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

A single historical substitution drives an increase in acetylcholine receptor complexity

Human adult muscle-type acetylcholine receptors are heteropentameric ion channels formed from four different, but evolutionarily related, subunits. These subunits assemble with a precise stoichiometry and arrangement such that two chemically distinct agonist-binding sites are formed between specific subunit pairs. How this subunit complexity evolved and became entrenched is unclear. Here we show that a single historical amino acid substitution is able to constrain the subunit stoichiometry of functional acetylcholine receptors. Using a combination of ancestral sequence reconstruction, single-channel electrophysiology, and concatenated subunits, we reveal that an ancestral β-subunit can not only replace the extant β-subunit but can also supplant the neighboring δ-subunit. By forward evolving the ancestral β-subunit with a single amino acid substitution, we restore the requirement for a δ-subunit for functional channels. These findings reveal that a single historical substitution necessitates an increase in acetylcholine receptor complexity and, more generally, that simple stepwise mutations can drive subunit entrenchment in this model heteromeric protein.

Unmasking coupling between channel gating and ion permeation in the muscle nicotinic receptor

Whether ion channel gating is independent of ion permeation has been an enduring, unresolved question. Here, applying single channel recording to the archetypal muscle nicotinic receptor, we unmask coupling between channel gating and ion permeation by structural perturbation of a conserved intramembrane salt bridge. A charge-neutralizing mutation suppresses channel gating, reduces unitary current amplitude, and increases fluctuations of the open channel current. Power spectra of the current fluctuations exhibit low- and high-frequency Lorentzian components, which increase in charge-neutralized mutant receptors. After aligning channel openings and closings at the time of transition, the average unitary current exhibits asymmetric relaxations just after channel opening and before channel closing. A theory in which structural motions contribute jointly to channel gating and ion conduction describes both the power spectrum and the current relaxations. Coupling manifests as a transient increase in the open channel current upon channel opening and a decrease upon channel closing.

Contingency between Historical Substitutions in the Acetylcholine Receptor Pore

Human adult muscle-type acetylcholine receptors incorporating a reconstructed ancestral β-subunit exhibit reduced single-channel conductance when compared to wild-type. The ancestral and wild-type β-subunits differ by 132 amino acids, including substitution of residues that line the lumen of the channel pore, near its narrowest constriction. Here we show that a single historical substitution in this region of the ancestral β-subunit accounts for the difference in conductance. Furthermore, the contribution of the substituted residue to conductance is dependent upon its ancestral or wild-type background, and it can be modulated by a neighboring residue that has also evolved throughout the β-subunit history. Using an expanded molecular phylogeny, we track the order in which these two mutations occurred and then show that the order in which they are installed upon the ancestral, but not the human, background determines their individual contribution to conductance. Our results show how the contribution of amino acids to acetylcholine receptor conductance is contingent upon their evolutionary history and that the order in which substitutions occurred was important for shaping conductance in the modern-day receptor.

How does the heart sense changes in oxygen tension: a role for ion channels?

SIGNIFICANCE

Oxygen plays a key role in cellular metabolism and function. Oxygen delivery to cells is crucial, and a lack of oxygen such as that which occurs during myocardial infarction can be lethal. Cells should, therefore, be able to respond to changes in oxygen tension.

RECENT ADVANCES

Since the first studies examining the acute cellular effect of hypoxia on activation of transmitter release from glomus or type I chemoreceptor cells, it is now known that virtually all cells are able to respond to changes in oxygen tension.

CRITICAL ISSUES

Despite advances made in characterizing hypoxic responses, the identity of the "oxygen sensor" remains debated. Recently, more evidence has evolved as to how cardiac myocytes sense acute changes in oxygen. This review will examine the available evidence in support of acute oxygen-sensing mechanisms providing a brief historical perspective and then more detailed insights into the heart and the role of cardiac ion channels in hypoxic responses.

FUTURE DIRECTIONS

A further understanding of these cellular processes should result in interventions that assist in preventing the deleterious effects of acute changes in oxygen tension such as alterations in contractile function and cardiac arrhythmia.

In glycine and GABA(A) channels, different subunits contribute asymmetrically to channel conductance via residues in the extracellular domain

Single-channel conductance in Cys-loop channels is controlled by the nature of the amino acids in the narrowest parts of the ion conduction pathway, namely the second transmembrane domain (M2) and the intracellular helix. In cationic channels, such as Torpedo ACh nicotinic receptors, conductance is increased by negatively charged residues exposed to the extracellular vestibule. We now show that positively charged residues at the same loop 5 position boost also the conductance of anionic Cys-loop channels, such as glycine (α1 and α1β) and GABA(A) (α1β2γ2) receptors. Charge reversal mutations here produce a greater decrease on outward conductance, but their effect strongly depends on which subunit carries the mutation. In the glycine α1β receptor, replacing Lys with Glu in α1 reduces single-channel conductance by 41%, but has no effect in the β subunit. By expressing concatameric receptors with constrained stoichiometry, we show that this asymmetry is not explained by the subunit copy number. A similar pattern is observed in the α1β2γ2 GABA(A) receptor, where only mutations in α1 or β2 decreased conductance (to different extents). In both glycine and GABA receptors, the effect of mutations in different subunits does not sum linearly: mutations that had no detectable effect in isolation did enhance the effect of mutations carried by other subunits. As in the nicotinic receptor, charged residues in the extracellular vestibule of anionic Cys-loop channels influence elementary conductance. The size of this effect strongly depends on the direction of the ion flow and, unexpectedly, on the nature of the subunit that carries the residue.

Spontaneous conformational change and toxin binding in alpha7 acetylcholine receptor: insight into channel activation and inhibition

Nicotinic AChRs (nAChRs) represent a paradigm for ligand-gated ion channels. Despite intensive studies over many years, our understanding of the mechanisms of activation and inhibition for nAChRs is still incomplete. Here, we present molecular dynamics (MD) simulations of the alpha7 nAChR ligand-binding domain, both in apo form and in alpha-Cobratoxin-bound form, starting from the respective homology models built on crystal structures of the acetylcholine-binding protein. The toxin-bound form was relatively stable, and its structure was validated by calculating mutational effects on the toxin-binding affinity. However, in the apo form, one subunit spontaneously moved away from the conformation of the other four subunits. This motion resembles what has been proposed for leading to channel opening. At the top, the C loop and the adjacent beta7-beta8 loop swing downward and inward, whereas at the bottom, the F loop and the C terminus of beta10 swing in the opposite direction. These swings appear to tilt the whole subunit clockwise. The resulting changes in solvent accessibility show strong correlation with experimental results by the substituted cysteine accessibility method upon addition of acetylcholine. Our MD simulation results suggest a mechanistic model in which the apo form, although predominantly sampling the "closed" state, can make excursions into the "open" state. The open state has high affinity for agonists, leading to channel activation, whereas the closed state upon distortion has high affinity for antagonists, leading to inhibition.

Channel opening by anesthetics and GABA induces similar changes in the GABAA receptor M2 segment

For many general anesthetics, their molecular basis of action involves interactions with GABA(A) receptors. Anesthetics produce concentration-dependent effects on GABA(A) receptors. Low concentrations potentiate submaximal GABA-induced currents. Higher concentrations directly activate the receptors. Functional effects of anesthetics have been characterized, but little is known about the conformational changes they induce. We probed anesthetic-induced conformational changes in the M2 membrane-spanning, channel-lining segment using disulfide trapping between engineered cysteines. Previously, we showed that oxidation by copper phenanthroline in the presence of GABA of the M2 6' cysteine mutants, alpha(1)T261Cbeta(1)T256C and alpha(1)beta(1)T256C resulted in formation of an intersubunit disulfide bond between the adjacent beta-subunits that significantly increased the channels' spontaneous open probability. Oxidation in GABA's absence had no effect. We examined the effect on alpha(1)T261Cbeta(1)T256C and on alpha(1)beta(1)T256C of oxidation by copper phenanthroline in the presence of potentiating and directly activating concentrations of the general anesthetics propofol, pentobarbital, and isoflurane. Oxidation in the presence of potentiating concentration of anesthetics had little effect. Oxidation in the presence of directly activating anesthetic concentrations significantly increased the channels' spontaneous open probability. We infer that activation by anesthetics and GABA induces a similar conformational change at the M2 segment 6' position that is related to channel opening.

Modulation of proton-induced current fluctuations in the human nicotinic acetylcholine receptor channel

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that switches upon activation from a closed state to a full conducting state. We found that the mutation delta S268K, located at 12' position of the second transmembrane domain of the delta subunit of the human nAChR generates a long-lived intermediate conducting state, from which openings to a wild-type like conductance level occur on a submillisecond time scale. Aiming to understand the interplay between structural changes near the 12' position and channel gating, we investigated the influence of various parameters: different ligands (acetylcholine, choline and epibatidine), ligand concentrations, transmembrane voltages and both fetal and adult nAChRs. Since sojourns in the high conductance state are not fully resolved in time, spectral noise analysis was used as a complement to dwell time analysis to determine the gating rate constants. Open channel current fluctuations are described by a two-state Markov model. The characteristic time of the process is markedly influenced by the ligand and the receptor type, whereas the frequency of openings to the high conductance state increases with membrane hyperpolarization. Conductance changes are discussed with regard to reversible transfer reaction of single protons at the lysine 12' side chain.

3Beta-hydroxysteroids and pregnenolone sulfate inhibit recombinant rat GABA(A) receptor through different channel property

3Beta-hydroxysteroids are pregnenolone sulfate-like GABA(A) receptor antagonists. The aim of the current study was to compare the functional differences between 3beta-hydroxysteroids and pregnenolone sulfate to inhibit GABA(A) receptors expressed in Xenopus oocytes. Recombinant rat GABA(A) receptors encoding wild type alpha1 beta2 gamma2L receptor, mutant alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors were examined using a two-electrode voltage-clamp technique. A homologous mutation of the residue at 2'position closest to the cytoplasmic end of the M2 helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, reduced the slow desensitization components of GABA-activated currents at saturating doses. Compared to the wild type receptor, the potency of GABA increased significantly in the alpha1V256S beta2 gamma2L receptor (P<0.05), whereas it decreased moderately in the alpha1 beta2A252S gamma2L receptor. We found that 5alpha-pregnan-3beta, 20(S)-diol (UC1019) and 5beta-pregnan-3beta, 20(R)-diol (UC1020) were the most effective blockers of maximal GABA responses among a panel of 3beta-hydroxysteroids. Pregnenolone sulfate, UC1019 and UC1020 were potent antagonists in the wild type receptor with calculated IC50s of 0.20+/-0.07 microM; 1.88+/-0.32 microM and 2.58+/-0.58 microM, respectively. The inhibitory effect of pregnenolone sulfate was significantly reduced in both mutants alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors (P<0.05), whereas the inhibitory effects of UC1019 and UC1020 were reduced only in the mutant alpha1V256S beta2 gamma2L receptor. Pregnenolone sulfate promoted slow desensitization with prolonged GABA application in a dose-dependent manner in the wild type receptor, but not mutant receptors. On the contrary, UC1019 and UC1020 (< or = 20 microM) did not promote desensitization in both wild type and mutant receptors. In conclusion, the GABA(A) receptor inhibition by pregnenolone sulfate, but not 3beta-hydroxysteroids, was dependent on desensitization kinetics of the Cl- channels. A point mutation at M2 helix of the beta2-subunit (beta2A252S) can dramatically reduce the inhibitory effect of pregnenolone sulfate on the GABA(A) receptors without affecting the inhibitory properties of 3beta-hydroxysteroids. These results are consistent with the hypothesis that pregnenolone sulfate-inhibition does not share with 3beta-hydroxysteroids the coincident channel property at the GABA(A) receptor.

Chimeric Mutations in the M2 Segment of the 5-Hydroxytryptamine-gated Chloride Channel MOD-1 Define a Minimal Determinant of Anion/Cation Permeability

The ionic selectivity of ligand-gated ion channels (LGICs) determines whether receptor activation produces an excitatory or inhibitory response. The determinants of anion/cation selectivity were investigated for a new member of the LGIC superfamily, MOD-1, a serotonin-gated chloride channel cloned from the nematode Caenorhabditis elegans. In common with other anionic LGICs (glycine receptors and GABA(A) receptors), the selectivity triple mutant in the pore-forming M2 segment (proline insertion, Ala --> Glu substitution at the central ring, and Thr --> Val at the hydrophobic ring) converted the selectivity of MOD-1 from anionic to cationic. Unlike other LGICs, however, this mutant in MOD-1 was highly selective for K+ over other cations. Subsets of this selectivity triple mutant were studied to define the minimal change required for conversion from anion-permeable to cation-permeable. The double mutant at the central ring of charge (deltaPro-269/A270E) produced a non-selective cation channel. Charge reversal at the central ring alone (A270E) was sufficient to convert MOD-1 to cation-permeable. These results refine the determinants of ion-charge selectivity in LGICs and demonstrate the critical role of the central ring of charge formed by the M2 segments.

Charge selectivity of the Cys-loop family of ligand-gated ion channels

The determinants of charge selectivity of the Cys-loop family of ligand-gated ion channels have been studied for more than a decade. The investigations have mainly covered homomeric receptors e.g. the nicotinic acetylcholine receptor alpha7, the glycine receptor alpha1 and the serotonin receptor 5-HT(3A). Only recently, the determinants of charge selectivity of heteromeric receptors have been addressed for the GABA(A) receptor alpha2beta3gamma2. For all receptor subtypes, the selectivity determinants have been located to an intracellular linker between transmembrane domains M1 and M2. Two features of the M1-M2 linker appear to control ion selectivity. A central role for charged amino acid residues in selectivity has been almost universally observed. Furthermore, recent studies point to an important role of the size of the narrowest constriction in the pore. In the present review, these determinants of charge selectivity of the Cys-loop family of ligand-gated ion channels will be discussed in detail.

Ligand-gated ion channels: mechanisms underlying ion selectivity

Anion/cation selectivity is a critical property of ion channels and underpins their physiological function. Recently, there have been numerous mutagenesis studies, which have mapped sites within the ion channel-forming segments of ligand-gated ion channels that are determinants of the ion selectivity. Site-directed mutations to specific amino acids within or flanking the M2 transmembrane segments of the anion-selective glycine, GABA(A) and GABA(C) receptors and the cation-selective nicotinic acetylcholine and serotonin (type 3) receptors have revealed discrete, equivalent regions within the ion channel that form the principal selectivity filter, leading to plausible molecular mechanisms and mathematical models to describe how ions preferentially permeate these channels. In particular, the dominant factor determining anion/cation selectivity seems to be the sign and exposure of charged amino acids lining the selectivity filter region of the open channel. In addition, the minimum pore diameter, which can be influenced by the presence of a local proline residue, also makes a contribution to such ion selectivity in LGICs with smaller diameters increasing anion/cation selectivity and larger ones decreasing it.

Structural effects of quinacrine binding in the open channel of the acetylcholine receptor

Noncompetitive inhibitors of the nicotinic acetylcholine (ACh) receptors suppress cation flux directly by binding in and blocking the open channel or indirectly by stabilizing closed states of the receptor. The lidocaine derivative QX-314 and the acridine derivative quinacrine act directly as open channel blockers, but can act indirectly as well. The binding site for quinacrine in the open channel of mouse-muscle ACh receptor was mapped in cysteine-substituted mutants of the alpha subunit expressed with wild-type beta, gamma, and delta subunits. In the open state, substituted cysteines in the inner half of the second membrane-spanning segment (M2), but not in the outer half, were protected by quinacrine from reaction with 2-aminoethyl methanethiosulfonate. In addition, an alkylating derivative, quinacrine mustard, affinity labeled a subset of the substituted cysteines in M2, but only in the open state. These results, mapped onto a model of the open channel surrounded by five alpha-helical M2s, imply that quinacrine binds midway down M2 in the same site previously mapped for QX-314. A cysteine substituted for a residue in the outer third of alphaM1, which reacted with 2-aminoethyl methanethiosulfonate only in the presence of ACh, reacted faster in the additional presence of quinacrine or QX-314. It is proposed that channel opening involves both the opening of the resting gate at the inner end of M2 and the removal of an obstruction formed by the outer end of M1 that retards diffusion of blockers into the closed channel. Blocker binding in the open channel causes a further change in structure.

Comparative surface accessibility of a pore-lining threonine residue (T6') in the glycine and GABA(A) receptors

The substituted cysteine accessibility method was used to probe the surface exposure of a pore-lining threonine residue (T6') common to both the glycine receptor (GlyR) and gamma-aminobutyric acid, type A receptor (GABA(A)R) chloride channels. This residue lies close to the channel activation gate, the ionic selectivity filter, and the main pore blocker binding site. Despite their high amino acid sequence homologies and common role in conducting chloride ions, recent studies have suggested that the GlyRs and GABA(A)Rs have divergent open state pore structures at the 6' position. When both the human alpha1(T6'C) homomeric GlyR and the rat alpha1(T6'C)beta1(T6'C) heteromeric GABA(A)R were expressed in human embryonic kidney 293 cells, their 6' residue surface accessibilities differed significantly in the closed state. However, when a soluble cysteine-modifying compound was applied in the presence of saturating agonist concentrations, both receptors were locked into the open state. This action was not induced by oxidizing agents in either receptor. These results provide evidence for a conserved pore opening mechanism in anion-selective members of the ligand-gated ion channel family. The results also indicate that the GABA(A)R pore structure at the 6' level may vary between different expression systems.

Emerging structure of the nicotinic acetylcholine receptors

The conversion of acetylcholine binding into ion conduction across the membrane is becoming more clearly understood in terms of the structure of the receptor and its transitions. A high-resolution structure of a protein that is homologous to the extracellular domain of the receptor has revealed the binding sites and subunit interfaces in great detail. Although the structures of the membrane and cytoplasmic domains are less well determined, the channel lining and the determinants of selectivity have been mapped. The location and structure of the gates, and the coupling between binding sites and gates, remain to be established.

Pregnenolone sulfate block of GABA(A) receptors: mechanism and involvement of a residue in the M2 region of the alpha subunit

Neurosteroids are produced in the brain, and can have rapid actions on membrane channels of neurons. Pregnenolone sulfate (PS) is a sulfated neurosteroid which reduces the responses of the [gamma]-aminobutyric acid A (GABA(A)) receptor. We analysed the actions of PS on single-channel currents from recombinant GABA(A) receptors formed from [alpha]1, [beta]2 and [gamma]2L subunits. Currents were elicited by a concentration of GABA eliciting a half-maximal response (50 microM) and a saturating concentration (1 mM). PS reduced the duration of clusters of single-channel activity at either concentration of GABA. PS had no discernable effect on rapid processes: no effects were apparent on channel opening and closing, nor on GABA affinity, and a rapidly recovering desensitised state was not affected. Instead, PS produced a slowly developing block which occurred at a similar rate for receptors with open or closed channels and with one or two bound GABA molecules. The rate of block was independent of membrane potential, implying that the charged sulfate moiety does not move through the membrane field. Change in a specific residue near the intracellular end of the channel lining portion of the [alpha]1 subunit had a major effect on the rate of block. Mutation of the residue [alpha]1 V256S reduced the rate of block by 30-fold. A mutation at the homologous position of the [beta]2 subunit ([beta]2 A252S) had no effect, nor did a complementary mutation in the [gamma]2L subunit ([gamma]2L S266A). It seems likely that this residue is involved in a conformational change underlying block by PS, instead of forming part of the binding site for PS.

Sex differences in the acetylcholine receptor kinetics of postnatal and denervated rat muscle

Single-channel recording from visualised endplates in freshly dissociated muscles from postnatal and denervated rat muscle revealed the presence of a low conductance, fetal type of acetylcholine receptor. Kinetic analysis showed a main component in the burst durations with a mean of 10.8 +/- 2.7 ms (n = 29). Receptors from female rats had an additional 27.3 +/- 5.5 ms (n = 5) kinetic component which was found in one-third of the 15 female endplates. Recordings from male and female denervated muscles gave more homogeneous kinetics with single time constants of 7.2 +/- 1.3 and 7.4 +/- 1.3 ms, respectively. It is concluded that the acetylcholine receptor channels present during early development are different from those of denervated muscle.

Location of the polyamine binding site in the vestibule of the nicotinic acetylcholine receptor ion channel

To map the structure of a ligand-gated ion channel, we used the photolabile polyamine-containing toxin MR44 as photoaffinity label. MR44 binds with high affinity to the nicotinic acetylcholine receptor in its closed channel conformation. The binding stoichiometry was two molecules of MR44 per receptor monomer. Upon UV irradiation of the receptor-ligand complex, (125)I-MR44 was incorporated into the receptor alpha-subunit. From proteolytic mapping studies, we conclude that the site of (125)I-MR44 cross-linking is contained in the sequence alpha His-186 to alpha Leu-199, which is part of the extracellular domain of the receptor. This sequence partially overlaps in its C-terminal region with one of the three loops that form the agonist-binding site. The agonist carbachol and the competitive antagonist alpha-bungarotoxin had only minor influence on the photocross-linking of (125)I-MR44. The site where the hydrophobic head group of (125)I-MR44 binds must therefore be located outside the zone that is sterically influenced by agonist bound at the nicotinic acetylcholine receptor. In binding and photocross-linking experiments, the luminal noncompetitive inhibitors ethidium and triphenylmethylphosphonium were found to compete with (125)I-MR44. We conclude that the polyamine moiety of (125)I-MR44 interacts with the high affinity noncompetitive inhibitor site deep in the channel of the nicotinic acetylcholine receptor, while the aromatic ring of this compound binds in the upper part of the ion channel (i.e. in the vestibule) to a hydrophobic region on the alpha-subunit that is located in close proximity to the agonist binding site. The region of the alpha-subunit labeled by (125)I-MR44 should therefore be accessible from the luminal side of the vestibule.

A single beta subunit M2 domain residue controls the picrotoxin sensitivity of alphabeta heteromeric glycine receptor chloride channels

This study investigated the residues responsible for the reduced picrotoxin sensitivity of the alphabeta heteromeric glycine receptor relative to the alpha homomeric receptor. By analogy with structurally related receptors, the beta subunit M2 domain residues P278 and F282 were considered the most likely candidates for mediating this effect. These residues align with G254 and T258 of the alpha subunit. The T258A, T258C and T258F mutations dramatically reduced the picrotoxin sensitivity of the alpha homomeric receptor. Furthermore, the converse F282T mutation in the beta subunit increased the picrotoxin sensitivity of the alphabeta heteromeric receptor. The P278G mutation in the beta subunit did not affect the picrotoxin sensitivity of the alphabeta heteromer. Thus, a ring of five threonines at the M2 domain depth corresponding to alpha subunit T258 is specifically required for picrotoxin sensitivity. Mutations to alpha subunit T258 also profoundly influenced the apparent glycine affinity. A substituted cysteine accessibility analysis revealed that the T258C sidechain increases its pore exposure in the channel open state. This provides further evidence for an allosteric mechanism of picrotoxin inhibition, but renders it unlikely that picrotoxin (as an allosterically acting 'competitive' antagonist) binds to this residue.

M2 pore mutations convert the glycine receptor channel from being anion- to cation-selective

Three mutations in the M2 transmembrane domains of the chloride-conducting alpha1 homomeric glycine receptor (P250Delta, A251E, and T265V), which normally mediate fast inhibitory neurotransmission, produced a cation-selective channel with P(Cl)/P(Na), = 0.27 (wild-type P(Cl)/P(Na) = 25), a permeability sequence P(Cs) > P(K) > P(Na) > P(Li), an impermeability to Ca(2+), and a reduced glycine sensitivity. Outside-out patch measurements indicated reversed and accentuated rectification with extremely low mean single channel conductances of 3 pS (inward current) and 11 pS (outward current). The three inverse mutations, to those analyzed in this study, have previously been shown to make the alpha7 acetylcholine receptor channel anion-selective, indicating a common location for determinants of charge selectivity of inhibitory and excitatory ligand-gated ion channels.

Kinetic, mechanistic, and structural aspects of unliganded gating of acetylcholine receptor channels: a single-channel study of second transmembrane segment 12' mutants

The spontaneous activity of adult mouse muscle acetylcholine receptor channels, transiently expressed in HEK-293 cells, was studied with the patch-clamp technique. To increase the frequency of unliganded openings, mutations at the 12' position of the second transmembrane segment were engineered. Our results indicate that: (a) in both wild type and mutants, a C <--> O kinetic scheme provides a good description of spontaneous gating. In the case of some mutant constructs, however, additional states were needed to improve the fit to the data. Similar additional states were also needed in one of six patches containing wild-type acetylcholine receptor channels; (b) the delta12' residue makes a more pronounced contribution to unliganded gating than the homologous residues of the alpha, beta, and straightepsilon subunits; (c) combinations of second transmembrane segment 12' mutations in the four different subunits appear to have cumulative effects; (d) the volume of the side chain at delta12' is relevant because residues larger than the wild-type Ser increase spontaneous gating; (e) the voltage dependence of the unliganded gating equilibrium constant is the same as that of diliganded gating, but the voltage dependences of the opening and closing rate constants are opposite (this indicates that the reaction pathway connecting the closed and open states of the receptor changes upon ligation); (f) engineering binding-site mutations that decrease diliganded gating (alphaY93F, alphaY190W, and alphaD200N) reduces spontaneous activity as well (this suggests that even in the absence of ligand the opening of the channel is accompanied by a conformational change at the binding sites); and (g) the diliganded gating equilibrium constant is also increased by the 12' mutations. Such increase is independent of the particular ligand used as the agonist, which suggests that these mutations affect mostly the isomerization step, having little, if any, effect on the ligand-affinity ratio.

The intrinsic electrostatic potential and the intermediate ring of charge in the acetylcholine receptor channel

A ring of aligned glutamate residues named the intermediate ring of charge surrounds the intracellular end of the acetylcholine receptor channel and dominates cation conduction (Imoto et al. 1988). Four of the five subunits in mouse-muscle acetylcholine receptor contribute a glutamate to the ring. These glutamates were mutated to glutamine or lysine, and combinations of mutant and native subunits, yielding net ring charges of -1 to -4, were expressed in Xenopus laevis oocytes. In all complexes, the alpha subunit contained a Cys substituted for alphaThr244, three residues away from the ring glutamate alphaGlu241. The rate constants for the reactions of alphaThr244Cys with the neutral 2-hydroxyethyl-methanethiosulfonate, the positively charged 2-ammonioethyl-methanethiosulfonate, and the doubly positively charged 2-ammonioethyl-2'-ammonioethanethiosulfonate were determined from the rates of irreversible inhibition of the responses to acetylcholine. The reagents were added in the presence and absence of acetylcholine and at various transmembrane potentials, and the rate constants were extrapolated to zero transmembrane potential. The intrinsic electrostatic potential in the channel in the vicinity of the ring of charge was estimated from the ratios of the rate constants of differently charged reagents. In the acetylcholine-induced open state, this potential was -230 mV with four glutamates in the ring and increased linearly towards 0 mV by +57 mV for each negative charge removed from the ring. Thus, the intrinsic electrostatic potential in the narrow, intracellular end of the open channel is almost entirely due to the intermediate ring of charge and is strongly correlated with alkali-metal-ion conductance through the channel. The intrinsic electrostatic potential in the closed state of the channel was more positive than in the open state at all values of the ring charge. These electrostatic properties were simulated by theoretical calculations based on a simplified model of the channel.

Mutational analysis of the charge selectivity filter of the alpha7 nicotinic acetylcholine receptor

In the alpha7 nicotinic acetylcholine receptors, we analyze the contribution of mutations E237A and V251T, together with the proline insertion P236', in the conversion of the charge selectivity from cationic to anionic. We show that the triple mutant exhibits spontaneous openings displaying anionic selectivity. Furthermore, at position 251, hydrophilic or even negatively charged residues are compatible with an anionic channel. In contrast, the additional proline yields an anionic channel only when inserted between positions 234 and 237; insertion before 234 yields a cationic channel and after 238 alters the receptor surface expression. The coiled 234-238 loop thus directly contributes to the charge selectivity filter of the alpha7 channel.

The 5-HT3B subunit is a major determinant of serotonin-receptor function

The neurotransmitter serotonin (5-hydroxytryptamine or 5-HT) mediates rapid excitatory responses through ligand-gated channels (5-HT3 receptors). Recombinant expression of the only identified receptor subunit (5-HT3A) yields functional 5-HT3 receptors. However, the conductance of these homomeric receptors (sub-picosiemens) is too small to be resolved directly, and contrasts with a robust channel conductance displayed by neuronal 5-HT3 receptors (9-17 pS). Neuronal 5-HT3 receptors also display a permeability to calcium ions and a current-voltage relationship that differ from those of homomeric receptors. Here we describe a new class of 5-HT3-receptor subunit (5-HT3B). Transcripts of this subunit are co-expressed with the 5-HT3A subunit in the amygdala, caudate and hippocampus. Heteromeric assemblies of 5-HT3A and 5-HT3B subunits display a large single-channel conductance (16 pS), low permeability to calcium ions, and a current-voltage relationship which resembles that of characterized neuronal 5-HT3 channels. The heteromeric receptors also display distinctive pharmacological properties. Surprisingly, the M2 region of the 5-HT3B subunit lacks any of the structural features that are known to promote the conductance of related receptors. In addition to providing a new target for therapeutic agents, the 5-HT3B subunit will be a valuable resource for defining the molecular mechanisms of ion-channel function.

The location of the gate in the acetylcholine receptor channel

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

Nature of the 5' residue in the M2 domain affects function of the human alpha 1 beta 1 GABAA receptor

The effects on the functional properties of the alpha 1 beta 1 GABAA receptor when the 5' (alpha 1 Val260; beta 1 Ile255) hydrophobic amino acids in the second transmembrane (M2) region were changed to threonine were examined. In response to a saturating concentration of GABA, the current evoked in mutant receptors showed a decreased rate of desensitization and at equilibrium was a greater fraction of the peak current than in wild-type receptors. The half-saturation concentration of the peak current response to GABA in mutant receptors was comparable to that in wild-type receptors, but the Hill coefficient was reduced to less than one. It was concluded that the 5' amino acids in the M2 region have a role in the conformational changes that occur within the alpha 1 beta 1 GABAA receptor in response to GABA.

Homologous mutations on different subunits cause unequal but additive effects on n-alcohol block in the nicotinic receptor pore

Hydrophobic antagonists of the nicotinic acetylcholine receptor inhibit channel activity by binding within the transmembrane pore formed by the second of four transmembrane domains (M2) on each of the receptor's subunits. Hydrophobic mutagenesis near the middle (10' locus) of the alpha-subunit M2 domain results in channels that are much more sensitive to block by long-chain alcohols and general anesthetics, indicating that the inhibitory site on wild-type receptors is nearby. To determine whether other receptor subunits also contribute to the blocker site, the hydrophobic mutagenesis strategy was extended to all four subunits at 10' loci. alpha S10'l causes the largest increase in apparent hexanol binding (4.3-fold compared to wild type), approximately twice the size of the change caused by beta T10'l (2.2-fold). gamma A10'l and delta A10'l mutations cause much smaller changes in apparent hexanol binding affinity (about 1.2-fold each), even when corrected for their smaller degree of side-chain hydrophobicity changes. When 10'l mutant subunits are coexpressed, the change from wild type in apparent hexanol binding energy (delta delta Gmixture) is roughly equal to the sum of hexanol binding energy changes for the constituent mutant subunits (sigma delta delta Gsubunits). The simplest model consistent with these results is one in which hydrophobic blockers make simultaneous contact with all five M2 10' residues, but the extent of contact is much greater for the alpha and beta than for gamma and delta side chains.

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.

The pore domain of the nicotinic acetylcholine receptor: molecular modeling, pore dimensions, and electrostatics

The pore domain of the nicotinic acetylcholine receptor has been modeled as a bundle of five kinked M2 helices. Models were generated via molecular dynamics simulations incorporating restraints derived from 9-A resolution cryoelectron microscopy data (Unwin, 1993; 1995), and from mutagenesis data that identify channel-lining side chains. Thus, these models conform to current experimental data but will require revision as higher resolution data become available. Models of the open and closed states of a homopentameric alpha 7 pore are compared. The minimum radius of the closed-state model is less than 2 A; the minimum radius of the open-state models is approximately 6 A. It is suggested that the presence of "bound" water molecules within the pore may reduce the effective minimum radii below these values by up to approximately 3 A. Poisson-Boltzmann calculations are used to obtain a first approximation to the potential energy of a monovalent cation as it moves along the pore axis. The differences in electrostatic potential energy profiles between the open-state models of alpha 7 and of a mutant of alpha 7 are consistent with the experimentally observed change in ion selectivity from cationic to anionic. Models of the open state of the heteropentameric Torpedo nicotinic acetylcholine receptor pore domain are also described. Relatively small differences in pore radius and electrostatic potential energy profiles are seen when the Torpedo and alpha 7 models are compared.

Size and selectivity of gap junction channels formed from different connexins

Gap junction channels have long been viewed as static structures containing a large-diameter, aqueous pore. This pore has a high permeability to hydrophilic molecules of approximately 900 daltons in molecular weight and a weak ionic selectivity. The evidence leading to these conclusions is reviewed in the context of more recent observations primarily coming from unitary channel recordings from transfected connexin channels expressed in communication-deficient cell lines. What is emerging is a more diverse view of connexin-specific gap junction channel structure and function where electrical conductance, ionic selectivity, and dye permeability vary by one full order of magnitude or more. furthermore, the often held contention that channel conductance and ionic or molecular selectivity are inversely proportional is refuted by recent evidence from five distinct connexin channels. The molecular basis for this diversity of channel function remains to be identified for the connexin family of gap junction proteins.

Functional and non-functional isoforms of the human muscle acetylcholine receptor

1. The properties of a recently identified isoform of the human muscle nicotinic acetylcholine receptor (AChR) alpha subunit (alpha +), which in muscle is expressed at similar levels to the alpha subunit, were investigated by both electrophysiological and biochemical approaches following expression in Xenopus laevis oocytes. The single-channel properties of adult (alpha 2 beta delta epsilon) and fetal (alpha 2 beta delta gamma) forms of the human AChR were also investigated. 2. The mean burst duration of adult channels (4.1 +/- 0.3 ms, mean +/- S.E.M., n = 5) is half that of fetal channels (7.9 +/- 0.6 ms, n = 4), while the single-channel conductance is larger (62.2 +/- 0.8 and 37.9 +/- 1.6 pS for adult and fetal channels, respectively), comparable to the developmental changes in single-channel properties observed for other mammalian species. 3. In contrast to the alpha isoform, the alpha + subunit does not bind 125I-labelled alpha-bungarotoxin or monoclonal antibodies directed against the AChR 'main immunogenic region' (MIR), illustrating why the alpha + subunit was first detected through screening of cDNA libraries. 4. By using site-directed mutagenesis to produce subunits that conferred different single-channel conductances on the AChR, we demonstrate that the alpha + isoform is not integrated into functional AChRs. 5. The mutagenesis experiments also revealed that the two alpha subunits within an AChR pentamer are not equivalent within the pore lining region.

Newly recognized congenital myasthenic syndrome associated with high conductance and fast closure of the acetylcholine receptor channel

We describe here a new congenital myasthenic syndrome associated with a kinetic abnormality of the acetylcholine receptor (AChR) channel. The propositus had poor suck and cry after birth. Subsequently, she had intermittent ocular symptoms and fatigued abnormally on exertion. At age 9 years, significant weakness was detected only in the frontalis, levator palpebrae, and neck flexor muscles. Electromyography showed no decrement in limb muscles but single-fiber examination of the facial muscles was consistent with a neuromuscular transmission defect. The ocular symptoms responded partially to pyridostigmine, but the abnormal fatigability did not. Tests for anti-AChR antibodies were negative. A younger sister had elements of the same disease. An intercostal muscle specimen was obtained from the propositus at age 9 years for endplate studies. The quantal content of the endplate potential was normal. Miniature endplate currents were abnormally large and their decay time constant was abnormally short. AChR channel properties were studied by analysis of acetylcholine-induced current noise. The mean single-channel conductance was increased 1.7-fold and the mean channel open time was 30% shorter than normal. The number of AChR per endplate was normal. Electron microscopy of most endplates showed no abnormality, but a few were degenerating or simplified. The channel abnormality may stem from a point mutation in an AChR subunit affecting a single amino acid residue lining the pore of the AChR channel. The mechanism by which the physiological abnormality produces clinical symptoms is not known, but possible explanations are considered.

Ion channels: molecular basis of ion selectivity

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

Control by asparagine residues of calcium permeability and magnesium blockade in the NMDA receptor

The N-methyl-D-aspartate (NMDA) receptor forms a cation-selective channel with a high calcium permeability and sensitivity to channel block by extracellular magnesium. These properties, which are believed to be important for the induction of long-term changes in synaptic strength, are imparted by asparagine residues in a putative channel-forming segment of the protein, transmembrane 2 (TM2). In the NR1 subunit, replacement of this asparagine by a glutamine residue decreases calcium permeability of the channel and slightly reduces magnesium block. The same substitution in NR2 subunits strongly reduces magnesium block and increases the magnesium permeability but barely affects calcium permeability. These asparagines are in a position homologous to the site in the TM2 region (Q/R site) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that is occupied by either glutamine (Q) or arginine (R) and that controls divalent cation permeability of the AMPA receptor channel. Hence AMPA and NMDA receptor channels contain common structural motifs in their TM2 segments that are responsible for some of their ion selectivity and conductance properties.