Ligand-gated ion channels in the brain: the amino acid receptor superfamily

[18F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging

Selective inhibition of glycine transporter 1 (GlyT1) has emerged as a potential approach to alleviate N-methyl-d-aspartate receptor (NMDAR) hypofunction in patients with schizophrenia and cognitive decline. ALX5407 is a potent and selective inhibitor of GlyT1 derived from the metabolic intermediate sarcosine (N-methylglycine) that showed antipsychotic potential in a number of animal models. Whereas clinical application of ALX5407 is limited by adverse effects on motor performance and respiratory function, a suitably radiolabeled drug could represent a promising PET tracer for the visualization of GlyT1 in the brain. Herein, [¹⁸F]ALX5407 and the corresponding methyl ester, [¹⁸F]ALX5406, were prepared by alcohol-enhanced copper mediated radiofluorination and studied in vitro in rat brain slices and in vivo in normal rats. [¹⁸F]ALX5407 demonstrated accumulation consistent with the distribution of GlyT1 in in vitro autoradiographic studies but no brain uptake in μPET experiments in naı̈ve rats. In contrast, the methyl ester [¹⁸F]ALX5406 rapidly entered the brain and was enzymatically transformed into [¹⁸F]ALX5407, resulting in a regional accumulation pattern consistent with GlyT1 specific binding. We conclude that [¹⁸F]ALX5406 is a promising and easily accessible PET probe for preclinical in vivo imaging of GlyT1 in the brain.

Ion channel profiling of the Lymnaea stagnalis ganglia via transcriptome analysis

BACKGROUND

The pond snail Lymnaea stagnalis (L. stagnalis) has been widely used as a model organism in neurobiology, ecotoxicology, and parasitology due to the relative simplicity of its central nervous system (CNS). However, its usefulness is restricted by a limited availability of transcriptome data. While sequence information for the L. stagnalis CNS transcripts has been obtained from EST libraries and a de novo RNA-seq assembly, the quality of these assemblies is limited by a combination of low coverage of EST libraries, the fragmented nature of de novo assemblies, and lack of reference genome.

RESULTS

In this study, taking advantage of the recent availability of a preliminary L. stagnalis genome, we generated an RNA-seq library from the adult L. stagnalis CNS, using a combination of genome-guided and de novo assembly programs to identify 17,832 protein-coding L. stagnalis transcripts. We combined our library with existing resources to produce a transcript set with greater sequence length, completeness, and diversity than previously available ones. Using our assembly and functional domain analysis, we profiled L. stagnalis CNS transcripts encoding ion channels and ionotropic receptors, which are key proteins for CNS function, and compared their sequences to other vertebrate and invertebrate model organisms. Interestingly, L. stagnalis transcripts encoding numerous putative Ca2+ channels showed the most sequence similarity to those of Mus musculus, Danio rerio, Xenopus tropicalis, Drosophila melanogaster, and Caenorhabditis elegans, suggesting that many calcium channel-related signaling pathways may be evolutionarily conserved.

CONCLUSIONS

Our study provides the most thorough characterization to date of the L. stagnalis transcriptome and provides insights into differences between vertebrates and invertebrates in CNS transcript diversity, according to function and protein class. Furthermore, this study provides a complete characterization of the ion channels of Lymnaea stagnalis, opening new avenues for future research on fundamental neurobiological processes in this model system.

ON selectivity in the Drosophila visual system is a multisynaptic process involving both glutamatergic and GABAergic inhibition

Sensory systems sequentially extract increasingly complex features. ON and OFF pathways, for example, encode increases or decreases of a stimulus from a common input. This ON/OFF pathway split is thought to occur at individual synaptic connections through a sign-inverting synapse in one of the pathways. Here, we show that ON selectivity is a multisynaptic process in the Drosophila visual system. A pharmacogenetics approach demonstrates that both glutamatergic inhibition through GluClα and GABAergic inhibition through Rdl mediate ON responses. Although neurons postsynaptic to the glutamatergic ON pathway input L1 lose all responses in GluClα mutants, they are resistant to a cell-type-specific loss of GluClα. This shows that ON selectivity is distributed across multiple synapses, and raises the possibility that cell-type-specific manipulations might reveal similar strategies in other sensory systems. Thus, sensory coding is more distributed than predicted by simple circuit motifs, allowing for robust neural processing.

New tricks for old dogs: improving the accuracy of biomolecular force fields by pair-specific corrections to non-bonded interactions

In contrast to ordinary polymers, the vast majority of biological macromolecules adopt highly ordered three-dimensional structures that define their functions. The key to folding of a biopolymer into a unique 3D structure or to an assembly of several biopolymers into a functional unit is a delicate balance between the attractive and repulsive forces that also makes such self-assembly reversible under physiological conditions. The all-atom molecular dynamics (MD) method has emerged as a powerful tool for studies of individual biomolecules and their functional assemblies, encompassing systems of ever increasing complexity. However, advances in parallel computing technology have outpaced the development of the underlying theoretical models-the molecular force fields, pushing the MD method into an untested territory. Recent tests of the MD method have found the most commonly used molecular force fields to be out of balance, overestimating attractive interactions between charged and hydrophobic groups, which can promote artificial aggregation in MD simulations of multi-component protein, nucleic acid, and lipid systems. One route towards improving the force fields is through the NBFIX corrections method, in which the intermolecular forces are calibrated against experimentally measured quantities such as osmotic pressure by making atom pair-specific adjustments to the non-bonded interactions. In this article, we review development of the NBFIX (Non-Bonded FIX) corrections to the AMBER and CHARMM force fields and discuss their implications for MD simulations of electrolyte solutions, dense DNA systems, Holliday junctions, protein folding, and lipid bilayer membranes.

Neurotransmitter receptors as signaling platforms in anterior pituitary cells

The functions of anterior pituitary cells are controlled by two major groups of hypothalamic and intrapituitary ligands: one exclusively acts on G protein-coupled receptors and the other activates both G protein-coupled receptors and ligand-gated receptor channels. The second group of ligands operates as neurotransmitters in neuronal cells and their receptors are termed as neurotransmitter receptors. Most information about pituitary neurotransmitter receptors was obtained from secretory studies, RT-PCR analyses of mRNA expression and immunohistochemical and biochemical analyses, all of which were performed using a mixed population of pituitary cells. However, recent electrophysiological and imaging experiments have characterized γ-aminobutyric acid-, acetylcholine-, and ATP-activated receptors and channels in single pituitary cell types, expanding this picture and revealing surprising differences in their expression between subtypes of secretory cells and between native and immortalized pituitary cells. The main focus of this review is on the electrophysiological and pharmacological properties of these receptors and their roles in calcium signaling and calcium-controlled hormone secretion.

Modulation of Glycine-Mediated Spinal Neurotransmission for the Treatment of Chronic Pain

Chronic pain constitutes a significant and expanding worldwide health crisis. Currently available analgesics poorly serve individuals suffering from chronic pain, and new therapeutic agents that are more effective, safer, and devoid of abuse liabilities are desperately needed. Among the myriad of cellular and molecular processes contributing to chronic pain, spinal disinhibition of pain signaling to higher cortical centers plays a critical role. Accumulating evidence shows that glycinergic inhibitory neurotransmission in the spinal cord dorsal horn gates nociceptive signaling, is essential in maintaining physiological pain sensitivity, and is diminished in pathological pain states. Thus, it is hypothesized that agents capable of enhancing glycinergic tone within the dorsal horn could obtund nociceptor signaling to the brain and serve as analgesics for persistent pain. This Perspective highlights the potential that pharmacotherapies capable of increasing inhibitory spinal glycinergic neurotransmission hold in providing new and transformative analgesic therapies for the treatment of chronic pain.

Voltage-Dependent Inhibition of Glycine Receptor Channels by Niflumic Acid

Niflumic acid (NFA) is a member of the fenamate class of nonsteroidal anti-inflammatory drugs. This compound and its derivatives are used worldwide clinically for the relief of chronic and acute pain. NFA is also a commonly used blocker of voltage-gated chloride channels. Here we present evidence that NFA is an efficient blocker of chloride-permeable glycine receptors (GlyRs) with subunit heterogeneity of action. Using the whole-cell configuration of patch-clamp recordings and molecular modeling, we analyzed the action of NFA on homomeric α1ΔIns, α2B, α3L, and heteromeric α1β and α2β GlyRs expressed in CHO cells. NFA inhibited glycine-induced currents in a voltage-dependent manner and its blocking potency in α2 and α3 GlyRs was higher than that in α1 GlyR. The Woodhull analysis suggests that NFA blocks α1 and α2 GlyRs at the fractional electrical distances of 0.16 and 0.65 from the external membrane surface, respectively. Thus, NFA binding site in α1 GlyR is closer to the external part of the membrane, while in α2 GlyR it is significantly deeper in the pore. Mutation G254A at the cytoplasmic part of the α1 GlyR pore-lining TM2 helix (level 2') increased the NFA blocking potency, while incorporation of the β subunit did not have a significant effect. The Hill plot analysis suggests that α1 and α2 GlyRs are preferably blocked by two and one NFA molecules, respectively. Molecular modeling using Monte Carlo energy minimizations provides the structural rationale for the experimental data and proposes more than one interaction site along the pore where NFA can suppress the ion permeation.

Binding site opening by loop C shift and chloride ion-pore interaction in the GABAAreceptor model

Molecular dynamics simulations of the shut α₁β₂γ₂GABA_Aheteropentamer receptor homology model reveal significant differences between intersubunit interfaces (ligand binding G1, G2 and non-binding) compared to homomeric receptor assemblies and possible ion interaction sites in the top part of the transmembrane domain (TMD).

Synthesis and Biological Evaluation of N-((1-(4-(Sulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide Inhibitors of Glycine Transporter-1

We previously disclosed the discovery of rationally designed N-((1-(4-(propylsulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide inhibitors of glycine transporter-1 (GlyT-1), represented by analogues 10 and 11. We describe herein further structure-activity relationship exploration of this series via an optimization strategy that primarily focused on the sulfonamide and benzamide appendages of the scaffold. These efforts led to the identification of advanced leads possessing a desirable balance of excellent in vitro GlyT-1 potency and selectivity, favorable ADME and in vitro pharmacological profiles, and suitable pharmacokinetic and safety characteristics. Representative analogue (+)-67 exhibited robust in vivo activity in the cerebral spinal fluid glycine biomarker model in both rodents and nonhuman primates. Furthermore, rodent microdialysis experiments also demonstrated that oral administration of (+)-67 significantly elevated extracellular glycine levels within the medial prefrontal cortex (mPFC).

Side chain flexibility and the pore dimensions in the GABAA receptor

Permeation of ions through open channels and their accessibility to pore-targeting drugs depend on the pore cross-sectional dimensions, which are known only for static X-ray and cryo-EM structures. Here, we have built homology models of the closed, open and desensitized α1β2γ2 GABAA receptor (GABAAR). The models are based, respectively, on the X-ray structure of α3 glycine receptor (α3 GlyR), cryo-EM structure of α1 GlyR and X-ray structure of β3 GABAAR. We employed Monte Carlo energy minimizations to explore how the pore lumen may increase due to repulsions of flexible side chains from a variable-diameter electroneutral atom (an expanding sphere) pulled through the pore. The expanding sphere computations predicted that the pore diameter averaged along the permeation pathway is larger by approximately 3 Å than that computed for the models with fixed sidechains. Our models predict three major pore constrictions located at the levels of -2', 9' and 20' residues. Residues around the -2' and 9' rings are known to form the desensitization and activation gates of GABAAR. Our computations predict that the 20' ring may also serve as GABAAR gate whose physiological role is unclear. The side chain flexibility of residues -2', 9' and 20' and hence the dimensions of the constrictions depend on the GABAAR functional state.

Functional sites involved in modulation of the GABAA receptor channel by the intravenous anesthetics propofol, etomidate and pentobarbital

GABAA receptors are the major inhibitory neurotransmitter receptors in the brain and are the target for many clinically important drugs. Among the many modulatory compounds are also the intravenous anesthetics propofol and etomidate, and barbiturates. The mechanism of receptor modulation by these compounds is of mayor relevance. The site of action of these compounds has been located to subunit interfaces in the intra-membrane region of the receptor. In α1β2γ2 GABAA receptors there are five such interfaces, two β+/α- and one each of α+/β-, α+/γ- and γ+/β- subunit interfaces. We have used reporter mutations located in the second trans-membrane region in different subunits to probe the effects of changes at these subunit interfaces on modulation by propofol, etomidate and pentobarbital. We provide evidence for the fact that each of these compounds either modulates through a different set of subunit interfaces or through the same set of subunit interfaces to a different degree. As a GABAA receptor pentamer harbors two β+/α- subunit interfaces, we used concatenated receptors to dissect the contribution of individual interfaces and show that only one of these interfaces is important for receptor modulation by etomidate.

Diversity in GABAergic signaling

GABA(A) receptor-mediated synaptic transmission is responsible for inhibitory control of neural function in the brain. Recent progress has shown that GABA(A) receptors also provide a wide range of additional functions beyond simple inhibition. This diversity of functions is mediated by a large variety of different interneuron classes acting on a diverse population of receptor subtypes. Here, I will focus on an additional source of GABAergic signaling diversity, caused by the highly variable ion signaling mechanism of GABA(A) receptors. In concert with the other two sources of GABAergic heterogeneity, this variability in signaling allows for a wide array of GABAergic effects that are crucial for the development of the brain and its function.

Block of GABA(A) receptor ion channel by penicillin: electrophysiological and modeling insights toward the mechanism

GABA(A) receptors (GABA(A)R) mainly mediate fast inhibitory neurotransmission in the central nervous system. Different classes of modulators target GABA(A)R properties. Penicillin G (PNG) belongs to the class of noncompetitive antagonists blocking the open GABA(A)R and is a prototype of β-lactam antibiotics. In this study, we combined electrophysiological and modeling approaches to investigate the peculiarities of PNG blockade of GABA-activated currents recorded from isolated rat Purkinje cells and to predict the PNG binding site. Whole-cell patch-сlamp recording and fast application system was used in the electrophysiological experiments. PNG block developed after channel activation and increased with membrane depolarization suggesting that the ligand binds within the open channel pore. PNG blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC50 value of 1.12mM at -70mV. The termination of GABA and PNG co-application was followed by a transient tail current. Protection of the tail current from bicuculline block and dependence of its kinetic parameters on agonist affinity suggest that PNG acts as a sequential open channel blocker that prevents agonist dissociation while the channel remains blocked. We built the GABA(A)R models based on nAChR and GLIC structures and performed an unbiased systematic search of the PNG binding site. Monte-Carlo energy minimization was used to find the lowest energy binding modes. We have shown that PNG binds close to the intracellular vestibule. In both models the maximum contribution to the energy of ligand-receptor interactions revealed residues located on the level of 2', 6' and 9' rings formed by a bundle of M2 transmembrane segments, indicating that these residues most likely participate in PNG binding. The predicted structural models support the described mechanism of PNG block.

P2X4 receptors (P2X4Rs) represent a novel target for the development of drugs to prevent and/or treat alcohol use disorders

Alcohol use disorders (AUDs) have a staggering socioeconomic impact. Few therapeutic options are available, and they are largely inadequate. These shortcomings highlight the urgent need to develop effective medications to prevent and/or treat AUDs. A critical barrier is the lack of information regarding the molecular target(s) by which ethanol (EtOH) exerts its pharmacological activity. This review highlights findings implicating P2X4 receptors (P2X4Rs) as a target for the development of therapeutics to treat AUDs and discusses the use of ivermectin (IVM) as a potential clinical tool for treatment of AUDs. P2XRs are a family of ligand-gated ion channels (LGICs) activated by extracellular ATP. Of the P2XR subtypes, P2X4Rs are expressed the most abundantly in the CNS. Converging evidence suggests that P2X4Rs are involved in the development and progression of AUDs. First, in vitro studies report that pharmacologically relevant EtOH concentrations can negatively modulate ATP-activated currents. Second, P2X4Rs in the mesocorticolimbic dopamine system are thought to play a role in synaptic plasticity and are located ideally to modulate brain reward systems. Third, alcohol-preferring (P) rats have lower functional expression of the p2rx4 gene than alcohol-non-preferring (NP) rats suggesting an inverse relationship between alcohol intake and P2X4R expression. Similarly, whole brain p2rx4 expression has been shown to relate inversely to innate 24 h alcohol preference across 28 strains of rats. Fourth, mice lacking the p2rx4 gene drink more EtOH than wildtype controls. Fifth, IVM, a positive modulator of P2X4Rs, antagonizes EtOH-mediated inhibition of P2X4Rs in vitro and reduces EtOH intake and preference in vivo. These findings suggest that P2X4Rs contribute to EtOH intake. The present review summarizes recent findings focusing on the P2X4R as a molecular target of EtOH action, its role in EtOH drinking behavior and modulation of its activity by IVM as a potential therapy for AUDs.

Contribution of P2X4 receptors to ethanol intake in male C57BL/6 mice

P2X receptors (P2XRs) are a family of cation-permeable ligand-gated ion channels activated by synaptically released extracellular adenosine 5'-triphosphate. The P2X4 subtype is abundantly expressed in the central nervous system and is sensitive to low intoxicating ethanol concentrations. Genetic meta-analyses identified the p2rx4 gene as a candidate gene for innate alcohol intake and/or preference. The current study used mice lacking the p2rx4 gene (knockout, KO) and wildtype (WT) C57BL/6 controls to test the hypothesis that P2X4Rs contribute to ethanol intake. The early acquisition and early maintenance phases of ethanol intake were measured with three different drinking procedures. Further, we tested the effects of ivermectin (IVM), a drug previously shown to reduce ethanol's effects on P2X4Rs and to reduce ethanol intake and preference, for its ability to differentially alter stable ethanol intake in KO and WT mice. Depending on the procedure and the concentration of the ethanol solution, ethanol intake was transiently increased in P2X4R KO versus WT mice during the acquisition of 24-h and limited access ethanol intake. IVM significantly reduced ethanol intake in P2X4R KO and WT mice, but the degree of reduction was 50 % less in the P2X4R KO mice. Western blot analysis identified significant changes in γ-aminobutyric acidA receptor α1 subunit expression in brain regions associated with the regulation of ethanol behaviors in P2X4R KO mice. These findings add to evidence that P2X4Rs contribute to ethanol intake and indicate that there is a complex interaction between P2X4Rs, ethanol, and other neurotransmitter receptor systems.

Presence of multiple binding sites on α9α10 nAChR receptors alludes to stoichiometric-dependent action of the α-conotoxin, Vc1.1

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in fast synaptic transmission. nAChRs are pentameric receptors formed from a combination of different or similar subunits to produce heteromeric or homomeric channels. The heteromeric, α9α10 nAChR subtype is well-known for its role in the auditory system, being expressed in cochlear hair cells. These nAChRs have also been shown to be involved in immune-modulation. Antagonists of α9α10 nAChRs, like the α-conotoxin Vc1.1, have analgesic effects in neuropathic pain. Unlike other nAChR subtypes there is no evidence that functional receptor stoichiometries of α9α10 exist. By using 2-electrode voltage clamp methods and maintaining a constant intracellular Ca(2+) concentration, we observed a biphasic activation curve for ACh that is dependent on receptor stoichiometry. Vc1.1, but not the α9α10 antagonists RgIA or atropine, inhibits ACh-evoked currents in a biphasic manner. Characteristics of the ACh and Vc1.1 activation and inhibition curves can be altered by varying the ratio of α9 and α10 mRNA injected into oocytes, changing the curves from biphasic to monophasic when an excess of α10 mRNA is used. These results highlight the difference in the pharmacological profiles of at least two different α9α10 nAChR stoichiometries, possibly (α9)₃(α10)₂ and (α9)₂(α10)₃. As a result, we infer that there is an additional binding site for ACh and Vc1.1 at the α9-α9 interface on the hypothesized (α9)₃(α10)₂ nAChR, in addition to the α10-α9 and or α9-α10 interfaces that are common to both stoichiometries. This study provides further evidence that receptor stoichiometry contributes another layer of complexity in understanding Cys-loop receptors.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

Structure and pharmacology of pentameric receptor channels: from bacteria to brain

Orthologs of the pentameric receptor channels that mediate fast synaptic transmission in the central and peripheral nervous systems have been found in several bacterial species and in a single archaea genus. Recent X-ray structures of bacterial and invertebrate pentameric receptors point to a striking conservation of the structural features within the whole family, even between distant prokaryotic and eukaryotic members. These structural data reveal general principles of molecular organization that allow allosteric membrane proteins to mediate chemoelectric transduction. Notably, several conformations have been solved, including open and closed channels with distinct global tertiary and quaternary structure. The data reveal features of the ion channel architecture and of diverse categories of binding sites, such as those that bind orthosteric ligands, including neurotransmitters, and those that bind allosteric modulators, such as general anesthetics, ivermectin, or lipids. In this review, we summarize the most recent data, discuss insights into the mechanism of action in these systems, and elaborate on newly opened avenues for drug design.

Structure, function, and modulation of GABA(A) receptors

The GABA(A) receptors are the major inhibitory neurotransmitter receptors in mammalian brain. Each isoform consists of five homologous or identical subunits surrounding a central chloride ion-selective channel gated by GABA. How many isoforms of the receptor exist is far from clear. GABA(A) receptors located in the postsynaptic membrane mediate neuronal inhibition that occurs in the millisecond time range; those located in the extrasynaptic membrane respond to ambient GABA and confer long-term inhibition. GABA(A) receptors are responsive to a wide variety of drugs, e.g. benzodiazepines, which are often used for their sedative/hypnotic and anxiolytic effects.

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.

Conformational changes in extracellular loop 2 associated with signal transduction in the glycine receptor

Ligand-gated ion channels efficiently couple neurotransmitter binding to the opening of an intrinsic ion channel to generate the post-synaptic potentials that are characteristic of fast synaptic transmission. In the Cys-loop family of ligand-gated ion channels, the ligand-binding site is approximately 60 Å above the channel gate. Structural modelling of related proteins and mutagenesis studies led to the hypothesis that loops 2 and 7 of the extracellular domain may couple ligand binding to receptor activation. Mutating loop 2 residues of the glycine receptor to cysteine reveals an alternating pattern of effect upon receptor function. Mutations A52C, T54C and M56C produced a threefold right-shift in EC(50) . In contrast, a 30-fold right-shift was seen for mutations E53C, T55C and D57C. Loop 2 conformational changes associated with ligand binding were assessed by measuring the rate of covalent modification of substituted cysteines by charged methane thiosulfonate reagents. We show for the first time state-dependent differences in the rate of reaction. A52C and T54C are more accessible in the resting state and M56C is more accessible in the activated state. These results demonstrate that loop 2 does undergo a conformational change as part of the mechanism that couples ligand binding to channel opening.

Glycine transporter 1 (GlyT1) inhibitors exhibit anticonvulsant properties in the rat maximal electroshock threshold (MEST) test

Glycine can act as either an inhibitory neurotransmitter or as a potentiator of NMDA-dependent excitatory neurotransmission. There is some evidence that glycine can have both pro- and anticonvulsant properties in various rodent models of epilepsy. In the present study we tested several glycine transporter 1 (GlyT1) inhibitors including NFPS, SSR 504734, Lu AA21279, Org 25935, SB-710622, GSK931145, as well as the glycine agonist d-serine, in the maximal electroshock threshold (MEST) test in the rat. In a series of experiments, male Sprague-Dawley rats (n=12/group) were pre-treated with a compound of interest and then received an electric shock delivered via corneal electrodes. A cohort of satellite animals (n=3/group) was also used to measure blood and brain levels of Org 25935. All GlyT1 inhibitors increased seizure thresholds dose-dependently, indicative of anticonvulsant activity. SB-710622 and GSK931145 had lower minimum effective doses (MEDs) in the MEST test than other GlyT1 inhibitors. At estimated t(max), increases in dose administered were paralleled by increases in blood and brain concentrations of Org 25935. Thus, increasing extracellular concentration of glycine via inhibition of its uptake protects from electroshock-induced seizures in the rat. Whether strychnine-sensitive or strychnine-insensitive glycine binding sites are involved in this effect remains to be determined.

Binding, activation and modulation of Cys-loop receptors

It is over forty years since the major neurotransmitters and their protein receptors were identified, and over twenty years since determination of the first amino-acid sequences of the Cys-loop receptors that recognize acetylcholine, serotonin, GABA and glycine. The last decade has seen the first structures of these proteins (and related bacterial and molluscan homologues) determined to atomic resolution. Hopefully over the next decade, more detailed molecular structures of entire Cys-loop receptors in drug-bound and drug-free conformations will become available. These, together with functional studies, will provide a clear picture of how these receptors participate in neurotransmission and how structural variations between receptor subtypes impart their unique characteristics. This insight should facilitate the design of novel and improved therapeutics to treat neurological disorders. This review considers our current understanding about the processes of agonist binding, receptor activation and channel opening, as well as allosteric modulation of the Cys-loop receptor family.

A tyramine-gated chloride channel coordinates distinct motor programs of a Caenorhabditis elegans escape response

A key feature of escape responses is the fast translation of sensory information into a coordinated motor output. In C. elegans, anterior touch initiates a backward escape response in which lateral head movements are suppressed. Here, we show that tyramine inhibits head movements and forward locomotion through the activation of a tyramine-gated chloride channel, LGC-55. lgc-55 mutant animals have defects in reversal behavior and fail to suppress head oscillations in response to anterior touch. lgc-55 is expressed in neurons and muscle cells that receive direct synaptic inputs from tyraminergic motor neurons. Therefore, tyramine can act as a classical inhibitory neurotransmitter. Activation of LGC-55 by tyramine coordinates the output of two distinct motor programs, locomotion and head movements that are critical for a C. elegans escape response.

Mysterious alpha6-containing nAChRs: function, pharmacology, and pathophysiology

Neuronal nicotinic acetylcholine receptors (nAChRs) are the superfamily of ligand-gated ion channels and widely expressed throughout the central and peripheral nervous systems. nAChRs play crucial roles in modulating a wide range of higher cognitive functions by mediating presynaptic, postsynaptic, and extrasynaptic signaling. Thus far, nine alpha (alpha2-alpha10) and three beta (beta2, beta3, and beta4) subunits have been identified in the CNS, and these subunits assemble to form a diversity of functional nAChRs. Although alpha4beta2- and alpha7-nAChRs are the two major functional nAChR types in the CNS, alpha6*-nAChRs are abundantly expressed in the midbrain dopaminergic (DAergic) system, including mesocorticolimbic and nigrostriatal pathways, and particularly present in presynaptic nerve terminals. Recently, functional and pharmacological profiles of alpha6*-nAChRs have been assessed with the use of alpha6 subunit blockers such as alpha-conotoxin MII and PIA, and also by using alpha6 subunit knockout mice. By modulating DA release in the nucleus accumbens (NAc) and modulating GABA release onto DAergic neurons in the ventral tegmental area (VTA), alpha6*-nAChRs may play important roles in the mediation of nicotine reward and addiction. Furthermore, alpha6*-nAChRs in the nigrostriatal DAergic system may be promising targets for selective preventative treatment of Parkinson's disease (PD). Thus, alpha6*-nAChRs may hold promise for future clinical treatment of human disorders, such as nicotine addiction and PD. In this review, we mainly focus on the recent advances in the understanding of alpha6*-nAChR function, pharmacology and pathophysiology.

Differential regulations of quercetin and its glycosides on ligand-gated ion channels

Quercetin, one of the flavonoids, is a compound of low molecular weight found in various plants and shows a wide range of diverse neuropharmacological actions. In fruits and vegetables, quercetin exists as monomer- (quercetin-3-O-rhamnoside) (Rham1), dimer- (Rutin), or trimer-glycosides [quercetin-3-(2G-rhamnosylrutinoside)] (Rham2) at carbon-3. In the previous studies, we demonstrated that quercetin inhibits both glycine and 5-hydroxytryptamine type 3, (5-HT3A) receptor channel activities expressed in Xenopus oocytes. However, the effects of quercetin glycosides on glycine and 5-HT3A receptor channel activities are not well known. In the present study, we investigated the effects of quercetin glycosides on the human glycine alpha1 receptor and mouse 5-HT3A receptor channel activities expressed in Xenopus oocytes using a two-electrode voltage clamp technique. In oocytes expressing glycine or 5-HT3A receptors, quercetin- or its glycosides-induced inhibitions on glycine- (IGly) and 5-HT-induced current (I5-HT) were dose-dependent and reversible. Applications of quercetin and its glycosides inhibited IGly in order of quercetin>Rutin> or =Rham1>Rham2. Applications of quercetin and its glycosides inhibited I5-HT in order of Rham2> or =quercetin>Rutin=Rham1. The inhibitions of IGly by quercetin glycosides were non-competitive and voltage-sensitive, whereas the inhibitions of I5-HT by quercetin glycosides were competitive and voltage-insensitive manners. These results also indicate that quercetin glycosides might regulate the human glycine alpha1 and mouse 5-HT3A receptors with differential manners.

Extrasynaptic and postsynaptic receptors in glycinergic and GABAergic neurotransmission: a division of labor?

Glycine and GABA mediate inhibitory neurotransmission in the spinal cord and central nervous system. The general concept of neurotransmission is now challenged by the contribution of both phasic activation of postsynaptic glycine and GABA(A) receptors (GlyRs and GABA(A)Rs, respectively) and tonic activity of these receptors located at extrasynaptic sites. GlyR and GABA(A)R kinetics depend on several parameters, including subunit composition, subsynaptic localization and activation mode. Postsynaptic and extrasynaptic receptors display different subunit compositions and are activated by fast presynaptic and slow paracrine release of neurotransmitters, respectively. GlyR and GABA(A)R functional properties also rely on their aggregation level, which is higher at postsynaptic densities than at extrasynaptic loci. Finally, these receptors can co-aggregate at mixed inhibitory postsynaptic densities where they cross-modulate their activity, providing another parameter of functional complexity. GlyR and GABA(A)R density at postsynaptic sites results from the balance between their internalization and insertion in the plasma membrane, but also on their lateral diffusion from and to the postsynaptic loci. The dynamic exchange of receptors between synaptic and extrasynaptic sites and their functional adaptation in terms of kinetics point out a new adaptive process of inhibitory neurotransmission.

Protons act as a transmitter for muscle contraction in C. elegans

Muscle contraction is normally mediated by the release of neurotransmitters from motor neurons. Here we demonstrate that protons can act as a direct transmitter from intestinal cells to stimulate muscle contraction. During the C. elegans defecation motor program the posterior body muscles contract even in the absence of neuronal inputs or vesicular neurotransmission. In this study, we demonstrate that the space between the intestine and the muscle is acidified just prior to muscle contraction and that the release of caged protons is sufficient to induce muscle contraction. PBO-4 is a putative Na+/H+ ion exchanger expressed on the basolateral membrane of the intestine, juxtaposed to the posterior body muscles. In pbo-4 mutants the extracellular space is not acidified and the muscles fail to contract. The pbo-5 and pbo-6 genes encode subunits of a "cys-loop" proton-gated cation channel required for muscles to respond to acidification. In heterologous expression assays the PBO receptor is half-maximally activated at a pH of 6.8. The identification of the mechanisms for release and reception of proton signals establishes a highly unusual mechanism for intercellular communication.

Modulation of 5-HT3 receptor desensitization by the light chain of microtubule-associated protein 1B expressed in HEK 293 cells

Regulation of ligand-gated ion channel (LGIC) function and trafficking by cytoskeleton proteins has been the topic of recent research. Here, we report that the light chain (LC1) of microtubule-associated protein 1B (MAP1B) specifically interacted with the 5-HT(3A) receptor, a predominant serotonin-gated ion channel in the brain. LC1 and 5-HT(3A) receptors were colocalized in central neurons and in HEK 293 cells expressing 5-HT(3A) receptors. LC1 reduced the steady-state density of 5-HT(3A) receptors at the membrane surface of HEK 293 cells and significantly accelerated receptor desensitization time constants from 3.8 +/- 0.3 s to 0.8 +/- 0.1 s. However, LC1 did not significantly alter agonist binding affinity and single-channel conductance of 5-HT(3A) receptors. On the other hand, application of specific LC1 antisense oligonucleotides and nocodazole, a microtubule disruptor, significantly prolonged the desensitization time of the recombinant and native neuronal 5-HT(3) receptors by 3- to 6-fold. This kinetic change induced by nocodazole was completely rescued by addition of LC1 but not GABA(A) receptor-associated protein (GABARAP), suggesting that LC1 can specifically interact with 5-HT(3A) receptors. These observations suggest that the LC1-5-HT(3A) receptor interaction contributes to a mechanism that regulates receptor desensitization kinetics. Such dynamic regulation may play a role in reshaping the efficacy of 5-HT(3) receptor-mediated synaptic transmission.

Stoichiometric analysis of the TM2 6' phenylalanine mutation on desensitization in alpha1beta2 and alpha1beta2gamma2 GABA A receptors

The presence of phenylalanine (F) at the 6' position of transmembrane domain 2 (TM2) in the alpha4 subunit of alpha4beta2 nicotinic receptors enhances desensitization. As the GABA A receptor affords the ability to study the influence of as few as one and as many as five Fs at this position, we have used it to investigate potential subunit- and stoichiometry-dependent effects of the TM2 6'F mutation on desensitization. Whereas the presence of one F at this position decreased extent of desensitization, desensitization was increased in all configurations that included two or more Fs at the TM2 6' position; desensitization was particularly rapid with 3 or 4 F residues present. Our results demonstrate the ability of F residues at the TM2 6' position to modulate desensitization is likely conserved in the cys-loop family of ligand-gated ion channels. Moreover, our findings demonstrate both stoichiometric- and subunit-dependent effects of the ability of this mutation to regulate desensitization in GABA A receptors.

Kinetics and functional characterization of the glycine receptor alpha2 and alpha3 subunit

In recent studies one serine residue (Ser-346) within the protein-kinase-A (R-E-S-R) consensus sequence of the GlyRalpha3 intracellular loop has proven to be an essential target for prostaglandin-E(2)-mediated phosphorylation, which further modulates spinal nociceptive transmission and central inflammatory pain sensitization. In the present study we investigated the effect of Ser-346 phosphorylation and Ser-346 mutation on receptor kinetics and function using whole-cell patch-clamp recordings in transfected HEK 293 T cells. We compared biophysical properties of wild type GlyRalpha3 and two site-directed mutants, where Ser-346 was replaced by alanine or aspartate, in the absence and presence of prostaglandin-E(2). The mutation to alanine was accompanied by significantly altered dose-response and desensitization properties. Mutation to aspartate had only minor effects on receptor kinetics and function. Phosphorylation of Ser-346 slowed desensitization and decreased glycinergic currents in GlyRalpha3/mEP2 transfected cells. In addition, we demonstrated that prostaglandin-E(2) also had an effect on the GlyRalpha2 subunit. Exposure to prostaglandin-E(2) decreased the maximum peak current amplitude of glycinergic currents in GlyRalpha2/mEP2 transfected cells in the same manner as phosphorylation of the GlyRalpha3 subunit. It led to a significant increase of the desensitization time constants and thus significantly affected the desensitization behaviour. These results indicate that the GlyRalpha2 and the GlyRalpha3 subunits act as important subunits for the modulation of glycine receptor kinetics and function.

Human glycine α1 receptor inhibition by quercetin is abolished or inversed by α267 mutations in transmembrane domain 2

Quercetin, one of the flavonoids, is a compound of low molecular weight found in fruits and vegetables. Besides its antioxidative effect, quercetin also shows a wide range of diverse neuropharmacological actions. However, the cellular mechanisms of quercetin's actions, especially on ligand-gated ion channels and synaptic transmissions, are not well studied. We investigated the effect of quercetin on the human glycine alpha1 receptor channel expressed in Xenopus oocytes using a two-electrode voltage clamp technique. Application of quercetin reversibly inhibited glycine-induced current (I(Gly)). Quercetin's inhibition depends on its dose, with an IC(50) of 21.5+/-.2 microM. The inhibition was sensitive to membrane voltages. Site-directed mutations of S267 to S267Y but not S267A, S267F, S267G, S267K, S267L and S267T at transmembrane domain 2 (TM2) nearly abolished quercetin-induced inhibition of I(Gly). In contrast, in site-directed mutant receptors such as S267 to S267I, S267R and S267V, quercetin enhanced I(Gly) compared to the wild-type receptor. The EC(50) was 22.6+/-1.4, 25.5+/-4.2, and 14.5+/-3.1 microM for S267I, S267R and S267V, respectively. These results indicate that quercetin might regulate the human glycine alpha(1) receptor via interaction with amino acid residue alpha267 and that alpha267 plays a key role in determining the regulatory consequences of the human glycine alpha1 receptor by quercetin.

Homology modeling and molecular dynamics simulations of the glycine receptor ligand binding domain

We present a homology based model of the ligand binding domain (LBD) of the homopentameric alpha1 glycine receptor (GlyR). The model is based on multiple sequence alignment with other members of the nicotinicoid ligand gated ion channel superfamily and two homologous acetylcholine binding proteins (AChBP) from the freshwater (Lymnaea stagnalis) and saltwater (Aplysia californica) snails with known high resolution structure. Using two template proteins with known structure to model three dimensional structure of a target protein is especially advantageous for sequences with low homology as in the case presented in this paper. The final model was cross-validated by critical evaluation of experimental and published mutagenesis, functional and other biochemical studies. In addition, a complex structure with strychnine antagonist in the putative binding site is proposed based on docking simulation using Autodock program. Molecular dynamics (MD) simulations with simulated annealing protocol are reported on the proposed LBD of GlyR, which is stable in 5 ns simulation in water, as well as for a deformed LBD structure modeled on the corresponding domain determined in low-resolution cryomicroscopy structure of the alpha subunit of the full-length acetylcholine receptor (AChR). Our simulations demonstrate that the beta-sandwich central core of the protein monomer is fairly rigid in the simulations and resistant to deformations in water.

The Evolution of GABAA Receptor–Like Genes

The inhibitory ligand-gated ion channel family of receptors, including the type A gamma-aminobutryic acid (GABA(A)) and glycine receptors, mediates inhibitory neurotransmissions in the central nervous system. In this study, GABA receptor (GABR) evolution was explored through comparative genomics using genomes that span divergent lineages. GABA(A)/Gly receptor-like (GRL) gene sequences were retrieved from the genomes of various species ranging from mammal to fish to worm and subjected to cross-species comparison. All vertebrate GRL gene sets in the study but no invertebrate ones exhibit the extensive and conserved pattern of gene clustering that is characteristic of human GABR genes, indicating that the gene clusters were established early in vertebrate evolution, after divergence from the invertebrates. Moreover, the vertebrate gene structure is highly conserved with a basic 9-coding exon structure, whereas, as well as being diverse in copy numbers and chromosomal loci, the invertebrate GRL genes display a variety of gene structures. Remarkably, the invertebrates each possess a unique GRL gene pair that lies in neighboring loci within their respective genomes: zc482.5 and zc482.1 in roundworm, CG8916 and CG17336 in fruitfly, Ci4249 and Ci4254 in Ciona, and these were revealed by phylogenetic analysis to be homologous to human GABR alpha and beta subunits, respectively. The phylogenetic classification of these genes is also corroborated by experimental ligand-binding measurements using recombinant gene products. Furthermore, the 3 invertebrate gene pairs harbor characteristic key residues and exhibit similarities in intron positions to their vertebrate counterparts. The results strongly indicate that such a gene pair originally existed in the bilaterian ancestor from which all 3 phyla evolved and suggest that the extant GABR clusters arose from an ancestral alpha-beta subunit gene pair gave rise to the extant GABR clusters.

Nmf11 is a novel ENU-induced mutation in the mouse glycine receptor alpha 1 subunit

Nmf11 is an N-ethyl-N-nitrosourea-induced recessive mouse mutation. In this article we show that the mutation is in the gene that encodes the glycine receptor alpha 1 subunit (Glra1). The new Glra1 mutation appears to affect glycine's inhibitory neurotransmission in the central nervous system (CNS) of the nmf11 homozygotes, which suffer from a severe startle disease-related phenotype and die by postnatal day 21. The nmf11 mutation involves a C-to-A transition of nucleotide 518, which results in the N46K substitution in the long extracellular NH(2) terminal or ligand-binding domain of the GLRA1 mature protein. The mutation does not result in reduced expression of GLRA1 at the mRNA or protein levels and the mutant glycine receptor localizes properly in synaptic sites of nmf11 homozygotes.

Glycinergic transmission

Inhibition in the mature central nervous system is mediated by activation of gamma-aminobutyric acid (GABA(A)) and glycine receptors. Both receptors belong to the same superfamily of ligand-gated ion channels and share common transmembrane topology and structural and functional features. Glycine receptors are pentameric ligand-gated anion channels composed of two different subunits, named alpha und beta, that assemble with a fixed stoichiometric ratio of two alpha to three beta subunits. Four genes encoding the alpha subunits exist, whereas only one gene encoding the beta subunit has been detected. Ligand binding occurs at the interface of alpha and beta subunits. The beta subunit, which is unable to form homo-oligomeric receptors, is responsible for assembly and channel properties. Moreover, this subunit carries a binding motif for the cytoplasmic protein gephyrin, which is believed to mediate synaptic clustering and anchoring at inhibitory synapses by interacting with the subsynaptic cytoskeleton. Synaptic gephyrin appears to restrict the mobility of glycine receptors diffusing in the plane of the plasma membrane, thereby generating dynamic plasma membrane domains contributing to the plasticity of inhibitory synapses. Glycine receptors are well established as playing important roles in controlling motor functions and sensory signaling in vision and audition and those in the dorsal horn of the spinal cord are now considered to be new targets for pain therapies. Like GABA(A) receptors, glycine receptors have been shown to be depolarizing during development. The functional meaning of the developmental switch from excitatory to inhibitory glycine receptor action remains to be elucidated.

Metal bridges between the PhoQ sensor domain and the membrane regulate transmembrane signaling

Bacterial histidine kinases respond to environmental stimuli by transducing a signal from an extracytosolic sensor domain to a cytosolic catalytic domain. Among them, PhoQ promotes bacterial virulence and is tightly repressed by the divalent cations such as calcium and magnesium. We have determined the crystal structure of the PhoQ sensor domain from Salmonella typhimurium in the Ca2+-bound state, which reveals a highly negatively charged surface that is in close proximity to the inner membrane. This acidic surface binds at least three Ca2+, which mediate the PhoQ-membrane interaction. Mutagenesis analysis indicates that structural integrity at the membrane proximal region of the PhoQ sensor domain promotes metal-mediated repression. We propose that depletion or displacement of divalent cations leads to charge repulsion between PhoQ and the membrane, which initiates transmembrane signaling through a change in orientation between the PhoQ sensor domain and membrane. Therefore, both PhoQ and the membrane are required for extracytosolic sensing and transmembrane signaling.

Pore conformations and gating mechanism of a Cys-loop receptor

Neurons regulate the propagation of chemoelectric signals throughout the nervous system by opening and closing ion channels, a process known as gating. Here, histidine-based metal-binding sites were engineered along the intrinsic pore of a chimeric Cys-loop receptor to probe state-dependent Zn(2+)-channel interactions. Patterns of Zn(2+) ion binding within the pore reveal that, in the closed state, the five pore-lining segments adopt an oblique orientation relative to the axis of ion conduction and constrict into a physical gate at their intracellular end. The interactions of Zn(2+) with the open state indicate that the five pore-lining segments should rigidly tilt to enable the movement of their intracellular ends away from the axis of ion conduction, so as to open the constriction (i.e., the gate). Alignment of the functional results with the 3D structure of an acetylcholine receptor allowed us to generate structural models accounting for the closed and open pore conformations and for a gating mechanism of a Cys-loop receptor.

Interaction of topiramate with glycine receptor channels

Glycine receptor channels are pentameric ligand-gated ion channels that respond to the application of inhibitory neurotransmitters by opening of a chloride-selective central pore. Topiramate (TPM) is a broad-spectrum antiepileptic drug used as add-on or monotherapy for focal seizures. In the present study the interaction of TPM with glycine receptor channels was studied on outside-out patches from HEK293 cells expressing alpha1beta glycine receptor channels. The patch clamp techniques combined with ultra fast solution exchange enabled us to investigate the kinetics of receptor channels in presence of TPM. Our study showed no agonistic or potentiating effect for TPM on glycine receptor channels. However, in presence of 1 mM glycine + 1 mM TPM, the desensitization got faster and the peak current amplitude decreased. After the end of glycine + TPM pulses, off-currents occurred, suggestive for a specific channel block mechanism.

Two different modes of action of pentobarbital at glycine receptor channels

Glycine receptor channels are pentameric ligand-gated ion channels which respond to the binding of inhibitory transmitters by opening of a chloride-selective central pore. Pentobarbital is widely used as an anticonvulsive, hypnotic and anaesthetic drug. In the present study, the interaction between pentobarbital and glycine receptor channels was studied on outside-out patches of human embryonic kidney (HEK) 293 cells expressing alpha(1)beta glycine receptor channels. Currents elicited by 0.03 mM glycine were enhanced by pentobarbital showing potentiation of alpha(1)beta glycine receptor channels. In the presence of 1 mM glycine+pentobarbital (1 and 3 mM), desensitization was faster and the peak current amplitude decreased. After the end of glycine+pentobarbital pulses, off-currents occurred suggestive for a channel block mechanism. Pentobarbital had no agonistic effects at glycine receptor channels.