Mapping the structural requirements for nicotinic acetylcholine receptor activation by using tethered alkyltrimethylammonium agonists and antagonists

Interactions of Nereistoxin and Its Analogs with Vertebrate Nicotinic Acetylcholine Receptors and Molluscan ACh Binding Proteins

Nereistoxin (NTX) is a marine toxin isolated from an annelid worm that lives along the coasts of Japan. Its insecticidal properties were discovered decades ago and this stimulated the development of a variety of insecticides such as Cartap that are readily transformed into NTX. One unusual feature of NTX is that it is a small cyclic molecule that contains a disulfide bond. In spite of its size, it acts as an antagonist at insect and mammalian nicotinic acetylcholine receptors (nAChRs). The functional importance of the disulfide bond was assessed by determining the effects of inserting a methylene group between the two sulfur atoms, creating dimethylaminodithiane (DMA-DT). We also assessed the effect of methylating the NTX and DMA-DT dimethylamino groups on binding to three vertebrate nAChRs. Radioligand receptor binding experiments were carried out using washed membranes from rat brain and fish (Torpedo) electric organ; [³H]-cytisine displacement was used to assess binding to the predominantly high affinity alpha4beta2 nAChRs and [¹²⁵I]-alpha-bungarotoxin displacement was used to measure binding of NTX and analogs to the alpha7 and skeletal muscle type nAChRs. While the two quaternary nitrogen analogs, relative to their respective tertiary amines, displayed lower α4β2 nAChR binding affinities, both displayed much higher affinities for the Torpedo muscle nAChR and rat alpha7 brain receptors than their respective tertiary amine forms. The binding affinities of DMA-DT for the three nAChRs were lower than those of NTX and MeNTX. An AChBP mutant lacking the C loop disulfide bond that would potentially react with the NTX disulfide bond displayed an NTX affinity very similar to the parent AChBP. Inhibition of [³H]-epibatidine binding to the AChBPs was not affected by exposure to NTX or MeNTX for up to 24 hr prior to addition of the radioligand. Thus, the disulfide bond of NTX is not required to react with the vicinal disulfide in the AChBP C loop for inhibition of [³H]-epibatidine binding. However, a reversible disulfide interchange reaction of NTX with nAChRs might still occur, especially under reducing conditions. Labeled MeNTX, because it can be readily prepared with high specific radioactivity and possesses relatively high affinity for the nAChR-rich Torpedo nAChR, would be a useful probe to detect and identify any nereistoxin adducts.

The activity of GAT107, an allosteric activator and positive modulator of α7 nicotinic acetylcholine receptors (nAChR), is regulated by aromatic amino acids that span the subunit interface

GAT107, the (+)-enantiomer of racemic 4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide, is a strong positive allosteric modulator (PAM) of α7 nicotinic acetylcholine receptor (nAChR) activation by orthosteric agonists with intrinsic allosteric agonist activities. The direct activation produced by GAT107 in electrophysiological studies is observed only as long as GAT107 is freely diffusible in solution, although the potentiating activity primed by GAT107 can persist for over 30 min after drug washout. Direct activation is sensitive to α7 nAChR antagonist methyllycaconitine, although the primed potentiation is not. The data are consistent with GAT107 activity arising from two different sites. We show that the coupling between PAMs and the binding of orthosteric ligands requires tryptophan 55 (Trp-55), which is located at the subunit interface on the complementary surface of the orthosteric binding site. Mutations of Trp-55 increase the direct activation produced by GAT107 and reduce or prevent the synergy between allosteric and orthosteric binding sites, so that these mutants can also be directly activated by other PAMs such as PNU-120596 and TQS, which do not activate wild-type α7 in the absence of orthosteric agonists. We identify Tyr-93 as an essential element for orthosteric activation, because Y93C mutants are insensitive to orthosteric agonists but respond to GAT107. Our data show that both orthosteric and allosteric activation of α7 nAChR require cooperative activity at the interface between the subunits in the extracellular domain. These cooperative effects rely on key aromatic residues, and although mutations of Trp-55 reduce the restraints placed on the requirement for orthosteric agonists, Tyr-93 can conduct both orthosteric activation and desensitization among the subunits.

Which agonist properties are important for the activation of 5-HT3A receptors?

PURPOSE

Why do anesthetics not activate excitatory ligand-gated ion channels such as 5-HT3 receptors in contrast to inhibitory ligand-gated ion channels? This study examines the actions of structural closely-related 5-HT derivatives and 5-HT constituent parts on 5-HT3A receptors with the aim of finding simpler if not minimal agonists and thus determining requirements for successful agonist action.

EXPERIMENTAL APPROACH

Responses to 5-HT derivatives of human 5-HT3A receptors stably expressed in HEK 293 cells have been examined with the patch-clamp technique in the outside-out configuration combined with a fast solution exchange system.

RESULTS

Phenol, pyrrole and alkyl amines, constituents of 5-HT, even at high concentrations, cannot activate 5-HT3A receptors but they can inhibit them. To date, tyramines are the smallest known agonists. However, an aromatic ring is not required for activation as acetylcholine is also an agonist of similar strength.

CONCLUSION

Simultaneous interactions of adequate strength at two separate subsites within the 5-HT binding domain appear to be essential for successful agonist function. Anesthetics either fail to achieve this or the activation they produce is so weak that it is masked by a comparatively very strong inhibition.

Experimental determination of the vertical alignment between the second and third transmembrane segments of muscle nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) are members of the Cys-loop ligand-gated ion channel superfamily. Muscle nAChR are heteropentamers that assemble from two α, and one each of β, γ, and δ subunits. Each subunit is composed of three domains, extracellular, transmembrane and intracellular. The transmembrane domain consists of four α-helical segments (M1-M4). Pioneering structural information was obtained using electronmicroscopy of Torpedo nAChR. The recently solved X-ray structure of the first eukaryotic Cys-loop receptor, a truncated (intracellular domain missing) glutamate-gated chloride channel α (GluClα) showed the same overall architecture. However, a significant difference with regard to the vertical alignment between the channel-lining segment M2 and segment M3 was observed. Here, we used functional studies utilizing disulfide trapping experiments in muscle nAChR to determine the spatial orientation between M2 and M3. Our results are in agreement with the vertical alignment as obtained when using the GluClα structure as a template to homology model muscle nAChR, however, they cannot be reconciled with the current Torpedo nAChR model. The vertical M2-M3 alignments as observed in X-ray structures of prokaryotic Gloeobacter violaceus ligand-gated ion channel and GluClα are in agreement. Our results further confirm that this alignment in Cys-loop receptors is conserved between prokaryotes and eukaryotes.

Tethered agonist analogs as site-specific probes for domains of the human α7 nicotinic acetylcholine receptor that differentially regulate activation and desensitization

Homomeric α7 nicotinic acetylcholine receptors represent an important and complex pharmaceutical target. They can be activated by structurally diverse agonists and are highly likely to enter and remain in desensitized states at rates determined by the structures of the agonists. To identify structural elements regulating this function, we introduced reactive cysteines into the α7 ligand-binding domain allowing us to bind sulfhydryl-reactive (SH) agonist analogs or control reagents onto specific positions in the ligand binding domain. We identified four α7 mutants (S36C, L38C, W55C, and L119C) in which the tethering of the SH reagents blocked further acetylcholine-evoked activation of the receptor. However, after selective reaction with SH agonist analogs, the type II allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl-3-isoxazolyl)-urea (PNU-120596) could reactivate L119C and W55C mutants and receptors with a reduced or modified C-loop. Modified S36C and L38C mutants were insensitive to reactivation by PNU-120596, whether they were reacted with agonist analogs or alternative SH reagents. Molecular modeling showed that in the W55C and L119C mutants, the ammonium pharmacophore of the agonist analog methanethiosulfonate-ethyltrimethylammonium would be in a similar but nonidentical position underneath the C-loop. The orientation assumed by the ligand tethered to 119C was approximately 3-fold more sensitive to PNU-120596 than the alternative pose at 55C. Our results support the hypothesis that a single ligand can bind within the receptor in different ways and, depending on the specific binding pose, may variously promote activation or desensitization, or, alternatively, function as a competitive antagonist. This insight may provide a new approach for drug development.

Cysteine accessibility analysis of the human alpha7 nicotinic acetylcholine receptor ligand-binding domain identifies L119 as a gatekeeper

A large number of structurally diverse ligands have been produced to selectively target α7 nicotinic acetylcholine receptors (nAChRs). We applied the method of scanning cysteine accessibility mutations (SCAM) to the ligand-binding domain of the α7 nAChR to identify subdomains of particular importance to the binding and subsequent activation by select agonists. We evaluated the activity of four structurally distinct α7 agonists on wild-type human α7 and 44 targeted mutants expressed in Xenopus oocytes. Responses were measured prior and subsequent to the application of the sulfhydryl reagent methanethiosulfonate ethylammonium (MTSEA). One mutant (C116S) served as a Cys-null control, and the additional mutants were made in the C116S background. In many cases, the insertion of free cysteines into the agonist-binding site had a negative effect on function, with 12 of 44 mutants showing no detectable responses to ACh, and with only 19 of the 44 mutants showing sufficiently large responses to permit further study. Several of the cysteine mutations, including W55C, showed selectively reduced responses to the largest agonist tested, 2-methoxy,4-hydroxy-benzylidene anabaseine. Interestingly, although homology models suggest that most of the introduced cysteine mutations should have had good solvent accessibility, application of MTSEA had no effect or produced only modest changes in the agonist response profile of most mutants. Consistent with previous studies implicating W55 to play important roles in agonist activation, MTSEA treatment further decreased the functional responses of W55C to all the test agonists. While the cysteine mutation at L119 itself had relatively little effect on receptor function, treatment of L119C receptors with MTSEA or alternative cationic sulfhydryl reagents profoundly decreased activation by all agonists tested, suggesting a general block of gating. The homologous mutation in heteromeric nAChRs produced similar results, provided that the mutation was placed in the beta subunit complementary surface of the ligand-binding domain. Structural models locate the L119 residue directly across the subunit interface from the C-loop of the primary face of the binding domain. Our data suggest that a covalent modification of L119C by MTSEA or other cationic reagents might block the binding of even small agonists such as TMA through electrostatic interactions. Reaction of L119C with small non-polar reagents increases activation by small agonists but can block the access of large ligands such as benzylidene anabaseines to the ligand-binding domain.

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.

Functional equivalence of the nicotinic acetylcholine receptor transmitter binding sites in the open state

The subunits of the muscle-type nicotinic acetylcholine receptor (AChR) are not uniformly oriented in the resting closed conformation: the two alpha subunits are rotated relative to its non-alpha subunits. In contrast, all the subunits overlay well with one another when agonist is bound to the AChR, suggesting that they are uniformly oriented in the open receptor. This gating-dependent increase in orientational uniformity due to rotation of the alpha subunits might affect the relative affinities of the two transmitter binding sites, making the two affinities dissimilar (functionally non-equivalent) in the initial ligand-bound closed state but similar (functionally equivalent) in the open state. To test this hypothesis, we measured single-channel activity of the alphaG153S gain-of-function mutant receptor evoked by choline, and estimated the resting closed-state and open-state affinities of the two transmitter binding sites. Both model-independent analyses and maximum-likelihood estimation of microscopic rate constants indicate that channel opening makes the binding sites' affinities more similar to each other. These results support the hypothesis that open-state affinities to the transmitter binding sites are primarily determined by the alpha subunits.

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

The two transmitter binding sites of the neuromuscular acetylcholine (ACh) receptor channel contain several aromatic residues, including a tryptophan located on the complementary, negative face of each binding pocket. These two residues, Trp-55 in the epsilon subunit and Trp-57 in the delta subunit, were mutated (AEFHILRVY), and for most constructs the rate constants for acetylcholine binding and channel gating were estimated by using single channel kinetic analyses. The rate constants for unliganded channel opening and closing were also estimated for some mutants. From these measurements we calculated all of the equilibrium constants of the "allosteric" cycle as follows: diliganded gating, unliganded gating, dissociation from the C(losed) conformation, and dissociation from the O(pen) conformation. The results indicate the following. (i) These aromatic side chains play a relatively minor role in ACh receptor channel activation. (ii) The main consequence of mutations is to reduce the affinity of the O conformation of the binding site for ACh, with the effect being greater at the epsilon subunit. (iii) In epsilon (but not delta) the aromatic nature of the side chain is important in determining affinity, to a slightly greater degree in the O conformation. Phi value analyses (of both tryptophan residues) show Phi approximately 1 for both the ACh binding and diliganded gating reactions. (iv) This suggests that the structural boundaries of the dynamic elements of the gating conformational change may not be subunit-delimited, and (v) the mutated tryptophan residues experience energy changes that occur relatively early in both the ligand-binding and channel-gating reactions.