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

Spontaneous activity, singly bound states and the impact of alpha1Phe64 mutation on GABAAR gating in the novel kinetic model based on the single-channel recordings

GABA_A receptor is the primary mediator of inhibition in the adult mammalian brain. Our recent studies revealed that a classic gating scheme for GABA_AR needed to be updated with an intermediate step (flipping) and that the α₁Phe64 mutation at the GABA binding site affects this transition. However, description of flipping at the single-channel level remains incomplete. In particular, its role in singly-bound and spontaneous activity remains unknown. We have performed thus single-channel recordings over wide range of agonist concentration for wild-type α₁β₂γ_2L receptors and α₁Phe64 mutants. For WT receptors we observed relatively frequent brief spontaneous openings which were also present at low [GABA]. However, closed times distributions for spontaneous activity and at low [GABA] were clearly different indicating that a proportion of short-lived openings were due to liganded, most likely singly bound receptors. Increasing [GABA] resulted in prolongation of bursts and increased occurrence of bursts with long openings and short closures. Mutations of α₁Phe64 residue dramatically affected the open and closed time distributions at high and saturating [GABA], especially in the case of cysteine mutants. However, this mutation weakly affected spontaneous or singly bound activity. Model fitting of our single-channel data led us to propose a novel and, to our knowledge, most complete GABA_AR kinetic model in which flipping occurs in singly and doubly bound states. However, spontaneous activity did not reveal involvement of flipping. Moreover, we report that α₁Phe64 mutation affects not only the flipping but also the opening/closing transitions indicating its generalized impact on the receptor gating.

Structural correlates of affinity in fetal versus adult endplate nicotinic receptors

Adult-type nicotinic acetylcholine receptors (AChRs) mediate signalling at mature neuromuscular junctions and fetal-type AChRs are necessary for proper synapse development. Each AChR has two neurotransmitter binding sites located at the interface of a principal and a complementary subunit. Although all agonist binding sites have the same core of five aromatic amino acids, the fetal site has ∼30-fold higher affinity for the neurotransmitter ACh. Here we use molecular dynamics simulations of adult versus fetal homology models to identify complementary-subunit residues near the core that influence affinity, and use single-channel electrophysiology to corroborate the results. Four residues in combination determine adult versus fetal affinity. Simulations suggest that at lower-affinity sites, one of these unsettles the core directly and the others (in loop E) increase backbone flexibility to unlock a key, complementary tryptophan from the core. Swapping only four amino acids is necessary and sufficient to exchange function between adult and fetal AChRs.

Adult and fetal nicotinic acetylcholine receptors (AChRs) have different functional requirements and affinity for ACh. Here, the authors use molecular dynamics and electrophysiology to investigate this affinity, and identify four amino acids that when swapped exchange function between adult and fetal AChRs.

Agonist activation of a nicotinic acetylcholine receptor

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

Functional differences between neurotransmitter binding sites of muscle acetylcholine receptors

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

Catch-and-hold activation of muscle acetylcholine receptors having transmitter binding site mutations

Agonists turn on receptors because their target sites have a higher affinity in the active versus resting conformation of the protein. We used single-channel electrophysiology to measure the lower-affinity (LA) and higher-affinity (HA) equilibrium dissociation constants for acetylcholine in adult-type muscle mouse nicotinic receptors (AChRs) having mutations of agonist binding site amino acids. For a series of agonists and for all mutations of αY93, αG147, αW149, αY190, αY198, εW55, and δW57, the change in LA binding energy was approximately half that in HA binding energy. The results were analyzed as a linear free energy relationship between LA and HA agonist binding, the slope of which (κ) gives the fraction of the overall binding chemical potential where the LA complex is established. The linear correlation between LA and HA binding energies suggests that the overall binding process is by an integrated mechanism (catch-and-hold). For the agonist and the above mutations, κ ∼ 0.5, but side-chain substitutions of two residues had a slope that was significantly higher (0.90; αG153) or lower (0.25; εP121). The results suggest that backbone rearrangements in loop B, loop C, and the non-α surface participate in both LA binding and the LA ↔ HA affinity switch. It appears that all of the intermediate steps in AChR activation comprise a single, energetically coupled process.

α1F64 Residue at GABA(A) receptor binding site is involved in gating by influencing the receptor flipping transitions

GABA receptors (GABAARs) mediate inhibition in the adult brain. These channels are heteropentamers and their ligand binding sites are localized at the β+ / α- interfaces. As expected, mutations of binding-site residues affect binding kinetics but accumulating evidence indicates that gating is also altered, although the underlying mechanisms are unclear. We investigated the impact of the hydrophobic box residue localized at α1(-), F64 (α1F64), on the binding and gating of rat recombinant α1β1γ2 receptors. The analysis of current responses to rapid agonist applications confirmed a marked effect of α1F64 mutations on agonist binding and revealed surprisingly strong effects on gating, including the disappearance of rapid desensitization, the slowing of current onset, and accelerated deactivation. Moreover, nonstationary variance analysis revealed that the α1F64C mutation dramatically reduced the maximum open probability without altering channel conductance. Interestingly, for wild-type receptors, responses to saturating concentration of a partial agonist, P4S, showed no rapid desensitization, similar to GABA-evoked responses mediated by α1F64C mutants. For the α1F64L mutation, the application of the high-affinity agonist muscimol partially rescued rapid desensitization compared with responses evoked by GABA. These findings suggest that α1F64 mutations do not disrupt desensitization mechanisms but rather affect other gating features that obscure it. Model simulations indicated that all of our observations related to α1F64 mutations could be properly reproduced by altering the flipped state transitions that occurred after agonist binding but preceded opening. In conclusion, we propose that the α1F64 residue may participate in linking binding and gating by influencing flipping kinetics.

Function of interfacial prolines at the transmitter-binding sites of the neuromuscular acetylcholine receptor

The neuromuscular acetylcholine (ACh) receptor has two conserved prolines in loop D of the complementary subunit at each of its two transmitter-binding sites (α-ε and α-δ). We used single-channel electrophysiology to estimate the energy changes caused by mutations of these prolines with regard to unliganded gating (ΔG0) and the affinity change for ACh that increases the open channel probability (ΔGB). The effects of mutations of ProD2 (εPro-121/δPro-123) were greater than those of its neighbor (εPro-120/δPro-122) and were greater at α-ε versus α-δ. The main consequence of the congenital myasthenic syndrome mutation εProD2-L was to impair the establishment of a high affinity for ACh and thus make ΔGB less favorable. At both binding sites, most ProD2 mutations decreased constitutive activity (increased ΔG0). LRYHQG and RL substitutions reduced substantially the net binding energy (made ΔGB(ACh) less favorable) by ≥2 kcal/mol at α-ε and α-δ, respectively. Mutant cycle analyses were used to estimate energy coupling between the two ProD2 residues and between each ProD2 and glycine residues (αGly-147 and αGly-153) on the primary (α subunit) side of each binding pocket. The distant binding site prolines interact weakly. ProD2 interacts strongly with αGly-147 but only at α-ε and only when ACh is present. The results suggest that in the low to-high affinity change there is a concerted inter-subunit strain in the backbones at εProD2 and αGly-147. It is possible to engineer receptors having a single functional binding site by using a α-ε or α-δ ProD2-R knock-out mutation. In adult-type ACh receptors, the energy from the affinity change for ACh is approximately the same at the two binding sites (approximately -5 kcal/mol).

An integrated catch-and-hold mechanism activates nicotinic acetylcholine receptors

In neuromuscular acetylcholine (ACh) receptor channels (AChRs), agonist molecules bind with a low affinity (LA) to two sites that can switch to high affinity (HA) and increase the probability of channel opening. We measured (by using single-channel kinetic analysis) the rate and equilibrium constants for LA binding and channel gating for several different agonists of adult-type mouse AChRs. Almost all of the variation in the equilibrium constants for LA binding was from differences in the association rate constants. These were consistently below the limit set by diffusion and were substantially different even though the agonists had similar sizes and the same charge. This suggests that binding to resting receptors is not by diffusion alone and, hence, that each binding site can undergo two conformational changes ("catch" and "hold") that connect three different structures (apo-, LA-bound, and HA-bound). Analyses of ACh-binding protein structures suggest that this binding site, too, may adopt three discrete structures having different degrees of loop C displacement ("capping"). For the agonists we tested, the logarithms of the equilibrium constants for LA binding and LA↔HA gating were correlated. Although agonist binding and channel gating have long been considered to be separate processes in the activation of ligand-gated ion channels, this correlation implies that the catch-and-hold conformational changes are energetically linked and together comprise an integrated process having a common structural basis. We propose that loop C capping mainly reflects agonist binding, with its two stages corresponding to the formation of the LA and HA complexes. The catch-and-hold reaction coordinate is discussed in terms of preopening states and thermodynamic cycles of activation.

Highly fatal fast-channel syndrome caused by AChR ε subunit mutation at the agonist binding site

OBJECTIVE

To characterize the molecular basis of a novel fast-channel congenital myasthenic syndrome.

METHODS

We used the candidate gene approach to identify the pathogenic mutation in the acetylcholine receptor (AChR) ε subunit, genetically engineered the mutant AChR into HEK cells, and evaluated the level of expression and kinetic properties of the mutant receptor.

RESULTS

An 8-year-old boy born to consanguineous parents had severe myasthenic symptoms since birth. He is wheelchair bound and pyridostigmine therapy enables him to take only a few steps. Three similarly affected siblings died in infancy. He carries a homozygous p.W55R mutation at the α/ε subunit interface of the AChR agonist binding site. The mutant protein expresses well in HEK cells. Patch-clamp analysis of the mutant receptor expressed in HEK cells reveals 30-fold reduced apparent agonist affinity, 75-fold reduced apparent gating efficiency, and strikingly attenuated channel opening probability (P(open)) over a range agonist concentrations.

CONCLUSION

Introduction of a cationic Arg into the anionic environment of α/ε AChR binding site hinders stabilization of cationic ACh by aromatic residues and accounts for the markedly perturbed kinetic properties of the receptor. The very low P(open) explains the poor response to pyridostigmine and the high fatality of the disease.

Sources of energy for gating by neurotransmitters in acetylcholine receptor channels

Nicotinic acetylcholine receptors (AChRs) mediate signaling in the central and peripheral nervous systems. The AChR gating conformational change is powered by a low- to high-affinity change for neurotransmitters at two transmitter binding sites. We estimated (from single-channel currents) the components of energy for gating arising from binding site aromatic residues in the α-subunit. All mutations reduced the energy (TyrC1>>TrpB≈TyrC2>TyrA), with TyrC1 providing ~40% of the total. Considered one at a time, the fractional energy contributions from the aromatic rings were TrpB ~35%, TyrC1 ~28%, TyrC2 ~28%, and TyrA ~10%. Together, TrpB, TyrC1, and TyrC2 comprise an "aromatic triad" that provides much of the total energy from the transmitter for gating. Analysis of mutant pairs suggests that the energy contributions from some residues are nearly independent. Mutations of TyrC1 cause particularly large energy reductions because they remove two favorable and approximately equal interactions between the aromatic ring and the quaternary amine of the agonist and between the hydroxyl and αLysβ7.

Mapping the sequence of conformational changes underlying selectivity filter gating in the K(v)11.1 potassium channel

The potassium channel selectivity filter both discriminates between K(+) and sodium ions and contributes to gating of ion flow. Static structures of conducting (open) and nonconducting (inactivated) conformations of this filter are known; however, the sequence of protein rearrangements that connect these two states is not. We show that closure of the selectivity filter gate in the human K(v)11.1 K(+) channel (also known as hERG, for ether-a-go-go-related gene), a key regulator of the rhythm of the heartbeat, is initiated by K(+) exit, followed in sequence by conformational rearrangements of the pore domain outer helix, extracellular turret region, voltage sensor domain, intracellular domains and pore domain inner helix. In contrast to the simple wave-like sequence of events proposed for opening of ligand-gated ion channels, a complex spatial and temporal sequence of widespread domain motions connect the open and inactivated states of the K(v)11.1 K(+) channel.

Acetylcholine receptor channels activated by a single agonist molecule

The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R<-->R). The R shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E(1) and the energy change associated with the R versus R change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E(1) approximately 4.3 x 10(-3) with ACh, and approximately 1.7 x 10(-4) with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E(0) approximately 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by approximately 5.2 kcal/mol for ACh and approximately 3.3 kcal/mol for choline.

Energetics of gating at the apo-acetylcholine receptor transmitter binding site

Acetylcholine receptor channels switch between conformations that have a low versus high affinity for the transmitter and conductance for ions (R↔R*; gating). The forward isomerization, which begins at the transmitter binding sites and propagates ∼50 Å to the narrow region of the pore, occurs by approximately the same sequence of molecular events with or without agonists present at the binding sites. To pinpoint the forces that govern the R versus R* agonist affinity ratio, we measured single-channel activation parameters for apo-receptors having combinations of mutations of 10 transmitter binding site residues in the α (Y93, G147, W149, G153, Y190, C192, and Y198), ε (W55 and P121), or δ (W57) subunit. Gating energy changes were largest for the tryptophan residues. The αW149 energy changes were coupled with those of the other aromatic amino acids. Mutating the aromatic residues to Phe reduces the R/R* equilibrium dissociation constant ratio, with αY190 and αW149 being the most sensitive positions. Most of the mutations eliminated long-lived spontaneous openings. The results provide a foundation for understanding how ligands trigger protein conformational change.

Molecular determinants for competitive inhibition of alpha4beta2 nicotinic acetylcholine receptors

The Erythrina alkaloids erysodine and dihydro-beta-erythroidine (DHbetaE) are potent and selective competitive inhibitors of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), but little is known about the molecular determinants of the sensitivity of this receptor subtype to inhibition by this class of antagonists. We addressed this issue by examining the effects of DHbetaE and a range of aromatic Erythrina alkaloids on [(3)H]cytisine binding and receptor function in conjunction with homology models of the alpha4beta2 nAChR, mutagenesis, and functional assays. The lactone group of DHbetaE and a hydroxyl group at position C-16 in aromatic Erythrina alkaloids were identified as major determinants of potency, which was decreased when the conserved residue Tyr126 in loop A of the alpha4 subunit was substituted by alanine. Sensitivity to inhibition was also decreased by substituting the conserved aromatic residues alpha4Trp182 (loop B), alpha4Tyr230 (loop C), and beta2Trp82 (loop D) and the nonconserved beta2Thr84; however, only alpha4Trp182 was predicted to contact bound antagonist, suggesting alpha4Tyr230, beta2Trp82, and beta2Thr84 contribute allosterically to the closed state elicited by bound antagonist. In addition, homology modeling predicted strong ionic interactions between the ammonium center of the Erythrina alkaloids and beta2Asp196, leading to the uncapping of loop C. Consistent with this, beta2D196A abolished sensitivity to inhibition by DHbetaE or erysodine but not by epierythratidine, which is not predicted to form ionic bonds with beta2Asp196. This residue is not conserved in subunits that comprise nAChRs with low sensitivity to inhibition by DHbetaE or erysodine, which highlights beta2Asp196 as a major determinant of the receptor selectivity of Erythrina alkaloids.

The gating isomerization of neuromuscular acetylcholine receptors

Acetylcholine receptor-channels are allosteric proteins that isomerize ('gate') between conformations that have a low vs. high affinity for the transmitter and conductance for ions. In order to comprehend the mechanism by which the affinity and conductance changes are linked it is of value to know the magnitude, timing and distribution of energy flowing through the system. Knowing both the di- and unliganded gating equilibrium constants (E(2) and E(0)) is a foundation for understanding the AChR gating mechanism and for engineering both the ligand and the protein to operate in predictable ways. In adult mouse neuromuscular receptors activated by acetylcholine, E(2) = 28 and E(0) approximately 6.5 x 10(7). At each (equivalent) transmitter binding site acetylcholine provides approximately 5.2 kcal mol(1) to motivate the isomerization. The partial agonist choline provides approximately 3.3 kcal mol(1). The relative time of a residue's gating energy change is revealed by the slope of its rate-equilibrium constant relationship. A map of this parameter suggests that energy propagates as a conformational cascade between the transmitter binding sites and the gate region. Although gating energy changes are widespread throughout the protein, some residues are particularly sensitive to perturbations. Several specific proposals for the structural events that comprise the gating conformational cascade are discussed.