Permeant but not impermeant divalent cations enhance activation of nondesensitizing alpha(7) nicotinic receptors

Recent Advances in the Discovery of Nicotinic Acetylcholine Receptor Allosteric Modulators

Positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent agonists, allosteric activating PAMs and neutral or silent allosteric modulators are compounds capable of modulating the nicotinic receptor by interacting at allosteric modulatory sites distinct from the orthosteric sites. This survey is focused on the compounds that have been shown or have been designed to interact with nicotinic receptors as allosteric modulators of different subtypes, mainly α7 and α4β2. Minimal chemical changes can cause a different pharmacological profile, which can then lead to the design of selective modulators. Experimental evidence supports the use of allosteric modulators as therapeutic tools for neurological and non-neurological conditions.

Nicotinic alpha 7 receptor expression and modulation of the lung epithelial response to lipopolysaccharide

Nicotine modulates multiple inflammatory responses in the lung through the nicotinic acetylcholine receptor subtype alpha7 (α7). Previously we reported that α7 modulates both the hematopoietic and epithelium responses in the lung to the bacterial inflammogen, lipopolysaccharide (LPS). Here we apply immunohistochemistry, flow cytometry and RNA-Seq analysis of isolated distal lung epithelium to further define α7-expression and function in this tissue. Mouse lines were used that co-express a bicistronic tau-green fluorescent protein (tGFP) as a reporter of α7 (α7^G) expression and that harbor an α7 with a specific point mutation (α7^E260A:G) that selectively uncouples it from cell calcium-signaling mechanisms. The tGFP reporter reveals strong cell-specific α7-expression by alveolar macrophages (AM), Club cells and ATII cells. Ciliated cells do not express detectible tGFP, but their numbers decrease by one-third in the α7^E260A:G lung compared to controls. Transcriptional comparisons (RNA-Seq) between α7^G and α7^E260A:G enriched lung epithelium 24 hours after challenge with either intra-nasal (i.n.) saline or LPS reveals a robust α7-genotype impact on both the stasis and inflammatory response of this tissue. Overall the α7^E260A:G lung epithelium exhibits reduced inflammatory cytokine/chemokine expression to i.n. LPS. Transcripts specific to Club cells (e.g., CC10, secretoglobins and Muc5b) or to ATII cells (e.g., surfactant proteins) were constitutively decreased in in the α7^E260A:G lung, but they were strongly induced in response to i.n. LPS. Protein analysis applying immunohistochemistry and ELISA also revealed α7-associated differences suggested by RNA-Seq including altered mucin protein 5b (Muc5b) accumulation in the α7^E260A:G bronchia, that in some cases appeared to form airway plugs, and a substantial increase in extracellular matrix deposits around α7^E260A:G airway bronchia linings that was not seen in controls. Our results show that α7 is an important modulator of normal gene expression stasis and the response to an inhaled inflammogen in the distal lung epithelium. Further, when normal α7 signaling is disrupted, changes in lung gene expression resemble those associated with long-term lung pathologies seen in humans who use inhaled nicotine products.

The nicotinic receptor Alpha7 impacts the mouse lung response to LPS through multiple mechanisms

The nicotinic acetylcholine receptor alpha7 (α7) is expressed by neuronal and non-neuronal cells throughout the body. We examined the mechanisms of the lung inflammatory response to intranasal (i.n.) lipopolysaccharide (LPS) regulated by α7. This was done in mice using homologous recombination to introduce a point mutation in the α7 receptor that replaces the glutamate residue 260 that lines the pore with alanine (α7E260A), which has been implicated in controlling the exceptional calcium ion conductance of this receptor. The α7E260A mice exhibit normal inflammatory cell recruitment to the blood in response to i.n. LPS administration. This differs from the α7knock-out (α7KO) in which upstream signaling to initiate the recruitment to the blood following i.n. LPS is significantly impaired. While hematopoietic cells are recruited to the bloodstream in the α7E260A mouse, they fail to be recruited efficiently into both the interstitium and alveolar spaces of the lung. Bone marrow reconstitution experiments demonstrate that the responsiveness of both CD45+ and CD45- cells of the α7E260A mouse are impaired. The expression of several pro-inflammatory cytokine and chemokine RNAs including TNFα, IL-1α, Ccl2 and Cxcl10 are decreased in the α7E260A mouse. However, there is a substantial increase in IL-13 expression by CD45- lung interstitial cells in the α7E260A mouse. Our results support the conclusion that α7 functional pleiotropy contributes to modulating the tissue response to an inflammatory insult through impacting upon a variety of mechanisms reflecting the individual cell composition of the lung.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Quercetin enhances human α7 nicotinic acetylcholine receptor-mediated ion current through interactions with Ca(2+) binding sites

The flavonoid quercetin is a low molecular weight substance found in fruits and vegetables. Aside from its anti-oxidative effect, quercetin, like other flavonoids, has a wide range of neuropharmacological actions. The α7 nicotinic acetylcholine receptor (α7 nAChR) has a Ca(2+)-binding site, is highly permeable to the Ca(2+) ion, and plays important roles in Ca(2+)-related normal brain functions. Dysfunctions of α7 nAChR are associated with a variety of neurological disorders. In the present study, we investigated the effects of quercetin on the ACh-induced inward peak current (I(ACh)) in Xenopus oocytes that heterologously express human α7 nAChR. I(ACh) was measured with the two-electrode voltage clamp technique. In oocytes injected with α7 nAChR cRNA, the effects of the co-application of quercetin on I(ACh) were concentration-dependent and reversible. The ED(50) was 36.1 + 6.1 μM. Quercetin-mediated enhancement of I(ACh) caused more potentiation when quercetin was pre-applied. The degree of I(ACh) potentiation by quercetin pre-application was time-dependent and saturated after 1 min. Quercetin-mediated I(ACh) enhancement was not affected by ACh concentration and was voltage-independent. However, quercetin-mediated I(ACh) enhancement was dependent on extracellular Ca(2+) concentrations and was specific to the Ca(2+) ion, since the removal of extracellular Ca(2+) or the addition of Ba(2+) instead of Ca(2+) greatly diminished quercetin enhancement of I(ACh). The mutation of Glu195 to Gln195, in the Ca(2+)-binding site, almost completely diminished quercetin-mediated I(ACh) enhancement. These results indicate that quercetin-mediated I(ACh) enhancement human α7 nAChR heterologously expressed in Xenopus oocytes could be achieved through interactions with the Ca(2+)-binding site of the receptor.

Calcium modulation of 5-HT3 receptor binding and function

Calcium modulates the 5-HT3 receptor response by reducing peak current amplitude and increasing rates of activation, deactivation and desensitisation, but the binding site(s) and mechanism(s) of this modulation are unknown. Here we study residues that may be involved in calcium binding in two partially overlapping regions of the extracellular domain (E213-E215-E218 and D204-E218-V219). The modulatory effects of calcium were assessed by radioligand binding and whole-cell patch-clamp. Comparisons of [3H]granisetron binding showed an increase in Kd in 10mM calcium that was abolished by the substitutions E213Q, E215Q, D204N and V219L. E218Q mutant receptors displayed no specific binding or function, and immunofluorescence showed that they did not reach the cell surface. E213Q increased inherent rates of desensitisation, but the relative effects of calcium on these rates, and on the reduction in current amplitude, were similar to wild type receptors. Current responses and calcium-mediated effects at E215Q mutant receptors were indistinguishable from wild type. D204N and V219L mutants were non-functional. A calcium impermeable mutant (E277A/S297R) revealed no changes in peak amplitude or kinetics with increased calcium. Our results are consistent with residues D204, E218 and V219 participating in receptor assembly, structure and/or trafficking to the plasma membrane, and we speculate that this might rely upon the stabilising effect of bound calcium. E213, E215, D204 and V219 may contribute to a calcium binding site that is responsible for the calcium-mediated effects on ligand binding. However, the major site for calcium-dependent modulation of the 5-HT3 current is located within the ion channel or cell interior.

Role of the Outer β-Sheet in Divalent Cation Modulation of α7 Nicotinic Receptors

alpha-7 Nicotinic acetylcholine receptors (AChRs) exhibit a positive modulation by divalent cations similar to that observed in other AChRs. In the chick alpha7 AChR, this modulation involves a conserved glutamate in loop 9 (Glu172) that undergoes agonist-dependent movements during activation. From these observations, we hypothesized that movements of the nearby beta-sheet formed by the beta7, beta9, and beta10 strands may be involved in agonist activation and/or divalent modulation. To test this hypothesis, we examined functional properties of cysteine mutations of the beta7 and beta10 strands, alone or in pairs. We postulated that reduced flexibility or mobility of the beta7/beta9/beta10-sheet as a result of introduction of a disulfide bond between the beta strands would alter activation by agonists. Using a nondesensitizing alpha7 mutant background (L247T), we identified one mutant pair, K144C + T198C, that exhibited a unique characteristic: it was fully activated by divalent cations (Ca2+, Ba2+, or Sr2+) in the absence of acetylcholine (ACh). Divalent-evoked currents were blocked by the alpha7 antagonist methyllycaconitine and were abolished when Glu172 was mutated to glutamine. When the K144C + T198C pair was expressed in wild-type alpha7 receptors, activation required both ACh and divalent cations. We conclude that the introduction of a disulfide bond into beta7/beta9/beta10 lowers the energetic barrier between open and closed conformations, probably by reducing the torsional flexibility of the beta-sheet. In this setting, divalent cations, acting at the conserved glutamate in loop 9, act as full agonists or requisite coagonists.

Divalent cation modulation of a-type potassium channels in acutely dissociated central neurons from wide-type and mutant Drosophila

Drosophila mutants provide an ideal model to study channel-type specificity of ion channel regulation in situ. In this study, the effects of divalent cations on voltage-gated K+ currents were investigated in acutely dissociated central neurons of Drosophila third instar larvae using the whole-cell patch-clamp recording. Our data showed that micromolar Cd2+ enhanced the peak inactivating current (I(A)) without affecting the delayed component (I(K)). The same results were obtained in Ca(2+)-free external solution, and from slo1 mutation, which eliminates transient Ca(2+)-activated K+ current. Micromolar Cd2+ and Zn2+, and millimolar Ca2+ and Mg2+ all shifted the steady-state inactivation curve of I(A) without affecting the voltage-dependence of I(A) activation, whereas millimolar Cd2+ markedly affected both the activation and steady-state inactivation curves for I(A). Divalent cations affected I(A) with different potency; the sequence was: Zn2+ > Cd2+ > Ca2+ > Mg2+. The modulation of I(A) by Cd2+ was partially inhibited in Sh(M), a null Shaker (one of I(A)-encoding genes) mutation. Taken together, the channel-type specificity, the asymmetric effects on I(A) activation and inactivation kinetics, and the diverse potency of divalent cations all strongly support the idea that physiological divalent cations modulate A-type K+ channels through specific binding to extracellular sites of the channels.

Biophysical and pharmacological characterization of nicotinic cholinergic receptors in rat cochlear inner hair cells

Before the onset of hearing, a transient efferent innervation is found on inner hair cells (IHCs). This synapse is inhibitory and mediated by a nicotinic cholinergic receptor (nAChR) probably formed by the alpha9 and alpha10 subunits. We analysed the pharmacological and biophysical characteristics of the native nAChR using whole-cell recordings from IHCs in acutely excised apical turns of the rat organ of Corti. Nicotine did not activate but rather blocked the acetylcholine (ACh)-evoked currents with an IC50 of 1 +/- 0.1 microM. Antagonists of non-cholinergic receptors such as strychnine, bicuculline and ICS-205930 blocked ACh-evoked responses with an IC50 of 8.6 +/- 0.8 nM, 59 +/- 4 nM and 0.30 +/- 0.02 microM, respectively. The IHC nAChR was both permeable to (P(Ca)/P(Na) = 8 +/- 0.9) and modulated by external Ca2+. ACh-evoked currents were potentiated by Ca2+ up to 500 microM but were reduced by higher concentrations of this cation. Ba2+ mimicked the effects of Ca2+ whereas Mg2+ only blocked these currents. In addition, elevation of extracellular Ca2+ reduced the amplitude of spontaneous synaptic currents without affecting their time course. The receptor had an EC50 for ACh of 60.7 +/- 2.8 microM in 0.5 mM Ca2+. In the absence of Ca2+, the EC50 for ACh increased, suggesting that potentiation by Ca2+ involves changes in the apparent affinity for the agonist. These pharmacological and biophysical characteristics of the IHC nAChR closely resemble those of the recombinant alpha9alpha10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear efferent-IHC synapse is composed of both the alpha9 and alpha10 subunits.

Unliganded human mutant alpha 7 nicotinic receptors are modulated by Ca2+ and trace levels of Zn2+

A large body of evidence indicates that ligand-gated channels may open spontaneously, exhibiting a basal activity in the absence of the neurotransmitter. In the present work, we were interested in studying the Ca(2+)-induced modulation of the basal channel activity of unliganded human L248Talpha7 receptors expressed in Xenopus oocytes. While the basal channel activity was blocked by either the nicotinic antagonist methyllycaconitine or the superfusion with a Ca(2+)-free medium, it was enhanced by increasing external Ca2+ concentrations. External Ca2+ significantly influenced the channel properties lengthening the channel duration and reducing the channel conductance, in a dose dependent manner. Furthermore, the basal channel activity in standard medium was blocked by N,N,N',N'-tetrakis-2-pyridylmethyl-ethylenediamine, the chelator of divalent cations with very high affinity for Zn2+, and was induced by Zn2+ when Ca2+ was present in the external medium. We conclude that basal activity of alpha7 mutant receptor-channels is caused by divalent cation contaminants present in the external medium, namely Zn2+; is positively modulated by the external Ca2+; and is inhibited when Ca2+ is absent from the medium. The patho-physiological consequences of these findings are discussed.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Agonist-induced conformational changes in the extracellular domain of alpha 7 nicotinic acetylcholine receptors

The molecular mechanisms that couple agonist binding to the gating of Cys-loop ionotropic receptors are not well understood. The crystal structure of the acetylcholine (ACh) binding protein has provided insights into the structure of the extracellular domain of nicotinic receptors and a framework for testing mechanisms of activation. Key ligand binding residues are located at the C-terminal end of the beta9 strand. At the N-terminal end of this strand (loop 9) is a conserved glutamate [E172 in chick alpha7 nicotinic acetylcholine receptors (nAChRs)] that is important for modulating activation. We hypothesize that agonist binding induces the movement of loop 9. To test this, we used the substituted-cysteine accessibility method to examine agonist-dependent changes in the modification of cysteines introduced in loop 9 of L247T alpha7 nAChRs. In the absence of agonist, ACh-evoked responses of E172C/L247T alpha7 nAChRs were inhibited by 2-trimethylammonioethylmethane thiosulfonate (MTSET). Agonist coapplication with MTSET reduced the extent and rate of modification. The dose-dependence of ACh activation was nearly identical with that of ACh-dependent protection from modification. ACh increased the inhibition by methanethiosulfonate reagents of N170C and did not change inhibition of G171C receptors. The antagonist dihydro-beta-erythroidine did not mimic the effects of ACh. Combined with a structural model, the data suggest that receptor activation includes subunit rotation and/or intrasubunit conformational changes that move N170 to a more accessible position and E172 to a more protected position away from the vestibule. Thus, loop 9, located near the junction between the extracellular and transmembrane domains, participates in conformational changes triggered by ligand binding.

Glutamate 172, essential for modulation of L247T alpha7 ACh receptors by Ca2+, lines the extracellular vestibule

Neuronal alpha7 nicotinic ACh receptors (nAChRs) are permeable to and modulated by Ca2+, Ba2+, and Sr2+. These permeant divalent cations interact with slowly desensitizing L247T alpha7 nAChRs to increase the potency and maximal efficacy of ACh, increase the efficacy of dihydro-beta-erythroidine (DHbetaE), and increase agonist-independent activity. Mutation of glutamate 172 (E172) to glutamine or cysteine eliminated these effects of permeant divalent cations. 2-(Trimethylammonium)ethyl methanethiosulfonate (MTSET), a cysteine-modifying reagent directed at water-accessible thiols, inhibited ACh-evoked currents of E172C/L247T alpha7 nAChRs by >90%, demonstrating that E172 was accessible to permeant ions. The data are consistent with a model of alpha7 receptors, derived from the crystal structure of the ACh binding protein (AChBP) from Lymnaea stagnalis, in which E172 projects toward the lumen of the extracellular vestibule. The observations that E172 was essential for divalent cation modulation of L247T alpha7 nAChRs and was accessible to permeating ions suggest that this residue participates in coupling ion permeation with modulation of receptor activity.