Gating of nicotinic ACh receptors; new insights into structural transitions triggered by agonist binding that induce channel opening

A Systematic Review on Drugs Acting as Nicotinic Acetylcholine Receptor Agonists in the Treatment of Dementia

Acetylcholine signaling is attenuated in early Alzheimer's disease (AD) and other dementias. A significant reduction in the expression of nicotinic acetylcholine receptors (nAChRs) in the brain of AD patients has also been reported in several molecular biological and in situ labeling studies. The modulation of the functional deficit of the cholinergic system as a pharmacological target could therefore have a clinical benefit, which is not to be neglected. This systematic review was conducted to identify clinical trials, which evaluated the safety and efficacy of nicotinic acetylcholine receptor agonists using Clinicaltrial (CT) and EudraCT databases. Structured searches identified 39 trials, which used 15 different drugs designed to increase the function of the nAChRs. Most of the identified clinical trials were phase II trials, with some of them classified as ongoing for several years. The systematic screening of the literature led to the selection of 14 studies out of the 8261 bibliographic records retrieved. Six trials reported detailed data on adverse events associated with the intervention, while twelve trials reported data on efficacy measures, such as attention, behavior and cognition. Overall, smost of the physical side effects of cholinergic agonists were reported to be well tolerated. Some trials also reported improvements in attention. However, the efficacy of these drugs in other cognitive and behavioral outcomes remains highly controversial.

Detection of differential selection pressure and functional-specific sites in subunits of vertebrate neuronal nicotinic acetylcholine receptors

The nicotinic acetylcholine receptors (nAChR) are made of subunits evolved from a common ancestor. Despite the similarity in their sequences and structures, the properties of these subunits vary significantly. Thus, identifying the evolution features and function-related sites specific to each subunit is essential for understanding the characteristics of the subunits and the receptors assembled by them. In this study, we examined the sequence features of the nine neuronal nAChRs subunits from representative vertebrate species. Analysis revealed that all the subunits were subject to strong purifying selection in evolution, and each was under a unique pattern of selection pressures. At the same time, the functional constraints were not uniform within each subunit, with different domains in the molecule being subject to different selection pressures. We also detected potential positive selection events in the subunits or subunit clusters, and identified the sites might be associated with the function specificity of each subunit. Furthermore, positive selection at some domains might contribute to the diversity of subunit function; for example, the β9 strand might be related to the agonist specificity of α subunit in heteromeric receptor and β4-β5 linker could be involved in Ca²⁺ permeability. Subunits α7, α4 and β2 subunits possess a strong adaptability in vertebrates. Our results highlighted the importance of tracking functional differentiation in protein sequence underlying functional properties of nAChRs. In summary, our work may provide clues on understanding the diversity and the function specificity of the nAChR subunits, as well as the receptors co-assembled by them.Communicated by Ramaswamy H. Sarma.

α1 Proline 277 Residues Regulate GABAAR Gating through M2-M3 Loop Interaction in the Interface Region

Cys-loop receptors are a superfamily of transmembrane, pentameric receptors that play a crucial role in mammalian CNS signaling. Physiological activation of these receptors is typically initiated by neurotransmitter binding to the orthosteric binding site, located at the extracellular domain (ECD), which leads to the opening of the channel pore (gate) at the transmembrane domain (TMD). Whereas considerable knowledge on molecular mechanisms of Cys-loop receptor activation was gathered for the acetylcholine receptor, little is known with this respect about the GABA_A receptor (GABA_AR), which mediates cellular inhibition. Importantly, several static structures of GABA_AR were recently described, paving the way to more in-depth molecular functional studies. Moreover, it has been pointed out that the TMD-ECD interface region plays a crucial role in transduction of conformational changes from the ligand binding site to the channel gate. One of the interface structures implicated in this transduction process is the M2-M3 loop with a highly conserved proline (P277) residue. To address this issue specifically for α₁β₂γ_2L GABA_AR, we choose to substitute proline α₁P277 with amino acids with different physicochemical features such as electrostatic charge or their ability to change the loop flexibility. To address the functional impact of these mutations, we performed macroscopic and single-channel patch-clamp analyses together with modeling. Our findings revealed that mutation of α₁P277 weakly affected agonist binding but was critical for all transitions of GABA_AR gating: opening/closing, preactivation, and desensitization. In conclusion, we provide evidence that conservative α₁P277 at the interface is strongly involved in regulating the receptor gating.

Cellular responses and functions of α7 nicotinic acetylcholine receptor activation in the brain: a narrative review

The α7 nicotinic acetylcholine receptor (α7nAChR) has been studied for many years since its discovery. Although many functions and characteristics of brain α7nAChR are widely understood, much remains to be elucidated. The α7nAChR is widely expressed in the central nervous system, not only in neurons but also in astrocytes, microglia, and endothelial cells. α7nAChR can be activated by endogenous agonist like acetylcholine or exogenous agonists like nicotine and PNU282987. Its agonists can be divided into selective agonists and non-selective agonists. The activation of α7nAChR results in a series of physiological processes which have both short-term and long-term effects on cells, for example, calcium influx, neurotransmitter release, synaptic plasticity, and excitatory transmission. It also induces other downstream events, such as inflammation, autophagy, necrosis, transcription, and apoptosis. The cellular responses to α7nAChR activation vary according to cell types and conditions. For example, α7nAChR activation in pyramidal neurons leads to long-term potentiation, while α7nAChR activation in GABAergic interneurons leads to long-term depression. Studies have also shown some contradictory phenomena, which requires further study for clarification. Herein, the cellular responses of α7nAChR activation are summarized, and the functions of α7nAChR in neurons and non-neuronal cells are discussed. We also summarized contradictory conclusions to show where we stand and where to go for future studies.

Functional divergence analysis of vertebrate neuronal nicotinic acetylcholine receptor subunits

Nicotinic acetylcholine receptors (nAChRs) are pentamers formed by subunits from a large multigene family and are highly variable in kinetic, electrophysiological and pharmacological properties. Due to the essential roles of nAChRs in many physiological procedures and diversity in function, identifying the function-related sites specific to each subunit is not only necessary to understand the properties of the receptors but also useful to design potential therapeutic compounds that target these macromolecules for treating a series of central neuronal disorders. By conducting a detailed function divergence analysis on nine neuronal nAChR subunits from representative vertebrate species, we revealed the existence of significant functional variation between most subunit pairs. Specifically, 44 unique residues were identified for the α7 subunit, while another 22 residues that were likely responsible for the specific features of other subunits were detected. By mapping these sites onto the 3 D structure of the human α7 subunit, a structure-function relationship profile was revealed. Our results suggested that the functional divergence related sites clustered in the ligand binding domain, the β2-β3 linker close to the N-terminal α-helix, the intracellular linkers between transmembrane domains, and the "transition zone" may have experienced altered evolutionary rates. The former two regions may be potential binding sites for the α7* subtype-specific allosteric modulators, while the latter region is likely to be subtype-specific allosteric modulations of the heteropentameric descendants such as the α4β2* nAChRs. Communicated by Ramaswamy H. Sarma.

Optogenetic studies of nicotinic contributions to cholinergic signaling in the central nervous system

Molecular manipulations and targeted pharmacological studies provide a compelling picture of which nicotinic receptor subtypes are where in the central nervous system (CNS) and what happens if one activates or deletes them. However, understanding the physiological contribution of nicotinic receptors to endogenous acetylcholine (ACh) signaling in the CNS has proven a more difficult problem to solve. In this review, we provide a synopsis of the literature on the use of optogenetic approaches to control the excitability of cholinergic neurons and to examine the role of CNS nicotinic ACh receptors (nAChRs). As is often the case, this relatively new technology has answered some questions and raised others. Overall, we believe that optogenetic manipulation of cholinergic excitability in combination with some rigorous pharmacology will ultimately advance our understanding of the many functions of nAChRs in the brain.

Nicotinic ACh receptors as therapeutic targets in CNS disorders

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability by acting on the cys-loop cation-conducting ligand-gated nicotinic ACh receptor (nAChR) channels. These receptors are widely distributed throughout the central nervous system (CNS), being expressed on neurons and non-neuronal cells, where they participate in a variety of physiological responses such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and cognitive functions. In the mammalian brain, nine different subunits have been found thus far, which assemble into pentameric complexes with much subunit diversity; however, the α7 and α4β2 subtypes predominate in the CNS. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders. Here we will briefly discuss the functional makeup and expression of the nAChRs in mammalian brain, and their role as targets in neurodegenerative diseases (in particular Alzheimer's disease, AD), neurodevelopmental disorders (in particular autism and schizophrenia), and neuropathic pain.

Tracking the molecular evolution of calcium permeability in a nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors are a family of ligand-gated nonselective cationic channels that participate in fundamental physiological processes at both the central and the peripheral nervous system. The extent of calcium entry through ligand-gated ion channels defines their distinct functions. The α9α10 nicotinic cholinergic receptor, expressed in cochlear hair cells, is a peculiar member of the family as it shows differences in the extent of calcium permeability across species. In particular, mammalian α9α10 receptors are among the ligand-gated ion channels which exhibit the highest calcium selectivity. This acquired differential property provides the unique opportunity of studying how protein function was shaped along evolutionary history, by tracking its evolutionary record and experimentally defining the amino acid changes involved. We have applied a molecular evolution approach of ancestral sequence reconstruction, together with molecular dynamics simulations and an evolutionary-based mutagenesis strategy, in order to trace the molecular events that yielded a high calcium permeable nicotinic α9α10 mammalian receptor. Only three specific amino acid substitutions in the α9 subunit were directly involved. These are located at the extracellular vestibule and at the exit of the channel pore and not at the transmembrane region 2 of the protein as previously thought. Moreover, we show that these three critical substitutions only increase calcium permeability in the context of the mammalian but not the avian receptor, stressing the relevance of overall protein structure on defining functional properties. These results highlight the importance of tracking evolutionarily acquired changes in protein sequence underlying fundamental functional properties of ligand-gated ion channels.

Mechanisms in the bed nucleus of the stria terminalis involved in control of autonomic and neuroendocrine functions: a review

The bed nucleus of the stria terminalis (BNST) is a heterogeneous and complex limbic forebrain structure, which plays an important role in controlling autonomic, neuroendocrine and behavioral responses. The BNST is thought to serve as a key relay connecting limbic forebrain structures to hypothalamic and brainstem regions associated with autonomic and neuroendocrine functions. Its control of physiological and behavioral activity is mediated by local action of numerous neurotransmitters. In the present review we discuss the role of the BNST in control of both autonomic and neuroendocrine function. A description of BNST control of cardiovascular and hypothalamus-pituitary-adrenal axisactivity at rest and during physiological challenges (stress and physical exercise) is presented. Moreover, evidence for modulation of hypothalamic magnocellular neurons activity is also discussed. We attempt to focus on the discussion of BNST neurochemical mechanisms. Therefore, the source and targets of neurochemical inputs to BNST subregions and their role in control of autonomic and neuroendocrine function is discussed in details.

Activation of the α7 nicotinic ACh receptor induces anxiogenic effects in rats which is blocked by a 5-HT₁a receptor antagonist

The α7 nicotinic acetylcholine receptor (nAChR) is highly expressed in different regions of the brain and is associated with cognitive function as well as anxiety. Agonists and positive allosteric modulators (PAMs) of the α7 subtype of nAChRs have been shown to improve cognition. Previously nicotine, which activates both α7 and non-α7 subtypes of nAChRs, has been shown to have an anxiogenic effect in behavioral tests. In this study, we compared the effects of the α7-selective agonist (PNU-282987) and PAM (PNU-120596) in a variety of behavioral tests in Sprague Dawley rats to look at their effects on learning and memory as well as anxiety. We found that neither PNU-282987 nor PNU-120596 improved spatial-learning or episodic memory by themselves. However when cognitive impairment was induced in the rats with scopolamine (1 mg/kg), both PNU-120596 and PNU-282987 were able to reverse this memory impairment and restore it back to normal levels. While PNU-120596 reversed the scopolamine-induced cognitive impairment, it did not have any adverse effect on anxiety. PNU-282987 on the other hand displayed an increase in anxiety-like behavior at a higher dose (10 mg/kg) that was significantly reduced by the serotonin 5-HT₁a receptor antagonist WAY-100135. However the α7 receptor antagonist methyllycaconitine was unable to reverse these anxiety-like effects seen with PNU-282987. These results suggest that α7 nAChR PAMs are pharmacologically advantageous over agonists, and should be considered for further development as therapeutic drugs targeting the α7 receptors.

Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability throughout the nervous system by acting on both the cys-loop ligand-gated nicotinic ACh receptor channels (nAChRs) and the G protein-coupled muscarinic ACh receptors (mAChRs). The hippocampus is an important area in the brain for learning and memory, where both nAChRs and mAChRs are expressed. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca, the activation of which can activate both nAChRs and mAChRs in the hippocampus and regulate synaptic communication and induce oscillations that are thought to be important for cognitive function. Dysfunction in the hippocampal cholinergic system has been linked with cognitive deficits and a variety of neurological disorders and diseases, including Alzheimer's disease and schizophrenia. My lab has focused on the role of the nAChRs in regulating hippocampal function, from understanding the expression and functional properties of the various subtypes of nAChRs, and what role these receptors may be playing in regulating synaptic plasticity. Here, I will briefly review this work, and where we are going in our attempts to further understand the role of these receptors in learning and memory, as well as in disease and neuroprotection.

Functional α7 nicotinic ACh receptors on astrocytes in rat hippocampal CA1 slices

Although much is known about the functional expression of the neuronal nicotinic acetylcholine receptors (nAChRs) in various neuronal populations in the brain and elsewhere, much less is known about their expression and functional relevance in glial cells. The expression of functional nAChRs has been reported for cultured astrocytes; however, previous work has failed to detect nAChR-mediated responses in astrocytes in acute slices. In the current study, functional α7 nAChRs on astrocytes in the CA1 region of the rat hippocampus were studied in situ using whole-cell patch-clamp recording and two-photon calcium imaging techniques in acute slices. We found that astrocytes and the chondroitin sulfate proteoglycan NG2-expressing (i.e., NG2) cells did express functional α7 nAChRs. Although the amplitudes of the responses were small, they could be enhanced by the α7-selective positive allosteric modulator PNU-120596. Under these conditions, we found that in comparing the properties of these responses between astrocytes, NG2, and interneurons, there were differences in the kinetics and increases in intracellular calcium levels. This is the first demonstration of functional α7 nAChR-mediated current responses in astrocytes in acute hippocampal slices, data which may shed light on the role of α7 nAChRs in neuroprotection.

Synaptic neurotransmitter-gated receptors

Since the discovery of the major excitatory and inhibitory neurotransmitters and their receptors in the brain, many have deliberated over their likely structures and how these may relate to function. This was initially satisfied by the determination of the first amino acid sequences of the Cys-loop receptors that recognized acetylcholine, serotonin, GABA, and glycine, followed later by similar determinations for the glutamate receptors, comprising non-NMDA and NMDA subtypes. The last decade has seen a rapid advance resulting in the first structures of Cys-loop receptors, related bacterial and molluscan homologs, and glutamate receptors, determined down to atomic resolution. This now provides a basis for determining not just the complete structures of these important receptor classes, but also for understanding how various domains and residues interact during agonist binding, receptor activation, and channel opening, including allosteric modulation. This article reviews our current understanding of these mechanisms for the Cys-loop and glutamate receptor families.

Isoflurane alters the structure and dynamics of GLIC

Pentameric ligand-gated ion channels are targets of general anesthetics. Although the search for discrete anesthetic binding sites has achieved some degree of success, little is known regarding how anesthetics work after the events of binding. Using the crystal structures of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC), which is sensitive to a variety of general anesthetics, we performed multiple molecular dynamics simulations in the presence and absence of the general anesthetic isoflurane. Isoflurane bound to several locations within GLIC, including the transmembrane pocket identified crystallographically, the extracellular (EC) domain, and the interface of the EC and transmembrane domains. Isoflurane also entered the channel after the pore was dehydrated in one of the simulations. Isoflurane disrupted the quaternary structure of GLIC, as evidenced in a striking association between the binding and breakage of intersubunit salt bridges in the EC domain. The pore-lining helix experienced lateral and inward radial tilting motion that contributed to the channel closure. Isoflurane binding introduced strong anticorrelated motions between different subunits of GLIC. The demonstrated structural and dynamical modulations by isoflurane aid in the understanding of the underlying mechanism of anesthetic inhibition of GLIC and possibly other homologous pentameric ligand-gated ion channels.

The GABRA6 mutation, R46W, associated with childhood absence epilepsy, alters 6β22 and 6β2 GABA(A) receptor channel gating and expression

A GABA(A) receptor α6 subunit mutation, R46W, was identified as a susceptibility gene that may contribute to the pathogenesis of childhood absence epilepsy (CAE), but the molecular basis for alteration of GABA(A) receptor function is unclear. The R46W mutation is located in a region homologous to a GABA(A) receptor γ2 subunit missense mutation, R82Q, that is associated with CAE and febrile seizures in humans. To determine how this mutation reduces GABAergic inhibition, we expressed wild-type (α6β2γ2L and α6β2δ) and mutant (α6(R46W)β2γ2L and α6(R46W)β2δ) receptors in HEK 293T cells and characterize their whole-cell and single-channel currents, and surface and total levels. We demonstrated that gating and assembly of both α6(R46W)β2γ2L and α6(R46W)β2δ receptors were impaired. Compared to wild-type currents, α6(R46W)β2γ2L and α6(R46W)β2δ receptors had a reduced current density, α6(R46W)β2γ2L currents desensitized to a greater extent and deactivated at a slower rate, α6(R46W)β2δ receptors did not desensitize but deactivated faster and both α6(R46W)β2γ2L and α6(R46W)β2δ single-channel current mean open times and burst durations were reduced. Surface levels of coexpressed α6(R46W), β2 and δ, but not γ2L, subunits were decreased. 'Heterozygous' coexpression of α6(R46W) and α6 subunits with β2 and γ2L subunits produced intermediate macroscopic current amplitudes by increasing incorporation of wild-type and decreasing incorporation of mutant subunits into receptors trafficked to the surface. Finally, these findings suggest that similar to the γ2(R82Q) mutation, the CAE-associated α6(R46W) mutation could cause neuronal disinhibition and thus increase susceptibility to generalized seizures through a reduction of αβγ and αβδ receptor function and expression.

Allosteric modulator Desformylflustrabromine relieves the inhibition of α2β2 and α4β2 nicotinic acetylcholine receptors by β-amyloid(1-42) peptide

Nicotinic acetylcholine receptors (nAChRs) are pentameric transmembrane proteins that belong to the cys-loop ligand-gated ion channel family. These receptors are widely expressed in the brain and implicated in the pathophysiology of many neurological conditions, including Alzheimer's disease (AD), where typical symptoms include the loss of cognitive function and dementia. The presence of extracellular neuritic plaques composed of β amyloid (Aβ(1-42)) peptide is a characteristic feature of AD. Desformylflustrabromine (dFBr) is a positive allosteric modulator (PAM) for α4β2 nAChRs since it increases peak ACh responses without inducing a response on its own. Previously, the effect of dFBr on the α2β2 nAChR subtype was not known. The action of dFBr was tested on α2β2 receptors expressed in Xenopus oocytes. It was found that dFBr is also a PAM for the α2β2 receptor. Next we tested whether dFBr had any effect on the previously known block of both the α4β2 and α2β2 receptors by Aβ(1-42). We found that the functional blockade of ACh-induced currents in oocytes expressing α4β2 and α2β2 receptors by Aβ(1-42) was prevented by dFBr. We conclude that dFBr is a positive allosteric modulator for both α4β2 and α2β2 subtypes of nAChRs and that it also relieves the blockade of these receptors by Aβ(1-42). This study demonstrates that PAMs for the non-α7 nAChRs have the potential to develop into clinically applicable drugs for AD and other disorders.

Role of key aromatic residues in the ligand-binding domain of alpha7 nicotinic receptors in the agonist action of beta-amyloid

Soluble β-amyloid (Aβ) resides in certain regions of the brain at or near picomolar concentration, rising in level during the prodromic stage of Alzheimer disease. Recently, we identified the homomeric α7 nicotinic acetylcholine receptor (α7-nAChR) as one possible functional target for picomolar Aβ. This study was aimed at addressing which residues in α7-nAChRs potentially interact with Aβ to regulate the presynaptic function of this receptor. Site-directed mutagenesis was carried out to study the key aromatic residues in the mouse α7-nAChR agonist-binding pocket. Mutations of tyrosine188 resulted in a decrease in activation of presynaptic α7-nAChRs by ACh and Aβ but with no change in response to nicotine, indicating the critical role of Tyr-188 in presynaptic regulation by Aβ. Coimmunoprecipitation additionally revealed direct binding of Aβ to α7-nAChRs and to the Tyr-188 mutant receptor. In contrast, mutations of Tyr-195 in α7-nAChR led to decreased activation by nicotine without apparent effects on ACh- or Aβ-induced responses. Agonist-induced responses of Tyr-93 mutant α7-nAChRs indicated possible interactions of nicotine and Aβ with its hydroxyl group, but there was no change in presynaptic responses after mutation of Trp-149. All of the mutants were shown to be expressed on the plasma membrane using cell surface labeling. Together, these results directly demonstrate an essential role for the aromatic residue Tyr-188 as a key component in the agonist binding domain for the activation of α7-nAChRs by Aβ.

Additional acetylcholine (ACh) binding site at alpha4/alpha4 interface of (alpha4beta2)2alpha4 nicotinic receptor influences agonist sensitivity

Nicotinic acetylcholine receptor (nAChR) α4 and β2 subunits assemble in two alternate stoichiometries to produce (α4β2)(2)α4 and (α4β2)(2)β2, which display different agonist sensitivities. Functionally relevant agonist binding sites are thought to be located at α4(+)/β2(-) subunit interfaces, but because these interfaces are present in both receptor isoforms, it is unlikely that they account for differences in agonist sensitivities. In contrast, incorporation of either α4 or β2 as auxiliary subunits produces isoform-specific α4(+)/α4(-) or β2(+)/β2(-) interfaces. Using fully concatenated (α4β2)(2)α4 nAChRs in conjunction with structural modeling, chimeric receptors, and functional mutagenesis, we have identified an additional site at the α4(+)/α4(-) interface that accounts for isoform-specific agonist sensitivity of the (α4β2)(2)α4 nAChR. The additional site resides in a region that also contains a potentiating Zn(2+) site but is engaged by agonists to contribute to receptor activation. By engineering α4 subunits to provide a free cysteine in loop C at the α4(+)α4(-) interface, we demonstrated that the acetylcholine responses of the mutated receptors are attenuated or enhanced, respectively, following treatment with the sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate or aminoethyl methanethiosulfonate. The findings suggest that agonist occupation of the site at the α4(+)/(α4(-) interface leads to channel gating through a coupling mechanism involving loop C. Overall, we propose that the additional agonist site at the α4(+)/α4(-) interface, when occupied by agonist, contributes to receptor activation and that this additional contribution underlies the agonist sensitivity signature of (α4β2)(2)α4 nAChRs.

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Rapid desensitization of the rat α7 nAChR is facilitated by the presence of a proline residue in the outer β-sheet

The rat α7 nicotinic acetylcholine receptor (nAChR) has a proline residue near the middle of the β9 strand. The replacement of this proline residue at position 180 (P180) by either threonine (α7-P180T) or serine (α7-P180S) slowed the onset of desensitization dramatically, with half-times of ~930 and 700 ms, respectively, compared to 90 ms for the wild-type receptor. To investigate the importance of the hydroxyl group on the position 180 side-chains, the mutant receptors α7-P180Y and α7-P180F were studied and showed half-times of desensitization of 650 and 160 ms, respectively. While a position 180 side-chain OH group may contribute to the slow desensitization rates, α7-P180S and α7-P180V resulted in receptors with similar desensitization rates, suggesting that increased backbone to backbone H bonding expected in the absence of proline at position 180 would likely exert a great effect on desensitization. Single channel recordings indicated that for the α7-P180T receptor there was a significantly reduced closed time without any change in single channel conductance (as compared to wild-type). Kinetic simulations indicated that all changes observed for the mutant channel behaviour were reproduced by decreasing the rate of desensitization, and increasing the microscopic affinity to resting receptors. Molecular dynamics (MD) simulations on a homology model were used to provide insight into likely H bond interactions within the outer β-sheet that occur when the P180 residue is mutated. All mutations analysed increased about twofold the predicted number of H bonds between the residue at position 180 and the backbone of the β10 strand. Moreover, the α7-P180T and α7-P180S mutations also formed some intrastrand H bonds along the β9 strand, although H bonding of the OH groups of the threonine or serine side-chains was predicted to be infrequent. Our results indicate that rapid desensitization of the wild-type rat α7 nAChR is facilitated by the presence of the proline residue within the β9 strand.

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.

Identification and functional characterization of a novel acetylcholine-binding protein from the marine annelid Capitella teleta

We identified a homologue of the molluscan acetylcholine-binding protein (AChBP) in the marine polychaete Capitella teleta, from the annelid phylum. The amino acid sequence of C. teleta AChBP (ct-AChBP) is 21-30% identical with those of known molluscan AChBPs. Sequence alignments indicate that ct-AChBP has a shortened Cys loop compared to other Cys loop receptors, and a variation on a conserved Cys loop triad, which is associated with ligand binding in other AChBPs and nicotinic ACh receptor (nAChR) alpha subunits. Within the D loop of ct-AChBP, a conserved aromatic residue (Tyr or Trp) in nAChRs and molluscan AChBPs, which has been implicated directly in ligand binding, is substituted with an isoleucine. Mass spectrometry results indicate that Asn122 and Asn216 of ct-AChBP are glycosylated when expressed using HEK293 cells. Small-angle X-ray scattering data suggest that the overall shape of ct-AChBP in the apo or unliganded state is similar to that of homologues with known pentameric crystal structures. NMR experiments show that acetylcholine, nicotine, and alpha-bungarotoxin bind to ct-AChBP with high affinity, with K(D) values of 28.7 microM, 209 nM, and 110 nM, respectively. Choline bound with a lower affinity (K(D) = 163 microM). Our finding of a functional AChBP in a marine annelid demonstrates that AChBPs may exhibit variations in hallmark motifs such as ligand-binding residues and Cys loop length and shows conclusively that this neurotransmitter binding protein is not limited to the phylum Mollusca.

Activation of alpha6-containing GABAA receptors by pentobarbital occurs through a different mechanism than activation by GABA

The GABA(A) receptors are ligand-gated chloride channels which are the targets for many clinically used sedatives, including the barbiturates. The barbiturate pentobarbital acts through multiple sites on the GABA(A) receptor. At low concentrations (muM), it acts as a positive allosteric modulator while at higher concentrations it can directly activate the receptor. This agonist action is influenced by the subunit composition of the receptor, and pentobarbital is a more effective agonist than GABA only at receptors containing an alpha6 subunit. The conformational change that translates GABA binding into channel opening is known to involve a lysine residue located in an extracellular domain between the 2nd and 3rd transmembrane domains. Mutations of this residue disrupt activation of the channel by GABA and have been linked to inherited epilepsy. Pentobarbital binds to the receptor at a different agonist site than GABA, but could use a common signal transduction mechanism to gate the channel. To address this question, we compared the effect of a mutating the homologous lysine residue in the alpha1 or alpha6 subunits (K278 or K277, respectively) to methionine on direct activation of recombinant GABA(A) receptors by GABA or pentobarbital. We found that this mutation reduced GABA sensitivity for both alpha1 and alpha6 subunits, but affected pentobarbital sensitivity only for the alpha1 subunit. This suggests that pentobarbital acts through a distinct signal transduction pathway at the alpha6 subunit, which may account for its greater efficacy compared to GABA at receptors containing this subunit.

Insights into docking and scoring neuronal alpha4beta2 nicotinic receptor agonists using molecular dynamics simulations and QM/MM calculations

A combined quantum mechanical (QM)-polarized docking and molecular dynamics approach to study the binding mode and to predict the binding affinity of ligands acting at the alpha4beta2-nAChR is presented. The results obtained in this study indicate that the quantum mechanical/molecular mechanics docking protocol well describes the charge-driven interactions occurring in the binding of nicotinic agonists, and it is able to represent the polarization effects on the ligand exerted by the surrounding atoms of the receptor at the binding site. This makes it possible to properly score agonists of alpha4beta2-nAChR and to reproduce the experimental binding affinity data with good accuracy, within a mean error of 2.2 kcal/mol. Moreover, applying the QM-polarized docking to an ensemble of nAChR conformations obtained from MD simulations enabled us to accurately capture nAChR-ligand induced-fit effects.

Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region

Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.

Mapping of disulfide bonds within the amino-terminal extracellular domain of the inhibitory glycine receptor

The strychnine-sensitive glycine receptor (GlyR) is a ligand-gated chloride channel and a member of the superfamily of cysteine loop (Cys-loop) neurotransmitter receptors, which also comprises the nicotinic acetylcholine receptor (nAChR). Within the extracellular domain (ECD), the eponymous Cys-loop harbors two conserved cysteines, assumed to be linked by a superfamily-specific disulfide bond. The GlyR ECD carries three additional cysteine residues, two are predicted to form a second, GlyR-specific bond. The configuration of none of the cysteines of GlyR, however, had been determined directly. Based on a crystal structure of the nAChRalpha1 ECD, we generated a model of the human GlyRalpha1 where close proximity of the respective cysteines was consistent with the formation of both the Cys-loop and the GlyR-specific disulfide bonds. To identify native disulfide bonds, the GlyRalpha1 ECD was heterologously expressed and refolded under oxidative conditions. By matrix-assisted laser desorption ionization time-of-flight mass spectrometry, we detected tryptic fragments of the ECD indicative of disulfide bond formation for both pairs of cysteines, as proposed by modeling. The identity of tryptic fragments was confirmed using chemical modification of cysteine and lysine residues. As evident from circular dichroism spectroscopy, mutagenesis of single cysteines did not impair refolding of the ECD in vitro, whereas it led to partial or complete intracellular retention and consequently to a loss of function of full-length GlyR subunits in human embryonic kidney 293 cells. Our results indicate that the GlyR ECD forms both a Cys-loop and a GlyR-specific disulfide bond. In addition, cysteine residues appear to be important for protein maturation in vivo.

An allosteric modulator of alpha7 nicotinic receptors, N-(5-Chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596), causes conformational changes in the extracellular ligand binding domain similar to those caused by acetylcholine

Nicotinic acetylcholine receptors are implicated in several neuropsychiatric disorders, including nicotine addiction, Alzheimer's, schizophrenia, and depression. Therefore, they represent a critical molecular target for drug development and targeted therapeutic intervention. Understanding the molecular mechanisms by which allosteric modulators enhance activation of these receptors is crucial to the development of new drugs. We used the substituted cysteine accessibility method to study conformational changes induced by the positive allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596) in the extracellular ligand binding domain of alpha7 nicotinic receptors carrying the L247T mutation. PNU-120596 caused changes in cysteine accessibility at the inner beta sheet, transition zone, and agonist binding site. These changes in accessibility are similar to but not identical to those caused by ACh alone. In particular, PNU-120596 induced changes in MTSEA accessibility at N170C (in the transition zone) that were substantially different from those evoked by acetylcholine (ACh). We found that PNU-120596 induced changes at position E172C in the absence of allosteric modulation. We identified a cysteine mutation of the agonist binding site (W148C) that exhibited an unexpected phenotype in which PNU-120596 acts as a full agonist. In this mutant, ACh-evoked currents were more sensitive to thiol modification than PNU-evoked currents, suggesting that PNU-120596 does not bind at unoccupied agonist-binding sites. Our results provide evidence that binding sites for PNU-120596 are not in the agonist-binding sites and demonstrate that positive allosteric modulators such as PNU-120596 enhance agonist-evoked gating of nicotinic receptors by eliciting conformational effects that are similar but nonidentical to the gating conformations promoted by ACh.

Gating mechanisms in Cys-loop receptors

The Cys-loop receptor superfamily of ligand-gated ion channels has a prominent role in neuronal signalling. These receptors are pentamers, each subunit containing ten beta-strands in the extracellular domain and four alpha-helical transmembrane domains (M1-M4). The M2 domain of each subunit lines the intrinsic ion channel pore and residues within the extracellular domain form ligand binding sites. Ligand binding initiates a conformational change that opens the ion-selective pore. The coupling between ligand binding in the extracellular domain and opening of the intrinsic ion channel pore located in the membrane is not fully understood. Several loop structures, such as loop 2, the Cys-loop, the pre-M1 region and the M2-M3 loop have been implicated in receptor activation. The current "conformational change wave" hypothesis suggests that binding of a ligand initiates a rotation of the beta-sheets around an axis that passes through the Cys-loop. Due to this rotation, the Cys-loop and loop 2 are displaced. Movement of the M2-M3 loop then twists the M2 domain leading to a separation of the helices and opening of the pore. The publication of a crystal structure of an acetylcholine binding protein and the refined structure of the Torpedo marmorata acetylcholine receptor have improved the understanding of the mechanisms and structures involved in coupling ligand binding to channel gating. In this review, the most recent findings on some of these loop structures will be reported and discussed in view of their role in the gating mechanism.

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.

Conformational changes in alpha 7 acetylcholine receptors underlying allosteric modulation by divalent cations

Allosteric modulation of membrane receptors is a widespread mechanism by which endogenous and exogenous agents regulate receptor function. For example, several members of the nicotinic receptor family are modulated by physiological concentrations of extracellular calcium ions. In this paper, we examined conformational changes underlying this modulation and compare these with changes evoked by ACh. Two sets of residues in the α7 acetylcholine receptor extracellular domain were mutated to cysteine and analyzed by measuring the rates of modification by the thiol-specific reagent 2-aminoethylmethane thiosulfonate. Using Ba²⁺ as a surrogate for Ca²⁺, we found a divalent-dependent decrease the modification rates of cysteine substitutions at M³⁷ and M⁴⁰, residues at which rates were also slowed by ACh. In contrast, Ba²⁺ had no significant effect at N⁵²C, a residue where ACh increased the rate of modification. Thus divalent modulators cause some but not all of the conformational effects elicited by agonist. Cysteine substitution of either of two glutamates (E⁴⁴ or E¹⁷²), thought to participate in the divalent cation binding site, caused a loss of allosteric modulation, yet Ba²⁺ still had a significant effect on modification rates of these residues. In addition, the effect of Ba²⁺ at these residues did not appear to be due to direct occlusion. Our data demonstrate that modulation by divalent cations involves substantial conformational changes in the receptor extracellular domain. Our evidence also suggests the modulation occurs via a binding site distinct from one which includes either (or both) of the conserved glutamates at E⁴⁴ or E¹⁷².

Congenital myasthenia-related AChR delta subunit mutation interferes with intersubunit communication essential for channel gating

Congenital myasthenias (CMs) arise from defects in neuromuscular junction-associated proteins. Deciphering the molecular bases of the CMs is required for therapy and illuminates structure-function relationships in these proteins. Here, we analyze the effects of a mutation in 1 of 4 homologous subunits in the AChR from a CM patient, a Leu to Pro mutation at position 42 of the delta subunit. The mutation is located in a region of contact between subunits required for rapid opening of the AChR channel and impedes the rate of channel opening. Substitutions of Gly, Lys, or Asp for deltaL42, or substitutions of Pro along the local protein chain, also slowed channel opening. Substitution of Pro for Leu in the epsilon subunit slowed opening, whereas this substitution had no effect in the beta subunit and actually sped opening in the alpha subunit. Analyses of energetic coupling between residues at the subunit interface showed that deltaL42 is functionally linked to alphaT127, a key residue in the adjacent alpha subunit required for rapid channel opening. Thus, deltaL42 is part of an intersubunit network that enables ACh binding to rapidly open the AChR channel, which may be compromised in patients with CM.

Aromatic residues at position 55 of rat alpha7 nicotinic acetylcholine receptors are critical for maintaining rapid desensitization

The rat alpha7 nicotinic acetylcholine receptor (nAChR) can undergo rapid onset of desensitization; however, the mechanisms of desensitization are largely unknown. The contribution of a tryptophan (W) residue at position 55 of the rat alpha7 nAChR subunit, which lies within the beta2 strand, was studied by mutating it to other hydrophobic and/or aromatic amino acids, followed by voltage-clamp experiments in Xenopus oocytes. When mutated to alanine, the alpha7-W55A nAChR desensitized more slowly, and recovered from desensitization more rapidly, than wildtype alpha7 nAChRs. The contribution of desensitization was validated by kinetic modelling. Mutating W55 to other aromatic residues (phenylalanine or tyrosine) had no significant effect on the kinetics of desensitization, whereas mutation to various hydrophobic residues (alanine, cysteine or valine) significantly decreased the rate of onset and increased the rate of recovery from desensitization. To gain insight into possible structural rearrangements during desensitization, we probed the accessibility of W55 by mutating W55 to cysteine (alpha7-W55C) and testing the ability of various sulfhydryl reagents to react with this cysteine. Several positively charged sulfhydryl reagents blocked ACh-induced responses for alpha7-W55C nAChRs, whereas a neutral sulfhydryl reagent potentiated responses; residue C55 was not accessible for modification in the desensitized state. These data suggest that W55 plays an important role in both the onset and recovery from desensitization in the rat alpha7 nAChR, and that aromatic residues at position 55 are critical for maintaining rapid desensitization. Furthermore, these data suggest that W55 may be a potential target for modulatory agents operating via hydrophobic interactions.