Action of nicotine and analogs on acetylcholine receptors having mutations of transmitter-binding site residue αG153

Agonist efficiency links binding and gating in a nicotinic receptor

Receptors signal by switching between resting (C) and active (O) shapes ('gating') under the influence of agonists. The receptor's maximum response depends on the difference in agonist binding energy, O minus C. In nicotinic receptors, efficiency (η) represents the fraction of agonist binding energy applied to a local rearrangement (an induced fit) that initiates gating. In this receptor, free energy changes in gating and binding can be interchanged by the conversion factor η. Efficiencies estimated from concentration-response curves (23 agonists, 53 mutations) sort into five discrete classes (%): 0.56 (17), 0.51(32), 0.45(13), 0.41(26), and 0.31(12), implying that there are 5 C versus O binding site structural pairs. Within each class efficacy and affinity are corelated linearly, but multiple classes hide this relationship. η unites agonist binding with receptor gating and calibrates one link in a chain of coupled domain rearrangements that comprises the allosteric transition of the protein.

Agonist efficiency from concentration-response curves: Structural implications and applications

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC₅₀) that indicates potency, a high-concentration asymptote that indicates efficacy, and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency (η), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that η can be calculated from EC₅₀ and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of η-values is bimodal with population means at 51% (including acetylcholine, nornicotine, and dimethylphenylpiperazinium) and 40% (including epibatidine, varenicline, and cytisine). The value of η is related inversely to the size of the agonist's headgroup, with high- versus low-efficiency ligands having an average volume of 70 vs. 102 Å³. Most binding site mutations have only a small effect on acetylcholine efficiency, except for αY190A (35%), αW149A (60%), and those at αG153 (42%). If η is known, the EC₅₀ and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC₅₀ of a CRC that has been normalized to 1. Given η, the level of constitutive activity can be estimated from a single CRC.

Characterization of nAChRs in Nematostella vectensis supports neuronal and non-neuronal roles in the cnidarian-bilaterian common ancestor

Background

Nicotinic and muscarinic acetylcholine receptors likely evolved in the cnidarian–bilaterian common ancestor. Both receptor families are best known for their role at chemical synapses in bilaterian animals, but they also have described roles as non-neuronal signaling receptors within the bilaterians. It is not clear when either of the functions for nicotinic or muscarinic receptors evolved. Previous studies in cnidarians suggest that acetylcholine’s neuronal role existed prior to the cnidarian–bilaterian divergence, but did not address potential non-neuronal functions. To determine the origins of neuronal and non-neuronal functions of nicotinic acetylcholine receptors, we investigated the phylogenetic position of cnidarian acetylcholine receptors, characterized the spatiotemporal expression patterns of nicotinic receptors in N. vectensis, and compared pharmacological studies in N. vectensis to the previous work in other cnidarians.

Results

Consistent with described activity in other cnidarians, treatment with acetylcholine-induced tentacular contractions in the cnidarian sea anemone N. vectensis. Phylogenetic analysis suggests that the N. vectensis genome encodes 26 nicotinic (nAChRs) and no muscarinic (mAChRs) acetylcholine receptors and that nAChRs independently radiated in cnidarian and bilaterian linages. The namesake nAChR agonist, nicotine, induced tentacular contractions similar to those observed with acetylcholine, and the nAChR antagonist mecamylamine suppressed tentacular contractions induced by both acetylcholine and nicotine. This indicated that tentacle contractions are in fact mediated by nAChRs. Nicotine also induced the contraction of radial muscles, which contract as part of the peristaltic waves that propagate along the oral–aboral axis of the trunk. Radial contractions and peristaltic waves were suppressed by mecamylamine. The ability of nicotine to mimic acetylcholine responses, and of mecamylamine to suppress acetylcholine and nicotine-induced contractions, supports a neuronal function for acetylcholine in cnidarians. Examination of the spatiotemporal expression of N. vectensis nAChRs (NvnAChRs) during development and in juvenile polyps identified that NvnAChRs are expressed in neurons, muscles, gonads, and large domains known to be consistent with a role in developmental patterning. These patterns are consistent with nAChRs functioning in both a neuronal and non-neuronal capacity in N. vectensis.

Conclusion

Our data suggest that nAChR receptors functioned at chemical synapses in N. vectensis to regulate tentacle contraction. Similar responses to acetylcholine are well documented in cnidarians, suggesting that the neuronal function represents an ancestral role for nAChRs. Expression patterns of nAChRs are consistent with both neuronal and non-neuronal roles for acetylcholine in cnidarians. Together, these observations suggest that both neuronal and non-neuronal functions for the ancestral nAChRs were present in the cnidarian–bilaterian common ancestor. Thus, both roles described in bilaterian species likely arose at or near the base of nAChR evolution.

HPO-Shuffle: an associated gene prioritization strategy and its application in drug repurposing for the treatment of canine epilepsy

Epilepsy is a common neurological disorder that affects mammalian species including human beings and dogs. In order to discover novel drugs for the treatment of canine epilepsy, multiomics data were analyzed to identify epilepsy associated genes. In this research, the original ranking of associated genes was obtained by two high-throughput bioinformatics experiments: Genome Wide Association Study (GWAS) and microarray analysis. The association ranking of genes was enhanced by a re-ranking system, HPO-Shuffle, which integrated information from GWAS, microarray analysis and Human Phenotype Ontology database for further downstream analysis. After applying HPO-Shuffle, the association ranking of epilepsy genes were improved. Afterward, a weighted gene coexpression network analysis (WGCNA) led to a set of gene modules, which hinted a clear relevance between the high ranked genes and the target disease. Finally, WGCNA and connectivity map (CMap) analysis were performed on the integrated dataset to discover a potential drug list, in which a well-known anticonvulsant phensuximide was included.

A single molecular distance predicts agonist binding energy in nicotinic receptors

Agonists turn on receptors because they bind more strongly to active (R*) versus resting (R) conformations of their target sites. Here, to explore how agonists activate neuromuscular acetylcholine receptors, we built homology models of R and R* neurotransmitter binding sites, docked ligands to those sites, ran molecular dynamics simulations to relax ("equilibrate") the structures, measured binding site structural parameters, and correlated them with experimental agonist binding energies. Each binding pocket is a pyramid formed by five aromatic amino acids and covered partially by loop C. We found that in R* versus R, loop C is displaced outward, the pocket is smaller and skewed, the agonist orientation is reversed, and a key nitrogen atom in the agonist is closer to the pocket center (distance dx) and a tryptophan pair but farther from αY190. Of these differences, the change in dx shows the largest correlation with experimental binding energy and provides a good estimate of agonist affinity, efficacy, and efficiency. Indeed, concentration-response curves can be calculated from just dx values. The contraction and twist of the binding pocket upon activation resemble gating rearrangements of the extracellular domain of related receptors at a smaller scale.

A mechanism for acetylcholine receptor gating based on structure, coupling, phi, and flip

Gupta et al. use single-channel electrophysiology to investigate the gating mechanism of acetylcholine receptor ion channels. They propose that channel opening starts at the M2–M3 linker and ligand-binding sites and proceeds through four brief intermediate conformations before ending with the collapse of a gate bubble.

Nicotinic acetylcholine receptors are allosteric proteins that generate membrane currents by isomerizing (“gating”) between resting and active conformations under the influence of neurotransmitters. Here, to explore the mechanisms that link the transmitter-binding sites (TBSs) with the distant gate, we use mutant cycle analyses to measure coupling between residue pairs, phi value analyses to sequence domain rearrangements, and current simulations to reproduce a microsecond shut component (“flip”) apparent in single-channel recordings. Significant interactions between amino acids separated by >15 Å are rare; an exception is between the αM2–M3 linkers and the TBSs that are ∼30 Å apart. Linker residues also make significant, local interactions within and between subunits. Phi value analyses indicate that without agonists, the linker is the first region in the protein to reach the gating transition state. Together, the phi pattern and flip component suggest that a complete, resting↔active allosteric transition involves passage through four brief intermediate states, with brief shut events arising from sojourns in all or a subset. We derive energy landscapes for gating with and without agonists, and propose a structure-based model in which resting→active starts with spontaneous rearrangements of the M2–M3 linkers and TBSs. These conformational changes stabilize a twisted extracellular domain to promote transmembrane helix tilting, gate dilation, and the formation of a “bubble” that collapses to initiate ion conduction. The energy landscapes suggest that twisting is the most energetically unfavorable step in the resting→active conformational change and that the rate-limiting step in the reverse process is bubble formation.

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'.

Activation of endplate nicotinic acetylcholine receptors by agonists

The interaction of a small molecule made in one cell with a large receptor made in another is the signature event of cell signaling. Understanding the structure and energy changes associated with agonist activation is important for engineering drugs, receptors and synapses. The nicotinic acetylcholine receptor (AChR) is a ∼300kD ion channel that binds the neurotransmitter acetylcholine (ACh) and other cholinergic agonists to elicit electrical responses in the central and peripheral nervous systems. This mini-review is in two sections. First, general concepts of skeletal muscle AChR operation are discussed in terms of energy landscapes for conformational change. Second, adult vs. fetal AChRs are compared with regard to interaction energies between ACh and agonist-site side chains, measured by single-channel electrophysiology and molecular dynamics simulations. The five aromatic residues that form the core of each agonist binding site can be divided into two working groups, a triad (led by αY190) that behaves similarly at all sites and a coupled pair (led by γW55) that has a large influence on affinity only in fetal AChRs. Each endplate AChR has 5 homologous subunits, two of α(1) and one each of β, δ, and either γ (fetal) or ϵ (adult). These nicotinic AChRs have only 2 functional agonist binding sites located in the extracellular domain, at αδ and either αγ or αϵ subunit interfaces. The receptor undergoes a reversible, global isomerization between structures called C and O. The C shape does not conduct ions and has a relatively low affinity for ACh, whereas O conducts cations and has a higher affinity. When both agonist sites are empty (filled only with water) the probability of taking on the O conformation (PO) is low, <10(-6). When ACh molecules occupy the agonist sites the C→O opening rate constant and C↔O gating equilibrium constant increase dramatically. Following a pulse of ACh at the nerve-muscle synapse, the endplate current rises rapidly to reach a peak that corresponds to PO ∼0.96.

Neurotransmitter GABA activates muscle but not α7 nicotinic receptors

Cys-loop receptors are neurotransmitter-activated ion channels involved in synaptic and extrasynaptic transmission in the brain and are also present in non-neuronal cells. As GABAA and nicotinic receptors (nAChR) belong to this family, we explored by macroscopic and single-channel recordings whether the inhibitory neurotransmitter GABA has the ability to activate excitatory nAChRs. GABA differentially activates nAChR subtypes. It activates muscle nAChRs, with maximal peak currents of about 10% of those elicited by acetylcholine (ACh) and 15-fold higher EC50 with respect to ACh. At the single-channel level, the weak agonism is revealed by the requirement of 20-fold higher concentration of GABA for detectable channel openings, a major population of brief openings, and absence of clusters of openings when compared with ACh. Mutations at key residues of the principal binding-site face of muscle nAChRs (αY190 and αG153) affect GABA activation similarly as ACh activation, whereas a mutation at the complementary face (εG57) shows a selective effect for GABA. Studies with subunit-lacking receptors show that GABA can activate muscle nAChRs through the α/δ interface. Interestingly, single-channel activity elicited by GABA is similar to that elicited by ACh in gain-of-function nAChR mutants associated to congenital myasthenic syndromes, which could be important in the progression of the disorders due to steady exposure to serum GABA. In contrast, GABA cannot elicit single-channel or macroscopic currents of α7 or the chimeric α7-serotonin-type 3 receptor, a feature important for preserving an adequate excitatory/inhibitory balance in the brain as well as for avoiding activation of non-neuronal receptors by serum GABA.

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.

Energy for wild-type acetylcholine receptor channel gating from different choline derivatives

Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.

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).

The energy and work of a ligand-gated ion channel

Ligand-gated ion channels are allosteric membrane proteins that isomerize between C(losed) and O(pen) conformations. A difference in affinity for ligands in the two states influences the C↔O "gating" equilibrium constant. The energies associated with adult-type mouse neuromuscular nicotinic acetylcholine receptor (AChR) channel gating have been measured by using single-channel electrophysiology. Without ligands, the free energy, enthalpy and entropy of gating are ΔG0=+8.4, ΔH0=+10.9 and TΔS0=+2.5kcal/mol (-100mV, 23°C). Many mutations throughout the protein change ΔG0, including natural ones that cause disease. Agonists and most mutations change approximately independently the ground-state energy difference; thus, it is possible to forecast and engineer AChR responses simply by combining perturbations. The free energy of the low↔high affinity change for the neurotransmitter at each of two functionally equivalent binding sites is ΔGB(ACh)=-5.1kcal/mol. ΔGB(ACh) is set mainly by interactions of ACh with just three binding site aromatic groups. For a series of structurally related agonists, there is a correlation between the energies of low- and high-affinity binding, which implies that gating commences with the formation of the low-affinity complex. Brief, intermediate states in binding and gating have been detected. Several proposals for the nature of the gating transition-state energy landscape and the isomerization mechanism are discussed.