect. 4. The current integral per burst was reduced by all the n-alcohols between pentanol and undecanol. The IC50S were as follows: hexanol, 0.53 +/- 0.14 mM; heptanol, 0.097 +/- 0.02 mM; octanol, 0.04 mM and nonanol, 0.16 +/- 0.035 mM. 5. The results were analysed in terms of an open channel block model with a long-lived closed-blocked state beyond the blocked state. Over the range of concentrations tested this describes the effects of all the n-alcohols (C5 to C12) on channel gating reasonably well. 6. Blocking rate constants (k+B) for pentanol through to nonanol were calculated to be between 2.8 and 5.7 X 10(6) M-1 S-1. These values are based on the assumption that the concentration of the alcohols at their site(s) of action was equal to the aqueous concentration applied to the membrane. 7. Equilibrium dissociation constants (KD), calculated from the blocking and unblocking rate constants (KD = k-B/k+B), decreased with increasing chain length from 8 mM for pentanol to 0.15 mM for octanol. The standard free energy per methylene group for adsorption to the site of action was calculated to be about -3.3 kJ mol-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Actions of n-alcohols on nicotinic acetylcholine receptor ion channels in cultured rat muscle cells

The effects of the n-alcohols from pentanol to dodecanol on nAChR channel function were resolved at the single channel level. ACh-activated channel activity was recorded from isolated membrane patches using the patch clamp method. The intermediate-chain alcohols (C5-C8) had two main effects: (1) They caused channel openings to be interrupted by brief shut or blocked periods, the duration of which was dependent on chain length of the alcohol but independent of concentration. (2) They caused a reduction in the duration of bursts of openings. The long-chain alcohols (C9-C11) produced only the second effect, and there was a decline in activity beyond undecanol. Results were consistent with a mechanism of channel blockade and were analyzed in terms of an open channel block model with a long-lived closed-blocked state beyond the blocked state. Affinity for the binding site increased with chain length up to octanol. The standard free energy per methylene group for adsorption to the site was calculated to be -3.3 kJ/mol, indicating the very hydrophobic nature of the site.

Single-channel properties of mouse-Torpedo acetylcholine receptor hybrids expressed in Xenopus oocytes

This report analyzes the contribution of individual nicotinic acetylcholine receptor (AChR) subunits to the single-channel properties of the AChR ion channel. By in vitro synthesis of mRNA from cDNA clones encoding each AChR subunit (alpha, beta, gamma, and delta) from mouse BC3H-1 cells and Torpedo electric organ and microinjection of appropriate mRNA combinations into Xenopus oocytes, we studied the single-channel properties of both 'homologous' (all subunits from the same species) and 'hybrid' (subunits from both species) AChRs as they were expressed in the oocyte membrane. AChR expression was determined by surface binding of 125I-labeled alpha-bungarotoxin to intact oocytes, and those with binding sites of 1 fmol/cell or more were chosen for patch-clamp studies. Our results indicate the following: (1) Species difference in single-channel conductance can be explained largely by the charge distribution flanking the M2 transmembrane domain. (2) The alpha and delta subunits from mouse AChR independently lengthen the channel open time, in some cases by 10-fold; the beta subunit from mouse shortens the channel open time; the mouse gamma subunit lengthens open time less dramatically. (3) Voltage sensitivity, as measured by the ratio of channel open times at -60 mV and +60 mV, is influenced by the beta and delta subunits, in agreement with our previous study by two-electrode voltage-clamp recording. We conclude that single-channel properties of the AChR are governed by multiple elements located on different AChR subunits.

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

To gain an insight into the molecular basis of the weak but significant selectivity among alkali metal cations of the nicotinic acetylcholine receptor (AChR) channel, we have determined single-channel conductance and permeability ratios for alkali metal cations on specifically mutated Torpedo californica AChR channels expressed in Xenopus oocytes. The mutations involved charged and polar side chains in the three anionic rings (extracellular, intermediate and cytoplasmic ring) which have previously been found to determine the rate of K+ transport through the AChR channel. The results obtained reveal that mutations in the intermediate ring exert much stronger effects on ion selectivity than do mutations in the extracellular and the cytoplasmic ring. The experimental results, together with simulations of the channel's energy profile, suggest that the amino acid residues forming the intermediate ring come into close contact with permeating cations and possibly represent part of the physical correlate of the postulated selectivity filter in the AChR channel.

Ion channel formation by synthetic transmembrane segments of the inhibitory glycine receptor--a model study

The inhibitory glycine receptor (GlyR) of rat spinal cord contains an intrinsic transmembrane channel mediating agonist-gated anion flux. Here, synthetic peptides modelled after the predicted transmembrane domains M2 and M4 of its ligand-binding subunit were incorporated into lipid vesicle membranes and black lipid bilayers to analyze their channel forming capabilities. Both types of peptides prohibited the establishment of, or dissipated, preexisting transmembrane potentials in the vesicle system. Incorporation of peptide M2 into the black lipid bilayer elicited randomly gated single channel events with various conductance states and life-times. Peptide M4 increased the conductance of the bilayer without producing single channels. Exchange of the terminal arginine residues of peptide M2 by glutamate resulted in a significant shift towards cation selectivity of the respective channels as compared to peptide M2. In conclusion, the peptide channels observed differed significantly from native GlyR in both conductivity and ion-selectivity indicating that individual synthetic transmembrane segments are not sufficient to mimic a channel protein composed of subunits with multiple transmembrane segments.

MK-801 inhibition of nicotinic acetylcholine receptor channels

MK-801 is a potent inhibitor of the NMDA subtype of glutamate receptors. Single-channel and macroscopic currents indicate that MK-801 also inhibits nicotinic acetylcholine receptors (nAChRs). MK-801 does not significantly increase desensitization of the nAChRs or compete for the ACh binding site. Although there is a slight inhibition of the closed nAChR, the main action of MK-801 is to enter and block the open channel. The voltage dependence for block is consistent with a single binding site within the channel that is 50% of the way through the membrane field. The IC50 for block is 3 microM at -70 mV for currents induced by 0.5 microM ACh. The data from both single-channel and macroscopic currents can be used to estimate a Kd (0) of 7 microM, which is about 40 times higher than the Kd (0) for MK-801 binding to the NMDA receptor. The relative potency of tricyclic compounds like MK-801 for various neurotransmitter systems points out that the pharmacologic action of these drugs could involve complicated interactions in vivo.

Beta-adrenergic stimulation induces intracellular Ca++ increase in human epidermal keratinocytes

Intracellular Ca++ ([Ca++]i) is one of the most important second messengers of extracellular signals that induce cellular responses. In epidermal keratinocytes, both extracellular and intracellular Ca++ are reported to be important to cell differentiation and proliferation. Several mechanisms that increase [Ca++]i have been elicited in various tissues; however, in epidermal keratinocytes they remain unknown. Thus, we investigated the [Ca++]i modulation in cultured human epidermal keratinocytes and the stimulation that increases the concentration. The [Ca++]i concentration of keratinocytes was increased immediately and transiently by epinephrine. Methoxamine hydrochloride and clonidine (alpha-1- and 2-adrenergic agonists) did not induce an increase in [Ca++]i. The beta-antagonist, propranolol, inhibited the [Ca++]i increase induced by epinephrine and salbutamol (a beta-2-agonist). These results reveal that the beta-adrenergic stimulation induces an immediate and transient [Ca++]i increase in human keratinocytes. Beta-adrenergic stimulation is known to induce adenylate cyclase activation, which results in cyclic AMP accumulation through stimulatory guanosine 5-triphosphate (GTP) binding proteins in the keratinocytes. Also, epinephrine is reported to inhibit cultured epidermal cell proliferation. The effect of epinephrine has been demonstrated by cyclic AMP accumulation; however, beta-adrenergic stimulation revealed a [Ca++]i increase in keratinocytes in our study. One of epinephrine's regulatory effects on epidermal cell proliferation is assumed to occur through the [Ca++]i increase as well.

Effects of pH on acetylcholine receptor function

We have examined the effects of changing extracellular pH on the function of nicotinic acetylcholine receptors from Torpedo californica using ion flux and electrophysiological methods. Agonist-induced cation efflux from vesicles containing purified, reconstituted receptors showed a monotonic dependence on external hydrogen ion concentration with maximal fluxes at alkaline pH and no agonist-induced efflux at pH's less than approximately 5. A similar pH dependence was measured for the peak agonist-activated membrane currents measured in microelectrode voltage-clamped Xenopus oocytes induced to express Torpedo receptor through mRNA injection. Half-maximal inhibition occurred at a similar pH in both systems, in the range of pH 6.5-7.0. Single-channel currents from Torpedo ACh receptors measured in patch-clamp recordings were also reduced in amplitude at acid pH with an apparent pKa for block of less than 5. Measurements of channel kinetics had a more complicated dependence on pH. The mean channel open time determined from patch-clamp measurements was maximal at neutral pH and decreased at both acid and alkaline pH's. Thus, both channel permeability properties and channel gating properties are affected by the extracellular pH.

Location of a threonine residue in the alpha-subunit M2 transmembrane segment that determines the ion flow through the acetylcholine receptor channel

By the combination of cDNA manipulation and functional analysis of normal and mutant acetylcholine receptor (AChR) channels of Torpedo expressed in Xenopus laevis oocytes determinants of ion flow were localized in the bends bordering the putative M2 transmembrane segment (Imoto et al. 1988). We now report that in the rat muscle AChR, substitution of a threonine residue in the alpha-subunit localized in the M2 transmembrane segment increases or decreases the channel conductance, depending on the size of the amino acid side chain located at this position. This threonine residue (alpha T264) is located adjacent to the cluster of charged amino acids that form the intermediate anionic ring (Imoto et al. 1988). This effect is pronounced for the large alkali cations Cs+, Rb+, K+ whereas for Na+ the effect is much smaller. Taken together the results suggest that the threonine residues at position 264 in the two alpha-subunits together with the amino acids of the intermediate anionic ring form part of a narrow region close to the cytoplasmic mouth of the AChR channel.

Role of a key cysteine residue in the gating of the acetylcholine receptor

We have examined changes in single-channel behavior that result from conservative amino acid substitutions at the Cys230 residue in the putative first transmembrane region (M1) of the murine nicotinic acetylcholine receptor. Mutations made in the gamma subunit altered the energy barrier for a single closing rate constant in proportion to the size of the substituted side chain. One of these substitutions, when made in the alpha subunits, had no effect on gating. No mutations altered permeation. We conclude that the region surrounding the M1 Cys is involved in the gating of the nicotinic acetylcholine receptor and that the gamma subunit contributes significantly to the control of channel closure.

Primary structure and functional expression of the alpha-, beta-, gamma-, delta- and epsilon-subunits of the acetylcholine receptor from rat muscle

The isolation and characterization of five clones carrying sequences of the alpha-, beta-, gamma-, delta- and epsilon-subunit precursors of the rat muscle acetylcholine receptor (AChR) are described. The deduced amino acid sequences indicate that these polypeptides contain 457-519 amino acids and reveal the structural characteristics common to subunits of ligand-gated ion channels. The pattern of subunit-specific mRNA levels in rat muscle shows characteristic changes during development and following denervation, suggesting that innervation of muscle reduces the expression of the alpha-, beta- and delta-subunit mRNAs, suppresses the expression of the gamma-subunit mRNA, and induces expression of epsilon-subunit mRNA. Subunit-specific cRNAs generated in vitro were injected into Xenopus laevis oocytes, resulting in the assembly of two functionally different AChR channel subtypes. The AChR gamma, composed of the alpha-, beta-, gamma- and delta-subunits, has functional properties similar to those of the native AChRs in fetal muscle. The AChR epsilon, composed of alpha-, beta-, delta- and epsilon-subunits, corresponds to the end-plate channel of the adult muscle. Thus in rat skeletal muscle the motor nerve regulates the expression of two functionally different AChR subtypes with different molecular composition by the differential expression of subunit-specific mRNAs.